Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen

Subscriber access provided by Uppsala universitetsbibliotek

Article

Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen generation Guojing Li, Jiquan Huang, Zhonghua Deng, Jian Chen, Qiufeng Huang, Zhuguang Liu, Wang Guo, and Rong Cao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00797 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Crystal Growth & Design

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

This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies.

Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen generation Journal: Crystal Growth & Design Manuscript ID cg-2019-007979.R1 Manuscript Type: Article Date Submitted by the 17-Jul-2019 Author: Complete List of Authors: Li, Guojing; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter Huang, Jiquan; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter, Key Lab of Optoelectronic Materials Chemistry and Physics Deng, Zhonghua; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter Chen, Jian; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter Huang, Qiufeng; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter, Key Lab of Optoelectronic Materials Chemistry and Physics Liu, Zhuguang; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter Guo, Wang; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter, Key Laboratory of Optoelectronic Materials Chemistry and Physics Cao, Rong; Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter, State Key Laboratory of Structural Chemistry; Devision of Structural Chemistry, Chinese Academy of Sciences Fujian Institute of Research on the Structure of Matter

ACS Paragon Plus Environment

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


Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen generation Guojing Li a, b, Jiquan Huang a,, Zhonghua Deng a , Jian Chen a, Qiufeng Huang a, Zhuguang Liu a, Wang Guo a, Rong Caoa, c, a Key

Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute

of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China b

University of the Chinese Academy of Sciences, Beijing 100039, People’s Republic

of China c State

Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, People’s Republic of China Abstract The photocatalytic performance and the involved mechanism of copper modified TiO2 have attracted a wide range of scientific interest in recent years. In this study, we fabricate CuOx/TiO2 photocatalysts by depositing Cu, Cu2O, and CuO on the surface area of hierarchical TiO2 arrays using magnetron sputtering method. These CuOx/TiO2 photocatalysts exhibit excellent photocatalytic performance, with H2 generation rate of 30 mmol m-2 h-1 for CuO/TiO2, 37 mmol m-2 h-1 for Cu2O/TiO2 and 53 mmol m-2 h-1 for Cu/TiO2. We further study the composition evolution of CuOx during the photocatalytic reaction process to illustrate the actual active copper species. It is found that CuO is easily photo-reduced to Cu2O and the latter can be further reduced to metallic Cu under irradiation in the solution. In the steady stage of photocatalytic reaction, the Corresponding author. Tel: +86-591-63179098; Fax: +86-591-83721039. Email: [email protected] 

Corresponding author. Tel: +86-591-63173698; Fax: +86-591-63173698. Email: [email protected] 


Page 2 of 18

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


Page 2 of 20

photo-induced TiO2/Cu2O/Cu three-phase system facilitates the separation of photo-generated carriers through the migration of electrons in both TiO2 and Cu2O to metallic Cu. This study demonstrates that the effective copper-based co-catalyst in various CuOx/TiO2 systems exists in the metallic form. KEYWORDS

Anatase titania; CuOx; water splitting; hydrogen generation 1. Introduction The ever-growing human activities are accompanied by the depletion of fossil energy and the deterioration of environment, which has led to a strong demand for the development of alternative clean and renewable energy. In this regard, sunlight is considered to be the most idea energy source because of its abundant reserves, free cost and environmental friendliness.1,

2

However,

there are also difficulties in direct use and bulk storage. Therefore, converting solar energy into an easy-to-use form has represented an interesting strategy in the last few decades.3-9 Among various solar energy converting strategies, utilizing photocatalyst to split water into hydrogen and oxygen without the emission of any byproducts is a hot spot. Such a photocatalyst system requires semiconductor materials that can provide rapid charge transfer to the surface active sites, long-term stability, and efficient harvesting of a wide range of the solar spectrum.10 One of the typical semiconductors is TiO2 which has been highly focused for a long time. Nevertheless, the conversion efficiency for single semiconductor remains low up to now. In this case, co-catalyst modified semiconductor has been put forward to improve the conversion efficiency by effectively separating the photon-generated carriers. The commonly used co-catalysts are noble metals, carbon materials, oxide and nonoxide semiconductors.5, 11-16 As well known, noble metals such as Pt and Au can greatly promote the phtocatalytic activity of TiO2, however, they are expensive. Recently, some studies found that copper species (Cu0, Cu+, and Cu2+, e.g. metallic Cu, Cu2O, CuO, Cu(OH)2 and Cu2(OH)2CO3) can significantly improve the H2 generation rate of TiO2 to a high level that is different to surpass for other non-noble metal-based co-catalysts.13, 17, 18 Driven by the outstanding photocatalytic activity, low cost and good environmental acceptability, copper-based co-catalysts have widely considered as promising alternatives to noble metals.19-22 In spite of this, the nature of the active copper species and the involved mechanism of the high


