Black Phosphorus Sensitized TiO2 Mesocrystal Photocatalyst for

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Black phosphorous sensitized TiO mesocrystals photocatalyst for hydrogen evolution with visible and near-infrared light irradiation Ossama Elbanna, Mingshan Zhu, Mamoru Fujitsuka, and Tetsuro Majima ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05081 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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ACS Catalysis

Black phosphorous sensitized TiO2 mesocrystals photocatalyst for hydrogen evolution with visible and near-infrared light irradiation Ossama Elbanna, Mingshan Zhu, 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 correspondence should be addressed. E-MAIL: [email protected] (M.F.), [email protected] (T.M.)

ABSTRACT: Wide absorption from ultraviolet (UV) to near-infrared (NIR) region and enhanced charge separation are two main requirements for promising semiconductor photocatalysts. Here, we studied visible-NIR driven photocatalytic hydrogen evolution over black phosphorus nanosheets/TiO2 mesocrystals loaded with Pt heterostructure (BP NS/ Pt (3 wt%)/TMC TMC). BP NS/Pt (3 wt%)/TMC can harvest photons from UV to NIR and simultaneously has enhanced charge separation to increase the generation of electrons for photocatalytic reduction of water. BP NS/Pt (3 wt%)/TMC exhibited photocatalytic H2 evolution rates of 1.9 and 0.41 µmol h-1 under visible (λ> 420 nm (420-1800 nm)) and NIR (λ> 780 nm (780-1800 nm) irradiation, respectively, compared with 0.3 and 0.10 μmol h-1 for BP NS/Pt (3 wt%)/P25. Moreover, a comparative study was made to examine the effect of thickness of BP NS on the photocatalytic H2 evolution. Femtosecond time-resolved diffused reflectance

spectroscopy

(fs-TRDRS)

was

integrated

together

with

photoelectrochemical measurement to shed the light on the importance of charge transfer and separation, confirming that decreasing the thickness of BP NS enhances electron injection from BP NS to TMC to increase the photocatalytic activity. Keywords: Black phosphorus, TiO2 mesocrystals, femtosecond time-resolved diffuse reflectance, charge carriers dynamics, hydrogen evolution, visible-light photocatalyst 1

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1. INTRODUCTION Design of suitable photocatalyst for effective transformation of sunlight to chemical energy is a hopeful way for the utilization of solar energy.1-2 After the pioneering study of photoelectrochemical splitting of water on n-type TiO2 electrode, photocatalytic H2 evolution has received enormous attention to overcome the shortage of traditional fossil fuels.3-4 Remarkable progress was attained and various photocatalysts have been prepared from mainly semiconductor materials such as TiO2, metal sulphides, and metal oxynitrides.5-7 Among these substances, TiO2 is widely applied semiconductors as photocatalyst owing to its photostability, non-toxicity, and low cost. Recently, TiO2 mesocrystals (TMC) which are characterized by highly ordered superstructure have attracted particular attention because of its large surface area and enhanced charge separation.8 However, TiO2 can respond only to UV light, which mainly retards its application as practical photocatalyst.9 Therefore, one of the important challenges for TiO2-based photocatalyst is to find a way to utilize the solar energy from ultraviolet (UV) to near-infrared (NIR) region. For this end, formation of heterojunction by consolidating TiO2 with other visible light active component such as metal sulphides and metal nanoparticles has been examined.10-11 However, most of TiO2 heterojunction with other semiconductor mainly respond to the visible light till 600 nm. Further red (> 600 nm) and NIR (> 800 nm) regions are not utilized. Recently, black phosphorus (BP) has received increasing concern as a hopeful semiconductor.12 BP can bind graphene and transition-metal dichalcognides in which graphene has zero bandgap and the transition-metal dichalcognides have reduced absorption in the NIR region due to their large band gaps (e.g., 1.29–1.8 eV for MoS2).13 BP can be exfoliated into twodimensional (2D) materials and it has unique band gap which is tuneable with thickness. The absorption spectrum of 2D BP can be extended from visible to near and mid infrared range.14 Recently, few papers have reported about applying BP as a photocatalyst.15-17 However, the photocatalytic activity is retarded by fast recombination between photogenerated electrons and holes. For example, our group reported BP/ graphitic carbon nitride as a metal free photocatalyst for hydrogen (H2) 2


