N-Codoped ZnGa2O4 Nanospheres

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C: Energy Conversion and Storage; Energy and Charge Transport 2

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Oxygen Vacancy-Modified B/N-Codoped ZnGaO Nanospheres With Enhanced Photocatalytic Hydrogen Evolution Performance in the Absence of Pt Cocatalyst Peng Zhao, Yanlu Li, Lili Li, Shulin Bu, and Weiliu Fan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01776 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

Oxygen

Vacancy-Modified

B/N-Codoped

ZnGa2O4

Nanospheres With Enhanced Photocatalytic Hydrogen Evolution Performance in the Absence of Pt Cocatalyst Peng Zhaoa, Yanlu Li*,b, Lili Lib, Shulin Buc and Weiliu Fan*,a a

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100,

China b

c

State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China

Joint Colleges of Science, Australian National University, Canberra, ACT, Australia

Corresponding author *E-mail: [email protected]

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Abstract: Here,

we

report

oxygen

vacancy

(VO)-modified

B/N-codoped

ZnGa2O4

(VO-B/N-ZGO) nanospheres, showing excellent photocatalytic H2 production even without Pt cocatalyst, which is better than that obtained with VO-modified B-doped ZnGa2O4 (VO-B-ZGO) or N-doped ZnGa2O4 (N-ZGO) and as high as about three times of that achieved with the undoped ZnGa2O4 (ZGO) photocatalyst. The dramatically enhanced photocatalytic activity of VO-B/N-ZGO predominately originates from the improvement of charge separation and surface activation. Experimental characterization combined with the theoretical calculation method demonstrates that VO-B/N-ZGO can show effective charge compensation more easily through the interaction of oxygen vacancies, interstitial boron, and substitutional nitrogen; especially for the formation of B-N bond, it avoids the presence of semi-occupied states as new recombination centers in doped photocatalyst. In addition, VO-B/N-ZGO rich in reactive sites is generated by oxygen vacancy-modified B/N-codoping, which overcomes the limitation for most semiconductors without high H2 evolution activities in the absence of a cocatalyst and provides a potentially new photocatalytic H2 generation research. KEYWORDS Charge compensation, surface activation, and cocatalyst free

1. INTRODUCTION Photocatalytic water splitting to obtain hydrogen energy is considered to be a promising and attractive strategy to address the environmental pollution and energy 2


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shortage issues.1 Zinc gallate (ZnGa2O4) is a binary metal oxide with a cubic spinel structure and is attracting wide attention as a photocatalyst for solar fuel production and organic contaminant degradation.2-4 Its conduction band minimum is predominately composed of Ga 4s4p and Zn 4s4p hybrid orbitals, inducing large dispersion, which produce photoexcited electrons with large mobility in the conduction band.5 However, a wide band gap of about 4.5 eV limits the light response range of ZnGa2O4. Nanomaterials are often used as the photocatalyst owing to its large specific surface areas and rich active sites, but structural imperfections (such as VO) as the carrier recombination center, are often generated with the synthesis process and they negatively impact photocatalysis.6 Since the report on N-doped TiO2 by Asahi et al.7, great efforts have been made to improve the photocatalytic activity by doping with non-metal atoms (such as B8, C9, N10, and S11). New recombination centers, originating from the semi-occupied states induced by the single electron of the dopant, were found to decrease the photocatalytic activity in some single-doped photocatalytic system. As a previous report12 revealed, the higher doping concentrations of boron in TiO2 induced better light absorption properties, accompanied with higher recombination rates. An effective approach to tackle this challenge is to dope semiconductor photocatalyst with

two or three types

of nonmetal atoms (such as N/S-codoping13,

N/B-codoping14-16, N/F-codoping17, and N/B/F-tridoping18), which can not only exhibit higher visible-light absorption performance because of the synergistic effect of different dopants, but also possess a lower electron-hole recombination rate owing to 3



their charge compensation. Liu et al.14 reported red anatase TiO2 with a full visible spectrum response by B/N-codoping. The pre-doped interstitial B effectively compensates for the charge difference between substitutional N3- and lattice O2-; hence, no new recombination centers (such as oxygen vacancy and Ti3+) were observed. Moreover, some other studies demonstrated that the oxygen vacancy as the donor could compensate for the single electron from acceptor dopants when they meet the exact quantitative relation that also decreases the new recombination centers and benefits the photocatalytic activity.19,20 Although some studies have been reported on polyatomic doping or multi-factor doping, it is still difficult to control the chemical states, which are sensitive to electronic structures and photocatalytic performance. There are different opinions on mechanism of polyatomic doping (and/or multi-factor doping) to achieve improved photocatalytic activity. Computational chemistry could simulate and clearly reveal the geometric structure and electronic properties that need to be used together with experimental determination to further discuss the polyatomic doping mechanism for photoreactivity. Moreover, facile and efficient approaches should be developed because of the increased difficulty and complexity of the polyatomic doping process. Oxygen vacancy is a common defect widely found in oxides and composite oxide nanomaterials.21 As these oxygen vacancies are potential recombination centers for electron-hole pairs, they adversely impact the photocatalytic performance.6 However, every coin has two sides. Some researchers take advantage of the defects in materials synthesis and electronic structure regulation by precisely controlling the amount of 4


