The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds

The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds The effects of Li/Nb ratio on the preparation of Li-Nb-O compounds by a hydrothermal method were studied deeply. Li/Nb ratio has a great impact on the formation of LiNbO ; the ratio smaller than 3:1 is beneficial to the formation of LiNbO , while larger than 3:1, forms no LiNbO at all and the morphology and chemical bond of 3 3 Nb O raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, 2 5 which is beneficial to form Li NbO not LiNbO , and also, even if LiNbO particle locally forms, it is easily dissolved 3 4 3 3 in LiOH solution with strong alkalinity. Pure LiNb O powders are obtained with two absolutely opposite Li/Nb 3 8 ratios: 8:1 and 1:3; the former shows a unique porous and hollow structure, quite different from the particle aggregation (the latter shows). Compared with Li/Nb = 1:3, the 4.2 times higher photocatalytic performance of LiNb O (Li/Nb = 8:1) are observed and it can be attributed to the unique porous and hollow structure, which 3 8 provides a high density of active sites for the degradation of MB. Compared to LiNbO , the improved photocatalytic performance of LiNb O can be attributed to its layered structure type with the reduced symmetry 3 8 enhancing the separation of electrons and holes. Keywords: Lithium Niobate, Hydrothermal, Porous Materials, Photocatalysis Background structures which favor a possible delocalization of charge Niobium compounds, a very versatile group of materials, carriers [12]. Secondly, the conduction bands consisting of including niobium oxides, alkali niobates, and columbite Nb4d orbitals located at a more negative state of redox niobates, exhibit many interesting physical properties and potential of H+/H promote the separation and transfer of have been widely studied in many fields, such as catalysis photo-induced charge carriers and result in high photo- [1–3], memristors [4], dye-sensitized solar cells [5], optical catalytic activities [13]. Among these materials, LiNb O 3 8 devices, and others [6, 7]. LiNbO , as one of the most fam- displays unique performances. As a novel lithium-ion bat- ous alkali niobates, presents prominent properties such as tery (LIB) anode material, the theoretical capacity of electro-optical and nonlinear optical behaviors, pyroelec- LiNb O is 389 mAh/g assuming two-electron transfers 3 8 5+ 3+ tricity, and piezoelectricity, and it is mainly used as optical (Nb → Nb ), larger than many other anode materials, modulators, waveguides, acoustic wave transducers, et al. such as Li Ti O [14, 15]. Used for supercapacitor de- 4 5 12 in optical devices. vices, LiNb O nanoflakes show excellent cycle stability 3 8 For environmental remediation and clean energy appli- with negligible specific capacitance decrease even after cations, niobates, such as (Na, K)NbO [8], BiNbO [9], 15,000 cycles [16]. Also, it is used as an efficient photoca- 3 4 LiNbO [10], and LiNb O [11], have been deeply investi- talyst in the applications of hydrogen generation and deg- 3 3 8 gated, owing to their special distorted [NbO ]octahedral radation of organic pollutants. Pure LiNb O is a highly 6 3 8 active UV-photocatalyst for water reduction producing * Correspondence: haifazhai@126.com 83.87 μmol of hydrogen in 1 h, and it does not produce Henan Key Laboratory of Photovoltaic Materials, College of Physics and hydrogen under visible-light irradiation due to its large Materials Science, Henan Normal University, Xinxiang 453007, People’s band gap (i.e., 3.9 eV) and inability to absorb visible light Republic of China National Laboratory of Solid State Microstructures, Nanjing University, [17, 18]. LiNb O nanoflakes show fast decolorization of 3 8 Nanjing 210093, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 2 of 7 toluidine blue O (TBO) dye under UV light compared to electron microscope (FESEM; JSM-6700F). Chemical commercial TiO powders [13]. bonds were analyzed by Fourier-transformed infrared −1 At most time, the appearance of LiNb O is recognized spectroscopy (FTIR) in the range of 2000–650 cm .X- 3 8 as an impurity phase during the preparation of LiNbO , ray photoelectron spectroscopy (XPS) analysis was per- especially in film samples, owing to high annealing formed on a Thermo-Fisher Escalab 250Xi instrument to temperature or inhomogeneous distribution of Li element characterize the chemical component of Li-Nb-O com- in precursors [19, 20]. Due to the difficulty to prepare a pounds. The specific surface area was measured on a pure phase, LiNb O has been rarely studied, while for surface area apparatus (Micromeritics ASAP 2460) at 3 8 LiNbO powders, the preparation technologies are various, 77 K by N adsorption/desorption method (BET 3 2 such as sol-gel [19], hydrothermal [21], and laser irradiation method). The photoluminescence (PL) spectra were de- methods [22]. Hydrothermal method is widely used to tected using an F-280 fluorescence spectrophotometer synthesize nanomaterials with advantages such as low with an excitation wavelength of 320 nm. temperature, environmental friendliness, and homogenous To evaluate the photocatalytic performance of Li-Nb-O particle-size distribution, which can efficiently avoid the compounds, the degradation of methylene blue (MB) variation of Li/Nb molar ratio without going through high aqueous solution (5 mg/L) was carried out under irradi- temperatures. As for hydrothermal method, the parameters ation of