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TiO2 synthesized with a defect migration strategy for superior photocatalytic activity
Journal Pre-proofs Full Length Article Surface Oxygen Vacancies Enriched Pt/TiO2 Synthesized with a Defect Migration Strategy for Superior Photocatalytic Activity Huan Qiu, Xujun Ma, Chunyu Sun, Bin Zhao, Feng Chen PII: DOI: Reference:
S0169-4332(19)33838-3 https://doi.org/10.1016/j.apsusc.2019.145021 APSUSC 145021
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Applied Surface Science
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11 October 2019 6 December 2019 8 December 2019
Please cite this article as: H. Qiu, X. Ma, C. Sun, B. Zhao, F. Chen, Surface Oxygen Vacancies Enriched Pt/TiO2 Synthesized with a Defect Migration Strategy for Superior Photocatalytic Activity, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145021
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Surface Oxygen Vacancies Enriched Pt/TiO2 Synthesized with a Defect Migration Strategy for Superior Photocatalytic Activity
Huan Qiu,a Xujun Ma,a Chunyu Sun,a Bin Zhao*b and Feng Chen*a
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of
Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China. b
College of Materials Science and Engineering, Shenzhen University, 1066 Xueyuan
Avenue, Shenzhen 518055, Guangdong Province, P.R. China.
* Corresponding authors: Dr. Bin Zhao, E-mail: [email protected], Tel: +86-755-2693 5509. Prof. Dr. Feng Chen, E-mail: [email protected], Tel: +86-21-6425 3056.
Abstract: Surface oxygen vacancies (Ovs) may act as growth centers for Pt nanoparticles (NPs) and stabilize Pt NPs to avoid spontaneous aggregation, which practically improves the photocatalytic activity of Pt/TiO2. In this work, a defect migration strategy is developed; that is, the amount of surface Ovs on Pt/TiO2 is effectively improved by migrating bulk Ovs to the surface through a vacuum annealing process. The Ovs enriched surface of Pt/TiO2 increases the proportion of active Pt species (Pt0 and Pt-OH), thus greatly facilitating the photocatalytic activity of the catalyst under visible-light irradiation. Specifically, the apparent rate constant for the typical sample of Pt-Ovs/Ti3+/TiO2 (PVTT) prepared with the defect migration strategy is about 4.44 times as much as that for the control sample (Pt-TiO2) without the contribution of Ovs. Keywords: Pt/TiO2; bulk oxygen vacancy; surface oxygen vacancy; defect migration; photocatalysis.
1. Introduction In recent years, developing the visible-light-active photocatalysts based on titanium dioxide has been enormously investigated due to its wide range of applications in energy and environment related fields.[1,2] Specially, loading Pt nanoparticles (NPs) onto the TiO2-based photocatalysts may greatly enhance the photocatalytic activity,[3-5] as Pt NPs can effectively build Schottky barrier at their interface with TiO2 to hinder the recombination of photogenerated electron-hole pairs.[6] The properties of Pt NPs, such as particle size, dispersion, and chemical state, have great influences on the photocatalytic performances of the noble metal/semiconductor nanomaterials.[7-9] In general, the metallic state of Pt (Pt0) shows the highest activity,[10,11] whereas its oxidized state (PtOx) exhibits a lower activity.[7] In addition, Pt linked with the adsorbed oxygen species (PtOads) has a positive effect on the catalytic oxidation of organic substances.[12] The surface defects on TiO2 has great influences on the deposited noble metals,[13-15] as it effectively regulates the morphology and the chemical state of Pt NPs. The surface defects play key roles on the nucleation, growth, and stabilization of metal clusters on metal oxide surfaces.[16] As the most common defects, surface oxygen vacancies (Ovs) can act as growth centers for the nucleation of noble metals.[17] Generally, the released electrons located in the Ovs contribute to the reduction of the adsorbed Pt ions, which simultaneously achieve the Ovs regeneration and noble-metal deposition.[18] Furthermore, Ovs can not only improve the dispersion of Pt NPs by stabilizing Pt clusters to avoid their aggregation but also optimize the
chemical states of Pt by enhancing the proportion of active Pt species, due to the strong interaction between defect sites and Pt NPs.[19] Due to the important roles of surface Ovs on regulating the physical and chemical properties of deposited noble metal, developing new strategies for synthesizing Ovs enriched TiO2 is meaningful for preparing noble-metal/TiO2 hybrid nanostructures with an optimized photocatalytic performance. The concentration of surface Ovs can be increased by increasing the temperature of vacuum thermal (or hydrogen thermal treatment) within a certain range (about 120-560 oC).[20] However, further increasing the temperature exceeding this range will result in the decrease of the surface Ovs concentration.[21,22] Furthermore, the concentration of surface Ovs can also be increased by the traditional impurity doping, but the introduction of impurity element is not conducive to preserve the intrinsic lattice structure of TiO2.[23,24] Although many efforts have been devoted, how to conveniently obtain abundant surface Ovs on TiO2 without introducing impurity disturbance is still a great challenge. Therefore, it is interesting to develop a facile route to achieve impurity-free TiO2 with high concentration of surface Ovs. Generally, when sufficient defect activation energy is provided, the bulk defects can constantly overcome the energy barrier between the lattice points and continue migrating. Theoretical calculations show that bulk defects diffuse to the surface on the appropriate surface preparation and temperature conditions.[25-27] This is also supported by the experimental data. Tait and Kasowski[28] proved the Ovs diffusion from bulk to surface during annealing on an ion sputtered TiO2. The existence of bulk Ovs in the lattice of TiO2 causes an
increase of the chemical potential in the region where bulk Ovs existed.[29,30] During the long period of annealing, driven by the concentration and chemical potential gradients, bulk Ovs diffuse in the direction of the upper part of the nanostructure (while oxygen ions diffuse towards the bulk of TiO2), lowering the overall free energy of the system.[31] Furthermore, bulk defects play a major role in restructuring and reconstruction processes of TiO2 surface when high temperatures annealing is necessary.[32-34] Bulk Ti3+ defects often coexist with bulk Ovs in the defective TiO2 due to the charge imbalance of the defects introduction.[35] A huge number of Ti3+-Ovs associates are introduced into the bulk of defective TiO2, which is reported by the previous studies.[36] Generally, the singly ionized center (Ov+·Ti3+)+ and crystal neutral center (Ov+·2Ti3+)0 are considered to be two types of Ti3+-Ovs associates, showing the different coordination surroundings.[37-39] Therefore, the deep investigation on the chemical states of bulk Ti3+ is critical for the insight into the migration behavior of bulk Ovs. This work aims to improve the concentration of surface Ovs to achieve a better Pt NPs modulation, thereby enhancing the photocatalytic activity of Pt/TiO2. For this purpose, a defect migration strategy is developed to obtain the surface Ovs enriched TiO2, in which the migration and transformation of the bulk Ovs are involved. Specifically, bulk Ti3+ defects enriched TiO2 catalysts are synthesized through a solvothermal reaction. Surface Ovs enriched TiO2 are achieved by migrating bulk Ovs under vacuum annealing at a temperature of 350 oC. In addition, the influence of
Ovs enriched surface on the chemical states of Pt NPs is also investigated for Pt/TiO2 hybrid nanostructures. The photocatalytic activities of various Pt/TiO2 nano-hybrid samples, obtained with/without applying bulk Ovs migration in the synthesis procedure, are investigated by photo-degradation of Acid Orange II (AO7), which evidently demonstrate the contribution of defect migration strategy to the improvement of photocatalytic performance.
