TiO2 Quantum Dot

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Synthesis of Cu2O Octadecahedron/TiO2 Quantum Dot Heterojunctions with High Visible Light Photocatalytic Activity and High Stability Xu Xu, Zhonghui Gao, Zhenduo Cui, Yanqin Liang, Zhaoyang Li, Shengli Zhu, Xianjin Yang, and Jianmin Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06536 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 14, 2015

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ACS Applied Materials & Interfaces

Synthesis of Cu2O Octadecahedron/TiO2 Quantum Dot Heterojunctions with High Visible Light Photocatalytic Activity and High Stability Xu Xu,a Zhonghui Gao,a Zhenduo Cui,a Yanqin Liang,a,b,∗ Zhaoyang Li,a,b Shengli Zhu,a,b Xianjin Yang,a,b and Jianmin Mac,d,∗ a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

b

c

Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China

Key Laboratory for Micro-/Nano-Optoelectronic Devices of the Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China

d

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia

∗

Corresponding author Tel: +86-022-27402494 E-mail: [email protected] (Y. Q. Liang)

∗

Corresponding author Tel: +86-0731-88822137 E-mail: [email protected] (J. M. Ma)

ACS Paragon Plus Environment

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ABSTRACT: Since p-n heterojunction photocatalysts with higher energy facets exposed usually possess greatly enhanced photocatalytic activities than single-phase catalysts, a novel Cu2O octadecahedron/TiO2 quantum dot (Cu2O-O/TiO2-QD) p-n heterojunctions composite was designed and synthesized in this study. Cu2O octadecahedra (Cu2O-O) with {110} facets and {100} facets exposed were synthesized firstly, then highly dispersed TiO2 quantum dots (TiO2QDs) were loaded on Cu2O-O by the precipitation of TiO2-QDs sol in the presence of absolute ethanol. The morphology, crystal structure, chemical composition, optical properties, photocatalytic activity, and stability of Cu2O-O/TiO2-QD heterojunctions were characterized and investigated. It was found that TiO2-QDs were firmly anchored on Cu2O-O single crystals with good dispersibility. The Cu2O-O/TiO2-QD heterojunctions with partial coverage of TiO2-QDs showed a strong absorbance of visible light and exhibited an effective transfer of photoexcited electrons. The degradation of methyl orange (MO) under visible light irradiation indicated that the photocatalytic activity of Cu2O-O/TiO2-QD heterojunctions was significantly enhanced compared with that of Cu2O-O. This Cu2O-O/TiO2-QD heterojunctions composite exhibited high stability in MO degradation process and after storage in air. The high visible light photocatalytic activity and good stability were attributed to high utilization of light, effective separation of photoexcited electron−hole pairs, and instant scavenging of holes in the unique heterojunction structure.

KEYWORDS: higher energy facets, heterojunctions, Cu2O octadecahedron/TiO2 quantum dot, photocatalytic activity, stability, photoexcited electron−hole pairs


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1. INTRODUCTION Semiconductor photocatalysts have attracted considerable attention for both fundamental research and practical application in energy and environment fields.1−5 Particularly, Cu2O is of great importance owing to its many unique physical and chemical properties,6−10 such as narrow band gap, reasonable cost, low toxicity, good environmental acceptability, and strong adsorption of molecular oxygen.11−13 These advantages have contributed to the application of Cu2O in the visible light photocatalytic degradation of organic pollutants. It is well known that the performances of a single-phase semiconductor are strongly dependent on its size, shapes, and crystallization.14−16 For low-index facets Cu2O crystals, it has been reported that the average surface energies follow the order as {110} > {111} > {100}.10 Recent studies suggested that photocatalysts with higher energy facets exposed were more active, and the slight difference in surface energy levels between different facets could accelerate the separation of photoexcited electron−hole pairs.17−19 Therefore, incorporation of different exposed facets is an effective strategy to enhance the photocatalytic activity of Cu2O.20,21 Furthermore, coupling narrow-bandgap p-type Cu2O with n-type TiO2 to form a heterojunction structure is another important approach to improve the photocatalytic efficiency.22−24 It has been reported that the matching energy-level alignments thermodynamically promote the transfer of photoexcited electrons from Cu2O to TiO2.25,26 In addition, the inner electrostatic field of p-n heterojunction also enhances the separation efficiency of electron−hole pairs.27,28 Thus, significant effort should be devoted to creating heterojunctions on Cu2O with controlled facets. However, investigations about creation of heterojunctions on controlled Cu2O facets were limited and some problems still exist in structural design, synthesis process, and practical application.29−32 The creation of heterojunctions on controlled Cu2O facets were dominated by


