A Crucial Role of Bond Covalency Competition in Determining the

Article pubs.acs.org/JPCC

A Crucial Role of Bond Covalency Competition in Determining the Bandgap and Photocatalytic Performance of Silver Oxosalts Xiaoyan Jin,† In Young Kim,† Yun Kyung Jo,† Jerry L. Bettis, Jr.,‡ Hyun-Joo Koo,§ Myung-Hwan Whangbo,†,‡ and Seong-Ju Hwang*,† †

Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Korea ‡ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States § Department of Chemistry and Research Institute of Basic Science, Kyung Hee University, Seoul 130-701, Korea S Supporting Information *

ABSTRACT: The optical bandgaps, the surface charges, and the photocatalytic activities of the silver oxosalts Ag3AsO4, Ag2CO3, Ag3PO4, Ag2SO4, and Ag2SeO4 are systematically investigated with several experimental techniques and first principles density functional theory calculations. The trends in the optical bandgaps and the surface charges of these silver oxosalts, Agx(XOy)z, are analyzed by considering how the X−O bond covalency affects the charge on the terminal oxygen atoms and the Ag−O bond covalency. The optical bandgaps of Agx(XOy)z are well-described by the bond-covalency competition in the Ag−O−X linkages because an increase in the overlap between the O 2s/2p and X ns/np orbitals decreases the overlap between the Ag 4d and O 2s/2p orbitals. The optical bandgap increases linearly with increasing the Z/r ratio of the atom X, a simple measure of the X−O bond covalency. In the photodegradation of charged molecules, the surface charge of Agx(XOy)z plays a prominent role and decreases with increasing the Z/r ratio. As expected from the present theoretical predictions, newly investigated Ag2SeO4 exhibits a promising photocatalytic activity under visible light. The Z/r ratio of the central atom X provides an effective measure for predicting the photocatalyst performance and the optical bandgap of silver oxosalts Agx(XOy)z.

1. INTRODUCTION Semiconductor-assisted photocatalytic reactions attract a great deal of research activity because of their usefulness as an environmentally benign option for the remediation of environment pollution and the harnessing of solar energy.1−6 Many kinds of semiconducting inorganic solids such as metal oxides, metal sulfides, and metal hydroxides are intensely studied as UV- and visible-light-active photocatalysts.7−13 Recently research activity on the exploration of novel photocatalysts is extended to metal oxosalts and metal halides.14−17 Of prime importance is that silver oxosalts such as Ag3PO4 and Ag2CO3 show promising photocatalyst functionality under visible light illumination.18,19 There are various oxosalt groups XOy (X = C, P, S, As, and other p-block elements; y = 3, 4) that can bind with various metal ions to form semiconductor lattice, so this family of materials provides opportunities to design new efficient photocatalysts. To establish a way of predicting the photocatalyst performance of unexplored silver oxosalts, it is necessary to understand the electronic structures of silver oxosalt-based photocatalysts, given that photocatalytic reactions are driven by photogenerated electrons and holes.20 In semiconducting compounds consisting of metal−ligand covalent bonds, the strength of their covalency strongly affects the optical bandgap as well as the widths of the valence band (VB) and conduction band (CB). For example, the Ag−O bond © 2013 American Chemical Society

covalency is a main factor determining the overall band structure of silver oxosalts, Agx(XOy)z, in which O atom in the XOy group makes bonds with both X and Ag to form Ag−O−X linkages (see Figure S1 of Supporting Information). It is noted that the Ag−O bond of each Ag−O−X linkage is strongly affected by the neighboring X−O bond through the covalency competition effect;21 an increase (a decrease) in the X−O bond covalency decreases (increases) the covalent character of the neighboring Ag−O bond, hence influencing the band structure of the silver oxosalts. Since the X−O bond covalency is sensitive to the charge-to-size (Z/r) ratio of the central X atom,22 it would be informative to correlate this Z/r ratio of the central atom with the bandgap and the photocatalytic activity of the silver oxosalts. In addition to optical band gaps, the surface charge of silver oxosalts is also crucial in enhancing the photodegradation of organic molecules,23,24 since the photocatalysis of substrate molecules begins with their adsorption on the surface of photocatalysts. The X−O bond covalency affects the partial charge on oxygen atoms in silver oxosalts and hence the surface charge and photocatalytic efficiency. To date, there is no systematic investigation on how the photocatalyst Received: October 14, 2013 Published: December 2, 2013 26509

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

filter (Milipore) to remove catalyst particles. To rule out the effect of the adsorption of substrate on the surface of catalyst, the variation of substrate concentration was measured after the equilibration under dark conditions. The time-dependent variation of the MB, MO, and RhB concentration was examined by measuring the absorbance with an UV−vis spectrophotometer (Agilent 8453). The concentration of phenol was estimated with a reverse-phase high performance liquid chromatograph (HPLC, Agilent 1100 series). The eluent solution was composed of acetonitrile (40%) and deionized water (60%).

