Cu2O Photocatalysts for Durable

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Defect Modulation of Z-Scheme TiO2/Cu2O Photocatalysts for Durable Water Splitting Tingcha Wei, Ya-Nan Zhu, Xiaoqiang An, Limin Liu, Xingzhong Cao, Huijuan Liu, and Jiuhui Qu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01786 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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Defect

Modulation

of

Z-Scheme

TiO2/Cu2O

Photocatalysts for Durable Water Splitting Tingcha Wei, +,# Yanan Zhu, + Xiaoqiang An, #,* Li-min Liu, & ,* Xingzhong Cao,※ Huijuan Liu, # Jiuhui Qu #

+

Beijing Computational Science Research Center, Beijing 100193, China.

# Center

for Water and Ecology, State Key Joint Laboratory of Environment Simulation

and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.

& School

of Physics, Beihang University, 100191, China.

※ Institute

of High Energy Physics, Chinese Academy of Science, Beijing 100049,

China.

ACS Paragon Plus Environment

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ABSTRACT: Although possessing high activity for solar hydrogen production, exploring robust Cu2O-based photocatalysts remains a challenging task due to its intrinsic drawback of susceptible oxidation. Herein, we present a strategy to stabilize Cu2O by modulating the exposed facets and structural defects of TiO2. Both experimental characterizations and theoretical calculations proved that surface oxygen vacancies in 101-faceted TiO2 could create conducting channel for denoting electrons to Cu2O, mimicking the Z-scheme charge transfer in natural photosynthesis. Due to the defectenhanced charge separation and the effective scavenging of oxidative holes in Cu2O, Cu2O/TiO2 heterostructures with exposed {101} facets and oxygen vacancies exhibited 251-fold increased activity for solar water splitting, together with unpredicted photostability. In contrast, defect-induced isolated states in the bulk of 001-faceted TiO2 led to the formation of Type II Cu2O/TiO2 junction with moderate photoactivity and poor stability. Thus, our work not only provides insights into the facet- and defect-dependent interfacial mechanism in heterostructured nanocatalysts, but also opens up a promising avenue into developing high-performance noble-metal-free photocatalysts for energy conversion applications.


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KEYWORDS: Charge transfer, photocatalytic water splitting, oxygen vacancy, crystal facet, Z-scheme mechanism. Introduction

Photocatalytic water splitting provides as a clean and effective way to address the aggravated energy and environmental problems. By far, the practical application of most promising TiO2 photocatalysts is seriously limited by the poor light utilization efficiency and fast photocarrier recombination.1 Tremendous efforts are being pursued to enhance the charge separation through coupling TiO2 with narrow-bandgap semiconductors or cocatalsysts.2-4 Among these materials, Cu2O has emerged as a promising candidate, due to the characteristics of earth abundance, environmental compatibility and high visible light activity.5-8 Unfortunately, durable hydrogen evolution over Cu2O remains a great challenge, as it suffers from serious photocorrosion.9-11 Since the redox potential of monovalent copper lies within its bandgap, accumulated photocarriers thermodynamically favored the transformation of Cu2O into CuO and Cu, resulting in the sharply deteriorated


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photoactivity for water splitting.12 Stabilizing Cu2O is an essential prerequisite for the construction of high-performance Cu2O/TiO2 photocatalysts.

Currently, creating protective layer has become the most common strategy to stabilize Cu2O. However, its photoactivity is unavoidably suppressed by the buried active sites and blocked electron transport. With respect to Cu2O/TiO2 junctions with Type II band alignment, holes are prone to accumulate on Cu2O, which might aggravate the photocorrosion.13 Water splitting based on Z-scheme mechanism has been considered as a promising strategy to overcome the limitations of single-component photocatalysts both kinetically and thermodynamically.14 Different from the conventional strategies, the direct recombination of electrons in the conduction band (CB) of TiO2 with holes in the valence band (VB) of Cu2O could precisely deprive oxidative holes from Cu2O.15,16 Thus, this Z-scheme charge transfer has a huge potential for suppressing the photocorrosion of incorporated Cu2O. Noted that our recent studies revealed the defect- and facetdependent behavior of charge carriers, strategically integrating facet engineering and defect modulation might provide a versatile approach to construct Z-scheme Cu2O/TiO2


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heterostructures.17-20 However, the impact of defect-involved Z-scheme process on the stability of Cu2O remains largely unknown.

