Synthesis of Efficient Nanosized Rutile TiO2 and Its Main Factors

Jul 19, 2012 - Nanosized TiO2 containing different contents of rutile phase was controllably synthesized by a hydrochloric acid-modified hydrothermal ...
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Synthesis of Efficient Nanosized Rutile TiO2 and Its Main Factors Determining Its Photodegradation Activity: Roles of Residual Chloride and Adsorbed Oxygen Yunbo Luan,†,‡ Liqiang Jing,*,† Qingqiang Meng,† He Nan,† Peng Luan,† Mingzheng Xie,‡ and Yujie Feng*,‡ †

Key Laboratory of Functional Inorganic Materials Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China ‡ State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, P. R. China S Supporting Information *

ABSTRACT: Nanosized TiO2 containing different contents of rutile phase was controllably synthesized by a hydrochloric acid-modified hydrothermal process. It is demonstrated that the formation of rutile phase in TiO2 mainly depends on the role of chlorine anions in the synthesis, and a certain amount of residual chloride would exist on the surfaces of the resulting nanocrystalline rutile TiO2. Interestingly, the as-prepared rutile shows high activity for photodegradation of rhodamine B dye compared with the as-prepared anatase, even superior to the P25 TiO2. It is mainly attributed to the residual chloride that could promote the dye adsorbed on the surfaces of TiO2, consequently accelerating the photosensitization oxidation reactions of the dye molecules. In the photodegradation of liquidphase phenol and gas-phase aldehyde, the as-prepared rutile TiO2 samples display low activity, which is attributed to the photogenerated electrons weakly captured by the adsorbed oxygen, since the residual chloride could effectively capture photoinduced holes based on the atmosphere-controlled surface photovoltage spectroscopy results. Further, the photoactivity of resulting rutile for degrading phenol and aldehyde is greatly enhanced by modifying a proper amount of phosphoric acids to increase the adsorption of O2, even higher than that of the P25 TiO2. This work would explore feasible routes to synthesize efficient nanosized rutile TiO2-based photocatalysts for degrading colored and colorless organic pollutants by investigating the rate-determining factors in the photodegradation processes.

1. INTRODUCTION In recent years, the semiconductor photocatalysis technique has attracted much attention for solving the increasingly serious problems of environmental pollution. Titania (TiO2) has been proven to be one of the most ideal photocatalyst materials, owing to its high efficiency, low cost, and availability.1,2 It mainly exists as three different polymorphs: anatase, rutile, and brookite. In general, anatase is considered to be better photoactivity than rutile due to both a high adsorption affinity for organic molecules3 and a low charge recombination rate.4 Thus, rutile TiO2 is widely neglected in the photocatalysis. However, rutile possesses several advantages over anatase, such as high chemical stability, narrow band gap, and high refractive index,5,6 implying that it might be a promising photocatalyst. Traditionally, the rutile phase of TiO2 is obtained via hightemperature calcination of anatase particles. However, such phase change process is accompanied by sintering, grain growth, and surface area loss, which is responsible for the weak photoactivity of rutile.7,8 Recently, several researchers have directly synthesized nanosized rutile TiO2 by hydrothermal processes or simple low-temperature methods, without high© 2012 American Chemical Society

temperature thermal treatments, aiming to improve its photocatalytic activity. Some researchers9−12 demonstrated that the obtained rutile TiO2 showed higher photocatalytic activity for decomposing pollutants than anatase, sometimes even superior to commercial P25 TiO2, which was attributed to the combination effects of small nanocrystal size, large surface area, and high rutile crystallinity, whereas others13−15 reported that the obtained rutile TiO2 exhibited poor photoactivity, which was mainly ascribed to the narrow energy band gap. Through a detailed comparative analysis, we find an interesting phenomenon that the rutile TiO2 obtained at low temperature exhibits different photoactivity, mainly depending on the selected pollutants. When some colored dyes, such as rhodamine B (RhB), methyl orange, and methylene green, are used as the pollutants, the rutile TiO2 exhibits high photoactivity. Contrarily, it displays poor photoactivity, if the colorless pollutants, such as phenol, benzene, and 4-nitroReceived: May 27, 2012 Revised: July 19, 2012 Published: July 19, 2012 17094

