Letter pubs.acs.org/JPCL
Enhanced Photocatalysis by Au Nanoparticle Loading on TiO2 SingleCrystal (001) and (110) Facets Ming-Yang Xing,† Bing-Xing Yang,‡ Huan Yu,† Bao-Zhu Tian,† Segomotso Bagwasi,† Jin-Long Zhang,*,† and Xue-Qing Gong‡ †
Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China ‡ State Key Laboratory of Chemical Engineering, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *
ABSTRACT: Although the preparation of TiO2 exposed with high-energy facets is a challenge, these facets have a large application potential in the loading of Au nanoparticles. We successfully prepared TiO2 single crystals exposed with (001) and (110) facets as an ideal support to load the highly dispersed nanosized Au particles to improve the stability of Au on the catalyst surface and expand its photocatalytic applications. The transfer of photoexcited electrons from the higher-surface-energy facet, that is, (001) and (110), to the lower-energy facet of (101) could make the electrons aggregate on the (101) facet and leave holes on the (001) and (110) facets, which would promote the separation of electrons and holes. The highly dispersed Au nanoparticles also could capture the electrons from active facets and further improve the photocatalytic activity of TiO2. Systematic density functional theory calculation was also carried out to investigate the formation of (001) and (110) facets. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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inhibited to minimize the overall surface energy of the crystal.12−14 For example, TiO2 single crystals with exposed high-energy (001) facets have been demonstrated to be quite reactive, and their syntheses have attracted considerable attention.15−18 Moreover, the average surface energy of the (110) facet was reported to be even much higher than that of the (001) facet (1.09 versus 0.90 J·m−2),19 indicating that the (110) facet could have a larger application potential in the loading of Au nanoparticles. However, its high surface energy also makes it more difficult to obtain in TiO2 synthesis. Recently, Liu et al.20 have successfully synthesized the anatase crystals with exposed (001) and (110) facets by using a modified hydrothermal technique in the presence of hydrogen peroxide and hydrofluoric acid solution. However, its formation mechanism is still largely unknown, and the activity of the (110) facet for photocatalysis purposes also remains unclear. Although much experimental research has been done on (001), (101), and (100) facets of TiO2, there are fewer reports on the selection of Au nanoparticle loading on different surfaces like (101), (001), and (110) facets, especially for mechanistic study. For instance, Liu et al.20 and Wu et al.21 reported an anatase TiO2 single crystal with exposed (001) and (110) facets, which has been successfully synthesized in the presence
reparation of Au and other noble metal particles loaded on catalysts has been widely studied since Haruta et al.1 reported that highly dispersed gold particles supported on metal oxides are extremely active for many reactions.2−9 Particularly, it has been suggested that Au nanoparticles loaded on a TiO2 surface could greatly increase the migration of photoelectrons, so as to promote the separation of electrons and holes.10 In addition, high dispersion of Au nanoparticles on a TiO2 support also plays an important role in enhancing its photocatalytic activity. Therefore, TiO2-supported highly dispersed Au nanoparticles have a large potential in photocatalysis applications. However, the detachment of Au nanoparticles from the catalyst surface during the washing and reaction process has been a big problem to constrain their application in photocatalysis and environmental engineering due to the low surface energy of the (101) facet in anatase TiO2. As we all know, the stability and reactivity of TiO2 single crystals are largely determined by their surface chemistry, which is also a key factor in the loading of Au particles on its surface. High-energy surface atoms exhibit high activity and are easy to combine with the foreign atoms such as the Au atoms to form a stable structure.11 Hence, in order to solve the above problem, the first step is to prepare the TiO2 single crystals with exposed high-energy facets. The preparation of crystals with exposed high-energy facets has been a well-known challenge because they usually have large formation energies and their occurrence is naturally © 2013 American Chemical Society
