Article pubs.acs.org/JPCC
Is Photooxidation Activity of {001} Facets Truly Lower Than That of {101} Facets for Anatase TiO2 Crystals? Qian Wu,†,‡ Min Liu,† Zhijiao Wu,† Yongliang Li,‡ and Lingyu Piao*,† †
National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, China Analytical and Testing Center, Beijing Normal University, No. 19 Xinjiekouwai St., Haidian District, Beijing 100875, China
‡
S Supporting Information *
ABSTRACT: The effect of particle size and active surfaces on photoreactivity of TiO2 crystals is investigated in this report. The clarification of highly active surfaces is the key to understanding the photoreactivity of anatase TiO2 crystal and also to morphological control of photocatalysts with well-defined crystal facets. The anatase TiO2 single crystals with different percentage of {001} facets in uniform size, as well as anatase TiO2 single crystals with different particles size in same percentage of {001} facets, are synthesized by carefully controlling the synthesis parameters. Their photooxiation reactivity results indicate that the underlying dominant factor for photooxidation activity of anatase TiO2 crystals is particle size. The photooxidation activity of {001} facets is greater than that of {101} facets when the crystals size of anatase TiO2 is identical. This work would be beneficial for better understanding the different photocatalytic performance of different facets of metal oxide crystals in photoreactivity processes.
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INTRODUCTION Semiconductor functional materials with photocatalytic properties have attracted much attention because of their potential in energy and environmental applications.1−6 Among all the factors, surface properties, especially the exposed surfaces with distinct crystal facets, are vital to a semiconductor’s photocatalytic performance. Therefore, controlled fabrication of various semiconducting functional materials with desired shapes to expose specific crystal facets is a common strategy to control their photocatalytic performance.7−11 Investigations on morphology-controlled preparation of TiO2 have facilitated significant advancements in this field.12−21 The order of average surface energies of anatase TiO2 is {110} (1.09 J m−2) > {001} (0.90 J m−2) > {100} and {010} (0.53 J m−2) > {101} (0.44 J m−2),22,23 so the {110} and {001} facets of anatase TiO2 are commonly considered to be more reactive than {101} facets.3,24−28 However, several reports have proposed the contradictive conclusions that {001} facets of anatase TiO2 crystals exhibit lower reactivity than {101} facets in photocatalysis reactions.6,10 This is an interesting result; nevertheless, we believe there should be something uncertain according to their data and analysis. First, the particle size of the anatase TiO2 is very different (about 3.1 × 1.1 μm2, 1.8 × 1.6 μm2, and 0.8 × 1.5 μm2 in length and thickness, respectively) when they compare the photoreactivity between TiO2 particles with different percentage of {001} and {101} facets.6,10 As we know, the reactivity of catalysts is directly related to their sizes, and many previous reports have confirmed that significant effect of particle size on the photoreactivity of anatase TiO2 crystals.29−31 Thus, the impact of the particle sizes on the photocatalytic performance could not be neglected. Furthermore, the electronic band structures of the anatase TiO2 © 2012 American Chemical Society
crystals determined by means of ultraviolet−visible (UV−vis) spectra and X-ray photoelectron spectra (XPS)10 indicated the different band gap of the anatase TiO2 crystals used in their work. They believed the difference, which arose from the different reactive facets, directly caused the lower photoreactivity of {001} facets than that of {101} facets. However, the influence of the particle size on band gap of semiconductor is again neglected in their work. In addition, we could also identify the preferential binding of Pt nanoparticles on (101) surface in Gordon’s work,6 which could be another factor affecting the photoreactivity of {101} facets of anatase TiO2 crystal. The clarification of highly active surfaces is the key to understanding the photoreactivity of anatase TiO2 crystal and also to morphological control of photocatalysts with welldefined crystal facets. In this report, we synthesize the anatase TiO2 single crystals with different percentages of {001} facets in uniform size, as well as anatase TiO2 single crystals with different particle sizes in same percentage of {001} facets by carefully controlling the synthesis parameters. Their photooxidation reactivity is investigated, while photoreduction reactivity is not. The previous reports have already confirmed that the photoreduction reactivity of anatase TiO2 crystals predominantly occurs on {101} facets of anatase TiO2 crystals.32,33 Received: September 3, 2012 Revised: November 20, 2012 Published: December 13, 2012 26800
