Synergistic Effect of Hydrogenation and Thiocyanate Treatments on

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A Synergistic Effect of Hydrogenation and Thiocyanate Treatments on Ag-loaded TiO2 Nanoparticles for Solar-to-Hydrogen Conversion Tsai-Te Wang, Putikam Raghunath, Yan-Gu Lin, and Ming-Chang Lin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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The Journal of Physical Chemistry

A Synergistic Effect of Hydrogenation and Thiocyanate Treatments on Ag-loaded TiO2 Nanoparticles for Solar-to-Hydrogen Conversion T. T. Wang1, P. Raghunath1, Y. G. Lin2 and M. C. Lin1* 1

Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan 2

Materials Science Group, Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

ABSTRACT: H2 evolution rate enhanced by Ag-loading on 25 nm TiO2 anatase nanoparticles (denoted as Ag/TiO2), Ag-loaded on hydrogenated TiO2 NPs (Ag/H:TiO2), as well as by the treatment of both NPs with potassium thiocyanate (KSCN) solution have been systematically investigated in conjunction with quantum-chemical calculations and XANES and EXAFS analyses with synchrotron radiation. We have observed a cumulative enhancement effect of these fabrication processes on solar to hydrogen (STH) conversion using a simulating light source. Ag/TiO2 shows an enhanced visible absorption with 4~5 time increase in H2 evolution over that of TiO2 or H:TiO2 prepared under mild hydrogenation conditions, while Ag/H:TiO2 exhibits an even greater UV-visible absorption, similar to that of AgSCN/H:TiO2, with 3.1 times higher STH than that of Ag/TiO2. The treatment of Ag/TiO2 and Ag/H:TiO2 NPs with 0.1 mM KSCN solution further increases their STHs by 3.6 and 2.8 times, respectively. Optimization of KSCN concentration up to 0.2 mM gave [H2] production rate rise to 2.75 mmol h-1g-1 under Xe lamp illumination for the AgSCN/H:TiO2 system, which has also been tested for its durability, showing a notable robustness. The observed synergistic effect of TiO2 hydrogenation and SCN treatment of the Ag/H:TiO2 NPs has been corroborated by the results of quantum chemical elucidation of H2 production mechanism and the photo-catalytic effects of Ag/H:TiO2 and AgSCN/H:TiO2 NPs revealed by appearances of new sub-band states within the TiO2 bandgap, as well as by the result of XANES and EXAFS analyses which support the electron-pulling effect of the SCN group attached to Ag. Finally, we have also compared the efficacies of H2, HCOOH and CH3OH as hydrogenation sources at 300 oC and the efficacies of CH3OH, C2H5OH and sucrose as sacrificial agent to facilitate the separation of the electron from the hole.

*Corresponding author: [email protected] 1

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INTRODUCTION TiO2, a non-toxic mid-band gap metal oxide semiconductor, is perhaps the most

versatile and widely used material for photocatalysis and solar energy harvesting, particularly in its different nanostructural forms such as nanoparticle (NP), nanowire (NW), nanotube (NT) and nanorod (NR). Modification of its band gap to enhance its absorption in the visible region of the solar spectrum has been very actively pursued in the past couple of decades through elemental doping and metal cluster loading; two very nice reviews on the subjects have been made by Chen and Mao1 and by Fujishima et al.2 One of the very effective techniques to narrow the band gap of TiO2 has also been achieved by Mao and co-workers3 by hydrogenation of TiO2 NPs carried out under 20 atm H2 pressure at 200 oC for 5 days. The blackened powders exhibit a much enhanced H2 evolution rate with a high durability. Similar studies have been made with TiO2 NWs4 and NTs5 through hydrogenation under atmospheric condition at 200-400 oC. A theoretical interpretation on the mechanism of TiO2 hydrogenation has been made by Rahunath et al.6; their result suggests that the process is controlled by the initial dissociation of H2 on the surface with a 48 kcal/mol barrier, to be followed by the migration of H atoms into the bulk with a much lower barrier. The H atoms inside the bulk form >TiOH groups which result in the creation of O-vacancies and the reduction of Ti4+ to Ti3+ with a concomitant lower band gap. This mechanism has prompted the group to utilize the high catalytic activity of Ni to promote the hydrogenation process through7 doping and loading8 which significantly lower the H2 dissociation barriers to 12-17 kcal/mol and as low as 1-3 kcal/mol, respectively, resulting in pronounced enhancement in H2 evolution rate.7,8 A similar metal loading experiment has been carried out by Choi and co-workers9 who compare the efficacies of loading Pt, Au and Ag clusters on TiO2 NPs followed by thiocyanate (SCN) ion 2


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treatment. Most interestingly, they observed a noticeable enhancement in H2 evolution rate for the least active Ag-catalyst by a factor of 4, whereas those of Pt and Au were found to be lowered notably. The enhancement observed in the Ag/TiO2 system was attributed to the electron pulling effect of the SCN group attached to the Ag cluster and thus facilitated the separation of the electron from the hole. In view of the pronounced increase in H2 evolution rate by the hydrogenation of TiO2 NPs in the Ni-loaded system8 as aforementioned, in this work, we have investigated the effects of the hydrogenation as well as SCN treatment for the Ag-loaded TiO2 system aided by microstructural characterization with synchrotron radiation and quantum-chemical calculations to elucidate the mechanism involved in the SCN interaction with the Ag/TiO2 surface and the hydrogen production process. Particularly, this present study performed the in-situ X-ray measurements to probe the interfacial electronic states of Ag-loaded TiO2 under simulated sunlight. The experimentally and theoretically observed synergistic effect of TiO2 hydrogenation and SCN treatment on solar to hydrogen conversion with notable durability, is reported herein.

