Theoretical and Experimental Investigations on Effects of Native Point

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Theoretical and Experimental Investigations on Effects of Native Point Defects and Nitrogen Doping on Optical Band Structure of Spinel ZnGaO 2

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Yuguo Xia, Ting Wang, Xiaozhou Zhao, Xiuling Jiao, and Dairong Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12326 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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The Journal of Physical Chemistry

Theoretical and Experimental Investigations on Effects of Native Point Defects and Nitrogen Doping on Optical Band Structure of Spinel ZnGa2O4 Yuguo Xia, Ting Wang, Xiaozhou Zhao, Xiuling Jiao* and Dairong Chen* National Engineering Research Center for Colloidal Materials, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, China

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Abstract Impurity states in semiconductors are so important for optical correlated properties that understanding the role of native and imported defects is essential to design highly active semiconductors. Here, the structural and electronic properties of ZnGa2O4 with native atomic substitution, oxygen vacancy as well as imported N doping are firstly investigated by first-principles calculations. It is demonstrated that native atomic substitution is energetically unfavorable while the most stable existence forms for N doping and O vacancy are N-1 and O2+ states in most cases under various chemical environment. The band structures and DOS reveal that the photochemical property is only significantly enhanced by 2Ns doping with a greatly increases of VBM relative to Fermi level while single N atom doping or import of O vacancy or simultaneously import of N doping and O vacancy just generates impurity states in the band gaps. Moreover, experimental characterizations including XPS and DRS spectra confirm above theoretical results, and optical calculations further illustrate the effects of defects for light absorption. Our results will be helpful to understand the effects of native point defects and external nitrogen doping on spinel ZnGa2O4, and design its band gap with desired optical property.

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1. Introduction Since the decisive role of band structure in optical related properties, such as optical, photocatalysis, photoluminescence and photoelectrocatalysis, rational design and controlled synthesis of semiconductors with proper valence band maximum (VBM) and conduction band minimum (CBM) are necessary to obtain semiconductors with ideal light responses

1-2

. Generally, the electronic band gap of pure semiconductor is

exactly equivalent to the optical band gap measured from experimental UV-Vis absorption while its defected structures are not 3. Thus, numerous experimental and theoretical researches have been done to investigate the influences of the key parameters of native and doped defects in semiconductors, such as formation energies, donor and acceptor levels, optical transition energies, as well as atomic and electronic structure on the final optical and electronic properties. For example, Wang et al prepared H-doped black titania with a core/shell structure, and the H-doping was found to be favorable to eliminate the recombination centers and induce the localized surface plasma resonance as noble metal 4. Gong’s group reported a controllable approach to fabricate N-doped graphene, in which the type of electronic transmission carrier was changed from traditional p-type to n-type, in favor of producing graphene-based devices 5. Therefore, understanding and controlling the native and doped defects in semiconductor are essential to tune corresponding optical and electronic properties. As a wide band gap semiconductor, ZnGa2O4 has received much attention due to its excellent persistent luminescence activity as phosphors and broad application as 3



photocatalyst for fuel production and organic contaminant degradation

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6-8

. However,

limited by its native large band gap (ca. 4.4 eV), ZnGa2O4 exhibits nearly no response in sunlight spectrum, hindering its practical utilization in photoelectric conversion as photocatalysts, such as, water splitting. Therefore, exploring a feasible strategy to reduce the optical band gap of ZnGa2O4 is necessary. Although nitriding process for the precursor or intermediate products of ZnGa2O4 is found to be an effective approach to improve the absorption and excitation properties of ZnGa2O4 in sunlight, the phase of final photocatalyst is mainly transferred into wurzitic (Ga1-xZnx) (N1-xOx) multiple-metal oxynitrides phase instead of retaining spinel ZnGa2O4 9-10. In regard to N-doped spinel ZnGa2O4 phase, Lobo’s group reported a combination method of sol-gel based precursors and low temperature nitriding to realize phase retention and N doping 11-12. In addition, the formation and effect of N-doping in ZnGa2O4 was investigated by only a few theoretical studies, which illustrated electronic structural changes are mainly due to the N impurity states

7,13

. To the best of our knowledge,

some basic questions about properties of defected spinel ZnGa2O4 are still unsolved, such as: (i) the effect of chemical environment to the formation of N doping is unrevealed; (ii) import of O-vacancies is confirmed the role of endowing new donor levels and increasing states of density in many other semiconductors