dx.doi.org/10.1021/jp3087495 | J. Phys. Chem. C 2012, 116, 26800−26804
The Journal of Physical Chemistry C
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EXPERIMENTAL SECTION
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RESULTS Figure 1 shows the SEM images of anatase TiO2 crystals. The low-magnification SEM images can be seen in Figure S1. The
Sample Synthesis. For the synthesis of sample A, 0.01 g of Ti powder was dissolved in aqueous solutions of HF (0.11 mL, 40 wt %) and 27 mL of H2O (18.0 MΩ cm). Then, 3 mL of H2O2 (30%) was transferred to the above solution. The mixture was transferred to a Teflon-lined autoclave, heated to 180 °C, and kept at 180 °C for 10 h. After the reaction, the products were collected by centrifugation and washed with deionized water several times until the pH of the solution was neutral. The samples were then dried at 80 °C for 10 h and calcined at 500 °C for 2 h in air, respectively. The surface fluorine was removed from as-prepared anatase TiO2 during the calcinations. For the synthesis of samples B and C, 0.2 g of P25 (Degussa, 99.5%) powder was dissolved in aqueous solutions of HF (2 mL, 40 wt %) and 80 mL of H2O (18.0 MΩ cm). The suspension was transferred to a Teflon-lined autoclave, heated to 180 °C, and kept at 180 °C for 10 h to prepare the precursor. Then the precursor was cooled to room temperature naturally. In the typical synthetic routes for samples B and C, 5 and 3 mL of precursor was placed into the autoclave with 17 mL H2O + 3 mL H2O2, and 22 mL H2O + 3 mL H2O2, respectively. Then the autoclave was kept at 180 °C for 10 h. And then the products were collected by centrifugation and washed with deionized water several times until the pH of the solution was neutral. The samples were then dried at 80 °C for 10 h and calcined at 500 °C for 2 h in air, respectively. For the synthesis of samples D, E, and F, the preparation process of the precursor is the same as that of samples B and C. In the typical synthetic routes for samples D, E, and F, 4, 5, and 10 mL of precursor was placed into autoclave with 25 mL H2O + 1.5 mL H2O2, 30 mL H2O + 1.5 mL H2O2, and 22 mL H2O + 3 mL H2O2, respectively. Then, the autoclave was kept at 180 °C for 10 h. Then, the products were collected by centrifugation and washed with deionized water several times until the pH of the solution was neutral. The samples were then dried at 80 °C for 10 h and calcined at 500 °C for 2 h in air, respectively. Characterizations. X-ray diffraction (XRD) patterns of the samples were recorded on a D/Max-TTR III (CBO) diffractometer with Cu Kα radiation. The morphology of the samples was determined by scanning electron microscopy (SEM) on a Hitachi S-4800 system. The BET specific surface area was determined by nitrogen adsorption at 77 K (ASAP 2020). The optical absorbance spectra of the samples were recorded on a UV−vis spectrophotometer (Hitachi U-3900). The valence-band spectra of anatase TiO2 crystals were analyzed by XPS spectrometer (Thermo Scientific Escalab 250, monochromatic Al Kα X-ray source). Raman spectra were collected with Renishaw Invia plus spectrometer operating at 514 nm. Photoreactivity Measurements. The photoreactivity of the TiO2 sample was evaluated by the degradation of methylene blue (MB) in a cylindrical quartz flask under UV light irradiation (365 nm) from a 300 W Xe lamp. In the typical reaction, 10 mg of anatase TiO2 crystal was suspended in 100 mL of aqueous solution containing 10 mg/L MB. Also, the suspension was stirred for 60 min in the dark. Then, the suspension was exposed to the UV light, and the sample was then taken out for analysis every 10 min. The UV−vis absorption spectra of the liquid sample were recorded after centrifugation at 8000 rpm.
Figure 1. Morphologies of anatase TiO2 crystals: (1-1) A, B, and C, schematic and SEM images of anatase TiO2 with different particles size in same percentage of {001} facets; (1-2) D, E, and F, schematic and SEM images of anatase TiO2 with different percentages of {001} facets in uniform size.
