Au-assisted methanol-hydrogenated titanium dioxide for photocatalytic evolution of hydrogen

Journal Pre-proof Au-assisted Methanol-hydrogenated Titanium Dioxide for Photocatalytic Evolution of Hydrogen Tsai-Te Wang, Yu-Chang Lin, Ming-Chan Lin, Yan-Gu Lin

PII:

S0920-5861(19)30610-8

DOI:

https://doi.org/10.1016/j.cattod.2019.11.003

Reference:

CATTOD 12548

To appear in:

Catalysis Today

Received Date:

7 March 2019

Revised Date:

26 October 2019

Accepted Date:

7 November 2019

Please cite this article as: Wang T-Te, Lin Y-Chang, Lin M-Chan, Lin Y-Gu, Au-assisted Methanol-hydrogenated Titanium Dioxide for Photocatalytic Evolution of Hydrogen, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.11.003

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Au-assisted Methanol-hydrogenated Titanium Dioxide for Photocatalytic Evolution of Hydrogen Tsai-Te Wang,1 Yu-Chang Lin,2 Ming-Chan Lin,3,4,* [email protected] , Yan-Gu Lin1,4,# [email protected] . National Synchrotron Radiation Research Center, 101 Hsin-Ann Rd., Hsinchu

30076, Taiwan 2

. Department of Materials Science and Engineering, National Chiao Tung

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University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan

. Department of Applied Chemistry, National Chiao Tung University, 1001 Ta-Hsueh

Rd., Hsinchu 30010, Taiwan

. Center for Emergent Functional Matter Science, National Chiao Tung University,

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1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan

Corresponding author.

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Corresponding author.

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*

Graphical abstract

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Highlights

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Au-assisted CH3OH-hydrogenated TiO2 was fabricated. Au-assisted CH3OH-hydrogenated TiO2 boosted photocatalytic H2

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production.

Extraordinary activity and robust stability was obtained.

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The synergistic effect was investigated with soft-XAS in situ.

ABSTRACT The development of efficient photocatalysts with extraordinary activity and robust stability is an eternal yet challenging goal for the conversion of solar energy to hydrogen. One effective and mild approach to facilitate the production of H atoms on TiO2 is to employ molecules such as CH3OH that can dissociate with much smaller energy barriers without the danger of contamination of the surface by C, which might decrease the photo-catalytic activity of TiO2. A facile strategy based on hydrogenation

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of TiO2 with CH3OH in an Au-loaded system is here proposed for the exploration of excellent photocatalysts with ultrahigh activity and robust stability. Under solar

illumination, Au loaded on hydrogenated TiO2 photocatalysts produced hydrogen at a

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rate 2.73 mmol h-1 g-1 with operational stability up to 50 h, significantly

outperforming photocatalysts at the state of the art. Systematic studies using UV-Vis,

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PL, Raman, XRD and XPS analyses indicated that, relative to conventionally

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hydrogenated TiO2 with H2 as the hydrogen source, Au loaded on hydrogenated TiO2 using CH3OH as the hydrogen source significantly enhanced the activity and stability. The synergistic effect of the hydrogenation using CH3OH and Au decoration was

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investigated systematically and confirmed with synchrotron-based soft X-ray absorption spectra in situ.

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Keywords: Hydrogenation, Hydrogen evolution, Plasmonic, Soft X-ray

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absorption spectra, Photocatalytic

1. Introduction Over the past decade, photocatalysis based on localized surface plasmon resonance (LSPR) has been studied both theoretically and experimentally [1-3]. Plasmonic energy conversion could be an especially promising candidate for an alternative path of solar-energy conversion once an effectively separating and

efficiently collecting strategy of the plasmon-excited electron–hole pairs is achieved. Unlike the conventional excitation mechanism of semiconductors in which electrons at the valence band of the semiconductor absorb incident radiation and are subsequently excited into its conduction band, there exists a totally distinct behavior for plasmon-induced hot carriers, according to which the plasmonic metallic nanostructures capture radiation with a wavelength characteristic of LSPR to generate highly energetic electron–hole pairs on the surface of nanostructures. As a result, a

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new strategy must be developed to achieve efficient photocatalysis in plasmonic solar- energy conversion.

