Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (5): 585-590

#### The article information

Mi Shi-yang, Liu Yuan-xu, Wang Wen-dong

Photo-depositing Ru and RuO2 on Anatase TiO2 Nanosheets as Co-catalysts for Photocatalytic O2 Evolution from Water Oxidation

Chinese Journal of Chemical Physics, 2016, 29(5): 585-590

http://dx.doi.org/10.1063/1674-0068/29/cjcp1603057

### Article history

Accepted on: April 24, 2016
Photo-depositing Ru and RuO2 on Anatase TiO2 Nanosheets as Co-catalysts for Photocatalytic O2 Evolution from Water Oxidation
Mi Shi-yang, Liu Yuan-xu, Wang Wen-dong
Dated: Received on March 26, 2016; Accepted on April 24, 2016
CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
Author: E-mail:wangwd@ustc.edu.cn
Abstract: TiO2 nanosheets mainly exposed (001) facet were prepared through a hydrothermal process with HF as the morphology-directing agent. Ru and RuO2 species were loaded by photo-deposition methods to prepare the photocatalysts. The structural features of the catalysts were characterized by X-ray di raction, transmission electron microscopy, inductively cou-pled plasma atomic emission spectrum, and H2 Temperature-programmed reduction. The photocatalytic property was studied by the O2 evolution from water oxidation, which was examined with respect to the in uences of Ru contents as well as the oxidation and reduction treatments, suggesting the charge separation effect of the Ru species co-catalysts on di erent facets of TiO2 nanosheets. In contrast to Ru/TiO2 and RuO2/TiO2 with the single deposited co-catalyst, the optimized catalyst 0.5%Ru-1.0%RuO2/TiO2 with dual co-catalysts achieved a much improved catalytic performance, in terms of the synergetic effect of dual co-catalysts and the enhanced charge separation effect.
Key words: Anatase TiO2 nanosheets     Photocatalytic O2 evolution     Crystal facet     Ru co-catalyst     Charge separation
I. INTRODUCTION

The photocatalytic splitting of water is considered as one of the promising techniques to convert solar light energy into clean and renewable chemical energy [1]. Among the vast semiconductor photocatalysts applied to the studies of photocatalytic water splitting, TiO$_2$ appears to be the most suitable material owing to its high activity, low cost, chemical stability, and nontoxicity [2-4]. Although much effort and great progress have been made, it is still a great challenge to overcome the disadvantages of conventional TiO$_2$-based materials, such as exposure of low activity crystal facets, fast recombination of the photogenerated electrons and holes, and low absorbance of visible light.

Theoretical calculation has demonstrated that the (001) surface of anatase TiO$_2$ is more active than the (101) surface [5], therefore conventional anatase TiO$_2$ nanoparticles prefer to expose the (101) crystal facets with low surface energy (0. 44 J/m$^2$) rather than the (001) facets with high surface energy (0. 90 J/m$^2$). To obtain TiO$_2$ mainly exposing high reactive crystal facets, hydrofluoric acid (HF) has been used as the structure-directing agent to fabricate nanocrystalline TiO$_2$ that exposed 47% (001) crystal facets and showed an excellent photocatalytic activity [6]. Based on this breakthrough, a number of studies on TiO$_2$-based materials with dominant highly reactive (001) facets and their enhanced photocatalytic properties have been reported [7-12].

It is regarded as one of the crucial aspects of photocatalytic activity to reduce the recombination of photogenerated carriers, and one of the effective tactics to improve the photogenerated charge separation efficiency by loading metal or metal oxide nanoparticles as co-catalysts to build heterojunctions on photocatalysts [3, 9, 13]. The metal and metal oxide nanoparticles loaded on TiO$_2$ can be served as a trap for the photogenerated electrons and holes, respectively. Among the various elements used as effective co-catalysts loading in the form of metal or metal oxide, Ru presents remarkable catalytic activities due to the unique properties of Ru and RuO$_2$ [3]. The enhancement of H$_2$ evolution by Ru on photocatalysts has been reported [14, 15], which may be ascribed to electronic structure of the interface between the Ru particles and photocatalysts facilitating electron transfers from photocatalysts to Ru. Meanwhile, RuO$_2$-loaded photocatalysts can promote the overall splitting of water [16, 17], as the holes would be trapped by RuO$_2$, resulting in efficient charge separation and improved photocatalytic activity. However, there are seldom reported photocatalysts with both Ru and RuO$_2$ nanoparticles as co-catalyst loading.

