Hexagonal Planar CdS Monolayer Sheet for Visible Light

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Hexagonal Planar CdS Monolayer Sheet for Visible Light Photo-Catalysis Priyanka Garg, Sourabh Kumar, Indrani Choudhuri, Arup Mahata, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01622 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 25, 2016

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The Journal of Physical Chemistry

Hexagonal Planar CdS Monolayer Sheet for Visible Light Photo-Catalysis Priyanka Garg,† Sourabh Kumar,† Indrani Choudhuri,† Arup Mahata,† Biswarup Pathak, †,#,* †

Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology (IIT) Indore, Indore, M.P. 452020, India

#

Centre for Material Science and Engineering, Indian Institute of Technology (IIT) Indore, Indore, M. P. 452020, India

Corresponding author. Contact Information: [email protected], +91-731-2438-772.

Abstract: Two-dimensional (2D) stable CdS monolayer sheets are proposed using the state-of-the-art theoretical calculations. Three different conformers (planar, distorted and buckled) are predicted which are separated by low energy barriers. These monolayer sheets are not only thermodynamically, mechanically and dynamically stable, but can withstand temperature as high as 1000 K. Band edge alignment of these monolayer sheets and bulk CdS is done with respect to the water oxidation and reduction potential to evaluate their photo-catalytic activities. Here we show, planar CdS monolayer sheet is the most promising material for visible light photo-catalysis and can be used for electronic and optoelectronic devices. 1. Introduction: Recently, two-dimensional (2D) materials have been drawn tremendous attentions due to their excellent electrical and optical properties.1 They are used for a wide range of applications (such as optoelectronics, spintronics, catalysts, sensors, super capacitors, solar cells, batteries and so on) owing to their large surface area, high mechanical strength and high thermal conductivity.2 The discovery of graphene has given the breakthrough to the researchers toward the innovation of a 2D monolayer sheet. However, the research interests on 2D materials have expanded beyond graphene.3-6 Recently, chalcogenide-based (typically

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S, Se, and Te) 2D materials have gained renewed interests due to their sizeable natural energy bandgap and electron mobility.7-12 Among all the chalcogenide-based materials, sulphurbased materials have drawn tremendous attentions due to their abundances and wide range of applications.13-16 Recently, hydrogen sulphide reported to be the warmest (at 230 K) ever superconductor.17 Lately, sulphide-based materials are emerging as promising materials for important applications.18-21 CdS is one of the most studied sulphur-based materials for various applications; thus, synthesized in all forms starting from nanowires,22 nanospheres,23 nanorods,24 nanflowers,25 nanotubes,26 nanocubes,27 petals28, nanobelts29,30 and so on. CdS quantum dots/nanorods31 have been used for the photo-degradation of waste materials, water treatments and air purifications.32 Similarly, CdS core-shell structures have been reported for visible light photocatalytic activities.33 Many such forms are reported for photovoltaic cells, electro-optic modulators, sensors, solar cells, electro-luminescent and photo luminescent devices.34-36 Thus, different types of CdS have been synthesized for tuning their valence and conduction band positions for various applications.37-39 Hence, the size dependent properties40 are substantially improved if the materials are synthesized at the nanoscale.41 Monolayer CdS has been synthesized in presence of surfactants through the free standing monolayer is yet to be isolated42 Recently Zhou et al. theoretically predicted the possibility of a stable free standing CdS monolayer sheet; thus encouraging for further experimental and theoretical explorations to recognise its importance.43 Recently, Xu et al. synthesized the ultrathin CdS nanosheets with a thickness of 4 nm through an ultrasonic-induced aqueous exfoliation method. They have demonstrated that CdS ultrathin showed remarkable visible light photocatalytic activity.44


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Here, we demonstrate a freestanding monolayer of cadmium sulphide (CdS) as an alternative chalcogenide based 2D material for electronic and optoelectronic applications. We have systematically investigated the stability of the planar, distorted, and buckled CdS monolayer sheets and their electronic properties. The stability of these sheets is confirmed from the energetic (cohesive energy), mechanical (stress vs. strain), dynamical (phonon dispersion), and thermal (molecular dynamics simulation) studies. Band edge alignment of all the monolayer sheets is done with respect to water oxidation and reduction potentials and compared with CdS bulk structure to understand their importance for visible light photocatalysis. Thus, here we propose that CdS monolayer sheet could be a promising material for visible light photo-catalysis and electronic and optoelectronic devices.