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


efficiency of copper-incorporated TiO2 have not been fully understood. In the early years, the outstanding performance is attributed to the doped copper in TiO2 lattice sites.23 Later, some studies on CuO/TiO2 ascribe the photoactivity to the formation of CuO-TiO2 p-n heterojunction improving the interfacial charge transfer, while some other researchers suggest that the real active species is Cu2O.24-27 Besides, metallic Cu can also be an effective co-catalyst as reported in literatures.28-30 An earlier work of our group on copper modified TiO2 found that Cu2+ is unstable and will be reduced to Cu+ and even Cu0 during the photocatalytic reaction, and thereby proposed that the excellent photocatalytic performance of CuOx (CuO, Cu2O or Cu) modified TiO2 origins from the formation of Cu2O/TiO2 and TiO2/Cu heterojunctions.31 This hypothesis has been confirmed by some subsequent studies on Cu2(OH)2CO3/TiO2 and Cu2O (or Cu2+)/TiO2.32, 33 Recently, we constructed a TiO2 array composed of feather-like bundles on silicon substrate, and found that this hierarchical structure was very beneficial to improve the hydrogen generation efficiency of TiO2 array.34 Herein, we further decorate the above TiO2 array with Cu, Cu2O, and CuO, respectively, and realize an ultra-high H2 generation rate up to 53 mmol m-2 h-1 (which is nearly 4 times that of pure TiO2 array, and 4000 times that of Degussa P25).35 Further mechanism study suggests that Cu2+ is easily reduced to Cu+ and the latter can be further reduced to Cu0 under light irradiation. This observation clearly indicates that the mainly responsible species for the outstanding hydrogen generation performance is metallic Cu obtained by photo-reduction.

2. Experimental 2.1. Catalysts preparation A conventional reactive magnetron sputtering method was employed to synthesize CuOx/TiO2 array catalysts using a RF magnetron sputtering system (Model JGP520D). The Si wafer was supplied as the deposition substrate. After cleaned ultrasonically in deionized water, acetone and alcohol, Si wafer was ready to be used. In a typical procedure, titanium target of 99.99% purity, copper target of 99.99% purity, Si wafer and sample stage were positioned and the system was vacuumed until the base pressure of the system reached 3.0 × 10-3 Pa. Then 40 sccm argon and 5 sccm oxygen was introduced to the system through a flowmeter while argon was acted as working atmosphere and oxygen was served as reaction gas. The deposition temperature, sample stage rotation and Ti sputtering rate were set as 550 oC, 10 rpm and 130 W respectively.


Page 4 of 18

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


After depositing for 12 h, TiO2 arrays were obtained. Subsequently, CuOx co-catalyst was deposited on the surface area of the as-synthesized TiO2 arrays by sputtering copper target for 2 min. During the deposition of CuOx, the flow rate of oxygen was controlled to obtain various Cu species, i.e., 0, 2, and 100 sccm for the fabrication of Cu/TiO2, Cu2O/TiO2, and CuO/TiO2, respectively. Moreover, the sputtering rate of Cu, the flow rate of argon, and the deposition temperature was set at 100 W, 40 sccm, and 550 oC, respectively. The schematic deposition procedures were showed in Scheme S1.

2.2. Catalyst characterization X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a Rigaku Miniflex 600 equipped with a radiation source of Cu Kα (λ = 1.5418 Å). XRD patterns of the photocatalysts were performed with an accelerating voltage of 40 kV and a current of 30 mA. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM 2010 operated at 200 keV. TEM sample was scraped off the silicon substrate to the Mo grid. The surface analysis was determined by X-ray photoelectron spectroscopy and auger electron spectroscopy (XPS and AES, ESCALAB 250, Thermo Scientific, USA) using an Al-Kα source. The efficiency of electron-cavity separation was evaluated by photoluminescence spectra (PL, FLS1000, Edingburgh, England) with various slit width (1 nm for TiO2 and 12 nm for CuOx/TiO2 photocatalysts).

