Synthesis of Zn₃V₂O₈/rGO Nanocomposite for Photocatalytic Hydrogen Production

Abstract

In this study, zinc vanadate/reduced graphene oxide (Zn₃V₂O₈/rGO) composite has been synthesized via a simple approach. Advanced characterization techniques (powder X-ray, scanning electron microscopy, energy dispersive X-ray spectroscopy and ultraviolet-visible (UV-vis) spectroscopy) have been used to authenticate the formation of Zn₃V₂O₈/rGO composite. Subsequently, Zn₃V₂O₈/rGO was applied as photo-catalyst for hydrogen generation using photo-catalysis. The Zn₃V₂O₈/rGO photo-catalyst exhibited a good hydrogen generation amount of 104.6 µmolg⁻¹. The Zn₃V₂O₈/rGO composite also demonstrates excellent cyclic stability which indicated better reusability of the photo-catalyst (Zn₃V₂O₈/rGO). This work proposes a new photo-catalyst for H₂ production application. We believe that the presence of synergistic interactions was responsible for the improved photo-catalytic properties of Zn₃V₂O₈/rGO composite. The Zn₃V₂O₈/rGO composite is an environmentally friendly and cost-effective photo-catalyst and can be used for photo-catalytic applications.

1. Introduction

Growing environmental degradation and a warming planet have an impact on life on earth and are serious issues for the present and the future [1,2,3]. A quick response or solution is needed to prevent impending crisis [1]. Researchers have discovered that the main cause of these problems, including the emission of greenhouse gases, is the over-use of fossil fuels for energy production [4,5,6]. Additionally, as fossil fuels are a key source of energy production today, their replacement with alternatives that are more efficient is crucial for both helping to reduce greenhouse gas emissions and ensuring enough energy supply [7].

Renewable energy sources such as wind and solar have severe limits due to low efficiency and high production costs [8]. Hydrogen may be an alternative energy source to overcome the energy crisis [9]. In recent years, hydrogen production has gained much attention because of its clean and efficient combustion [9]. Firstly, Fujishima and colleagues developed a TiO₂ based electrode for hydrogen production application under UV illumination [10]. There are several reports available on photo-catalytic hydrogen evolution [11,12,13]. It is a challenging task to develop a highly efficient photo-catalyst for this [13]. The band gap of the photo-catalyst plays a significant role in the hydrogen evolution process [14]. In addition, morphological features of the photo-catalyst may also affect the its performance in the hydrogen evolution process [15]. Surface area, mechanical stability and structural morphology may also play a significant role in photo-catalytic hydrogen production [15,16].

Nanostructured materials have the potential to separate electron-hole pairs and provide a better path for electron transportation [17]. Moreover, stability of the photo-catalyst is also an important factor for its potential application in large-scale hydrogen production [18]. Previous years have witnessed the fabrication of various novel catalysts such as TiO₂, C₃N₄, CdS, ZnO and MoS₂ for hydrogen evolution application [17,18,19,20,21]. It is still a challenge to design and develop a cost effective and eco-friendly photo-catalyst.

Recently, transition metal vanadates (TMVs) have drawn extensive attention for energy related applications (supercapacitor, batteries) due to their low cost, changeable oxidation state and availability [22,23,24,25]. A variety of TMV materials, including MV₂O₄, M₃V₂O₈, MVO₄, MV₃O₈, M₂V₂O₇ and MV₂O₆ (M = Fe, Ni, Mn, Zn) have been utilized in various optoelectronic applications [24,25,26,27,28,29,30,31,32]. Vanadium-based photo-catalysts are attracting much interest for their photo-catalytic performance due to their band gap, potential, and stable chemical properties [33]. Kudo and colleagues used the BiVO₄ monoclinic electrode material for O₂ evolution under visible light irradiation [34]. BiVO₄ was utilized by Liu and colleagues for photo-reduction of CO₂ in water [35]. Li et al. reported photo-reduction of CO₂ into sustainable hydrocarbon fuel using Fe₂VO₄ [36]. Ag₃VO₄ was prepared using the solid state reaction technique and its effectiveness as a photo-catalyst to generate O₂ was evaluated by Konta et al. [37]. However, few reports are available on the use of TMVs as photo-catalyst for hydrogen production. Sekar et al. [38] reported the synthesis of BiVO₄/graphene composite for hydrogen production and a hydrogen production rate of 11.5 μmol·g⁻¹·h⁻¹. Further, Dhabarde et al. [39] also reported on the facile conditions for the synthesis of BiVO₄/graphene composite. This synthesized BiVO₄/graphene composite was used as photo-catalyst towards hydrogen peroxide production [39]. This suggested that TMV based composite materials can be used as photo-catalyst in the hydrogen evolution process.

