TiO2 mesocrystals composited with gold nanorods for highly efficient visible-NIR-photocatalytic hydrogen production

Author’s Accepted Manuscript TiO2 mesocrystals composited with gold nanorods for highly efficient visible-NIR-photocatalytic hydrogen production Ossama Elbanna, Sooyeon Fujitsuka, Tetsuro Majima

Kim,

Mamoru www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(17)30144-1 http://dx.doi.org/10.1016/j.nanoen.2017.03.014 NANOEN1842

To appear in: Nano Energy Received date: 10 February 2017 Revised date: 6 March 2017 Accepted date: 6 March 2017 Cite this article as: Ossama Elbanna, Sooyeon Kim, Mamoru Fujitsuka and Tetsuro Majima, TiO 2 mesocrystals composited with gold nanorods for highly efficient visible-NIR-photocatalytic hydrogen production, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

TiO2 mesocrystals composited with gold nanorods for

highly

efficient

visible-NIR-photocatalytic

hydrogen production Ossama Elbanna, Sooyeon Kim, 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] (T.M.) Abstract TiO2 mesocrystals (TMC) have efficient charge transport properties and long-lived charges which cause high photoconductivity and photocatalytic activity. However, TMC have no absorption in the region of visible and near-Infrared (NIR) light. The optical resonance of Au nanorods (NRs) depending mainly on their length and width (aspect ratio) can be used to design panchromatic absorbers, covering most of the useful solar spectrum. Therefore, in this article, Au NRs and TMC composites were prepared by the ligand exchange method to show highly efficient H2 production (924 µmol h-1 g-1) under visible-NIR light irradiation. The efficient H2 production is explained by surface plasmon resonance (SPR) of Au NRs, electron injection to TMC, efficient charge transport in TMC due to the superstructure of TMC and water reduction on TMC. Single-particle confocal fluorescence microscopy and time-resolved diffuse reflectance measurements confirmed the efficient hot electron injection and charge separation from Au NRs with strong SPR to the superstructure of TMC.

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KEYWORDS: TiO2 mesocrystal, Au NRs, H2 production, Visible-NIR photocatalysis, hot electron, charge transfer 1. Introduction Solar photocatalysis based on semiconductor materials has been thoroughly investigated over the past decades as an essence technology for solar light harvesting and solar conversion process [1-2]. Since the discovery of the photocatalytic activity,[3] TiO2 has been the most extensively studied semiconductor for photocatalysis such as pollutant degradation, water splitting and organic synthesis because of its stability, low cost and nontoxicity [4]. As a novel class of TiO2 materials, TiO2 mesocrystals (TMC) have been prepared through topotactic conversion of NH4TiOF3 in the presence of the anionic surfactant [5]. Mesocrystals consist of highly ordered nanoparticles have large surface area and high crystallinity at the same time [6]. In our previous work, we have shown that TMC have long-lived charges compared to poly-crystalline materials, leading to high photoconductivity and photocatalytic activity [7]. Since the wide band gap of TiO2 (3.2 eV) greatly restricts its performance only in UV region (about 3% of solar light), various attempts have been made on the extension of the TiO2 photoresponsibility from UV to visible and NIR regions. Doping of metal and non-metal ions, [8-9] sensitization with organic dyes and coupling with narrow band gap semiconductors are examples of such attempts [10-11]. Noble metals such as Au and Ag exhibit surface plasmon resonance (SPR) in visible-NIR region to generate hot electrons under the irradiation. When such noble metals are composited with n-type semiconductor, the SPR of noble metal can inject hot electrons into the conduction band (CB) of the semiconductors via Schottky junction, and the holes remain in noble metal nanoparticles where oxidation reactions take place [12-13]. For example, hot electrons generated in Au nanoparticles (Au NPs) are injected to wide band-gap