2.3. Photocatalytic activity testing The photocatalytic activity of the CuOx/TiO2 catalysts was evaluated by monitoring H2 evolution from water splitting reaction in the presence of methanol. Methanol was employed as the sacrificial reagent to quench the hole during the reaction. Overall water splitting reactions were carried out using a piece of CuOx/TiO2 catalyst in an aqueous methanol solution (10 vol.% CH3OH, 100 mL).The corresponding photocatalytic reaction was conducted on an Online Photocatalytic Activity Evaluation System (CEL-SPH2N, AULTT, China). The reactant solution was evacuated for about one hour by a mechanical pump to remove air prior to irradiation under a 300 W xenon lamp (300 nm ≤ λ ≤ 2500 nm). Constant temperature of the reactant solution was


Page 4 of 20

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


maintained at 10 °C by a flow of cooling water during the reaction. The evolved H2 were analyzed by an online gas chromatograph (GC, TCD detector, and N2 carrier).

3. Results and discussion

Figure 1. XRD patterns of CuOx/TiO2 photocatalysts. Figure 1 shows the XRD patterns of the CuOx/TiO2 photocatalysts. It is found that the copper species transform from metallic Cu to Cu2O and then to CuO with increasing O2 flow rate during deposition, demonstrated by the fact that CuOx species correspond well to the standard X-ray diffraction pattern of Cu, Cu2O and CuO respectively. Besides the diffraction peaks of copper species, for all these CuOx/TiO2 photocatalysts, a strong reflection at 38.6o and five weak ones at 25.3o, 37.8o, 48.0o, 53.9o and 55.1o were also observed, which were assign to (112), (101), (004), (200), (105), (211) lattice planes of anatase TiO2 (JCPDS 21-1272), respectively. In addition, the strong (112) reflection indicates a preferred orientation which has been discussed in our previous papers.34


Page 6 of 18

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


Figure 2. HRTEM images and corresponding STEM-EDS elemental mapping images.

Figure 3. Morphology and structure characterization of CuO/TiO2 catalyst. TEM image (a); HRTEM images of the top region (c) and the bulk district (d) of the CuO/TiO2 catalyst.

The morphology and elements distribution of CuO/TiO2 are characterized by TEM, as shown in Figure 2 and Figure 3. Herein, only CuO/TiO2 is presented because all the CuOx/TiO2 catalysts have the similar morphologies (these catalysts are prepared by the same procedure and with the same experimental conditions except for the O2 flow rate). The STEM-EDS observation confirms the presence of copper species, as shown in Figure 2f. Elemental mapping images of Ti, O and Cu (Figure 2b-e) further demonstrate that CuO particles are anchored on the top of the TiO2 arrays. The array morphology of CuO/TiO2 catalyst shown in Figure 3a and Figure S1 can be illustrated


Page 6 of 20

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


Page 8 of 18

by Figure 3b for easy understanding. The top region and bulk district of the CuO/TiO2 catalyst are magnified to analyze the structural features. As shown in lattice images Figure 3c and Figure 3d, fringes with spacing of ca. 0.27 nm and ca. 0.35 nm correspond to (110) plane of monoclinic-phase CuO and (101) plane of tetragonal-phase anatase TiO2 perfectly. 36-38

Figure 4. High resolution XPS spectra of CuOx/TiO2 photocatalysts before (a-c) and after (d-f) irradiation. For further understanding the surface compositional features and the electronic structure of the CuOx/TiO2 photocatalysts, XPS is employed. Figure 4a and Figure 4d show the high resolution Ti 2p spectra of the photocatalysts before and after photocatalytic reaction, respectively. The peaks centered at 458.5 eV and 464.2 eV are corresponding to the Ti4+ 2p1/2 and Ti4+ 2p3/2 spin– orbital splitting. None of the catalysts shows significant binding energy peak shift or shoulder after irradiation, suggesting the decent photostability of TiO2.39,

40

Figure 4b shows the high

resolution Cu 2p spectra for the photocatalysts before photocatalytic reaction. Binding energy peaks located at 933.8 eV and 953.7 eV for CuO/TiO2 are nicely corresponded to Cu2+, which can be further demonstrated by the obvious shake-up satellite.