Received: October 1, 2013 Accepted: November 4, 2013 Published: November 4, 2013 3910
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agent in the preparation of the (001) facet, and its amount plays an important role in controlling the morphology of TiO2.26 As one can see, with 0.15 mL of HF, the prepared TiO2 grains appear to aggregate and do not show any regular morphology (Figure 2A). However, by increasing the amount of HF, TiO2 crystals can be obtained showing better morphology with an average size of about 3 μm and a thickness of 0.5 μm (Figure 2B). According to the SEM image, the major exposed facets for such 0.20-TiO2 are (001) and (101) facetsl the top and bottom two flat and square surfaces are the characteristic (001) facets,27 and the eight isosceles trapezoidal surfaces are ascribed to (101) facets. Figure 3A presents the TEM image of 0.20-TiO2, which clearly shows its single-crystal structure and the high percentage
of hydrogen peroxide and hydrofluoric acid solution; Murakami et al.22 reported a new synthesis method to obtain TiO2 exposed with (101) and (001) facets by using PTA with PVA as a shape-control reagent; and Tachikawa et al.23 reported an investigation of the electron transfer between (101) and (001) facets on a TiO2 surface. Zhu et al.24 developed a new nonhydrolytic approach to synthesize a TiO2 nanocrystal with dominant (001) facets. As an ideal support surface, the (001) facet has a high exposed percentage, high exposed area, and smooth surface, which can be loaded with noble metal. Yu et al.25 reported a Pt/TiO2 nanosheet with exposed (001) facets with enhanced photocatalytic activity. However, their obtained (001) faces are not large enough to load with highly dispersive Pt nanoparticles, and they did not investigate the selection of Pt particles on different facets. Compared with all abovementioned reports, herein, we successfully prepared TiO2 single crystals exposed with (101), (001), and (110) facets in the absence of H2O2 and only used HF as the facet-control reagent. The stability of Au nanoparticle loading on different faces and the selection of Au nanoparticles on (101) and (110) facets are investigated in this work. Additionally, systematic DFT calculation was carried out to investigate the formation of different active facets and the influence of HF dosage on the generation of (110) facets. From the X-ray diffraction (XRD) patterns (Figure 1), all of the peaks of the TiO2 single crystals prepared with different
Figure 1. XRD patterns of anatase TiO2 using different amounts of HF. The TiO2 single crystals without Au are denoted as m-TiO2, where m describes the volume amount of HF.
amounts of HF can be indexed to the anatase TiO2 phase with the lattice constant a = 3.7852 (JCPDS No. 21-1272). Figure 2 shows the SEM images of TiO2 single crystals with different amounts of HF. HF was usually used as the facet-controlling
Figure 3. TEM and FE-SEM images of anatase TiO2 single crystals. (A) TEM images of a representative anatase TiO2 single crystal for 0.20-TiO2. (B) HRTEM image recorded from the 0.20-TiO2. The inset of (B) is the corresponding FFT pattern. (C) FE-SEM of 0.25TiO2. (D) Schematic diagram of the 0.25-TiO2 single crystal. (E) Schematic diagram of the anatase single crystal and the formula of calculating the percentage of exposed (001). The TiO2 single crystals without Au are denoted as m-TiO2, where m describes the volume amount of HF.
Figure 2. SEM images of anatase TiO2 single crystals on exposed (001) and (110) facets. (A) 0.15-TiO2; (B) 0.20-TiO2.
of exposed (001) facets. An angle of 68.3° is also consistent with the interfacial angle between (001) and (101) facets, further confirming the exhibition of flat (001) and (101) facets on the 0.20-TiO2 single crystal.28 The high-resolution TEM image of 0.20-TiO2 shows the (101) and (004) atomic planes with lattice spacing of 0.352 and 0.238 nm, respectively (Figure 3B). It needs to be mentioned that the angle between these two planes is also 68.3°. Moreover, the same angle of 68.3° was detected in its corresponding fast-Fourier transform (FFT) image (Figure 3B) as well. Interestingly, when the amount of HF increases to 0.25 mL, the TiO2 samples can even expose the 3911
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adsorption, it is determined that two molecules can be adsorbed in each surface cell of (110)-1 × 1, while three molecules can be adsorbed at (110)-2 × 1 (see Figure 4). In Table 1, the calculated adsorption energies for each HCl/HF at
