Structural transformation of layered double hydroxides: an in situ TEM analysis

Structural transformation of layered double hydroxides: an in situ TEM analysis www.nature.com/npj2dmaterials https://doi.org/10.1038/10.1038/XXX Corrected: Author correction ARTICLE OPEN Structural transformation of layered double hydroxides: an in situ TEM analysis 1,2,3 2,4 2,3 2,3 5 5 1,3 Christopher Hobbs , Sonia Jaskaniec , Eoin K. McCarthy , Clive Downing , Konrad Opelt , Konrad Güth , Aleksey Shmeliov , 6 7,8 2,4 Maurice C. D. Mourad , Karl Mandel and Valeria Nicolosi A comprehensive nanoscale understanding of layered double hydroxide (LDH) thermal evolution is critical for their current and future applications as catalysts, flame retardants and oxygen evolution performers. In this report, we applied in situ transmission electron microscopy (TEM) to extensively characterise the thermal progressions of nickel-iron containing (Ni-Fe) LDH nanomaterials. The combinative approach of TEM and selected area electron diffraction (SAED) yielded both a morphological and crystallographic understanding of such processes. As the Ni-Fe LDH nanomaterials are heated in situ, an amorphization occurred at 250 °C, followed by a transition to a heterogeneous structure of NiO particles embedded throughout a NiFe O matrix at 850 °C, confirmed by high- 2 4 resolution TEM and scanning TEM. Further electron microscopy characterisation methodologies of energy-filtered TEM were utilised to directly observe these mechanistic behaviours in real time, showing an evolution and nucleation to an array of spherical NiO nanoparticles on the platelet surfaces. The versatility of this characterisation approach was verified by the analogous behaviours of Ni-Fe LDH materials heated ex situ as well as parallel in situ TEM and SAED comparisons to that of an akin magnesium-aluminium containing (Mg-Al) LDH structure. The in situ TEM work hereby discussed allows for a state-of-the-art understanding of the Ni-Fe material thermal evolution. This is an important first, which reveals pivotal information, especially when considering LDH applications as catalysts and flame retardants. npj 2D Materials and Applications (2018) 2:4 ; doi:10.1038/s41699-018-0048-4 INTRODUCTION areas, greater active sites and ability to interact with various catalytic supports. In fact, the calcination of LDH materials in In recent years, two-dimensional (2D) nanomaterials have been general has received an abundance of attention from both an described as strongholds across the fields of nanotechnology with 20–22 applications and scientific perspective. The associated extensive applications in electronics, catalysis, drug delivery, mechanisms of this thermal degradation have also been well photonics or magnetics. Layered double hydroxides (LDHs) are established and described via a complex procedure of dehydra- a particular class of 2D materials. Described as a member of the tion, dehydroxylisation and decarbonisation of the lamellar anionic clay family, LDHs are composed of ‘brucite-like’ cationic 23,24 25 26 material. While factors such as composition, morphology, layers where an inclusion of trivalent cations introduces an overall structure and atmospheric conditions can affect these calcina- positive charge to the nanosheets. Charge compensating anions tion behaviours, many studies have utilised the calcination are located in the interlayer galleries leading to the generalised 2+ 3+ x+ n− 2+ 3 procedures for applications in areas such as oxygen evolution LDH formula, [M M (OH) ] [A ] ·mH O, where M /M 1−x x 2 x/n 2 + n 2 and catalytic supports. In particular, it is the generation of metal = divalent/trivalent metal cation and A = interlayer. A repre- oxides and spinel structure composites derived from LDH sentation of a general LDH structure is displayed in Fig. 1. calcinations that contribute to the enhancement of catalytic LDHs are versatile materials with high industrial and academic 3 4 properties across several catalysis-related fields including nano- interest, having applications in drug delivery, water oxidation, 28,29 30 31 5 6 7 8 catalyst design, hydrogenation and photocatalysis. More- catalysis, supercapacitors, gas absorbents, nanocomposites, over, the application of LDH materials in flame retardant and transistors, . Two exciting members of the LDH family are composites has attracted interest in recent times. The incorpora- magnesium-aluminium (Mg-Al) LDH and nickel-iron (Ni-Fe) LDH. tion of the lamellar structures into polymer materials has shown The former has previously been applied in flame retardant 10,11 12 13 beneficial results of thermal stability, melting temperatures and studies, polymer composites and catalyst supports. The smoke suppression as well as an attenuation of peak heat release latter also exhibits broad applicability in flame retardancy, 15 16 17 18 32 rate in a number of cases. Evidently, the thermal decomposition sensors, electrocatalysis, water oxidation, oxygen evolution and energy storage. The application of Ni-Fe LDH in these fields of the LDH structures plays an important role in both catalytic is largely owed to their inherent material properties such as performance and flame retardant properties of LDH nanocompo- simplistic methods of fabrication, low cost, large specific surface sites. Despite this extensive work throughout such areas of 1 2 School of Physics, Trinity College Dublin (TCD), Dublin, Ireland; Advanced Materials and Bioengineering Research Centre (AMBER) and Centre for Research of Adaptive Nanostructures and Nanodevices (CRANN) Trinity College Dublin (TCD), Dublin, Ireland; The Advanced Microscopy Laboratory, CRANN Trinity College Dublin (TCD), Dublin, 4 5 Ireland; School of Chemistry Trinity College Dublin (TCD), Dublin, Ireland; Fraunhofer-Project Group Materials Recycling and Resource Strategies IWKS 63457 Hanau, Germany; 6 7 8 Department of Materials Solutions, TNO, Eindhoven, The Netherlands; Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Würzburg, Germany and Chair of Chemical Technology of Materials Synthesis, Department Chemistry and Pharmacy, Julius-Maximilians University Würzburg, Röntgenring 11, 97070 Würzburg, Germany Correspondence: Valeria Nicolosi (nicolov@tcd.ie) Received: 30 May 2017 Revised: 14 December 2017 Accepted: 15 January 2018 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. molecules. The sharp absorption band at approximately −1 2− 1350 cm is ascribed to the stretching modes of CO , originating from the intercalated carbonates. There is also evidence of C-N stretching modes coming from tertiary amines at −1 approximately 1,155 and 1,040 cm absorption bands in the FTIR −1 spectrum. Moreover, absorption bands below 1,000 cm are attributed to lattice stretching modes of metal-O sites. TGA analysis was also conducted on both LDH compositions (Figure S1b). Firstly, there is a significant mass loss observed (10%) in the 75–150 °C range and is attributed to the dehydration of surface adsorbed and interstitial water. Further mass losses (approxi- Fig. 1 Schematic representations of the as-synthesised Ni-Fe LDH mately 20%) in the 200–800 °C temperature range is also structure, viewed along the½ 001 and½ 010 directions. (Structures were visualised using CrystalMaker™ software.) Cationic layers observed for both compositions, and is related to the dehydrox- composed of Ni and Fe octahedrally surrounded by hydroxides ylation and decomposition of the counter ions and brucite-like are charge compensated by anionic interlayer moieties such as layers themselves. AAS analysis demonstrated a Ni:Fe molar 3− carbonate (CO ), water (H O) and triethanolamine (C H NO ) 2 2 6 15 3 ratio of 3.54:1. In situ and ex situ heating TEM experiments were comparatively carried out in parallel to fully understand the development of LDH research, the majority of these works have largely relied on thermally induced calcination, as well as the various crystal- macroscopic characterisation techniques such as X-ray diffraction lographic transitions occurring before that. For this purpose, the (XRD), Fourier transform infrared spectroscopy (FTIR) and thermo- as-synthesised LDH samples were subjected to identical heating 8,27,33–35 gravimetric analysis (TGA). In addition to these character- ramp conditions both in ex situ and in situ experimental set-ups isation techniques, pre-mortem and post-mortem electron (Figure S2). Further details can be found in the Methods section. microscopy can provide important information. However, none This parallel set-up determined if the behaviours of LDH of these pre-mortem and post-mortem methods can be used to transformations were independent of the different in situ and elucidate on how to optimise the processes that occur under real- ex situ experimental conditions. For example, the significant time conditions at single nanoparticle level. A detailed in situ TEM pressure variations as the samples were heated may have characterisation, observing thermal transformations at individual impacted the thermal behaviours of the LDH nanomaterials. More nanoparticle level, has yet to be established. Determining the importantly, there can be an appreciable contribution of radiation correct structure–property–function relationships requires a beam damage (in the form of radiolysis or knock-on) or Joule detailed description of the material in its working state. Recent sample heating when the electron beam interacts with the technological advances in EM allow us to image materials with a sample. In view of these possible issues, full TEM characterisation range of in situ techniques that have been developed to follow of the various LDH samples were performed pre and post ex situ the evolution of materials in the presence of such external stimuli. heating experiments, as well as during in situ analysis; this In addition to observing materials in their working state, in situ comparison was of crucial importance in order to rule out any techniques can also capture important intermediate transitional eventual electron beam-induced transition. forms that may be involved in phase transformations. As such, We investigated if the LDH degradation processes are previous research acknowledges that these exact transformations dependent on both the composition and platelet size of the in which the LDH decomposes are not fully understood. Utilising material. As such, the structural transformation of a Ni-Fe LDH such in situ techniques help to answer the underlying questions material was studied via the application of in situ heating TEM. Fig. regarding the thermal transformations of the LDH material from a 2 displays the BFTEM and associated SAED patterns at various nanoscale perspective. Ultimately, these behaviours can be stages of the in situ heating experiments for the Ni-Fe LDH monitored and analysed in real time, providing a further under- material. Initially, the material was synthesised to have a well- standing and optimisation of the related processes. Here, we defined hexagonal morphology with a lateral dimension on the report on the application of state-of-the-art in situ transmission order of microns. This starting material was found to have an LDH electron microscopy to reveal the processes by which LDHs crystallographic structure, with the associated SAED pattern thermally decompose and to characterise the stages at which revealing the (101), 011 ,112 and (110) crystallographic planes these morphological and crystallographic alterations occur. Due to (Fig. 2e). Upon heating, both a morphological and crystallographic their broad application and well-established calcination processes change was evidenced. As the sample reached 250 °C, the loss of in the previous literature, we elected to focus our attention on Mg- the (101), 011 and 112 reflections indicates a collapse in the Al LDH and Ni-Fe LDH for in situ TEM analysis. Undoubtedly, there crystal structure and a reduction in the non-basal crystallographic is an urgent need to fully characterise and understand these phases while still retaining its hexagonal morphology. The loss of materials from a single particle perspective, given their prospec- such planes is believed to be due to the dehydration of the tive nanoscale applications in flame retardancy, energy storage interstitial galleries. Moreover, the contrast variations of BFTEM and catalysis. images (Fig. 2a, b) indicates a surface