Bandgap Engineering of the g-ZnO Nanosheet via CationicâAnionic

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Bandgap Engineering of the G-ZnO Nanosheet via Cationic-Anionic Passivated Codoping for Visible-Light-Driven Photocatalysis Xu kai Luo, Guangzhao Wang, Yuhong Huang, Biao Wang, Hongkuan Yuan, and Hong Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03616 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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The Journal of Physical Chemistry

Bandgap Engineering of the g-ZnO Nanosheet via Cationic-Anionic Passivated Codoping for Visible-Light-Driven Photocatalysis Xukai Luo, Guangzhao Wang, Yuhong Huang, Biao Wang, Hongkuan Yuan, and Hong Chen∗ School of Physical Science and Technology, Southwest University, Chongqing 400715, China E-mail: [email protected] Abstract The graphene-like ZnO (g-ZnO) nanosheets were synthesized and shown to exhibit highly photocatalystic activity for the degradation of RhB under ultraviolet irradiation. In this work, we utilize cationic-anionic passivated codoping to explore the potential of g-ZnO nanosheet for the design of efficient water redox photocatalysts by employing density functional theory calculations with the hybrid HSE06 functional. Our calculations show that anion-cation passivated codoped systems are not only more favorable than the corresponding monodoping in g-ZnO nanosheet due to the Coulomb interactions, but also effectively reduce the band gap without introducing unoccupied states which accelerate the electron-hole recombination. The charge compensated P–Sc and C–Zr codoped g-ZnO nanosheets are energetically favorable for hydrogen evolution but not insufficient to produce oxygen, indicating that they could serve as Z-scheme photocatalysts. The C–Ti, N–Y, and P–Y codoped systems may be potential potocatalysts for photo-electrochemical water splitting to generate hydrogen due to their appropriate band gaps and band edge positions. In particular, the charge compensated P–Y

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codoped g-ZnO nanosheet has the most excellent stability and the largest absorption region of visible light among these codoped systems. Further, we show that P–Y passivated codoping can reduce the overpotentials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of g-ZnO nanosheet, indicating that the OER or HER on P-Y codoped g-ZnO nanosheet can be easier driven by the irradiation generated holes or electrons.

INTRODUCTION Development of efficient strategies for obtaining renewable energy sources has drawn significant attention in the field of science and technology in recent years. Hydrogen is an excellent energy carrier and is regarded as one of the new generation of clean energy fuels which can be used in fuel cells, chemical industries, etc. 1–3 Photocatalytic water splitting with various photocatalysts under visible light irradiation is a promising and potential technology for the production of hydrogen. 4,5 There have been taken numerous efforts to search for efficient photocatalysts for this intention during the past few decades. Zinc oxide (ZnO) with the band gap (3.37 eV) is one of the most promising materials for solar cells, photocatalysis, and photovoltaics due to its unique characteristics such as excellent optical and piezoelectric properties. 6–10 In the recent years, low dimensional ZnO nanostructures including nanowires, nanobelts, and nanotubes have been widely studied because of their potential applications in optoelectronic, nanoelectronic, and photonic devices. 11–15 Particularly, two-dimensional (2D) systems show extraordinary electronic and optical properties which are different from the corresponding bulk counterparts. 16 In general, 2D materials can minimize the migration distance of photogenerated carriers to facilitate their migration to the surface and have large surface areas available providing photocatalytic reaction sites. 17 Therefore, the research interests on environmentally friendly 2D ZnO nanosheet systems have significantly increased. Recently, some experimental studies show that the graphene-like ZnO nanosheets (g-ZnO) have been successfully synthesized. Tushce 2 ACS Paragon Plus Environment

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et al. 18 have found that planar ZnO sheets of two-layer-thickness can be deposited on a Ag (111) surface where Zn and O atoms are arranged in planar sheet with hexagonal graphiticlike structure. Claeyssens et al. 19 also have presented experimental evidence that the thin films of ZnO (001) surface display graphene-like structure. Interestingly, Kang et al. 20 have reported that the synthesized g-ZnO nanosheets exhibit excellent photocatalytic activity for the degradation of RhB under ultraviolet irradiation. However, the g-ZnO nanosheet has been reported to be a semiconductor with a direct wide band-gap of 3.25 eV, 21 which greatly restricts its visible light adsorption and the efficiency of hydrogen evolution. Therefore, several strategies should been adopted for the band gap effective reduction of g-ZnO nanosheet so as to increase its visible light absorption and utilization. Recently, doping of foreign elements into semiconductors is an effective strategy to tune the electronic structures and magnetic properties. 21–27 Typically, 2D g-ZnO nanosheets exhibit a nonmagnetic - antiferromagnetic - ferromagnetic transition by doping nonmetal atoms. 21 An et al. 25 have investigated the properties of SnS2 nanosheets doped with Cu, which indicates that the visible-light photocatalytic activity is much higher than pristine SnS2 nanosheets. Later on, Khokhra and coworkers 27 have synthesized Fe-doped g-ZnO nanosheets and observed that the ZnO monolayer nanosheets with low dopant concentration exhibit excellent photocatalytic activity in degradation of organic pollutants. However, monodoping usually introduces partially unoccupied donor states or acceptor states, which accelerates the electron-hole recombination process. Besides, uncompensated defects, which can trap efficient charge carriers, may form because of the charge imbalance. 28,29 To overcome the above-described issues, the charge compensated codoping has been found to be a promising strategy to modify the band structures of semiconductor photocatalysts. 30–33 To the best of our knowledge, there have been no theoretical reports about the effect of anion-cation passivated codoping on the photocatalytic activity of g-ZnO nanosheet. In this work, in order to obtain a systematic understanding of charge compensated codoped g-ZnO nanosheet, the density functional theory calculations have been adopted