performance of silver oxosalts depends on the nature of chemical bonding in their oxosalt group. In the present study, the optical bandgaps, the surface charges, and the photocatalytic activities under visible-light irradiation of some commercially available Agx(XOy)z salts, namely, Ag2CO3, Ag3PO4, Ag3AsO4, and Ag2SO4, as well as experimentally prepared Ag2SeO4 are systematically investigated on the basis of comparative experiments and electronic band structure calculations, and the findings from this investigation are analyzed in terms of the covalency competition effect of the oxosalt group XOy (= CO3, PO4, AsO4, SO4, and SeO4). We explain the qualitative trends in the photocatalytic activities of these silver oxosalts by invoking the covalency competition model on the basis of first-principles electronic band structure calculations.

3. RESULTS AND DISCUSSION 3.1. Diffuse Reflectance UV−vis Spectroscopy. Figure 1 represents the diffuse reflectance UV−vis spectra of several silver oxosalts of Ag2CO3, Ag3PO4, Ag3AsO4, Ag2SO4, and Ag2SeO4.

2. EXPERIMENTAL SECTION 2.1. Materials. All the present silver oxosalts except for Ag2SeO4 were purchased from Aldrich Co. and used without any further purification. Ag2SeO4 was synthesized by dropping the aqueous solution of AgNO3 to the aqueous solution of MgSeO4. The obtained precipitate was isolated by centrifugation, washed with distilled water, and dried at 110 °C in ambient atmosphere. The formation of single-phase Ag2SeO4 was confirmed by powder X-ray diffraction (XRD) measurements, as presented in Figure S2 of the Supporting Information. 2.2. Characterization. The diffuse reflectance UV−vis spectra of the silver oxosalts were obtained on the Agilent Technologies Cary 5000 spectrophotometer equipped with an integrating sphere using polytetrafluoroethylene (PTFE) as a reference. A zeta potential of the silver oxosalts was measured with the Zetasizer Nano ZS (Malvern Instruments). The silver oxosalts were well-dispersed at 25 °C and circulated into the zeta cell. After stabilization, the zeta potential of the silver oxosalts was measured. The photoluminescence (PL) spectra of the silver oxosalts were measured with a PerkinElmer LS55 fluorescence spectrometer. The excitation of these materials was achieved using UV radiation (λ = 325 nm). The crystal structures of the silver oxosalts before and after the photoreactions were examined with powder XRD analysis. The effect of photocatalytic reaction on the morphology of silver oxosalts was probed with field emission-scanning electron microscopy (FE-SEM, Jeol JSM-6700F). The surface area of the silver oxosalts was studied with N2 adsorption−desorption isotherm measurements at 77 K with a gas sorption analyzer (ASAP 2020). Before the measurements, all the samples except for Ag2CO3 were degassed at 100 °C in a vacuum of 420 nm) to eliminate IR and UV radiations, respectively. Sample aliquots were drawn by a 1 mL syringe intermittently during the photoreaction and filtered through a 0.45 μm PTFE

Figure 1. Diffuse reflectance UV−vis spectra of silver oxosalts: (a) Ag3AsO4 (black), (b) Ag3PO4 (red), (c) Ag2CO3 (blue), (d) Ag2SeO4 (pink), and (e) Ag2SO4 (green).

The bandgaps derived from these spectra are summarized in Table 1. Ag3PO4 shows a bandgap of 2.46 eV, which is quite Table 1. Bandgaps Eg (in eV) of Silver Oxosalts Determined from Reflectance Measurements and from WIEN2k and VASP Electronic Band Structure Calculations compounds Ag3AsO4 Ag2CO3 Ag3PO4 Ag2SO4 Ag2SeO4

reflectance 1.88 2.69 2.46 3.92 2.96

WIEN2ka −0.60 0.67 0.20 1.57 0.60

b

(1.88) (3.15) (2.68) (4.05) (3.08)

VASPa −0.60b (1.88) 0.65 (3.13) 0.20 (2.68) 1.35 (3.83) 0.52 (3.00)

a

The numbers in parentheses were obtained by adding 2.48 eV to the calculated bandgaps. bThe negative number −0.60 signifies that the bottom of the Ag 5s-block at Γ dips below the Fermi level by 0.6 eV.

suitable for visible-light-induced photocatalytic reactions.18 In comparison with Ag3PO4, Ag2SO4 shows a much larger bandgap of 3.92 eV, Ag3AsO4 a smaller bandgap of 1.88 eV, and Ag2SeO4 a value of 2.96 eV that lies between those of Ag2SO4 and Ag3AsO4. Ag2CO3 exhibits the bandgap of 2.69 eV, which is quite similar to that of Ag3PO4. Ag2SeO4 shows a bandgap of 2.96 eV. All the present oxosalts except Ag2SO4 possess bandgaps smaller than 3 eV, which is suitable for harnessing visible-light energy. It is clear that the optical bandgaps of the silver oxosalts are significantly affected by their 26510

