Composition and Band Gap Tailoring of Crystalline (GaN)1–x(ZnO)x

Apr 10, 2018 - State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Composition and Band Gap Tailoring of Crystalline (GaN)1−x(ZnO)x Solid Solution Nanowires for Enhanced Photoelectrochemical Performance Jing Li,†,‡ Baodan Liu,*,‡ Aimin Wu,*,† Bing Yang,‡ Wenjin Yang,‡ Fei Liu,§ Xinglai Zhang,‡ Vladimir An,∥ and Xin Jiang*,‡ †

Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72 Wenhua Road, Shenyang 110016, People’s Republic of China § State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China ∥ School of Advanced Manufacturing Technologies, National Research Tomsk Polytechnic University, 30 Lenin Ave., 634050 Tomsk, Russia S Supporting Information *

ABSTRACT: Photoelectrochemical water splitting has emerged as an effective artificial photosynthesis technology to generate clean energy of H2 from sunlight. The core issue in this reaction system is to develop a highly efficient photoanode with a large fraction of solar light absorption and greater active surface area. In this work, we take advantage of energy band engineering to synthesize (GaN)1−x(ZnO)x solid solution nanowires with ZnO contents ranging from 10.3% to 47.6% and corresponding band gap tailoring from 3.08 to 2.77 eV on the basis of the Au-assisted VLS mechanism. The morphology of nanowires directly grown on the conductive substrate facilitates the charge transfer and simultaneously improves the surface reaction sites. As a result, a photocurrent approximately 10 times larger than that for a conventional powder-based photoanode is obtained, which indicates the potential of (GaN)1−x(ZnO)x nanowires in the preparation of superior photoanodes for enhanced water splitting. It is anticipated that the water-splitting capability of (GaN)1−x(ZnO)x nanowire can be further increased through alignment control for enhanced visible light absorption and reduction of charge transfer resistance.

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INTRODUCTION

For PEC water splitting, the key problem is how to develop and prepare highly efficient photoelectrodes with stable and durable H2 and O2 production under visible light irradiation.5−7 As a promising visible light driven photocatalyst, (GaN)1−x(ZnO)x solid solution has been demonstrated to possess excellent overall PC water-splitting capability due to its appropriate conduction/valence band positions and controllable band gaps governed by ZnO contents.8−15 Moreover, powder-based (GaN)1−x(ZnO)x solid solution has realized a maximum IQE of 5.9% and a long-term operation of half a year, indicating the great potential of the (GaN)1−x(ZnO)x photocatalyst in preparing high-performance photoanodes.16,17 However, previous (GaN)1−x(ZnO)x photoanodes have generally been obtained through the transfer of as-prepared (GaN)1−x(ZnO)x particles on a conductive FTO substrate,

Photoelectrochemical (PEC) water splitting via suitable semiconductor photoelectrodes has emerged as an attractive and feasible strategy to produce H2 energy from abundant solar energy and seawater since the discovery of the TiO2 electrode by Honda and Fujishima in the early 1970s.1 In comparison with the extremely low internal quantum efficiency (IQE) of powder-type photocatalysts for photocatalytic (PC) water splitting, PEC water splitting based on electrode-type photocatalysts can achieve a higher IQE and solar conversion efficiency (SCE) through an applied external voltage compensation of the inappropriate conduction/valence band positions and insufficient overpotentials, as well as the simultaneous improvement of the separation efficiency of photogenerated electrons and holes.2,3 In particular, these electrodes will make a significant contribution in addressing the present energy and environmental problems in combination with state of the art photovoltaic technology.4 © XXXX American Chemical Society

Received: February 1, 2018

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DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

In our previous work, we successfully synthesized series of porous (GaN)1−x(ZnO)x powders with ZnO content covering from 0.25 to 0.8526 and then realized the growth of (GaN)1−x(ZnO)x nanorods with exposed facets by using ZnGa-O precursors.27 It is found that the Zn-Ga-O precursors obtained from the calcination of Zn-Ga sol−gel have a low nitridation temperature of phase transformation and low decomposition temperature to simultaneously generate the gas source of Ga and Zn. On the basis of the synthesis routes and experiences in the controllable growth of 1D nanomaterials28−30 such as GaP-ZnS24 and GaP-ZnSe,31 the substrate is placed directly above the Zn-Ga-O precursors for the preparation of (GaN)1−x(ZnO)x nanowires in this work. In this way, the short diffusion distance of the gas source and relatively high temperature can guarantee the thorough substitution of elements and avoid the phase separation. Correspondingly, series of Zn-Ga-O precursors with different Zn/(Zn + Ga) ratios are also used to tailor the ZnO composition and band gap. The growth temperature and time are optimized to be 850 °C and 30 min. As a result, (GaN)1−x(ZnO)x nanowires with controllable ZnO contents are first synthesized on the Si substrate via an Au-assisted VLS mechanism. Furthermore, the PEC performance of obtained (GaN)1−x(ZnO)x samples as photoanodes is investigated for water splitting in detail.