percentage of {001}, {101} facets and average particle size of samples A−F are given in Table 1 (the data are calculated Table 1. Average Particle Size, Percentages of {001} Facet, and Intensity Ration of {101} to {001} Facets in Samples A− F samples
A
intensity ratio of {101} to 6.5 {001} facets from XRD patterns percentage of {001} facet 20% particle size (μm2) 1.3 × 1.3
B
C
D
E
F
6.7
6.5
5.8
4.1
2.8
20% 2.5 × 2.5
20% 5.0 × 5.0
22% 3.0 × 2.5
34% 3.0 × 1.8
55% 3.0 × 1.0
according to the SEM images and XRD pattern). The statistic data of the size distribution of the samples A−F can be seen Table S1. First, the average particle thicknesses (the L1 in Figure 1-1, along [001] direction) and the widths (the L2 in Figure 1-1, along [100] directions) of samples A, B, and C are about 1.3 × 1.3 μm2, 2.5 × 2.5 μm2, and 5.0 × 5.0 μm2, respectively. The percentage of exposed {001} facets of samples A, B, and C are about 20%. The average particle widths and thicknesses of samples D, E, and F are about 3 × 2.5 μm2, 3 × 1.8 μm2, and 3 × 1.0 μm2, respectively. Therefore, the percentage of exposed {001} facets are 22%, 34%, and 55%, respectively (see Table 1). According to the symmetries of anatase TiO2 crystals, the two flat and square surfaces should be {001} facets, and the eight hexagonal surfaces should be {101} facets, respectively. An angle of 68.3° ± 0.3° (see Figure 1-2, E) which is consistent with the interfacial angle between (001) and 26801
dx.doi.org/10.1021/jp3087495 | J. Phys. Chem. C 2012, 116, 26800−26804
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exists in anatase TiO2 crystal.37 No blue shift of the Eg band confirmed that there is no impurity in the anatase TiO2 samples. The EDS results and Raman spectrum results confirmed the high purity of the anatase TiO2 samples. The photooxidation activity of the TiO2 samples was estimated by evaluating the degradation of MB under UV light irradiation. This typical photooxidation reaction has been commonly used in validating photoreactivity of photocatalysts.15,38−42 The photooxidation results are shown in Figure 3. The photooxidation activity order is A > B > C, and F > E > D, respectively. In other words, the smaller the size, the higher the photooxidation reactivity is, for the TiO2 crystals with same percentage of {001} facets. And the higher the percentage of {001} facets, the higher the photooxidation reactivity is, for the TiO2 crystals with same size. It means that the photooxidation activity of anatase TiO2 crystals decreases with the increasing of percentage of {101} facets.
(101) surfaces, is observed on the particles, suggesting that the particles exhibit flat facets of {001} and {101}.7,19 The energy disperse spectroscopy (EDS) was used to characterize the element composition of the anatase TiO2 crystals (see Figure S2). The result of EDS indicates that the anatase TiO2 samples in this work are only composed of Ti and O. The Si and Au elements come from Si wafer and sputtering film (for SEM samples preparation), respectively. The crystallographic structure of the anatase TiO2 crystals has been confirmed by XRD, and their XRD patterns are shown in Figure 2. The diffraction patterns of all TiO2 samples clearly
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DISCUSSION Electronic band structures of anatase TiO2 crystals with different crystal sizes and percentages of reactive facets were investigated (see Figure 4a,b). UV−vis absorption spectra (Figure 4a) demonstrate that the absorption edge has a obvious red shift when crystal size gradually decreases for samples A−C. At the same time, the absorption edge has almost no shift for samples D−F, which have different percentage of {001} facets in uniform size. The above results indicate that the band gap of anatase TiO2 crystals is nearly independent with the percentage of different reactive facets, while the band gap will be narrower with smaller particles size. The following XPS results ( 4b) demonstrate that the anatase TiO2 crystals with different percentage of {001} facets in uniform size (samples D−F) have nearly identical widths of the valence bands at about 6.42 eV. However, the widths of the valence bands for samples A−C, bearing different particle sizes with the same percentage of {001} facets, are 6.88, 6.87, and 6.31 eV, respectively. So, the valence-band maxima are different for samples A−C. By combining the UV−vis and XPS results we can draw a conclusion that the electronic band structures of anatase TiO2 crystals are nearly independent with their reactive facets. In contrast, the crystals size is a very important factor for their electronic band structures. It has been stated in Liu’s paper that the photoreactivity of anatase crystal is irrespective of both its surface atomic structure (the density of undercoordinated Ti atoms) and surface electronic band structure (the power of photoexcited charge
Figure 2. XRD patterns of TiO2 crystals. A, B, C: Anatase TiO2 with different particles size in same percentage of {001} facets. D, E, F: anatase TiO2 with different percentages of {001} facets in uniform size.