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to investigate if the anion-cation passivated codoping of C–Ti, C–Zr, N–Sc, N–Y, P–Y, and P–Sc could effectively improve the photocatalytic ability and modify the band structures of g-ZnO nanosheet. We have mainly calculated the geometry structures, defect binding energies, minimum defect formation energies, electronic structures, and optical properties of the doped g-ZnO monolayer nanosheets. In addition, the band edge positions of pure and codoped g-ZnO nanosheets with respect to the standard hydrogen electrode potentials and the red shift of optical absorption edge of codoped systems with reference to that of the pure g-ZnO system have been investigated in our work. At last, we also explore the effect of doping concentrations on the electronic structure of g-ZnO nanosheet.

COMPUTATIONAL METHOD In the present study, all the spin-polarized density functional theory (DFT) calculations have been employed to obtain optimized geometry structures by using the Vienna ab initio simulation package (VASP). 34,35 The exchange correlation effects and the interactions between valence electrons and ion cores have been described by the generalized gradient approximation (GGA) 36 of the Perdew-Burke-Ernzerhof (PBE) scheme 37 and the projector augmented wave (PAW) 38 potential, respectively. The valence states of Zr(4s2 4p6 5s2 4d2 ), Ti(3p6 3d2 4s2 ), O(2s2 2p4 ), N(2s2 2p3 ), P(3s2 3p3 ), C(2s2 2p2 ), Sc(3s2 3p6 4s2 3d1 ), Y(4s2 4p6 5s2 4d1 ), and Zn(3d10 4s2 ) have been considered to construct the potential. The Monkhorst-Pack 39 grid k points are adopted for the Brillouin zone sampling. The cutoff energy of 400 eV and the tolerance for energy convergence of 1.0×10−5 eV have been utilized to obtain the optimized geometry configurations. A 15 Å-thick vacuum layer has been used in the pure and modified ZnO monolayer models so as to wipe out the interactions among periodic images. The calculations of geometry optimization and electronic properties for ZnO monolayer adopt the 3×3×1 and 5×5×1 k-point meshes, respectively. The monodoped and codoped systems have been constructed with a (4×4×1) hexagonal supercell, which contains 16 O atoms and 16 Zn atoms.


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To model the cation and anion doped g-ZnO nanosheets, we use one metal atom (TM = Ti, Zr, Sc, and Y) to substitude one Zn atom and one nonmetal atom (NM = C, N, and P) to replace one O atom, respectively. For the charge compensated codoped systems, the doping metal atoms have been considered to substitute the Zn atom (A site in Figure 1), while the nonmetal atoms are to replace the O atom at the B site (Figure 1), which is the nearest position to the substitutional atom. This is because that this kind of configuration has the lowest energy. The corresponding doping concentrations for monodoped and codoped g-ZnO nanosheets are 3.125 at.% and 6.25 at.%, respectively. We would also like to point out that the van der Waals interaction has been not considered in the g-ZnO nanosheet systems due to they only have a monolayer. The Heyd-Scuseria-Ernzerhof (HSE06) 40,41 hybrid functional, a more accurate and time-consuming approach, has been adopted to calculate electronic and optical properties. The ion-core interaction consists of two parts in this functional: one for the short-range(SR), one for the long-range(LR). The exchange-correlation energy is expressed as

P BE,SR P BE,LR HSE SR EXC = χEX (µ) + (1 − χ)EX (µ) + EX (µ) + ECP BE ,

(1)

where χ and µ stand for the mixing coefficient and the screening parameter, respectively. The mixing exchange parameter of 0.25 and screening parameter of 0.2 Å

−1

are adopted in

all calculations. In addition, The more reliable tetrahedron method with Blochl correction 42 has been adopted to calculate the density of states (DOS) and projected density of states (PDOS).



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RESULTS AND DISCUSSION The Pristine g-ZnO Nanosheet The initial g-ZnO nanosheet structure is obtained by cleaving from the (001) surface of optimized wurtzite phase of ZnO bulk structure, consistent with the reported structure of the experimentally synthesized ZnO monolayer nanosheet. The 4×4×1 optimized supercell structure of g-ZnO nanosheet is presented in Figure 1. It possesses a planar graphitic honeycomb structure where the Zn-O bond distance is 1.88 Å, which is in good agreement with the experimental value of 1.92 Å 18 and other theoretical values of 1.86 Å by Tu et al. 43 and 1.90 Å by Topsakal et al. 44 The DOS and PDOS for pure g-ZnO nanosheet are displayed in Figure 2. It can be found that the pure g-ZnO monolayer nanosheet is a non-magnetic semiconductor with a band gap of 3.30 eV, which agrees well with the previous theoretical study, 21 indicating the availability of our calculational methods. The valence band maximum (VBM) is dominantly comprised of O 2p and Zn 3d states, while the conduction band minimum (CBM) is mainly contributed by Zn 4s states.