which inevitably resulted in a weak connection between adjacent particles and also poor substrate adherence. These issues retarded the interfacial charge transfer and increased the recombination efficiency of photogenerated electrons and holes. As a result, the produced photocurrents were only up to the level of 1−2 μA cm−2 and then decayed gradually because of the peeling off of photocatalysts.18,19 Therefore, to optimize PEC performance, it is essential and meaningful to establish good electric contact between (GaN)1−x(ZnO)x particles/films and the conductive substrate and simultaneously reduce the density of boundaries and weak point contacts between adjacent (GaN)1−x(ZnO)x particles. In this regard, Imanaka et al.20 developed an aerosol deposition method to fabricate dense (GaN)1−x(ZnO)x film with nanoceramic particulate structure and high crystallinity, which showed an enhanced photocurrent of 500 μA cm−2 under simulated sunlight irradiation (100 mW cm−2). Later, Wang et al.21 proposed a moisture-assisted nitridation approach for the fabrication of compact and crystalline (GaN)1−x(ZnO)x film photoanodes. With HCl acid treatment and CoP cocatalyst modification, an enhanced photocurrent of 2.0 mA cm−2 was achieved under an applied bias of 1.4 V vs RHE. To further increase the photocurrent and IQE for efficient PEC water splitting, one-dimensional (GaN)1−x(ZnO)x nanowires directly grown on the conductive substrates have been considered as promising candidates for superior photoanodes due to their sufficient surface area and efficient electron−hole pair separation. In this case, the photoexcited minority holes of (GaN)1−x(ZnO)x nanowires can quickly arrive at the electrolyte interface along the radial direction for water oxidation reaction and, correspondingly, the majority of the electrons can be easily transferred along the axial direction to the counter electrode for the water reduction reaction under an external voltage.22 In 2010, Han et al. 23 first prepared single-crystal (GaN)0.88(ZnO)0.12 nanowires on a silicon substrate based on the basis of a vapor−liquid−solid (V-L-S) process through the chemical vapor deposition (CVD) method, in which nanopatterned Au (2 nm) and ZnGa2O4 powders were used as catalyst and reaction precursor, respectively. Subsequently, we also successfully obtained crystalline (GaN)1−x(ZnO)x nanowires with a narrow ZnO solubility range (x ≲ 0.05) at a growth temperature of 1150 °C by nitridating the mixed Ga2O3/Zn/ZnO precursors.24 Yang and co-workers25 used ZnO nanowires as templates and epitaxial GaN as shells to synthesize single-crystal (GaN)1−x(ZnO)x nanotubes with a ZnO concentration of 10% by a flowing heat treatment process. Nevertheless, the synthesis of quaternary (GaN)1−x(ZnO)x nanowires with controllable compositions and tunable band gaps still remains challenging and has rarely been reported until now. The main problem is the substantial and serious loss of Zn content under the growth conditions of harsh reduction atmosphere and high temperature (>900 °C). As is wellknown, the band gap of (GaN)1−x(ZnO)x generally decreases with the monoclinic increase of ZnO content. Therefore, increasing the ZnO content can enable an efficient absorption of visible light for an enhanced photocurrent and IQE. To reach this goal and solve the aforementioned problem, it is important to develop a new synthesis strategy and further decrease the growth temperature of (GaN)1−x(ZnO)x nanowires as much as possible. Meanwhile, a decent substrate adherence of the obtained nanowires is still required to guarantee a good electric contact and efficient electron transportation for enhanced PEC water splitting.

