Cooperative Effect between Cation and Anion of Copper Phosphate

Oct 30, 2012 - Surface modification of TiO2 with CuO or calcium phosphate (CaP) can result in enhancement in the photocatalytic activity for organic d...
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Cooperative Effect between Cation and Anion of Copper Phosphate on the Photocatalytic Activity of TiO2 for Phenol Degradation in Aqueous Suspension Haihang Chen and Yiming Xu* State Laboratory of Silica Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Surface modification of TiO2 with CuO or calcium phosphate (CaP) can result in enhancement in the photocatalytic activity for organic degradation. In this work, we report on a synergism between the cation and anion of copper phosphate (CuP) on the photocatalytic activity of TiO2, for phenol degradation in aerated aqueous suspension under UV light at wavelengths longer than 320 nm. Photocatalysts were prepared by mixing TiO2 and CuP powders in isopropyl alcohol, followed by drying at 90 °C. As CuP loading increased, the activity of the modified TiO2 first increased and then decreased. The maximum activity was observed with the catalyst containing 0.1 wt % CuP, which was about 1.9−3.4 times that of bare TiO2 (anatase, rutile, and their mixture) and also exceeded that of the modified TiO2 with CuO or CaP. During five repeated tests, the catalyst activity was stable, without detectable leaching of cupric and phosphate ions into aqueous solution. Solid characterization with several techniques including electron paramagnetic resonance (EPR) spectroscopy revealed that CuP particles at low loading were highly dispersed onto TiO2 as a kind of clusters, whereas the TiO2 phase in different samples remained nearly unchanged in terms of the crystal structure, surface area, and crystallinity. Upon exposure to UV light, the EPR signal of Cu(II) in CuP or CuO-modifed TiO2 was unchanged in air but slightly decreased in N2. Moreover, CuP-modified TiO2 showed a higher capacity than bare TiO2 and CuO- or CaP-modified TiO2 for the uptake of 2,4-dichlorophenol from water. It is proposed that cupric and phosphate ions act as an electron scavenger and organic sorbent, which facilitate electron and hole transfer, respectively. Their co-operation would significantly improve the efficiency of charge separation, and thus increase the rate of phenol degradation.

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INTRODUCTION

promising approach for improvement in the overall efficiency of organic degradation. For instance, the addition of Cu2+ ions into the aerated aqueous suspension of TiO2 can result in enhancement in the rate of organic degradation under UV light.11−14 This observation is mainly attributed to the faster reduction of Cu2+ by the photogenerated electron, as compared to the reduction of O2. Interestingly, the resulting Cu (and Cu2O) particles can be reoxidized into Cu2+ by O2,15−17 together with the formation of H2O2.18 Moreover, the grafted or impregnated Cu2+ ions on TiO2, thermally treated at 80−500 °C, are also positive for organic degradation.19−26 A kinetic study of electron paramagnetic resonance (EPR) spectroscopy at low temperature reveals that CuO is reduced by an electron from the excited TiO2, followed by regeneration of Cu(II) with O2.23,26 On the other hand, there have been several studies focusing on the hole reactions. For example, addition of phosphate anions into the aqueous suspension of TiO2 can promote organic degradation.27−32 Since phosphate anion

Heterogeneous photocatalysis of TiO2 for air purification and water treatment has been widely studied for over 30 years.1−4 This is because nearly all organic compounds can degrade into CO2 and/or small fragments over TiO2 under UV light at normal temperature and pressure. In comparison with other semiconductors, TiO2 is highly photoactive, robust, cheap, and friendly to the environment. However, the quantum yield of organic disappearance over TiO2 is very low,5 mainly attributed to the fast recombination of photogenerated charge carriers. In principle, the conduction band electron and valence band hole, generated from UV-excited TiO2, can react with O2 and H2O/ OH− to produce O2−• and •OH radicals, respectively. Then, the overall efficiency of organic degradation will be determined by the competition between carrier recombination and interfacial charge transfer.2 Furthermore, it is generally held that O2 reduction is the rate-determining step,6−9 while the • OH radicals bound onto TiO2 slowly react with the organic substrate at the solid−liquid interface.10 Therefore, increase of TiO2 crystallinity and enrichment of both O2 and organic substrate onto the photocatalyst surface would be the © 2012 American Chemical Society