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

Figure 2. Schematic diagram showing how the Ag 4d and Ag 5s bandwidths, and hence the bandgap, of the silver oxosalt Agx(XOy)z are affected by the covalency competition of the Ag−O and O−X bonds in their Ag−O−X linkages.

oxosalt groups, even though the overall band structures of silver oxosalts depend mainly on the orbitals of silver and oxygen atoms.25−27 3.2. Covalency Bond Competition Model and Optical Bandgap. To understand the variation of the optical bandgap upon the change in the central atom X in the oxosalt group XOy, we note that the Ag−O bond covalency in silver oxosalts, Agx(XOy)z, is influenced by the X−O bonds because these bonds are fused to form the Ag−O−X linkages (see Figure S1 of Supporting Information). In the Ag−O−X linkages, the covalent character of the Ag−O bond and that of the O−X bond involve the participation of the oxygen 2s/2p orbitals. Thus, the covalency competition model predicts that an increase in the X−O bond covalency, i.e., the overlap of the O 2s/2p orbitals with the atomic orbitals of X, should decrease the Ag−O bond covalency, i.e., the overlap of the O 2s/2p orbitals with the atomic orbitals of Ag. As depicted in Figure 2, the VB top of a silver oxosalt is given by the Ag 4d band and the CB bottom of a silver oxosalt by the Ag 5s band. The Ag 4d and O 2s/2p orbitals are combined outof-phase in the Ag 4d band, and so are the Ag 5s and O 2s/2p orbitals in the Ag 5s band. Since Ag 4d orbital is much more contracted than the Ag 5s orbital, an increase in the Ag−O covalency would raise the top of the Ag 4d band much more than the bottom of the Ag 5s band, thereby decreasing the bandgap. The X−O bond covalency is expected to increase as the charge-to-size ratio Z/r of the central atom X is increased, which in turn should decrease the Ag−O bond covalency. Therefore, the bandgaps of the silver oxosalts are expected to increase with increasing their Z/r ratio. Figure 3 presents the plots of the bandgap energy vs the Z/r ratio of the central atoms for several silver oxosalts. Considering

that the high oxidation state of the central atoms X makes the X−O bond fairly covalent,22,28,29 the Z/r ratio of these compounds is calculated by using the covalent radius of the central atom. As can be seen from Figure 3, the bandgap indeed increases with increasing the Z/r ratio. Among the silver oxosalts, Ag2SO4 should possess the highest X−O bond covalency because the hexavalent sulfur ion has the highest oxidation state with very small radius.22 For this reason, Ag2SO4 has the largest bandgap and cannot absorb visible light. For the other silver oxosalts, the central atom in the oxosalt group has a lower oxidation state and/or a larger ionic radius than does the hexavalent sulfur ion in Ag2SO4. Thus, these compounds show a smaller bandgap and are suitable for harvesting visible-light energy. As shown in Figure 3, the regression analysis provides the linear relationship between the Z/r and ΔE, namely, ΔE = 111.75 × (Z/r) − 2.69, where the ΔE is in eV, Z is the formal charge, and r is the covalent radius in pm. In this regression analysis, the data point of Ag2CO3 was excluded because it has a trigonal-planar CO3 group,30,31 whereas the rest of the silver oxosalts consist of tetrahedral XO4 groups. The importance of the X−O bond covalency in determining the bandgap of the silver oxosalts is further evidenced by their Ag−O bond lengths, since the average metal−oxygen bond distance should decrease with increasing the bond covalency;32,33 d(Ag−O) = 2.344 Å for Ag3AsO4, 2.377 Å for Ag3PO4, 2.578 Å for Ag2SeO4, and 2.591 Å for Ag2SO4.34−37 3.3. Electronic Band Structures. On the basis of firstprinciples density functional theory (DFT) calculations, we investigate the electronic band structures of Ag2CO3, Ag3PO4, Ag3AsO4, Ag2SO4, and Ag2SeO4. Our calculations employ the frozen-core projector augmented wave method encoded in the Vienna Ab initio Simulation Package (VASP),38−40 the generalized-gradient approximation (GGA) of Perdue, Burke, and Ernzerhof for the exchange-correlation functional41 with a plane-wave cutoff energy of 400 eV, a 2 × 4 × 2 k-point mesh for the irreducible Brillouin zone, and the self-consistent-field convergence threshold of 10−5 eV for the total electronic energy. We also carry out calculations by employing the allelectron full-potential linearized augmented-plane-wave (FPLAPW) method encoded in the WIEN2k_13.1 package42,43 with RMT*Kmax = 7.0, Gmax = 12, the GGA for the exchangecorrelation functional41 and the energy threshold of −6.0 Ry for the separation of the core and valence states, and 500, 440, 500, 448, 210, and 210 k-points for the irreducible Brillouin zone for X = AsO4, CO3, PO4, SO4, and SeO4, respectively. We use the following sets of basis orbitals: [Kr]4d15s1 for Ag, [Ar]3d104s24p3 for As, [He]2s22p2 for C, [Ne]3s23p3 for P, [Ne]3s23p4 for S, and [He]2s22p4 for O. Figure 4 shows the band dispersion relations obtained for Ag3AsO4, Ag2CO3,