From the viewpoint of photochemical reactions, it is essential to consider the

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lifetime of the photo-induced charge carriers. The lifetime of plasmonic hot carriers is significantly shorter than the requirement for the occurrence of chemical reactions,

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which makes difficult the extraction of hot electrons and holes from the plasmonic

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materials. Toward this end, the charge extraction could be improved through building metal–metal or metal–semiconductor junctions. Hydrogenated TiO2 (H:TiO2) has been demonstrated to be an efficient photocatalyst for water splitting under visible

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light, on which research has continued since the first report by Chen in 2011 [4]. It is believed that the effects of hydrogenation on the photocatalytic performance arise

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from disorder in the structure, which results in changes to the electronic and optical

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properties of TiO2. Our group has found recently that the performance of photocatalytic hydrogen production for Ag loaded on hydrogenated TiO2 using CH3OH as the hydrogen source was appreciably enhanced relative to H2 as the hydrogen source [5]. Hence, constructing novel heterojunctions of Au loaded on hydrogenated TiO2 (Au/M:TiO2) using CH3OH as the hydrogen source might enhance the charge separation of M:TiO2 to improve further the photocatalytic performance.

As far as we are aware, there is no report of the photocatalytic performance of Au/M:TiO2. We designed an Au/M:TiO2 nanocomposite system to produce solar hydrogen such that the effect of hydrogenation with CH3OH allowed suppression of the carrier recombination at the interface between TiO2 and Au. The corresponding charge transfer of plasmonic Au combined with M:TiO2 was fundamentally studied to realize the LSPR-induced effects on photocatalytic hydrogen production. Furthermore, the

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evolution of the density of states (DOS) upon the TiO2 conduction band was probed in real time with X-ray absorption spectra (XAS) of the Ti L-edge, in which plasmoninduced irradiation with solar light was simultaneously utilized to reveal the LSPR

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effects on the electronic structure of M:TiO2.

2.

EXPERIMENTS

2.1 Sample preparation The TiO2 nanoparticle (NP) supports loaded with Au clusters were prepared using HAuCl4 as precursor [6]. Briefly, anatase TiO2 (0.1 g, ~25 nm, 99.7 %, Aldrich) NP were dispersed in DI water with sonication (30 min); HAuCl4 (0.1~1 M) was dissolved in the CH3OH/TiO2 mixture (1 M) with sonication (another 5 min) before illumination (30 min). The pale yellow mixture became dark purple in minutes under

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illumination with a Xe lamp (300 W), indicating that Au ions were reduced and

loaded on the TiO2 NP surface. The mixture was washed with ethanol solution and

centrifuged to eliminate unreacted chemicals; the dark-colored powders were obtained

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after drying in an oven overnight. To prepare hydrogenated substrates, based on our

earlier work [5][7], CH3OH (LCMS grade, Fluka) liquid served as an effective source

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of hydrogen. Briefly, CH3OH was first degassed twice with a freeze-pump- thaw

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cycle before the hydrogenation began. The TiO2 NP were placed in a ceramic bowl and evacuated first in a glass furnace. The sample was heated (3 h) in methanol vapour (120 Torr, 300 oC). This process turned the white pristine TiO2 powders pale

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grey after hydrogenation, which indicated that the hydrogenated TiO2 (M:TiO2) was readily obtained from the low-pressure pyrolysis. The Au clusters were deposited on

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M:TiO2 NP with the same procedure as aforementioned. In this work, the Au-loaded

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TiO2 substrate (Au/TiO2) and the Au-loaded M:TiO2 (Au/M:TiO2) were prepared on controlling the Au/Ti ratio in a range 0.6~6 % with detailed analyses of their photocatalytic activities.