41, 42

Similarly, for Cu/TiO2 and

Cu2O/TiO2, the binding energy peaks of Cu 2p3/2 and Cu 2p1/2 are found to be located at 932.6 eV and 952.3 eV respectively, which can be assigned to Cu+ or Cu0. To further identify the Cu+ and Cu0 in CuOx/TiO2 photocatalysts, AES is carried out. The signals appeared at 916.8, 917.7 and 918.6 eV in the AES spectra (Figure 4c) are attributed to Cu+, Cu2+ and Cu0 respectively.43-45 Therefore, the oxidation state of surface Cu is Cu2+ for CuO/TiO2 and Cu+ for Cu2O/TiO2, while


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


metal Cu and Cu+ co-existed in Cu/TiO2. Possible reason for the presence of Cu+ in Cu/TiO2 catalyst is the partial oxidation of the superficial Cu nanoparticles when exposed to air. Surface Cu species evolution after photocatalytic reaction is also studied, as shown in Figure 4e and 4f. After irradiation, the Cu 2p3/2 peaks are centered at 932.6 eV (Cu+ or Cu0) for all the catalysts, along with the disappearing of the characterized peak and distinctive satellite peaks for Cu2+. Meanwhile, Cu LMM auger spectra exhibit two peaks at 916.8 eV (Cu+) and 918.6 eV (Cu0) respectively for all samples, and the LMM auger peak for Cu0 was greatly enhanced after irradiation. All these results manifest that Cu2+ is completely reduced to Cu+ while Cu+ is partially reduced to Cu0.

Figure 5. PL (a) and PLE (b) spectra of CuOx/TiO2 catalysts. The effective separation of photo-generated electron-hole is of vital importance for photocatalysts. PL spectra are carried out to investigate the recombination efficiency of electron-hole pairs on the TiO2 and CuOx/TiO2 semiconductor during the photocatalytic process. Figure 5 shows that the emission intensity at 442 nm for TiO2 is far stronger than that of CuOx/TiO2 catalysts even under the condition of improving the slit width from 1 nm for TiO2 to 12 nm for CuOx/TiO2. It is manifest that electron-hole recombination can be effectively suppressed by introducing CuOx. For Cu/TiO2, the photo-generated electrons from the conduction band (CB) of TiO2 can be injected easily into Cu co-catalyst due to the higher electronic work function of Cu. For CuO/TiO2 and Cu2O/TiO2, the formation of p-n heterojuction facilitates the electron-hole separation.27, 46, 47 Effective separation of charge carrier ensures high photocatalytic performance.


Page 8 of 20

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


Figure 6. H2 yield from methanol aqueous solution over CuOx/TiO2 catalyst. Figure 6 shows the hydrogen generation from an aqueous methanol solution of the CuOx/TiO2 photocatalysts. The pristine TiO2 exhibits stable hydrogen production activity, as evidenced by the linearity of the plots. However, CuOx co-catalyst assisted TiO2 photocatalysts displays two-stage nature of the plots, which is consistent with previous report.32, 48 In the initial few hours of light irradiation, the hydrogen generation rate is relatively low. Especially, the hydrogen generation rates for CuO/TiO2 and Cu2O/TiO2 are even lower than that of pristine TiO2, implying that CuO and Cu2O may not be effective co-catalysts for improving the hydrogen generation performance of TiO2. As characterizing by XPS and AES analysis, Cu2+ is reduced to Cu+ and Cu+ is further reduced to Cu0 by photo-generated electrons when the catalyst is subjected to intensive irradiation. This Cu species reduction process competes with the water splitting reaction for photoinduced electrons, and thereby leading to the low hydrogen generation rate in the initial stage. After several hours of illumination, the photo-reduction of copper species is completed (reached equilibrium) and no longer consumes the photo-generated electrons. Consequently, a higher and stable (Figure S3) hydrogen generation rate is achieved. As shown in Figure 4, for all the samples, Cu2+ is completely eliminated while Cu+ is partially reduced to Cu0 during the photocatalytic reaction, and the ratio of Cu0/Cu+ increases in the order of CuO/TiO2 < Cu2O/TiO2 < Cu/TiO2. It is noticeable that the hydrogen generation rate is also increasing in the same order. The stable hydrogen generation rates are 30, 37 and 53 mmol m-2 h-1 for CuO/TiO2, Cu2O/TiO2 and Cu/TiO2, respectively. Obviously, the hydrogen production efficiency is positively correlated with Cu content (Cu0/Cu+). In other words, the enhanced photocatalytic performance can be mainly attributed to the metallic Cu co-catalyst. Based on the aforementioned experimental