For this challenging purpose, we hypothesized the stabilization of Cu2O by modulating the defects in faceted Cu2O/TiO2 heterostructures. Guided by this inspiration, Cu2O was rationally arranged onto the typical facets of TiO2 photocatalysts. We found that oxygen vacancies in 101-faceted TiO2 can create unique channel for Z-scheme charge transfer in Cu2O/TiO2 heterostructures, with a hydrogen production rate of 32.6 mmol h-1 g-1 and unpredicted photostability. However, Type II charge transfer in defective Cu2O/001faceted TiO2 was unfavorable for the durable and efficient water splitting. These findings validate the pronounced effect of defect modulation on the photoactivity and photostability of faceted heterostructures.

Experimental Section

Synthesis of Catalysts


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Synthesis of nonstoichiometric TiO2 nanostructures: Hydrothermal reactions reported in our previous studies were used to fabricate {001}-faceted TiO2 (T0) and {101}-faceted TiO2 (T1), respectively.21 A thermal reduction strategy was thereafter used to fabricate oxygen-deficient faceted TiO2. Typically, as-synthesized TiO2 were placed in the middle of a horizontal tube furnace. The powders were heat-treated at 300℃ for 2 h with H2 flow (20 mL/min). Nonstoichiometric {001}- and {101}-faceted TiO2 were denoted as T0-VO and T1-VO. The concentration of oxygen vacancies was regulated by changing the treatment temperature, such as 200, 300, 400℃.

Photodeposition of copper-related nanoclusters onto nonstoichiometric TiO2

Copper-related species were deposited onto the surface of different TiO2 photocatalysts via a photodeposition route. In a typical procedure, 5 mM CuCl2 and 25 mM sodium citrate were added into deionized water to prepare the precursor solution of CuII-citrate. Then, 50 mg of TiO2 was dispersed into a mixture of ethanol/water. 40 μL of 10 M NaOH and 240 μL of CuII-citrate solution were added, respectively. After stirring in the dark for 30 min, the solution was irradiated by a 300 W Xe lamp without filters for 1 h. After the


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filtrating, washing and drying procedures, copper loaded TiO2 samples were obtained. Composite photocatalysts synthesized from oxygen-deficient T0 and T1 were denoted as Cu2O/T0-VO and Cu2O/T1-VO, respectively. For comparison, perfect T0 and T1 were also used to fabricate Cu2O/T0 and Cu2O/T1 photocatalysts.

Catalyst Characterizations

The phase structure of photocatalysts was studied by X-ray diffraction (XRD, X’Pert Pro MPD), using Cu kα radiation with a scanning angle (2θ) of 20°-80°. X-ray photoelectron spectroscopy (XPS, ESCALAB MKII) was used to analyze the electronic structure. With Mg Kα excitation source (hν = 1253.6 eV), the charge of samples was corrected by the binding energy of adventitious carbon (284.6 eV). Scanning electron microscope (SEM, SIGMA) and transmission electron microscope (TEM, JEOL-2100F) were used to observe the morphology of products. The elemental components were determined by an energy dispersive X-ray spectrometer (EDXS) attached to the microscopes. We obtained the size distribution of Cu2O by analyzing 150 particles. Brunauer-Emmett-Teller (BET) measurements were carried out on a nitrogen adsorption apparatus (Micromeritics


