Preparation and photocatalytic activity of ZnGa₂O₄-β-Ga₂O₃ thin films

Abstract

ZnGa₂O₄ and ZnGa₂O₄-β-Ga₂O₃ thin films were prepared via aerosol-assisted chemical vapor deposition (AACVD) using various ratios of the Zn and Ga precursors, resulting in the formation of amorphous ZnGa₂O₄ and Ga₂O₃. The formation of crystalline zinc gallate and heterostructure zinc gallate thin films was achieved by annealing the resulting films at high temperatures under air. The ZnGa₂O₄-β-Ga₂O₃ thin films showed enhanced photocatalytic activity compared with ZnGa₂O₄. The photocatalytic enhancement of the ZnGa₂O₄-β-Ga₂O₃ is explained by the formation of type-II band alignment at the interfaces between ZnGa₂O₄ and Ga₂O₃, resulting in enhanced photoinduced charge separation in the material.

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1. Introduction

Oxide semiconductors have attracted much interest as an effective material for photocatalysts because of their high photocatalytic activity and mechanical and chemical durability.^1–3 Semiconductor-based photocatalysts have been applied for various applications, such as for self-cleaning, organic/inorganic pollutant decomposition, photoelectrochemical water splitting and antimicrobial coatings. Since the first study on p-block metal oxides of MIn₂O₄, M₂SnO₄ and M₂Sb₂O₇ (M = Ca, Sr) was reported by Inoue et al.,^4–7 semiconductors with a d¹⁰ electronic configuration have been widely studied and show promise as photocatalysts.^8–12 Among the p-block metal oxide photocatalysts, zinc gallate (ZnGa₂O₄) is also a promising photocatalyst and has been applied to water splitting,^13–15 organic pollutant degradation¹⁶ and CO₂ reduction^17,18 applications under UV irradiation. However, ZnGa₂O₄ has had limited practical success due to its wide band gap energy (4.1–4.5 eV) and high recombination rate of the electrons and holes,¹⁹ resulting in low photocatalytic performance. Therefore, broadening the light absorption and preventing the rapid recombination of the photogenerated electron–hole pairs of ZnGa₂O₄ photocatalysts, has become an area of focus.

Doping ZnGa₂O₄ with foreign ions is one approach to broaden the light absorption and improve the photocatalytic performance. Doping ZnGa₂O₄ with cations has been investigated showing improved visible light absorption.^20,21 Although the band gap was successfully reduced, some metal-doped ZnGa₂O₄ showed low photocatalytic efficiency due to rapid electron–hole recombination.²² The formation of heterojunction structures is an attractive strategy to improve the photocatalytic properties of ZnGa₂O₄ by increasing the charge separation in photocatalytic processes. Heterojunction structures, such as ZnGa₂O₄/N-rGO¹⁴ and ZnO/ZnGa₂O₄,²³ have been developed for improving the photocatalytic performance of ZnGa₂O₄.

Many methods have been used to synthesize ZnGa₂O₄ particles or thin films including hydrothermal,²⁴ solid-state,²⁵ sol–gel,²⁶ chemical vapor deposition,²⁷ and RF magnetron sputtering.²⁸ Among the large number of synthesis methods to produce ZnGa₂O₄ materials, the synthesis of thin films is widely convenient for practical applications compared with powders, due to the problems in the separation and recovery of powders. Aerosol-assisted chemical vapor deposition (AACVD) is a promising technique for thin film preparation because it is a simple, scalable, and cost-efficient technique that allows good control over physical and chemical properties.^29,30 In AACVD, precursors with low vapor pressure are dissolved in an appropriate solvent and the solution is aerosolized and transported into the CVD chamber using a carrier gas. The chemical composition of the starting material can play a crucial role in influencing the chemical and physical properties of the deposited films.

In this study, the formation of crystalline zinc gallate and heterostructure zinc gallate (ZnGa₂O₄-β-Ga₂O₃) thin films was studied. A deeper understanding of the effect of the heterostructure formation on band alignment, charge transfer and photocatalytic activity is required to explain the phenomena at the interfaces between ZnGa₂O₄ and Ga₂O₃.