Moreover, some researchers have found that photogenerated electrons and holes might voluntarily separate towards different crystal facets in the photochemical process [18, 19], and hence anisotropic-shaped semiconductor nanoparticles could display higher charge separation efficiency than spherical nanoparticles [7, 20, 21]. It has been revealed that loading reduction and oxidation co-catalysts on the right crystal facets of semiconductor would enhance the separation of electrons and holes. In the case of anatase TiO$_2$, (001) and (101) facets have been demonstrated as oxidative and reductive sites, respectively [22, 23]. Recently, it has been reported that the deposition of dual co-catalysts, namely both reduction and oxidation co-catalysts, onto a semiconductor photocatalyst can significantly improve its photocatalytic activity [24-26] due to the synergetic effect of rapid consumption of photogenerated electrons and holes as well as the facile charge separation.

In this work, anatase TiO$_2$ nanosheets with dominant (001) facets are synthesized by the hydrothermal method. Ru and RuO$_2$ nanoparticles were loaded by different photo-deposition processes. The photocatalytic O$_2$ evolution from water oxidation was examined to evaluate the performances of synthesized catalysts. The results may demonstrate the charge separation effect on crystal facets of anatase TiO$_2$ nanosheets, and high catalytic activity of the anatase TiO$_2$ nanosheets photocatalyst with both Ru and RuO$_2$ nanoparticles as co-catalyst loading is expected.

II. EXPERIMENTS

All chemicals employed in this work were analytical reagents and obtained from Sinopharm, including Ti(OBu)$_4$ (TBOT), 40wt% HF, ethanol, NaOH pellets, RuCl$_3$, and KIO$_3$ powder.

TiO$_2$ nanosheets with dominant (001) crystal facets are synthesized by hydrothermal method [10, 11]. In a typical procedure, 5 mL of TBOT was mixed with 20 mL of ethanol under strong stirring, and then 0. 9 mL of 40wt% HF solution was added. The resulting solution was transferred into a Teflon autoclave with a capacity of 50 mL and then kept at 160 ℃ for 24 h. When cooling to room temperature, the white precipitate was collected after centrifugation, washed with ethanol and distilled water for several times in turn, and dried at 80 ℃ for 12 h. In order to remove the surface residual fluoride, the powder was dispersed in 0. 1 mol/L NaOH solution and stirred overnight at room temperature, and then washed with distilled water several times to neutral and finally dried at 80 ℃ for 12 h.

The photo-deposition of Ru was conducted with RuCl$_3$ as precursor. Typically, 0. 15 g TiO$_2$ nanosheets were suspended in 50 mL of distilled water, and then the calculated RuCl$_3$ solution was added. The suspension was stirred for 2 h in the dark and then irradiated under a 500 W UV lamp with continuous stirring. After photo-deposition for 5 h, the suspension was filtered, washed with distilled water for at least three times and finally dried at 80 ℃. The obtained catalyst is denoted as Ru/TiO$_2$. The photo-deposition of RuO$_2$ was conducted by a similar method to prepare RuO$_2$/TiO$_2$ catalyst, and the only difference was that the solution used to suspend TiO$_2$ nanosheets was changed to 50 mL KIO$_3$ aqueous solution (0. 1 mol/L). The photo-deposition of dual co-catalysts Ru and RuO$_2$ on TiO$_2$ nanosheets was prepared by two steps for Ru-RuO$_2$/TiO$_2$ catalyst. Ru was firstly loaded after 5 h photo-deposition, and the suspension was moved to the dark. Then the calculated RuCl$_3$ solution and 5 mL of KIO$_3$ solution (1 mol/L) was added into the suspension and stirred for 2 h, which was subjected to another 5 h photo-deposition to deposit RuO$_2$.

The reduction treatment was performed at 150 ℃ for 2 h in a flow of 5% H$_2$/Ar with a heating rate of 5 ℃/min, while the oxidation treatment at 200 ℃ for 2 h in a muffle.

The contents of Ru and RuO$_2$ deposited on TiO$_2$ nanosheets were determined by an Optima 7300 DV inductively coupled plasma atomic emission spectrometer (ICP-AES). The phase compositions of the catalysts were analyzed by powder X-ray diffraction (XRD) with a Rigaku TTR-III diffractometer using Cu K$\alpha$ radiation ($\lambda$=0. 15405 nm). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL JEM-2100F instrument. Temperature-programmed reduction (TPR) was performed at a heating rate of 5 ℃/min from room temperature up to 200 ℃ in a flow of 5%H$_2$/Ar. The amount of H$_2$ consumption during TPR was estimated from the integrated peak area using AgO$_2$ as a standard.