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ASAP2020). UV-vis diffusion reflectance spectrophotometer (DRS, Cary 5000) were employed to evaluate the absorption ability in the range of 200-900 nm. The time-resolved and steady-state fluorescence spectra were collected by the FLS-980 fluorescence spectrometer. The excitation wavelength was 375 nm. A Bruker E500 spectrometer was used to collect the electron spin resonance (ESR) spectra at room-temperature and lowtemperature. Low temperature was measured under 77 K by liquid nitrogen. An inductively coupled plasma spectrometry (ICP-OES 710, Aglient) was used to determine the content of copper in the composites. Positron annihilation spectroscopy (PAS) were measured by a conventional fast-low coincidence spectroscope. The time resolution was 195 ps at room temperature and 16 μCi 22Na was used as positron source.

Photocatalytic experiments

Photocatalytic hydrogen evolution was conducted in a gas-closed circulation system. Typically, 20 mg of different copper-loaded photocatalysts were added into 100 mL of DI water, with 10 mL of methanol as hole scavenger. The light source was a 300 W Xe lamp (CEL-HXF300). Prior to irradiation, the reaction system was thoroughly evacuated to drive off the air. Before, the reaction system was degassed by evacuation for 30 min to. An online gas chromatograph (GC-7806, 5 Å molecular sieves) was used to analyze the evoluted H2. For comparison, 1 wt% Pt was


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photodeposited and used as noble metal cocatalyst for photocatalytic water reduction. AQY was calculated according to the following formula: Apparent quantum yield % =

[The number of reacted electrons or holes] × 100 [The number of incident photons]

The monochromatic light of 350 nm was obtained with band-pass filters.

Electrochemical measurements

A conventional three-electrode cell was employed to investigate the electrochemical properties. Working electrodes were fabricated by depositing sample suspensions onto FTO glasses. A piece of Pt plate and standard Ag/AgCl were used as counter and reference electrode, respectively. The electrolyte was Na2SO4 solution (0.5 M, pH=6.8). Electrical impedance spectroscopy (EIS) was collected by an electrochemical workstation (Gamry, Interface 1000), with an applied potential of 0 V vs. Ag/AgCl.

Computation method

First-principles calculations was carried out by using the CP2K/Quickstep package.22 The generalized gradient approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) exchange-correlation functional was used.23 Normconserving Goedecker, Hutter (GTH)


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pseudopotentials and Teter were used to describe the core electrons.24 For Ti 3d, Cu 3d orbitals, O 2p orbitals of TiO2 and O 2p orbitals of Cu2O, Hubbard U correction was carried out. The corresponding U values were set to 3.5, 6.8, 3.5 and 12 eV, respectively.25-27 Based on the Gaussian functions, the wave functions of the valence electrons were expanded, with molecularly optimized double-ζ polarized basis sets (m-DZVP).28 The plane waves with a 280 Ry cutoff were applied. The copper-modified TiO2 surface was modeled using three-trilayer 101-anatase and four-trilayer 111-Cu2O slabs, as shown in Figure S1. To clarify the effect of oxygen vacancies on the stability of Cu+, two oxygen atoms were removed from the subsurface of 101-anatase, because of the better stability of subsurface oxygen vacancies.29,30 To avoid the periodic interaction, the vacuum layers were set to 15 Å.

Results and Discussion

In order to verify the above hypothesis, 001-faceted (T0) and 101-faceted TiO2 (T1) were used to construct Cu2O/TiO2 heterostructures (Scheme 1). Hydrogen thermal reduction was used to create oxygen vacancies and the concentration of defects was regulated by


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changing the treatment temperature.31 Cu2O was thereafter grafted onto defective samples by a facile photodeposition method, achieving Cu2O/T0-VO and Cu2O/T1-VO. For comparison, 001- and 101-faceted TiO2 were also used to construct Cu2O/T0 and Cu2O/T1 photocatalysts.