EXPERIMENTAL STUDIES A. Sample preparation and characterization To prepare hydrogenated substrates, anatase TiO2 (~25nm, 99.7%, ALDRICH)

nanoparticles were used from the commercial source without further purification. H2 (99.999%, Chiah Lung) , HCOOH and CH3OH (LCMS grade, FLUKA) vapors were used as hydrogenation source based on our recent computational and experimental test results.10 Both HCOOH and CH3OH were purified twice by a freeze-pump-thaw cycle before the hydrogenation process. A Pyrex reactor in which TiO2 NPs were placed in a ceramic bowl was evacuated first and then filled with an H source at room 3



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temperature. The TiO2 NPs were hydrogenated after 3-hour heating at 300oC under static condition. The white powders were noted to change to light brown reflecting the completion of the hydrogenation process. Silver particles were reduced onto the hydrogenated TiO2 nanoparticles (H:TiO2) by a photoreduction process. Fifty mg of H:TiO2 NPs was dispersed in 100 ml DI water with 1 M methanol (acting as an electron donor for silver ion reduction9) and 0.14mM AgNO3 (99.9%, BAKER). Although the milky white AgNO3/TiO2 aqueous solution was turned to light brown immediately after illumination from a 500W Xe lamp system (SCIENCETECH Inc.), it took 30 min irradiation with stirring to fulfil the reduction process. The Ag-loaded H:TiO2 powders (Ag/H:TiO2) were collected after 2 times DI water washings, centrifuging and drying in a 65 oC oven. The adsorption of SCN- ligand was treated by dispersing 50mg of the Ag/H:TiO2 substrate in 0.1mM potassium thiocyanate (KSCN, 99%, SIGMA-ALDRICH) aqueous solution with 3 hours stirring at room temperature. The AgSCN/H:TiO2 sample was obtained by 2 times DI water washings, centrifuging and drying at 65oC. In Fig.1, the normalized absorption spectra of the collected powders including TiO2, H:TiO2, Ag/TiO2, Ag/H:TiO2 and AgSCN/H:TiO2 are presented. It is evident that a great enhancement was observed in the visible range by Ag loading and the SCN ligand attachment. Notably, the blue-shift of Ag absorption peak was observed upon thiocyanate treatment. This phenomenon was attributed to the plasmon-induced silver atom surface electron change.9,22 We will have a closer look through EXAFS spectroscopy analysis. In order to carefully examine the TiO2 hydrogenation effect on the solar to hydrogen conversion process, the same anatase TiO2 support (without hydrogenation) was used to prepare Ag/TiO2 and AgSCN/TiO2 samples with the same photodeposition and KSCN treatment procedures as discussed above. The morphology of Ag/H:TiO2 powder analysis had been carried out by a JEO:-2010F 4


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TEM system. In Figure S2, well separated dark round particles with diameter 7~15nm are seen on ~25nm anatase TiO2 particles. Figure S3 EDS point-scan accumulation data clearly show the difference between (B) TiO2 and (C) Ag. B. Catalyst characterization The local coordination of Ag/H:TiO2 samples was characterized using Ag K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The Ag/H:TiO2 powder samples were packed in a Teflon holder and sealed with Kapton tape at a thickness calculated to yield unit edge step across the Ag K-edge. The Ag K-edge spectra were collected respectively at Beamline 01C1 of the NSRRC. Ag L-edge X-ray absorption near edge structure (XANES) data were collected at Beamline BL16A1 of the NSRRC. The Ti L-edge spectra were collected from fine powder, dispersed over conducting copper tape, with the surface sensitive total electron yield at beamline 20A1 of the NSRRC with an energy resolution of ∆E/E ∼ 1/5000. The Ag L-edge and Ti L-edge in-situ XANES of Ag/H:TiO2 under dark and illumination conditions were collected at the NSRRC. The in-situ illumination experiments were carried out with the AM 1.5G simulated solar light at 100 mW cm-2. The Ti L3,2-edge XANES spectra of TiO2 and H:TiO2 are presented in Fig. 2(a). The shapes of the spectral features of TiO2 and H:TiO2 are very similar. The A, C and B, D features are respectively assigned as the t2g and eg states in 3d orbital. The eg states could be considered as dz2 (B1 peak) and dx2-y2 (B2 peak) orbitals which are directed toward ligand anions and therefore more sensitive to deviations from octahedral symmetry. Fig. 2(b) shows the ratio of B1/B2-peak integrated intensities, implying the degree of distortion from original octahedral symmetry. The increased ratio between dz2 and dx2-y2 orbitals revealed the significant distortion of Oh symmetry when TiO2 treated with hydrogenation process. Fig. 2(c) shows the ratio of A/(B1+B2)-peak integrated intensities.The decreased ratio between t2g and eg orbitals 5



