Synthesis of High-Activity TiO2-Based Photocatalysts by

ARTICLE pubs.acs.org/JPCC

Synthesis of High-Activity TiO2-Based Photocatalysts by Compounding a Small Amount of Porous Nanosized LaFeO3 and the Activity-Enhanced Mechanisms Liqiang Jing,* Yichun Qu, Haijiao Su, Changhao Yao, and Honggang Fu* Key Laboratory of Functional Inorganic Materials Chemistry, Heilongjiang University, Ministry of Education, School of Chemistry and Materials Science, Harbin 150080, People's Republic of China

bS Supporting Information ABSTRACT: In this work, a feasible strategy has been developed to fabricate highly active TiO2-based photocatalysts for degrading rhodamine B solution under ultraviolet or visible irradiation. The key of this strategy is to compound a small amount of porous LaFeO3 to nanocrystalline TiO2, leading to the delay of anatase-rutile phase tansformation and then to the formation of the anatase TiO2/rutile TiO2/LaFeO3 three-phase nanostructured heterojunctions after thermal treatment at a high temperature of 800 °C. The resulting three-phase heterojunctions greatly favor the photoinduced charge separation, especially promoting the visible induced electrons of LaFeO3 captured by the adsorbed O2, based on their surface photovoltage responses, which is well responsible for the ultraviolet or visible activity enhancement. This work would provide a feasible route to fabricate high-performance TiO2based nanomaterials.

1. INTRODUCTION TiO2 is considered to be an ideal semiconductor photocatalyst due to its photostability, environmental friendliness, low cost, and strong oxidizing ability.1
5 However, there are two main bottleneck factors hindering its practical application, including high photoinduced charge recombination rate and low efficiency for utilizing solar light. Numerous studies have been performed to handle these problems, including optimization of synthetic conditions,6
8 surface modification,9
11 impurity doping,12
14 photosensitization,15,16 coupling other semiconductors with narrow energy band gap,17
20 and so on. Widely accepted, high anatase crystallinity, mixed phase composition, and large surface area are favorable to enhance the photocatalytic activity of nanosized TiO2 under ultraviolet light.21,22 Recently, our group synthesized highly active TiO2-based photocatalysts with high anatase thermal stability using ethylenediamine21 and mesoporous amorphous SiO222 as the modifiers, attributed to the high anatase crystallinity and large surface area. Therefore, it is necessary for the high activity to enhance the thermal stability of nanocrystalline anatase TiO2. It has been proven that the phase transformation process is suppressed by keeping nanocrystalline anatase particles separated and/or unconnected directly.21,22 Thus, it is possible to enhance the thermal stability of anatase TiO2 by introducing porous materials if the porous structure can fix TiO2 crystallites. In addition, it is regrettable that TiO2 can exhibit high photocatalytic activity only under ultraviolet (UV) other than under visible (Vis) light irradiation. r 2011 American Chemical Society

LaFeO3 with a typical ABO3-type perovskite structure has many applications such as gas-sensitive characters,23,24 catalysis oxidation owing to its exceeding properties as high stability and nontoxicity.25,26 It is also a potential semiconductor materials as efficient visible photocatalyst due to its narrow band gap energy.27
29 Porous LaFeO3 with large surface area has been prepared by our group via a hard-template method,30 leading to the obvious increase in the photocatalytic activity under visible light. However, it is not ideal that the activity of the as-prepared porous LaFeO3 is not greatly enhanced under UV irradiation in place of Vis light. On the basis of the above analysis, we have the idea to fabricate high active TiO2-based nanophotocatalysts by introducing porous LaFeO3 under either UV or Vis illumination.

