Enhancing the Photocatalytic Performance of Commercial TiO2

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Enhancing the Photocatalytic Performance of Commercial TiO2 Crystals by Coupling with Trace Narrow-Band-Gap Ag2CO3 Changlin Yu,*,† Longfu Wei,† Jianchai Chen,† Yu Xie,*,‡ Wanqin Zhou,† and Qizhe Fan† †

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China College of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, Jiangxi, PR China

‡

ABSTRACT: Trace Ag2CO3 (0.5%, 1%, 2%, and 4%) was loaded on commercial TiO2 crystals by a facile precipitation method. The obtained Ag2CO3/TiO2 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), photolimunescence (PL) spectroscopy, and UV−vis diffuse reflectance spectroscopy (DRS). Under UV light (λ = 254 nm) or visible light irradiation, the photocatalytic performance of the samples was tested in degradation of methyl orange (MO). Results showed that loading of 1 wt % Ag2CO3 results in the best photocatalytic activity. Under UV or visible light irradiation, the degradation activity of 1%Ag2CO3/TiO2 composite is 6 times that of TiO2 and 4 times that of Ag2CO3 or 3.4 times that of TiO2 and 1.7 times that of Ag2CO3. Reasons for enhanced activity were found that the coupling of Ag2CO3 promoted the visible light absorption and suppressed the recombination rate of e−/h+ pairs. Moreover, more surface OH groups over the Ag2CO3/TiO2 composite can react with the photogenerated h+ and produce •OH radicals to decompose the dye.

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INTRODUCTION

In order to enhance the photocatalytic performance or prolong the light absorption of TiO2 from UV light to the visible light region, many studies have been made, which includes the deposition of noble metals over TiO2,20,21 nonmetal anion doping (e.g., C, N, S, F),22−25 and coupling with other semiconductors.26,27 The deposited noble metal particles (Ag, Pt, Pd, etc.) over TiO2 could effectively promote the separation of photogenerated e−/h+ because the photogenerated electrons could accumulate on the metal and holes would remain on the photocatalyst surface.1,28 Nonmetal element doping could also restrain the recombination of photogenerated e− and h+ or prolong the light absorption of TiO2 into the visible light region.29,30As for the semiconductor coupling, due to the suitable difference in band gap potentials, the formation of heterojunction between the coupled semiconductors often gives rise to effective separation of photogenerated e− and h+.31,32 For example, Yuan et al.33 reported a novel core−shell TiO2@ZnIn2S4 composite which is more efficient than pure TiO2 and ZnIn2S4 in the photocatalytic degradation of methylene blue (MB). At present, the obvious visible light activity of Ag2CO3 has received great attention.34−36 Our research37 found that the Ag2O/Ag2CO3 heterostructures prepared via phase transformation route exhibited extremely high efficiency in decomposition of methyl orange (MO) under visible light irradiation. The degradation rate of MO over Ag2O/Ag2CO3 is 66, 30, and 52 times higher than the activity of Ag2CO3, Ag2O, and Ag3PO4, respectively. In this work, trace of Ag2CO3 (0.5%, 1%, 2%, and 4%) was coupled into a commercial anatase TiO2. The effects of Ag2CO3 coupling on the surface property, light absorption, and

Nowadays, dye pollution in water is a major source of environmental pollution in waste fields. Dye wastewater usually exhibits high levels of chemical oxygen demand (COD) and deep color. Moreover, the conventional water treatment technologies cannot effectively eliminate dye pollutants in water. Over the past decades, a great deal of effort has been devoted to the investigation of photocatalytic degradation of organic water pollutants by photocatalysis. Photocatalytic degradation reaction has some advantages, such as extremely fast degradation rate, high mineralization efficiency, and low toxigenicity. The produced end products of CO2 and H2O in photocatalysis are environmentally friendly.1−5 Up to now, a great deal of wide-band-gap semiconductor photocatalysts have been developed, such as TiO2,6,7 ZnO,8 SnO2,9 ZnS,10 BiOCl,11 CuBO2,12 etc. Among these reported photocatalysts, TiO2 is the widely investigated photocatalyst. TiO2 is not toxic to the environment and has strong oxidizing power and good stability in photocatalytic reactions.13−19 Under UV light irradiation, the electrons (e−) in the valence band of TiO2 are excited, producing electron (e−)and hole (h+) pairs. Part of the photogenerated e− and h+ was consumed by producing thermal energy through the combination of e− and h+. Other photogenerated e− and h+ can migrate to the surface of TiO2 and react with the adsorbed species (OH, O2, etc.) at the surface of catalyst and generate free radicals (•OH, O2•−, etc). These radicals could degrade the adsorbed organic pollutants. Both the separation efficiency of photogenerated electrons and holes and the textured property of catalysts effectively influence the photocatalytic performance. The major disadvantage of TiO2 is that it can not absorb visible light (∼50% in solar light) due to its large band gap of 3.2 eV, which means that in photocatalysis reaction, TiO2 can only utilize UV light in solar light, a small fraction of solar light energy (2−3%). © 2014 American Chemical Society

