Synthesis, characterization and application of superhydrophobic low-cost Cu and Al nanoparticles

Synthesis, characterization and application of superhydrophobic low-cost Cu and Al nanoparticles Cu and Al nanoparticles were prepared using a simple chemical etching method followed by a chemical reduction method. The synthesized metal nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), electron-dispersive (EDX) spectrum and water contact angle (WCA) measurements. The application of Cu and Al nanoparticles was tested towards the catalytic reduction of 4-nitrophenol (NiP), hexavalent chro- mium {Cr(VI)} and rhodamine 6G (R6G) dye in the presence sodiumborohydride (NaBH ) as a reducing agent. From the UV–visible spectrum, the reduction rate constant (k ) and the induction time (T ) were determined and compared critically. app i The HRTEM analysis confirmed the nanosize of Cu and Al prepared by a simple chemical etching process followed by the chemical reduction method. Keywords Nanostructure · Characterization · Water contact angle · Catalyst · Reduction Introduction the open environment, they degrade the quality of soil and water highly. Hence, it is necessary to remove the pollut- The environment is spoiled due to the various activities ant or to reduce the pollution due to phenolic compounds, of human beings such as industrialization, modernization particularly NiP, dye effluents and Cr(VI) ion. On the other and civilization. Among these, the industrialization plays a hand, the reduced product of NiP plays a vital role in the vital role in disturbing the environmental quality because of pharmaceutical field as a starting material for the prepara - the effluents that are highly toxic and hazardous, and sim - tion of drugs and it is not possible to remove the NiP from ply disposed into the open environment, for example, the the chemical industry. Moreover, the catalytic reduction of nitrophenols (NiP) from chemical industries [1, 2], hexa- NiP is an interesting one because of its pseudo-first-order valent chromium [Cr(VI)] from chromium industry, metal kinetics. Hence, the reduction of NiP using various cata- finishing industry, tannery [ 3] and dye effluents from dye lyst systems is the only way to reduce the pollution caused and textile industries [4, 5]. When they are disposed into by the NiP. Currently, various catalyst systems such as iron niobate [6], hydroxyapatite [7], Au nanoparticles (NP) [8], Au–SiO [9], layered double hydroxides (LDH) [10], TiO 2 2 * R. Anbarasan [11], Al O [12], Pd–Ag [13], AC–Ag [14], Au–CeO [15], 2 3 2 anbu_may3@yahoo.co.in aminoclay–Fe NP [16], Clay–Pt [17], Fe O [18], Ni–Se [19] 3 4 * Kuo-Lun Tung and Ni–P [20] are used for the reduction of NiP. Through kltung@ntu.edu.tw study of the literature, any report based on the Cu and Al nanoparticles as a catalyst for the reduction of NiP in the Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, presence of NaBH is not found. Moreover, the Cu and Al Tamil Nadu 626 001, India nanoparticles formed as by-products during the chemical Department of Mechanical Engineering, MEMS Thermal etching of metal surface followed by a chemical reduction Control Lab, National Taiwan University, Taipei 10617, are converted into useful materials such as catalyst for the Taiwan, ROC reduction reaction. Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Vol.:(0123456789) 1 3 148 International Nano Letters (2018) 8:147–156 Cr(III) is less toxic when compared with Cr(VI). Hence, The main aim of the present investigation is fabrication it is better to reduce the Cr(VI) using different methodology. of low-cost S.H Cu and Al nanoparticles and extending their Photocatalytic reduction of Cr(VI) over TiO particles was application towards the catalysis field. For the synthesis of reported by Testa et al. [21]. In 2014, Sadik and co-workers S.H Cu or Al metal plates, a standard literature procedure [22] studied the catalytic reduction of Cr(VI) using Pd NP. was followed [35]. The Cu or Al plate with the area of Cu(II)-catalyzed reduction of Cr(VI) was reported in the 12 cm was taken as a source material. The metal plate was literature [23]. Goethite surface-assisted catalytic reduction cleaned with acetone and ultrasonicated for 1 h in an aque- of Cr(VI) was reported by Kim and research team [24]. In ous medium. Then the plate was removed from the ultra- 2015, microbial system-catalyzed reduction of Cr(VI) was sonic bath and dried at 110 °C for 2 h and it was weighed reported in the literature [25]. The literature survey indicates (W g) and subjected to the chemical etching reaction in the that the catalytic reduction of Cr(VI) by Cu and Al nano- presence of 0.10 g MA in 100 mL ethanol medium under structured catalyst system is not available so far. This urged mild stirring condition. After 7 days of chemical etching, the researchers to do the present investigation. the metal plate was removed from the reaction medium and Rhodamine 6G (R6G) is a photostable organic fluorescent dried at 80 °C for 2 h (W g). The content of the medium was dye and has a wide range of applications in textile industry. dried at 80 °C for a night. The metal with hierarchical struc- Unfortunately, the conventional biological methods are less ture was obtained as a powder, weighed (W g) and stored effective towards the degradation or color removal of R6G in a zip lock cover. During the fabrication of S.H Cu or Al dye. In order to remove the color and to reduce the toxicity plate, the etching agent-coated Cu or Al is formed as a by- of R6G, it is necessary to reduce the structure of R6G in the product without any use. Thus, obtained hierarchically struc- presence of