Reduction mechanism of hydrothermally synthesized wide band gap ZnWO 4 nanorods for HER application

. This manuscript presents a detailed investigation of the synthesis of zinc tungstate (ZnWO 4 ) nanoparticles and various characterizations of the as-synthesized sample to reveal its potential for hydrogen evolution reaction (HER). The study focuses on a simple and efficient hydrothermal method that facilitates the production of ZnWO 4 nanoparticles, which involves the controlled reaction between zinc and tungstate ions in a specific solution. The resulting ZnWO 4 nanoparticles were characterized by various characterization techniques which include XRD, UV-vis spectroscopy, TEM and electrochemical study to get insights into their size, structure, properties and electrochemical behaviour. The characterization includes the analysis of the nanoparticles' structural features and optical properties. The material’s electrochemical properties were also investigated by employing cyclic voltammetry (CV) and potentio electrochemical impedance spectroscopy (PEIS). The charge transfer process was studied for the material revealing its diffusion controlled behaviour and reduction peaks in the cathodic region. These properties suggest that the material is a potential candidate for HER catalysis.


Introduction
Zinc tungstate (ZnWO4) has received wide attention in the materials research because of its intriguing ceramic characteristics and potential applications.It is a crystalline compound which exhibits excellent optical and luminescent characteristics as observed in various studies [1].It is an optically transparent material which makes it suitable for various optical applications [2].This material possesses a high chemical stability, thermal resistance, and a wide bandgap, which make it suitable for applications in radiation detection, scintillation detectors, solar cells and phosphors for white light-emitting diodes (LEDs) [3], [4].
A variety of methods can be used to synthesize ZnWO4 nanoparticles like hydrothermal, chemical vapour deposition, solvothermal, etc. [5], [6] Researchers have synthesized ZnWO4 nanoparticles by a variety of physicals and chemical techniques and confirmed their suitability for different applications.G. Bhaskar Kumar et.al. used solid-state route to synthesize ZnWO4 ceramic powder presented its structural, optical, morphological and dielectric properties [1].Severo et.al. synthesized ZnWO4 nanoparticles by solvohydrothermal technique and revealed their application for inulinase immobilisation [7].Another research by Abubakar et.al. presented review of zinc tungstate as a photocatalyst [8].
The synthesis is a key component in the development of advanced radiation detectors used in medical imaging, homeland security, and nuclear physics research.Its scintillating properties enable the accurate detection of ionizing radiation, making it crucial in healthcare diagnostics and safety measures.Zinc tungstate has also shown promising applications in the field of LED technology, contributing to the production of energy-efficient lighting sources [9].As the world transitions towards sustainable energy solutions, materials like zinc tungstate play a vital role in reducing energy consumption and environmental impact.
Despite the significant progress made in understanding zinc tungstate, there remain research gaps that need to be addressed.One notable gap is the exploration of novel synthesis methods and the optimization of existing ones.The research gap also includes the study of ZnWO4 for hydrogen evolution reaction (HER).This manuscript has reported the synthesis of ZnWO4 by one step facile hydrothermal route.Another research by Siriwong et.al. [10] reported the synthesis of ZnWO4 by hydrothermal method using zinc nitrate and sodium tungstate as precursors and keeping it in autoclave at 200°C for 24 hours.They studied the optical and microstructural properties of ZnWo4 nanorods.Another group, Mashkani et.al.
[11] have used precipitation process to synthesize ZnWO4 nanorods and studied its optical, morphological and magnetic properties along with studying its photocatalytic applications.
The present work delves into the characterization of the material required to analyze the material's structure, composition, and properties comprehensively.The electrochemical properties of the as-synthesized zinc tungstate material are also investigated which show the diffusive behaviour of the material and reduction process involved revealing its possible usage for HER.It provides a holistic overview of this remarkable material's properties.This work is pivotal in advancing our knowledge and harnessing the full potential of zinc tungstate for technological advancements.This manuscript presents a comprehensive exploration of the synthesis and characterization of zinc tungstate, shedding light on its remarkable attributes, the importance of studying it, and addressing pertinent research gaps.