Page 10 of 18

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


results and discussion, the mechanism of improved hydrogen generation is tentatively proposed, and a schematic model is presented in Figure 7. According to the XPS results, in the steady stage (i.e., after being irradiated for several hours), Cu2+ is absent, and the real compositions of the CuOx/TiO2 photocatalysts are TiO2/Cu2O/Cu (with different Cu2O/Cu ratio). These three phases are in contact with each other. The photo-generated electrons in both TiO2 and Cu2O can easily migrate to metallic Cu due to high work function of Cu, and react with water to generate H2 there. Meanwhile, the holes migrate to the surface of the semiconductors and are consumed by the methanol sacrificial agent. On the basis of photocatalytic hydrogen generation experiments, the presence of metallic Cu can facilitate the separation of photo-generated electrons and holes, and efficiently enhance the photocatalytic activity. As shown in Figure 6, the hydrogen generation rate for Cu/TiO2 array is as high as 53 mmol m-2 h-1. For comparison, the hydrogen generation rate is 14 mmol m-2 h-1 for pristine TiO2 array, and 0.014 mmol m-2 h-1 for P25 film (with a similar thickness).35

Figure 7. Schematic photocatalytic reaction mechanism of CuOx/TiO2 catalyst.

4. Conclusions In summary, surface modification of TiO2 arrays with CuOx (Cu, Cu2O, and CuO) have been constructed by a simple RF reactive magnetron sputtering method. The synthesized CuOx/TiO2 catalysts exhibit outstanding photocatalytic performance that depends greatly on the copper species. The stable H2 generation rates in methanol solution are 14, 30, 37 and 53 mmol m-2 h-1 for pristine TiO2, CuO/TiO2, Cu2O/TiO2 and Cu/TiO2, respectively. XPS studies suggest that during the photocatalytic reaction process, CuO is unstable and complete photo-reduction, analogously, Cu2O also suffers partial reduction. As a result, the real composition of the CuOx/TiO2 catalyst in


Page 10 of 20

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


the steady stage of photocatalytic reaction is TiO2/Cu2O/Cu three-phase system. Photocatalytic study shows that the H2 generation rate increases with increased Cu/Cu2O ratio, and thereby indicates the metallic copper is the key factor that in charge of the improved photocatalytic activity of copper species modified TiO2 photocatalysts. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx. Schematic deposition process of CuOx/TiO2 arrays; SEM image of CuO/TiO2 catalyst; XRD pattern of catalysts after the photocatalytic reaction; photocatalytic stability of Cu2O/TiO2 catalyst. Corresponding Authors Email: [email protected]. Tel/Fax: +86-591-83721039. Email: [email protected]. Tel/Fax: +86-591-63173698. ORCID Jiquan Huang: 0000-0002-3983-3400 Rong Cao: 0000-0002-2398-399X

Acknowledgments This work was supported by the National Key R & D Program of China (2016YFB0701003), NSFC (21521061, 21331006), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

Reference (1) Wang, G. M.; Ling, Y. C.; Wang, H. Y.; Lu, X. H.; Li, Y., Chemically modified nanostructures for photoelectrochemical water splitting. J. Photoch. Photobio. C 2014, 19, 35-51. (2) Xing, Z.; Zong, X.; Pan, J.; Wang, L. Z., On the engineering part of solar hydrogen production from water splitting: Photoreactor design. Chem. Eng. Sci. 2013, 104, 125-146. (3) Zhang, W.; Chen, H.; Zhang, L. N.; Zhang, P.; Dong, E. L.; Ma, J. W.; Wang, G. Q., Design