alteration at this temperature, possibly as a consequence of the additional loss of water both from the surface of the material and the interlayer RESULTS AND DISCUSSION galleries. The irradiation of the electron beam perhaps also In both Mg-Al LDH and Ni-Fe LDH cases, a molar ratio of the contributed to the dehydration of the LDH materials; however, this cationic sites of 3 was selected as a representative of these LDH is difficult to avoid. As the Ni-Fe LDH was heated to 450 °C, the compositions due to its well-established behaviours and previous well-defined hexagonal morphology is retained (Fig. 2c). However, 36–40 successful applications. The as-synthesised materials were the evolution of spherical particles, <50 nm in size, were seen to first characterised by XRD, FTIR, TGA and atomic absorption generate and randomly distribute themselves onto the hexagonal spectroscopy (AAS). The FTIR spectrum of the precursor Ni-Fe and parent material (Fig. 2c). There appears to be minimal crystal- Mg-Al LDH samples is shown in Figure S1a. The broad bands at lographic tranformations from 250 °C to 450 °C (Fig. 2f, g). This is −1 approximately 3,400 and 1,645 cm are due to the various types indicated by the conservation of the (110) LDH plane. As the of O-H bonding in both materials such as the hydroxyl groups on sample was calcined to 850 °C, there is clear evidence of a further the brucite-like layers and the adsorbed/inter-gallery water generation of these particles (Fig. 2d). These nucleations become npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; Structural transformation of layered... C Hobbs et al. Fig. 2 BFTEM and associated SAED patterns from various stages of the in situ heating experiments of the Ni-Fe LDH. a–d BFTEM and e–h SAED patterns corresponding to column temperature, end of the 250 °C step, end of the 450 °C step and end of the 850 °C step, respectively. −1 Scale bars are a–d 200 nm and e–h 2nm , respectively more numerous, increase in size and randomly distribute environments and, in turn, would have impacted the true themselves across the hexagonal LDH platelet (Fig. 2d). The SAED reflection of the Ni-Fe LDH calcination behaviours. pattern of the calcined sample at 850 °C shows a crystal structure EFTEM revealed the distribution of Ni and Fe throughout the alteration (Fig. 2h). In contrast to the patterns recorded at 250 °C calcined products (Fig. 3a). In contrast to the Ni evolution, Fe was found to have a more regular distribution across the platelet, with and 450 °C, there is a generation of new reflections in the a localisation in the centre of the calcined material. Larger NiO diffraction pattern, corresponding to newly arranged crystal- particles tended to form on the central sections of the platelets, lographic planes. The evolution of such particles and a variance in with smaller NiO crystallites arranged on the platelets edge the diffraction patterns during the calcination procedures regions. This may have been due to the non-uniform local confirms a rearrangement of the LDH structures into a new crystallographic environments of the cationic sites, whereby the crystallographic phase. It is believed that this is a topotactic different environments at the edge regions of the platelets transformation caused by the in situ heating effects. Further affected the growth of both NiFe O trevorite and NiO oxide 2 4 analysis of the SAED patterns recorded at 850 °C from in situ TEM nanoparticles. Moreover, it is perceived that the catalytic activity at experiments (Fig. 2h) portrays the emergence of the {200} and these edge regions were not only influenced by Fe sites but {220} family of planes of the NiO and trevorite structures, −1 were also influenced by smaller sized NiO particles. We also used respectively. The reflections occurring at 13.1 nm may also the EFTEM technique to directly visualise the Ni-Fe thermal derive from the parent LDH structure. This suggests that certain transformations as they took place (Movie V1 Supplementary LDH planes may be retained even after thermal degradation Information). Our experimental set-up utilised signals from the procedures. In this regard, the SAED findings revealed an excitations of the electrons in the Ni 2p orbital (855 eV), that is, 3/2 evolution to a mixed phase of LDH, metal oxides and trevorite the Ni L EELS edge, to form the intensities in such video structures (Fig. 2e–h). 45 representations. As the platelets were subject to 850 °C, the To further understand the characteristics of the calcined Ni-Fe nucleation of Ni containing particles was clearly evidenced. structures, we post-analysed the Ni-Fe LDH samples using energy- Particles initially evolved and migrated in a random fashion on filtered TEM (EFTEM) and high-resolution TEM (HRTEM) (Figs. 3 and the platelet surfaces. Further exposure at this temperature led to a 4, respectively). Further details of the EFTEM technique can be sudden transformation where the remaining material transformed found in the Methods section. EFTEM was favoured over scanning to an array of smaller similar Ni containing particles. This TEM (STEM) for acquiring spectroscopic data in situ such as somewhat restricted size is perhaps due to the limited local energy-dispersive x-ray (EDX) and electron energy loss spectro- availability of Ni cations after the initial transformations occurred. scopy (EELS) to reduce the risk of inducing further transformations Complimentary to the EFTEM data, an elemental distribution of due to the beam-sample interaction and not as a consequence of the calcined Ni-Fe LDH material was further analysed in STEM. An applied thermal environments. Also, the application of EFTEM associated EDX elemental map confirmed that the spherical allowed for the direct analysis of the Ni and Fe from the whole nanoparticles were largely composed of nickel (green signal) platelet via the acquisition of two EFTEM maps (Fig. 3a). This (Figure S6B). The effect of the background was corrected for by application was preferred over STEM-EELS methods as a longer the subtraction of the average intensity value in the related Fe timescale for spectroscopic acquistions is required for such LDH map (red signal) (Figure S6B). It is also noted that small amounts of platelet regions. The STEM approach may have also introduced Fe may have been present either within or in the vicinity of the issues of drift and potential alterations of crystallographic particle, suggested by the feint red intensities across the Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. Fig. 3 a Energy-filtered TEM image of Ni-Fe LDH sample post-heating. Ni and Fe signals are represented by the colours green and red, respectively. b Scanning transmission electron microscopy image of the same platelet. Areas 1–4 indicated in b represent regions from which EDX spectra were acquired. Scale bar in a is 200 nm elemental map. Moreover, the low amounts of Fe was illustrated resolved the lattice spacings attributed to the {200} planes of the by the recorded EDX sum spectra from areas 1 to 3 across the NiO structure with an orientation along the [023] axis, hence particle (Figure S6C and D). Although this may have suggested confirming the evolution of metal oxide particles as a result of the that there is an existence of Fe migration into the NiO particles, Ni-Fe LDH calcination. However, there was a discrepancy in the the Fe-K EDX line peak at 6.4 keV was not statistically significant correlation of the STEM nanoprobe diffraction and HRTEM of across the particle and was deemed to be undetectable above similar particle regions, whereby the 111 family of planes found background counts. Hence, the Fe signals are speculated to be in the STEM nanoprobe acquisitions did not fully coincide with the due to EDX background counts. FFT analysis of our HRTEM data (Fig. 4c) and Supporting Furthermore, we employed electron diffraction in STEM mode Information S3f. We perceive this to be due to the evolution of to investigate the variation in the orientation of the metal oxides a non-stoichiometric NiO compound as opposed to a pure NiO (Supporting Information Figure S3). This overcame the drawback phase. The existence of non-stochiometric NiOs has been of SAED techniques in TEM, where the dimensions of the selected previously investigated by Da Rocha and Rougier. area apertures limit the recording of diffraction patterns from As aforementioned, the parallel TEM analysis of samples heated regions of minimum 100 nm in size. Whereas the application of ex situ investigated the applicability of our in situ EM methodol- electron diffraction using a STEM electron probe permitted the ogies as well as assessed the effect of the electron beam on the characterisation of features of approximately 10 nm, that is, the LDH thermal evolution. Figure 5 presents the TEM and SAED size range of the products of calcination procedures. In addition, studies applied to Ni-Fe LDH samples heated with ex situ the application of electron diffraction studies in STEM allowed us experimental conditions. to investigate the crystallographic orientations across the The samples heated to 250 °C ex situ presented an irregular calcined Ni-Fe structures. The variations in the recorded patterns platelet morphology (Fig. 5a). This evidently contrasts the show that the NiO particles assembled with random orientations precursor LDH platelets as well as the LDH materials heated to relative to the basal plane of the LDH platelet (Figure S3d and f). 250 °C using in situ methods (Fig. 2a, b, respectively). The This is also viewed in the NiFe O trevorite regions of the calcined corresponding SAED pattern showed that these irregular sheet- 2 4 structure (Figure S3c and e). It is deduced that the products of like materials display an LDH crystallographic structure, demon- mixed metal oxides derived from the Ni-Fe LDH calcinations strated by the {101} and {110} planes (Fig. 5d). Similarly, these arrange themselves randomly in a topotactic fashion throughout crystallographic planes were evidenced as the sample was heated the parent material, but confine themselves to within the platelet to 450 °C using ex situ methods (Fig. 5e). The BFTEM findings (Fig. domains. 5b) are also consistent with in situ studies at this temperature, in Energy-dispersive x-ray spectroscopy in STEM quantitatively particular the irregular morphology and formation of particles on assessed the calcined products of the Ni-Fe degradations. While the platelets surface (Fig. 5b). The morphology demonstrated an the Fe content appeared to be ubiquitously distributed, STEM-EDX arrangement of smaller crystallite regions at 850 °C (Fig. 5c) with revealed that there is a notable higher Ni content on the thermally similar crystallographic features (Fig. 5f). These observations of evolved spherical particles (Fig. 3b and Table 1 (Supplementary ex situ heated materials may also be perceived as a reconstructed Information)). This yielded additional evidence that the evolved LDH due to its exposure to aqueous environments, that is, the LDH particles are that of NiO, and also correlated well with the data ‘memory effect’. Moreover, the effect of rehydration due to acquired by EFTEM methods. exposure to air conditions can also be viewed from the analysis of An investigation by HRTEM showed the presence of the NiFe O the in situ samples, 40 days after experimentation (Figure S7). The 2 4 trevorite lattice structure surrounding the spherical particles hexagonal symmetry in the associated SAED patterns highlighted resulting in a heterogeneous structure (Fig. 4e). FFT analysis of a partial revert to the LDH structure (Fig. 5f and Figure S7B). the associated regions (Fig. 4g) confirmed the existence of the However, the existence of trevorite phases remain, as shown by {220} and {242} family of planes (indicated by red indices in Fig. HRTEM analysis in both ex situ and in situ experiments (Figure 4g), where the crystallite region had a ½111 orientation. This can S10B and Fig. 4, respectively). Combinative EFTEM images of the also be corroborated to the Fe signal as found by EFTEM methods ex situ heated sample up to 850 °C highlighted an even (Fig. 3a). Similar analysis of the spherical nanoparticles directly distribution of Ni and Fe throughout the material. This was npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. Fig. 4 a, e HRTEM images of various regions after in situ TEM heating