Structure and Properties of Cd4P2Cl3, an Analogue of CdS

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Structure and Properties of Cd4P2Cl3, an Analogue of CdS Anand Kumar Roy, Sandhya Shenoy, Krishnappa Manjunath, Pratap Vishnoi, Umesh V. Waghmare, and Chintamani Nagesa Ramachandra Rao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04058 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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The Journal of Physical Chemistry

Structure and Properties of Cd4P2Cl3, an Analogue of CdS Anand Roy1, U. Sandhya Shenoy2, K. Manjunath1, Pratap Vishnoi 1, Umesh V. Waghmare2 and C. N. R. Rao.1* 1

New Chemistry Unit, International Centre for Materials Science(ICMS) and CSIR Centre of Excellence in Chemistry, Sheikh Saqr Laboratory, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore2 560064, India. Theoretical Sciences Unit Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore- 560064. *To whom correspondence should be addressed (Email: [email protected]. Phone: 91-8023653075)) ABSTRACT: Substitution of the sulfide ions in CdS by aliovalent P3- and Cl- ions is known to markedly affect the electronic structure and properties, reducing the band gap of the semiconductor. The decrease in band gap arises because the P (3p) states occupy the top of the valence band while the Cl (3p) orbitals lie deep down in energy. Progressive substitution of S by equal proportions of P and Cl should result in CdP0.5Cl0.5 or Cd2PCl. We have investigated the electronic structure of this compound and carried out first principles calculations to understand the electronic structure and properties. Interestingly, Cd4P2Cl3 is a semiconductor with a band gap of 2.36 eV comparable to that of CdS, and exhibits a photoluminescence band at 580 nm similar to CdS. Its conduction band and valence band edges are appropriately placed with respect to the water redox potentials, for it to exhibit excellent hydrogen evolution by photochemical water splitting. Visible-light induced hydrogen evolution rate of 1007(± 23) and 54(±4) µmol h-1g-1 has been obtained in the presence and absence of sacrificial agent. Hydrogen evolution in the absence of any sacrificial agent and the absence of photocorrosion seems to be unique features of Cd4P2Cl3.

INTRODUCTION CdS is one of the most explored metal sulfide semiconductors due to its unique optical and electronic properties.1-3 A band gap of 2.4 eV and photoluminescence in the visible region makes it suitable for use in light emitting diode, solar cells, and other devices.4-7 CdS has also received considerable attention in solar water splitting to produce molecular hydrogen, because of the favorable conduction and valence band positions with respect to the water redox potential.8,9 Metal sulfides including CdS suffer from the problem of photocorrosion wherein S2ions get oxidized by the photo-generated holes, resulting in the elution of Cd2+ and metal sulfide cannot produce hydrogen from water without a sacrificial agent.10 A possible strategy to overcome the photocorrosion problem of metal sulfides, is to make use of valence band engineering by anion substitution.11 Substitution of aliovalent anions in metal oxides and sulfides brings about marked changes in the electronic structures and properties, unlike substitution of isovalent anions.11-13 Thus, substitution of P3- and Cl- in place of the sulfide ions in CdS causes a change in electronic properties along with a significant decrease in the band gap.11 Such changes are not brought about by substitution of S2- ions by Se2- ions. Progressive substitution of P and Cl in place of S, in CdS would be expected to ultimately yield a compound of the formula Cd2PCl. However, under different synthetic conditions one obtains a compound of the formula Cd2PCl1.5 or Cd4P2Cl3. Electronic and optical properties of some ternary semiconductor

compounds have been reported in US patent.14 Structural aspects of a few Cd-P-Cl compounds including Cd4P2Cl3 have been reported in the literature, but there has been little efforts to understand the electronic structure as well as properties.15-17 The main interest in the present study is to examine the electronic structure and related properties of Cd4P2Cl3 which is close to being an analogue of CdS. We have synthesized Cd4P2Cl3 by different methods and examined its electronic structure and properties after having confirmed its crystal structure. It has an optical band gap of 2.36 eV and a photoluminescence band in the visible region. From first-principles calculations, we find Cd4P2Cl3 to be a semiconductor with the valence band and conduction band positions appropriate for generation of hydrogen by photochemical water splitting. We have examined the photochemical hydrogen production from Cd4P2Cl3 experimentally and found it to be an effective and stable photocatalyst. Cd4P2Cl3 produces hydrogen from water in the absence of sacrificial agents unlike CdS which requires sacrificial agents to produce hydrogen. Use of sacrificial agents is economically and environmentally not favorable18 and to the best of our knowledge, this is the first report of photochemical hydrogen generation by these semiconductors with and without the use of a sacrificial agent.