Figure 3. Plots of the bandgap ΔE vs Z/r for Ag3AsO4, Ag2CO3, Ag3PO4, Ag2SO4, and Ag2SeO4. The solid line denotes the regression line for the data points excluding Ag2CO3. 26511

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

substrate molecules to enhance the photocatalyst-assisted degradation.47,48 Thus, the surface charge of the silver oxosalts should have a profound effect on their photocatalytic activities. The surface charges of silver oxosalts are estimated with zeta potential measurements for aqueous suspension of these compounds. All the four silver oxosalts possess a negative surface charge. Among them, Ag3PO4, Ag3AsO4, and Ag2CO3 show large zeta potential values of −50, −35, and −36 mV, respectively, reflecting that their surface is negatively charged. Conversely, a much smaller zeta potential of −2 mV is observed for Ag2SO4. This result clearly demonstrates that the surface charge of the silver oxosalts becomes less negative with increasing the Z/r ratio of the central atoms in the oxosalt groups and can also be explained in terms of the X−O bond covalency in the silver oxosalts. An increase in the Z/r ratio enhances the overlap of the X ns/np with the 2s/2p orbitals of the terminal O atoms exposed to the surface of the compound, hence decreasing the negative charge on the oxygen atoms. It should be noted that the negative surface charge of the silver oxosalts originates largely from their surface-exposed terminal oxygen atoms. Then, the smallest negative charge on the terminal oxygen is expected in the SO42− group because the strongest X−O bond covalency is expected for the S6+−O bonds. This explains the smallest negative charge on the surface of Ag2SO4. Ag2SeO4 has a small zeta potential of −6 mV, as in the case of Ag2SO4, which is also a result of the strong Se6+−O bond covalency. 3.5. Photoluminescence (PL) Spectroscopy. Important factors determining the photocatalytic efficiency of a semiconducting material are the lifetimes of its photogenerated electrons and holes are important as well as its bandgap ΔE.49,50 To probe the recombination behavior of silver oxosalts, the PL spectra of the present materials are examined and plotted in Figure 5. A weak PL peak is found for the salts Figure 4. Band dispersion relations calculated for (a) Ag3AsO4, (b) Ag2CO3, (c) Ag2SO4, (d) Ag2SeO4, and (e) Ag3PO4.

Ag3PO4, Ag2SO4, and Ag2SeO4 from the WIEN2k calculations. Quite similar results are obtained from the corresponding VASP calculations. The plots of the projected density of states (PDOS) calculated for the Ag 4d, Ag 5s, Ag 5p, X ns, X np, O 2s, and O 2p orbitals are summarized in Figures S3−S5 of the Supporting Information, which show that the main contributors to the top portion of the VB are the Ag 4d and O 2p orbitals and those to the bottom portion of the CB the Ag 5s orbitals. These orbital contributions are typical of many Ag+ (d10) systems.44 The bandgaps of Ag2CO3, Ag3PO4, Ag2SO4, and Ag2SeO4, which are indirect, are much smaller than the corresponding experimental values. Furthermore, the bandgap of Ag3AsO4 is predicted to be zero. Such an underestimation of bandgaps is a well-known problem of DFT calculations.45,46 Since the bandgaps of the silver oxosalts are determined by the top of the Ag 4d band and the bottom of the Ag 5s band, one might speculate if the bandgap underestimation is nearly constant for all of them. According to the experimental bandgap of 1.88 eV for Ag3AsO4, for which the calculated bandgap is zero, the bandgap underestimation is about 2.48 eV because the bottom of the Ag 5s band dips below the Fermi level by 0.60 eV. Once the calculated bandgaps are raised by 2.48 eV, they reproduce the experimental bandgaps fairly well, as can be seen from Table 1. 3.4. Zeta Potential Measurement. It is of prime importance to optimize the surface adsorption of organic

Figure 5. PL spectra of silver oxosalts: (a) Ag2SO4, (b) Ag2SeO4, (c) Ag2CO3, (d) Ag3PO4, and (e) Ag3AsO4.

