Charge Carrier Dynamics in TiO2 Mesocrystals ... - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Charge Carrier Dynamics in TiO Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation Ossama Elbanna, Mamoru Fujitsuka, Sooyeon Kim, and Tetsuro Majima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04026 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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The Journal of Physical Chemistry

Charge Carrier Dynamics in TiO2 Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation Ossama Elbanna, Mamoru Fujitsuka, Sooyeon Kim, and Tetsuro Majima*

* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

*Author to whom correspond should be addressed. E-MAIL: [email protected] (T.M.)

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ABSTRACT: There is a great concern about black TiO2 prepared by H2 treatment due to its ability to enhance light harvesting of TiO2. Black TiO2 shows different photocatalytic activity compared to white TiO2. However, the mechanism of photocatalytic reaction has not been clearly understood. Here, femtosecond time resolved

diffuse

reflectance

spectroscopy

(fs-TRDR)

and

single

particle

photoluminescence measurements were applied to gain better understanding about the relation between oxygen vacancy, charge transfer, lifetime of photogenerated charge and photocatalytic activity. We prepared reduced TiO2 mesocrystals (R-TMC) through simple solid state chemical reduction at moderate temperature 350 oC. R-TMC has nearly two times higher photocatalytic activity for H2 production under solar light irradiation. The presence of oxygen vacancies and Ti3+ was studied by electro paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy. This work confirms the findings of previous studies that enhancement of light absorption by formation of surface defects does not always lead to high photocatalytic activity. Femtosecond diffuse reflectance spectra (fs-TRDR) reveals that proper concentration of oxygen vacancies enhances the charge separation of the photogenerated carriers leading to high photocatalytic activity.

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1. INTRODUCTION TiO2 as the most widely used semiconductor photocatalyst has received higher interest because of their potential application for the degradation of organic pollutant, H2 evolution, and CO2 reduction.1,2 However, one of the main drawbacks of TiO2 is its low utilization of solar energy due to its large band gap. Moreover, the photocatalytic efficiency is limited by rapid recombination of photogenerated electrons and holes.3,4 To overcome above obstacles, several approaches have been used to modify TiO2 including doping of metal ions or non-metal elements in TiO2 lattice5,6 and coupling with narrow band gap semiconductors or carrier charge mediator such as graphene.7,8 Mao and his coworkers reported a breakthrough method to produce disordered surface layers of highly crystalized TiO2 nanoparticles by surface hydrogenation of TiO2 nano crystals providing a new way to increase the light absorption.9 To date, many papers were reported about the preparation reduced colored TiO2.10,11 However, to the best of our knowledge, there are no papers about the preparation of reduced TiO2 mesocrystals (R-TMC). TMC is a novel class of TiO2 which has ordered superstructure of TiO2 nano crystals leading to efficient charge separation.12 The reduced TiO2 exhibited high photocatalytic activity for H2 evolution under solar light irradiation.13,14,15 Naldoni et al. investigated the nature and location of defects formed in the lattice of black TiO2 nanoparticles.16 Also, Zhang et al. studied the structural, optical, electronic, and ultrafast properties of hydrogen treated TiO2 nanowires.17 In addition, Cowan et al. reported in details the factors determining the solar-to-hydrogen (STH) efficiencies for oxygen-deficient rutile TiO2 nanowires array.18 It has been demonstrated that high photocatalytic activity can be attributed to the presence of localized midgap states (oxygen vacancies and Ti3+) separated from the conduction band (CB) which prohibit electron hole recombination. Photocatalytic and photoelectrochemical properties have been investigated in terms of defects and oxygen vacancies.19-20 Defects can be considered as a recombination and trapping center of photogenerated charge carriers leading to lower photocatalytic activity.21 On the other hand, some researchers believed that the presence of defects leads to efficient charge 3