2. Experimental 2.1 Chemicals Titanium trichloride solution (TiCl3, 15.0-20.0%), isopropanol (analytical grade), tetrabutyl titanate (TBOT, analytical grade), hydroflfluoric acid (HF, analytical grade, 40%), methanol (analytical grade), and hydrochloroplatinic acid (H2PtCl6·6H2O). All chemicals were used as-received without further purification. 2.2 Catalysts preparation Preparation of Ti3+/TiO2. The preparation of Ti3+/TiO2 is similar to the previous report by Zhou et al.[38] In a typical experimental procedure, the Ti3+ self-doped TiO2 photocatalyst was synthesized with 25 mL of TiCl3 solution, 5 mL of TBOT, 1.2 mL of HF, and 20 mL of isopropanol as the precursors under solvothermal treatment at 180 oC for 48 h. Then the cooled precipitate was washed successively with ethanol and deionized water three times, and dried in an oven at 75 oC for 12 h. The obtained sample was denoted as TT.
In order to facilitate the comparison, while maintaining the above conditions, isopropanol was used to synthesize pure TiO2 instead of TiCl3 solution. Preparation of Ovs/Ti3+/TiO2. 0.15 g of Ti3+/TiO2 and 0.15 g of TiO2 were respectively heated to 350 oC for 5.0 h under vacuum condition (6 × 10-2 Pa) with a heating rate of 5 oC/min, so that Ovs were introduced on the surface of the samples. The obtained samples were denoted as VTT and V/TiO2, respectively. Preparation of Pt-Ovs/Ti3+/TiO2. Pt NPs deposited catalysts were prepared through a photo-deposition method via using VTT (TiO2, V/TiO2, or TT) as supports, respectively. Typically, 0.25 g of VTT (TiO2, V/TiO2, or TT), 3.312 mL of H2PtCl6·6H2O solution (1.0 g/L), and 50 mL of methanol solution (20%) were mixed in a quartz tube and treated under ultrasound for 1-5 min until no obvious granular precipitate was observed. Then the mixture was irradiated under ultraviolet light with stirring for 3.0 h. The suspension obtained by photodeposition was centrifuged and washed three times with deionized water, and dried in an oven at 75 oC for 8.0 h. The obtained sample was denoted as PVTT (Pt-TiO2, Pt-V/TiO2, or PTT), wherein the theoretical content of Pt was 0.5 wt.%. The actual Pt contents of four catalysts are 0.47 ± 0.01 wt.% measured by ICP-AES, which suggesting that surface Ovs and bulk Ti3+ defects have little effects on the loading amount of Pt NPs. Fig. 1 shows the schematic diagram of the experimental steps.
Fig. 1. Schematic diagram of material synthesis experiments. Abbreviations: IPA, isopropanol; TT, Ti3+ self-doped TiO2; HTT, higher-concentration Ti3+ self-doped TiO2; VTT, vacuum heated TT; VHTT, vacuum heated HTT; PTT, Pt-loaded TT; PVTT, Pt-loaded VTT; PVHTT, Pt-loaded VHTT.
2.3 Material characterizations Inductively coupled plasma atomic emission spectrometer (ICP-AES, Agilent 725) was used to measure the actual content of Pt in the photocatalysts. The X-ray diffraction (XRD) patterns of crystalline samples were carried out using a diffractometer (Rigaku Ultima IV Focus) with Cu Kα (λ = 0.154 nm) as radiation source. UV-Vis
diffused spectra were recorded on a UV-Vis
scanning
spectrophotometer (Shimadzu 2600). Raman spectra were obtained on a Renishaw invia Raman microscope at room temperature with 532 nm laser excitation. Electron paramagnetic resonance spectra were recorded on an EMX-8/2.7 EPR spectrometer. All EPR spectra of the catalysts were obtained at room temperature under atmospheric conditions. The morphology and particle sizes of the catalysts were acquired by a JEM-2100 high-resolution transmission electron microscope (HRTEM).
X-ray photoelectron spectra (XPS) were carried out using a Thermo Fisher Automatic 250 xi system equipped with an Al Kα radiation (1361 eV), and calibrated internally by C1s binding energy (Eb) at 284.6 eV from the carbon contaminants. Photoluminescence (PL) spectra were collected using 320 nm radiations as the excitation light at room temperature with a fluorescence spectrometer (LS-55 Lumine). Photocurrent test was measured by using an electrochemical workstation (CHI660E), which equipped with a standard three-electrode system. Ag/AgCl (saturated KCl) was used as a reference electrode. During the test, the electrolyte was 0.5 M Na2SO4 aqueous solution, and a metal halide lamp with a power of 35 W was the excitation light source. 2.4 Photocatalytic activity evaluation The activities of the photocatalysts were calculated from the rate of degradation of AO7 (20 mg/L) dye under visible light. A 35 W metal halide lamp (Philips, HID-CV 35/S CDM) with a 420 nm cutoff filter was used as the visible light source. 50 mg of the photocatalyst was dispersed into 50 mL AO7 aqueous solution in a quartz tube. Before illumination, the suspension was stirred in the dark for 30 min to reach an adsorption-desorption
equilibrium
and
then
the
Photocatalytic
degradation
experiment was carried out under visible light. Suspension was taken out every 20 min and centrifuged at 12000 rpm for 5 min. Then the AO7 concentration from the upper clear solution was analyzed according to its maximum absorption measured with Shimadzu 2600 spectrophotometer.
3. Results and discussion 3.1 Crystallinity and morphology of catalysts. Fig. 2 displays the XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT photocatalysts. All catalysts show the anatase phase (JCPDS, no. 78-2486) with good crystallinity. There is no remarkable change in the crystalline phase after vacuum annealing. Besides, the absence of the diffraction peak of Pt in the XRD patterns may be attributed to the small quantity and high dispersion of Pt NPs. Pt-TiO2
(101)
Pt-V/TiO2 PTT (200)
Intensity (a.u.)
(004)
20
40
PVTT
(211) (105) (204)
2 (degree)
(220) (215) (116)
60
80
Fig. 2. XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT.
The morphology of Pt NPs on four samples is characterized by TEM and shown in Fig. 3. Fig. 3d shows a high dispersion of Pt NPs on PVTT with a small particle size of about 3 nm. Interplanar lattice space of 0.35 nm in the magnified scale (Fig. S1, Supporting Information) should be indexed to the TiO2 (101) lattice planes.[40] This is well confirmed with the results from the XRD patterns in Fig. 2.
Fig. 3. TEM images of (a) Pt-TiO2 , (b) Pt-V/TiO2, (c) PTT, and (d) PVTT.