14-16

, but the

effect of O vacancy as well as other native defects on the final electronic property of ZnGa2O4 has not been presented; (iii) the optical properties have not been calculated, especially for the defected structures. Therefore, the structural, electronic and optical properties of spinel ZnGa2O4 with native point defects and N doping are necessary to 4


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be further investigated. Herein, the effect of native point defects involved in ZnGa2O4 and N-doping on the structural stability, electronic and optical properties is calculated. The chemical environment is considered in all the formation calculations of the defects. The effects of N doping and O vacancy in corresponding stable structures are further investigated in term of density of states and band structures, which provides a fundamental understanding between the electronic structures and imported defects. Moreover, optical properties of ZnGa2O4 with N-doping and O vacancy are studied, which also confirms the results of the electronic structural calculations. Meanwhile, ZnGa2O4 and its defected samples are experimentally prepared and characterized, which draws a detailed picture between the imported defects and final electronic and optical properties along with above theoretical results.

2. COMPUTATIONAL SECTION 2.1 Computational methods and models All the spin-polarized density functional theory calculations are performed using the Vienna ab initio simulation package 17-18. The generalized gradient approximation (GGA) in the scheme of Perdew-Bueke-Ernzerhof (PBE) is used for the exchange correlation functional, and the wave functions are expanded by the projector augmented wave (PAW) method

19-21

. Interactions between the valence electrons and

the ion core are represented in 4d105p2, 4d105s25p1, 2s22p4 and 2s22p3 orbital for Zn, Ga, O and N atoms, respectively. The cutoff energy is set to 500 eV which is high enough to ensure that no Pulay stresses occur within the cell during geometry

5



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relaxation. To achieve the accurate density of the electronic states, the k-space integrations are done with Monkhorst-Pack grids with a 6×6×6 k-points in the Brillouin zone of the cubic ZnGa2O4. Before single-point energy calculation and other N-doped structures optimization, geometry of ZnGa2O4 is first optimized. For all the geometry relaxations are performed until the residual forces on each ion converged to be smaller than 0.02 eV/Å. Herein, we only concern the geometry stability and electronic structure changes of ZnGa2O4 with the native atomic antisite, O vacancy and import of N doping. Therefore, the concentration of O vacancy or N impurity as well as possible phase transition produced by above defects is not considered in this paper. On the basis of the optimized geometry, √2×√2×1 ZnGa2O4 crystal cell is employed to construct the atomic antisite O vacancy and N doping configuration, corresponding 112-atoms supercell. 2.2 On-site Coulomb interaction parameter U Due to the well-known limitation of general GGA as well as the local-density approximation (LDA) method in describing the electronic and structural properties of semiconductors, the formation energy of vacancy and impurity energy state induced by foreign elements doping may not be correctly presented. Therefore, all DFT calculations in ZnGa2O4 system are performed with GGA+U method to correct the strongly electronic correlation

22

. To reproduce the experimental band gap, proper

on-site Coulomb interaction parameter U is essential. In this study, the U value is derived from first principle atomic calculations as described in the following.

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Firstly, the atomic U values for single ions (Uion) of Zn2+, Ga3+ and O2- are determined from formula (1) and (2), respectively 23.

U ion ( Zn 2+ or Ga 3+ d ) = [ E (3d 10 ) − E (3d 9 )] − [ E (3d 9 ) − E (3d 8 )]

(1)

U ion (O 2− p ) = [ E (2 p 6 ) − E (2 p 5 )] − [ E (2 p 5 ) − E (2 p 4 )]

(2)

Where E(3dn) represents the total energy of the isolated Zn and Ga atoms with n electrons occupying the d sub shell while E(2pn) represents the total energy of the isolated O atom with n electrons occupying the p sub shell. Noteworthily, convergence for O2- cannot be achieved and the Uion value for O2- is estimated from an extrapolation scheme as shown in Fig.1a. Based on above Uion values, the Hubbard parameter U to describe the strong onsite Columb repulsion among the localized transition 3d metal should be calculated by Uion/ε, where ε is the experimental optical dielectric constant. However, the ε value for a given material is sometimes unknown. Herein, the values of U for bulk ZnGa2O4 can be reasonably estimated from U = αUion where the multiplied parameter α is used to reproduce the experimental band gap of ZnGa2O4, and similar method to obtain rational U value has been successfully applied in the DFT+U calculations of TiO2, SrTiO3, Ag3PO4 and Ta3N5

24-27

. By multiplying

the optimum value of αopt (0.7) as shown in Fig.1b, final U values for Zn2+, Ga3+ and O2- are 12.6 eV, 17.3 eV and 4.4 eV, respectively.