2. Experimental section

2.1. Preparation of ZnGa₂O₄ and ZnGa₂O₄-β-Ga₂O₃ thin films

Zinc acetylacetonate [(Zn(C₅H₇O₂)₂)], 99.9% and gallium acetylacetonate [(Ga(C₅H₇O₂)₃], 99.9%) from Sigma Aldrich were used as the zinc and gallium source materials, respectively. In a round bottom flask, solutions for AACVD were prepared by dissolving 0.5 mmol of zinc acetylacetonate and different amounts of gallium acetylacetonate (0.5, 0.7 and 1.0 mmol) in 40 mL methanol. Using a piezoelectric ultrasonic humidifier, a precursor mist was created and delivered to the reaction chamber with a flow rate of 1.0 L min⁻¹ nitrogen gas. The deposition was carried out in a reactor at 400 °C. The obtained films were then annealed at 700 °C in air for 5 h, yielding clear thin films.

2.2. Analytical methods

Film morphology and thickness were studied using top- and side-view scanning electron microscopy (SEM) in a JEOL6301 instrument (10 kV). X-ray diffraction (XRD) analysis was carried out using a Bruker-Axs D8 (GADDS) diffractometer. The instrument operated with a monochromated Cu X-ray source with Cu K_α1 (λ = 1.54056 Å) and Cu K_α2 radiation (λ = 1.54439 Å) emitted with an intensity ratio of 2 : 1 and a 2D area X-ray detector with a resolution of 0.01°. Films were analyzed with a glancing incident angle (θ) of 1°. The diffraction patterns obtained were confirmed using database standards. UV/Vis spectroscopy was performed using a double monochromated PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer in the 300–800 nm range. X-Ray photoelectron spectroscopy (XPS) was performed using a Thermo K alpha spectrometer with monochromated Al Kα radiation, a dual beam charge compensation system and constant pass energy of 50 eV. Survey scans were collected in the range of 0–1200 eV. High-resolution XPS spectra were used for the principal peaks of Zn (2p) and Ga (2p), and deconvoluted using CasaXPS software with the calibration of C1s at 284.5 eV. The surface roughness of the films was characterized by atomic force microscopy (AFM) on a Keysight 5600LS scanning probe microscope taken at a scale of 5 μm × 5 μm.

2.3. Photocatalytic tests

The intrinsic photocatalytic properties of the films were investigated through the photodegradation of octadecanoic (stearic acid), which was used here as a model organic pollutant.³¹ Stearic acid is very stable under UV irradiation in the absence of a photocatalyst. In these experiments, the films were dip coated with a thin layer of stearic acid (0.05 M solution in chloroform) and then monitored under UVC irradiation over a period of 32 h using a PerkinElmer RX-I Fourier transform infrared (FTIR) spectrometer. Plots of integrated areas of characteristic C–H infrared bands at 2923 and 2853 cm⁻¹ were produced and the photodegradation rates were estimated from linear regression of the initial 30–50% of the curves. A conversion factor from the literature (1 cm⁻¹ = 9.7 × 10¹⁵ molecules of stearic acid)³² allowed for the estimation of the number of molecules of stearic acid degraded upon irradiation time. The light source used was a UVC (λ = 254 nm) Vilber-Lourmat BLB lamp (2 × 8 W, I = 1.0 mW cm⁻²). The irradiance of the lamp was measured using a UVX radiometer (UVP).