Received: July 21, 2012 Revised: October 24, 2012 Published: October 30, 2012 24582

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heating at 60 °C for 2 h, and storage overnight. The AT and RT precipitates were filtered and washed several times with water until no chloride ions were detected in the filtrate. Finally, solids were sintered at 450 °C in air for 3 h. The solid crystalline structure was confirmed by XRD. The BET surface areas of AT and RT were 66 and 33 m2/g, respectively. Calcium phosphate (CaP) was prepared by using Ca(NO3)2 and (NH4)2HPO4 as starting materials.39 Milli-Q ultrapure water was used throughout this work. Synthesis of Composite Catalyst. Typically, 1.00 g of TiO2 was dispersed into 40 mL of isopropyl alcohol under magnetic stirrer. Then, to this suspension, a certain amount of copper phosphate was added, followed by stirring for 30 min, and drying at 90 °C overnight. Then, the solid was ground into a fine powder. A similar procedure was also used to prepare the modified TiO2 with CuO and CaP. The final samples were named xCuP/TiO2, yCuO/TiO2, and zCaP/TiO2, where x, y, and z represent the weight percents of Cu3(PO4)2, CuO, and Ca3(PO4)2, respectively. For example, 2CuP/TiO2 refers to the sample containing 2 wt % Cu3(PO4)2 and 98 wt % TiO2. As a reference, the modifier loading in units of Cu/Ti and P/Ti atomic ratios and/or atomic percents is also presented in Tables S1−S3 of the Supporting Information. Characterization. XRD patterns were recorded on a D/ max-2550/PC diffractometer (Rigaku). Crystallite diameter (ds) was calculated by using the Scherrer equation, based on the integrated intensities of anatase (101) and rutile (110). Diffuse reflectance spectra were recorded on a Shimadzu UV-2550 using BaSO4 as a reference. Adsorption of N2 on solid at 77 K was measured on a Micromeritics ASAP2020 apparatus, from which the BET specific surface area (Asp) was calculated. Scanning electron microscopy (SEM) measurement was performed on a Hitachi S-4800, attached with energy-dispersive X-ray spectroscopy (EDS). EPR spectra were recorded at room temperature on a Bruker A300 spectrometer at X band. Photoluminescence (PL) spectra were recorded on a Shimadzu F-2500 spectrophotometer. X-ray photoelectron spectroscopy (XPS) data were recorded with a Kratos AXIS UItra DLD spectrometer and calibrated with C 1s at 284.8 eV. Photocatalysis. Reactions were carried out in a Pyrex-glass reactor (inner diameter of 2.9 cm and height of 9.1 cm), thermostatted at 25 °C through a recycle system. An aqueous suspension (50 mL) containing 0.43 mM phenol and 1.0 g/L catalyst was first stirred in the dark for 1 h and then irradiated with a high pressure mercury lamp (300 W, Shanghai Mengya) through a 320 nm cutoff filter. The distance between the reactor and the lamp was fixed at 10 cm. The light intensity reaching the external surface of the reactor was 1.41 mW/cm2, as measured by a UV-irradiance meter (UV-A, Instruments of Beijing Normal University, China). At given intervals, 2.0 mL of the suspension was withdrawn by a microsyringe and filtered through a membrane (0.22 μm in pore size). Organic substrates were analyzed by high-performance liquid chromatography (HPLC) on a Dionex P680 (Apollo C18 reverse column, and 50% CH3OH aqueous solution as an eluent). Inorganic anions were analyzed by ionic chromatography (IC) on a Dionex ISC90 (AS14A column, and 10 μM Na2CO3/NaHCO3 as an eluent). Catalyst stability was tested as follows. The suspension containing 50 mg of 0.1CuP/TiO2 and 50 mL of 0.215 mM phenol was stirred in the dark for 60 min. Then, it was irradiated and analyzed as described above. After each run was complete, 1 mL of phenol stock solution, 4 mL of distilled