separation and transportation which retards the recombination of photogenerated charge carriers.22 In this paper, we proposed a facile method to prepare R-TMC using solid state reaction of NaBH4 and evaluate the photocatalytic activity of TMC and RTMC for H2 production under solar light irradiation. To clarify the effects of defects on the dynamics of photogenerated charge carriers, we estimated the photocatalytic activities of R-TMC with varied concentration of oxygen vacancies. Then, we compared the activity with the population and lifetime of photogenerated charges estimated using single particle photoluminescence spectroscopy and time resolved absorption spectroscopy (TAS). The TAS spectroscopy was used to predict the role of trap state on hydrogen-treated TiO2 and hydrogen treated ZnO.18,23 As a result, we demonstrated that the photocatalytic activity of R-TMC is mainly relate to oxygen vacancy concentration. 2. EXPERIMENTAL SECTION 2.1.Materials. Ammonium nitrate (NH4NO3), ammonium fluoride (NH4F), absolute ethanol, and methanol were purchased from Wako. Titanium(IV) fluoride (TiF4) and sodium borohydride (NaBH4, ≥99%) were purchased from Sigma Aldrich. All chemicals were of analytical grade and used as received. 2.2. Preparation of TMC. TMC was synthesized according to our previous study.24A precursor solution of TiF4, H2O, NH4NO3, and NH4F (molar ratio = 1:117:6.6:4) was drippled on a silicon wafer to form a fine layer. The precursor was annealed in air by a heating rate of 10 oC min-1 at 500 oC for 2 h. The obtained product was annealed at 500 o

C in oxygen atmosphere for 8 h to detach surface impurities. 2.3. Preparation of R-TMC. R-TMC was obtained by a solid state method using

NaBH4 as a reducing agent.25 Briefly, 50 mg of TMC was mixed with different weight ratios of NaBH4 and the mixture was ground thoroughly in a mortar. Then, the mixture was annealed under Ar atmosphere at 300 oC. After that, the produced colored TMC was simply washed with distilled water and ethanol for several times to remove unreacted NaBH4 and dried at 60 oC. Different colored TiO2 samples from light blue to black were obtained by controlling the wt% of NaBH4. In the present paper, the samples 4


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were denoted as R-TMC 10, R-TMC 20, R-TMC 30, R-TMC 40, and R-TMC 50 according to the weight% of NaBH4. 2.4. Preparation of Pt/TMC. Pt/TMC was synthesized using the photochemical deposition method. 0.1 g of TMC, 50 mL of Milli-Q ultrapure water, 25 mL of methanol, and H2PtCl6 (Sigma-Aldrich) were mixed together. The suspension was then irradiated by UV light for 30 min at room temperature. After that, the solution was centrifuged at 10000 rpm to obtain the solid products. After that, the products were annealed in air (20 °C min−1) at 300 °C for 30 min. 2.5. Characterization of materials. The surface morphologies were investigated by a field-emission scanning electron microscopy (FESEM, JEOL, JSM-6330FT) and a transmission electron microscopy (TEM, JEOL, JEM-2100 operated at 200 kV). The diffuse reflectance spectra were determined using a UV-visible-NIR spectrophotometer (Jasco, V-570). X-ray diffraction (XRD) patterns of the samples were gained by a smartlab system with Cu Kα radiation operated at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was performed by a JEOL JPS-9010 MC spectrometer. EPR spectra were recorded on a JEOL JES-RE2X electron spin resonance spectrometer at 77 K. 2.6. Photocatalytic H2 evolution. Before, the photocatalytic test, Pt was deposited onto TMC and R-TMC as mentioned above. Specifically, 2 mg of photocatalyst was dispersed in 5 mL of aqueous solution including 20 vol % methanol and was fasten tightly with a rubber stopper in a glass tube. The suspension of the photocatalyst was deaerated by bubbling argon for 30 min. Then, the sample was irradiated with solar light irradiation (Asahi Spectra Hal-320, 200 mW cm-2) under magnetic stirring at room temperature. The H2 evolution was detected by a Shimadzu GC-8A gas chromatograph equipped with an MS-5A column and a thermal conductivity detector. The H2 evolution reaction for each sample was recorded for 3 times. In the cycling test, the used catalyst was recovered by centrifugation and re-dispersed into 5 mL methanol-H2O solution to check its stability. To get an action spectrum, H2 evolution of catalyst was determined using the same procedure but with controlling the light irradiation wavelength by a 5