a 500 W Hg lamp at a natural pH value. Fifty mil- of reaction temperature, raw material ratio, and holding ligrams of powders were dispersed into 50 mL of MB time play important roles in determining the as-obtained aqueous solution. Before the irradiation, the suspension materials, while the research of Li/Nb ratio much larger was stirred in dark for 1 h to achieve adsorption equilib- than 1:1 in the preparation of Li-Nb-O compounds has not rium. Then, the suspension was irradiated by the Hg lamp. been reported before. The concentration of residual MB was analyzed with an In this paper, the effects of Li/Nb ratio on the prepar- interval of 30 min using an ultraviolet-visible near- ation of Li-Nb-O compounds by a hydrothermal method infrared (UV-vis-NIR) spectrophotometer at 665 nm. were studied deeply. A series of analytical techniques were used to characterize the crystallinity, morphology, Results and Discussion and chemical composition of the Li-Nb-O samples, es- The XRD patterns of the products obtained after hydro- pecially the changes before and after the hydrothermal thermal reaction with different Li/Nb mole ratios are reaction. Pure LiNb O and LiNbO photocatalysts were shown in Fig. 1. It is obvious that pure LiNbO phase 3 8 3 3 prepared, and the photocatalytic performance was stud- (JCPDF, No. 20-0631) is obtained with Li:Nb = 2:1. For ied with the effect of Li/Nb ratio in raw materials. the ratio of Li/Nb smaller than 2:1, such as 1:1 or 1:3, the main phase is still LiNbO , accompanied with the re- Methods sidual of Nb O (JCPDF, No. 37-1468), which means 2 5 The preparation of Li-Nb-O compounds was carried out that the Li content is not sufficient to fully react with by the hydrothermal method using lithium hydroxide Nb O to form LiNbO . When we increase the Li con- 2 5 3 monohydrate (LiOH·H O; Aladdin, ACS, ≥ 98.0%) and tent largely, an amazing phenomenon occurs: there is no niobium pentaoxide (Nb O ; Aladdin, AR, 99.9%) as start- LiNbO formed at all after the hydrothermal reaction, as 2 5 3 ing materials. Firstly, 3.5 mmol of Nb O was dispersed 2 5 into 35 ml deionized water with a certain amount of LiOH·H O under magnetic stirring. The mole ratios of Li:Nb are 1:3, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, and 8:1; as the results of the samples prepared with ratios of 4:1, 5:1, 6:1, and 7:1 are similar, only the ratios of Li:Nb = 4:1 and 7:1 are shown below. The suspension solutions were put into 50-mL Teflon-lined hydrothermal synthesis autoclave re- actors and maintained at 260 °C for 24 h, then cooled down naturally to room temperature. The as-obtained powders were then washed with deionized water and etha- nol for several times and dried at 60 °C. Finally, the prod- ucts were calcined at various temperatures from 500 to 800 °C for 2 h with a ramp rate of 5 °C/min. The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Discover diffractometer with Cu Kα Fig. 1 XRD patterns of the Li-Nb-O powders obtained after hydrothermal radiation (40 kV, 40 mA). The morphologies of the sam- reaction with different Li/Nb mole ratios ples were characterized by field emission scanning Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 3 of 7 clearly shown in Fig. 1. When the ratio of Li/Nb is 4:1 or larger, only Nb O phase exists in XRD patterns, no 2 5 other impurities detected. Is the Li ion washed away during the washing process? Just like the former litera- ture reported [23]. To illustrate the phase evolution when the Li/Nb ratio is large enough, the products obtained by the hydrother- mal method, using Li/Nb = 8:1 as an example, are cal- cined at different temperatures and the XRD patterns are shown in Fig. 2. When the products are calcined at 500 and 600 °C, a new phase LiNbO appears which proves that a Li element truly exists in the products ob- tained just after the hydrothermal reaction, though not detected by XRD. Also, a diffraction peak at 30.26° ap- pears at 600 °C, which can be indexed as (410) plane of monoclinic LiNb O . The reaction can be described by 3 8 Fig. 3 FTIR spectra of Nb O raw materials and Li-Nb-O powders 2 5 the following Eq. (1) [24]: (mole ratio of Li:Nb = 8:1) calcined at different temperatures LiNbO þ Nb O →LiNb O ð1Þ 3 2 5 3 8 −1 at 891 cm appears, while disappears at 700 °C, consist- ent with the XRD results of the formation and reaction At 700 °C, the monoclinic LiNb O is the predominant of LiNbO . At 700 and 800 °C, the bands at 908 and 3 8 3 −1 phase with almost negligible impurity. The pure phase 828 cm correspond to the formation of LiNb O com- 3 8 of LiNb O is obtained at 800 °C with all the diffraction pounds [26, 27]. The FTIR results are well consistent 3 8 peaks indexed to the monoclinic phase (JCPDF, No. 36- with the XRD results of Fig. 2. 