Nanoplasmonically-Engineered Interfaces on Amorphous TiO2 for

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Nanoplasmonically-Engineered Interfaces on Amorphous TiO2 for Highly Efficient Photocatalysis in Hydrogen Evolution Huijun Liang, Qiuxia Meng, Xiaobing Wang, Hucheng Zhang, and Jianji Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00677 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Materials & Interfaces

Nanoplasmonically-Engineered Interfaces on Amorphous TiO2 for Highly Efficient Photocatalysis in Hydrogen Evolution Huijun Liang,†,‡ Qiuxia Meng,† Xiaobing Wang,† Hucheng Zhang,†,* and Jianji Wang†,* †

Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine

Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China ‡

College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang, Henan 453003, P. R.

KEYWORDS: Amorphous TiO2, Ag/TiO2 composite, photocatalysis, hydrogen evolution, ethylene glycol.

ABSTRACT: The Nanoplasmonic metal-driven photocatalytic activity depends heavily on the spacing between metal nanoparticles (NPs) and semiconductors, and this work shows that ethylene glycol (EG) is an ideal candidate for interface spacer. Controlling the synthetic systems at pH=3, the composite of Ag NPs with EG-stabilized amorphous TiO2 (Ag/TiO2-3) was synthesized by the facile light-induced reduction. It is verified that EG spacers can set up the

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suitable geometric arrangement in the composite: the twin hydroxyls act as the stabilizers to bridge Ag NPs and TiO2 together; and the non-conductive alkyl chains consisted only of two CH2 are able to separate the two building blocks completely from the direct contact, and more provide the shortest channels for efficient transfers of radiation energies into reach TiO2. As employed as photocatalysts in hydrogen evolution under visible light, amorphous TiO2 hardly exhibits the catalytic activity due to high defect density, whereas Ag/TiO2-3 represents the remarkably high catalytic efficiency. The enhancement mechanism of reaction rate is proposed by analysis of the composition, structure and optical properties from a series of Ag/TiO2 composites.

1. INTRODUCTION

The heterogeneous photocatalysts usually are semiconductors that can harvest and convert light energy to chemical energy, hence, play the important roles in sustainable chemistry and engineering, such as H2 evolution, CO2 reduction, and various pollutant degradation.1-4 In this regard, crystalline TiO2 (c-TiO2) with well-defined lattice planes has been intensively investigated due to its eminent characteristics in ultraviolet absorption, photocatalytic activity, eco-friendliness, and long term stability.5-8 By contrast, amorphous TiO2 (a-TiO2) with the short range ordered structure and high defect density received very little attention up to now. Recently, it is reported that a-TiO2 possesses lower synthesis temperature, facile synthetic route, larger specific surface area,9-11 as well as the excellent abilities in absorbing light and capturing photogenerated holes,12-14 and is a potential alternative to c-TiO2 in photocatalytic reactions. The nanostructured noble metals represent localized surface plasmon resonance (LSPR) that arises from the collective oscillation of valence electrons as the frequency of resonant photons


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matches with the intrinsic frequency of surface electrons.15-16 The unique performance allows the noble metal nanoparticles (NPs) for wide applications in solar cells, surface-enhanced Raman spectroscopy, chemical and biological sensors, and medicinal treatment and so on.17-19 In photocatalytic topics, LSPR of NPs on the semiconductors can be employed as the hot spots to converge light flux, extend absorption bands toward visible light, and enhance the intensity of surface electric fields. Therefore, the composites of plasmonic-metal and semiconductor are usually constructed in an attempt to improve the photocatalytic activity.20-26 It is well known that the resonance frequency of LSPR strongly depends on particle size, shape, composition, the refractive index of the surrounding medium, and the interparticle spacing.15, 27-28 Hence, the LSPR-induced rate enhancements in the composite photocatalysis rest with the geometric arrangements of noble metal NPs and semiconductor in addition to their optical properties. Several strategies have been developed to synthesize the composite photocatalysts, including chemical reduction, light-induced reduction, photodeposition, thermal decomposition, liquid impregnation, sol-gel or hydrothermal method.16,