experiments. b, f Zoomed in images of square regions as indicated in a and e. c, g Calculated FFT from square red region as indicated on the TEM micrograph in a and e. Blue and red annotations (c and g) represent labelled NiO and trevorite crystallographic planes, respectively. d, h Schematic representations of the respective crystalline structures, viewed along the [023] (d) and ½111 (h) directions. Scale bars are 10 nm for micrographs a and e and 5 nm for b and f. Structures d and h were TM visualised using CrystalMaker evidenced by the ubiquitous EFTEM intensities of Ni (green) and appearance of the (111), (200) and (220) of NiO occurred at 43.4°, Fe (red), respectively (Figure S9A). The non-specific location of 50.6° and 74.1°, respectively. This behaviour was also viewed via either Ni or Fe contrasts the EFTEM findings from in situ the in situ TEM experiments, where the birth of the spherical NiO experiments (Fig. 3a). This is acknowledged to be the effect of particles was observed at this temperature (Fig. 2c). At 850 °C, the rehydration of the heated LDH material due to its exposure to (111), (200) and (220) diffraction peaks of NiO became more moist air conditions as a consequence of sample preparation, pronounced. This is believed to be due to the growth and where the Ni and Fe sites could revert back to an LDH structure. A nucleation of further NiO particles, as observed in our in situ TEM potential reconstruction to an LDH structure was also verified by experiments (Fig. 2d). It is also possible that these diffraction peaks STEM-EDX, demonstrating a greater Ni content in the analysed could be attributed to (222), (400) and (440) of the NiFe O phase, 2 4 region (areas 3 and 4 in Figure S8B and Table 2 in Supporting but this is difficult to distinguish due to their ambiguity with the Information). NiO peaks. Parallel ex situ XRD experiments were also carried out Despite maintaining the focus of interest on the nanoscale on the Ni-Fe LDH samples. At room temperature, the XRD peaks at features responsible for the thermal evolution of these LDH 2θ angles 5.3°, 10.32°, 15.7°, 17.6° and 20.7° corresponded to the samples, it is acknowledged that the above findings may not have (003), (006), (101), (104) and (018) planes of the Ni-Fe LDH reflected their bulk material counterparts. Hence, we compared crystallographic planes (Figure S11B(a)). Similar traits relative to these behaviours via the application of in situ and ex situ XRD in situ XRD approaches were evidenced at 250 °C. The loss of the analysis (Figure S11A and B, respectively). Powder samples were (006), (101), (104) and (018) peaks indicate a breakdown in the Ni- measured at intermediate temperature stages of aforementioned Fe LDH crystal structure. At this temperature, the XRD patterns heating protocols (Figure S2). In particular, XRD was strategically also showed a decreasing and broadening of the (003) peaks conducted firstly at room temperature and then samples were indicating an alteration in basal spacings (Figure S11B (b)). The heated to 250 °C for 2 h followed by 450 °C for 2 h and finally to evolution of the NiO (111), (200) and (220) planes and the NiFe O 2 4 850 °C for 2 h and post-analysed at each stage (Figure S11A(a)–(d) (220), (311), (222), (400), (511), (440), (622) and (444) planes were and Figure S11B(a)–(d), respectively). At room temperature, the evidenced at 450 °C and 850 °C, respectively (Figure S11B(c) and in situ XRD analysis revealed typical LDH (003), (006), (101), (015) (d) respectively). The evolution of these NiO phases is also and (110) crystallographic planes at 2θ angles 16°, 26.6°, 39.4°, mirrored via our in situ XRD approach. Moreover, the NiO {200} 45.7° and 70.9°, respectively. Upon thermal treatment to 250 °C, and NiFe O {220} planar families were observed both in the SAED 2 4 a lateral shift of the (003) and loss of (006) basal planes suggests patterns recorded during in situ TEM experiments (Figs. 2h and 4f, an expansion of the unit cell and a reduction in basal plane respectively). However, ambiguities were also seen in selected periodicity. This is attributed to the dehydration of the associated ex situ XRD peaks of the calcined samples as they could H O content from interlayer LDH galleries. Moreover, the retention correspond to either oxide or trevorite structures. The XRD peaks of the LDH (110) plane is observed in both in situ TEM and in situ at 2θ angles 17.0°, 19.7° and 27.9° could be hypothesised to derive XRD approaches (Fig. 2g and Figure S11A(b)), validating how from the respective NiO (111), (200) and (220) or NiFe O (222), 2 4 single nanoparticle behaviours can affect bulk material character- (400) and (440) family of planes. This has also been observed by istics. As the samples were subject to 450 °C in situ, a phase previous work. Furthermore, this indistinctness in the possible transition took place. The loss of the LDH diffraction peaks and the source of XRD peaks validates our motivation of applying the Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. Fig. 5 BFTEM and associated SAED patterns of ex situ heating experiments of the Ni-Fe LDH. a–c BFTEM and d–f SAED patterns corresponding to the end of the 250 °C step, end of the 450 °C step and end of the 850 °C step of the applied heating ramp, respectively. Green annotations −1 (d and e) represent labelled LDH crystallographic planes. Scale bars for TEM micrographs a–c and SAED patterns d–f are 200 nm and 2 nm , respectively techniques of SAED and HRTEM. The observed correlations properties. Similar features of the Mg-Al LDH degradation, in between XRD macroscopic techniques and EM analysis indicates particular the evolution of a porous matrix, are also evidenced in that the bulk features derived from the LDH nanoscale the recent independent studies of Warringham et al. In contrast transformations. Hence, our findings highlight the importance of to the Mg-Al LDH thermal degradation, the Ni-Fe LDH does not understanding the nanoscale thermal degradation properties for develop a morphological variation along the borders of the the design and fabrication of LDH-based devices. platelets (Figs. 6d and 2d, respectively). Analogously, the In light of such findings related to the Ni-Fe LDH material, we development of a porous structure of the Ni-Fe LDH is not as applied an identical experimental approach and analysis to a pronounced as the Mg-Al counterpart. The Mg-Al LDH morphol- similar Mg-Al LDH structure. This yielded a comparison of both ogy and crystalline structures are maintained as they are subject size and composition of various LDH structures with regards to to in situ heating up to 250 °C, as demonstrated by the TEM their associated thermal degradations. Figure 6 displays the TEM micrographs and electron diffraction patterns in Fig. 6b, f. An findings from in situ heating of an Mg-Al LDH material, where the intensity attenuation in the {100} and {110} diffraction spots occurs aforementioned heating ramp was imposed (Figure S2). Similar to (Fig. 6f). The loss in periodicity of these planes suggests a the Ni-Fe LDH, the material appeared to have a well-defind crystallographic rearrangement of the LDH structures and is platelet structures; however, they portrayed a width of approxi- accredited to the loss of H O from the interstitial layers and a 23,53 mately 350 nm and rounded morphology, as shown by BFTEM dehydroxlisation of the octahedral layers. Interestingly, this (Fig. 6a). Moreover, the crystallographic resemblances of these transformation was also paralleled in the Ni-Fe LDH structures at hydrotalcite platelets to the Ni-Fe compositions were seen via the this temperature (Fig. 2f). existence of well-defined {100}, {110}, {120} and {330} planar As the continuation of the in situ heating ramp regime reached families of the LDH structure (Fig. 6e). 450 °C, Mg-Al platelets demonstrated increased porosity with an An overall examination of the LDH platelets as they are subject evident rearrangement taking place, as shown in BFTEM (Fig. 6c). to the in situ heating ramp revealed an inhomogeneous contrast The overall morphology of the LDH remained intact but an and a porous structure develops within the platelets (Fig. 6d and amorphization occurred. This was evidenced by in situ SAED Movie V2 Supplementary Information). These porous structures patterns via the loss of intensity and broadening of the {100}, {110} evolved in an irregular, somewhat random, fashion on the and {120} LDH diffraction spots (Fig. 6f, g). Moreover, there are hydrotalcite materials. The borders of the platelets were not parallels in this Mg-Al LDH crystallographic transition, however, as observed to have the same behaviours, possibly due to their amorphization was found to occur at approximately 450 °C when different crystallographic environments when compared the compared to the Ni-Fe degradations, which displayed an central domains of the materials. In contrast, they developed a amorphization tendencies at 250 °C (Fig. 2f). Hence, it is believed denser frame on the outer regions of the heated material. Indeed, that the Mg-Al LDH structural transformations are less susceptible it is indicative that the composition and dimension of LDH to degradations at higher temperatures, suggesting that LDH materials has an influential effect on its thermal degradative dehydration procedures exhibit a compositional dependence. This npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. Fig. 6 BFTEM and associated SAED patterns from various stages of the in situ heating experiments of the Mg-Al LDH. a–d BFTEM and e–h SAED patterns corresponding to column temperature 20 °C, end of 250 °C step, end of 450 °C step and end of 850 °C step, respectively. Scale −1 bars are 100 nm for images a–d and are 2 nm for e–h feature was also corroborated in our TGA, where greater mass calcinations. More generally, the application of in situ techniques losses were observed in the Ni-Fe case (Figure S12). elucidated the progressions of the structural alterations of the At the 850 °C step, the emergence of new diffraction spots in nanoplatelets. We have revealed a direct observation of the the associated SAED patterns provided direct experimental nanoscale nucleation behaviours and mechanisms involved in Ni- confirmation of a crystallographic transformation from an LDH Fe LDH and Mg-Al LDH thermal degradation. material to a combination of α-Al O and MgAl O spinel This study reports the application of in situ TEM to investigate 2 3 2 4 structures (Fig. 6h). This was evidenced by the existence of the and compare the thermal degradation behaviours of LDH associated {111} and {022} reflections of the α-Al O and {422}, 2 3 materials. The application of in situ heating techniques in the {531} and {444} of MgAl O spinel. This evolution to mixed metal 2 4 TEM revealed the morphological and crystallographic integrity of oxides was paralleled in the Ni-Fe degradations at this tempera- individual LDH platelets as well as the nanoscale behaviours and 24,55 ture (Fig. 2h) and was also similar to previous work. These progression mechanisms involved in Mg-Al LDH thermal degrada- in situ methods showed that a complete decomposition did not tion. Ni-Fe and Mg-Al LDH compounds exhibited similar beha- occur via the existence of the {110}, {120} and {300} LDH planes viours but also had unique signatures in their own right. The (Fig. 6h). in situ heating of Ni-Fe LDH material resulted in the transforma- The size dependence of the LDH platelets to their calcination tion into a coexistence of Ni oxide-type particles arranged behaviours was also investigated (Figure S14). We applied our throughout a NiFe O trevorite matrix, confirmed by TEM, HRTEM 2 4 in situ heating experiment conditions to Mg-Al LDH platelets of and STEM-EDX. In particular, the nucleation processes of NiO relatively larger size (approximately 3 μm in lateral dimension). In particles was directly visualised by EFTEM methods. The calcina- comparison to the smaller Mg-Al LDH material (Fig. 6), the larger tions were also paralleled via ex situ and XRD methods, exhibiting platelets were seen to have very similar degradation procedures. identical phase transformations, verifying the observed beha- The reduction in crystallinity up to 450 °C (Figure S14E–G) and viours by TEM are due to the inherent properties of the LDH evolution of pores across the material at 850 °C (Figure S14D) materials. It