3.2 Structure and chemical state of catalysts. Fig. 4a shows the Raman scattering peaks in the range of 100-800 cm-1, which can be assigned to the stretching modes of Ti-O bands in TiO2. As for Pt-TiO2, five typical Raman peaks at 142, 196, 398, 515, and 635 cm-1 are ascribed to the anatase phase. However, compared with Pt-TiO2, the principal peaks at 142 cm-1 of Pt-V/TiO2 and PVTT have no obvious shift, while that of the PTT sample has a hypsochromic shift of 5 cm-1 and an obvious peak broadening. The shift in Raman peak of PTT is associated with the lattice disorder in TiO2 crystal, which originates from the bulk Ti3+ centers.[38]
The UV-vis diffuse reflectance spectra of Pt-TiO2, PTT, Pt-V/TiO2, and PVTT are shown in Fig. 4b. All samples show visible light absorption. Previous studies proved bulk Ti3+ species could introduce isolated states in the bandgap of TiO2 and consequently decrease the excitation energy.[41,42] It is worth noting that the absorption of PVTT in the visible region is significantly lower than that of PTT, which is related to the decrease of bulk Ti3+ defects during vacuum annealing.
Fig. 4. (a) Raman spectra of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) UV-vis diffused reflectance spectra of catalysts. (c, d) EPR spectra and (e) Ti 2p XPS spectra of photocatalysts.
Considering the fact that there are unpaired electrons in the samples, EPR spectra are used to detect the presence of surface Ovs, bulk Ti3+, and the migration of Ovs. As indicated in Fig. 4c, the EPR measurement of PTT sample has only one signal (g = 1.93), which can be assigned to the bulk Ti3+ species,[43] suggesting the Ti3+ centers exist in the bulk of PTT rather than on the surface. EPR signal for Ti3+ species is not observed in Pt-V/TiO2 and PVTT samples. In addition, Pt-TiO2 sample does not show
any paramagnetic signal, whereas the Pt-V/TiO2 and PVTT samples show a strong EPR signal at g = 2.003, which suggests the occurrence of surface Ovs species.[37,44] Particularly, compared to PTT, the EPR signal for bulk Ti3+ species disappears, while that for surface Ovs appears in PVTT. The EPR signal for surface Ovs in PVTT is much stronger than that in Pt-V/TiO2, indicating that bulk Ti3+ defects are beneficial to the extra generation of surface Ovs. Results suggest that an inside-outside migration from bulk Ovs to surface Ovs occurs during vacuum annealing. When Ovs migrate from bulk to surface, oxygen ions simultaneously migrate towards the contrary direction.[31] To better illustrate the defect migration process, TiO2 sample with higher Ti3+ concentration (HTT) is also synthesized by further increasing the HF content. Then the HTT sample is vacuum-annealed at 350 oC to obtain VHTT. The EPR spectra in Fig. 4d show that the bulk Ovs in TT completely migrate to the surface of TiO2 as the EPR signal of bulk Ti3+ defects in VTT entirely disappears. However, the intermediate electronic state in VHTT (g = 1.967) that Ovs migrate from bulk to surface is captured, which is conclusively assigned to the reduced Ti3+ centers in regular, octahedrally coordinated site in the anatase lattice. This signal can be observed when electrons are introduced into the system by thermal depletion of oxygen or in the early stages of reductive process.[45] Furthermore, a shoulder peak (g = 2.001) is still observed on the EPR spectrum of VHTT, indicating an incomplete outward migration of bulk Ovs, which can be attributed to the excessively higher Ti3+ concentration in HTT under a same vacuum annealing condition as that for VTT.
The Ti 2p XPS spectra are shown in Fig. 4e. The doublet Ti 2p3/2 (binding energy 458.9 eV) and Ti 2p1/2 (binding energy 464.6 eV) of Pt-TiO2 arise from spin orbit-splitting. These peaks are consistent with Ti4+ in TiO2 lattice.[46] However, the XPS spectrum of PTT shows no evidence of Ti3+ on the surface of TiO2, which should appear in 463.6 and 457.9 eV for Ti3+ 2p1/2 and Ti3+ 2p3/2, respectively.[47] The Ti 2p3/2 peak of Pt-V/TiO2 is basically consistent with Pt-TiO2, which indicates the surface Ovs concentration of Pt-TiO2 generated under vacuum annealing is insufficient to shift the Ti 2p3/2 peak. The peak of PVTT is now located at binding energy 458.7 eV (Ti 2p3/2), showing a slight shift compared to Pt-TiO2, which indicates that the Ovs enriched surface has an effect on the electronic state of Ti element.[27] Such high surface Ovs concentration is caused by the migration of bulk Ovs during vacuum annealing. XPS analysis is further performed to investigate the chemical states of O and Pt in catalysts. The O 1s spectra of samples are shown in Fig. 5a, which are deconvoluted into three peaks at binding energies of 530.0, 531.2, and 532.1 eV, attributing to lattice oxygen (Ti-O), surface hydroxyl group of TiO2 (Ti-OH) and chemisorbed water (H2Oads), respectively. It is worth noting that a higher proportion of hydroxyl groups (11.8%) and H2Oads (11.4%) are found on the surface of PVTT (Fig. 5b and Table S1, Supporting Information) compared to those species on Pt-V/TiO2 (7.0% and 3.4%). In fact, it has been proved in the photo-induced superhydrophilic TiO2 that H2O molecules are more easily adsorbed onto the Ovs defect sites to consequently convert into hydroxyl radicals (·OH), which explains
why the catalyst surfaces are capable of forming more H2Oads and -OH.[48] Therefore, the results of O 1s XPS spectra support that the highly concentrated Ovs exist on the surface of PVTT, resulted by the migration of bulk Ovs. In addition, the Pt 4f XPS spectra of samples are shown in Fig. 5c, which are deconvoluted into three species (Table S2, Supporting Information),[49] of which Pt0 and Pt-OH show higher photocatalytic activity than Pt2+. The specific components of three Pt species are calculated in Table S3 (Supporting Information). As shown in Fig. 5d, a detailed statistical analysis shows that the relative content of Pt0 decreases (48.1%→38.9%, 47.6%→45.5%) and the relative content of Pt-OH increases (25.0%→33.3%, 31.7%→36.4%) after vacuum treatment. The introduction of Ovs effectively increases the total percentage of Pt0 and Pt-OH in the catalysts (72.2%→81.8%), signifying that the Ovs migration from bulk to the surface of TiO2 increases the concentration of surface Ovs and consequently improves the content of active Pt species.
Fig. 5. (a) O 1s, and (c) Pt 4f XPS spectra of photocatalysts; Histogram of the ratio of O species (b) and Pt species (d).