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exhibited a lower oxidation state of Ti for H:TiO2 as compared to pristine TiO2. The energy level of Ti3+ species is located between the valence band and the conduction band of TiO2, which could effectively promote the electrons in the new sub-band states to be excited to the conduction band of TiO2. 3 Since the photochemical property of the STH conversion reaction is greatly dependent on the band structure of the semiconductor,23,24 we suggest that the plasmon-induced charge-transfer may modify the electronic structure of H:TiO2 and Ag/H:TiO2. Therefore, XANES was utilized to provide valuable evidence and in-situ measurements were made simultaneously to collect the spectra with/without solar light illumination. Figure 3 displays the absorbance differences to compare dark and in-situ AM1.5 illumination states for Ag/H:TiO2 and AgSCN/H:TiO2. Ag/H:TiO2 showed a negative ∆A values of XANES under in-situ AM1.5 illumination for Ti L-edge. The negative value suggests that Ag can significantly induce the plasmonic hot electron and further lead to an increase in electron transition by soft X-ray. In contrast, AgSCN/H:TiO2 shows a positive ∆A values of Ti L-edge XANES and a negative ∆A values of Ag L-edge XANES under in-situ AM1.5 illumination. This considerably opposite behavior suggests that SCN treatment can additionally provide an electromagnetic field and further induce vacancies in the conduction band of H:TiO2. Notably, this observation reveals that the localized surface plasmon resonance (LSPR) induced electromagnetic field greatly dominates the conduction band nature of H:TiO2. A similar observation was also noticed in Au decorated TiO2 nanorod arrays, in which L-edge absorption of Ti was performed to monitor the CB of TiO2 owing to a desired transition from 2p to 3d orbitals of Ti.25 This finding implied SCN treatment could greatly facilitate the electron transport from H:TiO2 to Ag and decrease the accumulation of photoexcited electron in H:TiO2 when exposed to simulated solar light. AgSCN-H:TiO2 heterostructures thus yielded satisfactory 6


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performance. This report is the first of synchrotron-based in-situ analysis for AgSCN/H:TiO2 with solar light illumination. The Fourier transforms extended X-ray absorption fine structure (EXAFS) spectra of Ag K-edge and the fitting results were shown in Fig. 4. The Ag EXAFS spectra of AgSCN/H:TiO2 are very similar to Ag/H:TiO2 (Fig. 4a). However, the amplitudes of AgSCN/H:TiO2 showed a progressive decrease of the first-shell Ag-Ag contribution from the fcc structure at ∼2.6 Å. This phenomenon was accompanied with alteration on the spectral features between 1.5 Å and 2.5 Å. The data were fit (R-factor = 0.009) with an EXAFS model derived from a Ag and AgSCN structural model that accounted for the highest-amplitude, single-scattering Ag-Ag and Ag-S paths out to 3.0 Å (Fig. 4b-c). The best fitting EXAFS results and the fitting paths used in models are reported in Fig. 4b-c as solid black curves and the quantitative fit results in Table 1. The Ag coordination number (CN) for Ag/H:TiO2 was fixed to 12 and interatomic distance (R) was 2.864(0.006) Å. For AgSCN/H:TiO2, the corresponding CN and R were 9.2(0.8) and 2.856(0.007) Å, respectively. Although decreasing in CN for AgSCN/H:TiO2 accounted for the decrease in the FT magnitude (Fig. 4a), the increasing structural disorder and sulfur bonding for Ag would also diminish the high-shell back scattering signal. Fitting analyses evidenced that σ2 values for the Ag-Ag paths of AgSCN/H:TiO2 were increased from 0.0093 to 0.0102. During the fitting process, it was found that Ag-Ag shell alone was not sufficient to fit the data for AgSCN/H:TiO2 and the inclusion of Ag-S path was necessary for improving the fit. The fitting results displayed the CN and R for Ag-S path are 1.0(0.3) and 2.48 (0.016), respectively. This fitting result implied a chemical bonding between S and Ag was formed further, and thus increased the structural disorder of Ag significantly. C. H2 evolution experiment 7



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The production of H2 was measured volumetrically. For the reaction, 30 mg of prepared powders was first dispersed in 60 ml DI water with 20 % methanol as sacrificial reagent using a Pyrex reactor with a 2-mm thick wall, followed by 30 min sonication for complete particle dispersion. Before illumination, the reactor was evacuated with a freeze-pump-thaw cycle to remove residual air. A cold trap containing -78 oC ethanol/dry ice was used to trap methanol and water vapors escaping from the reactor. Although at dry-ice temperature, pure methanol vapor pressure is known to be 15 mTorr, the co-condensation of methanol and water should reduce it to a negligible level as indicated by the pressure reading prior to photolysis as shown in Fig. 5 and 6. For the H2 evolution reaction, an 80-90 min long H2 sampling was carried out, including 10 min before and 10-20 min after photolysis and the 1-h long irradiation using 3.25 W Xe lamp output. H2 pressures were recorded for different samples employed. The results were converted to mmoles of H2 per hour per gram of sample employed through the ideal gas law (Table 2). In Fig. 5, the hydrogen produced by 1-h illumination is compared with different substrates; the results reflect the enhanced UV-vis absorption shown in Fig. 1 by Ag-loading, the hydrogenation of TiO2 and the final SCN treatment. With Ag-loading, a factor of 5 increase was noted for Ag/TiO2 vs. TiO2. The hydrogenation of TiO2 using CH3OH as the H-source, a 12-time increase in H2 yield was observed for the Ag/H:TiO2 vs. H:TiO2. For the effect of SCN treatment, a 3.6 time enhancement by SCN ligand attachment to Ag/TiO2 was noted, in good agreement with the 4-time increase reported by Choi et al.9

With the hydrogenated TiO2, a further 2.8-time increase was also observed for

AgSCN/H:TiO2 over Ag/H:TiO2 and 3.3-time enhancement was further optimized by varying the KSCN concentration to 0.2 mM in preparing AgSCN/H:TiO2 NPs. The results summarized in Table 2 using the hydrogenated and nonhydrogenated TiO2 NPs show the synergistic effect of the hydrogenation and SCN treatment. 8


