Ag 6 Si 2 O 7 : a silicate photocatalyst for the visible region

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1 Supporting information Ag 6 Si 2 O 7 : a silicate photocatalyst for the visible region Zaizhu Lou, Baibiao Huang,*, Zeyan Wang, Xiangchao Ma, Rui Zhang, Xiaoyang Zhang, Xiaoyan Qin, Ying Dai and Myung-Hwan Whangbo State Key Laboratory of Crystal Materials and School of Physics, Shandong University Jinan , P. R. China, bbhuang@sdu.edu.cn Department of Chemistry, North Carolina State University, Raleigh, North Carolina , USA

2 1. Experimental details All reagents were analytical grade. AgNO 3, Na 2 SiO 3 9H 2 O, methylene blue, Na 2 -EDTA, NaOH, Na 3 PO 4, benzoquinone, tert-butanol were purchased and used without any further purification. Synthesis of the Ag 6 Si 2 O 7 : In a general procedure, g Na 2 SiO 3 was mixed with the 70 ml deionized water under constant stirring. Then the solution was added slowly into 30 ml 0.1M AgNO 3 solution to generate reddish brown precipitates. After 30 minutes stirring, the samples (i.e., the precipitates) were separated from the solution by filtering. The reddish brown powders were obtained after washing with deionized water and ethanol for three times, dried at room temperature for 6 h. For comparison, the Ag 2 O and Ag 3 PO 4 were prepared by a direct precipitation method. 20 ml 0.1 M AgNO 3 solution was mixed with the 70 ml solution containing 2 mmol NaOH under constant stirring for 30 min. 30 ml 0.1 M AgNO 3 solution was mixed with 70 ml solution containing 1 mmol Na 3 PO 4 under constant stirring for 30 min. The precipitates were separated from the solution by filtering. The Ag 2 O and Ag 3 PO 4 samples were collected after washing with deionized water and ethanol, dried at room temperature for 6 h. Photocatalytic reactions: 0.1 g Ag 6 Si 2 O 7 samples were mixed with 100 ml MB solution (20 mg/l) in a 200 ml breaker. The mixture solution was stirred for 30 min in the dark to achieve the dye absorption balance on the surface of catalysts, and then photocatalystic reactions were carried out with stirring under visible light provided by a 300 W Xe arclamp (PLS-SXE300, Beijing Trusttech Co. Ltd.) equipped with different cut-off filters to cut off lights shorter than 420, 570 and 700nm wavelengths. 5 ml solution was taken out at certain time intervals until the solution was blanched, and the concentration of MB was tested by measuring the absorption of aqueous MB solution with Shimadzu UV2550 recording spectrophotometer. Characterization: The X-ray diffraction patterns (XRD) of samples were obtained on Bruker D8-advanced X-ray powder diffractometer with Cu Kα radiation λ= Å. The scanning electron microscopy (SEM) measurements were carried out by Hitachi S-4800 microscope. The elemental composition was detected by X-ray photoelectron spectroscopy

3 measurements (VG MicroTech ESCA 3000 X-ray photoelectron spectroscope). UV-Vis diffuse reflectance spectra were recorded for the dry-pressed disk samples by a Shimadzu UV2550 recording spectrophotometer in the wavelength range nm. The surface areas of the samples were measured by using the Brunauer Emmett Teller method with a Builder 4200 instrument at liquid nitrogen temperature. The total of organic carbon (TOC) of MB solution was measured by using Total Organic Analyzer (TOC-VCPH, Shimadzu Corp.) Photoacoustic Field Gas-Monitor (INNOVA 1412) was used to measure acetone and CO 2 during the degradation of acetone, and GC for the measurement of O 2. Calculations of electronic structures: The electronic structures are obtained based on the spin-polarized density functional theory (DFT) calculations using the projector augmented wave (PAW) pseudopotentials as implemented in the Vienna ab initio Simulation Package (VASP) code [1, 2]. Heyd Scuseria Ernzerhof (HSE06) hybrid functional is used in our calculations, which can provide good physical descriptions of electronic structures of semiconductors [3-5]. A unit cell with the lattice parameters (a = Å, b = Å, c = Å; = o ) is used to simulate the bulk Ag 6 Si 2 O 7. The plane wave cutoff energy of 400 ev was employed to provide sufficient precision. The Brillouin zone is sampled with Г-centered k-points [6]. The atomic positions of all atoms are fully relaxed until the residual force is smaller than 0.02 ev/å. The total and projected densities of states were obtained using Blöchl et al. s method [7]. References [1] G. Kresse and J. Hafner, Phys. Rev. B 1993, 47, 558. [2] G. Kresse and J. Furthmüller, Phys. Rev. B 1996, 54, [3] J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 2003,118, [4] J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 2006,124, [5] M. Choi, F. Oba, Y. Kumagai, I. Tanaka, Adv. Mater., 2013, 25, [6] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, [7] P. E. Blöchl, O. Jepsen and O. K. Andersen, Phys. Rev. B 1994, 49,