Zinc vanadium-based metal oxides, particularly Zn₃V₂O_8, possess excellent optical properties and can be used as a photo-catalyst for hydrogen production application [30]. Zn₃V₂O₈ and its composite with rGO have been used in dye degradation and supercapacitor applications [27,28,29]. However, they have not been explored as photo-catalyst for hydrogen production applications.

In the present study, a novel photo-catalyst (Zn₃V₂O₈/rGO) has been prepared for hydrogen production applications. So far, no report has been found on the use of Zn₃V₂O₈/rGO as cost-effective photo-catalyst. According to our literature survey, this is the first report which demonstrated the use of Zn₃V₂O₈/rGO as environmentally friendly and cost-effective photo-catalyst for photo-catalytic hydrogen production application.

2. Experimental Section

2.1. Materials and Chemicals

Chemicals and reagents, ammonium vanadate (99.95% trace metals, Merck, Rahway, NJ, USA), 2-methyl imidazole (99%, Merck), zinc nitrate hexahydrate (Fischer Scientific, Waltham, MA, USA), lactic acid (Fischer Scientific), ethanol (99.8%, Sigma, Kawasaki, Japan) and graphene oxide (powder, Merck) were used as received.

2.2. Apparatus

A Rigaku powder X-ray diffractometer was used for the powder X-ray diffraction (PXRD) study. Scanning electron microscopic (SEM) pictures of the samples were conducted on Zeiss microscope. The energy dispersive X-ray (EDS) study was carried out on a Horiba Instrument. Ultraviolet-visible (UV-vis) absorption spectra were recorded on an Agilent Cary Instrument. Photo-catalytic investigations were carried out using gas chromatograph.

2.3. Synthesis of Zn₃V₂O₈ and Zn₃V₂O₈/rGO

The Zn₃V₂O₈ was obtained under facile conditions. As typical, 0.5 mmol of zinc nitrate hexahydrate and 0.33 mmol of ammonium vanadate were dissolved in 20 mL of distilled water and stirred for 30 min. Further, 4 mmol of 2-methyl imidazole was dissolved in distilled water and added to the above mixture and stirred for 5 min. The obtained precipitate was filtered, washed several times with water and ethanol and dried at 70 °C for 4 h and calcined at 450 °C for 3 h to obtain the Zn₃V₂O₈. To obtain the Zn₃V₂O₈/rGO composite, 50 mg of graphene oxide was dispersed in distilled water using an ultra-sonicator for 1 h. Further, Zn₃V₂O₈ was added to the graphene oxide dispersion and an appropriate amount of hydrazine hydrate was added to reduce the graphene oxide. The above mixture was sonicated for 2 h, filtered and washed and dried at 70 °C for 4 h. The graphene oxide was purchased as reported elsewhere. The reduced graphene oxide was also prepared under similar conditions except for the addition of Zn₃V₂O₈.

2.4. Photo-Catalytic Hydrogen Generation

For the photo-catalytic studies, an air-tight quartz tube was used as the photo-catalytic reactor system. First, 7 mL of lactic acid was poured in 100 mL of water. The photo-catalyst (50 mg; Zn₃V₂O₈ or Zn₃V₂O₈/rGO) was added to the prepared solution. Nitrogen (N₂) gas was purged for 30 min to remove dissolved gases. Visible light source (300 W LED, λ = 420 nm) was used for photo catalytic measurements. The generated H₂ gas was collected at fixed time intervals and checked by gas chromatograph.

