TiO2-Au-Cu2O Photocathodes: Au-Mediated Z-Scheme Charge

Subscriber access provided by Kaohsiung Medical University

Article

TiO2-Au-Cu2O Photocathodes: Au-Mediated Z-Scheme Charge Transfer for Efficient Solar-Driven Photoelectrochemical Reduction Jing-Mei Li, Chun-Wen Tsao, Mei-Jing Fang, Chun-Chi Chen, Chen-Wei Liu, and Yung-Jung Hsu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01678 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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.

Page 1 of 33 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

ACS Applied Nano Materials

TiO2-Au-Cu2O Photocathodes: Au-Mediated ZScheme Charge Transfer for Efficient Solar-Driven Photoelectrochemical Reduction Jing-Mei Li,a,b Chun-Wen Tsao,a Mei-Jing Fang,a Chun-Chi Chen,c Chen-Wei Liu,c Yung-Jung Hsua,d,* a

Department of Materials Science and Engineering, National Chiao Tung University, 1001

University Road, Hsinchu 30010, Taiwan b

Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue,

Columbus, Ohio 43210, United States c

Green Energy and Environment Research Laboratories, Industrial Technology Research

Institute, 195 Chung Hsing Road, Hsinchu 31040, Taiwan d

Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu

30010, Taiwan

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 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 33

Abstract

An Au-mediated Cu2O-based Z-scheme heterostructure system is demonstrated for use as efficient photocathodes in photoelectrochemical (PEC) reduction. The samples are prepared by electrodepositing a Cu2O layer on the surface of Au particle-coated TiO2 nanorods. For TiO2-AuCu2O, the embedded Au particles function as a charge transfer mediator to enhance the electron transportation from the conduction band of TiO2 to the valence band of Cu2O. Such a vectorial charge transfer leads to the concentration of electrons at the conduction band of Cu2O and the collection of holes at the valence band of TiO2, providing TiO2-Au-Cu2O with substantially high redox abilities for reduction applications. Time-resolved photoluminescence spectra and electrochemical impedance spectroscopy analysis suggest that interfacial charge transfer is significantly improved because of the Au-mediated Z-scheme charge transfer mechanism. By virtue of the high redox ability and improved interfacial charge transfer, TiO2-Au-Cu2O performs much better as a photocathode in H2 production and CO2 reduction than pure Cu2O and binary TiO2-supported Cu2O do. Remarkably, the photocurrent density of TiO2-Au-Cu2O toward PEC CO2 reduction can reach as high as -1.82 mA/cm2 at +0.11 V vs. RHE. The incident photon-tocurrent conversion efficiency data manifest that TiO2-Au-Cu2O surpasses both pure Cu2O and binary TiO2-supported Cu2O in PEC reduction across the whole photoactive region. The current study paves a valuable approach of devising Z-scheme photocathode for the construction of sophisticated artificial photosynthesis systems capable of solar-to-fuel conversion.

Keywords: photoelectrochemical reduction, Z-scheme, photocathode, Cu2O, Au-mediated


Page 3 of 33 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


1. Introduction Finding a practical long-term solution to the problem of ever-increasing global energy demand is one of the greatest challenges in the 21st century.1 Photocatalytic conversion of undesirable substances to valuable chemcial fuels using solar energy is one of the best solutions to both the fossil fuel shortage and global warming. The idea is based on the replication of natural photosynthetic processes that convert water to oxygen and useful carbohydrates. This brilliant approach is called artificial photosynthesis and has been consistently gaining attention for more than 30 years. Photoelectrochemical (PEC) cell combines the photocatalytic process with electrochemical technology to catalyze the desired redox reactions with the aid of external bias, which offers a feasible approach to the realization of artificial photosynthesis. Tremendous efforts have been devoted to seeking suitable photoelectrodes for carrying out the respective oxdiation and reduction reactions in order to construct a sophisticated PEC cell operated in a tandam configuration. For PEC oxidation reactions, a variety of n-type semiconductor photoanodes such as TiO2,2-4 ZnO,5-6 CdS,7-9 and BiVO410 have been proposed and proven effective. The develpment of p-type semiconductor photocathodes for PEC reduction on the other hand has been sluggish. To achieve practical PEC reduction, the semiconductor photocathodes must possess (i) sufficient reducing ability, meaning that the conduction band level should be cathodically high enough to drive reduction reaction, (ii) excellent light absorption to utilize the entire solar irradiation spectrum, and (iii) pronounced charge separation to maximize the carrier utilization efficiency. However, it is difficult for a single-component semiconductor to fulfill all the criteria. Therefore, semiconductor heterostructures comprising multiple components integrated into unique architectures have been devised and employed. Typical examples are type-II semiconductor/semiconductor11 and metal/semiconductor



Page 4 of 33

heterostructures,12 in which the charge carriers are spatially separated at the interface resulting in suppressed charge recombination and, thereby, enhanced photoconversion efficiency.11,13-18 Although the PEC reduction performance is improved by means of the heterostructure design, the reducing ability is usually sacrificed because of the descending electron transfer, especially for type-II semiconductor/semiconductor heterostructures.19 Additional energy, such as an external bias, is therefore required for typical semiconductor heterostructures to conduct PEC reduction reactions with a satisfactory efficiency. To achieve pronounced charge separation yet still retain high redox ability, scientists have learned from nature by introducing the Z-scheme charge transfer mechanism into the heterostructure design. Natural green plants contain two photosynthetic reaction centers, PS-I and PS-II, which work cooperatively and sequentially to absorb photons and convey their energy in an electron transfer chain reaction. The electron flow is initiated in PS-II, the electron acceptor, and then proceeds to PS-I, the electron donor. In PS-II, electrons are provided through the splitting of water into O2, while PS-I is responsible for delivering these electrons to produce high-energy carriers. The whole charge transfer process is termed Z-scheme which links two photosystems in a cooperative manner. To realize the Z-scheme charge transfer scenario, noble metal particles or reduced graphene oxides have been used as electron transfer mediators, which triggers the electron transfer chain reactions at the interface of two neighboring semiconductors nanostructures.19-24 As a representative example, ZnO-Au-SnO2 was utilized as an efficient Zscheme photoanode to split water into O2 5. For ZnO-Au-SnO2, the Au nanoparticles acted as an electron transfer bridge to enable electron transfer from SnO2, through Au, and then to ZnO. This artificial Z-scheme charge transfer generated superior charge separation in which the electrons and holes remained on ZnO and SnO2, respectively, providing ZnO-Au-SnO2 photoanode with