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In Fig. 6, we further illustrate the synergistic effect of hydrogenation and SCN treatment by comparing the hydrogen production rates using different hydrogenation sources, H2, HCOOH and CH3OH; although the degree of enhancement for AgSCN/H:TiO2 vs. Ag/H:TiO2 is not quantitatively the same, the trend is qualitatively similar. In terms of the H-source, CH3OH is noted to be more effective than H2 and HCOOH for the present system. We have also carried out the durability test; the reactor containing AgSCN/H:TiO2 solution was stored in dark after 1-h illumination once per week; the hydrogen production rate was observed to remain unchanged after a consecutive 5-week test, to be followed by a full week of outdoor solar irradiation as shown in Fig. 7; the result reveals the robustness and durability of the AgSCN/H:TiO2 system. The results of experiments presented above were obtained by using a Pyrex photolysis cell with a 2-mm thick wall employing 3.25 W Xe lamp output. The incident light has been compared with the AM1.5G light source. In this test, the Xe light intensity passing through a 2 mm thick Pyrex glass was measured as described in Supplemental Information Figure S1; we detected 10 % attenuation of the Xe light due to scattering and absorption by the Pyrex glass. An additional test using AM1.5G and the Xe light for H2 evolution using a quartz cell was made with the AgSCN/H:TiO2 sample prepared with 0.1 mM KSCN and H:TiO2 as was employed in most of the experiments carried out above. The result of this test indicated that the STH with the Xe lamp was about 10 % higher than that of the AM1.5G tested with 0.38 W intensity. This result suggests that the H2 evolution reported above using a Pyrex cell should be effectively the same as using AM1.5G with a quartz window which transmit all wavelength above 200 nm without much attenuation. Accordingly, our highest [H2] cited above should be effectively equivalent to the value obtained with the AM1.5G light source using a quartz window. 9



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In addition, we have also varied the Xe incident beam intensity from 0.8 W to 3.25 W, no clear power dependence was observed. (see Table S1). Furthermore, we have also compared the efficiencies of methanol, ethanol and sucrose as sacrificial agent which helps the separation of the electron from the hole during the photocatalytic reduction of H2O. The results summarized in Table S2 indicate that 20 % methanol is slightly better than 20 % ethanol, which is similar to the 0.5 M sucrose employed using the 0.1 mM KSCN treated H:TiO2 anatase NPs. H2 production rates with various alcohol solutions had been discussed by Chen et al. using their Au/TiO2 system20. The H2 production rate is expected to be correlated with the band gap of the electron donor oxidation potential Eoox and the TiO2 valence band potential EoVB(TiO2) (2.7 V vs. NHE).21 The standard oxidation potentials (vs. NHE) were calculated with the Gibbs free energies of the reactions, CxHyOz + (2x-z)H2O → xCO2 + nH+ + ne-

(1)

with Eoox (V) vs. NHE = - ∆Go/nF. Thus, with the Gibbs free energies of formation from Lange's handbook, the standard oxidation potential of methanol (0.016 V) is lower than those of sucrose (0.023 V) and ethanol (0.084 V). These vaues are consistent with the H2 evolution rates (in mmol h-1g-1) measured with methanol, sucrose and ethanol, 0.326, 0.290 and 0.286, respectively, as sacrificial agents. A further study on the efficacy of sucrose for the present Ag-loaded TiO2 as well as other photocatalysts will be pursued in the near future.

COMPUTATIONAL STUDY Computationally, we have investigated the adsorption of SCN and the reaction

of H on Ag-loaded anatase TiO2(101) surface by the first-principles calculations using the Vienna ab initio simulation package (VASP)11. For the total energy prediction, the 10


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exchange-correlation function treated by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof formulation (PBE)12 has been applied with spin-polarization throughout the system. The lattice constants predicted at the PBE level for the bulk TiO2 in the anatase phase, as reported in our previous study13, a=3.828 Å and c= 9.677 Å are in good agreement with the experimental values of a= 3.782 Å and c= 9.502 Å.14 In this study, we used the surface model consisting of 24 [TiO2] units with 11.088 x 15.287 Å2 surface area, extended along the < 111 > and

< 010 > directions, separated perpendicularly by a 15.0 Å vacuum space as shown in Figure S4. Brillouin zone integrals were computed over regular Monkhorst-Pack grids with 2 × 3 × 1 k-point for the anatase (101) surface. As shown in Figure S4, the geometrical parameters of anatase (101) surface are characterized by the presence of acidic–basic pairs of coordinative unsaturated ions, i.e. five fold coordinated Ti4+ ions (Ti5c) and twofold coordinated bridging oxygen O2− ions (O2c) including coordinated ions on the surface (O3c, Ti6c). The climbing-image nudged-elastic band (CINEB) method15 was applied to locate the transition states for H2 elimination processes. To analyze the electron correlations in transition metal oxides, the conventional DFT calculations based on the local density approximation and GGA failed to predict correctly the values of band gaps. In view of this, the DFT+U method16 with spin-polarization was applied in order to accurately correct the strong on-site Coulomb repulsion of Ti 3d states, where the value of U employed, 4 eV, is consistent with that reported by Finazziet al.17 In the present work, we first systematically investigated the 1 and 3 Ag atoms placed on the TiO2 (101) surface and its stable conformations depicted as Ag/TiO2, (Ag)3/TiO2 in the Figure S4. The stable configurations were utilized for adsorption of SCN and the reaction of H atoms. The adsorption energies of 1 and 3 Ag cluster/SCN: AgSCN/TiO2 and Ag(AgSCN)2/TiO2 were calculated using the following equation: 11