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Fig.1 (a) Estimated U values for single ions and atoms of O, F and Ne with several occupations. (b) Determination of αopt to reproduce experimental band gap. Using above obtained U values, the optimized lattice constants of bulk ZnGa2O4 are a = b = c = 8.042 Å, α = β = γ = 90 °, which agrees within the effective range comparing with experimental value (JCPDS: 38-1240, a = b = c = 8.335 Å, α = β = γ = 90 °). Moreover，the calculated band gap is 4.36 eV which agrees well with the experimental value

(ca. 4.4 eV)

28

. To verify the validity of the obtained U for

reproducing the band gap of bulk ZnGa2O4, the optical band gap and density of state are further calculated by GGA and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional with ω equals to 0.2 Å-1 as comparison 29. 2.3 Formation energy of point defects Based on the geometrical configuration of cubic bulk ZnGa2O4, all the Zn atoms have the same coordination environment with four neighboring oxygen atoms while all the Ga atoms are coordinated with six neighboring oxygen atoms. Herein, three categories of defects in ZnGa2O4 crystal can be expected, that is, atomic substitution and interstitial doping, atomic antisite, and atomic vacancy which are also considered in other compounds as point defects

30

. According to above defect category, native

8


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defect, namely oxygen vacancy V (q = 0, +1 and +2), substitutional nitrogen at an oxygen site N (q = -1 and 0, Note: considering the charge balance and electronegative of N element, only N atom with -1 and 0 charge is reasonable), interstitial nitrogen, and antisite atom (ZnGa, GaZn and ZnGa-GaZn) are considered. The stability of the charged defects, Xq, is investigated by formation energy, which is calculated as follows 31.

∆H f ( X q ) = Etot ( X q ) − E tot (bulk ) − µr + µa + q( EVBM + EF )

(3)

Where Etot(Xq) and Etot(bulk) are the total energy of the defect X with charge q and bulk ZnGa2O4, respectively, µr,a is the atomic chemical potential of the elements removed or added to the system. Here, EVBM and EF is the valence band maximum and Fermi level, respectively. Thus, the value of q, i.e., the charge state of a specific defect Xq, can be deduced from the dependence of ∆Hf on EF. The chemical potentials represent the growth conditions of atoms, which must be carefully treated to investigate relative stability of introduced defects. Under thermal equilibrium growth condition, ZnGa2O4 should satisfy equation (4). Moreover, precipitation of secondary phases such as ZnO, β-Ga2O3 (β-Ga2O3 is the most stable phase during α, β and γ-Ga2O3, with the smallest formation enthalpy

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should be

excluded. Simultaneously, the chemical potential of each element must not be larger than corresponding chemical potential of the bulk element. In equations, all the conditions can be summarized as follows. f ∆µZn + 2∆µGa + 4∆µO = H ZnGa 2O4

(4)

f ∆µZn + ∆µO ≤ H ZnO

(5) 9



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2∆µGa + 3∆µO ≤ H βf -Ga2O3

(6)

∆ µ Zn ≤ 0 ; ∆µ Ga ≤ 0; ∆µ O ≤ 0

(7)

3. RESULTS AND DISCUSSIONS 3.1 Electronic structure of bulk ZnGa2O4 To investigate the electronic structure of bulk ZnGa2O4 and evaluate the accuracy of GGA+U method on description of electronic orbital interactions, the band structure as well as density of states (DOS) is separately calculated by GGA, GGA+U and HSE06 methods. As shown in Fig.2a, the band gap calculated by GGA method is only 2.26 eV which confirms its well-known limitation in deal with strongly correlated system. The band gap values given by both GGA+U and HSE06 method are in good agreement with the experimental value. Moreover, the band edge of ZnGa2O4 is further determined by experimental ultraviolet photoelectron spectroscopy (UPS) to identify the accuracy of the results given by three methods as shown in Fig.S1, which illustrates GGA method gives the most accurate valence band position compared to the experimental value (-1.13 eV) while HSE06 method reproduces a relatively poor value in valence band position in spite of matching the best in optical band gap as shown in Fig.2c comparing with GGA and GGA+U methods (Fig.2b). Except for the difference in band gap, the dispersion of energy in Brillouin zone obtained by three methods is similar. The DOS plots corresponded to the band structures are shown in Fig.2(d-f), revealing that the valence band maximum (VBM) primarily derives from O atom while the conduct band minimum (CBM) basically originates from Ga and O atoms, mainly Ga 4s and O 2p orbitals to form dispersed sp-class band. Moreover, the 10


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CBM locates in the Γ point and VBM locates in the K-Γ region, illustrative of its indirect semiconductor nature, which is consistent with previous DFT results. Overall, considering the calculation accuracy in reproducing experimental results as well as expense in computing resource, herein, GGA+U method is adopted for all the calculations.