different band pass filters (Asahi Spectra Hal-320; 1 mW cm-2 ±5 nm). The apparent quantum efficiency (AQE) was estimated using the following equation: AQE = (2 ×number of H2 molecules/number of incident photons) × 100. 2.7 Photo-electrochemical measurements. Photo-electrochemical experiments (Photo current response , electrochemical impedance spectroscopy (EIS) and the Mott −Schottky plots) were performed in a three-electrode system with an electrochemical analyzer (ALS, 660B) using 0.1 M Na2SO4 as the electrolyte. A platinum (Pt) wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The photocatalyst film on a glassy carbon acted as the working electrode. Asahi Spectra Hal-320 (100 mW cm-2) with a 420 nm and 780 nm cut off filter was employed as a solar light source. 2.8. Single-Particle PL Measurements by Confocal Fluorescence Microscopy. Single-particle PL images and spectra were recorded by using an objective scanning confocal fluorescence microscope system (PicoQuant, MicroTime 200). In order to obtain PL images, the samples were excited using an oil-immersion objective lens (Olympus, UplanSApochromat, 100×, 1.4 NA) with a circular-polarized 375-nm pulsed laser (Spectra-Physics, Mai Tai HTS-W with an automated frequency doubler, Inspire Blue FAST-W; 0.8 MHz repetition rate) controlled by a PDL-800B driver (PicoQuant).Typical excitation powers for the PL measurements were 80 μW at the sample. The emission from the sample was gathered by the same objective and determined by a single-photon avalanche photodiode (Micro Photon Devices, PDM 50CT) through a dichroic beam splitter (Chroma, 405rdc) and long pass filter (Chroma, HQ430CP). For the spectroscopy, only the emission that passed through a slit entered the imaging spectrograph (Acton Research, SP-2356) that was equipped with an EMCCD camera (Princeton Instruments, ProEM). The spectra were typically integrated for 20 s. 2.9. Time-resolved diffuse reflectance spectral measurements. The femtosecond diffuse reflectance spectra (fs-TRDR) were determined by the pump and probe method using a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz) pumped by a Nd:YLF laser (Spectra-Physics, Empower 15). An optical 6


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parametric amplifier (Spectra Physics, OPA-800CF-1) was applied to produce the excitation pulse (340 nm, 2 μJ pulse-1). The white light continuum pulse, which was generated by concentrating the residual of the fundamental light on a sapphire crystal, was pointed to the sample powder coated on the glass substrate and the reflected lights were detected by a linear InGaAs array detector equipped with the polychromator (Solar, MS3504).

3. RESULTS AND DISCUSSION

Figure 1. TEM images of TMC and R-TMC-30 (a,d), HRTEM images of TMC and RTMC (b,e), and SEM images of TMC and R-TMC (c,f).

TEM and SEM images were taken to investigate the detailed information of the structures of prepared R-TMC. As shown in Figures 1a, d, low resolution TEM images confirmed that morphology (sheet structure) and thickness of TMC were not impacted by heat treatment with NaBH4. High resolution transmission electron microscopy 7