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EXPERIMENTAL SECTION

Preparation of (GaN)1−x(ZnO)x Solid Solution Nanowires. (GaN)1−x(ZnO)x solid solution nanowires were prepared through the combination of a chemical wetting process and an Au-assisted VLS process. First, a certain amount of gallium nitrate hydrate (Aladdin, 99.9%) and zinc acetate dihydrate (Aladdin, 99.9%) were dissolved in a solution of 2-methoxyethanol (Aladdin, 99.5%) and ethanolamine (Aladdin, 99.5%) with magnetic stirring at 70 °C for 60 min to form a transparent sol. The ethanolamine/2-methoxyethanol volume ratio was fixed at 5%. Then, the sol was transferred to the furnace and calcined at 550 °C for 8 h to generate a white powder precursor containing Zn-Ga-O. Finally, the precursor of Zn-Ga-O was evaporated to generate the gas source of Ga, Zn, and O atoms at an optimized temperature of 850 °C and reacted with NH3 (50 mL min−1) gas to lead to the nucleation of (GaN)1−x(ZnO)x nanowires on an n-type conductive Si substrate coated with Au layer (5 nm), which directly faced the reactant. The stable growth process of (GaN)1−x(ZnO)x nanowires was maintained at 850 °C for 30 min. To modulate the ZnO content in (GaN)1−x(ZnO)x nanowires, a series of Zn-Ga-O precursors with different Zn/Ga ratios were synthesized through designing the atomic ratio of Zn/Ga in the initial sol with Zn/ Ga = 10%, 30%, 50%, 60%, 70%, and 90% (atomic percentage). Microstructure and Optical Characterizations. The morphology and composition of as-synthesized nanowires were characterized using a field-emission scanning electron microscope (FE-SEM: FEI, Inspect F50) equipped with a Quanta 600 energy dispersive X-ray spectrometer (EDS). The microstructure, phase, crystallinity, and composition uniformity of single (GaN)1−x(ZnO)x nanowires were further examined by using X-ray diffraction (XRD; Rigaku RINT 2000) and a transmission electron microscope (TEM; Tecnai F20) under an accelerated voltage of 200 kV. The corresponding optical properties including absorption, band gap, and emission wavelength were analyzed by using UV−visible diffuse reflectance spectroscopy (DRS; Hitachi U-3900) and high-resolution cathodoluminescence (CL) spectroscopy (Horiba) under an accelerated voltage of 5 kV. Raman spectroscopy with a 532 nm laser (Horiba, HR Evolution) was used to analyze the vibrational states of bonding atoms in (GaN)1−x(ZnO)x nanowires. PEC Measurements. An electrochemical workstation (Autolab 302N) equipped with a 300 W xenon lamp was used to evaluate the PEC performance of (GaN)1−x(ZnO)x nanowires. Pt foil, a saturated B

DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Ag/AgCl electrode, and a (GaN)1−x(ZnO)x electrode were respectively used as the counter, reference, and working electrodes of a threeelectrode system. Linear sweep voltammetry with a scan rate of 10 mV s−1 was chosen as the running procedure, and the transient photocurrent was obtained in 0.5 M Na2SO4 (pH 6.0) electrolyte with chopping light irradiation (on/off switching interval of 5 s). AM 1.5G and cutoff (λ ≥400 nm) filters were used for the generation of visible light. The light intensity was fixed at 100 mW cm−2. The electrochemical impedance spectrum measurements were carried out at an applied voltage of 0 V under the same conditions as mentioned above. The different wavelength light was realized by a monochromatic filter and maintained at 15 mW cm−2.

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RESULTS AND DISCUSSION On the basis of a previous study on the synthesis of (GaN)1−x(ZnO)x solid solution, it has been found that the reaction temperature is critical to the nucleation and solubility tailoring.32,33 When the temperature is higher than 900 °C, the serious mass loss of ZnO (mainly Zn) content will appear. In this case, series of porous and nanocrystalline Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of 10%, 30%, 50%, 60%, 70%, and 90% have been designed and prepared to generate a Zn, Ga, and O gas source for the growth of (GaN)1−x(ZnO)x nanowires with controllable ZnO compositions on the basis of an Au-assisted VLS mechanism.23 Nanocrystalline Zn-Ga-O precursors have a lower decomposition temperature due to the size effect, which facilitates the growth of (GaN)1−x(ZnO)x nanowires at lower temperature and thus increases the ZnO content, correspondingly. Additionally, a short transportation distance of gas source is helpful in restraining the formation of GaN and ZnO clusters in the process of vapor phase transport and thus enhancing the ZnO solubility and uniformity in the nanowires. In order to establish the optimized growth temperature, the Zn/(Zn + Ga) ratio of Zn-Ga-O precursors was first fixed at 50% and then the growth temperature was gradually changed from 800 to 850 and finally 900 °C, respectively. As shown in Figure S1a, a large amount of nanowires with short length and irregular morphology directly grow on the Si substrate after reaction at 800 °C for 30 min. When the temperature was increased to 850 °C, the nanowires showed a longer length up to 1−2 μm and rather smooth surface morphology (Figure S1b). On further increase in the growth temperature to 900 °C, the shape and crystallinity of nanowires started to deteriorate (Figure S1c). Notably, the ZnO content in the nanowires synthesized at 900 °C dramatically decreased to 19.6% from the initial concentration of 37% at 850 °C. In fact, extending the reaction temperature to 1100 °C only induced the nucleation and growth of GaN nanowires (Figure S2). Therefore, it can be concluded that 850 °C is the most suitable temperature for the direct growth of (GaN)1−x(ZnO)x nanowires with the desired size and morphology, as well as acceptable ZnO content. Subsequently, various Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of 10%, 30%, 50%, 60%, 70%, and 90% were then used to realize the control of ZnO concentration and corresponding band gap tailoring of (GaN)1−x(ZnO)x nanowires. In Figure 1a−d and Figure S3a−d, it can be seen that Zn-Ga-O precursors with Zn/ (Zn + Ga) ratios of 10%, 30%, 50%, and 60% can promote the growth of numerous nanowires which possess an average diameter of ∼50 nm and a length of 0.5−2 μm. The corresponding cross-view SEM images demonstrate that the thickness of the nanowire film is about 100−200 nm and all (GaN)1−x(ZnO)x nanowires directly grow on the Si substrate without a transition layer (Figure S4). The composition