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Eads = E[slab] + E[adsorbate]− E[slab+adsorbate] where Eslab is the total energy of the clean slab, Eadsorbate is the energy of the AgSCN/TiO2 or Ag(AgSCN)2/TiO2, and E[slab+adsorbate] is the total energy of the slab with adsorbate. The Ag nanoclusters deposited on the anatase (101) surface have been shown in Figure S4. For the deposition of single Ag atom, we found the stable adsorption position placed in between the O2c site on the surface with bond length of 2.214 Å is noted at Ag/TiO2, with an adsorption energy of 27.7 kcal/mol. In the (Ag)3/TiO2 case, the three silver atoms form a triangle cluster structure, its adsorption energy was found to be 48.2 kcal/mol, favorably located on the O2c and Ti5c sites. The bond length of Ag-Ag on the surface is observed to be ~2.7 Å and that of the Ag and O2c site is observed to be ~2.3 Å. A recent studies providing insights into the binding mechanisms and geometries of Agn and Ptn Clusters (n=2,4,8) deposited on the anatase TiO2 (101) Surface were investigated using density functional theory.18,19 They also found that Ag-Ag bond length on the surface to be ~ 2.7 Å. The detailed geometrical parameters are shown in Figure S4. After obtaining the most stable adsorption configurations of the Ag clusters on anatase TiO2(101), the adsorption properties of 1 and 2 SCN were investigated and the related adsorption configurations are displayed in Fig. S4. We placed SCN molecule on the Ag clusters and TiO2 surface in order to identify the mode of the strongest adsorptions. The preferred and most stable SCN adsorption structures on the Ag site, Eads of 43.0 kcal/mol for 1 SCN on 1 Ag and 60.2 kcal/mol for 2 SCN on 3 Ag atoms, binding through S atom(s) to the Ag atom(s). Both AgSCN/TiO2 and Ag(AgSCN)2/TiO2, with Ag–S distances of ~2.4 Å. Our results showing that the SCN adsorption on the TiO2 surface via Ti site is less stable and its adsorption energy is 4.6 kcal/mol and Ti and S bond length is 2.829 Å. Therefore the adsorption energies of SCN would become smaller on the clean TiO2 as compared to that on the TiO2 12


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anatase supported Agn cluster, and this may help to enhance the selectivity and stability of SCN formation on the Ag cluster. The result is consistent with the observation that thiocyanate strongly interacts with Ag NPs and the surface complexation of thiocyanate with Ag/TiO2 is responsible for the enhancement of H2 production.9 In order to understand the nature of charge in the SCN adsorption on (Ag)3/TiO2 system, we have carried out Bader charge analyses using the DFT method as shown in Figure 8. According to the analysis, the charge of S and N atoms in the both AgSCN/TiO2 and Ag(AgSCN)2/TiO2 are around 0.1 e and -1.1 e, respectively, where e is the magnitude of the charge on an electron (see Figure 8). Based on our present work and the previous experimental finding9, we have calculated the H atom interactions with Ag-SCN surface complex via two exothermic reactions for the production of H2; the results are shown in Figure 9. The H atom can directly attach to N and S atoms of AgSCN/TiO2 giving AgSCNH/TiO2 with an exothermicity of -67.9 kca/mol and AgS(H)CN/TiO2 with an exothermicity of -46.9 kca/mol, respectively. As shown in Figure 9a, an additional H atom can directly abstract the H atom of AgSCNH/TiO2 or AgS(H)CN/TiO2 to release H2 through TS1 and TS2 with 10.2 or 4.6 kcal/mol barriers, respectively (see Fig. S4). In the case of 2 SCN on 3 Ag loaded TiO2, Ag(AgSCN)2/TiO2 (Figure 9b), similar to the 1 Ag case, 2 H atoms can directly form N−H bonds, Ag(AgSCNH)2/TiO2, with very large exothermicity of -138.6 kca/mol, comparing with the formation of S−H bonds, Ag(AgS(H)CN)2/TiO2, -91.4 kca/mol. From these two intermediates, H2 elimination from 2 N−H and 2 S−H required 52.8 kcal/mol and 6.9 kcal/mol, respectively. The Potential energy surfaces shown in Fig. 9 indicate the H2 can be formed exothermically through reactions involving the SCN antenna attached to the Ag particles.

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To further understand the effect of SCN with and without hydrogenated Ag/TiO2, we calculated the densities of states (DOS) and projected DOS (PDOS) of Ag, SCN and H by using the DFT + U with the value of U = 4 eV as mentioned before. The results are presented in Figure 10. Figure 10(a) shows the band structure of the pure anatase TiO2. We can see from the figure that the top of the valence band (VB) of pure TiO2 mainly consisting of the O 2p states is predominantly found between -4.5 and 0 eV and the lower part of the conduction band (CB) is dominated by Ti 3d states, consistent with our previous report.6 The DFT + U method can correct the energy level of Ti 3d states by adding the Hubbard-U to Ti d states, and the band gap can be improved to 3.0 eV (from 2.2 eV by DFT method). The calculated DOSs for the 1 and 3 Ag loaded TiO2, Ag/TiO2 and (Ag)3/TiO2, are shown in panel 10(b) and 10(c) which indicates the reduction of the band gap to 2.7 eV. Significantly, two new peaks appear above the VB at ~0.2 eV and an additional peak at 2.0 eV in the 3 Ag case. These new peaks in the band gap of the Ag-loaded TiO2 are mainly derived from the 3d states of Ag and 2p states of O with a minor contribution from Ti. The SCN effect on Ag loaded TiO2 results are shown in Panel 10(d), the DOS of one and two SCN on the Ag-loaded surface, AgSCN/TiO2 and Ag(AgSCN)2/TiO2. The band gap is observed to be 2.6 eV with 0.4eV reduction from that of the TiO2. The VB is originated at -2.6 eV below the Fermi level and the new peaks appear at 0.2 eV above the VB and another new peak 1.0 eV above the CB, the d electronic states can be mainly attributed to the Ag, while the p electronic states are due to the O 2p states. The results of DOS for the SCN effect on hydrogenated 1 and 3 Ag atom loaded TiO2, Ag/H:TiO2 and Ag3/H:TiO2, respectively. The 2H atoms incorporated into the subsurface with the 2 OH groups opposite to each other, AgSCN/2H:TiO2 and Ag(AgSCN)2/2H:TiO2, are shown in panel (f) and (g). Several new peaks appearing

14


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between the VB and CB are mainly originated from the Ag and O with a negligible contribution from 3d of Ti.

