Facile Synthesis of a Z-Scheme ZnIn2S4/MoO3 Heterojunction with Enhanced Photocatalytic Activity under Visible Light Irradiation.

Employing a visible-light-driven direct Z-scheme photocatalytic system for the abatement of organic pollutants has become the key scientific approach in the area of photocatalysis. In this study, a highly efficient Z-scheme ZnIn2S4/MoO3 heterojunction was prepared through the hydrothermal method, followed by the impregnation technique that facilitates the formation of an interface between the two phases for efficient photocatalysis. The structural, optical, and surface elemental composition and morphology of the prepared samples were characterized in detail through X-ray diffraction, UV-vis diffuse reflectance spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that the composite materials have a strong intimate contact between the two phases, which is beneficial for the effective separation of photoinduced charge carriers. The visible-light-mediated photocatalytic activity of the samples was tested by studying the degradation of methyl orange (MO), rhodamine B (RhB), and paracetamol in aqueous suspension. An optimum loading content of 40 wt % ZnIn2S4/MoO3 exhibits the best degradation efficiency toward the above pollutants compared to bare MoO3 and ZnIn2S4. The improved photocatalytic activity could be ascribed to the efficient light-harvesting property and prolonged charge separation ability of the Z-scheme ZnIn2S4/MoO3 catalyst. Based on reactive species determination results, the Z-scheme charge transfer mechanism of ZnIn2S4/MoO3 was discussed and proposed. This study paves the way toward the development of highly efficient direct Z-scheme structures for a multitude of applications.

Introduction

In recent years, sunlight-mediated photocatalysis technology has been regarded as a promising tool to eliminate organic pollutants and split water. This technology is considered to be a sustainable and green approach to solve the problems related to environmental and energy issues. However, most of the present research is focused on the large-band-gap-energy semiconductors such as ZnO, TiO2, and SrTiO3, which can respond only to UV light and suffer from inherent charge carrier recombination.[1,2] To address the above problems, researchers have devoted great efforts to develop novel photocatalysts with a narrow band gap that can respond to the visible region of the solar spectrum.[3] In this regard, a variety of visible-light-responsive photocatalysts have been employed for photocatalytic hydrogen production and degradation of organic pollutants.[4,5] However, the reported photocatalysts still suffer from low photocatalytic efficiency, which is far below the needs of practical applications.[6] The undesirable separation of the photogenerated charge carriers is the main reason for the poor photocatalytic performance. To overcome the above problem, heterojunctions of different photocatalysts have been designed and studied, and among the variously reported heterojunctions, Z-scheme photocatalytic systems having two or more semiconducting materials with suitable band-gap structures have attracted a great deal of interest in the field of photocatalysis.[7,8] The advantage of a Z-scheme system is that it could quench the weak oxidative holes and reductive electrons during the charge transfer among the different components of the Z-scheme.[9] The above process facilitates the efficient separation of photoinduced holes and electrons and meanwhile improves their oxidation and reduction capabilities.[10] To date, various kinds of Z-scheme systems have been developed for a multitude of applications, including hydrogen production, CO2 reduction, and organic pollutant degradation.[11−13] However, in most of the Z-scheme systems, graphene oxide and noble metals are employed as mediators to shuttle electrons.[14] The use of noble metals for the development of Z-scheme catalysts increases the cost sharply, and employing graphene oxide as a mediator increases the difficulty of fabricating the ternary systems. Therefore, the scientific community is looking for an alternative approach that can avoid the high-cost precursor and ease the synthesis process. A direct Z-scheme system is a solution to the above problems as it does not require a mediator to shuttle electrons. A direct Z-scheme photocatalytic system has many advantages over the mediated Z-scheme systems such as simple composition, low cost, and high efficiency.[15−17]

Molybdenum oxide (MoO3) is an n-type semiconductor having band-gap energy in the range of 2.8–3.2 eV and could utilize both visible and UV light for excitation to generate electron–hole pairs.[18] The high positive potential of the valence band (VB) makes MoO3 an attractive photocatalyst for the oxidation reactions.[19] MoO3 with distinct nanostructured morphologies has been synthesized and studied for the application in photocatalysis because such structures provide high surface area and exhibit higher photocatalytic performance than bulk MoO3.[20] However, the practical application of bare MoO3 as a photocatalyst is still limited due to some shortcomings, such as rapid recombination of charge carriers and insufficient utilization of visible light.[21] In addition, the high positive potential of the conduction band (CB) limits the effectiveness of MoO3 due to its insufficient potential for the reduction of the oxygen molecule, resulting in poor photocatalytic activity.[18] Therefore, several composites materials such as MoO3–CdS,[22] MoO3–SnS2,[23] ZnS–MoO3,[24] and MoO3–MoS2[25] have been developed with an aim to improve the photocatalytic performance of MoO3. However, the reduction and oxidation abilities of photogenerated electron and hole pairs in simple heterojunction photocatalytic systems are usually lower than those of their single components.[26] Therefore, the development of a Z-scheme system is a better option to utilize the high redox potential of semiconducting materials. To develop highly efficient photocatalysts for the abatement of organic pollutants, ZnIn2S4, a ternary chalcogenide, has been considered a good candidate due to its strong absorption of visible light and high reduction power of electrons at the CB.[27] Therefore, vast studies related to ZnIn2S4 with distinct morphologies have been explored and examined to utilize this material as a potential applicant for wastewater treatment and H2 production.[28−31] However, the quantum efficiency of ZnIn2S4 is still unsatisfactory from a practical application point of view due to its high recombination rate. Therefore, hybridization of ZnIn2S4 with other photocatalysts has been found to enhance the photocatalytic performance.[32] The energy-band structures of MoO3 and ZnIn2S4 are matched well, and it is expected that the hybridization between these two semiconductors may lead to the formation of a direct Z-scheme system, which can eventually overcome the drawbacks of both catalysts.[33,34] The heterojunction formation between these two catalysts (MoO3 and ZnIn2S4) can not only provide the high oxidative and reductive powers of holes and electrons in MoO3 and ZnIn2S4, respectively, but also increase the use of broad absorption of visible light.

