ZnGaNO Photocatalyst Particles Prepared from Methane-Based

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

ZnGaNO Photocatalyst Particles Prepared from Methane-Based Nitridation Using Zn/Ga/CO3 LDH as Precursor Yan-Ling Hu,*,† Shuhua Ou,‡ Jialiang Huang,† Huayu Ji,† Siwan Xiang,§ Yuqin Zhu,† Zhibang Chen,‡ Cheng Gong,§ Lan Sun,§ Jiqiong Lian,† Dongya Sun,† Yongsheng Fu,∥ and Tongmei Ma*,‡

Downloaded via DURHAM UNIV on July 21, 2018 at 11:41:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Fujian Provincial Key Laboratory of Functional Materials and Applications, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen, 361024, P. R. China ‡ School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510641, P. R. China § State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, 361005, P. R. China ∥ Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Ministry of Education, Nanjing, 210094, P. R. China S Supporting Information *

ABSTRACT: Methane-based nitridation was employed to produce wurtzite zincgallium oxynitride (ZnGaNO) photocatalyst particles using Zn/Ga/CO3 layered double hydroxides (LDHs) as precursor. Introduction of methane to nitridation would promote the formation of Zn−O bonding and suppress shallow acceptor complexes such as V(Zn)Ga(Zn) and Ga-Oi in ZnGaNO particles. On the other hand, high flow rate of methane would induce breaking of Ga−N bonding and enhance surface deposition of metallic Ga atoms. After loading with Rh and RuO2, ZnGaNO particles had free electron density in an order of S50 > S20 > S90 > S0, which correlated well with their photocatalytic performance upon visible-light irradiation. The best performance of the loaded S50 was ascribed to the relatively flat surface band bending of the particle. Methane-based nitridation of Zn/Ga/CO3 LDHs would provide a new route to tune the surface chemistry of ZnGaNO and enhance the photocatalytic performance to its full potential. behavior of ZaGaNO.22−24 For most of the reported nitridation processes, oxygen content in the precursors would decrease with prolonged nitridation because of a harsh reductive environment where large NH3 flow rates, high temperatures, and/or long nitridation periods were employed to realize a full phase transformation. Tremendous loss of oxygen atoms would also lead to volatilization of zinc atoms during nitridation, give rise to high density of zinc vacancies, and result in high recombination rate of the photogenerated carriers in the ZnGaNO particles. Oxygen has been added during the nitridation process and proved to enhance photocatalytic properties of the final product ZnGaNO by reducing Zn volatilization and increasing interconnection between the particles and the substrate.22,23 Safe and precise introduction of oxygen, however, remains a problem. It is difficult to quantitatively control the amount of oxygen in the form of moisture,23 and it is also dangerous to mix O2 and NH3 under high temperature because O2/NH3 could form a flammable and explosive mixture.22 An alternative of oxygen source is “intrinsic” water molecules contained in the Zn/Ga/CO3 LDHs precursor. The molecular formula of Zn/ Ga/CO3 LDHs can be expressed as [(Zn2+)1−x(Ga3+)x-

1. INTRODUCTION Zinc-gallium oxynitride solid solutions (ZaGaNO), also known as GaN:ZnO or (Ga1−xZnx)(N1−xOx), are promising photocatalysts capable of visible-light-driven overall water splitting,1 organic pollutant degradation,2 and CO2 reduction.3 ZaGaNO solid solutions are attractive for their tunable band-gap energy,4 high quantum efficiency (17.3%),5 reliable long-term durability,6 and expected long inherent carrier life.7 ZnGaNO solid solutions have been successfully synthesized by high temperature nitridation of different starting materials/precursors,8 most of which are oxides containing gallium and zinc, such as Ga2O3 and ZnO,1 GaN and ZnO,9 ZnGa2O4,10 Ga-ZnO nanoprecursor,11−13 Zn/Ga/CO3 layered double hydroxides (LDHs),14−16 ZnGa2O4 and ZnO nanoparticles,17−19 Ga2O3 and ZnO plus Zn,20 Ga2O3(ZnO)16,21 carbon-templated ZnGaO hollow spheres,4 etc. Although some of them exhibited impressive photocatalytic activity, the overall quantum efficiency of ZnGaNO is far from their theoretical limit. There are still many problems that remain unclear regarding to their photocatalytic mechanism. It has been widely considered that the photocatalytic performance of ZnGaNO solid solutions is determined by the concentration of zinc and gallium cations. Little attention is paid to oxygen and nitrogen anions. Only recently was oxygen reported to play an important role in the photocatalytic © XXXX American Chemical Society

Received: May 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b01415 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (OH)2]x+ (CO 32−)x/2·nH2O, where the water molecules structurally intercalated between positively charged brucitelike (Mg(OH)2-type) layers.25 The desorption of these interlayer water molecules in LDHs occurs only above certain high temperature, and dehydration of the brucite-like (Mg(OH)2-type) layers of LDHs would continue to supply oxygen upon increased temperature.24 Therefore, Zn/Ga/CO3 LDHs, attractive as a precursor with a short nitridation period, may also provide reliable and sustainable oxygen source for nitridation. The amount of oxygen released from LDHs can be controlled by nitridation parameters, such as quantity of LDHs, temperature, flow rate of carrier gas, etc. More oxygen would be introduced in terms of water when a larger amount of LDH precursor, higher heating rate, and shorter nitridation period were employed, leading to an oxidative rather than reductive environment for nitridation. On the basis of the above analysis, in the present work, Zn/ Ga/CO3 LDHs with [Zn]:[Ga] = 2:1 were nitridated under an oxidative environment to prepare ZnGaNO photocatalyst particles. Methane (CH4) was introduced during the nitridation to provide carbon, which can work as a reducing agent to enhance the conversion rate of Ga2O3 to GaN.26 The flow rates of methane were varied to adjust the redox environment of the nitridation process while the other nitridation parameters were kept the same. The microstructures, optical properties, electrochemical, and photocatalytic performances of the obtained oxynitrides were then characterized and compared. The contents and valence states of Zn, Ga, N, O on the surface of the oxynitride particles were analyzed, and photocatalytic activities of the ZnGaNO particles were evaluated against the photodegradation of organic pollutants including phenol and Rhodamine B (RhB) in aqueous solution under visible-light irradiation. It was demonstrated that the photocatalytic performance of the ZnGaNO particle was greatly enhanced with an optimum flow rate of methane. The origin of this improvement was discussed by correlating the microstructure, electronic band structure, and optical property of ZnGaNO particles with the flow rate of methane. And a possible mechanism is proposed to explain the best performance of loaded S50 using surface band bending theory.

