New Insight of Water-Splitting Photocatalyst: H2O2-Resistance

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New insight of water splitting photocatalyst: HO-resistance poisoning and photothermal deactivation in submicron CoO octahedrons Weilong Shi, Feng Guo, Huibo Wang, Sijie Guo, Hao Li, Yunjie Zhou, Cheng Zhu, Yanhong Liu, Hui Huang, Baodong Mao, Yang Liu, and Zhenhui Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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ACS Applied Materials & Interfaces

New insight of water splitting photocatalyst: H2O2-resistance poisoning and photothermal deactivation in submicron CoO octahedrons Weilong Shi1,3, Feng Guo1, Huibo Wang1, Sijie Guo1, Hao Li1, Yunjie Zhou1, Cheng Zhu1, Yanhong Liu2, Hui Huang1, Baodong Mao2*,Yang Liu1* and Zhenhui Kang1* 1

Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices,

Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China. Email: [email protected]; [email protected] 2

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, China. Email: [email protected] 3

School of Physics, Huazhong University of Science and Technology, Wuhan

430074, P.R. China. KEYWORDS:

CoO

octahedrons,

H2O2-resistance

poisoning,

photothermal

deactivation, overall water splitting, photocatalyst.

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ABSTRACT

Hydrogen production by photocatalytic overall water splitting represents an ideal pathway for clean energy harvesting, for which developing high-efficiency catalysts has been the central scientific topic. Nanosized CoO with high solar-to-hydrogen efficiency (5%) is one of the most promising catalyst candidates. However, poor understanding of this photocatalyst leaves the key issue of rapid deactivation unclear and severely hinders its wide application. Here, we report a submicron CoO octahedron photocatalyst with high overall-water-splitting activity and outstanding ability of H2O2-resistance poisoning. We show that the deactivation of CoO catalyst originates from the unintended thermo-induced oxidation of CoO during photocatalysis, with coexistence of oxygen and water. We then demonstrate that introduction of graphene, as a heat conductor, largely enhanced the photocatalytic activity and stability of the CoO. Our work not only provides a new insight of CoO for photocatalytic water splitting, but also demonstrates a new concept for photocatalyst design.

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INTRODUCTION As an ultimate solution for clean and renewable energy, hydrogen production from overall water splitting using sunlight has been regarded as the “holy-grail” in chemistry and materials science.1,2 In the past 40 years, tremendous effort has been contributed to the development of various water splitting photocatalysts, including single-component

semiconductors,3-5

solid

solutions,6-8

heterostructures,9-12

composites13,14 and Z-scheme systems.15,16 Among them, CoO holds the greatest potential as a simple single-component photocatalyst, not only with relatively narrow band gap (2.6 eV) for visible light absorption and very high solar-to-hydrogen efficiency (STH=5%), but also with high earth abundancy and low cost. Previous reports claim that only small size CoO nanoparticles (less than 10 nm) are capable of water splitting, while the bulk counterparts (micropowders) lack of activity.17 However, poor stability (deactivating within 1 hour) of the CoO nanocatalyst hinders its further development. To uncover the primary cause of its deactivation and subsequently to seek suitable solutions is still an urgent challenge. Here, we report the fabrication of single-crystalline phase-pure CoO submicron sized octahedrons with exposed active (111) facets, showing unexpected high water splitting activity without any cocatalysts or sacrificial reagents. The CoO octahedron photocatalysts posses outstanding H2O2-resistance poisoning and the deactiviation of CoO originates from the photothermal-induced CoO to Co3O4 transformation in present of oxygen and water. Further combining graphene (reduced graphene oxide, RGO; as the heat conductor) with CoO, the CoO/RGO composite shows increased water splitting activity (2.5 times of pristine CoO) and excellent stability over 15 days. RESULTS AND DISCUSSION A series of CoO samples were synthesized through a solvothermal route at 220 °C according to a literature method with revisions (see Supporting Information for details).18 All the eleven CoO samples (CoO-1 to CoO-11, listed in Table S1) show the same crystal structure and the following discussion will focus on the sample with the highest photocatalytic activity (e.g. CoO-3 in Supporting Information) unless 3



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otherwise mentioned. All the X-ray diffraction (XRD) peaks in Figure S1 can be indexed to the face-centered cubic phase CoO (JCPDS 71-1178).