EXPERIMENTAL SECTION Cd4P2Cl3 was prepared by three different methods. In method 1, Cd4P2Cl3 (Cd4P2Cl3-1) was synthesized by heat-

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The Journal of Physical Chemistry ing Cd3P2 and CdCl2 in a sealed ampoule. Pure Cd3P2 was first prepared. A mixture of Cd3P2 and CdCl2 (1:1 mole ratio) was ground inside a glove box for 30 min. The homogeneous mixture was sealed inside a quartz ampoule in vacuum and heated at 500 ◦C for 20 h, with the ramp rate of 2.3◦/min and allowed to cool down at the same rate. Along with yellow crystalline powder product, we obtained needle shaped crystals at the mouth of the ampoule (yield; 83%) In method 2, a mixture of Cd3P2, CdCl2 and NH4Cl (1:1:1 mole ratio) was ground inside a glove-box for 30 min, the mixture was sealed in a quartz ampoule under vacuum and heated in a muffle furnace at 500 ◦C for 20 h (yield 85%). In method 3, a mixture of Cdpowder, red-P, and NH4Cl (mole ratio of 2:1:1.5) was ground inside a glove box for 30 min. The mixture was sealed in a quartz ampoule under vacuum and heated in a muffle furnace at 500 ◦C for 72 h, with the ramp rate of 2.3◦/min and allowed to cool down with the same rate(yield; 82%). CdS was synthesized under similar condition by solid state reaction. In a typical synthesis 1:1 mole ratio of Cd(CH3COO)2. 2H2O and CS(NH2)2 was ground for 15 min. This homogenous mixture was heated under N2 atmosphere at 500 ◦C for 4 h inside a tube furnace. In order to prepare Cd4P2Cl3/Pt and CdS/Pt, Pt deposition was carried out by the reported literature where an aqueous solution of H2PtCl6 was used as a precursor of Pt.19 In a typical synthesis 1 wt% of Pt was photodeposited on the photocatalyst in 1 M NaOH solution under UVVisible irradiation for 1.5 h. X-ray diffraction (XRD) patterns were recorded with a Bruker D8 Diffraction system using a Cu Kα source (λ = 0.1541 nm). Single crystal XRD data were collected at room temperature on a Bruker Smart diffractometer equipped with a normal focus 2.4 kW sealed tube X-ray source with graphite monochromatized Mo Kα radiation (λ = 0.710 Å) operating at 50 kV and 30 mA in ω scan mode. UV-Vis absorption spectra were collected with Perkin Elmer Model Lambda 900 spectrometer. Photoluminescence spectra were collected by Horiba JobinYvon spectrometer (iHR 320). Solid state 31P magic angle spinning NMR spectrum was collected with a Bruker AVANCE 400 MHz spectrometer. Raman spectra were collected using a 514.5 nm Ar+laser with a HORIBA LabRam HR800 spectrometer. Photocatalytic H2 evolution measurements were carried out by dispersing 20 mg of the photocatalyst in 75 mL of water containing 0.18 M Na2S and 0. 24 M Na2SO3 in a 135 mL glass cell. A Xe arc lamp (New Port, 6279NS ozone-free, working at 400 watt) with a 395 nm cut-off filter was used to obtain light of wavelength λ> 395 nm. The amount of H2 evolved was measured by gas chromatography (Perkin Elmer, Clarus 580) equipped with TCD. Apparent quantum yields (AQY) were estimated by the formula, AQY (%) = (2R/I)*100 where, R is the rate of evolution of hydrogen per second and I is the rate of incident photon per second. The photon flux (6.3*1017 photons/s) was measured using a irradiance meter.