Page 5 of 33 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


sufficiently high oxidizing ability to boost PEC water oxidation. On the other hand, direct Zscheme charge transfer without the use of an electron transfer mediator has also been demonstrated in binary semiconductor heterostructures, such as ZnO-CdS,7 α-Fe2O3-Cu2O,25 PBiVO4,26 and BiVO4-SrTiO3.27 However, the effectiveness of charge separation and the redox capability were inferior to those attained by the three-component metal particle-mediated Zscheme heterostructures. There have been many examples of semiconductor heterostructures with an artificial Zscheme mechanism.20-21,25-29 Most of them have focused on employing powder-form photocatalysts, and relatively few efforts have been made on the design of Z-scheme photoelectrodes, especially those for conducting PEC reduction reactions. The development of Z-scheme photoelectrodes has been hindered by many technical limitations. For example, the difficulty of precise controls over the microstructure features at each component may arouse concerns regarding the effectiveness of Z-scheme charge transfer. Particularly, the validness of Z-scheme mechanism has been controversial due to the lack of direct evidence. How to design and construct a well-defined Z-scheme heterostructure system with verified, reliable charge transfer mechanism is therefore the key to pushing forward the practice of Z-scheme photoelectrodes. In fact, the charge transfer scenario of the Z-scheme mechanism is quite favorable for designing photocathodes for PEC reduction because the reduction kinetics can be significantly improved as a result of the substantially high reducing ability. Hence, the development of photocathodes with an artificial Z-scheme mechanism is practically essential. Regarding PEC reduction, p-type Cu2O has a favorable band structure with a suitable bandgap energy (Eg = 1.9~2.5 eV)30-32 and a conduction band edge (ECB = -1.04 V vs. NHE)32 that is more negative than the potentials for most of the technologically important reductions, such as H2



Page 6 of 33

production (EH+/H2 = 0 V vs. NHE) and CO2 reduction (ECO2/CO = -0.10 V vs. NHE; ECO2/HCOOH = -0.17 V vs. NHE).33 Particularly, Cu2O has demonstrated outstanding selectivity for CO2 reduction over other concurrent redox reactions under light illumination in CO2-saturated electrolyte,34 making Cu2O an ideal photoelectrode platform to realize practical PEC CO2 reduction.11,34-37 However, the prevalence of charge recombination caused by the short diffusion length of the minority carriers restricts the utility of Cu2O in the PEC applications.38 Herein we reported the demonstration of TiO2-Au-supported Cu2O heterostructures (denoted as TiO2-AuCu2O) as the Z-scheme photocathode for efficient PEC reduction reactions. The heterostructures were prepared by electrodepositing a Cu2O layer on the surface of Au particle-coated TiO2 nanorods. For TiO2-Au-Cu2O, the embedded Au had a Fermi level (+0.60 V vs. NHE) substantially lower than the conduction band level of TiO2 (-0.47 V vs. NHE),32 which promoted photoexcited electron transfer from TiO2 to Au. The accumulated electrons at Au further induced the recombination with the photogenerated holes at the valence band of Cu2O (+0.87 V vs. NHE), mediating an interfacial charge transfer pathway from TiO2, through Au, and then to Cu2O. Such a vectorial charge transfer led to electron concentration at the conduction band of Cu2O and hole collection at the valence band of TiO2. The electrons concentrated at the Cu2O had a sufficiently high reducing ability (ECB = -1.04 V vs. NHE) allowing efficient PEC reduction at the electrode/electrolyte interface. The collected holes at TiO2, on the other hand, were highly oxidative (EVB = +2.55 V vs. NHE), which can transfer to the counter electrode and effectively drive oxidation reactions. The Z-scheme charge transfer dynamics were examined with timeresolved photoluminescence (PL). The performances toward PEC reduction reactions for relevant samples including pure Cu2O, binary TiO2-Cu2O and Z-scheme TiO2-Au-Cu2O were compared and interpreted in terms of the distinct charge transfer scenarios.


Page 7 of 33 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


2. Experimental Section 2.1. Preparation of Z-Scheme TiO2-Au-Cu2O. Scheme 1 illustrates the synthetic procedures for sample preparation of TiO2-Au-Cu2O. Rutile TiO2 nanorod arrays were first grown on fluorine-doped tin oxide (FTO) glass substrates using a hydrothermal method reported in the literature.2,39 The deposition of Au particles on the rutile TiO2 was then conducted in the DC magnetron sputtering system. The sputtering time was suitably controlled to achieve complete coverage of the TiO2 surface with Au. The Au particle-coated TiO2 (denoted as TiO2-Au) was then subjected to electrochemical reduction for the further growth of Cu2O.30,40-42 The electrodeposition of Cu2O was performed at 45 °C using a constant current density of -0.9 mA/cm2 in a three-electrode cell. The cell consisted of a Pt counter electrode and an Ag/AgCl reference electrode (3 M KCl). The electrolyte (500 mL, pH = 11.0) was composed of CuSO4 (0.4 M) and lactic acid (C3H6O3, 1.0 M). After the electrodeposition, the samples were washed with deionized water, followed by an annealing treatment at 600 °C in N2 atmosphere for 4 h. In this work, various electrodeposition times (1.0, 1.5 and 2.0 h) were used to prepare TiO2-AuCu2O with various Cu2O thicknesses. The thus-obtained samples were respectively denoted as TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5 and TiO2-Au-Cu2O-2.0. Furthermore, pure Cu2O films and binary TiO2-supported Cu2O nanorods (denoted as TiO2-Cu2O) were also prepared by conducting the electrodeposition of Cu2O for 1.5 h on bare FTO substrates and pristine TiO2 nanorods, respectively. 2.2. Site-Selective Deposition of PbO2 and Ag. The selective deposition of PbO2 and Ag on specific sites of TiO2-Au-Cu2O and TiO2-Cu2O was carried out with a typical photodeposition method.5,43 Notably, these two samples were prepared using a slightly modified procedure in order to highlight the outcome of selective deposition. For example, for the case of TiO2-Au-



Page 8 of 33

Cu2O, an insulator tape was intentionally adhered to pristine TiO2 to mask half of the TiO2 surface. After the consecutive deposition of Au and Cu2O, the insulator tape was peeled off to expose the underlying TiO2. Similarly, TiO2-Cu2O with the TiO2 surface partly exposed was also prepared and used for the selective deposition experiment. To deposit PbO2 on the exposed TiO2 surface of TiO2-Au-Cu2O, the sample (TiO2-Au-Cu2O-1.5) was vertically immersed in 40 mL of deionized water, followed by injecting Pb(NO3)2 (30 μL, 0.01 M) solution. Afterwards, the reaction solution was irradiated with a xenon lamp (100 mW/cm2) normally incident upon the substrate surface at room temperature for 3 h. To deposit Ag on the Cu2O surface of TiO2-AuCu2O, the same photodeposition process was adopted except that AgNO3 (0.2 mM) and trisodium citrate (Na3C6H5O7, 4 mM) were respectively employed as the Ag source and stabilizer. A similar selective deposition approach was implemented on TiO2-Cu2O, in which PbO2 and Ag were respectively deposited on TiO2 and Cu2O surface to reveal the direct Z-scheme charge transfer mechanism. 2.3. Characterizations. The morphology, dimensions and compositions of the samples were examined with a scanning electron microscope (SEM, JEOL, JSM-6500) attached with the energy dispersive X-ray spectrometer (EDS) and an X-ray photoelectron spectroscope (XPS, VG Scientific Microlab 350). The crystallographic structure was studied with X-ray diffraction (XRD, Bruker, D2 phaser). The UV-visible diffuse reflectance spectra were acquired on a spectrophotometer equipped with an integrating sphere (Hitachi, U-3900H). The steady-state PL spectra were collected with 375 nm diode laser excitation at room temperature. The timeresolved PL data were measured in a customized single photon counting system,43-46 in which a sub-nanosecond pulsed diode laser (λex = 375 nm, PicoQuant, PLD 375) was installed as the excitation source. A three-electrode configuration using Pt foil as counter electrode and Ag/AgCl


Page 9 of 33 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


(3 M KCl) as reference electrode was employed to conduct PEC measurements. Prior to measurement, the 0.5 M Na2SO4 electrolyte was bubbled with CO2 (pH = 4.2) or N2 gas (pH = 7.0) to provide a favorable reaction medium for CO2 reduction or H2 production. All the PEC data including linear-sweep voltammograms (I-V curves), amperometric scans (I-t curves), Nyquist plots and Mott-Schottky analyses were recorded on a potentiostat (Autolab, PGSTAT204) under AM 1.5G illumination (100 mW/cm2). The incident photon-to-current conversion efficiency (IPCE) spectra were collected by recording photocurrent generation under monochromatic light irradiation according to the expression IPCE = 1240 I/(λPi ), where I is the photocurrent generation and Pi is the irradiation power at wavelength λ.