Figure 1. Physical structure characterization of the CoO octahedrons. (a) SEM image of the CoO octahedrons (inset is TEM image). (b) HRTEM image of the edge area of a single CoO octahedron. (c) TEM images (upper) and the corresponding SAED patterns (lower) over single CoO octahedrons (CoO-3) with different orientations. High-resolution XPS spectra of the CoO octahedrons: (d) Co 2p and (e) O 1s.

SEM and TEM images in Figure 1a indicate the surface-smoothing octahedral morphology of CoO particles with submicron sizes of around 300-700 nm and relatively narrow size distribution (Figure S2). A HRTEM image (Figure 1b) shows the clear lattice with an interplanar spacing of 0.25 nm, corresponding to the (111) facet of cubic phase CoO.19 TEM images acquired from different orientations (Figure 1c) together with the corresponding selective area electron diffraction (SAED) 4


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patterns clearly show the well-developed symmetric octahedron morphology (schematic cartoons, insets in Figure 1c). These SAED patterns recorded from different orientations further prove the single-crystalline nature and the preferred exposure of the (111) facets of the CoO octahedrons.20 The Co 2p XPS spectrum in Figure 1d (XPS survey spectrum see Figure S3) shows two main peaks centered at 780.1 eV and 796.2 eV with their satellite features (786.1 and 802.6 eV), proving that Co exists as Co2+ in CoO.21 The O 1s spectrum (Figure 1e) shows two peaks at 529.4 eV and 531.1 eV, well matching with the lattice oxygen (OL) and the chemisorbed and dissociated oxygen species (OC), respectively.22,23 Three characteristic peaks at 489, 540 and 690 cm-1 in the Raman spectrum in Figure S4 are identified as the Eg, T2g and A1g modes of CoO, respectively.24 Additional characterizations of other CoO samples are provided in Supporting Information including XRD (Figure S5) and SEM (Figure S6).

Figure 2. Optical property and band structure of the CoO octahedrons. (a) UV-vis absorption spectrum. Inset: Digital photograph (4 cm × 4 cm) of the sample indicating the characteristic clay-like light brown color of CoO. (b) The Tauc plot of (αhν)2 versus hν derived from UV-vis spectrum in (a) for band gap estimation. The horizontal dashed black line is the baseline and the other dashed line is the tangent of the curve. (c) UPS spectra of the CoO octahedrons for the determination of the ionization potential of the CoO octahedrons, corresponding to the valence 5



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band energy (Ev). (d) Band structure diagram for the CoO octahedrons along with the reduction level for H+ to H2, and the oxidation levels for H2O to H2O2 or O2. VB, valence band; CB, conduction band.

The CoO octahedrons exhibit light absorption over the entire wavelength range with two obvious absorption edges (Figure 2a). The first one is due to the band gap absorption, while the second absorption feature in longer wavelength (λ > 600 nm) is attributed to the electron excitation from the d orbital of Co2+ to the conduction band of CoO that may contribute to both photocatalysis and photo-thermal effect.25 The optical band gap (Eg) of the CoO octahedrons can be estimated from the Tauc of converted (αhv)r versus hv from the UV-vis spectrum, where α, h, and v are the absorption coefficient, Planck constant, and light frequency, respectively, and r = 2 for a direct band gap material here. Based on the Tauc plot (Figure 2b), Eg of CoO octahedrons was estimated to be 2.47 eV, which agrees with the reported values (2.6 eV) for CoO nanoparticles17 or nanowires26 with demonstrated water splitting ability. UV-vis spectra and Tauc plots of all other CoO samples are provided in Figure S7. Besides the band gap, proper band energy level alignment is another key parameter to ensure the redox ability for overall water splitting. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the ionization potential of the CoO octahedrons, corresponding to the valence band energy (Ev), from which the conduction band energy (Ec) is thus estimated by Ev-Eg.14 In Figure 2c, the dashed red lines mark the baseline and the tangents of the curve and the intersections of the tangents with the baseline give the edges of the UPS spectra, from which the UPS width of 14.43 eV is determined. The ionization potential (e.g. Ev) of the CoO octahedrons was estimated to be 6.79 eV (vs. vacuum) by subtracting the peak width of UPS spectrum from the excitation energy (He I, 21.22 eV), and consequently, Ec was calculated as 4.32 eV (vs. vacuum), which equals to -0.12 volts (V) versus RHE (reversible hydrogen electrode) after converting to electrochemical energy potentials in V, where 0 V versus RHE equals -4.44 eV versus vacuum level. Figure 2d shows the relative energy level alignment of CoO octahedrons with respect to the electrochemical redox potentials of H+/H2, O2/H2O and H2O2/H2O. It can be seen that the CoO octahedrons have theoretically appropriate reduction and oxidation energy 6