Keeping these points in mind, we have made an attempt to construct a composite material of MoO3 and ZnIn2S4 as a direct Z-scheme photocatalytic system for the degradation of organic pollutants under visible light irradiation. This direct Z-scheme ZnIn2S4/MoO3 photocatalytic system obtained via hydrothermal and impregnation techniques facilitates the path for the interfacial recombination between the electrons in the CB of MoO3 and holes in the VB of ZnIn2S4 in a manner that the charge separation efficiency is enhanced along with the accumulation of the highly reactive charge carrier in their respective bands. The developed direct Z-scheme photocatalytic systems were characterized by standard analytical techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis, UV–vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL), and their photocatalytic activity was investigated by studying the degradation of MO, RhB, and paracetamol under a visible light source. The photocatalytic mechanism of the Z-scheme system has been proposed based on the radical-trapping experiments.

Results and Discussion

XRD Analysis

The phases and crystal structures of pure ZnIn2S4 and MoO3 and composites of ZnIn2S4/MoO3 with varying ZnIn2S4 weight percentages are shown in Figure . The sharp and intense peaks observed in the XRD analysis show the crystalline nature of the materials. The pure MoO3 showed the diffraction peaks at 2θ angles of 13.07, 23.65, 25.9, 27.67, 23.07, 33.94, 35.86, 39.23, 46.28, 49.65, 52.66, and 59.17° which can be assigned to scattering from (020), (110), (040), (021), (130), (111), (041), (060), (200), (061), (002), and (081), respectively.[35] The diffraction peaks of pure MoO3 observed at different 2θ values correspond to an orthorhombic phase (JCPDS No. 05-0508).[18,36,37]However, the diffraction peaks of ZnIn2S4 are seen at 2θ values of 22.52, 32.02, 35.98, 39.53, 45.56, 47.7, 56.5, 66.6, 71.31, and 75.7°, which could be assigned to the (006), (105), (0012), (108), (0010), (112), (0012), (203), (0012), (0017), and (211), respectively.[38] The diffraction peaks of pure ZnIn2S4 show the hexagonal phase (JCPDS card no. 65-2023).[39−41] No impurity peak was observed in both cases, indicating that the samples are in the pure form. It is pointed out that in the case of composite materials, all diffraction peaks corresponding to MoO3 and ZnIn2S4 were observed, implying the incorporation of ZnIn2S4 over MoO3. The XRD spectra also showed a gradual decrease in the peak intensity of MoO3 on the incorporation of ZnIn2S4, which also signifies the formation of a heterojunction between both catalysts. In addition, the peak width of XRD spectra decreases with the increasing wt % of ZnIn2S4 onto the surface of MoO3, confirming the coupling of both catalysts during the synthesis. The heterojunction formation is later confirmed by the XPS and TEM analyses.

Figure 1

XRD patterns of pure (a) ZnIn2S4 and (b) MoO3 and (c–f) of different wt % of ZnIn2S4/MoO3 composites.

UV–vis Diffuse Reflectance Spectroscopy (UV–vis DRS) Analysis

The optical properties significantly influence the photocatalytic activity of the semiconducting materials.[42] The UV–vis diffuse reflectance studies were carried out to determine the role of ZnIn2S4 loading in changing the visible absorption. Figure a shows the UV–vis DRS spectra of pure ZnIn2S4 and MoO3 and different wt % of ZnIn2S4/MoO3 composites. It could be seen in the figure that both pure MoO3 and ZnIn2S4 show the absorption onset at 424 and 552 nm, respectively. It is also observed in the figure that the incorporation of ZnIn2S4 over MoO3 extends the optical response toward the higher wavelength, which could be attributed to the strong interaction between MoO3 and ZnIn2S4. The band-gap energy of synthesized materials was calculated using the following equationwhere α, ν, and Eg are the absorption coefficient, light frequency, and band-gap energy, respectively, and “A” is a constant. The estimation of n could depend on the type of electronic change, for instance, n = 1 for a direct and n = 4 for an indirect transition. The Eg values for samples were obtained by linearly extrapolating the curvature to the photon energy. The plot of (αhν)2 versus hν is shown in Figure b. The band-gap energies of pure ZnIn2S4 and MoO3 and ZnIn2S4/MoO3 composite materials were found in the range between 2.92 and 2.24 eV, as shown in Figure b. The calculated results for band-gap energy demonstrate that the prepared composite material could show proficiently high photocatalytic activity under visible light illumination.