strongly adsorbs on TiO2, it is thought that the negative electrostatic field formed in the surface layers of TiO2 can facilitate the hole transfer, thus improving the efficiency of carrier separation and increasing the rate of organic degradation via •OH attack.28,31,32 Moreover, the surface-modified TiO2 with a hardly water-soluble calcium phosphate (CaP) also shows a higher activity than the unmodified TiO2 for the photocatalytic degradation of phenol and organic dye under UV light, probably due to the increased organic adsorption on the catalyst surface.33,34 With all modified catalysts, the optimum loading of CuO or CaP that corresponds to the maximum activity are only a few percent in weight.22−24,34 In this work, copper phosphate (CuP) that contains both Cu(II) and phosphate ions has been used as a modifier of TiO2. It is likely that copper cation and phosphate anion of CuP may have a cooperative effect on the photocatalytic activity of TiO2 for organic degradation in aqueous solution. To make clear the effect of CuP, the TiO2 phase in different samples should be controlled the same as much as possible. It is often observed that the photocatalytic activity of TiO2 greatly depends on its physical properties, including crystal structure, phase composition, and crystallinity.1−4 Moreover, the model organic substrate, used for assessment of the catalyst activity, should be colorless and poorly adsorptive onto the catalyst in aqueous solution.35−37 The former will avoid possible degradation of the organic substrate through photolysis and/or dye sensitization while the latter will simplify the rate determination without consideration of the substrate adsorption effect possibly changing with irradiation time.37 With those points in mind, a commercial or presintered TiO2 was used as starting materials and mixed directly with CuP powders at different ratios in an alcoholic solvent, followed by drying at 90 °C. Since phenol only absorbs the light at wavelengths shorter than 300 nm and weakly adsorbs on TiO2 in water, it was used as a model substrate for evaluating the catalyst activity under UV light at wavelengths longer than 320 nm. For comparison, the modified TiO2 samples with CuO and CaP at different ratios were also prepared in parallel. Results showed that CuP-modified TiO2 was not only more active than the unmodified TiO2 but also superior to the modified TiO2 with CuO or CaP under similar conditions. Solids were characterized with several techniques, including X-ray diffraction (XRD), N2 adsorption, and EPR spectroscopy. Finally, a possible mechanism responsible for the enhanced photocatalytic activity of CuP-modified TiO2 is discussed.

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EXPERIMENTAL SECTION Materials. Copper phosphate (98%) was purchased from Alfa Aesar, and other chemicals in analytical grade were obtained from Shanghai Chemicals Inc., China. The atomic ratio of Cu to P in CuP, analyzed with X-ray fluorescence spectroscopy was 1.50, in agreement with that of Cu3(PO4)2. Except stated otherwise, a commercial TiO2 (Taixing Nanomaterials, China), which consisted of anatase (98%) and rutile (2%) and had a Brunauer−Emmett−Teller (BET) surface area of 140 m2/g, was used as starting material throughout this study. Pure anatase and rutile TiO2 samples, denoted as AT and RT in the text, were prepared by following literature procedure.38 Briefly, AT was synthesized by TiCl4 hydrolysis in water containing 0.1 M HCl and 3.3 M (NH4)2SO4 in an iced bath, followed by boiling for 2 h, and neutralization with ammonia at room temperature. RT was synthesized by the hydrolysis of TiCl4 in 1.2 M HCl in an iced bath, followed by 24583

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Figure 1. (A) XRD patterns of CuP/TiO2 samples: (a−e) containing CuP at 0, 1, 2.9, 4.8, and 9.1 wt %, respectively. The asterisks represent the crystal phase of Cu3(PO4)2 (PDF no. 21-0298). (B) BET surface area of (a) CuP/TiO2 and (b) CuO/TiO2. Curves (c) and (d) correspond to the calculated surface area for CuP/TiO2 and CuO/TiO2, respectively, based on the surface areas of the solvent-treated TiO2, CuP, and CuO (see the text for details).