high-energy (110) facets. Figure 3C shows the FE-SEM image of 0.25-TiO2 single crystals, which clearly exhibit such rhombus (110) facets. The schematic illustration of such an anatase single crystal is presented in Figure 3D. In addition, when the HF content is 0.20 mL, the percentage of the exposed (001) facet is approximately 67%, calculated by the formula in Figure 3E. With the increase of the amount of HF to 0.25 mL, the exposed (001) facet can be up to 73%; meanwhile, the highenergy (110) facet is also exposed, indicating that the HF concentration plays an important role in the growth inhibition of (001) and (110) facets. In this work, by using a simple one-step hydrothermal approach, TiO2 nanocrystals simultaneously exposing highenergy (001) and rarely observed (110) facets can be prepared. As we all know, in a clear system, the balance between the O− O repulsions and the attractive Ti−O π interactions is broken owing to the cleavage of surfaces, causing unsaturated O and Ti atoms to move outward.27 During the growth of TiO2 crystals, the facets such as (001) and (110) are absent due to their high exposed surface energies. However, in a system full of Cl and F ions, these foreign ions, especially F ions, are adsorbed on some high facet surface to form Ti−F bonds, which will make the surface O and Ti atoms move inward and outward due to the strong repulsive and attractive interactions of O−F and Ti−F bonds, respectively. On these facet surface, the Ti3d and O2p electrons can both interact strongly with the F2p electron, and then, a new balance can thereby be established between O−O/ F−O repulsions and Ti−O/Ti−F attractions, which stabilizes Ti and O atoms on the surfaces and results into the exposure of (001) and (110) facets,27 as shown in Figure 3. In order to understand these results, systematic DFT calculations for the energetics of different surfaces have been performed. Anatase TiO2 surfaces were modeled by periodic slabs, and c(1 × 2), c(2 × 2), and c(1 × 1) surface cells were used for (101), (001), and (110) surfaces, respectively. Accordingly, at (101) and (001) surfaces, at most four HCl or HF molecules can be adsorbed in each surface cell. Moreover, considering the highly acidic condition in the experimental work, the adsorption of HCl/HF in dissociative configuration was calculated. As we have shown in Figure 4, the dissociative adsorption of HCl/HF was calculated with all of the surface O2c binding with H and Ti5c with Cl/F. For dissociative HF/HCl
Table 1. Calculated Average Adsorption Energies and Structural Parameters of HCl and HF at Different Anatase TiO2 Surfaces HCl HF
Ea (eV) Cl−Ti (Å) Ea (eV) F−Ti (Å)
(101)
(001)
(110)-2 × 1
(110)-1 × 1
0.82 2.29 1.26 1.82
0.79 2.41 1.33 1.89
1.14
1.50 2.273 1.90 1.820
1.73
the different surfaces as well as the calculated distances between adsorbed Cl/F and surface Ti5c are listed. For the anatase TiO2(110) surface, two different termination structures were considered in this work. The bulk-truncated (110) shown in Figure 5A exhibits a compact dentate conformation with all of
Figure 5. Calculated structures (side view) of anatase (A) TiO2 (110)1 × 1 and (B) 2 × 1 surfaces. The Ti atoms are in gray, and O atoms are in red.
the O2c−Ti4c−O2c units closely sitting beside each other. As has been reported in an early work, the surface energy was estimated to be as high as 1.09 J·m−2, in line with the fact that it does not appear in natural anatase TiO2 crystals.29 Interestingly, it is also found that by removing half of the structural units from each surface cell, the (2 × 1) reconstructed (110) surface (see Figure 5B) can be actually obtained. Such termination with much less compact conformation can undergo drastic relaxation during optimization and give rise to a lower surface energy of 1.01 J·m−2. Therefore, it is confirmed that the actual (110) surface in real application would most likely take the structure in between these two extremes. As we can see from Table 1, the adsorption of HCl at the corresponding anatase TiO2 surface is generally much weaker than that of HF. Specifically, the average adsorption energies for each HCl at the two possible TiO2(110) surfaces are 1.14 and 1.50 eV, respectively, significantly higher than those of HCl at (101) and (001) surfaces, which is clearly due to the very low coordination number of exposed Ti at (110). It suggests that, compared to the (101) and (001) surfaces, the (110) covered by HCl is more difficult to be replenished by HF. One may also notice that the adsorption of HF at (110) gives much higher adsorption energies compared to that at (101) and (001) as well. For example, the coadsorption of various HF at the (110)2 × 1 has an overall adsorption energy as high as 1.59 J·m−2 (see Table 2). It needs to be mentioned that because the HF and HCl species were not consumed for the growth of TiO2 crystals and their amount should be high enough to cover all of the exposed surface sites, we only considered the full coverage adsorption in this work.30 It also needs to be mentioned that