4 2. Estimation of the VB and CB edges of Ag 6 Si 2 O 7 The CB edge of a semiconductor at the point of zero charge ( E 0 CB ) is empirically expressed as [1-3], 0 e 1 ECB comp 2.30RT (phzpc ph) / F E E g), (1) 2 where R is the gas contant, T the temperature, and F the Faraday constant. E g is the band gap of the semiconductor, and E e the energy of the free electrons on the hydrogen scale (i. e., e E 4.5 ev ). Under the reasonable assumption that the solution s ph value at the zero point of charge, ph ZPC, is very close to the solution s ph value, ph, we obtain comp 0 e 1 ECB ECB comp E Eg (2) 2 is the electronegativity of a compound which is given by the geometric mean of the electronegativities of the constituent atoms, that is [4],, (3) N r s p q comp 1 2 n 1 n where, n, and N are the electronegativity of the constituent atom, the number of the species, n and the total number of atoms in the compound, respectivity. The superscripts r, s, p,..., q refer to the numbers of the atoms 1, 2,..., n-1 and n, respectively in the molecule, respectively, so that r + s + + p + q = N. From its UV/Vis diffuse reflectance spectrum, the band gap of Ag 6 Si 2 O 7 is estimated to be 1.58 ev. The values of O, Si and Ag are 7.54, 4.77, 4.44, respectively [5]. Thus, from Eq. 2, the CB edge of Ag 6 Si 2 O 7 is estimated to be 0.44 ev with respect to the normal hydrogen electode (NHE), and ev with respect to the absolute vacuum scale (AVS). Consequently, on the basis of its band gap (1.58 ev), the VB edge of Ag 6 Si 2 O 7 is determined to be 2.02 ev with respect to the NHE, and ev with respect to the AVS.

5 References [1] S. R. Morrison, Electrochemistry at Semiconductor and Oxidized MetalElectrodes, Plenum Press, New York, [2] M. A. Butler, D. S. Ginley, J. Electrochem. Soc. 1978, 125, [3]Y. Xu, M. Schoonen, Am. Mineral, 2000, 85, [4] R. T. Sanderson, Chemical Periodicity, Reinhold, New York [5] D. Yu, Z.-D. Chen, F. Wang, S.-Z. Li, Acta Phys. Chim. Sin. 2001, 17,

6 Figure S1. Photograph of powder samples of Ag 6 Si 2 O 7. Figure S2. (A, B) SEM image, (C) XRD patterns and (D) EDS spectra of Ag 6 Si 2 O 7 samples prepared by using the traditional precipitation method.

7 Figure S3. XPS spectra of the Ag 3d states of Ag 6 Si 2 O 7 particles Figure S4. TOC (total organic carbon), TC (total carbon), and IC (inorganic carbon) of the MB solution over Ag 6 Si 2 O 7.

8 Figure S5. Comparison of the photo-degradation of MB over Ag 6 Si 2 O 7 and N-doped p25 as photocatalysts under visible light irradiation. Figure S6. UV/Vis absorbance spectra of a RhB solution (20 mg/l) during the photocatalytic degradation process over Ag 6 Si 2 O 7 as photocatalyst under visible light illumination. Figure S7. Concentrations of CO 2 and acetone during the photo-degradation of isopropanol.

