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Premixed Stagnation Flame Synthesized TiO2 Nanoparticles with Mixed Phases for Efficient Photocatalytic Hydrogen Generation Shuyang Wu, Weijing Wang, Wenguang Tu, Shengming Yin, Yuan Sheng, Manoel Manuputty, Markus Kraft, and Rong Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03142 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Sustainable Chemistry & Engineering

Premixed Stagnation Flame Synthesized TiO2 Nanoparticles with Mixed Phases for Efficient Photocatalytic Hydrogen Generation

Shuyang Wu,†, ‡ Weijing Wang,†, ‡ Wenguang Tu,† Shengming Yin,† Yuan Sheng,† , ‡ Manoel Y. Manuputty, ‡, § Markus Kraft, ‡, §, * and Rong Xu†, ‡, * †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore

‡

CARES, National Research Foundation, CREATE Tower 1 Create Way, 138602, Singapore §

Department of Chemical Engineering and Biotechnology, University of Cambridge, West Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, United Kingdom

Corresponding Author

* E-mail: [email protected] (M.K.)

* E-mail: [email protected] (R.X.)

ACS Paragon Plus Environment

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ABSTRACT: Mixed-phase TiO2 nanoparticles (10-20 nm) composed of rutile, anatase and srilankite phases are prepared through a one-step flame synthesis method. The phase composition of TiO2 nanoparticles can be easily tuned by changing the flame conditions such as the gas flow rates of the fuel, oxygen and argon carrier. The optimized anatase/rutile/srilankite TiO2 sample with as low as 0.1 wt% of Pt co-catalyst exhibits remarkable photocatalytic H2 generation rate of 21.9 mmol h-1 g-1 and an apparent quantum efficiency (AQE) of 39.4% at 360 nm, higher than those of anatase/rutile or anatase TiO2. The less studied srilankite phase is first time investigated for photocatalytic H2 generation. It is revealed that the relatively low content of srilankite phase in TiO2 could efficiently promote charge separation and transportation. It is remarkable that compared to the commercial P25 TiO2, the flame-made TiO2 significantly improves Pt reduction and dispersion owing to the oxygen vacancies and surface defects. The optimized TiO2 sample with surface defects facilitates the deposition of ultra-small Pt nanoclusters of around 0.63 nm and stabilizes the low valence state of Pt0, leading to efficient utilization of noble metal and remarkable enhancement of H2 generation rate.

Keywords: TiO2, flame synthesis, photocatalysis, hetero-phase junctions, H2 generation INTRODUCTION Energy shortage and environment deterioration have become a global concern over the past few decades. With the depletion of limited fossil fuels, numerous endeavours and interests have been focused on the exploitation of green alternative sources to address the energy crisis. Sunlight and water are abundant natural resources, which can be utilized to produce renewable energy in the form of H2 through water splitting. Titanium dioxide (TiO2) has been one of the most widely used semiconductors for photocatalytic processes such as CO2 reduction,1-3 water


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splitting for H2 generation,4-6 and dye degradation7, 8 due to its excellent photocatalytic activity, good stability and environmental friendliness. Although it has been studied for decades, it is still one of the benchmark materials for photocatalytic reactions. However, besides the limited visible light absorption capability, the other key issue that restricts the photocatalytic performance of TiO2 is the poor charge separation efficiency, which is mainly determined by the intrinsic properties of TiO2. Fabricating phase junctions in a single semiconductor material has been proven an effective way to improve charge separation in photocatalysis.9, 10 Generally, TiO2 can be present in four phases: anatase, rutile, brookite and srilankite. Anatase is commonly regarded as the most active phase for photocatalytic applications due to its lower charge recombination rate11, 12 while rutile phase is most thermodynamically stable.13 However, compared with the pure phase, mixed phases with hetero-phase junctions such as anatase-rutile14 and brookite-anatase15 exhibit better photocatalytic performances due to the efficient charge transfer caused by the band alignment. Commercial Degussa P25 (~80% anatase, ~20% rutile) is one of the most commonly used mixed-phase TiO2 for photocatalytic applications.16, 17 Apart from the commonly known anatase, rutile and brookite, the orthorhombic srilankite phase (space group: Pbcn) is believed to be the transitional phase between anatase and rutile under high pressure.18,