Figure 2. EPR spectra of (A) CuP/TiO2 and (B) CuO/TiO2 at different loading of (a) 0.1, (b) 0.5, (c) 1, (d) 2.9, (e) 4.8, and (f) 9.1 wt %. Note that the y axis scales for (A) and (B) are the same. As a reference, the Cu/Ti ratios in mole % from (a) to (f) were 0.0420, 0.210, 0.417, 1.24, 2.06, and 4.02 for CuP/TiO2, while they were 0.100, 0.498, 0.990, 2.913, 4.76, and 9.09 for CuO/TiO2.

by XRD is either due to its low concentration or small crystallite size. Similar changes were also observed with CuO/ TiO 2 and CaP/TiO 2 samples (Figure S1, Supporting Information). Moreover, in all modified samples, the TiO2 phase remained nearly unchanged in terms of its phase composition, crystal size, and crystallinity. Figure 1B shows the BET surface areas of CuP/TiO2 and CuO/TiO2, measured by N2 adsorption at 77 K. The solventtreated TiO2 had a surface area of 125 m2/g, much larger than that of either CuP (9.9 m2/g) or CuO (1.2 m2/g). Interestingly, with the modified TiO2, a volcano curve of surface area versus CuP or CuO loading was observed. In relation to the unmodified TiO2, the samples containing 0.05 wt % CuP and 0.5 wt % CuO showed 9.6% and 6.4% increase in surface area, respectively. If the composite was a simple mixture of TiO2 and CuP or CuO, its surface area would be the sum of the surface areas of two components, which is expected to linearly decrease with CuP or CuO loading (curves c and d, Figure 1B). It is highly possible that, during the sample synthesis in isopropyl alcohol, the large particles may have deaggregated into small

water, and 5 mL of the aqueous suspension of 0.1CuP/TiO2 (2.0 g/L) were supplied to renew the initial concentration of phenol and catalyst at 0.215 mM and 1.0 g/L, respectively. The new suspension was stirred in the dark for 30 min and then irradiated again. Such a procedure was repeated five times. Finally, the dissolved phosphate in the filtrate of the last irradiated suspension was analyzed by IC, while the dissolved cupric ions were spectrometrically analyzed at 435 nm through a complex with sodium diethyldithiocarbamatre.

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RESULTS AND DISCUSSION Characterization. Figure 1A showed the XRD patterns of CuP/TiO2 samples as a function of CuP loading (0−10 wt %). With bare TiO2, there was a set of peaks, attributed to anatase (98%, ds = 11.0 nm) and rutile (2%, ds = 13.7 nm). With CuPmodified samples, the diffraction patterns were similar to that of bare TiO2, but the peak intensity of TiO2 decreased with CuP loading, due to a decrease in the content of TiO2. The diffraction of copper phosphate was only observed with the samples containing more than 5 wt % CuP. No detectable CuP 24584