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semiconductors such as TiO2, causing photocatalytic hydrogen production [14-15]. Au NPs can be used as photosensitizers instead of common dye molecules in dye-sensitized solar cells [16]. The plasmon-induced charge-transfer mechanisms between excited Au NPs and TiO2 have been examined by many research groups [17-18]. Based on ultrafast pump-probe femtosecond laser transient absorption measurement, such electron transfer process was reported to proceed within less than 100 fs [19]. Among plasmonic composite photocatalysts, great efforts were concentrated on harvesting visible light using spherical Ag and Au NPs which have SPR at wavelength of 390-460 and 520640 nm, respectively [20]. However, harvesting longer wavelength in NIR region, has received little attention. One dimensional Au NRs with tunable SPR where the longitudinal SPR (LSPR) can be varied from visible (600 nm) to NIR (1200 nm) depending on the aspect ratio (defined as length divided by width of the nanorods) can extend the absorption range to longer wavelength [21]. In addition, Au NRs can be combined with efficient electron acceptor such as Pt and Pd to maximize charge separation [22-23]. Photoinduced electron transfer from Au to semiconductor is required for the efficient photocatalytic reactions, because the separated electrons and holes participate to the reductive and oxidative reactions, respectively. To date, there are some reports on Au NRs/TiO2 as plasmonic photocatalysts. Yen and his coworkers have demonstrated broadband visible-NIR light absorption by introducing Au NRs on TiO2 as an annetana [24]. Moreover, Au NRs/TiO2 core shell structure [25] was investigated by several groups and relatively high photocatalytic activities were achieved. Since TMC exhibit high electron flow and remarkably long-lived charges, TMC composited with Au NRs (Au NRs/TMC) are expected to have improved photocatalytic activity under visible-NIR light irradiation. However, there is no report on Au NRs/TMC.

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In the present work, Au NRs with controllable size and tunable SPR band were loaded onto anatase TMC superstructures. Au NRs/TMC were used as photocatalyst to produce H2 under visible-NIR light irradiation. Hot electron injection from Au NRs to TMC was studied by singleparticle photoluminescence (PL) spectroscopy. In addition, the injection of hot electrons was investigated by measuring the specific absorption of electrons in TMC by time-resolved diffuse reflectance spectroscopy. Au NRs/TMC have much higher photocatalytic activity than Au NRs/P25, confirming the important role of superstructure of TMC to enhance the electron transfer and the photocatalytic activity. 2. Experimental section 2.1. Materials: Hexadecyltrimethylammonium bromide (CTAB, >99%), hydrogen tetrachloroaurate (III) trihydrate (HAuCl4•H2O, ≥99.999%), L-ascorbic acid (BioUltra, ≥99.5%), silver nitrate (AgNO3, ≥99%), sodium borohydride (NaBH4, ≥99%), titanium (IV) fluoride (TiF4) and 3mercaptopropionic acid (3-MPA) were purchased from Sigma Aldrich. The ammonium nitrate (NH4NO3), ammonium fluoride (NH4F) and hydrochloric acid (HCl, 37% in water) were purchased from Wako Pure Chemical Industries. Sodium oleate (NaOL, >97%) was purchased from TCI America. All chemicals were used without further purification. 2.2. Material synthesis: 2.2.1. Preparation of Au NRs: Au NRs were prepared in a high yield according to a simple seed-growth method [26]. Seed solution was prepared by mixing CTAB solution (5 mL, 0.20 M) with HAuCl4 (5.0 mL, 0.50 mM). Then, ice-cooled NaBH4 solution (0.60 mL, 0.010 M) was injected to the stirring solution, which resulted in the formation of a brownish yellow solution. Vigorous stirring of the seed solution was continued for 2 min. The seed solution was aged at

