Communication pubs.acs.org/cm
Ag6Si2O7: 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 250100, P. R. China § Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *
S
ince the demonstration of photocatalytic water splitting,1 photocatalysis has attracted much attention. Extensive studies in this field during the last few decades have led to a variety of photocatalysts,2−10 but their photocatalytic activities are still low. The efficiency of a photocatalyst is determined mainly by its band gap and by its ability to separate photogenerated charge carriers (i.e., electron−hole pairs). To facilitate the separation of these charge carriers, it is desirable that a strong driving force for the charge separation be present throughout the entire photocatalyst.11 Our study on BiOIO3 showed12 that the separation of the charge carriers is enhanced when there exists internal polar electric field, while our study on Cu2(OH)PO4 showed13 that it is enhanced when there exist nonequivalent metal−oxygen polyhedra interconnected by metal−oxygen−metal bridges. Thus, it is expected that photocatalysts possessing both of the two structural features should have a synergetically enhanced, strong photocatalytic activity. In this Communication, we show that this is indeed the case. Silicates are ubiquitous materials with abundant reserves, and have been widely used as industrial catalysts.14 So far, however, no silicate photocatalyst has been reported. In the presence of transition-metal cations the SiO4 tetrahedra in silicates can be easily distorted and get polarized, so it would be possible to construct an internal polar electric field by controlling the arrangement of the polar SiO4 tetrahedra. Furthermore, the presence of several different coordination environments for transition-metal cations can enhance not only the preferential migration of photogenerated charge carriers from one metal− oxygen polyhedra to another but also the optical absorption at various wavelengths. Therefore, it is likely that photocatalysts possessing the aforementioned two structural features will be found among silicates. In addition, given the low cost and abundant reserves, silicate-based photocatalysts will be desirable for practical applications. Our search for such photocatalysts led to Ag6Si2O7, which has an internal polar electric field set along the b-direction (see below) and consists of two-, three- and four-coordinate Ag+ ions leading to AgO2, AgO3, and AgO4 units, respectively. Ag6Si2O7 is photocatalytic nearly in the whole visible-light region (λ < 740 nm) and exhibits a very strong photocatalytic activity. Ag6Si2O7 was prepared by slowly adding Na2SiO3 into an AgNO3 solution. The reddish brown powder samples of Ag6Si2O7, shown in Figure S1 in the Supporting Information (SI), were prepared by the hydrolysis and ion-exchange between Ag+ and SiO32− ions in the solution, which can be described as © 2014 American Chemical Society
6Ag + + 2SiO32 − + 2OH− → Ag 6Si 2O7 + H 2O
The as-prepared samples are irregular in shape with size of 100 nm, which can be observed in the SEM images (Figure S2 in the Supporting Information). Ag6Si2O7 crystallizes in a monoclinic system with space group P21 and the cell parameters, a = 5.304 Å, b = 9.753 Å, c = 15.928 Å and β = 91.165° (Figure 1a). The purity of the prepared Ag6Si2O7
Figure 1. Crystal structure of Ag6Si2O7. (a) Atoms of a unit cell. Red circle = O, gray circle = Si, green cylinder = Ag−O bond, gray cylinder = Si−O bond, pink circle = Ag atom of AgO2, yellow circle = Ag of AgO3, and green circle = Ag of AgO4. (b) The arrangements of the Si2O7 units.