a catalyst. The catalytic reduction of R6G in tured Cu or Al powder was dispersed in an aqueous medium the presence of Ag–SiO nanocatalyst was carried out [26]. with the aid of ultrasonication for 30 min. Once the dis- Other catalysts such as SiO @TiO [27], ammonium phos- persion was completed, excess of NaBH was added under 2 2 4 phomolybdate [28], Au–Pd nanoalloy [29], TiO [30], and ultrasonication. The hierarchically structured Cu or Al is Au–SiO [31] were employed. Recently, Huang et al. used now subjected to chemical reduction reaction and reduced to bismuth oxyiodide [32, 33] and bismuth oxycarbonate [34] Cu and Al nanoparticles. The reduction reaction was allowed for the photocatalytic degradation of various organic dyes. to continue for another 30 min. At the end of the reaction, Few reports are available on the catalytic reduction of R6G the dark-colored precipitate was filtered, dried, weighed and in the presence of a nanostructured catalyst in the literature. stored in a zip lock cover under nitrogen atmosphere. During the fabrication of superhydrophobic (S.H) metal sur- Catalytic reduction study was carried out by the stand- −5 −5 faces by a simple chemical etching process, hierarchically ard procedure [7]. 2 mL of 6.3 × 10 M NiP or 1.1 × 10 −5 structured metal nanoparticles are formed as by-products. M K Cr O or 2.17 × 10 M R6G solution was taken in 2 2 7 −3 Very few applications are found for such hierarchic struc- a 3-mL capacity quartz cuvette microreactor. 1 × 10 g of tured materials. The present investigation is made to extend Cu or Al nanoparticle catalyst was weighed and added to the application of the S.H hierarchically structured metal the micro-reactor. 15 mg of NaBH was added in excess after the chemical reduction method in the catalysis field. to the nanoreactor. While adding N aBH in the presence Catalysis is a developing field and considerable amount of of a nanocatalyst, the NiP was converted into aminophenol research work is going on. The novelties of the present inves- and is mentioned in Scheme 1. The reduction reaction was tigation are easy preparation, economically cheaper and no use of hazardous solvents. Experimental 4-Nitrophenol (NiP, S.D. Fine Chemicals, India), potassium dichromate (PDC, S.D. Fine Chemicals), rhodamine6G (R6G, CDH Chemicals, India), myristic acid (MA, CDH Chemicals, India, etching agent) and sodium borohydride (NaBH , Himedia chemicals, India, reducing agent) were purchased and used as received. Cu and Al metal plates (resource material) with 99% purity were purchased locally with the following dimensions: length 6 cm, breadth 2 cm, area 12 cm . Double-distilled (DD) water was used for labo- ratory work. Scheme 1 Reduction of 4-NP, Cr(VI) and R6G 1 3 International Nano Letters (2018) 8:147–156 149 quantitatively measured with the help of the UV–visible the rod shape was determined as 3–5 µm with the breadth spectrophotometer at the interval of 1 min. of less than 700 nm. The arrow mark confirmed the same. UV–visible spectrum (Shimadzu 3600 NIR, Japan) The sphere-shaped NP with the size of 150  nm is also for samples was recorded in aqueous medium from 250 seen. Figure 2b indicates the SEM image of MA-encapsu- to 550 nm. Water contact angle (WCA) was measured by lated Al. The morphology of MA-encapsulated Al NP is Kyowa DMs-200, Japan model instrument. The surface entirely different from the MA-encapsulated Cu NP. The morphology with EDX spectrum was scanned by SEM, Al–MA system exhibits gel-like morphology with more JSM 6300, JEOL model instrument. FTIR spectra for the number of micro-voids. One or two NP are also seen in samples were recorded with the help of Shimadzu 8400 S, the circled area. This type of material is very much use- Japan, instrument by KBr pelletization method from 400 ful in drug delivery and catalytic application. The size −1 to 4000 cm . HRTEM image of the Cu and Al salts were of the voids is varied from 2 to 5 µm. Hence, the SEM recorded with the help of TEM 3010, JEOL instrument. The images confirmed the presence of nanostructured Cu–MA binding energy was determined by XPS (XPS, Thermo Sci- and Al–MA systems [36]. For the sake of comparison, the entific, Theta Probe, UK). SEM image of pristine Cu and Al NP is given in Fig. 2c, d, respectively. Here also one can see the spherical and rod-shaped Cu NP. In the case of Al NP, some gel-like Results and discussion morphology is seen. This concludes that even after the chemical etching process with simultaneous encapsula- Figure  1 indicates the FTIR spectra of MA-encapsulated tion, the surface morphology of the Cu and Al NP is not metal nanoparticles. Figure 1a confirms the FTIR spectrum changed. of MA-encapsulated Cu nanoparticle. The C–H symmet- Figure 3a indicates the HRTEM image of Cu–MA sys- ric and anti-symmetric stretching appeared at 2845 and tem. Here one can see the fiber and plate-like morphology. −1 2910 cm , respectively [35]. The C=O stretching of long- Agglomerated structure is also observed. The length of the −1 chain aliphatic carboxylic acid appeared at 1710 cm . The fiber was found to be 1–4 µm with the breadth of ~ 150 nm. −1 C–O linkage appeared at 937 cm . The metal–oxide (M–O) The length and breadth of the plate were calculated as 800 −1 stretching appeared at 544 cm . Figure 1b represents the and 250 nm, respectively. Figure 3b represents the HRTEM FTIR spectrum of MA-encapsulated Al nanoparticle. Peaks image of Al–MA system with nanoparticle-like morphol- appeared as explained earlier. Thus, the FTIR spectrum ogy. The size of the particle varied between 20 and 80 nm. confirmed the MA-encapsulated Cu and Al nanoparticle Hence, the HRTEM images