COMPUTATIONAL DETAILS We used the Quantum Espresso package for firstprinciples calculations based on Density functional theory (DFT) to determine electronic structure and stability of Cd4P2Cl3.20-22 Scalar relativistic Vanderbilt ultra-soft pseudopotentials and a gradient corrected exchangecorrelation energy functional of Perdew-Burke-Ernzerhof (PBE) were employed in these calculations.23,24 Valence electrons of Cd, P and Cl in 4d10 5s2, 3s23p3 and 3s23p5 configurations respectively were simulated using pseudopotentials. Cd4P2Cl3 crystallizes in the cubic structure containing 72 atoms, with symmetry of the space group 205 (Pa-3) [with unit cell parameter of 12.13 Å]. Plane wave basis representing the wavefunctions was truncated with an energy cutoff of 50 Ry and that representing charge density with a cut off of 400 Ry. A uniform mesh of 4X4X4 of k points was used to sample integrations over the Brilloiun zone. Structure was relaxed through minimization of energy until the Hellman-Feynman forces on each of the atoms was 0.01 eV/°A or less, and the stresses within 1 kbar. The difference in the theoretical estimates of band gap and its experimental value is typically due to the approximation used in the calculations. DFT-GGA gives underestimation of band gap up to 40%. Such underestimation of the band gap by GGA is due to the existence of a discontinuity in the derivative of energy with respect to number of electron.25 On the other hand, HSE uses error function screened Coloumb potential to treat the exact exchange portion of the energy26 and hence gaps obtained from HSE calculation are typically higher than the GGA ones and are closer to experiment. Hence, HeydScuseria-Ernzerhof (HSE-06) hybrid functional was used to obtain more accurate estimates of the gap. We used Vienna ab-initio simulation package (VASP) with projector augmented wave method (PAW)27,28 for these calculations based on a generalized gradient approximation (GGA)-based energy functional of Perdew, Burke, and Erzenhoff (PBE).21,24 The structure was optimized to minimize energy using conjugate-gradients algorithm.

RESULTS AND DISCUSSION In Figure 1, we show powder X-ray diffraction patterns of CdS and as prepared Cd4P2Cl3 compounds. The PXRD patterns of all the Cd4P2Cl3 samples are identical and Cd4P2Cl3-3 Intensity (a.u.)

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Cd4P2Cl3-2 Cd4P2Cl3-1 CdS 10

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Figure 1. PXRD patterns of CdS and Cd4P2Cl3.


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The band at 493 cm-1 is due to P-P bond where as bands at 353, 371 and 294, 340 cm-1 are likely to be due to two different Cd-P and Cd-Cl bonds respectively described in the crystal structure discussion.29-31

Figure 2. Structure of Cd4P2Cl3-1 showing different polyhedra (orange CdP2Cl2 and blue CdPCl3).

UV-Visible diffuse reflectance spectrum of Cd4P2Cl3 is shown in Figure 3. Cd4P2Cl3 exhibit a semiconductor-like characteristic absorption in the visible region similar to CdS. It has a direct band gap of 2.36 eV as obtained by the Kubelk-Munk method. This band gap is comparable to that of CdS (2.34 eV) (Figure s5). This is close to the band gap reported by Suchow and Stemple.17 The photoluminescence spectrum of all Cd4P2Cl3 at 400 nm excitation (Figure 4a) shows a band at the 580 nm originating from recombination of electrons and holes generated by band edge excitation. Note that CdS exhibits a photoluminescence band close to 510 nm.32

a) Normalized intensity (a.u.)

Figure 3. UV-Visible absorption spectra of CdS and Cd4P2Cl3 samples. Colors of samples are also shown.

could be indexed in the cubic structure with a space group; Pa-3 (a = 12.13). The sharp diffraction peaks suggest the highly crystalline nature of Cd4P2Cl3. The experimental PXRD pattern match well with that simulated from single crystal X-ray diffraction data (Figure S1). The crystallite size of Cd4P2Cl3 was generally around 70 nm. Our single crystal study gave a structure (Figure 2) identical to that reported in the literature 15, 16. Essentials of the structure of Cd4P2Cl3 are as follows. The Cd atoms are distorted fcc packed with octahedral and tetrahedral interstices.15 The arrangement of P and Cl atoms bestows a tetrahedral coordination to the Cd atoms, wherein Cd(1) is coordinated by one P and three Cl, and Cd(2) is coordinated by two P and two Cl atoms. The P(1) atoms form PP pairs (dp-p = 2.18 Å), and involved in [P2Cd6] octahedra, where as the P(2) atoms form [PCd4] tetrahedron. These two polyhedra form the 3-D structure of Cd4P2Cl3 (see, Figure S2 for the ball and stick model). Detailed information on crystal structure refinement has been provided in Table S1 (see supporting information). The presence of two different types of P atoms in Cd4P2Cl3 was further confirmed by 31P solid state NMR. 31P solid state NMR spectra of Cd4P2Cl3 collected at 162.01 MHz shows the presence of two 31P signals corresponding to P-P bonded and non-bonded P atoms (Figure S3).We have characterized Cd4P2Cl3 using Raman spectral studies (Figure s4).

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Figure 5. a) Electronic structure and b) Density of electronic states of, Cd4P2Cl3 obtained by GGA based calculations. The energy is shifted with respect to Fermi energy which is set to zero.