Received: Revised: Accepted: Published: 5759

December 31, 2013 March 9, 2014 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/ie404283d | Ind. Eng. Chem. Res. 2014, 53, 5759−5766

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RESULTS AND DISCUSSION Crystallite Analysis. The crystallite property of the prepared samples was analyzed by XRD. Figure 1 displays the

photocatalytic performance of TiO2 were explored. The results indicate that the obtained Ag2CO3/TiO2 composite displayed great enhancement in photocatalytic performance in degradation of the MO under both UV and visible light irradiation.

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EXPERIMENTAL SECTION Sample Synthesis. The commercial anatase TiO2 crystals (surface area 25 m2/g) were provided by Hunan Zhuzhou Chemical Industry Group Co., Ltd. China. Other used reagents were analytic grade in this experiment. A 0.99 g portion of anatase TiO2 powder was first dispersed into 10 mL of deionized water under stirring, and 0.038 g Na2CO3 was added and stirred for 10 min. Then, 10 mL AgNO3 aqueous solution containing 0.0123 g AgNO3 was added dropwise to this TiO2 mixture. After stirring for 1 h at room temperature, the powder was collected by filtering, washed three times with deionized water, and dried in oven at 60 °C for 4 h; 1% Ag2CO3/TiO2 was obtained. Here, % means mass percent. Ag2CO3, 0.5% Ag2CO3/TiO2, 2% Ag2CO3/TiO2, and 4% Ag2CO3/ TiO2 samples were prepared via the same procedure. Sample Characterization. Powder X-ray diffraction (XRD) of the samples were recorded on a Bruker D8-Advance X-ray diffractometer using Cu Kα (λ = 0.154178 nm) radiation with scanning angles of 10−80°. The applied current and accelerating voltage were 40 kV and 40 mA, respectively. The morphology of the samples was tested by scanning electron microscopy (SEM) on a XL 30 scanning electron microscope. A transmission electron microscopy (TEM) image was also used to analyze the prime particle and the dispersed state of Ag2CO3. The TEM image and energy-dispersive X-ray spectroscopy (EDX) were obtained on a Tecnai 20 FEG microscope coupled with an energy-dispersive X-ray (Oxford Instrument) spectrometer. The X-ray photoelectron spectroscopy (XPS) was obtained on a Thermo ESCALAB 250XI XPS System with a monochromatic Al Ka source and a charge neutralizer. Fourier transform infrared (FT−IR) spectra were recorded with a Nicolet 470 FT−IR spectrometer (USA), and samples were pressed by a KBr disk preparation apparatus. The room temperature photoluminescence (PL) emission spectra of the samples were tested. The excitation light source is 325 nm He−Cd laser. The emission from the sample was measured by a spectrometer (Spex 500M, USA) equipped with a photon counter (SR400, USA). UV−vis diffuse reflectance spectra (UV−vis DRS) were recorded on UV−vis spectrophotometer (UV−2550, Shimadzu) referenced to BaSO4. Photocatalytic Activity Test. The photocatalytic activity was carried out in degradation of methyl orange (MO) in water solution under UV or visible light irradiation. In a UV light test, a 7 W lamp with 254 nm light wavelength was applied as the light resource. A 0.05 g portion of photocatalyst was suspended in 80 mL MO aqueous solution. The initial MO concentration is 0.020 g/L. Before light irradiation, the suspension was stirred in the dark for 40 min to attain physical adsorption/desorption equilibrium. The suspension was stirred by magnetic force during the degradation process, and the reaction temperature was maintained at 22 °C by circulation of water. After fixed intervals of illumination, a sample of the suspension was taken out and centrifuged. The upper clear solution was analyzed by UV-2550 spectrophotometer. The dye concentration was measured at λ = 464 nm, which is the characteristic absorption wavelength for MO. In the visible light activity test, a 300 W iodine tungsten lamp was substituted for the UV lamp.

