TiO2 Mesocrystals Composite for H2 Evolution under Visible

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g-CN/TiO Mesocrystals Composite for H Evolution under Visible Light Irradiation and Its Charge Carriers Dynamics Ossama Elbanna, Mamoru Fujitsuka, and Tetsuro Majima ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08548 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 16, 2017

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ACS Applied Materials & Interfaces

g-C3N4/TiO2 Mesocrystals Composite for H2 Evolution under Visible Light Irradiation and Its Charge Carriers Dynamics Ossama Elbanna, Mamoru Fujitsuka, 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.) ABSTRACT: The photocatalytic performance of graphitic carbon nitride (g-C3N4) has been limited to low efficiency due to the fast charge recombination. Here, we constructed g-C3N4 nanosheets/TiO2 mesocrystals metal-free composite (g-C3N4 NS/TMC) to promote the efficiency of charge separation. The photocatalytic H2 evolution experiments indicate that coupling g-C3N4 NS with TMC increases photogenerated charge carriers in g-C3N4 NS/TMC composite due to efficient charge separation. g-C3N4 NS (31 wt%)/TMC shows the highest photocatalytic activity and the corresponding H2 evolution rate is 3.6 µ mol h-1. This value is 20 times larger than that of g-C3N4 NS without any noble metal cocatalyst under visible light irradiation (λ> 420 nm). The photocatalytic activity of g-C3N4 NS/TMC (3.6 µmol h-1) is 7 times higher than that of g-C3N4 NS/P25 (0.5 µ mol h-1), confirming the importance of strong interface interaction between two dimensional g-C3N4 NS and plate-shape TMC. Femtosecond time-resolved diffuse reflectance (fs-TDR) was employed to study the fundamental photophysical processes of bulk g-C3N4, g-C3N4 NS, and g-C3N4/TMC composite which are essential to explain the photocatalytic activity. Using fs-TDR, we demonstrate that the photocatalytic activity depends on the increased driving force for photoinduced electron transfer and a higher percentage of photogenerated charges. Keywords: graphitic carbon nitride; TiO2 mesocystals; femtosecond time-resolved diffuse reflectance; charge carriers dynamics; hydrogen evolution; visible-light photocatalysis. 1

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1. INTRODUCTION The enormous depletion of fossil fuels has forced the humanity to search for highly efficient catalyst for the production of clean and renewable energy. Solar evolution of H2 from water by semiconductors represents one of the most attractive routes for solar energy conversion. Among semiconductor materials, TiO2 is one of the most commonly used materials as a photocatalyst due to its excellent chemical resistance, nontoxicity, and low cost.1,2 However, its obstacles in photocatalysis are the wide bandgap of pristine TiO2 (3.2 eV) which can be excited only by the UV irradiation and excessive recombination rate of photogenerated electrons and holes. To address these issues, it is of great significant to prepare TiO2 photocatalyst with visible light response and efficient photoinduced charge separation.3,4 Doping TiO2 with metals or non-metals5,6, deposition of plasmonic metals such as Au,7 and coupling with narrower bandgap semiconductors such as metal sulphides8 successfully increase the light absorption and enhance the charge separation. Recently, graphitic carbon nitride (g-C3N4), a new type of metal-free organic polymer semiconductor has received more attention because of its two dimensional structure, moderate band gap of 2.7 eV, outstanding stability, and low cost. 9 g-C3N4 is prepared through elevated temperature and elevated-pressure route utilizing melamine and cyanamide as precursors resulting in the formation of grain boundary defects. Consequently, the photogenerated charge carriers recombine at high rate, leading to lower photocatalytic activity.10 g-C3N4 nanosheets (NS) have also been developed in the recent years with large surface area and high charge transport ability along the in-plane direction compared to bulk g-C3N4.11 It has been reported that g-C3N4 exhibits adequate photocatalytic performance for H2 evolution from water.12-14 However, the photocatalytic activity is reduced by rapid recombination of photogenerated electrons and holes, leading to lower quantum efficiency. Recently, there is so much attention about coupling TiO2 with g-C3N4.15 When TiO2 contacts strongly with g-C3N4, a typeΠ heterojunction interface is established. In consequent to interband excitation, the photogenerated electrons can transfer to the conduction band (CB) of TiO2 and the 2