Figure 2

(a) UV–vis diffuse reflectance spectra of different samples; (b) their corresponding Tauc’s plot.

SEM, TEM, and Energy Dispersion Spectra (EDS) Analyses

The surface morphologies of MoO3, ZnIn2S4, and MoO3/ZnIn2S4 were elucidated by scanning electron microscopy, and their images are shown in Figure a–d. The beltlike morphology with smooth surfaces could be clearly observed in the case of pure MoO3, as shown in Figure a, and the structure of ZnIn2S4 with a marigold flower shape is displayed in Figure b. In the case of the composite material, MoO3 nanobelts are uniformly decorated over the surface of ZnIn2S4 with an intimate interfacial contact, as shown in Figure c,d. The EDS spectra and wt % of each element present in the 40 wt % ZnIn2S4/MoO3 are shown in Figure e, which indicates that all of the elements are present in the composite. The data show that the elemental compositions of all elements were found to be close to those of the wt % of all metals. To confirm the elemental distribution over the nanocomposite, elemental mapping of the 40 wt % ZnIn2S4/MoO3 composite was typically performed to probe the elemental distribution patterns. As shown in Figure f–j, evidently, all elements such as Zn, In, S, Mo, and O are uniformly distributed in the whole structure. The results demonstrate that MoO3 and ZnIn2S4 are intimately bound together, which may increase the separation of the charge carrier and enhance the photocatalytic activity.

Figure 3

SEM images of (a) MoO3 and (b) ZnIn2S4, (c, d) low- and high-resolution SEM images, (e) EDS spectrum, and (f–j) elemental mapping of 40 wt % ZnIn2S4/MoO3.

Further investigation of the morphology and intimate contact between MoO3 and ZnIn2S4 was carried out by transmission electron microscopy (TEM) analysis. Figure a,b shows the representative TEM images of pure MoO3 and ZnIn2S4, which indicate the beltlike and marigold-flower-shaped morphologies, respectively. Interestingly, a clear interaction between MoO3 and ZnIn2S4 was confirmed by the TEM images given in Figure c,d. The TEM image in Figure d shows a clear contact between MoO3 and ZnIn2S4, which could be beneficial for the photoinduced electron transfer through the heterojunction. Also, the belt of MoO3 has a porous nature and may help adsorption of more pollutants onto its surface during photocatalysis. The porous character of MoO3 was further confirmed by the BET measurement, and the pore size distribution graph is given in the Supporting Information. The Barrett–Joyner–Halenda (BJH) plot (Figure S1) shows the pore size distribution plot of MoO3. The maximum average pore diameter was found to be 2.67 nm, and the total pore volume of MoO3 was determined to be 0.026 cm3/g. The different pore sizes and adequate pore volume may encourage the adsorption of organic pollutants on the surface of the catalyst.

Figure 4

TEM image of a pure MoO3 nanobelt at (a) high magnification and (inset) low magnification. (b) TEM image of a pure flowerlike ZnIn2S4. (c, d) TEM images of the 40 wt % ZnIn2S4/MoO3 composite and heterojunction between photocatalysts.

FTIR Analysis

To understand the interaction between MoO3 and ZnIn2S4 in a better way, FTIR analysis was conducted, and the results are shown in Figure . The FTIR spectra of pure MoO3 and ZnIn2S4 and the MoO3/ZnIn2S4 composite were recorded in the range of 400–4000 cm–1. It could be seen in the figure that pure MoO3 shows five major distinct peaks at around 553, 876, 995, 1630, and 3445 cm–1, which are in good agreement with those in the previous literature.[20] The terminal Mo–O bond peak appearing at 996 cm–1 is an indicator of the layered orthorhombic MoO3 phase. The absorption bands appearing at 565 and 872 cm–1 are assigned to the asymmetric M–O bonds and bending vibration of Mo–O–Mo units, respectively. The absorption band appearing at 3445 cm–1 is likely due to the O–H stretching mode of water, and an asymmetric band centered at 1630 cm–1 could be ascribed to the bending vibration of hydroxyl groups adsorbed on the surface of MoO3. In the case of pure ZnIn2S4, only two peaks at 1396 and 1610 cm–1 are found due to the surface hydroxyl and water molecules adsorbed on the surface of the catalyst.[21] After the incorporation of ZnIn2S4 onto the surface of MoO3, the absorption bands appear with a decrease in their intensity. The results confirm that ZnIn2S4 is successfully integrated with MoO3 during the impregnation method.

Figure 5

FTIR spectra of pure MoO3 and ZnIn2S4 and the 40 wt % ZnIn2S4/MoO3 composite at room temperature.