Synthesis of ZnWO4 nanorods
ZnO nanoparticles of average particle size 10-30 nm and Sodium tungstate dihydrate AR grade (Na2WO4.2H2O;>99) were procured from M/s Nanoshel LLC and M/s Thomas Baker Pvt. Ltd. respectively.HNO3 (70 %) of M/s CDH ltd. was also used during the synthesis.ZnWO4 nanoparticles were synthesized by hydrothermal method.100 mg ZnO powder was ultrasonically dispersed in 20 ml DI water.Subsequently, 0.2 mmol Na2WO4 was dissolved in 30 ml DI water.The former solution was mixed dropwise into the later one and further stirred.Later few drops of 5M HNO3 was added into the solution to make pH acidic.Later, the solution was kept in autoclave at 180°C for 24 hours.It was then left to cool down naturally followed by washing and drying at 60°C.

Deposition of thin film and electrochemical study
The thin film of the as-synthesized ZnWO4 sample was deposited on a fluorine doped tin oxide (FTO) glass substrate.A uniformly dispersed solution of 50 mg/ml concentration was prepared in ethanol and dispensed onto the substrate once, spinning at 1500 rpm for a minute to prepare a uniformly deposited thin film.The electrochemical behavior of ZnWO4 nanorods to study its usage for HER has been studied by using three electrode electrochemical workstation (Biologic make, SP-240 model).The setup includes three electrodes which includes working, reference and counter electrode.Ag|AgCl was used as the reference electrode in the present study while Pt was used as the counter electrode.The deposited film of the sample was used as the working electrode.

X-ray Diffraction
The structural properties of the as-synthesized sample were investigated by using X-ray diffraction technique.The obtained XRD spectra (fig. 2 (a)) revealed polycrystalline material indexed to zinc tungstate as matched with JCPDS file 98-008-7933.The diffraction peaks corresponding to the (hkl) planes as depicted in fig. 2. were found to match with the monoclinic crystal structure of ZnWO4 with the space group P12/c1 and space group number 13.The lattice parameters of the as-synthesized ZnWO4 material were calculated by wellknown Bragg's law [12] and were found to be a=4.64A, b=5.60A and c=4.93A The values were compared with the standard values of JCPDS file as shown in table 1 which revealed little deviation from the standard values inferring negligible strain in the synthesized ZnWO4 material.
The crystallite size and strain of ZnWO4 were estimated by two different approaches which include Williamson-Hall (W-H) plot [13] (fig. 2 (b)) and size-strain (S-S) plot [14](fig.2 (c)).The crystallite size calculated using intercept of W-H plot and slope of S-S plot was found out to be 31.86nm and 24 nm respectively.Alternatively, the strain was found out to be 0.00136 and 0.0109 by W-H and S-S plot respectively.The crystallite size and strain values are summarized in table 2.

UV-vis Spectroscopy
UV-visible spectroscopy was utilised to study the optical properties of the as-obtained ZnWO4 sample.The absorbance spectrum (fig.3 (a)) was obtained using Shimadzu make 2600i model UV-visible Spectrophotometer.The spectrum revealed the absorbance by the material in UV region with maximum prominent absorbance peaks at 220, 266 and 360 nm.
The spectrum was further used to estimate the optical band gap of the material by employing Tauc plot method [15], [16].The linear part of the Tauc plot was extrapolated as shown in fig. 3 (b) to get the value of optical band gap to be 3.23 eV which makes it a wide band gap semiconductor.The refractive index of the material was also investigated based on the optical band gap and found out to be 2.34 [17].
Furthermore, HOMO and LUMO levels of the material were identified using the obtained band gap value and 6.31 eV as absolute electronegativity of ZnWO4.[12].Using these values, the valence and conduction bands were observed to be 3.426 eV and 0.194 eV respectively.The whole data is summarized in table 3.

Transmission Electron Microscopy
The microstructural characteristics of the ZnWO4 sample were studied by TEM of JEOL make JEM-1400 model microscope.TEM image (fig.4 (a)) reveals the formation of nanorods of varied lengths from 50 nm to 200 nm.The corresponding area's selected area electron diffraction pattern (SAEDP) was captured, as seen in fig.3(b).The different spots of varying intensities forming the circles were observed revealing the polycrystalline nature of the material with fine crystallites.To study the d-spacing and further determine the appropriate (hkl) planes, one of the spots was investigated.It was indexed to the (100) plane, which is well in line with the XRD spectra displayed in fig.2(a).