Figure 4. Calculated structures of HF dissociative adsorption at anatase TiO2 (A) (110), (B) (001), (C) (110)-1 × 1, and (D) (110)-2 × 1 surfaces. The F atoms are in light blue, and H atoms are in white. 3912
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four corners in the middle is 0.67, which is significantly smaller than the length of equivalent [110] vectors if the stabilization effect of HCl adsorption at corresponding facets is considered (γ′(001) = 0.43; Table 2). Interestingly, once the (110)-2 × 1 surface fully covered by HF is taken into consideration, a large portion of each corner would be expected to be missing for the creation of micro (110) facets because its modified surface energy of 0.43 J·m−2 is much smaller than 0.67 J·m−2. However, in the real case, the (110) surface may take some intermediate structure between (110)-2 × 1 and (110)-1 × 1, and it may not be completely replenished by HF either. Therefore, a small (110) facet would be most likely to occur (see Figure 3). For the (001) and (110) facets, their high surface energies induce high surface activities, which will make a strong combination with the foreign atoms such as the Au atoms, to improve the stability of loading Au nanoparticles on TiO2 surfaces. Hence, the prepared 0.25-TiO2 single crystal exposed with (001) and (110) facets was used as the support to load the nanoscaled Au particles, through an impregnation−calcination method. According to the literature data,31 the mechanism of adsorption of gold hydroxy chlorides is not an electrostatic interaction but a surface reaction with OH to form an innersphere complex, as shown in eq 1.31−33
Table 2. Calculated Formation Energies (γ), the Total Adsorption Energies (Ea; Ea′ obtained with the Ea of (101) as the reference), and the Modified Formation Energies (γ′, γ′ = γ − Ea′) of the Anatase TiO2(101), (001), and (110) Surfaces (in J·m−2) γ Ea Ea′ γ′
(101)
(001)
(110)-2 × 1
(110)-1 × 1
0.44 1.01 (HF) 0 0.44
0.90 1.48 (HF) 0.47 0.43
1.01 1.59 (HF) 0.58 0.43
1.09 1.17 (HF) 0.16 0.93
the adsorption energies reported here were estimated with the gas-phase HCl or HF molecule as the reference. Though they may not precisely reflect the energies in the solution, these calculated adsorption energies still clearly indicate that the (110) originally covered by HCl, which represents the surface condition of precursors built by titanium chloride, would be efficiently replenished and stabilized by HF only when its concentration is high enough. It seems to be consistent with our experimental findings that for the anatase TiO2 nanocrystals prepared with TiCl3 as the titanium source, the capping agent of HF can favor the exposure of the (110) facet after its amount is increased to 0.25 mL, while the typical anatase TiO2 with exposed (101) and (001) facets can be obtained when the amount of HF is lower than 0.20 mL. In other words, it can be predicted from the theory that the highly activated facetcontrolled TiO2 could be achieved by changing the dosage of HF. To further verify the above explanation, the Wulff construction analysis was performed to predict the equilibrium shape of anatase TiO2. As one can see from Table 2 and Figure 6, once the stabilization effect of adsorption of HF at (101) and
TiOH + AuCl3(OH) ↔ TiOAuCl 2 + H 2O + Cl
(1)
From the eq 1, we can deduce that the amount of OH on the TiO2 surface plays an important role in the loading of Au. In our preparation, the amount of neutral OH species on TiO2 is low at low pH, and we used NaOH solution to adjust the pH value to 8.0. The Au was bound to oxygen as gold hydroxide, which would decompose at high calcination temperature and form metallic gold. Thereby, it can be confirmed that gold species should exist in the form of metallic gold on the TiO2 surface after the catalysts are calcined at 300 °C for 4 h, as shown in Figure 7A. Au nanoparticles can be seen as dark contrasts, which are highly dispersed on the various facets of
Figure 6. Schematic illustration of the formation mechanism for a TiO2 single crystal exposed with (001) and (110) facets.