2. EXPERIMENTAL SECTION 2.1. Materials. TiO2 nanoparticles are prepared by sol-hydrothermal processes.31 A 5 mL portion of Ti(OBu)4 is dissolved in 5 mL of anhydrous C2H5OH (99.7%) to produce Ti(OBu)4C2H5OH solution. Meanwhile, 5 mL of water and 1 mL of HNO3 (67%) are added to another 20 mL of anhydrous C2H5OH in turn to form an ethanol
HNO3
water solution. After the two resulting solutions are stirred for 30 min, respectively, the Ti(OBu)4
C2H5OH solution is slowly added Received: April 17, 2011 Revised: May 8, 2011 Published: June 07, 2011 12375

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The Journal of Physical Chemistry C dropwise to the ethanol
HNO3
water solution under vigorously stirring to carry out a hydrolysis. Thus, a semitransparent sol is gained after continuously stirring for 1 h. After the resulting semitransparent sol is kept at 160 °C for 6 h in a high-pressure stainless steel vessel, subsequently cooled to room temperature, the sol-hydrothermal production is obtained, and the T-80 powder was obtained by drying the production under 80 °C. Nanosized LaFeO3 was synthesized according to the literature.30 First, one gram of freshly as-synthesized SBA-16 was dispersed in 2 mL of ethanol solution, containing 0.0041 mol of La(NO3)3 3 6H2O and 0.0041 mol of Fe(NO3)3 3 9H2O. After ultrasonication for 10 min, the mixture obtained was kept at 60 °C under stirring until the solvent was completely evaporated. Subsequently, the LaFeO3
SBA-16 composite was obtained after the above mixture was dried in a vacuum at 80 °C and then calcined in air at needed temperature for 2 h. Finally, the template-free LaFeO3 nanopowder was gained by washing, drying and grinding after the silica in the composite was dissolved in the 2 M NaOH solution. LaFeO3
TiO2 samples were synthesized by a simple strategy. 0.03 g of as-prepared porous LaFeO3 was added to 5 mL anhydrous C2H5OH, after ultrasonication and dispersed 1 h, the mixture was poured to one pot (about 1 g pure TiO2) of TiO2 sol-hydrothermal production. After ultrasonication for 15 min and then stirring for 4 h, the solvent was evaporated under continuously stirring in 80 °C hot water bath to produce dry powder. At last, the samples are obtained and represented by T-X, in which T means TiO2, X is the calcination temperature, and by L-X, in which L means LaFeO3, X is the calcination temperature, and by LT-Y-X, in which LT means LaFeO3
TiO2 composite, Y is the mass percentage of porous LaFeO3 in the composite, and X is the calcination temperature. In addition, similar process was employed to fabricate the composite of nonporous LaFeO3 prepared by the transitional citrate acid method and nanocrystalline TiO2. 2.2. Characterization. The crystal structure of the samples was determined by X-ray diffraction (XRD) method (Rigaku D/MAXrA powder diffractormeter, Japan). Transmission electron microscopy (TEM) observation of the sample was performed on a JEM3010 electron microscope (JEOL, Japan), with an acceleration voltage of 300 kV. The BET surface area was evaluated by a ST2000 constant volume adsorption apparatus. The UV
vis diffuse reflection spectrum (UV
vis DRS) of the samples was recorded with Shimadzu UV-2550 Spectrophotometer, using BaSO4 as reference. The SPS measurement of the samples was carried out with a home-built apparatus that had been described elsewhere.41 2.3. Photodegradation. Rhodamine B (C28H31ClN2O3) is a common dye that is extensively used in a variety of industrial applications. Therefore, the photocatalytic activity of the assynthesized samples was evaluated by photodegradation of Rhodamine B under irradiation of UV
vis and visible light (λ > 400 nm). The light source was a high-pressure Xe lamp (150 W). Light directly or passed through a cutoff filter focused onto a 100 mL beaker. The reaction was maintained at ambient temperature. In a typical experiment, 40 mL of 20 mg/L Rhodamine B and 0.10 g of the photocatalyst powder were placed together in the beaker. Prior to irradiation, the suspension was magnetically stirred in the dark to ensure the establishment of an adsorption/desorption equilibrium. At a given irradiation time, 10 mL of the suspension was collected and centrifuged to remove the particles, then the dye concentration was determined by measuring the characteristic absorbance of the dye solution.

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Figure 1. XRD patterns of different samples.