24, 29-33

To render the

plasmonic-metal/ semiconductor composites with the high photocatalystic activity, however, there are still great challenges in the controlled synthesis of the composites in which the building blocks are suitably arranged and critically separated by a non-conductive organics with the optimal spacing to depress the decay process of irradiation energy.34 In this work, we report a facile protocol to load Ag NPs on a-TiO2 by means of light-induced chemical reduction of AgNO3 aqueous solution for the reasons that Ag NPs have strong LSPR effect in response to visible light, relatively low work function and more inexpensive than other noble metals. It is shown that the content of ethylene glycol (EG) on a-TiO2 is controlled only by the pH in reaction mixtures, and in turn exerts a significant influence on the optical properties of


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Ag/a-TiO2 composites due to the different spacing between Ag NPs and a-TiO2. When employed as photocatalysts, a-TiO2 with high defect density hardly exhibits hydrogen evolution, but Ag/aTiO2 composite has the photocatalytic efficiency much higher as compared with the Ag/c-TiO2 composites reported in literature. The efficient photocatalysis is ascribed to the optimal spacing of EG between Ag NPs and a-TiO2, the extending visible absorption bands, and the resulting enhancement in LSPR-driven near-field electromagnetic effect, scattering of resonant photons and Förster resonance energy transfer (FRET) on the semiconductor.

2. EXPERIMENT SECTION Tetrabutyl titanate (TBT, ≥98%), and ethylene glycol (EG, ≥99%), AgNO3 (≥99.5%), absolute ethanol, acetic acid (HAc, ≥99.5%) and NH3·H2O (25wt.%) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were analytical grade and without further purification. 2.1 Preparation of Ag/TiO2 composite photocatalysts In a typical procedure, 2.5 mL TBT was mixed with 50 mL EG, and then 0.17 mL KCl of 0.4 mM was added under vigorously magnetic stirring for 30 min to obtain a transparent solution. After that, the reaction mixture was transferred into a flask, and heated in water bath at 85 oC for 120 min to form a white suspension of a-TiO2. Under vigorously magnetic stirring, the as-prepared a-TiO2 colloidal solution was added into 250 mL deionized water of pH=3. Then, the suspension was transferred into a quartz reactor that was held at 10 oC by a water-cooled jacket, and irradiated by a 500W Hg lamp for 1h until the colour of a-TiO2 solution changes from white to black blue. After turning off the irradiation, 10 mL AgNO3 solution of 19.4 mM was added under vigorously magnetic stirring for 4 h, and the colour of suspension evolved from black blue to brown yellow. Finally, the precipitate was


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centrifuged and washed with deionized water and absolute ethanol three times, and dried at 80 oC for 10 h to obtain Ag/TiO2 composite photocatalyst that was denoted as Ag/TiO2-3. Following the same procedures as done for Ag/TiO2-3 composite, Ag/TiO2-2, Ag/TiO2-4, Ag/TiO2-5, Ag/TiO2-6, and Ag/TiO2-7 photocatalysts were respectively synthesized only when the pH value of deionized water in reaction mixtures was adjusted to 2, 4, 5, 6, and 7 by HAc or NH3·H2O solution. 2.2 Photocatalytic experiment The photocatalytic experiments were carried out in an airtight quartz reactor with a circulating cooling water to hold the reaction system at 25 oC, and a 300W xenon lamp was adopted as the light source. In a typical procedure, the aqueous solution containing 20 vol.% methanol was used as reactant, and 50 mg Ag/a-TiO2 photocatalyst was dispersed in 100 mL aqueous methanol under magnetic stirring. During the irradiation, the hydrogen evolution was monitored by a gas chromatograph (GC7900, Techomp) with high purity N2 as the carrier gas. In order to convert the recorded peak area into the volume of the H2 produced by photocatalysis at atmospheric pressure, the integrated peak area from GC curve was compared to that of the standard H2 curve of the certain volume. 2.3 Characterization Methods The Microstructures and morphologies investigations of as-synthesized composites were characterized by X-ray diffraction (XRD, Bruker advance-D8 XRD with Cu Kα radiation, λ=0.154178 nm, the accelerating voltage was set at 40 kV with a 100 mA flux), transmission electron microscope (TEM, JEM-2100), X-ray photoelectron spectroscopy (XPS, on a VG Scientific ESCALABMKLL spectrometer using Al Kα X-ray source with 10 mA at 15 kV), and thermogravimetric analysis (TGA, STA449C Analyzer, NETZSCH, Germany, the heating rate of


