Hydrogen Production Through Water Splitting Reactions Using Zn-Al-In Mixed Metal Oxide Nanocomposite Photocatalysts Induced by Visible Light

Abstract

In this study, the synthesis of hybrid photocatalysts of Zn-Al-In mixed metal oxides were activated by using visible light, derived from Zn-Al-In layered double hydroxide (ZnAlIn-LDH), and these nanocomposites demonstrated high efficiency for photocatalytic H₂ production under UV light when using methanol as a sacrificial agent. The most active photocatalytic material produced 372 μmol h⁻¹ g⁻¹ of H₂. The characterization of these materials included X-ray diffraction (DRX), infrared spectroscopy (FTIR), X-ray fluorescence spectroscopy (XRF), X-ray spectroscopy (XEDS), scanning electron microscopy analysis (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy, and N₂- physisorption. In addition, the materials were characterized by photoelectrochemical techniques to explain the photocatalytic behavior. Subsequently, the photocatalytic performance for the water-splitting reactions under visible irradiation was evaluated. The ZnAlIn-MMOs with an In/(Al + In) molar ratio of 0.45 exhibited the highest photocatalytic activity in tests under visible light, attributed to the efficient separation and transport of photogenerated charge carriers originating from the new nanocomposite. This discovery indicates a method for developing new types of heteronanostructured photocatalysts which are activated by visible light.

1. Introduction

Currently, global energy systems heavily depend on the combustion of fossil fuels such as coal and natural gas, which together account for over 85% of the world’s primary energy production. This growing global demand for fuel has spurred research into alternative energy sources, positioning hydrogen as an ideal fuel for the future [1,2,3]. Hydrogen is non-toxic, and its combustion produces no pollution or greenhouse gases. Various techniques for hydrogen production are currently being explored, including the use of nuclear reactors to power hydrolysis reactors, combustion of fossil fuels, electrocatalytic processes, electrolysis, thermolysis, and photocatalytic water-splitting reactions [4,5,6,7,8].

Hydrogen-based energy systems are an effective, eco-friendly, and sustainable solution for reducing carbon in various sectors where significantly lowering carbon emissions is challenging. Hydrogen contains 2.5 times the energy per weight of any other standard fuel, and it is well known that its combustion products are harmless to the environment [1,9]. Photocatalytic hydrogen production has garnered interest due to its potential to generate green fuel in an environmentally friendly manner, making it one of the most promising strategies among various hydrogen production methods [10,11,12].

Photocatalysis is regarded as an efficient and cost-effective key technology for generating hydrogen fuel. It utilizes the absorption and energy conversion mechanisms by irradiating semiconductor materials to produce electron-hole pairs, which split water molecules into hydrogen and oxygen. Since the pioneering work of Fujishima and Honda [13,14] on water splitting, thousands of papers have been published on this topic. Various semiconductor oxides, including TiO₂, ZnO₂, RuO₂, NaTaO₃, and CdS, as well as transition metal oxides, have been studied as catalysts. Photocatalytic hydrogen production fundamentally relies on the photogeneration of electron-hole pairs, with the condition that the conduction band (CB) and valence band (VB) levels meet the energy requirements set by the reduction and oxidation potentials of the H₂O molecule, respectively [10,15,16,17].

Layered double hydroxides (LDHs) are a family of highly ordered, two-dimensional layered anionic clays composed of positively charged layers, containing divalent and trivalent cations alternately distributed within the layers and charge-balancing anions between the layers [18]. Since LDHs can host a wide variety of tunable M²⁺ and M³⁺ ions within their layers or in the interlayer space in the form of metal complexes, calcining LDHs at intermediate temperatures (450–600 °C) can result in poorly crystallized mixed metal oxides [19,20]. This structural transformation endows LDH materials with an extraordinary capacity as catalysts [21,22]. LDH-based photocatalysts have been extensively investigated in recent decades due to their applicability to the photocatalytic splitting of water molecules [16,23,24].