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photogenerated holes can transfer to the valence band (VB) of g-C3N4, leading to efficient charge carrier separation in the heterojunction.16 To date, many research groups reported the modification of TiO2 with g-C3N4, using bulk g-C3N4 and various shaped TiO2 (particles, mesopores, thin films, spherical particles, and porous network).17 Among various TiO2 structures, TiO2 mesocrystals (TMC) composed of highly ordered nanoparticles are particularly attractive for its unique superstructure. This superstructure significantly promotes the charge separation to increase the lifetime of photogenerated charges, and significantly improve the photocatalytic activity.18 Notably, both g-C3N4 NS and TMC have sheet-like structures with long-lived charge carriers. In addition, g-C3N4 NS have absorption in the visible region. Therefore, it is reasonable to expect that g-C3N4 NS/TMC composite has visible light harvesting ability with efficient charge separation. It is of scientific interest to study the charge carrier dynamics in g-C3N4 and its composites.19,20 However, our knowledge of the photophysics, charge transfer, and charge trapping is still limited. There are only few photophysical studies performed using the transient absorption spectroscopy (TAS).21 The TAS study of TiO2-related materials are limited, because low light transmittance through opaque sample makes the measurements difficult. Time-resolved diffuse reflectance spectroscopy (TDR), where diffuse reflected light is employed as monitoring light instead of transmitted light in conventional spectroscopic methods, is very strong to analyze the charge carrier dynamics in optically opaque systems.22 In this study, to find a better TiO2 candidate for loading g-C3N4 sensitizer, TMC was hybridized with bulk g-C3N4 or g-C3N4 NS. Moreover, for the sake of comparison P25 was combined with g-C3N4 NS. g-C3N4 NS/TMC exhibits highly efficient photocatalytic performance for H2 evolution compared to bulk-g-C3N4/TMC and g-C3N4 NS/P25 without any noble metal cocatalyst. These results suggest that g-C3N4 NS/TMC produces charges with prolonged lifetime, leading to enhance the photocatalytic activity. Charge carrier dynamics based on fs-TDR of bulk g-C3N4, g-C3N4 NS, g-C3N4 NS/TMC, and g-C3N4 NS/P25 were studied under the visible-NIR light irradiation to 3



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provide detailed information about the behaviours of both electrons and holes.

2. EXPERIMENTAL SECTION 2.1. Materials. Melamine (99%), ammonium nitrate (NH4NO3), ammonium fluoride (NH4F), absolute ethanol, and methanol were purchased from Wako. Isopropyl alcohol (IPA) (99%) and titanium(IV) fluoride (TiF4) were purschased from Sigma Aldrich. All chemicals were of analytical grade and used without additional purification. 2.2. Preparation of bulk g-C3N4 and g-C3N4 NS. Bulk g-C3N4 was prepared by heating of melamine under elevated temperature. 2 g of melamine was added to a crucible with a cover and heated at 550 °C for 3 h under air condition (the sample was heated at 2.3 °C/min). The obtained yellow product (bulk g-C3N4) was ground into the powder. g-C3N4 NS were prepared by exfoliation of bulk g-C3N4 in IPA. In brief, 0.1 g of bulk g-C3N4 powder was distributed in 50 mL of IPA and the mixture was sonicated for 10 h. The resulting suspension was centrifuged at 3000 rpm to get rid of the residual unexfoliated g-C3N4. Finally, g-C3N4 NS were collected by centrifuging at 10,000 rpm for 5 min.23 The yield of g-C3N4 NS was 8.8%. 2.3. Preparation of TMC. TMC were prepared following our former study.24 A precursor solution of TiF4, H2O, NH4NO3, and NH4F (molar ratio = 1:117:6.6:4) was dropped on a silicon wafer to form a thin layer. The precursor was annealed in air using a ramping 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 remove surface impurities. 2.4. Preparation of g-C3N4/TMC. Firstly, an appropriate amount of bulk g-C3N4 or

g-C3N4 NS and TMC were dispersed into methanol to have uniform solution by ultrasonic treatment for 30 min. Then, the suspension was stirred in a fume hood for 24 h. After that, the mixture was dried at 60 oC for 12 h. Finally, the powders were gathered and annealed at 400 0C for 2 h in an air atmosphere to form a robust contact between g-C3N4 and TMC. The weight percentage (wt%) of g-C3N4 NS against TMC was 11, 20, 31, 41, and 63.

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2.5. Characterization of materials. The morphologies were examined using a fieldemission scanning electron microscopy (FESEM) (JEOL, JSM-6330FT) and a transmission electron microscopy (TEM) (JEOL, JEM-2100 managed at 200 kV). The diffuse reflectance spectra were measured with a UV-visible-NIR spectrophotometer (Jasco, V-570). Thermogravimetric (TGA) measurements were measured with a Rigaku, Thermo plus EVO II/TG–DTA, TG8120. The AFM images were measured on a nanoscale hybrid microscope (Keyence VN-8010). X-ray diffraction (XRD) patterns of the samples were measured with a smartlab system with Cu Kα radiation operated at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was measured using a JEOL JPS-9010 MC spectrometer. FT-IR measurements were measured on a PerkinElmer spectrometer. The photoluminescence (PL) spectra were measured at room temperature on a Horiba FluoroMax with an excitation wavelength of 370 nm. Timeresolved fluorescence decay spectra were measured on PicoQuant MicroTime 200 by time-correlated single photon counting. The samples were excited using an oilimmersion objective lens (Olympus, UplanSApochromat, 100×, 1.4 NA) with a circular-polarized 405-nm laser in continuous wave (CW) mode managed by a PDL800B driver (PicoQuant). 2.6. Photocatalytic H2 evolution. Specifically, 3 mg of photocatalyst was dispersed in 5 mL of aqueous solution including 20 vol % methanol, and was sealed with a rubber stopper in a tube. The suspension of the photocatalyst was purged with argon for 30 min to get rid of dissolved oxygen. Then, the tube was irradiated with visible light (Asahi Spectra Hal-320, 200 mW cm-2) with magnetic stirring at room temperature. The wavelength of incident light was controlled using a 420-nm cut off filter. The H2 evolution was recorded during the irradiation by utilizing a Shimadzu GC-8A gas chromatograph with an MS-5A column and a thermal conductivity detector. The H2 evolution reaction for each sample was measured 3 times. In the cycling test, the used catalyst was separated by centrifugation to check its stability. To get an action spectrum, H2 evolution of catalyst was measured using the irradiation of monochromatic light (Asahi Spectra Hal-320; 1 mW cm-2 ±5 nm). The apparent quantum efficiency (AQE) was 5