Fig.2 Band structures and DOS of pure bulk ZnGa2O4 obtained by GGA (a, d), GGA+U (b, e) and HSE06 (c, f) methods, repectively. The primitive bulk ZnGa2O4 is adopted and Kpath in the reciprocal space for the band structure calculation is generated by Aflow-online tool as shown in Fig.S2 33. The photochemical reduction and oxidation ability is highly correlative with its effective masses of electron and hole. To give insight into the photochemical property of bulk ZnGa2O4, effective masses of eletron and hole along possibly directions connecting with high-symmetry points in the Brillouin zone (Fig.S3, ESI) are calculated. Although the effective masses of eletron and hole of ZnGa2O4 have been calculated by other reports

34-35

, the localized d orbitals involved in the Zn and Ga 11



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atoms are not considered. Herein, since the CBM locates at the high-symmetry point Γ, effective masses of the electron along three directions, Γ→X, Γ→K and Γ→L are calculated. Due to the VBM locating in the K-Γ region instead of a high-symmetry point, effective masses of the hole are calculated along Γ→K, Γ→L and L→U direction. The effective mass of carriers relative to the electron rest mass m0 can be calculated from the following equation.

1 1 ∂2 E = 2 2 m * ћ ∂k

(8)

where ћ is the reduced Planck constant and

can be obtained from the second

derivative value of the curve in valence band or conduction band. The effective masses for carriers are summarized in Table 1, revealing that effective mass of electron is an order of magnitude lower than effective mass of hole and this larger discrepancy in effective masses can be ascribed to the anisotropy of the band structure of ZnGa2O4. Effective mass of electron along Γ→L direction is smaller than Γ→X and Γ→K, illstrative of fastest electron migration rate. In constrast, effective mass of hole along L→U is relative small, suggesting excellent mobility of hole along this direction and inferring the good oxidation abillity of (121) as exposed plane. ZnGa2O4 nanocrystal with low index (100), (110) and (111) are experimentally synthesized to investigate oxidation property in our previous report

36

. Herein, this

problem can be interpreted into comparsion of the effective mass of hole along certain directions, where (100), (110) and (111) correponds to the effective masses of hole along Γ→K, Γ→X and Γ→L directions, respectively. The order of effective masses of hole is (110) (Γ→X, 13.14m0) > (111) > (100), which illustrate crystal with (100) as 12


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exposed plane should exhibit optimal oxidation property and is consistent with previously reported experimental results. Carriers electron hole

0.660 (Γ-X) 5.906 (K-Γ)

Effective mass 0.664 (Γ-K) 9.480 (Γ-L)

0.658 (Γ-L) 3.876 (L-U)

Table 1 Effective masses of eletron (me) and hole (mh) relative to the eletron rest mass (m0) along different directions in the Brillouin zone. 3.2 Relative stability and optical band gaps of ZnGa2O4 with point defects To sustain conditions for the formation of ZnGa2O4, certain conditions as described in equations (4-8) must be kept, and these conditions determine the phase space of atomic chemical potential for Zn, Ga and O under which it is possible to synthesize ZnGa2O4 crystals as indicated by the shaded area in Fig.3. Whereas addtional competing phases must be considered, ZnO, β-Ga2O3 and GaN as induced by N doping are enough to give a reliable estimate of the phase space of allowed chemical potentials. The upper boundary in Fig.3 is given by the equations: ∆ = (H − ∆ ) and ∆ = ((H − H −3∆ )/2) . Therefore, point A in Fig.3

corresponds to the condition in equilibrium with Zn bulk (∆µZn = 0 eV, zinc rich) and the poorest oxygen possible (∆µO = -2.897 eV, ∆µGa = -1.912 eV). Point C is oxygen rich (∆µO = 0 eV), but is the poorest possible condition in zinc, with ∆µZn = -2.897 eV and ∆µGa = -4.809 eV. The lower boundary of the shaded area is defined by following conditions: ∆ = ((H!" −3$ )/2) and ∆ = (H − H!" − # #