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room temperature for 30 min before use. To prepare the growth solution, 2.4 g of CTAB and 0.44 g of NaOL were dissolved in 100 mL of warm water (~50oC). The solution was allowed to cool down to 30oC and different volumes of 10 mM AgNO3 solution were added to control the aspect ratios. The mixture was kept undisturbed at 30oC for 15 min, then, 100 mL of 1 mM HAuCl4 solution was added. After 90 min of stirring (700 rpm), a definite volume of HCl (37 wt % in water, 12.1 M) was introduced. After another 15 min of slow stirring at 400 rpm, 0.32 ml of 0.1 M ascorbic acid was added and the solution was vigorously stirred for 30 s. Finally, a small amount of seed solution was added to the growth solution. The resultant mixture was stirred for 30 s and left undisturbed at 30oC for 12 h for nanorods growth. The final products were separated by centrifugation at 7000 rpm for 30 min followed by removal of the supernatant. 2.2.2. Preparation of TMC: TMC were prepared according to our previous report [27]. A precursor solution of TiF4, H2O, NH4NO3 and NH4F (molar ratio = 1:117:6.6:4) was placed on a silicon wafer to form a thin layer. The precursor was calcined in air with a heating rate of 10oC min-1 at 500oC for 2 h. The obtained samples were calcined at 500oC in oxygen atmosphere for 8 h to ensure removal of surface residue. 2.2.3. Preparation of Au NRs/TMC: For the preparation of Au NRs/TMC, 50 mg of TMC were dispersed in aqueous solution containing 0.1 ml of ammonia (28%) and 0.2 ml of 3-MPA for 12 h. Then, 5 ml of Au NRs solution was added under vigorous stirring/sonication in a period of 20 min. The mixture was kept for 12 h. Then, the obtained product was centrifuged, washed with ethanol and water to remove the remaining reactants, and dried at 60°C in vacuum [28]. 2.3. Characterization of materials: The morphologies were examined using a field-emission scanning electron microscopy (FESEM) (JEOL, JSM-6330FT) and a transmission electron microscopy (TEM) (JEOL, JEM-2100 operated at 200 kV). Extinction spectra were measured

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using quartz cuvettes on a Shimadzu UV-3600 UV-visible-NIR spectrophotometer. The UVvisible diffuse reflectance spectra (UV-visible DRS) were measured by a UV-visible spectrophotometer (Jasco, V-570). The Au content of the photocatalysts was determined with an inductively coupled plasma optical emission spectrometer (Shimadzu, ICPS-8100). 2.4. Photocatalytic H2 Production: Au NRs/TMC photocatalysts were further treated with 60% HClO4 under ultrasonic for 10 min, then thoroughly washed and redispersed in equivalent MilliQ ultrapure water. The same procedure was used for preparing the sample for single-particle PL measurements and time-resolved diffuse reflectance spectral measurements. In a typical photocatalytic experiment, Au NRs/TMC photocatalysts were dispersed in 5 mL of aqueous solution of 20 vol % methanol, sealed with a rubber stopper in a tube. The suspension was purged with argon for 30 min to remove dissolved oxygen. Then the tube was irradiated with visible-NIR light (Asahi Spectra, LAX-C100, 200 mW cm-2) with magnetic stirring at room temperature. A 420-nm cutoff filter was used to remove UV light. H2 evolution was determined by using a Shimadzu GC-8A gas chromatograph equipped with an MS-5A column and a thermal conductivity detector. In the cycling test, the used catalyst was collected by centrifugation to check its stability. To get an action spectrum, H2 formation in an aqueous suspension of catalyst was performed under the irradiation of monochromatic light with wavelengths of ± 5 nm. The apparent quantum efficiency (AQE) was calculated using the following equation: AQE = (2 × number of H2 molecules / number of incident photons) × 100. 2.5. Sample preparation for single-particle photoluminescence (PL) experiments: The grid cover glasses (Matsunami Glass Ind., thickness 0.15–0.18 mm) were cleaned by sonication in a 20% detergent solution (As One, Cleanace) for 6 h, followed by washings with warm water for 5 times. Finally, the cover glasses were washed again with Milli-Q ultrapure water. The as-