2.2 Sample characterization

UV-Vis spectra (Hitachi spectrophotometer, U-3010) of compact TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2 powders are presented in Fig. 1(A). The photoluminescence (PL) spectra (Edinburgh Instruments, FS5) are presented in Fig. 1(B). Figure 2(A) shows the X-ray diffraction pattern from NSRRC BL17B and SP8 12B2 of pristine TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2 NP for characterization of the bulk crystal. Raman spectra in Fig. 2(B), were recorded with excitation from an Ar-ion laser (λ= 514.5 nm). X-ray photoelectron spectra (XPS) were recorded at NSRRC BL24A1. Ti

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L-edge XAS were recorded for a fine powder, dispersed over a conducting copper tape, with the surface-sensitive total- electron yield (TEY) detector at NSRRC

beamline 20A1 of Taiwan Light Source (energy resolution ΔE/E ∼ 1/5000). Ti L-

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edge XAS of CH3OH-hydrogenated TiO2 NP were recorded in situ under darkness

and with illumination. The illumination experiments were also performed in situ with

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a Xe lamp (300 W) incorporating a filter (AM 1.5). Figure 5 presents the Ti L3,2-edge

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X-ray-absorption near-edge-structure (XANES) spectra of Au (4 %) loaded on TiO2

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and M:TiO2.

3. Results and discussion 3.1 UV-Vis absorption spectra and Electron Microscope To characterize the Au-loaded substrates, we undertook several analyses that are presented here. Figure 1(A) shows the normalized UV-Vis absorption spectra (320 800 nm) of the collected powders including bare anatase TiO2 NP, M:TiO2, Au/TiO2 and Au/M:TiO2, both with Au (4 %) loaded on the substrates. These NP were packed in a glass vessel for the absorption measurement (Hitachi U-3010 integrating sphere).

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First, M:TiO2 exhibits absorption through the visible range that is noticeably

enhanced relative to pristine TiO2 NP. The oxygen vacancy generated appears to

improve the absorption in the visible range as discussed in our earlier work [5][7]. For

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the Au- loaded substrates, a broad absorption throughout the entire visible range

exceeding our detection limit was obtained. Strong absorption features at 542 and 543

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nm were observed for Au/TiO2 and Au/M:TiO2, respectively. The surface-plasmonic

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effect of the Au-loaded TiO2 NP was discussed by Bahnermann et al [9]. The Au NPs with the size of 8-18 nm were observed in the STEM image as shown in Figure S3. Moreover, the uniform separation of Au and Ti element were observed in the SEM

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elemental mapping analyses as shown in Figure S2.

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3.2 Photoluminescence

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The PL results of pristine TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2 are presented in Fig. 1(B). The large PL intensity observed for TiO2 represents generally the rapid recombination of photo-induced electrons and holes[10,11]. The emission features of TiO2 have been previously assigned. The emission at 407 nm is considered to be due to a self-trapped exciton (STE) in bulk anatase TiO2 [12,13]. The emission at 478 nm was assigned to a de-excitation from the lower vibronic level of Ti3+ 3d states [14-16]. Ti3+ was formed through a lost oxygen atom in the TiO2

lattice. The emissions at 559 and 635 nm are attributed to de-excitations from the oxygen vacancy to the ground state [17,18]. In contrast, the PL intensity decreased noticeably with M:TiO2, reflecting an effective charge separation. The PL decreased to zero with Au loaded on both TiO2 and M:TiO2. The formation of a Schottky barrier at the Au/TiO2 interface would act as an electron sink to prevent the recombination of charge carriers, as discussed in Lim’s work on

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Au@TiO2 [19].