3. Results and Discussion

3.1. Characterization

Phase purity of the prepared powder samples (rGO, GO, Zn₃V₂O₈ and Zn₃V₂O₈/rGO) was determined by using the powder X-ray diffraction (PXRD) technique (RINT:2500V X-ray diffractometer (Rigaku, Japan; Cu-Ka irradiation and 𝜆 = 1.5406 Å)). The PXRD diffractograms of the prepared GO and rGO samples were obtained at the 2Theta range of 5–80°. The PXRD pattern of GO exhibits the well-defined diffraction plane (001) of GO which indicated the formation of GO. In the case of rGO, a broad diffraction peak appeared which corresponded to the (002) diffraction plane of rGO and confirmed the successful conversion of GO to rGO (Figure 1a).

The PXRD patterns of the Zn₃V₂O₈ and Zn₃V₂O₈/rGO were also recorded at the 2Theta range of 10–80°. Figure 1b shows the PXRD patterns of the prepared Zn₃V₂O₈ and Zn₃V₂O₈/rGO. The obtained PXRD results for Zn₃V₂O₈ showed the presence of (220), (251), (213) and (004) diffraction planes of Zn₃V₂O₈. The PXRD pattern of Zn₃V₂O₈ was well-matched with previous JCPDS no. 034-0378. No detected signal for (002) the diffraction plane in the PXRD pattern of Zn₃V₂O₈/rGO was attributed to the low content of rGO or poor crystalline nature of rGO. Previous studies also reported that rGO was visible in the PXRD data of the synthesized composite materials due to the poor crystalline nature of rGO and low rGO content [40]. Thus, it can be considered that Zn₃V₂O₈/rGO has been formed successfully.

Previous reports suggested that morphological features of the photo-catalyst play a vital role in photo-catalytic applications [41]. Thus, it is interesting to observe and study the top surface morphological properties of the prepared Zn₃V₂O₈ and Zn₃V₂O₈/rGO composite. In this regard, scanning electron microscopy (SEM; Supra 55 Zeiss microscope; 10 keV) has been utilized to study the morphological properties of the different prepared powder samples (rGO, GO, Zn₃V₂O₈, and Zn₃V₂O₈/rGO). Figure 2a show the obtained SEM graph of GO. It is observed from the SEM investigations that GO consists of a sheet-like surface. Similarly, a SEM image of rGO is displayed in Figure 2b. The observations confirmed that rGO was comprised of a sheet-like surface which is the characteristic feature of rGO. The morphological characteristics of the Zn₃V₂O₈ and Zn₃V₂O₈/rGO photo-catalysts were also studied. Figure 2c,d show the collected SEM graphs of the Zn₃V₂O₈ photo-catalyst and suggest that Zn₃V₂O₈ consists of an interconnected nanowire shaped surface morphology. The SEM graph of Zn₃V₂O₈/rGO was also collected at different magnifications. Figure 2e,f demonstrate the SEM graph of the Zn₃V₂O₈/rGO composite. According to the observations, it is clear that interconnected Zn₃V₂O₈ nanowires have been grown on to the top surface of the rGO sheets. Although rGO could not be observed in the PXRD results, SEM images clearly show the presence of rGO sheets in the synthesized Zn₃V₂O₈/rGO composite. Thus, it is confirmed that Zn₃V₂O₈/rGO was synthesized successfully.

Energy-dispersive X-ray spectroscopy (EDS) plays a very important role in determining the elemental composition of the synthesized nanostructured materials. It is widely used as one of the important characterization tools for the determination of phase purity of the synthesized nanostructured materials. The EDS spectra of Zn₃V₂O₈ and Zn₃V₂O₈/rGO composite were collected via an energy-dispersive X-ray spectroscope (Oxford Instruments X-max, Aztec, MA, USA). Figure 3a shows the EDS spectrum of the prepared Zn₃V₂O₈ and obtained results show the signals for the presence of Zn, V and O elements. No other signal was detected, which indicated good phase purity of Zn₃V₂O₈. Figure 3b exhibits the EDS spectrum of Zn₃V₂O₈/rGO composite and shows the presence of signals for C, Zn, V and O elements. The presence of a signal for C element in the EDS spectrum authenticated the presence of rGO in the Zn₃V₂O₈/rGO composite. The band gap of the photo-catalyst is one of the most important features of the photo-catalytic hydrogen production process.