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It has been rarely

investigated for photocatalytic applications, except a few studies reporting that srilankite phase has inferior photocatalytic performance in dye degradation compared to anatase phase due to its more disordered and amorphous structure.20, 21 However, surface defects associated with such features might be beneficial towards anchoring single sites or small clusters of metal catalysts, as demonstrated in some recent studies.22-24 For example, Maicu et al. reported that the sulfate pretreatment of TiO2 support can generate surface defects and influence both the level of


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distribution and the size of deposited noble-metal (Pt, Pd, Au) nanoparticles.23 Pan et al. prepared noble-metal nanoparticles with an average size of 2 nm evenly dispersed on defective TiO2 with oxygen vacancies.24 Therefore, there is great room to further explore the effect of surface defects on the dispersion of metal catalysts, such as the expensive Pt catalyst for H2 evolution. Flame synthesis is a continuous one-step and solvent-free process with high throughput and less waste produced.25, 26 It accounts for the fabrication of various metal oxides (e.g., ZnO, Al2O3, ZrO2, WO3, etc.), carbon black and composite materials for industrial applications, such as sewage treatment, toxic gas sensing and solar energy conversion.27-29 The production rate can be easily several grams per hour in lab-scale burners and around one kilogram per hour in industrial-scale burners.30 The products need no post treatment and can be directly used for different applications. In this study, we prepared TiO2 samples consisting of three mixed phases through the one-step flame stabilized on a rotation surface (FSRS) method.27, 31-33 In a recent study by Manuputty et al., it has been newly discovered that the metastable srilankite (TiO2-II) phase can be successfully synthesized by flame synthesis at ambient pressure under oxygen-lean condition.34 In the FSRS method, a laminar one-dimensional premixed flame is stabilized by the aerodynamic gas flow in the middle of the nozzle and rotating disk. In a typical run, the carrier gas, gaseous metal precursor, oxygen and fuel are preheated to the same temperature before entering the nozzle and kept at constant flow rates. The temperature of the flame is typically above 2000 K and the temperature of the rotating surface is maintained at around 350 K due to the convective heat transfer with surrounding air. The large temperature difference generates a strong thermophoretic force to push the nanoparticles towards the cold disk. Due to the rotation, particles form a thin film on the substrates placed on the disk. The precursor vapor (e.g. titanium tetraisopropoxide) at the flame position is rapidly oxidized to TiO2 in a liquid-like droplet state


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due to the high temperature which subsequently undergoes the process of nucleation and coagulation to form TiO2 nanoparticles. The short growth time within a few milliseconds in the flame limits the agglomeration of primary particles and the chilled rotating surface prevents the particles from growing into larger sizes. The nanoparticles produced by this one-dimensional flame method can thus preserve the particle sizes within a narrow range and the size in the range of 5 nm to 20 nm can be directly controlled by the concentration of the precursor.35 In this work, four types of TiO2 samples which are anatase (A), anatase/rutile (A/R), rutile/srilankite (R/S), anatase/rutile/srilankite (A/R/S) were successfully synthesized by one-step flame synthesis method. The phase composition can be easily tuned by varying the flame condition. The optimized three-phase A/R/S sample exhibits the highest photocatalytic H2 evolution rate followed by the two-phase and then single phase TiO2. The role of srilankite in the mixed phases for photocatalytic H2 production was studied for the first time. It is revealed that the relatively low content of srilankite can promote photogenerated electron-hole separation and transportation. The surface defects in the amorphous area facilitate the photo-reduction of Pt precursor to Pt0 and promote the dispersion of Pt nanoclusters of around 0.63 nm on the surface of TiO2 nanoparticles with a very low loading of 0.1 wt%. Thus, the superior dispersion and reduced size of metallic Pt clusters contribute to the efficient utilization of noble metal and enhancement of the photocatalytic activity. EXPERIMENTAL SECTION Fabrication of mixed-phase TiO2 nanoparticles with flame synthesis method. As shown in Figure 1, the burner is an aerodynamically designed nozzle with an exit outlet of 1 cm in diameter. The mixed gas stream flows out of the nozzle and forms a laminar flat flame impacting against the rotating disk. The disk with a diameter of 30.5 cm is positioned 1.4 cm below the