confirmed the nanostructure of formation. Cu–MA and Al–MA systems. The surface morphology of the MA-encapsulated Cu In the present investigation, water-insoluble Cu–MA and nanoparticle is given in Fig. 2a. The image indicates the Al–MA were prepared. To confirm the heterogeneity of the presence of both rod and spherical particles. The length of catalysts, water contact angle (WCA) was measured. Fig- ure 4a represents the WCA image of Cu–MA and Fig. 4b represents WCA image of Al–MA systems. The WCA was determined as 147° and 140.1°, respectively, for Cu and Al systems. Among these two systems, the Cu–MA system exhibited the S.H character. The fluorinated long-chain ali- phatic acid also exhibited similar WCA [36]. Hence, the WCA measurement confirmed the SH and heterogeneous catalytic nature of Cu–MA and Al–MA systems. The percentage of elements present in the systems can be determined by the EDX spectrum. Figure 5a indicates the EDX spectrum of Cu–MA system and Fig. 5b represents the EDX spectrum of Al–MA system. From Fig. 5a, the content of Cu was determined as 1.84%. Figure 5b shows the content of Al as 2.68%. The XPS gives an idea about the outer most energy level of a material. Figure 6a represents the XPS of Cu–MA sys- tem. The appearance of Cu2p (953.1  eV) and Cu2p 1/2 3/2 (932.2  eV) confirmed that Cu is in nanosize. This is in accordance with the literature report [33]. The XPS of nano- sized Al–MA is given in Fig. 6b. The Al2p is appeared at Fig. 1 FTIR spectrum of (a) Cu salt, (b) Al salt system 1 3 150 International Nano Letters (2018) 8:147–156 Fig. 2 SEM image of a Cu–MA, b Al–MA system, c Cu, d Al system Fig. 3 TEM image of a Cu– MA, b Al–MA system 1 3 International Nano Letters (2018) 8:147–156 151 72.6 eV [37]. Thus, the XPS confirmed the formation of nano-sized Cu–MA and Al–MA. In the present investigation, the above-synthesized Cu and Al nanoparticles were tested towards the catalytic reduc- tion of NiP, PDC and R6G. The catalytic activities of metal nanoparticles toward three different reactions were tested. Catalytic reduction of NiP involves the conversion of nitro group into amino group. Catalytic reduction of PDC involves the conversion of Cr(VI) into Cr(III). Similarly, the cata- Fig. 4 WCA image of a Cu–MA, b Al–MA systems lytic reduction of R6G involves the conversion of R6G into reduced form of R6G, i.e., the reduction of extended double bond. The catalytic reduction activity is identified with the help of UV–visible spectrophotometer. Generally, metal or metal oxide NP is considered as an efficient catalyst system for the catalytic reduction reaction. The common example is the catalytic reduction of NiP at room temperature. The reduction follows a pseudo-first- order kinetics and it is considered as a model compound [38]. In the present investigation, an attempt is made towards the catalytic reduction of NiP using Cu and Al nanoparticles. The catalytic reduction was quantitatively followed with the help of UV–visible spectrophotometer. The UV–visible spectrum of NiP exhibits an absorbance peak at 401 nm (Fig. 7a). After the addition of the catalyst and NaBH , the absorbance reduced drastically at 401 nm within 6 min. This confirmed that the Cu–MA system is an efficient catalyst towards the reduction of NiP in Fig. 7b–g. The reduction of NiP into aminophenol proceeded through the formation of phenolate anion. To find out the reduction rate constant, the plot of time against ln(A/A ) (Fig. 7h) was drawn. The plot showed a straight line with negative trend. From the slope value, the apparent rate constant (k ) was calculated as app −3 −1 17.8 × 10  s (Table 1). The intercept value showed the induction time as 0.22 s. To compare the catalytic activity of Al nanoparticle, similar type of reduction study was carried out (Fig. 8a–k). The k value was determined from Fig. 8l app −3 −1 as 16.2 × 10  s with an induction time of 0.46 s (Table 1). The catalytic reduction study confirmed that the Cu nanopar - ticle is more efficient than the Al nanoparticle towards the Fig. 5 EDX spectrum of a Cu–MA, b Al–MA systems catalytic reduction of NiP in the presence of N aBH . Both Fig. 6 XPS of a Cu2p, b Al2p 1 3 152 International Nano Letters (2018) 8:147–156 Fig. 7 UV–visible spectrum of NiP taken at 1-min time interval in the presence of Cu–MA system a–g, the plot of time vs. ln(A/A ) h −3 −1 3.83 × 10  s . Hence, when compared with the literature, Table 1 The k and T values of the catalysts app i the present investigation yielded better results. −1 System k (s ) T (s) app i The catalytic reduction of Cr(VI) into Cr(III) was carried −3 Cu–MA–PDC 9.32 × 10 5.59 out in the presence of Cu and Al nanoparticles as an individ- −3 Cu–MA–R6G 21.2 × 10 4.19 ual catalyst using NaBH as a reducing agent. Figure 9a–m −3 Cu–MA–NiP 17.8 × 10 0.22 indicates the UV–visible spectrum of Cr(VI) measured at −3 Al–MA–PDC 6.44 × 10 2.32 time interval of 1 min in the given experimental conditions. −3 Al–MA–R6G 11.7 × 10 2.06 For Cu–MA system, it was found that while increasing −3 Al–MA–NiP 16.2 × 10 0.46 the reduction time the concentration of Cr(VI) at 372 nm reduced slowly. It means the Cr(VI) is converted into Cr(III). To find out k , the plot of time vs ln (A/A ) (Fig. 9n) was app o drawn. The plot showed a decreasing trend. The slope and the k and induction time confirmed the same. The nano- app intercept values were noted from which the k value was structured system yielded higher k when compared with app app −3 −1 calculated as 9.32 × 10  s . The T was determined as the literature value [7]. This proved the high catalytic nature i 5.59 s. When compared with the literature [24], the present of