Monodoping in g-ZnO Nanosheet The case of anionic monodoping (C, N, and P) and cationic monodoping (Ti, Zr, Sc, and Y) have been investigated for giving insight into the effect of monodopants on the characteristics of electronic structure of g-ZnO nanosheet. The calculated DOS and PDOS of g-ZnO nanosheet monodoped with nonmetal atoms are illustrated in Figure 3. For the NM (C, N, and P) monodoped g-ZnO nanosheet, the valence electron numbers for these systems have one or two fewer than that of pure system, so the spin-polarization effect has been considered in our calculations. In the case of C doping, the calculated DOS and PDOS of C doped g-ZnO nanosheet is displayed in Figure 3a. It is found that the VBM is mainly contributed by the mixtures between C 2p and O 2p states whereas the CBM is still dominated by Zn 4s states. Moreover, the impurity states are close to the valence band of g-ZnO 6 ACS Paragon Plus Environment

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monolayer that mainly derived from hybridization between C 2p and O 2p states, which is due to the C 2p orbital’s energy higher than the O 2p orbital’s energy. Herein, we define that the effective band gap is the energy difference between these unoccupied states and the CBM, which is calculated as 1.16 eV that is beneficial to absorb visible light. However, the unoccupied impurity states may act as carrier recombination centers. In the case of N doping, the DOS and PDOS for N doped system are displayed in Figure 3b, which displays that substituting the O atom with one N atom does not result in a significant change in band gap of g-ZnO nanosheet. The occupied N 2p states couple with O 2p states in the valence band and some unoccupied N 2p states appear in conduction band. We believe that a higher doping concentration of N atom may lead to the formation of unoccupied impurity states in band gap. The characteristics of DOS and PDOS of P doped in g-ZnO nanosheet are presented in Figure 3c. The impurity states originated from the mixing of P 3p and O 2p states appear in the band gap. Therefore, the effective band gap is 1.71 eV. Next, we have studied the case of cationic monodoping. In the case of Ti or Zr monodoped g-ZnO nanosheet, as Ti or Zr atom has two more valence electrons than Zn atom, the Fermi level of Ti or Zr doped system steps into conduction band, which indicates that this system is a n-type semiconductor. The DOS and PDOS for Ti and Zr doped g-ZnO nanosheets are plotted in Figure 4a,b, indicating that the valence band edges for them are both mainly contributed by O 2p and Zn 3d states, while the bottom of the conduction band of Ti doped system is mainly dominated by Zn 4s states and that of Zr doped g-ZnO nanosheet is dominantly comprised of Zn 4s and Zr 3d states. However, though the effective band gaps ( VBM to occupied impurity states ) for Ti and Zr monodoped g-ZnO nanosheets are 3.2 and 2.69 eV, respectively, Ti or Zr doped system suffers from partially occupied states appearing at the CBM, which can increase the electron-hole recombination and suppress the photocatalytic activity. For the Sc or Y doping system, due to the presence of one more electron in the valence shell of Sc or Y than that in Zn, so Sc and Y doped systems are both n-type semiconductors. The calculated DOS and PDOS of Sc and Y doped g-ZnO nanosheets



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are presented in Figure 4c,d. Although the effective band gaps for Sc and Y doped g-ZnO nanosheets are reduced by these occupied states, the presence of these impurity states and the formation of charge uncompensated defects are harmful for photocatalytic performance.

Passivated Codoping in g-ZnO Nanosheet To overcome above-mentioned these shortages, the anion-cation passivated codoping simultaneously has been considered to maintain the charge balance. In our work, we consider six different codoping configurations, viz.: (C+Ti), (C+Zr), (N+Sc), (N+Y), (P+Sc), and (P+Y). The DOS and PDOS for C–Ti, C–Zr, N–Sc, N–Y, P–Sc, and P–Y codoped systems are respectively presented in Figure 5a–f. It is interesting to observe that the dopant levels are fully occupied in the band gaps of codoped g-ZnO nanosheets, which leads to a significant reduction of band gaps in these systems. The calculated band gaps of C–Ti, C–Zr, N–Sc, N–Y, P–Sc, and P–Y codoped g-ZnO nanosheets are 2.44, 2.31, 2.99, 2.63, 2.40, and 2.35 eV, respectively. Here, we take P–Y codoped g-ZnO nanosheet as an example. It is worth notice that the fully occupied level is contributed by the mixing of P 3p, O 2p, Zn 3d, and Y 3d states and the effective band gap (occupied impurity level to CBM) is 2.34 eV. The VBM is dominated by O 2p and Zn 3d states, while CBM consists mainly of Zn 4s, Zn 3d, and Y 3d states. Due to the existence of the impurity level between the valence band and conduction band, the electrons can be easily excited from the VBM to dopant level and subsequently to CBM. Therefore the the existence of these dopant levels is beneficial for extending the light adsorption into the visible region.


