Accepted Manuscript Nickel Oxide Decorated Zinc Oxide Composite Nanorods: Excellent Catalyst for Photoreduction of Hexavalent Chromium Simranjeet Singh, Imtiaz Ahmed, Krishna Kanta Haldar PII: DOI: Reference:
S0021-9797(18)30254-6 https://doi.org/10.1016/j.jcis.2018.03.012 YJCIS 23362
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
6 November 2017 6 March 2018 6 March 2018
Please cite this article as: S. Singh, I. Ahmed, K. Kanta Haldar, Nickel Oxide Decorated Zinc Oxide Composite Nanorods: Excellent Catalyst for Photoreduction of Hexavalent Chromium, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.03.012
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Nickel Oxide Decorated Zinc Oxide Composite Nanorods: Excellent Catalyst for Photoreduction of Hexavalent Chromium
Simranjeet Singh, Imtiaz Ahmed and Krishna Kanta Haldar* Department of Chemical Science, Central University of Punjab, Bathinda, Punjab, 151001, India.
*Author to whom correspondence should be addressed, electronic mail: [email protected]
Abstract In light of the growing interest and ability to search for new materials, we have synthesized Nickel oxide (NiO) nanoparticles decorated Zinc (ZnO) nanorods composite (NiO/ZnO) nanostructure. The NiO/ZnO heterostructure formation was confirmed by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). The fabricated environmental friendly NiO/ZnO composite nanostructure shows a well-defined photoreduction characteristic of hexavalent Chromium (Cr) (VI) to tri-valent Chromium (Cr) (III) under UVlight. Such an enhanced photoreduction property is attributed due to the decreased electron-hole recombination process which was proved by photoluminescence (PL) spectroscopy, photocurrent study, and electrochemical impedance spectroscopy. Furthermore, the photocatalytic activity rate of the NiO decorated ZnO nanorods was much higher than that of bare ZnO nanorods for the reduction of chromium (VI) and the rate is found to be 0.306 min -1. These results have demonstrates that suitable surface engineering may open up new opportunities in the development of high-performance photocatalyst.
1. Introduction: High amount of heavy metal ions such as cobalt, copper, nickel, chromium, zinc, cadmium, mercury and arsenic are detected in aquatic streams present in mining operations, tanneries, petrochemical industries, electroplating, and also in textile industries . According to the Agency for Toxic Substances and Disease Registry (ATSDR), Cr (VI) salt shows higher mobility than Cr (III) and hence is considered to be more toxic to humans. In aqueous solution, Cr (VI) exists in the form of chromate (CrO24-), dichromate (Cr2O7–) and hydrogen chromate (HCrO4 which is of great concern due to its toxicity which is 500 times more toxic than the trivalent one. It causes skin irritation, lung cancer, as well as damage to kidney and liver. Chromium exposure also affects the aquatic animals such as Whales. Recently, it has been reported that Free-ranging North Atlantic Right Whales and Sperm Whales are highly influenced by Chromium. According to WHO, the recommended limit of Cr (VI) in potable water is only 0.05 mg/l. There are various chemical and physical techniques for remediation of Cr(VI), such as; using a low cost fertilizer industry[6,7] waste material, Seaweed Biosorbent, Membrane Filtration Techniques, using sphagnum moss peat, Montmorillonite-supported magnetite nanoparticles, nanofiltration, UV irradiation in presence of TiO 2 etc. Reduction of Cr (VI) to Cr (III) has also been achieved via photo-catalytic reduction  which is effectively used to remediate other heavy metal ions like Cd(II) , Hg(II)  and As(V) . Among the various remediation techniques, the semiconductor-based photocatalytic approach is of particular interest for the reduction of Cr (VI) to Cr (III). The most commonly used semiconductor photocatalysts have been metal oxides like TiO2, which shows ultraviolet absorption ability because of its large band gap energy. Ku et al. have shown that the Cr(VI) adsorbed on the surface of TiO2 particles undergoes complete photoreduction to Cr(III) . Shao et al. successfully demonstrated the photocatalytic reduction of Cr(VI) to Cr(III) in an aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition by using oxalate as a model organic compound in the natural environment. The visible light photoreduction of toxic Cr(VI) over TiO2 has been shown by Wang et al. through surface modification with small 3