TGA Analysis

In addition, thermogravimetric analysis was conducted to get information about weight loss during heat treatment. Figure S2 shows the weight loss of MoO3, ZnIn2S4, and the 40 wt % MoO3/ZnIn2S4 composite material as a function of temperature. Pure MoO3 shows a slight weight loss between 30 and 200 °C, which could be ascribed to the desorption of physically adsorbed water molecules on the surface of the catalyst.[18] The weight loss observed between 300 and 550 °C in the case of pure MoO3 is due to the liberation of volatile byproducts associated with nitrates and ammonium decomposition.[43,44] It is reported well in the literature that pure ZnIn2S4 undergoes oxygenolysis in the temperature range of 300–800 °C.[45] ZnIn2S4 shows an around 16.4% weight loss, and the result is consistent with that of the previous literature.[45] The weight loss below 300 °C shown in the figure is due to the loss of water molecules adsorbed on the surface of ZnIn2S4.[45] After heterojunction formation, the stability of the sample is retained, as shown in Figure S2.

XPS Analysis

To determine the surface chemical composition and chemical state of the prepared samples, XPS analysis was carried out, and the results of the survey scan and high-resolution XPS spectra are shown in Figure . The survey scan spectra of MoO3 and ZnIn2S4 show the presence of MO, O, Zn, In, and S at their respective binding energies. However, the survey scan of 40 wt % ZnIn2S4/MoO3 demonstrates the same elements such as Zn, In, S, Mo, and O at their respective binding energies, as shown in Figure a. The high-resolution XPS spectra of the individual components in pure and composite materials are shown in Figure b–f. The high-resolution spectra of Zn present in pure ZnIn2S4 and 40 wt % ZnIn2S4/MoO3 (Figure b) display the two peaks at binding energies of 1020.08 and 1042.86 eV, which correspond to Zn 2p3/2 and Zn 2p1/2, respectively.[46] The energy difference between the two peaks was found to be 22.78 eV, indicating that Zn exists in the +2 oxidation state in both pure and composite materials.[47] In Figure c, the spectra of In 3d show a pair of symmetrical peaks with binding energies centered at 442.96 and 450.63 eV for In 3d5/2 and In 3d3/2, respectively. The S 2p spectrum (Figure d) displays a peak at the binding energy of 168.93 eV, indicating that the S particles exist as S2–.[48] From the high-resolution XPS spectrum of the Mo 3d scan (Figure e), two major peaks at 228.9 and 232 eV are observed and can be assigned to MoVI 3d5/2 and MoVI 3d3/2, respectively. In addition, the O 1s XPS signal shown in Figure f consists of a sharp peak at a binding energy of 530.03 eV, which is ascribed to the −2 oxidation state of oxygen.[18] However, a shift to higher binding energy in elements was observed upon the incorporation of ZnIn2S4 over MoO3, resulting in the strong electronic interaction between both catalysts. In addition, the peak intensity of all elements was found to decrease after heterojunction formation, indicating the intimate contact between both MoO3 and ZnIn2S4 catalysts.

Figure 6

(a) XPS survey scan spectra of pure MoO3 and ZnIn2S4 and the ZnIn2S4/MoO3 composite; high-resolution XPS spectra of (b) Zn 2p, (c) In 3d, (d) S 2p, (e) Mo 3d, and (f) O 1s.

Photoluminescence (PL) Study

The photoluminescence technique is a useful tool that reveals the charge trapping and separation of excitons in the semiconductors. It is well known that the exciton recombination is directly related to the PL emission signal, which, in turn, predicts the photocatalytic activity. Hence, we recorded the PL spectra of pure MoO3 and ZnIn2S4 and the 40 wt % ZnIn2S4/MoO3 composite with a fixed excitation wavelength of 330 nm, as shown in Figure . Both pure MoO3 and ZnIn2S4 show intense peaks compared to the composite material, indicating the high degree of recombination rate in the case of pure samples. In contrast, the PL signal of 40 wt % ZnIn2S4/MoO3 was found to show the less intense signal, suggesting that the recombination rate is delayed after heterojunction formation.

Figure 7

PL emission spectra of MoO3, ZnIn2S4, and 40 wt % ZnIn2S4/MoO3.