samples was determined by XRD and EDS measurements. As shown in Figure S2 in the Supporting Information, the XRD peaks of the samples match those of the standard Ag6Si2O7 structure (JCPDS no. 85−281). The EDS spectrum (see Figure S2 in the Supporint Information) shows the presence of only three elements, O, Si and Ag, with atomic percentages 58.03, 10.25, and 31.72%, respectively. The resulting atom ratio of Si to Ag is around 1:3, which further confirms that the as-prepared samples are Ag6Si2O7. The XPS measurements (see Figure S3 in the Supporting Information) show that two individual peaks exist in the Ag 3d spectrum of Ag6Si2O7 at about 374.3 and 368.2 eV, which can be attributed to the Ag 3d3/2 and Ag 3d5/2 binding energies, respectively. Received: February 24, 2014 Revised: June 10, 2014 Published: June 12, 2014 3873
dx.doi.org/10.1021/cm500657n | Chem. Mater. 2014, 26, 3873−3875
Chemistry of Materials
Communication
in 5 and 9 min, respectively. However, only 78 and 42% of MB are decomposed over Ag 2 O and Ag 3PO 4 in 40 min, respectively. Thus, the degradation of MB over Ag6Si2O7 is faster than that over Ag2O by a factor of 5−9, and that over Ag3PO4 by a factor of 9−11. The main cause for the high photocatalytic activity of Ag6Si2O7 is not the surface area but lies in its intrinsic electronic structure because the surface areas of the Ag6Si2O7, Ag2O and Ag3PO4 samples are 17.726, 4.535, and 27.831 m2/g, respectively, according to our nitrogen absorption−desorption isotherm measurements. To confirm a complete degradation of MB during the photocatalytic process, the total organic carbon of the MB solution was measured over the photocatalyst Ag6Si2O7. Figure S4 in the Supporting Information reveals that the total organic carbon and the total carbon decrease with increasing the inorganic carbon (i.e., CO2) during the photocatalytic process. In 30 min 50% organic carbon was decomposed into inorganic carbon, indicating that MB is decomposed completely into inorganic carbon, which is vaporized out of the solution. Ag6Si2O7 has a much stronger photocatalytic activity than does the traditional N-doped p25, as shown in Figure S5 in the Supporting Information. In addition, Ag6Si2O7 photocatalytically degrades rhodamine B, isopropanol, phenol, and 2,4-DCP, and reduces Cr6+ ions (see Figures S6−S9 in the Supporting Information). With broad absorption in the visible-light region (λ < 740 nm), Ag6Si2O7 should be photocatalytically active nearly in the whole visible-light region. We test this implication by investigating the wavelength dependence of the photocatalytic activity with different optical filters to cut off lights with λ > 420, 570, and 700 nm (Figure 2c). As shown in Figure 2d, under the irradiation of visible light (λ > 420 nm), MB is decomposed completely in 10 min as discussed above. When the light with λ > 570 nm is supplied, the degradation of MB is slow and over 80% of MB is decomposed in 66 min. MB is also decomposed, though slowly, under the illumination of red lights (λ > 700 nm). The absorption peak of MB (650 nm, see Figure S11 in the Supporting Information) and the results of our experiments with light filters for λ (>420, 570, and 700 nm) confirm that the dye sensitization is not the main cause for the MB degradation. The stability of Ag6Si2O7 as photocatalysts was studied, as summarized in Figures S12 and S13 in the Supporting Information (for further discussions, see the Supporting Information). Ag6Si2O7 is photocatalytically active in a wide visible-light region between 400 and 740 nm. Our photocatalysis experiments with added hole- and electronquenchers show that the active species in the degradation of MB are the photogenerated holes (see Figure S10 in the Supporting Information). To examine the probable cause for the high photocatalytic activity, the electronic structure of Ag2Si2O7 was studied by DFT calculations, and the bond dipole moments of the system were investigated based on the bond-valence model,16 as described in the Supporting Information. From the bond dipole moments of the SiO4 and AgOn (n = 2−4) units (Tables S3 and S4 in the Supporting Information), Ag6Si2O7 is found to possess a nonzero electric polarization along the b-axis direction, as expected from the fact that it belongs to the space group of P21. This can be readily seen from Figure 1b, which shows that the Si−O bond dipole moments are canceled out along the a- and c-directions, but are not along the bdirection. The existence of a polar electric field should promote the separation of photogenerated charge carriers effectively enhancing the photocatalytic performance of Ag6Si2O7. As
The SiO4 tetrahedra of Ag6Si2O7 are corner-shared to form Si2O7 units (Figure 1b), which provide 12 nonequivalent Ag+ (d10) ion sites (Figure 1a). Provided that the Ag−O bonds are taken to be shorter than 2.65 Å, the 12 different Ag+ ions form two different AgO2 units, seven different AgO3 units, and three different AgO4 units (Figure 1a). Hereafter, the Ag atoms leading to AgO2, AgO3, and AgO4 units are referred to as the Ag(a), Ag(b), and Ag(c), respectively, for the convenience of our discussion. The Ag(c)O4 units share corners to form chains running along the a-direction with Ag(c)−O−Ag(c) bridges, whereas the Ag(b)O3 units share corners to form a threedimensional lattice with Ag(b)−O−Ag(b) bridges. The Ag(a)O2 units are corner-shared with the Ag(b)O3 units, but not with the Ag(c)O4 units. The UV/vis diffuse reflectance spectrum of Ag6Si2O7 is compared with those of Ag2O and Ag3PO4 in Figure 2a.