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estimated using the following equation: AQE = (2 ×number of H2 molecules/number of incident photons) × 100. 2.7 Photo-electrochemical measurements. Photo-electrochemical experiments were carried out using a three-electrode system with an electrochemical analyzer (ALS, 660B) using 0.1 M Na2SO4 aqueous solution 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 (300 mW cm-2) with a 420 nm cut off filter was employed as a visible light source. The photocurrent response and electrochemical impedance spectroscopy (EIS) were measured at room temperature. 2.8. Time-resolved diffuse reflectance spectral measurements. The fs-TDR was measured by the pump and probe method applying a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz). An optical parametric amplifier (Spectra Physics, OPA-800CF-1) was utilized to generate the excitation pulse (420 nm, 2.2 μJ pulse-1). The white light continuum pulse, which was created by concentrating the residual of the fundamental light on a sapphire crystal, was directed to the sample powder coated on the glass substrate and a linear InGaAs array detector with the polychromator (Solar, MS3504) detected the reflected light.

3. RESULTS AND DISCUSSION 3.1. Characterization of the prepared photocatalysts. The crystalline phase and composition of the prepared samples were investigated by XRD (Figure 1a). The peaks have 2θ values of 25.2°, 37.8°, 47.9°, 53.9°, and 62.6° are corresponding to the (101), (004), (200), (105), and (204) plane diffractions of anatase TiO2. For the pure g-C3N4 sample, two characteristics peaks appeared at 2θ = 13.2° and 27.3°. These peaks are indexed to the (100) plane from crystal plane of tri-s-triazine units and the (002) plane diffraction resulting from the stacking of the conjugated aromatic system of g-C3N4.25 Similar diffraction peaks of anatase TMC and g-C3N4 were observed in all the gC3N4/TMC composites, confirming the presence of anatase TMC and g-C3N4 in the 6


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composite. Compared to the XRD patterns of TMC, weak diffraction peaks for g-C3N4 were observed in g-C3N4/TMC composite This is probably ascribed to low crystallinity of pure g-C3N4 compared to TMC. There are no other peaks, affirming the purity of the prepared composites.

Figure 1. XRD patterns (a) and FTIR spectra (b) for pure TMC, bulk g-C3N4, g-C3N4 NS, bulk g-C3N4 (31 wt%)/TMC, and g-C3N4 NS (31 wt%)/TMC.

To assert the existence of g-C3N4 in the composite, the FTIR technique was applied. As shown in Figure 1b, three main absorption regions were detected in pure TMC. The broadband peak at 3300–3500 cm−1 is originated from the O-H stretching of physisorbed water on the surface of TMC and the relatively sharp band at 1637 cm −1 due to O-H bending mode of water molecules. The strong absorption below 850 cm−1 is ascribed to the absorption of Ti-O-Ti.26 Also, three main absorption domains were noticed for pristine g-C3N4 where the broad peak at 3000–3300 cm−1 is assigned to the stretching vibration of N-H and surface adsorbed OH, the powerful band at 1240–1640 cm−1, with the typical peaks at the absorption band at 1640 cm–1 is attributed to C–N heterocycle stretching vibration modes, whereas the four strong peaks at 1241, 1320, 1410, and 1571 cm–1 represent aromatic C–N stretching vibration modes. The peak at 7



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807 cm–1 is related to the particular breathing mode of triazine units.27 In order to determine the contents of g-C3N4 and confirm their thermal stability, the TGA was measured from room temperature to 800 oC at 10 oC min-1. Figure S1 exhibits the TG plots of the samples. The little weight loss at low temperature (200-300 oC) is assigned to desorption of physically adsorbed water molecules. It can be seen that pure TMC are stable through the temperature range of measurement. For pure g-C3N4 NS, a 100% weight loss was observed indicating that g-C3N4 completely decomposed at approximately 600 oC in air.25 Therefore, the second weight loss represents wt% of gC3N4 in the composite. The wt% of g-C3N4 in the prepared samples was calculated to be 31 wt% for bulk g-C3N4/TMC, g-C3N4 NS/TMC, and g-C3N4 NS/P25. In addition, g-C3N4 NS/TMC samples with different wt% (11, 20, 41, and 63) were prepared. The C/N molar ratio was estimated by elemental analysis (Table S1) to be 0.67, which is smaller than the theoretical C/N ratio of 0.75. This is ascribed to the existence of uncondensed amino groups. XPS technique was used to investigate the surface chemical composition and chemical status of elements in g-C3N4 NS (31 wt%)/TMC. Figure 2a shows the XPS spectra of C 1s for g-C3N4 NS (31 wt%)/TMC and g-C3N4 NS, which display two peaks at 284.6 and 288.0 eV. The peak appeared at 284.6 eV is attributed to C-C and C=C, which originate from the adventitious carbon. However, another peak at 288.0 eV is ascribed to sp2-hybridized carbon in N-containing aromatic ring (N-C=N).28 N 1s XPS binding energy (Figure 2b) has four peaks centred at 398.5, 399.1, 400.5, and 404.4 eV. These peaks represent sp2-hybridized nitrogen (C-N=C), the tertiary nitrogen N-(C)3 groups, the free amino groups (C-N-H), and π-excitations, respectively.29 In Figure 2c, Ti 2p spectrum has two peaks at binding energies of 458.6 (Ti 2p3/2) and 464.3 eV (Ti 2p1/2), which are attributed to Ti4+. O 1s high-resolution spectrum in Figure 2d was fitted with two peaks to indicate the Ti-O bond (529.9 eV) and O-H bond (531.6 eV), suggesting the existence of a hydroxyl group or water molecule.30