∆ ). Point B corresponds to equilibrium with Ga bulk and more oxygen poor than Point A, where ∆µO = -3.087 eV and ∆µZn = -0.168. Finally, point D corresponds to 13



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the condition poorest in zinc with ∆µO = 0, ∆µGa = -4.630 eV and ∆µZn= -3.255 eV. Due to many defects involving nitrogen, its atomic chemical potential from N2 gas is also calculated, which is µN = -3.125 eV and taken as constant in all conditions (always nitrogen rich).

Fig.3 Range of possible stoichiometric regimes (shaded area) for the atomic chemical potentials of zinc, gallium and oxygen to sustain stable growth of ZnGa2O4. The formation energies of the native and nitrogen-related defects in ZnGa2O4 are calculated using eqn.9 for the conditions indicated by points A, B, C and D as shown in Fig.4. To minimize the effect of spurious electrostatic interactions in charged defect calculations due to the perodic cell approximation, Leslie-Gillan correction is included to evalute the Madelung energy of a point charge in our supercell screened by the dielectric constant of ZnGa2O4 (eqn. S1)

37

. The upper limit for these

conrrection for our supercell varies from 0.1 eV for single charged defects to 0.4 eV for defects of charge 2, and detailed computional procedure is suppled in suportting information. Possible structures of native and N-related defects in ZnGa2O4 involve interstitial N, substitutional atom GaZn and ZnGa, atomic antisite ZnGa-GaZn, double substitutional N, N substation in O site, O vacancy and coexist of N substitution and 14


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O vacancy are shown in Fig.S4. The formation energies of all possible defected structures without charge and formation energies of charged defects versus Fermi level in different chemical potential are shown in Fig.4. Several important features of ZnGa2O4 with native or N defects are domonstrated. First, as shown in Fig .4a, all three

structures

with

interstitual

N

occupying

different

interstices

are

thermodynamically unstable. As one of the structural degrees of freedom for spinel structure(AB2O4), getting knowledge of the inversion phenomena, that is, cation exchange between A- and B- site is fundamental to understand the variation in the physical and chemical property of different spinel composition. Herein, all the formation energy for ZnGa and ZnGa-GaZn substitutional structures are positive, indicating their thermodynamically unstable feature and low possibility to form native metal substitution. In addition, although substitutional GaZn structure is energetically favorable only in Ga-poor conditions (-0.207 eV in both A and C), the chemical potential of Ga has reached the limit, which required nearly no Ga elements involved. That is obviously impractical in the synthesis process of ZnGa2O4. All the band structures of metallic ions substitutional geometries are calculated as shown in Fig.S5, which indicates the approach to generate defects of metallic ions substitution involved in ZnGa2O4 is infeasible. Besides, substitutional N doping is energetically more favorable than interstitial one in our calculation, therefore, substitutional N is adopted as the existence form of N doping. Based on above elementary results of points defects in ZnGa2O4, herein, formation energies of oxygen vacancy (V = 0, +1 and +2) and substitutional N (N = 15



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-1 and 0) are mainly investigated as shown in Fig. 4(b-d). The thermodynamic transition levels ε (q1/q2) for oxygen vacancy and substitutional N, which are denoted as kinks on crossing lines, are determined as ε (2+ /0) and ε (0/-1) with respect to the VBM, respectively, and detailed transition energies are labeled in the inset figures. Notably, the formation energy of substituted N defects is fairly low in formation energy relative to the native defect throughout most of the band gap. The Fermi level is determined by a charge balance between donors and acceptor. The most plausible donor that compensates the acceptors in defected ZnGa2O4 seems to be oxygen vacancy with 2+ charge state acting as a double donor and most stable substituted N defects in all conditions are negatively charged throughout the entire band gap, which are also shallow donors of electrons to the crystal. However, charge neutrality must be satisfied, which requires charged oxygen vacancy and N substitution must exist in certain region of Fermi level without considering charged metal vacancies on account of no metallic ions peaks in different valence state found. Considering the possibility of coexistence of N doping and oxygen vacancy, the formation energy of this situation is also calculated, which reveals only O-poor conditions (condition A, -0.26 eV and condition B, -0.45 eV) are favorable.