3.3 Defect migration mechanism. On the basis of the as-above experimental observations, a defect migration mechanism during vacuum annealing is proposed in Fig. 6. Theoretically, one Ov has effective charge of +2. Three different EPR signals are displayed on the EPR spectra. The free vacancy (doubly ionized center, Ov2+) appears on the TiO2 surface or at the interface between metal and TiO2, showing a EPR signal at g = 2.003.[37] However, the concentration of Ov2+ is too low to produce a large optical red-shift. The singly ionized center (Ov+·Ti3+)+ in the bulk of TiO2 shows a EPR signal at g = 1.967,[39,42] and the crystal neutral center (Ov+·2Ti3+)0 in the bulk shows a EPR signal at g = 1.93, which are the fingerprint of an extended reductive process. The defect migration occurs when the sufficient defect activation energy is provided. That is, bulk Ovs continue migrating to the surface while oxygen ions migrate to the bulk of TiO2 in the
contrary direction during vacuum annealing.[33] Therefore, the main characterization techniques, such as EPR and XPS, signify the electronic state changes of bulk Ovs through defect migration process under vacuum annealing as follows: Bulk (Ov+·2Ti3+)0 → bulk (Ov+·Ti3+)+ → surface Ov2+. Since the concentration of bulk defects is much larger than that of surface, more Ovs are generated for PVTT sample in the defect migration process, compared with the bulk Ti3+ absent samples (Pt-TiO2, or Pt-V/TiO2). Furthermore, the Ovs enriched surface of PVTT optimizes the proportion of active Pt species (Pt0 and Pt-OH). During the photo-deposition process, Pt4+ ions are initially nucleated and reduced, forming a Pt2+-Ti4+ connection on the surface Ovs sites. Photogenerated electrons in the surface Ovs sites construct a local reductive region surrounding Ovs, which subsequently destroys the Pt2+-Ti4+ connection and reduces Pt species.[19]
Fig. 6. The mechanism diagram of defect migration on PTT.
3.4 Photocatalytic performance. Fig. 7a reveals the photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. The photocurrent response can be used to analyze the photo-generated carrier separation efficiency of the catalyst. PTT and Pt-TiO2 samples show a weak
photocurrent density of only ca. 0.18 μA/cm2. After vacuum annealing, the photocurrent density of Pt-V/TiO2 is slightly improved (ca. 0.26 μA/cm2), whereas the PVTT sample shows the strongest photocurrent density, ca. 0.70 μA/cm2. The photocurrent density of PVTT is about 3.9 times higher than that of Pt-TiO2 and PTT, 2.7 times higher than that of Pt-V/TiO2. The above facts indicate that the introduction of surface Ovs effectively optimizes the chemical states of Pt, thereby improving the separation of photo-generated carriers. PL is used to investigate the recombination behavior of the photogenerated carriers in the photocatalysts. As shown in Fig. 7b, the fluorescence intensity of vacuum-annealed catalysts (PVTT and Pt-V/TiO2) shows much more decrease than those without vacuum-annealing samples (PTT and Pt-TiO2), respectively. The fluorescence intensity of PVTT shows much more decrease than Pt-V/TiO2, which suggests that defect migration further benefits the photo-generated charge separation by effectively modulating the deposited Pt clusters. Abundant oxygen vacancies only at the Pt-TiO2 interface can locate hole-driven oxidation sites in proximity to electron-driven reduction sites for triggering unusual reactions.[50] To evaluate the photocatalytic performance of the samples under visible light irradiation, AO7 is chosen as the target pollutant. As observed in Fig. 7c, AO7 is completely decomposed in 80 min when PVTT is used as the photocatalyst. However, only 50.9%, 54.8%, and 82.9% of AO7 molecules are degraded when Pt-TiO2, PTT, and Pt-V/TiO2 are used as the photocatalysts, respectively. The reaction kinetics in all reaction systems follow the first-order kinetics equation: ln(C/C0) = kt, where k is the
apparent rate constant. Fig. 7d shows a comparison of the apparent rate constants for the PTT (0.0123 min-1), Pt-V/TiO2 (0.0229 min-1), and PVTT (0.0391 min-1), which are about 1.40, 2.60, and 4.44 times as much as that over Pt-TiO2 (0.0088 min-1), respectively. The excellent photocatalytic performance of PVTT can be attributed to the improved content of active Pt components, which is modulated by the surface Ovs enriched TiO2. Besides, the existence of residual Ovs at the Pt-TiO2 interface of as-synthesized catalyst is also an important reason for the improvement of photocatalytic activity.[50] In addition, the photocatalytic performance of PHTT and PVHTT are given in Fig. S6 (Supporting Information), indicating that excessively higher concentration of bulk Ti3+ defects is not advantageous to the generation of surface Ovs, nor to the photocatalytic activity of the catalyst. Under the same vacuum annealing condition, results indicate sample with high concentration of bulk Ti3+ requires high activation energy for bulk Ovs migration. Namely, only a small part of bulk Ovs migrate to the surface and most of Ti3+ defects still present in the bulk after the vacuum annealing. It leads to an increase in electron-hole recombination rate and a deterioration of visible light activity.[51]
Fig. 7. (a) Photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) PL spectra under 320 nm excitation for the samples. (c) Photocatalytic degradation curves of AO7 under visible light irradiation. (d) Histogram of the photocatalytic activity rate constants (k).
4. Conclusions. In conclusion, a defect migration strategy is developed to promote the concentration of surface Ovs for TiO2. Specifically, Ti3+ self-doped anatase TiO2 photocatalysts are facilely prepared under hydrothermal treatment. The outward migration of bulk Ovs and the inward migration of O ions are then achieved to obtain highly concentrated surface Ovs under vacuum annealing. The Ovs enriched surface of PVTT has a positive effect on regulating active Pt components (Pt0 and Pt-OH), which greatly enhances the photocatalytic activity of the catalysts compared with the defect migration absent samples. The work presents a new idea for the design of surface Ovs enriched semiconductive catalysts.
Conflict of interest The authors declare no competing interests.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21677049, 21876051).
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Figure Captions Fig. 1. Schematic diagram of material synthesis experiments. Abbreviations: IPA, isopropanol; TT, Ti3+ self-doped TiO2; HTT, higher-concentration Ti3+ self-doped TiO2; VTT, vacuum heated TT; VHTT, vacuum heated HTT; PTT, Pt-loaded TT; PVTT, Pt-loaded VTT; PVHTT, Pt-loaded VHTT. Fig. 2. XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. Fig. 3. TEM images of (a) Pt-TiO2 , (b) Pt-V/TiO2, (c) PTT, and (d) PVTT. Fig. 4. (a) Raman spectra of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) UV-vis diffused reflectance spectra of catalysts. (c, d) EPR spectra and (e) Ti 2p XPS spectra of photocatalysts. Fig. 5. (a) O 1s, and (c) Pt 4f XPS spectra of photocatalysts; Histogram of the ratio of O species (b) and Pt species (d). Fig. 6. The mechanism diagram of defect migration on PTT. Fig. 7. (a) Photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) PL spectra under 320 nm excitation for the samples. (c) Photocatalytic degradation curves of AO7 under visible light irradiation. (d) Histogram of the photocatalytic activity rate constants (k).
Author Contribution Statement Huan
Qiu:
Conceptualization,
Methodology,
Validation,
Investigation, Data Curation, Writing-Original Draft Xujun
Formal
Ma:
analysis,
Methodology,
Formal analysis Chunyu Sun: Validation, Investigation Bin Zhao: Writing-Review & Editing, Supervision Feng Chen: Conceptualization, Resources, Writing-Review & Editing, Project administration, Funding acquisition
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights
Bulk Ovs would diffuse to the surface under vacuum annealing.