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particles. Since the sample at low loading shows a large increase in surface area, there would be a strong interaction between TiO2 and CuP (CuO), favoring the deaggregation of CuP (CuO) particles. Control experiments with individual TiO2, CuP, and CuO showed that, after solvent treatment, only CuP had an increased surface area from 7.4 to 9.9 m2/g, while a decreased surface area was observed with TiO2 (from 142 to 125 m2/g) and CuO (from 2.8 to 1.2 m2/g), respectively. Since no filtration was used for sample preparation, the decreased surface area is probably due to the solvent-induced aggregation of particles. This result implies that, in the modified samples, the deaggregation of TiO2 particles is less likely. Therefore, the observed increase in surface area can be attributed to a reduction in the particle size of CuP or CuO. SEM images showed that the parent CuP was large in size (50−500 nm) but no such large particles were observed with CuP-loaded TiO2 even at 4.8 wt % (Figure S2, Supporting Information). EDS analysis for the distribution of Ti, O, Cu, and P elements showed that, at low loading, CuP particles were homogenously distributed, while at high loading (>5 wt %), both small and large particles of CuP were present (Figure S3, Supporting Information). This result is in agreement with the above hypothesis that only CuP particles at low loading undergo efficient deaggregation into fine particles. Moreover, the estimated atomic ratios of Cu/Ti and P/Ti for 0.1CuP/ TiO2 were 0.00568 and 0.00656, respectively, which were about 1 order of magnitude higher than those calculated (Cu/Ti = 0.000628; and P/Ti = 0.000419). This observation indicates that CuP particles are mainly located on the TiO2 surface. However, the estimated atomic ratio of Cu/P (0.866) was lower than that calculated (1.5) in the form of Cu3(PO4)2, probably due to Cu(HPO4) and Cu(H2PO4)2 also present in CuP/TiO2. XPS was then employed to examine the states of phosphate in CuP/TiO2, and the result is shown in Figure S4 of the Supporting Information. The binding energy of P 2p was located at around 133.8 eV, indicative of a P5+ state in the sample.28 However, the P 2p spectrum was asymmetric. By fitting with the Gauss equation, two symmetric peaks at 134.1 and 133.4 eV were obtained, the positions very close to those of NaH2PO4 (133.9 eV) and Na2HPO4 (133.1 eV).28 This result suggests that, in CuP/TiO2, phosphate anions are bound to Ti(IV) sites through both monodentate and bidentate coordination, while the negative charges are compensated by cupric cations. Figure 2 shows the EPR spectra of CuP/TiO2 and CuO/ TiO2, recorded at room temperature. With all samples, a hyperfine structure of Cu2+ ions with g// = 2.34 and g⊥ = 2.08 was observed, similar to the spectrum of CuO/TiO2 reported in literature.23,26 However, as CuP loading increased, not only did the signal intensity increase but also the hyperfine structure became poorly resolved. The increases of signal intensity and line width are attributed to an increase in the long-range dipolar interactions between neighboring Cu(II) sites within CuP or CuO clusters. The line broadening may also result from large particles of CuP or CuO present in the sample, because the EPR spectra of bare CuO and CuP were silent and poorly resolved, respectively (Figure S5, Supporting Information). We conclude that both CuP and CuO particles at low loading are highly dispersed onto TiO2, in agreement with the results obtained with N2 adsorption and element distribution. Figure 3 shows the diffuse reflectance spectra of CuP/TiO2 powders. With bare TiO2, there was a strong absorption band at 200−400 nm, due to the ligand-to-metal charge transfer.

Figure 3. Diffuse reflectance spectra of (a) TiO2, (b) 1CuP/TiO2, (c) 2.9CuP/TiO2, (d) 4.8CuP/TiO2, (e) 9.1CuP/TiO2, and (f) CuP. The y axis is expressed by Kubelka−Munk unit, FR = (1 − R)2/2R, where R is the reflectance.

After CuP loading, there was also a weak absorption band at 400−800 nm, attributed to the d−d transitions of Cu2+ in copper phosphate. With all samples, the spectral onsets of TiO2 were approximately located at 400 nm. Similar spectral changes were also observed with CuO/TiO2 (Figure S6, Supporting Information). However, at given loading, CuO/TiO2 showed a higher absorbance than CuP/TiO2 in the visible light region. In fact, CuO was black in color, while CuP was blue. Moreover, CaP-modified TiO2 only absorbed UV light, because CaP was white in color. The optical differences among the samples may have some influence on the photocatalytic activity for organic degradation under UV or visible light. Photocatalytic Activity Measurement. Figure 4A shows the results of phenol degradation obtained with different catalysts (note that 0.1CuP/TiO2 and 1CuO/TiO2 are the best in activity respectively among individual sets of the catalysts, see below). With each catalyst, phenol concentration exponentially decreased with time, with the kinetics well fitting to the firstorder rate equation. Among three modified samples, CuP/TiO2 showed the highest activity (columns a−d, Figure 4B). In comparison with P25 TiO2, an international standard photocatalyst, CuP/TiO2 was also slightly more active (column e, Figure 4B). Moreover, a similar trend in activity among the catalysts was also observed with the formation and degradation of hydroquinone, the main intermediates of phenol degradation detected in aqueous solution (Figure S7, Supporting Information). The primary result indicates that CuP is probably a better modifier of TiO2 than CuO for improving the photocatalytic activity for organic degradation. In order to further confirm the positive effect of CuP, other TiO2 samples in the forms of pure anatase (AT) and rutile (RT) were loaded with 0.10 wt % CuP. For the photocatalytic degradation of phenol in aqueous suspension, both the CuPmodified AT and RT were more active than the unmodified ones (columns f−i, Figure 4B). Interestingly, the photocatalytic activity of AT was increased by 234%, much higher than that of RT (89%). This difference in activity is not simply attributed to the difference in surface area, measured by N2 adsorption. For example, P25 TiO2 has a lower surface area (50 m2/g) than those of TiO2 (125 m2/g) and AT (66 m2/g), but its activity is much higher than those of the latter catalysts. Furthermore, the different increase in activity due to CuP loading is also not 24585