Figure 1. SEM images of (GaN)1−x(ZnO)x nanowires obtained by using Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of (a) 10%, (b) 30%, (c) 50%, (d) 60%, (e) 70%, and (f) 90%.

analysis using EDS in SEM mode reveals that the actual Zn/ (Zn + Ga) ratios of as-prepared nanowires correspond to 10.3%, 25%, 37%, and 47.6%, respectively (Figure S5). However, a Zn content of Zn-Ga-O precursors up to 70% and 90% causes the disappearance of the nanowire morphology (Figure 1e,f and Figure S3e,f). In Figure 1e, it can be observed that there are only a few nanowires lying on the substrate along with Au alloy nanoparticles. The 90% sample shows a film-like appearance covered with Au nanoparticles (Figure 1f). As a result, it can be noted that (GaN)1−x(ZnO)x solid solution nanowires with tunable ZnO contents ranging from 10.3% to 47.6% can be obtained using our designed growth strategy. In a GaN-ZnO solid solution system, it is easy to form separated phases during the nanowire growth due to the multicomponent elements.34 Therefore, it is important to verify the formation of a (GaN)1−x(ZnO)x solid solution rather than mixed phases of GaN and ZnO. For this reason, the crystal structure and composition of as-prepared nanowires were thoroughly analyzed by X-ray diffraction (XRD) and TEM equipped with EDS. It can be observed in Figure 2 that all the XRD peaks of as-prepared nanowire samples are located in the range of WZ-GaN and WZ-ZnO and gradually shift from GaN to ZnO as the ZnO content increases. More importantly, there is no peak splitting of (GaN)1−x(ZnO)x solid solution nanowires, demonstrating the formation of a single-phase (GaN)1−x(ZnO)x solid solution rather than a mixture of

Figure 2. XRD patterns of (GaN)1−x(ZnO)x nanowires obtained by using Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of 10%, 30%, 50%, and 60%. C

DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and N elements are confirmed to exist inside the (GaN)1−x(ZnO)x solid solution nanowire and no obvious areas of ZnO and/or GaN aggregation are observed from the elemental mappings. In addition, the ratios of Ga to N and of Zn to O are approximately close to 1:1, as given in Table S1. In combination with the above structural analysis, we can confirm that the as-prepared nanowires are a (GaN)1−x(ZnO)x solid solution free of separated GaN and ZnO phases. To unravel the effect of Au catalyst on the nucleation and growth of nanowires, the ex situ Au alloy nanoparticle on the top of the nanowire was also investigated in detail. According to the HRTEM and FFT analysis in Figure 3d,e, it can be demonstrated that the crystal structure of the catalyst matches well with the hexagonal Au0.87Ga0.13. Furthermore, a direct epitaxial growth between the catalyst and (GaN)1−x(ZnO)x nanowires has been observed from the HRTEM image and their crystallographic relationship can be defined as [0001]GaN‑ZnO//[1−21−3]Au−Ga and (10−10)GaN‑ZnO//(10− 10)Au−Ga (Figure 3f). The corresponding lattice mismatch of 10.3% is released by the formation of periodic dislocations, as reported in heteroepitaxial GaN nanowires on sapphire substrate.35 Therefore, we deduce that the formation of an Au-Ga alloy catalyst plays an important role in the nucleation and growth of (GaN)1−x(ZnO)x nanowires. In fact, it has been proved that decreasing the Ga content of Zn-Ga-O precursors to 30% and even 10% directly results in the problems of nucleation and growth of nanowires (Figure 1e,f). The elemental mapping of the Au-Ga alloy catalyst in Figure 4 clearly exhibits the coexistence of Au, Ga, Zn, N, and O elements due to the possible contamination/involvement in the catalyst, as observed in similar quaternary solid solution nanowires made of GaP and ZnS.24 In addition, a rich distribution of Ga and N elements with respect to the weak intensity of Zn and O signals is obviously discovered at the section close to Au catalyst. Table S1 shows that the Zn/(Zn + Ga) ratio in the top area 1 is about 19.52%, which is much lower than the value of 32.22% in area 2 and 36.10% in area 3 in Figure 4. These phenomena are universally observed in all of the nanowires investigated in this work, as shown in Figure S7 and Table S2. The main reason is tentatively attributed to the cooling process of (GaN)1−x(ZnO)x nanowires from 850 °C to room temperature. During this period, the (GaN)1−x(ZnO)x nanowires actually continue to crystallize and grow when the cooling temperature is still above the eutectic temperature of the Au-alloy phase. It is easy to understand that the Ga component from the Ga-Au alloy droplet is continuously consumed for the slow growth of (GaN)1−x(ZnO)x nanowires without sufficient and extra supplement from the gas source due to its difficult volatilization from Zn-Ga-O precursors at low temperature. As a result, the final Ga content in the alloy catalyst is relatively lower in comparison to other elements (Figure 4 and Figure S7). The low concentration of ZnO at the end of the nanowires can be attributed to the reduced ZnO solubility in the Au-alloy droplet at lower temperature, as well as the limited provision of Zn gas precursor. In spite of the aforementioned results, we also discovered the graded composition in (GaN)1−x(ZnO)x nanowires along the growth direction which may be caused by the gradual decrease of ZnO concentration in the precursors during the growth process (Figure 4 and Table S1).36,37 Importantly, EDS analysis in TEM mode revealed that the average Zn/(Zn + Ga) ratios for the 60% and 30% samples are about 35.6% and 22% (Tables S1 and 2 and Figure S8). This further indicates that we have