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nozzle outlet, acting as a stagnation surface to stabilize the flame. The round flame with a diameter of 2 cm is stabilized by flow divergence and stretch force at the position of 0.7 cm from the exit outlet. Argon was used as carrier gas for the vaporized reactants before entering the nozzle. The reactant, titanium tetraisopropoxide (TTIP, Aldrich, 97%) was heated at 150 oC for vaporization and injected into the carrier gas using a syringe controlled by a syringe pump (ColeParmer 100). The volumetric flow rate of TTIP was maintained at 5 mL/h. Argon carried with vaporized TTIP, C2H4, O2 and N2 were all held at 150 oC. The mixed gas was then injected into the nozzle and ignited to form the flame stabilized on a rotating disk. The flow rate of each gas was controlled individually. Through adjusting the flow rate of each gas, the equivalence ratios (φ) of the flame can be varied systematically, as shown in Table S1. The equivalence ratio is defined as the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio.36 If φ = 1, the flame is in the stoichiometric condition. If φ < 1, the fuel-lean flame is combusted with excess air, and if φ > 1, the fuel-rich flame undergoes incomplete combustion. The sample name is labeled as TiO2-φ. The flame was stabilized and kept in a regular shape by the N2 gas in the outer layer of concentric nozzle. The rotating disk was motivated by a stepper motor (Applied Motion Products HT23-554) with the speed controller. The spin rate for the disk was held constant at 300 rpm. Several slots were machined on the disk to place the substrates from which the flame-made nanoparticles can be collected. With the disk rotating, a thin continuous film was deposited on the substrates. At the end of 20 min deposition time, the TiO2 nanoparticles were scraped off from the substrates and directly used as the photocatalyst without post treatment.


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Figure 1. Schematic of the flame stabilized on a rotation surface (FSRS) equipment. Characterization. Field emission scanning electron microscopy (FESEM) was conducted on JEOL JSM 6701F microscope to analyze the particle size of TiO2. The morphology, lattice fringe, particle size measurement and high-angle annular dark-field (HAADF) images were obtained using a scanning transmission electron microscope (JEOL JEM-2100F TEM/STEM and JEOL JEM-ARM200F equipped with a Cs corrector STEM). The crystal phase of as-prepared samples was obtained by an X-ray diffractometer (XRD, Bruker D2 Phaser) with Cu Kα (λ=1.54184 Å) radiation at 30 kV and 10 mA. X-ray photoelectron spectroscopy (XPS) analysis was acquired by Kratos AXIS Ultra DLD spectrometer. The binding energy was calibrated by C 1s peak at 284.9


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eV. The UV-visible absorption spectra were collected on a UV-2450 spectrophotometer (Shimadazu). The Brunauer–Emmett–Teller (BET) surface area of TiO2 samples was analyzed by N2 adsorption and desorption at 77 K using Quantachrome Autosorb-6 sorption system. Electron paramagnetic resonance (EPR) was performed on a Bruker EMX-10/12 EPR spectrometer at 9.363 GHz. The time-resolved photoluminescence (PL) spectra was collected on an Edinburgh Instruments FLS920 PL spectrometer with excitation at 375 nm. Photoelectrochemical measurement. The catalyst ink was prepared by dispersing 5 mg of TiO2 and 20 µL of Nafion solution (5 wt %) in 1 mL of ethanol solution. The mixture was ultrasonically dispersed to form a homogeneous solution. To prepare the working electrode, 40 µL of the ink was deposited on the ITO conductive glass with an area of 0.25 cm2 and then dried in a 40 oC oven. Photoelectrochemical measurement was performed in a three-electrode configuration system consisting of the ITO working electrode, Pt as the counter electrode, and Ag/AgCl as the reference electrode. The Na2SO4 (0.5 M) solution was used as the electrolyte. The photocurrent generated under the illumination of 300 W Xenon lamp was measured at an applied potential of 0.6 V versus Ag/AgCl. Electron impedance spectroscopy (EIS) analysis was conducted at an applied potential of 0 V and frequency range of 0.1 Hz-105 Hz under illumination. Photocatalytic measurement. The photocatalytic H2 evolution reaction was performed in a 300 mL Pyrex glass vessel connected to a closed gas circulation system. In a typical photocatalytic experiment, 20 mg of the photocatalyst was dispersed in 120 mL aqueous methanol solution (25 vol %), where methanol served as a sacrificial reagent to consume photogenerated holes. Next, 20 µL of 1 mg Pt/ml H2PtCl6 solution corresponding to 0.1 wt% Pt was added and subsequently in situ photodeposited on the surface of photocatalysts. The reaction