3. Results and Discussion

Scheme 1. Synthetic procedures for preparation of TiO2-Au-Cu2O and TiO2-Cu2O.

Figure 1 first shows the morphological investigations on pristine TiO2, TiO2-Au, pure Cu2O and TiO2-Au-Cu2O. Pristine TiO2 was characterized by bundles of partially aligned nanorods, a typical structural feature of a nanorod array. Upon the Au particle deposition by sputtering, the



Page 10 of 33

TiO2 surface was covered by a nearly continuous Au particle film, while the framework of the nanorod array was still preserved. On the other hand, pure Cu2O prepared by electrodeposition showed polyhedral architectures with a fairly smooth surface. Figures 1 (D)-(F) show the topview SEM images for TiO2-Au-Cu2O with increasing Cu2O electrodeposition time. The sample surface possessed a microstructure resembling that of pure Cu2O, indicating that the deposited Cu2O fully covered the TiO2-Au nanorods. As revealed from the cross-sectional SEM images, the deposited Cu2O infilled the interstices of the nanorod array and grew as an impact film with the thickness increasing with increased electrodeposition time. Furthermore, TiO2-Cu2O displayed a similar morphology to that of pure Cu2O, showing a polyhedral Cu2O structure on the surface as well.


Page 11 of 33 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 1. Top-view SEM images of (A) pristine TiO2, (B) TiO2-Au, (C) pure Cu2O, (D) TiO2Au-Cu2O-1.0, (E) TiO2-Au-Cu2O-1.5, and (F) TiO2-Au-Cu2O-2.0. (G), (H), (I) show the crosssectional SEM images of TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5, and TiO2-Au-Cu2O-2.0, respectively. The top of the TiO2 nanorods is marked with dashed lines.

The XRD patterns of the relevants samples were presented in Figure 2(A). For pristine TiO2, in addition to the signals from the FTO, two diffraction peaks at 36.6° and 63.5° were recorded, which can be respectively identified as the (101) and (002) planes of rutile TiO2.41 On the other hand, pure Cu2O exhibited diffraction peaks that can be assigned to cubic Cu2O. The deposition of Au particles on TiO2, however, did not generate the observable diffraction of Au since the Au content was fairly low (around 5.18 at% as determined by SED-EDS analysis). The successful loading of Au on the surface of TiO2 can be further confirmed by XPS measurement. As shown



Page 12 of 33

in Figure S1 (Supporting Information), the recorded binding energies of the core levels of Ti 2p, O 1s and Au 4f for TiO2-Au can be readily recognized as TiO2 and Au components.32 Further electrodeposition of Cu2O produced the Cu2O diffraction pattern, which overpowered those of TiO2 and FTO. This outcome can be understood from the considerably large thickness of the deposited Cu2O. XPS analysis confirmed the chemical states of Cu2O grown in the electrodeposition process. Figure 2(B) shows the XPS Cu 2p spectra of pure Cu2O, TiO2-Cu2O and the three TiO2-Au-Cu2O samples. The recorded Cu 2p at 951.6 and 931.6 eV with a spinorbit splitting of 20.0 eV suggested the prevalence of the Cu+ state for these samples,18,32,43 confirming the Cu2O composition. Furthermore, the absence of satellite peaks ruled out the formation of CuO within the samples.

Figure 2. (A) XRD patterns and (B) XPS Cu 2p spectra for all the samples. In (A), the standard patterns of rutile TiO2 (JCPDS #88-1175), cubic Cu2O (JCPDS #05-0667), fcc Au (JCPDS #652870) and tetragonal FTO (JCPDS #41-1445) were also included for comparison.

Figure 3(A) shows the UV-visible diffused reflectance spectra for all the samples. Both pristine TiO2 and TiO2-Au had a sharp absorption onset at approximately 410 nm, corresponding to the bandgap energy of TiO2. A considerably weak yet identifiable absorption band from 550 to


Page 13 of 33 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


650 nm was present for TiO2-Au, which was attributed to the surface plasmon resonance (SPR) of the deposited Au. For TiO2-Cu2O and the three TiO2-Au-Cu2O samples, the light absorption features resembled that of pure Cu2O, showing a moderate absorption onset at 650 nm and an apparent bandgap of 1.91 eV.38,43 Most importantly, the observations from absorption spectra suggested that the primary photoactive constituent of TiO2-Cu2O and TiO2-Au-Cu2O was Cu2O, a promising component that can demonstrate efficient reduction reactions in the PEC cells. Figure 3(B) further shows the steady-state PL spectra for the relevant samples. To observe the emissions behavior of the Cu2O constituent, only Cu2O-based samples were tested, and the spectra were collected in the wavelength range of 500-850 nm, a region where the emissive transitions of Cu2O may occur. As presented in Figure 3(B), the spectra exhibited three main emissions peaks at 625, 750 and 810 nm. The broad low-intensity emission at 625 nm was assigned to the yellow 1s (Y1) excitonic emission of Cu2O.38,47,48 The sharp, yet low-intensity, red emission band at 750 nm was attributed to the defect-associated trap states of doubly ionized oxygen vacancies (Vo2+) of Cu2O. On the other hand, the sharp and most prominent emission band centered at 810 nm can be assigned to defect-associated trap states of singly ionized oxygen vacancies (Vo+) of Cu2O. Compared to pure Cu2O, TiO2-Cu2O and TiO2-Au-Cu2O both showed significantly dampened excitonic and defect-band emissions. Here, a Z-scheme charge transfer model, in which the electrons transferred from the conduction band of TiO2 to the valence band of Cu2O, was considered as the primary cause of the notable PL quenching for both TiO2-Cu2O and TiO2-Au-Cu2O. Especially for TiO2-Au-Cu2O, the embedded Au mediated the electron transfer from TiO2 through Au and then to Cu2O, therefore consolidating the effectiveness of the Z-scheme mechanism.5,23,43 The much stronger emission quenching of the three TiO2-Au-Cu2O samples than that of TiO2-Cu2O manifested more effective charge carrier separation in the Au-