separated GaN and ZnO phases. The strong diffraction peaks of Au mainly come from the Au catalyst. Figure 3 shows the

Figure 3. (a) Bright-field TEM, (b) HRTEM, and (c) FFT images of (GaN)1−x(ZnO)x nanowires obtained by using Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of 60%. (d, e) HRTEM and FFT images of Au alloy catalyst. (f) Corresponding HRTEM image of interface between nanowires and Au alloy catalyst.

typical TEM result of a representative (GaN)1−x(ZnO)x solid solution nanowire with a nominal composition ratio of Zn/(Zn + Ga) = 60%. Obviously, there exists a catalyst particle on the top of the nanowire according to the bright-field TEM image in Figure 3a, implying the VLS growth mechanism of as-prepared nanowires. A high-resolution TEM image taken from the edge part of the nanowire, shown in Figure 3b, reveals the regular atomic arrangement of lattice fringes without any stacking fault and dislocation, demonstrating the comparatively excellent crystal quality of (GaN)1−x(ZnO)x solid solution nanowires. The corresponding fast Fourier transform (FFT) of the HRTEM image shown Figure 3b matches well with the diffraction patterns of a hexagonal WZ-GaN/ZnO along the [0001] zone axis (Figure 3c). This indicates again that the asprepared nanowires belong to a single-crystal and hexagonal wurtzite structure. Additionally, the measured d spacing of 0.278 nm from the neighboring lattice planes along the length direction is between the interplanar distance of {10−10} planes of GaN (0.276 nm) and ZnO (0.281 nm), which implies the successful and random substitution of Zn, O atoms to Ga, N atoms within the wurtzite-type structure and demonstrates the preferential growth direction of ⟨10−10⟩ (Figure 3b). The TEM results taken from another nanowire and zone axis of [2− 1−10] also prove the same conclusions again (Figure S6). To further clarify the formation of (GaN)1−x(ZnO)x solid solution nanowires, the spatially resolved elemental distribution inside the as-prepared nanowires was identified from high-resolution elemental mapping in scanning transmission electron microscopy (STEM) mode. As shown in Figure 4, all four Ga, Zn, O

Figure 4. High angle annular dark field (HADDF) image of (GaN)1−x(ZnO)x nanowire for 60% sample and corresponding elemental maps of Ga, Zn, O, N, and Au. The calculated EDS results in the rectangular areas of 1−3 of the Zn image are given in Table S1. D

DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry successfully synthesized (GaN)1−x(ZnO)x nanowires with controllable ZnO content. Additionally, the Raman spectroscopy was also utilized to get further insight into the bonding structure and disorder of (GaN)1−x(ZnO)x nanowires. In previous work, Han et al.11 discovered two additional peaks at 198 and 164 cm−1 along with the typical Raman peaks of GaN and ZnO in (GaN)1−x(ZnO)x nanocrystals indirectly obtained through the nitridation of Zn-Ga-O precursors. They speculated that these peaks belonged to a surface Ga−O stretching mode. In fact, the peaks at 198 and 164 cm−1 have been commonly observed in GaN materials synthesized from Ga2O3 reactant,38 which matches well with the typical Raman peaks of standard βGa2O3 (Figure 5). The strongest intensity of these peaks