the nano-structured systems. The bio-based nanospheres investigation yielded better results due to the nanostructure [39] consumed 40 min for the complete reduction of NiP, of the catalyst. but the present investigation consumed 11 min only. In 2017, A similar experimental procedure was followed for the Majumdar et al. [40] reported the Pd nanoparticle-mediated catalytic reduction of Cr(VI) using Al nanoparticle as a catalytic reduction NiP and the k value was reported as app catalyst. Figure 10a–j indicates the UV–visible spectrum of Fig. 8 UV–visible spectrum of NiP taken at 1-min time interval in the presence of Al–MA system a–k, plot of time vs. ln(A/A ) l 1 3 International Nano Letters (2018) 8:147–156 153 Cr(VI) at 372 nm. Here, also while increasing the reduc- The pollution or the toxicity of a material can be low- tion time, the absorbance reduced at 372 nm. Again, this ered by reduction or oxidation reaction. In the present confirmed the reduction of Cr(VI) into Cr(III). The k was investigation, the reduction reaction is considered. R6G app calculated from the plot of time vs ln (A/A ) (Fig. 10k) as contains one quinone ring and one eneiminium chlo- −3 −1 6.44 × 10  s . The T was noted as 2.32 s. The Cu nano- ride functional group. During the reduction reaction, the particle yielded the highest k and T values. The highest eneiminium chloride reduced. This can be quantitatively app i k value explained that the Cu nanoparticle has high cata- measured with the help of UV–visible spectrophotom- app lytic activity rather than the Al nanoparticle due to smaller eter. An absorbance peak reduced drastically at 526 nm in size of Cu nanoparticle (Table 1). Both the Cu and Al (Fig. 11a–e) for the Cu–MA system-catalyzed reduction nanoparticles produced better results when compared with of R6G. The spectrum was recorded at the time interval of the goethite-like catalyst [24]. 1 min. The k value was calculated from the plot of time app Fig. 9 UV–visible spectrum of Cr(VI) taken at 1-min time interval in the presence of Cu– MA system a–m, plot of time vs. ln(A/A ) n Fig. 10 UV–visible spectrum of Cr(VI) taken at 1-min time interval in the presence of Al– MA system a–j, plot of time vs. ln(A/A ) k Fig. 11 UV–visible spectrum of R6G taken at 1-min time interval in the presence of Cu– MA system a–e, plot of time vs. ln(A/A ) f 1 3 154 International Nano Letters (2018) 8:147–156 Fig. 12 UV–visible spectrum of R6G taken at 1-min time interval in the presence of Al– MA system a–g, plot of time vs. ln(A/A ) h value of the reaction while repeating the experiments. This confirmed the catalytic efficiency of the Cu and Al salts toward the reduction of Cr(VI), R6G and NiP-like pollut- ants. Figure 13a–f indicates the plots for Cu and Al salt catalyst systems. In an overall study, the Cu–MA system exhibited the highest k value due to the small size of Cu atom. The app important point to be noted in the study is T value. The T i i value of the Cu–MA system or Al–MA system for differ - ent reduction reaction is different. For example, the Cu–MA system, towards the reduction of Cr(VI), NiP and R6G, the T value was determined as 5.59, 0.22 and 4.19 s, respec- tively. This proved that the T value depends on the nature of the reaction medium, size and charge of the material to be reduced and the catalyst. −1 Fig. 13 Plot of number of cycles against k (s ), a Cu–MA–PDC, app b Cu–MA–R6G, c Cu–MA–NiP, d Al–MA–PDC,e Al–MA–R6G, f Al–MA–NiP systems Conclusions −2 −1 From the study, the following niche points are presented as against ln(A/A ) (Fig. 11f) as 21.2 × 10  s and the T as o i conclusion. The FTIR spectrum confirmed the metal nano- 4.19 s (Table 1). The Ag–SiO catalyst system consumed particle formation. The WCA measurement confirmed the longer time to complete the reduction of R6G [26]. The SH nature of metal nanoparticles (147°). The SEM results present catalyst systems yield a good result within a short declared the presence of nanometer-sized materials in the period of reduction time. A similar type of plot was made MA-encapsulated metal nanoparticles. The HRTEM image for the Al–MA system (Fig. 12a–g). From Fig. 12 g, the −2 −1 of Al nanoparticle declared the size as 20–80 nm. The EDX k value was determined as 11.7 × 10  s and the T was app i spectrum informed the higher percentage content of Al in determined as 2.06 s. The Cu–MA system exhibited the the Al–MA system. The XPS showed the Al2p at 72.6 eV. highest k value and high T value for Cu–MA system. app i −2 −1 The Cu–MA system exhibited high k (2.12 × 10  s ) and app To confirm the catalytic effect of Cu and Al salts toward high T (5.5 s) values for all the systems. The present investi- the reduction of Cr(VI), R6G dye and NiP, the reduction gation proved that a nanostructured material can effectively study was carried out for six repeated times for each sys- act as a catalyst for the reduction study. tem and their k values were determined. It is very inter- app esting to note that even after the sixth cycle the k value app Acknowledgements This research work was supported by DRDO, New is not decreased, i.e., the catalyst is stable towards the Delhi, ERIPR/ER/1104580/M/01/1445, 2012. Dr. N. Sundararajan, experimental conditions. This can be explained as follows: Associate Professor of English Department, KCET, Virudhunagar, is gratefully acknowledged for his valuable help during this manuscript during the reduction reaction, the Cu and Al NP are simul- preparation work. taneously reduced and maintained their shape and size in each repeating cycle. 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Synthesis, characterization and application of superhydrophobic low-cost Cu and Al nanoparticles