9 Figure S8. Photocatalytic degradation of phenol and 2,4-DCP (20 mg/l) over Ag 6 Si 2 O 7 photocatalyst under visible-light irradiation. Figure S9. UV/Vis absorbance spectra of a Cr 6+ solution over Ag 6 Si 2 O 7 as photocatalyst under visible-light illumination.

10 Figure S10 How the photo-degradation of MB over Ag 6 Si 2 O 7 depends on adding the 1 mm solution of AgNO 3 (red), Na 2 EDTA (black), benzoquinone (green) and tert-butanol (reddish brown) into the MB solution. To identify the major active species in the degradation of MB over Ag 6 Si 2 O 7, radical-trapping experiments were carried out by using benzoquinone (a scavenger of superoxide anion radicals), disodium ethylenediaminetetraacetate (Na 2 EDTA) (a hole scavenger), AgNO 3 (an electron scavenger) and tert-butanol (an OH radical scavenger). As shown in Fig. S10, when the Na 2 EDTA was added into the MB solution, the concentration of MB does not decrease but increases with the evaporation of water, indicating that the photo-generated hole is the main active species for the degradation of MB. The degradation rate changes slightly in the presence of benzoquinone, suggesting that superoxide anion radicals are not main active species in degrading MB. By the addition of 1 mm AgNO 3 or tert-butanol, the degradation rate is hardly changed. The reduction potential of Ag + /Ag and O 2 /H 2 O is ev and 1.23 ev, respectively. Thus, the photo-generated electrons are trapped by the oxygen dissolved in the solution. However, some Ag + ions are reduced to Ag, as can be seen from the color change in the samples undergoing reaction.

11 Figure S11 Transmission % of the optical filters for λ (> 420, 570 and 700 nm) and absorbance spectra of MB(black). Figure S12 Reapeted experiments for the decomposition of MB over Ag 2 Si 2 O 7

12 Figure S13 XPS spectra of the Ag 3d states of Ag 6 Si 2 O 7 samples (a) before and (b-d) after first, second and third cycle photocatalytic reactions Ag + ions in most Ag-based compound are easily reduced to be Ag 0, so the color of such compounds will change after their photocatalytic reaction. We measured the contents of Ag formed in Ag 6 Si 2 O 7 after three cycles of consecutive photocatalytic reactions by the XPS. As shown in Fig. S14, peaks of Ag 0 3d are present in the XPS spectra of the samples, and the content of Ag 0 is about 5%, 5% and 6% of the total Ag after first, second and third cycles, respectively. The repeated photocatalytic reactions were also carried as shown in Fig. S13; the activities of the samples in the recycle time 2 and 3 decrease somewhat, which can be attributed to the formation of Ag 0 on surface and the loss of samples in the recycle experiment. The formed Ag 0 reduces the specific surface area and light absorption of samples, resulting in the decrease in the activity of a photocatalytic reaction (W. G. Wang, B. Cheng, J. G. Yu, G. Liu and W. H. Fan, Chemistry-An Asian Journal, 2012, 7, ).

13 Figure S14 Crystal structures of Ag 2 O, Ag 3 PO 4 and Ag 6 Si 2 O 7 3. Calculations of the Ag-O bond valences and dipole moments of the AgO n (n = 2, 3, 4) polyhedra and the Si-O bond valences and dipole moments of the SiO 4 tetrahedra in Ag 6 Si 2 O 7 We note that the bond-valence-sum V i for each atom i is defined as (1) V S exp[(r R ) / B] i ij 0 ij j j where B = R 0 and R ij are the reference and actual lengths of the bond i-j that the atom i makes with the surrounding atoms j, respectively, and S ij is the corresponding bond valence [1]. Let R represent the difference between the centroids of the positive (r + ) and negative charges (r - ) Then, for a given Ag i -O j bond with the actual bond length R ij, R = r r + (2) Given that the nuclear charges of Ag and O are 47 and 8, respectively, we have (47 V i ) r = (8 + S ij ) (R ij r ) (3) 47 r + = 8(R ij r + ) (4)