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evolution.18 Yang’s group reported the visible light photocatalytic H2 evolution from BP nanosheets (BP NS).19 Tian’s group reported a single-step solvothermal procedure for the direct synthesis of BP NS which exhibited 24 folds higher photocatalytic activity for H2 production than C3N4 nano sheets.20 In addition, an efficient H2 evolution photocatalyst was prepared using BP NS supported by amorphous Co-P nanoparticles, synthesized by a facile, cheap, and scalable solvothermal route.21 As far as we know, no report has been published for the preparation of BP NS/TMC for photocatalytic H2 evolution and their charge carrier dynamics based on transient absorption spectroscopy. Transient absorption spectroscopy is recently applied to examine the charge carrier dynamics in semiconductors and 2D materials.22-23 In this article, we study the synthesis of BP NS/Pt (3 wt%)/TMC heterostructure which shows higher photocatalytic efficiency under visible and NIR irradiation compared with BP NS/Pt (3 wt%)/P25. In addition, we studied the effect of thickness of BP NS on photocatalytic H2 evolution. Based on fs-TRDRS, we demonstrated that visible light excitation can induce electron transfer from BP NS to TMC with a very short time scale (a few picoseconds) prohibiting electron hole recombination. More importantly, we confirmed that decreasing the thickness of BP NS enhances the electron injection to TMC to increase the photocatalytic activity.

2. EXPERIMENTAL DETAILS 2.1. Materials. Bulk BP crystals were obtained from Nanjing XFNANO Materials Tech Co., Ltd. N-methyl-2-pyrrolidone (NMP (99.5%)), sodium hydroxide (NaOH), ammonium nitrate (NH4NO3), ammonium fluoride (NH4F), absolute ethanol, and methanol were provided by Wako. Titanium (IV) fluoride (TiF4) and H2PtCl6 were obtained from Sigma Aldrich. All chemicals were used without additional purification. 2.2. Synthesis of BP NS. The BP NS were synthesized through a basic NMP solvent exfoliation. 24 Bulk BP (15 mg) was smashed and mixed with saturated NaOH/NMP solution (30 mL). The mixture was dispersed for 4 hours by an ultrasonic probe at a temperature below 25 oC by ice cooling to perform the liquid exfoliation of bulk BP. After exfoliation, the 3


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solution was centrifuged for 2 times at 2000 rpm for 20 min to detach the remaining bulk BP. Then, the supernatant was centrifuged at 10000 rpm to prepare BP NS. In addition, the supernatant can be centrifuged at different centrifugation speeds to prepare BP NS with different thickness and lateral size. BP NS were stored in NMP to protect it from the degradation. 2.3. Synthesis of TMC. TMC were synthesized according to our previous report.25 A precursor solution of TiF4, H2O, NH4NO3, and NH4F (molar ratio = 1:117:6.6:4) was dripped on a silicon wafer to obtain a fluffy layer, and heated in air with increasing temperature (10 oC

min-1) at 500 oC for 2 h. Then, the resulted material was heated at 500 oC in oxygen

atmosphere for 8 h to eliminate surface impurities. 2.4. Synthesis of Pt/TMC. Pt/TMC was prepared by the photochemical deposition method. 0.1 g of TMC, 50 mL of Milli-Q ultrapure water, 25 mL of methanol, and H2PtCl6 were dispersed together. The suspension was irradiated by UV light for 30 min at room temperature. Then, the solutions were centrifuged at 10000 rpm. Finally, the resulting material was heated in air (20 °C min−1) at 300 °C for 30 min. Pt/TMC with different wt% of Pt were prepared using different amounts of Pt. 2.5. Synthesis of BP NS/TMC and BP NS/Pt (3 wt%)/TMC. Firstly, 20 mg TMC was distributed in 10 ml NMP and sonicated till forming uniform dispersion. Then, a certain amount of BP NS in NMP solution was mixed with TMC dispersion. The sample was filled by Ar for 1 h to remove air. After that, the sample was sonicated in water bath for 3h. Then, the mixture was stirred overnight under Ar atmosphere. The suspension was centrifuged and cleaned with ethanol for many times. Then, the precipitate was vacuum dried. Different wt% of BP NS/TMC can be prepared by adding different amount of BP NS to TMC suspension. BP NS/Pt (3 wt%)/TMC was prepared by the same method but we used Pt (3 wt%)/TMC instead of TMC. 2.6. Characterization of samples. The morphology of surface was studied by a fieldemission scanning electron microscopy (FESEM, JEOL, JSM-6330FT) and a transmission electron microscopy (TEM, JEOL, JEM-2100 managed at 200 kV). The steady-state diffuse reflectance spectra (DRS) were recorded using a UV-visible-NIR spectrophotometer (Jasco, 4