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Materials Science; Nanotechnology; Nanochemistry; Nanoscale Science and Technology
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Abstract

Cu and Al nanoparticles were prepared using a simple chemical etching method followed by a chemical reduction method. The synthesized metal nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), electron-dispersive (EDX) spectrum and water contact angle (WCA) measurements. The application of Cu and Al nanoparticles was tested towards the catalytic reduction of 4-nitrophenol (NiP), hexavalent chro- mium {Cr(VI)} and rhodamine 6G (R6G) dye in the presence sodiumborohydride (NaBH ) as a reducing agent. From the UV–visible spectrum, the reduction rate constant (k ) and the induction time (T ) were determined and compared critically. app i The HRTEM analysis confirmed the nanosize of Cu and Al prepared by a simple chemical etching process followed by the chemical reduction method. Keywords Nanostructure · Characterization · Water contact angle · Catalyst · Reduction Introduction the open environment, they degrade the quality of soil and water highly. Hence, it is necessary to remove the pollut- The environment is spoiled due to the various activities ant or to reduce the pollution due to phenolic compounds, of human beings such as industrialization, modernization particularly NiP, dye effluents and Cr(VI) ion. On the other and civilization. Among these, the industrialization plays a hand, the reduced product of NiP plays a vital role in the vital role in disturbing the environmental quality because of pharmaceutical field as a starting material for the prepara - the effluents that are highly toxic and hazardous, and sim - tion of drugs and it is not possible to remove the NiP from ply disposed into the open environment, for example, the the chemical industry. Moreover, the catalytic reduction of nitrophenols (NiP) from chemical industries [1, 2], hexa- NiP is an interesting one because of its pseudo-first-order valent chromium [Cr(VI)] from chromium industry, metal kinetics. Hence, the reduction of NiP using various cata- finishing industry, tannery [ 3] and dye effluents from dye lyst systems is the only way to reduce the pollution caused and textile industries [4, 5]. When they are disposed into by the NiP. Currently, various catalyst systems such as iron niobate [6], hydroxyapatite [7], Au nanoparticles (NP) [8], Au–SiO [9], layered double hydroxides (LDH) [10], TiO 2 2 * R. Anbarasan [11], Al O [12], Pd–Ag [13], AC–Ag [14], Au–CeO [15], 2 3 2 anbu_may3@yahoo.co.in aminoclay–Fe NP [16], Clay–Pt [17], Fe O [18], Ni–Se [19] 3 4 * Kuo-Lun Tung and Ni–P [20] are used for the reduction of NiP. Through kltung@ntu.edu.tw study of the literature, any report based on the Cu and Al nanoparticles as a catalyst for the reduction of NiP in the Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar, presence of NaBH is not found. Moreover, the Cu and Al Tamil Nadu 626 001, India nanoparticles formed as by-products during the chemical Department of Mechanical Engineering, MEMS Thermal etching of metal surface followed by a chemical reduction Control Lab, National Taiwan University, Taipei 10617, are converted into useful materials such as catalyst for the Taiwan, ROC reduction reaction. Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Vol.:(0123456789) 1 3 148 International Nano Letters (2018) 8:147–156 Cr(III) is less toxic when compared with Cr(VI). Hence, The main aim of the present investigation is fabrication it is better to reduce the Cr(VI) using different methodology. of low-cost S.H Cu and Al nanoparticles and extending their Photocatalytic reduction of Cr(VI) over TiO particles was application towards the catalysis field. For the synthesis of reported by Testa et al. [21]. In 2014, Sadik and co-workers S.H Cu or Al metal plates, a standard literature procedure [22] studied the catalytic reduction of Cr(VI) using Pd NP. was followed [35]. The Cu or Al plate with the area of Cu(II)-catalyzed reduction of Cr(VI) was reported in the 12 cm was taken as a source material. The metal plate was literature [23]. Goethite surface-assisted catalytic reduction cleaned with acetone and ultrasonicated for 1 h in an aque- of Cr(VI) was reported by Kim and research team [24]. In ous medium. Then the plate was removed from the ultra- 2015, microbial system-catalyzed reduction of Cr(VI) was sonic bath and dried at 110 °C for 2 h and it was weighed reported in the literature [25]. The literature survey indicates (W g) and subjected to the chemical etching reaction in the that the catalytic reduction of Cr(VI) by Cu and Al nano- presence of 0.10 g MA in 100 mL ethanol medium under structured catalyst system is not available so far. This urged mild stirring condition. After 7 days of chemical etching, the researchers to do the present investigation. the metal plate was removed from the reaction medium and Rhodamine 6G (R6G) is a photostable organic fluorescent dried at 80 °C for 2 h (W g). The content of the medium was dye and has a wide range of applications in textile industry. dried at 80 °C for a night. The metal with hierarchical struc- Unfortunately, the conventional biological methods are less ture was obtained as a powder, weighed (W g) and stored effective towards the degradation or color removal of R6G in a zip lock cover. During the fabrication of S.H Cu or Al dye. In order to remove the color and to reduce the toxicity plate, the etching agent-coated Cu or Al is formed as a by- of R6G, it is necessary to reduce the structure of R6G in the product without any