Defect migration improves the concentration of surface Ovs.
Ovs enriched surface of Pt/TiO2 increases the proportion of active Pt species.
S0169-4332(19)33838-3 https://doi.org/10.1016/j.apsusc.2019.145021 APSUSC 145021
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
11 October 2019 6 December 2019 8 December 2019
Please cite this article as: H. Qiu, X. Ma, C. Sun, B. Zhao, F. Chen, Surface Oxygen Vacancies Enriched Pt/TiO2 Synthesized with a Defect Migration Strategy for Superior Photocatalytic Activity, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145021
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Surface Oxygen Vacancies Enriched Pt/TiO2 Synthesized with a Defect Migration Strategy for Superior Photocatalytic Activity
Huan Qiu,a Xujun Ma,a Chunyu Sun,a Bin Zhao*b and Feng Chen*a
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of
Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China. b
College of Materials Science and Engineering, Shenzhen University, 1066 Xueyuan
Avenue, Shenzhen 518055, Guangdong Province, P.R. China.
* Corresponding authors: Dr. Bin Zhao, E-mail: [email protected], Tel: +86-755-2693 5509. Prof. Dr. Feng Chen, E-mail: [email protected], Tel: +86-21-6425 3056.
Abstract: Surface oxygen vacancies (Ovs) may act as growth centers for Pt nanoparticles (NPs) and stabilize Pt NPs to avoid spontaneous aggregation, which practically improves the photocatalytic activity of Pt/TiO2. In this work, a defect migration strategy is developed; that is, the amount of surface Ovs on Pt/TiO2 is effectively improved by migrating bulk Ovs to the surface through a vacuum annealing process. The Ovs enriched surface of Pt/TiO2 increases the proportion of active Pt species (Pt0 and Pt-OH), thus greatly facilitating the photocatalytic activity of the catalyst under visible-light irradiation. Specifically, the apparent rate constant for the typical sample of Pt-Ovs/Ti3+/TiO2 (PVTT) prepared with the defect migration strategy is about 4.44 times as much as that for the control sample (Pt-TiO2) without the contribution of Ovs. Keywords: Pt/TiO2; bulk oxygen vacancy; surface oxygen vacancy; defect migration; photocatalysis.
1. Introduction In recent years, developing the visible-light-active photocatalysts based on titanium dioxide has been enormously investigated due to its wide range of applications in energy and environment related fields.[1,2] Specially, loading Pt nanoparticles (NPs) onto the TiO2-based photocatalysts may greatly enhance the photocatalytic activity,[3-5] as Pt NPs can effectively build Schottky barrier at their interface with TiO2 to hinder the recombination of photogenerated electron-hole pairs.[6] The properties of Pt NPs, such as particle size, dispersion, and chemical state, have great influences on the photocatalytic performances of the noble metal/semiconductor nanomaterials.[7-9] In general, the metallic state of Pt (Pt0) shows the highest activity,[10,11] whereas its oxidized state (PtOx) exhibits a lower activity.[7] In addition, Pt linked with the adsorbed oxygen species (PtOads) has a positive effect on the catalytic oxidation of organic substances.[12] The surface defects on TiO2 has great influences on the deposited noble metals,[13-15] as it effectively regulates the morphology and the chemical state of Pt NPs. The surface defects play key roles on the nucleation, growth, and stabilization of metal clusters on metal oxide surfaces.[16] As the most common defects, surface oxygen vacancies (Ovs) can act as growth centers for the nucleation of noble metals.[17] Generally, the released electrons located in the Ovs contribute to the reduction of the adsorbed Pt ions, which simultaneously achieve the Ovs regeneration and noble-metal deposition.[18] Furthermore, Ovs can not only improve the dispersion of Pt NPs by stabilizing Pt clusters to avoid their aggregation but also optimize the
chemical states of Pt by enhancing the proportion of active Pt species, due to the strong interaction between defect sites and Pt NPs.[19] Due to the important roles of surface Ovs on regulating the physical and chemical properties of deposited noble metal, developing new strategies for synthesizing Ovs enriched TiO2 is meaningful for preparing noble-metal/TiO2 hybrid nanostructures with an optimized photocatalytic performance. The concentration of surface Ovs can be increased by increasing the temperature of vacuum thermal (or hydrogen thermal treatment) within a certain range (about 120-560 oC).[20] However, further increasing the temperature exceeding this range will result in the decrease of the surface Ovs concentration.[21,22] Furthermore, the concentration of surface Ovs can also be increased by the traditional impurity doping, but the introduction of impurity element is not conducive to preserve the intrinsic lattice structure of TiO2.[23,24] Although many efforts have been devoted, how to conveniently obtain abundant surface Ovs on TiO2 without introducing impurity disturbance is still a great challenge. Therefore, it is interesting to develop a facile route to achieve impurity-free TiO2 with high concentration of surface Ovs. Generally, when sufficient defect activation energy is provided, the bulk defects can constantly overcome the energy barrier between the lattice points and continue migrating. Theoretical calculations show that bulk defects diffuse to the surface on the appropriate surface preparation and temperature conditions.[25-27] This is also supported by the experimental data. Tait and Kasowski[28] proved the Ovs diffusion from bulk to surface during annealing on an ion sputtered TiO2. The existence of bulk Ovs in the lattice of TiO2 causes an
increase of the chemical potential in the region where bulk Ovs existed.[29,30] During the long period of annealing, driven by the concentration and chemical potential gradients, bulk Ovs diffuse in the direction of the upper part of the nanostructure (while oxygen ions diffuse towards the bulk of TiO2), lowering the overall free energy of the system.[31] Furthermore, bulk defects play a major role in restructuring and reconstruction processes of TiO2 surface when high temperatures annealing is necessary.[32-34] Bulk Ti3+ defects often coexist with bulk Ovs in the defective TiO2 due to the charge imbalance of the defects introduction.[35] A huge number of Ti3+-Ovs associates are introduced into the bulk of defective TiO2, which is reported by the previous studies.[36] Generally, the singly ionized center (Ov+·Ti3+)+ and crystal neutral center (Ov+·2Ti3+)0 are considered to be two types of Ti3+-Ovs associates, showing the different coordination surroundings.[37-39] Therefore, the deep investigation on the chemical states of bulk Ti3+ is critical for the insight into the migration behavior of bulk Ovs. This work aims to improve the concentration of surface Ovs to achieve a better Pt NPs modulation, thereby enhancing the photocatalytic activity of Pt/TiO2. For this purpose, a defect migration strategy is developed to obtain the surface Ovs enriched TiO2, in which the migration and transformation of the bulk Ovs are involved. Specifically, bulk Ti3+ defects enriched TiO2 catalysts are synthesized through a solvothermal reaction. Surface Ovs enriched TiO2 are achieved by migrating bulk Ovs under vacuum annealing at a temperature of 350 oC. In addition, the influence of
Ovs enriched surface on the chemical states of Pt NPs is also investigated for Pt/TiO2 hybrid nanostructures. The photocatalytic activities of various Pt/TiO2 nano-hybrid samples, obtained with/without applying bulk Ovs migration in the synthesis procedure, are investigated by photo-degradation of Acid Orange II (AO7), which evidently demonstrate the contribution of defect migration strategy to the improvement of photocatalytic performance.