0307), a space group of P21/a, which provides a new Based on the results, we can conclude that Li/Nb ratio way to prepare LiNb O compounds. has a great impact on the formation of LiNbO ; the ratio 3 8 3 FTIR test is also performed to study the phase evolu- smaller than 3:1 is beneficial to the formation of tion of the products with Li:Nb = 8:1, as shown in Fig. 3. LiNbO , while larger than 3:1, no LiNbO forms at all. 3 3 The raw material Nb O is tested as a reference. In Fig. Based on the diagram, the congruent Li content is 2 5 −1 3, the band at 962 cm due to the stretching vibrations 97.2 mol% of the Nb content for the preparation of per- of Nb = O in Nb O is existent until 700 °C [25]. After fect single-phase LiNbO , and the excess or deficiency of 2 5 3 hydrothermal reaction, no other bands detected at this the Li content is compensated by the formation of range means the only niobate is still Nb O . When the Li NbO or LiNb O phase [28]. The large excess of 2 5 3 4 3 8 calcination temperature is 500 and 600 °C, a new band LiOH is beneficial to form Li NbO not LiNbO , while 3 4 3 no Li NbO phase is observed after hydrothermal reac- 3 4 tion due to the insufficient reaction condition; even if the LiNbO particle locally formed, it is easily dissolved in LiOH solution with strong alkalinity [29]. As discussed above, the Li element is not detected after the hydrothermal reaction without further calcin- ation, while it truly exists in the products with Li:Nb = 8:1. For Nb O , is it still the same as the raw 2 5 material after the hydrothermal reaction? The XPS test is carried out to characterize the chemical component of Nb O raw material and the products obtained after 2 5 hydrothermal reaction, as shown in Fig. 4. The differ- ence of Nb 3d and 3d is the 2.7 eV for both sam- 3/2 5/2 5+ ples, indicating the Nb state in both samples without other reduced Nb oxides species [3]. The binding ener- gies of Nb 3d shift towards the low binding-energy state after the hydrothermal reaction, about 0.5 eV difference. Fig. 2 XRD patterns of the Li-Nb-O powders (mole ratio of Li:Nb = 8:1) It means that the chemical environment of Nb changes, calcined at different temperatures for 2 h while no other compounds are formed. The change may Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 4 of 7 Fig. 4 XPS spectra of Nb O raw materials and the products (mole 2 5 Fig. 6 XRD patterns of three typical Li/Nb ratios products calcined at ratio of Li:Nb = 8:1) obtained after hydrothermal method 800 °C for 2 h be due to the existence of Li ions in the product; though no obvious Li-Nb-O compound is formed, the existence absolutely opposite Li/Nb ratios: 8:1 and 1:3 (designated of Li ions with larger iconicity has strong attraction of O as LiNb O -8:1 and LiNb O -1:3). For other Li/Nb ratios 3 8 3 8 ions around Nb, resulting in the chemical shift of Nb 3d not shown in Fig. 6, the products calcined at 800 °C result binding energy. in the formation of two mixed phases: LiNb O and 3 8 The influence of Li ions on Nb O is also observed in LiNbO . Based on the XRD results, pure LiNb O pow- 2 5 3 3 8 SEM images, as shown in Fig. 5. Figure 5a is the image ders are prepared with two different Li/Nb ratios, while is of Nb O raw material, with irregular shape, dense there any differences between the two products? 2 5 structure, and length of several micrometers. After the The SEM images of the two products are displayed as hydrothermal reaction, the large crystal particle is di- Fig. 7b, c, respectively. As shown in Fig. 7, the morph- vided into small particles with the maximum size of ology of LiNb O -1:3 are quite different with that of 3 8 about 200 nm, though the small particles still aggregate LiNb O -8:1. LiNb O -8:1 has a porous and hollow 3 8 3 8 together. From the XRD and XPS results, we know that structure formed by LiNb O nanoparticles with the 3 8 the small particles are still Nb O . The change of the length of several micrometers, similar as that of a honey- 2 5 morphology of Nb O can be attributed to the hydro- comb. It is quite different with the particle aggregation 2 5 thermal condition and large content of LiOH·H Oin of solid-state reaction, as LiNb O -1:3 shown. The BET 2 3 8 raw materials. areas of LiNb O -8:1 and LiNb O -1:3 are 4.46 and 3 8 3 8 The products obtained after hydrothermal reaction are 0.96 m /g, respectively, the larger surface area of the calcined at 800 °C with different Li/Nb ratios. Hereafter, we choose three typical Li/Nb ratios as examples: 1:3, 2:1, and 8:1. The XRD patterns of the three samples are shown in Fig. 6. From the XRD results, pure LiNbO are pre- pared with Li/Nb = 2:1 and has shown no change even when calcined at 800 °C. For the preparation of another Li-Nb-O compound LiNb O , it can be obtained with two 3 8 Fig. 5 SEM images of a Nb O raw materials and b the products Fig. 7 SEM images of three typical Li/Nb ratios products calcined at 2 5 (mole ratio of Li:Nb = 8:1) obtained after hydrothermal method different temperatures: a 2:1 at 500 °C, b 1:3, c 8:1, and d 2:1 at 800 °C Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 5 of 7 former results from the porous and hollow structure. The photocatalytic performances of LiNb O and 3 8 The morphology difference can be attributed to the dif- LiNbO are shown in Fig. 8. From the UV-vis absorption ferent morphologies of the reactants: for LiNb O -8:1, spectra of MB at the varied irradiation time (Fig. 8a–d), it is 3 8 the reactant of LiNbO is formed based on the calcina- observed that the maximum absorption band (665 nm) be- tions of the products after hydrothermal reaction, the comes weak with the increase of the irradiation time. The morphology of the products is shown in Fig. 5b, while degradation rate of MB is largely improved with the cata- for LiNb O -1:3, the morphology of LiNbO obtained lysts of LiNb O and LiNbO under UV light, especially for 3 8 3 3 8 3 directly after the hydrothermal reaction is hexahedron- LiNb O -8:1, about 85% of MB degraded after 30 min ir- 3 8 like, as shown in Fig. 7a [21]. The formation of the por- radiation, as shown in Fig. 8e. As the photo-degradation of ous and hollow structure for LiNb O -8:1 can be attrib- MB