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defects that are crucial for its effect. For example, Ye et al.22 reported a facile method that makes use of the weak reductivity of the oxygen-deficient WO2.72 to in situ redox reaction with oxidative metal salts for direct growth of noble-metal particles on WO3. Moreover, the defects originating from the vacancy of bridging oxygen in TiO2 could also serve as active sites for N2 reduction in NH3 production photosynthesis.21 Therefore, oxygen vacancies with reasonable regulation may play a positive role for heteroatom-doped photocatalyst in the electronic structure modification, and thus, help improve carrier separation and surface activation, which should be investigated further. The water splitting reaction is a thermodynamically uphill reaction in which the Gibbs free energy increases by 237 kJ mol-1, which generally manifests low performance without a cocatalyst.23 Platinum with some other noble metals (such as Ru, Pd, and Rh), serving as electron sinks and reaction sites for the water reduction reaction, was considered to be the most active cocatalyst for the photocatalytic H2 generation reaction.1 However, noble metals are very rare, which limits their practical applications. Recently, it was reported that for photocatalytic CO2 reduction24 and NH3 production21 systems, oxide semiconductors with boron doping or the presence of VO show improved cocatalyst-free photocatalytic activity because of the surface activation effect. Therefore, in addition to broadening the light response range and suppressing the recombination, heteroatomic doping and/or VO introduction could contribute to active sites for the semiconductor photocatalyst that play an important role as the platinum cocatalyst in the photocatalytic reaction. Therefore, it would be 5



highly useful development if platinum cocatalysts could not be necessary in heteroatomic or VO introduced semiconductor photocatalysts for photocatalytic proton reduction reactions. Herein, we present a facile method to prepare B/N-codoped ZnGa2O4 nanospheres accompanied with oxygen vacancy modification. In this work, NaBH4 not only served as the boron source but also generated oxygen vacancies at a mild temperature. These preformed oxygen vacancies decreased the formation energy for the incorporation of boron and nitrogen atoms into ZnGa2O4. Furthermore, this VO-modified B/N-codoped ZnGa2O4 exhibits excellent photocatalytic hydrogen production efficiency even without any additional cocatalyst, which is better than the VO-B-ZGO or N-ZGO samples and as high as about three times that of undoped ZnGa2O4. This superior photocatalytic performance of VO-B/N-ZGO could be explained by the following major factors. For one, the VO-modified B/N-codoping obviously enhances carrier separation of ZnGa2O4 photocatalyst. For another, there are also rich reactive sites produced in VO-B/N-ZGO, which promotes the surface reaction efficiency. In addition, the theoretical calculation was combined with experimental characterization to explain the mechanism of ZnGa2O4 photocatalytic reaction. We hope that this work could provide some reference for the interested researchers.

2. EXPERIMENTAL SECTION Materials. All chemicals in this work were analytical-grade reagents and used as received without further purification. Zinc oxide (ZnO) and gallium nitrate hydrate (Ga(NO3)3•xH2O) were purchased from Aladdin Chemical Reagents Corp. Sodium 6


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citrate and sodium borohydride (NaBH4) were purchased from Sinopharm Chemicals. Preparation of photocatalysts. Preparation of ZnGa2O4 nanospheres: Stoichiometric amounts of zinc oxide and gallium nitrate were diluted in 40 mL deionized water with vigorous stirring. Then, 230 mg sodium citrate was added to the above solution. After continuously stirring for 30 min, the mixed solution was transferred to a 50 mL Teflon-lined autoclave and autoclaved at 200 °C for 24 h. The product was collected by centrifugation, and washed with deionized water and alcohol three times. Preparation of VO-modified B/N-codoped ZnGa2O4 photocatalyst (VO-B/N-ZGO): The mixture of as-prepared ZnGa2O4 nanospheres and NaBH4 in a molar ratio of 1:1 was calcined at 500 °C in an NH3 atmosphere for 3 h. After naturally cooling, VO-B/N-ZGO was obtained by simply washing with deionized water and dried at 60 °C. For comparison, B-doped ZnGa2O4 with oxygen vacancy modification (VO-B-ZGO) was synthesized in a similar manner, but was calcined in the Ar atmosphere; N-doped (N-ZGO) and undoped ZnGa2O4 photocatalysts (ZGO) were obtained by annealing as-prepared ZnGa2O4 nanospheres in NH3 or air atmosphere. Both the undoped and monatomic-doped ZnGa2O4 were also calcined at 500 °C for 3 h before characterizing for comparison. Characterization of photocatalysts. Powder X-ray diffraction (XRD) patterns were recorded in a Brüker Advance D8 system using CuKα radiation in the range 10°< 2θ < 80°. X-ray photoelectron spectroscopy (XPS) analysis was carried out 7



using an ESCALAB 250 spectrometer equipped with an Al Kα source. All the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. Electron spin resonance (ESR) spectra were recorded at room temperature using a JEOL JES-X320 ESR spectrometer. Transmission electron microscopy (TEM) and high resolution electron microscopy (HRTEM) images were taken with a JEOL JEM-2100 electron microscope. Scanning electron microscope (SEM) images were obtained on S-4800 field emission SEM (FESEM, Hitachi, Japan). The Brunauer-Emmett-Teller (BET) surface area was measured using a JW-BK112T nitrogen absorption/desorption apparatus. UV–vis diffuse reflectance spectra (DRS) were performed at room temperature between 200 and 800 nm using a Shimadzu UV2550 spectrophotometer. Photoluminescence (PL) spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer to observe the combination rates of the electron–hole pairs. The fluorescence lifetimes of samples were measured by fluorescence spectrometer (FLS920, Edinburgh instrument Ltd.), with excitation wavelength at 280 nm and emission wavelength at 460 nm. Photoelectrochemical experiments. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky measurements were conducted on a CHI1660E electrochemistry workstation (Chenhua, Shanghai, China) at room temperature. All experiments were performed in a standard three-electrode cell using platinum foil as the counter electrode and a saturated calomel electrode as the reference electrode, respectively. The working electrodes were prepared by depositing catalysts on the ITO transparent conductive glass substrate by electrophoretic deposition. Here, 0.5 M 8