(HRTEM) of TMC (Figure 1b) exhibits characteristic lattice fringe of 0.35 nm which can be indexed to (010) plane of anatase TMC.25 After reduction, a disordered layer appears on the surface of R-TMC 30 as in Figure 1e and it displayed a core-shell structure with crystallized core and highly disordered surface. The lattice fringe of crystalline core is 0.35 nm in accordance to (010) plane of anatase TMC. TEM and HRTEM images of other R-TMC are shown in Figure S1. It can be noticed that there is a slight increase of thickness of disordered layer with the increase in wt% of NaBH4. Moreover, SEM images of R-TMC (Figures 1f and S2) retained the sheet structure of TMC (Figure 1c) indicating that the morphology of R-TMC doesn’t change after reduction with NaBH4. The XRD patterns of TMC and R-TMC samples are shown in Figure S3. TMC has diffraction peaks at 25.3o, 37.8o, 47.9o, 53.8o, 55.1o, 62.7o, 68.7o 70.0 o, and 75.0o which can be attributed to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) plane diffractions of anatase TiO2.26 R-TMC exhibits the same diffraction peaks of TMC indicating that R-TMC conserved the pure anatase phase. However, the peak intensity and the full widths at half maximum (FWHM) of main anatase peak positioned at 25.8 are gradually reduced and broaden, respectively with the increase of wt% of NaBH4 Moreover, the N2 adsorption-desorption isotherms depict typical type-IV isotherm (Figure S4) with clear hysteresis loop indicating mesoporous feature.11 The surface area of TMC, R-TMC 10, R-TMC 30, and R-TMC 50 is 62, 61, 59, and 58 m2/g, respectively, revealing that reduction treatment has no effect on the surface area. XPS spectra were measured to study the chemical state of TMC and R-TMC. As shown in Figure 2a, the high resolution XPS spectra of Ti 2P for TMC has two major peaks at 459.2 eV and 465.0 eV which are due to Ti4+-O in TiO2. After reduction with small amount of NaBH4, the peaks shifted to lower binding energy at 459.1 eV and 464.8 eV, respectively. In addition, a new peak appeared at 457.7 eV indicating the presence of Ti3+ on the surface of R-TMC 10. With the increase in NaBH4 (R-TMC30), the peaks shifted to lower binding energies (459.1 and 464.6 eV) and the peak due 8


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to Ti3+ at 457.3 eV increased in intensity. However, additional increase of NaBH4 (RTMC 50), the Ti peaks shifted to higher binding energy than bare TMC. The peak of Ti3+ shifted from 457.6 to 458.0 eV and its intensity decreased indicating that Ti3+ concentration decrease. The shift of peak from 459.2 to 460.1 eV could be ascribed to the presence of more oxygen vacancies or the presence of lower valence Ti species like Ti2+ in TiO2.25 To clarify the presence of Ti3+ by XPS, we subtracted the normalized Ti 2p of TMC from R-TMC 30. Obviously, two peaks positioned at 462.9 and 457.7 eV which correspond to Ti 2p 3/2 and Ti 2p ½ peaks of Ti3+ as shown in Figure S5. The O 1s XPS spectrum (Figure 2b) shows a broad peak with a shoulder at higher binding energy. This peak can be deconvoulated into two peaks at 529.8 eV corresponding to Ti-O bond and 531.6 eV which is related to the formation of Ti-OH group. The Ti-OH peak of R-TMC has high intensity compared with bare TMC due to the increase of population of OH by reduction.

Figure 2. XPS spectra of Ti (a) and O (b) for TMC and R-TMC, (c) EPR spectra of TMC and R-TMC, and (d) valence band spectra of TMC and R-TMC.

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The presence of Ti3+ and oxygen vacancies was further studied by EPR as shown in Figure 2c. The bare TMC has a strong peak at g = 2.00‒2.01 which can be ascribed to superoxide radical related to compensated surface oxygen vacancies on TMC and molecular adsorbed oxygen.26 After treatment with NaBH4, R-TMC 30 exhibited strong signal at g = 1.97 indicating the formation of Ti3+ in bulk. The high intensity of signal at g = 1.96‒1.97 compared to signal at g = 2.00‒2.01 confirmed the presence of high concentration of Ti 3+ compared with surface compensated oxygen vacancies. However, strongly reduced sample (R-TMC 50) showed strong signal at g = 1.96‒1.97 similar to R-TMC 30. In addition, the signal at g = 2.00‒2.01 exhibited noticeable increase due to reduction of adsorbed O2 by surface Ti3+.