molecular weight organic acids as sacrificial organics. Krishnan et al. have reported the heterogeneous photocatalytic reduction of Cr (VI) in presence of UV-irradiated TiO2 suspensions. Thus most of the efforts have been expanded to develop TiO 2 based photocatalyst. Therefore, while the metal oxide comprising of single-component is extensively studied and implemented as photocatalysts for promoting for the reduction of Cr (VI) to Cr (III) reaction, composite metal oxides are not widely explored. Among these composite materials, the NiO/ZnO heterostructural nanomaterials have attracted much interest in particular, because of NiO has a p-type semiconductor (3.5 eV), high hole mobility, and low lattice mismatch with ZnO, which is beneficial for the formation of p-n heterojunction with ZnO (16-18). ZnO is a typical n-type semiconductor metal oxide having a wide direct band gap (3.37 eV), another most important nanomaterials for UV photodetection for the photocatalytic applications. ZnO shows higher electron mobility property compared to other wide band gap oxides[22-24]. While ZnO and NiO nanostructures individually have been studied extensively, the composite structure of NiO/ZnO nanocrystals has not been widely explored.[ 2528] Here, we have proposed to design NiO nanoparticles decorated ZnO nanorods composite nanostructures following the sol-gel process and microwave route. These NiO nanoparticles decorated ZnO nanorods composite nanostructures manifest excellent UV-visible light directed photocatalytic performance for the photoreduction of toxic heavy metals of Cr (VI) to Cr (III) in aqueous solution. 2. Experimental: Chemicals: Zinc nitrate hexahydrate (SDFCL, LR) , nickel nitrate hexahydrate(Otto, 98%), potassium dichromate (Loba Chemie, 99.9 %), ethylenediamine (SDFCL, AR) , sodium alginate (SDFCL, AR ), 1, 5 – diphenylcarbazide (Loba Chemie, 98%), Na2SO4 (Loba Chemie, 99 %), NH3 solution (Loba Chemie, 25%), HCl (SDFCL, 85%), sodium hydroxide (Loba Chemie, 98%), ethanol(AR, 99.9%). The chemicals were used in this work without further purification. Methods: For the fabrication of NiO nanoparticles decorated ZnO nanorods composite nanostructure, first ZnO nanorods were synthesized, then NiO was deposited on ZnO nanorods. ZnO nanorods were prepared by the reported methods with slight modification . Briefly, 4
7.45g zinc nitrate hexahydrate and 20g sodium hydroxide were dissolved in 50ml double distilled water. Then, 4ml of the alkali solution of zinc was mixed with 15 ml pure ethanol and 3 ml ethylenediamine, followed by sonication for 30 min.
Next, the solution mixture was
transferred to a Teflon-lined autoclave for hydrothermal synthesis and reactions were conducted at 180°C for 24hrs in an electric hot air oven. After the reactions, the ZnO nanorods were collected by centrifugation and followed by repeated washings with ethanol and deionized water for NiO nanoparticles decoration. In a typical NiO nanoparticles decoration process, 50mg ZnO nanorods and 54 mg Ni(NO3)2·6H2O were dissolved in de-ionized water (100 ml) and 0.1M sodium alginate was added to the solution to serve as stabilizing agent. Next, the NH3 solution was added to maintain the pH below 6 to avoid the formation of
Ni(OH)2 and then placed in a microwave oven for
microwave irradiation for 30 min. After this, the solid gel-like product was washed by centrifugation at 6000 rpm and re-dispersed in deionized water for further characterization. The final products were found to be blackish in color. 2.1 Instrumentation The crystalline phase of the obtained ZnO nanorods and NiO/ZnO composite was studied by Xray diffraction patterns (Bruker's Eco D8 advance). The transmission electron microscopy (TEM) images were taken using a JEOL-TEM-2010 transmission electron microscope with an operating voltage of 200kV. Room temperature optical absorption spectra were obtained with a UV-Vis spectrophotometer (Shimadzu). The emission spectra of all samples were recorded in a Fluoro Max-P (HORIBA JOBIN YVON) Luminescence Spectrometer. The photoreduction of Cr (VI) was done in a cylindrical annular batch photoreactor fitted with a medium pressure mercury vapor lamp of 125W and lamp was surrounded with a double-walled borosilicate immersion well and cooled by circulating water constantly. For the photoreduction study, 20ppm of potassium dichromate solution in 100ml and 30 mg of catalyst were taken in the reaction vessel. The pH of the solution was maintained at slightly above 2 by using 0.1M HCl and 0.1M NaOH before starting the experiment. The solution was subjected to adsorptiondesorption equilibrium under the dark condition for 15 minutes. The aliquots were collected at regular time intervals during the photoreduction. The analysis of Cr (VI) was carried out by forming a complex with 1, 5 - diphenylcarbazide. 5