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Na2SO4 aqueous solution served as the electrolyte for photoelectrochemical measurement. In electrochemical impedance spectroscopy (EIS) measurements, the frequency was scanned from 0.01 to 100,000 Hz at an applied potential under Xe lamp irradiation. Mott–Schottky plots were obtained at a frequency of 1 kHz in the dark. Photocatalytic activity measurements. Photocatalytic hydrogen production of ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO samples was carried out in a vessel connected to a glass-closed gas circulation system with 300 W Xe lamp (λ ≥ 250 nm, Beijing Trusttech Co. Ltd., PLS-SXE-300C) irradiation on the top. Photocatalyst powder (20 mg) was dispersed in 100 mL Na2SO3 aqueous solution (0.05 M) and deposited without any co-catalysts. For determining the role of surface defects, the Pt cocatalyst was deposited in different amounts by directly adding H2PtCl6 in the above 100 mL reaction solution. The amounts of H2 released by water splitting were determined using gas chromatography. The apparent quantum yield (QY) of as-prepared samples was measured under the similar photocatalytic reaction conditions using a low power UV-LED (10 W, 313 nm, Shenzhen LAMPLIC Science Co. Ltd. China). The optical power density was measured to be 57.4 mW/cm2 by an optical power meter (CEL-NP 2000, Beijing China Education Au-light Co., Ltd). The light area was detected to be 9.4 cm2. The quantum yield was estimated according to the following equation6: QY =

number of evolved H molecules × 2 × 100% number of incident photons

Theoretical calculation. The Vienna Ab Initio Simulation Package (VASP)25,26 9



based on the first-principles and the projector-augmented-wave (PAW) method27 was implemented in the present calculation. Zn 3d104s2, Ga 4s24p1, O 2s22p4, N 2s22p3, and B 2s22p1 states were treated as valence electrons. The kinetic energy cutoff for the plane-wave basis was set to be 800 eV. The general gradient approximation (GGA) of Perdew Burke and Ernzerhof (PBE)28 is employed to optimize the crystal structure with the force convergence criterion of 0.01 eV/Å. All the atoms and lattice constants are fully relaxed by using the conjugate gradient techniques. The optimized lattice parameters were a = b = c = 8.386 Å, in good agreement with the experimental value of 8.334 Å.29 The energetics and electronic structures were determined by using the screened-exchange hybrid density functional of Heyd, Suseria, and Ernzerhof (HSE06).30,31 In this approach, the long-range exchange potential and the correlation potential were calculated with the PBE functional, while the short-range exchange potential was calculated by mixing a fraction of nonlocal Hartree-Fock exchange with PBE. The screening length and mixing parameter were fixed at 10 Å and 0.25, respectively. Then, 4×4×4 and 2×2×2 Monkhorst-Pack k-point meshes were performed to sample the Brillouin zones for PBE and HSE06 calculations. Considering the calculation cost and accuracy, the cubic supercells with eight ZnGa2O4 units containing 56 atoms were used to model various N and B doping defect configurations.

3. RESULTS AND DISCUSSION 3.1. Structural and chemical characterizations B/N-codoped ZnGa2O4 with VO modification was prepared successfully by facile 10


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one-step calcination. In our method, NaBH4 plays a dual role; it acts as a B source, and also behaves as an oxygen scavenger because it has strong reducibility, because of which it can remove the oxygen atoms from the ZnGa2O4 surface to produce VO at mild temperature.32,33 Moreover, NH3 was used as the N source, and the N-doped and VO-modified B-doped ZnGa2O4 photocatalysts were synthesized for comparison.

Figure 1. XRD patterns of (a) full spectra and (b) magnified spectra of ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO samples.

To investigate possible changes in the phase and crystal structure after doping, XRD patterns of the as-synthesized ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO samples were carried out as presented in Figure 1. Figure 1a shows that the diffraction 11



peaks of all the samples are sharp and intense, indicating their highly crystalline nature. No impurity peaks were observed, which implies the high purity for both undoped and doped ZnGa2O4. In addition, compared with pristine ZnGa2O4, the doped samples exhibit a diffraction shift of varied degrees as shown in the magnified spectra (Figure 1b), indicating lattice deformation probably induced by the incorporation of dopants or oxygen vacancies.18,34 As for N-ZGO and VO-B-ZGO, the obvious shift to high angles is a typical phenomenon for lattice contraction12, which probably results from substitutional doping and oxygen vacancy generation owing to the reaction environment offered by NH3 and NaBH4. However, the diffraction peak positions for VO-B/N-ZGO show very slight shifts, probably caused by the compromise between lattice expanding and shrinking when the dopants are incorporated into both substitutional and interstitial sites.

12


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Figure 2. SEM, TEM, and HRTEM images of samples: (a), (e), and (i) pristine ZnGa2O4; (b), (f), and (j) N-doped ZnGa2O4; (c), (g), and (k) VO-modified B-doped ZnGa2O4; (d), (h), and (l) VO-modified B/N-codoped ZnGa2O4.

The microstructure of all the ZnGa2O4 photocatalysts were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 2. We can see from the SEM graphs that all samples with spherical morphology show good dispersibility. These nanospheres with a uniform diameter of 13



approximately 300 nm are composed of round-shaped grains of about 10 nm, as estimated from the TEM images. The grain sizes (Table 1) are in agreement with the XRD results calculated by the Scherrer formula. The obvious lattice fringes, as shown in high-resolution TEM (HRTEM) images, confirm the high crystallinity for all the samples. In order to compare the changes of crystal structure after doping, the crystal face with same orientation were selected for all samples. The lattice spacing of about 0.4808 nm for all samples corresponds to the (111) plane of cubic ZnGa2O4. For the doped samples, the d-spacing of the lattice finger slightly shrinks in contrast to that of pristine ZnGa2O4, which is consistent with the diffraction shift (Figure 1b) and may result from the influence of heteroatoms and oxygen vacancies in the crystal structure. As shown in the HRTEM images of doped samples, there are only some differences of lattice spacing compared with pristine ZnGa2O4. But no amorphous layer was found as previous work about surface defect11,32,33, which may indicate that the doped factors (B atom, N atom and VO) did not distribute on the surface intensively. Moreover, the specific surface areas of the samples were also measured by the nitrogen absorption/desorption method, as shown in Table 1. No obvious difference is observed among the ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO in the BET surface areas, which are in the range of 20–30 m2/g and are negatively related with the crystal sizes.

14


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Figure 3. XPS spectra of (a) N 1s, (b) B 1s, (c) Zn 2p, (d) Ga 3d, and (e) O 1s; (f) ESR spectra of ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO.