The life time of charge carriers decides their probability of involvement in photocatalytic reactions before their recombination with a longer life time suggesting a greater probability of involvement of charge carriers in photocatalytic reaction.33-35 In order to find out the lifetime of photogenerated charge carriers in Cd4P2Cl3 compounds, we recorded the time-resolved photoluminescence spectra (Figure 4b). Cd4P2Cl3 samples prepared by different methods exhibit almost identical life times. The life time is found to be 3 ns which is higher than that of bulk CdS.31 We have examined the electronic structure of Cd4P2Cl3 along the path Γ-X-M-Γ-R-X (Figure 5a) in the Brillouin Zone, by employing GGA based calculations. The calculations reveal a band gap of 0.61 eV, which is an underestimate, as this is typical of DFT-based calculations. In order to understand the orbital character of the bands, we have analyzed the density of states by projecting them onto the atomic orbitals. The valence band is constituted primarily of P 3p orbitals with some mixing with Cd 5s orbitals, while the lowest energy conduction band has a predominant character of P 3p orbitals (Figure 5b). The P atoms contributing the 3p orbitals to the valence band posses a 3- charge (denoted as P2 in the Figure 5b) and the ones contributing to the conduction band are the ones with 2- charge (denoted as P1 in the Figure 5b), forming P-P bonds (with bond length of ~2.1 Å), connected to the cadmium atom coordinated to two other Cl atoms. To overcome the under estimation of the band gap in the calculations, we have used the HSE functional as implemented in VASP. From the total density of states, we note that the band gap is 0.91 eV (Figure 6). However, this is not the gap relevant to optical absorption. Visualization of the wavefunctions of states at the Γ point and of the energy around the Fermi level (Figure 7) shows that VB1 (penultimate valence band) and VB (topmost valence band) have major contributions from the 3p orbitals of phosphorus, resulting in odd parity. The conduction band (CB, lowest energy conduction band) has contributions from the phosphorus 3p orbitals and is odd under inver-

sion. Hence, a dipole-mediated transition from VB to CB is forbidden. CB1 (second higher energy conduction band) has contributions mainly from the s orbitals of Cd, resulting in even parity. Thus, a transition from VB to CB1 is an allowed transition according to the dipole selection rule. Thus, the optical band gap in the GGA-based electronic structure is 1.19 eV, while it is 1.8 eV in the electronic structure obtained with the HSE functional, which is closer to the observed experimental band gap. To determine the potential of Cd4P2Cl3 for use in photocatalytic water splitting, we aligned its valence and conduction band edges with respect to vacuum, and compared with the redox potentials of water splitting. Though our calculations with the HSE functional give a better estimate of the band gap than the GGA, the improvement is not quite complete.

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Figure 6. The density of state of Cd4P2Cl3 obtained using VASP calculations based on the HSE functional. The energies are shifted with respect to the valence band maximum which is set to zero.


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Figure 7. Visualization of the wavefunctions of states at the Γ point of a) CB, b)CB , c) VB, d) VB [Electronic structure revealing the specific points of visualization is shown in Figure 5a] (Color code: grey-Cd, Yellow-P, and Green-Cl).

To this end, we use a scissor correction in aligning the conduction band by taking experimental band gap in shifting it. We find that the valence band and the conduction band edges straddle the redox potentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We therefore expect, Cd4P2Cl3 to be suitable for use as a photo-catalyst for water splitting. In the light of the above theoretical predictions we have studied photocatalytic water splitting from Cd4P2Cl3 samples under visible-light irradiation. Figure 9a shows H2 generation from Cd4P2Cl3/Pt along with CdS/Pt under visible-light irradiation with Na2S-Na2SO3 as the sacrificial agent. CdS synthesized at the same temperature by solid state reaction shows much less activity than all the Cd4P2Cl3 samples. A H2 evolution rate of 285 (±11), 555(±25) and 336(±9) μmol h-1g-1 was exhibited by Cd4P2Cl3-1, Cd4P2Cl3-2 and Cd4P2Cl3-3 respectively. Cd4P2Cl3-2 exhibits a hydrogen evolution rate of 555(±25) μmol h-1g-1, which is 25 times higher than the H2 evolution rate from CdS (22.0±9 μmol h-1g-1). An enhancement of nearly two times in the catalytic activity of all Cd4P2Cl3 samples was observed after an induction period of 3h. Cd4P2Cl3-2 shows an enhancement in the photocatalytic H2 generation activity from 555 to 1007 (± 26) μmol h-1g-1

after an induction period of 3h, and remains nearly constant afterwards. Differences in HER activity of the three samples needs to be examined further, and it is possible that surface properties could be an important factor. The apparent quantum yield (AQY) obtained from Cd4P2Cl3-2 under visible-light is ~1.07 (± 0.04) which is higher than that of CdS (AQY~ 0.023).