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2 oC min-1 in the air ambience. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were performed using an ELAN DRC-e apparatus (PerkinElmer, USA). Nitrogen adsorption/desorption isotherms were determined with an ASAP 2020 (Micromeritics Instruments). Surface-area determination and pore analysis were respectively performed by using the Brunauer-Emmet-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method. The UV–visible (vis) diffuse reflectance spectra (DRS) were obtained for the dry-pressed disk samples using a UV–vis spectrometer (Lambd 950, PerkinElmer Inc, America), and BaSO4 was used as a reflectance standard. UV-vis extinction spectra were recorded with a TU-1810 UV–vis spectrophotometer (Beijing Purkinje General). Photoluminescence (PL) measurements of the suspensions containing the same mass of a-TiO2 were performed on an FLS980 spectrometer (Edinburgh, UK) at room temperature using the excitation wavelength of 290 nm.

3. RESULTS AND DISCUSSION The photocatalyst of a-TiO2 was prepared by the hydrolysis of tetrabutyl titanate at a low temperature. The TEM images exhibit the as-synthesized a-TiO2 is well dispersed suspension, and the lattice fringe-free HRTEM images and the featureless XRD patterns imply the a-TiO2 with the most probable amorphous phase (Figure 1A and S1). Ag NPs could be loaded on the aTiO2 by light-induced reduction of AgNO3 in aqueous solution of pH=3. The growth of Ag NPs with average grain sizes of 3±0.5 nm is observed from the TEM image of as-prepared Ag/TiO2-3 composite, and confirmed from the fringe spacing of 0.24 nm in the HRTEM images that is indexed to the (111) crystal face of Ag metal (Figure 1B).35 Under the exact same experimental conditions, Ag/TiO2-2, Ag/TiO2-4, Ag/TiO2-5, Ag/TiO2-6, and Ag/TiO2-7 composites were respectively prepared by adjusting pH at 2, 4, 5, 6, and 7 in the initial reaction mixtures. Except Ag/TiO2-2，the Ag NPs on the a-TiO2 surfaces can be observed from the TEM and HRTEM


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images of the as-synthesized Ag/TiO2 composites (Figure 1C-F), and exhibit the average size that fells into the range from 3 to 5nm for all samples (Figure S1), a particle size for generating remarkable LSPR effect. Because the contrast of organics is different from a-TiO2, the thicknesses of organic layers (nm) can be identified and determined respectively from the selected nanoparticles to be about 0.81 (Ag/TiO2-3), 0.65 (Ag/TiO2-4), 0.57 (Ag/TiO2-5), 0.34 (Ag/TiO2-6), and 0.31 (Ag/TiO2-7). Furthermore, the XRD patterns of these composites represent the typical amorphous characteristics, and the almost unrecognizable diffraction peaks of Ag NPs due to the low metal loading (Figure S2). Exceptionally, the Ag nanosheets instead of Ag NPs were formed on the Ag/TiO2-2 composite (Figure S3), inherently has the very weak LSPR, and hence are not addressed in this study.