Indium oxide (In₂O₃) has been explored as a potentially valuable material for hydrogen production via photocatalysis due to its unique properties [25,26,27]. In₂O₃ has a bandgap of approximately 2.9 eV, making it responsive to visible light, which is beneficial for solar energy absorption. It is chemically stable and resistant to corrosion in aqueous solutions, which is essential for the longevity of the photocatalyst. It has good charge carrier mobility, facilitating the separation of light-generated electrons and holes, reducing recombination, and enhancing photocatalytic efficiency [25,26,27,28].

Taking this into account, this study showcases the results of the synthesis and characterization of Zn-Al-In mixed metal oxides (ZnAlIn-MMOs) derived from ZnAlIn-LDH layered double hydroxides, as well as the photocatalytic behavior of the obtained heterojunctions induced by visible light in the water-splitting reaction. It was found that an appropriate In/(Al + In) molar ratio of 0.52 can exhibit enhanced photocatalytic activity for the water-splitting reaction process.

2. Results

2.1. Crystalline Structure

Figure 1 presents the XRD diffraction patterns. All samples exhibited intense characteristic reflections corresponding to the two-dimensional LDH family which are similar to hydrotalcite, namely (0 0 3), (0 0 6), (0 1 2), (0 1 5), (0 1 8), (1 1 0), and (1 1 3) for the hydrotalcite-like LDH phase (JCPDS no. 38-0487) [28]. In each case, the basal spacing value (d 0 0 3) of the LDH phase was approximately 0.76 nm, and its higher-order diffractions are indicative of the highly crystalline nature and excellent layered characteristic of the samples [29]. The peak of (0 0 3), centered at 2θ = 11.9, suggests the intercalation of CO₃²⁻ anions within the layers. LDH-0.3, 0.5, and 0.7 exhibited several characteristic reflections of the In(OH)₃ phase at approximately (2 0 0), (2 2 0), and (2 2 2) (Figure 1), in addition to the reflections of the ZnAl precursor. Figure 2 displays the spectra for the samples calcined at 500 °C. The calcination process resulted in a complete collapse of the original structure, with all reflections clearly indexed to the hexagonal wurtzite structure of ZnO (JCPDS no. 36-1451). The broadening of the ZnO phase reflections suggests that the ZnAlIn-MMO samples were not highly crystalline. Additionally, reflections (2 2 2) and (4 0 0) from the crystalline phase of In₂O₃ (JCPDS no. 06-0416) were observed at 2θ = 32° and 37°, respectively, in the pattern of the calcined composites. There were no detectable reflections for the Al₂O₃ phase, indicating the presence of amorphous Al₂O₃. This amorphous Al₂O₃ likely resulted from the collapse of the ZnAlIn-LDH layered structure during calcination. Consequently, the amorphous Al₂O₃ may act as a dispersing agent, preventing the aggregation of ZnO or In₂O₃ particles.

2.2. Fourier Transform Infrared Spectroscopy

In the FTIR analyses of its precursors (Figure 3A), the NO₃⁻ group dominated the spectrum, with the strongest vibrations near the 1330–1390 cm⁻¹ regions. The metal (Zn, Al, or In) showed barely noticeable signals (500–600 cm⁻¹) due to the strong absorption from the nitrate. The signals appearing between 1600 and 1640 cm⁻¹ corresponded to vibrations from the present adsorbed water or moisture. The bands between 3000 and 3550 cm⁻¹ were associated with the asymmetric stretching vibration of the O–H bonds in the water molecules. This is usually a broad band due to the hydrogen bonding interactions between water molecules.