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Defect Formation Energy To explore the proper growth conditions of doped g-ZnO nanosheets, the defect formation energy (Ef ) has been calculated by using the following equation 45,46 Ef (X) = E(X) − E(pure) −

∑

ni µi ,

(2)

i

where E(X) and E(pure) represent the total energies of the g-ZnO nanosheets with and without dopants, respectively. ni is the number of atoms added to (ni > 0), or taken from (ni < 0) the pure system to form doped g-ZnO nanosheets. µi is the chemical potential of the corresponding atom, whose value depends on the synthetic environment. Under the conditions of equilibrium between g-ZnO nanosheet and the reservoirs of the Zn and O atoms, their chemical potential must follow the relation:

µZn + µO = µZnO(monolayer) ,

(3)

where µZnO(monolayer) is the chemical potential of the g-ZnO monolayer, which can be obtained from the total energy of the g-ZnO monolayer per unit cell. As the chemical potentials of elements Zn and O can not exceed the those of Zn bulk and O2 molecule, thus the following relationships must be obeyed:

µmin Zn ≤ µZn ≤ µZn(bulk) ,

(4a)

µmin ≤ µO ≤ µO(gas) . O

(4b)

Moreover, to form the g-ZnO nanosheet spontaneously, the minima of µZn and µO satisfy µmin Zn = E(Znn On ) − E(Znn−1 On ),

(5a)

µmin = E(Znn On ) − E(Znn On−1 ), O

(5b)



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where E(Znn On ), E(Znn−1 On ), and E(Znn On−1 ) are, repectively, the total energies of Znn On system constructed by n primitive cells without and with Zn and O defect. Here, only the minimum defect formation energies of doped g-ZnO nanosheets have been calculated in this work, i.e., the defect formation energies take the minimum value when µZn is equal to µZn(bulk) . For dopants, we suppose that the N2 molecule, P4 molecule, graphite, bulk Ti, bulk Zr, bulk Sc, bulk Y, and bulk Zn act as N, P, C, Ti, Zr, Sc, Y, and Zn reservoirs, respectively. The calculated minimum defect formation energies for different doped systems are presented in Figure 6 and summarized in Table 1. It has been found that the minimum defect formation energies of the NM-monodoping is positive whereas these of TM-monodoping is negative, indicating metal atoms should be much easier than nonmetal atoms to dope into g-ZnO nanosheet. Due to the strong Coulomb interaction between the n- and p-type dopants, the formation energies of charge compensated codoping are lower than those of monodoping counterparts, which indicates that the codoping is feasible experimentally. Importantly, the solution of C, N, and P impurities in the host lattice position can be improved via doping metal atoms. Moreover, the P–Y codoped system has the lowest defect formation energy among all the codoped configurations, indicating that P–Y atom pairs have the strongest electrostatic attraction. We also calculated the defect pair banding energies of codoped systems to explore the relative stability of codoped g-ZnO nanosheets with respect to their monodoping systems, which is defined as Eb = E(A) + E(B) − E(pure) − E(A + B),

(6)

where E(pure), E(A), and E(B) are total energies of pure, A, and B monodoped systems, and E(A + B) denotes the total energies of the (A–B) codoped systems with the same supercell, respectively. As shown in Table 1, the defect binding energies of all the codoped g-ZnO nanosheets are positive, which means that anion-cation passivated codoping are more favorable in energy in comparison with the corresponding monodoping in g-ZnO nanosheet. This is due to the strong Coulomb interaction between the dopants and other atoms in those 10 ACS Paragon Plus Environment

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systems.

Bandedge Alignment While modifying the band gap of a photocatalyst, it should note that the band edge of the system must be located in appropriate position, i.e., the water reduction level and oxidation level must lie between the CBM and VBM to satisfy the thermodynamic criterion. On the basis of the band edge positions, some materials are suitable for only hydrogen production and some materials are good for only oxygen production, which is called the Z-scheme photocatalysis system. Here, the calculated CBM and VBM positions of pure and doped g-ZnO nanosheets with respect to standard hydrogen electrode potentials are illustrated in Figure 7. For doped systems, we only consider codoped g-ZnO nanosheets, as these cases lead to a significant reduction of band gaps without bringing unoccupied impurity states in band gaps. In this calculation, the band alignment is determined adopting the vacuum potential as a common reference. From the graph, it can be seen that the change of standard hydrogen electrode potentials with pH values. Due to the standard reduction potential of H+ /H2 is EH + /H2 = −4.44 eV+pH × 0.059 eV with respect to the vacuum level 47 and the difference between the hydrogen reduction potential and the water oxidation potential is 1.23 eV, 48 so the water oxidation potential of O2 /H2 O is EO2 /H2 O = −5.67 eV+pH × 0.059 eV. The CBM and VBM of pure g-ZnO nanosheet straddle the water redox level, indicating that pure g-ZnO nanosheet can split water into hydrogen and oxygen. For the P–Sc and C–Zr codoped systems, the VBM are above the water oxidation level, which shows that these systems are unfavorable for oxidation production. However, the P–Sc and C–Zr codoped systems may be considered for Z-scheme photocatalysis. The VBM of the N–Y, P–Y, N–Sc, and C–Ti codoped systems are below the water oxidation level when the pH greater than 8.5, 10, 4.5, and 11, respectively. Although these systems can produce oxygen and hydrogen in proper pH value of the electrolyte, the band gap of N–Sc codoped g-ZnO nanosheet is still too large, which is not beneficial to absorb visible light. N–Y, P–Y, and C–Ti codoped 11 ACS Paragon Plus Environment