Figure 1. TEM (subscript 1) and HRTEM (subscript 2) images for the as-synthesized composites: a-TiO2 (A), Ag/TiO2-3 (B), Ag/TiO2-4 (C), Ag/TiO2-5 (D), Ag/TiO2-6 (E), and Ag/TiO2-7 (F).


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Figure 2. Photographs of Ag/TiO2 composites under indoor natural light. Although the chemical constitutions in the series of as-prepared Ag/TiO2 composites are expected to be similar, their colors are significantly different from each other in indoor natural light (Figure 2), and presumably depends on the LSPR effect that is related to the chemical surroundings of Ag NPs and the interface interactions between a-TiO2 and Ag NPs.27-28 To rationalize the observation, the contents of organic residues and Ag metal on Ag/TiO2 composites were respectively analyzed by TGA, FT-IR, and ICP-MS techniques. In each TGA curve (Figure 3A), there is a significant mass loss about at 273oC. Below 273oC, the mass loss is attributed to the removal of physically adsorbed solvents on the composite surfaces,34, 36 and these relatively “free” solvents hardly affect the interface interactions between a-TiO2 and Ag NPs. When the temperature is higher than 273oC, the mass loss is ascribed to the degradation of chemically bonded organic residues.34,

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For the as-prepared a-TiO2, the content of organic

residues should be equal to the difference of mass loss between 273 and 600 oC, and comes up to 25.5%. This high content is ascribed to the favorable chemisorption environment due to the large specific surface area of 393 m2·g-1, which is determined from the N2 adsorption-desorption experiment (Figure S4). For the Ag/TiO2 composites, the content of organic residues decreases significantly due to the occurrence of Ag NPs. The FT-IR spectra indicate that the organic residues are mainly consisted of EG molecules (Figure 3B). The bands at 3366, 2928 and 2859, 1648, and 1083 cm-1 are observed from a-TiO2,


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Figure 3. (A)TGA curves, and (B) FT-IR spectra respectively for a-TiO2 (black), Ag/TiO2-3 (red), Ag/TiO2-4 (pink), Ag/TiO2-5 (purple), Ag/TiO2-6 (orange), and Ag/TiO2-7 (green). and respectively attributed to O-H stretching, C-H stretching, O-H bending, and C-O stretching in EG molecules.38-39 As Ag NPs are loaded on a-TiO2, the O-H stretching band becomes narrow and shifts to 3424 cm-1, C-H stretching vibrations red-shift to 2943 and 2866 cm-1, whereas O-H bending and C-O stretching blue-shift respectively to 1634 and 1072 cm-1, suggesting that the twin hydroxyls in EG act as the stabilizer both a-TiO2 and Ag NPs. In this situation, one end of EG molecules anchors on the a-TiO2 surfaces, and the other end on Ag NPs. As a result, the EG molecules in the composites bridge a-TiO2 and Ag NPs together, moreover, separate the two building blocks from direct contact through building the non-conductive organic spacers. In the context, the thickness of organic spacing on the composites is impossibly longer than a EG molecular chain ( 11.0 (Ag/TiO2-4) ~ 10.7 (Ag/TiO2-5) > 8.5 (Ag/TiO2-6) ~ 8.2 (Ag/TiO2-