Figure 3B presents the analysis results of the calcined LDH samples. The ZnAlIn-0.3-C, ZnAlI-0.5-C, and ZnAlIn-0.7-C samples exhibited absorption peaks from 500 to 700 cm⁻¹. This region showed the characteristic vibrations of the Zn–O, Al–O, and In–O bonds, indicating the formation of metal oxides [30]. Additionally, the absorption band observed at 1065 cm⁻¹ can be attributed to the characteristic lattice vibration of the metal oxides (MOs) associated with the LDH layers (M = Zn, Al). The FTIR spectra of the calcined LDH samples revealed a peak at 1400 cm⁻¹, which can be attributed to the asymmetric stretching of carbonate species located in the interlayer regions [31]. Therefore, the FTIR spectra validated the presence of carbonate ions in the structures of all samples. The region of 3000–3600 cm⁻¹ can be attributed to hydroxyl groups or residual adsorbed water.

The main difference between the FTIR spectra of the precursors and the mixed oxide is elimination of the nitrate-related bands and the appearance of the metal-oxygen bond bands in the mixed oxide.

2.3. Elemental Chemical Composition

The elemental chemical composition in bulk was determined from the corresponding XEDS spectrum of each sample, and the results are summarized in

Table 1. Elemental chemical compositions of the samples by XEDS.

Sample	Atomic %					Zn/Al	In/(Al + In)
	Zn	Al	O	C	In
ZnAl-C	25.02	8.08	31.57	35.33	----	3.09	0
ZnAlIn-0.3-C	42.13	13.09	27.77	9.96	7.05	3.22	0.35
ZnAlIn-0.5-C	45.05	14.70	19.88	4.38	15.99	3.06	0.52
ZnAlIn-0.7-C	50.07	14.89	10.13	1.04	23.87	3.36	0.62

. It was found that the ZnAl-C material was mainly composed of Zn, Al, O, and C (Figure 4A). In addition, the presence of In was corroborated (Figure 4B) by the In₂O₃ formation, as demonstrated by X-ray diffraction. Elemental analysis via a JSX-1000S (X-ray fluorescence spectrometer, JEOL, Tokyo, Japan) showed that the actual molar ratios of Zn/Al/In in the final products aligned the molar ratios closely with the theoretical values (

Table 2. Chemical, textural, and optical properties of the samples.

Sample	Zn/Al Molar Ratio	In/(Al + In) Molar Ratio	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Average Pore Diameter (nm)	Band Gap (eV)
ZnAl-C	3.12	0	34.19	0.06	8.17	3.45
ZnAlIn-0.3-C	3.30	0.26	73.23	0.16	10.12	2.91
ZnAlIn-0.5-C	2.77	0.49	80.16	0.25	12.15	2.77
ZnAlIn-0.7-C	3.45	0.64	66.53	0.10	9.73	2.67

), indicating that nearly all of the metal ions were fully coprecipitated.

2.4. Scanning Electron Microscopy (SEM) and Elemental Mapping (XEDS)

SEM was employed to closely examine the microstructure of the synthesized MMO nanocomposite systems. Figure 5a–d and Figure 6a–d display representative SEM images of ZnAl-C and ZnAlIn-0.5-C, respectively. A typical morphology was noted, showing layers of platelets within a single particle, with the sizes and shapes of this LDH varying for the ZnAl-C matrix (Figure 5a–d). The observed morphology aligned with the XRD findings, indicating that the material was well crystallized and possessed the structural and morphological traits characteristic of LDH [32]. The ZnAlIn-0.5-C sample exhibited a morphology of sharp-edged hexagonal platelets (Figure 6a–d). This variation might be related to differences in nucleation, growth, and the LDH mechanism under various reaction environments (metal and salts) and conditions. Through elemental mapping (see Figure 5a–g and Figure 6e–h for ZnAl-C and ZnAlIn-0.5-C, respectively), it was possible to observe that the analyzed particles of the ZnAlIn-0.5-C material were composed mainly of Zn, Al, and In.