Figure 1. XRD patterns of the samples.

XRD patterns of the TiO2, Ag2CO3, and Ag2CO3/TiO2 samples with different Ag2CO3 concentrations. From this figure, we can see that, over Ag2CO3, the obvious diffraction peaks at 2θ = 18.61°, 20.66°, 32.76°, 33.76°, 37.09°, 39.75°, 41.91°, and 44.45° can be indexed to crystallite planes of (020), (110), (−101), (−130), (200), (031), (220), and (131), which are corresponding to monoclinic phase Ag2CO3 (JCPDS card No. 26-0339). The high intensity of the diffraction peaks of Ag2CO3 implies that even at room temperature the produced Ag2CO3 has good crystallinity. As for the commercial TiO2, the strong diffraction peaks at 2θ of 25.3°, 38.2°, 48.1°, 53.5°, and 55.6° are corresponding to (101), (004), (200), (105), and (211) planes, respectively, which are systematically ascribed to the anatase-TiO2 (JCPDS card No. 84-1285). These sharp and strong diffraction peaks suggest the high crystallinity of the commercial TiO2 crystals. Scherrer equation: D = 0.89λ/(β cos θ)14 was applied to calculate the average crystallite sizes of TiO2. The (101) plane of TiO2 was applied in these calculations. The calculation result indicates that the average grain of TiO2 is around 75 nm. However, over Ag2CO3/TiO2 composite samples, no characteristic diffraction peak of Ag2CO3 is observed. The possible reasons could be that the loaded Ag2CO3 exists as highly dispersed state or the low content of Ag2CO3 can not be detected by XRD test. Morphology Analysis. The commercial TiO2 and asprepared Ag2CO3/TiO2 composite samples were analyzed by SEM. The images of the typical samples are presented in Figure 2. Figure 2a clearly shows that the commercial TiO2 sample has good dispersion and exhibits sphere-like morphology. Figure 2b displays the photograph of Ag2CO3 which is composed of microcuboids with a length of 1−5 μm. Figure 2c indicates that the morphology of 1% Ag2CO3/TiO2 is similar to TiO2. The presence of trace Ag2CO3 has no effects on the morphology of TiO2. TEM and EDX Analysis. To further determine the grain size of TiO2 and the dispersed state of Ag2CO3 in Ag2CO3/ TiO2, we carried out the TEM and EDX tests. The obtained results are presented in Figures 3 and 4, respectively. Figure 3 shows that the grain size of TiO2 is in the range of 50−100 nm, which is in accordance with the XRD test. Over the plate-like TiO2 crystals, some very small dispersed particles are observed. 5760

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Figure 2. SEM images of the samples: (a) TiO2; (b) Ag2CO3; (c) 1% Ag2CO3/TiO2.

Figure 4 gives the selected area EDX spectrum of one small particle over TiO2. Five elements of C, O, Ti, Ag, and Cu are observed. The Cu peaks come from the supporting copper grid. TEM and EDX tests further confirm that the high dispersed Ag2CO3 was loaded over TiO2 nanoplates. FT-IR Analysis. The pure TiO2, Ag2CO3, and Ag2CO3/ TiO2 samples were analyzed by FT-IR techniques. Figure 5

Figure 3. TEM image of 1% Ag2CO3/TiO2.

Figure 5. FT-IR spectra of the samples.

gives the obtained FT-IR spectra. One peak at 3440 cm−1 over all the samples was observed. This peak is assigned to the stretching vibration and bending vibration of surface •OH groups on TiO2. Another peak at 1630 cm−1 is attributed to the bending vibration of H−O−H on adsorbed water. The peaks around 500−700 cm−1 are the characteristic peaks of the TiO2. Over pure Ag2CO3, the absorption bands of CO32− could be found around 1449, 1382, 883, and 705 cm−1. Moreover, in

Figure 4. EDX spectrum of the 1% Ag2CO3/TiO2.

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Figure 6. High resolution XPS spectra of 1% Ag2CO3/TiO2: (a) C 1s; (b) O 1s; (b) Ti 2p; (d) Ag 3d.