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activity for degrading phenol and aldehyde of the rutile TiO2 is greatly improved by increasing the adsorbed O2 after phosphate modification. This work would help us well understand the photodegradation rate-determining steps of pollutants over rutile TiO2 and provide new ideal for synthesis of high-activity rutile TiO2-based photocatalysts.

phenol, are employed. Thus, the above phenomenon spurs us to explore the main factors determining the photodegradation activity of rutile TiO2, for reasonably elucidating the photoactivity change. Besides the factors suggested by those researchers,7−15 mainly involved with small crystal size, large surface area, high rutile crystallinity, and narrow energy band gap, it is assumed that there are other key factors to influence the photoactivity of rutile TiO2. It is widely accepted that the photodegradation of organic pollutants over TiO2 is mainly involved with the photocatalytic oxidation (PCO) degradation through photoholes and photosensitization oxidation (PSO) degradation. In the PCO degradation, it is pivotal that the adsorbed O2 captures photogenerated electrons from the conduction band of TiO2 to produce typically O2− and/or HO2, effectively avoiding the buildup of negative charges so as to further suppress electron− hole recombination and then to enhance photoactivity,16,17 whereas it is important for the PSO degradation that the rutile TiO2 possesses strong ability to adsorb colored dye molecules. Surprisingly, the adsorption of the O2 and pollutant molecules on rutile are often neglected in the photodegradation reactions. In addition, it is noticed that the direct formation of rutile TiO2 often depends on the presence of Cl−.18,19 Thus, it is naturally expected that there are some residual chloride in the formed rutile, and the residual chloride would possibly display obvious effects on the adsorption of organic pollutants and the capture of photogenerated charges, further determining its final photodegradation activity. However, to the best of our knowledge, the studies about the roles of the residual chloride in the photodegradation of pollutants have not been reported up to date. Therefore, it is very meaningful to explore the roles of the residual chloride in rutile and adsorbed O2 for understanding the rate-determining factors of photodegradation reactions over rutile. Surface photovoltage spectroscopy (SPS) is one of effective methods for characterizing the charge separation and recombination and charge transfer.20,21 It has been well used for the study of the charge transfer in photosimulated surface interactions, dye sensitization process, and photocatalysis. Especially, the SPS method is recently modified by an atmosphere-controlled testing cell in our group, much different from the only air-atmosphere one previously.22,23 It is much beneficial to deep reveal the properties of photogenerated charges by the origins from photogenerated electrons or holes under different atmosphere conditions. In addition, to evaluate the amount of adsorbed oxygen, it is necessary to carry out O2 temperature-programmed desorption (TPD) measurements widely employed in the transitional thermal catalysis, while seldom in the photocatalysis.22 Herein, we controllably synthesized nanosized TiO2 with different ratio of rutile to anatase by a HCl-modified hydrothermal process directly and explored main factors determining the photoactivity of the resulting rutile TiO2 for degrading three kinds of representative pollutants, RhB dye, phenol, and aldehyde, principally involved with the roles of residual chloride and adsorbed oxygen by means of SPS techniques and oxygen TPD curves. It is well demonstrated for the first time that the residual chloride in rutile TiO2 could enhance the adsorption of dye, leading to its efficient photodegradation, and the low ability to adsorb O2 of the resulting rutile is not much favorable for the photocatalytic degradation of phenol and aldehyde since the residual chloride could capture photogenerated holes. Moreover, the photo-