High-resolution XPS spectra were performed to further confirm the presence and chemical states of nitrogen and boron dopants in ZnGa2O4 crystals, as shown in Figure 3. After curve fitting, three peaks were obtained for both N-ZGO and VO-B/N-ZGO in the N 1s spectra (Figure 3a). A stronger peak at 396.3 eV and a very

15



weak peak at 399.7 eV could be found in N-ZGO, which are attributed to the substitutional and interstitial nitrogen, respectively.15,35 The nitrogen predominately incorporated into substitutional sites probably results from the anaerobic experiment condition provided by NH3. However, the binding energies of substitutional and interstitial nitrogen in VO-B/N-ZGO shift to 397.6 eV and 400.9 eV, respectively14, which may originate from the electron density around nitrogen that is affected by the interstitial boron present in VO-B/N-ZGO. The peaks at 393.1 eV and 394.9 may be aroused by the Auger peak of Ga 2p according to an earlier former report.36 The XPS spectra of B 1s in VO-B-ZGO and VO-B/N-ZGO are shown in Figure 3b. The B 1s spectra shows two peaks for both VO-B-ZGO and VO-B/N-ZGO; however, no peak at 193.0 eV corresponding to B-O from B2O3 is found in these samples.37 The binding energy at 188.1 eV is assigned to the form of substituted boron.38 The peak at 192.3 eV can be assigned to the interstitial boron, which forms Zn-O-B bond.14,39 The boron dopant also replaced the lattice oxygen in B-doped ZnGa2O4. According to previous reports40, the peak at 190.2 eV may be assigned to the B-N bond. The N 1s binding energy at 397.6 eV for VO-B/N-ZGO also favors the formation of the B-N bond.16 The weak peak at 191.7 eV could be assigned to the interstitial boron.14 No peak was detected below 190.0 eV, indicating that the boron dopant without any substitutional state exists in the VO-B/N-ZGO sample. For N-ZGO and VO-B-ZGO samples, the boron or nitrogen dopants are predominantly introduced in substitutional sites, which may be influenced by the reaction environment41 provided by NH3 and NaBH4; for substitutional boron 16


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particularly, a previous report12 suggested that it is difficult to acquire and easily transform substitutional boron to interstitial boron with an oxygen vacancy. In our study, we observed that the generation of oxygen vacancies may promote the formation of substitutional boron and the remaining electrons originating from VO enhance its stability. Moreover, theoretical calculation also demonstrated that the presence of VO reduces the formation energy (Ef) of B-doping, N-doping, and B/N-codoping, as shown in Table S1. Notably, most of the substitutional boron in the VO-B-ZGO sample transformed to interstitial boron in VO-B/N-ZGO. Thus, we could infer that substitutional nitrogen with smaller formation energy will be preformed in the VO-B/N-ZGO photocatalyst, and would influence the chemical state of the doped boron change from the substitutional state (for VO-B-ZGO) to interstitial boron (for VO-B/N-ZGO). The calculated formation energies (Table S1) show that the Ef of interstitial B doping with substitutional N and VO is 5.85 eV, while that of substitutional B doping is 11.91 eV. The significant difference of 6.06 eV revealed that the preformed oxygen vacancies and substitutional nitrogen lead to the more easy and stable incorporation of boron in the interstitial site than substitution of lattice oxygen. Considering that the proportion of the doping concentration for boron and nitrogen is close to 1: 1 (Table 1), we could infer that the B-N bond was formed by the substitutional nitrogen with interstitial boron in the adjacent location for VO-B/N-ZGO. Moreover, the positive shift of N 1s binding energy for VO-B/N-ZGO compared with N-ZGO could be explained by the formation of the Zn-N-B or Ga-N-B structure. The higher electronegativity of B than 17



that of Ga (Zn) decreases the electron density around nitrogen in B/N-codoped ZnGa2O4.17 The binding energy shift could also be observed on Zn 2p, Ga 3d, and O 1s spectra (Figure 3c, 3d, 3e), owing to the electron density change caused by the incorporation of nitrogen, boron, and oxygen vacancies.33 In N-ZGO, a negative shift for Zn 2p and Ga 3d binding energy was observed, in contrast with pristine ZnGa2O4. The substitution of nitrogen for lattice oxygen generates a Zn-N-Ga structure, resulting in an increase in the electron density around Zn and Ga owing to the smaller electronegativity of the N atom than the O atom. The electron density around O in N-ZGO also decreased compared with that in pristine ZnGa2O4 because the lattice oxygen in the initial O-Zn-O and/or O-Ga-O was substituted by nitrogen with smaller electronegativity, which manifest as a negative shift for O 1s binding energy in N-doped ZnGa2O4. The shifts for Zn 2p, Ga 3d, and O 1s binding energy of N-ZGO were in accordance with the result of substitutional N-doping obtained from the N 1s spectra. However, the binding energy of Zn 2p, Ga 3d, and O1s manifests a positive shift for VO-B-ZGO and VO-B/N-ZGO, probably owing to the influence of oxygen vacancy. According to a previous report of Sun et al.33, after NaBH4 treatment, the peaks of Sr, Ti, and O shift to higher binding energy compared with that of pristine SrTiO3 and the binding energy increased gradually with the increasing reaction time and temperature. This implies that oxygen vacancies result in a change in the equilibrium electron density change for the entire system and these changes increased with the increasing amount of oxygen vacancies. Based on the above analysis, the 18