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Moreover, as displayed in the valence

band (VB) XPS spectra (Figure 2d), the VB of R-TMC exhibited a gradual decrease with an increase of NaBH4 wt%. R-TMC 30 and R-TMC 50 exhibited blue shifts to be 2.59 and 2.55 eV, respectively, compared to 2.7 eV for bare TMC. This indicates that the VB shift is due to the reduction treatment.

Figure 3. UV-diffuse reflectance spectra of TMC and R-TMC (a) and Tauc plots for TMC and R-TMC (b). 10


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Color change was noticed after NaBH4 treatment. The color turned from white to gray and finally black with the increase of wt% of NaBH4 as shown in Figure S6. UV–vis diffuse reflectance spectra (DRS) were measured to study the optical characteristics of TMC and R-TMC. As shown in Figure 3a, the sharp increase in the absorption at wavelength below 400 nm can be ascribed to intrinsic band gap absorption of crystalline anatase TiO2. In contrast to pure TMC, an additional absorption band spreading to the near-infrared region is noticed for R-TMC analogously to other reports for black TiO2 which is related to bulk Ti3+ defects. The absorption in the visible light was elevated as wt% of NaBH4 increase. This enhanced absorption in the visible range can be attributed to the increase of oxygen vacancies concentration on the surface of RTMC and the presence of disorder in the surface layer of R-TMC by the reduction treatment. The band gap energies of semiconductors were estimated according to Tauc’s equation: αhv = A (hv-Eg)n/2 where α, h, ν, Eg, and A are the absorption coefficient, Planck's constant, light frequency, band gap energy, and a constant, respectively.28 From the plot of (αhν)2 versus (hν) in Figure 3b, the Eg values of TMC, R-TMC 10, R-TMC 20, R-TMC 30, R-TMC 40, and R-TMC 50 were calculated to be 3.20, 3.15, 3.07, 2.95, 2.85, and 2.46 eV, respectively. Figure 4a shows the photocatalytic activities of samples for H2 evolution under solar light irradiation with light intensity of 100 mW cm-2. The H2 evolution of platinized RTMC 30 (TEM image of platinized TMC is depicted in Figure S7) is 24 μmol h-1 after 3h of photocatalytic reaction compared with 11 μ mol h-1 for platinized pure TMC. This enhancement in H2 evolution is comparable with other black TiO2.29 The H2 evolution activity is R-TMC 30 > R-TMC 20 > R-TMC 40 > R-TMC 10 > R-TMC 50. According to the UV results in Figure 3, R-TMC 50 has the strongest absorption in the visible range but it shows low photocatalytic activity compared with others. Therefore, it is concluded that the enhanced optical absorption in the visible range caused by hydrogenation is not the key factor for the increased H2 production.16-18 Moreover, TMC and the R-TMC have nearly the same surface area so the increase in photocatalytic activity cannot be correlated with surface area. The reduction in 11



photocatalytic activity for R-TMC 50 can be attributed to excess oxygen vacancies which work as recombination center.

Figure 4. Photocatalytic hydrogen evolution of TMC and R-TMC (loaded with 1 wt % Pt) as a function of time under the solar light irradiation (a) and UV diffuse reflectance spectra and AQE of TMC and R-TMC 30 (b).