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7), coincident with the thickness order of organic layers as observed from the HRTEM images. In addition, the loadings of Ag metal (wt.%) are determined by ICP-MS analysis in the order: 1.15 (Ag/TiO2-3) < 1.67 (Ag/TiO2-4) ~ 1.83 (Ag/TiO2-5) < 2.68 (Ag/TiO2-6) ~ 2.71 (Ag/TiO27). These analyses give a profile of the geometric arrangements for Ag NPs and EG on a-TiO2 (Figure 4A). Evidently, EG molecules involve in the growth of Ag NPs on a-TiO2, and the growth kinetics depend on pH in the reaction mixtures, and mediate the loading of Ag NPs and thickness of EG under the preparation conditions. The more Ag NPs are loaded, the more the EG are depleted, as a result, thinner the thickness of the organic spacers become. Accordingly, the physicochemical surroundings of Ag NPs on the composites can be controlled by adjusting the pH in reaction mixture, as a result, exert an important effect on the interface interactions of aTiO2 with Ag NPs, and hence on the LSPR effect and photocatalytic efficiency. The geometric arrangements of Ag NPs on a-TiO2 can be further illustrated by XPS analysis. The survey spectra show that the composites are comprised of C, Ag, Ti, and O elements (Figure 4B). As shown in Figure 4C, the Ti 2p peaks in XPS spectra of Ag/TiO2-3 and a-TiO2 locate the almost same positions.40-41 This indicates that Ag NPs on Ag/TiO2-3 do not affect the Ti binding energies, and are separated by the thin EG spacers well from a-TiO2. With the loading increase of Ag metal and the decrease of EG, however, the Ti 2p3/2 peak shifts from 459.4eV in Ag/TiO23 to 458.3eV in Ag/TiO2-7, and correspondingly the Ti 2p1/2 peak from 465.1eV to 464.1eV. Similarly, the Ag 3d peak in high-resolution XPS spectra shifts to lower binding energy with increasing Ag loading from Ag/TiO2-3 to Ag/TiO2-7 (Figure 4D).16 Compared with the binding energies of Ti element in TiO2 and Ag metal in Ag/TiO2-3, the lowering binding energies imply the direct contact between Ag NPs and a-TiO2 to some degrees, as the results, the stronger coupling and the more photogenerated electron communications occur in two building blocks.


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Therefore, it is believed that the non-conductive EG spacers can completely prevent the two building blocks from direct contact in Ag/TiO2-3, but partly in Ag/TiO2-4, Ag/TiO2-5, Ag/TiO26, and Ag/TiO2-7.

Figure 4. (A) Content-based geometric arrangements of Ag NPs (red), EG (green), and a-TiO2 (gray), (B) XPS survey spectra, (C) High-resolution XPS spectra of Ti 2p, and (D) highresolution XPS spectra of Ag 3d in Ag/TiO2 composites.


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Figure 5. (A) Hydrogen evolution in the presence of the different Ag/TiO2 composites. (B) Hydrogen evolution in the presence of the Ag/TiO2-3 composites with four different Ag loadings (wt.%): 0.63% Ag (green), 0.88% Ag (blue), 1.15%Ag (red), and 1.68% Ag (black). (C) Dependence of the mmol number of hydrogen evolution on the Ag content in composites during 4h illumination. The plasmonic Ag/TiO2 composites with different geometric arrangements were respectively employed as photocatalyst for hydrogen evolution. When the aqueous solution is irradiated by a 300W xenon lamp, the a-TiO2 does not exhibit measurable hydrogen evolution, but all Ag/TiO2 composites represent the significant photocatalytic activity (Figure 5A). Obviously, the nanosized Ag with strong LSPR is the critical promoter of a-TiO2 photocatalysis, and hence the mmol number of the evolved hydrogen is normalized by the Ag-loaded mass in the composites. It is shown that Ag/TiO2-3 has the greatest photocatalytic activity, and Ag/TiO2-6 and Ag/TiO2-7 have the similar and lowest photocatalytic efficiencies. For 4h illumination, the H2 output on Ag/TiO2-3 evolves to 102.1 mmol per gram of Ag, and correspondingly to 476.5 µmol h-1 per gram of photocatalyst, indicating that Ag NPs work more efficiently than the nanostructured Ag