Photocatalytic Activity

The photocatalytic activities of the as-prepared MoO3, ZnIn2S4, and ZnIn2S4/MoO3 composites were evaluated by studying the degradation of two dye derivatives, methyl orange (MO) and rhodamine B (RhB), and a drug derivative, paracetamol, under visible light irradiation (λ ≥ 420 nm) in aqueous suspension with the continuous bubbling of air. Figure a,b shows the decrease in absorption intensity at different time intervals on irradiation of an aqueous suspension of MO and RhB, in the presence of 40 wt % ZnIn2S4/MoO3 under visible light irradiation. It could be seen in the figure that the absorption intensity of both dyes decreased with an increase in irradiation time and was found to be decolorized in 80–100 min. Figure c,d shows the change in concentration as a function of time of irradiation of MO and RhB in the absence and presence of different catalysts such as MoO3, ZnIn2S4, and 20, 30, 40, and 50 wt % ZnIn2S4/MoO3. The concentration of dye derivatives/paracetamol was calculated by a standard calibration curve obtained from the absorbance of dyes/paracetamol at different concentrations. In the absence of a catalyst, the self-photodegradation of MO is almost negligible in 100 min of illumination, indicating that the catalyst is indispensable for the degradation of MO, as shown in Figure c. Furthermore, pure MoO3 and ZnIn2S4 seem to show lower photocatalytic activity with the degradation efficiencies of 44 and 65%, respectively, within 100 min of illumination time. In contrast, the 40 wt % ZnIn2S4/MoO3 composite exhibited excellent photocatalytic activity, affording 98% degradation within the same irradiation time. On the other hand, in the case of RhB, self-degradation of RhB is insignificant without the catalyst, demonstrating that RhB is a stable dye and could not be degraded under direct photolysis. In addition, slow degradation was observed with pure MoO3 and ZnIn2S4 photocatalysts, and almost 99 percent degradation was observed in 80 min in the presence of the most active 40 wt % ZnIn2S4/MoO3 photocatalyst composite material, as shown in Figure d. In the series of ZnIn2S4/MoO3 composites, the 40 wt % composite has shown the highest photocatalytic activity due to efficient absorption of visible light and better separation of charge carriers (as a result of the formation of a heterojunction between ZnIn2S4 and MoO3). It is pertinent to mention here that the photocatalytic activity of high loading content (above 40 wt %) shows the low activity compared to the 40 wt % ZnIn2S4/MoO3 composite. There are two reasons explaining the low photocatalytic activity of the high-content-ZnIn2S4-loaded MoO3 composite: (1) High loading content of ZnIn2S4 may hinder MoO3 from absorbing visible light radiation and decrease the active sites of MoO3 by covering its surface.[49,50] (2) The overloading of ZnIn2S4 onto MoO3 may also turn into a recombination center of photogenerated charge carriers, which can eventually cause a reduction in photocatalytic activity.[50] Therefore, an appropriate amount of ZnIn2S4 loading is crucial in the increasing photocatalytic activity of the ZnIn2S4/MoO3 composite.

Figure 8

Changes in UV–vis absorption spectra at different time intervals on irradiation of an aqueous suspension of (a) MO and (b) RhB in the presence of 40 wt % ZnIn2S4/MoO3; changes in the concentration of (c) MO and (d) RhB in the absence and presence of different catalysts under a visible light source.

In addition, photodegradation of a colorless pollutant, a drug derivative like paracetamol, was also investigated using synthesized catalysts under analogous conditions. Figure S3 shows the time-dependent UV–vis absorption spectra of irradiation of an aqueous suspension of paracetamol in the presence of the 40 wt % ZnIn2S4/MoO3 photocatalyst composite under visible light. It could be seen in the figure that the absorption intensity decreases with increasing irradiation time and about 87% of paracetamol degradation was observed within 100 min of irradiation time under a similar condition. The substantial enhancement in the photocatalytic activity of 40 wt % ZnIn2S4/MoO3 is attributed to the extended visible light response and prolonged separation of photogenerated electron–hole pairs, which is achieved with the help of the direct Z-scheme system. Furthermore, the degradation of paracetamol in the presence of 40 wt % ZnIn2S4/MoO3 was also monitored using the high-performance liquid chromatography (HPLC) analysis technique. Figure S4 shows the HPLC of the irradiated mixture at different time intervals on irradiation of paracetamol in the presence of the 40 wt % ZnIn2S4/MoO3 composite under visible light. The figure clearly demonstrates that the starting material peak appearing at a retention time of nearly 100 min gradually decreases with increasing irradiation time, indicating the degradation of paracetamol.

Kinetics Study of the Degradation of MO

Degradation kinetics of MO was further studied by comparing the photocatalytic activities of synthesized photocatalysts using the Langmuir Hinshelwood rate equation.[51] The degradation of MO followed pseudo-first-order kinetics and is expressed in eq .where C0 and Ct are the initial and final concentrations at a fixed irradiation time, respectively, and k is the pseudo-first-order rate constant. The kinetic fits for the degradation of MO using pure MoO3 and ZnIn2S4 and different wt % loadings of ZnIn2S4/MoO3 are shown in Figure . The rate constants of pure ZnIn2S4 and MoO3 and different wt % ZnIn2S4-loaded MoO3 composites and the corresponding correlation coefficients (R2) are given in Table , which were determined by the linear fitting of ln(C0/C) vs irradiation time. The apparent rate constant value of composite materials was found to be higher than those of pure ZnIn2S4 and MoO3, and among all composite materials, the 40 wt % ZnIn2S4/MoO3 composite showed the highest rate constant within 100 min of irradiation time. The outcomes show that the MO degradation rate increases with the increasing loading content of ZnIn2S4 up to 40% and a further increase in loading content leads to a decrease in the photocatalytic activity. The incident light intensity effect on the removal of MO was tested at three different levels of incident light intensity. To conduct this experiment, we varied the light intensity to see the effect of incident light intensity on the removal of MO in the presence of the best catalyst (40 wt % ZnIn2S4/MoO3). It could be seen in Figure S5 that the photocatalytic activity of 40 wt % ZnIn2S4/MoO3 was found to be higher at 9500 lumens and any decrease or increase in light intensity leads to a decrease in degradation rate.

Figure 9

Plot between −ln C/C0 and irradiation time showing a kinetic study of MO degradation with and without photocatalysts.