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synthesized Au NRs suspensions were centrifuged at 12000 rpm (Hitachi, himac CF16RX) to remove excess surfactant and washed with Milli-Q ultrapure water for 2 times. Au NRs with a low concentration were spin-coated on the pre-cleaned grid cover glass, which was subsequently annealed at 100oC for 0.5 h to immobilize Au NRs on the glass surface. For Au NRs/TMC, welldispersed aqueous suspensions of Au NRs/TMC after washing with HClO4 were subsequently spin-coated on the cleaned grid cover glasses. The grid cover glasses were annealed at 100oC for 1 h to immobilize the particles on the glass surface. 2.6. 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). Before proceeding to single-particle PL measurement, the exact place of each coated sample on the grid cover glass was confirmed by a bright-field microscopy with an electron-multiplying charge-coupled device (EM-CCD) camera (Hamamatsu Photonics, ImagEM) operated by a MetaMorph software (Molecular Device). In order to obtain PL images, the samples were excited through an oil-immersion objective lens (Olympus, UplanSApochromat, 100×, 1.4 NA) with a circular-polarized 485-nm laser (PicoQuant, LDH-D-C-485) in continuous wave (CW) mode 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 detected by a single-photon avalanche photodiode (Micro Photon Devices, PDM 50CT) through a dichroic beam splitter (Semrock, Di03-R488-t1-25x36) and a long pass filter (Semrock, BLP01-488R-25). For the spectroscopy, only the emission that passed through a slit was detected with an EM-CCD camera (Princeton Instruments, ProEM) equipped with imaging spectrograph (Acton Research, SP-2356). The spectra were typically integrated for 20 s.

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2.7. Time-resolved diffuse reflectance spectral measurements The femtosecond diffuse reflectance spectra (TDR) were measured by the pump and probe method using a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz). An optical parametric amplifier (Spectra Physics, OPA-800CF-1) was used to produce the excitation pulse (520 nm, 2.2 μJ pulse-1). The white light continuum pulse, which was generated by focusing the residual of the fundamental light on a sapphire crystal, was directed 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). All measurements were carried out at room temperature. 3. Results and Discussion 3.1. Material preparation. Four Au NRs with different aspect ratios (2.3, 3.2, 4.8 and 7.5) were prepared by the seedmediated method, using of CTAB and NaOL as stabilizers [26]. The sizes were relatively uniformed as shown in the TEM images of Au NRs-660, Au NRs-715, Au NRs-859 and Au NRs-975 (Fig. S1), where numbers denote the wavelength of the LSPR absorption peak. The SPR absorption spectrum of Au NRs is characterized by two peaks corresponding to the oscillation of free electrons along and perpendicular to the long axis of the rods. The transversal SPR (TSPR) has resonance around 520 nm, while LSPR appears at longer wavelength, depending on the aspect ratio. By controlling the aspect ratio of Au NRs, the LSPR absorption changes from 660 to 975 nm covering visible to NIR region as shown in Fig. S2. Four Au NRs/TMC were obtained by loading Au NRs with different aspect ratios onto TMC using ligand exchange method [28] as shown in Fig. 1a. Plate TMC were obtained, similar to our pervious report, by annealing thin layer of aqueous solution containing TiF4, NH4F and NH4NO3 at 500oC

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[27]. TMC were immersed in 3-MPA which acted as a bifunctional surface modifier attaching COOH group to the OH-group on the surface of TMC and thiol groups with highest affinity to noble metal surface particularly Au were exposed outwards. Then, specific volume of Au NRs was added to the functionalized TMC to form Au-S bond leading to effective connection between TMC and Au-NR. 3-MPA at the Au-TMC interface was further segregated by vacuum evaporation and annealing treatment to enhance the interfacial contact. Au NRs/P25 were prepared with the same method as for Au NRs/TMC. The TEM image and UV-visible diffuse reflectance spectra are shown in Fig. S8. The bare TMC have only strong absorption below 400 nm owing to the large bandgap of TMC. After loading of Au NRs onto TMC, two SPR peaks (TSPR and LSPR) as in the corresponding Au NRs appear in the visible and NIR regions. In addition, the LSPR absorption ranged from 700 to 1030 nm as shown in Fig. 1b depending on the aspect ratio of Au NRs, indicating that photocatalysts with tunable light harvesting range were prepared by controlling the aspect ratio of Au NRs. The corresponding photocatalysts are denoted as Au NRs/TMC-780, Au NRs/TMC-890 and Au NRs/TMC-1030. The LSPR peak in Au NRs/TMC was red shifted compared to bare Au NRs.

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Fig. 1. (a) Preparation scheme for Au NRs/TMC. (b) UV−visible diffuse reflectance spectra of TMC and Au NRs/TMC with different aspect ratios.