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Fig.4 Plots of formation energies of defects in ZnGa2O4 versus Fermi level for uncharged defects (a) and charged defects (b-d) in conditions A to C (both condition C and D are O-rich) indicated in Figure 3. Native defects involve substitutional atom (GaZn and ZnGa), atomic antisite (ZnGa-GaZn) and oxygen vacancy (V = 0, +1 and +2). N-related defects involve interstitial N, single substitutional N (N = -1 and 0), double substitutional N, coexist of N substitution and oxygen vacancy (NsOv). The specific element related to these defects is indicated next to the corresponding curve. Condition A is intermediate Ga/O poor and Zn rich; condition B corresponds oxygen-poorest, Ga rich; condition C is Ga poorest, O-rich; and condition D is Zn poorest, O-rich. The vertical dashed lines indicate the calculated band edges of the crystal with EVBM shift to 0. The slope of the curves indicated the charge state and the kinks in the curves indicate transition of charge states. To get insight into the effects of substitutional nitrogen defect and oxygen vacancy 17



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in the band structure of ZnGa2O4, projected density of states of defected ZnGa2O4 and corresponding band structures are calculated. Based on above analysis of defect formation energy in various conditions, herein, only the most stable situations are considered which involves substitutional nitrogen defect in -1 charge (N-1), oxygen vacancy in +2 charge (O2+), and coexistence of N substitution and oxygen vacancy (NsOv). Besides, the situations of ZnGa2O4 doped with two substituted or interstitial N atoms have been investigated by Fan in details

13

. Here, only ZnGa2O4 with two

substituted N atoms is considered for comparison. As shown in Fig. 5a, three isolated impurity levels localize in the band gap of N-1 substituted ZnGa2O4 below the Fermi level, which are mainly contributed by N 2p states and O 2p states. Although the host band gap is about 4.33 eV which is nearly unchanged comparing with that of ZnGa2O4 bulk (4.36 eV), the impurity states can serve as light absorption center to reduce the band gap. For structure with O2+ vacancy in Fig. 5b, only one isolated level localizes in the band gap close to the CBM which is mainly comprised by O 2p states and empty states. The oxygen vacancy narrows the band gap as in other oxide semiconductors

14

, which decreases to 3.54 eV, indicating that import of oxygen

vacancy is beneficial to reduce the optical band gap of ZnGa2O4. For structure simultaneously with substituted N and oxygen vacancy in Fig. 5c, two isolated spin asymmetric levels formed in both sides of Fermi level separately, which are mainly consisted by empty states mixing with a little O 2p states. Noteworthily, the impurity states located in band gap are spin asymmetric, which implies the bulk polarity may assist the conversion of orbital angular momentum promote the electron separation 18


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during light excitation, considering the indirect band gap semiconductor nature of ZnGa2O4. Besides, the energy gap of impurity states is only 0.63 eV, which may serve as electron excited center, however, as well as recombination center for excited electron and hole. The host optical band gap is still up to 3.89 eV, and the electrons in VBM are deep donors in consider of its position relative to the Fermi level. Furthermore, as shown in Fig. 5d, the DOS reveals semi-metallic property in ZnGa2O4 with two N atoms substitution, in which the band gap reduces to 2.91 eV, indicating its good photoexcited property.

19



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Fig.5 Projected DOS and corresponding band structures of most stable defected structures N-1 substitution (a), O2+ vacancy (b), NsOv (c) and 2Ns (d). 3.3 Experimental investigation of ZnGa2O4 with N defect and O vacancy Except for above DFT simulation results, experimental verification is still demanded due to the possibility of impurity states serving as both excited center and recombination center for electron and hole. Four samples are prepared to simulate

20


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single N atom doping, multiple N atom doping, O vacancy, and simultaneously with N doping and oxygen vacancy, which is named as ZGO_Ns, ZGO_xN, ZGO_Ov and ZGO_NsOv, respectively. (Detailed synthesis procedures are supplied in supporting information).