2.5. High-Resolution Transmission Electron Microscopy (HRTEM)

Figure 7a shows the material ZnAlIn-0.5-C, where three different lattice fringes can be observed. The first (of 0.26 nm) was found in zone 1 (Figure 7b), corresponding to the distance between planes (0 0 2) in the ZnO crystal lattice. The second one, marked in zone 2 (Figure 7c), presented an interplanar distance of 0.24 nm, which could correspond to the phase In₂O₃, although this was also a value close to the distance between the planes of the ZnO. Finally, the lattice observed in zone 3 (Figure 7d) showed a heterojunction of ZnO and In₂O₃. The interface between ZnO and In₂O₃ observed in the micrograph confirmed the presence of the two phases in that material.

2.6. Diffuse Reflectance Spectroscopy

As indicated by the band gap values of the ZnAlIn-MMO in Table 2, increasing the concentration of indium resulted in a decrease in the band gap (Figure 8). The band gap energies (Eg) for the samples, derived from the intersection points of the UV-Vis spectra using the equation Eg = 1240/λ [33], were about 3.45 eV for ZnAl-C, 2.91 eV for ZnAlIn-0.3-C, 2.77 eV for ZnAlIn-0.5-C, and 2.67 eV for ZnAlIn-0.7-C. These results demonstrate that the ZnAlIn-MMO samples possessed visible light absorption properties, attributed to the heterostructured nature of the nanocomposite.

2.7. N₂ Adsorption-Desorption

Figure 9A presents the nitrogen adsorption-desorption isotherms for the calcined samples: ZnAl-C, ZnAlIn-0.3-C, ZnAlIn-0.5-C, and ZnAlIn-0.7-C. All of the samples showed type IV isotherms. The samples of ZnAl-C, ZnAlIn-0.3-C, and ZnAlIn-0.7-C exhibited H₃ type hysteresis loops which lacked any limiting adsorption at high P/P₀, suggesting the mesoporous nature of these materials with an interconnected 3D pore structure. This phenomenon is typically associated with particle aggregates forming slit-shaped pores due to irregular gaps between particles. The ZnAlIn-0.5-C sample displayed a H₂ type hysteresis loop, which is typical for most inorganic oxides. Furthermore, the BET-specific surface area of the samples increased from 34.19 m²/g for ZnAl-C to 80.16 m²/g for ZnAlIn-0.5-C (Table 2), which was attributed to the heteronanostructure present in the ZnAlIn-0.5-C sample. As shown in Figure 9B, the samples exhibited a pore size distribution ranging from 6 to 50 nm, with a peak near 10 nm (Table 2). This typically corresponds to larger pores with a moderate surface area due to a layered structure, placing the pore diameters within the mesoporous range (~3–10 nm). The introduction of indium slightly reduced the pore size, likely due to the creation of more compact structures. However, the surface area tended to increase, potentially due to a higher density of active sites, as shown in Table 2.

2.8. Photoelectrochemical Characterization

Impedance has been widely used to measure the resistance to charge transfer when the photocatalyst is illuminated. This technique is advantageous since it is possible to obtain information about the processes taking place at the electrolyte/photocatalyst interface under illumination. Considering that the magnitude of the semicircles in the EIS Nyquist plot is directly related to the rate of charge transfer, the Nyquist plots are shown in Figure 10a. For all hydrotalcite-type materials, the diameter of the semicircle formed was smaller than that presented by the reference material ZnAl-C. The samples, in order from the smallest to the largest diameter, were ZnAlIn-0.5-C, ZnAlIn-0.7-C, ZnAlIn-0.3-C, and ZnAl-C. This indicates that the hydrotalcite-type materials exhibited the lowest resistance to charge transfer and thus the lowest recombination rate of photogenerated charge carriers [31].