Figure 2a shows a TEM image of Au NRs-660 with average length and width of 74 ± 3 and 33 ± 3 nm, respectively. TEM images of other three Au NRs are shown in Fig. S1. After loading Au NRs onto TMC, the length and width of Au NRs were nearly the same (69 ± 3 and 28 ± 3 nm, respectively) for Au NRs/TMC-690. Length and width of Au NRs before and after loading onto TMC are summarized in Figs. S3, S5 and Table S1, suggesting that the morphology of Au NRs has been conserved after loading onto TMC and that the red shift of LSPR peak Au NRs/TMC is mainly due to the larger refractive index of TMC compared to air [29]. The TEM images for Au NRs/TMC-690 (Figs. 2b and c) reveal the uniform distribution of Au NRs onto TMC and the good connection between them after washing for several times and drying under vacuum. The high resolution TEM (HRTEM) image taken at the interface of Au NRs and TMC (Fig. 2d) clearly reveals lattice fringes of anatase (200) or (020) with the lattice spacing around 0.189 nm for TMC and the (002) plane (perpendicular to (200) plane in face-centered cubic metals) with

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lattice spacing around 0.201 nm for Au NRs. Such close contact at the interface between Au NRs and TMC is considered to be effective for injection of hot electrons generated in Au NRs to TMC. Figure S4 shows TEM images for Au NRs/TMC-780, Au NRs/TMC-890 and Au NRs/TMC-1030 revealing that Au NRs were loaded onto plate-shaped TMC for all prepared samples. In addition, SEM images in Fig. S6 confirm the loading of Au NRs onto TMC for Au NRs/TMC-780 and Au NRs/TMC-890. Moreover, elemental mapping in Fig. S7 confirmed that Au NRs were attached to the surface of TMC.

Fig. 2. TEM images of Au NRs-615 (a) and Au NRs/TMC-690 (b). HRTEM images of Au NRs/TMC-690 (c, d).

3.2. Photocatalytic activity. The photocatalytic H2 production was performed in the presence of methanol as a sacrificial reagent under visible-NIR light irradiation using a 300-W Xenon lamp equipped with a 420 nm cutoff filter. As shown in Fig. 3a very small amount of H2 was detected with pure Au NRs and TMC, while H2 production was observed for Au NRs/TMC under visible and NIR light. As

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shown in Fig. S9, the H2 production rate increased nearly double after treatment with HClO4 to remove the surfactants, indicating that the surfactant quenched holes in Au NRs by the adsorbed methanol (electron donor). As shown in Fig. 3a, the amounts of H2 produced were 924 and 65 µmol h-1 g-1 for Au NRs/TMC-780 and Au NRs/P25-780, respectively. The amounts of H2 produced for other Au NRs/TMC are summarized in Table S1. It should be noted that the photocatalytic reaction occurred without cocatalysts and observed activity is surprisingly higher than those of AuNPs /TMC/Pt and Au NPs/TMC (Table S2) [7] . This can be ascribed to the broader plasmon band of Au NRs that enables the harvesting of a wide range of visible and NIR light. Also, it has been reported that the optical near-field on the edge of Au NRs enhanced by SPR assists the electron excitation of Au NRs even with NIR light irradiation, [30] leading to efficient electron transfer from Au NRs to the CB of TMC. The great difference in the photocatalytic activities of Au NRs/TMC-780 and Au NRs/P25-780 under visible-NIR light irradiation suggests that the TMC superstructure facilitates electron transfer between nanocrystals, leading to inhibition of the charge recombination, enhancing the charge separation, and consequently higher photocatalytic activity than Au NRs/P25-780. Au NRs/TMC-780 exhibited higher photocatalytic activity than Au NRs/TMC-690, Au NRs/TMC-890 and Au NRs/TMC-1030. In this respect, it is supposed that increasing the aspect ratio favours the adsorption of the reactants due to higher ratio of higher energy plane (100) than (110) plane, thus improving the oxidation (reaction between methanol and holes) on Au NRs after visible light excitation, retarding electron hole recombination. However, further increasing the ratio from 3.2 to 7.5 shifts the SPR band to a longer wavelength with a lower photon energy which results in decreasing the photocatalytic activity [31]. In addition, the influence of the amount of Au on the photocatalytic activity has been studied for Au NRs/TMC-780 as shown in