A suitable photo-catalyst should have narrow band gap with good stability. The optical band gap of the prepared Zn₃V₂O₈ and Zn₃V₂O₈/rGO was also checked using ultraviolet-visible (UV-vis) absorption spectroscopy. Figure 4 demonstrated the UV-vis spectra of the prepared Zn₃V₂O₈ and Zn₃V₂O₈/rGO. The band gaps of the Zn₃V₂O₈ and Zn₃V₂O₈/rGO were calculated using Tauc-relation as given below,

αhv = A (hv − E_g)ⁿ

(1)

herein, α is absorption coefficient, h = plank’s constant, v is frequency, E_g = band gap energy, and n = transition value. The n = ½ has been taken for the direct band gap of Zn₃V₂O₈ and Zn₃V₂O₈/rGO. The observation shows that Zn₃V₂O₈ and Zn₃V₂O₈/rGO have band gaps of 3.5 eV (Figure S1) and 3.48 eV (Figure S2), respectively.

3.2. Hydrogen Generation

The hydrogen (H₂) production studies were carried out with lactic acid in water solution in presence of photo-catalyst (Zn₃V₂O₈ or Zn₃V₂O₈/rGO). In the first step, N₂ gas was passed into the prepared solution for 0.5 h. In previous studies [42], lactic acid has been widely used as scavenger or sacrificial reagent for H₂ production applications. Herein, we have adopted lactic acid for this purpose. Subsequently, this prepared solution was irradiated with LED lamp and after 1, 2, 3, 4, 5 and 6 h of light irradiations, the produced H₂ was taken out from the quartz tube using a syringe and examined by gas chromatograph (TCD). Figure 5a demonstrates the photo-catalytic H₂ generation amount using Zn₃V₂O₈ as photo-catalyst under light irradiation and without light irradiation. The obtained H₂ generation results indicated that no H₂ was produced in the absence of light irradiation using Zn₃V₂O₈ photo-catalyst. However, the Zn₃V₂O₈ photo-catalyst exhibited the generation of 24.56 µmolg⁻¹ H₂. It is obvious that Zn₃V₂O₈ acted as an efficient photo-catalyst in presence of light irradiation only. In further investigations, the effect of light irradiation on H₂ generation was also studied. The H₂ generation was checked in the absence and presence of photo-catalyst (Zn₃V₂O₈) under light irradiation. Figure 5b shows that 24.56 µmolg⁻¹ H₂ was produced in the presence of photo-catalyst (Zn₃V₂O₈) under light irradiation whereas no H₂ was generated in the absence of photo-catalyst (Zn₃V₂O₈). This clearly indicates that the absence of photo-catalyst does not produce H₂. Furthermore, we have also employed Zn₃V₂O₈/rGO as photo-catalyst under similar conditions. The obtained results are summarized in Figure 5c.

Figure 5c shows that improved H₂ evolution was observed for Zn₃V₂O₈/rGO compared to Zn₃V₂O₈. This may be attributed to the excellent physiochemical properties of rGO and synergistic effects/interactions between Zn₃V₂O₈ and rGO. The highest H₂ generation amount of 104.6 µmolg⁻¹ was obtained using Zn₃V₂O₈/rGO photo-catalyst whereas Zn₃V₂O₈ photo-catalyst produced a lower amount of 24.56 µmol·g⁻¹. From the photo-catalytic investigations, it can be clearly understood that Zn₃V₂O₈/rGO has excellent photo-catalytic behavior compared to Zn₃V₂O₈. In previous reports, BiVO₄/rGO was adopted as photo-catalyst for hydrogen evolution reaction and good photo-catalytic activity of 0.75 μmol·h⁻¹ was reported [43]. BiVO₄ based photo-catalyst also exhibited hydrogen production activity of 195.6 μmol·h⁻¹ [44]. Another work also reported hydrogen production activity of 0.92 μmol/h using carbon dot/BiVO₄ quantum dot composite [45]. Quantum sized BiVO₄ photo-catalyst showed decent hydrogen production activity [46]. Sekar et al. [38] have used BiVO₄, and BiVO₄/rGO as photo-catalyst for hydrogen generation. BiVO₄ and BiVO₄/rGO exhibited hydrogen production activity of 0.03 and 11.5 μmol·g⁻¹·h⁻¹, respectively [38]. The obtained hydrogen production activity in the present study is comparable with the previous reports as discussed above.