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potentials for water splitting into H2 and O2 or H2O2.

Figure 3. Photocatalytic water-splitting performance of the CoO octahedrons. (a) Typical time course of H2 and O2 evolution from water under visible light irradiation (λ > 400 nm) and (b) the cycle stability of photocatalytic H2 production rate (each cycle is 24 hours) over the CoO octahedrons. (c) XRD patterns of the catalysts before and after 10 photocatalytic reaction cycles showing decreased peak intensity of CoO. Insets are corresponding SEM images indicating the increased surface roughness after 10 cycles. Scale bar: 100 nm. (d) Co 2p XPS of the CoO octahedrons before (fresh sample) and after 10 cycles of photocatalytic reactions. The grey shadows highlight the decrease of the two characteristic satellite peaks at 786.1 and 802.6 eV attributed to Co2+ in CoO.

Photocatalytic overall water splitting over the CoO octahedrons suspended in neutral water was evaluated under visible light irradiation (λ > 400 nm). It was observed that the catalyst dispersed in ultrapure water is able to produce nearly stoichiometric H2 and O2 (around 2:1) at rates of 0.266 and 0.146 µmol/h, simultaneously over a 24-hour reaction period (Figure 3a). It is worthwhile mentioning that trace amount of H2O2 was also detected in the photocatalytic system using UV-vis absorption spectroscopy (Figure S8). The photocatalytic H2/O2 evolutions of all the other CoO octahedron samples are shown in Figure S9. The results demonstrate that with the addition of different ratios of n-octanol and ethanol, photocatalytic performances of CoO have been greatly changed, implying that the 7



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morphology of CoO has a significant influence on the photocatalytic activity, especially the transition from the octahedral structure of the exposed (111) surface to irregular nanoparticles (Figure S6c-k). That is to say, the exposed (111) surfaces of CoO photocatlaysts gradually disappeared, thus resulting in the decreased photocatalytic activity. Previous report suggests that only quite small sized CoO particles (less than 10 nm) are capable of water splitting, whereas large CoO microparticles are difficult to realize water splitting due to the restrict of conduction band energy.17 In sharp contrast, here our result proves that large submicron CoO particles can split water to stoichiometric H2 and O2, though the direct hydrogen evolution rate is lower than that of the 10 nm CoO nanoparticles. However, an important fact needs to be considered is that the surface-to-volume ratio of our submicron CoO octahedrons (~500 nm) is about 50 times lower than the nanosized CoO nanoparticles (e.g. ~10 nm).27 There is plenty of room to further increase the activity by reducing the particles of CoO octahedrons. Anyway, the reason of submicron CoO particles with relative high photocatalytic activity may attribute to the exposed highly actives (111) facets on the octahedral CoO particle.28 Next, the CoO octahedron catalyst was repeatedly examined over a total of 10 cycles with an intermittent evacuation every 24 h under visible light irradiation in Figure 3b. Compared with the state-of-the-art CoO nanoparticle photocatalysts (deactivating within 1 h), our submicrometer CoO octahedrons show prominently higher stability over days. With more cycle runs, noticeable decrease of the photocatalytic H2 evolution was clearly observed in the pristine CoO octahedrons. Figure 3c shows the XRD patterns and SEM images before and after 10 cycles of photocatalysis experiments. All the XRD peaks of CoO become apparently weaker and increased etching and surface roughness was observed from the SEM images (insets of Figure 3c and Figure S10) of the CoO sample after 10 cycles compared to that of the fresh one. The two characteristic XPS satellite peaks at 786.1 and 802.6 eV (Figure 3d; survey spectrum see Figure S11) and Raman peak at 540 cm-1 of CoO all decreased dramatically after 10 photocatalysis cycles (Figure S12). 8