Ag3PO4, Ag3AsO4, and Ag2CO3 with bandgap smaller than 3 eV, but a much more intense PL peak for Ag2SO4 with bandgap greater than 3 eV. This finding indicates that the recombination of electrons and holes in Ag3PO4, Ag3AsO4, and Ag2CO3 is considerably depressed, reflecting the extended lifetime of the photogenerated carriers. The PL feature of Ag2SeO4 with bandgap close to 3 eV is comparable to that of Ag2CO3. The photogenerated electrons and holes of all the present silver oxosalts, except for Ag2SO4, possess a sufficient lifetime for their participation in photocatalytic reactions. 26512

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

Although there are some fluctuations in the order of photocatalytic activity depending on the type of the substrate molecules, the overall photocatalytic efficiency of the present silver oxosalts decreases in the order Ag3PO4 > Ag3AsO4, Ag2CO3 > Ag2SeO4 > Ag2SO4. The relative efficiency of the photocatalyst performance of the other compounds roughly matches with the relative orders of their bandgap energy and PL intensity. In addition, the surface areas of the present silver salts are investigated, since the surface area can affect the photocatalytic activity of semiconducting material. The N2 adsorption−desorption isotherm measurements demonstrate that all the present silver oxosalts are nonporous with small surface area of less than 2 m2 g−1. This allows us to rule out the contribution of the surface area to the difference in the photocatalytic activities of these materials. 3.7. XRD and FE-SEM Analyses after the Photoreactions. To probe the photostability of the silver oxosalts, we carry out powder XRD measurements for the materials used for the photocatalyst tests before and after their use (Figure 7). Except for Ag2SO4, which has only a negligible photocatalytic activity, all the other materials clearly show the occurrence of the XRD peaks corresponding to Ag metal after the photocatalyst test. This result reflects the partial reduction of these photocatalytically active silver compounds under the

3.6. Photocatalytic Activity Measurements. We examine the photocatalytic activities of the silver oxosalts for the degradation of the organic substrates MB, MO, RhB, and phenol while their solutions with suspended photocatalysts are irradiated by visible light (λ > 420 nm). Their decompositions as a function of the irradiation time are summarized in Figure 6, showing that except for Ag2SO4, all the silver oxosalts including newly investigated Ag2SeO4 are fairly active.

Figure 6. Photocatalytic degradation of (a) MB, (b) MO, (c) RhB, and (d) phenol by silver oxosalts (i) Ag2CO3, (ii) Ag3PO4, (iii) Ag3AsO4, (iv) Ag2SO4, and (v) Ag2SeO4.

The photocatalytic efficiency of the silver oxosalts decreases in the order of Ag3PO4 > Ag2SeO4 > Ag2CO3, Ag3AsO4 > Ag2SO4 for the decomposition of MB; Ag3AsO4 > Ag3PO4, Ag2CO3 > Ag2SeO4 > Ag2SO4 for the decomposition of MO; Ag3PO4 > Ag3AsO4 > Ag2CO3, Ag2SeO4 > Ag2SO4 for the decomposition of RhB; and Ag3PO4, Ag2CO3 > Ag3AsO4 > Ag2SeO4 > Ag2SO4 for the decomposition of phenol. The newly investigated Ag2SeO4 is fairly active for the photocatalytic decomposition of the dye molecules (MB, MO, and RhB) under visible light irradiation. In the case of MB decomposition, Ag2SeO4 shows much higher photocatalytic activity than do the Ag2CO3 and Ag3AsO4. Since the MB molecules can be decomposed by both oxidative and reductive pathways,51 the high photocatalytic activity of Ag2SeO4 for MB decomposition would be attributable to its lower CB position, which facilitates the reduction of organic substrate molecules by photogenerated electrons. It is worthwhile to note that in contrast to the other organic substrates, MO can be more effectively decomposed by Ag3AsO4 than by Ag3PO4. The MO molecule with carboxyl group can readily form anionic species.52 Thus, the higher photocatalytic efficiency of Ag3AsO4 for the MO decomposition is attributable to its weaker negative surface charge than that of Ag3PO4. The resulting weak electrostatic repulsion facilitates the surface adsorption of MO molecules on the surface of Ag3AsO4 and hence their decomposition. As can be clearly seen from Figure 6, Ag2SO4 induces only a weak degradation for all the substrate molecules, indicating its poor photocatalytic activity. This is mainly attributable to its large bandgap caused by the high covalency of the S−O bond.

Figure 7. Powder XRD patterns of silver oxosalts (i) before and (ii) after the photocatalytic reaction for (a) Ag2CO3, (b) Ag3PO4, (c) Ag3AsO4, (d) Ag2SO4, and (e) Ag2SeO4. The circles denote the Bragg reflections of Ag metal. 26513

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

irradiation of visible light. Among the photocatalytically active materials, both Ag3PO4 and Ag3AsO4 show a lower XRD intensity for the Ag-related peaks than do the Ag2CO3 and Ag2SeO4 and hence have a higher stability against the photoreduction of Ag+ ions to Ag0. Finally, we examine the morphological change of the silver salts upon the photocatalytic FE-SEM analysis. As presented in Figure 8, both Ag3PO4 and Ag3AsO4 experience a less

prominent variation of the crystal morphology and crystal size than do the other oxosalts, confirming their higher photostability. This finding is well-consistent with the XRD results (Figure 7). In contrast to the XRD results showing no formation of Ag metal, the notable surface corrosion is discernible for Ag2SO4, which is attributed to the high solubility of this compound.53