cooled naturally with the protection of ammonia (150 sccm) until 700 °C and then argon (60 sccm) to room temperature. ZnGaNO powders were then loaded with RuO2 (5 wt %) and Rh as cocatalysts to promote the photo-oxidation and photoreduction reactions, respectively. 28 Deposition of RuO2 cocatalyst was performed by immersing and stirring ZnGaNO particles in a RuCl3 ethanol solution at 80 °C until all the ethanol was completely evaporated. Resulting mixed powders were heated in air at 350 °C for 1 h to convert the Ru species to RuO2. Metallic Rh was then applied to the RuO2-loaded samples using a photodeposition process, in which the RuO2-loaded ZnGaNO particles (0.4 g) were dispersed and stirred in a quartz chamber containing 200 mL of an aqueous solution of 23.39 mg of Na3[RhCl6]·xH2O (Rh 17.1 wt %) and 20 mL of methanol. The chamber was then pumped down for 30 min and subsequently irradiated by visible light (λ > 400 nm) for 4 h to obtain Rh-RuO2-loaded samples.29 2.2. Material Characterization. The structure of the prepared powders was identified by X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.5417 Å) (Philips, PANalytical X’pert). The morphology of powders was examined using a field emission scanning electron microscope (SEM) (Hitachi-S4800, Japan). Transmission electron microscopy (TEM) was performed with a TEM/STEM system (FEI Talos F200X) equipped with 2 Super-X SDDs. The chemical compositions of zinc and gallium in the Zn/Ga/CO3 LDH precursor and ZnGaNO particles were determined by inductively coupled plasma optical emission spectroscopy (ICP-AES) (Prodigy, Leeman Laboratories). The contents of nitrogen in the ZnGaNO powders were measured with an elemental analyzer (Vario EL III, Elementar), and the concentrations of oxygen were tested with an oxygen and nitrogen determinator (ONH836, LECO Corporation). Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were carried out on the LDH precursor under N2 (LabSys Evo, Setaram). Zn metal was used as a calibration standard. Approximately 50 mg (TGA) and 15 mg (DTA) quantities of the LDH sample, predried at 60 °C for 10 h in an oven, were heated at rates of 5 K/min (TGA) and 20 K/min (DTA). Valences of elements on the surface of ZnGaNO specimens were analyzed with X-ray photoelectron spectroscopy (XPS) using Al-Ka X-ray (1486.6 eV) operated at 15 kV, 10 mA, and 150 W radiation (Thermo ESCALAB 250X). The binding energies were normalized to the signal for adventitious C 1s at 284.8 eV. The laser Raman spectra were recorded at a resolution of 2 cm−1 in backscattering (180°) configuration using 638 nm excitation (XploRA, Horiba). UV−vis diffuse refection spectra (DRS) were recorded in the range of 200− 800 nm at room temperature (Varian Cary-5000). Photoluminescence (PL) measurements were carried out at room temperature using a fluorescence spectrophotometer (Hitachi F-7000) with a xenon lamp as the excitation source (λ = 325 nm). 2.3. Photocatalytic Activities. A 0.1 g sample of ZnGaNO powder loaded with both RuO2 and Rh was dispersed in 100 mL of an aqueous solution with pH of 4.5 (adjusted by H2SO4 solution) containing phenol or RhB. The initial concentration of phenol was 2 mg/L, and the initial concentration of RhB is 10 mg·L−1. The mixture was then stirred in the dark for 60 min to attain adsorption equilibrium on the surface of the catalyst. It was then stirred and irradiated under visible light from a 300 W xenon light source (PLSSXE300C, Perfectlight) equipped with a cutoff filter to provide visible light (λ > 400 nm). The average light intensity was 100 mW/cm2 for degradation of phenol and RhB. The curves of absorbance vs wavelength for phenol and RhB solutions were recorded using a UV−vis spectrophotometer (UV9000A, Metash). The concentration of RhB was measured at a wavelength of 554 nm. Quantitative analysis of phenol, however, was performed on an HPLC-UVD system consisting of a high performance liquid chromatograph (1260 Agilent) and ultraviolet (UV) detection. The chromatographic separation was achieved using a Zorbax Eclipse XDB-C18 column (Agilent). For HPLC, a binary solvent system consisting of water and methanol at a ratio of 1:1 was used for isocratic elution at a flow rate of 0.8 mL/min. The detection wavelength for the UV detection is 275 nm.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Zn/Ga/CO3 LDH precursor with a [Zn2+]/[Ga3+] mole ratio of 2:1 was prepared using the Constant-pH Method as described in our previous work.27 Briefly, a mixed aqueous solution with 0.6 M Zn(NO3)2·6H2O and 0.3 M Ga(NO3)3·9H2O was prepared to give a metallic nitrate solution (Solution A), and 2 M NaOH and 1 M Na2CO3 were mixed to form a base solution (Solution B). Solution A and Solution B were simultaneously added dropwise to a vessel containing stirred deionized water at such a rate that the pH of the reaction mixture was maintained at 8. After aging the reaction mixture at 80 °C for 12−24 h with good stirring, the precipitate was collected by repeatedly centrifuged and washed with deionized water, and finally dried at 70 °C for 24 h. The obtained Zn/ Ga/CO3 LDH precursors (2.0 g) were then nitrided to synthesize wurtzite ZnGaNO particles in a quartz tubular furnace (Φ 50 mm) at 800 °C under a mixture of ammonia and methane for 30 min. The flow rate of ammonia was fixed at 300 standard-state cubic centimeters per minute (sccm), while the flow rate of methane was 0, 20, 50, and 90 sccm, respectively. The obtained oxynitride samples were named as S0, S20, S50, and S90 accordingly. Before each nitridation, the tubular furnace was pumped and purged with argon three times, and then heated to 800 °C within 70 min under a flow of argon (60 sccm). After the nitridation, the oxynitride powders were B


Article

Inorganic Chemistry

(CO3)1/6·nH2O. Theoretical weight percentage of intrinsic water in the LDH should be 10.5% with nmax = 1 − 3x/2 = 0.5, where x corresponds to atomic percentage of gallium.25 The experimental data of 16% obviously exceed the theoretical maximum for intrinsic water, indicating that the “extracted” water includes both the intrinsic water and “extrinsic” water at external surfaces.25 The second endothermic maximum in DTA is located at around 240 °C, which may correspond to the decarbonation−dehydroxylation of the LDH. A total of 27% weight loss was obtained when the temperature was increased to 800 °C, indicating that oxygen in the form of water or OH− would be released from the Zn/Ga/CO3 LDH precursor. The data can be used to precisely control the amount of introduced oxygen by fixing the weight of the LDH precursor before the nitridation. Zn/Ga/CO3 LDH precursor was then subjected to nitridation with introduction of different flow rates of methane. XRD patterns for these obtained ZnGaNO particles are almost the same (Figure 2). All XRD peaks correspond to a single

2.4. Photoelectrochemical Measurements. The photoelectrochemical measurements were carried out in an electrochemical cell containing 0.2 M Na2SO4 aqueous solution. The electrochemical cell consisted of three electrodes, including a ZnGaNO working electrode, a platinum counter electrode (5 mm × 5 mm), and a saturated calomel reference electrode. The oxynitride/FTO working electrodes were fabricated using a similar method developed by Hashiguchi.30 Briefly, dilute mixtures were prepared using oxynitride powders (0.02 g), water (2 mL), acetylacetone (100 mL), and surfactant (Triton X100, 20 mL). The suspensions were dropped onto a cleaned FTO surface, dried, and repeated several times until a uniform area of 1.5 cm2 was achieved. The slides were heated at 623 K in air for 0.5 h in order to remove organic solvent and surfactant and to enhance the attachment of particles to the substrate. Photoelectrochemical measurements of the ZnGaNO electrodes were measured with an electrochemical system (Ivium Technologies BV). The Mott− Schottky plots were obtained in the dark at a fixed frequency of 1 kHz. Electrochemical impedance spectra (EIS) were performed under both dark and irradiation conditions at open circuit potentials (OCP) over the frequency range of 0.01−10 Hz with an AC voltage of 10 mV.

3. RESULTS AND DISCUSSION 3.1. Microstructure. The precursor particles synthesized from the Constant-pH Method are smooth platelets with a diameter of 300−500 nm and thickness less than 50 nm (Figure 1a). The XRD pattern of the precursor shows major

Figure 2. XRD patterns of ZnGaNO particles of S0, S20, S50, and S90.

phase of wurtzite structure with lattice parameters very close to ZnO (JCPDS No. 36-1451, a = 3.249 Å, c = 5.207 Å). The morphogies of the obtained particles are also very similar to each other (Figure 3). According to SEM images, ZnGaNO particles have been subjected to severe surface etching during nitridation. For S20, S50, and S90 samples, particularly, the original platelets collapsed into smaller particles 100−200 nm in diameter, and the small particles tended to coalesce to form clusters.

