Well-dispersed Ruthenium in Mesoporous Crystal TiO2

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Well-dispersed Ruthenium in Mesoporous Crystal TiO as an Advanced Electrocatalyst for Hydrogen Evolution Reaction Shuying Nong, Wujie Dong, Junwen Yin, Bowei Dong, Yue Lu, Xiaotao Yuan, Xin Wang, Kejun Bu, Mingyang Chen, Shangda Jiang, Li-Min Liu, Manling Sui, and Fuqiang Huang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Journal of the American Chemical Society

Well-dispersed Ruthenium in Mesoporous Crystal TiO2 as an Advanced Electrocatalyst for Hydrogen Evolution Reaction Shuying Nong†, #, Wujie Dong†, #, Junwen Yin ¶, #, Bowei Dong†, Yue Lu§, Xiaotao Yuan†, Xin Wang†, Kejun Bu‡, Mingyang Chen¶, Shangda Jiang †, Li-Min Liu*, ¶, ǁ,Manling Sui *,§,Fuqiang Huang*, †, ‡ †

State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡

State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China ¶

Beijing Computational Science Research Center, Beijing 100084, China

ǁ

School of Physics, Beihang University, 100191, China Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing, 100124, China §

ABSTRACT: TiO2 mesoporous crystal has been prepared by one-step corroding process via an oriented attachment (OA) mechanism with SrTiO3 as precursor. High resolution transmission electron microscopy (HRTEM) and nitrogen adsorption-desorption isotherms confirm its mesoporous crystal structure. Well-dispersed ruthenium (Ru) in the mesoporous nanocrystal TiO2 can be attained by the same process using Ru-doped precursor SrTi1-xRuxO3. Ru is doped into lattice of TiO2, which is identified by HRTEM and super energy dispersive spectrometer (super-EDS) elemental mapping. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) suggest the pentavalent Ru but not tetravalent, while partial Ti in TiO2 accept an electron from Ru and become Ti3+, which is observed for the first time. This Ru-doped TiO2 performs high activity for electrocatalytic hydrogen evolution reaction (HER) in alkaline solution. First-principles calculations simulate the HER process and prove TiO2:Ru with Ru5+ and Ti3+ holds high HER activity with appropriate hydrogen-adsorption Gibbs free energies (ΔGH).

INTRODUCTION

high conductivity can favor the HER activity, the effective carrier transportation within the substance optimized the efficiency of the device.14,15 Grain boundaries play a dominant factor in limiting carrier transportation and reduce conductivity, which is especially true to the nano-scale polycrystal materials. Therefore, single-crystal structure has higher conductivity and more robust structure for HER compared to the polycrystal, showing better catalytic activity and stability.16-18 From another point of view, to improve the performance of the catalytic means to upsurge the active sites. For this reason, high porosity materials have the advantages as catalyst because of their high surface area and high active site exposing.19-21 Therefore, if the material combines the property of single-crystal and mesoporous, it can possess both advantages of these two properties, simultaneously.

The growing apprehensions of the energy crisis and environmental pollution have stimulated global effort to explore sustainable and clean energy sources as the candidates to substitude the fossil fuels. Amongst them, hydrogen energy is recognized as a promising alternative due to its zero carbon footprints and high gravimetric energy density.1,2 Consequently, electrocatalytic hydrogen evolution reaction (HER), which is the most widely adopted approach for hydrogen production, has been a research hotspot nowadays. Generally, platinum (Pt) is regarded as the most efficient electrocatalyst because of its low over potential (η).1 However, the high cost of Pt has driven researchers to explore the high performance but low-cost and robust catalysts.3-7 It is well known that the electrocatalytic efficiency is largely affected by the morphology and electrical conductivity of the catalyst.8-10 In addition, well-dispersed noble metal on the carrier can harvest much heightened performance and reduce its consumption.11-13 Normally,

Here we synthesized rutile nanocrystal TiO2 by corroding SrTiO3 precursor. Each particle of the TiO2 was a tree trunk-like single crystalline of ~20 nm × 50 nm with many channels and mesopores, which were identified by 1

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Figure 1. (a) Schematic diagram of the oriented attachment (OA) mechanism growing process of the mesoporous crystal TiO2. Strontium was corroded out from the bulk SrTiO3, followed by the recombination of titanium and oxygen atoms into monodispersed rutile TiO2 nanoparticles (NPs). These monodispersed nanoparticles were highly aggregated and then grew into mesoporous single crystals by sharing common crystallographic orientations. (b) TEM image of monodispersed TiO2 NPs before oriented attachment. Insets: the corresponding selected area electron diffraction (SAED, left top) and the HRTEM images (right top). (c1) TEM image of mesoporous crystal TiO2 particles. (c2) HRTEM image of mesoporous crystal TiO2 particle. Pores are marked in red circles and the direction consistent lattice fringes prove it to be a single crystal but not polycrystal. Inset: SAED of an isolated particle, whose lattices confirm that the particle is a single crystal. (d) Nitrogen adsorption-desorption isotherms of the mesoporous crystal TiO2.