Photocurrent response was performed for NiO/ZnO composite and ZnO nanorods in a photoelectrochemical system under white light irradiation. The spin-coated film of NiO/ZnO composite and ZnO nanorods on Indium tin oxide coated glass (ITO) substrate was used as the photoanode in the three-electrode cell which consisted of Pt counter electrode, Ag/AgCl reference electrode, and 0.15 M Na2SO4 redox couple. The photocurrent transient responses were recorded by illuminating the sample with 365nm light for 10sec and then switching off the light for another 10sec. All measurements were carried out in ambient condition. Electrochemical impedance spectroscopy (EIS) studies were obtained on a Metrohm Autolab 204/ PGSTAT electrochemical workstation, and the measurements were acquired in the light at an open circuit voltage over a frequency range from 0.1 Hz to 100 kHz with an AC voltage of 50 mV. 3. Results and discussion The powder X-ray diffraction (XRD) patterns of ZnO nanorods and NiO decorated ZnO nanorods composite structures are shown in Figure 1. The powder XRD patterns are well matched with the standard JCPDS Card. The XRD pattern (refer spectra ‘a’ of figure 1) clearly shows single crystalline phase and hexagonal structure of ZnO nanorods (JCPDS card no. 361451). However, in the case of NiO decorated ZnO nanorods composite samples, the XRD pattern exhibits patterns for both NiO and ZnO structures. The diffraction peaks at 36°, 43.1° and 63.5o (Figure 1b) are for NiO (JCPDS card no. 44-1159) and ZnO respectively, and the remaining peaks in the NiO decorated ZnO nanorods composite specimens are indexed to be the hexagonal structure of ZnO. This result indicates that the precursor has completely transformed into NiO decorated ZnO nanorods composite structure. The hexagonal structure of ZnO nanorods was not modified by the decorating of NiO nanoparticles into the ZnO surface. The as-prepared NiO decorated ZnO nanorods composite structures were further characterized by transmission electron microscopy (TEM) and is shown in Figure 2. Figure 2a reveals a wide view of the TEM images showing NiO/ZnO nanorods. They could easily assemble together to form a 1D structure when dropped on the carbon-supported copper grid with an appropriate concentration. Without size sorting, the 1D structured nanocrystals obtained here showed good monodispersity with the existence of well-resolved lattice planes. The decorated ZnO nanorods composite structures possessed a highly crystalline structure. The NiO/ZnO heterostructures
were further confirmed HRTEM analysis. Figure 2b shows a typical HRTEM image of a longitudinal of NiO/ZnO nanorods where both ZnO and NiO are visualized. This result clearly reveals that the favored growth directions of ZnO nanorods are along the  direction. From the HRTEM, FFT, inverse FFT images, and line-scan height profile (Figure 2e-h), the d-spacing was obtained as 2.81Å, which corresponds to the (100) planes of hexagonal ZnO. The typical dspacing was also calculated for NiO onto the ZnO nanorods and the value is 2.41Å corresponds to the (100) planes of NiO. From the lattice parameters, it is evident that the lattice mismatch between NiO and ZnO are very close, resulting in the difficulties in identifying the interface between NiO and ZnO both simultaneously. However, the NiO particle and ZnO nanorods combine together easily and form a heterojunction. We have correlated the photocurrent response of ZnO nanorods and NiO decorated ZnO nanorods composite counterpart electrodes by inserting them in a photoelectrochemical cell to prove the effect of NiO decoration onto ZnO on the charge separation of ZnO for the present heterostructure. The photocurrent response of ZnO nanorods and NiO decorated ZnO nanorods composite structures are shown in Figure 3, under illumination of light irradiation. Both of them shows effective response towards the on/off cycle of light depicting the electron transfer process. From the Figure, it is clearly seen that photocurrent response of NiO decorated ZnO nanorods crystal is higher compared to ZnO nanorods. The following responses for the improvement in photocurrent may be due to electron transfer from NiO to ZnO increasing the lifetime of the electron. The enhanced photoelectrochemical activities for the NiO decorated ZnO nanorods composites could be ascribed to the larger surface area of NiO nanoparticles which can minimize quick charge recombination process in the ZnO films. Thus, the enhancement of the photoresponse speed can widen the opportunity of exploiting NiO decorated ZnO nanorods heterostructures for superior photocatalytic activities. The photogenerated electron separation was
photoluminescence (PL) spectra of the ZnO nanorods in absence and presence of NiO nanoparticles, respectively. Figure 4 represents the PL spectra of ZnO nanorods and NiO decorated ZnO nanorods, respectively. From the figure, it is found that the NiO decorated ZnO nanorods exhibited much lower emission intensity than ZnO nanorods, indicating that the recombination of the photogenerated charge carrier was greatly inhibited in presence NiO 7