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Fig. S10. The photocatalytic activity increased with increasing the Au amount till 1 wt %. A further increase in Au content lead to decrease in the photocatalytic activity because excess Au acts as the recombination center for photogenerated electrons and holes. To obtain an action spectrum, the AQE was calculated as function of wavelength using monochromatic light with width of ± 5 nm and intensity of 4.8 mW cm-2. The action spectrum of Au NRs/TMC-780 in Fig. 3b follows the trend of extinction spectrum confirming that H2 production is induced by photoabsorption due to the SPR of Au NRs loaded onto TMC. The AQE was 0.53 and 0.41% at 780 and 520 nm, respectively. Similarly, the action spectra for Au NRs/TMC-690, Au NRs/TMC-890 and Au NRs/TMC-1030 are shown in Fig. S11, confirming that H2 production is induced by the photoabsorption. The stability of Au NRs/TMC-780 was checked by filtering the catalyst and using it for the second run under the same condition. After two consecutive uses, Au NRs/TMC-780 showed good stability without significant loss in the photocatalytic activity as shown in Fig. S12.

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Fig. 3. Time courses of H2 production from water in the presence of methanol (20 vol %) using TMC, different Au NRs/TMC, or Au NRs/P25 as a photocatalyst under visible-NIR light irradiation (λ > 420 nm, 200 mW cm-2) (a). Extinction spectrum and action spectrum of AQE for the photocatalytic H2 production using Au NRs/TMC-780 (b).

3.3. Single particle PL measurement. To clarify the electron injection from Au NRs to TMC under visible light irradiation, singleparticle PL spectra were measured using 485-nm laser excitation. Single-particle PL can avoid the complication due to the sample inhomogeneity and help to establish the morphology-property relationship [32]. Because of the high excitation rate in the single-particle PL measurements using a laser excitation source, enough signals were obtained for recording single-particle PL spectra. SEM image in Fig. S13 confirmed that Au NRs were monodispersed on the grid cover glass. The typical single-particle PL images of individual Au NRs and Au NRs/TMC-780 spin coated on grid cover glasses are shown in Figs. 4a and b. The PL spectra of six representative Au

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NRs are shown in Fig. S14, which have two bands at 520 and 720 nm assigned to TSPR and LSPR, respectively. The strong LSPR, even when the excitation wavelength is far away from the LSPR peak, indicates that the photon energy is transferred to LSPR because the excitation of the LSPR occurs less efficiently than that of the TSPR. For Au NRs/TMC-780, a pattern-matching method was used to relate the morphology of each AuNR observed in SEM image (Fig. S15) and its PL spectra obtained from the confocal microscope (Fig. S16), indicating that Au NRs are monodispersed onto the surface of TMC. PL spectra of seven Au NRs loaded onto TMC are shown in Fig. S17. Figure 4c shows the PL spectra of representative Au NRs before and after loading onto TMC exhibiting strong quenching of the LSPR peak. The PL intensities of LSPR for 30 particles of bare Au NRs and 30 particles of Au NRs loaded onto TMC are summarized in Fig. 4d. The quenching of LSPR of Au NRs after loading onto TMC was clearly observed which indicates efficient electron transfer from Au NRs to TMC, while TSPR was nearly equal for Au NRs and Au NRs/TMC. We observed various LSPR PL intensities from Au NRs because they have morphological heterogeneity during chemical synthesis. In order to analyze the data statistically, we define the intensity ratio of LSPR against TSPR (Int LSPR/IntTSPR). The TSPR PL changed slightly after loading onto TMC while LSPR PL was quenched significantly. The IntLSPR/IntTSPR decreased significantly after loading Au NRs onto TMC as shown in Fig. 4e. In our single-particle PL measurement, the interband transition of Au NRs was excited using 485-nm laser to generate electron-hole pairs which lose their energies through nonradiative decays because of the large density of states of the sp-CB and d-valence band (VB). However, the TSPR from a fast interconversion between electron-hole pairs decays radiatively to show the PL peak at a short wavelength [33]. This process is followed by energy transfer to LSPR, which subsequently emits a photon, leading to the LSPR PL as shown in Fig. 5b. The TSPR has a