2. Experimental 2.1 Chemicals Titanium trichloride solution (TiCl3, 15.0-20.0%), isopropanol (analytical grade), tetrabutyl titanate (TBOT, analytical grade), hydroflfluoric acid (HF, analytical grade, 40%), methanol (analytical grade), and hydrochloroplatinic acid (H2PtCl6·6H2O). All chemicals were used as-received without further purification. 2.2 Catalysts preparation Preparation of Ti3+/TiO2. The preparation of Ti3+/TiO2 is similar to the previous report by Zhou et al.[38] In a typical experimental procedure, the Ti3+ self-doped TiO2 photocatalyst was synthesized with 25 mL of TiCl3 solution, 5 mL of TBOT, 1.2 mL of HF, and 20 mL of isopropanol as the precursors under solvothermal treatment at 180 oC for 48 h. Then the cooled precipitate was washed successively with ethanol and deionized water three times, and dried in an oven at 75 oC for 12 h. The obtained sample was denoted as TT.
In order to facilitate the comparison, while maintaining the above conditions, isopropanol was used to synthesize pure TiO2 instead of TiCl3 solution. Preparation of Ovs/Ti3+/TiO2. 0.15 g of Ti3+/TiO2 and 0.15 g of TiO2 were respectively heated to 350 oC for 5.0 h under vacuum condition (6 × 10-2 Pa) with a heating rate of 5 oC/min, so that Ovs were introduced on the surface of the samples. The obtained samples were denoted as VTT and V/TiO2, respectively. Preparation of Pt-Ovs/Ti3+/TiO2. Pt NPs deposited catalysts were prepared through a photo-deposition method via using VTT (TiO2, V/TiO2, or TT) as supports, respectively. Typically, 0.25 g of VTT (TiO2, V/TiO2, or TT), 3.312 mL of H2PtCl6·6H2O solution (1.0 g/L), and 50 mL of methanol solution (20%) were mixed in a quartz tube and treated under ultrasound for 1-5 min until no obvious granular precipitate was observed. Then the mixture was irradiated under ultraviolet light with stirring for 3.0 h. The suspension obtained by photodeposition was centrifuged and washed three times with deionized water, and dried in an oven at 75 oC for 8.0 h. The obtained sample was denoted as PVTT (Pt-TiO2, Pt-V/TiO2, or PTT), wherein the theoretical content of Pt was 0.5 wt.%. The actual Pt contents of four catalysts are 0.47 ± 0.01 wt.% measured by ICP-AES, which suggesting that surface Ovs and bulk Ti3+ defects have little effects on the loading amount of Pt NPs. Fig. 1 shows the schematic diagram of the experimental steps.
Fig. 1. Schematic diagram of material synthesis experiments. Abbreviations: IPA, isopropanol; TT, Ti3+ self-doped TiO2; HTT, higher-concentration Ti3+ self-doped TiO2; VTT, vacuum heated TT; VHTT, vacuum heated HTT; PTT, Pt-loaded TT; PVTT, Pt-loaded VTT; PVHTT, Pt-loaded VHTT.
2.3 Material characterizations Inductively coupled plasma atomic emission spectrometer (ICP-AES, Agilent 725) was used to measure the actual content of Pt in the photocatalysts. The X-ray diffraction (XRD) patterns of crystalline samples were carried out using a diffractometer (Rigaku Ultima IV Focus) with Cu Kα (λ = 0.154 nm) as radiation source. UV-Vis
diffused spectra were recorded on a UV-Vis
scanning
spectrophotometer (Shimadzu 2600). Raman spectra were obtained on a Renishaw invia Raman microscope at room temperature with 532 nm laser excitation. Electron paramagnetic resonance spectra were recorded on an EMX-8/2.7 EPR spectrometer. All EPR spectra of the catalysts were obtained at room temperature under atmospheric conditions. The morphology and particle sizes of the catalysts were acquired by a JEM-2100 high-resolution transmission electron microscope (HRTEM).
X-ray photoelectron spectra (XPS) were carried out using a Thermo Fisher Automatic 250 xi system equipped with an Al Kα radiation (1361 eV), and calibrated internally by C1s binding energy (Eb) at 284.6 eV from the carbon contaminants. Photoluminescence (PL) spectra were collected using 320 nm radiations as the excitation light at room temperature with a fluorescence spectrometer (LS-55 Lumine). Photocurrent test was measured by using an electrochemical workstation (CHI660E), which equipped with a standard three-electrode system. Ag/AgCl (saturated KCl) was used as a reference electrode. During the test, the electrolyte was 0.5 M Na2SO4 aqueous solution, and a metal halide lamp with a power of 35 W was the excitation light source. 2.4 Photocatalytic activity evaluation The activities of the photocatalysts were calculated from the rate of degradation of AO7 (20 mg/L) dye under visible light. A 35 W metal halide lamp (Philips, HID-CV 35/S CDM) with a 420 nm cutoff filter was used as the visible light source. 50 mg of the photocatalyst was dispersed into 50 mL AO7 aqueous solution in a quartz tube. Before illumination, the suspension was stirred in the dark for 30 min to reach an adsorption-desorption
equilibrium
and
then
the
Photocatalytic
degradation
experiment was carried out under visible light. Suspension was taken out every 20 min and centrifuged at 12000 rpm for 5 min. Then the AO7 concentration from the upper clear solution was analyzed according to its maximum absorption measured with Shimadzu 2600 spectrophotometer.
3. Results and discussion 3.1 Crystallinity and morphology of catalysts. Fig. 2 displays the XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT photocatalysts. All catalysts show the anatase phase (JCPDS, no. 78-2486) with good crystallinity. There is no remarkable change in the crystalline phase after vacuum annealing. Besides, the absence of the diffraction peak of Pt in the XRD patterns may be attributed to the small quantity and high dispersion of Pt NPs. Pt-TiO2
(101)
Pt-V/TiO2 PTT (200)
Intensity (a.u.)
(004)
20
40
PVTT
(211) (105) (204)
2 (degree)
(220) (215) (116)
60
80
Fig. 2. XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT.
The morphology of Pt NPs on four samples is characterized by TEM and shown in Fig. 3. Fig. 3d shows a high dispersion of Pt NPs on PVTT with a small particle size of about 3 nm. Interplanar lattice space of 0.35 nm in the magnified scale (Fig. S1, Supporting Information) should be indexed to the TiO2 (101) lattice planes.[40] This is well confirmed with the results from the XRD patterns in Fig. 2.
Fig. 3. TEM images of (a) Pt-TiO2 , (b) Pt-V/TiO2, (c) PTT, and (d) PVTT.