To evaluate the efficiency of charge carrier separation in the synthesized materials, the current generation was measured under illumination by imposing an appropriate potential (Figure 10b). Initially, the material caused an increase in the recorded current, and then the irradiation was interrupted, causing a rapid decrease in the currents to zero. All photocatalysts showed an increase in current upon illumination and a decrease to zero upon stopping the passage of current. The material exhibiting the lowest photocurrent was ZnAl-C, which could reflect the presence of traps which hindered electron transport to the substrate. On the other hand, the ZnAlIn materials exhibited higher and faster photocurrent values with respect to ZnAl-C. Additionally, the ZnAlIn-0.5-C material presented the best behavior associated with higher pair separation (e⁻-h⁺). The presence of methanol facilitated hole trapping and increased the photocurrent, which led to a higher number of electrons capable of performing the reduction reaction.

2.9. Photocatalytic H₂ Production

The hybrid materials were tested as photocatalysts for a hydrogen evolution reaction using a 50 Vol.% methanol aqueous solution under visible light for 5 h. Figure 11 shows the H₂ production profiles for these photocatalysts. As the indium content increased, the H₂ production also increased, with ZnAlIn-0.5-C (372 $\mathsf{\mu }$mol h⁻¹ g⁻¹) exhibiting the highest photocatalytic activity compared with ZnAl-C (42 $\mathsf{\mu }$mol h⁻¹ g⁻¹) (Figure 12). Lower H₂ production of photocatalysts with the In content was observed (Figure 12) in ZnAlIn-0.3-C (255 $\mathsf{\mu }$mol h⁻¹ g⁻¹), likely due to an excessive amount of In₂O₃ in the composite resulting in poor charge separation. This indicates an optimal In/(Al + In) ratio in ZnAlIn-LDH, which aids in the separation and transport of photogenerated charge carriers, enhancing H₂ evolution and reducing electron-hole recombination. When the ZnAlIn-0.5-C was illuminated, electrons efficiently transferred from the conduction band of amorphous In₂O₃ to that of ZnO, while photogenerated holes moved from the valence band of ZnO to that of In₂O₃. This charge migration improved hydrogen production efficiency, as the ZnAlIn-MMO structures promoted electron-hole separation. However, a higher In₂O₃ content in ZnAlIn-0.7-C (289 $\mathsf{\mu }$mol h⁻¹ g⁻¹) resulted in a decrease in photocatalytic activity, likely due to a rapid recombination of the photogenerated electron-hole pairs. This suggests that an optimal In₂O₃ content in ZnAl interfaces directly enhances the photocatalytic H₂ evolution rate.

ZnO nanocrystallites interacting with amorphous In₂O₃ components can act as electron traps, aiding in electron-hole separation and transferring trapped electrons to adsorbed O₂. Additionally, the amorphous Al₂O₃ phase in ZnAlIn-MMO may prevent aggregation of the ZnO-In₂O₃ heteronanostructure, enhancing the photocatalytic activity of ZnAlIn-MMO photocatalysts. In contrast, rapid recombination of photogenerated electrons and holes can easily occur in the monophase ZnAl sample. The low photocatalytic activity of ZnAlIn-0.7-C (289 $\mathsf{\mu }$mol h⁻¹ g⁻¹) was due to an excessively high In₂O₃ content, resulting in poor charge separation. Thus, the ZnAlIn-MMO structure is crucial for the photocatalytic H₂ evolution rate under visible light.

Consequently, the well-designed structure of ZnAlIn-MMO nanocomposites ensures a close interaction between In₂O₃ domains and ZnO nanocrystallites, significantly enhancing photocatalytic activity. Future studies could develop more efficient visible light-induced photocatalysts based on these findings. These results demonstrate that ZnAlIn-MMO nanocomposites derived from LDH precursors show improved photocatalytic activity under visible light irradiation. The ZnAl-C (42 $\mathsf{\mu }$mol h⁻¹ g⁻¹) material failed to generate H₂ because of the large band gap of 3.45 eV and the insufficient reduction potential of H₂O required for the reduction of H⁺ to H₂ by ZnO [34].