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cell was maintained at 18 °C by the external cooling water circulation. The reaction system was then sealed up, vacuumed and purged with argon gas for several times to remove residual air and finally purged with argon gas to reach about 30 Torr. A 300 W Xenon lamp (Newport) was used as the light source irradiating from the top. The generated H2 gas was analyzed by an online gas chromatograph (GC, Agilent 6890N, 5 Å molecular sieve column, TCD detector, argon as carrier gas). The apparent quantum efficiency was measured by adding a band-pass filter (Newport) centered at 360 nm at the same reaction condition. The AQE was calculated according to eq 1: AQE =

2×(the number of evolved H2 molecules) × 100% the number of incident photons

(1)

Computational method. To investigate the effect of flame condition on the crystal phase equilibrium in TiO2 nanoparticles, the adiabatic flame temperature and equilibrium O2 mole fraction were calculated using the software package kinetics37 and the USC mechanism for H2/CO/C1-C4 chemistry proposed by Wang et al.38 RESULTS AND DISCUSSION


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Figure 2. XRD patterns of as-prepared TiO2 samples and commercial P25 TiO2. Inset: enlarged patterns between 2θ value of 20o-50o. Physicochemical properties of flame synthesized TiO2. XRD analysis was used to investigate the phase composition of TiO2 synthesized with different equivalence ratios and the commercial Degussa P25 as shown in Figure 2. The characteristic peaks of anatase TiO2 appear at 25.3o and 48.4o which can be attributed to (101) and (200) planes, respectively (JCPDS, 010562). Peaks at 27.5o, 36.0o, 41.2o can be ascribed to the (110), (101) and (111) planes of rutile TiO2 (JCPDS, 01-1292). The characteristic peaks of srilankite TiO2 was observed at 25.4o, 31.5o,


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42.7o, which correspond to (110), (111) and (121) planes, respectively (JCPDS, 21-1236). When oxygen is in excess (φ = 0.38), the XRD pattern of sample TiO2-0.38 shows that TiO2 with predominant anatase phase is formed with good crystallinity. After the flame condition was switched to oxygen lean with φ at 1.8, TiO2-1.8 sample consists of both rutile and srilankite phases while anatase phase cannot be observed. Figure 2 shows that the srilankite (111) plane at 31.5o can be clearly seen. Since the characteristic peaks of anatase and srilankite phases at 25.3o and 25.4o are very close to each other, the peak of anatase (200) plane at 48.4o is used to identify the existence of anatase. No anatase phase can be observed for TiO2-1.8, TiO2-2.0 and TiO2-2.1 samples. The appearance of srilankite phase is accompanied by the reduction of crystallinity indicated by the weak and broad peaks compared with those of anatase phase in TiO2-0.38. As the value of φ is increased from 1.8 to 2.1, the percentage of srilankite phase reduces and that of rutile phase increases. This is accompanied by the improvement of crystallinity. With φ from 2.19 to 2.7, the anatase phase re-appears and its percentage increases with the value of φ, while the percentage of srilankite phase continues to drop in this range. The phase percentage of TiO22.30 was calculated to be 70.2% of rutile, 17.5% of anatase and 12.3% of srilankite using the matrix-flushing method with α-Al2O3 as the internal standard.39 At φ of 2.7, no srilankite phase can be observed. In summary, TiO2 with predominant anatase phase can be formed under fuel lean condition with φ value less than one. Under the fuel rich condition with φ from 1.8 to 2.7, two-phase and three-phase TiO2 can be synthesized with transition from rutile-srilankite (φ from 1.8 to 2.1) to rutile-srilankite-anatase (φ from 2.19 to 2.47), followed by rutile-anatase (φ = 2.7) phases. Based on the recent studies on the flame synthesis, the main factor determining the TiO2 phase composition is found to be oxygen availability.31, 34, 40, 41 Thermodynamic analysis by Wang and