The photocatalytic O$_2$ evolution from water oxidation was examined to evaluate the performances of synthesized catalysts. The photocatalytic reaction was carried out in a closed quartz glass reaction vessel at room temperature. 10 mg of photocatalyst was dispersed into 40 mL of KIO$_3$ aqueous solution (0. 02 mol/L), which was magnetically stirred throughout the whole photocatalytic reaction. Before irradiation, Ar was introduced to replace the air in the reaction system. The reaction was initiated by irradiation with a 500 W UV lamp, and the UV light was irradiated from the side. The evolved O$_2$ was analyzed by a Shimadzu GC-14C gas chromatograph equipped with a thermal conductivity detector.

III. RESULTS AND DISCUSSION

The actual Ru content determined by ICP-AES is listed in Table I for Ru/TiO$_2$ and RuO$_2$/TiO$_2$ catalysts with different Ru loading. The result indicates the presence of Ru species and confirms the actual Ru content is very close to the nominal one.

Table I The Ru content of nominal and actual catalysts.

The XRD patterns of TiO$_2$ nanosheets and the photocatalysts that loaded with different co-catalysts are compared in Fig. 1. TiO$_2$ nanosheets only shows the typical diffraction patterns of anatase TiO$_2$ (JCPDS No. 21-1272). However, the XRD patterns of all the photocatalysts with different Ru loading (not shown), almost identical to those of pure anatase TiO$_2$ nanosheets and the three typical catalysts as shown in Fig. 1, are in absence of any diffraction peak related to either metallic Ru or ruthenium oxides. This result is consistent with the presence of very tiny nanoparticles of Ru species whose sizes may be beyond the detection limitation of XRD, as previously perceived in the case of Ru supported on TiO$_2$ and carbon nanotubes support [9, 27-29].

 FIG. 1 XRD patterns of (a) TiO$_2$ nanosheets, (b) 0. 5%Ru/TiO$_2$, (c) 1. 0%RuO$_2$/TiO$_2$, and (d) 0. 5%Ru-1. 0%RuO$_2$/TiO$_2$ catalysts.

Figure 2 shows the TEM and HRTEM images of the obtained TiO$_2$ nanosheets and typical photocatalysts to verify the formation of their morphology features. It is observed that the obtained TiO$_2$ nanosheets are composed of rectangular nanosheets with a length of 15-40 nm and thickness of 3-6 nm featuring a compressed truncated octahedral bipyramid shape [12]. The HRTEM image indicates that two sets of lattice fringes with spacing of 0. 235 and 0. 189 nm may be identified. This result suggests two mainly exposed facets corresponding to (001) facet and other eight facets corresponding to (101) facet, respectively, and the percentages of (001) facet can be estimated to be about 70% in this work according to the previous studies [6, 11, 12]. However, the presence of Ru and RuO$_2$ particles are hardly to be discovered from the TEM images of three typical photocatalysts, which may be in line with the highly dispersed co-catalyst nanoparticles of Ru species beyond the detection limitation of XRD, and could be ascribed to the absence of sufficient contrast between for the detection of highly dispersed Ru species deposited on the TiO$_2$ nanosheets as well.

 FIG. 2 TEM and HRTEM images of TiO$_2$ nanosheets and catalysts. (a) Pure TiO$_2$, (b) 0. 5%Ru/TiO$_2$, (c) 1. 0%RuO$_2$/TiO$_2$, (d) 0. 5% Ru-1. 0%RuO$_2$/TiO$_2$, and (e) HR-TEM image of pure TiO$_2$.

The H$_2$-TPR profiles of the selected catalysts are plotted in Fig. 3 to compare their redox properties. There is not any reduction peak in the exaimed temperature region for the TiO$_2$ nanosheets (Fig. 3(a)). For 0. 5%Ru/TiO$_2$ catalyst, almost no peak appears except a trace of H$_2$ consumption near 100 ℃ (Fig. 3(b)), while an obvious peak centered at 131 ℃ is observed after the oxidation treatment (Fig. 3(c)) but it totally disappears after the subsequent reduction treatment (Fig. 3(d)) as expected, which suggests the Ru species can be effectively loaded on TiO$_2$ nanosheets by photo-deposition. With respect to 1. 0%RuO$_2$/TiO$_2$ catalyst, it features a H$_2$ consumption peak at 104 ℃ (Fig. 3(e)) due to the reduction of RuO$_2$ [30], and then the peak disappears after the reduction treatment (Fig. 3(f)); however, another peak located about 128 ℃ is identified after the subsequent oxidation treatment (Fig. 3(g)). It is verified that the content of Ru species is basically in accordance with the amount of H$_2$ consumption estimated from the integrated peak area.