To confirm the solar energy conversion efficiency, the AQE at different wavelength was measured (Figure 4b). The AQE of platinized R-TMC 30 is 72.3 % compared to 46.1 % for platinized TMC at 365 nm indicating the efficient usage of photogenerated charges of R-TMC 30. The AQE of platinized R-TMC 30 is 2.2% at 420 nm and 0.7% at 460 nm. This small AQE in the visible range affirms that the absorption in the UV region is the main reason for high photocatalytic activity. Also, R-TMC shows photocatalytic activity for H2 production without the use of any cocatalyst (Figure S8). Non-platinized R-TMC 30 exhibited H2 evolution about 7.4 μ mol h-1, AQE= 27.3 % at 360 nm, and 0.4% at 420 nm which are quiet comparable with other noble metal free black TiO2. These findings can be attributed to superstructure of TMC which can retard 12


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electron hole recombination and allow electrons to participate in photocatalytic reaction. The stability of R-TMC 30 was confirmed by 4 cycling experiments as shown in Figure S9 . Over four cycles, H2 was evolved at constant rate and there was no evidence of activity bleaching. XPS and EPR were measured (Figure S10) to confirm the stability of R-TMC 30. The XPS exhibited a peak at 457.7 eV assigned to Ti3+, and the EPR depicted a signal at g= 1.97.These results confirm the stability of R-TMC 30 after the photocatalytic reaction. The charge transfer of TMC and R-TMC was investigated by a series of photoelectrochemical experiments using a three-electrode system under solar light irradiation. Linear sweep voltammetry of TMC and R-TMC (Figure S11a) exhibited photocurrent density of -77, -54, -32, and -20 μA cm-2 for R-TMC 30, R-TMC 10, RTMC 50, and TMC, respectively. It is known that the photocurrent resulted from the transfer of the photogenerated electrons to the back contact, and at the same time, the photoinduced holes are captured by the hole acceptor in the electrolyte. 30 The higher photocurrent density of R-TMC 30 indicates higher charge separation and transport in the sample. This assumption is also confirmed by photo-chronamperometry measurements as depicted in Figure S11b. R-TMC 30 has the highest photocurrent density about 0.70 μA cm-2 which is nearly 6 fold higher than TMC (0.11 μA cm-2). RTMC 30 has low current density (0.18 μA cm-2) compared to R-TMC 10 (0.40 μA cm2

). The observed current densities suggest that charge recombination was enhanced

in R-TMC 50.

31

In addition, the flat band potential (Ef) of TMC and R-TMC 30 was

calculated using the following Mott–Schottky equation: 1/Csc2 = [2/eoεoεNdA2] [(E-Ef) (KbT/eo)] where Csc is the space charge capacitance (F cm-2), eo is the elementary charge (1.62 × 10−19 C), εo is the permittivity of vacuum (8.85×10-14 F cm-1), ε is the relative dielectric constant of the semiconductor, Nd is the donor density, A is the area, E is the applied potential (V), Kb is the Boltzmann constant (1.38×10−23 J K-1), and T is the temperature. Both TMC and R-TMC 30 (Figure S11c ) have positive slope as expected for n-type semiconductor. In addition, the smaller slope of R-TMC 30 compared to TMC indicates an increase in donor density for R-TMC 30. The donor density can be 13



estimated from the slope of Mott–Schottky plot. The donor densities (Nd) of TMC and R-TMC 30 are 1.09×1018 and 2.36×1018, respectively. The electrochemical impedance spectra (EIS) Nyquist plots (Figure S12 ) shows a smaller semicircle for R-TMC 30 compared to other samples indicating the smaller interfacial resistance for R-TMC 30.

Figure 5. Optical emission image of TMC and R-TMC 30 (a,d), the average emission spectra observed at the center and edge positions for TMC and R-TMC 30 (b,e), and emission decay profiles noticed at the center and edge positions on TMC and R-TMC 30 after excitation at 375 nm (c,f).