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Figure 4. (A) Time profiles of phenol degradation in aerated aqueous suspension under UV light and (B) the corresponding apparent rate constant. Catalysts were (a) TiO2, (b) 0.1CuP/TiO2, (c) 1CuO/TiO2, (d) 0.1CaP/TiO2, (e) P25, (f) AT, (g) 0.1CuP/AT, (h) RT, and (i) 0.1CuP/RT.

simply ascribed to the different activity of bare TiO2. For instance, a mixed phase TiO2 was more active than AT (columns a vs f, Figure 4B), but it showed a lower increase in activity (110%) than AT (234%) after 0.1 wt % CuP was loaded. In a recent study,9,40 we have proposed that the photocatalytic activity of TiO2 for organic degradation in aqueous solution is mainly determined by two factors of crystallinity and sorption capacity toward the dissolved O2 and that anatase usually more photoactive than rutile is due to its larger capacity for the uptake of O2 from water. In the present study, CuP loading onto TiO2 may lead to decrease in the amount of O2 adsorption, detrimental to the surface reaction. Such a negative effect of CuP is probably more profound with rutile than with anatase. As a result, RT shows a lower increase in activity than anatase after CuP loading. However, this hypothesis needs to be proved by measurement of O 2 adsorption, which is difficult to follow at the present. Nevertheless, these observations show that, whatever TiO2 is in the form of anatase, rutile, and/or the mixture, the surface modification with 0.10 wt % CuP can result in significant improvement in the photocatalytic activity for organic degradation in aqueous solution under UV light. Effect of Cu Loading. Since CuP and CuO were both positive to the photcatalytic reaction, their optimum loadings onto TiO2 were then examined, and the result is shown in Figure 5. As Cu loading onto TiO2 increased, the rate of phenol disappearance first increased and then decreased. The maximum rate of phenol loss was observed with 0.1CuP/ TiO2 and 0.5CuO/TiO2, respectively. Interestingly, 0.1CuP/ TiO2 was more active than 0.5CuO/TiO2, even though the former had a much lower Cu/Ti mole ratio (0.00063) than the latter (0.005). Since 0.1CuP/TiO2 contains phosphate (P/Ti = 0.042 mol %), its activity higher than that of 0.5CuO/TiO2 may be due to the unique effect of phosphate.27−34 For this concern, TiO2 was loaded with 0−9.1 wt % CaP (P/Ti = 0−4.9 mol %). However, the CaP-modified TiO2 was only slightly more photoactive than the unmodified TiO2 (Figure S8, Supporting Information). Therefore, the higher activity of 0.1CuP/TiO2 than 0.5CuO/TiO2 is not simply due to the effect of phosphate. Note that, when the rate constant of phenol loss was replotted against the Cu/Ti atomic ratio (Figure S9A, Supporting Information), the trend in activity among the catalysts was similar to that shown in Figure 5. Moreover, the BET surface area normalized rate constants of phenol

Figure 5. Effect of Cu loading on the rate of phenol degradation under UV light in the aerated aqueous suspension of (a) CuP/TiO2 and (b) CuO/TiO2.