The phase structure of as-synthesized photocatalysts was studied by XRD. As shown in Figure S1, the strong diffraction peaks can be well indexed to anatase TiO2 (JCPDS 01-078-2486). No peaks related to Cu, CuO, Cu2O and Cu(OH)2 are observed in copper loaded samples. This can be ascribed to the small amounts present and the high dispersion of Cu-related species on the surface of TiO2.

Scheme 1. The schematic illustration of synthetic routes for Cu2O/TiO2 heterostructures.


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According to the field emission SEM observation (Figure S2), both T0 and T1 are found to be uniform throughout the product. TEM image displays a well-defined square-shaped morphology of T0 (Figure 1a). The side length and thickness of nanosheets is about 100 nm and 5 nm, respectively. High-resolution TEM presents the fringe spacing of 0.35 nm (Figure 1b), which agrees well with the (101) lattice plane of anatase TiO2. In contrast, rhombic morphology with an average apex-to-apex diameter of 15 nm is observed for T1 (Figure 1c). Based on Wulff construction, these nanoparticles are tetragonal nanobipyramids composed of eight (101) facets (Figure 1d).32 High-angle-annular-darkfield scanning transmission electron microscopy (HAADF-STEM) was further performed to study the structure of copper loaded TiO2. In Figure 1e and 1f, the appearance of large amount

of

bright

points

confirms

that

successful

formation

of

TiO2/Cu2O

heterostructures.33 The average diameter of Cu2O nanoclusters is determined to be 2 nm, and control experiments demonstrate that the deposition conditions exhibits ignorable influence on the size distribution. EDXS mapping results suggest the uniform distribution of Cu2O on the surface of TiO2 (Figure S3). The total mass content of copper-based species was about 1.5 percent according to the ICP-OES results (Table S1).


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Figure 1. TEM and HR-TEM images of T0-VO (a, b) and T1-VO (c, d); HAADF-STEM images of Cu2O/T0-VO (e) and Cu2O/T1-VO (f).

To study the electronic structure of TiO2 supports, samples reduced at various temperatures were analyzed. For pristine T0, typical peaks are observed at g =1.98 and


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1.96 in the ESR spectra (Figure 2a), which are ascribed to Ti3+ in the bulk of TiO2.34 In contrast, pristine T1 shows a strong and symmetrical signal at the g value of 2.002, indicating the trapped electrons by surface oxygen vacancies (Figure 2b).35 The remarkable increase of ESR signals of hydrogen-treated samples proves the successful creation of defects in faceted TiO2. Furthermore, the concentration of these defects can be effectively tuned by changing treatment temperature (Figure S4a and S4b). Both Ti3+ and oxygen vacancies are detected in T0-VO, while only oxygen vacancies are observed in T1-VO. The deposition of Cu2O onto defective T0-VO and T1-VO results in the remarkable decrease of signal intensity. In the enlarged spectra, weak ESR peak around g=2.05 suggests the presence of small amount of Cu2+ in the composites (Figure S4c and S4d).

PAS was further used to reveal the defect states of TiO2 supports. Based on the fitted parameters in Table S2, three lifetime components (τ1, τ2 and τ3) are observed for both T0 and T1. According to the literatures, the shortest lifetime component (τ1) is the weighted average of free positrons and those trapped by small bulk defects. The medium


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lifetime component (τ2) is ascribe to positrons trapped by larger defect clusters. The longest component (τ3) is assigned to the annihilation of orthopositronium atoms in the voids of TiO2.36 Compared to T0, T1 possesses shorter positron lifetime of fast component (τ1) and longer lifetime of slow component (τ3). It implies that {101} facets of TiO2 is more favourable for the formation and accumulation of surface defects, while T0 is prone to form isolated defects in the bulk.37 Noted that the ratio of τ1 to τ2 (I1/I2) can embody the relative concentration of bulk and surface defects, the much smaller value of T1 further validate the above deduction.38 Apparently, T1-VO exhibits further decreased I1/I2 and shortened positron lifetime. It evidences the facilitated formation of surface oxygen vacancies and the dissociation of defect-related associates for charge separation.39 All these results evidence the difference in electronic structures of T0 and T1 supports.