3.3 XRD and Raman spectra

Figure 2(A) presents XRD diagrams of TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2

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with Au loading (4 %). The diffraction pattern of anatase TiO2 fits well with JCPDS:

021-1272 in TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2, separately. The size of particles

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was estimated with the Scherrer equation from the TiO2 (101) signal of the TiO2

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crystal. In Table S1(A), 2θ of TiO2 (101) signals remained unshifted and the particle size was found to remain constant among TiO2, M:TiO2, Au/TiO2 and Au/M:TiO2. This result implies that the TiO2 crystallinity was well preserved during the

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hydrogenation and Au loading. The Au diffraction signals were determined with JCPDS: 04-0784 in Au/TiO2 and Au/M:TiO2. The Au (200) signals evaluated for Au

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particles are listed in Table S1(B). Neither a clear shift of the features nor a

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broadening was observed, indicating that the crystallinity of TiO2 NP was well preserved during the Au loading. The Raman spectra of TiO2 vibrations are presented in Fig. 2(B) for TiO2,

M:TiO2, Au/TiO2 and Au/M:TiO2. With respect to the well preserved TiO2 and M:TiO2 substrates, the Eg vibrational line was clearly blue-shifted to 151.3 cm-1 for the Au- loaded samples from 144.2 cm-1 of TiO2 and M:TiO2 samples. The Eg mode is attributed mainly to the O-Ti-O symmetric-stretching vibration. The

blue shift was caused by O-Ti bond shortening, as discussed by Dal Santo et al [20] and Li et al [21]. In this work, the result of analysis of the size of TiO2 bulk crystals is presented in Fig. 2(A) X-ray diffractograms. The particle size of anatase TiO2 and the crystallinity were well preserved after the Au loading. Surface disordering is considered to be responsible for the blue shift observed [21]. The width (full width at half maximum signal) of the TiO2 Eg lines was clearly increased upon Au loading for both TiO2 (14.7→20.2 cm-1) and M:TiO2

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(15.1→18.4 cm-1) systems. In the Au-TiO2 (anatase) work of Strunk et al [22], the formation of rutile was proposed at the Au and TiO2 interface [9], whereas, in Fig. 2(B), the formation of rutile crystallinity was not observed. The TiO2

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vibrational mode broadened in the Au-loaded substrate: because of a surface

interaction, small particles were formed at the interface of Au and TiO2 upon Au

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loading.

3.4 XPS

The results of analyses of XPS for Au/TiO2 and Au/M:TiO2 are presented in

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Fig. 3. In Fig. 3(A), the binding energy of the lattice-bound Ti4+-O 2p3/2 line remains constant at 458.42 eV, compared with our previous results on TiO2 and

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M:TiO2 samples [7]. The energy of the 2p1/2 line is 5.69 eV greater than that of

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the 2p3/2 line, which fits well with the binding-energy difference 5.7 eV. A slight shift, 0.1 eV, toward a smaller binding energy in Au/M:TiO2 might be attributed to a charging effect. A fixed Ti3+ (~1 %) in 457 eV was found for Au/M:TiO2 but not observed for Au-loaded TiO2. The oxygen 1s spectra are shown in Fig.3 (B) for Au/TiO2 (black) and Au/M:TiO2 (red). The lines for lattice Ti-O-Ti binding and a surface OH group/oxygen vacancy (O-) are labelled as 529.63 and 530.55 eV with

accumulated areas 75 and 25 % for Au/TiO2. Relative to Au/TiO2, the signal intensity of OH group/O- increased on Au/hydrogenated TiO2, reflecting that the proportion of surface OH group and/or O- significantly increased due to methanol hydrogenation treatment. Therefore, it can be expected methanol hydrogenation can induce a much high concentration of surface OH group and/or O- so that the performance in photocatalysis is improved. Moreover, the abundant oxygen vacancies can result in band-gap narrowing and an enhanced

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photocatalytic activity [25], which might contribute to the effective photocatalytic reaction of Au/M:TiO2.

Figure 3(C) of the Au 4f XPS shows that Au0 4f7/2 and 4f5/2 lines are assigned

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at 82.94 and 86.62 eV, respectively. The decrease from 84 eV for bulk metallic

Au was considered to be due to (i) a decreased coordination number observed for

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surface Au in Au/SiO2 [26], Au/ZrO2 [27] and Au/TiO2 [26,28], and (ii) defects

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generated from an intimate contact between the Au surface and the substrate [29,30]. The proportion of Au on the catalyst was estimated to be 0.009; the Au/Ti atomic ratio calculated from the RSF correction was 0.032 in Au/TiO2.