3. Discussion

As previously noted, the most photoactive material for H₂ production using UV light and methanol as a sacrificial agent was the ZnAlIn-0.5-C photocatalyst. This photocatalytic activity resulted from the formation of the ZnO-In₂O₃ composite on the surface. The reaction mechanism for H₂ evolution by photocatalysis using the ZnO-In₂O₃/ZnAl material is illustrated in Figure 13. The active phase was the ZnO-In₂O₃ composite, where the heterojunction at the interface of In₂O₃ and ZnO promoted efficient separation of the e⁻-h⁺ pairs.

An increase in the In₂O₃ content on the ZnO significantly boosted the hydrogen yield. The superior photocatalytic performance of the ZnO-In₂O₃/ZnAl can be attributed to several factors. First, the heterojunction between In₂O₃ and ZnO enhanced the charge transfer process [27]. The conduction band position (CB) of In₂O₃ is more negative than the CB of ZnO, while the valence band (VB) of ZnO is more positive than the VB of In₂O₃ [35]. Under visible light, electrons from the VB were excited to the CB of In₂O₃ and ZnO. Then, the photogenerated electrons from the CB of In₂O₃ transferred to the CB of ZnO, and the holes from the VB of ZnO transferred to the VB of In₂O₃. The photogenerated holes were trapped by surface hydroxyl groups or water molecules, forming highly oxidative hydroxyl radicals (·OH) which oxidized the sacrificial molecule (methanol) to CO₂, while the electrons were captured by H⁺ on the surface of ZnO, resulting in H₂ production [36].

Second, the separated electrons and holes from ZnO-In₂O₃ increased the yield and lifetime of the charge carriers and reduced the recombination of electrons and holes. Third, the band gap energy between the VB of In₂O₃ and the O₂/H₂O energy level was much lower than that of ZnO and the O₂/H₂O energy level, and thus the high oxidative capability of surface hydroxyl chemisorption accelerated the capture of photogenerated holes [37].

4. Materials and Methods

4.1. Synthesis Procedure

Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, and In(NO₃)₃·4H₂O with a Zn/(Al + In) molar ratio of 3.0 and different In/(Al + In) molar ratios (0.3, 0.5, and 0.7, respectively) were dissolved in 30 mL of deionized water to form a clear salt solution ([Zn²⁺] = 0.15 M; [Al³⁺] + [In³⁺] = 0.05 M). Then, 50 mL of an alkaline solution of NaOH (0.24 M) and Na₂CO₃ (0.1 M) was added dropwise to the salt solution under vigorous stirring at room temperature.

The formation of ZnAlIn-LDHs and the conversion into mixed metal oxides involved several key reactions. The metal nitrates (Zn(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, and In(NO₃)₃·4H₂O) were dissolved in water to release their respective metal cations:

Zn(NO₃)₂·6H₂O → Zn²⁺ + 2NO₃⁻ + 6H₂O

Al(NO₃)₃·9H₂O → Al³⁺ + 3NO₃⁻ + 9H₂O

In(NO₃)₃·4H₂O → In³⁺ + 3NO₃⁻ + 4H₂O

As the NaOH was added, the hydroxide ions (OH⁻) reacted with the metal cations to form the respective metal hydroxides:

Zn²⁺ + 2OH⁻ → Zn(OH)₂

Al³⁺ + 3OH⁻ → Al(OH)₃

In³⁺ + 3OH⁻ → In(OH)₃

These hydroxides then arranged themselves into layered structures, stabilized by the intercalation of carbonate ions (CO₃²⁻) between the positively charged layers.