3.2 Structure and chemical state of catalysts. Fig. 4a shows the Raman scattering peaks in the range of 100-800 cm-1, which can be assigned to the stretching modes of Ti-O bands in TiO2. As for Pt-TiO2, five typical Raman peaks at 142, 196, 398, 515, and 635 cm-1 are ascribed to the anatase phase. However, compared with Pt-TiO2, the principal peaks at 142 cm-1 of Pt-V/TiO2 and PVTT have no obvious shift, while that of the PTT sample has a hypsochromic shift of 5 cm-1 and an obvious peak broadening. The shift in Raman peak of PTT is associated with the lattice disorder in TiO2 crystal, which originates from the bulk Ti3+ centers.[38]
The UV-vis diffuse reflectance spectra of Pt-TiO2, PTT, Pt-V/TiO2, and PVTT are shown in Fig. 4b. All samples show visible light absorption. Previous studies proved bulk Ti3+ species could introduce isolated states in the bandgap of TiO2 and consequently decrease the excitation energy.[41,42] It is worth noting that the absorption of PVTT in the visible region is significantly lower than that of PTT, which is related to the decrease of bulk Ti3+ defects during vacuum annealing.
Fig. 4. (a) Raman spectra of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) UV-vis diffused reflectance spectra of catalysts. (c, d) EPR spectra and (e) Ti 2p XPS spectra of photocatalysts.
Considering the fact that there are unpaired electrons in the samples, EPR spectra are used to detect the presence of surface Ovs, bulk Ti3+, and the migration of Ovs. As indicated in Fig. 4c, the EPR measurement of PTT sample has only one signal (g = 1.93), which can be assigned to the bulk Ti3+ species,[43] suggesting the Ti3+ centers exist in the bulk of PTT rather than on the surface. EPR signal for Ti3+ species is not observed in Pt-V/TiO2 and PVTT samples. In addition, Pt-TiO2 sample does not show
any paramagnetic signal, whereas the Pt-V/TiO2 and PVTT samples show a strong EPR signal at g = 2.003, which suggests the occurrence of surface Ovs species.[37,44] Particularly, compared to PTT, the EPR signal for bulk Ti3+ species disappears, while that for surface Ovs appears in PVTT. The EPR signal for surface Ovs in PVTT is much stronger than that in Pt-V/TiO2, indicating that bulk Ti3+ defects are beneficial to the extra generation of surface Ovs. Results suggest that an inside-outside migration from bulk Ovs to surface Ovs occurs during vacuum annealing. When Ovs migrate from bulk to surface, oxygen ions simultaneously migrate towards the contrary direction.[31] To better illustrate the defect migration process, TiO2 sample with higher Ti3+ concentration (HTT) is also synthesized by further increasing the HF content. Then the HTT sample is vacuum-annealed at 350 oC to obtain VHTT. The EPR spectra in Fig. 4d show that the bulk Ovs in TT completely migrate to the surface of TiO2 as the EPR signal of bulk Ti3+ defects in VTT entirely disappears. However, the intermediate electronic state in VHTT (g = 1.967) that Ovs migrate from bulk to surface is captured, which is conclusively assigned to the reduced Ti3+ centers in regular, octahedrally coordinated site in the anatase lattice. This signal can be observed when electrons are introduced into the system by thermal depletion of oxygen or in the early stages of reductive process.[45] Furthermore, a shoulder peak (g = 2.001) is still observed on the EPR spectrum of VHTT, indicating an incomplete outward migration of bulk Ovs, which can be attributed to the excessively higher Ti3+ concentration in HTT under a same vacuum annealing condition as that for VTT.
The Ti 2p XPS spectra are shown in Fig. 4e. The doublet Ti 2p3/2 (binding energy 458.9 eV) and Ti 2p1/2 (binding energy 464.6 eV) of Pt-TiO2 arise from spin orbit-splitting. These peaks are consistent with Ti4+ in TiO2 lattice.[46] However, the XPS spectrum of PTT shows no evidence of Ti3+ on the surface of TiO2, which should appear in 463.6 and 457.9 eV for Ti3+ 2p1/2 and Ti3+ 2p3/2, respectively.[47] The Ti 2p3/2 peak of Pt-V/TiO2 is basically consistent with Pt-TiO2, which indicates the surface Ovs concentration of Pt-TiO2 generated under vacuum annealing is insufficient to shift the Ti 2p3/2 peak. The peak of PVTT is now located at binding energy 458.7 eV (Ti 2p3/2), showing a slight shift compared to Pt-TiO2, which indicates that the Ovs enriched surface has an effect on the electronic state of Ti element.[27] Such high surface Ovs concentration is caused by the migration of bulk Ovs during vacuum annealing. XPS analysis is further performed to investigate the chemical states of O and Pt in catalysts. The O 1s spectra of samples are shown in Fig. 5a, which are deconvoluted into three peaks at binding energies of 530.0, 531.2, and 532.1 eV, attributing to lattice oxygen (Ti-O), surface hydroxyl group of TiO2 (Ti-OH) and chemisorbed water (H2Oads), respectively. It is worth noting that a higher proportion of hydroxyl groups (11.8%) and H2Oads (11.4%) are found on the surface of PVTT (Fig. 5b and Table S1, Supporting Information) compared to those species on Pt-V/TiO2 (7.0% and 3.4%). In fact, it has been proved in the photo-induced superhydrophilic TiO2 that H2O molecules are more easily adsorbed onto the Ovs defect sites to consequently convert into hydroxyl radicals (·OH), which explains
why the catalyst surfaces are capable of forming more H2Oads and -OH.[48] Therefore, the results of O 1s XPS spectra support that the highly concentrated Ovs exist on the surface of PVTT, resulted by the migration of bulk Ovs. In addition, the Pt 4f XPS spectra of samples are shown in Fig. 5c, which are deconvoluted into three species (Table S2, Supporting Information),[49] of which Pt0 and Pt-OH show higher photocatalytic activity than Pt2+. The specific components of three Pt species are calculated in Table S3 (Supporting Information). As shown in Fig. 5d, a detailed statistical analysis shows that the relative content of Pt0 decreases (48.1%→38.9%, 47.6%→45.5%) and the relative content of Pt-OH increases (25.0%→33.3%, 31.7%→36.4%) after vacuum treatment. The introduction of Ovs effectively increases the total percentage of Pt0 and Pt-OH in the catalysts (72.2%→81.8%), signifying that the Ovs migration from bulk to the surface of TiO2 increases the concentration of surface Ovs and consequently improves the content of active Pt species.
Fig. 5. (a) O 1s, and (c) Pt 4f XPS spectra of photocatalysts; Histogram of the ratio of O species (b) and Pt species (d).