Figure 5, it is obviously observed that over the Ag2CO3/TiO2 composites most of the peaks of surface OH groups become broader and stronger, indicating that more abundant OH groups exist over the surface of the composite. In degradation of the dyes, OH groups can react with the photogenerated h+ and produce •OH radicals. •OH radicals are believed to the main free radicals in decomposition of the dye molecules. Therefore, it is reasonable to infer that the coupling of Ag2CO3 can effectively improve the photocatalytic activity. XPS. The chemical state of the elements in 1% Ag2CO3/ TiO2 was analyzed by XPS. The XPS spectra for C 1s, O 1s, Ti 2p, and Ag 3d for 1% Ag2CO3/TiO2 are shown in Figure 6. The carbon element was mainly attributed to the Ag2CO3. However, we could not exclude the hydrocarbon from XPS instrument. Figure 6b indicates that the O 1s peak can be divided into two small peaks (530.3 and 531.4 eV). The small peak at 530.3 eV is attributed to the O2− in the TiO2 or Ag2CO3, and the other peak at 531.4 eV arises from hydroxyl groups on the surface of sample. The 2p3/2 peak at 458 eV and 2p1/2 peak at 464 eV (Figure 6c) indicates that Ti exists in the Ti4+ form.38 The binding energies of 367.8 eV (Ag 3d3/2 peak) and 373.8 eV (Ag 3d5/2 peak) of Ag 3d are in good accordance with the values reported for Ag+.39 UV−vis DRS Analysis. The light absorption of the prepared samples at different light wavelength was determined by UV−vis diffuse reflectance spectroscopy. As shown in Figure 7, TiO2 exhibits poor absorption ability to visible light due to its large band gap, but Ag2CO3 has a broad absorption from UV to visible region (over 500 nm). After loading Ag2CO3, the Ag 2CO 3/TiO2 composite exhibits obvious visible light absorption (from 500 to 650 nm). The visible light absorption of the Ag2CO3/TiO2 composite is attributed to the presence of Ag2CO3. The band gap energy (Eg) for the prepared samples was determined using the equation Eg = 1240/λg (eV), where λg

Figure 7. UV−vis diffuse reflectance spectra of the samples.

is the light absorption edge. To obtain light absorption edge, the intercept was made between the tangent of the absorption curve and the abscissa. The calculated band gap energies for Ag2CO3 and TiO2 are 2.3 and 2.89 eV, respectively. Photoluminescence Analysis. PL emission spectroscopy can be used to analyze the separation efficiency of photogenerated e− and h+ in semiconductor particles. Figure 8 shows the PL spectra of the samples. It is observed that the intensity of the PL spectra of Ag2CO3/TiO2 composites is much lower than that of pure TiO2. According to the literature,40,41 the observed PL spectrum of TiO2 could arise from the radiative recombination process of self-trapped excitations, or hydroxylated Ti3+ surface complexes,42 from the charge transfer excited state of the highly dispersed titanium oxide species. Therefore, the reduction of PL intensity in the Ag2CO3/TiO2 suggests a low rate of radiative recombination process. As a result, more 5762

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photocatalytic activity. The optimal concentration of Ag2CO3 is found as 1%. Under visible light irradiation, TiO2 exhibits poor activity due to its weak absorption under near visible light region. Ag2CO3 coupling obviously enhances the visible light activity. The good visible light activity of Ag2CO3/TiO2 composite mainly comes from the contribution of Ag2CO3. A pseudo-first-order model expressed by the equation ln(C0/ C) = kt43,44 (C0 the initial concentration; C the final concentration, k the pseudo-first-order rate constant) was applied to investigate the MO degradation kinetics. The obtained results are present in Table 1. Table 1 indicates a Table 1. Results of the First-Order Reaction Rate Constant in Degradation of Methyl Orange UV light irradiation

Figure 8. PL spectra of the samples.

hydroxyl radicals could be produced because more holes are available to react with the surface OH groups. Photocatalytic Performance Test. The photocatalytic performance of the samples was tested in decomposition of methyl orange (MO) under UV light (λ = 254 nm) or visible light irradiation. The results for the photocatalytic activity over different samples are present in Figure 9. Figure 9a indicates that MO molecule is stable under UV or visible light irradiation without catalyst because the concentration of MO shows no variations. Due to the high crystallinity and good light absorption, both the commercial TiO2 and the prepared Ag2CO3 exhibit obvious UV light activity. More importantly, the loading of trace of Ag2CO3 largely increases the