transmission electron microscopy (TEM), selected area electron diffraction (SAED) and nitrogen adsorption-desorption isotherms analysis. This unique structure was obtained via an uncommon oriented attachment (OA) growth mechanism without helping of surfactant. To functionalize this material, ruthenium (Ru) was doped by the same etching processing of SrTi1-xRuxO3 and TiO2:Ru was obtained. Confirmed by the high resolution transmission electron microscopy (HRTEM) and super energy dispersive spectrometer (super-EDS) elemental mapping, Ru was well dispersed in TiO2 forming RuO2 -TiO2 solid solution. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) designate the pentavalent Ru but not tetravalent, while partial Ti in TiO2 receive an electron from Ru and become the Ti3+, which is not once found before. The obtained mesoporous crystal TiO2:Ru-5 % (at.% Ru:(Ru+Ti)) showed great HER activity with onset potential of only 82 mV in 0.1 M alkaline solution, which was as superior as pure commercial RuO2. What’s more, the performance was greatly enhanced after further aluminum reduced processing because of its fine-tuned electron structure. The onset potential was only 45 mV, the over potential was 150 mV at a current density of 10 mA cm-2 and the Tafel slope was 97 mV dec-1. First-principles

calculations simulate this structure of Ru-doped TiO2 with Ru5+, Ti3+ and reveal the mechanism of HER process, proving TiO2:Ru holds high HER activity with appropriate hydrogen-adsorption Gibbs free energies (ΔGH). To the best of our knowledge, the HER performance of our Ru-doped mesoporous crystal TiO2 is at the advanced level in HER catalysts.2,22-24 Since the lower mass loading (~5 %) and cheaper price (~60 dollars ounce-1) of Ru than the Pt (~930 dollars ounce-1), it is a promising HER catalyst for large-scale application.

RESULTS AND DISCUSSION Figure 1a interprets the process of preparing the mesoporous nanocrystal TiO2 via an oriented attachment (OA) mechanism. A bulk SrTiO3 having typical perovskite structure, whose SEM image is shown in Figure S1, was used as precursor. Sr was etched out from the lattice by hydrochloric acid. Simultaneously, titanium and oxygen atoms recombined and formed into monodispersed rutile TiO2 nanoparticles (NPs), as shown in Figure 1b. The size of the TiO2 NPs are ~8 nm. HRTEM shows that inter planar spacing is 0.25 nm, corresponding to rutile TiO2 (110), which is additionally confirmed by selected area electron diffraction (SAED) rings and x-ray diffraction (XRD) pattern (Figure S2). Since the etching reaction is 2


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vigorous and rapid beneath hydrothermal condition, these monodispersed TiO2 NPs are formed rapidly and highly aggregated within a limited space volume, resulting a large amount of TiO2 nanocrystals being side-by-side and collided with each other frequently. Driving by van der Waals forces, interatomic columbic interactions, and most significantly, the force of elimination of the high energy surfaces, those TiO2 NPs grow into bigger crystals by sharing a common crystallographic orientation.25-28 Meanwhile, numerous pores and channels are created inside of the newly-formed crystals via this OA process because of the large number of unattached facets. Figure 1c1 and 1c2 show that the assembled crystal TiO2 had a tree trunk-like single crystal of ~20 nm × 50 nm with many channels and mesoporous. Although these assembled particles look like polycrystalline, the SAED from an isolated particle is not the polycrystalline diffraction rings but dot matrix (Inserting of Figure 1c2). Together with the direction consistent lattice fringes, it can be established that these tree trunk-like TiO2 are single crystals. The inter planar spacing shown in Figure 1c2 was 0.25nm (110), indicating that TiO2 NPs were assembled along the (110) direction. Nitrogen adsorption measurement was applied to represent properties of the mesoporous crystal TiO2, which is shown in Figure 1d. The isotherm pattern for the crystal TiO2 sample is IUPAC type IV with hysteresis loops, indicating the existent of mesopores. The hysteresis loop is type H3, signifying the presence of slit-shaped pores. Brunauer–Emmett–Teller (BET) and Barrett–Joyner– Halenda (BJH) methods were employed to analyze the specific surface area and the pore size distribution, respectively. Corresponding pore size distribution plot is shown in Figure S3. The peak is obtained at about 3 nm with the pore-size distribution relatively wide, which is consistent with the HRTEM image (Figure 1c2). The BET surface area is 80 m2/g, which is much larger than the rutile TiO2 in reported research with the similar size but without pores.29

or where particles have abundant surface-bound water) and probably occurs in nature. It may apply when particles nucleate side-by-side and coalesce during growth.25,36,37 The OA mechanism usually results in particles with irregular morphologies such as pores,38 twins,39 zigzag nanowires40 and multi-pod nanocrystals.41 In this paper, OA mechanism grew the single crystals. The bulk SrTiO3 was corroded and monodispersed TiO2 NPs were designed, which were rapidly nucleated and highly aggregated and then self-assembled by sharing a common crystallographic orientation, grew into a tree trunk-like crystal. Normally, we etched SrTiO3 by hydrochloric acid under hydrothermal condition and obtained the tree trunk-like TiO2 crystal. As described above, the key point of this OA mechanism is that abundant nanoparticles are formed rapidly and highly aggregated within a limited space volume, so that they can collide with each other frequently and finally grow into bigger crystals by sharing a common crystallographic orientation. Therefore, if we break the aggregation status before the small nanoparticles grow into the bigger crystal, we can detect the transition stage and obtained the TiO2 NPs. To prove the above suspect, we put the sealed hydrothermal reactor into an oil bath and heated up to the same temperature but kept magnetic stirring all the time. The continuous stirring broke the aggregation status of TiO2 NPs so that the OA processing could not happen, thus the monodispersed TiO2 NPs can be obtained, as shown in Figure 1b. Through this experiment, our growing mechanism suspect of the tree trunk-like TiO2 single crystalline can be confirmed. This mesoporous crystal TiO2 has a unique structure