XPS was used to investigate the chemical environment of component elements in defective heterostructures. Survey spectra suggests the existence of Ti, O and Cu elements in Cu2O/TiO2 heterostructures (Figure S5). Compared to T0, Ti 2p peaks of T0VO and Cu2O/T0-VO shift toward higher binding energies. It indicates the transformation


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of surface Ti3+ into bulk defects during defect modulation (Figure 2c).40 Differently, opposite peak shift is observed for T1-VO and Cu2O/T1-VO, suggesting the facilitated formation of surface Ti3+ caused by oxygen vacancy-induced electrons.41-43 O 1s spectra of typical samples were further collected and deconvoluted. In Figure S6a and S6b, the strong peak at 529.8 eV can be assigned to the lattice oxygen in metal oxides. The weak peaks located at 531.2 eV and 532.5 eV are attributed to hydroxyl groups and surface defects, respectively.44,45 For T1-Vo, the intensity of the latter peak is relatively higher than that of pristine T1. The deposition of Cu2O results in the further increase of peak intensity from 4.0% to 6.4%. It suggests the beneficial role of hydrogen reduction and junction construction for the formation of oxygen vacancies. As-formed defects present obvious impact on the chemical state of copper. In Figure 2d, the two dominant peaks at 932.4 eV and 952.2 eV can be assigned to surface Cu(0) or Cu(I), which cannot be distinguished by Cu 2p spectrum.46-48 Auger electron spectroscopy is often employed to investigate the oxidation state of Cu-related species. It has been reported that Cu LMM Auger peaks of Cu(0), Cu(I) and Cu(II) are located at 918.4, 916.8 and 917.7 eV, respectively.49-51 Figure S6c well evidences that copper species in the heterostructures


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are existed in the form of Cu2O. Deconvoluted Cu 2p spectra suggest the presence of Cu(II) in both Cu2O/T0 and Cu2O/T1 (Figure S6d). However, the utilization of defective TiO2 results in the sharply decrease of Cu(II) content. It suggests the strong interfacial interactions between Cu2O and defective TiO2, which is consistent with the quenched ESR signal of copper-grafted heterostructures. Overall, these characterizations validate the contribution of structural defects in TiO2 to the stability of Cu2O, i.e. inhibiting the oxidation of Cu(I) into Cu(II).


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Figure 2. (a) ESR spectra of T0, T0-VO and Cu2O/T0-VO. (b) ESR spectra of T1, T1-VO and Cu2O/T1-VO. (c) Ti 2p XPS spectra of faceted TiO2 and copper grafted faceted TiO2. (d) Cu 2p XPS spectra of copper grafted faceted TiO2 with oxygen vacancies.

Photocatalytic

water

splitting

reaction

was

carried

out

using

TiO2/Cu2O

heterostructures with and without defects as photocatalysts. Under AM 1.5 irradiation, T0 exhibits 8-fold higher H2 production rate than T1 (Figure 3a). Although junction constraction increases the activity of T0 to a certain extent, the inferior performance of defective heterostructures suggests the detrimental role of these defects in charge separation (Figure S7). Amazingly, the synergetic integration of surface defects and Cu2O nanoclusters with T1 results in the significantly increased photoactivity (Figure 3b). With a quantum efficiency of 53.5% at 350 nm, the maximum H2 production rate of Cu2O/T1VO reaches 32.6 mmol/h/g. This value is 251 and 39 times higher than pristine T1 and Cu2O/T0-VO. To demonstrate the preponderance of our photocatalysts, the benchmark P25 was also used to construct copper grafted heterostructures (Figure 3c). In Figure S7c, the 3.5-fold higher hydrogen evolution rate of Cu2O/T1-VO than Cu2O/P25-VO suggests


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the significant contribution of facet engineering to the photoactivity. More importantly, Cu2O/T1-VO with modulated facets and defects even exhibits much superior activity than conventional platinized TiO2 and other excellent TiO2-based catalysts reported previously in the literatures (Table S3). Therefore, our synergistic strategy not only offers a promising way to enhance photocatalytic efficiency, but also marks an important step toward exploring noble-metal-free system for hydrogen production.