14 Then, in units of Debye, in which R is measured by Å and the charge by statcoulomb, the net bond dipole moment μ ij of the Ag i -O j bond is then calculated by using the expression [2]. μ ij = n ij er (5) n ij =(47 V i ) + (8 + S ij ) (6) where n ij is the number of electrons forming the Ag i -O j, and e the electron charge (i.e., statcoulombs in cgs unit). References [1] I. Brown, D. Altermatt, Acta Crystallographica Section B: Structural Science, 1985, 41, 244. [2] A. M. Paul, S. N. Tiffany, L. S. Charlotte, R. P. Kenneth, Journal of Solid State Chemistry, 2003, 175, 27. Table S1. Bond lengths (R ij ), bond valence (S ij ), bond valence sum (V i ) and net dipole moment (μ ij ) of the Si-O bonds in SiO 4 O1 O2 O3 O4 Vi R ij S ij μ ij O5 O6 O7 O4 R ij S ij μ ij O8 O9 O10 O11 R ij S ij μ ij O12 O13 O14 O11 R ij

15 S ij μ ij

16 Table S2. Bond lengths (R ij ), bond valence (S ij ), bond valence sum (V i ) and net dipole moment (μ ij ) of the Ag-O bonds i-j in the Ag(a)O 2, Ag(b)O 3 and Ag(c)O 4 units O3 O12 O14 Vi Ag(b)O 3 R ij S ij μ ij O5 O6 O9 R ij S ij μ ij O5 O10 O14 R ij S ij μ ij O2 O4 O6 O13 Ag(c)O 4 R ij S ij μ ij O1 O2 O3 Ag(b)O 3 R ij S ij μ ij O2 O3 O7 R ij S ij μ ij

17 O9 O10 O14 R ij S ij μ ij O6 O8 O13 O14 Ag(c)O 4 R ij S ij μ ij O7 O12 Ag(a)O 2 R ij S ij μ ij O8 O9 O10 Ag(b)O 3 R ij S ij μ ij O8 O10 O13 R ij S ij μ ij Ag(a)O 2 O1 O7 R ij S ij μ ij

18 Table S3. Net dipole moment (μ ij ) of SiO 4 units along the a, b,c -axises O1 O2 O3 O4 μ i (a,b, c) μ ij μ ij (a) μ ij (b) μ ij (c) O5 O6 O7 O4 μ ij μ ij (a) μ ij (b) μ ij (c) O8 O9 O10 O11 μ ij μ ij (a) μ ij (b) μ ij (c) O12 O13 O14 O11 μ ij μ ij (a) μ ij (b) μ ij (c)

19 Table S4. Net dipole moments (μ ij ) of the Ag(a)O 2, Ag(b)O 3 and Ag(c)O 4 units along the a, b,c axises in terms of their Ag-O bond dipole moments O3 O12 O14 μ ij (a, b, c) Ag(b)O 3 μ ij μ ij (a) μ ij (b) μ ij (c) O5 O6 O9 μ ij μ ij (a) μ ij (b) μ ij (c) O5 O10 O14 μ ij μ ij (a) μ ij (b) μ ij (c) O2 O4 O6 O13 Ag(c)O 4 μ ij μ ij (a) μ ij (b) μ ij (c) O1 O2 O3 Ag(b)O 3 μ ij μ ij (a) μ ij (b) μ ij (c)

20 O2 O3 O7 μ ij μ ij (a) μ ij (b) μ ij (c) O9 O10 O14 μ ij μ ij (a) μ ij (b) μ ij (c) O6 O8 O13 O14 Ag(c)O 4 μ ij μ ij (a) μ ij (b) μ ij (c) O7 O12 Ag(a)O 2 μ ij μ ij (a) μ ij (b) μ ij (c) Ag(b)O 3 O8 O9 O10 μ ij μ ij (a) μ ij (b) μ ij (c)

21 O8 O10 O13 μ ij μ ij (a) μ ij (b) μ ij (c) Ag(a)O 2 O1 O7 μ ij μ ij (a) μ ij (b) μ ij (c) In terms of the bond dipole moments listed in Tables S1 S4, the dipole moment of a unit cell is calculated as: ( ) [ ( )] ( ) [ ( )] ( ) [ ( )]