using Li-Nb-O catalysts obeys the pseudo-first-order 3 8 uted to the lithium volatilization during the calcinations kinetics, described by the modified Langmuir-Hinshelwood process, which is beneficial to the formation of new kinetics mode [30], the constants of the pseudo-first-order LiNb O particles and networks between the particles rate (k) are calculated, displayed in Fig. 8f. The obtained 3 8 [11]. For LiNbO calcined at 800 °C (i.e., Li/Nb = 2:1), first-order rate constants of MB without catalysts, −2 its grain size is about 200 nm and the shape seems ir- LiNb O -1:3, LiNbO ,and LiNb O -8:1 are 0.71 × 10 , 3 8 3 3 8 −2 −2 −2 −1 regular, as shown in Fig. 7d; the BET area is about 1.61 × 10 ,4.18× 10 ,and6.73 ×10 min ,respect- 3.91 m /g. ively. The higher the first-order rate constant is, the more Fig. 8 UV-vis absorption spectra of the degradation of MB: a without catalyst and catalyzed by b LiNb O -1:3, c LiNbO ,and d LiNb O -8:1, respectively. 3 8 3 3 8 e Photo-degradation of MB and f kinetic fit with respect to the irradiation time using Li-Nb-O powders Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 6 of 7 outstanding the photocatalytic performance is. The k of LiNbO , while larger than 3:1, forms no LiNbO at all 3 3 LiNb O -8:1 is 9.5 times of MB without catalysts, 4.2 times and the morphology and chemical bond of Nb O raw 3 8 2 5 of LiNb O -1:3, and 1.6 times of LiNbO .Compared with material are totally modified by Li ions. The reason can 3 8 3 LiNb O -1:3, the higher photocatalytic performance of be attributed to the large content of LiOH, which is 3 8 LiNb O -8:1 can be attributed to the unique porous and beneficial to form Li NbO not LiNbO , and also, even 3 8 3 4 3 hollow structure, which provides a high density of active if the LiNbO particle locally forms, it is easily dissolved sites for the degradation of MB [31]. in LiOH solution with strong alkalinity. Pure LiNb O 3 8 Compared to LiNbO , the improved photocatalytic per- powders are obtained with two absolutely opposite Li/ formance of LiNb O -8:1, which has almost the same ab- Nb ratios: 8:1 and 1:3; the former shows a unique porous 3 8 sorption ability of MB as that of LiNbO ,can be and hollow structure, quite different with the particle ag- attributed to its layered structure type with the reduce gregation (the latter shows). Compared with Li/Nb = 1:3, symmetry. The layered structure can enhance the separ- higher photocatalytic performance of LiNb O (Li/ 3 8 ation of electrons and holes [32], consistent with the PL Nb = 8:1) are observed and it can be attributed to the spectra, as shown in Fig. 9. At the same time, the LiNb O unique porous and hollow structure, which provides a 3 8 framework is constructed by three different niobate octa- high density of active sites for the degradation of MB. hedrons and Li atoms share partial octahedral sites; the Compared to LiNbO , the improved photocatalytic per- higher niobate octahedral site is expected to provide more formance of LiNb O can be attributed to its layered 3 8 active sites for photocatalysis. Finally, the smaller energy structure type with the reduced symmetry enhancing the band gap of LiNb O (about 3.9 eV) than that of LiNbO separation of electrons and holes. 3 8 3 (4.14 eV) means that it can utilize more incident light to Acknowledgements participate in the photocatalytic process [33]. This work was financially supported by the National Natural Science The separation efficiency of photogenerated carries of Foundation of China (No. 51202107) and the Foundation of Henan Educational Committee (No. 16A140028). Li-Nb-O catalyst are investigated by PL spectra, as shown in Fig. 9. As we know, PL emission spectra mainly result Authors’ contributions from the recombination of free carriers. As seen in Fig. 9, HZ and HL conceived and designed the experiments; HL and LZ prepared the samples; CH and ZW performed the XRD and SEM measurements; JQ LiNb O shows smaller emitting peaks around 470 nm 3 8 performed the XPS; JY participated in the photocatalytic test; HZ wrote the than LiNbO . It means that LiNb O has longer charge 3 3 8 paper. All of the authors read and approved the final manuscript. carrier lifetime and improved efficiency of interfacial Competing interests charge transfer, which can be attributed to its layered The authors declare that they have no competing interests. structure with the reduced symmetry enhancing the separ- ation of electrons and holes. Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in Conclusions published maps and institutional affiliations. From the results above, we can conclude that Li/Nb ra- Received: 3 June 2017 Accepted: 11 August 2017 tio has a great impact on the formation of LiNbO ; the ratio smaller than 3:1 is beneficial to the formation of References 1. Nico C, Monteiro T, Graça MPF (2016) Niobium oxides and niobates physical properties: review and prospects. Prog Mater Sci 80:1–37 2. Zhai HF, Li AD, Kong JZ, Li XF, Zhao J, Guo BL, Yin J, Li ZS, Wu D (2013) Preparation and visible-light photocatalytic properties of BiNbO and BiTaO 4 4 by a citrate method. J Solid State Chem 202:6–14 3. Zhai HF, Shang SY, Zheng LY, Li PP, Li HQ, Luo HY, Kong JZ (2016) Efficient visible-light photocatalytic properties in low-temperature Bi-Nb-O system photocatalysts. Nanoscale Res Lett 11:383 4. 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The Effects of Li/Nb Ratio on the Preparation and Photocatalytic Performance of Li-Nb-O Compounds