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V-570). The thickness of BP NS was determined using a nanoscale hybrid atomic force microscope (Keyence VN-8010). The sample was prepared for AFM measurements by transferring the exfoliated BP NS into isopropyl alcohol (IPA) after washing 3 times and the sample was highly diluted (nearly transparent to the eye). Then, it was dropped on a newly cleaved mica surface and dried in air. X-ray diffraction (XRD) patterns of the prepared materials were acquired by a Smart lab system with Cu Kα radiation operated at 40 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was measured by a JEOL JPS-9010 MC spectrometer. 2.7. Photocatalytic H2 evolution. Prior to the photocatalytic reaction, Pt was loaded on TMC and P25. 2 mg of photocatalyst was distributed in 5 mL methanol-H2O solution (1 methanol: 4 H2O) and was sealed with a rubber stopper in a glass tube. The dispersion of the photocatalyst was purged with Ar for 30 min to confirm the removal of air. After that, the sample was irradiated with visible light (Asahi Spectra Hal-320, 200 mW cm-2) provided with a 420- and 780-nm cut off filter to control the wavelength of incident light under magnetic stirring at room temperature. The photocatalytic activity was measured during the irradiation by utilizing a Shimadzu GC-8A gas chromatograph with an MS-5A column and a thermal conductivity detector. The photocatalytic reaction for each sample was measured for 3 times. During the cycling test, the recycled photocatalyst was detached by centrifugation and re-suspended into 5 mL methanol-H2O solution to confirm its stability. To obtain an action spectrum, photocatalytic H2 evolution was recorded by the similar method. However, we control the light irradiation wavelength with different band pass filters (Asahi Spectra Hal-320; 1 mW cm-2 ± 5 nm). The apparent quantum efficiency (AQE) was calculated using the equation: AQE = (2 × number of H2 molecules/number of incident photons) × 100. 2.8. Photoelectrochemical experiments. Photoelectrochemical measurements were performed in a three-electrode system with an electrochemical analyzer (ALS, 660B) using 0.1 M NaOH ethanol solution as the electrolyte. A platinum (Pt) wire and an Ag/AgCl electrode were applied as the counter and reference electrodes, respectively. The working electrode was assembled as follows: 4 mg of photocatalyst was suspended in 100 µl of ethanol and then 50 µl of Nafion (Sigma Aldrich) was added to the above mixture to form 5


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homogenous suspension by ultrasonication for 30 minutes. Then, 10 µl of above suspension was drop casted onto a glassy carbon electrode followed by air drying before measurement. Asahi Spectra Hal-320 (300 mW cm-2) with 420- or 780-nm cut off filter was applied for a visible light source. The photocurrent response and electrochemical impedance spectroscopy (EIS) were recorded at room temperature. 2.9. Time-resolved diffuse reflectance spectral measurements. The fs-TRDRS was performed by the pump and probe method with a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz). The excitation pulse (520 nm, 2.2 μJ pulse-1) was generated using an optical parametric amplifier (Spectra Physics, OPA-800CF-1). A white light continuum pulse, which was produced by focusing the fundamental light on a sapphire crystal following the computer controlled optical delay, was used as the probe pulse. The powder samples were prepared as a dry solid thin film on cleaned glass cover slip. The probe pulse was focused on the sample and the reflected lights were measured by a linear InGaAs array detector connected with the polychromator (Solar, MS3504).

3. RESULTS AND DISCUSSION 3.1. Characterization of the synthesized materials. The morphology and thickness were distinguished by TEM, SEM, and AFM analyses. The TEM image of BP NS (Figure 1a) confirmed its two dimensional layered structure. BP NS were synthesized by liquid exfoliation in saturated NaOH/NMP. After ultrsonication for 4 hours, BP NS were separated by centrifugation at 2000 rpm where bulk and unexofliated BP were discarded. Then, a second centrifugation step was performed at 10000 rpm to obtain BP NS with lateral size between 100-900 nm (The mean lateral size is 200 nm) as shown in Figure S1. The thickness of BP NS was measured by AFM to be 3-5 nm (Figure S2). We used a 3 different centrifugation speed (2000-4000, 4000-7000, and 7000-10000 rpm) to obtain BP NS 4000 (the mean lateral size is 680 nm and the mean thickness is 12 nm), BP NS 7000 (the mean lateral size is 365 nm and and the mean thickness is 7 nm), and BP NS 10000 (the mean lateral size is 170 nm and the mean thickness is 2.5 nm) respectively, as shown in Figures S1-2. As depicted in Figure 1b, 6


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TMC have a sheet-like structure with size of several micrometres. The crystallinity of BP NS and TMC was studied by HRTEM as shown in Figure S3. TMC exhibited lattice spacing of 0.19 nm according to 001 facets. The lattice spacing of BP NS is 0.322 and 0.221 nm belonging to (012) and (014) facets, respectively.24 TEM image of BP NS/TMC in Figure 1c confirms the strong contact between BP NS and TMC. Moreover, the intimate contact between BP NS and TMC and the high crystallinity of BP NS/TMC can be confirmed from HRTEM image with lattice spacing