Figure 3. (a) Hydrogen production rates of T0, T0-VO and Cu2O/T0-VO. (b) Hydrogen production rates of T1, T1-VO and Cu2O/T1-VO. (c) Hydrogen production rates of P25,


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Cu2O/P25 and Cu2O/P25-VO. (d) Cycling measurements of photocatalytic hydrogen generation over Cu2O/T1-VO under simulated solar light.

Due to the occurrence of photocorrosion, Cu2O is usually not stable under light irradiation conditions. Unexpectedly, our Cu2O nanoclusters immobilized on defective T1 turn to be stable. As shown in Figure 3d, Cu2O/T1-VO keeps extremely efficient and stable performance for hydrogen evolution, with no noticeable decrease in the photoactivity even up to 32 hours. Noted that obviously decreased photoactivity is achieved for Cu2O/T0-VO (Figure S7d), our strategy indeed plays a pronounced effect on stabilizing Cu2O. It should be mentioned that no Cu(II)-related species was detected in the recycled photocatalysts (Figure S8), suggesting the effective inhibition of hole-induced photooxidation. This pronounced effect well validates the hypothesis of defect-stabilized Cu2O photocatalysts.

The fundamental reasons for the unprecedented photocatalytic performance of Cu2O/T1-VO was thereafter investigated. The photocatalytic activity of T1 and T1-Vo is poor, although they possess two times higher BET surface areas than T0 and T0-Vo (Table S4). However, the deposition of Cu2O onto T1-Vo results in the significantly


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increase of hydrogen evolution rate, with slightly changed surface area. Therefore, surface area should not be the fundamental reason for the superior photoactivity of Cu2O/T1-VO. Therefore, we looked into the photochemical processes involved in TiO2/Cu2O heterostructures. According to the diffusion reflection measurements, photoactivity of Cu2O/T1-VO should be attributed to the visible light absorbed by Cu2O and defect states in TiO2, together with the UV light absorbed by TiO2 (Figure S9). Based on the Tauc plots, the bandgaps of T1 and T0 are determined to be about 3.0 eV. In accordance with previous reports, the formation of defect levels leads to the remarkable visible light absorption of T0-Vo and T1-Vo. Besides the further decreased bandgap, additional absorption edges are observed for Cu2O/TiO2 heterosteuctures, corresponding to the visible light absorption of Cu2O (Figure S9c and S9d). The photoexcitation process was thereafter studied by ambient photoluminescence (PL) and time-resolved photoluminescence spectroscopies. As displayed in Figure 4a and Figure 4b, prompt PL spectra recorded under 300 K exhibit a facet-dependent feature. For T0, a broad emission at ~495 nm is observed, which can be assigned to excitons bound to partially reduced bulk Ti3+.52 In comparison, T1 shows a PL band in the region of 400-450 nm. Based on


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previous reports, this emission is attributed to the self-trapped excitons located at the TiO6 octahedron.53 The suppressed PL emission of all Cu2O grafted samples confirms the beneficial role of heterostructure formation for interfacial charge separation.54 As expected, the impact of defect modulation on the radiative emission is facet-dependent. In consistent with the negligible change of Ti3+ in the ESR results, PL emission of T0 keeps unchanged after hydrogen reduction. Differently, significant PL quenching is observed for T1-VO, demonstrating the efficient charge separation facilitated by surface oxygen vacancies.