For practical purposes, the photo-catalyst should have excellent reusability. In this regard, reusability of the synthesized Zn₃V₂O₈/rGO photo-catalyst was investigated regarding photo-catalytic H₂ generation. The obtained reusability results can be seen in Figure 6.

The observations reveal that no significant changes were seen in H₂ evolution after 4 cycles. This clearly suggested that Zn₃V₂O₈/rGO possess excellent cyclic stability up to 24 h with reusability up to four cycles. The SEM image of the Zn₃V₂O₈/rGO was also recorded after the reusability study. The SEM image is presented in Figure S3 which suggests good stability of the prepared Zn₃V₂O₈/rGO. The probable mechanism of H₂ production has been illustrated in Scheme 1, occurring through water splitting and lactic acid reforming over the prepared Zn₃V₂O₈/rGO nanocomposite under light irradiation. The light irradiation of the material results in the generation of a photogenerated electron-hole pair (Equation (2)). The photogenerated electron can move from the conduction band (CB) of Zn₃V₂O₈ to the CB of rGO and thereby it can be trapped due to the resonance effect. Thus, rGO with Zn₃V₂O₈ assists in enhancing the separation between photogenerated electron and hole, thus improving the photocatalytic activity of the resulting material. The holes created at valence band (VB) react with surrounding water molecules to form H⁺ ions and reactive hydroxyl radicals (^•OH) Equation (3). The H⁺ ions formed in VB move to CB and thereby interact with electrons on the surface of rGO and thus become reduced to H₂ gas (Equation (4)). The lactic acid (sacrificial reagent) interacts with ^•OH radicals and thus transforms into oxidized products and H+ ions (Equation (5)) which is further reduced to H₂ gas on the rGO surface. Thus, both photocatalytic water splitting and the lactic acid reforming process contribute to hydrogen production synergistically [47,48].

$${\mathrm{Zn}}_3{\mathrm{V}}_2{\mathrm{O}}_8/\mathrm{rGO}\stackrel{hv}{\to }{\mathrm{Zn}}_3{\mathrm{V}}_2{\mathrm{O}}_8{/\mathrm{rGO}}^{*}{(\mathrm{h}}_{\mathrm{VB}}^{+}+e_{CB}^{-})$$

(2)

$${\mathrm{Zn}}_3{\mathrm{V}}_2{\mathrm{O}}_8{/\mathrm{rGO}}^{*}{(\mathrm{h}}_{\mathrm{VB}}^{+}{)+\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{}^{?}\mathrm{O}\mathrm{H}$$

(3)

$${\mathrm{Zn}}_3{\mathrm{V}}_2{\mathrm{O}}_8{/\mathrm{rGO}}^{*}{(\mathrm{e}}_{CB}^{-}{)+\mathrm{H}}^{+}\to {1/2\mathrm{H}}_2$$

(4)

$${}^{•}\mathrm{O}\mathrm{H}+\mathrm{Lactic}\mathrm{Acid}\to {\mathrm{H}}_2{\mathrm{O}+\mathrm{CO}}_2$$

(5)

4. Conclusions

In conclusion, a hybrid composite of zinc vanadate and reduced graphene oxide (Zn₃V₂O₈/rGO) has been obtained using simple strategies. The fabricated Zn₃V₂O₈/rGO composite possesses excellent photo-catalytic properties compared to Zn₃V₂O₈. The Zn₃V₂O₈/rGO composite exhibits a good hydrogen generation amount of 104.6 µmolg⁻¹. Moreover, Zn₃V₂O₈/rGO showed good cyclic stability up to 24 h, which suggested that Zn₃V₂O₈/rGO has excellent reusability features. Although Zn₃V₂O₈/rGO showed good photo-catalytic performance for H₂ production, its wide band gap of 3.48 eV limited its potential application for large scale production. The photo-catalytic properties of the Zn₃V₂O₈/rGO can be further improved by incorporating unique and novel materials or methods. The Zn₃V₂O₈/rGO photo-catalyst possesses good optical properties which suggests its further potential for waste water treatment, dye sensitized solar cells and photo-detector applications.