was found that the calcined Mg-Al LDH evolved into compares to that of their smaller counterparts (Fig. 6). Overall, it Al O and MgAl O structures, effectively characterised by HRTEM 2 3 2 4 was seen that the degradations were not platelet size dependent, and SAED. However, the transition of the Mg-Al material occurred rather the materials composition having a more profound effect via a different mechanism, with a development of a porous matrix on such mechanistic behaviours. as opposed to the generation of spherical particles. The Our results yield important impacts in relation to the employ- application of SAED in situ effectively characterised the crystal- ment of LDHs as dye adsorbents, flame retardants and catalysts. lographic transitions involved in the degradations of both Ni-Fe For example, our findings potentially impart further information and Mg-Al of LDH materials. into the role of LDH platelets and their calcined derivatives have The applicability of our experimental methodologies was on the thermal stability, toxicity and structural morphology of confirmed by our analysis of LDH materials of a different when incorporated into polymers or resins. Moreover, the compositions, establishing a suitable in situ approach to distribution of calcined products could be of interest for the characterise the LDH thermal transformation behaviours. Ulti- design and testing of future nanocatalysts in relation to the mately, our successful nanoscale assessment of such mechanisms dispersion and confinement of catalytic sites with enhanced catalytic activity or selectivity. The impacts of our analysis may has an important bearing on their further development and further be extended to dye sorbents, possibly revealing insights as strategic designs of catalysts and flame retardants as well as many to how sorption capacities are affected by the extent of LDH additional current areas of nanotechnology. Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. METHODS microelectromechanical systems chip design with electron transparent silicon nitride windows onto which the sample is deposited. The heating of Ni-Fe LDH synthesis the chip is conducted via a metallic spiralling system with suitable contacts Ni(NO ) ·6H O, Fe(NO ) ·9H O, TEA and urea were dissolved in 80 ml of 3 2 2 3 3 2 and interpolated with an electron transparent SiN window support film, deionized water giving a final concentration of 7.5, 2.5, 10 and 17.5 mM, onto which the sample is directly deposited. Hence this set-up exposes the respectively, and stirred at room temperature for 24 h. Then, the reaction sample to thermal environments, and as such, the nanoscale properties mixture was heated to 100 °C in an oil bath for 48 h and the obtained and features of the LDH materials can be characterised as they are subject yellow precipitate was washed with deionized water several times. to specific elevated temperatures. Furthermore, the TEM set-up was carefully monitored to reduce the beam effects discussed above. As the Mg-Al LDH synthesis sample was subject to heating, the electron beam was blanked to avoid To prepare a 3:1 Mg-Al LDH with intercalated carbonate, 343 g (1.68 mol) unwarranted interactions. By doing this, we can be reassured that the MgCl ·6H O and 136 g (0.56 mol) AlCl ·6H O were dissolved in 100 ml discussed findings are indeed effect of the applied heating treatment 2 2 3 2 deionized water as reactant mixture A. A second reactant mixture B was rather than undesired beam-induced effects. EDX and EFTEM studies were prepared by dissolving 180 g (4.5 mol) NaOH pellets and 30 g (0.28 mol) carried out using a EDAX single SiLi EDX detector and GIF Tridiem energy Na CO in 100 ml deionized water. Subsequently, the reactant mixtures A 2 3 filter (Gatan, USA) with a spectrometer, respectively. Our SAED experi- and B were pumped together within 30 min with a syringe pump into a mental set-up utilised a selected area aperture to assist in measuring the beaker, containing another 150 ml deionized water, under heavy shear- relative diffraction patterns, allowing us to analyse crystallographic mixing using an ultraturrax rotor-stator-mixer. The reaction product was properties from regions much smaller than the individual hydrotalcites subsequently washed five times with deionized water via dispersing the themselves. An analysis of such materials exposed to in situ heating particles in fresh deionized water and solid–liquid separating the particles environments yielded a direct visualisation of how these elevated via gravitational sedimentation (centrifugation). Eventually, the whole temperatures affect the properties of LDH materials. In addition, the product of the washed LDH particles was redispersed in 1,500 ml of accompanying SAED methods permitted an investigation into the crystal- deionized water and heated at 80 °C for 4 days to yield well-ordered lographic features that occur at the various thermal stages in real time. This crystallites. approach was used to directly observe the thermal degradations of To synthesise large platelets of carbonate containing Mg-Al LDH with hydrotalcites and assess the nanoscale features that play a significant role Mg:Al ratio of 3, Mg(NO ) ·6H O, Al(NO ) ·9H O and urea were dissolved in 3 2 2 3 3 2 in these processes such as size, morphology and localised composition. 60 ml of deionised water giving a final concentration of 0.125, 0.042 and TEM images and SAED patterns were analysed using Digital Micrograph 1.67 M, respectively. Then, the solution was transferred to 100 ml round (Gatan Inc., California, USA). The EFTEM experimental technique extends bottom flask and immersed in an oil bath previously heated to 90 °C and from EELS in the electron microscope. Through collection and analysis of continuously stirred for 48 h under reflux. After this time, the flask was the inelastically scattered electrons transmitted through the sample, an cooled in a water bath for 30 min, the precipitate was separated by EEL spectrum can be acquired. At higher energies of these spectra, core- centrifugation (3,000 rpm/10 min) and washed three times with water. loss signals in EELS originate from the ionisation of inner shells (e.g. K, L or M) due to energy transfer from the incident electron beam, and are Experimental methods characteristic of which element they come from. By selecting certain Heating conditions of LDH materials for in situ and ex situ experiments. The energies using an energy slit width to contain such ionisation edge signals, LDH samples were initially exposed to a temperature of 30 °C for 10 min. the unwanted electrons are filtered out, resulting in images containing This was followed by a ramp to 250 °C at 10 °C/min and kept at this vital elemental information. temperature for 120 min. Subsequently, the same sample was heated to 450 °C at 10 °C/min and kept at 450 °C for 120 min. Finally, the sample was Data availability elevated to 850 °C at 10 °C/min and maintained at 850 °C for 2 h. The data related to the findings of this work are available from the corresponding author subject to reasonable request. TEM sample preparation Samples were prepared for in situ TEM experiments by suspending the synthesised LDH materials in deionized water and sonicating for 30 min. ACKNOWLEDGEMENTS Subsequently, 5 μl of the associated samples were subsequently placed We would like to acknowledge the following funding supports: SFI AMBER, SFI PIYRA, TM DENS Solutions Nano-chips for in situ experiments, respectively. Excess ERC StG 2D NanoCaps, ERC CoG 3D2DPrint and Horizon2020 NMP Co-Pilot. We would sample was wicked using filter paper and then air dried for approximately like to thank the Advanced Microscopy Laboratory at CRANN, Trinity College Dublin. 30 min each before TEM analysis. Ex situ heated Ni-Fe samples were The Microanalytical Laboratory, School of Chemistry, University College Dublin is prepared for TEM analysis by directly depositing the powder forms of the acknowledged for bulk ICP-AAS analysis. materials onto lacey Carbon TEM Cu grids (TED Pella Inc., USA). Characterisation methods AUTHOR CONTRIBUTIONS Powder XRD was used to characterise the precursor LDH bulk sample as C.H., K.M., M.C.D.M. and V.N. discussed the proposed plans of this work. S.J. and M.C.D. well as LDH samples subject to various thermal conditions. A Bruker M synthesised the associated Ni-Fe LDH and Mg-Al LDH samples, respectively. C.H. Advance Powder X-ray diffractometer (Bruker, MA, USA) operating with a and E.K.McC. performed in situ TEM characterisation experiments. C.H. and C.D. −1 molybdenum K-α emission source (λ ¼ 0:7107 Å ) was used to record the performed post in situ analysis via (S)TEM methods. C.H. performed ex situ analysis associated diffractograms. In situ XRD experiments were conducted using a using TEM methods. S.J. performed ex situ heating experiments along with XRD and PANalytical EMRYREAN XRD with θ–θ Bragg–Brentano geometry. A cobalt TGA characterisations. K.O. and K.G. performed in situ XRD experiments C.H., M.C.D.M. −1 K-α emission source (λ ¼ 1:789 Å ). To exclude any possibility of 1,2 and A.S. interpreted electron diffraction results. C.H., K.M. and V.N. discussed and reconstruction to an LDH structure due to water exposure, we performed structured the paper content and results. C.H. wrote the paper with all authors XRD using samples in powder form. contributing to preparation of the manuscript. TGA was also conducted as a measure of the mass losses when the LDH was subject to thermal environments. TGA studies were performed using a PerkinElmer Pyris 1 TGA (PerkinElmer, MA, USA). ADDITIONAL INFORMATION TEM was used to characterise the overall morphology and crystal- Supplementary information accompanies the paper on the npj 2D Materials and lographic structure of the associated LDH samples. TEM and SAED were Applications website (https://doi.org/10.1038/s41699-018-0048-4). conducted on a FEI Titan300 (FEI, Oregon, USA), operated at 300 kV. For comparative purposes of the in situ heating experiments, TEM magnifica- Competing interests: The authors declare no competing financial interests. tions and electron diffraction camera lengths were kept constant. In situ heating TEM experiments were conducted using a DENS Solutions TM Wildfire TEM holder (DENS solutions, Delft, The Netherlands). This Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims experimental in situ TEM sample holder set-up consists of a in published maps and institutional affiliations. npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. REFERENCES 27. Perez-Ramirez, J., Mul, G., Kapteijn, F. & Moulijn, J. A. In situ investigation of the thermal decomposition of Co±Al hydrotalcite in different atmospheres. J. Mater. 1. Coleman, J. N. et al. 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A activation of layered hydroxides. ACS Chem. Mater. 29, 4052–4062 (2017). 399,87–92 (2011). Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. adaptation, distribution and reproduction in any medium or format, as long as you give 53. Constantino, V. R. L. & Hnnavaia, T. J. Basic properties of Mg2+1,Al3+, layered appropriate credit to the original author(s) and the source, provide a link to the Creative double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate Commons license, and indicate if changes were made. The images or other third party anions. Inorg. Chem. 34, 883–892 (1995). material in this article are included in the article’s Creative Commons license, unless 54. Allmann, R. Magnesium aluminium carbonate hydroxide tetrahydrate: a discus- indicated otherwise in a credit line to the material. If material is not included in the sion. Am. Mineral. 53, 1057 (1968). article’s Creative Commons license and your intended use is not permitted by statutory 55. Yang, W., Kim, Y., Liu, P. K. T., Sahimi, M. & Tsotsis, T. T. A study by in situ tech- regulation or exceeds the permitted use, you will need to obtain permission directly niques of the thermal evolution of the structure of a Mg-Al-CO layered double from the copyright holder. To view a copy of this license, visit http://creativecommons. hydroxide. Chem. Eng. Sci. 57, 2945–2953 (2002). org/licenses/by/4.0/. 56. Willams, D. B. & Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science. 