3. Results and discussion

Amorphous thin films were grown on quartz substrates using aerosol-assisted chemical vapor deposition (AACVD) at 400 °C, as detailed in the experimental section. On the heated substrates, the precursors react and deposit on the substrates. The unreacted precursors and waste products are carried away to the exhaust, as shown in Fig. S1 (ESI†). A series of zinc gallate thin films were deposited using different molar ratios of Zn/Ga in the precursor mixture, resulting in the Zn/Ga mole ratios of 0.45, 0.26 and 0.17 in the films, as determined by elemental analysis (EDS), henceforth referred to as ZG-[Zn/Ga mole ratio], namely ZG-0.45, ZG-0.26 and ZG-0.17, respectively. X-ray diffraction (XRD) of the as-deposited films from AACVD showed no patterns and therefore they were presumed to be amorphous (Fig. S2, ESI†).

The amorphous phase was analyzed by XPS, where the energy separation between the Zn 2p_3/2 and Ga 2p_3/2 peaks was studied. The energy separation between Zn 2p_3/2 and Ga 2p_3/2 peaks (ΔE) has been used as a tool to distinguish whether the obtained product is a complete ZnGa₂O₄ or a composite of ZnO and Ga₂O₃, where the formation of ZnGa₂O₄ provides lower ΔE compared with a mixture of ZnO and Ga₂O₃.^33,34 In this work, ΔE of a mixture of commercial ZnO and Ga₂O₃ was studied providing a value of 96.6 eV. It was found that ΔE of the commercial mixture is higher than ΔE of the as-deposited amorphous thin films giving a value of 96.2 eV (Table S1, ESI†). This indicates the formation of zinc gallate under AACVD. The amorphous thin films were annealed under air at 700 °C, resulting in the formation of crystalline zinc gallate, which was confirmed by XRD as shown in Fig. 1. For those samples with low Ga content (ZG-0.45) only the pattern of cubic-phase ZnGa₂O₄ (JCPDS no. 86-0415)³⁵ was found. Relatively high Ga contents (ZG-0.26 and ZG-0.17), showed additional reflection peaks that were consistent with β-Ga₂O₃ (JCPDS no. 43-1012).^22,36

Fig. 1 XRD patterns of thin films annealed under air on quartz substrates at 700 °C of ZG-0.45, ZG-0.26 and ZG-0.17, respectively. Triangle symbols correspond to cubic-phase ZnGa₂O₄ (JCPDS no. 86-0415) and circle symbols correspond to β-Ga₂O₃ (JCPDS no. 43-1012).

The morphology and cross-section SEM images of the samples are shown in Fig. 2(a)–(c). SEM analysis showed similar surface morphologies in all thin films, with the thickness of ZG-0.45, ZG-0.26 and ZG-0.17 being 230, 280 and 340 nm, respectively. The surface topography of the annealed films (ZG-0.45, ZG-0.26 and ZG-0.17) was investigated by AFM, taken with a scale of 5 μm × 5 μm, as shown in Fig. 2.

Fig. 2 Scanning electron microscopy (SEM) of (a) ZG-0.45, (b) ZG-0.26 and (c) ZG-0.17. Atomic Force Microscopy (AFM) images showing the surface morphology (top) and 3D images (bottom) of (d), (g) ZG-0.45, (e), (h) ZG-0.26 and (f), (i) ZG-0.17 thin films on quartz substrates prepared by the AACVD method at different mole ratios of Zn and Ga in the precursor mixture. The surface topography of the annealed films investigated by AFM, taken with a scale of 5 μm × 5 μm.

Inspection of these images shows that the surface area and surface roughness of the thin films was not significantly affected by the ratio of Zn and Ga in the films. The surface roughness (rms roughness) of the annealed films (ZG-0.45, ZG-0.26 and ZG-0.17) was 63, 51 and 73, respectively, with surface areas of 26.7, 25.6 and 25.7 μm², respectively, as depicted in Table 1.

