Efficient Strategy for Enhancement of Visible Light Photocatalytic

Article

Efficient Strategy for Enhancement of Visible Light Photocatalytic Activity of NaTaO by a Significant Extent 3

Brindaban Modak, Pampa Modak, and Swapan K Ghosh J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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The Journal of Physical Chemistry

Efficient Strategy for Enhancement of Visible Light Photocatalytic Activity of NaTaO3 by a Significant Extent

Brindaban Modak,1,2 Pampa Modak3 and Swapan K. Ghosh1,2,4*

1

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai – 400 085, India 2

3

4

Homi Bhabha National Institute, Mumbai – 400 094, India

Radiological Safety Division, Atomic Energy Regulatory Board, Mumbai-400094, India

UM-DAE Centre of Excellence in Basic Sciences, Kalina Campus, Mumbai-400098, India

Email: [email protected] Phone: 91-22-25595092 Fax: 91-22-25505151

ACS Paragon Plus Environment


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ABSTRACT Among different photocatalysts developed so far for the generation of hydrogen through water splitting, NaTaO3 has been at the forefront due to its excellent stability, and tunable electronic and optical properties. However, to extend its applicability to the range of visible light, the band gap has to be reduced significantly. In this study, we propose an efficient way to improve its visible light photocatalytic activity by doping with carbon in presence of Cr, or Mo, or W. Although, monodoping with either C or Cr/Mo is able to reduce the band gap, the presence of localized defect states limits their applicability for photocatalytic purpose. This can be avoided by using dopant pair leading to formation of codoped systems. In the present study, two different types of codoped systems for each pair [(Cr, C), (Mo, C) and (W, C)] have been considered by varying the relative proportions of the dopant elements (1:1 and 2:1). Unfortunately, electronic structure analysis of 1:1 type of codoped systems show limitation similar to that of monodoped system. The advantage of 2:1 type of codoped system is that spontaneous formation of vacancy defects, which are efficient source for charge carrier recombination centres, can be minimum due to charge compensated nature. Besides, synthesis of 2:1 type of codoped system is found to be more feasible than the monodoped as well as 1:1 codoped system, as indicated by the formation energy calculation. Among the three 2:1 types of codoping, both (Mo, C) and (W, C) pairs lead to formation of favourable band structure with significantly reduced band gap (2.01 eV and 2.23 eV, respectively). The calculation of frequency dependent dielectric function has been carried out to get an idea about the shift in optical spectrum towards visible region due to codoping. Finally, the feasibility of water splitting involving the two codoped systems have been checked by aligning their band edge positions with respect to water redox levels.


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1. INTRODUCTION Increasing energy demand coupled with limited availability of fossil fuel and their environmental impact leads to search for efficient route to for clean energy generation. Hydrogen is considered to be one of the most promising energy carrier. Among different techniques to generate hydrogen, photocatalytic water splitting generates immense interest in recent times. For this purpose wide range of photocatalysts have been developed and demonstrated successfully till date.1-14 Particularly, perovskite type NaTaO3 has been extensively studied due to its excellent performance in the field of photocatalytic water splitting and destruction of environmental toxic pollutants.15-20 However, its wide spread application is largely restricted by poor activity under visible light due to the large band gap (4.10 eV) associated with these materials. It is worthwhile to mention that doping with foreign element is one of the promising strategies to improve the visible light photocatalytic activity of semiconductor based materials.21-29 It has been reported that doping of Bi, Cu, Cr, Nb can improve the visible light activity.30-37 Recently, it has been demonstrated that doping with lanthanide ion can reduce the effective band gap by introducing defect states.38-40 Wang et al synthesized self (Ta+4) doped NaTaO3 to enhance the photocatalytic activity under visible light.41 In the anion category, N doping has been attempted by several groups,42-45 and several other nonmetal dopants like sulphur46 and iodine47 have also been shown to be effective in reducing the band gap. Doping of carbon into oxide based photocatalytic materials has been shown to be interesting in recent days due to enhancement of visible light activity by a significant extent.28,

48-52

However, doping with single element has several

drawbacks. It is known that, doped material often show poor photoconversion efficiency in comparison to the parent material in spite of significant improvement of visible light activity. This may be due to the introduction of localized acceptor or donor states, which although reduces the effective band gap, but can hinder the mobility of photogenerated charge carriers,