Figure 2. (a) UV/vis absorption spectra of Ag6Si2O7, Ag2O, and Ag3PO4. (b) Photocatalytic degradation of MB under visible light irradiation (λ ≥ 420 nm) over Ag6Si2O7 (red), Ag2O (black), and Ag3PO4 (blue); adsorption of MB over Ag6Si2O7 in dark (green). (c) Transmission % of the optical filters for λ > 420, 570, and 700 nm. (d) Photocatalytic degradation of MB over Ag6Si2O7 under illumination of different light λ > 420 (blue), 570 (green), and 700 nm (red) using with C0 of MB = 20 mg/L. (e) Photocatalytic reduction of Cr6+ and photocatalytic oxidization of MB over Ag6Si2O7.
Ag3PO4 absorbs strongly in the region below 520 nm,15 but Ag6Si2O7 absorbs nearly in the full visible-light region (λ < 740 nm). The absorption edge of Ag6Si2O7 is 740 nm, and the band gap Eg is estimated to be 1.58 eV (see Figure 2e in the Supporting Information). The photocatalytic activity of Ag6Si2O7 was tested by measuring the degradation of methylene blue (MB). The decreases in the MB concentrations in the photocatalytic reactions over Ag2O, Ag3PO4, and Ag6Si2O7 are compared in Figure 2b. Over Ag6Si2O7 under visible light (λ > 420 nm), 90 and 99% of MB are decomposed 3874
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Tables S1−S4. These materials are available free of charge via the Internet at http://pubs.acs.org
shown in Figure 3a, our DFT calculations show a band gap of 1.5 eV. Moreover, the local electronic structures of the 12
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by research grants from the National Basic Research Program of China (the 973 Program; 2013CB632401), the National Natural Science Foundation of China (21333006, 11374190, and 51021062)
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Figure 3. Electronic structure of Ag6Si2O7 obtained from DFT calculations: (a) plots of the total density of states (TDOS) and projected density of states (PDOS) for the O 2p states. (b) PDOS plots for the 5s and 4d states of all Ag atoms. (c) Comparison of the PDOS plots for Ag(c) and Ag(b). (d) Comparison of the PDOS plots for Ag(b) and Ag(a). Note that Ag(a), Ag(b), and Ag(c) have two, seven, and three nonequivalent Ag atoms, respectively. The PDOS plots for Ag(a), Ag(b), and Ag(c) represent the average of the different PDOS plots belonging to each group.
nonequivalent AgOn (n = 2, 3, 4) units are different from one another. That is, the 12 nonequivalent AgOn units give rise to different energy gaps between the Ag 4d and Ag 5s states, which explains why Ag6Si2O7 absorbs light nearly in the full visible-light spectrum. To the bottom of the CB, the Ag(b)O3 units contribute in average slightly more than do the Ag(c)O4 units (Figure 3c), suggesting the accumulation of photogenerated electrons in the Ag(b)O3 units via the migration through the Ag(b)−O−Ag(c) bridges. Likewise, to the bottom of the CB, the Ag(a)O2 units contribute in average slightly more than do the Ag(b)O3 units (Figure 3d). This suggests the accumulation of photogenerated electrons in the Ag(a)O3 units via the migration through the Ag(b)−O−Ag(a) bridges. All these features are absent in Ag2O and Ag3PO4, because they possess an identical coordinate environment for Ag+ in their crystal structures (see Figure S14 in the Supporting Information). In summary, Ag6Si2O7 is photocatalytically active nearly in the full visible-light spectrum (λ < 740 nm), and has a much stronger photocatalytic activity than do Ag2O, Ag3PO4, and Ndoped p25. This strong photocatalytic activity of Ag6Si2O7 is a consequence of the synergetic effect of possessing the two structural features enhancing the separation of photogenerated charge carriers, namely, the presence of an internal polar electric field and that of nonequivalent metal−oxygen polyhedra linked by corner-sharing. Given that silicates are abundant and inexpensive, it is worthwhile to further explore silicate-based photocatalysts.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, estimation of the Ag6Si2O7 band edges, Figures S1−S14, the bond valences/bond dipole moments, 3875
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