3. RESULT AND DISCUSSION For the pure TiO2, T-80 sample (Figure 1) is completely anatase33,34 with poor crystallinity, and its crystallite size is about 5 nm. Its anantase crystallinity increases after the sample is calcined at high temperature (Supporting Information (SI)-1). However, when the thermal treatment temperature rises to 550 °C, the rutile phase (about 12%) appears (SI-2) and the anantase crystallite size grows to about 13 nm, indicating the phase transformation begins to take place. Continuously increasing the thermal treatment temperature, the content of the rutile phase increases evidently and the anatase almost completely transforms to rutile phase up to 850 °C (SI-1). For the LaFeO3
TiO2 composite, the LT-3
600 (Figure 1) is entire anantase with lower crystallinity. As the thermal treatment temperature increases, its anantase crystallinity increases gradually. It is worth noting that the content of the rutile phase increases a little compared with pure TiO2 treated at the same temperature, and the anantase phase still keeps in the dominating percentage, even in the sample calcined at 900 °C. In addition, the characteristic diffraction peaks of LaFeO3 do not appear in the LaFeO3
TiO2 composite, due to its low content. These results demonstrate that compounding a small amount of porous LaFeO3 can effectively inhibit the phase transformation of TiO2 from anatase to rutile, and it is also found that the larger is the amount of LaFeO3 used, the more obvious is the inhibition effect (SI-3). However, the nonporous LaFeO3 prepared by the traditional citrate acid method28 shows little ability to inhibit the phase transformation (SI-4) of TiO2 compared with the porous LaFeO3, implying that the proper porous structure plays important roles in the inhibition phase transformation. The disordered porous structure and the relative small particle size are responsible for the high surface area of the as-prepared LaFeO3 (L-600 and L-800) (SI-2), base on the TEM image (Figure 2A) and the previous results.30 Interestingly, TiO2 in the LT-3-600 still maintains small particle size (about 5 nm) and is uniformly dispersed onto the porous LaFeO3 particle (Figure 2B). It is speculated that the T-80 sample with 5 nm particle size is fixed partly in the pores of LaFeO3 with pore sizes from 2 to 5 nm,30 which effectively holds back the nanocrystalline anatase agglomeration, further inhibiting its phase transformation since the rutile phase starts to occur easily at the interfaces between the anatase particles in the agglomerated TiO2 particles.35 This is responsible for the important roles of the proper porous structure. It is also seen that a small part of TiO2 12376

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Figure 2. TEM and HRTEM photographs. A: L-600 (inset: LaFeO3 (200) lattice plane with d spacing of 0.29 nm). B: LT-3-600 (inset: Anatase (101) lattice plane with d spacing of 0.35 nm and LaFeO3 (200) lattice plane). C: LT-3-800 (inset: Rutile (110) lattice plane with d spacing of 0.32 nm, LaFeO3 (200) lattice plane and Anatase (101) lattice plane).

Figure 3. UV
vis DRS spectra of different samples.

nanoparticles are relatively large in the LT-3-800 composite (Figure 2C), attributed mainly to the small amount of porous LaFeO3 used. However, most of TiO2 nanoparticles are small, leading to the large surface area (SI-2). As expected, compounding a small amount of porous LaFeO3 can markedly inhibit the phase transformation of TiO2 from

anatase to rutile. As a result, the TiO2-based nanoparticles with high anatase crystallinity and large surface area are successfully synthesized. Moreover, the HRTEM images show that there are two-phase (anatase-LaFeO3) and three-phase (anatase, rutile and LaFeO3) heterojunctions in the LT-3-600 and LT-3-800, respectively, which are consistent with the XRD results. The built two-phase and three-phase heterjunctions in the resulting composites will favor the separation of photoinduced charges. UV
vis diffuse reflectance spectra (DRS) are employed to characterize the optical properties of as-prepared samples (Figure 3). There is a significant absorption lower than 400 nm for pure TiO2, attributed to its electron transitions from the valence band to the conduction band (O2p f Ti3d).36,37 And, the absorption edge of pure LaFeO3 occurs at about 600 nm, attributed to its electron transitions from valence band to conduction band (O2p f Fe3d).27 According to the respective optical absorption edge, it is estimated that the band gap energies of the T-550 and L-800 are about 3.1 and 2.0 eV, close to the reports.19,27 For the resulting composite sample LT-3-800, it not only absorbs UV light, but also a certain amount of visible light. The surface photovoltage spectroscopy (SPS) is an effective technique to study the photophysics of excited states generated 12377

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Figure 4. SPS responses of different samples.