2. Computational Details: First-principles calculations are performed using a projected augmented wave (PAW) method,45 as implemented in the Vienna ab initio simulation package (VASP).46 The exchange-correlation potential is treated at the level of the GGA using Perdew-BurkeErnzerhof (GGA-PBE)47-48 whereas a hybrid functional (HSE06)49 is used for high accuracy electronic structure calculations. Plane waves with a kinetic energy cut off of 392 eV are used to expand the valence shell electron wave functions. In all the calculations, the convergence criteria for total energy and atomic force components are set at 10-6 eV and 10-6 eV/Å, respectively. The Brillouin zone is sampled with 12 × 12 × 3 and 36 × 36 × 9 Gamma-pack kpoint grid for geometry optimization and density state calculations, respectively. About 16 Å of vacuum is employed in the z-direction to avoid any interaction between the periodic images. Molecular dynamics (MD) simulations are performed to verify the thermal stability of the CdS monolayer. MD simulations are carried out at 200, 300, and 1000 K with a time step of 1 fs for 5 ps. Temperature control is achieved by the Nose thermostat model.50 Supercell geometries with the dimensions of 2×2, 4×4 and 5×5 are considered to take into 3 ACS Paragon Plus Environment


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account the long-range interatomic interactions for dynamic (phonon), mechanical and thermal studies (MD simulations), respectively. The charge density difference51-54 is plotted to understand the nature of bonding in CdS monolayer. The charge density difference (ρCDD) is calculated using the following equation: (1)

ρCDD = ρitotal - i ρifragments where

ρtotal is the total charge density and ρifragments is the charge density of the fragments.

Bader charge analysis is done using the Henkelman code55-56 with near-grid algorithm refineedge method.

3. Results and Discussion: A hexagonal unit cell of CdS monolayer consisting two-atoms (Cd =1 and S =1) is constructed from the (001) plane of the wurtzite phase of CdS,57 which is equivalent to the (111) plane of zinc blende phase.43 The relaxed CdS monolayer sheet (Figure 1a-b) possess a graphene like planar honeycomb structure.58-59 Interestingly, the lattice parameters (a = b = 4.24 Å) of the planar sheet are elongated compared to the bulk (a = b = 4.13 Å) CdS structure. The Cd-S bond distance (2.44 Å) is 0.07 Å shorter than the Cd-S bond distance (2.51 Å) in the bulk structure.


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Figure 1: (a) Top and side views of the monolayer cadmium sulphide. The black dotted line indicates two-atom unit cell. (b) Charge density difference (Isosurface value: 0.004 e/Å3) and (c) total electron density (Isosurface value: 0.07 e/Å3) of planar CdS monolayer sheet. Blue and green refer to regions of electron depletion and accumulation, respectively. Pink and yellow colour balls represent cadmium and sulphur atoms, respectively.

The chemical bonding of the Cd-S monolayer can be understood from its charge density difference and electron density plots (Figure 1b-c). In Figure 1(b), blue and green colours represent the charge depletion (on Cd) and accumulation (on S), respectively. Therefore, a significant amount of electron transfers from Cd to S atoms as can be seen from Figure 1b-c. Our Bader charge analysis suggests that the Cd-S bonds are ionic in nature as net charges on Cd and S atoms are +0.86 |e| and -0.86 |e|, respectively.

Figure 2: Top and side views of (a) distorted and (b) buckling phases of cadmium sulphide monolayer sheets. We have performed DFT calculations to find out whether buckling phases of CdS structures are stable or not? Two more phases (distorted in Figure 2a and buckled in Figure 2b) are calculated to be minima in the potential energy surface with buckling of 1.4 (Figure 2a) and 5 ACS Paragon Plus Environment


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1.0 Å (Figure 2b), respectively.43 Interestingly, the distorted (Figure 2a) and buckling (Figure 2b) phases are 0.0405 and 0.0403 eV/atom more stable than the planar CdS sheet. Therefore, both the distorted and buckled structures show similar stability with respect to the planar sheet.