3.3 Defect migration mechanism. On the basis of the as-above experimental observations, a defect migration mechanism during vacuum annealing is proposed in Fig. 6. Theoretically, one Ov has effective charge of +2. Three different EPR signals are displayed on the EPR spectra. The free vacancy (doubly ionized center, Ov2+) appears on the TiO2 surface or at the interface between metal and TiO2, showing a EPR signal at g = 2.003.[37] However, the concentration of Ov2+ is too low to produce a large optical red-shift. The singly ionized center (Ov+·Ti3+)+ in the bulk of TiO2 shows a EPR signal at g = 1.967,[39,42] and the crystal neutral center (Ov+·2Ti3+)0 in the bulk shows a EPR signal at g = 1.93, which are the fingerprint of an extended reductive process. The defect migration occurs when the sufficient defect activation energy is provided. That is, bulk Ovs continue migrating to the surface while oxygen ions migrate to the bulk of TiO2 in the
contrary direction during vacuum annealing.[33] Therefore, the main characterization techniques, such as EPR and XPS, signify the electronic state changes of bulk Ovs through defect migration process under vacuum annealing as follows: Bulk (Ov+·2Ti3+)0 → bulk (Ov+·Ti3+)+ → surface Ov2+. Since the concentration of bulk defects is much larger than that of surface, more Ovs are generated for PVTT sample in the defect migration process, compared with the bulk Ti3+ absent samples (Pt-TiO2, or Pt-V/TiO2). Furthermore, the Ovs enriched surface of PVTT optimizes the proportion of active Pt species (Pt0 and Pt-OH). During the photo-deposition process, Pt4+ ions are initially nucleated and reduced, forming a Pt2+-Ti4+ connection on the surface Ovs sites. Photogenerated electrons in the surface Ovs sites construct a local reductive region surrounding Ovs, which subsequently destroys the Pt2+-Ti4+ connection and reduces Pt species.[19]
Fig. 6. The mechanism diagram of defect migration on PTT.
3.4 Photocatalytic performance. Fig. 7a reveals the photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. The photocurrent response can be used to analyze the photo-generated carrier separation efficiency of the catalyst. PTT and Pt-TiO2 samples show a weak
photocurrent density of only ca. 0.18 μA/cm2. After vacuum annealing, the photocurrent density of Pt-V/TiO2 is slightly improved (ca. 0.26 μA/cm2), whereas the PVTT sample shows the strongest photocurrent density, ca. 0.70 μA/cm2. The photocurrent density of PVTT is about 3.9 times higher than that of Pt-TiO2 and PTT, 2.7 times higher than that of Pt-V/TiO2. The above facts indicate that the introduction of surface Ovs effectively optimizes the chemical states of Pt, thereby improving the separation of photo-generated carriers. PL is used to investigate the recombination behavior of the photogenerated carriers in the photocatalysts. As shown in Fig. 7b, the fluorescence intensity of vacuum-annealed catalysts (PVTT and Pt-V/TiO2) shows much more decrease than those without vacuum-annealing samples (PTT and Pt-TiO2), respectively. The fluorescence intensity of PVTT shows much more decrease than Pt-V/TiO2, which suggests that defect migration further benefits the photo-generated charge separation by effectively modulating the deposited Pt clusters. Abundant oxygen vacancies only at the Pt-TiO2 interface can locate hole-driven oxidation sites in proximity to electron-driven reduction sites for triggering unusual reactions.[50] To evaluate the photocatalytic performance of the samples under visible light irradiation, AO7 is chosen as the target pollutant. As observed in Fig. 7c, AO7 is completely decomposed in 80 min when PVTT is used as the photocatalyst. However, only 50.9%, 54.8%, and 82.9% of AO7 molecules are degraded when Pt-TiO2, PTT, and Pt-V/TiO2 are used as the photocatalysts, respectively. The reaction kinetics in all reaction systems follow the first-order kinetics equation: ln(C/C0) = kt, where k is the
apparent rate constant. Fig. 7d shows a comparison of the apparent rate constants for the PTT (0.0123 min-1), Pt-V/TiO2 (0.0229 min-1), and PVTT (0.0391 min-1), which are about 1.40, 2.60, and 4.44 times as much as that over Pt-TiO2 (0.0088 min-1), respectively. The excellent photocatalytic performance of PVTT can be attributed to the improved content of active Pt components, which is modulated by the surface Ovs enriched TiO2. Besides, the existence of residual Ovs at the Pt-TiO2 interface of as-synthesized catalyst is also an important reason for the improvement of photocatalytic activity.[50] In addition, the photocatalytic performance of PHTT and PVHTT are given in Fig. S6 (Supporting Information), indicating that excessively higher concentration of bulk Ti3+ defects is not advantageous to the generation of surface Ovs, nor to the photocatalytic activity of the catalyst. Under the same vacuum annealing condition, results indicate sample with high concentration of bulk Ti3+ requires high activation energy for bulk Ovs migration. Namely, only a small part of bulk Ovs migrate to the surface and most of Ti3+ defects still present in the bulk after the vacuum annealing. It leads to an increase in electron-hole recombination rate and a deterioration of visible light activity.[51]
Fig. 7. (a) Photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) PL spectra under 320 nm excitation for the samples. (c) Photocatalytic degradation curves of AO7 under visible light irradiation. (d) Histogram of the photocatalytic activity rate constants (k).
4. Conclusions. In conclusion, a defect migration strategy is developed to promote the concentration of surface Ovs for TiO2. Specifically, Ti3+ self-doped anatase TiO2 photocatalysts are facilely prepared under hydrothermal treatment. The outward migration of bulk Ovs and the inward migration of O ions are then achieved to obtain highly concentrated surface Ovs under vacuum annealing. The Ovs enriched surface of PVTT has a positive effect on regulating active Pt components (Pt0 and Pt-OH), which greatly enhances the photocatalytic activity of the catalysts compared with the defect migration absent samples. The work presents a new idea for the design of surface Ovs enriched semiconductive catalysts.
Conflict of interest The authors declare no competing interests.
Acknowledgements This work was supported by the National Natural Science Foundation of China (21677049, 21876051).
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Figure Captions Fig. 1. Schematic diagram of material synthesis experiments. Abbreviations: IPA, isopropanol; TT, Ti3+ self-doped TiO2; HTT, higher-concentration Ti3+ self-doped TiO2; VTT, vacuum heated TT; VHTT, vacuum heated HTT; PTT, Pt-loaded TT; PVTT, Pt-loaded VTT; PVHTT, Pt-loaded VHTT. Fig. 2. XRD patterns of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. Fig. 3. TEM images of (a) Pt-TiO2 , (b) Pt-V/TiO2, (c) PTT, and (d) PVTT. Fig. 4. (a) Raman spectra of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) UV-vis diffused reflectance spectra of catalysts. (c, d) EPR spectra and (e) Ti 2p XPS spectra of photocatalysts. Fig. 5. (a) O 1s, and (c) Pt 4f XPS spectra of photocatalysts; Histogram of the ratio of O species (b) and Pt species (d). Fig. 6. The mechanism diagram of defect migration on PTT. Fig. 7. (a) Photocurrent responses of Pt-TiO2, Pt-V/TiO2, PTT, and PVTT. (b) PL spectra under 320 nm excitation for the samples. (c) Photocatalytic degradation curves of AO7 under visible light irradiation. (d) Histogram of the photocatalytic activity rate constants (k).
Author Contribution Statement Huan
Qiu:
Conceptualization,
Methodology,
Validation,
Investigation, Data Curation, Writing-Original Draft Xujun
Formal
Ma:
analysis,
Methodology,
Formal analysis Chunyu Sun: Validation, Investigation Bin Zhao: Writing-Review & Editing, Supervision Feng Chen: Conceptualization, Resources, Writing-Review & Editing, Project administration, Funding acquisition
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Highlights
Bulk Ovs would diffuse to the surface under vacuum annealing.
Defect migration improves the concentration of surface Ovs.
Ovs enriched surface of Pt/TiO2 increases the proportion of active Pt species.