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The atomic ratio was at a reasonable level with respect to Au (4 %) used in preparation of the substrate through photo-reduction. Notably, the extent of Au0

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in the Au/M:TiO2 intensity was distinctly greater than that in Au/TiO2. The

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signal was shifted 0.1 eV toward smaller binding energy relative to 82.94 eV in Au/TiO2. The decrease of the Au0 signal was attributed to an electron transferred from the surface Ti3+ to the Au cluster [31]. The proportion of Au was greater in Au/M:TiO2 (0.011); the Au/Ti atomic ratio was estimated to be 0.038. As the same amount of HAuCl4 was used in the photo-reduction of Au ions on TiO2 and on M:TiO2, this difference of Au/Ti atomic ratio reflects a more effective photoreduction of Au ion on the hydrogenated TiO2 substrate. The presence of Ti3+ on

the TiO2 surface would reduce Au ions more effectively, as discussed by Liu et al [32].

3.5 H2 evolution experiment and IPCE The system setup and the evolution of H2 under illumination with a Xe lamp was presented by Wang et al [5]. In this work, the rate of hydrogen evolution (HER) was obtained from both Au/TiO2 and the hydrogenated Au/M:TiO2 NP with varied Au/Ti

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ratio; the results were carefully compared. Experimentally, catalysts (30 mg) were dispersed in a vessel (Pyrex) with the CH3OH-H2O mixture (60 mL, 20 %). In the

experiment, methanol served as a sacrificial reagent. After ultrasonic application (30

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min) for complete mixing, the reactor was connected to the HER system. The rate of hydrogen evolution was measured in a closed static chamber in vacuo. A freeze-

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pump-thaw cycle was undertaken to decrease the pressure to less than 1 mTorr before illumination. The hydrogen generated from water splitting under irradiation (1 h) with

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a Xe lamp (300 W, Pin = 100 mW cm-2), incorporating a filter (AM1.5), is reported here. In Fig. 4(A), the slash column represents the rate of hydrogen evolution of

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pristine TiO2 and Au/TiO2 with varied Au loading (0.6~6 %) on TiO2 NP. Relative to pristine TiO2, a HER enhancement 52~67 times was obtained with Au/TiO2. The

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optimized rate, 1.07 mmol h-1 g-1, of H2 evolution was attained with Au/TiO2 NP (4

The effect of hydrogenated TiO2 with methanol treatment on H2 generation is

shown in Fig. 4(A) and S1, in which the HER of Au/M:TiO2 samples with varied Au loading is demonstrated. The Pt/TiO2 was also listed as a control experiment. Relative to the M:TiO2 substrate, an even greater improvement, 29~85 times, of HER was attained. The optimized HER, 2.73 mmol h-1 g-1, was obtained with Au/M:TiO2 NP (4 %), 15% higher than Pt/TiO2 was observed. The hydrogenation of TiO2 hence

enhanced the HER of Au-loaded TiO2 significantly. In Fig. 4(A), hydrogen evolution (1 h) under illumination (AM 1.5G) is demonstrated with TiO2, M:TiO2, Au/TiO2, Au/M:TiO2 (Au loading 4 %) and Pt/TiO2 (Pt loading 2%) Before illumination, no hydrogen was produced. Hydrogen was clearly observed after illumination with a Xe lamp (incorporating filter AM1.5G). An enhancement 67 times was observed for Au/TiO2 (4 %) with respect to the TiO2 NP. The H2 evolution was further enhanced 2.6 times with Au loaded on hydrogenated TiO2. An enhancement 171 times in total

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for the hydrogen evolution was achieved with Au/M:TiO2 (4 %) over that with pure TiO2.