3.1 Energetic stability: In order to measure the stability of the monolayer sheet, the energy difference between the sheet and their constituents are calculated using the following equation:60 ECoh = [ECdS – {nCdECd+nsES}]/N

(2)

where ECoh is the cohesive energy (per atom), ECdS is the total energy of the unit cell, nCd and nS are the number of atoms in the unit cell, ECd and ES are the energy of the isolated elements and N is the total number of atoms in the unit cell. Here, negative value indicates the stability of the system. The calculated cohesive energies are -2.65, -2.69, and -2.69 eV/atom for planar, distorted and buckled phases, respectively. Hence, all the monolayer sheets are thermodynamically stable and their relative stabilities are very much comparable. Similarly, the formation energy (Ef/atom) is calculated to evaluate their thermodynamic stabilities. Formation energy is calculated using the following equation: Ef = [ECdS – {nCdµCd)+nsµS}]/N

(3)

where µCd and µS represent the chemical potentials of Cd and S in their bulk structures, respectively.61-62 The negative formation energy (Ef) means, the formation of the monolayer sheets is thermodynamically favourable from the constituents at their stable bulk structures. The calculated formation energies are -0.54, -0.56, and -0.56 eV/atom for the planar, distorted and buckled monolayer sheets, respectively. Ding et al.63 concluded from their 6 ACS Paragon Plus Environment

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cohesive energy (Ecoh/atom) calculations that SiX and XSi3 (X = B, C, N, Al and P) can possibly be synthesized by the chemical vapour deposition and molecular beam epitaxy methods. Similarly, we find that our calculated cohesive and formation energies are negative. Thus, there is a possibility that these monolayer sheets can be synthesized using chemical vapour deposition and molecular beam epitaxy methods.

3.2 Dynamics Stability: The dynamic stability of the planar CdS sheet (unitcell) is confirmed from the phonon frequency calculations using the density functional perturbation theory (DFPT)64 as implemented in VASP.46 The lattice dynamics of the sheet is examined from its phonon dispersion plot (Figure 3). The Phonopy code65 is used to plot the dispersion of phonon modes. As can be seen from Figure 3, no imaginary vibrational frequency is found, which indicates the dynamic stability of the CdS monolayer. We have also studied the phonon dispersion of other two phases (distorted and buckled) and these monolayer sheets show small imaginary frequencies in the order of C12 and C44 > 0. The value of C11 can be obtained under uniaxial deformation whereas C12 can be calculated by polynomial fitting under biaxial deformation. For the planar CdS, the calculated values of C11 and C12 are 42 N/m and 23 N/m respectively. The value of C44 [C44=(C11-C12)/2] is calculated to be positive 10 N/m. Thus the calculated elastic constants of the planar CdS sheet satisfy all the criteria to be a mechanically stable sheet. Young’s modulus (Y) and Poisson’s ratio (PR) can be calculated from the elastic constants using the following equations:63 Y = (C211-C212)/ C11

(6)

PR = C12/C11

(7)

The in plane Young’s modulus calculated to be 29 N/m, which is again comparable with the value (43.6 N/m) of g-ZnS though much smaller than graphene (341 N/m).69 Our calculated Poisson’s ratio is 0.54 which is again quite close to the potion ratio of g-ZnS (0.51). Therefore, our calculated PR is higher than graphene and g-ZnS which indicates that it has higher shear motion than graphene and g-ZnS under strain.69 3.5 Electronic Properties: The electronic properties of the 2D sheet make it attractive compared to their bulk structures. Here, we have investigated the electronic properties (band structure and density of states) of the three CdS monolayer sheets and compared with their bulk structure. Firstly, GGA–PBE level of theory is used to describe the electronic structure (band structure and density of