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www.nature.com/npj2dmaterials https://doi.org/10.1038/10.1038/XXX Corrected: Author correction ARTICLE OPEN Structural transformation of layered double hydroxides: an in situ TEM analysis 1,2,3 2,4 2,3 2,3 5 5 1,3 Christopher Hobbs , Sonia Jaskaniec , Eoin K. McCarthy , Clive Downing , Konrad Opelt , Konrad Güth , Aleksey Shmeliov , 6 7,8 2,4 Maurice C. D. Mourad , Karl Mandel and Valeria Nicolosi A comprehensive nanoscale understanding of layered double hydroxide (LDH) thermal evolution is critical for their current and future applications as catalysts, flame retardants and oxygen evolution performers. In this report, we applied in situ transmission electron microscopy (TEM) to extensively characterise the thermal progressions of nickel-iron containing (Ni-Fe) LDH nanomaterials. The combinative approach of TEM and selected area electron diffraction (SAED) yielded both a morphological and crystallographic understanding of such processes. As the Ni-Fe LDH nanomaterials are heated in situ, an amorphization occurred at 250 °C, followed by a transition to a heterogeneous structure of NiO particles embedded throughout a NiFe O matrix at 850 °C, confirmed by high- 2 4 resolution TEM and scanning TEM. Further electron microscopy characterisation methodologies of energy-filtered TEM were utilised to directly observe these mechanistic behaviours in real time, showing an evolution and nucleation to an array of spherical NiO nanoparticles on the platelet surfaces. The versatility of this characterisation approach was verified by the analogous behaviours of Ni-Fe LDH materials heated ex situ as well as parallel in situ TEM and SAED comparisons to that of an akin magnesium-aluminium containing (Mg-Al) LDH structure. The in situ TEM work hereby discussed allows for a state-of-the-art understanding of the Ni-Fe material thermal evolution. This is an important first, which reveals pivotal information, especially when considering LDH applications as catalysts and flame retardants. npj 2D Materials and Applications (2018) 2:4 ; doi:10.1038/s41699-018-0048-4 INTRODUCTION areas, greater active sites and ability to interact with various catalytic supports. In fact, the calcination of LDH materials in In recent years, two-dimensional (2D) nanomaterials have been general has received an abundance of attention from both an described as strongholds across the fields of nanotechnology with 20–22 applications and scientific perspective. The associated extensive applications in electronics, catalysis, drug delivery, mechanisms of this thermal degradation have also been well photonics or magnetics. Layered double hydroxides (LDHs) are established and described via a complex procedure of dehydra- a particular class of 2D materials. Described as a member of the tion, dehydroxylisation and decarbonisation of the lamellar anionic clay family, LDHs are composed of ‘brucite-like’ cationic 23,24 25 26 material. While factors such as composition, morphology, layers where an inclusion of trivalent cations introduces an overall structure and atmospheric conditions can affect these calcina- positive charge to the nanosheets. Charge compensating anions tion behaviours, many studies have utilised the calcination are located in the interlayer galleries leading to the generalised 2+ 3+ x+ n− 2+ 3 procedures for applications in areas such as oxygen evolution LDH formula, [M M (OH) ] [A ] ·mH O, where M /M 1−x x 2 x/n 2 + n 2 and catalytic supports. In particular, it is the generation of metal = divalent/trivalent metal cation and A = interlayer. A repre- oxides and spinel structure composites derived from LDH sentation of a general LDH structure is displayed in Fig. 1. calcinations that contribute to the enhancement of catalytic LDHs are versatile materials with high industrial and academic 3 4 properties across several catalysis-related fields including nano- interest, having applications in drug delivery, water oxidation, 28,29 30 31 5 6 7 8 catalyst design, hydrogenation and photocatalysis. More- catalysis, supercapacitors, gas absorbents, nanocomposites, over, the application of LDH materials in flame retardant and transistors, . Two exciting members of the LDH family are composites has attracted interest in recent times. The incorpora- magnesium-aluminium (Mg-Al) LDH and nickel-iron (Ni-Fe) LDH. tion of the lamellar structures into polymer materials has shown The former has previously been applied in flame retardant 10,11 12 13 beneficial results of thermal stability, melting temperatures and studies, polymer composites and catalyst supports. The smoke suppression as well as an attenuation of peak heat release latter also exhibits broad applicability in flame retardancy, 15 16 17 18 32 rate in a number of cases. Evidently, the thermal decomposition sensors, electrocatalysis, water oxidation, oxygen evolution and energy storage. The application of Ni-Fe LDH in these fields of the LDH structures plays an important role in both catalytic is largely owed to their inherent material properties such as performance and flame retardant properties of LDH nanocompo- simplistic methods of fabrication, low cost, large specific surface sites. Despite this extensive work throughout such areas of 1 2 School of Physics, Trinity College Dublin (TCD), Dublin, Ireland; Advanced Materials and Bioengineering Research Centre (AMBER) and Centre for Research of Adaptive Nanostructures and Nanodevices (CRANN) Trinity College Dublin (TCD), Dublin, Ireland; The Advanced Microscopy Laboratory, CRANN Trinity College Dublin (TCD), Dublin, 4 5 Ireland; School of Chemistry Trinity College Dublin (TCD), Dublin, Ireland; Fraunhofer-Project Group Materials Recycling and Resource Strategies IWKS 63457 Hanau, Germany; 6 7 8 Department of Materials Solutions, TNO, Eindhoven, The Netherlands; Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Würzburg, Germany and Chair of Chemical Technology of Materials Synthesis, Department Chemistry and Pharmacy, Julius-Maximilians University Würzburg, Röntgenring 11, 97070 Würzburg, Germany Correspondence: Valeria Nicolosi (nicolov@tcd.ie) Received: 30 May 2017 Revised: 14 December 2017 Accepted: 15 January 2018 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. molecules. The sharp absorption band at approximately −1 2− 1350 cm is ascribed to the stretching modes of CO , originating from the intercalated carbonates. There is also evidence of C-N stretching modes coming from tertiary amines at −1 approximately 1,155 and 1,040 cm absorption bands in the FTIR −1 spectrum. Moreover, absorption bands below 1,000 cm are attributed to lattice stretching modes of metal-O sites. TGA analysis was also conducted on both LDH compositions (Figure S1b). Firstly, there is a significant mass loss observed (10%) in the 75–150 °C range and is attributed to the dehydration of surface adsorbed and interstitial water. Further mass losses (approxi- Fig. 1 Schematic representations of the as-synthesised Ni-Fe LDH mately 20%) in the 200–800 °C temperature range is also structure, viewed along the½ 001 and½ 010 directions. (Structures were visualised using CrystalMaker™ software.) Cationic layers observed for both compositions, and is related to the dehydrox- composed of Ni and Fe octahedrally surrounded by hydroxides ylation and decomposition of the counter ions and brucite-like are charge compensated by anionic interlayer moieties such as layers themselves. AAS analysis demonstrated a Ni:Fe molar 3− carbonate (CO ), water (H O) and triethanolamine (C H NO ) 2 2 6 15 3 ratio of 3.54:1. In situ and ex situ heating TEM experiments were comparatively carried out in parallel to fully understand the development of LDH research, the majority of these works have largely relied on thermally induced calcination, as well as the various crystal- macroscopic characterisation techniques such as X-ray diffraction lographic transitions occurring before that. For this purpose, the (XRD), Fourier transform infrared spectroscopy (FTIR) and thermo- as-synthesised LDH samples were subjected to identical heating 8,27,33–35 gravimetric analysis (TGA). In addition to these character- ramp conditions both in ex situ and in situ experimental set-ups isation techniques, pre-mortem and post-mortem electron (Figure S2). Further details can be found in the Methods section. microscopy can provide important information. However, none This parallel set-up determined if the behaviours of LDH of these pre-mortem and post-mortem methods can be used to transformations were independent of the different in situ and elucidate on how to optimise the processes that occur under real- ex situ experimental conditions. For example, the significant time conditions at single nanoparticle level. A detailed in situ TEM pressure variations as the samples were heated may have characterisation, observing thermal transformations at individual impacted the thermal behaviours of the LDH nanomaterials. More nanoparticle level, has yet to be established. Determining the importantly, there can be an appreciable contribution of radiation correct structure–property–function relationships requires a beam damage (in the form of radiolysis or knock-on) or Joule detailed description of the material in its working state. Recent sample heating when the electron beam interacts with the technological advances in EM allow us to image materials with a sample. In view of these possible issues, full TEM characterisation range of in situ techniques that have been developed to follow of the various LDH samples were performed pre and post ex situ the evolution of materials in the presence of such external stimuli. heating experiments, as well as during in situ analysis; this In addition to observing materials in their working state, in situ comparison was of crucial importance in order to rule out any techniques can also capture important intermediate transitional eventual electron beam-induced transition. forms that may be involved in phase transformations. As such, We investigated if the LDH degradation processes are previous research acknowledges that these exact transformations dependent on both the composition and platelet size of the in which the LDH decomposes are not fully understood. Utilising material. As such, the structural transformation of a Ni-Fe LDH such in situ techniques help to answer the underlying questions material was studied via the application of in situ heating TEM. Fig. regarding the thermal transformations of the LDH material from a 2 displays the BFTEM and associated SAED patterns at various nanoscale perspective. Ultimately, these behaviours can be stages of the in situ heating experiments for the Ni-Fe LDH monitored and analysed in real time, providing a further under- material. Initially, the material was synthesised to have a well- standing and optimisation of the related processes. Here, we defined hexagonal morphology with a lateral dimension on the report on the application of state-of-the-art in situ transmission order of microns. This starting material was found to have an LDH electron microscopy to reveal the processes by which LDHs crystallographic structure, with the associated SAED pattern thermally decompose and to characterise the stages at which revealing the (101), 011 ,112 and (110) crystallographic planes these morphological and crystallographic alterations occur. Due to (Fig. 2e). Upon heating, both a morphological and crystallographic their broad application and well-established calcination processes change was evidenced. As the sample reached 250 °C, the loss of in the previous literature, we elected to focus our attention on Mg- the (101), 011 and 112 reflections indicates a collapse in the Al LDH and Ni-Fe LDH for in situ TEM analysis. Undoubtedly, there crystal structure and a reduction in the non-basal crystallographic is an urgent need to fully characterise and understand these phases while still retaining its hexagonal morphology. The loss of materials from a single particle perspective, given their prospec- such planes is believed to be due to the dehydration of the tive nanoscale applications in flame retardancy, energy storage interstitial galleries. Moreover, the contrast variations of BFTEM and catalysis. images (Fig. 2a, b) indicates a surface alteration at this temperature, possibly as a consequence of the additional loss of water both from the surface of the material and the interlayer RESULTS AND DISCUSSION galleries. The irradiation of the electron beam perhaps also In both Mg-Al LDH and