 FIG. 3 H$_2$-TPR profiles of (a) TiO$_2$ nanosheets, (b) 0. 5%Ru/TiO$_2$, (c) 0. 5%Ru/TiO$_2$ after oxidation, (d) 0. 5%Ru/TiO$_2$ after oxidation reduction, (e) 1. 0%RuO$_2$/TiO$_2$, (f) 1. 0%RuO$_2$/TiO$_2$ after reduction, and (g) 1. 0%RuO$_2$/TiO$_2$ after reduction and subsequent oxidation.

The photocatalytic property of the Ru species loaded TiO$_2$ nanosheets was evaluated by the O$_2$ evolution from water oxidation. The dependence of catalytic performance on the Ru content for Ru/TiO$_2$ catalyst is shown in Fig. 4. There is no oxygen evolution for the only TiO$_2$ nanosheets without Ru species loading. The O$_2$ evolution rate increased apparently from 10. 59 mmol/(g$\cdot$h) to 18. 48 mmol/(g$\cdot$h) catalyst with the Ru content from 0. 1wt% to 0. 5wt%. However, the activity drastically decline to 5. 43 mmol/(g$\cdot$h) when the Ru content further inceases to 1. 5wt%. The similar dependence of O$_2$ evolution rate on the Ru content is also noticed for RuO$_2$/TiO$_2$ catalyst as indicated in Fig. 5, where the optimum Ru content of 1. 0wt% can be identified with the highest O$_2$ evolution rate of 20. 25 mmol/(g$\cdot$h). It suggests that the photo-deposition of Ru or RuO$_2$ onto TiO$_2$ nanosheets may both be the effective way to promote its ptotocatalytic activity.

 FIG. 4 Oxygen evolution rates over Ru/TiO$_2$ catalysts with different Ru contents.

It has been revealed that noble metals and metal oxides may be selectively deposited on the exposed (101) and (001) facets of TiO$_2$ [31], since the photogenerated electrons and holes mainly accumulate on the (101) and (001) facets during the photo-deposition process and then are involved in the photocatalytic reduction and oxidation reactions, respectively. In the present study, it is reasonable to infer that Ru and RuO$_2$ nanoparticles are selectively deposited on (101) and (001) facets of the obtained TiO$_2$ nanosheets with the simultaneous exposure of the two facets for Ru/TiO$_2$ and RuO$_2$/TiO$_2$ catalysts, respectively. The effect of Ru species on the O$_2$ evolution from photocatalytic water oxidation loaded might be explained by the charge separation effect on different facets of the TiO$_2$ nanosheets [25, 26, 32, 33]. For the Ru/TiO$_2$ catalysts, Ru nanoparticles deposited on the exposed (101) facets of TiO$_2$ act as centers for trapping electrons when the content of Ru species is at a lower stage, which may enhance the separation of electrons and holes. However, the excessive Ru loading might hinder the incident light from irradiating TiO$_2$ and serve as the recombination centers for electrons and holes, which leads to the decrease of charge separation efficiency [33, 34]. A similar situation may also be applied for the RuO$_2$/TiO$_2$ catalysts, the main difference is the RuO$_2$ nanoparticles deposited on the exposed (001) facets of TiO$_2$ act as centers for trapping holes.

 FIG. 5 Oxygen evolution rates over RuO$_2$/TiO$_2$ catalysts with different Ru content