The effect of Ti3+ and oxygen vacancies in a micrometer-sized single crystal of anatase TMC was studied by a single particle PL microscopy and imaging as in Figures 5a,d. The visible PL emission of anatase TMC is broader and centered at 530 nm which can be ascribed to the recombination of mobile free electrons with trapped holes.32 TMC has strong PL emission at the edge compared with the center due to the superstructure of TMC (Figure S13) which facilitates long distance electron transfer to the edge. After the reduction, the PL intensity at the center and edge of TMC is reduced in the following order: TMC > R-TMC 10 > R-TMC 30 > R-TMC 50 as shown in Figures 5b,e. In addition, the spatial distribution of PL changes from being strongly localized at the edge to simply increase around the center for R-TMC 10, R-TMC 30, and R-TMC 50. 14


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However, the intensity at the edge is still higher than that at the center for R-TMC 10 but the difference between the intensity at the edge and the center is smaller than TMC as shown in Figure S14. Moreover, the intensity at the edge is also slightly higher than that at the center for R-TMC 30 (Figure S15), while the intensities at the center and at the edge are nearly equal for R-TMC 50 (Figure S16). These results can be ascribed to the presence of mid gap states induced by oxygen vacancies and Ti3+ which behave as trapping sites for photogenerated charges and hinder the recombination of photogenerated electrons and holes. Moreover, it retards the transfer of photogenerated charges to the edge leading to reduction in PL emission intensity at the edge with increasing reduction time. The lifetime is recorded for TMC and R-TMC using time-correlated single photon counting (TCSPC). The lifetime (τPL) measured at the edge and center of TMC and R- TMC decreased following the order R-TMC 50 > R-TMC 30 > R-TMC 10 > TMC (Figures 5c,f and Table 1). These results demonstrate that the charge separation efficiency was enhanced in R-TMC. In addition, τPL at the edge is longer than at the center confirming efficient charge separation at the edge. Table 1. PL lifetimes and fractional intensities of TMC and R-TMC. Samples

τ1 (ns) (%)

τ1 (ns) (%)

τ2 (ns) (%)

τ2 (ns) (%)

ϰ2

ϰ2

(center)

(edge)

(center)

(edge)

(center)

0.6 (66)

0.8 (72)

2.7 (34)

4.1 (28)

0.97

0.99

R-TMC 10

0.8 (74)

1.3 (76)

4.1 (26)

7.5 (24)

0.98

0.99

R-TMC 30

1.3 (77)

1.6 (79)

6.4 (23)

10.1 (21)

0.98

0.99

R-TMC 50

2.1 (81)

2.6 (82)

9.1 (19)

13.4 (18)

0.96

0.97

TMC

15


(edge)


Figure 6. Fs-TRDR spectra observed at different delay times in NIR region for RTMC-30 (a), R-TMC 30/Pt (b), R-TMC 50 (c), and R-TMC 50/Pt (d) after excitation at 340 nm.

TiO2 has been extensively investigated with TRDR spectroscopy. However, a little difference in the charge carrier spectra was noticed. It is approved that trapped holes absorb light at 450-550 nm and trapped electrons at 700-800 nm (The charge carrier spectra in the visible region can be considered as overlap between trapped holes and trapped electrons). Shallowly trapped and free electrons have absorption at longer than 900 nm that increase with the increase of wavelength.33-35 As shown in Figures 6 (ad), S17, and S18, TMC and R-TMC have absorption band in the NIR region which can be ascribed to shallowly trapped and free electrons. Figures S19 and S20 depict that reduced TMC and TMC has strong absorption intensity in the wavelength region of 500-800 nm confirming the presence of deeply trapped electrons.

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Figure 7. Comparison of time profiles of TA for R-TMC (a,c) and platinized R-TMC (b,d) monitored at 1100 nm (a,b) and 750 nm (c,d).

To clarify these results we probed the TA kinetic of TMC and R-TMC at 750 and 1100 nm as shown in Figure 7. The photoexcited electrons in bare TMC were noticed to decay through trapping into natural defect site and recombination with holes. The slow decay of all R-TMC compared to TMC in NIR region )Figure 7a) indicates that reduction of TMC increases the lifetime of charge carriers most likely in shallow and deep localized state which were formed below the CB leading to slow recombination with holes. The decay curve was also measured in presence of Pt as electron scavenger as shown in Figure 7b. R-TMC 10 and R-TMC 30 undergo fast decay after modification with Pt and the lifetime decreases from 753 ps for R-TMC 30 to 273 ps for R-TMC 30/Pt as shown in Table S1 confirming the efficient electron capture by Pt. However, R-TMC 50 show less decay compared R-TMC 30 where the lifetime was reduced from 546 ps to 432 ps for R-TMC 50/Pt because of more electrons are deeply trapped which cannot be captured capture by Pt. The depth of electron trap rely upon how the defects 17