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and thus accelerate recombination rates.53 Besides, doping in many cases is accompanied by spontaneous formation of charge compensating vacancy defects, which are well known source for charge carrier trapping.54 More importantly, shifting of valence band maxima (VBM) and conduction band minima (CBM) by large extent may affect the redox behaviour significantly. Therefore, choice of dopant element is very crucial for controlled band gap engineering. The limitation of monodoping can be overcome by introducing another dopant element. As for example, codoping of La into Cr-doped, Co-doped, and N-doped NaTaO3 has been found to be successful for this purpose.55-58 However, narrowing of the band gap is still far from the ideal limit, which is considered to be around 2 eV. Therefore, the dopant pair should be chosen more critically. In this study, a detailed systematic density functional theory (DFT) based calculation has been carried out to find out the most suitable dopant pair among (Cr, C), (Mo, C), and (W, C) for enhancing the visible light photocatalytic activity of NaTaO3 in a controlled way. Since, the band gap is highly underestimated in calculation using standard DFT, we perform electronic structure calculation using Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional,59 which has been found to successfully reproduce the band gap of wide range of semiconductor materials.60-63

2. COMPUTATIONAL DETAILS In this study, all the calculations have been carried out using projector augmented wave (PAW)64 based electronic structure code, Vienna ab initio simulation (VASP) package.65 To define the interaction between the ion and electron we have considered pseudopotential generated under the framework of generalized gradient approximation (GGA).66 The configuration for valence set considered during construction of pseudopotential are, Na (3s12p6), Ta (6s25d3), Cr (4s23d4), Mo (5s24d4), W (6s25d4), O (2s22p4), and C (2s22p2). An


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energy cutoff of 500 eV has been chosen to expand the electronic wave function in the plane wave. To model the doped system, 2 × 1 × 2 supercell (80 atoms) of orthorhombic (Pcmn, 62) crystal structure for NaTaO3 has been considered. Firstly, geometry optimization of all the model structure have been carried out by relaxation of both ionic positions and cell parameter. We have involved Monkhorst and Pack scheme67 for sampling the k-points of the Brillouin zone. The energy convergence for self-consistent iteration has been chosen to be 106

eV. The optimized geometry has been considered during electronic structure calculation.

We have employed the HSE hybrid functional59 for the calculation of electronic properties. According to this functional, the ion-core interaction is divided into short range (SR) part and long range (LR) part. The interaction in short range is calculated by mixing a fraction of the exact Hatree-Fock exchange with Perdew-Burke-Ernzerhof (PBE) exchange, while the long range part is fully treated by PBE exchange. For the correlation part, only the PBE functional HSE has been employed. Thus, the exchange correlation energy ( E XC ) can be expressed as

HSE E XC =

1 SR 3 E X ( µ ) + E XPBE , SR ( µ ) + E XPBE , LR ( µ ) + ECPBE 4 4

(1)

where, µ indicates the screening parameter, defining the limit of short range and long range interaction. In our previous study, we have shown that the standard exchange mixing parameter of 25% and the screening parameter value of 0.15 Å-1 successfully reproduce experimental band gap of orthorhombic NaTaO3.11, 62 To calculate the density of states (DOS) and projected density of states (PDOS) we have adopted the tetrahedron method with Blöchl correction.68 Frequency dependent dielectric function calculation has been carried out for the optical property.69



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3. RESULTS AND DISCUSSION At room temperature, the most stable form of NaTaO3 is orthorhombic crystal structure (space group: 62). For carbon doping one of the oxygen atoms in the 2 × 1 × 2 supercell has been replaced by carbon (dopant concentration: 2.08%), while for metal doping one of the Ta atoms has been replaced by metal dopant (dopant concentration: 6.25%). In all the cases, at first step the structural optimization has been carried out, and then the defect formation energy calculated to find out the feasibility of their synthesis. Electronic structure is described by analyzing DOS and PDOS obtained from the optimized structure. Then we checked the extent of shifting of the absorption curve due to doping. Final conclusion has been drawn only after inspecting their band edge alignment with respect to water redox levels.

3.1. NaTaO3 The calculated lattice parameters for the optimized geometry of NaTaO3 (a= 5.52 Å, b= 7.86 Å, c= 5.52 Å) are found to be close to the earlier reported values. The DOS plot has been shown in Figure 1. The calculated band gap (4.05 eV) is very close to the experimentally reported value (4.10 eV). Figure 1 indicates that O 2p states and Ta 5d states mainly contribute to the VBM and CBM of NaTaO3, respectively. Now, we investigate the effect of doping on the geometry and electronic structure of NaTaO3.

3.2. C-Doped NaTaO3 C-Doped NaTaO3 system has been constructed by replacing one of the oxygen atoms in the 2 × 1 × 2 supercell by carbon (dopant concentration: 2.08 %). The optimized lattice parameter of C-doped NaTaO3 has been shown in Table 1. The cell dimension along 'a'- direction is slightly decreased (0.31%), while in the other two directions it shows a tendency to increase by 0.16% (along b axis) and 0.25% (along c axis). The calculated Ta-C bond is found to be


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significantly larger in comparison to the Ta-O bond in the undoped NaTaO3. The

Efficient Strategy for Enhancement of Visible Light Photocatalytic

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