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shorter lifetime (~5 fs) while the LSPR PL does longer lifetime (9–18 fs) [32]. This indicates that the LSPR PL is more easily influenced by other decay processes, while the TSPR PL is mainly related to its intrinsic features. When Au NRs were loaded onto TMC, Schottky barrier (ФSB) is formed at the Au NRs-TMC interface. After excitation of Au NRs with λ > 420 nm, SPRenhanced electromagnetic field is formed on the Au NRs to increase the yield of interfacial hot electrons with higher potential energy than ФSB, inducing fast and efficient transfer of hot electrons to TMC. Such hot electrons are available for photoreduction competing with the LSPR emission and resulting in the PL quenching (Fig. 5c) [34]. The Schottky barrier at the interface also helps hot electrons injection to be accumulated in the CB of TMC, prohibiting them from returning back to Au NRs [35].

Fig. 4. PL images of Au NRs and Au NRs/TMC-780 supported on grid cover glass (a and b, respectively). Representative PL spectra of single Au NR before and after loading onto TMC (c). PL intensity of LSPR (IntLSPR) for 30 Au NRs before and after loading onto TMC (d). PL intensity ratio of LSPR against TSPR (Int LSPR/IntTSPR) for 30 Au NRs before and after loading onto TMC (e).

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3.4. Time-Resolved Diffuse Reflectance Measurements. The lifetime of the generated electrons was measured in order to confirm the hot electron injection from the Au NRs to TMC. It is known that the dynamics of photoexcited Au NPs give three lifetimes, which are assigned to electron-electron scattering (10–100 fs) leading to a hot thermalized Fermi distribution of the CB electrons, cooling of hot electrons due to the electronphonon scattering (1−10 ps), and heat dissipation from the Au NPs to the surrounding through phonon-phonon scattering (∼100 ps) [36]. The hot electrons generated in Au NRs are injected to the CB of TMC before or during the electron-electron scattering process (10–100 fs). Upon the 520-nm laser excitation of Au NRs/TMC-780 (80 fs pulse width and 2 μJ pulse-1 intensity), a broad absorption band was observed in the 800–1200 nm region as shown in Fig. S18 to be assigned to trapped electrons (600–1000 nm) and free electrons (increasing monotonically from the visible to NIR regions) [37]. However, TMC has no absorption in the visible region due to the wide band gap, showing no generation of charges with the visible light excitation as observed in Fig. S18 (C). Figure 5a exhibits the normalized transient absorption decay profiles up to 1.5 ns for Au NRs/TMC-780 and Au NRs/P25-780 probed at 900 nm. Much faster electron injection and slower charge recombination were observed for Au NRs/TMC-780 than for Au NRs/P25780. The transient absorption profile for Au NRs/TMC-780 was fitted to a three-exponential function with time constants of 4.2 ps (29%), 244 ps (29%) and 4840 ps (42%) (Table 1) where τ1 is assigned to e-trapped below the CB of TMC, τ2 and τ3 are responsible for the diffusion of electrons to the reaction site through the nanocrystals structure and photocatalytic reaction in the μs time scale, which is beyond the time window measured here. Three decay lifetimes of 3.5 ps (28%), 209 ps (44%) and 418 ps (28%) for Au NRs/P25-780. The obtained results suggest that the superstructure of TMC consisting of highly ordered junction between the subunits (TiO2

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nanocrystals) has important role to improve the photocatalytic activity. The junction is much more ordered than that in P25. Therefore, the electrons can transfer easily through the well matched junction of TMC to increase the lifetime of electrons, enabling the free electrons in CB to diffuse much longer distance up to the reaction site

Fig. 5. Normalized transient absorption profiles observed at 900 nm for Au NRs/TMC-780 and Au NRs/P25-780 (a). Schematic diagram of SPR excitation of Au NR, PL and electron transfer to TMC (b). Scheme for the photocatalytic reaction involving electron transfer between Au NR and TMC, and charge trapping by H2O and MeOH (c). Transverse SPR (TSPR), longitudinal SPR (LSPR), photolumiscence (PL), charge separation (CS), EF,M, EF,S are Fermi level of metal and semiconductor, respectively, Evac is the vacuum energy, ϕ M is the work function of metal, ϕ SB is the Schottky barrier, χs is the electron affinity of the semiconductor, Ev and Ec is valence band (VB) and conduction band (CB) of anatase TMC.