Page 14 of 33

mediated Z-scheme mechanism. A similar phenomenon was also observed in ZnO-Au-CdS Zscheme heterostructure system, in which the charge separation efficiency in the three-component ZnO-Au-CdS was higher than in the case of binary ZnO-CdS.7 To further quantify the charge separation efficiency, time-resolved PL spectra were collected by recording the emissive photon signals of Cu2O at λem = 750 nm. As Figure 3(C) shows, TiO2Cu2O and the three TiO2-Au-Cu2O samples displayed different PL decay kinetics from those of pure Cu2O, suggesting that the charge transfer dynamics were fundamentally different in the heterostructure samples. The emission decay kinetics were analyzed with a biexponential function to obtain two lifetime values, τ1 and τ2, and the corresponding amplitudes, A1 and A2, which were respectively assigned to the radiative and non-radiative charge recombination pathways. The intensity-average emission time, , was determined and compared. As shown in Table 1, the values for pure Cu2O, TiO2-Cu2O, TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5 and TiO2-Au-Cu2O-2.0 were respectively calculated as 6.13, 5.57, 1.98, 1.43 and 2.24 ns. Importantly, both TiO2-Cu2O and TiO2-Au-Cu2O showed a shorter value than pure Cu2O, signifying the occurrence of pronounced charge separation due to the prevalence of interfacial charge transfer. By using the equation

Cu2O →TiO2)

TiO2-Cu2O

Cu2O , the

rate constant of charge transfer at the TiO2/Cu2O interface for TiO2-Cu2O can be calculated,5 which was approximately 0.15 × 108 s-1. Similarly, the interfacial charge transfer rate constant of the three TiO2-Au-Cu2O could be computed, approximately 0.23 × 108, 5.36 × 108, and 2.83 × 108 s-1 for TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5 and TiO2-Au-Cu2O-2.0, respectively. Obviously, all the three TiO2-Au-Cu2O samples had higher kct values than TiO2-Cu2O, corroborating that the Au-mediated Z-scheme mechanism was superior to the direct Z-scheme model for facilitating charge carrier separation. Furthermore, among the three TiO2-Au-Cu2O


Page 15 of 33 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


samples tested, the TiO2-Au-Cu2O-1.5 showed the largest kct, signifying that there existed an optimal Cu2O electrodeposition time, i.e., an optimal Cu2O thickness, for promoting the most effective interfacial charge transfer from the Au-mediated Z-scheme mechanism. It was obvious from the SEM investigations that the thickness of the grown Cu2O layer for TiO2-Au-Cu2O increased with increasing electrodeposition time. For TiO2-Au-Cu2O-2.0, the thickness of the grown Cu2O layer reached as thick as 5.31 μm, which greatly exceeded the diffusion length of minority carriers of Cu2O (around 200 nm38). The substantially large thickness of Cu2O would encourage intrinsic charge carrier recombination, compromising the effectiveness of the interfacial charge transfer from the Z-scheme mechanism. Therefore, an otherwise reduced kct value was obtained for TiO2-Au-Cu2O-2.0. This contention can be evidenced by the fact that TiO2-Au-Cu2O-2.0 displayed an increased contribution from radiative recombination component (A1 = 67.7 %).



Page 16 of 33

Figure 3. (A) UV-visible diffuse reflectance spectra, (B) steady-state PL spectra, and (C) timeresolved PL spectra for the relevant samples. Table 1. Fitting results of time-resolved PL spectra for the relevant samples. A1 (%)

τ1 (ns)

A2 (%)

τ2 (ns)

(ns)

χ2

kct (s-1)

pure Cu2O

61.8

9.61

38.2

0.50

6.13

1.09

-

TiO2-Cu2O

61.3

8.81

38.4

0.37

5.57

1.06

0.15×108

TiO2-Au-Cu2O-1.0

59.5

3.08

40.5

0.36

1.98

1.00

0.23×108

TiO2-Au-Cu2O-1.5

40.2

3.02

59.8

0.36

1.43

1.07

5.36×108

TiO2-Au-Cu2O-2.0

67.7

3.13

32.3

0.37

2.24

1.02

2.83×108

entry

To demonstrate the beneficial features of the Z-scheme mechanism in PEC reduction, the asprepared samples were employed as photocathodes to conduct PEC measurements. Note that


Page 17 of 33 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


TiO2 and TiO2-Au were not compared here because both of them functioned as photoanodes. Figure 4 shows a set of I-V curves for pure Cu2O, TiO2-Cu2O and the three TiO2-Au-Cu2O electrodes recorded under chopped AM 1.5G illumination in the electrolyte under either N2- or CO2-saturated condition. In the N2 atmosphere, the recorded photocurrents were associated with H2 generation from water splitting. The performance toward H2 production was evaluated by comparing the photocurrent values recorded at -0.35 V vs. Ag/AgCl (+0.27 V vs. RHE, pH = 7.0). Note that this potential was less cathodic than the electrochemical potential of H2 evolution, suggesting that the reaction was mainly driven by solar power. The recorded photocurrents were -0.15, -0.22, -0.28, -0.38, and -0.28 mA/cm2 for pure Cu2O, TiO2-Cu2O, TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5, and TiO2-Au-Cu2O-2.0, respectively. On the other hand, under CO2saturated condition, the photocurrents were mostly related to CO2 reduction. For all the samples tested, the recorded photocurrents in the CO2-saturated electrolyte were substantially higher than the values recorded in the N2-saturated electrolyte. This outcome suggests that CO2 was kinetically more favorable for capturing the photoexcited electrons from these Cu2O-based photocathodes than from water. Such a supposition can be further validated from the more anodic onset potential observed in the CO2-saturated electrolyte. For example, for pure Cu2O electrode, the onset potential recorded in the CO2-saturated electrolyte was -0.15 V vs. Ag/AgCl (+0.31 V vs. RHE, pH = 4.2), which was essentially more anodic than the -0.35 V vs. Ag/AgCl (+0.27 V vs. RHE, pH = 7.0) potential recorded under the N2-saturated condition. On the other hand, the possible effect of electrolyte pH on the PEC performance of the electrodes was also considered. Previous studies reported that at higher electrolyte pH, the Cu2O-based photocathodes exhibited enhanced PEC performance in terms of increased photocurrent generation and positive onset potential shift.49,50 In this work, because the pH of the CO2-



Page 18 of 33

saturated electrolyte (pH = 4.2) was lower than the N2-saturated electrolyte (pH = 7.0), the otherwise enhanced PEC performance obtained at the CO2-saturated condition was unlikely related to the variation of electrolyte pH. In fact, the H2 evolution on Cu2O-based photocathodes would be largely suppressed at reduced electrolyte pH due to the increased H2 bonding energy.49 Although H2 evolution was thermodynamically more favorable than CO2 reduction, the suppressed kinetics of hydrogen atom combination at reduced pH facilitated the reaction of CO2 reduction. Therefore, the PEC performance obtained in the CO2-saturated electrolyte was better than that obtained in the N2-saturated electrolyte. The PEC CO2 reduction performance was further evaluated by comparing the photocurrent values recorded at -0.35 V vs. Ag/AgCl (+0.11 V vs. RHE, pH = 4.2). Again, this potential was not cathodic enough to electrochemically drive CO2 reduction (-0.31 V vs. RHE for CO2/HCHO, -0.36 V vs. RHE for CO2/CO, and -0.44 V vs. RHE for CO2/HCOOH at pH = 4.2).51 Nevertheless, significant cathodic photocurrents still appeared as a result of the bias compensation by light illumination. The recorded photocurrents were -0.75, -1.10, -1.71, -1.82, and -1.63 mA/cm2 for pure Cu2O, TiO2-Cu2O, TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5, and TiO2-Au-Cu2O-2.0, respectively. Several important points can be concluded from these data. First, both TiO2-Cu2O and TiO2-Au-Cu2O showed substantially larger photocurrent density values than pure Cu2O did, reflecting the indispensable strategy of utilizing Z-scheme heterostructures for efficient PEC reduction reactions. Second, the three TiO2-Au-Cu2O photocathodes displayed higher photocurrents than TiO2-Cu2O, revealing that PEC reduction was more favorable with the Au-mediated Z-scheme mechanism than with the direct Z-scheme charge transfer. This result agreed with the charge dynamics data, confirming that the embedded Au in TiO2-Au-Cu2O can mediate interfacial charge transfer to promote effective charge