DISCUSSION

A. Effects of TiO2 hydrogenation and SCN ion treatment of Ag/TiO2 As alluded to in the introduction, the hydrogenation of TiO2 nano-materials led to enhanced H2 evolution rate.3-5 The effect was theoretically attributed to the migration of H atoms into the bulk forming >TiOH groups and the creation of O-vacnancies.6 Prolong hydrogenation under a high pressure condition also led to the disordering of the surface which helps the narrowing of the band gap.3 Under our mild hydrogenation condition using H2, HCOOH or CH3OH, TiO2 absorption in the visible region is only slightly red-shifted as shown in Fig. 1 with a negligible enhancement in H2 evolution as indicated by the result given in Fig. 6 and Table 2. The loading of Ag particles on TiO2 NPs, however, led to a factor of 5 increase in H2 evolution and the loading of Ag on the hydrogenated TiO2 (H:TiO2) led to a factor of 12 increase (see Table 2 and Fig. 5). The color of the Ag/H:TiO2 is seen to be black in Fig. 1, similar to that of the SCN ion treated Ag/H:TiO2 (or AgSCN/H:TiO2), which is a factor of 32 more efficient than H:TiO2 in terms of STH for the 0.1 mM KSCN treated sample. Further optimization of the KSCN concentration led to a factor of 39 increase in H2 evolution rate(see Table 2). The above results are consistent with our theoretical calculations using VASP; the binding energy for SCN with Ag/TiO2 and Ag3/TiO2 with 2 SCN were predicted to be 43 and 60 kca/mol, respectively, which are consistent with the robustness of the AgSCN/ TiO2 system under solar irradiation. In Fig. 10 the predicted densities of states the Ag loaded TiO2 and H:TiO2 indicate the appearance of new impurity states within the band gap. The adsorption of SCN on Ag/H:TiO2 in particular gives rise to 15



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very prominent new peaks at the band edge of the valence band with several smaller peaks inside the gap. These results reflect the very significant enhancement in the absorption in the visible region as shown in Fig. 1 for the SCN optimized AgSCN/H:TiO2 NPs. In Fig. 9 we present the predicted potential energy surfaces for H atoms interaction with AgSCN/TiO2 and Ag(AgSCN)2/TiO2 to elucidate the mechanism for H2 elimination. In the former system, H binds exothermically with 47 kcal/mol forming an S-H bond and also exothermically with 68 kcal/mol forming an N-H bond; both bonds react readily with another H giving H2 with an overall exothermicity of 105 kcal/mol. Similarly, in the 2H + Ag(AgSCN)2/ TiO2 → H2 + Ag(AgSCN)2/TiO2 reaction, occurring with much higher exotherimicities via the intermediate states with the same overall exotherimicity of 105 kcal/mol. These reactions are highly favored kinetically; however, because of the high electron densities carried by the terminal N atoms and the slightly positive charges of the S atoms according both models (see Fig. 8), the H+ ion is believed to react more readily with the terminal N atoms, which therefore should be the major catalytic hydrogen evolution sites during the photo-reduction process. It should be mentioned that the mechanism proposed by Choi and co-workers9 on hydrogen evolution is essentially the same as the 2H + Ag(AgSCN)2/TiO2 → Ag(AgS(H)CN)2/TiO2 → H2 + Ag(AgSCN)2/TiO2 channel. Their mechanism is therefore theoretically corroborated by our quantum chemical calculations. The mechanisms presented in Fig. 9 should be equally valid for H+ ion reactions because once the H+ ion approaches the SCN group with a positive charge, it should immediately pull an electron from the substrate to neutralize it. The electron pulling effect of the SCN group should therefore help the electron-hole pair separation as suggested by Choi and co-workers.9 The results presented above are also fully corroborated by the results of synchrotron analyses summarized below. 16


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B. Synchrotron data analyses. The above experimental and computational analyses show that SCN is selectively chemisorbed on the Ag surface and uniquely influences the solar to hydrogen conversion process on it. A more convincing evidence for the direct charge transfer between AgSCN and H:TiO2 was obtained from the synchrotron-based in-situ analysis. In Fig. 3, the results reveal that the formation of Ag-SCN surface complex significantly enhances the interfacial electron transfer rate as compared to bare Ag. The presence of the SCN group, which is highly electron-withdrawing, seems to be essential to withdraw electrons from Ag and reduce the interfacial charge transfer resistance on Ag/H:TiO2. Note that the increased structural disorder and strong bonding of sulfur in the local coordination of Ag was observed through SCN modification in Fig. 4. It gave an evidence for the strong interaction between Ag and the SCN group. Furthermore, SCN treatments were employed here to manage the interfacial nature between H:TiO2 and the surface of plasmonic Ag nanoparticles, which allowed us to strategically improve the electron-hole separation as well as to accelerate the sluggish kinetics of photochemical reaction. Meanwhile, the evolution in electronic structure upon H:TiO2 was probed at real time by X-ray absorption spectroscopy of Ti L-edge, in which a plasmon-induced irradiation with a desired wavelength was simultaneously utilized to reveal the LSPR effects over the electronic structure of H:TiO2. Therefore, the role of SCN to accelerate the electron transfer and to facilitate the H2 production exothermically via the intermediate states is highly critical on the Ag surface. To the best of our knowledge, this is the first ever reported in-situ X-ray study to clarify the interfacial behavior of charge transfer for Ag-loaded TiO2 under simulated sunlight. Furthermore, the approach to engineer the disorder in the surface layers of TiO2 through hydrogenation by Mao and coworkers3 is confirmed in this work. Combined 17



Page 18 of 30

with the results from computational data that suggest large amounts of lattice distortion in TiO2 could yield new sub-band states whose energy distributions differ from that of the bare TiO2 (see Fig. 2 and Fig. 10). Charges associated with the lower energy sub-band states distributed around every O or Ti atom are indicative of the overall impact of the disorder. Particularly, the reduction in the optical transition between new sub-band states and the conduction band tail via AgSCN loading is presumably responsible for effective visible-light absorption. This new energy state can become the dominant centers for photogenerated carriers and prevent them from rapid recombination, thus promoting photocatalytic reactions. The synergistic effect of TiO2 hydrogenation and SCN treatment, thus yielded the observed satisfactory performance of the AgSCN/H:TiO2 system.