Figure 1. TEM images of BP NS (a), TMC (b), BP NS/TMC (c), HRTEM image of BP NS/TMC (d), STEM image (e), and EDX elemental mappings of Ti (f), O (g), and P (h) for BP NS/TMC. of 0.322 and 0.189 nm which are identified to BP NS and TMC, respectively (Figure 1d). In addition, the HAADF-STEM image of BP NS/TMC and its EDX elemental mapping (Figures 1e-h and S4) confirm the strong interaction between BP NS and TMC. TEM, HRTEM, HAADF-STEM images of BP NS/TMC and corresponding EDX elemental mapping are depicted in the supporting information (Figures S5-S12) to confirm the strong interaction between BP NS and TMC. Figure S13 shows SEM images of BP NS 4000 and BP NS 10000. Figure S14 shows the SEM images of BP NS/TMC confirming the strong contact between BP NS and TMC. XPS is a powerful tool to confirm the chemical properties of element in composite. 7


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Figure 2a is the high-resolution XPS spectra of P 2p. It clearly shows the presence of P 2p in BP NS and BP NS/TMC whereas it is absent in TMC. The P 2p in BP NS can be deconvoluted into two peaks at 129.8 and 130.6 eV assigned to P 2p3/2 and P 2p1/2, respectively, which characterize the crystalline BP. Small oxidized phosphorus (Pox) are observed at 134.2 eV which can be ascribed to defects of oxygen or surface sub oxide in the BP which are formed from solvent exfoliation process.14 After BP NS were hybridized with TMC, the P 2p3/2 and P 2p1/2 were shifted to higher binding energy (130.8 and 131.5 eV, respectively) confirming the strong interaction between BP NS and TMC. The increase of binding energy is resulted from the decrease of electron concentration in BP NS indicating effective electron transfer from BP NS to TMC.26 Ti 2p spectra were measured as shown in Figure 2b. Ti4+-O in TMC is characterized by main peaks at 459.1 and 465 eV.27 Subsequent to the treatment with BP NS, these two peaks exhibited shift to smaller binding energies (458.7 and 464.6 eV, respectively) compared to pure TMC. The decrease of binding energy can correspond to higher electron concentration in titanium confirming electron injection from BP NS to TMC.

Figure 2. High-resolution XPS spectra and fitted curve of P 2p (a), Ti 2p (b), and O1s (c) of TMC, BP NS, and BP NS/TMC.

8


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The high-resolution O 1s XPS spectra of TMC, BP NS, and BP NS/TMC are shown in Figure 2C. TMC show two peaks at 529.8 eV assigned to Ti-O bond and 531.8 eV, suggesting the formation of Ti-OH groups. The high-resolution O 1s XPS spectra of the BP NS exhibit three peaks at 530.8, 532.0, and 533.3 eV assigned to P=O, P-O-P, and P-OH bonds, respectively.26 In addition, BP NS/TMC shows the characteristic peaks of Ti-O bond at 529.8 eV, whereas P=O (530.6 eV), P-O-P (531.8 eV), and POH bonds (533 eV) are still present in the sample but it exhibit slight shifts to lower binding energies. Therefore, we can deduce that increase and decrease in binding energy can be ascribed to decreasing of electron density around BP NS and increasing it around TMC. Also, XPS spectra of bulk BP/TMC is shown in Figure S15 to exhibit no change compared to bulk BP and TMC indicating less reactivity of bulk BP to interact with TMC.

Figure 3. XRD patterns (a) and DRS (b) of TMC, BP NS, and BP NS/TMC.

The XRD patterns of TMC, BP NS, and BP NS/TMC are shown in Figure 3a. TMC have diffraction peaks at 25.3o, 37.8o, 47.9o, 53.8o, 55.1o, 62.7o, 68.7o, 70.0o, and 75.0o according to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) plane 9