by photon absorption, resulting from the changes of surface potential barriers before and after illumination.38
40 Two SPS response peaks appear in the T-550 (Figure 4) positioned at about 350 and 400 nm, closely related to the photoinduced electronic transitions from the valence band to the conduction band of anatase and rutile in the sample, respectively.27,41 Notably, the response edges of SPS and DRS of the TiO2 sample are similar. For the L-800, its SPS peak appears at about 400 nm, attributed to the photoinduced electronic transitions from the valence band to the conduction band.28,30 Different from the TiO2, its SPS response edge (about at 520 nm) is not similar to its DRS one (at about 600 nm). This dissimilarity is attributed to the low bottom level of conduction band of the LaFeO3 based on the principle of SPS method. The valence band potential of a semiconductor at the point of zero charge can be calculated by the following empirical equation EVB = X
Ee þ 0.5Eg,19,42 where EVB is the valence band (VB) top potential via the standard H2 electrode(SHE), and the conduction band (CB) bottom potential can be determined by ECB = EVB
Eg. Thus, the EVB levels for TiO2 and LaFeO3 stand at 2.9 and 2.2 eV, respectively, and then their ECB levels are positioned at
0.3 and 0.2 eV. In general, the O2 affinity level (O2/O2
) stands at about 0 eV (
0.046 eV vs SHE).32 Thus, it is deduced that the photoinduced electrons at the ECB of TiO2 could be effectively captured by the adsorbed O2, leading to the beginning SPS response at the DRS edge, while that of LaFeO3 might not be captured by the adsorbed O2, leading to the beginning SPS response at the blue-shift position compared to its DRS edge. To overcome the energy difference (about 0.2 eV) between the conduction band bottom level (0.2 eV) of LaFeO3 and the affinity level (about 0 eV) of O2, it is evaluated in theory that the electrons induced by the light with less than 550 nm wavelength can be captured by the adsorbed O2. However, it has been found that there is nearly no SPS response at 550 nm for pure LaFeO3, which is possibly because these photoinduced electrons, with a little higher energy than the ECB, rapidly transfer to the ECB before being captured by the adsorbed O2. If many high energy electrons are photoexcited, then they might be captured by the adsorbed O2, contributing to a SPS signal. This is well responsible for its SPS response edge at about 520 nm other than at 550 nm. Compared with the SPS responses of T-550 and L-800, the SPS response of the constructed LaFeO3
TiO2 nanocomposite (LT-3-800) displays two features. One is that the SPS intensity is enhanced markedly, and another is that the SPS response range is remarkably expanded. It is assumed that the two SPS features are

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closely related to the three-phase heterojunctions in the sample. According to the energy band levels, possible photoinduced charge transfer processes are shown in the Figure 5, to explain the SPS responses of the nanocomposite. The SPS response of the LT-3-800 in UV region is much stronger than that of T-550, which is attributed to the transfers of the photogenerated electrons from anatase to rutile and/or LaFeO3. Moreover, the high anatase crystallinity in the nanocomposite is favorable for the transportation and separation of photoinduced charges. Similarly, the SPS response of the LT-3-800 in the visible region is much stronger and wider than that of the L-800, which is attributed to the transfers of the photogenerated electrons from LaFeO3 to anatase and/or rutile. According to the conduction band bottom level of TiO2, the electrons of TiO2 transferring from the LaFeO3 under visible light should be easily captured by the adsorbed O2, which would greatly promote the charge separation of LaFeO3. This is responsible for the markedly enhanced visible SPS response of LaFeO3. It should be noted that, although the LT-3-800 sample contains a small amount of LaFeO3 (3%), it results into a much stronger SPS response than the pure LaFeO3. On the basis of the above results, there are two interesting implications. One is that narrow-gap oxide semiconductors obviously absorbing visible light have rather high recombination rates of visible induced charges. This is mainly because their conduction band bottom levels are so low that it is difficult for the photoinduced electrons to be captured by the adsorbed O2. In general, even if the high-energy photoinduced electrons can be produced, they will quickly transfer to the conduction band bottom. Another is that wide-gap oxide semiconductors with proper conduction band bottom levels, which only absorbs UV light, can greatly improve the separation situation of visible induced charges of narrow-gap oxide semiconductors by constructing heterojunctions. This is mainly because the high-energy electrons produced under visible light can transfer to the widegap oxide semiconductors, favorable to be captured by the adsorbed O2. Therefore, the implications are useful to understand and enhance the separation rates of visible induced charges of narrow-gap oxide semiconductors, further developing efficient photofunctional nanomaterials. The activity of the samples involved was evaluated by photocatalytic degradation of Rhodamine B (RhB) solution under UV
vis or visible light (Figure 6). For the TiO2 samples calcined at different temperature (SI-1), it was demonstrated that the T-550 exhibited high photocatalytic activity under UV light, which is attributed to its mixed phase composition and high anatase crystallinity.22 Expectedly, it is seen from Figure 6 that the T-550 displays a weak visible photocatalytic activity. Also, the L-800 has a relative high visible activity. However, its UV activity is a little higher than its visible one. As discussed previously, even if the much high-energy photoinduced electrons can be produced under UV light, they will quickly transfer to the low conduction band bottom of LaFeO3, which remarkably influence the utilization efficiency of UV light. Interestingly, the LaFeO3
TiO2 composite samples show high photocatalytic activity compared with pure TiO2 (T-550) under UV
vis light, and with pure porous LaFeO3 (L-800) under visible light, especially for the LT-3-800. This is attributed to the high anatase crystallinity and effective heterojunctions in the samples by means of their SPS responses. It is demonstrated above that the three-phase heterojunctions in the LT-3-800 obviously promote the separation of visible or UV-induced 12378