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g-ZnO nanosheets have appropriate band gaps and desirable band edge positions, which can act as excellent photocatalysts for splitting water. In the experimental study, the g-ZnO nanosheet has been successfully synthesized at room temperature in a pH value of 13.2 which prepared by the mixture of 0.1 M Zn(NO3 )2 and 0.4 M NaOH aqueous solutions. 49 The band edge potentials of pure and doped g-ZnO nanosheets have been calculated in our work. The calculated CB and VB edges of pure g-ZnO nanosheet are located at -2.67 eV and -5.97 eV, respectively. The CBM is 0.99 eV above the water reduction level, whereas the VBM is 1.08 eV below the water oxidation level in the pH value of 13.2. For the doped systems, we have just discussed three cases of N–Y, C–Ti, and P–Y passivated codoped g-ZnO nanosheets because of the appropriate band gaps and band edge positions. In the case of N–Y (C–Ti) codoped g-ZnO nanosheet, the VBM is located at -5.17 eV (-5.02 eV), and the CBM is located at -2.54 eV (-2.60 eV). Thus, the VB edge and the CB edge are respectively upshift by 0.8 eV (0.95 eV) and 0.13 eV (0.07 eV) compared with the pure system, which indicates that the reduction capacity has improved obviously. In the case of P–Y codoped system, the CB edge is 0.94 eV above the water reduction potential, and the VB edge is 0.18 eV below the water oxidation potential. Thus, these band alignments show that both the oxidation and reduction reactions of water are thermodynamically feasible in N–Y, C–Ti, and P–Y passivated codoped systems.

Optical Properties We further investigate optical adsorption spectra of N–Y, P–Y, and C–Ti codoped and pure g-ZnO nanosheet. The frequency dependent dielectric function, which is expressed by the equation ε(ω) = ε1 (ω)+iε2 (ω), has been calculated. In our calculation, the real part (ε1 (ω)) is obtained by using Kramers-Kronig transformation and imaginary part (ε2 (ω)) has been calculated by summing over a large amount of empty states. The adsorption coefficient α


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(ω) satisfies the following relation 51

α(ω) =

√

√√ 2ω

ε21 (ω) + ε22 (ω) − ε1 (ω).

(7)

Figure 8 displays the calculated adsorption coefficient of four different configurations with the doping concentrations of both 6.25 at.% and 4 at.%. The absorption curve of pure g-ZnO nanosheet is restricted to the ultraviolet region, while these codoped systems (N–Y, P–Y, and C–Ti) can enhance the adsorption behavior of g-ZnO nanosheet due to reduction in band gap. Moreover, the enhancement of adsorption behavior is more obvious in the case of P–Y codoped system than other codoped systems. There are two obvious optical absorption peaks in the wavelength ranging from 300 to 500 nm in this codoped system because of the presence of more empty states in the conduction band by P–Y codoping.

Effect of Concentration To explore the effect of doping concentration on the electronic structure, a larger 5×5×1 supercell has been carried out. In this case, three codoped configurations (N–Y, P–Y, and C–Ti) have been considered because these codoped systems have appropriate band gaps without introducing unoccupied impurity states in band gaps. The dopant concentration corresponds to 4 at.%. In order to facilitate discussion, we call them as N–Y (I), P–Y (I), and C–Ti (I). The calculated minimal defect formation energies for N–Y (I), P–Y (I), and C–Ti (I) codoped systems are -2.17, -2.48, and 1.08 eV, respectively. The minimal defect formation energies of these codoped cases are smaller than that of the corresponding codoped systems with the doping concentration of 6.25 at.%, indicating these codoped systems are easier to form in lower doping concentration. On the other hand, the formation energies almost remain unchanged with the increase of supercell size, which indicates that 4×4×1 supercell structure adopted in our work is reasonable. The DOS and PDOS of (N–Y (I), P–Y (I), and C–Ti (I)) codoped g-ZnO nanosheets with the doping concentration of 4 at.%



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are indicated in Figure 9. The calculated band gaps are 2.66, 2.36, and 2.48 eV, respectively. The VBM of all cases are dominantly composed of the O 2p and Zn 3d states. The CBM for C–Ti (I) codoped g-ZnO nanosheet consists mainly of Zn 4s and Ti 3d states, while for other two codoped systems that are dominated by Zn 4s and Zn 3d states. As shown in Figure 7, the band edge positions with respect to standard hydrogen electrode potentials are almost similar in both dopant concentrations, which makes them be promising phtotcatalytic materials for water splitting under visible-light irradiation. In addition, the adsorption shifts of N–Y (I), P–Y (I), and C–Ti (I) codoped g-ZnO nanosheets with doping concentration of 4 at.% toward visible region are also quite significant (Figure 8).