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Figure 4. Photo-thermal effect of the CoO octahedrons. The photocatalytic H2 generation over CoO octahedrons under (a) UV-light irradiation (λ=365 nm, 28 mW cm-2) and (b) Infrared-light irradiation (λ=630 nm, 45 mW cm-2). The pictures of (c1) pure water (20 mL) and (d1) CoO solution (50 mg photocatalyst, 20 mL pure water). Infrared thermography images of (c2) pure water and (d2) CoO solution before irradiation. Infrared thermography images of (c3) pure water and (d3) CoO solution after 2 h visible light irradiation (λ > 400 nm).

For exploration of the reason for the CoO deactivation, two crucial factors (photocorrsion and photothermal) should be preferentially considered into the photocatalytic reaction. It can be seen that there is no apparent deactivation of the photocatalytic activity over CoO after three cyclings under UV-light irradiation (Figure 4a), thus eliminating the photocorrsion. Conversely, Figure 4b shows that the photocatalytic activity of CoO decreased obviously under the infrared light irradiation. This phenomenon spurs us to get a possible assumption that the apparently decreased H2 production may be attributed to the unintended photo-thermal effect during the photocatalysis. Thus, the Infrared (IR) thermal imaging technology was applied to probe the change of solution temperature before and after light irradiation. Figure 4c1 and d1 represent the images of pure water and CoO solution and the rectangular red dashed lines are the target areas. The IR thermography images showed that without 9



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the light irradiation, the temperatures of pure water (17.3 °C) and CoO solution (17.4 °C) are nearly negligible difference (Figure 4c2 and d2). But a notable difference between pure water and CoO solution was obtained after visible light irradiation for 2 h. As shown in Figure 4c3 and d3, the additional temperature of pure water increased from 17.3 °C to 23.1 °C and that of CoO even reached 27.2 °C. This enhanced temperature illustrates that light irradiation indeed generates heat during the photocatalysis. Another interesting finding is that CoO solution is 4 °C higher than pure water under same conditions, which implies that CoO could generate more heat energy by absorbing long wavelength light during the irradiation. This also indicates that the devastation of CoO should be associated with the thermal effect. From the view of further application, it is significant and necessary to explore temperature effect for CoO during the photocatalytic reaction. However, up to now, the microscopic detection for the precise temperature change of photocatalysts during the photocatalytic reaction still limited. In addition, other possible factors including the photogenerated O2, H2O2 should also be taken into account the deactivation of CoO.

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Figure 5. Thermal stability and H2O2-resistance poisoning effect of the CoO octahedrons. (a) Thermogravimetric analysis curves of the CoO octahedrons recorded with air or N2 flow rate of 100 mL min −1 and a heating rate of 10 °C min−1. Typical (b) XRD patterns, (c) Raman spectra and (d) Co 2p XPS spectra of the CoO octahedrons after treatment at different temperatures in water under air or N2 atmospheres. The grey shadow areas mark the characteristic XPS and Raman peaks of CoO. (e) The variation of H2 production (in 24 hours) from water over the CoO octahedrons heating treated at different temperatures. (f) The variation of H2 production after adding different amounts of H2O2 indicating the outstanding H2O2-resistance poisoning ability of the CoO octahedrons. The photocatalytic reactions were performed with 50 mg of the CoO octahedrons dispersed in 20 mL of ultrapure water under visible light irradiation (λ > 400 nm) and all data points represent the average of three independent data sets.

To further identify the critial cause of the gradual deactivation of CoO octerhedron catalyst, possible factors were introduced controllably to simulate the complex photocatalysis reaction environment, including the photogenerated O2, H2O2, and heat as well as the largely existing water. Next, thermogravimetric analysis (TGA), XRD, Raman and XPS were carried out on the CoO octahedrons upon systimatic treatments to investigate these factors of stability. First, TGA proves that 11



