Photo-depositing Ru and RuO2on Anatase TiO2Nanosheets as Co-catalysts for Photocatalytic O2Evolution from Water Oxidation

Shi-yang Mi,Yuan-xu Liu,Wen-dong WangCAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics,University of Science and Technology of China,Hefei 230026,China.

Photo-depositing Ru and RuO2on Anatase TiO2Nanosheets as Co-catalysts for Photocatalytic O2Evolution from Water Oxidation

Shi-yang Mi,Yuan-xu Liu,Wen-dong Wang?
CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics,University of Science and Technology of China,Hefei 230026,China.

TiO2nanosheets mainly exposed(001)facet were prepared through a hydrothermal process with HF as the morphology-directing agent.Ru and RuO2species were loaded by photodeposition methods to prepare the photocatalysts.The structural features of the catalysts were characterized by X-ray diffraction,transmission electron microscopy,inductively coupled plasma atomic emission spectrum,and H2Temperature-programmed reduction.The photocatalytic property was studied by the O2evolution from water oxidation,which was examined with respect to the influences of Ru contents as well as the oxidation and reduction treatments,suggesting the charge separation effect of the Ru species co-catalysts on different facets of TiO2nanosheets.In contrast to Ru/TiO2and RuO2/TiO2with the single deposited co-catalyst,the optimized catalyst 0.5%Ru-1.0%RuO2/TiO2with 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.

Anatase TiO2nanosheets,Photocatalytic O2evolution,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,TiO2appears 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 TiO2-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 TiO2is more active than the (101)surface[5],therefore conventional anatase TiO2nanoparticles prefer to expose the(101)crystal facets with low surface energy(0.44 J/m2)rather than the (001)facets with high surface energy(0.90 J/m2). To obtain TiO2mainly exposing high reactive crystal facets,hydrofluoric acid(HF)has been used as the structure-directing agent to fabricate nanocrystalline TiO2that exposed 47%(001)crystal facets and showed an excellent photocatalytic activity[6].Based on this breakthrough,a number of studies on TiO2-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 cocatalysts to build heterojunctions on photocatalysts[3, 9,13].The metal and metal oxide nanoparticles loaded on TiO2can 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 RuO2[3].The enhancement of H2evolution 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,RuO2-loaded photocatalysts can promote the overall splitting of water[16,17],as the holes would be trapped by RuO2,resulting in efficient charge separation and improved photocatalytic activity.However, there are seldom reported photocatalysts with both Ru and RuO2nanoparticles as co-catalyst loading.

Moreover,some researchers have found that photogenerated electrons and holes might voluntarily separate towards different crystal facets in the photochemi-cal 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 TiO2,(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.

?Author to whom correspondence should be addressed.E-mail: wangwd@ustc.edu.cn,Tel.:+86-551-63603683

In this work,anatase TiO2nanosheets with dominant(001)facets are synthesized by the hydrothermal method.Ru and RuO2nanoparticles were loaded by different photo-deposition processes.The photocatalytic O2evolution 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 TiO2nanosheets,and high catalytic activity of the anatase TiO2nanosheets photocatalyst with both Ru and RuO2nanoparticles 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, RuCl3,and KIO3powder.

TiO2nanosheets 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°C 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°C 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°C for 12 h.

The photo-deposition of Ru was conducted with RuCl3as precursor.Typically,0.15 g TiO2nanosheets were suspended in 50 mL of distilled water,and then the calculated RuCl3solution 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°C.The obtained catalyst is denoted as Ru/TiO2.The photo-deposition of RuO2was conducted by a similar method to prepare RuO2/TiO2catalyst,and the only difference was that the solution used to suspend TiO2nanosheets was changed to 50 mL KIO3aqueous solution(0.1 mol/L). The photo-deposition of dual co-catalysts Ru and RuO2on TiO2nanosheets was prepared by two steps for Ru-RuO2/TiO2catalyst.Ru was firstly loaded after 5 h photo-deposition,and the suspension was moved to the dark.Then the calculated RuCl3solution and 5 mL of KIO3solution(1 mol/L)was added into the suspension and stirred for 2 h,which was subjected to another 5 h photo-deposition to deposit RuO2.

The reduction treatment was performed at 150°C for 2 h in a flow of 5%H2/Ar with a heating rate of 5°C/min,while the oxidation treatment at 200°C for 2 h in a muffle.