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changes in the electronic structure were believed to be induced during the doping process, which probably influence the optical and dynamic properties of the photocatalysts. To further demonstrate the presence of oxygen vacancies in the ZnGa2O4 photocatalysts, ESR studies of undoped, N-doped, VO-modified B-doped, and VO-modified B/N-codoped ZnGa2O4 were conducted as shown in Figure 3f. No obvious signal peak was observed for undoped ZnGa2O4, indicating that little oxygen vacancies occur in pristine ZnGa2O4 that is negligible compared with that of the doped ZnGa2O4 photocatalysts. However, symmetric signal peaks at g = 2.0046 were detected in N-doped ZnGa2O4, VO-modified B-doped ZnGa2O4, and VO-modified B/N-codoped ZnGa2O4. According to a report by Serpone et al.42, the signal at g = 2.003–2.005 could be ascribed to an electron trapped in the oxygen vacancy (F+ center) in both the reduced and doped photocatalysts. Moreover, the ESR signal for VO-B-ZGO and VO-B/N-ZGO are strong and sharp, while for N-ZGO it is relatively weak. This is probably because oxygen vacancies are formed by N-doping because of the weak reducibility of NH3 or charge imbalance. However, oxygen vacancies in VO-B-ZGO and VO-B/N-ZGO mainly arose from the strong reductivity of NaBH4, and thus, a greater concentration of oxygen vacancies manifests as a stronger spin response signal. The remaining electrons from oxygen vacancies may compensate the electron deficiency arising from substitutional boron and nitrogen doping to some degree that help decrease the formation of new recombination centers. Furthermore, according to previous reports, the oxygen vacancies serve as electron donors33, and 19



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hence, they may help increase the donor density in VO-B-ZGO and VO-B/N-ZGO, which would enhance the charge transport and photocatalytic activity. Table1. Crystallite size from XRD result, specific surface value, dopant concentration measured by XPS, and energy band gaps (Eg) obtained from plots of ahv vs. hv.

ZnGa2O4 sample

crystallite size (nm)

specific surface(m2/g)

concentration of dopants (%) nitrogen

Eg(eV)

boron

Undoped

13.6

26.67

--

--

4.17

N-doped

11.5

29.69

12.0

--

4.10

VO-B-doped

12.4

29.16

--

3.32

3.96

VO-B/N-codoped

14.0

23.94

12.5

8.95

3.95

3.2.Optical properties of ZnGa2O4 photocatalysts

20


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2

Figure 4. (a) UV–vis diffuse reflectance spectra (DRS) and (b) plots of (αhν) vs. (hν) for ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO.

The optical properties of ZnGa2O4 photocatalysts may be affected by changes in the electronic structure after the introduction of boron, nitrogen, and oxygen vacancies. UV-vis DSR spectra were carried out to characterize the optical absorption of ZnGa2O4 photocatalysts, as shown in Figure 4a. Compared with undoped ZnGa2O4, the absorption edge of N-doped ZnGa2O4 could see a slight red shift and a weak shoulder absorption could be observed approximately at 350–550 nm. The energy bandgap (Eg) of all samples was calculated based on the Kubelka-Munk equation.8,44 The Eg values were obtained from the interception of the tangent to the X axis in the plot of (αhν)2 vs. (hν) (Figure 4b); few changes were observed after heteroatomic doping as shown in Table 1. In addition, it is noteworthy that an additional absorption band from 350 nm extending to the entire visible spectrum was observed for VO-B-ZGO and VO-B/N-ZGO in comparison with the pristine and N-doped ZnGa2O4. This may because NaBH4 with strong reducing capability generates many oxygen vacancies in the doping process, which contribute to the increased absorption intensity in the full visible light spectrum. Similar phenomena were reported by Sun et al.33 Compared with pristine SrTiO3, the additional absorption band was also observed with NaBH4 treated samples; the absorption band increased with increase in the reaction temperature and time, which indicated that the additional band result from the effect of oxygen vacancies and is positively related with the oxygen vacancy concentration. 21



Also, only negligible changes were observed for absorption onset in the SrTiO3 system. Moreover, there are two additional bands at about 420 nm and 730 nm in VO-B/N-ZGO, which may result from the electron excitation of impurity levels in the bandgap or plasmon resonance generated from high defect density44-46. Taking ESR results into consideration, we may conclude that the additional absorption for doped ZnGa2O4 was mainly caused by oxygen vacancies and increased with increasing VO concentration. 3.3. Dynamics characterizations

22


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Figure 5. (a) Photoluminescence (PL) spectra, (b) electrochemical impedance spectroscopy (EIS) Nyquist plots, and (c) Mott-Schottky plots of pristine, N-doped, VO-modified B-doped, and VO-modified B/N-codoped ZnGa2O4 samples.

The change in the electronic structure owing to doping will influence the separation and transfer efficiency of the carriers. Thus, photoluminescence (PL) spectra and

23



electrochemical impedance spectroscopy (EIS) Nyquist plots of the samples were carried out, as shown in Figure 5. A lower PL intensity implies a lower carrier recombination efficiency under irradiation, because PL emission results from the recombination of excited electrons and holes.47,48 As shown in Figure 5a, compared with pristine ZnGa2O4, the doped samples have lower PL intensity, indicating their enhanced separation efficiency of electron-holes; VO-B/N-ZGO shows the best carrier separation with the weakest PL emission. The PL lifetimes of samples were conducted to further investigate the efficiency of carrier separation. As shown in Figure S2, the mean lifetimes were obtained as 5.82 ns, 10.07 ns, 21.26 ns and 26.32 ns for ZGO, N-ZGO, VO-B-ZGO and VO-B/N-ZGO respectively. The longer lifetime indicates the lower carrier recombination, which is consistent with the steady-state fluorescence measurement. EIS Nyquist plots also illustrate that VO-B/N-ZGO shows the fastest interfacial charge transfer characteristic compared with ZGO, N-ZGO, and VO-B-ZGO samples (Figure 5b). Although it is inevitable that an unpaired electron exists in N or B-doped ZnGa2O4, because N and B atoms have 5 and 3 valence electrons, respectively, which may lead to new recombination centers for carriers in the photocatalyst. However, oxygen vacancies serving as electrons donor generated in our work may provide remaining electrons to passivate the single electrons present in monoatomic-doped samples to some extent. This charge compensation would weaken the formation of new recombination centers and benefit the photocatalytic activity.19 However, the passivation to a single electron in monoatomic-doped photocatalysts is seriously limited by concentration and position relationship between the heteroatoms 24


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and oxygen vacancies. Single electrons from substitutional nitrogen and interstitial boron in VO-B/N-ZGO could be effectively passivated by both the formation of B-N bond and compensation of the remaining electrons from oxygen vacancies, which may play a great role to achieve the best separation and migration efficiency for the VO-B/N-ZGO sample. The carrier density could influence the dynamic property and photocatalytic performance of the photocatalysts, which may be affected by doping. Therefore, the Mott-Schottky measurement, which revealed the differences in the donor density of several photocatalysts, was carried out, as shown in Figure 5c. According to the Mott-Schottky relationship in the linear part of Csc-2(E) plot9: # !"