The pH of the resulting solution was adjusted to 10.0 by further titration with NaOH solution (0.24 M). The suspension was aged at 60 °C for 6 h and then filtered and washed with distilled water until the pH of the filtrates was nearly 7. For a comparative study, ZnAl was prepared under identical experimental conditions in the absence of In³⁺ ions. The synthesized LDHs were calcined at 500 °C for 4 h using a heating rate of 5 °C/min. During the calcination at 500 °C, the hydroxides decomposed to form mixed metal oxides:

Zn(OH)₂ → ZnO + H₂O

Al(OH)₃ → AlO₃ + 3H₂O

In(OH₃) → In₂O₃ + 3H₂O

The resulting mixed metal oxides were referred to as ZnAlIn-X-C, where X represents the three different molar ratios of In (0.3, 0.5, and 0.7) and C represents the samples calcined at 500 °C.

4.2. Characterization Techniques

The diffraction patterns for the fresh ZnAlIn-LDH-X powders and the calcined ZnAlIn-MMO-X were acquired. These materials were analyzed at room temperature by using a Bruker D₂ PHASER, which was equipped with a LynxEye detector and used a CuKα source (λ = 0.154 nm). The diffraction peaks in the XRD patterns were identified using the JCPDS database. The FTIR spectra were obtained using a Shimadzu spectrophotometer (IRTracer-100) in the range of 500–4000 cm⁻¹. Images were obtained after sample metallization with gold using a JEOL JSM-6010/LA microscope. The SEM equipment was equipped with an energy dispersive spectrometry system (XEDS) for analysis of the samples’ chemical compositions. High-resolution transmission electron microscopy (HR-TEM) of the samples was performed on a JEM-ARM 200CF microscope operated at 200 kV. Solid state UV-Vis diffuse reflectance spectra were obtained using a Varian Cary 100 UV-Vis spectrometer fitted with an integrating sphere for diffuse reflectance measurements of solid samples. X-ray fluorescence spectroscopy (XRF) was used to detect and quantify the chemical elements present in the samples. The XFR spectra was obtained using a JEOL-1000S spectrometer. The specific surface area was determined from the nitrogen adsorption isotherms obtained with a 3P Instruments GmbH & Co. Meso 222 model, using N₂ as an adsorbate and liquid nitrogen (−196 °C) as a cooler. The BET and BJH methods were used for calculations of the specific surface area and mean pore diameter, respectively.

4.3. Photoelectrochemical Tests

A conventional three-electrode cell was employed to perform the photoelectrochemical characterization of the photocatalysts. A Ag/AgCl/0.1 M KCl electrode was used as the reference electrode. The counter electrode was a graphite rod. To prepare the photocatalyst films (working electrodes), 100 μL of a 30 mg mL⁻¹ ZSx suspension in ethanol was deposited onto a clean 1.25 × 2.5 cm ITO coated substrate (Aldrich, Rs = 15–25 Ω cm²), which was placed inside of a spin coater at 1000 rpm for 30 s. The as-prepared photocatalyst films were dried at 80 °C for 2 h to evaporate as much solvent as possible. Finally, a squared area of 0.5 × 0.5 cm was delimited to perform the measurements [38]. The electrochemical characterization was performed using a 0.03 M NaClO₄ in water or 1:1 water:methanol electrolyte. Prior to each measurement, N₂ was bubbled for 15 min. The illumination was performed using a Newport Q Housing (model 60025) equipped with a 100 W Hg arc lamp. EIS measurements were performed at the open circuit potential using an AC perturbation of ±25 mV. All measurements were carried out using an AUTOLAB 302 N potentiostat.

4.4. Photocatalytic Tests

For the hydrogen determinations of the different mixed metal oxides (MMO) under visible light irradiation, four blue LED lamps (3 W) with λ = 540 nm were used as the irradiation source. These LED lamps were properly distributed to ensure that the suspended solids were fully illuminated. The amount of hydrogen produced was measured using a Shimadzu G-08 gas chromatograph equipped with a thermal conductivity detector (TCD) and a Shincarbon column pack (2 m in length, 1 mm in inner diameter, and 25 mm in outer diameter), with N₂ as the carrier gas.