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diffractions of anatase TiO2, respectively. The XRD patterns of BP NS agreed with orthorhombic phase: two main peaks at 16.8 o and 34.2o can be attributed to (020) and (040), respectively, according to the layered planes of BP.28 All diffraction peaks due to anatase TMC can be observed in BP NS/TMC. However, diffraction peaks of BP NS at 16.8o and 34.2o became weak in intensity due to the small wt% of BP NS on BP NS/TMC and the well distribution of BP NS onto the surface of TMC. Moreover, Raman spectrum of BP NS/TMC is shown in Figure S16 to have several characteristics peaks at 354.7, 435.1, and 460.4 cm-1 identified to Ag1, B2g, and Ag2 modes, respectively, of BP NS. These peaks exhibit slight blue shifts compared to the corresponding peaks of pristine BP NS [14], indicating the electronic interactions between BP NS and TMC. Moreover, the vibration modes located at 145 cm-1 (Eg), 200 cm-1 (Eg), 394 cm-1 (B1g), 515 cm-1 (B1g+A1g), and 634 cm-1 (Eg) confirm the presence of anatase TMC. The peaks related to BP NS show weaker intensity compared to the TMC peaks because of the small wt% of BP NS in BP NS/TMC and lower crystallinity of BP NS compared to TMC. As the photocatalytic activity is firstly limited by the ability of light absorption, we studied the optical absorption characteristics of the prepared samples (Figure 3b). TMC do not exhibit absorption in the visible light region because of its wide band gap about 3.2 eV. On the other hand, BP NS have a wide absorption from UV to NIR region. After hybridizing TMC with BP NS, BP NS/TMC show strong absorption in the visible and NIR. In addition, DRS of BP NS and BP NS/TMC with different lateral size are depicted in Figure S17. The band gap energies of BP NS and BP NS/TMC were estimated from Tauc plot using the 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.29 As shown in Figures S18-19, the band gap of BP NS is larger with smaller thickness: 1.25, 0.81, 1.08, and 1.43 eV for BP NS, BP NS 4000, BP NS 7000, and BP NS 10000. In addition, the band gap of BP NS/TMC was calculated to be 1.34, 0.88, 1.14, and 1.51 eV for BP NS/TMC, BP NS 4000/TMC, BP NS 7000/TMC, and BP NS 10000/TMC. 10


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3.2. Photocatalytic H2 evolution. The H2 evolution from the prepared samples was studied in the presence of methanol as a hole scavenger. Prior to the photocatalytic reaction, the Pt nanoparticles were loaded on the surface of TMC (Figure S20). Pt (3 wt%)/TMC depicted the highest photocatalytic H2 evolution under UV light irradiation among different wt% of Pt/TMC as depicted in Figure S21. For the sake of comparison, we prepared BP NS/P25. TEM and DRS are provided in Figure S22. The photocatalytic H2 evolution of the photocatalysts under visible light irradiation is plotted in Figure 4a. TMC don’t show any noticeable H2 evolution due to absence of visible-NIR light absorption whereas a trace amount of H2 was found with BP NS owing to the fast combination between photogenerated charges. On the other hand, we observed photocatalytic H2 evolution rate was 1.9 µmol h-1 for BP NS/Pt (3 wt%)/TMC while it for BP NS/Pt (3 wt%)/P25 was 0.4 µmol h-1. Bulk BP/Pt (3 wt%)/TMC does not show any photocatalytic activity under visible light irradiation. Moreover, different sacrificial reagents including lactic acid and triethanol amine can drive the photocatalytic reaction as shown in Figure S23. However, they exhibited lower efficiency compared to methanol. Figure S24 shows the effect of wt% of BP NS on the photocatalytic activity where the maximum photocatalytic activity was obtained at 20 wt%. Under simulated solar light irradiation which excites both BP NS and TMC. BP NS/Pt (3 wt%)/TMC shows photocatalytic activity about 17.4 μmol h-1 (Figure 4b), which is larger than 11.7 μmol h-1 for TMC indicating the effective utilization of all solar light due to wide absorption and the strong interaction between BP NS and TMC. On the other hand, P25 and BP NS/Pt (3 wt%)/P25 exhibited photocatalytic activity of 2.8 and 4.1 μmol h-1, respectively, under simulated solar light irradiation. Interestingly, under the irradiation in the NIR region (λ> 780 nm) as depicted in Figure 4c, BP NS/Pt (3 wt%)/TMC and BP NS/Pt (3 wt%)/P25 show photocatalytic H2 evolution rates of 0.4 and 0.1 μmol h-1, respectively. BP NS and TMC showed trace and negligible H2 evolution. These facts confirm the important role of BP NS as a photosenstizer which has wide absorption in visible and NIR region. As shown in Figure 4d, the photocatalytic activity of BP NS/Pt (3 wt%)/TMC is robust and highly reproducible after 4 reaction cycles. TEM, XPS, 11


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DRS, and Raman spectra were measured after the photocatalytic reaction to confirm the stability of the photocatalyst as shown in Figures S25-28.