Table 1 Structural, optical, and functional parameters of ZnGa₂O₄ and ZnGa₂O₄-β-Ga₂O₃ heterojunction films prepared with different ratios of zinc acetylacetonate and gallium acetylacetonate in the precursor mixture

Sample	Zn/Ga atomic ratio		ZnGa2O4 : Ga2O3	E g (eV)	Film thickness (nm)	Surface area (μm^2)	RMS roughness (nm)	Surface area (μm^2)	ξ × 10^−4 (molecules photon^−1)
	In precursor solution	In as-deposited thin films
ZG-0.45	1.0	0.45	9 : 1	5.2	230	26.69	63	26.69	0.50
ZG-0.26	0.7	0.26	1 : 1	5.1	280	25.59	51	25.59	1.64
ZG-0.17	0.5	0.17	1 : 2	4.9	340	25.67	74	25.67	0.80

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High-resolution XPS was further employed to investigate the composition and surface electron state of the thin films. Fig. 3 shows the XPS spectra revealing (a) Ga 2p and (b) Zn 2p of the films ZG-0.45, ZG-0.26 and ZG-0.17, respectively. Binding energy (BE) calibration was carried out using the C1s peak located at 284.5 eV. The XPS spectrum of the ZG-0.45 reveals a symmetric peak for the Ga 2p orbital, with a BE of 1118.5 eV, while the spectra of ZG-0.26 and ZG-0.17 can be deconvoluted into two peaks, with binding energy values of Ga 2p_3/2 at 1118.4 and 1119.8 eV, respectively, which are associated with Ga³⁺ species in Ga₂O₃ and ZnGa₂O₄, respectively.³⁷

Fig. 3 High-resolution XPS of (a) Ga 2p and (b) Zn 2p spectra of ZG-0.45, ZG-0.26 and ZG-0.17 deposited by AACVD on quartz.

The XPS spectrum of the Zn 2p_1/2 peaks in the films was ∼1022 eV, which could be related to the formation of ZnGa₂O₄.³⁸ To investigate the vertical distribution of ZnGa₂O₄ and Ga₂O₃ in the films, the Zn/Ga ratio was calculated using XPS depth profiling shown in Fig. S3 (ESI†). The Zn/Ga atomic ratio of thin films with etching shows that there is a homogenous mix of ZnGa₂O₄ and Ga₂O₃ in the thin films (ZG-0.45, ZG-0.26 and ZG-0.17).

The light absorption properties of the materials were studied by using UV-vis spectroscopy. The absorption edges of ZG-0.45, ZG-0.26 and ZG-0.17 were located at around 250 nm. The band gap energy of the materials was determined from the Tauc plot for indirect band gap absorption, as shown in Fig. 4(a). The samples (ZG-0.45, ZG-0.26 and ZG-0.17) had E_g values of 5.2, 5.1 and 4.9 eV, respectively.

Fig. 4 (a) Tauc plot allowing an estimation of the band gap energies and the inset figure (b) shows the UV-Vis total transmittance of the annealed samples (ZG-0.45, ZG-0.26 and ZG-0.17) deposited by the AACVD method at different mole ratios between Zn/Ga in the precursor mixture of 1, 0.7 and 0.5, respectively.

The photocatalytic activity of the thin films was evaluated by testing the degradation rate of stearic acid, a model organic pollutant under UVC irradiation (BLB lamp, 1.0 mW cm⁻¹), as shown in Fig. 5 and Table 1. In the absence of catalyst, the detected concentration of stearic acid was stable under illumination, while the presence of catalyst (ZG-0.45, ZG-0.26 and ZG-0.17) leads to obvious degradation of stearic acid. The degradation of steric acid under irradiation results from the redox reactions of steric acid by photogenerated electrons and holes on the photocatalytic surface, providing the production of non-toxic CO₂ and H₂O.^39,40 The corresponding degradation curves are plotted against irradiation time in Fig. 5(a). The trend of degradation curves was as expected with the ZnGa₂O₄-β-Ga₂O₃ heterostructure thin film (ZG-0.26 and ZG-0.17) being a more effective photocatalyst than ZnGa₂O₄ (ZG-0.45). In the case of ZG-0.26, with the ratio between ZnGa₂O₄ and Ga₂O₃ being 1 : 1, this provided higher photocatalytic performance, compared with the other thin films. As mentioned above, the surface area of the thin films was similar, and the photocatalytic efficiency was independent of the thickness of the thin films (Table 1). The apparent enhancement could then be related to the optimal ratio between ZnGa₂O₄ and Ga₂O₃ in the thin film.