To be clear, here we present OA mechanism for a split second. Oriented attachment (OA) is a special case of particle-mediated crystal growth in which primary crystallites assemble into secondary crystals. Unlike the classical Ostwald ripening (OR) mechanism which involves larger grains grow at the expense of smaller grains with relatively higher solubility,30,31 oriented attachment (OA) involves the spontaneous self-organization of adjacent nanocrystals, resulting in crystal growth by addition of solid particles that share a common crystallographic orientation.26 The driving force for this spontaneous oriented attachment is to eliminating the pairs of high energy surfaces, which will lead to a substantial reduction in the surface free energy from the thermodynamic viewpoint.28,32,33 Van der Waals forces and interatomic columbic interactions are important factors to facilitate the process.25,28,34,35 OA mechanism is relevant in cases where particles are free to move (such as in solution

Figure 2. (a) Digital Photographs of mesoporous single-crystal TiO2:Ru (at. % Ru:(Ru+Ti) = 0 %, 1 %, 2.5 %, 5 %, 10 %) powders. (b) X-Ray Diffraction (XRD) patterns of mesoporous single-crystal TiO2:Ru. (c) Ultraviolet visible (UV-Vis) absorption spectra of TiO2:Ru. (d) Conductivity ratio of TiO2:Ru (5 %), reduced-TiO2:Ru (5 %) (R-TiO2:Ru , 5 %) and TiO2. 3



uniting single crystalline and mesoporous structure, which is supposed to be a catalyst carrier. To functionalize this material, we initially prepared Ru doped SrTiO3 precursors (SrTi1-xRuxO3, x=0 %, 1 %, 2.5%, 5%, 10 %) since the SrRuO3 is also perovskite structure. The phases are pure without any impurities or phase separation, whose XRD patterns are shown in Figure S4. After the same etching process, different molar ratios of Ru-doped mesoporous crystal TiO2 (TiO2:Ru, at. % Ru:(Ru+Ti) = 0 %, 1 %, 2.5%, 5%, 10 %) were obtained, whose digital photographs are in Figure 2a. Colors are gradually going deeper along with the increasing ratios of Ru, which are well coincident with Ultraviolet visible (UV-Vis) absorption spectra (Figure 2c). Compared with pure TiO2, whose absorption is mainly in the UV region, the absorption spectrum of TiO2:Ru has a great expansion to the visible region. Corresponding XRD patterns of these samples are shown in Figure 2b, which is rutile phase when Ru-doped amount within 5 at. % and become anatase up to 10 at. %. We speculate the reason is that surface energy changed by doping atoms affects the phase transformation. As reported research, doped anatase can stand higher temperature (about1000 ºC) compared to pure anatase (about 550 ºC), meaning anatase can be more stable through doping.42,43 Similarly, phase of 10% Ru-doped TiO2 is changed because of surface energy affected by Ru, which makes anatase more stable in a high doping content. Content of Ru were check by inductively coupled plasma atomic emission spectra (ICP-AES) and EDS. We use 5 % molar ratio sample as example. The precursor SrTi0.95Ru0.05O3 was characterized to have the atomic ratio of Ru:(Ti+Ru) of 4.9 % from ICP measurements, which is very close to the nominal composition of 5 %. The Ru:(Ti+Ru) in the product TiO2:Ru is 4.7 at.%, which is not much change compared to that of precursor. EDS measurement is well consistent with ICP-AES results (Figure S5). All molar ratio of TiO2:Ru were further treated with aluminum (Al) reduction process to improve their conductivity. Al reduction is a method to created oxygen vacancy in TiO2. The schematic process for aluminum reduction is illustrated in Figure S6. This method is suitable for raising the conductivity of TiO2.44,45 Corresponding XRD patterns of the Al reduced TiO2:Ru (R-TiO2:Ru) samples are in Figure S7, showing little difference with that of TiO2:Ru (Figure 2b). Digital photographs for all molar ratios of R-TiO2:Ru are displayed in Figure S8, whose colors are deeper compared to TiO2:Ru caused by oxygen vacancy. Direct evidence for the raise of conductivity was carried out by implementing conductivity of TiO2, TiO2:Ru (5 %) and R-TiO2:Ru (5 %) in Figure 2d, where the conductivity of the TiO2:Ru (5 %) is more than three times higher than TiO2, and further largely improved for two orders of magnitude after the Al reduction processing, which is similar with our previous work.46,47 Super-EDS elemental mapping and HRTEM were implemented to characterize dispersion of Ru in the mesoporous crystal TiO2:Ru (5 %).

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Figure 3. (a) Super-EDS elemental mapping of mesoporous crystal TiO2:Ru (5 %). (b, c) HRTEM images of TiO2:Ru (5 %) with consistent lattice fringes, indicating well-doped Ru in TiO2 lattice. (d) HRTEM of R-TiO2:Ru (5 %). Inserting: SAED of R-TiO2:Ru (5 %)

Figure 3a shows the super-EDS elemental mapping of scanning transmission electron microscopy (STEM) images. Together with the HRTEM images in Figure 3b and c, lattice fringes are well consistent and no RuO2 small particle is found on the TiO2 particles, indicative of Ru is doped into the lattice of TiO2 to form RuO2-TiO2 solid solution. While Ru distribution does not look very uniform from Figure 3a, one possible explanation is that the mesoporous structure makes the texture and Ru look inhomogeneous. Another explanation is that some Ru-rich regions have formed and well align on TiO2. XRD (Figure S9) and HRTEM (Figure 3d) of R-TiO2:Ru (5 %) show no obvious change in comparison with TiO2:Ru (5 %), remaining its mesoporous-crystal and solid-solution structure. N2 adsorption-desorption isotherms and pore size distribution of R-TiO2:Ru (5 %) are shown in Figure S10, which is also IUPAC type IV with hysteresis loops, confirming the remaining of mesopores. Its BET surface area is 62 m2 g-1.