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Table 1. Kinetic Parameters of Decays Observed for Au/TiO2 Samples. Sample

τ1 (ps)

τ2 (ps)

τ3 (ps)

Au NRs/TMC-780

4.2 (29%)

244 (29%)

4840 (42%)

Au NRs/P25-780

3.5 (28%)

209 (44%)

418 (28%)

Conclusions Broad band visible and even NIR light harvesting over TMC has been successfully achieved by loading Au NRs onto TMC. TEM and SEM confirmed the loading of Au NRs onto the plate structure of TMC. Using four Au NRs with different aspect ratios is beneficial for the preparation of Au NRs/TMC with adjustable light absorption from visible to NIR-region. All Au NRs/TMC exhibit photocatalytic activity for H2 production from aqueous methanol solution. However, with the increase of the aspect ratio, the LSPR peak shift toward longer wavelength region with lower photon energy. Therefore, Au NRs/TMC-780 shows the highest photocatalytic activity (924 µmol h-1 g-1) compared to (65 µmol h-1 g-1) for Au NRs/P25-780 indicating efficient charge transfer on TMC. Hot electron injection from SPR excited AuNRs to TMC at the interface between Au NRs and TMC, followed by electron transfer in TMC, were verified using single-particle PL measurement and femtosecond time resolved diffuse reflectance spectroscopy. The strong PL quenching at LSPR peak of Au NRs loaded onto TMC compared to bare Au NRs confirmed the electron transfer from Au NRs to TMC. Au NRs/TMC have a prolonged lifetime (4840 ps) compared to Au NRs/P25 (418 ps) confirming efficient charge separation of superstructure TMC. This work represents promising platform to prepare co-catalyst free visible-

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NIR light sensitive photocatalyst for H2 production. The findings in this work will provide a new way for the design and preparation of efficient and economical plasmonic photocatalysts. 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.

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Biosketch

Ossama Elbanna received his B.S and M.S degrees from Mansoura University, Egypt in 2008 and 2012, respectively. He worked as an assistant lecturer in Mansoura University for 20122014. From 2014, he is Ph. D. student under the supervision of Professor Tetsuro Majima at the Institute of Scientific and Industrial Research (SANKEN), Osaka University. His research subject focuses on the design of metal semiconductor hybrid nanomaterials for energy and environmental applications.

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Sooyeon Kim received her B.S degree from Hanyang University, Korea in 2010, and M.S. and Ph.D. degrees in 2012 and 2015, respectively, under the supervision of Professor Tetsuro Majima at SANKEN, Osaka University. From 2016, she is a specially-appointed assistant professor in SANKEN. Her scientific interests include photochemistry, photobiology, and photodynamic therapy.

Mamoru Fujitsuka received his doctoral degree from Kyoto University in 1994. After two years work as a postdoc, he joined the Institute for Chemical Reaction Science, Tohoku University, as a research associate in 1996. In 2003, he moved to SANKEN, Osaka University, as an associate professor. His scientific interests include the photoexcitation dynamics of functional molecules.

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Tetsuro Majima received his doctoral degree from Osaka University in 1980. He worked as a research associate in the University of Texas at Dallas for two years (1980-1982) before moving to RIKEN, Japan. In 1994 he moved to SANKEN, Osaka University, as an associate professor. He was promoted to a full professor in 1997. His current research interests include photochemistry of supramolecules, DNA, protein, metal nanoparticles, and photocatalysts.

Highlights  Au NRs/TMC with tuneable light harvesting covering visible-NIR region has been prepared.  Au NRs/TMC shows high photocatalytic activity for H2 production under visible–NIR irradiation.  Single-particle photolumiscence and femtosecond time resolved diffuse reflectance spectroscopy confirmed hot electron transfer from Au NRs to TMC.

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 The enhancement of H2 production is due to the synergetic effect of LSPR of Au NRs and improved charge separation due to superstructure of TMC.

Graphical Abstract TiO2 mesocrystals composited with gold nanorods for highly efficient visible-NIR-photocatalytic hydrogen production.

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