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Figure 2. High resolution XPS spectra of C 1s (a),N 1s (b), Ti 2p (c) ,and O 1s (d) for g-C3N4 NS and g-C3N4 NS (31 wt%)/TMC.

The structures and morphologies of the as-prepared samples were illustrated by SEM and TEM images. Figure 3a exhibits the crumpled sheet-like morphology of pure bulk g-C3N4, which contains many stacking layers, indicating the planar graphitic-like structure. After bulk g-C3N4 was exfoliated by ultrasonication, it turned into thin nanosheets with a few hundreds of nm as illustrated in Figure 3b. Figure S2 shows the AFM image of g-C3N4 NS with thickness of 2-3 nm. As depicted in Figure3c, TMC which has the plate like structure with size of several micrometres was wrapped by gC3N4 NS. The HRTEM image in Figure S3 reveals an extremely close interface between TMC and g-C3N4 NS. TMC have lattice fringes of anatase (200) or (020) with the lattice spacing of approximately 0.189 nm for TMC.31 It is hard to recognize the crystal lattice of g-C3N4 NS by HRTEM because of the organic nature of g-C3N4 NS which causes fast deterioration under irradiation of electron beam.16 These results indicate that TMC and g-C3N4 NS were contacted tightly with each other favoring efficient charge transfer. The coupling between TMC and g-C3N4 NS occurred on TMC 9



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{001} facets and g-C3N4 NS {002} facets due to the presence of planar matching. Therefore, stable heterojunction is easily formed by annealing the g-C3N4 NS on the surface of TMC with the exposed 001 facets.32 The elemental mapping of g-C3N4 NS (31 wt%)/TMC in Figure 3 (e-h) depicted that C, N, Ti, and O were well defined, which confirms that g-C3N4 NS homogenously attaches to TMC.

Figure 3. TEM images of bulk g-C3N4 (a), g-C3N4 NS (b), g-C3N4 NS (31 wt%)/TMC (c), STEM image (d), and EDX elemental mappings of C, N, Ti, and O for g-C3N4 NS (31 wt%) /TMC (e-h).

TEM images of TMC, bulk g-C3N4 (31 wt%)/TMC, and g-C3N4 NS (31 wt%)/P25 are shown in Figure S4. Figure S5 is the SEM images of bulk g-C3N4, g-C3N4 NS, bulk gC3N4 (31 wt%)/TMC, and g-C3N4 NS (31 wt%)/TMC. The SEM image of bulk g-C3N4 confirms the chunky and layered structure, which is in size of several micrometers. gC3N4 NS conserve loose and irregular tissue-like two dimensional nanosheet morphology. SEM images of bulk g-C3N4 (31 wt%)/TMC and g-C3N4 NS (31 wt%)/TMC confirm the intimate interfacial contact between g-C3N4 NS and TMC. The textural properties of the prepared photocatalysts were investigated by nitrogen adsorption–desorption isotherms (Figure S6). The prepared photocatalysts show type IV sorption isotherms according to IUPAC classification suggesting the presence of 10


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mesopores.30 BET specific surface area (SBET) of bulk g-C3N4 is about 8.4 m2 g-1. Exfoliation of bulk g-C3N4 to g-C3N4 NS increases the surface area nearly nine times to be 72.2 m2 g-1. After combination with TMC, the surface area of g-C3N4 NS (31 wt%)/TMC is about 57.4 m2 g-1 which is slightly lower than surface area of g-C3N4 NS (72.2 m2 g-1) owing to slightly low surface area of TMC (61.2 m2 g-1 ) compared to gC3N4 NS. On the other hand, coupling TMC with bulk g-C3N4 increases the surface area to 34.2 m2 g-1. The corresponding textural parameters of the samples were summarized in Table S2. 3.2. Optical Absorption Properties. The optical characteristics of the prepared samples were studied by using UV-vis diffuse reflectance spectroscopy as illustrated in Figure 4a. The absorption of pure TMC is under 400 nm indicating that pure TMC responses only to UV light. Both bulk g-C3N4 and g-C3N4 NS have strong absorption in the visible region. The absorption edge of g-C3N4 NS exhibits blue shift compared to that of bulk g-C3N4 which is assigned to the quantum confinement effect.33 In comparison with pure TMC, the optical absorption of g-C3N4 NS (31 wt%)/TMC is enhanced in the visible light region. This suggests that g-C3N4 NS modify the visible light absorption to trigger the photocatalytic reaction on the surface of TMC. Moreover, a weak visible-light absorption peak appears around 429 and 410 nm for bulk g-C3N4 (31 wt%)/TMC and g-C3N4 NS (31 wt%)/TMC, respectively. This red shift, compared to the strongest absorption peak of g-C3N4 NS at 380 nm, is probably attributed to the interaction between g-C3N4 NS and the TMC.34 The band gap energies of semiconductors were estimated by Kubelka–Munk transformation according to the following 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. The value of n depends on the characteristics of the transition in a semiconductor. The n values of g-C3N4 and TiO2 are both 4.35 From the plot of (αhν)2 versus (hν) in Figure 4b, the Eg values of bulk g-C3N4, g-C3N4 NS, bulk g-C3N4 (31 wt%)/TMC, g-C3N4 NS (31 wt%)/TMC, g-C3N4 NS (31 wt%)/P25, and TMC were estimated to be 2.61, 2.73, 2.66, 2.79, 2.79, and 3.16 eV, respectively. To determine the 11