=

2 E − +,- − kT/q $%& εε(

where Csc is the space charge capacitance of the semiconductor, q is the elementary charge (for electron or hole behaving as +e or –e, respectively), Nq is the doping density (as donors or accepters for n-type or p-type semiconductor, respectively), ε is the dielectric constant of the semiconductor, ε0 is the permittivity of free space, E is the applied potential, Efb is the flat band potential, k is the Boltzmann constant, and T is the temperature. According to the above expression, the positive slope of all samples is the typical characterization for the n-type semiconductor34; the magnitude of the slope therefore reflects the donor density, and the slope decrease with the increase in donor density.23 There are little difference between the slopes of N-ZGO and ZGO, possibly indicating charge compensation of substitutional N and oxygen vacancies to some degree. The slopes of VO-B-ZGO and VO-B/N-ZGO were much 25



smaller than those of N-ZGO and ZGO, indicating that the donor density in VO-B-ZGO and VO-B/N-ZGO are much greater owing to the many oxygen vacancies generated by NaBH4 reduction, which promote carrier transfer and photocatalytic activity. 3.4. Theoretical calculation In order to better understand the effect of impurity states in doped ZnGa2O4, DFT calculations of the equilibrium structures and the corresponding electronic properties of all samples were conducted. The nitrogen and boron dopants are mainly introduced in the substitutional sites of monatomic-doped ZnGa2O4, and in the substitutional and interstitial sites, respectively, in VO-B/N-ZGO. Oxygen vacancies were introduced along with the doping process. Moreover, all of the samples manifest the characterization of the n-type semiconductor. Based on the semiconductor physics, the substitutional N atom , VO, substitutional B atom and interstitial B atom doping would provide one accepter (-1e-), two donors (+2e-), three accepters (-3e-) and five donors (+5e-) for ZnGa2O4 semiconductor respectively. According to the above overall consideration of experimental results, the doped models were constructed with NO+VO, BO+2VO, and NO+Bi+VO as doping unit of N-ZGO, VO-B-ZGO, and VO-B/N-ZGO respectively. The GGA-PBE functional was employed to achieve the equilibrium structure of the cubic spinel ZnGa2O4 (56 atoms). All of the doped models were constructed based on the fully relaxed pristine ZnGa2O4. For N-ZGO, one oxygen atom was replaced by one nitrogen atom and one oxygen atom was removed from the ZnGa2O4 crystal 26


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structure, which was labeled as NO+VO. For VO-B-ZGO, two oxygen atoms were removed to form vacancies, and one substitutional boron atom was introduced, which was labeled as BO+2VO. For VO-B/N-ZGO, substitutional nitrogen, interstitial boron, and one oxygen vacancy were introduced in ZnGa2O4 lattice labeled as NO+Bi+VO. Almost all possible codoping positions were tested, and the most stable structures were obtained as shown in Figure S1. For the NO + VO model, the length of three N-Ga bonds are 1.916 Å for two N-Ga bonds and 1.961 Å for one N-Ga bond, respectively; the length of the N-Zn bond is 1.929 Å. The bond lengths of N-Ga and N-Zn are shorter than those of the O-Ga and O-Zn bonds with 2.008 Å and 1.976 Å in the pristine ZnGa2O4, respectively. Because of the attraction exerted by the adjacent VO, the substitutional N atom shifted toward the VO direction, which results in different N-Ga bond lengths. Moreover, owing to the strong attraction of two VO, the BO+2VO model showed a larger lattice distortion and one of the initial B-Ga bond disappeared along with a new B-Ga bond formed at a different location. For the NO+Bi+VO model, the substitutional N atom bonded with three Ga atoms (with N-Ga bond lengths of 2.071 Å, 2.066 Å, and 2.502 Å) and one Zn atom (with the N-Zn bond length of 1.969 Å); furthermore, the N atom also bonded with the adjacent B atom at the interstitial site, with a B-N bond length of 1.462 Å. The band structure and partial density of states (PDOS) were calculated based on the corresponding optimized configurations.

27



Figure 6. (a) Energy bands and (b) partial density of states (PDOS) of pristine, N-doped, VO-modified B-doped, and VO-modified B/N-codoped ZnGa2O4 samples.

For pristine ZnGa2O4, the valence band maximum (VBM) consists mainly of O 2p states; the conduction band minimum (CBM) is contributed by Ga 4s states with a minority of Zn 4s and O 2p states (Figure 6). It is consistent with the previously reported results20, and the calculated energy band gap (4.28 eV) is in good agreement with the experimental results (4.1–4.5 eV).2-4,49 In contrast with pristine ZnGa2O4, several impurity levels are incorporated in the doped samples, as shown in Figure 6, which may contribute to the extended light response range. Moreover, the impurity states induced by doping decrease the transition energy of electron to the conduction band in the doped samples, which may result in more electrons participating in photoreaction under the same irradiation condition. 28