use. Thus, obtained hierarchically struc- presence of a catalyst. The catalytic reduction of R6G in tured Cu or Al powder was dispersed in an aqueous medium the presence of Ag–SiO nanocatalyst was carried out [26]. with the aid of ultrasonication for 30 min. Once the dis- Other catalysts such as SiO @TiO [27], ammonium phos- persion was completed, excess of NaBH was added under 2 2 4 phomolybdate [28], Au–Pd nanoalloy [29], TiO [30], and ultrasonication. The hierarchically structured Cu or Al is Au–SiO [31] were employed. Recently, Huang et al. used now subjected to chemical reduction reaction and reduced to bismuth oxyiodide [32, 33] and bismuth oxycarbonate [34] Cu and Al nanoparticles. The reduction reaction was allowed for the photocatalytic degradation of various organic dyes. to continue for another 30 min. At the end of the reaction, Few reports are available on the catalytic reduction of R6G the dark-colored precipitate was filtered, dried, weighed and in the presence of a nanostructured catalyst in the literature. stored in a zip lock cover under nitrogen atmosphere. During the fabrication of superhydrophobic (S.H) metal sur- Catalytic reduction study was carried out by the stand- −5 −5 faces by a simple chemical etching process, hierarchically ard procedure [7]. 2 mL of 6.3 × 10 M NiP or 1.1 × 10 −5 structured metal nanoparticles are formed as by-products. M K Cr O or 2.17 × 10 M R6G solution was taken in 2 2 7 −3 Very few applications are found for such hierarchic struc- a 3-mL capacity quartz cuvette microreactor. 1 × 10 g of tured materials. The present investigation is made to extend Cu or Al nanoparticle catalyst was weighed and added to the application of the S.H hierarchically structured metal the micro-reactor. 15 mg of NaBH was added in excess after the chemical reduction method in the catalysis field. to the nanoreactor. While adding N aBH in the presence Catalysis is a developing field and considerable amount of of a nanocatalyst, the NiP was converted into aminophenol research work is going on. The novelties of the present inves- and is mentioned in Scheme 1. The reduction reaction was tigation are easy preparation, economically cheaper and no use of hazardous solvents. Experimental 4-Nitrophenol (NiP, S.D. Fine Chemicals, India), potassium dichromate (PDC, S.D. Fine Chemicals), rhodamine6G (R6G, CDH Chemicals, India), myristic acid (MA, CDH Chemicals, India, etching agent) and sodium borohydride (NaBH , Himedia chemicals, India, reducing agent) were purchased and used as received. Cu and Al metal plates (resource material) with 99% purity were purchased locally with the following dimensions: length 6 cm, breadth 2 cm, area 12 cm . Double-distilled (DD) water was used for labo- ratory work. Scheme 1 Reduction of 4-NP, Cr(VI) and R6G 1 3 International Nano Letters (2018) 8:147–156 149 quantitatively measured with the help of the UV–visible the rod shape was determined as 3–5 µm with the breadth spectrophotometer at the interval of 1 min. of less than 700 nm. The arrow mark confirmed the same. UV–visible spectrum (Shimadzu 3600 NIR, Japan) The sphere-shaped NP with the size of 150  nm is also for samples was recorded in aqueous medium from 250 seen. Figure 2b indicates the SEM image of MA-encapsu- to 550 nm. Water contact angle (WCA) was measured by lated Al. The morphology of MA-encapsulated Al NP is Kyowa DMs-200, Japan model instrument. The surface entirely different from the MA-encapsulated Cu NP. The morphology with EDX spectrum was scanned by SEM, Al–MA system exhibits gel-like morphology with more JSM 6300, JEOL model instrument. FTIR spectra for the number of micro-voids. One or two NP are also seen in samples were recorded with the help of Shimadzu 8400 S, the circled area. This type of material is very much use- Japan, instrument by KBr pelletization method from 400 ful in drug delivery and catalytic application. The size −1 to 4000 cm . HRTEM image of the Cu and Al salts were of the voids is varied from 2 to 5 µm. Hence, the SEM recorded with the help of TEM 3010, JEOL instrument. The images confirmed the presence of nanostructured Cu–MA binding energy was determined by XPS (XPS, Thermo Sci- and Al–MA systems [36]. For the sake of comparison, the entific, Theta Probe, UK). SEM image of pristine Cu and Al NP is given in Fig. 2c, d, respectively. Here also one can see the spherical and rod-shaped Cu NP. In the case of Al NP, some gel-like Results and discussion morphology is seen. This concludes that even after the chemical etching process with simultaneous encapsula- Figure  1 indicates the FTIR spectra of MA-encapsulated tion, the surface morphology of the Cu and Al NP is not metal nanoparticles. Figure 1a confirms the FTIR spectrum changed. of MA-encapsulated Cu nanoparticle. The C–H symmet- Figure 3a indicates the HRTEM image of Cu–MA sys- ric and anti-symmetric stretching appeared at 2845 and tem. Here one can see the fiber and plate-like morphology. −1 2910 cm , respectively [35]. The C=O stretching of long- Agglomerated structure is also observed. The length of the −1 chain aliphatic carboxylic acid appeared at 1710 cm . The fiber was found to be 1–4 µm with the breadth of ~ 150 nm. −1 C–O linkage appeared at 937 cm . The metal–oxide (M–O) The length and breadth of the plate were calculated as 800 −1 stretching appeared at 544 cm . Figure 1b represents the and 250 nm, respectively. Figure 3b represents the HRTEM FTIR spectrum of MA-encapsulated Al nanoparticle. Peaks image of Al–MA system with nanoparticle-like morphol- appeared as explained earlier. Thus, the FTIR spectrum ogy. The size of the particle varied between 20 and 80 nm. confirmed the MA-encapsulated