In Fig. 4(B), the durability of Au/M:TiO2 is presented. A HER experiment (50 h)

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was performed. During the durability test, we refilled methanol to the reactor after observing a decreased rate of hydrogen production because of methanol depletion

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after each 10 h. The rate of production attained the same level after each methanol

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refill, but a long-time HER decrease was noted, about 15 % after illumination for 50 h. Furthermore, incident-photon-to-electron conversion efficiency (IPCE) is one of the good approaches to evaluate the activities of Au modified TiO2 photocatalyst

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under visible-light. Therefore, the IPCE spectra of Au/TiO2 and Au/M:TiO2 were carried out and supplemented in the revised manuscript (Figure S4). Apparently, the

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activities on Au/TiO2 under visible-light can be confirmed due to plasmonic effect of

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Au. This is matched with the results of UV-VIS absorption spectra (Figure 1A). 3.6 XAS analyses and AQE To reveal further the plasmonic-induced charge transfer in the conduction band of

TiO2, we investigated the electron occupancy states in the conduction band of TiO2 with XAS of the Ti L-edge, because L-edge excitation results from electrons in 2p orbitals becoming excited to 3d orbitals composed of t2g and eg orbitals [23,24]. The conduction band of TiO2 is, typically, mainly contributed from the DOS of Ti. Figure

5 thus displays the absorbance differences to compare the states between darkness and illumination in situ for Au/M:TiO2 and Au/TiO2 samples. The negative ∆A values of XAS at the Ti L-edge are attributed to the fewer photoelectrons that were excited from plasmonic Au NP. The intensity difference of XAS spectra under illumination in situ for Au/M:TiO2 was similar to that for Au/TiO2, indicating that Au/M:TiO2 would significantly create additional photocarriers under illumination. This observation reveals, notably, that plasmonic-Au-induced hot-electron injection dominates the

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conduction-band nature of TiO2 rather than an effect of the electromagnetic field. In particular, Au/M:TiO2 showed much more negative ∆A values in XAS under

irradiation in situ than Au/TiO2, indicating that hydrogenated TiO2 with plasmonic Au

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can lead to an increased electron transition with soft X-rays. This observation

indicates that the hydrogenation of TiO2 can greatly improve the hot-electron kinetics

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and modify the energy levels at the interface between Au and TiO2, which allows hot

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electrons to transfer rapidly toward M:TiO2, leading to hot carriers separating from each other. The subsequent recombination of hot carriers was restrained so that the hot electrons survived; the amount of hot-electron injection became augmented. The

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hot electrons in Au would thus further inject into the conduction band of M:TiO2, and thereby yield a more satisfactory photocatalytic performance. For this reason we

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believe that Au/M:TiO2 is optically active and plays a major role in the photocatalytic

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enhancement in our system. The apparent quantum efficiencies (AQE) for various Au/TiO2 systems have also been carried out in this work (Table S2). After decorating Au NPs, the Au/M:TiO2 displayed the highest AQE, suggesting that the interface of Au/M:TiO2 has been greatly facilitated and contributed the efforts to the photocatalytic hydrogen production.

4. Conclusion This work presents a robust and effective Au-loaded hydrogenated TiO2 photocatalyst. A HER 2.73 mmol h-1 g-1 was optimized with Au (4 %) loaded on M:TiO2 using CH3OH as the hydrogen source under solar illumination with a Xe lamp incorporating AM1.5 filters. A promising improvement 171 times is reported relative to pristine anstase TiO2. Repeated long-time illuminations over 50 h demonstrated the great stability of the photocatalysts. In the PL analysis, the alteration

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of the charge recombination depicts the effective electron transfer in Au reduction.

The vibrational line broadened because of the surface interaction between a loaded Au atom and TiO2 was demonstrated with Raman spectra. A more effective Au reduction

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because of surface Ti3+ on hydrogenated TiO2 is discussed. The abundant oxygen vacancies result in a more highly efficient photocatalytic reaction in the water

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splitting observed in our Au/M:TiO2 (4 %) system. A mechanism of the plasmonic

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Au/M:TiO2 enhancing the performance of the photocatalytic reaction with the localized surface plasmon resonance is proposed and characterized in situ with X-ray absorption-near-edge spectra. The insight from the X-ray spectra and associated

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mechanistic interpretation and correlation with solar-to-hydrogen performances are presented herein, which serve as an informative guide to rational strategies for further

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processing of highly efficient production of hydrogen.