photodegradation as a function of Cu loading (Figure S9B, Supporting Information) also followed a similar trend to that in Figure 5. According to this specific rate, the maximum activity of CuP/TiO2 was approximately 1.4 times that of CuO/TiO2. These observations indicate that the higher activity of CuP/ TiO2 than CuO/TiO2 is not simply ascribed to the effect of phosphate and surface area. There should be a synergetic effect between the cation and anion of copper phosphate on the photocatalytic activity of TiO2 for phenol degradation, which will be discussed below. It has been reported that CuO/TiO2 has a higher photocatalytic activity than TiO2, ascribed to the electron transfer from the irradiated TiO2 to CuO, which improves the efficiency of charge separation.23,26 If this mechanism holds here, both CuP/TiO2 and CuO/TiO2 would show an increasing activity with Cu loading. However, an optimum Cu loading corresponding to the maximum activity was observed with either CuO/TiO2 or CuP/TiO2 (Figure 5). This observation is related to the fact that CuP and CuO are colored surface modifiers, which may reduce the number of the incident photons reaching TiO2, consequently decreasing the rate of phenol degradation at high Cu loading. However, as the Cu/Ti atomic ratio increases, the activity of CuP/TiO2 decreases more rapidly than that of CuO/TiO2 (Figure S9A, Supporting Information). This result reminds us of the 24586

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Figure 6. (A) Phenol degradation in the aerated aqueous suspension of 0.1CuP/TiO2 under UV light. (B) Ionic chromatographs of the filtrates obtained (a) in the dark and (b) after 2 h of the photoreaction. Curve (c) was obtained for 15 μM Na3PO4. The anions of chloride and sulfate resulted from the impurities in the commercial products of TiO2 and copper phosphate.

phosphate effect. It has been reported that when P-containing TiO2 is prepared by adsorption of H3PO4 or Na3PO4 in aqueous solution, followed by filtration, and washing and drying, its photocatalytic activity for phenol degradation first increases and then decreases with phosphate content.28,32 With pure CaP, there is an absorption band at wavelengths shorter than 350 nm (Figure S6A, Supporting Information). It is highly possible that the surface adsorbed phosphate has a screening effect on the UV light for excitation of TiO2. In fact, as the Cu/ Ti atomic ratio was higher than 0.0063, the photocatalytic activity of CuP/TiO2 became lower than that of bare TiO2 or CuO/TiO2 (Figure S9A, Supporting Information). On the other hand, Cu loading may lead to a decrease in the amount of O2 adsorption, consequently detrimental to phenol degradation. Therefore, the observed decrease in activity with Cu loading is due to both the negative effects of Cu loading on the light intensity and O2 adsorption. Catalyst Stability. A recycling test was carried out by using 0.1CuP/TiO2 as a photocatalyst. Figure 6A shows the result of phenol degradation in aerated aqueous suspension under UV light. From the first run to the last, the rate constant of phenol disappearance only decreased from 0.0191 min−1 to 0.0183 min−1. Such a decrease in the activity with time is mostly attributed to the reaction intermediates such as hydroquinone competing with phenol for reactive species. Moreover, no phosphate and cupric ions were found in the irradiated aqueous suspension by IC (Figure 6B) and spectral analysis, respectively. These observations show that CuP/TiO2 is stable and can be repeatedly used without significant loss in the photocatalytic activity. Possible Mechanism. EPR study has shown that Cu(II) species at low loading in CuO/TiO2 and CuP/TiO2 are highly dispersed as a kind of clusters (Figure 2). However, the electrochemistry of Cu(II) in those samples is not known. The standard redox potential for the Cu2+/Cu+ couple in aqueous solution is 0.166 V versus NHE, which is more positive than the conduction band edge of TiO2 (−0.12 V vs NHE at pH 0). If Cu(II) species are present like Cu2+, then the electron transfer from the band gap excited TiO2 to CuO23,26 and to CuP would be thermodynamically possible. In order to confirm this, the EPR spectra of CuP/TiO2, before and after exposure to UV light (λ > 320 nm) for 30 min, were recorded at room temperature, and the result is shown in Figure 7. In the atmosphere of N2, the signal intensity of Cu2+ was somewhat

Figure 7. EPR spectra of 1CuP/TiO2 obtained (a) under N2 and (b) in air. Dark and red lines represent the signals recorded in the dark and after UV irradiation for 30 min, respectively.