Page 19 of 33 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


separation to facilitate PEC reduction. Lastly, among the three TiO2-Au-Cu2O electrodes tested, TiO2-Au-Cu2O-1.5 showed the highest photocurrent toward H2 production and CO2 reduction. In terms of H2 production, TiO2-Au-Cu2O-1.5 showed a photoconversion efficiency of 1.10 % at +0.13 V vs. RHE,52 much higher than 0.69 % of TiO2-Cu2O and 0.28 % of pure Cu2O at the same bias condition. This outcome was again consistent with the carrier dynamics results, in which TiO2-Au-Cu2O-1.5 displayed the highest kct value and, therefore, showed the best photoactivity toward PEC reduction.

Figure 4. I-V profiles collected on pure Cu2O, TiO2-Cu2O, and the three TiO2-Au-Cu2O photocathodes under chopped AM 1.5G illumination in N2-saturated or CO2-saturated electrolyte.



Page 20 of 33

EIS measurements were further performed to learn the charge transfer kinetics information at interface, which is an important factor affecting the resultant PEC performance. Figure 5(A) compares the typical Nyquist plots for pristine Cu2O, TiO2-Cu2O, and the three TiO2-Au-Cu2O electrodes. All curves displayed the characteristic semicircular spectra, in which the diameter of the semicircle represents the charge transfer resistance of the tested electrode. Obviously, both TiO2-Cu2O and TiO2-Au-Cu2O displayed a more compressed semicircle than pure Cu2O did, suggesting improved charge transfer kinetics caused by Z-scheme mechanism. These plots were then fitted with an equivalent circuit shown as the inset of Figure 5(A), which consisted of an overall series resistance of the circuit (Rs), an internal resistance of the electrode (Ri) along with a constant phase element (CPEbulk,trap), and another charge transfer resistance from the surface states to the solution (Rct) with the corresponding CPE (CPEs). The fitting results were summarized in Table 2. Compared to pure Cu2O, TiO2-Cu2O and TiO2-Au-Cu2O showed smaller Ri and Rct values, attributable to the Z-scheme mechanism which not only increased the overall charge separation of the electrode but also enhanced the charge transfer kinetics at the electrode/electrolyte interface. More importantly, the three TiO2-Au-Cu2O electrodes displayed a highly reduced Ri and Rct relative to TiO2-Cu2O, corroborating the superiority of the Aumediated Z-scheme mechanism over the direct Z-scheme mechanism in promoting charge carrier separation as well as facilitating the charge transfer kinetics. Furthermore, the relative reaction time τ, which characterizes the duration for charge carrier transfer from surface states to electrolyte,53 was also determined according to τ

, where fmax represents the frequency at

max

the peak of the Nyquist plot. The fmax value of TiO2-Cu2O, TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O1.5, and TiO2-Au-Cu2O-2.0 were 37.3, 1599.9, 7196.9, and 754.3 Hz, respectively. The τ values were respectively determined to be 4.26, 0.10, 0.02, and 0.21 ms. Much shorter τ of TiO2-Au-


Page 21 of 33 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


Cu2O verified again the Au-mediated Z-scheme was superior to direct Z-scheme in enhancing charge transfer kinetics. The density of charge carriers that can possibly participate in PEC reactions can be evaluated by Mott-Schottky analysis. In Figure 5(B), the linear fits to the MottSchottky plots showed a negative slope associated with the p-type behavior. The charge carrier density can be calculated with the Mott-Schottky equation

,

where C is the capacitance, e0 is the electron charge, ε is the dielectric constant (ε = 7.5 for Cu2O),38,45 ε0 is the vacuum permittivity, Nd is the carrier density, VFB is the flat-band potential, kB is the Boltzmann constant, and T is the temperature. By computing the slope of the C-2 vs. V plot in Figure 5(B), Nd can be computed as 1.28 × 1017, 5.03 × 1017, 9.12 × 1017, 8.19 × 1018, and 1.59 × 1018 cm-3 for pure Cu2O, TiO2-Cu2O, TiO2-Au-Cu2O-1.0, TiO2-Au-Cu2O-1.5, and TiO2Au-Cu2O-2.0, respectively. The greatly enlarged Nd for TiO2-Au-Cu2O over pure Cu2O and TiO2-Cu2O again supported the contention that the Au-mediated Z-scheme mechanism was highly effective at promoting charge carrier separation and, thus, providing abundant available charge carriers. It is worth noting that the results of the Nyquist and Mott-Schottky analyses complied with the trend of the PEC performance variation. Among the different samples tested, TiO2-Au-Cu2O-1.5 exhibited the smallest Ri and Rct, and the largest Nd, which corresponded to the highest photocurrent generation observed in the PEC measurements. This correspondence illustrated that the improved PEC performance of TiO2-Au-Cu2O was associated with the enhanced charge separation, the increased charge carrier density, and the improved electron injection kinetics at the electrode/electrolyte interface.



Page 22 of 33

Figure 5. (A) Nyquist plots and (B) Mott-Schottky plots collected on pure Cu2O, TiO2-Cu2O, and the three TiO2-Au-Cu2O electrodes. Inset is the corresponding equivalent circuit.

Table 2. Fitting results of Nyquist plots for pure Cu2O, TiO2-Cu2O and the three TiO2-Au-Cu2O electrodes. a CPEbulk,trap Rs (Ω) Ri (kΩ) Rct (Ω) Cs (μF) n Qo (μS·sn) pure Cu2O 22 7.95 530 121 0.66 688 TiO2-Cu2O 42 5.00 380 902 0.59 129 TiO2-Au-Cu2O-1.0 30 4.05 132 562 0.44 2.62 TiO2-Au-Cu2O-1.5 30 2.94 10.5 561 0.48 2.32 TiO2-Au-Cu2O-2.0 26 3.29 42.4 422 0.50 2.58 a o CPE is the constant phase element with the impedance ZCPE Q . Here, Qo is the numerical value of the admittance, ω is the angular frequency, n is the CPE expoenet, and j states the imaginary number.