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Four impurity levels are introduced in the midgap for N-doped ZnGa2O4. The three ones at the top of VBM predominately originate from the N 2p states, which are fully occupied at about 4.3 eV from CBM with negligible Eg change compared with that of pristine ZnGa2O4. The higher impurity level was contributed by the mixed states of N 2p, Zn 4s, and Ga 4s states, which crossing the Fermi level is a half-filled level. The presence of Zn 4s and Ga 4s states in the midgap may be induced by the effect of oxygen vacancy and the remaining electrons of VO are redistributed to the Zn and Ga orbitals. The hybridization of Zn 4s, Ga 4s and N 2p orbitals may imply the charge compensation that Zn 4s and Ga 4s serving as donors provide electron to the electron-deficient N 2p orbital. However, this charge compensation is inefficient and a single electron still could not be passivated, which would capture electron transition from the valence band serving as the recombination center detrimental to the photocatalytic activity. Like N-ZGO, the semi-occupied level also occurs in VO-B-ZGO; it basically originates from the mixed states of Zn 4s and Ga 4s states, and is only about 0.7 eV from CBM. In comparison with the half-filled level in N-ZGO, the location closer to CBM for the one in VO-B-ZGO makes it easier to release the captured electrons and then transit to the conduction band to participate in the reduction reaction.50 The other three impurity levels are fully occupied states and mainly contributed by B 2p states and a few Zn 4s and O 2p states. The hybridization of B 2p, Zn 4s, and O 2p orbitals may also result from the partial charge compensation caused by the electrons transferred from oxygen vacancies donors to boron atom at the substitutional site. 29



However, the interaction of VO and boron in this system still does not passivate the single electron; however, the semi-occupied state closer to the conduction band would serve as electron sinks, which may help enhance the charge separation and photocatalytic activity. As for VO-modified B/N-codoped ZnGa2O4, four impurity levels were formed, as shown in Figure 6a. The three levels above the VBM mainly arise from the mixed states of N 2p states with small Ga 4s and B 2p states, and because of ~3.95 eV energy difference from the CBM, which are very close to the Eg value measured by DRS spectra. The one below the CBM is predominately contributed by the mixed states of Ga 4s and Zn 4s states, which may also originate from the charge redistribution induced by the oxygen vacancy that the remaining electrons are redistributed to the Ga 4s and Zn 4s orbitals. It is noteworthy that this impurity level is fully filled and is located below the Fermi level and no half-filled state is formed in VO-B/N-ZGO. This is probably because of the effective charge compensation among NO, Bi, and VO, especially for the formation of the B-N covalent structure with a B-N bond length of 1.462 Å, which passivate the single electrons from NO and Bi. The recombination centers generated from the half-filled state with a single electron were efficiently depleted in VO-B/N-ZGO, which are more favorable for the photocatalytic performance. 3.5. Photocatalytic H2 production

30


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Figure 7. (a) Photocatalytic hydrogen production rate of four samples (ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO), and (b) cycling test of VO-B/N-ZGO.

The photocatalytic activities of all ZnGa2O4 samples were evaluated by the ability of photocatalytic hydrogen generation from the Na2SO3 aqueous solution (Na2SO3 serves as the electron donor to sacrifice the holes) under UV-visible light and without any additional cocatalyst. As shown in Figure 7a, the hydrogen production rates of ZGO, N-ZGO, VO-B-ZGO and VO-B/N-ZGO are increased in turn; the rate of VO-B/N-ZGO is more than three times of that of pristine ZnGa2O4. The apparent quantum yield (QY) of samples was measured in the similar photocatalytic reaction conditions under 313 nm UV light. QY was calculated to be 0.17%, 0.30%, 0.51%, and 0.71% for ZGO, N-ZGO, VO-B-ZGO and VO-B/N-ZGO respectively. The doped samples, especially for VO-B/N-ZGO, manifest the dramatically enhanced quantum yield. However, it is very difficult for the doped samples to achieve efficiently photocatalytic hydrogen evolution under visible light, which probably because the photon energy absorbed in this region is smaller than their intrinsic bandgap (about 4.17 eV) and the photo-generated electrons involved in photocatalytic hydrogen

31



production is very few. According to the above analysis, the improved photocatalytic performance of doped samples may be mainly contributed by the enhanced carrier separation. As for monatomic-doped ZnGa2O4, VO-B-ZGO shows a better carrier separation and transfer efficiency compared with N-ZGO. The half-filled state was located near the conduction band in VO-B-ZGO, owing to the interaction of the oxygen vacancy with the B atom. Thus, the electrons trapped by this sink could be easily released to the conduction band with the available energy, which may be beneficial to the carrier separation. The semi-occupied state in N-ZGO lies approximately in the middle of the bandgap, and hence, more energy is needed to release the electrons captured by this impurity state to the conduction band participating in the proton reduction process. Moreover, oxygen vacancies generated in VO-B-ZGO contribute much more to the donor density, which facilitates carrier transfer and photocatalytic H2 production. For VO-modified B/N-codoped ZnGa2O4, owing to the efficient compensation effect caused by the formation of the B-N bond, VO-modified B/N-codoped ZnGa2O4 shows better charge separation efficiency than VO-B-ZGO. In addition, the largest donor density contributed by oxygen vacancies as well as interstitial boron would greatly promote the transfer efficiency. The excellent efficiencies of charge separation and transfer favor more electrons reached surface of VO-B/N-ZGO, which may involve in proton reduction reaction. Moreover, the recycling experiments (Figure 7b) reveal good stability for VO-modified B/N-codoped ZnGa2O4, showing a negligible reduction in photocatalytic H2 generation performance after four recycles. 32


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Figure 8. (a) photocatalytic H2 production with and without Pt deposition for ZGO, N-ZGO,

VO-B-ZGO, and VO-B/N-ZGO; (b) photocatalytic hydrogen production activity of VO-B/N-ZGO with different amount of cocatalyst Pt. (the loading amount of Pt was expressed in weight percent)

Photocatalytic H2 production involves three sequential steps: the semiconductor generates electron-hole pairs under light irradiation; electrons separate from holes and migrate to the surface of the semiconductor; and the electrons participate in water reduction on the surface.23 To increase the photocatalytic reaction efficiency, platinum was often used as a cocatalyst, which could provide active sites for proton reduction as previous studies have reported and improve the carrier separation owing to the Mott-Schottky barrier on the interface.1,23 According to a former report50, we deposited about 1.0 wt % platinum on the samples by photoreduction. However, as shown in Figure 8a, the four samples exhibited different trends after 1.0 wt % Pt deposition. The Pt cocatalysts obviously promote the photocatalytic hydrogen production performance of both pristine and N-doped ZnGa2O4, while almost no effect was observed on the hydrogen production of the VO-B-ZGO catalyst. More incredibly, Pt cocatalysts deposited on the