Table 1

Pseudo-First-Order Rate Constants and Corresponding R2 Values of Different Samples

sample	apparent rate constant (Kapp) (min–1)	R2
blank	5.06714 × 10–6	0.95731
MoO3	0.00296	0.98381
ZnIn2S4	0.00745	0.99538
20 wt % ZnIn2S4/MoO3	0.01359	0.98713
30 wt % ZnIn2S4/MoO3	0.01818	0.99629
40 wt % ZnIn2S4/MoO3	0.02509	0.99463
50 wt % ZnIn2S4/MoO3	0.02079	0.99609

Recyclability of the Photocatalyst

The recyclability of a catalyst has consistently been a significant factor in determining its stability. Accordingly, in this experiment, the stability was assessed by reusing the most active photocatalysts (40 wt % ZnIn2S4) for four back-to-back cycles for the degradation of RhB in aqueous suspension under the conditions similar to those in the photocatalytic experiment. Every next cycle was done after washing the catalyst with water and ethanol to remove the undesirable materials, and afterward, the material was dried at 120 °C for 6 h in an oven. As shown in Figure , the photocatalyst shows a slight loss in the photocatalytic performance after four progressive cycle runs, exhibiting that the catalyst has good stability during the photocatalytic response under the visible light source.

Figure 10

Recyclability of 40 wt % ZnIn2S4/MoO3 in the photocatalytic degradation of RhB for four successive cycles.

Proposed Mechanism of ZnIn2S4/MoO3 Nanocomposites

It is well known that the band-edge potential level being an intrinsic property of a semiconductor determines the migration process of photogenerated charge carriers and hence the photocatalytic activity. The band-gap energies of MoO3 and ZnIn2S4 were found to be 2.92 and 2.24 eV, respectively, as calculated by drawing a tangent in the graph of (αhν)2 vs photon energy. To explain the transfer mechanism of photogenerated electron–hole pairs in the case of the ZnIn2S4/MoO3 composite, the band-edge potential levels of MoO3 and ZnIn2S4 were calculated by the empirical formulas given in eqs and 4where X is the absolute electronegativity, EVB and ECB are the valence band and conduction band potentials of semiconductors, respectively, and Eg is the band-gap energy. The values of X for MoO3 and ZnIn2S4 were 6.4 and 4.86 eV, respectively, and are given on the basis of the previous literature.[49,52] The bottom of the CB and the top of the VB potentials of MoO3 were calculated to be 0.44 and 3.36 eV, respectively. For ZnIn2S4, the CB and VB potentials were found to be −0.76 and 1.48 eV, respectively, as calculated from the above equations.

It is well documented in the literature that irradiated semiconductors in the presence of water and oxygen generate reactive species such as O2•–, •OH, and h+, which may, in turn, react with organic pollutants to degrade them to CO2 and water. To prove the reaction mechanism of the ZnIn2S4/MoO3 composite in detail, trapping studies were carried out to quench the main reactive species involved in the degradation of MO. In this connection, an aqueous suspension of MO was irradiated under analogous conditions in the presence of the 40 wt % ZnIn2S4/MoO3 composite containing different quenchers such as benzoquinone (BQ), disodium ethylenediaminetetraacetate (EDTA-2Na), and isopropyl alcohol (IPA). As usual, the samples were collected at different time intervals and monitored spectrophotometrically. Figure shows the change in the concentration of MO as a function of irradiation time in the absence and presence of different quenchers. The degradation was not influenced by EDTA, indicating that the hole is not the main reactive species for the oxidation of the organic pollutants. Conversely, BQ and IPA are capable of inhibiting the reaction, implying the role of superoxide and hydroxyl radicals in the degradation of MO. Because the CB potential of ZnIn2S4 was found to be −0.76 eV, as mentioned above, the photogenerated electron of ZnIn2S4 has enough potential to reduce O2 to –•O2 through a one-electron reduction reaction. On the other hand, the VB potential of MoO3 was calculated to be 3.36 eV, which can convert water molecules into hydroxyl radicals. To further verify whether hydroxyl radicals were formed during the photocatalytic reaction, a quantification experiment of hydroxyl radical formation was conducted by the photoluminescence technique using terephthalic acid as a probe molecule. It could be seen in Figure S6 that the PL intensity observed at 425 increases with the increasing irradiation time, indicating the generation of hydroxyl radicals. The intensity observed at 425 is mainly originated due to the formation of 2-hydroxyterephthalic acid, which is formed by the reaction of hydroxyl radicals with terephthalic acid.

Figure 11

Effect of different scavengers on photodegradation of MO in the presence of 40 wt % ZnIn2S4/MoO3.