nanoparticles onto ZnO nanorods. At the same time, this commanding visible emission is associated with the increase of surface oxygen vacancy-related defects of ZnO NRs in the NiO decorated ZnO heterostructures. The present result suggests a high separation efficiency of photogenerated electron-hole pairs for NiO decorated ZnO nanorods compared to bare ZnO nanorods. For a better understanding the origin of the admirable photocatalytic performance of NiO decorated ZnO nanostructures, and to support Photoluminescence (PL) results, we utilized electrochemical impedance spectroscopy (EIS) studies. The Nyquist plots, a complex impedance plot has been presented here to figure out the different electrochemical characteristics of the electrode-electrolyte interface. Figure 5 shows the analysis of the impedance spectra of NiO decorated ZnO nanostructures and ZnO nanorods. A larger impedance semicircle radius in the high-frequency region is revealed for NiO decorated ZnO composite nanostructures compared to bare ZnO nanorods, which corresponds to a larger recombination. It is well known that the recombination resistance is inversely proportional to the recombination rate of the photoinduced charge carriers. By the way, the NiO decorated ZnO composite nanostructures have a lower recombination rate of the photoinduced charge carriers over bare ZnO nanorods. Thus, NiO deposition on ZnO nanorods significantly hindered the recombination rate of the photoinduced charge carriers which leads to an excellent visible light directed photocatalytic application of NiO decorated ZnO composite nanostructures. This indicates the persistent by our EIS results, which also consists of the photoluminescence analysis. Finally, the photocatalytic activity of the NiO decorated ZnO nanorods composites (NiO/ ZnO) for the reduction of inorganic pollutants has been demonstrated by carrying out experiments of the photocatalytic reduction of Cr (VI) ion as a test reaction. Therefore photocatalytic reduction of Cr (VI) was explored by using both bare ZnO nanorods and NiO decorated ZnO nanorods composite heterostructure. Figure 6 shows the photoreduction of Cr (VI) in aerobic condition under UV light exposure against time and the concentration was measured at 356 nm at pH slightly above 2. The pH of the aqueous solution is crucial for the adsorption of Cr(VI) possibly because of the presence of various Cr(VI) species. Oxalic acid was used as a hole scavenger for the photoreduction studies. It is found that the absorbance is gradually reduced with increased UV irradiation time for the NiO decorated ZnO nanorods composite heterostructure. It clearly 8
shows that the process is slower in the case of ZnO nanorods. Figure 6(i) represents the Cr (VI) reduction rate as a function of UV irradiation time. It is found that the absorbance of Cr (VI) is gradually reduced with increased duration of UV irradiation time for the NiO decorated ZnO nanorods composite heterostructure (Figure SI-1). The ZnO and NiO decorated ZnO nanorods composite heterostructures were tested for 11 min under identical conditions. It is interesting to observe that 98.7% of Cr (VI) is reduced to Cr (III) by the NiO decorated ZnO nanorods composites after 11 min of UV irradiation, whereas 10.3% reduction is observed in the case of ZnO nanorods. The photoreduction of Cr (III) under the same UV irradiation condition is negligible in the presence of NiO nanoparticles, and in absence of both ZnO nanorods and NiO decorated ZnO nanorods composites. Therefore, the large reduction efficiency of Cr (VI) to Cr (III) in the solution can be attributed to a photocatalytic effect that happens due to the presence of the NiO decorated ZnO nanorods composites. Degradation rate kD is calculated using following equation: kD = (C0-C)/C
where C0 and C are the initial absorbance and the sample absorbance, respectively. The kinetic profiles of Cr (VI) to Cr (III) reduction under UV light are also investigated. The photoreduction of Cr (VI) to Cr (III) follows apparently first-order kinetics. Its kinetics can be expressed as ln(I0/It) = kt, where k is the apparent reaction rate constant, I0 is the concentration of Cr (VI) at adsorption equilibrium, and It is the residual concentration of Cr (VI) at different illumination time intervals. To optimize the NiO decorated ZnO nanorods catalyst concentration, the effect of catalyst concentration on the Cr (VI) to Cr(III) photoreduction in aqueous solution was investigated. The results are illustrated in Figure SI.2. The lifetime of a catalyst is an important parameter for any catalytic process because of its further use for a long span of time leading to a significant cost diminution. For this reason, the NiO decorated ZnO nanorods composites catalyst was recycled, which indicate a slowdown in efficiency only from 100% (first run) to 95.0% (sixth run) as shown in Figure SI.3. These results suggested that NiO decorated ZnO nanorods composites catalyst sustained effective and reusable under UV light. Figure 6(ii) shows a linear relationship between ln(I0/It) and reaction time (t), indicating that the photoreduction of Cr (VI) follows a first-order kinetics model. The apparent rate constants as kinetic evidence for the samples are determined to be 0.0108 and 0.306 min -1 for ZnO nanorods 9
and NiO decorated ZnO nanorods composite heterostructure, respectively. Thus, the photoreduction of Cr(VI) to Cr(III) enhances due to NiO decorated onto ZnO nanorods. This is because of the promotion of an electron from NiO to ZnO when UV-light was illuminated and is responsible for possible photoreduction Cr(VI) to Cr(III) which is shown in Scheme 1.