Figure 6 illustrates the Z-scheme charge transfer scenarios for TiO2-Cu2O and TiO2-AuCu2O, with the TiO2-Au-Cu2O showing consolidated Z-scheme effectiveness as a result of the Au mediation. Upon light irradiation, the interfacial charge recombination led to the electron concentration at Cu2O and hole collection at TiO2, providing the samples with substantially high reduction (-1.04 V vs. NHE) and oxidation powers (+2.55 V vs. NHE). By performing siteselective photodeposition experiments, the Z-scheme mechanism of TiO2-Cu2O and TiO2-Au-


Page 23 of 33 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


Cu2O can be validated. For example, for TiO2-Cu2O, under light illumination the Cu2O surface was enriched with photoexcited electrons, while the underlying TiO2 was charged with photogenerated holes. These localized charge carriers can respectively drive the reduction and oxidation reactions, which can be experimentally examined by conducting the photoreduction of Ag+ and the photooxidation of Pb2+. The results of the site-selective photodeposition were characterized with SEM-EDS elemental mapping analysis. As shown in Figue 7(A), the recorded Pb elements were mostly distributed in the underlying TiO2 region, suggesting that PbO2 was preferentially grown on the TiO2 surface as a result of hole accumulation. On the other hand, the preferential growth of Ag on the Cu2O surface was also evident, as revealed from the matching of the Ag elemental distribution with Cu from Figure 7(B). Such elemental distribution matching of Pb with Ti and Ag with Cu can also be identified on TiO2-Au-Cu2O, suggesting that PbO2 and Ag can be grown respectively on the TiO2 and Cu2O surface due to the Z-scheme mechanism. As Figures 7(C) and (D) show, the selective growth of PbO2 on TiO2 and Ag on Cu2O was highly pronounced for TiO2-Au-Cu2O, illustrating that the Z-scheme mechanism can be greatly consolidated with Au mediation. As to the TiO2-Cu2O, the direct contact of TiO2 with Cu2O may induce the formation of p-n junction to arouse electron transfer from Cu2O to TiO2 and hole transfer from TiO2 to Cu2O. This type-II charge transfer pathway to some extent competed with the direct Z-scheme mechanism to deteriote the redox powers for TiO2-Cu2O, which was accountable for the inferior PEC performance as observed.



Page 24 of 33

Figure 6. Proposed charge transfer mechanism for (A) TiO2-Cu2O and (B) TiO2-Au-Cu2O.


Page 25 of 33 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 7. SEM images and the corresponding EDS elemental mapping taken on TiO2-Cu2O after (A) photooxidation of Pb2+ and (B) photoreduction of Ag+. Results taken on TiO2-Au-Cu2O were shown in (C) and (D), respectively.

The IPCE spectra were collected and compared in Figure 8. All samples exhibited prominent photoactivity which was matched with the corresponding absorption spectra. Significantly, the TiO2-Au-Cu2O surpassed both pure Cu2O and TiO2-Cu2O in photocurrent generation across the whole photoactive region. This outcome, combined with conclusions from PEC measurements, time-resolved PL data and EIS study, illustrated the beneficial feature of the Au-mediated Zscheme mechanism for advancing the overall PEC performance of TiO2-Au-Cu2O. In order to evaluate the plasmonic effect on the PEC performance for TiO2-Au-Cu2O, we monitored the



Page 26 of 33

photocurrent generation under additional visible irradiation (λex = 550 nm) exciting the SPR of Au.54 As shown in Figure S2 (Supporting Information), TiO2-Au-Cu2O-1.5 displayed an unambiguous photocurrent rise upon additional visible irradiation. On the contrary, TiO2-Cu2O barely responded to additional visible irradiation. This outcome suggested that SPR excitation of Au can to some extent contribute to photoactivity enhancement for TiO2-Au-Cu2O in PEC reduction reactions. As a remarkable note, the overall PEC performance of the present TiO2-AuCu2O was superior to most of the state-of-the-art photocathode systems developed for PEC reduction, particularly CO2 reduction. As summarized in Table S1 (Supporting Information), the TiO2-Au-Cu2O not only exhibited the relatively low onset potential and high photocurrent generation of CO2 reduction, but also displayed extended photoactivity across the whole visible range. Noticeably, the photocurrent density recorded at +0.11 V vs. RHE (-1.82 mA/cm2) was the highest value ever reported for semiconductor photocathodes at the same applied potential. This comparison further highlights the valuable use of TiO2-Au-Cu2O photocathodes for solarto-fuel energy conversion. In addition to high photocurrent generation, the long-term stability was also an improtant requirement for the practical use of photoelectrodes. We further performed photocurrent stability tests to evaluate the reusability of the electrodes. As Figure S3 (Supporting Information) shows, both TiO2-Cu2O and TiO2-Au-Cu2O exhibited rapidly reduced photocurrent generation under continuous light illumination. This poor stability was expectable and can be ascribed to the anodic and cathodic corrosions of the exposed Cu2O with the aqueous electrolyte.55 Nevertheless, the effectiveness of Z-scheme mechanism in enhancing the photoactivity of PEC reduction was still relevant for the present TiO2-Au-Cu2O. The practice of Z-scheme mechanism is expected to make much significant impact as the stability issue of the constructed photoelectrodes is appropriately addressed.55,56


Page 27 of 33 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 8. IPCE spectra of pure Cu2O, TiO2-Cu2O and the three TiO2-Au-Cu2O.

4. Conclusions The use of TiO2-Au-Cu2O as the Z-scheme photocathode in the PEC cell has been demonstrated. Compared with pure Cu2O and TiO2-Cu2O, the TiO2-Au-Cu2O photocathode exhibited enhanced photoactivity toward H2 production and CO2 reduction under AM 1.5G illumination. The results of time-resolved PL and EIS analyses suggested that the superior PEC performance of TiO2-Au-Cu2O derived from the Au-mediated Z-scheme mechanism, which enhanced the overall charge separation, increased the density of available charge carriers, and improved the kinetics of electron injection into electrolyte. The present study represents a potentially universal strategy for the construction of Z-scheme semiconductor heterostructures as valuable photocathodes in the PEC system for solar-to-fuel energy conversion.



Page 28 of 33

Associate Content Supporting Information XPS data, photocurrent response to SPR excitation and photocurrent stability tests of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author * E-mail: [email protected] (Y.-J. Hsu) Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the Ministry of Science and Technology (MOST) of Taiwan under grants MOST 105-2119-M-009-003 and MOST 106-2113-M-009-025. Y.-J. Hsu also acknowledges the budget support from the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan.

References 1. Climate Change 2013: The Physical Science Basis - Summary for Policy Makers. IPCC: Switzerland, 2013. 2. Pu, Y.-C.; Wang, G.; Chang, K.-D.; Ling, Y.; Lin, Y.-K.; Fitzmorris, B. C.; Liu, C.-M.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y.-J.; Li, Y. Au Nanostructure-Decorated TiO2 Nanowires