Cu and Al nanoparticle Hence, the HRTEM images confirmed the nanostructure of formation. Cu–MA and Al–MA systems. The surface morphology of the MA-encapsulated Cu In the present investigation, water-insoluble Cu–MA and nanoparticle is given in Fig. 2a. The image indicates the Al–MA were prepared. To confirm the heterogeneity of the presence of both rod and spherical particles. The length of catalysts, water contact angle (WCA) was measured. Fig- ure 4a represents the WCA image of Cu–MA and Fig. 4b represents WCA image of Al–MA systems. The WCA was determined as 147° and 140.1°, respectively, for Cu and Al systems. Among these two systems, the Cu–MA system exhibited the S.H character. The fluorinated long-chain ali- phatic acid also exhibited similar WCA [36]. Hence, the WCA measurement confirmed the SH and heterogeneous catalytic nature of Cu–MA and Al–MA systems. The percentage of elements present in the systems can be determined by the EDX spectrum. Figure 5a indicates the EDX spectrum of Cu–MA system and Fig. 5b represents the EDX spectrum of Al–MA system. From Fig. 5a, the content of Cu was determined as 1.84%. Figure 5b shows the content of Al as 2.68%. The XPS gives an idea about the outer most energy level of a material. Figure 6a represents the XPS of Cu–MA sys- tem. The appearance of Cu2p (953.1  eV) and Cu2p 1/2 3/2 (932.2  eV) confirmed that Cu is in nanosize. This is in accordance with the literature report [33]. The XPS of nano- sized Al–MA is given in Fig. 6b. The Al2p is appeared at Fig. 1 FTIR spectrum of (a) Cu salt, (b) Al salt system 1 3 150 International Nano Letters (2018) 8:147–156 Fig. 2 SEM image of a Cu–MA, b Al–MA system, c Cu, d Al system Fig. 3 TEM image of a Cu– MA, b Al–MA system 1 3 International Nano Letters (2018) 8:147–156 151 72.6 eV [37]. Thus, the XPS confirmed the formation of nano-sized Cu–MA and Al–MA. In the present investigation, the above-synthesized Cu and Al nanoparticles were tested towards the catalytic reduc- tion of NiP, PDC and R6G. The catalytic activities of metal nanoparticles toward three different reactions were tested. Catalytic reduction of NiP involves the conversion of nitro group into amino group. Catalytic reduction of PDC involves the conversion of Cr(VI) into Cr(III). Similarly, the cata- Fig. 4 WCA image of a Cu–MA, b Al–MA systems lytic reduction of R6G involves the conversion of R6G into reduced form of R6G, i.e., the reduction of extended double bond. The catalytic reduction activity is identified with the help of UV–visible spectrophotometer. Generally, metal or metal oxide NP is considered as an efficient catalyst system for the catalytic reduction reaction. The common example is the catalytic reduction of NiP at room temperature. The reduction follows a pseudo-first- order kinetics and it is considered as a model compound [38]. In the present investigation, an attempt is made towards the catalytic reduction of NiP using Cu and Al nanoparticles. The catalytic reduction was quantitatively followed with the help of UV–visible spectrophotometer. The UV–visible spectrum of NiP exhibits an absorbance peak at 401 nm (Fig. 7a). After the addition of the catalyst and NaBH , the absorbance reduced drastically at 401 nm within 6 min. This confirmed that the Cu–MA system is an efficient catalyst towards the reduction of NiP in Fig. 7b–g. The reduction of NiP into aminophenol proceeded through the formation of phenolate anion. To find out the reduction rate constant, the plot of time against ln(A/A ) (Fig. 7h) was drawn. The plot showed a straight line with negative trend. From the slope value, the apparent rate constant (k ) was calculated as app −3 −1 17.8 × 10  s (Table 1). The intercept value showed the induction time as 0.22 s. To compare the catalytic activity of Al nanoparticle, similar type of reduction study was carried out (Fig. 8a–k). The k value was determined from Fig. 8l app −3 −1 as 16.2 × 10  s with an induction time of 0.46 s (Table 1). The catalytic reduction study confirmed that the Cu nanopar - ticle is more efficient than the Al nanoparticle towards the Fig. 5 EDX spectrum of a Cu–MA, b Al–MA systems catalytic reduction of NiP in the presence of N aBH . Both Fig. 6 XPS of a Cu2p, b Al2p 1 3 152 International Nano Letters (2018) 8:147–156 Fig. 7 UV–visible spectrum of NiP taken at 1-min time interval in the presence of Cu–MA system a–g, the plot of time vs. ln(A/A ) h −3 −1 3.83 × 10  s . Hence, when compared with the literature, Table 1 The k and T values of the catalysts app i the present investigation yielded better results. −1 System k (s ) T (s) app i The catalytic reduction of Cr(VI) into Cr(III) was carried −3 Cu–MA–PDC 9.32 × 10 5.59 out in the presence of Cu and Al nanoparticles as an individ- −3 Cu–MA–R6G 21.2 × 10 4.19 ual catalyst using NaBH as a reducing agent. Figure 9a–m −3 Cu–MA–NiP 17.8 × 10 0.22 indicates the UV–visible spectrum of Cr(VI) measured at −3 Al–MA–PDC 6.44 × 10 2.32 time interval of 1 min in the given experimental conditions. −3 Al–MA–R6G 11.7 × 10 2.06 For Cu–MA system, it was found that while increasing −3 Al–MA–NiP 16.2 × 10 0.46 the reduction time the concentration of Cr(VI) at 372 nm reduced slowly. It means the Cr(VI) is converted into Cr(III). To find out k , the plot of time vs ln (A/A ) (Fig. 9n) was app o drawn. The plot showed a decreasing trend. The slope and the k and induction time confirmed the same. The nano- app intercept values were noted from which the k value was structured system yielded higher k when compared with app app −3 −1 calculated as 9.32 × 10  s . The T was determined as the literature value [7]. This proved the high catalytic nature i 5.59 s. When