In order to further explore the charge separation effect on the different facets of TiO$_2$ nanosheets, the catalysts were subjected to oxidation and reduction treatments and the photocatalytic activities were examed as shown in Fig. 6. For 0. 5%Ru/TiO$_2$ catalyst, the O$_2$ evolution rate deeply decreases after the oxidation treatment from 18. 48 mmol/(g$\cdot$h) to 6. 04 mmol/(g$\cdot$h) for 0. 5%Ru/TiO$_2$(Oxy). Since the oxidation transforms Ru deposited on TiO$_2$ (101) facets into RuO$_2$ as confirmed by TPR result, the incompatible configuration of co-catalyst RuO$_2$ (holes trapped) on the TiO$_2$ (101) facets (electrons accumulated) may result in the faster recombination of electrons and holes and thus much lower photocatalytic activity. It is noticed that the O$_2$ evolution rate then greatly recovers to 14. 07 mmol/(g$\cdot$h) for 0. 5%Ru/TiO$_2$ (Oxy-Red) after the subsequent reduction treatment. On the other hand, the difference in O$_2$ evolution rate for 0. 5%Ru/TiO$_2$ (Oxy) and 0. 5%RuO$_2$ /TiO$_2$ also implies that Ru and RuO$_2$ may be selectively deposited on the different TiO$_2$ facets. The analogous tendency can also be observed for 1. 0%RuO$_2$/TiO$_2$ catalyst after the similar treatments, during which the O$_2$ evolution decreases from 20. 25 mmol/(g$\cdot$h) to 8. 84 mmol/(g$\cdot$h) after the reduction for 1. 0%RuO$_2$/TiO$_2$(Red), and recovers to 17. 19 mmol/(g$\cdot$h) after the subsequent oxidation treatment for 1. 0%RuO$_2$/TiO$_2$(Red-Oxy). The evident decrease in the photocatalytic activity for 1. 0%RuO$_2$/TiO$_2$(Red) may also be due to the incompatible configuration of co-catalyst Ru (electrons accumulated) on the TiO$_2$ (001) facets (holes trapped). The difference between the recovered and the original activity is mainly ascribed to the possible calcination and loss of Ru species during the oxidation and reduction treatments.

 FIG. 6 Oxygen evolution rates over different Ru-loaded TiO$_2$ nanosheets catalysts. (a) 0. 5%Ru/TiO$_2$, (b) 0. 5%Ru/TiO$_2$ (Oxy), (c) 0. 5%Ru/TiO$_2$ (Oxy-Red), (d) 1. 0%RuO$_2$/TiO$_2$, (e) 1. 0%RuO$_2$/TiO$_2$ (Red), (f) 1. 0%RuO$_2$/TiO$_2$ (Red-Oxy), and (g) 0. 5%Ru-1. 0%RuO$_2$/TiO$_2$.

To fulfill a promising route to engineer the efficient photocatalyst by taking advantage of the charge separation effect, the dual co-catalysts on TiO$_2$ nanosheets was fabricated by two steps of photo-deposition for 0. 5%Ru-1. 0%RuO$_2$/TiO$_2$ catalyst, where it may be inferred that Ru and RuO$_2$ are simultaneously and selectively deposited on (101) and (001) facets of the TiO$_2$ nanosheets, respectively. As compared in Fig. 6, 0. 5%Ru-1. 0%RuO$_2$/TiO$_2$ sample deposited with dual co-catalysts features the highest O$_2$ evolution rate of 31. 8 mmol/(g$\cdot$h), which is not only superior to 0. 5%Ru/TiO$_2$ and 1. 0%RuO$_2$/TiO$_2$ with the single co-catalyst at the optimum Ru content, but also much boosted in comparison with 1. 5%Ru/TiO$_2$ and 1. 5%RuO$_2$/TiO$_2$ with the single co-catalyst at the same Ru content. The synergetic effect of dual co-catalysts may be due to the enhanced charge separation effect, achieved by both Ru and RuO$_2$ selectively deposited on (101) and (001) facets of TiO$_2$ nanosheets as the trapping centers of electrons and holes, which could further facilitate the charge separation and thus promote the photocatalytic reaction.

IV. CONCLUSION

In this work, anatase TiO$_2$ nanosheets with mainly exposed (001) facet of about 70% have been obtained by the hydrothermal process. Ru or RuO$_2$ nanoparticles are successfully loaded on the obtained TiO$_2$ nanosheets by photo-deposition methods to fabricate the photocatalysts. The structural characterizations suggest highly dispersed Ru species on the TiO$_2$ nanosheets. According to the photocatalytic O$_2$ evolution from water oxidation, the optimum Ru contents were identified to be 0. 5wt% and 1. 0wt% for Ru/TiO$_2$ and RuO$_2$/TiO$_2$ catalysts, respectively. It may be explained by the charge separation effect of the Ru species co-catalysts on the different facets of TiO$_2$ nanosheets. Combined with the redox property and the influence of oxidation and reduction treatments on the photocatalytic behavior, it may be inferred that the co-catalysts of Ru and RuO$_2$ are selectively deposited on (101) and (001) facets of the TiO$_2$ nanosheets, respectively. The optimal photocatalytic activity was achieved for 0. 5%Ru-1. 0%RuO$_2$/TiO$_2$ sample deposited with dual co-catalysts, which may be provn to be a promising route to engineer the efficient photocatalyst by fulfilling the enhanced charge separation effect.

V. ACKNOWLEDGMENTS

This work is supported by the Anhui Provincial Natural Science Foundation (No. 1408085MB25).

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