Figure. 8 Alignment of valence and conduction band of Cd4P2Cl3 obtained from calculations based on GGA functional (blue), HSE functional (Indigo) and experimental band gap (magenta) to position the conduction band.



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Figure 9. a) Photocatalytic H2 generation from CdS/Pt and Cd4P2Cl3 /Pt and b) Time course of photocatalytic H2 generation from Cd4P2Cl3-2/Pt, under visible-light irradiation with Na2S-Na2SO3 as sacrificial agent.

Figure 10. a) Visible-light induced H2 evolution from CdS/Pt and Cd4P2Cl3/Pt in alkaline medium without any sacrificial agent, b) Effect of NaOH concentration on H2 evolution from Cd4P2Cl3-2/Pt.

We have examined the catalytic performance of Cd4P2Cl32/Pt over 9 h in 3 cycles. Figure 9b shows recyclibity of these photocatalyst for hydrogen production under visible-light irradiation with Na2S-Na2SO3 as the sacrificial agent. Cd4P2Cl3-2/Pt exhibits a H2 evolution rate of rate of 555(±25), -1 -1 1007(± 26), and 993(±23) μmol h g in first, second and third cycle, respectively. We did not observe any decline in the photocatalytic activity of samples up to 9 h. In order to examine the stability of Cd4P2Cl3 under reaction conditions, we have collected the powder X-ray diffraction pattern after the full course of photocatalytic reaction. Figure S6 shows the PXRD patterns of the Cd4P2Cl3-2 photocatalyst before and after reaction. We see no change in the PXRD pattern of the photocatalyst indicating its stability.

samples along with CdS/Pt (without any sacrificial agent). It is noteworthy that Cd4P2Cl3 shows hydrogen evolution even without any sacrificial agent, unlike CdS which requires a sacrificial agent to produce hydrogen. The Cd4P2Cl3/Pt samples under visible-light irradiation shows a linear variation in the hydrogen evolution activity with time, suggesting stability of Cd4P2Cl3 against photocorrosion. This is probably linked with the fact that the lowest energy conduction band is flat (See Figure 5a).36 A hydrogen evolution rate of 54(±4) μmol h-1g-1 is exhibited by Cd4P2Cl3-2/Pt in alkaline medium without any sacrificial agent. Figure 10b shows the effect of NaOH concentration on the hydrogen evolution from Cd4P2Cl3-2/Pt sample under visible-light irradiation. With increase in NaOH concentration hydrogen evolution activity of the photocatalyst increases up to an optimum concentration of 3 M. The cause of this is yet to be understood. This finding

We have studied hydrogen evolution in alkaline medium from Cd4P2Cl3/Pt under visible-light irradiation without any sacrificial agent. Figure 10a shows visiblelight induced hydrogen evolution from Cd4P2Cl3/Pt


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suggests that Cd4P2Cl3 can be used in place of CdS to avoid photocorrosion.

CONCLUSIONS We have been able to examine the electronic structure of Cd4P2Cl3, which is close in composition to Cd2PCl which would be obtained by replacing S2- in CdS by equal proportions of P3- and Cl-. Cd4P2Cl3 with a cubic structure is a semiconductor, with a band gap of 2.36 eV close to that of CdS (2.34 eV) and a photoluminescence band at 580 nm. First principles calculations enable us to understand the experimental results clearly. Cd4P2Cl3 exhibits excellent hydrogen generation activity under visible-light induced water splitting as predicted by theory and does not exhibit any photocorrosion unlike CdS.

ASSOCIATED CONTENT Supporting Information. Materials; preparation of Cd3P2, simulated PXRD pattern of Cd4P2Cl3; Crystal structure of Cd4P2Cl3; Crystal information 31 file of Cd4P2Cl3; P NMR spectra of Cd4P2Cl3; Raman spectra of Cd4P2Cl3; Tauc plot of samples; PXRD pattern of photocatalyst after reaction; Crystal data Table. This material is avaliable free of charge via the internet at http://pubs.acs.org/.

Corresponding Author * E-Mail : [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT AR thankful to CSIR, India for fellowship. AR and KM thankful to Sheik Saqr Laboratory for the fellowship. PV thanks to DST for fellowship. USS and UVW thank DST, government of india for support through TUE-CMS and a JC Bose National Fellowship respectively.

ABBREVIATIONS AQY: Apparent quantum yield, HER: Hydrogen evolution reaction.

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Structure and Properties of Cd4P2Cl3, an Analogue of CdS

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