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Figure 4. XPS spectra for TiO2:Ru (5 %) of (a) Ti 2p1/2 and Ti 2p3/2 peaks with binding energy at 464.4eV and 458.7 eV attributed to 4+ 5+ Ti and (b) Ru 3d5/2 and Ru 3d3/2 peaks with binding energy at 282.1eV and 286.2eV attributed to Ru . XPS spectra for R-TiO2:Ru 4+ 5+ 4+ (5 %) of (c) Ti 2p1/2 and Ti 2p3/2 peaks attributed to Ti and (d) Ru 3d5/2 and Ru 3d3/2 peaks attributed to both Ru and Ru . 4+ Binding energy at 280.7eV and 284.9eV are assigned to Ru . (e) Electron paramagnetic resonance (EPR) spectra of TiO2:Ru (5 %) , R-TiO2:Ru (5 %) and SS-5 % RuO2/TiO2. Insert: EPR spectra of TiO2 and TiO2-x.

Binding states and quantitative chemical compositions of the mesoporous crystal TiO2:Ru(5 %) were investigated by x-ray photoelectron spectroscopy (XPS). Ti4+ yields binding energies of 464.4 and 458.7 eV assigned to Ti 2p1/2 and Ti 2p3/2, respectively (Figure 4a). The spin−orbit splitting is found to be 5.7eV, which is in good agreement with the one expected for the Ti4+ oxidation state in TiO2-based nanocomposites. The peak at about 462.4 eV superimposed between the Ti 2p3/2 and Ti 2p1/2 in Figure 4a is contribution from the Ru 3p3/2. Figure 4b showed the binding energies of Ru 3d5/2 and Ru 3d3/2 at 282.1eV and 286.3eV, which are attributed to Ru5+.48,49 Some preceding literatures also involve in such high valence state of Ru in RuO2/TiO2 system.50,51 However, there was no evidence confirming the above suspect since the Ti3+ species are difficult to be detected by XPS.52 Similarly, we couldn’t find the visible peak of Ti3+ in XPS, but we can see the Ti4+ peaks shifted toward lower binding energies, indicating fewer O neighbors around Ti on average.46,47 The three peaks located at 284.4eV, 287.5eV and 288.6 eV have been attributed to C1s due to carbon contamination on the surface of the sample.51,53 R-TiO2:Ru (5 %) was also analyzed of the binding energies in Figure 4c and 4d. No obviously change is found in the peaks of Ti after Al-reduced processing (Figure 4c), consistent with our previous work.52,54 However, there were two extra peaks of Ru compared to TiO2:Ru (5 %) with Ru 3d5/2 and Ru 3d3/2 at 280.7eV and 284.9eV, which are coincident to RuO2, indicating some of Ru5+ was reduced into Ru4+. By calculating the integration of the peaks’ area, the ratio of Ru4+:Ru5+ was about 2:1. To make the binding environment

of the Ti and Ru atoms clearer, electron paramagnetic resonance (EPR) was employed for Ru5+ and Ti3+ since it is highly sensitive to detect the paramagnetic species containing unpaired electrons like Ti3+ and Ru5+, but silence for Ti4+ and Ru4+. No signal was observed for the pure TiO2, while strong peaks for black TiO2 (TiO2-x) because of Ti3+ (Figure 4e inset), which is consistent with our previous works.55,56 The EPR spectrum for TiO2:Ru (5 %) and R-TiO2:Ru (5 %) both exhibit the signal of Ti3+ (g=1.99 Figure 4e),55,56 confirming the existence of trivalent Ti. Moreover, strong signals of Ru5+ are detected both in TiO2:Ru (5 %) and R-TiO2:Ru (5 %),48,57 supporting our previous speculation of charge transfer. For comparison, we synthesized RuO2/TiO2 composite by solid-state method under 1000 ºC using TiO2 and RuO2 as precursor, and yielded phase-separated sample RuO2/TiO2 (SS-5 % RuO2/TiO2). The phases was checked by XRD pattern in Figure S11. Different from well-dispersed TiO2:Ru (5 %) and R-TiO2:Ru (5 %), SS-5 % RuO2/TiO2 shows no EPR signal because no electron transfers between the Ru and Ti. The electrochemical activities of the mesoporous crystal TiO2:Ru and R-TiO2:Ru were examined using a three-electrode configuration in 0.1 M KOH electrolyte. TiO2:Ru (0 %, 1 %, 2.5 %, 5 %, 10 %) were implemented of the HER performance. When Ru-dopant is less than 5 %, the performance improves with the increase of doping amount. However, the performance declines when the amount of Ru is over 5 %, as shown in Figure S12. The best-performed sample TiO2:Ru (5%) shows high activity with low onset potential of only 82 mV (Figure 5b), which