Figure 1. (a) SEM image and (b) XRD pattern of the Zn/Ga/CO3 LDH powder.

peaks that fit well to the structure of a typical LDH structure (JCPDS No. 30-1835, a = 3.114 Å, c = 22.41 Å) (Figure 1b). DTA and TGA curves in Figure S1 reveal that, upon heating, Zn/Ga/CO3 LDH precursor experienced weight loss, most of which occurred under 300 °C with a sharp endothermic maximum at 175 °C. The percentage of the weight loss at 175 °C is around 16%, which should correspond to loss of water in LDH. For Zn/Ga/CO3 LDH with a [Zn]:[Ga] mole ratio of 2:1, its nominal formula should be [Zn2/3Ga1/3(OH)2]-

Figure 3. SEM images of ZnGaNO particles of S0, S20, S50, and S90. C


Article

Inorganic Chemistry Table 1. Average Composition for S0, S20, S50, and S90 wt % Zn Zn/Ga/CO3 LDH S0 S20 S50 S90 (ZnO)0.66(GaN)0.34

35.44 53.30 52.84 54.18 56.45 53.05

at. %

O

Ga

7.54 9.13 9.53 10.98 12.98

18.44 34.26 33.06 31.36 29.27 28.29

N

Zn/Ga

Zn/O

Ga/N

4.90 4.98 4.93 3.30 5.68

2.04 1.65 1.66 1.83 2.04 2.00

1.73 1.42 1.39 1.26 1.00

1.41 1.33 1.28 1.78 1.00

other hand, S90 exhibits heterogeneous element distribution as shown in Figure 5. The segregation trend in S90 can be

Compositions of the LDH precursor, S0, S20, S50, and S90 were measured using different techniques, and the results were normalized and are listed in Table 1. It has to be noted that, before normalization, the weight percentages of Ga, Zn, O, and N in ZnGaNO add up very close to 100% with errors less than 2 wt %, indicating the accuracy of the measurements. Zn/Ga/ CO3 LDH precursor has a Zn/Ga atomic ratio of 2.04, very close to the mole ratio of Zn2+ and Ga3+ ions (2.00) in the solution. The Zn/Ga atomic ratio of S0, however, is much lower than that of the precursor, suggesting a large loss of zinc during the nitridation. Except for S20, which has a slightly lower percentage of Zn than S0, the Zn/Ga atomic ratio increases monotonously with the flow rate of methane. The Zn/Ga atomic ratio becomes 2.04 again in S90, suggesting no zinc loss in this case. Similar to zinc, oxygen contents in obtained ZnGaNO particles also increase monotonously with methane. Violation of ZnO in the oxynitride during the nitridation was thereby suppressed by the introduction of methane. In contrast to Zn and O, evaporation of Ga and N in ZnGaNO during the nitridation appeared to be accelerated by methane. According to Table 1, Ga percentage decreases with the flow rate of methane, and the content of N is much lower in S90 than the other three samples. It is also worth noting that all Zn/O and Ga/N atomic ratios in Table 1 are larger than 1, suggesting that O and N are deficient in stoichiometry for all four samples. With the increase in the flow rate of methane, Zn/O and Ga/N ratios decrease to approach the unit except for S90, whose Ga/N ratio suddenly rises again. Typical STEM images for S0, S20, and S50 are demonstrated in Figure 4, which was taken from S50. Despite the rough surface with many cavities, no element segregation can be discerned in STEM images of S0, S20, and S50. On the

Figure 5. STEM images of S90 sample (from left to right and top to bottom): HAADF image, EDS/STEM mapping (net intensity) for elements Ga, N, Ga/Zn, Zn, and O.

observed more clearly by replotting Zn/Ga EDS mapping in the atomic percentage as Figure 6. Quantitative EDS analysis

Figure 6. EDS/STEM mapping for elements Ga and Zn (at. %) in S90.

for regions A and B in Figure 6 are listed in Table 2. Region A, the interior of a segregated particle in S90, contains much higher percentage of ZnO than GaN; region B, a relatively uniform particle, has a [Zn]:[Ga] mole ratio of 1.5:1 with lean N and O in stoichiometry. The STEM/EDS results indicate a heavy interior GaN depletion and metallic Ga deposited on the surface of S90

Figure 4. STEM images of S50 sample (from left to right and top to bottom): HAADF image, EDS/STEM mapping (net intensity) for elements Ga, N, Ga/Zn, Zn, and O. D


Article

Inorganic Chemistry

XPS spectra of Ga, N, Zn, and O. The area ratio of each peak with respect to all the peaks in one XPS spectrum, which represents the relative density of each bonding, was calculated and is listed in Tables 3−6.

Table 2. Compositions of Two Regions in S90 Measured from EDS/STEM region A region B

at. wt at. wt

% % % %

Zn

O

Ga

N

50.46 73.04 34.10 49.84

34.26 13.52 25.34 10.09

7.12 10.73 22.28 33.94

8.16 2.71 18.28 6.13

Table 3. Area Ratios for Fitted Peaks in Ga 3d XPS Spectra

particles. Similar phenomena have been reported for GaN film etched by methane-based ECR plasma,31 and for the (Ga1−xZnx)(N1−xOx) nanocrystals synthesized via the solidstate nitridation of a mixture of nanoscale ZnO and ZnGa2O4 nanocrystals.32 To clarify the mechanism of segregation in our case, ZnGaNO specimens were tested with high-resolution XPS spectra as shown in Figure 7. Peak areas were fitted for

binding energy (eV)

20.4

19.7

19.1

bonding

Ga−Oi

Ga−N

Ga−(N-H)

18.5 Ga

S0 S20 S50 S90

0.23 0.05 0.03 0

0.32 0.27 0.25 0.02

0.40 0.47 0.41 0.44

0.05 0.21 0.31 0.54

The Ga 3d spectra (Figure 7a) can be divided in four symmetric peaks at 20.4, 19.7, 19.1, and 18.5 eV,

Figure 7. Ga 3d, N 1s, Zn 2p, and O 1s XPS spectra for ZnGaNO particles of S0, S20, S50, and S90. The black hollow circles are the experimental data, whereas the colored lines are the fitting results. E


Article

Inorganic Chemistry

total for the relative densities of both Zn−O and Zn−O(N) bonding still increases in the order of S0, S20, S50, and S90. Zn−V(O), Zn−O, Zn−O(N)···V(Ga) bonding were also identified in the XPS spectra of O 1s in Figure 7d. The peak located at 532.5 eV is likely to be Ga−Oi bonding, whose relative density decreases in the order of S0, S20, S50, and S90 in Table 6, which is consistent with Figure 7a. The peak at

corresponding to Ga bonded to interstitial oxygen (Ga−Oi), to nitrogen (Ga−N), to H-N (Ga−(N-H)), and to metallic Ga atoms, respectively.33,34 According to peak area ratios in Table 3, relative density of Ga−Oi bonds decreases but that of metallic Ga atoms increases with an increasing flow rate of methane. Relative density of Ga−(N-H) bonding is almost the same for four specimens with small fluctuation. Relative density of Ga−N bonding, however, decreases gradually in the order of S0, S20, and S50, and plummets to near zero in S90, indicating that most of Ga−N bonds were destroyed on the surface of S90. N 1s XPS spectra are shown in Figure 7b. According to Table 4, area ratios increase for the broad shoulders related to

Table 6. Area Ratios for Fitted Peaks in O 1s XPS Spectra

Table 4. Area Ratios for Fitted Peaks in N 1s XPS Spectra binding energy (eV) bonding S0 S20 S50 S90

397.7

396.3

N−H2 N−Ga 0.07 0.01 0 0

0.73 0.69 0.61 0

395.8 O(N)···V(Ga) 0 0.04 0.06 0.56

394.8

393.0

binding energy (eV)

532.2

531.2

530.4

529.7

bonding

Ga−Oi

Zn−V(O)

Zn−O

Zn−O(N)

S0 S20 S50 S90

0.26 0.19 0.15 0

0.49 0.41 0.27 0.08

0.25 0.40 0.43 0.26

0 0 0.15 0.66

Ga LMM Ga LMM 0.14 0.19 0.24 0.26

0.06 0.07 0.09 0.18

531.2 eV can be assigned to zinc bonded to O from hydroxide groups or to O vacancies (V(O)).24,37,38 Zn-V(O) may stem from Zn−(OH) bonding in the LDH precursor. The peak at 530.4 eV corresponds to Zn−O bonding in a well-crystallized wurtzite environment.24,37,38 Both decreased Zn−V(O) bonding and increased Zn−O bonding with methane suggest better wurtzite structure of ZnGaNO. Peaks near 529.7 eV should represent the Zn−O(N) bond based on the fact that the peak emerged in S50 and maximized in S90, and its binding energy has a negative shift by 0.7 eV compared to Zn−O bonding. In agreement of the Zn 2p XPS spectrum, the total areas for peaks for Zn−O and Zn−O(N)···V(Ga) bonding increase with methane. It has been reported that the local crystal structure of ZnGaNO nanoparticles deviates significantly from a simple hexagonal wurtzite structure.39 In our case, normal nitridation of LDH would produce ZnGaNO with a large density of defect complexes such as Ga(Zn)-V(Zn), Zn-V(O), and Ga-Oi, which can be suppressed by suitable introduction of methane during the nitridation. It has to be noted that a small amount of methane (20 sccm) would slightly accelerate violation of zinc in ZnGaNO. Further increase in flow rate of methane, however, would enhance the formation of Zn−O bonding. Meanwhile, Ga ions from Ga−Oi bonding and Ga−N bonding in ZnGaNO can be gradually reduced to metallic Ga atoms, almost all of which would deposit on the surface due to the low vapor pressure of gallium. When the methane flow rate was as high as 90 sccm, overall depletion of GaN took place, leaving O(N) and V(Ga) in the interior of S90 and heavy Ga atoms on the surface of ZnGaNO particles. Therefore, it can be deduced that the phase transformation from LDH to ZnGaNO is a multistep process, where the major reactions are proposed as follows:

the Ga Auger L2M45M45 peak (BE = 394.8 eV, 393.0 eV)34 in the order of S0, S20, S50, and S90, suggesting an increased deposition of metallic gallium on the surface. Similar to the Ga 3d XPS spectrum, peak area ratios for N−Ga bonding in the N 1s XPS spectrum (BE = 396.3 eV)33 decrease slightly in the order of S0 > S20 > S50, and then diminish to zero in S90. A new peak centered at 395.8 eV rises up quickly in the N 1s XPS spectrum of S90. The new peak can be related to Zn− O(N) bonding, which could originate from an intermediate Zn−N bonding where N was replaced by oxygen due to hydrolysis.35 O(N) is positively charged and would strongly attract a Ga vacancy forming balanced complex Zn-O(N)··· V(Ga).36 Zn 2p3/2 core-level XPS spectra are shown in Figure 7c. There are four symmetric peaks with binding energy of 1023.2, 1022.5, 1021.7, and 1021.0 eV, which are attributed to Zn vacancy bonded to substituting Ga (V(Zn)−Ga(Zn)), Zn bonded to oxygen vacancy (Zn−V(O)), Zn bonded to oxygen (Zn−O), and Zn bonded to substituting oxygen (Zn−O(N)), respectively.37,38 According to Table 5, relative density of V(Zn)− Table 5. Area Ratios for Fitted Peaks in Zn 2p XPS Spectra binding energy (eV)

1023.2

1022.5

1021.7

1021.0

bonding

V(Zn)−Ga(Zn)

Zn−V(O)

Zn−O

Zn−O(N)

S0 S20 S50 S90

0.24 0.30 0 0

0.50 0.37 0.29 0.08

0.26 0.33 0.57 0.26

0 0 0.14 0.66

(Zn, Ga)(OH)2 (LDH) + H 2(g)

Ga(Zn) bonds increases slightly from S0 to S20, and then decreases to zero in both S50 and S90. With an increasing flow rate of methane, relative densities of Zn−V(O) bonds decrease while those of Zn−O bonding increase. Binding energy of Zn− O(N) is lower than that of Zn−O by 0.7 eV, probably because O(N) formed a stronger bond with V(Ga) in the Zn-O(N)···V(Ga) complex. For S90, shrinkage of Zn−O bonding and rise of Zn−O(N) bonding happened at the same time, which means that a high concentration of O(N) and V(Ga) formed in S90. The

→ Zn(s) + Ga(s) + 2H 2O(g)

F

(1)

Zn(s) + ·O → ZnO(wurtzite)

(2)

Ga(s) + ·N → GaN(wurtzite)

(3)

H 2O → 2 ·H + ·O

(4)

2NH3(g) → 2 ·N(g) + 3H 2(g)

(5) DOI: 10.1021/acs.inorgchem.8b01415 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. DRS for ZnGaNO particles of S0, S20, S50, and S90.

conductor photocatalyst. The absorption edges are all at around 530 nm, suggesting visible-light responses for four specimens. The high baseline for S90 in Figure 8a may relate to the deposition of Ga on the surface. The absorption coefficients (α) were obtained from a Kubelka−Munk transformation of the reflectance data, and (αhν)2−hν curves were drawn in Figure 8b. All four samples have band gaps of 2.8−2.9 eV, at which the slopes of the (αhν)2−hν curves increase in the order of S0, S20, S50, and S90, indicating better crystalline quality with increased flow rate of methane. Room temperature PL spectra with an excitation wavelength of 325 nm are shown in Figure 9. All spectra show distinct

4(Zn, Ga)(OH)2 (s) + CH4(g) → 4Zn(s) + 4Ga(s) + CO2 (g) + 6H 2O(g)

(6)

CH4(g) → ·CH 2 + H 2(g)

(7)

·CH 2(g) + 2 ·H → CH4(g)

(8)

Reactions 1−5 take place during the nitridation process without methane. Reaction 1 shows that Zn−(OH) and Ga− (OH) bonds in LDH would be reduced to metallic zinc and gallium atoms, which would then be quickly oxidized and nitrided to obtain wurtzite GaN:ZnO solid solution according to reactions 2 and 3, respectively. Reactions 4 and 5) show that the oxygen radical ·O and nitrogen radical ·N formed due to decomposition of NH3 and H2O, respectively. Two problems would arise for the nitridation without methane. The first problem is the sublimation of the intermediate metallic zinc atoms, leaving high density of V(Zn) in S0; the second problem is that hydrogen cannot reduce LDH completely, leading to large amount of residual Zn−(OH) and Ga−(OH) bonds, which may correspond to Zn−V(O) and Ga−Oi bonding in XPS. With the introduction of methane, reducing agents would be both methane and hydrogen, and reaction 6 would take place in addition to reaction 1, resulting in more complete reduction of LDH and therefore decreased number of Zn-V(O) and Ga-Oi defect complexes. Also, decomposition of methane would produce ·CH2 radical and H2 through reaction 7. Radicals of ·CH2 would scavenge hydrogen radicals ·H in the chamber through reaction 8 and accelerate the decomposition of H2O. Thus, the introduction of methane would increase the partial pressure of oxygen radical ·O and promote the formation of ZnO. It could be the main reason why methane can suppress the violation of zinc in ZnGaNO. Decomposition of methane would also produce H2, which would be involved in reaction 1 and promote the reduction of metallic ions in LDH. However, H2 from the decomposition of methane would suppress the decomposition of NH3 and result in less ·N radicals in the chamber, leaving more intermediate metallic Ga unreacted. When the methane flow rate is really high, ·N radicals in the chamber would be quite low. Depletion of GaN would therefore occur in the interior of the ZnGaNO particle and heavy metallic Ga would form on the surface, as we have observed in S90. 3.2. Optical Properties. Presented in Figure 8a are DRS of the obtained ZnGaNO particles, which is a useful tool for evaluating the light absorption characteristics of a semi-

Figure 9. Room-temperature PL spectra for ZnGaNO particles of S0, S20, S50, and S90.

peaks at 470 nm, 483 nm, 493 nm, and a broad peak at 523− 560 nm, which should be characteristic wavelengths of the ZnGaNO phase. The intensities at these characteristic wavelengths are generally low for four samples. Compared to S0 and S20, there is a slight increase in PL intensities at characteristic wavelengths for S50. An obvious increase in PL intensities can be observed for S90, probably due to the formation of a high density of O(N)···V(Ga) complexes inside or metallic Ga atoms on the surface, which can act as recombination centers for photogenerated carriers. Raman spectra can provide global information about the local bonding structures about the ZnGaNO particles. Figure 10 shows the Raman spectra for four specimens. All curves look quite similar, with the sharp strong peak at 274 cm−1, G