Page 12 of 18

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


and fabrication of an alpha-Fe2O3/TiO2/Si 3D hierarchical photoanode for improved photoelectrochemical water splitting. J. Alloys Compd. 2019, 773, 597-604. (4) Wang, Y.; Zhang, Z.; Zhang, L.; Luo, Z.; Shen, J.; Lin, H.; Long, J.; Wu, J. C. S.; Fu, X.; Wang, X.; Li, C., Visible-light driven overall conversion of CO2 and H2O to CH4 and O2 on 3D-SiC@2D-MoS2 heterostructure. J. Am. Chem. Soc. 2018, 140, 14595-14598. (5) Liu, Q.; Mo, R.; Li, X. L.; Yang, S.; Zhong, J. X.; Li, H. X., Cobalt phosphate modified 3D TiO2/BiVO4 composite inverse opals photoanode for enhanced photoelectrochemical water splitting. Appl. Surf. Sci. 2019, 464, 544-551. (6) Wang, S.; Guan, B. Y.; Lu, Y.; Lou, X. W. D., Formation of hierarchical In2S3-CdIn2S4 heterostructured nanotubes for efficient and stable visible light CO2 reduction. J. Am. Chem. Soc. 2017, 139, 17305-17308. (7) Sheridan, M. V.; Hill, D. J.; Sherman, B. D.; Wang, D.; Marquard, S. L.; Wee, K. R.; Cahoon, J. F.; Meyer, T. J., All-in-one derivatized tandem p+ n-Silicon-SnO2/TiO2 water splitting photoelectrochemical cell. Nano Lett. 2017, 17, 2440-2446. (8) Wang, D.; Pierre, A.; Kibria, M. G.; Cui, K.; Han, X.; Bevan, K. H.; Guo, H.; Paradis, S.; Hakima, A. R.; Mi, Z., Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy. Nano Lett. 2011, 11, 2353-7. (9) Konta, R.; Ishii, T.; Kato, H.; Kudo, A., Photocatalytic activities of noble metal ion doped SrTiO3 under visible light irradiation. J. Phys. Chem. B 2004, 108, 8992-8995. (10) Chen, H. M.; Chen, C. K.; Liu, R. S.; Zhang, L.; Zhang, J.; Wilkinson, D. P., Nano-architecture and material designs for water splitting photoelectrodes. Chem. Soc. Rev. 2012, 41, 5654-71. (11) Dahl, M.; Liu, Y.; Yin, Y., Composite titanium dioxide nanomaterials. Chem. Rev. 2014, 114, 9853-89. (12) Yu, H. J.; Zhao, Y. F.; Zhou, C.; Shang, L.; Peng, Y.; Cao, Y. H.; Wu, L. Z.; Tung, C. H.; Zhang, T. R., Carbon quantum dots/TiO2 composites for efficient photocatalytic hydrogen evolution. J. Mater. Chem. A 2014, 2, 3344-3351. (13) Xu, H.; Dai, D.; Li, S.; Ge, L.; Gao, Y., In situ synthesis of novel Cu2CO3(OH)2 decorated 2D TiO2 nanosheets with efficient photocatalytic H2 evolution activity. Dalton Trans. 2018, 47, 348-356.


Page 12 of 20

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


(14) Nguyen, C.-C.; Nguyen, D. T.; Do, T.-O., A novel route to synthesize C/Pt/TiO2 phase tunable anatase–Rutile TiO2 for efficient sunlight-driven photocatalytic applications. Appl. Catal., B 2018, 226, 46-52. (15) Liu, C.; Wang, F.; Zhang, J.; Wang, K.; Qiu, Y.; Liang, Q.; Chen, Z., Efficient photoelectrochemical water splitting by g-C3N4/TiO2 nanotube array heterostructures. Nano-micro Lett. 2018, 10, 37. (16) Zhang, J.; Yu, Z.; Gao, Z.; Ge, H.; Zhao, S.; Chen, C.; Chen, S.; Tong, X.; Wang, M.; Zheng, Z.; Qin, Y., Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx cocatalysts produced by atomic layer deposition for photocatalytic hydrogen production. Angew. Chem. Int. Ed. Engl. 2017, 56, 816-820. (17) Chen, S.; Li, X.; Zhou, W.; Zhang, S.; Fang, Y., Carbon-coated Cu-TiO2 nanocomposite with enhanced photostability and photocatalytic activity. Appl. Surf. Sci.2019, 466, 254-261. (18) Trofimovaite, R.; Parlett, C. M. A.; Kumar, S.; Frattini, L.; Isaacs, M. A.; Wilson, K.; Olivi, L.; Coulson, B.; Debgupta, J.; Douthwaite, R. E.; Lee, A. F., Single atom Cu(I) promoted mesoporous titanias for photocatalytic methyl orange depollution and H2 production. Appl. Catal. B-Environ. 2018, 232, 501-511. (19) Li, G.; Dimitrijevic, N. M.; Chen, L.; Rajh, T.; Gray, K. A., Role of surface/interfacial Cu2+ sites in the photocatalytic activity of coupled CuO−TiO2 nanocomposites. J. Phys. Chem. C 2008, 112, 19040-19044. (20) Lalitha, K.; Sadanandam, G.; Kumari, V. D.; Subrahmanyam, M.; Sreedhar, B.; Hebalkar, N. Y., Highly stabilized and finely dispersed Cu2O/TiO2: A promising visible sensitive photocatalyst for continuous production of hydrogen from glycerol:Water mixtures. J. Phys. Chem. C 2010, 114, 22181-22189. (21) Lei, X. W.; Yue, C. Y.; Wei, J. C.; Li, R. Q.; Mi, F. Q.; Li, Y.; Gao, L.; Liu, Q. X., Novel 3D semiconducting open-frameworks based on cuprous bromides with visible light driven photocatalytic properties. Chemistry 2017, 23, 14547-14553. (22) Yue, C. Y.; Hu, B.; Lei, X. W.; Li, R. Q.; Mi, F. Q.; Gao, H.; Li, Y.; Wu, F.; Wang, C. L.; Lin, N., Novel three-dimensional semiconducting materials based on Hybrid d10 transition metal halogenides as visible light-driven photocatalysts. Inorg. Chem. 2017, 56, 10962-10970. (23) Nishikiori, H.; Sato, T.; Kubota, S.; Tanaka, N.; Shimizu, Y.; Fujii, T., Preparation of