Page 29 of 33 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


Exhibiting Photoactivity across Entire Uv-Visible Region for Photoelectrochemical Water Splitting. Nano Lett. 2013, 13, 3817-3823. 3. Uddin, M. T.; Babot, O.; Thomas, L.; Olivier, C.; Redaelli, M.; D’Arienzo, M.; Morazzoni, F.; Jaegermann, W.; Rockstroh, N.; Junge, H.; Toupance, T. New Insights into the Photocatalytic Properties of RuO2/TiO2 Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition. J. Phys. Chem. C 2015, 119, 7006-7015. 4. Hu, J.; Zhang, S.; Cao, Y.; Wang, H.; Yu, H.; Peng, F. Novel Highly Active Anatase/Rutile TiO2 Photocatalyst with Hydrogenated Heterophase Interface Structures for Photoelectrochemical Water Splitting into Hydrogen. ACS Sustainable Chem. Eng. 2018, 6, 10823-10832. 5. Li, J.-M.; Cheng, H.-Y.; Chiu, Y.-H.; Hsu, Y.-J. ZnO-Au-SnO2 Z-Scheme Photoanodes for Remarkable Photoelectrochemical Water Splitting. Nanoscale 2016, 8, 15720-15729. 6. Baek, M.; Kim, D.; Yong, K. Simple but Effective Way to Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)2 Annihilation and Oxygen Vacancy Generation by Vacuum Annealing. ACS Appl. Mater. Interfaces 2017, 9, 2317-2325. 7. Zhang, N.; Xie, S.; Weng, B.; Xu, Y.-J. Vertically Aligned ZnO-Au@CdS Core-Shell Nanorod Arrays as an All-Solid-State Vectorial Z-Scheme System for Photocatalytic Application. J. Mater. Chem. A 2016, 4, 18804-18814. 8. Chang, Y.-S.; Choi, M.; Baek, M.; Hsieh, P.-Y.; Yong, K.; Hsu, Y.-J. CdS/CdSe CoSensitized Brookite H:TiO2 Nanostructures: Charge Carrier Dynamics and Photoelectrochemical Hydrogen Generation. Appl. Catal., B 2018, 225, 379-385. 9. Wang, R.; Yan, J.; Zu, M.; Yang, S.; Cai, X.; Gao, Q.; Fang, Y.; Zhang, S.; Zhang, S. Facile Synthesis of Interlocking G-C3N4/CdS Photoanode for Stable Photoelectrochemical Hydrogen Production. Electrochim. Acta 2018, 279, 74-83. 10. Van, C. N.; Chang, W. S.; Chen, J.-W.; Tsai, K.-A.; Tzeng, W.-Y.; Lin, Y.-C.; Kuo, H.H.; Liu, H.-J.; Chang, K.-D.; Chou, W.-C.; Wu, C.-L.; Chen, Y.-C.; Luo, C.-W.; Hsu, Y.-J.; Chu, Y.-H. Heteroepitaxial Approach to Explore Charge Dynamics across Au/BiVO4 Interface for Photoactivity Enhancement. Nano Energy 2015, 15, 625-633. 11. Ghadimkhani, G.; de Tacconi, N. R.; Chanmanee, W.; Janaky, C.; Rajeshwar, K. Efficient Solar Photoelectrosynthesis of Methanol from Carbon Dioxide Using Hybrid CuOCu2O Semiconductor Nanorod Arrays. Chem. Commun. 2013, 49, 1297-1299. 12. Zheng, X.; Zhang, L. Photonic Nanostructures for Solar Energy Conversion. Energy Environ. Sci. 2016, 9, 2511-2532. 13. Chen, Y.-C.; Pu, Y.-C.; Hsu, Y.-J. Interfacial Charge Carrier Dynamics of the ThreeComponent In2O3–TiO2–Pt Heterojunction System. J. Phys. Chem. C 2012, 116, 2967-2975. 14. Chen, M.-Y.; Hsu, Y.-J. Type-II Nanorod Heterostructure Formation through One-Step Cation Exchange. Nanoscale 2013, 5, 363-368. 15. Lin, Y.-F.; Hsu, Y.-J. Interfacial Charge Carrier Dynamics of Type-II Semiconductor Nanoheterostructures. Appl. Catal., B 2013, 130, 93-98. 16. Quynh, L. T.; Van, C. N.; Bitla, Y.; Chen, J.-W.; Do, T. H.; Tzeng, W.-Y.; Liao, S.-C.; Tsai, K.-A.; Chen, Y.-C.; Wu, C.-L.; Lai, C.-H.; Luo, C.-W.; Hsu, Y.-J.; Chu, Y.-H. SelfAssembled BiFeO3-ε-Fe2O3vertical Heteroepitaxy for Visible Light Photoelectrochemistry. Adv. Energy Mater. 2016, 6, 1600686.



Page 30 of 33

17. Do, T. H.; Nguyen Van, C.; Tsai, K.-A.; Quynh, L. T.; Chen, J.-W.; Lin, Y.-C.; Chen, Y.C.; Chou, W.-C.; Wu, C.-L.; Hsu, Y.-J.; Chu, Y.-H. Superior Photoelectrochemical Activity of Self-Assembled NiWO4–WO3 Heteroepitaxy. Nano Energy 2016, 23, 153-160. 18. Pal, J.; Sasmal, A. K.; Ganguly, M.; Pal, T. Surface Plasmon Effect of Cu and Presence of N–P Heterojunction in Oxide Nanocomposites for Visible Light Photocatalysis. J. Phys. Chem. C 2015, 119, 3780-3790. 19. Zhou, P.; Yu, J.; Jaroniec, M. All-Solid-State Z-Scheme Photocatalytic Systems. Adv. Mater. 2014, 26, 4920-4935. 20. Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. All-Solid-State Z-Scheme in CdS-Au-TiO2 Three-Component Nanojunction System. Nat. Mater. 2006, 5, 782-786. 21. Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J. Am. Chem. Soc. 2011, 133, 11054-11057. 22. Maeda, K. Z-Scheme Water Splitting Using Two Different Semiconductor Photocatalysts. ACS Catal. 2013, 3, 1486-1503. 23. Yu, Z. B.; Xie, Y. P.; Liu, G.; Lu, G. Q.; Ma, X. L.; Cheng, H.-M. Self-Assembled CdS/Au/ZnO Heterostructure Induced by Surface Polar Charges for Efficient Photocatalytic Hydrogen Evolution. J. Mater. Chem. A 2013, 1, 2773-2776. 24. Kim, D.; Sakimoto, K. K.; Hong, D.; Yang, P. Artificial Photosynthesis for Sustainable Fuel and Chemical Production. Angew. Chem. Int. Ed. 2015, 54, 3259-3266. 25. Wang, J.-C.; Zhang, L.; Fang, W.-X.; Ren, J.; Li, Y.-Y.; Yao, H.-C.; Wang, J.-S.; Li, Z.-J. Enhanced Photoreduction CO2 Activity over Direct Z-Scheme α-Fe2O3/Cu2O Heterostructures under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2015, 7, 8631-8639. 26. Zhu, M.; Sun, Z.; Fujitsuka, M.; Majima, T. ZScheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light. Angew. Chem. Int. Ed. 2018, 57, 2160-2164. 27. Jia, Q.; Iwase, A.; Kudo, A. BiVO4-Ru/SrTiO3:Rh Composite Z-Scheme Photocatalyst for Solar Water Splitting. Chem. Sci 2014, 5, 1513-1519. 28. He, Y.; Zhang, L.; Teng, B.; Fan, M. New Application of Z-Scheme Ag3PO4/g-C3N4 Composite in Converting CO2 to Fuel. Environ. Sci. Technol. 2015, 49, 649-656. 29. Wang, Q.; Hisatomi, T.; Jia, Q.; Tokudome, H.; Zhong, M.; Wang, C.; Pan, Z.; Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y.; Sharp, I. D.; Kudo, A.; Yamada, T.; Domen, K. Scalable Water Splitting on Particulate Photocatalyst Sheets with a Solar-to-Hydrogen Energy Conversion Efficiency Exceeding 1%. Nat. Mater. 2016, 15, 611. 30. Zhang, J.; Ma, H.; Liu, Z. Highly Efficient Photocatalyst Based on All Oxides WO3/Cu2O Heterojunction for Photoelectrochemical Water Splitting. Appl. Catal., B 2017, 201, 84-91. 31. Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nat. Mater. 2011, 10, 456-461. 32. Chiu, Y.-H.; Hsu, Y.-J. Au@Cu7S4 Yolk@Shell Nanocrystal-Decorated TiO2 Nanowires as an All-Day-Active Photocatalyst for Environmental Purification. Nano Energy 2017, 31, 286295. 33. Chang, X.; Wang, T.; Zhang, P.; Wei, Y.; Zhao, J.; Gong, J. Stable Aqueous Photoelectrochemical CO2 Reduction by a Cu2O Dark Cathode with Improved Selectivity for Carbonaceous Products. Angew. Chem. 2016, 128, 8986-8991.