HER and OER Free Energy It is known that the band alignment is the basic requirement for water splitting. Though the photocatalyst satisfies the basic criterion, it might not act as efficient photocatalyst for H2 and O2 evolution due to the existence of high overpotentials. 52 The P–Y passivated codoped g-ZnO nanosheet has been demonstrated that it has the most excellent photocatalytic behavior in our work. So it is necessary to evaluate the overpotentials for individual half-cell reactions for the pristine and P–Y codoped g-ZnO nanosheet. Here, the model proposed by Norskov and coworkers 53 has been adopted to estimate the overall water splitting reaction. The free energy change in the oxidation and reduction reactions has been calculated as 52,53

△G = △E + △ZP E − T △S + △GpH + △GU ,

(8)

where the △E is reaction energy which can be obtained by DFT calculations. The △ZP E and △S are the differences in zero point energy and entropy, respectively. The △GpH is the free energy correction which is computed as △GpH = −kB T In(10)·pH ≈ −0.06 eV·pH, and the △GU = −eU (U is the electrode potential). The oxygen evolution reaction (OER) proceeds in four elementary reaction steps as fol-


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lows: A : ∗ + H2 O → ∗OH + (H+ + e− ), B : ∗OH → ∗O + (H+ + e− ),

(9)

C : ∗O + H2 O → ∗OOH + (H+ + e− ), D : ∗OOH → ∗ + O2 + (H+ + e− ), where * represents the adsorption site on the g-ZnO nanosheet and the *O, *OH, and *OOH stand for adsorbed intermediates. For each reaction step, the free energy difference is written as follows: 54

△GA = E∗OH − E∗ − (EH2 O − 1/2EH2 ) + (△ZP E − T △S 0 )A + △GpH − eU, △GB = E∗O − E∗OH + 1/2EH2 + (△ZP E − T △S 0 )B + △GpH − eU, △GC = E∗OOH − E∗O − (EH2 O − 1/2EH2 ) + (△ZP E − T △S 0 )C + △GpH − eU,

(10)

(11) (12)

△GD = E∗ − E∗OOH + (2EH2 O − 3/2EH2 ) + 4.92 + (△ZP E − T △S 0 )D + △GpH − eU. (13) The zero point energy and entropic corrections to the free energies for adsorbed intermediates and gas phase molecules at 298 K are shown in Table 2. The theoretical overpotential for OER or HER is independent of the pH and the potential values. This is because that the free energies obtained by adopting above equations (10)–(13) vary in the same way with pH and U, therefore the potential determining step remains the same. The theoretical overpotential for oxygen evolution at standard condition has been calculated by adopting the following equation: 54 ηOER = (GOER /e) − 1.23V,

(14)

where the GOER is the maximum free energy difference for the reaction steps at U = 0 (pH = 0 and T = 298 K). The calculated free energy diagram during OER at different potentials of pristine and P–Y codoped g-ZnO nanosheet is presented in Figure 10. We have firstly



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considered the situation without any external potential (U = 0) to simulate the condition that without any irradiation on the sample. In the first reaction step, the H2 O adsorbed g-ZnO nanosheet is transferred to *OH via releasing one protonated electron (H+ + e− ). For the pure and P–Y codoped g-ZnO nanosheet, the free energies are +1.74 and +1.15 eV, respectively. In the next step, both systems require relatively lower energy than the first step. For pure and P–Y codoped g-ZnO nanosheet, these are respectively +1.71 and +0.79 eV. In the third reaction step, the free energy change for the pure g-ZnO nanosheet is +1.36 eV and that of P–Y codoped system is 1.51 eV. In the final step, the *OOH releases one electron-proton pair along with the release of dioxygen (O2 ) from the g-ZnO nanosheet. It is found that the free energy change for this step is +0.11 eV (+1.41 eV) for the pure (P-Y codoped) g-ZnO nanosheet. Therefore, the overpotential of pure system is ηOER = 0.51 V and that of P–Y codoped g-ZnO nanosheet is 0.28 V, which indicates that P–Y passivated codoping can slightly reduce the overpotential for OER. As shown in Figure 10, the four elementary reaction steps for both systems are uphill in the situation without any external potential (U = 0.0 V). At an equilibrium potential of U = 1.23 V, the first, second, and third reaction steps still remain uphill for pure g-ZnO nanosheet though the fourth step becomes downhill. For the P–Y codoped system, the third and fourth steps are uphill at this potential. The increase of U can make all reaction steps favorable. For instance, the potential (U) is 1.74 and 1.51 eV, respectively, for the pure and P–Y codoped g-ZnO nanosheet, in which the four reaction steps are exothermic. In addition, we have also studied the hydrogen evolution reaction (HER) on g-ZnO nanosheet, and the HER mechanism satisfies the following steps: E : ∗ + (H+ + e− ) → ∗H,

(15)

−

F : ∗H + (H + e ) → H2 + ∗. +

In order to provide a quantitative estimate for the thermodynamic requirement for applied voltage for hydrogen evolution reaction (HER), the overpotential for hydrogen evolution has 16 ACS Paragon Plus Environment

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been calculated by taking the minimum absolute value of the *H binding free energy (△GH ) at 0 V vs. RHE. The free energy of the adsorption atomic hydrogen (△GH ) is calculated as follows: △GH = △EH + △EZP E − T △SH ,