Ni-Fe LDH cases, a molar ratio of the contributed to the dehydration of the LDH materials; however, this cationic sites of 3 was selected as a representative of these LDH is difficult to avoid. As the Ni-Fe LDH was heated to 450 °C, the compositions due to its well-established behaviours and previous well-defined hexagonal morphology is retained (Fig. 2c). However, 36–40 successful applications. The as-synthesised materials were the evolution of spherical particles, <50 nm in size, were seen to first characterised by XRD, FTIR, TGA and atomic absorption generate and randomly distribute themselves onto the hexagonal spectroscopy (AAS). The FTIR spectrum of the precursor Ni-Fe and parent material (Fig. 2c). There appears to be minimal crystal- Mg-Al LDH samples is shown in Figure S1a. The broad bands at lographic tranformations from 250 °C to 450 °C (Fig. 2f, g). This is −1 approximately 3,400 and 1,645 cm are due to the various types indicated by the conservation of the (110) LDH plane. As the of O-H bonding in both materials such as the hydroxyl groups on sample was calcined to 850 °C, there is clear evidence of a further the brucite-like layers and the adsorbed/inter-gallery water generation of these particles (Fig. 2d). These nucleations become npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS 1234567890():,; Structural transformation of layered... C Hobbs et al. Fig. 2 BFTEM and associated SAED patterns from various stages of the in situ heating experiments of the Ni-Fe LDH. a–d BFTEM and e–h SAED patterns corresponding to column temperature, end of the 250 °C step, end of the 450 °C step and end of the 850 °C step, respectively. −1 Scale bars are a–d 200 nm and e–h 2nm , respectively more numerous, increase in size and randomly distribute environments and, in turn, would have impacted the true themselves across the hexagonal LDH platelet (Fig. 2d). The SAED reflection of the Ni-Fe LDH calcination behaviours. pattern of the calcined sample at 850 °C shows a crystal structure EFTEM revealed the distribution of Ni and Fe throughout the alteration (Fig. 2h). In contrast to the patterns recorded at 250 °C calcined products (Fig. 3a). In contrast to the Ni evolution, Fe was found to have a more regular distribution across the platelet, with and 450 °C, there is a generation of new reflections in the a localisation in the centre of the calcined material. Larger NiO diffraction pattern, corresponding to newly arranged crystal- particles tended to form on the central sections of the platelets, lographic planes. The evolution of such particles and a variance in with smaller NiO crystallites arranged on the platelets edge the diffraction patterns during the calcination procedures regions. This may have been due to the non-uniform local confirms a rearrangement of the LDH structures into a new crystallographic environments of the cationic sites, whereby the crystallographic phase. It is believed that this is a topotactic different environments at the edge regions of the platelets transformation caused by the in situ heating effects. Further affected the growth of both NiFe O trevorite and NiO oxide 2 4 analysis of the SAED patterns recorded at 850 °C from in situ TEM nanoparticles. Moreover, it is perceived that the catalytic activity at experiments (Fig. 2h) portrays the emergence of the {200} and these edge regions were not only influenced by Fe sites but {220} family of planes of the NiO and trevorite structures, −1 were also influenced by smaller sized NiO particles. We also used respectively. The reflections occurring at 13.1 nm may also the EFTEM technique to directly visualise the Ni-Fe thermal derive from the parent LDH structure. This suggests that certain transformations as they took place (Movie V1 Supplementary LDH planes may be retained even after thermal degradation Information). Our experimental set-up utilised signals from the procedures. In this regard, the SAED findings revealed an excitations of the electrons in the Ni 2p orbital (855 eV), that is, 3/2 evolution to a mixed phase of LDH, metal oxides and trevorite the Ni L EELS edge, to form the intensities in such video structures (Fig. 2e–h). 45 representations. As the platelets were subject to 850 °C, the To further understand the characteristics of the calcined Ni-Fe nucleation of Ni containing particles was clearly evidenced. structures, we post-analysed the Ni-Fe LDH samples using energy- Particles initially evolved and migrated in a random fashion on filtered TEM (EFTEM) and high-resolution TEM (HRTEM) (Figs. 3 and the platelet surfaces. Further exposure at this temperature led to a 4, respectively). Further details of the EFTEM technique can be sudden transformation where the remaining material transformed found in the Methods section. EFTEM was favoured over scanning to an array of smaller similar Ni containing particles. This TEM (STEM) for acquiring spectroscopic data in situ such as somewhat restricted size is perhaps due to the limited local energy-dispersive x-ray (EDX) and electron energy loss spectro- availability of Ni cations after the initial transformations occurred. scopy (EELS) to reduce the risk of inducing further transformations Complimentary to the EFTEM data, an elemental distribution of due to the beam-sample interaction and not as a consequence of the calcined Ni-Fe LDH material was further analysed in STEM. An applied thermal environments. Also, the application of EFTEM associated EDX elemental map confirmed that the spherical allowed for the direct analysis of the Ni and Fe from the whole nanoparticles were largely composed of nickel (green signal) platelet via the acquisition of two EFTEM maps (Fig. 3a). This (Figure S6B). The effect of the background was corrected for by application was preferred over STEM-EELS methods as a longer the subtraction of the average intensity value in the related Fe timescale for spectroscopic acquistions is required for such LDH map (red signal) (Figure S6B). It is also noted that small amounts of platelet regions. The STEM approach may have also introduced Fe may have been present either within or in the vicinity of the issues of drift and potential alterations of crystallographic particle, suggested by the feint red intensities across the Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. Fig. 3 a Energy-filtered TEM image of Ni-Fe LDH sample post-heating. Ni and Fe signals are represented by the colours green and red, respectively. b Scanning transmission electron microscopy image of the same platelet. Areas 1–4 indicated in b represent regions from which EDX spectra were acquired. Scale bar in a is 200 nm elemental map. Moreover, the low amounts of Fe was illustrated resolved the lattice spacings attributed to the {200} planes of the by the recorded EDX sum spectra from areas 1 to 3 across the NiO structure with an orientation along the [023] axis, hence particle (Figure S6C and D). Although this may have suggested confirming the evolution of metal oxide particles as a result of the that there is an existence of Fe migration into the NiO particles, Ni-Fe LDH calcination. However, there was a discrepancy in the the Fe-K EDX line peak at 6.4 keV was not statistically significant correlation of the STEM nanoprobe diffraction and HRTEM of across the particle and was deemed to be undetectable above similar particle regions, whereby the 111 family of planes found background counts. Hence, the Fe signals are speculated to be in the STEM nanoprobe acquisitions did not fully coincide with the due to EDX background counts. FFT analysis of our HRTEM data (Fig. 4c) and Supporting Furthermore, we employed electron diffraction in STEM mode Information S3f. We perceive this to be due to the evolution of to investigate the variation in the orientation of the metal oxides a non-stoichiometric NiO compound as opposed to a pure NiO (Supporting Information Figure S3). This overcame the drawback phase. The existence of non-stochiometric NiOs has been of SAED techniques in TEM, where the dimensions of the selected previously investigated by Da Rocha and Rougier. area apertures limit the recording of diffraction patterns from As aforementioned, the parallel TEM analysis of samples heated regions of minimum 100 nm in size. Whereas the application of ex situ investigated the applicability of our in situ EM methodol- electron diffraction using a STEM electron probe permitted the ogies as well as assessed the effect of the electron beam on the characterisation of features of approximately 10 nm, that is, the LDH thermal evolution. Figure 5 presents the TEM and SAED size range of the products of calcination procedures. In addition, studies applied to Ni-Fe LDH samples heated with ex situ the application of electron diffraction studies in STEM allowed us experimental conditions. to investigate the crystallographic orientations across the The samples heated to 250 °C ex situ presented an irregular calcined Ni-Fe structures. The variations in the recorded patterns platelet morphology (Fig. 5a). This evidently contrasts the show that the NiO particles assembled with random orientations precursor LDH platelets as well as the LDH materials heated to relative to the basal plane of the LDH platelet (Figure S3d and f). 250 °C using in situ methods (Fig. 2a, b, respectively). The This is also viewed in the NiFe O trevorite regions of the calcined corresponding SAED pattern showed that these irregular sheet- 2 4 structure (Figure S3c and e). It is deduced that the products of like materials display an LDH crystallographic structure, demon- mixed metal oxides derived from the Ni-Fe LDH calcinations strated by the {101} and {110} planes (Fig. 5d). Similarly, these arrange themselves randomly in a topotactic fashion throughout crystallographic planes were evidenced as the sample was heated the parent material, but confine themselves to within the platelet to 450 °C using ex situ methods (Fig. 5e). The BFTEM findings (Fig. domains. 5b) are also consistent with in situ studies at this temperature, in Energy-dispersive x-ray spectroscopy in STEM quantitatively particular the irregular morphology and formation of particles on assessed the calcined products of the Ni-Fe degradations. While the platelets surface (Fig. 5b). The morphology demonstrated an the Fe content appeared to be ubiquitously distributed, STEM-EDX arrangement of smaller crystallite regions at 850 °C (Fig. 5c) with revealed that there is a notable higher Ni content on the thermally similar crystallographic features (Fig. 5f). These observations of evolved spherical particles (Fig. 3b and Table 1 (Supplementary ex situ heated materials may also be perceived as a reconstructed Information)). This yielded additional evidence that the evolved LDH due to its exposure to aqueous environments, that is, the LDH particles are that of NiO, and also correlated well with the data ‘memory effect’. Moreover, the effect of rehydration due to acquired by EFTEM methods. exposure to air conditions can also be viewed from the analysis of An investigation by HRTEM showed the presence of the NiFe O the in situ samples, 40 days after experimentation (Figure S7). The 2 4 trevorite lattice structure surrounding the spherical particles hexagonal symmetry in the associated SAED patterns highlighted resulting in a heterogeneous structure (Fig. 4e). FFT analysis of a partial revert to the LDH structure (Fig. 5f and Figure S7B). the associated regions (Fig. 4g) confirmed the existence of the However, the existence of trevorite phases remain, as shown by {220} and {242} family of planes (indicated by red indices in Fig. HRTEM analysis in both ex situ and in situ experiments (Figure 4g), where the crystallite region had a ½111 orientation. This can S10B and Fig. 4, respectively). Combinative EFTEM images of the also be corroborated to the Fe signal as found by EFTEM methods ex situ heated sample up to 850 °C highlighted an even (Fig. 3a). Similar analysis of the spherical nanoparticles directly distribution of Ni and Fe throughout the material. This was npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. Fig. 4 a, e HRTEM images of various regions after in situ TEM heating experiments. b, f Zoomed in images of square regions as indicated in a and e. c, g Calculated FFT from square red region as indicated on the TEM micrograph in a and e. Blue and red annotations (c and g) represent labelled NiO and trevorite