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Figure 5. (a) LSV curves of mesoporous crystal R-TiO2:Ru (0 %, 1 %, 2.5 %, 5 %, 10 %) in 0.1M KOH (b) LSV curves of TiO2:Ru (5 %) , R-TiO2:Ru (5 %), TiO2, Pt/C and commercial RuO2. (c) The Tafel slops of TiO2:Ru (5 %) , R-TiO2:Ru (5 %), Pt/C and commercial RuO2, (d) The Nyquist plots of TiO2:Ru (5 %), R-TiO2:Ru (5 %) and commercial RuO2 at the overpotential of −330 mV. (e) LSV curves of Ti and Ru composite oxides prepared by different methods in 0.1M KOH. TiO2:Ru (5 %) was prepared by the method described in Figure 1. 5 % RuO2/TiO2 was heterostructure with RuO2 particles deposited on mesoporous TiO2 by impregnated method. Cp-TiO2:Ru (5 %) was prepared by coprecipitation of RuCl3 and TiCl4 solution with ammonia. (f ) Stability of R-TiO2:Ru(5 %).

commercial RuO2 (149 mV dec−1), and the Tafel slope of R-TiO2:Ru (5 %) significantly decreases to 97 mV dec−1 (Figure 5c). Volmer-Heyrovsky mechanism is proceeded according to the Tafel slopes, with the rate-determining step of the absorption of hydrogen.62 To further reveal the electrocatalytic kinetics, electrochemical impedance measurement was implemented. The Nyquist plots of TiO2:Ru (5 %), R-TiO2:Ru (5 %) and RuO2 at the overpotential of −330 mV are shown in Figure 5d. The entire plots show two semicircles. One in the high frequency domain is attributed to the electrode texture, another one in the middle and low frequency domain is attributed to the charge transfer resistance (Rct) of the HER. Bode plots are shown in Figure S15, containing two-time constants, which are well consistent to Nyquist plots. Equivalent circuit used for approximation of the EIS data is shown in Figure S16. A two-CPE model containing a series connection of two parallel R-constant phase element (CPE) in series with uncompensated resistance due to the cell and electrodes configuration, has been used to approximate the EIS data. R-TiO2:Ru (5 %) displays lowest Rct among all the samples, indicating its fast charge transfer, consistent with the result obtained from polarization measurements (Figure 5b). The stability of the R-TiO2:Ru (5 %) is valued by comparing the initial curve with the data after 1,000 cycles, which only shifts

is analogous to pure commercial RuO2 (65 mV). Besides, it shows even higher current density than RuO2 at the potential below −0.21 V. Hence, higher activity but less noble metal usage is achieved. To adjust the electron structure and obtain optimum activity, Al-reduction treatment was implemented. Different temperature of Al-reduced processing for TiO2:Ru (5%) were implemented and applied to HER, which were displayed in Figure S13. Optimum temperature was obtained at 500 ºC. Al-reduced treatment in 500 ºC for TiO2:Ru (0 %, 1 %, 2.5 %, 5 %, 10 %) were studied, with HER best-performed sample of R-TiO2:Ru (5 %) (Figure 5a). The onset potential of R-TiO2:Ru (5 %) is only 45 mV. The overpotential at 10 mA cm−1 is 150 mV, which is only 41 mV higher than Pt/C (Figure 5b). It is superior in low-cost HER catalysts.2,24 According to the literature, RuO2 are rarely used in HER, but rather in oxygen evolution reaction (OER) catalyst.22,23,58-61 However, Ru-doped TiO2 here show poor activity in OER (Figure S14), which can be an indirect evidence that RuO2 particle/domain does not exist, as we mentioned above. The Tafel slope of the amperometric response for a catalyst-modified electrode is an inherent property indicative of the rate-limiting step for HER. The Tafel slope of TiO2:Ru (5 %) is 113 mV dec−1, which is lower that of the 6


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about 18 mV at j = 10 mA cm−2 (Figure 5f). What’s more, XPS of R-TiO2:Ru (5 %)was implemented after LSV sweep (Figure S17), showing no change of Ru valence. Other molar ratios of TiO2:Ru and R-TiO2:Ru also show good stabilities with potential shift no more than 18 mV at j = 10 mA cm−2 as displayed in Figure S18, S19.

Ru5+. Since this corresponding electronic structure of this configuration does not agree with our experiment, and we show some data only for reference in the following. However, when the system contains a row of -RuO3 in the surface region of TiO2, defined as TiO2:Rusurf, Ru5+ and Ti3+ appear at the same time, and the electronic structure of this configurations agree well with our experiments. The alike reconstruction on rutile-like RuO2 was also proposed before.63 Figures 6a and 6b show the specific atomic structure of TiO2:Rusurf, every Ru atom stays between two surface O atoms, which is anchored over the surface Ti atoms, meanwhile the Ru atom also binds with two other dangling oxygen atoms. Thus every Ru atom binds to 4 oxygen atoms. Two of them are OTi-Ru (the oxygen bonds with both Ru and Ti atoms) and the rest two are single coordinated. In order to consider the effect of the oxygen vacancy, four oxygen vacancies are included in the calculations of TiO2:Rusurf. The detailed electronic structure is shown in Figure 6b. First of all, three electrons were identified for each Ru atom, thus the electronic configuration is Ru 4d3, or Ru5+. Meanwhile, excess electrons localized over Ti4+ result into Ti3+ in TiO2:Rusurf. As shown in Figure 6b and 6d, the excess electrons localized at Ti and Ru make the formation of Ti3+ and Ru5+. The partial density of states of TiO2:Rusurf and TiO2 are shown in Figure 6d. The gap states between 0 eV and 1.7eV correspond to Ru5+ and Ti3+. Such result is consistent with the experimental data, as discussion above. TiO2:Rusurf can reproduce well with the experimental