results clearly demonstrate that NiO decorated ZnO nanorods composites have significantly improved photoreduction of Cr(VI) in comparison with bare ZnO nanorods. Thus, the NiO decorated ZnO nanorods composite heterostructures improve the separation of photogenerated electron-hole pairs due to the potential energy differences between NiO and ZnO, thus enhancing the photoreduction activity of Cr(VI). 4. Conclusion In summary, by using a sol-gel process and microwave technology, NiO/ZnO nanorods composite structure were successfully fabricated with NiO were decorated on the ZnO nanorod surface. The examination of photoreduction ability shows that the NiO nanoparticles decorated ZnO nanorods composite structure possess higher photoreduction ability than the bare ZnO nanorods for the reduction of Cr (VI) to Cr (III) under UV-light irradiation due to the enhanced separation efficiency of photogenerated electron-hole pairs and exhibited excellent stability (about 5% loss after six cycles). Furthermore, the photocatalytic activity rate of the NiO decorated ZnO nanorods for the reduction of chromium (VI) is much higher than that of bare ZnO nanorods and the rate is found to be 0.306 min-1. Comparing our results with literature, it is found that NiO decorated ZnO nanorods composite shows much better photoreduction of Cr(VI) to Cr(III) to most of the other photocatalyst reported by ref 2 and 12. These results have demonstrated that suitable surface engineering may open up new opportunities in the development of high-performance photocatalyst. This work will be carried out in our lab for developing new NiO/ZnO nanocomposite based photoanode for the photoelectrochemical cell.
Acknowledgments This work was financially supported by the research seed money from the Central University of Punjab, Bathinda [Project GP-25] and UGC Start-up grant. Supporting Information 10
Illustration of photoreduction of Cr (VI), photocatalyst loading optimization and photocatalytic cycle for its reusability are available in supporting information.
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Figure Captions Figure 1. The powder X-ray diffraction (XRD) patterns of (a) ZnO nanorods and (b) NiO embedded ZnO nanorods composite structure.
Figure 2. Represents the TEM images of (a) NiO decorated ZnO nanorods, (b) typical HRTEM images of NiO decorated ZnO nanorods where NiO (c) and ZnO (d) areas are marked to achieve the FFT of the selected area (e and f). And (g and h) represents the corresponding d-profile of ZnO and NiO, respectively. Figure 3. Photocurrent potential versus time scans for ZnO nanorods (black) and for NiO decorated ZnO nanorods (red). The upward arrow (↑) and downward arrow (↓) represent the switching on and off the step of the illumination, respectively. Figure 4. Photoluminescence(PL) spectra of (a) ZnO nanorods and (b) NiO decorated ZnO nanorods. Figure 5. The Nyquist plots of (a) NiO decorated ZnO nanorods and (b) ZnO nanorods catalysts. Figure 6. (i) The absorbance spectral changes of Cr(VI) solution in the presence NiO decorated ZnO nanorods heterostructures, and (ii) Rate of photocatalytic reduction of Cr(VI) in presence of ZnO nanorods (a) and presence NiO decorated ZnO nanorods heterostructures under UV light (b)[ u 5% (error)].