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{111} and {100} facets.29,31,32 However, Cu2O polyhedron with {110} facets exposed is more active and thus it should be applied for higher photocatalytic activity. Several research groups synthesized Cu2O with {110} facets exposed in different methods, but these Cu2O required rigorous synthesis processes and faced problem of instability.33−35 Therefore the synthesis of Cu2O polyhedron need to be improved. In addition, designing heterojunction structure is another effective strategy to enhance photocatalytic activity. However, most reported heterojunctions were based on TiO2/Cu2O layered film structure or Cu2O/TiO2 core−shell structure.36−38 Although these structures could accelerate the separation of photoexcited charge carrier, the most significant drawback was that they failed to fulfill the high energy facets of Cu2O. The interface area of Cu2O/TiO2 heterojunctions was so large that Cu2O could not in direct contact with surrounding solution.39 As a result, photoexcited holes could not be scavenged instantly, and thus photocorrosion would occur in Cu2O catalysts. Therefore, it is reasonable to convince that Cu2O with partial coverage of TiO2 is favorable for enhancing the photocatalytic activity and stability. Furthermore, the problems of serious agglomeration and recycling process should be solved for photocatalysis applications.25,40 Therefore, it is feasible to improve photocatalytic activity by synthesizing a stable Cu2O/TiO2 heterojunction with favorable structure. In this work, Cu2O octadecahedra (Cu2O-O) single crystals with {110} and {100} facets exposed were synthesized preferentially. Then Cu2O octadecahedron/TiO2 quantum dot (Cu2OO/TiO2-QD) heterojunctions with partial coverage of TiO2 quantum dots (TiO2-QDs) were created by the precipitation of TiO2-QDs sol on Cu2O-O in the presence of absolute ethanol. The morphology, structure, chemical composition, and optical properties of the Cu2O-O/TiO2-QD heterojunctions were characterized and analyzed in detail. Their visible light photocatalytic activity was demonstrated by the degradation of methyl orange (MO). The stability of the Cu2O-


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O/TiO2-QD heterojunctions during the degradation process and storage were also investigated. Finally, structural advantages of the heterojunctions were discussed and analyzed. 2. EXPERIMENTAL SECTION 2.1 Chemicals. Copper(II) chloride dehydrate (CuCl2•H2O; 99%), sodium hydroxide (NaOH; 96%), and absolute ethanol (EtOH, 99.7%) were acquired from Tianjin Kemiou Chemicals Co., Ltd. Hydroxylamine hydrochloride (NH2OH•HCl; 99%), titanium(IV) n-butoxide (TBT; 99%) and methyl orange (MO) were purchased from Alfa Aesar. Cyclohexane (C6H12; 99.5%) and oleylamine (OM; 80-90%) were acquired from Acros. Sodium dodecyl sulfate (SDS; 98.5%) and oleic acid (OA; 95%) were purchased from Sigma-Aldrich and Beijing HWRK Chem Co., Ltd., respectively. All chemicals were used as received without further purification. 2.2 Synthesis of Cu2O-O. Cu2O-O were synthesized according to a reported process with some modifications.35 In a typical experiment, 100 mL deionized water was added into a 300 mL beaker and placed in a water bath at 30 °C. 7.5 mL of 0.1 M CuCl2 solution and 1.3 g SDS powder were added into the beaker with vigorous magnetic stirring. After complete dissolution of SDS, 2.7 mL of 1 M NaOH solution was quickly added into the mixture under stirring, which resulted in a light blue colored solution containing Cu(OH)2 precipitate. Then, 35 mL of 0.1 M NH2OH•HCl was quickly injected into the mixed solution under vigorous magnetic stirring. The solution was kept in the water bath for 1 h for crystal growth before being centrifuged at 5000 rpm for 3 min. The obtained orange precipitates were washed with the same water/ethanol mixture several times to remove unreacted chemicals and SDS surfactant. Finally, the obtained Cu2O-O was dried at 50 °C for 6 h in a vacuum oven. 2.3 Synthesis of TiO2-QDs. We refer to some previous studies and provide a facile synthesis procedure for TiO2-QDs here.41−44 Firstly, 10 mL cyclohexane, 3 mL oleic acid, 0.1 mL TBT,