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air oxidation of CoO octahedrons does not occur up to 200 °C as shown in Figure 5a. and further increase to 800 °C results in a weight increase of 6.5%, which agrees well with the theoretical value of CoO oxidation to Co3O4 (7.1%).29 No noticeable weight increase was observed in N2 atmosphere up to 800 °C. It is safe to conclude that, without water, the CoO octahedrons are stable in air up to 200 °C. Meanwhile, XRD patterns (Figure 5b) and Raman spectra (Figure 5c) of the CoO samples under different conditions (oxygen/nitrogen atmospheres, water and temperature) were further compared in an effort to simulating the photocatalytic reaction conditions. First, when exposed to nitrogen atmosphere in water, XRD, Raman and XPS spectra (Figure 5d) of the CoO samples are almost the same with that of the fresh sample, no martter the temperature reached 50 °C (red race) or 70 °C (blue race). In striking contrast, when exposed to air with increased temperature, dramatic difference from the fresh sample was oberved. In Figure.5b, though the XRD of 50 °C-treated CoO octahedrons with air and water (purple race) hardly differed from that of the fresh one, the diffraction peaks of Co3O4 emerged in the as-prepared CoO sample when the temperature reached at 70 °C (orange curve). With more sensitive Raman and XPS techniques, we found that the structure variation already started upon 50 °C heating in air and water condition. The Raman peaks of CoO initially located at 489, 540 and 690 cm−1 (Figure 5c) almost disappears after 50 °C or 70 °C boiling in air. In Figure 5d, the two characteristic satellite features at 786.1 and 802.6 eV of CoO were obviously reduced with 50°C heating and completely disappeared with 70 °C heating under air condition. More detailed investigation of the transition process of heating treated CoO octahedrons in air can be found in Supporting Information (SEM, XRD and XPS mages in Figure S13-15). Based on the above results, the deactivation of CoO photocatalyst can be mainly attributed to the transition of CoO to Co3O4 with the combined effect of elevated temperature, oxygen and water. This extremely easy conversion from CoO to Co3O4 at relatively low temperatures in water may be due to the high structure similarity of the two oxides that have a comparable oxygen sublattice with closest packed fcc structure, where the nearest-neighbor O2--O2distances match within 5%.30 In the coexistence of oxygen and water, the cobalt (II) 12


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oxidation to cobalt (III) would be accelerated easily with temperature increase. The photocatalytic H2 evolution was further performed using the CoO samples heating treated from 25 °C to 80 °C for 6 h under air condition. In Figure 5e, the photocatalytic H2 evolution evidently decreased with the increasing temperature. When treated at 80 °C, the 24-hour photocatalytic H2 evolution is hardly to be detected (0.053 µmol), 120 times lower than that of ambient temperature (the pristine CoO octahedron, 6.384 µmol). This solid result demonstrated the photocatalytic activity of CoO is strongly correlated with structure variation upon heating treatment with coexistence of air and water. Previous literature reported that octahedral CoO with the (111) surface facet is quite unstable due to the fact that Co2+ cations and O2anions alternate from one plane to another, and subsequently, the CoO would be oxidized to attain the spinel structure of Co3O4 to avoid this polar catastrophe and attain the thermodynamic stability.31-35 Furthermore, based on our experimental results above, we can conclude that the photo-thermal effect produced by CoO photocatalyst could accelerate this phase transformation from CoO to Co3O4 during the photocatalytic process. Another factor must be considered is that the photocatalytic evolved H2O2 in water splitting often poisons the photocatalysts. As mentioned above, H2O2 was detected in our CoO octahedron photocatalytic system (Figure S8). Surprisingly, photocatalytic H2 evolution activity of the CoO octahedrons was stable even with intentionally added H2O2 from 0.01 to 0.24 mol/L (Figure 5f). The excellent H2O2-resistance poisoning property of the CoO octahedrons may be attributed to the efficient H2O2 decomposition catalyzed by the Fenton-like fast Co(II)/Co(III) transformation in CoO.36-38 As known, many photocatalysts can be poisoned by H2O2 easily.39 Here, our H2O2-resistance CoO octahedrons may provide a promising single-component candidate for overall water splitting under visible light. More importantly, all the above results indicate that the key factor for deactivation of CoO sample is not the photo-produced H2O2, but the combined effect of heating, air and water as mentioned above. Here, the unintentional heating may come from the photo-induced internal temperature increase of the catalyst that can be correlated with 13



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the second absorption feature at long wavelengths (Figure 2a).