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on Ag/TiO2 composites reported in literature (Table S1). In the control experiment, the EG layer in Ag/TiO2-3 was removed and simultaneously a-TiO2 was turned into c-TiO2 by annealing at 300 °C for 4 h,42 and the resulting composite of Ag/c-TiO2 represents the H2 output as low as 4.76 mmol per gram of Ag in the photocatalytic reaction (Figure S5). For LSPR-driven hydrogen evolution of Ag/TiO2-3, therefore, the rate enhancement results not only from the very short spacers of EG but also from the complete separation of Ag NPs with a-TiO2. Besides the EG layer, the photocatalytic efficiency is associated to the interparticle spacing of Ag NPs that could be roughly evaluated on the metal content in composites (Figure 5B). By means of changing the added volume of AgNO3 solution in the reaction mixture with pH=3, Ag/TiO2-3 composites with four different Ag loadings (wt.%): 0.63, 0.88, 1.15 and 1.68 were prepared using the same processes, and correspondingly the H2 evolution efficiencies were measured under the same experimental conditions, respectively. It is the Ag/TiO2-3, in which Ag loading is 1.15% according to ICP-MS analysis, represents the greatest catalytic activity, but neither Ag(0.63%)/TiO2-3 nor Ag(1.68%)/TiO2-3. As mentioned above, the more loading of Ag NPs implies thinner EG spacers in the composites. In spite of the fact, the Ag/TiO2-3 composites with low Ag content are expected to possess similar thickness of EG layer, but the different efficiencies suggest that the interparticle spacing of Ag NPs exerts an effect on the H2 evolution. The H2 evolution efficiency increases with Ag loading up to 1.15%, and decreases with further increasing in Ag content owing to the LSPR interfering effect among Ag NPs (Figure 5C). Moreover, it is noted that Ag(1.68%)/TiO2-3 contains the roughly same Ag metal as Ag/TiO2-4 or Ag/TiO2-5 (dotted-line frame marked in Figure 5C), but displays the highest photocatalytic activity, which suggest that the non-conductive organic spacers dominate the enhancement of H2 evolution rate to greater extent than Ag NPs content. That is to say, it is the EG layers that act as


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the spacers to separate completely or partly Ag NPs from contact with a-TiO2 in the composites, and exert the significant effects on the photocatalytic efficiencies. The Ag/TiO2-3 was cycled for 4 times to evaluate the photocatalyst stability in the hydrogen evolution (Figure S6). Each run after reaction for 2 h, the Ag/TiO2-3 catalyst was separated by evaporator without other treatment, and redispersed in the freshly prepared water-methanol solution for the next cycle. It is seen that the H2 output decreases from 46.4 in the first run to 45.3 mmol per gram of Ag in the fourth run. The slight drop in catalytic activity indicates the high durability of Ag/TiO2-3 in the illumination and the reaction media.

Figure 6. (A) UV-vis diffuse reflectance spectrum, the inset shows the Tauc plot of a-TiO2. (B) UV−vis spectrum and (C) PL spectrum for the suspension of a-TiO2 and Ag/TiO2 composites, respectively. For the LSPR-enhanced photocatalytic activity of a-TiO2 in hydrogen evolution, the possible mechanism can be reflected by the optical performances of Ag/TiO2 composites. a-TiO2 in the DRS has low visible light absorption (Figure 6A), and the band gap (Eg) is estimated to be 3.6 eV by Tauc plot of (Ahν)2 vs. hν,40-41,

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where A is the absorption, hν is photon energy.