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Abstract

The effects of Li/Nb ratio on the preparation of Li-Nb-O compounds by a hydrothermal method were studied deeply. Li/Nb ratio has a great impact on the formation of LiNbO ; the ratio smaller than 3:1 is beneficial to the formation of LiNbO , while larger than 3:1, forms no LiNbO at all and the morphology and chemical bond of 3 3 Nb O raw material are totally modified by Li ions. The reason can be attributed to the large content of LiOH, 2 5 which is beneficial to form Li NbO not LiNbO , and also, even if LiNbO particle locally forms, it is easily dissolved 3 4 3 3 in LiOH solution with strong alkalinity. Pure LiNb O powders are obtained with two absolutely opposite Li/Nb 3 8 ratios: 8:1 and 1:3; the former shows a unique porous and hollow structure, quite different from the particle aggregation (the latter shows). Compared with Li/Nb = 1:3, the 4.2 times higher photocatalytic performance of LiNb O (Li/Nb = 8:1) are observed and it can be attributed to the unique porous and hollow structure, which 3 8 provides a high density of active sites for the degradation of MB. Compared to LiNbO , the improved photocatalytic performance of LiNb O can be attributed to its layered structure type with the reduced symmetry 3 8 enhancing the separation of electrons and holes. Keywords: Lithium Niobate, Hydrothermal, Porous Materials, Photocatalysis Background structures which favor a possible delocalization of charge Niobium compounds, a very versatile group of materials, carriers [12]. Secondly, the conduction bands consisting of including niobium oxides, alkali niobates, and columbite Nb4d orbitals located at a more negative state of redox niobates, exhibit many interesting physical properties and potential of H+/H promote the separation and transfer of have been widely studied in many fields, such as catalysis photo-induced charge carriers and result in high photo- [1–3], memristors [4], dye-sensitized solar cells [5], optical catalytic activities [13]. Among these materials, LiNb O 3 8 devices, and others [6, 7]. LiNbO , as one of the most fam- displays unique performances. As a novel lithium-ion bat- ous alkali niobates, presents prominent properties such as tery (LIB) anode material, the theoretical capacity of electro-optical and nonlinear optical behaviors, pyroelec- LiNb O is 389 mAh/g assuming two-electron transfers 3 8 5+ 3+ tricity, and piezoelectricity, and it is mainly used as optical (Nb → Nb ), larger than many other anode materials, modulators, waveguides, acoustic wave transducers, et al. such as Li Ti O [14, 15]. Used for supercapacitor de- 4 5 12 in optical devices. vices, LiNb O nanoflakes show excellent cycle stability 3 8 For environmental remediation and clean energy appli- with negligible specific capacitance decrease even after cations, niobates, such as (Na, K)NbO [8], BiNbO [9], 15,000 cycles [16]. Also, it is used as an efficient photoca- 3 4 LiNbO [10], and LiNb O [11], have been deeply investi- talyst in the applications of hydrogen generation and deg- 3 3 8 gated, owing to their special distorted [NbO ]octahedral radation of organic pollutants. Pure LiNb O is a highly 6 3 8 active UV-photocatalyst for water reduction producing * Correspondence: haifazhai@126.com 83.87 μmol of hydrogen in 1 h, and it does not produce Henan Key Laboratory of Photovoltaic Materials, College of Physics and hydrogen under visible-light irradiation due to its large Materials Science, Henan Normal University, Xinxiang 453007, People’s band gap (i.e., 3.9 eV) and inability to absorb visible light Republic of China National Laboratory of Solid State Microstructures, Nanjing University, [17, 18]. LiNb O nanoflakes show fast decolorization of 3 8 Nanjing 210093, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 2 of 7 toluidine blue O (TBO) dye under UV light compared to electron microscope (FESEM; JSM-6700F). Chemical commercial TiO powders [13]. bonds were analyzed by Fourier-transformed infrared −1 At most time, the appearance of LiNb O is recognized spectroscopy (FTIR) in the range of 2000–650 cm .X- 3 8 as an impurity phase during the preparation of LiNbO , ray photoelectron spectroscopy (XPS) analysis was per- especially in film samples, owing to high annealing formed on a Thermo-Fisher Escalab 250Xi instrument to temperature or inhomogeneous distribution of Li element characterize the chemical component of Li-Nb-O com- in precursors [19, 20]. Due to the difficulty to prepare a pounds. The specific surface area was measured on a pure phase, LiNb O has been rarely studied, while for surface area apparatus (Micromeritics ASAP 2460) at 3 8 LiNbO powders, the preparation technologies are various, 77 K by N adsorption/desorption method (BET 3 2 such as sol-gel [19], hydrothermal [21], and laser irradiation method). The photoluminescence (PL) spectra were de- methods [22]. Hydrothermal method is widely used to tected using an F-280 fluorescence spectrophotometer synthesize nanomaterials with advantages such as low with an excitation wavelength of 320 nm. temperature, environmental friendliness, and homogenous To evaluate the photocatalytic performance of Li-Nb-O particle-size distribution, which can efficiently avoid the compounds, the degradation of methylene blue (MB) variation of Li/Nb molar ratio without going through high aqueous solution (5 mg/L) was carried out under irradi- temperatures. As for hydrothermal method, the parameters ation of a 500 W Hg lamp at