2. EXPERIMENTAL SECTION All substances used in this study were analytical grade and used without further purification. Deionized water was used in all experiments. 2.1. Preparation of TiO2 Nanopowders. Tetrabutyl titanate was used as the main starting material. Initially, this reagent was dropwise added to a desired concentrated hydrochloric acid (HCl) solution, maintained below 10 °C by an ice−water bath. Then, the mixture was heated in a water bath for 4 h at 80 °C so as to produce white suspension. Subsequently, the suspension was placed in Teflon-lined hydrothermal reactors and heated at 160 °C for 6 h. After that, a white precipitate was collected and washed repeatedly with isopropanol and distilled water to remove all the organics and chloride ions. Finally, the TiO2 sample referred to as DSX was obtained by drying the white precipitate at 100 °C for 12 h, in which DS means direct synthesis, and X represents the molar concentration of HCl aqueous solution used. In addition, the rutile sample was modified with phosphoric acid (H3PO4) in order to probe the rate-determining factors for photodegradation of pollutants. 0.5 g of DS2.0 powder with pure rutile was put into 20 mL of different molar concentration of H3PO4 solution. Then, the resulting suspension was stirred for 2 h, centrifuged, and washed with water. Finally, the H3PO4modified TiO2 was obtained by drying and calcining at 450 °C for 0.5 h. The modified sample is defined as DS2.0-YP, in which Y is the concentration of H3PO4 solution used. 2.2. Characterizations of the Samples. The prepared samples were analyzed by various methods. The crystal structure of the samples was determined by X-ray diffraction (XRD) method (Rigaku D/MAXrA powder diffractormeter, Japan), using Cu K radiation (α = 0.154 18 nm), and an accelerating voltage of 30 kV and emission current of 20 mA were employed. Transmission electron microscopy (TEM) observation of the sample was performed on a JEM-3010 electron microscope (JEOL, Japan), with an acceleration voltage of 300 kV. The specific surface areas of the samples were measured by a Brunauer−Emmett−Teller (BET) instrument (Micromeritics automatic surface area analyzer Gemini 2360, Shimadzu), with nitrogen adsorption at 77 K. The surface composition and elemental chemical state of the samples were examined by X-ray photoelectron spectroscopy (XPS) using a Model VG ESCALAB apparatus with Mg K Xray source, and the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). The UV− vis diffuse reflection spectrum (UV−vis DRS) of the samples was recorded with Shimadzu UV-2550 spectrophotometer, using BaSO4 as reference. The surface photovoltage spectroscopy (SPS) measurements of the samples were carried out with a home-built apparatus that had been described in detail elsewhere.20,22 The powder sample was sandwiched between two ITO glass electrodes, which were arranged in an atmosphere-controlled sealed container. The SPS signals were the potential barrier change of the electrodes surface between in the presence of light and in the dark. Temperatureprogrammed desorption (TPD) of oxygen was conducted in a 17095

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flow apparatus built by ourselves, which was described in our previous report.22 30 mg of the powder sample was pretreated at 450 °C for 30 min in an ultrahigh-purity He flow, and then the sample was activated at 450 °C for 30 min in an ultrahighpurity O2 flow. After that, the sample sequentially adsorbed O2 for 120 min at 25 °C. Finally, the desorbed O2 amount was monitored by a gas chromatograph (GC-2014, Shimadzu) with a TCD detector. 2.3. Evaluation of Photodegradation Activity. The activities of the samples were evaluated by photodegradation of Rhodamine B (RhB), phenol solution, and aldehyde gas under xenon lamp similar to solar light. 0.05 g of TiO2 was dispersed in 30 mL of 10 mg/L pollutant solution, and then the irradiation lasted for 60 min for photodegradation of RhB and phenol. The RhB concentration was determined by the solution absorbance at 553 nm, and phenol concentration was measured by the 4-aminoantipyrine spectrophotometric method at the characteristic optical adsorption (510 nm) of phenol, both with a Shimadzu UV-2550 spectrophotometer after centrifugation. For photodegradation of aldehyde gas, 0.1 g of TiO2 was placed in a mixed gas system containing 810 ppm of aldehyde, 20% of O2, and 80% of N2 to carry out photodegradation reaction for 60 min. The determination of aldehyde concentration was performed with a gas chromatograph (GC-2014, Shimadzu) equipped with a flame ionization detector. The photodegradation activity, given as a percentage, refers to the difference in the pollutant concentration before irradiation, C(0), and after light irradiation for 60 min, C(60), divided by the C(0), (i.e., 100[C(0) − C(60)]/C(0)).

Table 1. XRD Data and BET Surface Areas of DS0, DS1.0, DS1.8, DS2.0, and DS2.5 TiO2 Samples crystallite size (nm) TiO2 samples

rutile content (%)

anatase

DS0 DS1.0 DS1.8 DS2.0 DS2.5

0 11.2 88.3 100 100

8.4 10.7

rutile

surface area (m2 g−1)

21.4 28.7 38.5

128.2 96.6 50.3 38.6 32.4

phase mainly depends on the high-concentration HCl solution used. The average crystallize sizes (Table 1) of the as-prepared samples can be calculated by employing Debye−Scherrer formula based on the XRD diffraction peaks of anatase (101) and rutile (110).26 It is found that the crystallite size grows from 8.4 to 38.5 nm with increasing the rutile content. The TEM images were used to further investigate the size and morphology of the samples, shown in Figure 2. It can be seen

3. RESULTS AND DISCUSSION 3.1. Structural Characterization and Surface Composition. Figure 1 shows the XRD patterns of the synthesized

Figure 1. XRD patterns of TiO2 samples prepared by varying molar concentrations of HCl solution from 0 to 2.5 M.