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the positions of the CB edges of g-C3N4 NS and TMC, Mott-Schottky (MS) analysis was performed. The MS plots (Figure S7) show n-type characteristics (positive slope) for g-C3N4 NS and TMC. Based on the MS analysis, the values of flat band potential were calculated to be -0.25 and -1.08 for TMC and g-C3N4 NS, respectively.

Figure 4. Diffuse-reflectance spectra (a) and plots of (αhν)2 versus (hν) (b) for pure TMC, bulk g-C3N4, g-C3N4 NS, bulk g-C3N4 (31 wt%)/TMC, g-C3N4 NS (31 wt%)/P25, and g-C3N4 NS (31 wt%)/TMC.

3.3 Measurement of Photocatalytic Activity for H2 Evolution. The photocatalytic performance was tested by H2 evolution from H2O including 20% methanol as a sacrificial reagent under visible light irradiation (λ> 420 nm) without any cocatalyts as shown in Figure 5a. Controllable experiments showed that no H2 was evolved in the absence of catalyst or light irradiation. Very small amount of H2 (0.1µ mol h-1) was detected when bulk g-C3N4 was used as a photocatalyst. After exfoliation of bulk g-C3N4 into g-C3N4 NS, the photocatalytic activity increased to 0.2 µ mol h-1 which is about 12


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two times higher than bulk g-C3N4. This increase is explained by the high surface area of g-C3N4 NS compared to bulk g-C3N4 and lower electron-hole recombination rate. There was no H2 evolution from pure TMC because they do not absorb visible light. However, after combining with small wt% of g-C3N4, H2 evolution was observed. The photocatalytic activity of g-C3N4 NS /TMC increased with increasing wt% of g-C3N4 NS from (10-31) and decreased with higher wt% of g-C3N4 NS (40-63) as depicted in Figure S8. This behaviour is ascribed to the enhancement of electron hole recombination rate for higher wt% of g-C3N4 NS which prevented the electrons from approaching the reaction sites on the surface of TMC.36

Figure 5. Visible-light photocatalytic activities for H2 evolution (λ> 420 nm) for TMC, bulk g-C3N4, g-C3N4 NS, bulk g-C3N4 (31 wt%)/TMC, g-C3N4 NS (31 wt%)/P25, and g-C3N4 NS (31 wt%)/TMC (a) and action spectrum for g-C3N4 NS (31 wt%)/TMC(b).

The g-C3N4 NS (31 wt%)/TMC has the highest photocatalytic activity (3.6 µ mol h-1) which is 18 times higher than g-C3N4 NS. Moreover, for the sake of comparison the photocatalytic activities of bulk g-C3N4 (31 wt%)/TMC and g-C3N4 NS (31 wt%)/P25 13



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were measured as shown in Figure 5a. The H2 evolution rate of g-C3N4 NS (31 wt%)/TMC (3.6 µ mol h-1) is about six times higher than that of bulk g-C3N4 (31 wt%)/TMC (0.6 µ mol h-1) and even seven times higher than that g-C3N4 NS (31 wt%)/P25 (0.5 µ mol h-1). Difference in the surface area is a possible factor for explaining the observed catalytic activities, while g-C3N4 NS (31 wt%)/TMC and g-C3N4 NS (31 wt%)/P25 have nearly the same surface area (Table S2). In addition, the surface area of gC3N4 NS (31 wt%)/TMC is only 1.7 times higher than that of bulk g-C3N4 (31 wt%)/TMC. Moreover, bulk g-C3N4 (31 wt%)/TMC has strong absorption in visible region compared to g-C3N4 NS (31 wt%)/TMC (Figure 4a). Therefore, the photocatalytic activity is not mainly related to either surface area or optical absorption. Under identical conditions, the wavelength dependant apparent quantum efficiency (AQE) was measured using different long-pass cut off optical filters to investigate whether the activity was due to photoexcited electrons or not. The AQE of g-C3N4 NS (31 wt%)/TMC decreased as the wavelength of monochromatic light increased tracking the characteristic absorption of g-C3N4 NS (31 wt%)/TMC. The AQE of g-C3N4 NS (31 wt%) /TMC is about 4.1 at 420 nm which is comparable with other systems loaded non-noble cocatalyts.37 The stability of the photocatalyst was confirmed by recycling the photocatalyst for H2 evolution. Slight decrease in H2 evolution was observed after 3 cycles (Figure S9). The g-C3N4 NS/TMC is stable and conserve the yellow color for 1 week after the photocatalytic reaction confirming the high stability of g-C3N4 NS without noticeable dissolving. The TEM, TGA, XRD, and XPS of g-C3N4 NS (31 wt%)/TMC measured after photocatalytic reaction showed no change (Figures S10-12), confirming that g-C3N4 NS (31%)/TMC was stable during photocatalytic reaction. 3.4. Photoinduced Charge Carrier Dynamics. The photocatalytic activity is strongly dependent on the efficiency of electron–hole pairs trapping, migration, and transfer. Therefore, photoelectrochemical measurement, steady state (PL), time-resolved PL (TRPL), and fs-TDR were used to investigate the charge separation behavior. Figure 6a exhibits the amperometric I−t curves of the samples coated on a carbon electrode in several on-off cycle. Photocurrent reflects the ability of semiconductor to generate 14