Based on the above results and discussion, a tentative mechanism for the degradation of MO, RhB, and paracetamol in the Z-scheme ZnIn2S4/MoO3 system was proposed, which is shown in Scheme . If the heterojunction between ZnIn2S4 and MoO3 is traditional, an electron would transfer from the CB of ZnIn2S4 to the CB of MoO3 and, simultaneously, a hole in the VB of MoO3 would migrate to the VB ZnIn2S4, as shown in Scheme a. The accumulated electron in the CB of MoO3 would not be able to generate superoxide radicals as the CB potential of MoO3 was found to be more positive than the potential of the one-electron reduction reaction, i.e., Eo (O2/–•O2 = 0.046 eV vs NHE).[53,54] Likewise, hydroxyl radicals would not be generated from the hole of the VB of ZnIn2S4 because the VB potential of ZnIn2S4 is not negative enough to produce hydroxyl radicals, as discussed in the previous literature.[55,56] However, the expected reactive species generated by the traditional mechanism does not coincide with the results of trapping experiments. Therefore, it is necessary for the photoinduced carriers to be separated in a Z-scheme manner to produce superoxide and hydroxyl radicals for the ZnIn2S4/MoO3 composite. The generation of hydroxyl and superoxide radicals confirmed by trapping experiments can be strong evidence to confirm that the ZnIn2S4/MoO3 system follows the direct Z-scheme during photocatalysis. In the Z-scheme mechanism, the electron transfer from the CB of ZnIn2S4 to the CB of MoO3 is excluded since the CB of the potential of ZnIn2S4 is more negative to reduce oxygen molecules to superoxide radicals. Therefore, the electron transfer in ZnIn2S4/MoO3 would follow the Z-scheme mechanism. Under visible light irradiation, both visible-light-absorbing semiconductors can be excited to generate electron and hole pairs at their respective bands, as shown in Scheme b. The photogenerated electrons in the CB of MoO3 recombine with the holes of ZnIn2S4, while the separated holes remain in the VB of MoO3. This interface charge carrier recombination between ZnIn2S4 and MoO3 results in the accumulation of electrons in the CB of ZnIn2S4 and holes in the VB of MoO3. These separated electrons are more reductive to produce superoxide radicals after reacting with oxygen molecules via a one-electron reduction reaction. Similarly, the holes separated in the VB of MoO3 would react with water molecules to produce hydroxyl radicals as the VB of MoO3 is lower than the potential required to produce hydroxyl radicals. Thus, the superoxide and hydroxyl radicals are the major species produced by the MoO3/ZnIn2S4 composite and these reactive species are responsible for the degradation of organic pollutants, as confirmed by reactive species determination results. Therefore, we can certainly conclude that the photocatalytic mechanism in ZnIn2S4/MoO3 follows the direct Z-scheme system without any mediator.

Scheme 1

Schematic Diagram Showing (a) Simple Heterojunction Photocatalytic System and (b) Direct Z-Scheme Photocatalytic System of ZnIn2S4/MoO3 for Pollutant Degradation

Conclusions

In summary, we have developed a unique direct Z-scheme heterojunction between MoO3 and ZnIn2S4 by a hydrothermal-cum-impregnation technique. The developed MoO3/ZnIn2S4 composite has been systematically studied as a visible-light-responsive Z-scheme system for the degradation of organic pollutants. Compared to pristine MoO3 and ZnIn2S4, the obtained composite material showed extremely high activity for the degradation of MO, RhB, and paracetamol under visible light irradiation. The trapping studies clearly indicate the involvement of hydroxyl radical and superoxide radical anions as the main species in the photooxidation process. The outstanding photocatalytic properties are attributed to extended light response and prolonged separation rate due to the formation of the Z-scheme ZnIn2S4/MoO3 heterostructure. A direct Z-scheme mechanism is proposed based on radical-trapping experiments. The exceptionally high photocatalytic activity shows that our material is suitable for practical applications.

Experimental Section

Chemicals

The chemicals used in this study were of analytical grade and used as received without further purification and processing. Zinc acetate dihydrate Zn(CH3CO2)2·2H2O, indium(III) chloride (InCl3), ammonium molybdate tetrahydrate (NH4)6Mo7O24·4H2O, and organic pollutants such as methyl orange (MO), rhodamine B (RhB), and paracetamol were purchased from Sigma-Aldrich. Sodium sulfide (Na2S) and nitric acid were acquired from Fisher Scientific India, Pvt. Ltd. The other reagent-grade compounds utilized in this investigation, such as isopropyl alcohol (IPA) and benzoquinone (BQ), were obtained from Merck India Pvt. Ltd. Water used in all experiments was double distilled.

Preparation of Marigold-Flower-like ZnIn2S4

The marigold-flower-like ZnIn2S4 was prepared using the hydrothermal method according to the procedure reported in the previous literature.[57] In a typical synthesis, 0.5 mmol Zn(CH3CO2)2·2H2O, 1 mmol InCl3, and 2 mmol of Na2S were dissolved in 40 mL of double-distilled water (DDW) and the mixture was stirred for 60 min. Afterward, the solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 24 h. The autoclave was allowed to cool down at room temperature, and then, the precipitate was collected through centrifugation and washed several times with ethanol and water and dried at 100 °C 12 h to get the final product.

Preparation of the MoO3 Nanobelt

The MoO3 nanobelt was prepared according to the method reported in a previous article.[43] During the preparation process of the MoO3 nanobelt, 0.005 mol (NH4)6Mo7O24·4H2O was dissolved in 100 mL of water, followed by the addition of 15 mL of 3 M HNO3 solution into the above solution. After being stirred for 2 h, the above solution was transferred to a Teflon-lined autoclave for hydrothermal treatment at 180 °C for 20 h. The white product was centrifuged, filtered, and dried at 100 °C for 10 h.