33



VO-B/N-ZGO produce a lower hydrogen production rate, which is quite different from the previously reported phenomena.23 To eliminate the possible impact of residual species (such as H2PtCl6, HCl/Cl-) after Pt deposition, photocatalytic H2 production of VO-B/N-ZGO (with Pt deposition) after adequate washing by deionized water was measured with Na2SO3 as sacrificial agent. As shown in Figure S3, there is almost no change for the photocatalytic H2 production of VO-B/N-ZGO (with Pt deposition) before and after washing, which all performed much lower H2 evolution rate compared with VO-B/N-ZGO without Pt deposition. The photocatalytic hydrogen production by VO-B/N-ZGO with different amounts of Pt depositions was measured to investigate the role of Pt deposition amount (as presented in Figure 8b). On increasing the amount of platinum, photocatalytic hydrogen production performance deteriorates gradually. Thus, we infer that some defects may be formed on the VO-B/N-ZGO surface, which can serve as reduction sites. In the platinum deposition process, Pt (+4) would be more likely to get electrons at the active sites and then would be reduced to Pt (0) covering the reactive centers. Therefore, more surface active sites will be consumed with the increase in Pt deposition on VO-B/N-ZGO, which deteriorates the photocatalytic activity. Similar phenomena were observed for photocatalytic CO2 reduction and photocatalytic conversion of nitrogen to ammonia system.21,24 Wang and Qu et al.24 reported that layered polyhedron SrTiO3 showed improved photocatalytic CO2 conversion in the absence of a co-catalyst because of the enhanced charge separation and surface activation by boron doping. Shiraishi et al.21 found that the defects (Ti3+ 34


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species on the vacancy of bridging oxygen) on the surface of TiO2 serve as active sites for N2 reduction. The photocatalyst exhibits very high photocatalytic activity without any noble metal cocatalysts and produce much smaller amounts of NH3 after the noble metal particles were loaded on it. ESR measurements of VO-B/N-ZGO with/without Pt deposition were carried out to investigate the effect of VO on surface activation as shown in Figure S4. There is a slight reduction of VO after 1 wt% Pt deposition, which may result from Pt covering some VO on the surface. Considering that VO-B/N-ZGO with Pt deposition produces a much lower hydrogen production rate, VO may have some contributions but not the key factor to surface activation. The active species possibly originate from the synergy of VO and other defects that need to be further studied in our future work.

4. CONCLUSIONS In summary, we prepared VO-modified B/N-codoped ZnGa2O4 using NaBH4 as the origin of B and oxygen vacancy and using NH3 as the N source. It appears that the additional absorption band extends the full spectrum of visible light, owing to the synergistic effect of impurity atoms and oxygen vacancies. Because of the efficient charge compensation by B-N bond formation, there is no semi-occupied state in VO-B/N-ZGO that could serve as the recombination center, which promotes charge separation. The large donor density contributed by oxygen vacancies and interstitial boron atoms facilitate the transfer of carriers. Rich active centers generated during the doping process enhance the efficient proton reduction, which may result in excellent

35



photocatalytic H2 production even without any additional noble metal cocatalyst. The obvious improvement of carrier separation efficiency and surface reaction efficiency is the main reason for the excellent photocatalytic H2 evolution for VO-B/N-ZGO.

AUTHOR INFORMATION Corresponding author

*E-mail: [email protected]

Notes

The author declares no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21173131) and the Taishan Scholar Project of Shandong Province.

Supporting Information Supporting Information Available: (Structure models, Calculation of formation energy, PL lifetime, Photocatalytic H2 evolution of VO-B/N-ZGO with Pt deposition before/after washing, and ESR spectra of VO-B/N-ZGO with/without Pt deposition) This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC GRAPHICS

45



Figure 2. SEM, TEM, and HRTEM images of samples: (a), (e), and (i) pristine ZnGa2O4; (b), (f), and (j) Ndoped ZnGa2O4; (c), (g), and (k) VO-modified B-doped ZnGa2O4; (d), (h), and (l) VO-modified B/Ncodoped ZnGa2O4. 160x176mm (300 x 300 DPI)


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Figure 3. XPS spectra of (a) N 1s, (b) B 1s, (c) Zn 2p, (d) Ga 3d, and (e) O 1s; (f) ESR spectra of ZGO, NZGO, VO-B-ZGO, and VO-B/N-ZGO. 160x180mm (300 x 300 DPI)



Figure 4. (a) UV–vis diffuse reflectance spectra (DRS) and (b) plots of (αhν)2 vs. (hν) for ZGO, N-ZGO, VOB-ZGO, and VO-B/N-ZGO. 80x120mm (300 x 300 DPI)


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Figure 5. (a) Photoluminescence (PL) spectra, (b) electrochemical impedance spectroscopy (EIS) Nyquist plots, and (c) Mott-Schottky plots of pristine, N-doped, VO-modified B-doped, and VO-modified B/N-codoped ZnGa2O4 samples. 80x178mm (300 x 300 DPI)



Figure 6. (a) Energy bands and (b) partial density of states (PDOS) of pristine, N-doped, VO-modified Bdoped, and VO-modified B/N-codoped ZnGa2O4 samples. 160x102mm (300 x 300 DPI)


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Figure 7. (a) Photocatalytic hydrogen production rate of four samples (ZGO, N-ZGO, VO-B-ZGO, and VOB/N-ZGO), and (b) cycling test of VO-B/N-ZGO. 160x60mm (300 x 300 DPI)



Figure 8. (a) photocatalytic H2 production with and without Pt deposition for ZGO, N-ZGO, VO-B-ZGO, and VO-B/N-ZGO; (b) photocatalytic hydrogen production activity of VO-B/N-ZGO with different amount of cocatalyst Pt. (the loading amount of Pt was expressed in weight percent) 160x61mm (300 x 300 DPI)


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TOC GRAPHICS 140x84mm (300 x 300 DPI)


N-Codoped ZnGa2O4 Nanospheres

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