Figure 2. TEM images of DS0, DS1.0, DS1.8, and DS2.0 TiO2 samples.

TiO2 nanocrystals. The relative phase percentages of TiO2 can be routinely evaluated as a function of XRD peak intensities of different crystallographic forms (anatase and rutile) on the basis of formulas,24,25 shown in Table 1. It can be seen that the anatase is the only phase observed in the absence of HCl. As the concentration of used HCl increases from 1.0 to 1.8 M, the anatase content decreases and rutile phase elevates from 11.2 to 88.3%. When the concentration is up to 2.0 M, or over, anatase disappears entirely, and subsequently only pure rutile phase presents in the resulting TiO2. It is demonstrated that the phase composition of TiO2 nanocrystals can be controlled by simply varying the HCl concentration, and the formation of the rutile

that TiO2 samples with main anatase phase consist of spherelike nanoparticles with small size, while those with main rutile do rodlike nanoparticles with relatively large size. As seen in Table 1, the pure anatase TiO2 sample has high surface area of 128.2 m2 g−1. As the rutile content increases, the surface area of resulting TiO2 obviously decreases. When the sample becomes pure rutile, the surface areas of DS2.0 and DS2.5 go down as small as about 38.6 and 32.4 m2 g−1, respectively. Although the resulting rutile sample displays 17096

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smaller surface area than the anatase one, it does much large surface area compared with the rutile TiO2 obtained by traditional thermal treatment at high temperature in the previous works.27,28 XPS measurements were carried out to investigate the chemical states of all of the elements in the TiO2 samples in Figure 3 and Supporting Information Figure 1. The pure

Figure 4. SPS responses of DS0, DS1.0, DS1.8, DS2.0, and DS2.5 TiO2 samples.

found between 300 and 425 nm. This is attributed to the electron transitions from the valence to conduction band (band-to-band transitions, O 2p−Ti 3d) on the basis of DRS spectra and TiO2 band structure.33,34 Noticeably, the SPS response gradually becomes strong with increasing rutile content. This is obviously different from the SPS orders of TiO2 samples prepared by the common thermal treatment method,20,35 for which the anatase-based samples usually show stronger SPS responses than the rutile-based ones, and the pure rutile always exhibits a weak SPS response, as shown in SIFigure 4. Why do the TiO2 samples with similar phase composition obtained by different methods exhibit differentstrength SPS response orders? In addition, the DS2.5 exhibits strong SPS response compared with the DS2.0, although they both are pure rutile. Thus, it is assumed that the residual chloride in the rutile-containing TiO2 would greatly influence the separation of photogenerated charges and then the SPS responses. To probe the assumption, the SPS responses of as-prepared TiO2 in different oxygen-concentration atmosphere were measured. For the pure-anatase DS0, there is no SPS response in the nitrogen, while a marked SPS response appears in the presence of oxygen, and its SPS response becomes strong with increasing oxygen concentration, as shown in SI-Figure 5. It is concluded that the SPS response of the pure anatase results from the adsorbed oxygen. The O2 affinity level (O2/O2−) stands at about 0 eV,36,37 lower than the conduction band potential of TiO2, favorable for the photoinduced electrons to be captured by the adsorbed oxygen so as to make the photoinduced holes diffuse preferentially to the surfaces of used electrode to produce the SPS response. However, it is found that the SPS response of pure-rutile DS2.0 is completely different from the anatase DS0, it displays an obvious SPS response in absence of oxygen, and its SPS intensity gradually becomes weak with increasing oxygen concentration shown in Figure 5. Therefore, it is deduced that the residual chloride could effectively capture the photoinduced holes, promoting the photoinduced electrons diffuse to the surfaces of used electrode to give rise to the SPS response. This is further supported by the SPS results of the rutile TiO2 obtained by calcining DS0 at 800 °C, as shown in SI-Figure 6. It is noticed that the obtained rutile TiO2 exhibits similar SPS responses to the pure anatase DS0 in different oxygen-concentration atmospheres, implying that they have same SPS attributes from the photogenerated electrons to be captured by the adsorbed O2. On the basis of the above results, it is hypothesized that the residual chloride in the rutile-containing