solid solution nanowires have superior crystal quality and more uniform element distribution for expected PEC performance in comparison to other (GaN)1−x(ZnO)x materials obtained from an indirect nitridation process.11,18,26 As shown in Figure 5, there are some peaks appearing in the positions of 135, 301, 434, 616, and 713 cm−1 for as-prepared (GaN)1−x(ZnO)x nanowires with different ZnO concentrations. In comparison with the standard Raman spectrum of single-crystal GaN film, the peaks located at 135 and 713 cm−1 can be assigned to the E2(low) (144 cm−1) and A1(LO) (734 cm−1) vibrational modes of a Ga−N bonding structure.39 The obvious red shifts of Raman peaks are mainly attributed to the doping of Zn and O into the GaN lattice. Notably, the intensity of the corresponding two peaks gradually decreases with an increase in ZnO content in the (GaN)1−x(ZnO)x nanowires. Meanwhile, the peaks at 303 and 434 cm−1 related to ZnO exhibit an increasing tendency in intensity, which are due to the multiphonon and E2(high) (437 cm−1) modes of the Zn−O bonding structure. In addition, a peak at 616 cm−1 and other small peaks with low intensity are also observed in the spectra. They may be attributed to the Ag and Bg modes of the Ga−O bonding structure, as indicated in Figure 5.40 All of these Raman peaks are broadened due to the nanosize effects and the increasing disorder in the complex (GaN)1−x(ZnO)x solid solution. Considering the important role of elemental supersaturation on the initial nucleation, an additional coating of Zn-Ga-O precursors on Si substrates was used to improve the nucleation density of (GaN)1−x(ZnO)x nanowires. Figure S9 shows the corresponding SEM images of obtained samples. It can be seen that the density of (GaN)1−x(ZnO)x nanowires is obviously increased for samples with Zn/(Zn + Ga) ratios of 10%, 30%, 50%,, and 60% under the synergistic effect of Zn-Ga-O precursor powders and corresponding films. Nevertheless, we have not yet observed nanowires in the case of 90%, indicating the harsh nucleation/crystallization of (GaN)1−x(ZnO)x nanowires under the conditions of higher Zn content. This is in good accordance with the growth behavior of (GaN)1−x(ZnO)x nanowires without Zn-Ga-O precursor films. Even though the Zn-Ga-O medium layer can improve the nucleation density, its

Figure 5. Room-temperature Raman spectra of as-prepared (GaN)1−x(ZnO)x nanowires and reference samples of GaN film, ZnO, and β-Ga2O3 powders.

implies the possible existence of a β-Ga2O3 phase on the surface of (GaN)1−x(ZnO)x nanocrystals, which would hinder the transfer of photogenerated electrons and holes to the surface reaction sites due to its wide band gap. In contrast, these Raman signals are not detected in the directly grown (GaN)1−x(ZnO)x nanowires in this work (Figure 5). Therefore, it can be demonstrated that our synthesized (GaN)1−x(ZnO)x

Figure 6. (a) CL spectra of as-prepared (GaN)1−x(ZnO)x nanowires obtained by using Zn-Ga-O precursors with Zn/(Zn + Ga) ratios of 10%, 30%, 50%, and 60%. (b-e) Corresponding analysis of peak differentiating and imitating. E

DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. (a) Photocurrent−potential curves of as-prepared (GaN)1−x(ZnO)x photoanodes measured under chopped visible light in 0.5 M Na2SO4 solution. (b) Corresponding electrochemical impedance spectra measured at 0 V (vs Ag/AgCl) in 0.5 M Na2SO4 solution. The inset shows an equivalent circuit for the photoanodes.

continuously increasing energy levels originating from defects restrain the radiative recombination of charges in the near band edge. As a result, both UV−vis and CL spectral analyses demonstrate that the substitution of ZnO in the GaN lattice is capable of generating visible light absorption for the solardriven PEC performance of (GaN)1−x(ZnO)x nanowires. Figure 7a shows the PEC performance of (GaN)1−x(ZnO)x nanowires as photoanodes related to different ZnO contents or band gaps. The photocurrent density was measured under AM 1.5G simulated solar light with a cutoff filter of 400 nm (light intensity: 1 sun or 100 mW cm−2) and 0.5 M Na2SO4 solution (at pH 6). To exclude the effect of the substrate, an n-type Si as a reference electrode was also investigated under the same conditions. It can be observed that n-type Si shows a negligible photocurrent. In contrast, all photoanodes of (GaN)1−x(ZnO)x nanowires with different ZnO contents display obvious photocurrents and correspondingly increasing tendency with respect to applied anodic potential, which represents a typical n-type semiconductor behavior and a good electrical contact between nanowires and substrates. The sharp transient photocurrents simultaneously demonstrate the excellent photoresponse of the PEC process. Especially, the 10% sample exhibits the lowest photocurrent density due to the largest band gap of 3.08 eV and the worst visible light absorption. Increasing the ZnO content gradually leads to the enhancement of overall photocurrent density, matching well with the shrinking tendency of the band gap (Figure S10 and Figure 6). Among those photoanodes, the 60% sample possesses the highest photocurrent density, which can reach up to 30 μA cm−2 at an applied potential of 1.23 V vs RHE. The corresponding solar conversion efficiency is roughly estimated to be 0.01% at 0.75 V vs RHE. Even though the photocurrent density is much higher than that of powder-based photoanodes, it is still far from that expected and lower than that for the film-based photoanodes reported by Imanaka and Wang and co-workers.18,20,21,26 The main problem lies in the lower density of prepared nanowires and higher charge transfer resistance. In fact, electrochemical impedance spectroscopy (EIS) in Figure 7b reveals the charge transfer behavior in detail. The Nyquist plots in the frequency range of 100 kHz to 100 mHz show deformed arcs which demonstrate the rate-determining step of charge transfer in the overall PEC process, and the equivalent circuit is displayed in the inset.5 In this equivalent Randle circuit, Rs is the solution resistance, Q1 is the constant phase element (CPE) for the