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states) of the bulk CdS and then compared with the electronic structure of the planar monolayer sheet. The projected partial density of states (PDOS) reveals how the orbitals of the individual atoms (Cd and S) anticipated in the total density of states (TDOS). The PDOS shows [Figure S7, Supporting Information] that the 4p and 4d orbitals of Cd are mostly present at the valence band edge (VB) and these orbitals are mixed with S 3p orbital (p-d mixing). In the 0 to -1 eV energy region, a broad orbital density is found due to the p-d mixing and can be well confirmed by comparing the TDOS. In the case of conduction band, the partial DOS divulges that large splitting occur due to vacant 5s orbital density of Cd and it is present near the conduction band minima. In the similar manner, the band structure plot represents that the band below the Fermi level is valence band maxima and it just touches the Fermi level at Γ-point and band above the Fermi level is conduction band. Band structure also confirms the direct band gap at Γ-point. Total density of states (TDOS) and band structure [Figure S7, Supporting Information] of planar CdS sheet clearly show a direct band gap of 1.60 eV which is higher than the bulk CdS system (1.10 eV).74 However, the experimental band gap of CdS wurtzite75 bulk structure is reported to be 2.42 eV. As GGA underestimates the band gap and thus we have performed hybrid functional (HSE06)48 calculations to get a more accurate electronic structure of CdS. Interestingly, our calculated (using HSE06) band gap (2.50 eV) of bulk CdS matches with experimental band gap of 2.42 eV. This suggests that the hybrid functional (HSE06) is good enough for the electronic structure calculations of CdS.76 Thus, from here on we have used HSE06 functional for the electronic structure calculations. The total electron density of states and band structure are shown in Figure 7. As can be seen from Figure 7, the band gap of planar CdS sheet is 2.77 eV, which is 0.27 eV higher than the band gap of bulk CdS. Thus, the band gap increases for the planar sheet compared to the bulk structure.



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Figure 7: (a) Electronic band structure and total density of states (TDOS) of CdS monolayer by HSE06 level of theory showing a direct band gap at 2.77 eV. The Fermi level (EF) is shifted to zero and indicated by green dotted line. Similarly, we have investigated the electronic properties of the other monolayer sheets. The GGA PBE calculated band gap for distorted and buckled CdS sheets are 2.11 eV and 2.02 eV, respectively [Figure S8, Supporting Information]. Thus, the calculated band gaps are higher than the planar CdS. The HSE06 functional is used to get the more accurate electronic structure and we find the band gap values are 3.09 eV and 2.97 eV for the distorted and buckled structures, respectively [Figure S9, Supporting Information]. Therefore, the planar CdS monolayer has a lower band gap compared to the distorted and buckled monolayer sheets. Thus, we believe that such planar monolayer sheet could be a better material for visible light photo-catalysis.


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3.6 Photo-Catalytic Properties: We have used HSE06 functional to calculate the work function of the materials to locate the Fermi position with respect to the water oxidation and reduction potentials. Work function (ɸ) is calculated using the following equation: (8)

Work-function (ɸ) = E (vacuum) - EF

Where, E (vacuum) and EF are the vacuum level and Fermi level, respectively. The calculated work function values for the planar, distorted and buckled monolayer sheet are 6.18, 5.61 and 5.38 eV, respectively. Thus, the work function is higher for the planar sheet compared to the distorted and buckled sheets. According to the Mulliken’s electronegativity theory,77 the conduction band potential can be calculated using the following equation: ECB = χ – Ec – 0.5Eg

(9)

Where, χ is the absolute electronegativity of the semiconductor, Ec is the energy of free electron on the hydrogen scale (~4.50 eV)78-79 and Eg is the band gap of the semiconductor. The band gap values are 2.77, 3.09 and 2.97 eV for planar, distorted and buckled CdS monolayer sheet respectively. Therefore, the valence band potential (EVB) can be calculated from EVB = ECB – Eg. The absolute electronegativity (χ) for CdS is 5.18 eV.80 Thus, the conduction band potentials are -0.70, -0.86, and -0.80 eV for planar, distorted and buckled CdS sheets, respectively. The band potential value can be calculated at the absolute vacuum scale (AVS) using EAVS = ENHE – ECB. Hence at the AVS scale, the conduction band potentials (Figure 8) are -3.80, -3.63 and -3.69 eV for planar, distorted and buckled structures, respectively.76