indicate the pure anatase phase (tetragonal, I41/amd, JCPDS 21-1272). The very sharp diffraction peaks of the samples in Figure 2 indicate the perfect crystallographic structure of these samples. The intensity ratio of {101} to {001} facets gradually decreases from the right panel of Figure 2 (also can be seen in Table 1), which indicates the gradually increased percentage of {001} facet from samples D to F. Meanwhile, anatase structure of TiO2 crystals was also identified by Raman spectrum as shown in Figure S3, and the detailed Raman shift data of anatase TiO2 crystals can be seen in Table S2. All Raman bands can be assigned to anatase TiO2 crystals.34−36 The Eg (144 cm−1) band will shift up to higher frequency if any impurity
Figure 3. Photooxidation of anatase TiO2 samples from A to F (MB, C0 = 10 mg/L) under UV-light radiation. A, B, C: anatase TiO2 with different particles size in same percentage of {001} facets. D, E, F: anatase TiO2 with different percentages of {001} facets in uniform size. 26802
dx.doi.org/10.1021/jp3087495 | J. Phys. Chem. C 2012, 116, 26800−26804
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Figure 4. Surface electronic and atomic structure: (a) UV−vis absorption spectra of sample A−F, (b) valence-band XP spectra of samples A−F, (c) schematic of atomic structure of {001}, {101}, and {010} facets.
carriers).10 First, the commonly used criteria to predict the photoreactivity of crystal facets is the density of surface undercoordinated atoms.43,44 The different facets have different density of undercoordinated Ti atoms. Figure 4c shows the atomic structural models of {001} and {101} facets of anatase TiO2. The surface energy of {001} facets is higher than that of {101} facets due to the 100% surface unsaturated Ti5C atoms on {001} facets while only 50% for the {101} facets. So, the {001} facets are theoretically considered more reactive than {101} and {010} facets,43,44 which is in agreement with our experimental results. So, the anatase TiO2 crystals with small particle size have both a favorable surface atomic structure and a surface electronic structure. The higher photoreactivity of {101} facets than that of {001} facets in Liu’s and Gordon’s work6,10 could be associated with the much smaller particle size of the sample with high percentage of {101} facets. Second, there is no direct evidence that confirms that the power of photoexcited charge carriers is related to the specific facets, because the UV−vis or XPS data are collected from the bulk powder of anatase TiO2 crystals while not from the specific facets. The information about electronic band structure of anatase TiO2 crystals consists of all different crystal facets. At the same time, the electronic band structures are highly dependent on particle size of anatase crystals. Considering the above discussion, the photooxidation reactivity of anatase TiO2 crystal depends not only on the crystal facets, but also mainly on the crystal size. The same size is a prerequisite for comparison of photocatalytic reactivity of different facets of anatase TiO2 crystals. Therefore, we can draw the conclusion that the photooxidation activity of {001} facets is greater than that of {101} facets when the crystal size of anatase TiO2 was similar. The conclusion is in accordance with the previous theoretical calculations and experimental results in this regard.17,18,31,45−48
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CONCLUSION
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ASSOCIATED CONTENT
In conclusion, we have synthesized a set of anatase TiO2 single crystals with different percentages of {001} facets in uniform size, as well as a set of anatase TiO2 single crystals with different sizes in same percentage of {001} facets. The properties and structures of anatase TiO2 crystals were characterized by SEM, XRD, Raman, XPS, and UV−vis absorption spectra. The photooxidation reactivity of the anatase TiO2 crystals was estimated by degradation of MB dye under UV light irradiation. The smaller the size, the higher the photooxidation reactivity is, for the anatase TiO2 crystals with same percentage of {001} facets. The higher the percentage of {001} facets, the higher the photooxidation reactivity is, for the TiO2 crystals with same size. The same size is the prerequisite for comparison of photoreactivity of different facets of anatase TiO2 crystal. This work would be beneficial for better understanding of the different photocatalytic performance of different facets of anatase TiO2 crystals. Although this work focuses on the properties of TiO2, we expect that this effort may provide important criteria for a more effective evaluation on the role of metal oxide crystal facets in photocatalytic processes.
* Supporting Information S
Experimental procedures and detailed characterization, including SEM, XRD, Raman, XPS, and UV−vis absorption spectra, and photocatalysis reaction. 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. 26803
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (2011CB932802). The authors would also like to acknowledge Prof. H. T. Gao for useful discussions.
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