Preparation of the ZnIn2S4/MoO3 Composites

The ZnIn2S4/MoO3 binary composites with different mass ratios of ZnIn2S4 to MoO3 were prepared by following a well-known impregnation technique mentioned in the previous literature.[18] Briefly, 1 g of the MoO3 nanobelt was dispersed in 30 mL of water containing the desired amount of already prepared ZnIn2S4 (200 mg) and stirred for 30 min to get a suspension. Afterward, the suspension was heated at 90 °C for 2 h and subsequently washed with water to remove the unreacted chemical species. The resulting yellow product was allowed to dry at 100 °C for 12 h, and the material was termed the 20 wt % ZnIn2S4/MoO3 composite. The different wt % ZnIn2S4-loaded MoO3 composites were prepared using a similar procedure by fixing the wt % of MoO3 and varying the percentage of ZnIn2S4 from 20 to 50 wt %. These different wt % of ZnIn2S4 loaded on MoO3 were R = 30, 40, and 50, and the composites were labeled as 30 wt % ZnIn2S4/MoO3, 40 wt % ZnIn2S4/MoO3, and 50 wt % ZnIn2S4/MoO3, respectively.

Material Characterization

An X-ray beam diffractometer (Shimadzu XRD, model 6100) was used to examine the crystal structure and phase of the prepared samples using graphite monochromatic copper radiation [Cu Kα radiation (1.540 Å)] operated at a voltage of 30 kV with a current of 15 mA at a 2θ value between 5 and 80° at the output speed of 10 degrees per minute. Fourier transform infrared (FTIR) spectroscopy was used to get information about the bonding in the molecular structure. The spectra of the samples were recorded in the range of 400–4000 cm–1 using a PerkinElmer Spectrum Two. The surface morphology and elemental analyses were performed with a JSM-6510 (JEOL) scanning electron microscope with the help of energy dispersion spectra (EDS) analysis. The intimate contact and heterostructure between both catalysts were confirmed by transmission electron microscopy (TEM, model JEOL-JEM 2100). The binding energy of all elements present in the samples was recorded on a PHI5000 (Versa Prob II, FEI Inc) X-ray photoelectron spectrometer using a monochromatic Al Kα (1486.6 eV) source. UV–vis diffuse reflectance spectroscopy (DRS) was performed using a PerkinElmer Lambda 35 with BaSO4 as a reflectance standard having a spectrum window of 200–700 nm. The photoluminescence study was conducted at room temperature using Hitachi F-2500 in the reflection mode at an excitation wavelength of 330 nm. The thermogravimetric analysis of the material was conducted using a Shimadzu 60H over the temperature range of 30–800 °C.

Photocatalytic Activity and Determination of Active Species

An immersion well photochemical reactor made of Pyrex glass was used to measure the photocatalytic activity of the prepared samples. The model pollutants such as methyl orange (MO), rhodamine B (RhB), and paracetamol were chosen to test the photocatalytic activity of the catalysts, and the degradation was conducted under a visible light source (λ ≥ 420 nm). The light source used in this study was a commercial halogen lamp (Philips 500-watt T3 double-ended). The light intensity of the lamp during degradation was recorded using a light radiometer and was found to be 9500 lumens. In a typical procedure, 0.2 g of the photocatalyst was dispersed in 200 mL of an aqueous solution of 10 ppm MO and RhB and 30 ppm paracetamol separately and then the solutions were kept in the dark for 30 min to attain the adsorption–desorption equilibrium. Thereafter, the suspension was exposed to the visible light with the constant bubbling of air, stirring, and proper cooling to avoid any thermal reaction. At certain time intervals, 5 mL aliquots were taken out from the solution and centrifuged (5000 rpm, 15 min) to remove the catalysts and analyzed using UV–vis spectrophotometry by measuring the change in absorbance as a function of time at their Lambda maximum.[58] The degradation of paracetamol was monitored using a chromatograph obtained from high-performance liquid chromatography (HPLC). The HPLC setup included a high pressure pump (model 515) and a C18 column (Waters India Ltd) with a UV–visible photodiode array detector (model 2489). A linear gradient elution (methanol/water 60:40) was used as the mobile phase with an injection volume of 20 μL.

To evaluate the involvement of active species in the degradation of methyl orange, trapping experiments were conducted by employing different scavengers such as 1,4-benzoquinone (BQ), isopropyl alcohol (IPA), and disodium ethylenediaminetetraacetate (EDTA-2Na) using 40 wt % ZnIn2S4/MoO3. In this study, a 2 mM concentration of different scavengers was used in the trapping experiments to check the inhibitory effect of scavengers during the photocatalytic reaction under analogous irradiation experimental conditions. These scavengers such as BQ, IPA, and EDTA were employed to trap the superoxide, hydroxyl radicals, and hole, respectively, during the photocatalytic process. The terephthalic acid photoluminescence probe method was employed to determine the generation of hydroxyl radicals during photocatalysis. In this experiment, the desired amount of the 40 wt % ZnIn2S4/MoO3 composite was suspended in a basic solution of terephthalic acid (5 × 10–4 M in 2 × 10–3 M NaOH solution). The above solution was stirred in the dark for 30 min to achieve the adsorption–desorption equilibrium, and afterward, the irradiation was started. At certain time intervals, the sample was collected and centrifuged to analyze the generation of hydroxyl radicals. The emission spectra of 2-hydroxyterephthalic acid were recorded at 425 nm at an excitation wavelength of 325 nm.