a natural pH value. Fifty mil- of reaction temperature, raw material ratio, and holding ligrams of powders were dispersed into 50 mL of MB time play important roles in determining the as-obtained aqueous solution. Before the irradiation, the suspension materials, while the research of Li/Nb ratio much larger was stirred in dark for 1 h to achieve adsorption equilib- than 1:1 in the preparation of Li-Nb-O compounds has not rium. Then, the suspension was irradiated by the Hg lamp. been reported before. The concentration of residual MB was analyzed with an In this paper, the effects of Li/Nb ratio on the prepar- interval of 30 min using an ultraviolet-visible near- ation of Li-Nb-O compounds by a hydrothermal method infrared (UV-vis-NIR) spectrophotometer at 665 nm. were studied deeply. A series of analytical techniques were used to characterize the crystallinity, morphology, Results and Discussion and chemical composition of the Li-Nb-O samples, es- The XRD patterns of the products obtained after hydro- pecially the changes before and after the hydrothermal thermal reaction with different Li/Nb mole ratios are reaction. Pure LiNb O and LiNbO photocatalysts were shown in Fig. 1. It is obvious that pure LiNbO phase 3 8 3 3 prepared, and the photocatalytic performance was stud- (JCPDF, No. 20-0631) is obtained with Li:Nb = 2:1. For ied with the effect of Li/Nb ratio in raw materials. the ratio of Li/Nb smaller than 2:1, such as 1:1 or 1:3, the main phase is still LiNbO , accompanied with the re- Methods sidual of Nb O (JCPDF, No. 37-1468), which means 2 5 The preparation of Li-Nb-O compounds was carried out that the Li content is not sufficient to fully react with by the hydrothermal method using lithium hydroxide Nb O to form LiNbO . When we increase the Li con- 2 5 3 monohydrate (LiOH·H O; Aladdin, ACS, ≥ 98.0%) and tent largely, an amazing phenomenon occurs: there is no niobium pentaoxide (Nb O ; Aladdin, AR, 99.9%) as start- LiNbO formed at all after the hydrothermal reaction, as 2 5 3 ing materials. Firstly, 3.5 mmol of Nb O was dispersed 2 5 into 35 ml deionized water with a certain amount of LiOH·H O under magnetic stirring. The mole ratios of Li:Nb are 1:3, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, and 8:1; as the results of the samples prepared with ratios of 4:1, 5:1, 6:1, and 7:1 are similar, only the ratios of Li:Nb = 4:1 and 7:1 are shown below. The suspension solutions were put into 50-mL Teflon-lined hydrothermal synthesis autoclave re- actors and maintained at 260 °C for 24 h, then cooled down naturally to room temperature. The as-obtained powders were then washed with deionized water and etha- nol for several times and dried at 60 °C. Finally, the prod- ucts were calcined at various temperatures from 500 to 800 °C for 2 h with a ramp rate of 5 °C/min. The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Discover diffractometer with Cu Kα Fig. 1 XRD patterns of the Li-Nb-O powders obtained after hydrothermal radiation (40 kV, 40 mA). The morphologies of the sam- reaction with different Li/Nb mole ratios ples were characterized by field emission scanning Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 3 of 7 clearly shown in Fig. 1. When the ratio of Li/Nb is 4:1 or larger, only Nb O phase exists in XRD patterns, no 2 5 other impurities detected. Is the Li ion washed away during the washing process? Just like the former litera- ture reported [23]. To illustrate the phase evolution when the Li/Nb ratio is large enough, the products obtained by the hydrother- mal method, using Li/Nb = 8:1 as an example, are cal- cined at different temperatures and the XRD patterns are shown in Fig. 2. When the products are calcined at 500 and 600 °C, a new phase LiNbO appears which proves that a Li element truly exists in the products ob- tained just after the hydrothermal reaction, though not detected by XRD. Also, a diffraction peak at 30.26° ap- pears at 600 °C, which can be indexed as (410) plane of monoclinic LiNb O . The reaction can be described by 3 8 Fig. 3 FTIR spectra of Nb O raw materials and Li-Nb-O powders 2 5 the following Eq. (1) [24]: (mole ratio of Li:Nb = 8:1) calcined at different temperatures LiNbO þ Nb O →LiNb O ð1Þ 3 2 5 3 8 −1 at 891 cm appears, while disappears at 700 °C, consist- ent with the XRD results of the formation and reaction At 700 °C, the monoclinic LiNb O is the predominant of LiNbO . At 700 and 800 °C, the bands at 908 and 3 8 3 −1 phase with almost negligible impurity. The pure phase 828 cm correspond to the formation of LiNb O com- 3 8 of LiNb O is obtained at 800 °C with all the diffraction pounds [26, 27]. The FTIR results are well consistent 3 8 peaks indexed to the monoclinic phase (JCPDF, No. 36- with the XRD results of Fig. 2. 