4. CONCLUSION The oxygen atoms of silver oxosalts, Agx(XOy)z, make two chemical bonds with adjacent Ag and X atoms, leading to the formation of Ag−O−X linkages. We examine how the covalency of the X−O bonds affects the optical bandgap, the surface charge, and the photocatalytic activity of Agx(XOy)z by studying Ag3AsO4, Ag2CO3, Ag3PO4, Ag2SO4, and Ag2SeO4. The VB top has the Ag 4d and O 2s/2p orbitals combined outof-phase, and the CB bottom has the Ag 5s and O 2s/2p orbitals combined out-of-phase. A shortening of the Ag−O bond decreases the optical bandgap since the Ag 4d orbital is much more contracted than the Ag 5s orbital. The optical bandgaps of these salts are well described by the bond covalency competition in the Ag−O−X linkages. In particular, the optical bandgap is found to increase linearly with increasing the Z/r ratio of the atom X, which is a simple measure of the X−O bond covalency. The surface charge of Agx(XOy)z, a crucial factor governing its photocatalytic activity for the degradation of charged substrate molecules, decreases with increasing the covalency of the X−O bond due to the terminal O atoms of the XOy groups. The silver oxosalt, Ag2SeO4, newly synthesized and characterized in this work, exhibits a promising photocatalytic activity under visible light. The present results clearly demonstrate the Z/r ratio of central atom provides a facile and effective measure for predicting the photocatalyst performance and the optical bandgap of silver oxosalts Agx(XOy)z. It would be of interest to further optimize the photocatalyst performance of silver oxosalts by tuning their electronic structures in terms of partially substituting the cations and anions.

■

ASSOCIATED CONTENT

S Supporting Information *

Crystal structure models of silver oxosalts, the experimental and theoretical XRD patterns of Ag2SeO4, and the PDOS plots of Ag-, X-, and O-related states for silver oxosalts. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-J.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by Korea Ministry of Environment as “Converging Technology Project” (191-101-001) and by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. The work at NCSU was supported by the computing resources of the NERSC center and the HPC center of NCSU.

Figure 8. FE-SEM images of silver oxosalts (left) before and (right) after the photocatalytic reaction: (a) Ag2CO3, (b) Ag3PO4, (c) Ag3AsO4, (d) Ag2SO4, and (e) Ag2SeO4. 26514

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

■

Article

(19) Dai, G.; Yu, J.; Liu, G. A New Approach for Photocorrosion Inhibition of Ag2CO3 Photocatalyst with Highly Visible-LightResponsive Reactivity. J. Phys. Chem. C 2012, 116, 15519−15524. (20) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (21) Kim, T. W.; Hur, S. G.; Han, A. R.; Hwang, S.-J.; Choy, J.-H. Effect of Bond Covalency on the Lattice Stability and Fatigue Behavior of Ferroelectric Bismuth Transition-Metal Oxides. J. Phys. Chem. C 2008, 112, 3434−3438. (22) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry; Harper Collins: New York, 1993. (23) Tsuji, I.; Kato, H.; Kudo, A. Photocatalytic Hydrogen Evolution on ZnS−CuInS2−AgInS2 Solid Solution Photocatalysts with Wide Visible Light Absorption Bands. Chem. Mater. 2006, 18, 1969−1975. (24) Barakat, M. A.; Schaeffer, H.; Hayes, G.; Shah, S. I. Photocatalytic Degradation of 2-Chlorophenol by Co-Doped TiO2 Nanoparticles. Appl. Catal. B: Environ. 2004, 57, 23−30. (25) Tan, A.; Harris, S. Extension of the Fenske-Hall Molecular Orbital Approach to Tight Binding Band Structure Calculations: Bulk and Surface Electronic Structure of MoS2. Inorg. Chem. 1998, 37, 2205−2214. (26) Malinowski, P. J.; Mazej, Z.; Derzsi, M.; Jagličić, Z.; Szydłowska, J.; Gilewski, T.; Grochala, W. Silver(II) Triflate with One-Dimensional [Ag(II)(SO3CF3)4/2]∞ Chains Hosting Antiferromagnetism. CrystEngComm 2011, 13, 6871−6879. (27) Ouyang, S.; Zhang, H.; Li, D.; Yu, T.; Ye, J.; Zou, Z. Electronic Structure and Photocatalytic Characterization of a Novel Photocatalyst AgAlO2. J. Phys. Chem. B 2006, 110, 11677−11682. (28) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Covalent Radii Revisited. Dalton Trans. 2008, 2832−2838. (29) Schrock, R. R. High Oxidation State Multiple Metal−Carbon Bonds. Chem. Rev. 2002, 102, 145−179. (30) Kang, S. O.; Begum, R. A.; James, K. B. Amide-Based Ligands for Anion Coordination. Angew. Chem., Int. Ed. 2006, 45, 7882−7894. (31) Masse, R.; Guitel, J. C.; Durif, A. Structure du Carbonate d’Argent. Acta Crystallogr., Sect. B 1979, 35, 1428−1429. (32) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A 1976, A32, 751−767. (33) Umezawa, N.; Shuxin, O.; Ye, J. Theoretical Study of High Photocatalytic Performance of Ag3PO4. Phys. Rev. B 2011, 83, 035202(1−8). (34) Helmholtz, L.; Levine, R. A Determination of Parameters in Potassium Dihydrogen Arsenate and Silver Arsenate. J. Am. Chem. Soc. 1942, 64, 354−358. (35) Masse, R.; Tordjman, I.; Durif, A. Affinement de la Structure Cristalline du Monophosphate d’Argent Ag3PO4. Existence d’une Forme Haute Temperature. Z. Kristallogr., Kristallgeometr., Kristallphys., Kristallchem. 1976, 144, 76−81. (36) Weil, M. Investigations in the Systems Ag−Hg−X−O (X = As(V), Se(IV), Se(VI)): Hydrothermal Single Crystal Growth of Ag3AsO4, AgHg(I)AsO4, AgHg(II)AsO4, Ag2SeO4 and the Crystal Sturcture of Ag2Hg(II)(SeO3)2. Z. Naturf., Teil B: Anorg. Chem., Org. Chem. 2003, 58, 1091−1096. (37) Zachariasen, W. H. Note on the Crystal Structure of Silver Sulphate, Ag2SO4. Z. Kristallogr., Kristallgeometr., Kristallphys., Kristallchem. 1932, 82, 161−162. (38) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561. (39) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (40) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.

REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C 2007, 111, 2834−2860. (3) Yadav, R. K.; Baeg, J.-O.; Oh, G. H.; Park, N.-J.; Kong, K.; Kim, J.; Hwang, D. W.; Biswas, S. K. A Photocatalyst−Enzyme Coupled Artificial Photosynthesis System for Solar Energy in Production of Formic Acid from CO2. J. Am. Chem. Soc. 2012, 134, 11455−11461. (4) Ranjit, K. T.; Willner, I.; Bossmann, S.; Braun, A. Iron(III) Phthalocyanine-Modified Titanium Dioxide: A Novel Photocatalyst for the Enhanced Photodegradation of Organic Pollutants. J. Phys. Chem. B 1998, 102, 9397−9403. (5) Rodrigues, S.; Ranjit, K. T.; Uma, S.; Martyanov, I. N.; Klabunde, K. J. Single-Step Synthesis of a Highly Active Visible-Light Photocatalyst for Oxidation of a Common Indoor Air Pollutant: Acetaldehyde. Adv. Mater. 2005, 17, 2467−2471. (6) Das, J.; Khushalani, D. Nonhydrolytic Route for Synthesis of ZnO and Its Use as a Recyclable Photocatalyst. J. Phys. Chem. C 2010, 114, 2544−2550. (7) Kim, I. Y.; Lee, J. M.; Kim, T. W.; Kim, H. N.; Kim, H.; Choi, W.; Hwang, S.-J. A Strong Electronic Coupling between Graphene Nanosheets and Layered Titanate Nanoplates: A Soft-Chemical Route to Highly Porous Nanocomposites with Improved Photocatalytic Activity. Small 2012, 8, 1038−1048. (8) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. Enhancement of Photocatalytic H2 Evolution on CdS by Loading MoS2 as Cocatalyst under Visible Light Irradiation. J. Am. Chem. Soc. 2008, 130, 7176−7177. (9) Arai, T.; Senda, S. -i.; Sato, Y.; Takahashi, H.; Shinoda, K.; Jeyadevan, B.; Tohji, K. Cu-Doped ZnS Hollow Particle with High Activity for Hydrogen Generation from Alkaline Sulfide Solution under Visible Light. Chem. Mater. 2008, 20, 1997−2000. (10) Niishiro, R.; Konta, R.; Kato, H.; Chun, W.-J.; Asakura, K.; Kudo, A. Photocatalytic O2 Evolution of Rhodium and AntimonyCodoped Rutile-Type TiO2 under Visible Light Irradiation. J. Phys. Chem. C 2007, 111, 17420−17426. (11) Gunjakar, J. L.; Kim, T. W.; Kim, H. N.; Kim, I. Y.; Hwang, S.-J. Mesoporous Layer-by-Layer Ordered Nanohybrids of Layered Double Hydroxide and Layered Metal Oxide: Highly Active Visible Light Photocatalysts with Improved Chemical Stability. J. Am. Chem. Soc. 2011, 133, 14998−15007. (12) Mohapatra, L.; Parida, K.; Satpathy, M. Molybdate/Tungstate Intercalated Oxo-Bridged Zn/Y LDH for Solar Light Induced Photodegradation of Organic Pollutants. J. Phys. Chem. C 2012, 116, 13063−13070. (13) Teramura, K.; Iguchi, S.; Mizuno, Y.; Shishido, T.; Tanaka, T. Photocatalytic Conversion of CO2 in Water over Layered Double Hydroxides. Angew. Chem., Int. Ed. 2012, 51, 8008−8011. (14) Tang, J.; Zou, Z.; Ye, J. Efficient Photocatalysis on BaBiO3 Driven by Visible Light. J. Phys. Chem. C 2007, 111, 12779−12785. (15) Wang, P.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Wei, J.; Whangbo, M.-H. Ag@AgCl: A Highly Efficient and Stable Photocatalyst Active under Visible Light. Angew. Chem., Int. Ed. 2008, 120, 8049−8051. (16) Zhu, M.; Chen, P.; Liu, M. Graphene Oxide Enwrapped Ag/ AgX (X = Br, Cl) Nanocomposite as a Highly Efficient Visible-Light Plasmonic Photocatalyst. ACS Nano 2011, 5, 4529−4536. (17) Ye, L.; Liu, J.; Gong, C.; Tian, L.; Peng, T.; Zan, L. Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge. ACS Catal. 2012, 2, 1677−1683. (18) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; Williams, H. S.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. An Orthophosphate Semiconductor with Photooxidation Properties under Visible-Light Irradiation. Nat. Mater. 2010, 9, 559−564. 26515