Article

Inorganic Chemistry

photogenerated holes (h+) to leave ZnGaNO and promote the oxidation reaction at the interface of RuO2/solution.27 Rh, on the other hand, can work as electron traps to facilitate reduction reaction of O2 to form active species O2−•.28 O2−• was proved to be the dominant species determining photoactivity of ZnGaNO, which can oxide both phenol and its intermediate BQ, and holes (h+) also play an important role in the photodegradation of phenol.27 Therefore, in this Article, S0, S20, S50, and S90 loaded with both RuO2 and Rh were used as photocatalysts to degrade phenol and RhB in aqueous solution under visible illumination. The curves of concentrations of phenol vs irradiation time are shown in Figure 11a, where phenol in the solutions was quantified using the HPLC-MSD system. Pseudo-first-order reaction kinetics ln(C/C0) = −kt was used to fit for data (Figure 11b) to give a quantitative estimation of the photoactivity of the oxynitrides, where k, C0, and C are the apparent first-order reaction constant, the initial and the reaction concentrations of phenol, respectively. After 120 min of irradiation, the reactivity of the catalysts for phenol degradation decreased in the order of S50 > S20 > S90 > S0. The curves for S20 and S50 in Figure 11b fit well to pseudo-first-order reaction kinetics with apparent rate constants k of 4.50 × 10−3 and 9.79 × 10−3 min−1, respectively. The curves for S0 and S90 samples, however, deviate from first-order kinetics, suggesting decreased photocatalytic activities with prolonged irradiation for these two specimens. Photodegradation of RhB was then used as a model reaction to further test the photocatalytic activities of the four catalysts (Figure 12a). RhB phtodegradation shows a same order of photoactivity as phenol, which is S50 > S20 > S90 > S0 with k = 0.015, 0.005, 0.004, and 0.003 min−1, respectively, as shown in Figure 12b. 3.4. Photoelectrochemical Properties. According to photodegradation results, it is obvious that loaded S50 exhibited much better photocatalytic activity than the other three samples. In order to elucidate the underlying mechanism, Mott−Schottky plots and EIS measurements were also performed on ZnGaNO/FTO electrodes, with ZnGaNO particles both loaded and unloaded (Figure 13). The Mott− Schottky equation reads

Figure 10. Raman spectra for ZnGaNO particles of S0, S20, S50, and S90.

broad peaks near 440 cm−1, 547−573 cm−1, 652 cm−1, and 709 cm−1, matching quite well with the literatures.11,12 The broadened peaks at 555 and 709 cm−1 correspond to Ga−N bonds ([A1(TO)] [E1(TO)] modes, and [A1(LO)] mode, respectively).The strong peak at 274 cm−1 is very close to 271.7 cm−1 which should represent the Ga-O-Zn local structure; and peaks at about 440 and 658 cm−1 relate to the O-Zn-O local structure.11,12 Peaks near 440 and 658 cm−1 for S0 are not as distinct as the other three specimens, suggesting rather low density of O−Zn−O bonds in S0, which is consistent with XPS spectra. The difference among S20, S50, and S90, however, is quite subtle in Raman spectra. 3.3. Photocatalytic Activities. Bare ZnGaNO powders had negligible activities for photodegradation of phenol as demonstrated by Figure S2a, showing UV−vis absorption spectra of phenol in the presence of unloaded S50. No significant improvement in photoactivity can be observed for S50 after loading with Rh alone (Figure S2b). S50 deposited with RuO2, however, showed much better performance for phenol photodegradation, despite an accumulation of 1,4benzoquinone (BQ) in the solution (Figure S2c). Figure S2d reveals that loading of RuO2, followed by photodeposition of Rh on S50, enhanced the photodegradation of phenol greatly and reduced the buildup of BQ. RuO2 has been treated as an effective oxidation site for the evolution of O2 gas.40,41 In our previous work, RuO2 had been demonstrated to enable

ij 2 yzi 1 zzjjφ − φ − kT zyzz + 1 = jjj FB 2 2 j eε0εr ND zzjk e { CFTO C k {

Figure 11. Photodegradation rates of phenol under visible-light irradiation (λ > 400 nm) in the presence of Rh- and 5 wt % RuO2-loaded ZnGaNO particles of S0, S20, S50, and S90. H


Article

Inorganic Chemistry

Figure 12. Photodegradation rates of RhB under visible-light irradiation (λ > 400 nm) in the presence of Rh- and 5 wt % RuO2-loaded ZnGaNO particles of S0, S20, S50, and S90.

Figure 13. (a, c) Mott−Schottky plots and (b, d) EIS Nyquist plots of FTO electrodes covered with (a, b) unloaded and (c, d) Rh- and 5 wt % RuO2-loaded S0, S20, S50, and S90.

where C is the total capacitance, CFTO is the capacitance of FTO, e is the electron charge, ε0 is the permittivity of free space, εr is the dielectric constant of ZnGaNO, ND is the number of donors per unit volume, φ is the applied voltage, φFB is the flat band potential, k is Boltzmann’ s constant, and T is the temperature. The contribution of Helmholtz capacitance (CH) is omitted by assuming much larger value than spacecharge capacitance of ZnGaNO.42

Almost all Mott−Schottky plots exhibit two slopes as shown by Figure 13a,c, which can be expected for dense oxynitride films on top of FTO. The larger slope on the left part of Mott− Schottky plot should correspond to the ZnGaNO film, while the smaller slope at larger potentials can be attributed to FTO.42 Positive slopes in Mott−Schottky plots indicate n-type behaviors for all ZnGaNO particles. In Figure 13a, φFB of ca. −0.27 V versus Ag/AgCl (pH = 7) can be determined from the X-intercept potential of the I


Article

Inorganic Chemistry

Figure 14. Schematic illustrations of the surface band bending, charge transfer, and surface reactions of ZnGaNO particles in the phenol solution.

tangent lines. The same φFB for all unloaded specimens suggests an equal Fermi level (EF) for all ZnGaNO particles. The slopes of the Mott−Schottky plots, however, decrease in the order of S20, S0, S50, and S90. By assuming a dielectric constant εr equal to 10, free electron densities (ND) can be calculated to be 1.87 × 1020 cm−3, 2.29 × 1020 cm−3, 2.51 × 1020 cm−3, and 3.71 × 1020 cm−3 for S20, S0, S50, and S90, respectively. ND thereby increases in the order of S20, S0, S50, and S90, which suggests that both conduction band minimum (EC) and valence band maximum (EV) would shift slightly to lower energy level in the same order. This speculation is verified by the values of EV measured from the spectrum ends (point of intersection with the background) in XPS spectra (Figure S3). The order of ND indicates that free electron density in ZnGaNO is more likely related to the concentration of shallow acceptors complexes including VZn-Ga(Zn) and GaOi.16,38,43,44 ND would increase if the density of VZn-Ga(Zn) or Ga-Oi complex decreases. O(N)···V(Ga), however, would not contribute to the density of free carrier because it is a deep acceptor complex and only works as recombination centers. Figure 13b presents EIS Nyquist plots for FTO electrodes covered with bare ZnGaNO particles. The semicircle at high frequencies is characteristic of the charge transfer process, and the diameter of the semicircle is equal to the charge transfer resistance.45 For the curves connecting the hollow marks in Figure 13b, which represent EIS Nyquist plots of electrodes under illumination, the diameter of the semicircles decreased in the order of S20, S0, S50, and S90, which is exactly the sequence for an increasing ND. The result suggests that the resistance of the overall charge-transfer process upon irradiation is dominated by the ZnGaNO particle itself. Easier charge transfer may arise from lower resistance of ZnGaNO with higher ND. For the bare ZnGaNO/FTO electrodes in the dark, arches of Nyquist plots are much larger than those under illumination, which is well expected. However, the diameters of semicircles show a different order of S0 > S90 > S20 ≈ S50. Charge transfer of the electrode in the dark, therefore, is more likely determined by the interfacial resistance between ZnGaNO/FTO or ZnGaNO/electrolyte, rather than the ZnGaNO particle itself, which will be discussed in detail later. After loading of cocatalysts, ND in ZnGaNO was greatly affected as demonstrated by Mott−Schottky plots for the unloaded (Figure 13a), the ones loaded with RuO2 (Figure S4), and the ones loaded with both RuO2 and Rh (Figure 13c).