Figure 4. Time-dependent photocatalytic H2 evolution curves of the synthesized photocatalyst under visible-light irradiation (a), solar light irradiation (b), NIR light irradiation (c), and cycle stability test for photocatalytic H2 evolution of BP NS/TMC at wavelengths (λ> 420 nm) (d). The action spectrum of BP NS/Pt (3 wt%)/TMC in Figure S29 tracks the absorption spectrum indicating that H2 evolution is prompted by photoabsorption of electrons. The AQE are 1.2 and 7.1 % at 780 and 420 nm, respectively, which are comparable with or better than other BP NS photocatalysts (Table S1). The thickness of BP NS influences the performance as catalyst for oxygen evolution reactions.30 To study the effect of thickness on the photocatalytic H2 evolution, we compared the photocatalytic activity of BP NS 4000/Pt (3 wt%)/TMC, BP NS 7000/Pt (3 wt%)/TMC, BP NS 10000/Pt (3 wt%)/TMC, and BP NS/Pt (3 wt%)/TMC. Under visible light irradiation (λ> 420 nm) (Figure S30a), the photocatalytic performance of BP NS 4000/Pt (3 wt%)/TMC was lower than BP NS/Pt (3 wt%)/TMC. Whereas for BP NS 7000/Pt (3 wt%)/TMC, the photocatalytic was higher than BP NS 4000/Pt (3 12


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wt%)/TMC. BP NS 10000/Pt (3 wt%)/TMC has higher photocatalytic activity compared to BP NS/TMC. These results can be explained on the basis of the thickness of BP NS. BP NS/TMC includes a wide range of thickness distribution (mostly 3-5 nm) but BP NS 4000/TMC contains thick NS (12 nm) which hinder the photocatalytic activity by limiting the charge transfer from BP NS 4000 to TMC. On the other hand, BP NS10000/TMC contains thinner BP NS which enhances charge transfer and increases the activity of photocatalytic of H2 evolution. Moreover, the photocatalytic H2 evolution under solar light irradiation follows the same trend of visible light irradiation as shown in Figure S30b. However, under NIR light irradiation (Figure S30c) BP NS/Pt (3 wt%)/TMC has photocatalytic activity of 0.41 μmol h-1 while the corresponding values were estimated to be 0.25, 0.45, and 0.18 μmol h-1 for BP NS 4000/Pt (3 wt%)/TMC, BP NS 7000/Pt (3 wt%)/TMC, and BP NS 10000/Pt (3 wt%)/TMC. Low photocatalytic activity was observed for BP NS 10000/Pt (3 wt%)/TMC under NIR light irradiation because the band gap is about 1.52 eV which limits the ability to absorb NIR light. On the other hand, BP NS 7000/Pt (3 wt%)/TMC has the highest photocatalytic activity under NIR light irradiation because it has less thickness compared to BP NS 4000/Pt (3 wt%)/TMC and narrow band gap compared to BP NS 10000/Pt (3 wt%)/TMC and BP NS/Pt (3 wt%)/TMC. 3.3. Mechanism of photocatalytic reaction. To understand the mechanism of photocatalytic reaction, we calculate the conduction band (CB) and valence band (VB) energy levels of bulk BP and BP NS through VB XPS spectra and Mott-Schottky plots as shown in Figures S31-S32. The detailed calculations are shown in the supporting information. The CB energy levels of bulk BP, BP NS, and TMC are calculated to be 0.20, -1.06, and -0.45 V, respectively. The VB energy levels of bulk BP, BP NS, and TMC were estimated to be 0.1, 0.23, and 2.75 V, respectively (Figure 5). According to these results, no photocatalytic activity of bulk BP/Pt (3 wt%)/TMC under visible light irradiation is rationalized because CB level of bulk BP is less negative than TMC. However, after liquid phase exfoliation, BP NS has more negative CB than TMC which allows effective electron injection from BP NS to TMC. Therefore, when BP NS/TMC 13


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is irradiated with visible light (λ> 420 nm), type II heterostructure is expected at the interface between BP NS and TMC.

Figure 5. Energy diagram and schematic illustration of electron injection from BP NS to TMC.