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516

The Journal of Physical Chemistry C

Article

(42) Blahs, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2k_13.1, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties; Techn. Univ.: Wien, 2013. (43) Singh, D. J. Planewaves, Pseudopotentials and the LAPW Method; Kluwer Academic: Boston, 1994. (44) Wang, L.; Cao, B.; Kang, W.; Hybertsen, M.; Maeda, K.; Domen, K.; Khalifah, P. G. Design of Medium Band Gap Ag−Bi−Nb− O and Ag−Bi−Ta−O Semiconductors for Driving Direct Water Splitting with Visible Light. Inorg. Chem. 2013, 52, 9192−9205. (45) Rocquefelte, X.; Goubin, F.; Koo, H.-J.; Whangbo, M.-H.; Jobic, S. Investigation of the Origin of the Empirical Relationship between Refractive Index and Density on the Basis of First Principles Calculations for the Refractive Indices of Various TiO2 Phases. Inorg. Chem. 2004, 43, 2246−2251. (46) Rocquefelte, X.; Goubin, F.; Montardi, Y.; Viadere, N.; Demourgues, A.; Tressaud, A.; Whangbo, M.-H.; Jobic, S. Analysis of the Refractive Indices of TiO2, TiOF2, and TiF4: Concept of Optical Channel as a Guide To Understand and Design Optical Materials. Inorg. Chem. 2005, 44, 3589−3593. (47) Miyauchi, M.; Ikezawa, A.; Tobimatsu, H.; Irie, H.; Hashimoto, K. Zeta Potential and Photocatalytic Activity of Nitrogen Doped TiO2 Thin Films. Phys. Chem. Chem. Phys. 2004, 6, 865−870. (48) Zhao, J.; Wu, T.; Wu, K.; Oikawa, K.; Hidaka, H.; Serpone, N. Photoassisted Degradation of Dye Pollutants. 3. Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO2 Dispersions under Visible Light Irradiation: Evidence for the Need of Substrate Adsorption on TiO2 Particles. Environ. Sci. Technol. 1998, 32, 2394−2400. (49) Liu, Z.; Sun, D. D.; Guo, P.; Leckie, J. O. An Efficient Bicomponent TiO2/SnO2 Nanofiber Photocatalyst Fabricated by Electrospinning with a Side-by-Side Dual Spinnere Method. Nano Lett. 2007, 7, 1081−1085. (50) Kim, H. N.; Kim, T. W.; Kim, I. Y.; Hwang, S.-J. Cocatalyst-Free Photocatalysts for Efficient Visible-Light-Induced H2 Production: Porous Assemblies of CdS Quantum Dots and Layered Titanate Nanosheets. Adv. Funct. Mater. 2011, 21, 3111−3118. (51) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J.-M. Photocatalytic Degradation Pathway of Methylene Blue in Water. Appl. Catal. B: Environ. 2001, 31, 145−157. (52) Luan, J.; Ma, K.; Wang, S.; Hu, Z.; Li, Y.; Pan, B. Research on Photocatalytic Degradation Pathway and Degradation Mechanisms of Organics. Curr. Org. Chem. 2010, 14, 645−682. (53) Lide, D. E., Ed.; Handbook of Chemstry and Physics, 77th ed.; CRC Press: NewYork, 1996.

26516

dx.doi.org/10.1021/jp4101784 | J. Phys. Chem. C 2013, 117, 26509−26516