Fig. 5 (a) Integrated area of the IR spectra obtained from the initial rates of photodegradation of stearic acid upon irradiation time under UV illumination (UVC, λ = 254 nm, I = 1.0 mW cm⁻²), with thin films (ZG-0.45, ZG-0.26 and ZG-0.17) acting as photocatalysts in the photodegradation. (b) Corresponding formal quantum efficiencies (ξ), given as molecules degraded per incident photon (units, molecules photon⁻¹), upon cycling experiments. A blank reference corresponds to the quartz substrate without the photocatalytic coating.

The fitting of the initial degradation steps (zero-order kinetics) allowed for the estimation of the formal quantum efficiencies (ξ, units molecules photon⁻¹), given as the number of acid molecules degraded per incident photon (Fig. 5(b)). The cycle experiments resulted in a slight drop of the initial ξ values of the thin films. The drop of photocatalytic performance in the cycling experiments might be due to carbon contamination during the photodegradation of organic pollutants.

The higher photocatalytic performance of heterostructures (ZG-0.26 and ZG-0.17) might result from the formation of a heterojunction, which is an interface between the two regions of dissimilar semiconductors. In order to classify the type of band alignment in the interface, the valence band potential and band gap energy of ZnGa₂O₄ and β-Ga₂O₃ were considered. The band structures of ZnGa₂O₄ and β-Ga₂O₃ were analyzed using XPS showing that the valence band potential of β-Ga₂O₃ was 1.5 eV more positive than that of ZnGa₂O₄.²² ZnGa₂O₄ has a wide bandgap energy of ∼4.1–4.5 eV being similar to the bandgap energy of β-Ga₂O₃ (4.8 eV). Therefore, the band alignment of the ZnGa₂O₄-β-Ga₂O₃ heterojunction could be a type-II band alignment, which can enhance charge separation, as shown in Scheme 1.

Scheme 1 A schematic description of charge transfers across the ZnGa₂O₄-β-Ga₂O₃ heterojunction.

The photogenerated electrons tend to migrate from the conduction band (CB) of ZnGa₂O₄ to that of β-Ga₂O₃, while the generated holes transfer from β-Ga₂O₃ to ZnGa₂O₄. Consequently, the recombination process in this material is decreased, benefiting the enhancement of the photocatalytic performance. The higher photocatalytic performance of ZG-0.26 than ZG-0.17 might result from achieving an ideal ratio between ZnGa₂O₄ and β-Ga₂O₃ (1 : 1) in the ZnGa₂O₄-β-Ga₂O₃ heterojunction, resulting in the increase of the heterojunction interface in the material.

4. Conclusion

In this research, the ability to tune the composition of ZnGa₂O₄ and β-Ga₂O₃ thin films by varying the ratios of Zn and Ga precursors in the starting mixture under aerosol-assisted chemical vapor deposition (AACVD) has been demonstrated. The optimal ratio between the Zn and Ga precursor was found to be 0.7, which showed promising photocatalytic performance of the final product (ZG-0.26). The ZnGa₂O₄-β-Ga₂O₃ heterojunction possesses type-II band alignment, resulting in enhanced photocatalytic properties of the material. This result is a step forward toward the fabrication of an optimized photocatalytic material and calls for the implementation of synthesis strategies of zinc gallate.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by The Development and Promotion of Science and Technology Talents Project for a fellowship of P. Promdet. I. P. P. and C. J. C. thank EPSRC for grant EP/L0177709/1. The authors would like to thank Prof. Andreas Kafizas and Dr Raul Quesada-Cabrera for useful discussions on photocatalysis. The authors would also like to thank Prof. Robert Palgrave and Dr Sanjayan Sathasivam for XPS and Jian Guo for AFM.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ma01016j

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