The photocurrent and EIS analyses were applied to confirm the charge separation in BP NS/TMC.31As shown in Figure 6a, BP NS exhibited a small photocurrent density due to fast combination between photogenerated charges. High photocurrent density of BP NS/TMC compared to BP NS and BP NS/P25 reveals that TMC can act as an electron sink enhancing charge separation due to its superstructure. Similar results were obtained under solar and NIR light irradiation as depicted in Figure S33. In addition, the photocurrent of BP NS/TMC including BP NS with different lateral size and thickness were studied under visible, solar, and NIR light irradiation as depicted in Figures S34-36. The high photocurrent density of BP NS 10000/TMC compared to other samples under visible and solar light irradiation confirms that decreasing the thickness enhances the electron injection from BP NS to TMC. However, under NIR light irradiation BP NS 7000/TMC shows high photocurrent because of its strong absorption in NIR compared to BP NS 10000/TMC and BP NS/TMC and its small thickness compared to BP NS 4000/TMC. Moreover, the migration of photogenerated electrons in BP NS, BP NS/TMC, and BP NS/P25 was studied by EIS (Figure 6b). The semicircle of the Nyquist plots is attributed to the migration resistance of photogenerated charge carrier. The smaller radius of BP 14


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NS/TMC compared to BP NS/P25 and BP NS indicates the enhanced electron transfer between BP NS and TMC. EIS data for BP NS 4000/TMC, BP NS 7000/TMC, and BP NS 10000/TMC are shown in Figures S34-36 under visible, solar, and NIR light irradiation. Small arc radius of BP NS 10000/TMC compared to other samples demonstrates low resistance for electron transfer with the decrease in thickness. To clarify the mechanism of photocatalytic reaction, we used fs-TRDRS as a strong tool to track the charge carrier dynamics of BP NS/TMC. All of the samples were

Figure 6. Photocurrent responses (a) and EIS (b) of BP NS, BP NS/TMC, and BP NS/P25 under visible light irradiation (λ> 420 nm). excited at 520 nm since the cooresponding energy of photons is below the band gap of TMC and can excite all BP NS with different thickness. Upon excitation, BP NS, BP NS/TMC, and BP NS/P25 (Figures 7a-c) exhibited a broad absorption in NIR region due to free and shallowly trapped electrons. However, TMC has no transient absorption signal due its wide band gap (Figure S37). Upon excitation, BP NS exhibited a fast decay of transient absorption signal due to the electron-hole recombination. 15


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Surprisingly, after BP NS combined with TMC, the transient absorption signal decayed slowly and faded within thousands of picoseconds. This finding demonstrates that in BP NS/TMC, the excited electrons on BP NS can transfer to the adherent TMC instead of electron-hole recombination. To emphasize these results, we plotted the transient absorption kinetic of pure BP NS, BP NS/TMC, and BP NS/P25 at 1000 nm in Figure 7d.

Figure 7. Transient absorption spectra observed during fs-TRDRS of BP NS (a), BP NS/TMC (b), and BP NS/P25 (c). Normalized transient absorption decays recorded at 1000 nm for the samples under 520-nm light irradiation (d). A comparison between normalized transient absorption spectra of BP NS and BP NS/TMC reveals that BP NS shows fast decay within a few picoseconds compared to slow decay for BP NS/TMC and BP NS/P25 revealing the suppression of electron-holes recombination. To gain better understanding about the electron transfer on BP NS/TMC, we applied an exponential decay function to fit the decay curve. The decay curve of BP NS can be fitted with biexponential decay: TA = A1exp(−t/τ1) + A2exp(−t/τ2) + TAt=0 where TA, A and τ refer the transient absorption, amplitudes, and lifetimes, respectively, 16


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as in Table 1. Table 1. Kinetic parameters (lifetimes (τ) and amplitudes) from exponential fittings of transient absorption decays at NIR region. The amplitudes of each component are given in parentheses. Sample

τ1 (ps)

τ2 (ps)

τ3 (ps)

τav (ps)

BP NS

0.5 (95 %)

3.4 (5 %)

-

1.23

BP NS/TMC

0.5 (91 %)

11 (7 %)

385 (2 %)

333

BP NS/P25

0.4 (91 %)

9 (7 %)

205 (2 %)

166

17


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The fast lifetime of 0.5 ps can be ascribed to electron captured by the surface defects

18


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of BP NS while the second lifetime of 3.4 ps is assigned to interband recombination of

19


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photogenerated electrons and holes. This fast decay and short lifetime suggest that

20


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recombination of photogenerated electrons and holes is the dominant pathway.

21


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However, the decay curves of BP NS/TMC and BP NS/P25 can be fitted with

22


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triexponential function: TA = A1exp(−t/τ1) + A2exp(−t/τ2) + A3exp(−t/τ3) + TAt=0. In the

23


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case of BP NS/TMC, the decay was fitted with τ1 = 0.5 ps (91%), τ2 = 11 ps (7%), and τ3 = 385 ps (2%) whereas BP NS/P25 exhibited τ1 = 0.4 ps (91%), τ2 = 9 ps (7%), and τ3 = 205 ps (2%). τ1 is assigned to electron captured in BP NS. τ2 is attributed to charge distribution at the boundary between BP NS and TMC and electron injection from BP NS to TMC. τ3 is the time demanded for the electrons to move to the reactive sites on the surface of TMC for photocatalytic H2 evolution which possibly took place in µs-ms time scale. The longer τ2 for BP NS/TMC compared to BP NS/P25 demonstrates