Page 14 of 18

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


Cu-doped TiO2 via refluxing of alkoxide solution and its photocatalytic properties. Res. Chem. Intermediat. 2012, 38, 595-613. (24) Cheng, Y. Q.; Gao, X.; Zhang, X. X.; Su, J. X.; Wang, G. Q.; Wang, L. Y., Synthesis of a TiO2-Cu2O composite catalyst with enhanced visible light photocatalytic activity for gas-phase toluene. New J. Chem. 2018, 42, 9252-9259. (25) Xie, Y.; Yin, Y. L.; Zeng, S. H.; Gao, M. Y.; Su, H. Q., Coexistence of Cu+ and Cu2+ in star-shaped CeO2/CuxO catalyst for preferential CO oxidation. Catal. Commun. 2017, 99, 110-114. (26) Bharad, P. A.; Nikam, A. V.; Thomas, F.; Gopinath, C. S., CuOx-TiO2 composites: Electronically integrated nanocomposites for solar hydrogen generation. Chemistryselect 2018, 3, 12022-12030. (27) Liao, Y.; Deng, P.; Wang, X.; Zhang, D.; Li, F.; Yang, Q.; Zhang, H.; Zhong, Z., A facile method for preparation of Cu2O-TiO2 NTA heterojunction with visible-photocatalytic activity. Nanoscale Res. Lett. 2018, 13, 221. (28) Reddy, N. L.; Kumar, S.; Krishnan, V.; Sathish, M.; Shankar, M. V., Multifunctional Cu/Ag quantum dots on TiO2 nanotubes as highly efficient photocatalysts for enhanced solar hydrogen evolution. J. Catal. 2017, 350, 226-239. (29) Kainthla, I.; Gurram, V. R. B.; Bhanushali, J. T.; Kamaraju, S. R. R.; Keri, R. S.; Gosavi, S. W.; Jadhav, A. H.; Nagaraja, B. M., In Situ Generation of Cu0 Supported on TiO2 Aerogel as a Catalyst for the Vapour Phase Hydrogenation of Nitrobenzene to Aniline. Catal. Lett. 2018, 148, 2891-2900. (30) Maldonado, M. I.; Lopez-Martin, A.; Colon, G.; Peral, J.; Martinez-Costa, J. I.; Malato, S., Solar pilot plant scale hydrogen generation by irradiation of Cu/TiO2 composites in presence of sacrificial electron donors. Appl. Catal. B-Environ. 2018, 229, 15-23. (31) Hu, Q.; Huang, J.; Li, G.; Jiang, Y.; Lan, H.; Guo, W.; Cao, Y., Origin of the improved photocatalytic activity of Cu incorporated TiO2 for hydrogen generation from water. Appl. Surf. Sci. 2016, 382, 170-177. (32) Polliotto, V.; Livraghi, S.; Krukowska, A.; Dozzi, M. V.; Zaleska-Medynska, A.; Selli, E.; Giamello, E., Copper-modified TiO2 and ZrTiO4: Cu oxidation state evolution during photocatalytic hydrogen production. ACS Appl. Mater. Interfaces 2018, 10, 27745-27756.