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charge carriers under irradiation.38 Bulk g-C3N4 exhibited low average photocurrent about 0.05 µA cm-2. The average value of photocurrent increased to be 0.16, 0.24, 0.30, and 0.88 µA cm-2 for g-C3N4 NS, g-C3N4 NS (31 wt%) /P25, bulk g-C3N4 (31 wt%)/TMC, and g-C3N4 NS (31 wt%)/TMC, respectively. Figure S13 depicts the photocurrent density of g-C3N4 NS/TMC with different wt% of g-C3N4 NS. g-C3N4 NS (31 wt%)/TMC has the highest photocurrent density which agrees with results of photocatalytic activity. The increased charge separation was further confirmed by (EIS) under visible light irradiation (λ> 420 nm) (Figure 6b). g-C3N4 NS, g-C3N4 NS (31 wt%)/p25, bulk g-C3N4 (31 wt%) /TMC, and g-C3N4 NS (31 wt%)/TMC have smaller arc radii compared to bulk g-C3N4. Especially, g-C3N4 NS (31 wt%)/TMC has the smallest arc radii indicating that the chargetransfer resistance was obviously decreased.39

Figure 6. Photocurrent response (a), Nyquist plots, (b) Room-temperature PL emission spectra (c), and TRPL spectra (d) for bulk g-C3N4, g-C3N4 NS, bulk g-C 3 N 4 (31 wt%)/TMC, g-C3N4 NS (31 wt%)/P25, and g-C3N4 NS (31 wt%)/TMC.

The steady state PL was used to analyse the separation and transfer of photogenerated charge carrier. Figure 6c shows that an emission peak of bulk g-C3N4 is located at 475 15



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nm, while the emission peak of g-C3N4 NS is located at 450 nm which agrees with the spectral shift observed in UV-vis diffuse reflectance spectra. The PL peak is assigned to multiple transition including π conjugated state and N lone pair.40 The PL of g-C3N4 NS was strongly quenched compared to bulk g-C3N4 due to low surface defect density (surface defects can work as recombination centers for the photogenerated electrons and holes) indicating the efficient charge separation. The PL of g-C3N4 NS (31 wt%)/TMC was strongly suppressed compared to bulk g-C3N4 (31 wt%)/TMC and gC3N4 NS (31wt%)/P25 confirming the efficient electron transfer from g-C3N4 NS to TMC superstructure. Also, the PL of bulk g-C3N4 (31 wt%)/TMC was quenched compared to bulk g-C3N4 suggesting that TMC superstructure facilitates electron-hole separation and transportation from bulk g-C3N4. In addition as depicted in Figure S14, PL of g-C3N4 NS (31 wt%)/TMC was strongly quenched compared to g-C3N4 NS/TMC with other wt% following the same trend of the photocatalytic activity results. To interpret the photoexcited charge carriers, the time resolved fluorescence decays of the samples were measured as shown in Figure 6d. All of them decayed exponentially to be well fitted with two radiative lifetimes: I(t) = A1e-t/τ1 + A2e-t/τ2. Table S3 summarized all the fitting parameters. It is already known that the increased lifetime of charge carrier is associated with slower recombination process. Theτ2 is directly attributed to recombination of photogenerated electrons and holes.41 The τ2 of g-C3N4 NS (31 wt%)/TMC (4.38 ns) is longer than other samples as shown in Table S3. This prolonged lifetime confirms the efficient charge transfer between g-C3N4 NS and TMC superstructure. In order to probe intransient behaviours of photoexcited g-C3N4, we performed fsTDR studies. TDR is a strong tool to monitor the population of photogenerated charges and track their kinetics on relevant time scale. Generally, the complex carrier dynamics need to be tracked on a broad spectral and temporal window to understand the overall performance of the photocatalyst and reveal the behavior of photogenerated charge carriers. We studied the transient absorption spectrum into two domains, i.e., shorter and longer than 800 nm. Firstly, in the NIR region (longer than 800 nm), the fs-TDR 16