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Compared with the Eg of 3.2 eV in c-TiO2, it is evident that the enlarged Eg in a-TiO2 is not a factor that promotes the photocatalytic hydrogen evolution under visible light. All Ag/TiO2 composites represent the DRS similar to Ag/TiO2-3 with the strong absorption from 360 to 750 nm (Figure 6A). The plasmonic Ag NPs can remarkably improve the visible absorption of aTiO2, and realize the photocatalysis enhancement over a wide spectral range. However, it is difficult to discriminate the enhanced photochemical activity from the relative absorption intensity in the DRS. It is believed that the short spacers of EG on Ag/TiO2 composites offer very favorable surroundings for the occurrences of near-field electromagnetic effect, scattering of resonant photons and FRET,44-45 and hence are responsible for the efficient LSPR-driven hydrogen evolution. The loaded Ag NPs on a-TiO2 exhibit the LSPR effect with the broadened extinction band between 370 and 550nm (Figure 6B), and the PL emission peak of Ag/TiO2 composites appears in the region from 300 to 450nm (Figure 6C), accordingly, the band overlap of LSPR absorption with PL emission, that leads to FRET, occurs to a certain extent (Figure S7). As irradiated by a xenon lamp, the plasmonic Ag NPs can concentrate the photons in a very small volume from illumination source, and transfer the irradiative energy into a-TiO2 not only through the near-field electromagnetic effect and the scattering of resonant photons, but also through the nonradiative FRET. In this situation, a-TiO2 is excited to generate more electron–hole (e-/h+) pairs, and induce the photochemical reactions. Synchronously, the Ag NPs collect the photons from the emission of Ag/TiO2 as well, and continuously give the resonant photons many chances to transfer energy into the photocatalysts. As a result, the Ag NPs on Ag/TiO2 can efficiently convert incident photons into e-/h+ pairs in the semiconductor, therefore, significantly enhance the photocatalytic activity of a-TiO2.


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The emission intensity in PL spectrum is proportional to the concentration of e-/h+ pairs in aTiO2. As shown in Figure 6C, the PL intensity of Ag/TiO2 composites is larger than that of aTiO2 with high defect density, and suggests the LSPR effect of Ag NPs can stimulate the increase in the formation rate of e-/h+ pairs. Moreover, Ag/TiO2-3 represents the strongest emission peak, indicating the Ag NPs on a-TiO2 are well separated by the thin EG spacers to prevent any direct photogenerated electron transfer between the two building blocks, and simultaneously to provide effective channels for resonant photons. By contrast, the low emission intensities of Ag/TiO2-4, Ag/TiO2-5, Ag/TiO2-6, and Ag/TiO2-7 result from the decrease in lifetime of e-/h+ pairs, because the photogenerated electron exchange between Ag NPs and aTiO2 increases the probability of e-/h+ recombination. Thus, Ag/TiO2-3 with high concentration of charge carriers becomes the preferred photocatalyst for photocatalytic hydrogen evolution.

4. CONCLUSIONS As loaded with plasmonic Ag NPs, the studies show that a-TiO2 with little photocatalytic activity can be employed as an efficient photocatalyst for photocatalytic hydrogen evolution. By means of light-induced chemical reduction, Ag/TiO2-3 was prepared using the EG-stabilized aTiO2 in aqueous solution with pH=3. In the architecture of Ag/TiO2-3, the Ag NPs in the small size possess the strong LSPR effect, and the EG molecules on the interfaces of composite bridge Ag NPs and a-TiO2 together. Particularly, the very thin EG spacers not only effectively choke the direct communication of photogenerated electrons between Ag NPs and a-TiO2, but also provide the shortest channels to efficiently transfer irradiative energies into a-TiO2 through the near-field electromagnetic effect, the scattering of resonant photons and the FRET, and constructively contribute to the remarkable enhancement of photocatalytic efficiency in hydrogen evolution. This report opens a space to produce the high-efficient photochemical


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catalysts

through

EG-regulated

spacing

between

plasmonic

metals

and

amorphous

semiconductors. ASSOCIATED CONTENT Supporting Information. XRD patterns, Grain size distributions, TEM images of Ag/TiO2-2, N2 adsorption–desorption isotherms, etc, AUTHOR INFORMATION Corresponding Author * [email protected], [email protected] Author Contributions H. L. and Q. M. contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21573059, and U1704251), Dr. Start-up Project Funding of Henan Normal University (qd16114). Innovation Funds for Postgraduate (No.YL201512, YL2017xx). ABBREVIATIONS c-TiO2, crystalline TiO2; a-TiO2, amorphous TiO2; LSPR, localized surface plasmon resonance; NPs, nanoparticles; TBT, Tetrabutyl titanate; EG, ethylene glycol; HAc, acetic acid;


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