Comparing the slopes in the Mott−Schottky plots, it is noted that only Rh-RuO2-loaded S50 has almost the same ND as the unloaded S50, while the other three samples have much less ND, especially after loading with Rh. For S50, however, the more the Rh loaded, the lower the slope of the Mott−Schottky plot (Figure S5). Thus, surface Rh nanoparticles are beneficial for the separation of photogenerated carriers in S50, but not for other three specimens. Figure 13c also revealed that both φFB and slopes of Mott− Schottky plots of the Rh-RuO2-loaded ZnGaNO decrease in the order of S90, S0, S20, and S50, suggesting that ND would follow the same order of S90 < S0 < S20 < S50. The sequence of ND resembles the order of the arch of the EIS Nyquist plot for unloaded electrodes in the dark (Figure 13b). ND in the loaded ZnGaNO, thereby, is likely related to the interfacial properties of the electrodes. The EIS measurements were then carried out on the loaded ZnGaNO electrodes as shown in Figure 13d, where two distinct features can be observed. The first is that EIS Nyquist plots overlapped for each specimen either irradiated or in the dark, which could be attributed to the Rh nanoparticles on the surface of the ZnGaNO. With a work function of 4.98 eV, Rh nanoparticles can form a Schottky junction with ZnGaNO and trap all the photogenerated electrons in ZnGaNO. Upon illumination, no photocurrent would therefore arise between the FTO electrode and the counter electrode.46 The second feature in Figure 13d is that the diameters for semicircles decrease in the order of S50 > S20 > S0 > S90, which is exactly opposite to the order of ND and photoactivity of the loaded ZnGaNO particles. The order of ND agrees with that of photoactivity for loaded ZnGaNO particles. Higher ND would give rise to better photocatalytic activity of loaded ZnGaNO particles. For most of the reported photocatalysts, an increase in carrier density is normally accompanied by a decrease in the diameter of the semicircle in the EIS Nyquist plot. The unusual behavior of our loaded ZnGaNO particles could be explained using surface band bending theory as illustrated by Figure 14. Surface band bending forms when the Fermi level of the particle aligns with those of surrounding materials. For the unloaded ZnGaNO particles, downward surface band bending (solid blue lines in Figure 14) can be expected in S20, S50, and S90 for two possible reasons. The most important reason should be Ga deposition on the surface, which can cause an upward shift of the Fermi level in ZnGaNO owing to a low J


Article

Inorganic Chemistry

upward surface band bending would create an additional energy barrier for electron to transfer to Rh nanoparticles, where reduction reaction took place to form ·O2− radicals. Since redox reactions occur conjugately, the overall rate for the redox reaction would be determined by the half-reaction with a lower rate, as illustrated by the thickness of arrows in Figure 14. Higher degree of band bending would lead to a much lower overall reaction rate.52 Loaded S50 particles, with the least degree of surface band bending, would exhibit the highest rate for overall redox reaction. According to the degree of surface band bending, the photocatalytic performance of ZnGaNO particles would decrease in the order of S50 > S20 > S0 > S90, which matches perfectly with the experiment results for the order of photodegradation rate of phenol and RhB. In summary, introduction of a suitable amount of methane during nitridation is beneficial to ZnGaNO, because it can accelerate the decomposition of the intrinsic water and prevent Zn and O from being volatilized, thereby suppressing the formation of shallow acceptors and facilitating electrons to transfer to interfaces upon illumination. On the other hand, methane would also accelerate decomposition of Ga−N bonds, promote surface deposition of metallic gallium atoms, and induce a downward surface band bending of ZnGaNO. The downward surface band bending can be reduced by cocatalyst loading such as RuO2 and Rh, as described in this Article. To synthesize Zn-rich ZnGaNO photocatalyst, Zn/Ga/CO3 LDHs are attractive precursors, not only because it only takes half an hour to be transformed into wurtzite structure but also because it can provide sustainable oxygen during the nitridation. Furthermore, using a different coprecipitaiton method, Zn/Ga/CO3 LDHs can easily lead to a ZnO/ ZnGaNO heterostructure with better separation efficiency of photogenerated carriers, which showed an apparent rate constant k of 5.12 × 10−3 min−1 against the photodegradation of phenol upon visible-light irradiation.27 In our case, an apparent rate constant k of 9.79 × 10−3 min−1 has been obtained for the loaded S50, which is almost double the k for the ZnO/ZnGaNO heterostructure, proving the potential application of methane-based nitridation. The photocatalytic performance of the obtained ZnGaNO could be further improved by optimizing the cocatalysts, adjusting the Zn:Ga mole ratios in the LDH precursor, or constructing heterostructures containing ZnGaNO. Methane-based nitridation of Zn/Ga/CO3 LDHs provides a new perspective to fully explore photoactivity of ZnGaNO.

work function of 4.2 eV for Ga. Downward surface band bend could also be ascribed to heterogeneous distribution of shallow acceptor-like complexes such as V(Zn)-Ga(Zn) and Ga-Oi due to the preferential nitridation reaction on the surface. There are less defect complexes and more ND on the surface than the interior. Given both reasons, the degree of downward surface band bending should have followed an increased order of S0 < S20 < S50 < S90. For unloaded S90, however, phase separation occurred. STEM images in Figure 5 suggest that a thin layer of GaN also formed on the surface of S90, probably due to the reaction between deposited gallium and residual NH3 upon the cooling process of nitridation. According to the energy diagram of GaN and ZnGaNO alloys,47−51 the ZnGaNO/GaN core− shell structure should have had upward surface band bending as illustrated by the green lines in Figure 14. Heavy deposition of Ga on the surface of S90 would offset the upward bending, leading to a downward surface band again but to a lesser degree as shown in Figure 14. Taking all of these factors into account, the degree of downward surface band bending should increase in the order of S0 < S90 < S20 < S50. Larger downward surface band bending would lead to easier electrons transfer across ZnGaNO/FTO interface, resulting in a smaller semicircle diameter in EIS Nyquist plots. According to the order of surface band bending, the arch diameter of the EIS Nyquist plots for the unloaded in the dark should decrease in an order of S0 > S90 > S20 > S50, as confirmed by Figure 13b. The downward surface band bending for the bare ZnGaNO particles, however, would also cause an energy barrier for holes to transfer across ZnGaNO/electrolyte interfaces, which could be the main reason why RuO2 has to be loaded for ZnGaNO to show photocatalytic activities. Upon loading of Rh, the degree of downward band bending would be reduced due to the large work function of Rh. The more the amount of photodeposited Rh nanoparticles, the more the EF of ZnGaNO shift downwardly. According to Figure 13b, the amount of photodeposited Rh would increase in the order of S20 < S0 < S50 < S90, which should also be the order for the downward movement in EF. Positions of EF were then schematically illustrated in Figure 14, where dashed red lines represent Fermi levels for the loaded ZnGaNOs and dashed blue lines for the unloaded samples. Relative positions of EF in Figure 14 were experimentally confirmed by dark OCP measurements for the loaded ZnGaNOs as 0.344 V for S0, 0.315 V for S20, 0.283 V for S50, and 0.321 V for S90 (vs Ag/AgCl). Figure 14 revealed that, after deposition of Rh nanoparticles, the original downward band bending for the bare ZnGaNO could be turned to an upward surface band bending. The order for the degree of the upward surface band bending would be changed to be S50 < S20 < S0 < S90. In this case, the rate-determining step for charge transfer would be the transportation of holes across the ZnGaNO/electrolyte interface rather than that of electrons across the ZnGaNO/FTO interface. The reason is unclear yet. A tunneling effect induced by Rh nanoparticles could play a role, leading to a much faster transfer for electrons than holes. The upward surface band bending would facilitate holes to leave ZnGaNO. The larger the degree of upward band bending, the easier the hole to transfer, and the smaller the arch in the EIS Nyquist plot. The diameter of the semicircle in the Nyquist plots would therefore decrease in the order of S50 > S20 > S0 > S90, consistent with Figure 13d. For a loaded ZnGaNO particle dispersed in the phenol solution, upward surface band bending would accelerate holes to oxide phenol directly via RuO2.27 On the other hand,

4. CONCLUSION Methane-based nitridation was employed to transform Zn/Ga/ CO3 LDH precursor to ZnGaNO photocatalyst particles. The obtained ZnGaNO particles exhibited a wurtzite structure with a band-gap energy of 2.8−2.9 eV. It was demonstrated that, during nitridation, methane addition can work with the intrinsic water in the LDH precursor to prevent Zn and O from being volatilized in the ZnGaNO particles, and to suppress the formation of defect complexes such as Zn-V(O), V(Zn)-Ga(Zn), and Ga-Oi. On the other hand, introduction of methane would also break Ga−N bonding, enhance surface deposition of metallic Ga atoms, and induce a downward surface band bending of the catalyst. In the unloaded ZnGaNO particles, ND decreased in the order of S90 > S50 > S20 > S0. A different order of S50 > S20 > S90 > S0 was followed by ND in the Rh-RuO2-loaded ZnGaNO particles, which correlated with their photocatalytic K