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co-workers reveals that oxygen desorption at high temperature critically influences the surface free energy, which plays an important role in crystal phase equilibrium of TiO2.40 They found that in an oxygen-rich environment, anatase is the predominant and stable phase while rutile is favored in the oxygen-lean condition (φ: 1.13-1.33). The phase composition results obtained in this study further verify such a finding. Similar to the findings from Manuputty et al.,34 it is further observed in this work that in a more oxygen-lean environment with φ from 1.8 to 2.47 and the calculated equilibrium O2 mole fraction a few tens of magnitude lower than those reported elsewhere (Table S2), a small percentage of metastable srilankite phase is co-present with rutile phase in the final products. This confirms that srilankite is a transition phase from anatase to rutile under the oxygen-lean condition. It is interesting to discover that under deep oxygen-lean condition where the concentration of O2 is much lower and the presence of other burned gases (CO, H2, etc.) is probably much more significant, this phase can be frozen and present in the final product. The reappearance of anatase with φ from 2.19 to 2.7 is most likely due to the reduced flame temperature (Table S2) since anatase is a kinetically preferred phase forming at lower temperatures.42, 43


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Figure 3. Morphology and hetero-phase junctions of TiO2-2.3. (a) FESEM image. (b) TEM image. (c) HRTEM image of hetero-phase junctions. A: anatase, R: rutile, and S: srilankite. (d) HRTEM image of surface defects. The morphology of flame-made TiO2-2.3 was analyzed by FESEM and TEM. The FESEM image (Figure 3a) shows that TiO2-2.3 consists of rather uniformly sized nanoparticles. Based on the TEM analysis, the average diameter of TiO2 nanoparticles was measured to be around 15 nm (Figure 3b). The high resolution TEM (HRTEM) image of TiO2-2.3 is shown in Figure 3c. The lattice spacing of 0.322 nm, 0.235 nm, and 0.284 nm can be attributed to (110) plane of rutile TiO2, (001) plane of anatase TiO2, and (111) plane of srilankite TiO2, respectively, which is consistent with the XRD result. It is evident that the three phases are co-present and randomly


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distributed, with junctions observed at the phase boundaries. The high temperature of the flame (above 2000 K) exceeds the melting point of TiO2 nanoparticles which is reported to be around 1800 K40 and enables the particles to experience the early nucleation stage in the form of TiO2 liquid droplets. Coalesce and solidification of the droplets at lower temperature results in close interfacial contact between different phases.31, 40 Moreover, Figure 3d shows that some TiO2 crystalline nanoparticles are surrounded by a thin amorphous layer which could be defects rich. The surface area of as-fabricated products was measured by nitrogen physisorption method. As shown in Table S2, there is no clear trend in surface area with the change of φ, which is due to similar particle size and texture present in all samples. The BET surface area of all products is in the range of 120 to 160 m2/g which is much larger compared with P25 TiO2 (50 m2/g). Such structural features are expected to influence the light absorption, deposition of co-catalyst and the photocatalytic performances.


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Figure 4. (a) UV-vis DRS spectra of flame-made TiO2 with different equivalence ratio and P25 TiO2. (b) EPR spectra of TiO2 with different equivalence ratio measured at 100 K, and XPS spectra of (c) Ti 2p and (d) O 1s for flame-made TiO2-2.3. UV-Vis diffuse reflectance spectroscopy (DRS) was used to study the light absorption properties of the samples. Figure 4a shows absorption spectra of flame-made TiO2 with different equivalence ratios and P25 TiO2. The absorption edge of P25 and flame-made TiO2-0.38 is at around 390 nm which corresponds to a bandgap energy of 3.2 eV for these two anatase dominated samples. When the φ value is in the range of 1.8 to 2.7, the rutile phase predominates