visible light irradiation

samples

k/h−1

R2

k/h−1

R2

TiO2 Ag2CO3 0.5% Ag2CO3/TiO2 1% Ag2CO3/TiO2 2% Ag2CO3/TiO2 4% Ag2CO3/TiO2

0.19 0.28 0.67 1.12 0.95 0.50

0.98 0.99 0.98 0.93 0.95 0.97

0.07 0.14 0.22 0.24 0.18 0.15

0.99 0.98 0.98 0.98 0.98 0.98

good correlation with pseudo-first-order reaction kinetics. Under UV light irradiation, 1% Ag2CO3/TiO2 shows the largest reaction rate constant (1.12 h−1), which is 6 times that of TiO2 (0.19 h−1) and 4 times that of Ag2CO3 (0.28 h−1). The visible light activity of Ag2CO3/TiO2 is 3.4 times that of TiO2 and 1.7 times that of Ag2CO3. Due to the poor stability of the silver-based semiconductor photocatalysts in photocatalysis process,37,45 the stability of 1% Ag2CO3/TiO2 was tested in recycling reaction. In the fourth reaction the degradation rate (D = C0 − C/C0, D the degradation rate, C0 the initial concentration of MO) of MO over 1% Ag2CO3/TiO2 decreased from 71% to 50%, which suggests that the increase of recycling times caused a decrease in activity. Reaction Mechanism. It is interesting to explore the plausible reaction mechanism for the enhanced photocatalytic performance of Ag2CO3/TiO2 composite. First, using the theory of electronegativity, the band gap structures of Ag2CO3 and TiO2 were estimated. VB (valence band) and CB (conductor band) positions are obtained by calculation using the empirical formula EVB = X −(E ×100.5)Eg,46,47 where EVB, X, Ee, and Eg are the energy of the VB edge potential, the absolute electronegativity, free electrons on the hydrogen scale (4.5 eV), and the band gap energy, respectively. The X values for TiO2 and Ag2CO3 are ca. 5.89 and 6.02 eV. On the basis of the above equations, the top of the VB and the bottom of the CB of TiO2 are calculated to be ca. 2.89 and −0.11 eV, respectively. The VB and CB of Ag2CO3 are estimated to be 2.67 and 0.37 eV, respectively. According to our calculation, the CB position of Ag2CO3 is more anodic than TiO2 and the difference in VB position of Ag2CO3 (2.67 eV) and TiO2 (2.89 eV) is small. The schematic energy band model of the composite can be depicted as shown in Figure 10. Under UV light irradiation, the excited electrons in the CB of Ag2CO3 can transfer to the CB of Ag2CO3. Therefore, the formation Ag2CO3/TiO2 heterojunction over the surface of Ag 2 CO 3 could inhibit the recombination of photogenerated e− and h+, resulting in the enhanced performance of the Ag2CO3/TiO2 composite as compared to Ag2CO3, which could be similar to the report of

Figure 9. Comparison of the photocatalytic activity of the different photocatalysts under UV light (a) and visible light (b) irradiation. 5763

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the composite also exhibits better dispersion, which could benefit the dye adsorption and light absorption.

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CONCLUSIONS Ag2CO3/TiO2 composite was synthesized by loading trace Ag2CO3 on commercial anatase TiO2 crystals via a facile precipitation process. The coupling of Ag2CO3 with TiO2 enriched the surface OH groups of the sample and restrained the recombination rate of photogenerated electrons (e−) and holes (h+). The formation of Ag2CO3/TiO2 heterojunction over the surface of TiO2 greatly enhances the photocatalytic activity under both UV and visible light irradiation. Under UV light irradiation, the activity of Ag2CO3/TiO2 is 6 times that of TiO2 and 4 times that of Ag2CO3. The visible light activity of Ag2CO3/TiO2 is 3.4 times that of TiO2 and 1.7 times that of Ag2CO3.

Figure 10. Suggested mechanism for the enhanced activity of Ag2CO3/TiO2 heterojunction photocatalyst.