CONCLUSION An efficient solar hydrogen system is developed based on SCN assisted

Ag-loaded H:TiO2 photocatalysts. 0.83 mmol h-1g-1 hydrogen was produced under one hour illumination from our Ag/H:TiO2, much better than bare TiO2 and Ag/TiO2, and subsequent surface modification with SCN further enhances production rate to 2.75 mmol h-1g-1. The improvement in photoactivity of solar to hydrogen conversion can be attributed to the enhanced visible-light absorption and improved interfacial charge-transfer kinetics due to the synergistic effects of hydrogenation and SCN treatment contributing to photocatalysis. The crucial roles of the hydrogenation and SCN treatment were investigated systematically and confirmed by quantum-chemical calculations with complementary synchrotron-based X-ray techniques. The formation of Ag-SCN surface complex enhances the interfacial electron transfer rate and facilitates the H2 production exothermically via the intermediate states on Ag/TiO2. Furthermore, the creation of new sub-band states within the TiO2 via hydrogenation and AgSCN loading is effectively responsible for effective visible-light absorption 18


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due to the reduction of the optical transition. The X-ray insights and associated mechanistic interpretation and correlation with STH performances are presented herein, which informatively guide the rational strategies for further processing of high efficiency H2 production. It is also worthnoting that sucrose is equally effective as a sacrificial agent as ethanol, which has been commonly employed to assist the separation of the electron from the hole in photo-catalytic studies of nano-materials. Aside from the enhanced hydrogen production, what are the oxidation products which remain in the solution? This is the question we will try to answer in the near future using the current AgSCN/H:TiO2 and other effective photo-catalysts.

SUPPORTING INFORMATION

Table S1 shows the Power dependence study under 1 h Xe lamp irradiation. Table S2 shows the H2 production rates under AM1.5G simulator illumination using different sacrificial agents. Figure S1 showing the comparing the Normalized AM1.5G and the Xe lamp. Figure S4 shows the detailed geometrical parameters are related to Figure 9.

ACKNOWLEDGEMENTS

The authors acknowledge the supports from the ATU Plan of the Ministry of Education, Taiwan, and also from the National Center for High-performance Computing for providing the computer time. MCL thanks the Ministry of Science and Technology of Taiwan for the distinguished visiting professorship at NCTU.

REFERENCES

(1) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891-2959. (2) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surface Science Reports. 2008, 63, 515-582. 19



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(3) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science. 2011, 331, 746-750. (4) Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang,

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Photoelectrochemical Water Splitting. Nano. Lett. 2011, 11, 3026-3033. (5) Li, J. C, The Application of Hydrogenated TiO2 Nanotubes for Water Splitting. M.S Thesis, National Chiao Tung University, Taiwan, 2012. (6) Raghunath, P.; Huang, W. F.; Lin, M. C. Quantum Chemical Elucidation of the Mechanism for Hydrogenation of TiO2 Anatase Crystals. J. Chem. Phys. 2013, 138, 154705. (7) Chuang, C. C.; Lin, C. K.; Wang, T. T.; Srinivasadesikan, V.; Raghunath, P.; Lin, M. C. Computational and Experimental Studies on the Effect of Hydrogenation of Ni-doped TiO2 Anatase Nanoparticles for the Application of Water Splitting. RSC Adv. 2015, 8, 81371-81377. (8) Lin, C. K.; Chuang, C. C.; Raghunath, P.; Srinivasadesikan, V.; Wang, T. T.; Lin, M. C. Quantum-chemical Prediction of the Effects of Ni-loading on the Hydrogenation and Water-splitting Efficiency of TiO2 Nanoparticles with an Experimental Test. Chem. Phys. Lett. 2017, 667, 278–283. (9) Choi, Y.; Kim, H-i.; Moon, G-h.; Jo, S.; Choi, W. Boosting up the Low Catalytic Activity of Silver for H2 Production on Ag/TiO2 Photocatalyst: Thiocyanate as a Selective Modifier. ACS Catal. 2016, 6, 821−828. (10)

Raghunath, P.; Wang, T. T.; Li, J. C.; Lin, M. C. Lin. Manuscript Under

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Mazheika, A. S.; Bredow, T.; Matulis, V. E.; Ivashkevich, O. A. Theoretical