0307), a space group of P21/a, which provides a new Based on the results, we can conclude that Li/Nb ratio way to prepare LiNb O compounds. has a great impact on the formation of LiNbO ; the ratio 3 8 3 FTIR test is also performed to study the phase evolu- smaller than 3:1 is beneficial to the formation of tion of the products with Li:Nb = 8:1, as shown in Fig. 3. LiNbO , while larger than 3:1, no LiNbO forms at all. 3 3 The raw material Nb O is tested as a reference. In Fig. Based on the diagram, the congruent Li content is 2 5 −1 3, the band at 962 cm due to the stretching vibrations 97.2 mol% of the Nb content for the preparation of per- of Nb = O in Nb O is existent until 700 °C [25]. After fect single-phase LiNbO , and the excess or deficiency of 2 5 3 hydrothermal reaction, no other bands detected at this the Li content is compensated by the formation of range means the only niobate is still Nb O . When the Li NbO or LiNb O phase [28]. The large excess of 2 5 3 4 3 8 calcination temperature is 500 and 600 °C, a new band LiOH is beneficial to form Li NbO not LiNbO , while 3 4 3 no Li NbO phase is observed after hydrothermal reac- 3 4 tion due to the insufficient reaction condition; even if the LiNbO particle locally formed, it is easily dissolved in LiOH solution with strong alkalinity [29]. As discussed above, the Li element is not detected after the hydrothermal reaction without further calcin- ation, while it truly exists in the products with Li:Nb = 8:1. For Nb O , is it still the same as the raw 2 5 material after the hydrothermal reaction? The XPS test is carried out to characterize the chemical component of Nb O raw material and the products obtained after 2 5 hydrothermal reaction, as shown in Fig. 4. The differ- ence of Nb 3d and 3d is the 2.7 eV for both sam- 3/2 5/2 5+ ples, indicating the Nb state in both samples without other reduced Nb oxides species [3]. The binding ener- gies of Nb 3d shift towards the low binding-energy state after the hydrothermal reaction, about 0.5 eV difference. Fig. 2 XRD patterns of the Li-Nb-O powders (mole ratio of Li:Nb = 8:1) It means that the chemical environment of Nb changes, calcined at different temperatures for 2 h while no other compounds are formed. The change may Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 4 of 7 Fig. 4 XPS spectra of Nb O raw materials and the products (mole 2 5 Fig. 6 XRD patterns of three typical Li/Nb ratios products calcined at ratio of Li:Nb = 8:1) obtained after hydrothermal method 800 °C for 2 h be due to the existence of Li ions in the product; though no obvious Li-Nb-O compound is formed, the existence absolutely opposite Li/Nb ratios: 8:1 and 1:3 (designated of Li ions with larger iconicity has strong attraction of O as LiNb O -8:1 and LiNb O -1:3). For other Li/Nb ratios 3 8 3 8 ions around Nb, resulting in the chemical shift of Nb 3d not shown in Fig. 6, the products calcined at 800 °C result binding energy. in the formation of two mixed phases: LiNb O and 3 8 The influence of Li ions on Nb O is also observed in LiNbO . Based on the XRD results, pure LiNb O pow- 2 5 3 3 8 SEM images, as shown in Fig. 5. Figure 5a is the image ders are prepared with two different Li/Nb ratios, while is of Nb O raw material, with irregular shape, dense there any differences between the two products? 2 5 structure, and length of several micrometers. After the The SEM images of the two products are displayed as hydrothermal reaction, the large crystal particle is di- Fig. 7b, c, respectively. As shown in Fig. 7, the morph- vided into small particles with the maximum size of ology of LiNb O -1:3 are quite different with that of 3 8 about 200 nm, though the small particles still aggregate LiNb O -8:1. LiNb O -8:1 has a porous and hollow 3 8 3 8 together. From the XRD and XPS results, we know that structure formed by LiNb O nanoparticles with the 3 8 the small particles are still Nb O . The change of the length of several micrometers, similar as that of a honey- 2 5 morphology of Nb O can be attributed to the hydro- comb. It is quite different with the particle aggregation 2 5 thermal condition and large content of LiOH·H Oin of solid-state reaction, as LiNb O -1:3 shown. The BET 2 3 8 raw materials. areas of LiNb O -8:1 and LiNb O -1:3 are 4.46 and 3 8 3 8 The products obtained after hydrothermal reaction are 0.96 m /g, respectively, the larger surface area of the calcined at 800 °C with different Li/Nb ratios. Hereafter, we choose three typical Li/Nb ratios as examples: 1:3, 2:1, and 8:1. The XRD patterns of the three samples are shown in Fig. 6. From the XRD results, pure LiNbO are pre- pared with Li/Nb = 2:1 and has shown no change even when calcined at 800 °C. For the preparation of another Li-Nb-O compound LiNb O , it can be obtained with two 3 8 Fig. 5 SEM images of a Nb O raw materials and b the products Fig. 7 SEM images of three typical Li/Nb ratios products calcined at 2 5 (mole ratio of Li:Nb = 8:1) obtained after hydrothermal method different temperatures: a 2:1 at 500 °C, b 1:3, c 8:1, and d 2:1 at 800 °C Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 5 of 7 former results from the porous and hollow structure. The photocatalytic performances of LiNb O and 3 8 The morphology difference can be attributed to the dif- LiNbO are shown in Fig. 8. From the UV-vis absorption ferent morphologies of the reactants: for LiNb O -8:1, spectra of MB at the varied irradiation time (Fig. 8a–d), it is 3 8 the reactant of LiNbO is formed based on the calcina- observed that the maximum absorption band (665 nm) be- tions of the products after hydrothermal reaction, the comes weak with the increase of the irradiation time. The morphology of the products is shown in Fig. 5b, while degradation rate of MB is largely improved with the cata- for LiNb O -1:3, the morphology of LiNbO obtained lysts of LiNb O and LiNbO under UV light, especially for 3 8 3 3 8 3 directly after the hydrothermal reaction is hexahedron- LiNb O -8:1, about 85% of MB degraded after 30 min ir- 3 8 like, as shown in Fig. 7a [21]. The formation of the por- radiation, as shown in Fig. 8e. As the photo-degradation of ous and hollow structure for LiNb O -8:1 can be attrib- MB using Li-Nb-O