Time-resoled PL spectra monitored at these emission peaks enable us to study the excitation kinetics of charge carriers. The decay profiles of faceted TiO2 before and after Cu2O modification are analyzed to be tri-exponential decays (Figure S10), and the corresponding lifetimes and fractional intensities are summarized in Table 1. Based on previous reports, the fast component originates from the radiative recombination of photoinduced electrons and holes, whereas the slow component arises from the selftrapped excitons.55 The delayed fluorescence lifetimes of T0 and T1 are determined to be


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13.6 and 9.76 ns, respectively. After the deposition of Cu2O onto T0, the prolonged lifetime of fast component and decreased lifetime of slow component implies the formation of Type II junction. In this case, Cu2O acts as photosensitizer to inject electrons into TiO2. T1 behaves totally different with respect to the charge transfer mechanism. The significantly shortened lifetime of fast component indicates that Z-scheme charge transfer efficiently restrained the recombination of charge carriers.56


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Figure 4. (a) Ambient PL spectra of T0, Cu2O/T0 and Cu2O/T0-VO. (b) Ambient PL spectra of T1, Cu2O/T1 and Cu2O/T1-VO. (c) Low-temperature PL spectra of T0, Cu2O/T0 with and without defects. (d) Low-temperature PL spectra of T1, Cu2O/T1 with and without defects. (e) Low-temperature Time-resolved PL decay curves of Cu2O/T0 and Cu2O/T0VO. (f) Low-temperature Time-resolved PL decay curves of Cu2O/T1 and Cu2O/T1-VO.

To obtain a fundamental understanding of the charge transfer process, we further performed low-temperature photoluminescence measurements. In contact with liquid nitrogen, all heterostructures exhibit a colour center emission at ~560 nm, which is ascribed to the recombination between mobile electrons (those in the CB or in the shallow band-tail traps) and trapped holes.57 As a consequence of bulk defect formation, Cu2O/T0-VO shows stronger PL emission than Cu2O/T0 (Figure 4c), together with prolonged decay lifetime (Figure 4e). This further confirms the Type II mechanism of charge transfer in defective Cu2O/T0 heterostructures.58 Conversely, the shortened PL lifetime of Cu2O/T1-VO well validates the fundamental role of surface oxygen vacancies as charge transfer channel to withdraw dissociated electrons from TiO2 (Figure 4d and


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Figure 4f).59 The facilitated charge transfer in Cu2O/T1-VO is further confirmed by the decreased semi-circle in the Nyquist plots (Figure S11). This unique feature will definitely contribute to the Z-scheme mechanism of defective Cu2O/T1-VO.

Table 1. Lifetime components of faceted TiO2 before and after Cu2O modification

τ1(ns)

τ2(ns)

τ3(ns)

I1

I2

I3

τave(ns)

T0 (496nm)

3.03±0.02

12.73±0.14

42.87±0.74

9.74

36.74

53.51

13.6

Cu2O/T0 (496nm)

40.22±0.23

9.78±0.07

1.63±0.03

52.69

37.15

10.15

8.8

T1 (405nm)

50.26±0.44

12.77±0.13

2.49±0.01

36.63

49.21

14.16

9.76

Cu2O/T1 (405nm)

3.11±0.02

14.16±0.09

52.71±0.58

16.21

49.22

34.56

10.7

Density functional theory (DFT) calculations were conducted to verify the above Zscheme mechanism. It is revealed that T1 is apt to form oxygen vacancy defects on the subsurface, with much lower formation energy than T0. Benefited from the defect-induced 5-fold Ti, T1 exhibits a strong polaronic effect and would localize excess electrons near the surface.60 In principle, these trapped electrons should influence the interfacial transfer of charge carriers.61 Two oxygen atoms are thereby removed from the subsurface of