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exhibits structureless broad absorptions, which are assigned to bulk carriers (free and surface shallowly trapped electrons) 42 as illustrated in Figure 7. Figure 7. Time-resolved diffuse reflectance spectra of bulk g-C3N4 (a), g-C3N4 NS (b), bulk g-C3N4 (31 wt%)/TMC (c), and g-C3N4 NS (31wt%)/TMC (d) in NIR region. However, TMC exhibits no transient absorption upon visible light excitation because of its wide band gap (Figure S15). The absorption-time profiles at 1000 nm (Figure 8a) were fitted by multi-exponential function. Table 1 summarized the fitting parameters for the prepared samples. The signal from bulk g-C3N4 was fitted by bi-exponential

function with time decay components of τ1= 240 ps (48%) and τ2 = 1200 ps (52%). The slow decay was attributed to deep electron trapping in the defects of bulk g-C3N4, which may hamper electron transfer leading to low photocatalytic activity.43 This deeply trapped electrons have lifetime about (micro- to millisecond) which is beyond time scale measured here. The g-C3N4 NS exhibit faster decay compared to bulk g-C3N4 with time constants of τ1= 30 ps (35%) and τ2= 970 ps (65%) indicating the increase in population of free and shallowly trapped electrons. These results indicate that exfoliation of bulk g-C3N4 reduces electron trapping into deep and photocatalytically

17



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inactive electron trap state,44 and increase their chance to transfer to other electron acceptors such as TiO2 and Pt.

Figure 8. (a,b) normalized fs-TDR decay kinetics at 1000 nm and 650 nm respectively for bulk g-C3N4, g-C3N4 NS, bulk g-C3N4 (31 wt%)/TMC, g-C3N4 NS (31 wt%)/P25, and gC3N4 NS (31 wt%)/TMC The decay curves observed for bulk g-C3N4 (31 wt%)/TMC and g-C3N4 NS (31 wt%)/TMC have an additional decay component compared to bulk g-C3N4 and g-C3N4 NS. In addition, electrons generated in g-C3N4 NS (31 wt%)/TMC seem to disappear much faster than those in g-C3N4 NS. Therefore, we conclude that TMC can trap the electrons providing a new pathway for electron transfer from g-C3N4 to TMC instead of deep trapping in the defects of g-C3N4. The g-C3N4 NS (31 wt%)/TMC has τ1= 3.8 ps, τ2=38 ps which are shorter than τ1= 5.4 ps , τ2=52.4 ps for g-C3N4 NS (31 wt%)/P25 and τ1= 13 ps , τ2=75 ps for bulk g-C3N4 (31 wt%)/TMC. (As τ1 and τ2 are attributed to the time required for electrons to move from the interior of g-C3N4 to surface of g-C3N4 and then transferred to TMC. The short τ1 and τ2 for g-C3N4 NS (31 wt%)/TMC indicate fast and the efficient electron transfer from g-C3N4 NS to the plate structure of TMC confirming the intimate interfacial contact between g-C3N4 NS and TMC. The longer τ1 and τ2 for bulk g-C3N4 (31 wt%)/TMC are due to the layered structure of bulk g-C3N4, which slowed down the electron transfer to TMC. τ3 is assigned to the time required for the electrons to diffuse to the reaction site on the surface of TiO2 for photocatalytic reactions which possibly occurred in µs-ms. τ3 for g-C3N4 NS (31 wt%)/TMC is 662 ps which is longer than 453 ps and 498 ps for g-C3N4 NS (31 wt%)/P25 and bulk g-C3N4 18


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(31 wt%)/TMC, respectively. The longest τ3 for g-C3N4 NS (31 wt%)/TMC is explained in terms of easier transfer of electrons through the well-matched ordered junction TiO2 nanocrystals compared to less ordered junction of P25. The longest τ3 for g-C3N4 NS (31 wt%)/TMC compared to bulk g-C3N4 (31 wt%)/TMC may be attributed to higher electron density in g-C3N4 NS (31 wt%)/TMC compared to bulk g-C3N4 (31 wt%)/TMC due to more efficient electron injection from g-C3N4 NS to the plate structure of TMC. Therefore, electrons will demand more time for diffusing to the reaction sites for photocatalytic reaction. Table 1 Fitting parameters of transient absorption decays at NIR region. Sample τ1 (ps) τ2 (ps) τ 3 (ps) bulk g-C3N4

240 (48%)

1200 (52%)

g-C3N4 NS

30 (35%)

970 (65%)

g-C3N4 NS (31 wt%)/P25

5.4 (31%)

52 (19%)

453 (50%)

bulk g-C3N4 (31 wt%)/TMC

13 (72%)

75 (77%)

498 (15%)

g-C3N4 NS (31 wt%)/TMC

3.8 (72%)

38 (71%)

662 (53%)

Secondly, in the visible region (Figure 9), the fs-TDR reveals bleaching signal below 550 nm, which is attributed to simulated emission.45 Moreover, positive absorption was observed in wavelength region longer than 600 nm. Analogous positive absorption peak was noticed in previous studies.46, 43, 21 This peak was assigned to either photogenerated holes or electrons, or also electron–hole pairs. The visible absorption band was well fitted by bi-exponential decay for all samples as shown in Figure 8b and the data were summarized in Table 2. The prolonged lifetimes for bulk g-C3N4 with τ2= 1804 ps are explained as following: when electrons were deeply trapped in the defects of bulk gC3N4, they became less mobile and thereby electrons and holes recombination was hindered. Therefore, the lifetimes of deeply trapped electrons and surface trapped holes are expected to increase. On the other hand, g-C3N4 NS has τ2= 625 ps which is shorter than bulk g-C3N4. This is attributed to the increase of the population of free electrons and shallowly trapped electrons, resulting in an enhancement of electrons-holes recombination. These results of carrier lifetimes for bulk g-C3N4 and g-C3N4 NS suggest that recombination via PL was not the dominant mechanism because PL emission of g19