Page 14 of 20

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


(33) He, Z.; Fu, J.; Cheng, B.; Yu, J.; Cao, S., Cu2(OH)2CO3 clusters: Novel noble-metal-free cocatalysts for efficient photocatalytic hydrogen production from water splitting. Appl. Catal. B-Environ. 2017, 205, 104-111. (34) Li, G.; Huang, J.; Xue, C.; Chen, J.; Deng, Z.; Huang, Q.; Liu, Z.; Gong, C.; Guo, W.; Cao, R., Facile Synthesis of oriented feather-like TiO2 bundle catalysts for efficient photocatalytic water splitting. Cryst. Growth Des. 2019, 19, 3584-3591. (35) Hu, Q.; Li, J.; Li, Q.; Wang, G.; Huang, J.; Huang, X., One-step synthesis of nonstoichiometric TiO2 nanorod films for enhanced photocatalytic H2 evolution. Dalton Trans. 2018, 47, 4478-4485. (36) Hu, L.; Xia, Y. T.; Wang, Q. Q.; Yang, H.; Zhang, Q. L., Core-shell structured dendritic CuO@TiO2 for high-k P(VDF-HFP) composites with suppressed dielectric loss and enhanced thermal conductivity. J. Mater. Sci.-Mater. El. 2018, 29, 1269-1279. (37) Shi, L.; Fan, C.; Sun, C.; Ren, Z.; Fu, X.; Qian, G.; Wang, Z., Synthesis of different CuO nanostructures from Cu(OH)2 nanorods through changing drying medium for lithium-ion battery anodes. Rsc. Adv. 2015, 5, 28611-28618. (38) Shen, L.; Xu, C.; Qi, X.; Cao, Y.; Tang, J.; Zheng, Y.; Jiang, L., Highly efficient CuxO/TiO2 catalysts: controllable dispersion and isolation of metal active species. Dalton Trans. 2016, 45, 4491-5. (39) Li, G.; Huang, J.; Chen, J.; Deng, Z.; Huang, Q.; Liu, Z.; Guo, W.; Cao, R., Highly active photocatalyst of Cu2O/TiO2 octahedron for hydrogen generation. ACS Omega 2019, 4, 3392-3397. (40) Patel, N.; Jaiswal, R.; Warang, T.; Scarduelli, G.; Dashora, A.; Ahuja, B. L.; Kothari, D. C.; Miotello, A., Efficient photocatalytic degradation of organic water pollutants using V-N-codoped TiO2 thin films. Appl. Catal. B-Environ. 2014, 150, 74-81. (41) Brookshier, M. A.; Chusuei, C. C.; Goodman, D. W., Control of CuO particle size on SiO2 by spin coating. Langmuir 1999, 15, 2043-2046. (42) Navas, J.; Sanchez-Coronilla, A.; Aguilar, T.; Hernandez, N. C.; de los Santos, D. M.; Sanchez-Marquez, J.; Zorrilla, D.; Fernandez-Lorenzo, C.; Alcantara, R.; Martin-Calleja, J., Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO2. Phys. Chem. Chem. Phys. 2014, 16, 3835-45. (43) Zhen, W.; Jiao, W.; Wu, Y.; Jing, H.; Lu, G., The role of a metallic copper interlayer during


Page 16 of 18

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


visible photocatalytic hydrogen generation over a Cu/Cu2O/Cu/TiO2 catalyst. Catal. Sci. Technol. 2017, 7, 5028-5037. (44) Schön, G., ESCA studies of Cu, Cu2O and CuO. Surface Science 1973, 35, 96-108. (45) Young, K. T.; Phillips, S. S.; Coley, J. T. T.; Perini, C. J.; Hitchcock, D. A.; Serkiz, S. M.; Vogel, E. M., The impact of defect density, grain size, and Cu orientation on thermal oxidation of graphene-coated Cu. Appl. Surf. Sci.2019, 478, 959-968. (46) Hou, H.; Shang, M.; Gao, F.; Wang, L.; Liu, Q.; Zheng, J.; Yang, Z.; Yang, W., Highly efficient photocatalytic hydrogen evolution in ternary hybrid TiO2/CuO/Cu thoroughly mesoporous nanofibers. ACS Appl. Mater. Interfaces 2016, 8, 20128-37. (47) Chen, S.; Qi, Y.; Li, C.; Domen, K.; Zhang, F., Surface strategies for particulate photocatalysts toward artificial photosynthesis. Joule 2018, 2, 2260-2288. (48) Valero, J. M.; Obregón, S.; Colón, G., Active site considerations on the photocatalytic H2 evolution performance of Cu-doped TiO2 obtained by different doping methods. Acs Catal. 2014, 4, 3320-3329.


Page 16 of 20

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


For Table of Contents Use Only Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen generation Guojing Li a, b, Jiquan Huang a, *, Zhonghua Deng a , Jian Chen a, Qiufeng Huang a, Zhuguang Liu a, Wang Guo a, Rong Caoa, c, *

Unique CuOx/TiO2 catalysts were constructed by surface modifying TiO2 arrays with CuOx co-catalysts and the synthesized CuOx/TiO2 catalysts exhibited ultra-high H2 generation activities. Further exploration manifested that photo-reduction occurred during the photocatalytic process, meanwhile, the reduced Cu acted as the active site could enhance the photocatalytic performance of CuOx/TiO2 to a great extent.


Page 18 of 18

Highly active photocatalyst of CuOx modified TiO2 arrays for hydrogen

Recommend Documents