(001) is considered only, the anatase TiO2 takes the wellknown trapezoid-like shape. We use the modified formation energies (γ′) for the facet to represent the distance from this facet to the center of the crystal. For instance, the value of γ′(101) is 0.44 J·m−2, as shown in Table 2, which could indicate that the distance from the (101) facet to the center of the crystal is also 0.44, as shown in Figure 6. Moreover, combined with the interfacial angle between (001) and (101) facets of 68.3° (Figure 3A,B and Figure 6), it can be easily determined that the distance between the center of the crystal to any of the
Figure 7. (A) Low- and (B) high-resolution bright-field TEM images of the anatase TiO2 prepared using 0.25 mL of HF and loading nanoscaled Au particles (Au/TiO2). The inset of (A) is a [001] projected geometrical model of the anatase single crystal loaded with Au particles. (C) Amplification of the TEM image of (001) and (101) facets on Au/TiO2. (D) Amplification of the TEM image of the (110) facet on Au/TiO2. 3913
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TiO2 single crystals (Figure 7A), that is, (001), (110), and (101) facets are all dispersely loaded with nanoscaled Au particles. Figure 7B is the high-resolution TEM image of Au/ TiO2, which indicates that the average particle size of Au is about 8 nm. The (111) atomic plane with lattice spacing of 0.235 nm is identical to the theoretical value for the Au crystal (JCPDS No. 65-2870). As we all know, the detachment of Au nanoparticles from the catalyst surface during the washing and photocatalysis process has been a big problem for constraining its application in photocatalysis and environmental engineering due to the low surface energy of the (101) facet in anatase TiO2. The low surface energy of the (101) facet determines whether it is difficult to load enough Au nanoparticles on the (101) surface. On the other hand, the stability and reactivity of TiO2 single crystals are largely determined by their surface chemistry, which is also a key factor in the loading of Au particles on its surface. High-energy surface atoms such as (001) and (110) exhibit high activity and are easy to combine with the foreign atoms such as the Au atoms to form a stable structure and significantly increase the number of loading Au nanoparticles on the TiO2 surface. For the catalyst of 0.25-TiO2, compared with the small area of the (110) facet, the (001) facet exhibits a large surface area and can easily be loaded with a large number of Au nanoparticles, as shown in Figure 7A. However, in the per unit area, the density of Au particles on the (110) facet is obviously larger than that on (001) and (101) facets, as shown in Figure 7C,D. A seen from Figure S1(A) and (B) (Supporting Information), the nanosized Au particles are loaded on (101) and (001) facets, and the loading concentration of Au is approximately 0.2 atom %. The amplified spectrum in Figure S1(B) (Supporting Information) indicates that the size of Au is 8.0 nm. 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. When the HF dosage was increased to 0.25 mL, the sharp corner was cut away, and four small vertical rhombus (110) facets were observed in the corners of the TiO2 single crystals. The exposure of (110) attracted much more highly dispersed Au nanoparticles loaded on its surface, and the Au loading concentration could be up to 0.4%, as shown in Figure S1(C) and (D) (Supporting Information). The superficial area of exposed vertical rhombus (110) facets is lower than the area of the original sharp corner, which is the interface of four (101) facets. However, the concentration of Au loaded on the (110) facets is much higher than that loaded on the original (101) facets, that is, the density of Au particles on the (110) facet is obviously higher than that on (001) and (101) facets. Compared with the (001) and (101) facets, the high-energy surface atoms on the (110) facets exhibit high activity and are easy to combine with the foreign atoms such as the Au atoms to form a stable structure. Hence, we can conclude that the TiO2 single crystal exposed with high-surface-energy facets of (001) and (110) is beneficial to the loading stability of Au on a catalyst surface, which could be an ideal support for the loading of noble metals to be used in the catalytic field. To investigate the compositions and the binding states of derived anatase TiO2 single crystals, high-resolution XPS was used to detect the surface F1s, Ti2p, and Au4f core levels (Figure 8). The loading of Au can also induce the TiO2 surface lattice distortion, which results in the generation of surface oxygen vacancy. For example, the XPS peak of Ti2p3/2 shifts from 458.8 to 458.4 eV after Au nanoparticle deposition, owing
Figure 8. High-resolution XPS spectra of (A) Ti2p, (B) O1s, (C)F1s, and (D) Au 4f for (a) 0.25-TiO2 and (b) 0.25-TiO2 with Au loading (Au/TiO2). (E) Wide-scan survey spectra for the samples before and after Au loading. The inset of (E) is the corresponding amplification image of (E).