Fig.6 (a) XRD patterns, (b) SEM images, (c) EDX elemental mapping image and (d) UV-vis diffuse reflectance spectra of ZGO_Ns, ZGO_xN, ZGO_Ov and ZGO_NsOv. The rhombus labeled in (a) are assigned to hexagonal ZnO phase, and inset of (d) is corresponding optical image. As shown in Fig.6a, XRD patterns show that all the samples maintain ZnGa2O4 phase and small amount of ZnO phase appears in ZGO_Ov and ZGO_NsOv samples which is mainly due to sublimation feature of Zn element under high temperature. The morphology and microstructure of as-prepared sample are investigated by SEM as shown in Fig. 6b, which reveals the primary structure units in ZGO_Ns and ZGO_xN are nanoparticles with a size of ca.20 nm while it increases to ca.150 nm in ZGO_Ov and ZGO_NsOv after calcination under 800 °C. The elemental distribution in four samples is analyzed by elemental mapping of EDX spectrometry as shown in Fig. 6c, 21



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revealing that the elements of Zn, Ga and O are distributed uniformly across final product while the N element shows obvious distribution only in ZGO_xN sample. To verify whether the N doping and O vacancy are imported in our synthesis condition, XPS measurement is performed as shown in Fig. 7. The binding energy of O1s of four samples locates nearly the same positions, while the binding energy of Zn 2p and Ga 2p in ZGO_Ns and ZGO_xN shifts towards high energy direction and it shifts towards low energy direction in ZGO_Ov and ZGO_NsOv, which illustrates the import of N doping in ZGO_xN and O vacancy in ZGO_Ov and ZGO_NsOv, because of the Coulomb effect. Besides, the N 1s XPS spectrum also illustrates the import of N doping in ZGO_Ns and ZGO_xN. Interestingly, no N1s signal is detected in ZGO_NsOv even the experimental condition for N doping is the same as preparing ZGO_xN. The formation energy of N doping in ZGO_Ov with different charged states in various chemical condition is further calculated as shown in Fig.S6, which illustrates it is energetically unfavorable to import N doping in the existence of O vacancy. The optical image in Fig. 6d reveals only ZGO_xN sample exhibits visible light absorption and the peak labeled by arrow is typical absorption peak of ZnO

38

.

Although the optical band gap cannot be fitted through Kubelka-Munk transform due to the interference of ZnO, it still can be estimated by the extension of absorption edge, where the optical band gaps are ca.4.01 eV, 3.65 eV, 3.59 eV and 2.64 eV, respectively, and are in accordance with above DFT results. To further reveal the effect of N doping and O vacancy on electronic structure of ZnGa2O4, room-temperature photoluminescence (PL) spectra were measured. As shown in Fig.S7, the lowest PL intensity in ZGO_xN electrocatalyst illustrates that most of defect states in ZGO_xN are nonradioactive recombination center, and indicates its smallest recombination rate of electrons and holes. The gradually red shift of PL spectra for ZGO_xN, ZGO_NO and ZGO_Ov relative to ZGO_Ns illustrates the decrease of band gap, which corresponded to the DRS results in Figure 6d. Moreover, the similar intensity and peaks position for ZGO_NO and ZGO_Ov also indicates that N doping cannot be imported under the condition of oxygen vacancy. 22


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Fig.7 XPS spectra of ZGO_Ns, ZGO_xN, ZGO_Ov and ZGO_NsOv compared with original ZGO, (a) Zn 2p, (b) Ga 2p, (c) O 1s, (d) N 1s. To further investigate the influence mechanism of N doping and O vacancy on the optical absorption of ZnGa2O4, the optical properties involving absorption index curve and dielectric function are calculated as shown in Fig.8. Except for the position of absorption peak in Fig.8a, the N doping and O vacancy do not change a lot of the relative absorption intensity. The absorption peak of ZGO_xN exhibits obvious red shift, while peaks of ZGO_Ns, ZGO_Ov and ZGO_NsOv are not, which is consistent with above DRS result. Besides, the absorption index curve of ZGO_NsOv also illustrates even simultaneously import of N doping and O vacancy cannot improve the light absorption of ZGO in theory, despite the ZGO_NsOv sample is not successful prepared due to the unfavorable in formation energy. The dielectric function in Fig.8b 23



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illustrates only in the ZGO_Ns the essential excitation energy remarkably decreases, which is also consistent with above DOS analysis.