crystallographic planes, respectively. d, h Schematic representations of the respective crystalline structures, viewed along the [023] (d) and ½111 (h) directions. Scale bars are 10 nm for micrographs a and e and 5 nm for b and f. Structures d and h were TM visualised using CrystalMaker evidenced by the ubiquitous EFTEM intensities of Ni (green) and appearance of the (111), (200) and (220) of NiO occurred at 43.4°, Fe (red), respectively (Figure S9A). The non-specific location of 50.6° and 74.1°, respectively. This behaviour was also viewed via either Ni or Fe contrasts the EFTEM findings from in situ the in situ TEM experiments, where the birth of the spherical NiO experiments (Fig. 3a). This is acknowledged to be the effect of particles was observed at this temperature (Fig. 2c). At 850 °C, the rehydration of the heated LDH material due to its exposure to (111), (200) and (220) diffraction peaks of NiO became more moist air conditions as a consequence of sample preparation, pronounced. This is believed to be due to the growth and where the Ni and Fe sites could revert back to an LDH structure. A nucleation of further NiO particles, as observed in our in situ TEM potential reconstruction to an LDH structure was also verified by experiments (Fig. 2d). It is also possible that these diffraction peaks STEM-EDX, demonstrating a greater Ni content in the analysed could be attributed to (222), (400) and (440) of the NiFe O phase, 2 4 region (areas 3 and 4 in Figure S8B and Table 2 in Supporting but this is difficult to distinguish due to their ambiguity with the Information). NiO peaks. Parallel ex situ XRD experiments were also carried out Despite maintaining the focus of interest on the nanoscale on the Ni-Fe LDH samples. At room temperature, the XRD peaks at features responsible for the thermal evolution of these LDH 2θ angles 5.3°, 10.32°, 15.7°, 17.6° and 20.7° corresponded to the samples, it is acknowledged that the above findings may not have (003), (006), (101), (104) and (018) planes of the Ni-Fe LDH reflected their bulk material counterparts. Hence, we compared crystallographic planes (Figure S11B(a)). Similar traits relative to these behaviours via the application of in situ and ex situ XRD in situ XRD approaches were evidenced at 250 °C. The loss of the analysis (Figure S11A and B, respectively). Powder samples were (006), (101), (104) and (018) peaks indicate a breakdown in the Ni- measured at intermediate temperature stages of aforementioned Fe LDH crystal structure. At this temperature, the XRD patterns heating protocols (Figure S2). In particular, XRD was strategically also showed a decreasing and broadening of the (003) peaks conducted firstly at room temperature and then samples were indicating an alteration in basal spacings (Figure S11B (b)). The heated to 250 °C for 2 h followed by 450 °C for 2 h and finally to evolution of the NiO (111), (200) and (220) planes and the NiFe O 2 4 850 °C for 2 h and post-analysed at each stage (Figure S11A(a)–(d) (220), (311), (222), (400), (511), (440), (622) and (444) planes were and Figure S11B(a)–(d), respectively). At room temperature, the evidenced at 450 °C and 850 °C, respectively (Figure S11B(c) and in situ XRD analysis revealed typical LDH (003), (006), (101), (015) (d) respectively). The evolution of these NiO phases is also and (110) crystallographic planes at 2θ angles 16°, 26.6°, 39.4°, mirrored via our in situ XRD approach. Moreover, the NiO {200} 45.7° and 70.9°, respectively. Upon thermal treatment to 250 °C, and NiFe O {220} planar families were observed both in the SAED 2 4 a lateral shift of the (003) and loss of (006) basal planes suggests patterns recorded during in situ TEM experiments (Figs. 2h and 4f, an expansion of the unit cell and a reduction in basal plane respectively). However, ambiguities were also seen in selected periodicity. This is attributed to the dehydration of the associated ex situ XRD peaks of the calcined samples as they could H O content from interlayer LDH galleries. Moreover, the retention correspond to either oxide or trevorite structures. The XRD peaks of the LDH (110) plane is observed in both in situ TEM and in situ at 2θ angles 17.0°, 19.7° and 27.9° could be hypothesised to derive XRD approaches (Fig. 2g and Figure S11A(b)), validating how from the respective NiO (111), (200) and (220) or NiFe O (222), 2 4 single nanoparticle behaviours can affect bulk material character- (400) and (440) family of planes. This has also been observed by istics. As the samples were subject to 450 °C in situ, a phase previous work. Furthermore, this indistinctness in the possible transition took place. The loss of the LDH diffraction peaks and the source of XRD peaks validates our motivation of applying the Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. Fig. 5 BFTEM and associated SAED patterns of ex situ heating experiments of the Ni-Fe LDH. a–c BFTEM and d–f SAED patterns corresponding to the end of the 250 °C step, end of the 450 °C step and end of the 850 °C step of the applied heating ramp, respectively. Green annotations −1 (d and e) represent labelled LDH crystallographic planes. Scale bars for TEM micrographs a–c and SAED patterns d–f are 200 nm and 2 nm , respectively techniques of SAED and HRTEM. The observed correlations properties. Similar features of the Mg-Al LDH degradation, in between XRD macroscopic techniques and EM analysis indicates particular the evolution of a porous matrix, are also evidenced in that the bulk features derived from the LDH nanoscale the recent independent studies of Warringham et al. In contrast transformations. Hence, our findings highlight the importance of to the Mg-Al LDH thermal degradation, the Ni-Fe LDH does not understanding the nanoscale thermal degradation properties for develop a morphological variation along the borders of the the design and fabrication of LDH-based devices. platelets (Figs. 6d and 2d, respectively). Analogously, the In light of such findings related to the Ni-Fe LDH material, we development of a porous structure of the Ni-Fe LDH is not as applied an identical experimental approach and analysis to a pronounced as the Mg-Al counterpart. The Mg-Al LDH morphol- similar Mg-Al LDH structure. This yielded a comparison of both ogy and crystalline structures are maintained as they are subject size and composition of various LDH structures with regards to to in situ heating up to 250 °C, as demonstrated by the TEM their associated thermal degradations. Figure 6 displays the TEM micrographs and electron diffraction patterns in Fig. 6b, f. An findings from in situ heating of an Mg-Al LDH material, where the intensity attenuation in the {100} and {110} diffraction spots occurs aforementioned heating ramp was imposed (Figure S2). Similar to (Fig. 6f). The loss in periodicity of these planes suggests a the Ni-Fe LDH, the material appeared to have a well-defind crystallographic rearrangement of the LDH structures and is platelet structures; however, they portrayed a width of approxi- accredited to the loss of H O from the interstitial layers and a 23,53 mately 350 nm and rounded morphology, as shown by BFTEM dehydroxlisation of the octahedral layers. Interestingly, this (Fig. 6a). Moreover, the crystallographic resemblances of these transformation was also paralleled in the Ni-Fe LDH structures at hydrotalcite platelets to the Ni-Fe compositions were seen via the this temperature (Fig. 2f). existence of well-defined {100}, {110}, {120} and {330} planar As the continuation of the in situ heating ramp regime reached families of the LDH structure (Fig. 6e). 450 °C, Mg-Al platelets demonstrated increased porosity with an An overall examination of the LDH platelets as they are subject evident rearrangement taking place, as shown in BFTEM (Fig. 6c). to the in situ heating ramp revealed an inhomogeneous contrast The overall morphology of the LDH remained intact but an and a porous structure develops within the platelets (Fig. 6d and amorphization occurred. This was evidenced by in situ SAED Movie V2 Supplementary Information). These porous structures patterns via the loss of intensity and broadening of the {100}, {110} evolved in an irregular, somewhat random, fashion on the and {120} LDH diffraction spots (Fig. 6f, g). Moreover, there are hydrotalcite materials. The borders of the platelets were not parallels in this Mg-Al LDH crystallographic transition, however, as observed to have the same behaviours, possibly due to their amorphization was found to occur at approximately 450 °C when different crystallographic environments when compared the compared to the Ni-Fe degradations, which displayed an central domains of the materials. In contrast, they developed a amorphization tendencies at 250 °C (Fig. 2f). Hence, it is believed denser frame on the outer regions of the heated material. Indeed, that the Mg-Al LDH structural transformations are less susceptible it is indicative that the composition and dimension of LDH to degradations at higher temperatures, suggesting that LDH materials has an influential effect on its thermal degradative dehydration procedures exhibit a compositional dependence. This npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. Fig. 6 BFTEM and associated SAED patterns from various stages of the in situ heating experiments of the Mg-Al LDH. a–d BFTEM and e–h SAED patterns corresponding to column temperature 20 °C, end of 250 °C step, end of 450 °C step and end of 850 °C step, respectively. Scale −1 bars are 100 nm for images a–d and are 2 nm for e–h feature was also corroborated in our TGA, where greater mass calcinations. More generally, the application of in situ techniques losses were observed in the Ni-Fe case (Figure S12). elucidated the progressions of the structural alterations of the At the 850 °C step, the emergence of new diffraction spots in nanoplatelets. We have revealed a direct observation of the the associated SAED patterns provided direct experimental nanoscale nucleation behaviours and mechanisms involved in Ni- confirmation of a crystallographic transformation from an LDH Fe LDH and Mg-Al LDH thermal degradation. material to a combination of α-Al O and MgAl O spinel This study reports the application of in situ TEM to investigate 2 3 2 4 structures (Fig. 6h). This was evidenced by the existence of the and compare the thermal degradation behaviours of LDH associated {111} and {022} reflections of the α-Al O and {422}, 2 3 materials. The application of in situ heating techniques in the {531} and {444} of MgAl O spinel. This evolution to mixed metal 2 4 TEM revealed the morphological and crystallographic integrity of oxides was paralleled in the Ni-Fe degradations at this tempera- individual LDH platelets as well as the nanoscale behaviours and 24,55 ture (Fig. 2h) and was also similar to previous work. These progression mechanisms involved in Mg-Al LDH thermal degrada- in situ methods showed that a complete decomposition did not tion. Ni-Fe and Mg-Al LDH compounds exhibited similar beha- occur via the existence of the {110}, {120} and {300} LDH planes viours but also had unique signatures in their own right. The (Fig. 6h). in situ heating of Ni-Fe LDH material resulted in the transforma- The size dependence of the LDH platelets to their calcination tion into a coexistence of Ni oxide-type particles arranged behaviours was also investigated (Figure S14). We applied our throughout a NiFe O trevorite matrix, confirmed by TEM, HRTEM 2 4 in situ heating experiment conditions to Mg-Al LDH platelets of and STEM-EDX. In particular, the nucleation processes of NiO relatively larger size (approximately 3 μm in lateral dimension). In particles was directly visualised by EFTEM methods. The calcina- comparison to the smaller Mg-Al LDH material (Fig. 6), the larger tions were also paralleled via ex situ and XRD methods, exhibiting platelets were seen to have very similar degradation procedures. identical phase transformations, verifying the observed beha- The reduction in crystallinity up to 450 °C (Figure S14E–G) and viours by TEM are due to the inherent properties of the LDH evolution of pores across the material at 850 °C (Figure S14D) materials. It was found that the calcined Mg-Al LDH evolved into compares to that of their smaller counterparts (Fig. 6). Overall, it Al O and MgAl O structures, effectively characterised by HRTEM 2 3 2 4 was seen that the degradations were not platelet size dependent, and SAED. However, the transition of the Mg-Al material occurred rather the materials composition having a more profound effect via a different mechanism, with a development of a porous matrix on such mechanistic behaviours. as opposed to the generation of spherical particles. The Our results yield important impacts in relation to the employ- application of SAED in situ effectively characterised the crystal- ment of LDHs as dye adsorbents, flame retardants and catalysts. lographic transitions involved in the degradations of both Ni-Fe For example, our findings potentially impart further information and Mg-Al of LDH materials. into the role of LDH platelets and their calcined derivatives have The applicability of our experimental methodologies was on the thermal stability, toxicity and structural morphology of confirmed by our analysis of LDH materials of a different when incorporated into polymers or resins. Moreover, the compositions, establishing a suitable in situ approach to distribution of calcined products could be of interest for the characterise the LDH thermal transformation behaviours. Ulti- design and testing of future nanocatalysts in relation to the mately, our successful nanoscale assessment of such mechanisms dispersion and confinement of catalytic sites with enhanced catalytic activity or selectivity. The impacts of our analysis may has an important bearing on their further development and further be extended to dye sorbents, possibly revealing insights as strategic designs of catalysts and flame retardants as well as many to how sorption capacities are affected by the extent of LDH additional current areas of nanotechnology. Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. METHODS microelectromechanical systems chip design with electron transparent silicon nitride windows onto which the sample is deposited. The heating of Ni-Fe LDH synthesis the chip is conducted via a metallic spiralling system with suitable contacts Ni(NO ) ·6H O, Fe(NO ) ·9H O, TEA and urea were dissolved in 80 ml of 3 2 2 3 3 2 and interpolated with an electron transparent SiN window support film, deionized water giving a final concentration of 7.5, 2.5, 10 and 17.5 mM, onto which the sample is directly deposited. Hence this set-up exposes the respectively, and stirred at room temperature for 24 h. Then, the reaction sample to thermal environments, and as such, the nanoscale properties mixture was heated to 100 °C in an oil bath for 48 h and the obtained and features of the LDH materials can be characterised as they are subject yellow precipitate was washed with deionized water several times. to specific elevated temperatures. Furthermore, the TEM set-up was carefully monitored to reduce the beam effects discussed above. As the Mg-Al LDH synthesis sample was subject to heating, the electron beam was blanked to avoid To prepare a 3:1 Mg-Al LDH with intercalated carbonate, 343 g (1.68 mol) unwarranted interactions. By doing this, we can be reassured that the MgCl ·6H O and 136 g (0.56 mol) AlCl ·6H O were dissolved in 100 ml discussed findings are indeed effect of the applied heating treatment 2 2 3 2 deionized water as reactant mixture A. A second reactant mixture B was rather than undesired beam-induced effects. EDX and EFTEM studies were prepared by dissolving 180 g (4.5 mol) NaOH pellets and 30 g (0.28 mol) carried out using a EDAX single SiLi EDX detector and GIF Tridiem energy Na CO in 100 ml deionized water. Subsequently, the reactant mixtures A 2 3 filter (Gatan, USA) with a spectrometer, respectively. Our SAED experi- and B were pumped together within 30 min with a syringe pump into a mental set-up utilised a selected area aperture to assist in measuring the beaker, containing another 150 ml deionized water, under heavy shear- relative diffraction patterns, allowing us to analyse crystallographic mixing using an ultraturrax rotor-stator-mixer. The reaction product was properties from regions much smaller than the individual hydrotalcites subsequently washed five times with deionized water via dispersing the themselves. An analysis of such materials exposed to in situ heating particles in fresh deionized water and solid–liquid separating the particles environments yielded a direct visualisation of how these elevated via gravitational sedimentation (centrifugation). Eventually, the whole temperatures affect the properties of LDH materials. In addition, the product of the washed LDH particles was redispersed in 1,500 ml of accompanying SAED methods permitted an investigation into the crystal- deionized water and heated at 80 °C for 4 days to yield well-ordered lographic features that occur at the various thermal stages in real time. This crystallites. approach was used to directly observe the thermal degradations of To synthesise large platelets of carbonate containing Mg-Al LDH with hydrotalcites and assess the nanoscale features that play a significant role Mg:Al ratio of 3, Mg(NO ) ·6H O, Al(NO ) ·9H O and urea were dissolved in 3 2 2 3 3 2 in these processes such as size, morphology and localised composition. 60 ml of deionised water giving a final concentration of 0.125, 0.042 and TEM images and SAED patterns were analysed using Digital Micrograph 1.67 M, respectively. Then, the solution was transferred to 100 ml round (Gatan Inc., California, USA). The EFTEM experimental technique extends bottom flask and immersed in an oil bath previously heated to 90 °C and from EELS in the electron microscope. Through collection and analysis of continuously stirred for 48 h under reflux. After this time, the flask was the inelastically scattered electrons transmitted through the sample, an cooled in a water bath for 30 min, the precipitate was separated by EEL spectrum can be acquired. At higher energies of these spectra, core- centrifugation (3,000 rpm/10 min) and washed three times with water. loss signals in EELS originate from the ionisation of inner shells (e.g. K, L or M) due to energy transfer from the incident electron beam, and are Experimental methods characteristic of which element they come from. By selecting certain Heating conditions of LDH materials for in situ and ex situ experiments. The energies using an energy slit width to contain such ionisation edge signals, LDH samples were initially exposed to a temperature of 30 °C for 10 min. the unwanted electrons are filtered out, resulting in images containing This was followed by a ramp to 250 °C at 10 °C/min and kept at this vital elemental information. temperature for 120 min. Subsequently, the same sample was heated to 450 °C at 10 °C/min and kept at 450 °C for 120 min. Finally, the sample was Data availability elevated to 850 °C at 10 °C/min and maintained at 850 °C for 2 h. The data related to the findings of this work are available from the corresponding author subject to reasonable request. TEM sample preparation Samples were prepared for in situ TEM experiments by suspending the synthesised LDH materials in deionized water and sonicating for 30 min. ACKNOWLEDGEMENTS Subsequently, 5 μl of the associated samples were subsequently placed We would like to acknowledge the following funding supports: SFI AMBER, SFI PIYRA, TM DENS Solutions Nano-chips for in situ experiments, respectively. Excess ERC StG 2D NanoCaps, ERC CoG 3D2DPrint and Horizon2020 NMP Co-Pilot. We would sample was wicked using filter paper and then air dried for approximately like to thank the Advanced Microscopy Laboratory at CRANN, Trinity College Dublin. 30 min each before TEM analysis. Ex situ heated Ni-Fe samples were The Microanalytical Laboratory, School of Chemistry, University College Dublin is prepared for TEM analysis by directly depositing the powder forms of the acknowledged for bulk ICP-AAS analysis. materials onto lacey Carbon TEM Cu grids (TED Pella Inc., USA). Characterisation methods AUTHOR CONTRIBUTIONS Powder XRD was used to characterise the precursor LDH bulk sample as C.H., K.M., M.C.D.M. and V.N. discussed the proposed plans of this work. S.J. and M.C.D. well as LDH samples subject to various thermal conditions. A Bruker M synthesised the associated Ni-Fe LDH and Mg-Al LDH samples, respectively. C.H. Advance Powder X-ray diffractometer (Bruker, MA, USA) operating with a and E.K.McC. performed in situ TEM characterisation experiments. C.H. and C.D. −1 molybdenum K-α emission source (λ ¼ 0:7107 Å ) was used to record the performed post in situ analysis via (S)TEM methods. C.H. performed ex situ analysis associated diffractograms. In situ XRD experiments were conducted using a using TEM methods. S.J. performed ex situ heating experiments along with XRD and PANalytical EMRYREAN XRD with θ–θ Bragg–Brentano geometry. A cobalt TGA characterisations. K.O. and K.G. performed in situ XRD experiments C.H., M.C.D.M. −1 K-α emission source (λ ¼ 1:789 Å ). To exclude any possibility of 1,2 and A.S. interpreted electron diffraction results. C.H., K.M. and V.N. discussed and reconstruction to an LDH structure due to water exposure, we performed structured the paper content and results. C.H. wrote the paper with all authors XRD using samples in powder form. contributing to preparation of the manuscript. TGA was also conducted as a measure of the mass losses when the LDH was subject to thermal environments. TGA studies were performed using a PerkinElmer Pyris 1 TGA (PerkinElmer, MA, USA). ADDITIONAL INFORMATION TEM was used to characterise the overall morphology and crystal- Supplementary information accompanies the paper on the npj 2D Materials and lographic structure of the associated LDH samples. TEM and SAED were Applications website (https://doi.org/10.1038/s41699-018-0048-4). conducted on a FEI Titan300 (FEI, Oregon, USA), operated at 300 kV. For comparative purposes of the in situ heating experiments, TEM magnifica- Competing interests: The authors declare no competing financial interests. tions and electron diffraction camera lengths were kept constant. In situ heating TEM experiments were conducted using a DENS Solutions TM Wildfire TEM holder (DENS solutions, Delft, The Netherlands). This Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims experimental in situ TEM sample holder set-up consists of a in published maps and institutional affiliations. npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS Structural transformation of layered... C Hobbs et al. REFERENCES 27. Perez-Ramirez, J., Mul, G., Kapteijn, F. & Moulijn, J. A. In situ investigation of the thermal decomposition of Co±Al hydrotalcite in different atmospheres. J. Mater. 1. Coleman, J. N. et al. 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A activation of layered hydroxides. ACS Chem. Mater. 29, 4052–4062 (2017). 399,87–92 (2011). Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2018) 4 Structural transformation of layered... C Hobbs et al. adaptation, distribution and reproduction in any medium or format, as long as you give 53. Constantino, V. R. L. & Hnnavaia, T. J. Basic properties of Mg2+1,Al3+, layered appropriate credit to the original author(s) and the source, provide a link to the Creative double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate Commons license, and indicate if changes were made. The images or other third party anions. Inorg. Chem. 34, 883–892 (1995). material in this article are included in the article’s Creative Commons license, unless 54. Allmann, R. Magnesium aluminium carbonate hydroxide tetrahydrate: a discus- indicated otherwise in a credit line to the material. If material is not included in the sion. Am. Mineral. 53, 1057 (1968). article’s Creative Commons license and your intended use is not permitted by statutory 55. Yang, W., Kim, Y., Liu, P. K. T., Sahimi, M. & Tsotsis, T. T. A study by in situ tech- regulation or exceeds the permitted use, you will need to obtain permission directly niques of the thermal evolution of the structure of a Mg-Al-CO layered double from the copyright holder. To view a copy of this license, visit http://creativecommons. hydroxide. Chem. Eng. Sci. 57, 2945–2953 (2002). org/licenses/by/4.0/. 56. Willams, D. B. & Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science. Springer, Boston, MA, USA (2009). © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, npj 2D Materials and Applications (2018) 4 Published in partnership with FCT NOVA with the support of E-MRS

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