(16)

where △EH , △EZP E , and △SH represent the hydrogen adsorption energy, the difference in zero point energy (ZPE) between the adsorbed hydrogen and hydrogen in the gas phase, and the entropy difference between the adsorbed state and the gas phase, respectively. The △SH is obtained by △SH = - 21 SH2 , where SH2 is the entropy of H2 in the gas phase at standard condition. Therefore, the △GH can be expressed as △GH = △EH + 0.24 eV. The calculated free energy diagram for HER of P-Y codoped and pure g-ZnO nanosheet is shown in Figure 11. The overpotential for the hydrogen evolution on the g-ZnO nanosheet is ηHER = 0.36 V and P–Y codoping reduces the overpotential to 0.05 V. A good catalyst for HER should satisfy the criterion that the |△GH | should closer to zero. Hence, we predict that the HER catalytic activity of g-ZnO nanosheet can be improved by P–Y passivated codoping.

CONCLUSIONS We have employed hybrid density functional calculations to explore the potential of modified g-ZnO nanosheet for the design of water splitting photocatalysts via anionic or cationic monodoping and anion-cation passivated codoping strategy. Our calculations indicate that the anionic or cationic monodoping will lead to the unoccupied states serving as recombination centers appear in the band gaps of doped g-ZnO nanosheets, which reduce the photocatalytic efficiency and suppress photocatalytic performance of g-ZnO nanosheets, while the anioncation passivated codoping can remove these unwanted states, and moreover, the codoped systems are energetically more favorable than their corresponding monodoped systems. The charge compensated P–Sc and C–Zr codoped g-ZnO nanosheets are energetically favorable for hydrogen evolution but not insufficient to produce oxygen, and thus, they may serve as 17 ACS Paragon Plus Environment


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Z-scheme photocatalysts. Even more interest is that the C–Ti, N–Y, and P–Y codoped systems may be potential photocatalysts for photo-electrochemical water splitting to generate hydrogen because of their appropriate band gaps, suitable band edge positions, and excellent visible light adsorption behaviors. In particular, the P–Y codoped g-ZnO nanosheet has a maximum shift toward the visible light region among these codoped systems even at lower doping concentration. Furthermore, our analysis on the OER and HER processes shows that the P–Y passivated codoping can reduce the overpotentials for OER and HER of gZnO nanosheet, which indicates that the OER or HER on P–Y codoped g-ZnO nanosheet can be easier driven by the irradiation generated holes or electrons. Overall, our simulation results may encourage experimentalists to synthesize highly efficient potocatalysts for water splitting.

Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant Nos.11645002, 11175146 and 10904125, and the Natural Science Foundation of Chongqing under Grant Nos. CSTC-2011BA6004 and CSTC-2008BB4253.

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Table 1: The calculated minimum defect formation energies (Ef ), the defect binding energies (Eb ), and the corresponding chemical potentials of O and Zn for doped g-ZnO nanosheets (in eV). Structure Ef Eb N 1.91 P 2.03 C 4.60 Y -2.71 Sc -2.65 Ti -1.56 Zr -1.12 N–Y -2.11 1.32 P–Y -2.42 1.75 N–Sc -2.21 1.47 P–Sc -0.74 0.11 C–Ti 1.06 1.97 C–Zr 1.19 2.29

△ µO -2.45 -2.45 -2.45 0 0 0 0 0 0 0 0 0 0

△ µZn 0 0 0 -1.95 -1.95 -1.95 -1.95 -1.95 -1.95 -1.95 -1.95 -1.95 -1.95



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Table 2: Zero point energy corrections and entropic contributions to the free energies at 298 K. Species H2 O H2 *O *OH *OOH

TS 0.67 0.41 0.00 0.00 0.00

ZPE 0.56 0.27 0.05 0.35 0.41


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Figure 1: 4×4×1 optimized supercell structure of ZnO nanosheet.


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DOS and PDOS (arb. unit)


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Total O 2p Zn 4s Zn 3d

-1

0

1

2

3

4

5

6

Energy (eV)

Figure 2: DOS and PDOS for pure g-ZnO nanosheet. The vertical black dashed line denotes the Fermi level.


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(a) DOS and PDOS(arb. unit)

Total C 2p O 2p Zn 4s Zn 3d

(b) DOS and PDOS(arb. unit)

Total N 2p O 2p Zn 4s Zn 3d

(c) DOS and PDOS(arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Total P 3p O 2p Zn 4s Zn 3d

-1

0

2

1

3

4

5

Energy (eV)

Figure 3: Calculated DOS and PDOS of g-ZnO nanosheets with (a) C (b) N, and (c) P monodoping. The vertical black dashed line denotes the Fermi level.




Total O 2p Ti 4s Ti 3d Zn 4s Zn 3d

(b) DOS and PDOS(arb. unit)

Total O 2p Zr 5s Zr 4d Zn 4s Zn 3d

(c) DOS and PDOS(arb. unit)

Total O 2p Sc 4s Sc 3d Zn 4s Zn 3d

(d)

Total

DOS and PDOS(arb. unit)

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O 2p Y 5s Y 4d Zn 4s Zn 3d

-5

-4

-3

-2

-1

0

1

2

3

Energy (eV)

Figure 4: The DOS and PDOS of g-ZnO nanosheets with (a) Ti (b) Zr (c) Sc, and (d) Y monodoping. The vertical dashed line indicates the Fermi level.