Acknowledgments MCL acknowledges support from Ministry of Science and Technology, Taiwan (grant MOST 107-3017-F009-003). YGL acknowledges support from Ministry of Science and Technology, Taiwan (grant MOST 105-2112-M-213-004-MY3 and 108-2112-M213-002-MY3). MCL and YGL also acknowledge the support from Ministry of Education, Taiwan (SPROUT project Center for Emergent Functional Matter Science

in National Chiao Tung University). YGL and TTW thank National Synchrotron

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Radiation Research Center for providing beam time.

References

[1] J. Li, S. K. Cushing, P. Zheng, T. Senty, F. Meng, A. D. Bristow, A. Manivannan, N. Wu, J. Am. Chem. Soc. 136 (2014) 8438-8449. [2] C. Clavero, Nat. Photonics 8 (2014) 95-103. [3] S. Liu, G. Chen, P. N. Prasad, M. T. Swihart, Chem. Mater. 23 (2011) 4098-4101. [4] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 331 (2011) 746–750.

ro of

[5] T. T. Wang, P. Raghunath, Y. G. Lin, M. C. Lin, J. Phys. Chem. C 121 (2017) 9681-9690.

[6] Y. Choi, H-i Kim, G-h. Moon, S. Jo, W. Choi, ACS Catal. 6 (2016) 821−828.

-p

[7] T. T. Wang, P. Raghunath, Y.C. Lin, Y. G. Lin, M. C. Lin, App. Surf. Sci. 488 (2019) 546-554.

re

[8] J. Tang, N. Qin, Y. Chong, Y. Diao, Yiliguma, Z. Wang, T. Xue, M. Jiang, J.

lP

Zhang, Nature Comm. 9 (2018) 786-798.

[9] J. Nie, J. Schneider, F. Sieland, L. Zhou, S. Xia, D. W. Bahnermann, RSC Adv. 8 (2018) 25881-25887.

na

[10] K. Khore, S. R. Kadam, S. D. Naik, B. B. Kale, R. S. Sonawane, New J. Chem. 42 (2018) 10958-10968.

ur

[11] W. J. Ong, L. L. Tan, S. P. Chai, S. T. Yong, A. R. Mohamed, Nanoscale 6

Jo

(2014) 1946-2008.

[12] S. A. Abdullah, M. Z. Sahdan, N. Nadarizal, H. Saim, A. S. Bakri, C. H. Cik Rohaida, F. Adriyanto, Y. Sari, J. Phys.: Conf. Ser. 995 (2018) 012067. [13] N. D. Abazovic, M. I. Comor, M. D. Dramicanin, D. J. Jovanovic, S. P. Ahrenkiel, J. M. Nedeljkovic, J. Phys. Chem. B 110 (2006) 25366-25370. [14] B. Prajapati, S. Kumar, M. Kumar, S. Chatterjee, A. K. Ghosh, J. Mater. Chem. C 5 (2017) 4257-4266.

[15] L. Chetibi, T. Busko, N. P. Kulish, D. Hamana, S. Chaieb, S. Achour, J. Nanopart. Res. 19 (2017) 129-138. [16] H. Zhang, M. Zhou, Q. Fu, B. Lei, W. Lin, H. Guo, M. Wu, Y. Lei, Nanotech. 25 (2014) 275603-275612. [17] N. D. Abazovic, M. I. Comor, M. D. Dramicanin, D. J. Jovanovic, S. P. Ahrenkiel, J. M. Nedeljkovic, J. Phys. Chem. B 110 (2006) 25366-25370.

Kshnyakin, J. Nano-electron. Phys. 5 (2013) 3047-3053.

ro of

[18] L. Kernazhitsky, V. Shymanovska, T. Gavrilko, V. Naumov, L. Fedorenko, V.

[19] S. P. Lim, A. Pandikumar, N. M. Huang, H. N. Lim, RSC Adv. 5 (2015) 4439844407.