Fig.8 The absorption coefficient (a) and imaginary part of dielectric functions (b) of ZGO_Ns, ZGO_xN, ZGO_Ov and ZGO_NsOv. Combining above theoretical and experimental results, the optical absorption can be greatly improved by multiple N atom doping which increases the position of VBM relative to the Fermi level, reflecting in the decrease of optical band gap. Compared with ZGO_xN, single N atom doping only generates impurity states between VBM and Fermi level, and these states do not enhance the optical absorption of ZGO as proved by dielectric functions calculation and may also serve as the combination center for electron and hole. Although import of O vacancy generates impurity states which are consisted of O 2p states and empty states, the band gap between VBM and impurity states is still too large to improve the visible light absorption, which indicates importing oxygen vacancy is not an effective approach to enhance the photocatalytic property of ZGO. Moreover, import of N doping under the condition of oxygen vacancy is calculated to be energetically unfavorable, which is also confirmed by the EDS and XPS analysis, and the optical absorption will not increase even if 24


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simultaneously import of N doping and oxygen vacancy as indicated by calculation of absorption coefficient and dielectric functions.

4. CONCLUSIONS In summary, first-principle calculations followed by experimental validation are carried out to investigate the effect of native point defects and imported N doping on the final photochemical property of ZnGa2O4. The calculations reveal the metallic atom substitutions involved in ZnGa2O4 are energetically unfavorable and have no effects for enhancing photocatalytic property. Besides, Oxygen vacancy in the states of O2+ and N doping in the states of N-1 are found to be most stable states under various chemical environments. Importing of oxygen vacancy, single N doping do not decrease the band gaps while only multiple N atom doping will greatly enhance photocatalytic property, which are confirmed by both calculation and experimental characterization. Moreover, theoretical calculations also indicate it is energetically unfavorable to import N doping in the existence of O vacancy, giving a proper explanation of our DRS results. We hope the calculated results in this paper are helpful to further investigate the ZnGa2O4 with defects.

SUPPORTING INFORMATION Experimental procedure for synthesis of ZGO_Ns, ZGO_xN, ZGO_Ov and ZGO_NsOv samples, physicochemical characterization, possible geometries with native and N-related point defects and band structures of atomic substitution and antisite for Zn and Ga atoms.

AUTHOR INFORMATION 25



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Corresponding Author Email: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 21701099), Natural Science Foundation of Shandong Province (Grant ZR2016BQ22) and the Taishan Scholars Climbing Program of Shandong Province (tspd20150201). REFERENCES 1. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W., Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919-9986. 2. Chen, Y.; Xi, J.; Dumcenco, D. O.; Liu, Z.; Suenaga, K.; Wang, D.; Shuai, Z.; Huang, Y.-S.; Xie, L., Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. ACS Nano 2013, 7, 4610-4616. 3. Malingowski, A. C.; Stephens, P. W.; Huq, A.; Huang, Q.; Khalid, S.; Khalifah, P. G., Substitutional Mechanism of Ni into the Wide-Band-Gap Semiconductor Intao4 and Its Implications for Water Splitting Activity in the Wolframite Structure Type. Inorg. Chem. 2012, 51, 6096-6103. 4. Wang, Z., et al., H-Doped Black Titania with Very High Solar Absorption and Excellent Photocatalysis Enhanced by Localized Surface Plasmon Resonance. Adv. Funct. Mater. 2013, 23, 5444-5450. 5. Guo, B.; Liu, Q.; Chen, E.; Zhu, H.; Fang, L.; Gong, J. R., Controllable N-Doping of Graphene. Nano Lett. 2010, 10, 4975-4980. 6. Yixi, Z.; Jumpei, U.; Setsuhisa, T., Enhancement of Red Persistent Luminescence in Cr 3+-Doped ZnGa2O4 Phosphors by Bi2O3 Codoping. Appl. Phys. Express 2013, 6, 052602. 7. Liu, Q.; Wu, D.; Zhou, Y.; Su, H.; Wang, R.; Zhang, C.; Yan, S.; Xiao, M.; Zou, Z., Single-Crystalline, Ultrathin ZnGa2O4 Nanosheet Scaffolds to Promote Photocatalytic Activity in CO2 Reduction into Methane. ACS Appl. Mater. Inter. 2014, 6, 2356-2361. 8. Teng, Y.; Song, L. X.; Liu, W.; Xu, Z. Y.; Wang, Q. S.; Ruan, M. M., Monodispersed Hierarchical ZnGa2O4 Microflowers for Self-Powered Solar-Blind Detection. J. Mater. Chem. C 2016, 4, 3113-3118. 9. Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K., Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295-295. 10. Ohno, T.; Bai, L.; Hisatomi, T.; Maeda, K.; Domen, K., Photocatalytic Water Splitting Using Modified Gan:Zno Solid Solution under Visible Light: Long-Time 26


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Table of Contents Graphic

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Theoretical and Experimental Investigations on Effects of Native Point

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