preserved the trapped electrons through structural relaxation (As the lattice become more deformed the trapped electrons tend to be more stable. 36 The electron transfer rate in TMC/Pt can be calculated using the following equation: ket = 1/τav (TMC/Pt) 1/τav (TMC). ket for R-TMC 30 is 2.3×109 s-1 compared to 1.1×109 s-1 , 0.7×109 s-1 , and 0.5×109 s-1 for R-TMC 10, R-TMC 50, and R-TMC confirming efficient electron transfer for R-TMC 30 (All the data are summarized in (Table S1). Figure 7c exhibits the time decay probed at visible region, R-TMC 50 has the slowest decay and the longest lifetime of 1222 ps, indicating higher percentage of deeply trapped electrons with the increase of deep localized state and oxygen vacancies. As shown in Figure 7d and Table S2, the use of Pt as electron scavenger has little effect on the decay time probed at 750 nm because most of electrons are deeply trapped and cannot be captured by Pt. The lifetime of R-TMC 50/Pt is 1162 ps. Also, in R-TMC 50, the rate of decay of free and shallowly trapped electrons is much faster than that of deeply trapped electrons (Figure S21) suggesting that most of free electrons are deeply trapped in oxygen vacancies and localized state below the CB. Most of these electrons cannot participate in photocatalytic reaction. However, for RTMC 30, the number of surviving shallow and free electrons is slightly higher than that of deeply trapped electrons (Figure S21) due to less amount of oxygen vacancies and localized state below the CB. The values of ket are calculated and summarized in Table S2 confirming inefficient electron transfer from R-TMC to Pt because most of electrons are deeply trapped as probed at 750 nm. These results are summarized with a simplified schematic diagram in figure S22 which illustrates the important role of free and shallowly trapped electrons in the photocatalytic H2 production.

4. CONCLUSION We have presented a simple method to prepare superstructure R-TMC through chemical reduction with NaBH4. The Ti3+ doping, oxygen vacancies, and disordered layer increase the light absorption through interband transition into Ti3+ and defect state 18


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and defect state-conduction band transition. The single particle PL and TA experiments demonstrate that defects can have positive and negative effect on the lifetime and reactivity of photogenerated charge carriers. When R-TMC has small amount of surface defects, the electrons are shallowly trapped and they are in equilibrium with free electrons. These electrons are still active and can participate in photocatalytic reaction. However, for R-TMC with high amount of defects, most of electrons are deeply trapped and cannot participate in photocatalytic reaction. This study provides detailed understanding of the mechanism of photocatalytic reduction in R-TMC. This can provide an effective path to enhance the photocatalytic activity of metal oxide with appropriate defects. Acknowledgements This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. We are thankful for the help of the Comprehensive Analysis Center of SANKEN, Osaka University. O. E. gratefully acknowledges financial support from the Egyptian Cultural Affairs and Missions Sector.

Supporting Information Supporting Information Available: Additional figures and data. This material is available free of charge via the Internet at http://pubs.acs.org. TEM images, HRTEM images, SEM images, Mott-Schottky plots, photocatalytic hydrogen generation experiments, transient photocurrent, single particle PL measurements and time-

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resolved diffuse reflectance spectra for TiO2 mesocrystals and reduced TiO2 mesocrystals

AUTHOR INFORMATION *E-mail: [email protected] Conﬂict of Interest: The authors declare no competing ﬁnancial interest.

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Charge Carrier Dynamics in TiO2 Mesocrystals ... - ACS Publications

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