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anatase to simulate the Cu2O/T1 heterointerface with high concentration defects (Figure 5a and Figure 5b). Based on the calculated density of states (DOS), the VB edge of perfect Cu2O/TiO2 is mainly composed of Cu 3d states, while the CB edge is caused by Ti 3d states (Figure 5c). Interestingly, after the creation of oxygen vacancies, several new localized states composed of Ti3+ form in the band gap of T1 (Figure 5d). Since these gap states are closed to the valence band maximum (VBM) of Cu(I), trapped electrons in TiO2 would spontaneously inject into the Cu 3d orbitals. Band alignment between TiO2 and Cu2O is further considered to support the charge transfer process. According to the Tauc plots and VB XPS spectra (Figure S12), T1 with lower band positions should be more favorable for the formation of Z-scheme Cu2O/TiO2 heterostructures. In accord with experimental characterizations, this unique process can theoretically retard the holeinduced oxidization of Cu(I) into Cu(II).

Upon elucidating the charge carrier behaviors, the photocatalytic mechanism of Cu2O/TiO2 heterostructures for water splitting is illustrated. In general, photo-induced free electrons are prone to be trapped by Ti ions in T0, forming bulk Ti3+ defects. When


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coupled with Cu2O, oxygen vacancies with positive charge states tend to accept electrons from Cu2O. This means the favorable formation of Type II junctions between Cu2O and T0 (Figure 5e).62 Although hydrogen reduction can create additional isolated states, the role of bulk defects as recombination center inevitably deteriorates the photocatalytic performance.63 Differently, both DFT simulation and structural characterizations reveal the favorable formation of oxygen vacancies on the surface of T1. Efficient trapping of free electrons on these defective sites generates electron-rich interface. By virtue of the recombination of these electrons with holes in the VB of Cu2O, the Z-scheme mechanism can effectively consume oxidative holes in Cu2O and avoid its self-oxidation into Cu(II) (Figure 5f). With the maximum amount of surface oxygen vacancies, Cu2O/T1-Vo fabricated at 300 ℃ exhibits excellent photoactivity and photostability for solar water splitting.


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Figure 5. DFT models of perfect (a) and defective (b) Cu2O/T1 heterostructures. The gray, blue and red balls represent the Ti, Cu and O atoms, respectively. The excess electrons induced by oxygen vacancies are shown as yellow contour. (b) The calculated density of states (DOS) of perfect (c) and defective (d) Cu2O/T1 heterostructures. The blue, red, purple and green lines stand for projected DOS for Ti, O of TiO2, Cu and O of Cu2O, respectively. The energy of VBM was set to zero. Schematic diagrams of interfacial charge transfer in defective Cu2O/T0 (e) and Cu2O/T1 (f) heterostructures.

Conclusion


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In summary, Z-scheme Cu2O/TiO2 heterostructures with modulated defects were constructs by depositing Cu2O clusters onto faceted TiO2. The combination of facet engineering and defect modulation not only resulted in the efficient separation of photocarriers, but also contributed to the suppressed self-oxidation of Cu2O. Benefitted from the Z-scheme charge transfer, 101-faceted TiO2 modified by Cu2O exhibited significantly improved activity and unprecedented photostability for solar hydrogen evolution. Noted that TiO2 protective layer has been widely used to alleviate the photocorrosion of Cu2O, these findings provide new insight into the development of Cu2Obased photocatalysts for efficient and durable water splitting.

AUTHOR INFORMATION

Corresponding Author * [email protected]

* [email protected]

Author Contributions


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Dr. Tingcha Wei and Dr. Yanan Zhu contributed equally to this work.

Notes The authors declare no competing financial interest.

Supporting Information The following files are available free of charge.

XRD patterns, SEM images, ESR spectra, Auger and UV-vis spectra, Time-resolved fluorescence spectra, hydrogen evolution performance, ICP and positron annihilation results.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (grant No. 51538013, 51578531). This work was also supported by the National Key R&D Program of China (Grant No. 2016YFC0400502). The computation supports from Tianhe-2JK computing time award at the Beijing Computational Science Research Center (CSRC)


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and the Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (the second phase) were also acknowledged.

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Graphical Abstract


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Cu2O Photocatalysts for Durable

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