To further demonstrate the superiority of this well-dispersed TiO2:Ru, we used other different methods to prepare Ti and Ru composite oxides and tested their HER activities for comparison. Methods of impregnation and coprecipitation were used to prepare samples of 5 % RuO2/TiO2 and coprecipitated-TiO2:Ru (5 %) (Cp-TiO2:Ru, 5 %), respectively. Their BET surface areas are presented in Table S1 and HER performances are in Figure 5e. Compared to well-dispersed mesoporous crystal TiO2:Ru (5 %), 5 % RuO2/TiO2 and Cp-TiO2:Ru(5 %) show much higher onset potential of 340 mV and 315 mV, respectively. The Tafel slops are shown in Figure S20, further indicating their inferiority compared to well-dispersed TiO2:Ru (5 %). To understand the effect of Ru doping on the TiO2 HER reactivity, first-principles calculations were performed. As mentioned above, XPS and EPR data confirm the existence of Ru5+, Ti3+ and O vacancies in TiO2:Ru. Several computational structures were considered to simulate the electronic structure of the TiO2:Ru system. First of all, substituting Ru for Ti4+ in the bulk, designated as TiO2:Rusub, results in Ru4+ instead of

Figure 6. The atomic and electronic structures of TiO2:Ru and the corresponding free energy profiles for HER. (a) Top and (b) side views of TiO2:Rusurf; (c) The side view of the atomic and electronic structure of the TiO2:Rusub with the hydrogen adsorption; (d) Density of states of TiO2 (top) and TiO2:Rusurf (bottom); (e) ΔGH profiles for the HER of rutile(110), TiO2:Rusurf, compared with pure Pt. Here TiO2:Rusurf (ORu) indicates the ΔGH of hydrogen adsorbed on ORu which were fully saturated, TiO2:Rusurf(Obr) indicates the ΔGH of hydrogen adsorbed on Obr while ORu were fully saturated. RI-TiO2:Rusurf indicates the ΔGH of TiO2:Rusurf(Obr) with one ORu vacancy and RII-TiO2:Rusurf indicates TiO2:Rusurf(Obr) with two Obulk vacancies, respectively. 7



results, and we only examined the other properties of TiO2:Rusurf in the following except we noted.

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crystal TiO2:Ru can be attributed to the following reasons. Firstly, the small size of the TiO2:Ru nanocrystals and highly porosity favor the exposure of more active sites for the HER.19,64 Secondly, single-crystal structure raise its conductivity and enhances electrocatalytic efficiency.14,15 Third, well-dispersed of Ru in solid solution improves the efficiency of heterogeneous catalysts though low loading of noble metal.1,11-13,65 Fourth, the conductivity is raised after Ru doping and Al reduction. Moreover, our first-principles results reveals that the HER superiority of the TiO2:Ru origin from valences of Ru and Ti, regulating the appropriate ΔGH for HER. An Al reduction process can further modulate the electronic structure and enhance HER activity.

In order to know how Ru doping affects the free energy profile of HER. The hydrogen binding energy (ΔEH) and hydrogen-adsorption Gibbs free energies (ΔGH) of pure rutile (110) and TiO2:Rusurf are explored. The ΔGH of TiO2 is 0.45 eV, indicating an inhibition of hydrogen absorption. When Ru atoms are introduced into the surface of the system, hydrogen atoms prefer to firstly adsorb on ORu (the oxygen bonds only with Ru atoms) of the –RuO3 with a ΔEH of -1.3 eV, and the corresponding ΔGH is -0.99 eV, which is not suitable for HER because a strong binding energy inhibits the desorption of H2. Our results show that Ru5+ is gradually reduced into Ru3+ upon the hydrogen adsorption on ORu. With the increase of hydrogen coverage, the ORu are fully saturated and all Ru5+ are reduced into Ru3+. The further hydrogen adsorption on TiO2:Rusurf chooses Obr (the bridge-bonded oxygen of TiO2(110)). Under this circumstance, the ΔEH on Obr becomes -0.63 eV, and the ΔGH is -0.28 eV, which is appropriate for HER. This phenomenon was also observed in TiO2:Rusub by substituting Ru for Ti4+ in the bulk. The initial Ru atom is tetravalent in the bulk, and Ru4+ can even be transformed into Ru2+ upon hydrogen adsorption (Figure 6c). The corresponding ΔEH is -0.53 eV, and the ΔGH is -0.18 eV (Figure S21). It should be noted that tetravalent is not observed in our experiment, thus we only show this data of TiO2:Rusub only for reference. As discussed above, the TiO2:Rusurf plays a vital role in modulating ΔGH of HER to enhance the HER activity of TiO2 compared with the pure TiO2.