catalysts obeys the pseudo-first-order 3 8 uted to the lithium volatilization during the calcinations kinetics, described by the modified Langmuir-Hinshelwood process, which is beneficial to the formation of new kinetics mode [30], the constants of the pseudo-first-order LiNb O particles and networks between the particles rate (k) are calculated, displayed in Fig. 8f. The obtained 3 8 [11]. For LiNbO calcined at 800 °C (i.e., Li/Nb = 2:1), first-order rate constants of MB without catalysts, −2 its grain size is about 200 nm and the shape seems ir- LiNb O -1:3, LiNbO ,and LiNb O -8:1 are 0.71 × 10 , 3 8 3 3 8 −2 −2 −2 −1 regular, as shown in Fig. 7d; the BET area is about 1.61 × 10 ,4.18× 10 ,and6.73 ×10 min ,respect- 3.91 m /g. ively. The higher the first-order rate constant is, the more Fig. 8 UV-vis absorption spectra of the degradation of MB: a without catalyst and catalyzed by b LiNb O -1:3, c LiNbO ,and d LiNb O -8:1, respectively. 3 8 3 3 8 e Photo-degradation of MB and f kinetic fit with respect to the irradiation time using Li-Nb-O powders Zhai et al. Nanoscale Research Letters (2017) 12:496 Page 6 of 7 outstanding the photocatalytic performance is. The k of LiNbO , while larger than 3:1, forms no LiNbO at all 3 3 LiNb O -8:1 is 9.5 times of MB without catalysts, 4.2 times and the morphology and chemical bond of Nb O raw 3 8 2 5 of LiNb O -1:3, and 1.6 times of LiNbO .Compared with material are totally modified by Li ions. The reason can 3 8 3 LiNb O -1:3, the higher photocatalytic performance of be attributed to the large content of LiOH, which is 3 8 LiNb O -8:1 can be attributed to the unique porous and beneficial to form Li NbO not LiNbO , and also, even 3 8 3 4 3 hollow structure, which provides a high density of active if the LiNbO particle locally forms, it is easily dissolved sites for the degradation of MB [31]. in LiOH solution with strong alkalinity. Pure LiNb O 3 8 Compared to LiNbO , the improved photocatalytic per- powders are obtained with two absolutely opposite Li/ formance of LiNb O -8:1, which has almost the same ab- Nb ratios: 8:1 and 1:3; the former shows a unique porous 3 8 sorption ability of MB as that of LiNbO ,can be and hollow structure, quite different with the particle ag- attributed to its layered structure type with the reduce gregation (the latter shows). Compared with Li/Nb = 1:3, symmetry. The layered structure can enhance the separ- higher photocatalytic performance of LiNb O (Li/ 3 8 ation of electrons and holes [32], consistent with the PL Nb = 8:1) are observed and it can be attributed to the spectra, as shown in Fig. 9. At the same time, the LiNb O unique porous and hollow structure, which provides a 3 8 framework is constructed by three different niobate octa- high density of active sites for the degradation of MB. hedrons and Li atoms share partial octahedral sites; the Compared to LiNbO , the improved photocatalytic per- higher niobate octahedral site is expected to provide more formance of LiNb O can be attributed to its layered 3 8 active sites for photocatalysis. Finally, the smaller energy structure type with the reduced symmetry enhancing the band gap of LiNb O (about 3.9 eV) than that of LiNbO separation of electrons and holes. 3 8 3 (4.14 eV) means that it can utilize more incident light to Acknowledgements participate in the photocatalytic process [33]. This work was financially supported by the National Natural Science The separation efficiency of photogenerated carries of Foundation of China (No. 51202107) and the Foundation of Henan Educational Committee (No. 16A140028). Li-Nb-O catalyst are investigated by PL spectra, as shown in Fig. 9. As we know, PL emission spectra mainly result Authors’ contributions from the recombination of free carriers. As seen in Fig. 9, HZ and HL conceived and designed the experiments; HL and LZ prepared the samples; CH and ZW performed the XRD and SEM measurements; JQ LiNb O shows smaller emitting peaks around 470 nm 3 8 performed the XPS; JY participated in the photocatalytic test; HZ wrote the than LiNbO . It means that LiNb O has longer charge 3 3 8 paper. All of the authors read and approved the final manuscript. carrier lifetime and improved efficiency of interfacial Competing interests charge transfer, which can be attributed to its layered The authors declare that they have no competing interests. structure with the reduced symmetry enhancing the separ- ation of electrons and holes. Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in Conclusions published maps and institutional affiliations. From the results above, we can conclude that Li/Nb ra- Received: 3 June 2017 Accepted: 11 August 2017 tio has a great impact on the formation of LiNbO ; the ratio smaller than 3:1 is beneficial to the formation of References 1. Nico C, Monteiro T, Graça MPF (2016) Niobium oxides and niobates physical properties: review and prospects. Prog Mater Sci 80:1–37 2. Zhai HF, Li AD, Kong JZ, Li XF, Zhao J, Guo BL, Yin J, Li ZS, Wu D (2013) Preparation and visible-light photocatalytic properties of BiNbO and BiTaO 4 4 by a citrate method. J Solid State Chem 202:6–14 3. Zhai HF, Shang SY, Zheng LY, Li PP, Li HQ, Luo HY, Kong JZ (2016) Efficient visible-light photocatalytic properties in low-temperature Bi-Nb-O system photocatalysts. Nanoscale Res Lett 11:383 4. 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Nanoscale Research LettersSpringer Journals

Published: Aug 15, 2017

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