Li48 that the formation of anatase/rutile heterojunction over the surface of TiO2 greatly increased the photocatalytic activity in H2 production. Moreover, the abundant OH groups over the surface of the composite catalyst can react with the photogenerated h+ and produce •OH radicals which can effectively decompose the dye. To confirm our deduction that hydroxyl radicals could be the major radical to decompose the MO dye, hydroxyl radicals (•OH) generated during the photocatalysis process were detected by the reported photoluminescence (PL) technique.49 Coumarin (COU) can easily reacted with •OH to produce the highly fluorescent product 7-hydroxycoumarin (7HC). Figure 11 shows that after 5 h of visible light irradiation, the PL signal

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86(797) 8312334. Fax: +86(797) 8312334. Email:[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the fund from the National Natural Science Foundation of China (21067004, 21263005), Young Science and Technology Project of Jiangxi Province Natural Science Foundation China (20133BAB21003), Young Scientist Training Project of Jiangxi Province China (20122BCB23015), and Science Foundation of Jiangxi Province Education Office China (GJJ12344).

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REFERENCES

(1) Yu, C. L.; Li, G.; Kumar, S.; Kawasaki, H.; Jin, R. C. Stable Au25(SR)18/TiO2 Composite Nanostructure with Enhanced Visible Light Photocatalytic Activity. J. Phys. Chem. Lett. 2013, 4, 2847−285. (2) Lucíad, C. C.; Maríad, C. G.; Acosta, E. O.; Berta, G. Removal of Cr (VI) and Humic Acid by Heterogeneous Photocatalysis in a Laboratory Reactor and a Pilot Reactor. Ind. Eng. Chem. Res. 2012, 51, 9468−9474. (3) Yu, C. L.; Cao, F. F.; Li, G.; Wei, L. F.; Yu, J. C.; Jin, R. C.; Fan, Q. Z.; Wang, C. Y. Novel Noble Metal (Rh, Pd, Pt)/BiOX(Cl, Br, I) Composite Photocatalysts with Enhanced Photocatalytic Performance in Dye Degradation. Sep. Purif. Technol. 2013, 120, 110−122. (4) Frontistis, Z.; Drosou, C.; Tyrovola, K.; Mantzavinos, D.; FattaKassinos, D.; Venieri, D.; Xekoukoulotakis, N. P. Experimental and Modeling Studies of the Degradation of Estrogen Hormones in Aqueous TiO2 Suspensions under Simulated Solar Radiation. Ind. Eng. Chem. Res. 2012, 51, 16552−16563. (5) Danwittayakul, S.; Jaisai, M.; Koottatep, T.; Chowdhury, J. D. P.; Malekshoar, G.; Ray, M. B.; Zhu, J.; Ray, A. K. Enhancement of Photocatalytic Degradation of Methyl Orange by Supported Zinc Oxide Nanorods/Zinc Stannate (ZnO/ZTO) on Porous Substrates. Ind. Eng. Chem. Res. 2013, 52, 13629−13636. (6) He, F.; Li, J. L.; Li, T.; Li, G. X. Solvothermal Synthesis of Mesoporous TiO2: The Effect of Morphology, Size and Calcination Progress on Photocatalytic Activity in the Degradation of Gaseous Benzene. Chem. Eng. J. 2014, 237, 312−321. (7) Radeka, M.; Markov, S.; Loncar, E.; Rudic, O.; Vucetic, S.; Ranogajec, J. Photocatalytic Effects of TiO2 Mesoporous Coating Immobilized on Clay Roofing Tiles. J. Eur. Ceram. Soc. 2014, 34, 127− 136.

Figure 11. PL spectral changes observed during illumination of TiO2 and 1% Ag2CO3/TiO2 dispersed in a 10−3 M coumarin aqueous solution.

at 456 nm of 7HC in the 1% Ag2CO3/TiO2/water system is much stronger than that at the TiO2/water interface, which indicated that more hydroxyl radicals were produced in 1% Ag2CO3/TiO2 aqueous solution. Due to the small difference in VB position of Ag2CO3 (2.67 eV) and TiO2 (2.89 eV), the transfer of holes from the VB of TiO2 to the VB of Ag2CO3 could be negligible. However, the high concentration of Ag2CO3 could decrease the dispersion of Ag2CO3 and the redundant Ag2CO3 could become recombination centers, resulting in an adverse effect on photocatalytic activity. As for the higher visible light activity of the composite than pure Ag2CO3, the possible reason could be the synergistic effect between Ag2CO3 and TiO2 which benefits the inctease in activity. Moreover, surface area test show that larger surface area (24 m2/g) was observed over 1% Ag2CO3/TiO2 than Ag2CO3 (19 m2/g). The SEM analysis further dindicates that the particles of Ag2CO3/TiO2 are smaller that of Ag2CO3 and 5764

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