Article

Inorganic Chemistry

(6) Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K. Photocatalytic water splitting using modified GaN: ZnO solid solution under visible light: long-time operation and regeneration of activity. J. Am. Chem. Soc. 2012, 134, 8254−8259. (7) Chuang, C.-H.; Lu, Y.-G.; Lee, K.; Ciston, J.; Dukovic, G. Strong visible absorption and broad time scale excited-state relaxation in (Ga1‑xZnx)(N1‑xOx) nanocrystals. J. Am. Chem. Soc. 2015, 137, 6452− 6455. (8) Adeli, B.; Taghipour, F. A review of synthesis techniques for gallium-zinc oxynitride solar-activated photocatalyst for water splitting. ECS J. Solid State Sci. Technol. 2013, 2, Q118−Q126. (9) Chen, H.; Wang, L.; Bai, J.; Hanson, J. C.; Warren, J. B.; Muckerman, J. T.; Fujita, E.; Rodriguez, J. A. In situ XRD studies of ZnO/GaN mixtures at high pressure and high temperature: synthesis of Zn-rich (Ga1‑xZnx)(N1‑xOx) photocatalysts. J. Phys. Chem. C 2010, 114, 1809−1814. (10) Yan, S.; Wang, Z.; Li, Z.; Zou, Z. Two-step reactive template route to a mesoporous ZnGaNO solid solution for improved photocatalytic performance. J. Mater. Chem. 2011, 21, 5682−5686. (11) Han, W.-Q.; Liu, Z.; Yu, H.-G. Synthesis and optical properties of GaN/ZnO solid solution nanocrystals. Appl. Phys. Lett. 2010, 96, 183112. (12) Li, J.; Liu, B.; Wu, A.; Yang, B.; Yang, W.; Liu, F.; Zhang, X.; An, V.; Jiang, X. Composition and Band Gap Tailoring of Crystalline (GaN)1−x(ZnO)x Solid solution nanowires for enhanced photoelectrochemical performance. Inorg. Chem. 2018, 57, 5240−5248. (13) Li, J.; Liu, B.; Yang, W.; Cho, Y.; Zhang, X.; Dierre, B.; Sekiguchi, T.; Wu, A.; Jiang, X. Solubility and crystallographic facet tailoring of (GaN)1‑x(ZnO)x pseudobinary solid-solution nanostructures as promising photocatalysts. Nanoscale 2016, 8, 3694−3703. (14) Wang, J.; Huang, B.; Wang, Z.; Wang, P.; Cheng, H.; Zheng, Z.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M.-H. Facile synthesis of Znrich (GaN)1−x(ZnO)x solid solutions using layered double hydroxides as precursors. J. Mater. Chem. 2011, 21, 4562−4567. (15) Wang, J.; Yang, P.; Wang, Z.; Huang, B.; Dai, Y. Effect of temperature on the transformation from Zn-Ga layered double hydroxides into (GaN)1‑x(ZnO)x solid solution. J. Alloys Compd. 2015, 652, 205−212. (16) Huang, H.; Sklute, E. C.; Lehuta, K. A.; Kittilstved, K. R.; Glotch, T. D.; Liu, M.; Khalifah, P. G. Influence of thermal annealing on free carrier concentration in (GaN)1−x(ZnO)x semiconductors. J. Phys. Chem. C 2017, 121, 23249−23258. (17) Sun, X.; Maeda, K.; Le Faucheur, M.; Teramura, K.; Domen, K. Preparation of (Ga1−xZnx)(N1−xOx) solid-solution from ZnGa2O4 and ZnO as a photo-catalyst for overall water splitting under visible light. Appl. Catal., A 2007, 327, 114−121. (18) Lee, K.; Tienes, B. M.; Wilker, M. B.; Schnitzenbaumer, K. J.; Dukovic, G. (Ga1‑xZnx)(N1‑xOx) nanocrystals: visible absorbers with tunable composition and absorption spectra. Nano Lett. 2012, 12, 3268−3272. (19) Lee, K.; Lu, Y.-G.; Chuang, C.-H.; Ciston, J.; Dukovic, G. Synthesis and characterization of (Ga1−xZnx)(N1−xOx) nanocrystals with a wide range of compositions. J. Mater. Chem. A 2016, 4, 2927− 2935. (20) Yang, M.; Huang, Q.; Jin, X. Microwave synthesis of porous ZnGaNO solid solution for improved visible light photocatalytic performance. Solid State Sci. 2012, 14, 465−470. (21) Reinert, A. A.; Payne, C.; Wang, L.; Ciston, J.; Zhu, Y.; Khalifah, P. G. Synthesis and characterization of visible light absorbing (GaN)1‑x(ZnO)x semiconductor nanorods. Inorg. Chem. 2013, 52, 8389−8398. (22) Wang, Z.; Han, J.; Li, Z.; Li, M.; Wang, H.; Zong, X.; Li, C. Moisture-assisted preparation of compact GaN:ZnO photoanode toward efficient photoelectrochemical water oxidation. Adv. Energy Mater. 2016, 6, 1600864. (23) Chen, D. P.; Skrabalak, S. E. Synthesis of (Ga1−xZnx)(N1−xOx) with Enhanced visible-light absorption and reduced defects by suppressing Zn volatilization. Inorg. Chem. 2016, 55, 3822−3828.

performances under visible-light irradiation. S50 exhibited the best photoactivity with apparent rate constant k = 0.015 min−1 for the photodegradation of RhB, and k = 9.79 × 10−3 min−1 for the photodegradation of phenol. The best performance of loaded S50 is ascribed to the relatively flat surface band bending of the particle.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01415. DTA and TGA curves for the Zn/Ga/CO3 LDH precursor powder, UV−vis absorption spectra of phenol solutions in the presence of ZnGaNOs upon visible-light irradiation, XPS spectra for EV identifications of ZnGaNOs, Mott−Schottky plots and EIS Nyquist plots of 5 wt % RuO2-loaded ZnGaNO electrodes, and Mott−Schottky plots for FTO electrodes covered with S50 loaded with different amounts of Rh (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.-L.H.). *E-mail: [email protected] (T.M.). ORCID

Yan-Ling Hu: 0000-0001-8892-6534 Dongya Sun: 0000-0002-6490-0038 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51402249 and No. 21773073), the Natural Science Foundation of Fujian Province (No. 2017J01593), the Research Fund Programs for Returnees of Xiamen (No. 2016166), the Fundamental Research Funds for the Central Universities (No. 30916014103), and the Research Fund Program of Guangdong Provincial Key Lab of Green Chemical Product Technology.

■

REFERENCES

(1) Maeda, K.; Teramura, K.; Takata, T.; Hara, M.; Saito, N.; Toda, K.; Inoue, Y.; Kobayashi, H.; Domen, K. Overall water splitting on (Ga1−xZnx)(N1−xOx) solid solution photocatalyst: relationship between physical properties and photocatalytic activity. J. Phys. Chem. B 2005, 109, 20504−20510. (2) Wang, L.; Ouyang, S.; Ren, B.; Ye, J.; Wang, D. Enhanced photocatalytic degradation of 2-propanol over macroporous GaN/ ZnO solid solution prepared by a novel sol-gel method. APL Mater. 2015, 3, 104414. (3) Yu, S.-H.; Chiu, C.-W.; Wu, Y.-T.; Liao, C.-H.; Nguyen, V.-H.; Wu, J. C.S. Photocatalytic water splitting and hydrogenation of CO2 in a novel twin photoreactor with IO3−/I− shuttle redox mediator. Appl. Catal., A 2016, 518, 158−166. (4) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN: ZnO solid solution as a photocatalyst for visiblelight-driven overall water splitting. J. Am. Chem. Soc. 2005, 127, 8286−8287. (5) Li, Y.; Zhu, L.; Yang, Y.; Song, H.; Lou, Z.; Guo, Y.; Ye, Z. A full compositional range for a (Ga1‑xZnx)(N1‑xOx) nanostructure: high efficiency for overall water splitting and optical properties. Small 2015, 11, 871−876. L