Page 31 of 33 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


34. Rajeshwar, K.; de Tacconi, N. R.; Ghadimkhani, G.; Chanmanee, W.; Janáky, C. Tailoring Copper Oxide Semiconductor Nanorod Arrays for Photoelectrochemical Reduction of Carbon Dioxide to Methanol. ChemPhysChem 2013, 14, 2251-2259. 35. An, X.; Li, K.; Tang, J. Cu2O/Reduced Graphene Oxide Composites for the Photocatalytic Conversion of CO2. ChemSusChem 2014, 7, 1086-1093. 36. Mistry, H.; Varela, A. S.; Bonifacio, C. S.; Zegkinoglou, I.; Sinev, I.; Choi, Y.-W.; Kisslinger, K.; Stach, E. A.; Yang, J. C.; Strasser, P.; Cuenya, B. R. Highly Selective PlasmaActivated Copper Catalysts for Carbon Dioxide Reduction to Ethylene. Nat. Commun. 2016, 7, 12123. 37. Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A Review of Catalysts for the Electroreduction of Carbon Dioxide to Produce Low-Carbon Fuels. Chem. Soc. Rev. 2014, 43, 631-675. 38. Luo, J.; Steier, L.; Son, M.-K.; Schreier, M.; Mayer, M. T.; Grätzel, M. Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Lett. 2016, 16, 1848-1857. 39. Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985-3990. 40. Luo, J.; Tilley, S. D.; Steier, L.; Schreier, M.; Mayer, M. T.; Fan, H. J.; Grätzel, M. Solution Transformation of Cu2O into CuInS2 for Solar Water Splitting. Nano Lett. 2015, 15, 1395-1402. 41. Ren, S.; Wang, B.; Zhang, H.; Ding, P.; Wang, Q. Sandwiched ZnO@Au@Cu2O Nanorod Films as Efficient Visible-Light-Driven Plasmonic Photocatalysts. ACS Appl. Mater. Interfaces 2015, 7, 4066-4074. 42. Xu, Q.; Qian, X.; Qu, Y.; Hang, T.; Zhang, P.; Li, M.; Gao, L. Electrodeposition of Cu2O Nanostructure on 3D Cu Micro-Cone Arrays as Photocathode for Photoelectrochemical Water Reduction. J. Electrochem. Soc. 2016, 163, H976-H981. 43. Pu, Y.-C.; Lin, W.-H.; Hsu, Y.-J. Modulation of Charge Carrier Dynamics of NaxH2−XTi3O7-Au-Cu2O Z-Scheme Nanoheterostructures through Size Effect. Appl. Catal., B 2015, 163, 343-351. 44. Chen, Y.-C.; Liu, T.-C.; Hsu, Y.-J. ZnSe-0.5(N2H4) Hybrid Nanostructures: A Promising Alternative Photocatalyst for Solar Conversion. ACS Appl. Mater. Interfaces 2015, 7, 1616-1623. 45. Pu, Y.-C.; Chou, H.-Y.; Kuo, W.-S.; Wei, K.-H.; Hsu, Y.-J. Interfacial Charge Carrier Dynamics of Cuprous Oxide-Reduced Graphene Oxide (Cu2O-rGO) Nanoheterostructures and Their Related Visible-Light-Driven Photocatalysis. Appl. Catal. B 2017, 204, 21-32. 46. Lin, W.-H.; Chiu, Y.-H.; Shao, P.-W.; Hsu, Y.-J. Metal-Particle-Decorated ZnO Nanocrystals: Photocatalysis and Charge Dynamics. ACS Appl. Mater. Interfaces 2016, 8, 32754-32763. 47. Ito, T.; Yamaguchi, H.; Okabe, K.; Masumi, T. Single-Crystal Growth and Characterization of Cu2O and CuO. J. Mater. Sci. 1998, 33, 3555-3566. 48. Li, J.; Mei, Z.; Ye, D.; Liang, H.; Liu, L.; Liu, Y.; Galeckas, A.; Kuznetsov, A. Y.; Du, X. Engineering of Optically Defect Free Cu2O Enabling Exciton Luminescence at Room Temperature. Opt. Mater. Express 2013, 3, 2072-2077. 49. Pan, L.; Kim, J. H.; Mayer, M. T.; Son, M.-K.; Ummadisingu, A.; Lee, J. S.; Hagfeldt, A.; Luo, J.; Grätzel, M. Boosting the Performance of Cu2O Photocathodes for Unassisted Solar Water Splitting Devices. Nat. Catal. 2018, 1, 412-420. 50. Yang, Y.; Xu, D.; Wu, Q.; Diao, P. Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochemical Hydrogen Evolution Reaction. Sci. Rep. 2016, 6, 35158.



Page 32 of 33

51. Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 8999. 52. Chiu, Y.-H.; Lai, T.-H.; Chen, C.-Y.; Hsieh, P.-Y.; Ozasa, K.; Niinomi, M.; Okada, K.; Chang, T.-F. M.; Matsushita, N.; Sone, M.; Hsu, Y.-J. Fully Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production. ACS Appl. Mater. Interfaces 2018, 10, 22997-23008. 53. Li, J.-M.; Wang, Y.-T.; Hsu, Y.-J. A More Accurate, Reliable Method to Evaluate the Photoelectrochemical Performance of Semiconductor Electrode without under/over Estimation. Electrochim. Acta 2018, 267, 141-149. 54. Chiu, Y.-H.; Chang, K.-D.; Hsu, Y.-J. Plasmon-Mediated Charge Dynamics and Photoactivity Enhancement for Au-Decorated ZnO Nanocrystals. J. Mater. Chem. A 2018, 6, 4286-4296. 55. Bae, D.; Seger, B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Strategies for Stable Water Splitting via Protected Photoelectrodes. Chem. Soc. Rev. 2017, 46, 1933-1954. 56. Maijenburg, A. W.; Zoontjes, M. G. C.; Mul, G. Insight into the Origin of the Limited Activity and Stability of p-Cu2O Films in Photoelectrochemical Proton Reduction. Electrochimica Acta 2017, 245, 259-267.


Page 33 of 33 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


Table of Contents Graphic


TiO2-Au-Cu2O Photocathodes: Au-Mediated Z-Scheme Charge

Recommend Documents