Figure 3. Cl 2p XPS spectra of DS0, DS1.0, DS1.8 and DS2.0 TiO2 samples.

anatase TiO2 only contains Ti and O elements. Unexpectedly, the Cl 2p XPS composed of two peaks is detected in the containing-rutile TiO2, except for Ti and O. The weak one at 199.3 eV might be assigned to the doped Cl− incorporated into the crystal lattice. The strong one centering at 197.4 eV is assigned to Cl− chemically adsorbed on the surfaces of the resulting rutile.29,30 On the basis of the shift to high energy direction of corresponsive binding energy of Ti 2p and O 1s, it is deduced that the adsorbed Cl− is located on the TiO2 surfaces as the form of O−Ti−Cl. In addition, as the rutile content of TiO2 increases, the concentration of doped Cl− almost does not change, whereas the amount of chemically adsorbed Cl− gradually rises. It seems that the presence of Cl− in the synthesis is crucial for forming the rutile phase. To prove this point, a designed experiment was performed with H2SO4 and HNO3 instead of HCl as acidic mediums, and the molar ratio of Ti4+ to H+ was fixed at two, for keeping the same acidity condition. It can be seen from SI-Figure 2 that the product is pure rutile from HCl medium, while it is a mixture of rutile and anatase from a H2SO4 or HNO3 medium. It is confirmed that the Cl− should play important roles in the synthesis of rutile TiO2. This is in good agreement with the literature.18,19 In addition, it is naturally expected that the residual chloride on the surfaces of resulting rutile would display obvious effects on the adsorption of pollutants and the capturing of photogenerated charges, similar to the surface modification with fluorine and nitrogen.31,32 3.2. Photoinduced Charge Separation. As seen from SIFigure 3, the rutile TiO2 shows extended absorption in the visible region compared with anatase sample. The absorption edge gradually shifts to long wavelength with increasing HCl concentration, mainly attributed to the ratio change of rutile to anatase based on the XRD results. According to the respective optical absorption edge, it is estimated that the band gap energies of pure anatase TiO2 (DS0) and pure rutile TiO2 (DS2.0) are about 3.12 and 2.98 eV, consistent with the reports.4,5 The SPS responses of as-prepared TiO2 samples in air are shown in Figure 4. For all samples, an obvious SPS response is 17097

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Figure 5. SPS responses of DS2.0 in different O2-concentration atmospheres.

TiO2 greatly affects the photodegradation activity of pollutants since it could capture photogenerated holes. 3.3. Photodegradation Activities. The degradations of all the synthesized samples and commercial P25 TiO2 used as reference were investigated by monitoring concentration changes in RhB, phenol, and aldehyde under simulative solar light irradiation. The direct photolysis of the three pollutants mentioned above is neglectable compared with the photodegradation in the presence of TiO2. The photodegradation rate is equal to the difference between the total degradation rate under illumination and the adsorption degradation rate without light, shown in Figure 6. One can see that the photodegradation rate of RhB on the as-prepared TiO2 gradually becomes large as the rutile content in TiO2 increases, and the DS2.0 sample with pure rutile phase exhibits a much high photoactivity, which is about 30% higher in the photodegradation rate than the DS0 sample with pure anatase, even superior to the commercial P25 TiO2 (over 50%). The high photoactivity of DS2.0 is also further proved by the degradation evolution curve of RhB solution under natural sunlight in SI-Figure 7. Unexpected, differently from in the photodegradation of RhB, the activity of the as-prepared TiO2 gradually decreases in the photodegradation of phenol or aldehyde as the rutile content in TiO2 increases, and the DS2.0 shows a much low photodegradation activity. 3.4. Discussion. In general, the SPS response could reflect the separation of photoinduced charges, and the stronger is the SPS response, the higher is the photoinduced charge separation rate, which should be favorable for the enhancement of photocatalytic activity.38−40 Why does the as-prepared rutilebased TiO2, which usually corresponds to a strong SPS response, exhibit low activity for degradation of phenol and aldehyde, compared with the anatase-based TiO2? And also, why does the as-prepared rutile TiO2 perform different photodegradation activity for different pollutants? For RhB as a dye pollutant, it is naturally expected that its photodegradation is attributed to the photosensitization oxidation41 and photohole oxidation two ways. In the photosensitization oxidation process, the photoinduced electrons resulting from the excited dye by the visible absorption are immediately injected into the conduction band of TiO2, leading to the oxidation of dye. Thus, it is favorable for the photosensitization oxidation degradation of dye to enhance its adsorption on TiO2 surfaces in advance.42 As seen from the SIFigure 8, the as-prepared rutile-containing TiO2 exhibits high capacity to adsorb the RhB. This is attributed to the residual adsorbed chloride in TiO2, by which the RhB dye molecules are