-p

[20] A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C. L. Bianchi, R. Psaro, V. D. Santo, J. Am. Chem. Soc. 134 (2012) 7600-7603.

re

[21] L. Song, Z. Lu, Y. Zhang, Q. Su, L. Li, Catalysts 8 (2018) 26-43.

lP

[22] A. Pougin, A. Lüken, C. Klinkhammer, D. Hiltrop, M. Kauer, K. Tölle, M. Havenith-Newen, K. Morgenstern, W. Grünert, M. Muhler, J. Strunk, Top Catal. 60 (2017) 1744-1753.

na

[23] Y.C. Lin, L.C. Hsu, C.Y. Lin, C.L. Chiang, C.M. Chou, W.W. Wu, S.Y. Chen, Y.G. Lin, ACS Appl. Mater. Interfaces 11 (2019) 38625-38632.

ur

[24] X. Bu, C.L. Chiang, R. Wei, Z. Li, Y. Meng, C.K. Peng, Y.C. Lin, Y. Li, Y.G.

Jo

Lin, K.S. Chan, J.C. Ho, ACS Appl. Mater. Interfaces 11 (2019) 38633-38640. [25] J. Wang, Z. Wang, B. Huang, Y. Ma, Y. Liu, X. Qin, X. Zhang, Y. Dai, ACS Appl. Mater. Interfaces 4 (2012) 4024-4030. [26] S. Arrii, F. Morfin, A. J. Renouprez, J. L. Rousset, J. Am. Chem. Soc. 126 (2004) 1199-1205. [27] S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A. Brückner, J. Radnik, H. Hofmeister, P. Claus, Catal. Today 72 (2002) 63-78.

[28] J. Radnik, C. Mohr, P. Claus, Phys. Chem. Chem. Phys. 5 (2003) 172-177. [29] X. Pan, Y. Xu, Appl. Catal. A 459 (2013) 34-40. [30] Z. Zheng, B. Huang, X. Qin, X. Zhang, Y. Dai, M. Whangbo, J. Mater. Chem. 21 (2011) 9079-9087. [31] A. Zwijnenburg, A. Goossens, W. G. Sloof, M. W. J. Crajé, A. M. van der Kraan, L. Jos de Jongh, M. Makkee, J. A. Moulijn, J. Phys. Chem. B 106 (2002) 9853-9862. [32] Z. X. Pei, L. Y. Ding, W. H. Feng, S. X. Weng, P. Liu, Phys. Chem. Chem. Phys.

ro of

16 (2004) 21876-21881.

[33] Z. Jin, X. Zhang, Y. Li, S. Li, G. X. Lu, Catal. Commun. 8 (2007) 1267-1273.

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[34] X. Wang, B. Chen, G. Chen, X. Sun, RSC Advances 6 (2016) 87978-87987.

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Fig. 1 (A) Normalized UV-VIS absorption spectra and (B) PL spectra of pristine TiO2 (black), M:TiO2 (blue), Au/TiO2 (orange) and Au/M:TiO2 (red) with 4 % Au

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Fig. 2 X-ray diffractograms (A) and Raman spectra (B) of pristine TiO2 (black), M:TiO2 (blue), Au/TiO2 (orange) and Au/M:TiO2 (red) with 4% Au loaded. The crystallinity of anatase TiO2 (◆JCPDS: 021-1272) and Au (▼JCPDS: 04-0784) was confirmed.

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Fig. 4 (A) Rate of hydrogen evolution of TiO2 (black slash column), M:TiO2 (blue slash column), Au/TiO2 (orange slash column), Au/M:TiO2 (red slash column) with 4% Au loaded and Pt/TiO2 (green slash column) with 2% Pt loaded. (B) Durability experiment with 4 % Au/M:TiO2 as photocatalyst.

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Fig. 5 Intensity difference of Ti L-edge spectra for Au/TiO2 and Au/M:TiO2 between conditions of illumination (AM1.5) in situ and darkness (∆A = Aillumination – Adarkness).