compared with the literature [24], the present of the nano-structured systems. The bio-based nanospheres investigation yielded better results due to the nanostructure [39] consumed 40 min for the complete reduction of NiP, of the catalyst. but the present investigation consumed 11 min only. In 2017, A similar experimental procedure was followed for the Majumdar et al. [40] reported the Pd nanoparticle-mediated catalytic reduction of Cr(VI) using Al nanoparticle as a catalytic reduction NiP and the k value was reported as app catalyst. Figure 10a–j indicates the UV–visible spectrum of Fig. 8 UV–visible spectrum of NiP taken at 1-min time interval in the presence of Al–MA system a–k, plot of time vs. ln(A/A ) l 1 3 International Nano Letters (2018) 8:147–156 153 Cr(VI) at 372 nm. Here, also while increasing the reduc- The pollution or the toxicity of a material can be low- tion time, the absorbance reduced at 372 nm. Again, this ered by reduction or oxidation reaction. In the present confirmed the reduction of Cr(VI) into Cr(III). The k was investigation, the reduction reaction is considered. R6G app calculated from the plot of time vs ln (A/A ) (Fig. 10k) as contains one quinone ring and one eneiminium chlo- −3 −1 6.44 × 10  s . The T was noted as 2.32 s. The Cu nano- ride functional group. During the reduction reaction, the particle yielded the highest k and T values. The highest eneiminium chloride reduced. This can be quantitatively app i k value explained that the Cu nanoparticle has high cata- measured with the help of UV–visible spectrophotom- app lytic activity rather than the Al nanoparticle due to smaller eter. An absorbance peak reduced drastically at 526 nm in size of Cu nanoparticle (Table 1). Both the Cu and Al (Fig. 11a–e) for the Cu–MA system-catalyzed reduction nanoparticles produced better results when compared with of R6G. The spectrum was recorded at the time interval of the goethite-like catalyst [24]. 1 min. The k value was calculated from the plot of time app Fig. 9 UV–visible spectrum of Cr(VI) taken at 1-min time interval in the presence of Cu– MA system a–m, plot of time vs. ln(A/A ) n Fig. 10 UV–visible spectrum of Cr(VI) taken at 1-min time interval in the presence of Al– MA system a–j, plot of time vs. ln(A/A ) k Fig. 11 UV–visible spectrum of R6G taken at 1-min time interval in the presence of Cu– MA system a–e, plot of time vs. ln(A/A ) f 1 3 154 International Nano Letters (2018) 8:147–156 Fig. 12 UV–visible spectrum of R6G taken at 1-min time interval in the presence of Al– MA system a–g, plot of time vs. ln(A/A ) h value of the reaction while repeating the experiments. This confirmed the catalytic efficiency of the Cu and Al salts toward the reduction of Cr(VI), R6G and NiP-like pollut- ants. Figure 13a–f indicates the plots for Cu and Al salt catalyst systems. In an overall study, the Cu–MA system exhibited the highest k value due to the small size of Cu atom. The app important point to be noted in the study is T value. The T i i value of the Cu–MA system or Al–MA system for differ - ent reduction reaction is different. For example, the Cu–MA system, towards the reduction of Cr(VI), NiP and R6G, the T value was determined as 5.59, 0.22 and 4.19 s, respec- tively. This proved that the T value depends on the nature of the reaction medium, size and charge of the material to be reduced and the catalyst. −1 Fig. 13 Plot of number of cycles against k (s ), a Cu–MA–PDC, app b Cu–MA–R6G, c Cu–MA–NiP, d Al–MA–PDC,e Al–MA–R6G, f Al–MA–NiP systems Conclusions −2 −1 From the study, the following niche points are presented as against ln(A/A ) (Fig. 11f) as 21.2 × 10  s and the T as o i conclusion. The FTIR spectrum confirmed the metal nano- 4.19 s (Table 1). The Ag–SiO catalyst system consumed particle formation. The WCA measurement confirmed the longer time to complete the reduction of R6G [26]. The SH nature of metal nanoparticles (147°). The SEM results present catalyst systems yield a good result within a short declared the presence of nanometer-sized materials in the period of reduction time. A similar type of plot was made MA-encapsulated metal nanoparticles. The HRTEM image for the Al–MA system (Fig. 12a–g). From Fig. 12 g, the −2 −1 of Al nanoparticle declared the size as 20–80 nm. The EDX k value was determined as 11.7 × 10  s and the T was app i spectrum informed the higher percentage content of Al in determined as 2.06 s. The Cu–MA system exhibited the the Al–MA system. The XPS showed the Al2p at 72.6 eV. highest k value and high T value for Cu–MA system. app i −2 −1 The Cu–MA system exhibited high k (2.12 × 10  s ) and app To confirm the catalytic effect of Cu and Al salts toward high T (5.5 s) values for all the systems. The present investi- the reduction of Cr(VI), R6G dye and NiP, the reduction gation proved that a nanostructured material can effectively study was carried out for six repeated times for each sys- act as a catalyst for the reduction study. tem and their k values were determined. It is very inter- app esting to note that even after the sixth cycle the k value app Acknowledgements This research work was supported by DRDO, New is not decreased, i.e., the catalyst is stable towards the Delhi, ERIPR/ER/1104580/M/01/1445, 2012. Dr. N. Sundararajan, experimental conditions. This can be explained as follows: Associate Professor of English Department, KCET, Virudhunagar, is gratefully acknowledged for his valuable help during this manuscript during the reduction reaction, the Cu and Al NP are simul- preparation work. taneously reduced and maintained their shape and size in each repeating cycle. 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