Electrochemical Studies
CV and PEIS techniques were used to study the electrochemical characteristics of the assynthesised ZnWO4 nanomaterial.Fig. 5 (a) depicts the cyclic voltammograms at varying scan rates ranging from 40 mV/s to 140 mV/s.The significant reduction peaks were observed in the voltammograms.The peak current was observed to be increasing with increased scan rate which is in good agreement with Randle Sevick equation.Fig. 5 (c) shows the variation of peak current with scan rate.It showed the linear relationship between the two revealing diffusion controlled behaviour of the working electrode which is thin film of ZnWO4.
Another plot utilized to verify the diffusion controlled behaviour is log (peak current) vs log (scan rate) plot (fig.5 (d)).The slope of this plot was obtained as 0.39 which verifies the diffusive behaviour of the working electrode [18].The diffusive behaviour of ZnWO4 makes it a potential candidate to be used as a catalyst in hydrogen evolution reaction.Another research group of Shad et.al. [19] have also studied the cyclic voltammetry study of ZnWO4 and observed no oxidation.Alternatively, the reduction peaks were observed at approximately 1-2 μA which is quite less than the value obtained in the present work.
PEIS technique was analysed to look into the redox process of the working electrode and thus resistances and capacitances involved.Fig. 5 (b) shows nyquist plots at varying potential values ranging from 0 mV to -400 mV.The potential applied to an electrochemical system affected the kinetics of electrode reactions [20].As the potential changed, the rate of charge transfer processes at the electrode-electrolyte interface also changed.This, in turn, affected the impedance response observed in the Nyquist plot.At small |V| values, large impedance was observed leading to larger radii semicircles.Alternatively, at larger |V| values, impedance was observed to be decreasing as the radii of semicircles decreased.The Nyquist plots at -350 mV and -400 mV were of same radii suggesting the saturation.The smaller radii semicircles showed the presence of Warburg impedance also suggested by a slant line followed by the semicircle which shows the formation of infinite layer of ions at the interface [21].It shows good diffusion of charge carriers in the system [22], [23].

Conclusion
A comprehensive study on the synthesis and characterization of zinc tungstate nanoparticles using the hydrothermal route is presented in the manuscript.The findings from various characterization techniques shed light on the material's properties and potential applications.XRD confirmed the polycrystalline nature of the synthesized ZnWO4, matching its monoclinic crystal structure.UV-Visible spectroscopy revealed significant absorbance in UV region, indicating material's potential for applications in UV-sensitive devices.The estimated band gap of 3.23 eV and refractive index of 2.34 indicates the material's optical properties.The determination of the HOMO -LUMO levels at 3.323 and 0.195 eV, respectively, provides valuable information for potential electronic and photonic applications.TEM demonstrated the formation of nanorods, showcasing the control over morphology achieved through the hydrothermal synthesis method.SAED pattern analysis confirmed the presence of ZnWO4 crystals, with one of the dots corresponding to the (100) plane, further revealing polycrystalline material.Cyclic voltammetry revealed the presence of reduction peaks, indicating the material's electrochemical activity.The diffusion-controlled behaviour observed suggests its potential in HER catalysis.PEIS at various potential values demonstrated the presence of Warburg impedance, highlighting the material's suitability for photocatalytic activity.
The authors acknowledge the support of Prof. Anand Srivastava, Vice Chancellor, NSUT, Delhi for providing the essential resources required for the research work.
All the data used for this research work is included within the manuscript.

Fig. 4 .
Fig. 4. (a) TEM image at 50 nm scale showing nanorods and (b) SAED pattern of the corresponding area.

Table 1 .
Lattice parameters of the as-synthesized ZnWO4 as matched with JCPDS data.

Table 2 .
Crystallite size and strain values of as-synthesized ZnWO4 using W-H and S-S plot.

Table 3 .
Optical properties of as-synthesized ZnWO4 sample.