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in the product. As a result, the absorption edge of these samples red shifts to around 410 nm due to the smaller bandgap energy of rutile phase (3.0 eV) than that of anatase phase. Since the phase compositions of TiO2 with φ values from 1.8 to 2.7 are not significantly different based on the XRD results, the bandgap edges of these samples are at the similar positions. Electron paramagnetic spectroscopy (EPR) was performed to determine the presence of Ti3+. In Figure 4b, EPR signal was not observed for TiO2-0.38 due to the use of excess oxygen during flame synthesis. Ti3+ is formed in the reductive flame condition when φ is larger than 1. The EPR signal (g = 1.93) observed for TiO2 with φ from 2.3 to 2.7 can be attributed to the existence of Ti3+.44, 45 Under such an oxygen deficient condition, it is not surprising that Ti4+ is partially reduced to Ti3+ and surface oxygen vacancy is formed. Furthermore, XPS was applied to analyze the chemical valence states of the samples. Consistent observations were made from the high-resolution XPS spectra of Ti 2p and O 1s, respectively, as shown in Figure 4c,d. In the spectrum of Ti 2p, the two main peaks at 458.7 eV and 464.4 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 of Ti4+ oxidation state. The presence of Ti3+ is evidenced by the two smaller peaks at 457.6 eV and 463.3 eV. Correspondingly, oxygen vacancies are present to maintain the charge equilibrium. As shown in Figure 4d, the spectrum of O 1s can be deconvoluted into three peaks located at 529.7 eV, 531.8 eV, 533.4 eV, which are attributed to bulk Ti-O lattice bond,46 O-atoms in the vicinity of an O-vacancy,47-49 and H-O-H bond from physically adsorbed water.50


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Figure 5. (a) Photocatalytic H2 generation after 6 h over flame synthesized TiO2 with different equivalence ratios and P25 TiO2 under light irradiation (20 mg of TiO2, 0.1 wt% Pt, 25 vol% methanol, 300 W Xe lamp). (b) Stability test of TiO2-2.3 under the same condition. (c) Transient photocurrent density and (d) Nyquist plots of the EIS spectra of flame synthesized TiO2 and P25 TiO2 under a 300 W Xe lamp illumination. Photocatalytic activity. Figure 5a shows the amount of photocatalytic H2 evolved in 6 h, with 0.1 wt% of Pt co-catalyst loaded on TiO2. Since the specific surface area of all flame synthesized TiO2 samples does not vary significantly (Table S2), the difference in the activity should arise


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from other properties such as phase composition and the state of defects. Among all the samples, TiO2 comprised of all the three phases (TiO2-2.19, TiO2-2.3, TiO2-2.47) of A/R/S are more active than the other samples. The highest activity is displayed by TiO2-2.3 with 21.9 mmol h-1 g1

of H2 generated and AQE of 39.4% at 360 nm. On the other hand, TiO2-0.38 which consists of

anatase phase alone exhibits the lowest photocatalytic activity. This could be due to the larger bandgap of anatase phase and thus less efficient light absorption (Figure 4a), and the poor charge separation efficiency which is revealed by the photoelectrochemical and impedance studies (Figure 5c). Enhanced photocatalytic performance is observed when the equivalence ratio increases from 0.38 to 1.8 till 2.3. In these samples prepared under oxygen lean condition, srilankite phase is co-present with rutile phase, and all three phases are present in TiO2-2.19 and TiO2-2.3. It is believed that the heterojunction between different phases in these samples plays an important role in enhancing the charge transfer efficiency. The density functional theory (DFT) calculation of mixed-phase TiO2 reveals that a thin layer of srilankite formed between anatase and rutile can effectively facilitate single-way migration of photogenerated holes from anatase to rutile but suppresses the flow of electrons in the reverse direction, which enhance the charge separation efficiency.51 In addition, Ti3+ species (Figure 4b) and surface defects in the amorphous layer (Figure 3d) lead to efficient reduction of platinum and ultrafine dispersion Pt nanoclusters as indicated by XPS and TEM analysis results to be discussed shortly (Figures 6 and 7). It is observed that the photocatalytic performance decreases with φ further increased from 2.3 to 2.7. This could be due to excess oxygen vacancies as revealed from the increased intensity of EPR signal (Figure 4b) which might act as charge recombination centers. It is also noted that the activities of flame synthesized TiO2 samples except TiO2-0.38 are all higher than that of P25 TiO2 under the same reaction condition.