Article

Inorganic Chemistry (24) Cheng, J.; Wang, Y.; Xing, Y.; Shahid, M.; Pan, W. Surface defects decorated zinc doped gallium oxynitride nanowireswith high photocatalytic activity. Appl. Catal., B 2017, 209, 53−61. (25) Yun, S. K.; Pinnavaia, T. J. Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides. Chem. Mater. 1995, 7, 348−354. (26) Zhang, C. G.; Chen, W. D.; Bian, L. F.; Song, S. F.; Hsu, C. C. Preparation and characterization of GaN films by radio frequency magnetron sputtering and carbonized-reaction technique. Appl. Surf. Sci. 2006, 252, 2153−2158. (27) Hu, Y.-L.; Wu, Z.; Zheng, X.; Lin, N.; Yang, Y.; Zuo, J.; Sun, D.; Jiang, C.; Sun, L.; Lin, C.-J; Fu, Y. ZnO/ZnGaNO heterostructure with enhanced photocatalytic properties prepared from a LDH precursor using a coprecipitation method. J. Alloys Compd. 2017, 709, 42−53. (28) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Noble-Metal/Cr2O3 core/shell nanoparticles as a cocatalyst for photocatalytic overall water splitting. Angew. Chem., Int. Ed. 2006, 45, 7806−7809. (29) Maeda, K.; Xiong, A.; Yoshinaga, T.; Ikeda, T.; Sakamoto, N.; Hisatomi, T.; Takashima, M.; Lu, D.; Kanehara, M.; Setoyama, T.; Teranishi, T.; Domen, K. Photocatalytic overall water Splitting promoted by two different cocatalysts for hydrogen and oxygen evolution under visible light. Angew. Chem., Int. Ed. 2010, 49, 4096− 4099. (30) Hashiguchi, H.; Maeda, K.; Abe, R.; Ishikawa, A.; Kubota, J.; Domen, K. Photoresponse of GaN: ZnO electrode on FTO under visible light irradiation. Bull. Chem. Soc. Jpn. 2009, 82, 401−407. (31) Jin, Z.; Hashizume, T.; Hasegawa, H. Effects of nitrogen addition on methane-based ECR plasma etching of gallium nitride. Appl. Surf. Sci. 2002, 190, 361−365. (32) Tongying, P.; Lu, Y.-G.; Hall, L. M.G.; Lee, K.; Sulima, M.; Ciston, J.; Dukovic, G. Control of elemental distribution in the nanoscale solid-state eeaction that produces (Ga1−xZnx)(N1−xOx) nanocrystals. ACS Nano 2017, 11, 8401−8412. (33) Hattori, A. N.; Kawamura, F.; Yoshimura, M.; Kitaoka, Y.; Mori, Y.; Hattori, K.; Daimon, H.; Endo, K. Chemical etchant dependence of surface structure and morphology on GaN (0001) substrates. Surf. Sci. 2010, 604, 1247−1253. (34) Hasegawa, H.; Koyama, Y.; Hashizume, T. Properties of metalsemiconductor interfaces formed on n-Type GaN. Jpn. J. Appl. Phys. 1999, 38, 2634−2639. (35) Zong, F.; Ma, H.; Xue, C.; Zhuang, H.; Zhang, X.; Xiao, H.; Ma, J.; Ji, F. Synthesis and thermal stability of Zn3N2 powder. Solid State Commun. 2004, 132, 521−525. (36) Neugebauer, J.; Van de Walle, C. G. Gallium vacancies and the yellow luminescence in GaN. Appl. Phys. Lett. 1996, 69, 503−505. (37) Marrani, A. G.; Caprioli, F.; Boccia, A.; Zanoni, R.; Decker, F. Electrochemically deposited ZnO films: an XPS study on the evolution of their surface hydroxide and defect composition upon thermal annealing. J. Solid State Electrochem. 2014, 18, 505−513. (38) Hamza Taha, M. K.; Boisron, O.; Canut, B.; Melinon, P.; Penuelas, J.; Gendry, M.; Masenelli, B. Control of the compensating defects in Al-doped and Ga-doped ZnO nanocrystals for MIR plasmonics. RSC Adv. 2017, 7, 28677−28683. (39) Feygenson, M.; Neuefeind, J. C.; Tyson, T. A.; Schieber, N.; Han, W.-Q. Average and local crystal structures of (Ga1−xZnx)(N1−xOx) solid solution nanoparticles. Inorg. Chem. 2015, 54, 11226− 11235. (40) Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K. Photocatalytic water splitting using modified GaN:ZnO solid solution under visible light: long-time operation and regeneration of activity. J. Am. Chem. Soc. 2012, 134, 8254−8259. (41) Teramura, K.; Maeda, K.; Saito, T.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Characterization of ruthenium oxide nanocluster as a cocatalyst with (Ga1‑xZnx)(N1‑xOx) for photocatalytic overall water splitting. J. Phys. Chem. B 2005, 109, 21915−21921.

(42) Kavan, L.; Tétreault, N.; Moehl, T.; Grätzel, M. Electrochemical characterization of TiO2 blocking layers for dye-sensitized solar cells. J. Phys. Chem. C 2014, 118, 16408−16418. (43) Ievtushenko, A.; Khyzhun, O.; Shtepliuk, I.; Bykov, O.; Jakieła, R.; Tkach, S.; Kuzmenko, E.; Baturin, V.; Karpenko, O.; Olifan, O.; Lashkarev, G. , X-ray photoelectron spectroscopy study of highlydoped ZnO:AlN films grown at O-rich conditions. J. Alloys Compd. 2017, 722, 683−689. (44) Demchenko, D. O.; Earles, B.; Liu, H. Y.; Avrutin, V.; Izyumskaya, N.; Ö zgür, Ü .; Morkoc, H. Impurity complexes and conductivity of Ga-doped ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 075201. (45) Zhang, Z.; Yu, Y.; Wang, P. Hierarchical top-porous/bottomtubular TiO2 nanostructures decorated with Pd nanoparticles for efficient photoelectrocatalytic decomposition of synergistic pollutants. ACS Appl. Mater. Interfaces 2012, 4, 990−996. (46) Yilmaz, P.; Lacerda, A. M.; Larrosa, I.; Dunn, S. Photoelectrocatalysis of rhodamine B and solar hydrogen production by TiO2 and Pd/TiO2 catalyst systems. Electrochim. Acta 2017, 231, 641−649. (47) Liu, J.; Fernández-Serra, M. V.; Allen, P. B. Special quasiordered structures: Role of short-range order in the semiconductor alloy (GaN)1−x(ZnO)x. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 054207. (48) Wang, Z.; Zhao, M.; Wang, X.; Xi, Y.; He, X.; Liu, X.; Yan, S. Hybrid density functional study of band alignment in ZnO−GaN and ZnO-(Ga1‑xZnx)(N1‑xOx)-GaN heterostructures. Phys. Chem. Chem. Phys. 2012, 14, 15693−15698. (49) Imanaka, Y.; Anazawa, T.; Manabe, T.; Amada, H.; Ido, S.; Kumasaka, F.; Awaji, N.; Sánchez-Santolino, G.; Ishikawa, R.; Ikuhara, Y. An artificial photosynthesis anode electrode composed of a nanoparticulate photocatalyst film in a visible light responsive GaNZnO solid solution system. Sci. Rep. 2016, 6, 35593. (50) Jensen, L. L.; Muckerman, J. T.; Newton, M. D. First-Principles Studies of the Structural and Electronic Properties of the (Ga1‑xZnx)(N1‑xOx) Solid Solution Photocatalyst. J. Phys. Chem. C 2008, 112, 3439−3446. (51) Wu, A.; Li, J.; Liu, B.; Yang, W.; Jiang, Y.; Liu, L.; Zhang, X.; Xiong, C.; Jiang, X. Band-gap tailoring and visible-light-driven photocatalytic performance of porous (GaN)1−x(ZnO)x solid solution. Dalton Trans. 2017, 46, 2643. (52) Kibria, M. G.; Zhao, S.; Chowdhury, F. A.; Wang, Q.; Nguyen, H. P. T.; Trudeau, M. L.; Guo, H.; Mi, Z. Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting. Nat. Commun. 2014, 5, 3825.

M


ZnGaNO Photocatalyst Particles Prepared from Methane-Based

Recommend Documents