CONCLUSION In conclusion, we developed rutile TiO2 which is highly porous, but each particle is single crystal. It was grown by corroding off the Sr from the SrTiO3 precursor with the growth mechanism of OA. In this growth process, Sr was etched out by HCl from SrTiO3 and the Ti and O atoms recombined to form monodispersed TiO2 nanoparticles (TiO2 NPs), which were highly aggregated and subsequently self-assembled into tree trunk-like crystals by sharing a common crystallographic orientation. TEM and SAED images proved each of these final crystal to be single crystal. BET and BJH results show it has a high surface area of 80 m2 g−1 with many pores and channels. Possessing these special morphologies, it was used as a catalytic carrier with Ru well doped into the lattice. XPS and EPR peaks suggest the pentavalent Ru but not tetravalent, while partial Ti in TiO2 accept an electron from Ru and become the Ti3+, which is observed for the first time. The material showed high activity of HER performance as the specification of its morphology and structure. First-principles calculations reveal the superiority of the TiO2:Ru, which show reducibility of Ru and Ti favor the HER process, regulating the appropriate ΔGH for HER. After being reduced to justify its electron structure, better performance can be achieved. This material also has the potential in other applications such as photocatalyst, supercapacitors, electrochemical sensors, and so on.

Next we explore how the reduced TiO2 (R-TiO2) induced by Al reduction further enhances HER activity of TiO2:Rusurf. The amount of oxygen vacancies in the TiO2:Ru system were further increased to mimic the reduced TiO2 by removing either one more ORu or two more Obulk (the oxygen in the bulk TiO2) in TiO2:Rusurf, and the two systems are denoted as RI-TiO2:Rusurf and RII-TiO2:Rusurf in the following, respectively. As discussed above, oxygen vacancies introduce excess electrons into the system, and the increasing amount of Ti3+ was observed. As shown in Figure 6e, the ΔGH of RI-TiO2:Rusurf and RII-TiO2:Rusurf become -0.24 eV and -0.19 eV, respectively. The corresponding absolute values of ΔGH are about 0.04 eV and 0.09 eV smaller than that of TiO2:Ru. Such results clearly show that the reduced TiO2 could further decrease ΔGH and enhance the HER activity by inducing more Ti3+. The main reason should originate from the coulomb repulsion between the excess electrons and hydrogen, which can effectively decrease the hydrogen adsorption.

EXPERIMENTAL SECTION Synthesis of SrTiO3 (SrTi1-xRuxO3). All reagents were analytically pure and used without further purification. The hydrothermal reactions were carried out in a stainless-steel autoclave with a Teflon liner (100 ml in total capacity) under autogenous pressure. To prepare SrTiO3, stoichiometric ratio of TiCl4 and SrCl2 were used. 5 mmol TiCl4 was dropped on ice under stirring and form 10 ml solution, then 2 ml SrCl2 solution (2.5 M) was added. 20 ml KOH solution (4 M) was dripped into the mixed solution and got a slurry. The slurry was put into an

Combining experimental and first-principles results, the observation of the superior activity of the mesoporous 8


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autoclave, sealed and heated at 100 ºC for one day. After cooling, the product was filtered, washed with distilled water for several times and dried at 70 ºC for 4 hours. For preparing SrTiO3:Ru, stoichiometric ratio of RuCl3 was add into the ice following TiCl4, and all other steps were the same as pure SrTiO3.

rpm to remove the hydrogen gas bubbles formed at the catalyst surface. LSV scan rate was 0.01 V s −1. The potential was converted to potential versus reversible hydrogen electrode (RHE) according to E (vs. RHE)=E (vs. Hg/HgO) + 0.098 (Hg/HgO vs. SHE) + 0.059×pH-iR. The measured current density cannot reveal the inherent features of electrocatalysts as a result of the ohmic resistance effect. All of the polarization curves were 90 % iR-corrected using the data from electrochemical impedance spectroscopy (EIS) (see Figure 5d).

Synthesis of porous crystal TiO2 (TiO2:Ru), R-TiO2:Ru and TiO2 NPs. 0.5 g SrTiO3 was put into 50 ml autoclave equipped with a Teflon liner, then 25 ml H2O and 2.5 ml HCl was successively added into the autoclave. The autoclave was heating at 130 ºC for 2.5 hours under autogenous pressure and naturally cooled to room temperature. The mixture taking from the autoclave was dialyzed and freeze-dried. The powder of TiO2 was finally obtained. TiO2:Ru was prepared via the same process but using SrTi1-xRuxO3 as precursor. R-TiO2:Ru was synthesized by putting TiO2:Ru and aluminum separately in a two-zone tube furnace. The schematic process for aluminum reduction is illustrated in Figure S6. Aluminum was heated at 850 ºC, and Ru-TiO2 samples were heated at 500 ºC for both 4 h simultaneously. 4g Al was used corresponding to 100mg TiO2. A pump was used to maintain vacuum at about 1.4 Pa. TiO2 NPs was obtained by the same condition as porous crystal TiO2 but using magnetic stir as descripted above.

Material Characterization.The phase purity of as-prepared products was evaluated using a Bruker D2 X-ray diffractometer with Cu Kα radiation (k = 1.5418 Å) at a scanning rate of 2° min−1 in the 2θ range of 5°–80°. The morphology of the products was characterized with a transmission electron microscope (TEM, JEM2010-HR and JEOL 2010F, equipped with an Oxford link-ISIS energy-dispersive Spectroscopy (EDS)) and a scanning electron microscope (SEM, S4800, hν =5 eV). The BET analysis was applied with (Micrometrics ASAP 2010). XPS (Axis Ultra of Kratos Analytical Ltd. Al Kα radiation, hν = 1486.7 eV) survey was carried out to analysis the binding energy. Ultraviolet–visible (UV–vis) absorption spectroscopy in the wavelength range of 250–1000 nm was implemented using a Hitachi UV-4100 spectrophotometer. The X-band electron paramagnetic resonance (EPR) spectra were recorded at room temperature (Bruker's WIN-EPR). To measure the conductivity, pressed pellets (≈0.2 g samples, Φ = 13 mm, thickness ≈0.5 mm) were first fabricated to the devices of Stainless steel/Indium/sample pellet/Indium/ Stainless steel, where the soft and thin indium layer decrease interface resistance between stainless steel and sample pellet. Current-voltage (I-V) curves of above devices was measured by an electrochemical workstation. The conductivity was