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The stability of TiO2-2.3 (0.1 wt% Pt) photocatalyst was evaluated by three consecutive runs as shown in Figure 5b. The photoreactor system was evacuated after 12 h before the next run. After 36 h reaction, the photocatalytic performance decreased by only 7%, which is mainly due to the consumption and evaporation of methanol sacrificial reagent in prolonged reaction time. The turnover number (TON) calculated at 36 h with respect to Pt loaded is more than 13500. To further investigate the charge separation efficiency and transfer resistance, the measurement of transient photocurrent density and EIS study were carried out with the photoelectrochemical settings. Generally, higher photocurrent density indicates superior charge separation ability.52 Figure 5c shows the photocurrent for TiO2 with different phase composition. Consistent with the photocatalytic performance, TiO2-2.3 comprised of all the three phases (A/R/S) exhibits the most remarkable photocurrent among the flame-made samples and P25 TiO2. The two-phase samples such as TiO2-2.7 (A/R), TiO2-1.8 (R/S) and P25 TiO2 (A/R) present inferior performance, but still better than TiO2-0.38 with anatase phase alone. Such observations indicate that the heterojunctions associated with the right balance of the three phases effectively promote the electron-hole separation. The transient time-resolved photoluminescence (PL) spectra (Figure S1) shows TiO2-2.3 (A/R/S) exhibits longer fluorescence decay time than the two-phase TiO2 samples (Table S3). The fluorescence decay is attributed to the recombination of trapped electrons and holes. The longer decay time of TiO2-2.3 suggests the heterojunctions formed among the three phases can significantly suppress the charge recombination and prolong the lifetime of the charges. The EIS spectra were further obtained under irradiation to determine the resistance of charge transportation. Smaller arc radius in the spectra suggests lower charge migration resistance, efficient electron-hole separation and faster charge transfer rate at the interface of photoelectrode and electrolyte.53 As shown in Figure 5d, TiO2-2.3 (A/R/S) has the


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smaller arc radius compared with other samples, which suggests the better electron-hole separation and lower interfacial charge transfer resistance. The results of photocurrent density, transient PL spectra and EIS spectra all reveal the similar trend with photocatalytic performance.

Figure 6. TEM and HAADF images of (a, b) 0.1 wt% Pt loaded TiO2-2.3, and (c, d) 0.1 wt% Pt loaded P25 TiO2.


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State of Pt co-catalyst. The property of cocatalysts critically affects the photocatalytic activity for H2 evolution. In general, the deposition of noble-metal cocatalysts on the semiconductor surface can significantly improve the electron-hole separation, extend the lifetime of excited charge carriers and serve as active sites, thus enhancing the photocatalytic performance.54, 55 On the other hand, the properties of the semiconductor should influence the state of the cocatalysts loaded on its surface. In this study, it was found that the flame-made TiO2 containing phase heterojunctions, Ti3+ and oxygen defects effectively promote the reduction of platinum species and anchoring of small clusters of Pt by simple in situ photodeposition. Remarkably, the flamemade TiO2 with an ultra-low Pt loading of 0.1 wt% outperforms the commercial benchmark P25 TiO2 with the same Pt loading (Figure 5a). As shown in the TEM and HAADF images (Figure 6a,b), Pt clusters with an average size of 0.63 nm (Figure 6b) are evenly dispersed on TiO2-2.3, and even some single Pt atoms can be identified (Figure S2). In contrast, Pt nanoparticles are poorly dispersed on P25 TiO2 and the particle sizes are generally larger than 2 nm with much greater polydispersity (Figure 6c,d). Such a distinct contrast reveals that flame-made TiO2 possessing rich defects and phase junctions is advantageous in more efficient utilization of expensive metals like Pt. In fact, when the loading of Pt was increased 10 times to 1 wt%, the activity of the photocatalyst is almost the same as that when the loading is 0.1 wt% (Figure S3). Such seemingly unexpected results can be easily explained by the properties of Pt revealed by TEM, HAADF, and XPS analyses. With 1 wt% Pt loaded on TiO2-2.3, the size of Pt nanoparticles is obviously larger with an average size of 2.5 nm based on HAADF image (Figure S4a,b) and the dispersion is poorer. At higher concentration of Pt precursor in the solution, the kinetic growth of Pt nanoparticles predominates the thermodynamic control of the cluster size by the surface chemistry. In contrast, no obvious change of Pt size on P25 TiO2 was