to the generation of a Ti3+ neighbor to the oxygen vacancy (Figure 8A).34 After Au loading, O1s XPS spectra of Au/TiO2 showed a strong peak at 532.1 eV, which results from the generation of surface oxygen vacancies (Figure 8B).35 Moreover, before Au loading, there is only one peak at 684.5 eV in the F1s XPS spectrum of 0.25-TiO2, which is ascribed to the surface Ti−F structures (Figure 8C).36 After loading Au on the TiO2 single-crystal surfaces, a new peak at 690.0 eV was determined, which can be explained by the fact that the strong electron-withdrawing ability of surface oxygen vacancies leads to the decrease of electron density of the adjacent Ti−F. The Au4f XPS spectra of Au/TiO2 show two peaks at 86.2 and 82.6 eV, which indicates the existence of Au in the form of a simple substance without the occurrence of gold oxides (Figure 8D).37 A seen from Figure 8E, the wide-scan survey spectra for the samples before and after Au loading, there is a significant peak in the range of 50−100 eV ascribed to the Au4f after the Au nanoparticle loading, and the relative atomic amount of Au on the TiO2 surface is 0.61%. Interestingly, after the Au loading, the surface relative atomic amount of F has a distinct decrease from 2.94 to 0.68%, which suggests the existence of interaction between the loaded Au nanoparticles and the TiO2 surface atoms. The atoms on high-surface-energy facets such as (001) and (110) facets exhibit high activity and are easy to combine with the Au atoms to form a stable structure. This strong 3914
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Figure 9. (A) Photodegradation of RhB (20 mg/L) over different samples under UV light irradiation. (B) TEM image for Au/TiO2 after photocatalytic experiment.
the density functional electronic structure calculations. As a result, if there are only (101) facets exposed on TiO2, the large and similar trapping energies and adjacent trapping sites for electrons and holes will result in high electron−hole recombination rates. However, when many high-energy facets exist, the large difference of trapping energy for electrons and holes on different surfaces leads to spatial gathering of electrons and holes on different surfaces, which will promote the separation of electrons and holes. Additionally, they also investigated the charge separation between the well-formed {001} and {101} facets.40 They predicted that the enhanced photocatalytic efficiency was largely attributed to the efficient separation of photogenerated charges among the crystal facets co-exposed by a DFT calculation method. Their agreement between experiment and theory indicated that the difference in the energy levels between crystal facets could drive electron and hole transfer to different crystal facets, resulting in the separation of electrons and holes. Under the UV light irradiation, the photogenerated electrons on {001} facets will migrate to {101} facets, which is energetically favorable. According to the DOS results in refs 40 and 41, we can verify that the valence band of TiO2(110) extends to somewhat higher energies with respect to (101) facets due to its higher surface energy. In our investigation, although we did not calculate the trapping energy for electrons and holes for different facets, we could deduce that the electrons will migrate from the (110) facet to the adjacent (101) facet, owing to the higher surface energy and formation energy of (110) facet. This migration of electrons between different facets will improve the separation of electrons and holes. Additionally, it is rather remarkable that a large number of Au nanoparticles still load on the TiO2 surface after a process of photocatalytic experiment (Figure 9B), which suggests that the exposure of a high-surface-energy facet such as the (001) and (110) facets is indeed beneficial to the stabilization of Au on the catalyst surface. In order to further demonstrate the stability of Au nanoparticles during the UV light irradiation, the photocatalytic cycle test was carried out as follows. The commercial pure anatase TiO2 (produced by Shanghai Kangyi Co. Ltd.) was treated in the same way to load Au nanoparticles and was considered as a reference, which was without highenergy facets and denoted as Au/TiO2-R. Using the same method, the cycle test was carried out five times, and the photocatalysis results are shown in Figure S2 (Supporting Information). For the prior two cycle times, the photoactivity for the Au-loaded TiO2 with high-energy facets (Au/TiO2) has a decreased degradation of RhB, maybe owing to the dropping
interaction between Au and surface atoms will definitely decrease the number of Ti−F bonds on the TiO2 surface. The photocatalytic activities of the anatase TiO2 single crystal before and after Au loading were evaluated in terms of the degradation of rhodamine B (RhB, 20 mg/L) under UV light irradiation (Figure 9A). The concentration of a blank RhB solution without any catalysts does not change under irradiation conditions, hence ruling out the contribution of self-degradation of RhB. The 0.20-TiO2 single crystal with a high percentage of exposed (001) facets besides (101) shows a good photocatalytic activity under UV light irradiation, which could be due to the special electronic and surface structures of (001) facets.27 In particular, the 2p states on the (001) facets surface oxygen atoms were proposed to be destabilized and become very reactive,38 which may lead to formation of more oxygen vacancies on the (001) facets. Such defects can act as an active center in the photocatalysis process through adsorption of OH•, which can help increase UV light photocatalytic activities of 0.20-TiO2 (see Figure 10). Interestingly, for the
Figure 10. Schematic diagram of electron transfer among different crystal faces and Au particles. (a−d) The four major transfer pathways for the photoexcited electrons.