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Total


(a)


-2

O 2p

Sc 4s

Sc 4s

Sc 3d

Sc 3d

Zn 4s

Zn 4s

Zn 3d

Zn 3d

P 3p

O 2p

O 2p

Y 5s

Y 5s

Y 4d

Y 4d

Zn 4s

Zn 4s

Zn 3d

Zn 3d

0

Total

(f)

C 2p

-1

Total

(e)

N 2p

Total

(c)

P 3p

O 2p

Total

(b)

Total

(d)

N 2p


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 2p

O 2p

O 2p

Ti 4s

Zr 5s

Ti 3d

Zr 4d

Zn 4s

Zn 4s

Zn 3d

Zn 3d

1

2

3

4

5

-2

-1

0

1

2

3

4

5

Energy (eV)

Figure 5: DOS and PDOS of (a) N–Sc, (b) N–Y, (c) C–Ti, (d) P–Sc, (e) P–Y, and (f) C–Zr codoped g-ZnO nanosheets. The vertical black dashed line denotes the Fermi level.


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Minimum formation energy (eV)


5 4 3 2 1 0

N

P

C-Ti C-Zr

C

-1

Zr Ti

-2 -3

P-Sc

N-Y Y

Sc

P-Y

N-Sc

-4

Figure 6: The calculated minimum defect formation energies of TM or NE monodoped and codoped g-ZnO nanosheets.


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-2.0

-2.0 C-Ti (I)

N-Y (I)

2.66eV

2.36eV

P-Y (I)

P-Y

2.44eV

2.31 eV

2.40 eV

2.63 eV

-3.5

2.99 eV

-3.0

C-Ti

2.35eV

N-Y

ZnO

3.30 eV

-2.5

C-Zr

2.48eV

P-Sc

N-Sc

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3.5 -4.0

-4.5

-4.5

-5.0

-5.0

-5.5

-5.5

-6.0

-6.0 1

2

3

4

5

6

7

8

9

10

11

12

13

VB

-3.0

-4.0

-6.5 0

CB

-2.5

+

(H /H ) 2

(O /H O) 2

2

-6.5 14

pH

Figure 7: The band edge positions of pure and codoped g-ZnO nanosheets with respect to standard hydrogen electrode potentials. The standard hydrogen electrode potentials varying with pH values are indicated by the red axis and the VBM and CBM positions are indicated by the black axis.


6.25 at.% Optical Adsorbance (abr. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Optical Adsorbance (abr. unit)


200

300

Page 34 of 38

ZnO

N-Y

C-Ti

P-Y

4 at.%

ZnO P-Y (I) N-Y (I) C-Ti (I)

200

400

300

400

500

500

600

600

Wavelength (nm)

Figure 8: The optical adsorption spectra of pure, C–Ti, N–Y, and P–Y passivated codoped g-ZnO nanosheets with the doping concentrations of 6.25 at.% and 4 at.%.


Page 35 of 38

Total


N 2p O 2p Y 5s Y 4d Zn 4s Zn 3d

Total


(b)

P 3p

(c)

Total

O 2p Y 5s Y 4d Zn 4s Zn 3d

C 2p


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2

O 2p Ti 4s Ti 3d Zn 4s Zn 3d

-1

0

1

2

3

4

5

Energy (eV)

Figure 9: The calculated DOS and PDOS of (a) N–Y (I), (b) P–Y (I), and (c) C–Ti (I) codoped g-ZnO nanosheets. The vertical black dashed line denotes the Fermi level.



(a)

5

(b)

U = 0.0 V

5

U = 1.23 V

4

U = 0.0 V U = 1.23 V

Free energy (eV)

Free energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

U = 1.74 V

3 2 1 0 -1

4

U = 1.51 V

3 2 1 0 -1

-2 A

B

C

A

D

B

C

D

Reaction coordinate

Reaction coordinate

Figure 10: The calculated free energy diagram during OER at different potentials of (a) pristine and (b) P–Y codoped g-ZnO nanosheet.


Page 37 of 38

0.4 Free energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZnO

H*

P-Y

0.3 0.2 0.1 0.0

+

H

+ e

1/2 H 2

-0.1 -0.2 Reaction coordinate

Figure 11: The calculated free energy diagram for HER of P–Y codoped and pristine g-ZnO nanosheet.



P-Y

-2.5

CB

-3.0

VB

-3.5 (H+/H

) 2

-4.0 -4.5 -5.0

(O /H O) 2 2

-5.5 -6.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Optical Adsorbance (abr. unit)

C-Ti

2.35 eV

N-Y

2.44 eV

ZnO

2.63 eV

Graphical TOC Entry 3.30 eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

200

pH

300

ZnO

N-Y

C-Ti

P-Y

400

500

Wavelength (nm)


600

Bandgap Engineering of the g-ZnO Nanosheet via CationicâAnionic

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Bandgap Engineering of the g-ZnO Nanosheet via CationicâAnionic