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observed with different loadings because of good crystallinity and lack of surface defects (Figure 6c,d and Figure S4c,d). Similarly, TiO2-0.38 of predominate anatase phase and relatively better crystallinity results in larger Pt clusters of 2.4 nm in average size (Figure S5a). Other flame-made TiO2 samples prepared under oxygen lean condition also lead to smaller Pt cluster sizes due to their amorphous feature, surface defects and hetero-phase junctions (Figure S5b,c,d). Correspondingly, these photocatalysts all have higher activities than P25 TiO2 to various extents (Figure 5). Furthermore, XPS analysis was used to investigate the valence state of Pt on TiO2 after 6 h of photocatalytic reaction. As shown in Table 1 and Figure 7, the percentage of metallic Pt0 in 0.1 wt% Pt loaded TiO2-2.3 is 76.2%, considerably higher than that of P25 TiO2 (45.5%) at the same loading. XPS peak fitting shows the remaining platinum in both samples is the partially reduced Pt2+ species. Since metallic Pt0 serves as the active site for H2 evolution, TiO2-2.3 (0.1 wt% Pt) exhibits higher activity than P25 TiO2 (0.1 wt% Pt). It is noticed that the Al 2p peak at 75.9 eV is obvious in the spectrum of Pt 4f with 0.1 wt% Pt loading, probably due to the presence of minute amount of Al element impurities in the TTIP precursor, which can be verified by the XPS spectrum of TiO2-2.3 and P25 TiO2 without Pt loading (Figure S6). The oxidation state of Pt on other flame made samples are shown in Table S4. Significant enhancement of Pt0 percentage was observed on TiO2-2.3 compared with other TiO2 samples with the same Pt loading (Table S4). Therefore, the property of TiO2 support plays very important role in the state of Pt. It was found that the percentage of Pt0 reduces when Pt loading was increased to 1 wt% in both flame-made and P25 TiO2 (Figure S7). Furthermore, there is significant percentage of high valence Pt4+ in both samples. Such results demonstrate the utilization of Pt is inefficient at higher loadings, due to the formation of larger Pt nanoparticles and the presence high valance Pt species. Hence, this


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study has presented important new insights into the strategies for enhancing the utilization of noble metals by tuning the properties of metal oxide substrates.

Figure 7. XPS results of Pt 4f, in 0.1 wt% Pt loaded (a) TiO2-2.3 and (b) P25 TiO2. Table 1. Percentage of Pt species on TiO2-2.3 and P25 observed from XPS spectra. Pt0 (%)

Pt2+ (%)

Pt4+ (%)

TiO2-2.3 0.1% Pt

76.2

23.8

0

TiO2-2.3 1% Pt

42.0

37.7

20.3

P25 0.1% Pt

45.5

54.5

0

P25 1% Pt

38.8

31.4

29.8

CONCLUSIONS In summary, we have successfully synthesized TiO2 nanoparticles with mixed phases through the facile one-step flame synthesis method. The phase composition can be tuned by changing the flame condition. The optimal anatase/srilankite/rutile TiO2 sample exhibits superior photocatalytic performance for H2 evolution. With only 0.1 wt% Pt cocatalyst loaded, a high AQE of 39.4% at 360 nm was obtained. The as-formed three-phase TiO2 nanoparticles of 10-15


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nm in diameter contains multiple heterojunctions that promote charge separation and transfer. In addition, the nanoparticles were found rich in defects with Ti3+ species and surface amorphous layers. The combination of these properties in flame-made TiO2 facilitates the efficient reduction and excellent dispersion of Pt cocatalyst with an ultra-small average size of around 0.63 nm. This work has demonstrated that vapor fed flame synthesis is a promising method for the fabrication of metal oxide nanoparticles with tunable properties. Remarkably, through the photocatalytic H2 evolution study, it is revealed that flame-made TiO2 presents great opportunities to control the state of Pt catalyst, leading to optimum utilization of expensive noble metal. Such a strategy might be extended to a wide range of metal oxide support and active metal catalyst systems for important industrial applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org Experimental details about flame synthesis and additional figures and tables (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (M.K.) * E-mail: [email protected] (R.X.) ORCID Shuyang Wu: 0000-0003-4398-3417


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Wenguang Tu: 0000-0002-0800-9777 Markus Kraft: 0000-0002-4293-8924 Rong Xu: 0000-0002-7562-2627 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge the ﬁnancial support from National Research Foundation through the Cambridge Centre for Carbon Reduction in Chemical Technology (C4T) CREATE Programme, and Ministry of Education through AcRF Tier 1 grant (RG116/16).


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For Table of Contents Use Only Synopsis Mixed-phase TiO2 nanoparticles prepared by the continuous one-step flame synthesis method exhibit excellent performance of renewable energy production. TOC figure


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Premixed Stagnation Flame Synthesized TiO2 ... - ACS Publications

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