Figure 6. Photodegradation rates of RhB (A), phenol (B), and aldehyde (C) on different TiO2 samples under simulative solar light.

easily linked on the TiO2 surfaces with −N(Et)2 group.43 Therefore, it is concluded that the residual adsorbed chloride in TiO 2 is favorable for the photosensitization oxidation degradation by promoting the dye adsorption, leading to the efficient photodegradation of RhB. It is further supported that the DS0-T800-Cl sample, obtained by the addition of extra Cl− into the reaction suspension composed of RhB dye solution and the rutile-phase DS0-T800 sample, displays higher photoactivity than the DS0-T800 one without Cl− on the surfaces (shown in SI-Figure 9). Therefore, the pure rutilephase DS2.0 sample with more amount of the adsorbed chloride exhibits a much higher photoactivity in the photodegradation RhB than the DS0 with pure anatase. In addition to the residual chloride, the surface area would affect the photodegradation reactions by influencing the adsorption of O2 as oxidant. This is responsible for the result that the DS2.5 displays a lower photoactivity in the photodegradation RhB than the DS2.0. In addition, the above conclusion was further supported by the confirmatory experiments. As shown in SIFigure 8, the rutile-based samples exhibit good photoactivity for 17098

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degrading RhB under visible irridation (>420 or 460 nm), while do weak photoactivity under ultraviolet irradiation (360 nm), clearly indicating that the photodegradation of RhB on the asprepared rutile-containing TiO2 mainly results from the contribution of the photosensitization oxidation way rather than the photohole-oxidation one. For phenol or aldehyde as pollutant, its photodegradation should be attributed to the photohole-oxidation way. In this case, it is important to improve the separation of photogenerated charges. On the basis of the strong SPS response, it seems that the as-prepared rutile-containing TiO2 samples exhibit high separation of photogenerated charges since the residual chloride could effectively capture photogenerated holes, compared with the anatase TiO2. However, the photoactivity for degrading phenol or aldehyde on the resulting rutile-based TiO2 is lower than that on the resulting anatasebased one. This implies that the step that the photogenerated holes are captured is not rate-determining one in the photodegradation process. Therefore, it is possible that the rate-determining step is the other half-reaction induced by the photogenerated electrons. In general, it is widely accepted that the adsorbed O2 should play an important roles in the halfreaction on TiO2, and the rutile TiO2 usually possesses a weak ability to adsorb O2 compared with the anatase one.16 Thus, it is assumed that the step that the photogenerated electrons are captured by the adsorbed O2 is the rate-determining one, which is attributed to the low adsorbed amount of O2 on the resulting rutile-based TiO2. To further prove the assumption, we modified the asprepared rutile TiO2 (DS2.0) with an appropriate amount of phosphoric acid, aiming to promote the adsorption of O2 on TiO2, based on our previous investigation.22 Notably, the phosphate modification does not change the phase composition and the residual chloride amount based on the XRD patterns (SI-Figure 10) and XPS measurements (SI-Figure 11), respectively. As expected, the amount of adsorbed O2 on the DS2.0 is successfully increased by phosphate medication by means of the TPD curves shown in Figure 7A, which is well responsible for the result that the SPS response is obviously decreased after phosphate modification since the residual chloride in DS2.0 could still capture photogenerated holes effectively so as to make the photoelectrons migrate preferentially to the tested electrode (SI-Figure 12). It is demonstrated that the photogenerated charge separation of DS2.0 is improved by phosphate modification. Consequently, the photoactivity for degrading phenol or aldehyde of rutile DS2.0 is markedly improved after phosphate modification (Figure 7B and SI-Figure 13), higher than that of anatase DS.0 (even superior to that of P25 TiO2). Therefore, it is suggested that the poor photoactivity of the as-prepared rutile-based TiO2 is attributed to its weak ability to adsorb O2 in the degradation of phenol and aldehyde and would be greatly enhanced by increasing the adsorption of O2.