5. Conclusions

This study presented a straightforward method for creating visible light-induced ZnAlIn-MMO photocatalysts from ZnAl-LDH precursors, synthesized using a coprecipitation technique with varying In/(Al + In) molar ratios. In the synthesized ZnAlIn-MMOs, well-dispersed amorphous In₂O₃ domains could closely interact with ZnO nanocrystals, as observed by HRTEM, forming a mixed metal oxide (MMO) heterostructure. The ZnAlIn-calcined nanocomposites, with band gaps ranging from 2.6 to 2.9 eV, exhibited higher specific surface areas compared with pure ZnAl-C. Under visible light irradiation, the ZnAlIn-0.5-C photocatalyst demonstrated significantly higher photocatalytic activity compared with pure ZnAl-C, representing improvement by a factor of 8.8.

The superior photocatalytic activity of the ZnAlIn-0.5-C photocatalyst for hydrogen production was primarily due to the formation of a ZnO-In₂O₃ composite on the ZnAl-LDH surface. The heterojunction between ZnO and In₂O₃ played a critical role in enhancing charge separation, leading to efficient electron-hole pair separation and minimizing recombination during photocatalytic reactions. The difference in conduction and valence band positions between ZnO and In₂O₃ facilitated the transfer of photogenerated electrons and holes, which in turn improved the oxidation and reduction reactions necessary for hydrogen evolution.

When exposed to visible light, the electrons in the valence band of In₂O₃ were excited to its conduction band. These electrons then transferred to the conduction band of ZnO, while the holes in the valence band of ZnO transferred to the valence band of In₂O₃. The holes reacted with water or surface hydroxyl groups, generating highly oxidative hydroxyl radicals (·OH), which aid in oxidizing methanol into CO₂. Meanwhile, the electrons reduced the protons on the ZnO surface, producing hydrogen gas.

Indium’s role in the ZnAlIn photocatalyst extends beyond creating efficient heterojunctions. Doping with indium reduces the material’s bandgap through several mechanisms, enhancing its visible light absorption capacity. Indium’s 5s and 5p orbitals interact with Zn and Al atoms in the crystal structure, creating hybridized energy levels in the conduction and valence bands. This hybridization narrows the bandgap, allowing the semiconductor to absorb light at lower energies (longer wavelengths), which is crucial for improving photocatalytic efficiency. The introduction of indium also generates intermediate energy levels within the bandgap, acting as electron and hole traps which facilitate electronic transitions with lower-energy photons, further reducing the energy barrier for electron excitation from the valence to the conduction band.

Moreover, indium introduced distortions in the crystal lattice due to its ionic size difference relative to Zn and Al, affecting electronic interactions within the material. These structural distortions increased the density of the states near the conduction band, facilitating charge carrier mobility and further reducing the bandgap. As a result, the energy required to excite electrons from the valence band to the conduction band was lowered, enhancing the photocatalyst’s ability to generate electron-hole pairs under visible light exposure. These electron-hole pairs are essential for the water-splitting reaction, where electrons in the conduction band reduce protons to produce hydrogen and holes in the valence band participate in water oxidation to produce oxygen.

In addition to reducing the bandgap and improving charge separation, indium enhances the chemical stability of the semiconductor, protecting it against corrosion and degradation. This stability is crucial for maintaining photocatalytic performance over time, ensuring that the ZnAlIn-MMO remains efficient throughout repeated cycles of the water-splitting reaction.

In summary, the enhanced photocatalytic activity of ZnAlIn-MMO-0.5 for hydrogen production is attributed to the synergistic effects of ZnO and In₂O₃ in creating efficient heterojunctions, improving charge separation, and reducing electron-hole recombination. Indium’s ability to modify the semiconductor’s electronic structure, reduce the bandgap, and enhance visible light absorption further contributes to the superior performance of the ZnAlIn photocatalyst in water-splitting applications. Additionally, indium provides increased chemical stability.