TiO2 prepared with a HF amount of up to 0.25 mL, its UV light activity toward degradation of RhB is enhanced (Figure 9A). This might be due to the transfer of photoexcited electrons from the (110) facet to the (101) facet because the transfer of electrons from the higher-surface-energy facets, that is, (001) and (110), to the lower-energy facet of (101) can improve the separation of electrons and holes and inhibit their recombination. Recently, Baibiao Huang and Ying Dai investigated the ability of gathering electrons and holes on different surfaces of TiO2, such as (101), (100), and (001) facets.39 They examined the trapping energy, trapping sites, and relative oxidation and reduction potentials of simulated photogenerated holes and electrons in the form of more realistic polaronic states based on 3915
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Letter
and transfer its electron from the F ion to the Au particle via the adjacent oxygen defects.44 These different photoelectrontransfer pathways are all beneficial to the separation of photoexcited electrons and holes as well as the generation of mobile free OH radicals and other active groups, which results in the enhanced UV light photocatalytic activity of Au/TiO2 single crystals. In conclusion, anatase TiO2 single crystals with exposed active (001) and (110) facets have been successfully prepared through a one-step hydrothermal method with the TiCl3 as the precursor and HF as the facet-controlling agent. The percentage of exposed (001) facets can be up to 73%, and a significant amount of (110) facet, which is very rare at usual anatase crystals, also exists. These exposed high-energy (001) and (110) facets were taken as an ideal support for highly dispersed Au nanoparticles to improve the stability of Au on the catalyst surface. The exposed active (001) and (110) facets together with the loaded Au nanoparticles dramatically enhanced the UV light photocatalytic activity of the Au/TiO2 single crystal. The prepared Au/TiO2 catalyst is expected to have attractive applications in dye-sensitized solar cells, photocatalysis, opotoelectronic devices, and sensors. The finding of our work provides a new way to steadily improve photocatalytic efficiency through loading of stable noble metals on the TiO2 single crystal with high-energy surfaces, and the preparation of small-sized or mesoporous TiO2 single crystals exposed with high-energy facets and loaded with different noble metals is under investigation in our laboratory.
of some physical adsorbed Au nanoparticles from the TiO2 surface. After three cycles of photocatalysis, the surface chemical adsorbed Au nanoparticles become stable, and the photocatalytic degradation rate becomes nonpeeling. During the UV light irradiation, the photogenerated active groups may react with the surface-bonding Au to induce the dropping of Au particles from the TiO2 surface. The ICP-AES results for Au/ TiO2 indicated that the weight percent of Au on a TiO2 single crystal was decreased from 1.3 to 0.87% after five cycle tests. The corresponding expulsion rate is 33.1%. However, for the anatase TiO2 without high-energy facets, its photocatalytic activity keeps decreasing after five cycle tests due to the low stability of Au nanoparticles on the TiO2 surface. The weight percent of Au was decreased from 2.0 to 0.84%, and its corresponding expulsion rate was 58.0%. Compared with the theoretical 1.0 wt %, the higher original Au weight percent of 2.0 for Au/TiO2-R indicated the local agrregation and low despersity of Au nanoparticles on the anatase TiO2 surface, and the 1.3 wt % loading concentration on the TiO2 single crystal suggested the highly dispersed Au particles on high-energy facets. Combining the lower Au expulsion rate of 33.1% for Au/ TiO2 with the stable photocatalytic cycle test, we could conclude that the TiO2 single crystal exposed with a highsurface-energy facets of (001) and (110) is beneficial to the loading stability of Au on the catalyst surface. After the Au nanoparticles were loaded on 0.25-TiO2, the photocatalytic activity was further improved, and a degradation rate even slightly higher than P25 was obtained (Figure 9A). It is rather remarkable that the particle size of the prepared Au/ TiO2 single crystal is much larger than that of commercial P25, which suggests that the catalyst of Au/TiO2 possesses a lower specific surface area but a higher photocatalytic activity than P25. As an acknowledged UV light-driven photocatalyst, nanosized P25 has a small particle size, high dispersibility, and high specific surface area (>50 m2/g), which results in the high photocatalytic activity under UV light irradiation. Compared with P25, our prepared Au/TiO2 has a large particle size (3.0−4.0 μm) and much smaller specific surface area (