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C3N4 only probes electronic states close to the band edge (shallow traps) rather than deeply trapped states. Table2. Fitting parameters of transient absorption decays in visible region. Sample

τ1 (ps)

τ2 (ps)

bulk g-C3N4

132 (46%)

1804 (54%)

g-C3N4 NS

39 (53%)

625 (47%)

g-C3N4 NS (31 wt%)/P25

35 (32%)

732 (68%)

bulk g-C3N4 (31 wt%)/TMC

56 (53%)

1490 (47%)

g-C3N4 NS (31 wt%)/TMC

49 (51%)

1420 (49%)

Figure 9. Time-resolved diffuse reflectance spectra of bulk g-C3N4 (a), g-C3N4 NS (b), bulk g-C3N4 (31 wt%)/TMC (c), and g-C3N4 NS (31 wt%)/TMC (d) in the visible region.

The g-C3N4 NS (31 wt%)/P25 and g-C3N4 NS (31 wt%)/TMC showed τ2= 732 ps and τ2= 1420 ps, respectively. The longer τ2= 1420 ps for g-C3N4 NS (31 wt%)/TMC compared to that of g-C3N4 NS confirms the efficient electron transfer through the 20


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superstructure of TMC as shown in Figure 10 resulting in longer lifetime of surface trapped holes on g-C3N4 NS/TMC. Moreover, bulk g-C3N4 (31 wt%)/TMC has τ2= 1490 ps because electrons were transferred to TMC or deeply trapped on bulk g-C3N4 leading to longer lifetime of the holes. In order to confirm that the positive absorption peak is mainly assigned to surface trapped holes, we measured fs-TDR for g-C3N4 NS where the holes and electrons were scavenged by methanol and Pt (g-C3N4 NS/MeOH and gC3N4 NS/Pt) (Figure S16). The decay of g-C3N4 NS was accelerated after loading of MeOH as expected (τ2= 488 ps). In contrast, the decay of g-C3N4 NS was deaccelerated after loading of Pt (1451 ps). As MeOH and Pt act as hole and electrons scavenger respectively, these results (Figure S16) confirm that the positive absorption band is mainly attributed to surface trapped holes.

Figure 10. Representative Scheme of electron injection and movement in g-C3N4 NS (31 wt%)/TMC during visible light irradiation.

4. Conclusion In conclusion, a series of g-C3N4 NS/TMC with different component of g-C3N4 NS were fabricated. The results of FT-IR, XRD, XPS, SEM, and TEM confirm the successful preparation of the g-C3N4 NS/TMC and the intimate interfacial contact between two dimensional g-C3N4 NS and plate superstructure of TMC. The photocatalytic activity of g-C3N4 NS (31 wt%)/TMC for H2 evolution (3.6 µ mol h-1) under visible 21



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light irradiation (λ> 420 nm) is nearly 20 times and 7 times higher than that of g-C3N4 NS and g-C3N4 NS (31 wt%)/P25 composite, respectively, without any cocatalyst. Electrochemical and PL measurements indicate that this increase in photocatalytic activity is mainly ascribed to the improved separation of photogenerated charge carriers. The fs-TDR was performed to study charge carrier kinetic. The lifetime of electrons and holes increase and the recombination of photogenerated charge carriers is decreased due to the tight interface between g-C3N4 NS and plate TMC which facilitates the charge transfer. The fs-TDR confirms that TMC acts as electron transfer channel to promote the charge separation. Since the poor efficiency of charge separation is mainly considered as the bottleneck of photocatalytic activity of g-C3N4, we believe that the design of composite with strong contact between two dimensional g-C3N4 NS and plate TMC can provide flexible choice to enhance solar energy utilization of g-C3N4 photocatalyst. Supporting Information Supporting Information Available: Additional figures and data. This material is available free of charge via the Internet at http://pubs.acs.org. Thermogravimetric (TGA) measurements, AFM image, TEM images, HRTEM image of,

SEM

images,

N2

adsorption-desorption

isotherm,

Mott-Schottky plot,

photocatalytic hydrogen generation reaction, durability performance, TEM, XPS, XRD, and TGA analyses for the recycling experiment, transient photocurrent, and PL emission spectra, Time-resolved diffuse reflectance spectra and elemental analysis for different samples (g-C3N4 NS, g-C3N4 NS/TMC with different wt% of g-C3N4 NS, bulk g-C3N4, bulk g-C3N4 /TMC, g-C3N4 NS/P25, g-C3N4 NS/Pt, and g-C3N4 NS/MeOH. 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 22


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Comprehensive Analysis Center of SANKEN, Osaka University. O. E. gratefully acknowledges financial support from the Egyptian Cultural Affairs and Missions Sector. AUTHOR INFORMATION *E-mail: [email protected] Conﬂict of Interest: The authors declare no competing ﬁnancial interest.

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TiO2 Mesocrystals Composite for H2 Evolution under Visible

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