decreased, while in the air the signal remained almost unchanged. Slight decrease in signal intensity might be due to fast reoxidation of Cu(I) by residual O2 present in the capillary tube. Similar changes in EPR spectra upon UV light irradiation were also observed with CuO/TiO2 (Figure S10, Supporting Information). Moreover, the PL emission of CuP/ TiO2 was weaker than that of bare TiO2, presumably indicative of the decreased recombination of charge carriers (Figure S11, Supporting Information). These observations give support of the proposal that Cu(II) is reduced to Cu(I) by the conduction band electron of TiO2 and then regenerated by O2. However, the one-electron reduction potential of O2 (−0.05 V vs NHE at pH 0) is more negative than that for the Cu2+/Cu+ couple, unfavorable to the formation of superoxide radicals. Therefore, only a multielectron reduction of O2 by Cu(I) with the formation of either H2O2 or H2O would be thermodynamically possible (Eo(O2/H2O2) = 0.68 V, Eo(O2/H2O) = 1.23 V vs NHE). However, at low Cu loading, CuP/TiO2 is more photoactive than CuO/TiO2 (Figure 5). Since CaP/TiO2 is only slightly more active than TiO2 (Figure S8, Supporting Information),34 it suggests that there is a synergetic effect between Cu(II) and phosphate. Several groups have reported that the phosphatemodified TiO2 is photocatalytically more active than the unmodified TiO 2 for organic degradation in aqueous 24587

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solution.27−32 Zhao and co-workers have proposed that a negative electrostatic field is formed in the surface layers of TiO2 as a result of phosphate adsorption, which would facilitate the hole migration and subsequently increase the rate of •OH production for organic degradation.28 By means of the transient spectroscopy, Jing and co-workers31 have recently shown that phosphate- or OH−-adsorbed TiO2 film has a longer lifetime of the photogenerated charges than the unmodified TiO2 film in oxygen-free water. This observation appears to support the model of the enhanced OH generation over the phosphatemodified TiO2.28 However, for organic degradation in a solid− gas phase, Jing and co-workers32 have attributed the increased activity of phosphate-modified TiO 2 to the enhanced adsorption of O2, because in atmosphere, the surface phosphate groups (−Ti−O−P−OH) would hardly deprotonate to produce the expected negative electrostatic field. As CuP is concerned, the strong electrostatic interaction between the phosphate anion and the cupric cation would not favor the formation of a negative electrostatic field in the surface layers of TiO2. We speculate that phosphate-modified TiO2 has a higher sorption capacity than bare TiO2 toward organic pollutant in aqueous solution.34 Since phenol weakly adsorbs on TiO2 in water, 2,4-dichloprhenol (DCP) has been used as a model sorbate. The result showed that the amount of DCP adsorbed on TiO2 in aqueous solution increased in the order of CuP/ TiO2 > CuO/TiO2 > CaP/TiO2 > TiO2 (Figure S12, Supporting Information). Enrichment of organic substrate onto the catalyst surface would increase the rate of organic degradation. Therefore, we propose that Cu(II) and phosphate act as an electron scavenger and organic sorbent, facilitating the electron and hole transfer, respectively. Since electron and hole are formed in a pair, the cooperation between Cu(II)- and phosphate-mediated processes would result in significant enhancement in the efficiency of charge separation and in the rate of phenol degradation. This proposed mechanism is considered to be responsible for the observed synergetic effect between cations and anions of copper phosphate on the photocatalytic activity of TiO2.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 program of China (nos. 2009CB825300 and 2011CB936003).

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REFERENCES

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CONCLUSIONS In this work, CuP-modified TiO2 has been prepared by a simple mixing method, which allows CuP particles highly dispersed onto TiO2. Because of that, the modified TiO2 with 0.10 wt % CuP shows a much higher photocatalytic activity than the unmodified TiO2, for phenol degradation in aerated aqueous suspension. We propose that Cu(II) acts as an electron scavenger and phosphate as an organic sorbent. Their cooperation would promote the charge separation and thus accelerate organic degradation at the solid−liquid interface. Although the CuP-modified catalyst is blue in color, its visible light activity (λ > 420 nm) for phenol degradation in aerated aqueous is very low. We conclude that CuP/TiO2 may find application for water treatment with UV light and molecular oxygen.

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Article

ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, SEM photos, element mapping, EPR, diffuse reflectance, and PL spectra of different catalysts, evaluation of CaP/TiO2 activity, surface area normalized rate constants of phenol degradation, and adsorption isotherm of DCP. This material is available free of charge via the Internet at http:// pubs.acs.org. 24588

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