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Figure 5. Scheme for the energy bang structure and the possible transfer processes of photoinduced electrons between heterojunction under visible light (A) and UV light (B) irradiation.

Figure 6. Photocatalytic degradation rates of RhB on different samples.

charges. By comparing different LaFeO3
TiO2 composites, it is suggested that the rutile content in the three-phase heterojunction is an important factor influencing the charge separation situation and then photocatalytic activity. On the basis of the XRD results (SI-3), if the amount of porous LaFeO3 used is too small, a high content of rutile in the LT-1-800 is formed, and if the amount of porous LaFeO3 used is too large, then a low content of rutile in the LT-10-800 is produced. Therefore, it is deduced that the highest activity of LT-3-800 among the composite samples is attributed to its proper content of rutile in the constructed three-phase heterojunctions.

4. CONCLUSIONS In summary, a feasible strategy was developed to fabricate high active TiO2-based photocatalysts for degrading rhodamine B solution under ultraviolet or visible irradiation, by compounding a small amount of porous LaFeO3 to nanocrystalline TiO2 synthesized in advance by sol-hydrothermal processes. The proper porous structure of LaFeO3 plays important roles in the thermal stability enhancement of the nanocrystalline anatase TiO2, leading to the formation of the anatase TiO2/ rutile TiO2/LaFeO3 three-phase nanostructured heterojunctions after thermal treatment at high temperature of 800 °C.

The resulting three-phase heterojunctions greatly favor the photoinduced charge separation based on their surface photovoltage responses, especially promoting the visible induced high-energy electrons of LaFeO3 captured by the adsorbed O2. Moreover, the proper rutile content in the constructed threephase nanostructured heterojunctions is favorable for the photoinduced charge separation. These are well responsible for the high photocatalytic activity of the LT-3
800. It is suggested that the SPS method is an efficient tool to investigate photophysical processes of oxide semiconductors, especially reflecting the reaction ability between the photoinduced electrons and the absorbed O2. This work would provide a simple synthetic route to fabricate high-performance TiO2-based photocatalysts under not only UV light but also visible light. Moreover, it would give us a feasible idea to greatly improve the separation rates of visible induced charges of narrow-gap oxide semiconductors, which is very useful to develop new, efficient, and visible oxidebased photocatalysts.

’ ASSOCIATED CONTENT

bS

Supporting Information. XRD Patterns of TiO2 samples, Mass percentage of anatase in TiO2 and BET surface areas, XRD Patterns of different LaFeO3
TiO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-451-86608610; Fax: þ86-451-86673647; E-mail: Jinglq@ hlju.edu.cn.

’ ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China (No. 21071048), the Science Foundation of Harbin City of China (No. 2011RFXXG001), the Program for Innovative Research Team in Heilongjiang University (Hdtd2010-02), the programme for New Century Excellent Talents in universities (NCET-07-259), and the Science Foundation of Excellent Youth of Heilongjiang Province of China (JC200701), for which we are very grateful. 12379

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