The contents of Ru and RuO2deposited on TiO2nanosheets 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 Kα radiation (λ=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°C/min from room temperature up to 200°C in a flow of 5%H2/Ar.The amount of H2consumption during TPR was estimated from the integrated peak area using AgO2as a standard.

The photocatalytic O2evolution 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 KIO3aqueous 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 O2was 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/TiO2and RuO2/TiO2catalysts 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.

The XRD patterns of TiO2nanosheets and the photocatalysts that loaded with different co-catalysts are compared in Fig.1.TiO2nanosheets only shows thetypical diffraction patterns of anatase TiO2(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 TiO2nanosheets 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 TiO2and carbon nanotubes support [9,27?29].

TABLE I The Ru content of nominal and actual catalysts.

FIG. 1 XRD patterns of (a) TiO2 nanosheets, (b)0.5%Ru/TiO2,(c)1.0%RuO2/TiO2,and(d)0.5%Ru-1.0%RuO2/TiO2catalysts.

Figure 2 shows the TEM and HRTEM images of the obtained TiO2nanosheets and typical photocatalysts to verify the formation of their morphology features. It is observed that the obtained TiO2nanosheets 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 RuO2particles 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 TiO2nanosheets as well.

The H2-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 TiO2nanosheets(Fig.3(a)).For 0.5%Ru/TiO2catalyst,almost no peak appears except a trace of H2consumption near 100°C(Fig.3(b)),while an obvious peak centered at 131°C 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 TiO2nanosheets by photo-deposition. With respect to 1.0%RuO2/TiO2catalyst,it features a H2consumption peak at 104°C(Fig.3(e))due to the reduction of RuO2[30],and then the peak disappears after the reduction treatment(Fig.3(f));however,another peak located about 128°C 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 H2consumption estimated from the integrated peak area.

The photocatalytic property of the Ru species loaded TiO2nanosheets was evaluated by the O2evolution from water oxidation. The dependence of catalytic performance on the Ru content for Ru/TiO2catalyst is shown in Fig.4.There is no oxygen evolution for the only TiO2nanosheets without Ru species loading.The O2evolution rate increased apparently from 10.59 mmol/(g·h)to 18.48 mmol/(g·h)catalyst with the Ru content from 0.1wt%to 0.5wt%. However,the activity drastically decline to 5.43 mmol/(g·h)when the Ru content further inceases to 1.5wt%.The similar dependence of O2evolution rate on the Ru content is also noticed for RuO2/TiO2catalyst as indicated in Fig.5,where the optimum Ru content of 1.0wt%can be identified with the highest O2evolution rate of 20.25 mmol/(g·h).It suggests that the photodeposition of Ru or RuO2onto TiO2nanosheets may both be the effective way to promote its ptotocatalytic activity.

It has been revealed that noble metals and metal oxides may be selectively deposited on the exposed(101) and(001)facets of TiO2[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 RuO2nanoparticlesare selectively deposited on(101)and(001)facets of the obtained TiO2nanosheets with the simultaneous exposure of the two facets for Ru/TiO2and RuO2/TiO2catalysts,respectively.The effect of Ru species on the O2evolution from photocatalytic water oxidation loaded might be explained by the charge separation effect on different facets of the TiO2nanosheets[25,26,32,33]. For the Ru/TiO2catalysts,Ru nanoparticles deposited on the exposed(101)facets of TiO2act 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 TiO2and 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 RuO2/TiO2catalysts,the main difference is the RuO2nanoparticles deposited on the exposed(001)facets of TiO2act as centers for trapping holes.

FIG.2 TEM and HRTEM images of TiO2nanosheets and catalysts.(a)Pure TiO2,(b)0.5%Ru/TiO2,(c)1.0%RuO2/TiO2, (d)0.5%Ru-1.0%RuO2/TiO2,and(e)HR-TEM image of pure TiO2.

FIG.3 H2-TPR profiles of(a) TiO2 nanosheets, (b) 0.5%Ru/TiO2, (c) 0.5%Ru/TiO2 after oxidation, (d) 0.5%Ru/TiO2 after oxidation reduction, (e)1.0%RuO2/TiO2,(f)1.0%RuO2/TiO2after reduction, and(g)1.0%RuO2/TiO2after reduction and subsequent oxidation.