poor conductivity will also dramatically affect the transfer efficiency of electrons. In this case, the (GaN)1−x(ZnO)x nanowires grown on the Zn-Ga-O layer are not suitable as photoanodes for the PEC test. Figure S10 shows the UV−vis spectra of as-prepared nanowires on the Zn-Ga-O film, which reveals the obvious change in light absorption edge and corresponding band gap after forming solid solution. In comparison with the strong UV light absorption of pure GaN and ZnO, all samples of (GaN)1−x(ZnO)x nanowires show obvious absorption in the visible light region and their absorption edges gradually shift to the long-wavelength direction as the ZnO content increases (Figure S10a). Correspondingly, the band gaps derived from the modified Kubelka−Munk function are 3.08, 2.88, 2.8, and 2.77 eV, respectively, for the 10%, 30%, 50%, and 60% samples (Figure S10b). Apart from the strong light absorption of band edge levels, an extra absorption peak with broadened width covering from 500 to 650 nm is also found in all samples. It can be attributed to the synergistic effect of surface plasmon resonance (SPR) of residual Au nanoparticles41,42 and the structure defects of the solid solution. It has been demonstrated that the position of the SPR absorption peaks is closely related to the size, shape, and support of Au nanoparticles.41 For a spherical Au nanoparticle with a size of 20 nm, the SPR absorption peak is about 550 nm, which is in accordance with our results.42 Moreover, the high-resolution cathodoluminescence (CL) analysis in Figure 6 also demonstrated the existence of obvious emission peaks ranging from 519 to 672 nm which may be caused by defect energy levels such as Zn and Ga vacancies.43 In addition, an extra emission peak with strong intensity in the range of 400−450 nm can still be observed in the CL spectrum. These peaks should correspond to the near band edge luminescence. According to peak differentiating and imitating, the position of these peaks shifts from 410 to 426, 435, and finally 450 nm with the nominal ZnO content increasing from 10% to 30%, 50%, and 60%, as reported in the ternary CdS1−xSex solid solution.44 The CL spectra further reveal the tunable band gap engineering of (GaN)1−x(ZnO)x nanowires through the control of ZnO solubility. It should be noted that the light emission in the range of yellow and red regions has also been significantly enhanced owing to the increase of ZnO content in (GaN)1−x(ZnO)x nanowires, which in turn causes the weakening of near band edge luminescence, especially for the 60% sample. In other words, this means that the F

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Inorganic Chemistry electrode/electrolyte interface, and Rct is the charge transfer resistance of the electrode and interface. According to the electrochemical fit and simulation, the fitted values of Rct are 8.15, 13.3, 2.74, and 2.39 MΩ, respectively, for the 10%, 30%, 50% and 60% samples with light irradiation. The great resistance which mainly comes from the inner part and interface part of the nanowires dramatically limits the PEC performance. The incident photon to current conversion efficiency (IPCE) of (GaN)1−x(ZnO)x nanowires with different band gaps has also been studied by irradiating with monochromatic light, as shown in Figure 8. It can be seen

consisting of Zn, Ga, O, and N gases makes it somewhat challenging to obtain a homogeneous (GaN)1−x(ZnO)x solid solution due to the structural and chemical differences during its complicated formation process. As a result, the generation of structure defects is inevitable, which would dramatically affect the electron transfer and accelerate the electron−hole recombination. Additionally, the electrical resistance of (GaN)1−x(ZnO)x solid solution will significantly increase even though a small fraction of ZnO goes into the GaN lattice. Therefore, it can be expected that the PEC performance of (GaN)1−x(ZnO)x solid solution nanowires can be further improved if the above problems are properly solved. For example, the gaseous Zn and Ga precursors can be used to promote the growth of (GaN)1−x(ZnO)x solid solution nanowires with controllable length and ZnO content and higher crystal quality; the (GaN)1−x(ZnO)x nanowire arrays can be epitaxially grown on a conductive GaN substrate, etc. In addition, as is well-known, the PEC water splitting involves the two half-reactions including water oxidation at the anode and water reduction at the cathode. The achieved anodic photocurrent directly reflects the performance of the (GaN)1−x(ZnO)x photoanode in the production of oxygen. Correspondingly, hydrogen is produced at the Pt counter electrode. As a result, improving the performance of the photoanode appears to be significantly important for H2 production in the PEC process.