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Figure 8: Schematic presentation of the band edge alignment of CdS bulk and monolayer sheets with respect to the water oxidation (H2O/O2) and reduction (H+/H2) potentials. Here, ECB, EVB, EF, Eg, and Φ represent conduction band, valence band, Fermi energy, band gap (eV), and work function (eV), respectively. Figure 8 shows the band edge alignment of the cadmium sulphide monolayer sheets with respect to the water oxidation and reduction potentials. This is done to evaluate their photocatalytic activities with respect to their bulk structure. We find that the band gap of the CdS bulk structure and the conduction band minimum (CBM) are higher in energy by 2.50 and 0.57 eV with respect to the water reduction potential whereas, valance band maximum (VBM) is 0.75 eV lower than the water oxidation potential. Thus, the band gap and band edge positions are excellent for the photo-catalytic activities. However, hole transfer is very important to improve the efficiency of CdS-based photo-catalysts.81 This can be efficiently achieved by building a Z-scheme photo-catalyst. CdS is a very important for the Z-scheme 18 ACS Paragon Plus Environment

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photo-catalysts.82 On the other hand, two dimensional materials are highly flexible for building such hybrid photo-catalysts. We find (Figure 8) that the valence band edges are stabilized (with respect to CdS bulk) by 0.14, 0.29 and 0.13 eV for the planar, distorted and buckled sheets, respectively. Thus, the valence band edge is stabilized most (Figure 8) for the distorted sheet than the buckled and planar sheets, respectively. So for a Z-scheme photocatalyst, such stabilization of the valence band of the monolayer sheets will facilitate the hole transfer process, which in turn will reduce the electron-hole recombination. This is very important for the photo-stability of the material. Thus, these monolayer sheets are more promising photo catalytic materials compared to its bulk structures. Furthermore, their band edge energies are perfectly aligned with respect to the water oxidation and reduction potential and thus important for the visible light photo-catalytic activities. Thus we predict, the planar CdS monolayer sheet will be the most efficient visible light photo-catalyst as it has a lower band gap than others. In fact, our findings are very much in agreement with a previous report, where they have demonstrated remarkable visible light photocatalytic activity of an ultrathin CdS nanosheet.44 4. Conclusions: Density functional calculations are carried out for predicting the structures of the CdS monolayer sheets. Three monolayer sheets (planar, distorted and buckled) are calculated to be thermodynamically stable. We have accessed their thermodynamic stabilities based on the cohesive and formation energy calculations. Interestingly, all these monolayer sheets are stable and their relative stabilities are very much comparable. As the relative stabilities of these sheets are very much comparable, we have chosen planar sheet for further studies. The absence of imaginary modes confirms the dynamic stability of the planar sheet. Similarly, the planar CdS sheet calculated to be a mechanically stable and its mechanical stability is very much comparable with a system such as g-ZnS sheet. AIMD simulations are performed to 19 ACS Paragon Plus Environment


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check their thermal stabilities and we find these monolayer sheets are very stable and can withstand temperature as high as 1000 K though they might interconvert to each other as they have a very low inter-conversion energy barrier. Band edge alignment is done with respect to water oxidation and reduction potentials and we find such monolayer sheets are promising materials for visible light photo catalysis as they have perfect band gap and band energy for photo catalytic activities. The planar CdS sheet has a lower band gap than others and thus it can be a better material for visible light photo-catalysis. On the other hand, such materials can be very promising for Z-scheme photo-catalysis due to their low dimensionality and flexibility. 5. Acknowledgments: We thank IIT Indore for the lab and computing facilities. This work is supported by Council of Scientific and Industrial Research [CSIR, Grant number: 01(2723)/13/EMR(II)], New Delhi. P.G., I. C. and A. M thank MHRD and S. K. thanks UGC for the research fellowship.

6. Associated Content: * Supporting Information: Total energy verses time step of the AIMD Simulations carried out at different temperatures (200K, 300K, and 1000K) are presented for 2×2 and 5×5 CdS supercell geometries. Structures and TDOS under different (5%, 10%, 15% and 20%) strains (tensile and compressive) in the uniaxial and biaxial directions are preseneted. TDOS and band structure of the planar, distorted and buckled CdS monolayer are presented calculated at GGA PBE level of theory. TDOS of buckled and distorted CdS structures are shown calculated using


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HSE06 functional. Electrostatic potentials of the planar, buckled and distorted CdS are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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Hexagonal Planar CdS Monolayer Sheet for Visible Light

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