FIG.4 Oxygen evolution rates over Ru/TiO2catalysts with different Ru contents.

In order to further explore the charge separation effect on the different facets of TiO2nanosheets,the catalysts were subjected to oxidation and reduction treatments and the photocatalytic activities were examed as shown in Fig.6. For 0.5%Ru/TiO2catalyst,the O2evolution rate deeply decreases after the oxidationtreatment from 18.48 mmol/(g·h)to 6.04 mmol/(g·h) for 0.5%Ru/TiO2(Oxy). Since the oxidation transforms Ru deposited on TiO2(101)facets into RuO2as confirmed by TPR result,the incompatible configuration of co-catalyst RuO2(holes trapped)on the TiO2(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 O2evolution rate then greatly recovers to 14.07 mmol/(g·h)for 0.5%Ru/TiO2(Oxy-Red) after the subsequent reduction treatment. On the other hand,the difference in O2evolution rate for 0.5%Ru/TiO2(Oxy)and 0.5%RuO2/TiO2also implies that Ru and RuO2may be selectively deposited on the different TiO2facets.The analogous tendency can also be observed for 1.0%RuO2/TiO2catalyst after the similar treatments,during which the O2evolution decreases from 20.25 mmol/(g·h)to 8.84 mmol/(g·h) after the reduction for 1.0%RuO2/TiO2(Red),and recovers to 17.19 mmol/(g·h)after the subsequent oxidation treatment for 1.0%RuO2/TiO2(Red-Oxy).The evident decrease in the photocatalytic activity for 1.0%RuO2/TiO2(Red)may also be due to the incompatible configuration of co-catalyst Ru(electrons accumulated)on the TiO2(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.5 Oxygen evolution rates over RuO2/TiO2catalysts with different Ru content

To fulfill a promising route to engineer the efficient photocatalyst by taking advantage of the charge separation effect,the dual co-catalysts on TiO2nanosheets was fabricated by two steps of photo-deposition for 0.5%Ru-1.0%RuO2/TiO2catalyst,where it may be inferred that Ru and RuO2are simultaneously and selectively deposited on(101)and(001)facets of the TiO2nanosheets,respectively. As compared in Fig.6,0.5%Ru-1.0%RuO2/TiO2sample deposited with dual co-catalysts features the highest O2evolution rate of 31.8 mmol/(g·h),which is not only superior to 0.5%Ru/TiO2and 1.0%RuO2/TiO2with the single co-catalyst at the optimum Ru content,but also much boosted in comparison with 1.5%Ru/TiO2and 1.5%RuO2/TiO2with 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 RuO2selectively deposited on (101)and(001)facets of TiO2nanosheets as the trapping centers of electrons and holes,which could further facilitate the charge separation and thus promote the photocatalytic reaction.

FIG.6 Oxygen evolution rates overdifferent Ruloaded TiO2nanosheets catalysts. (a)0.5%Ru/TiO2, (b)0.5%Ru/TiO2(Oxy),(c)0.5%Ru/TiO2(Oxy-Red), (d) 1.0%RuO2/TiO2, (e) 1.0%RuO2/TiO2 (Red), (f) 1.0%RuO2/TiO2 (Red-Oxy), and (g) 0.5%Ru-1.0%RuO2/TiO2.

IV.CONCLUSION

In this work,anatase TiO2nanosheets with mainly exposed(001)facet of about 70%have been obtained by the hydrothermal process.Ru or RuO2nanoparticles are successfully loaded on the obtained TiO2nanosheets by photo-deposition methods to fabricate the photocatalysts.The structural characterizations suggest highly dispersed Ru species on the TiO2nanosheets.According to the photocatalytic O2evolution from water oxidation,the optimum Ru contents were identified to be 0.5wt%and 1.0wt%for Ru/TiO2and RuO2/TiO2catalysts,respectively. It may be explained by the charge separation effect of the Ru species co-catalysts on the different facets of TiO2nanosheets. 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 RuO2are selectively deposited on(101) and(001)facets of the TiO2nanosheets,respectively. The optimal photocatalytic activity was achieved for 0.5%Ru-1.0%RuO2/TiO2sample 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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(Dated:Received on March 26,2016;Accepted on April 24,2016)