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Figure 8. Incident photon to current conversion efficiency (IPCE) of (GaN)1−x(ZnO)x photoanodes. The IPCE was measured at 0.7 V (vs Ag/AgCl) in 0.5 M Na2SO4 solution.

CONCLUSION In summary, we present an optimized strategy to prepare crystalline (GaN)1−x(ZnO)x nanowires as photoanodes for PEC water splitting. This method has been demonstrated to be quite efficient in tailoring the ZnO content in the nanowires from ∼10.3% to 47.6% and maintaining electrical contact between the nanowires and substrate. An insight into the mechanism of the Au catalyst reveals that the formation of AuGa alloy catalyst in the initial stage plays an important role in the nucleation and growth of (GaN)1−x(ZnO)x nanowires. In addition, optical analysis using UV−vis and CL demonstrates the enhanced visible light absorption and narrowing band gaps of 3.08, 2.88, 2.8, and 2.77 eV, verifying the successful substitution of ZnO into the GaN lattice. Correspondingly, the PEC measurements show increasing photocurrent density with respect to the decrease in band gap. The highest photocurrent density of ∼30 μA cm−2 is achieved at the applied potential of 1.23 V vs RHE for the 60% sample. This work highlights the potential of (GaN)1−x(ZnO)x nanowires in the preparation of superior photoanodes for water splitting, and it is believed that further optimization of the crystal quality and decrease in inner resistance caused by defects will make a big difference, even though the performance of our prepared photoanode is still far from that expected. Meanwhile, the aligned nanowire array on a conductive substrate is thought to be an ideal candidate for a photoanode because of its rational utilization of active area and light absorption.

that all samples show obvious visible light driven PEC performances. Although the IPCE is very low in the range of 380−650 nm, we can roughly estimate band gaps of ∼3.02 eV (410 nm), 2.92 eV (425 nm), and 2.76 eV (450 nm) for the 10%, 30%, and 60% samples, respectively. In addition, IPCE also shows a peak in the range of 500−600 nm. It is derived from the enhanced visible light absorption of defects and the SPR effect of Au nanoparticles mentioned above. All of these are in accordance with the analysis of UV−vis and CL results. The stability of as-prepared (GaN)1−x(ZnO)x nanowire photoanodes has been evaluated as shown in Figure S11. In the first 3 h, the photocurrent density appears a serious degradation which is mainly caused by the peeling off of nanowires with weak substrate adhesion, as shown in Figure S12. It can be further improved by using highly doped GaN substrate and the modification of proper cocatalysts. Excellent lattice matching and close thermal expansion efficients would guarantee better mechanical and electrical contacts of epitaxial (GaN)1−x(ZnO)x solid solution nanowires. (GaN)1−x(ZnO)x solid solution has been regarded as a promising photocatalyst for water splitting to produce H2 under visible light. However, the performance of (GaN)1−x(ZnO)x solid solution nanowires in PEC water splitting for H2 production is still far from that expected. The main problems left for further improvements are considered to be the preparation of (GaN)1−x(ZnO)x nanowires with higher density, wide range of ZnO content, and good crystal quality, as well as excellent electrical conductivity. Even though the Zn-Ga-O precursor has been synthesized for the generation of homogeneous Zn, Ga, and O atoms at a lower temperature, the vapor pressure of Ga or Zn gases is not yet enough for the nucleation and continuous growth of high-density (GaN)1−x(ZnO)x nanowires. Meanwhile, the quaternary system

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00277. G

DOI: 10.1021/acs.inorgchem.8b00277 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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SEM images, EDS spectra, TEM results, and UV−vis diffuse reflectance spectra of (GaN)1−x(ZnO)x solid solution (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for B.L.: [email protected]. *E-mail for A.W.: [email protected]. *E-mail for X.J.: [email protected]. ORCID

Baodan Liu: 0000-0001-8141-8940 Fei Liu: 0000-0001-7603-9436 Funding

This work was mainly supported by the National Natural Science Foundation of China (No. 51702326) and the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No. 2017EP05 and 2017RP25). In addition, the research was partly funded from Tomsk Polytechnic University Competitiveness Enhancement Program grant. The authors also thank Prof. Andriy Lotnyk for his kind help in STEM analysis. Notes

The authors declare no competing financial interest.

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