Figure 7. SPS responses in the air and the oxygen-TPD curves (A) and photodegradation rates of aldehyde gas (B) for the unmodified and phosphoric acid modified rutile TiO2.

As a result, the resulting rutile TiO2 exhibits high photoactivity for degrading RhB, even superior to P25 TiO2. Since the residual chloride could effectively capture photogenerated holes based on the atmosphere-controlled SPS measurements, the low photoactivity of the rutile TiO2 for degrading phenol or aldehyde is attributed to the weak capture of photogenerated electrons by the adsorbed O2. Based on our previous study, the modification with phosphoric acid is made to increase the adsorption of O2, leading to a great increase of the rutile in the photoactivity for degrading phenol or aldehyde, even superior to P25 TiO2. Therefore, it is concluded that the photodegradation activity of resulting rutile TiO2 is mainly determined by the residual chloride and the adsorbed O2. This work would help us understand well what to determine the photoactivity of rutile and provide us with a feasible route to design and fabricate high-performance rutile TiO2-based photocatalyst materials.

■

ASSOCIATED CONTENT

S Supporting Information *

SI-Figure 1: XPS spectra of DS0, DS1.0, DS1.8, DS2.0, and DS2.5 TiO2 samples for Ti 2p and O 1s core levels; SI-Figure 2: XRD patterns of TiO2 samples prepared with HCl, HNO3, and H2SO4; SI-Figure 3: UV−vis DRS spectra of DS0, DS1.0, DS1.8, DS2.0, and DS2.5 TiO2 samples; SI-Figure 4: XRD patterns (A) and SPS responses (B) of TiO2 samples prepared by calcining DS0 at different temperature from 0 to 800 °C; SIFigure 5: SPS responses of DS0 in different O2-concentrations atmosphere; SI-Figure 6: SPS responses of DS0-T800 in different O2-concentration atmospheres; SI-Figure 7: photodegradation rates of RhB solution on DS2.0 and P25 TiO2 under natural sunlight; SI-Figure 8: photodegradation rates of RhB on different TiO2 samples under visible (A) and single wavelength (B) light; SI-Figure 9: photodegradation rates of

4. CONCLUSIONS In summary, nanosized TiO2 with different ratio of rutile to anatase was successfully fabricated by a low-temperature hydrothermal process. The key of this controllable synthesis lies at the concentration change of used HCl solution. It is demonstrated that the residual chloride would exist on the surfaces of the resulting rutile-containing TiO2, favorable to increase the adsorption of colored RhB to promote principally its photodegradation via the photosensitization oxidation way. 17099

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The Journal of Physical Chemistry C

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RhB on the rutile-phase DS0-T800 sample prepared by calcining DS0 at 800 °C and the DS0-T800-Cl sample obtained by the addition of a certain amount of NaCl into the reaction suspension composed of RhB dye solution and the DS0-T800 sample; SI-Figure 10: XRD patterns of DS2.0 samples unmodified and modified with H3PO4 of 0.05, 0.1, and 0.3 M ; SI-Figure 11: XPS spectra of unmodified and phosphate modified TiO2 for Ti 2p, O 1s, Cl 2p, and P 2p core levels; SIFigure 12: SPS responses of the modified TiO2 in different O2concentration atmospheres; SI-Figure 13: photodegradation rates of phenol solution on DS2.0, DS2.0−0.05P, DS2.0−0.1P, and DS2.0−0.3P TiO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 is financially supported from the National Nature Science Foundation of China (No. 21071408), the Chang Jiang Scholar Candidates Programme for Provincial Universities in Heilongjiang, the Science Foundation of Harbin City of China (No. 2011RFXXG001), and the Program for Innovative Research Team in Heilongjiang University (Hdtd2010-02), for which we are very grateful.

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dx.doi.org/10.1021/jp305142j | J. Phys. Chem. C 2012, 116, 17094−17100