Synthesis of 5 % RuO2/TiO2, Cp-TiO2:Ru (5 %) and SS-5 % RuO2/TiO2. The 5 % RuO2/TiO2 sample was prepared by impregnated method. 0.08g porous crystal TiO2 (this paper) was impregnated into RuCl3 solution, which contained 0.011g RuCl3. This mixture was ultrasonically dispersed for half an hour and then dry in a 100 ºC oven. The resulting powder was annealed at 500 ºC for 2 h to get heterostructures 5 % RuO2/TiO2. Cp-TiO2:Ru (5 %) was synthesized by coprecipitation method. 5ml TiCl4 and 0.496 g RuCl3 were dissolved in ice water and then stirred for 5 minutes. 20ml ammonia was added under vigorously stir. The resulting precipitate is filtered, washed, dried, and then annealed at 500 ºC for 2 h to get the product. SS-5 % RuO2/TiO2 was synthesis by solid-state method using TiO2 and RuO2 as precursor. Stoichiometric ratio of commercial TiO2 and RuO2 were well mixed by grinding in a mortar and then transferred in to a muffle furnace to anneal for 6 hours in 1100 ºC.

calculated from the I-V curves by R= Δ U/ Δ I, σ =L/RS. where R is the resistance, σ is the conductivity, L and S is the thickness and cross-sectional area of the tablet, respectively. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) was performed on a Prodigy 7 (Leeman). Computational method. The first-principles calculations were performed using the CP2K/Quickstep package.66 The Perdew–Burke–Ernzerhof functional was used to describe the nonlocal exchange and correlation energies.67 Hubbard U correction was applied to evaluate the on-site Coulomb interactions in localized orbitals and exchange interactions. The corresponding U values of Ru 4d, Ti 3d and O 2p orbitals were set to 2.9 eV, 3.5 eV and 3.5 eV.68 The norm-conserving Goedecker, Teter, and Hutter (GTH) pseudopotentials were used to describe core electrons.69 Gaussian function with molecularly optimized double-zeta polarized basis sets (m-DZVP) were adopted for expanding the wave function of Ru 4s24p64d75s1, and m-TZVP for Ti 3s23p63d24s2 and O 2s22p4 electrons.70 A 500

Electrochemical Test. The as-prepared TiO2 (TiO2:Ru) powder was first ultrasonically dispersed in absolute ethanol containing 0.1 wt. % of Nafion and then stirred for 4 hours. 10 µL of aqueous dispersion of the catalyst (5.0 mg/mL) was applied onto the glass carbon rotating disk electrode (RDE, 0.196) as a working electrode. The reference electrode was an Hg/HgO in 1 M KOH solution, and the reticulated vitreous carbon (RVC) was used as the counter electrode. 0.1 M KOH solution was used as electrolyte. A flow of N2 was introduced and maintained until the testing ended into the electrolyte eliminates O2. The working electrode was rotated at 2000 9



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Ry cut off energy was used for auxiliary basis set of plane waves. For bulk rutile, the calculated lattice parameters for a and c are 4.59 Å and 2.95 Å. A four-layer 4×2 rutile (110) model was adopted for all calculations. During the calculations, all the atomic positions were fully relaxed until the force is smaller than 0.05 eV/Å. The hydrogen-adsorption Gibbs free energies, ΔGH, were calculated according to the following equation,71 ΔEH = E (n+1)H/TiO2 – EnH/TiO2 – ½ EH2 (1) ΔGH = ΔEH + ΔEZPE – TΔS (2) ΔEH is defined as the hydrogen binding energy on TiO2 substrate. E (n+1)H/TiO2 and EnH/TiO2 refer to the total energies of n+1 and n hydrogen atoms adsorption on TiO2, respectively. EH2 refers to the total energy of a hydrogen molecule in vacuum. ΔEZPE is the difference of the zero-point energy with and without hydrogen adsorption, T is the temperature (300 K), and ΔS is the entropy change between an adsorbed hydrogen and gas-phase hydrogen at 101,325 Pa. Here, the calculated correction term of ΔEZPE and TΔS is 0.35 eV.

ASSOCIATED CONTENT Figures giving details of SEM image of SrTiO3, XRD patterns of precursors and intermediate TiO2 NPs, XRD and HER details for TiO2:Ru and R-TiO2:Ru, stability of TiO2:Ru, R-TiO2:Ru, Tafel slopes of 5 % RuO2/TiO2 and Cp-TiO2:Ru (5 %). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] * [email protected] Author Contributions #

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These authors contributed equally.

ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB0901600), the National Science Foundation of China (Grant Nos. 51402334 and 51502331), the Science and Technology Commission of Shanghai (Grant No. 14520722000), the Key Research Program of Chinese Academy of Sciences (Grant No. KGZD-EW-T06) and National Natural Science Foundation of China (Grant Nos. 51572016, U1530401). We acknowledge the computational support from the Beijing Computational Science Research Center (CSRC) and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund. Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under the grant no. U1501501 were also acknowledged.

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12 ACS Paragon Plus Environment

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Well-dispersed Ruthenium in Mesoporous Crystal TiO2

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