Figure 8. Transient absorption spectra observed during fs-TRDRS of BP NS 4000/TMC (a), BP NS 7000/TMC (b), BP NS 10000/TMC (c). Normalized transient absorption decays recorded at 1000 nm for the samples under 520-nm light irradiation (d). improved charge separation at the interface of BP NS/TMC leading more electrons to be injected to TMC. The longer τ3 for BP NS/TMC than that of BP NS/P25 reveals the enhanced charge separation through highly ordered superstructure of TMC compared to less ordered P25. The average lifetimes were calculated from the following equation: 𝑖=𝑛

𝑖=𝑛

τ = ∑𝑖 = 1𝐴𝑖𝜏2𝑖 / ∑𝑖 = 1𝐴𝑖τi.32 The increase of average lifetime and the appearance of 24


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additional component τ3 for BP NS/TMC confirm electron transfer from BP NS to TMC. To understand deeply the mechanism of photocatalytic H2 evolution, we studied fsTRDRS of BP NS and BP NS/TMC with different lateral size and thickness. The fsTRDRS showed slower decay with the increase of thickness and lateral size (Figure S38). There is still signal for BP NS 4000 at 8 ps while for BP NS 10000 is almost zero after 2 ps. In addition, the carrier lifetime increased by the increase of thickness and flake size of BP NS (Table S2): τ1 = 0.9 ps and τ2 = 8 ps for BP NS 4000 and τ1 = 0.4 and τ2 = 1.3 ps for BP NS 10000. The recombination is mainly dominated by defects within the band gap where in 2D BP NS the most common defects are at the edge. As excited carrier can transfer to the edge quickly in smaller (thin) flake compared to larger (thick) flake NS, smaller BP NS will have short lifetime.33 Figures 8a-c show transient absorption signals of BP NS/TMC samples in NIR region. The decay kinetics of BP NS/TMC are shown in Figure 8d. BP NS 10000/TMC has the shortest τ2 = 7 ps compared to 11 ps for BP NS/TMC, 15 ps for BP NS 7000/TMC, and 20 ps for BP NS 4000/TMC as shown in Table S3. These results confirmed that enhanced charge separation and electron transfer at the interface of BP NS/TMC are strongly related to the thickness. Moreover, the longer τ3 for BP NS 10000/TMC (444 ps) than that of BP NS/TMC (385 ps), BP NS 7000/TMC (312 ps), and BP NS 4000/TMC (284 ps) reveals the accumulation of electrons on TMC due to enhanced electron transfer in BP NS 10000/TMC. 4. CONCLUSIONS We successfully synthesized visible-NIR light absorbing BP NS sensitized TMC which owns almost full spectrum absorption for photocatalytic water reduction. The results of XRD, XPS, SEM, and TEM indicate the strong interaction between two dimensional BP NS and plate superstructure of TMC. The photocatalytic H2 evolution rate of BP NS/Pt (3 wt%)/TMC is 1.9 and 0.41 µmol h-1 at wavelength longer than 420 and 780 nm, respectively. Moreover, thickness dependant photocatalytic activities of BP NS/Pt (3 wt%)/TMC has been studied where the photocatalytic activities of BP NS 25


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exhibited an improved performance along with the decrease in thickness of BP NS by selective centrifugation. The photostability was clearly observed in the reproducibility test for H2 evolution. Based on fs-TRDRS, we have a clear evidence for electron transfer from BP NS to TMC under visible light excitation. Also, fs-TRDRS demonstrated that decreasing the thickness of BP NS can increase the rate of electron injection from BP NS to TMC. The present work provides a detailed study about the photocatalytic activities and charge carrier dynamics of BP NS/TMC paving the way of engineering BP NS and metal oxide semiconductors in photocatalytic and optoelectronic applications.

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. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] Notes: The authors declare no competing ﬁnancial interest. Supporting Information Supporting Information Available: Additional figures and data. This material is available free of charge via the Internet at http://pubs.acs.org. AFM images, TEM images, HRTEM images, SEM images, Mott-Schottky plots, photocatalytic H2 evolution experiments, TEM, XPS, and DRS analyses for the stability experiment, photocurrent, and fs-TRDRS for different samples (BP NS, BP NS 4000, BP NS 7000, BP NS 10000, BP NS/TMC, BP NS 4000/TMC, BP NS 7000/TMC, and 26


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Black Phosphorus Sensitized TiO2 Mesocrystal Photocatalyst for

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