Study of Adsorption of Ag Clusters on the Anatase TiO2(100) Surface. J. Phys. Chem. C, 2011, 115, 17368–17377. (20) Chen W.-T.; Chan A.; Al-Azri Z. H.N.; Dosado A. G.; Nadeem M. A.; Sun-Waterhouse D.; Idriss H.; Waterhouse G. I.N. Effect of TiO2 polymorph and Alcohol Sacrificial Agent on the Activity of Au/TiO2 Photocatalysts for H2 Production in Alcohol-Water Mixtures. J. Catalysis, 2015, 329, 499-513. (21) Balzani V.; Scandola F.; in: Gratzel M. Energy Resources through Photochemistry and Catalysis. Academic Press Inc. New York, 1983, 2-48. (22) Henglein, A.; Meisel, D. Spectrophotometric Observation of the Adsorption of Organosulfur Compounds on Colloidal Silver Nanoparticles. J Phy Chem B 1998, 102, 8364-8366. (23) Wu K.; Chen J.; McBride J. R.; Lian T. Efficient Hot-Electron Transfer by a Plasmon-Induced Interfacial Charge-Transfer Transition. Science 2015, 349, 632-635. (24) Wood A.; Giersig M.; Mulvaney . Fermi Level Equilibration in Quantum Dot-Metal Nanojunctions. J Phy Chem B 2001 105, 8810-8815. (25) Hung, S. F.; Xiao, F. X.; Hsu, Y. Y.; Suen, N. T.; Yang, H. B.; Chen, H. M.; Liu, B. Iridium Oxide-Assisted Plasmon-Induced Hot Carriers: Improvement on Kinetics and Thermodynamics of Hot Carriers. Adv. Energy Mater. 2016, 6, 1501339-1501350.

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Figure 1. Normalized absorption spectra of anatase TiO2, H:TiO2, Ag/TiO2 Ag/H:TiO2, AgSCN/H:TiO2; 150 Torr CH3OH vapor was used for TiO2 hydrogenation at 300 oC for 3 hours.

Figure 2. (a) Ti L3,2-edge XANES spectra of various TiO2. (b) The ratio of B1/B2 and (c) A/(B1+B2) integrated intensities with various TiO2. 23



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Figure 3. The intensity difference between in-situ AM1.5 illumination and dark states of (a) Ti L3-edge and (b) Ag L-edge. (∆A = Aillumination – Adark)

Figure 4.(a) Ag K-edge EXAFS spectra for Ag/H:TiO2 and AgSCN/H:TiO2. Magnitude of Fourier transformed k 3 χ(k) data; Ag K-Edge EXAFS Fitting Analysis for (b) Ag/H:TiO2 and (c) AgSCN/H:TiO2. 24


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100 TiO2 H:TiO2

80

Ag/TiO2 AgSCN/TiO2

60 [H2] /µmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ag/H:TiO2

40

AgSCN/H:TiO2

a

AgSCN/H:TiO2

b

20

0 0

10

20

30

40

50

60

70

80

90

Time /min

Figure 5. H2 production from 1 h hydrogen conversion in a Pyrex cell using 3.25 W Xe light. a AgSCN/H:TiO2 sample prepared with 0.1 mM KSCN solution. b

AgSCN/H:TiO2 sample prepared with 0.2 mM KSCN solution.

Figure 6. H2 pressures recorded in solar to hydrogen conversion with different TiO2 hydrogenation sources. The base line (-●-) was taken with 20 % methanol/DI water mixture only.

25



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Figure 7. Durability test for AgSCN/H:TiO2 NPs prepared with 0.1 mM KSCN treated sample carried out with 3.25 W Xe light in a Pyrex cell. In week 6, the reactor was placed outdoor for the full week under natural solar illumination.

Figure 8. Bader atomic charges (e) are in parenthesis for (a) 1 and (b) 2 SCN adsorbed on the 3 Ag loaded TiO2 anatase (101) surface.The bond lengths are given in Å. 26


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Figure 9. Predicted potential energy surfaces of H atoms binding on SCN and H2 elimination (a) on the 1 Ag loaded TiO2, (b) on the 3 Ag loaded TiO2.

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Figure 10. Density of states (DOS) for (a) clean TiO2, (b) 1 Ag loaded TiO2, (c) 3 Ag loaded TiO2, (d) SCN adsorbed on 1 Ag loaded TiO2, (e) 2 SCN adsorbed on 3 Ag loaded TiO2, (f) SCN adsorbed on 2 H inside the bulk of 1 Ag loaded TiO2, (g) 2 SCN adsorbed on 2 H inside the bulk of 3 Agloaded TiO2 calculated at the DFT + U level (U = 4.0 eV for Ti) (for clarity, Ag and SCN PDOS peaks (dotted lines) are magnified by 10 times).

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Table 1. Structural Parameters Obtained from Ag K-Edge EXAFS Fitting Analysis for Ag-H-TiO2 and KSCN-Ag-H-TiO2a,b

a

Fitting was done across the k range of 3.0 to 12.0 Å–1 and an R range of 1.0 to 3.0 Å.

Numbers in parentheses are uncertainties calculated for the EXAFS model. b

All samples were fit simultaneously, yielding a normalized sum of squared residuals

[R-factor = ∑(data-fit)2/∑data2) of 0.009 (0.9 %)].Values of other EXAFS model parameters not shown above were either fixed or fit to a common value across all samples as follows: S02 = 0.66(0.09) (fixed amplitude reduction factor based on first-shell fitting of Ag-H-TiO2). ∆E = 1.72 (0.89) and eV (fitted energy shift parameter). The data were fit with an EXAFS model derived from Ag and AgSCN structural model.

Table 2. H2 produced under 1 h irradiation with 3.25 W Xe light in a Pyrex cell.a

[H2]

TiO2

H:TiO2b

Ag/TiO2

Ag/H:TiO2b

AgSCN/TiO2

AgSCN/H:TiO2b

0.053

0.073

0.267

0.828

0.967

2.33/2.75c

mmol h-1g-1 a.

30 mg NPs were used in solar to hydrogen conversion experiments with

[MeOH]=20 vol % as sacrificial reagent. b. c

CH3OH vapour was used as H source for TiO2 hydrogenation.

.H2 production rate from the AgSCN/H:TiO2 sample prepared with 0.2 mM

KSCN solution.

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TOC GRAPHIC


Synergistic Effect of Hydrogenation and Thiocyanate Treatments on

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