Figure 6. Photocatalytic activities of the CoO/RGO composite. (a) TEM image of CoO/RGO composite. (b) photocatalytic activities of the CoO/RGO composite: typical time course of H2 and O2 production from pure water under visible light irradiation (λ > 400 nm) and (c) the stability of photocatalytic activity (one cycle is 24 hours) over the CoO/RGO composite. (d) Cycle stability of H2 production using the 70 °C treated CoO/RGO composite and CoO octahedrons (each cycle is 24 hours).

Considering graphene is an excellent heat conductor,40-44 reduced graphene oxide (RGO) was introduced to form the CoO/RGO composite to dredge the photo-induced heating of CoO during photocatalysis. TEM image indicates that the octahedral morphology of CoO was maintained in the CoO/RGO composite (Figure 6a). As expected, the H2 evolution rate of CoO/RGO is increased to 0.675 µmol/h (Figure 6b), 2.53 times higher than that of pristine CoO octahedrons. More importantly, cycle photocatalytic experiments confirm that the stability of CoO/RGO is dramatically improved (Figure 6c) with no noticeable deactivation even after fifteen successive cycles under visible light irradiation. Further insight into the cycle stability suggests that the 70 oC-treated CoO/RGO lasts over five 24-hour cycles with no noticeable deactivation (red curves, Figure 6d), while the 70 oC-treated CoO octahedrons almost completely deactivated within two cycles (black curve, Figure 6d). Based on the 14


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above results, the introduction of RGO resulted in the dramatic increase the photocatalytic activity (Figure 6b), cycle stability (Figure 6c), as well as the resistivity to the heating induced deactivation (Figure 6d). The introduction of graphene in the CoO/RGO composite brings several prominent advantages: (i) the excellent thermal conductivity to relieve the photo-induced heat accumulation; (ii) the flexible 2D structure to stabilize the photocatalyst; (iii) to increase the separation of the photo-generated electron-hole pairs; (iv) to improve light absorption.45-48 Among these factors, the thermal conductivity and structural flexibility mainly contributed to the improved stability, while the rest two contributed to the increase of photocatalytic activity.

CONCLUSIONS In this work, we synthesized a series of single-crystalline phase-pure submicron CoO octahedrons through a facile solvothermal method. These CoO octahedrons are capable of overall water splitting under visible light (λ > 400 nm) with high H2O2-resistance poisoning ability. We confirm that the deactivation, the most restricting issue for CoO photocatalysts, is resulted from the thermo-induced oxidation of CoO to Co3O4 at elevated temperatures with coexistence of oxygen and water. Further introduction of graphene as heat conductor largely enhanced the photocatalytic activity and thermal stability of the CoO octahedron catalyst. Our work provides an interesting insight of photocatalytic water splitting and a new concept for high-efficiency photocatalyst design.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details, supplemental figures and a table, including XRD patterns, SEM images, UV-vis absorption spectra and H2/O2 evolution of as-prepared CoO samples (CoO-1 to CoO-11), comparison of XRD patterns, XPS spectra, Raman spectra and 15



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SEM images of CoO octahedrons before and after photocatalytic reaction, the size of CoO with different volume ratios of components (PDF).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]; [email protected]. NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Natural Science Foundation of China (51422207, 51572179, 21471106, 21501126), the Natural Science Foundation of Jiangsu Province (BK20161216) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

REFERENCES (1) Mubeen, S.; Lee, J.; Singh, N.; Kramer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in which all Charge Carriers Derive from Surface Plasmons. Nat. Nanotechnol. 2013, 8, 247-251. (2) Liu, C.; Tang, J.; Chen, H. M.; Liu, B.; Yang, P. A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting. Nano Lett. 2013, 13, 2989-2992. (3) Tanigawa, S.; Irie, H. Visible-Light-Sensitive Two-step Overall Water-splitting based on Band Structure Control of Titanium Dioxide. Appl. Catal., B 2016, 180, 1-5. (4) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.; Domen, K.; Antonietti, M. A Metal-free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80. 16


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TOC 335x245mm (72 x 72 DPI)


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New Insight of Water-Splitting Photocatalyst: H2O2-Resistance

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