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and 1 mL oleylamine were sequentially added into a beaker with vigorous magnetic stirring to from a solution. Then, the mixed solution was transferred into a 20 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 24 h. Finally, a clear brown sol containing monodisperse TiO2-QDs was obtained. 2.4 Synthesis of Cu2O-O/TiO2-QD. In a typical synthesis, 10 mL EtOH and 10 mg Cu2O-O were added into a 20 mL vial with vigorous magnetic stirring. Then, 360 µL TiO2-QD sol was quickly injected into the vial. The mixture was stirred for 5 min before being centrifuged at 8000 rpm for 3 min. Finally, the Cu2O-O/TiO2-QD composite was washed with EtOH for several times to remove the organics, and then dried at 50 °C for 6 h in a vacuum oven. 2.5 Material Characterization. The crystal structures of the synthesized samples were analyzed by X-ray diffraction (XRD, D8 Advanced, Bruker, Germany) with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA. Surface morphologies of the synthesized samples were observed by field emission scanning electron microscopy (FE-SEM, 4800S, Hitachi, Japan). Transmission electron microscope (TEM) images, high-resolution TEM (HR-TEM) images, and the corresponding selected-area electron diffraction (SAED) patterns were collected on a fieldemission transmission electron microscope (FE-TEM, JEM-2011F, JEOL, Japan) operated at 200 kV with point-to-point resolution of 0.19 nm. TEM samples were prepared by dispersing the powders on carbon-coated Ni grids. The elemental distribution of synthesized samples was studied by bright field scanning transmission electron microscopy (BF-STEM) with X-ray energy dispersive spectroscopy (EDS). Surface chemical analyses of synthesized samples were performed by X-ray photoelectron spectroscopy (XPS) using a PHL1600ESCA instrument equipped with a monochromatic Al Kα X-ray source (E = 1486.6 eV) operated at 250 W. Ultraviolet−visible (UV−vis) spectra of samples were collected on a spectrophotometer (UV-


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2700, Shimadzu, Japan). Photoluminescence (PL) emission spectra of the samples were collected on a fluorescence spectrophotometer (FluoroLog 3-21, Horiba Jobin Yvon, France) with the excitation source at 392 nm. The Brunauer-Emmett-Teller (BET) surface areas of the samples were measured by nitrogen adsorption isotherms at 77 K, using an AutosorbiQ instrument (Quantachrome U.S.) with a 6 h outgas at 80 °C. 2.6 Photocatalytic Experiment. Photocatalytic activity of the Cu2O-O/TiO2-QD heterojunctions was evaluated by the degradation of methyl orange (MO) under visible light illumination. 25 mg of photocatalyst was dispersed in 100 mL of 30 mg/L MO aqueous solution by ultrasonication for 2 min. Before illumination, the suspension was constantly stirred for 30 min in the dark to ensure the establishment of an adsorption−desorption equilibrium between the photocatalyst and the MO. A 500 W xenon lamp (CHF-XM-500W, Beijing Trusttech Co., Ltd., China) was used as the light source, which has a glass filter to provide zero light intensity below 400 nm. The light intensity reaching the MO solution was measured to be 100 mW/cm2 by a radiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University, China). During the photocatalytic experiment, the solution was exposed to the light irradiation under magnetic stirring and surrounded by a circulating water bath for cooling. 2 mL of the suspension was collected at each time interval to separate the photocatalysts, and light absorption of the clear MO solution at 464 nm was used to evaluate its degradation as measured by the UV-2700 spectrophotometer. Cu2O-O, TiO2-QDs, and their physical mixture (Cu2O-O@TiO2-QD) were also used in the experiments on the photocatalytic degradation of MO for comparison purposes under the same experimental conditions. Recycling experiments on the degradation of MO by Cu2O-O/TiO2-QD heterojunctions were carried out to evaluate the degradation stability. At the end of every cycle, the sample was separated by allowing it to stand, washed with deionized


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water and absolute ethanol, and then dried in a vacuum oven for the next cycle. The effect of storage time on the photocatalytic properties of the composite was also investigated. All photocatalytic experiments were in triplicate. 3. RESULTS AND DISCUSSION Scheme 1. Schematic illustration of the synthesis process for the Cu2O-O/TiO2-QD.

Scheme 1 demonstrates the synthesis process of Cu2O-O/TiO2-QD. Firstly, the synthesis of Cu2O-O was based on the reduction of Cu2+ with NH2OH•HCl. As demonstrated in Figure 1a and Figure S1 in the Supporting Information, Cu2O-O with controlled facets were successfully synthesized via the modified process. These Cu2O-O had smooth surfaces and were octadecahedral in shape, with an average size of 2 µm. These Cu2O-O were formed by truncating rhombic dodecahedra along the [100] direction, thus it had twelve hexagonal {110} faces and six square {100} faces.45,46 The SAED pattern of Cu2O-O viewed along the [100] direction confirmed its single-crystal structure. These Cu2O-O with {110} faces exposed were more active, and the slight difference in surface energy levels between {110} and {100} faces provided driving force for the separation of photoexcited electron−hole pairs. Therefore, the


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synthesized Cu2O-O single crystals were expected to exhibit high photocatalytic activity and good dispersibility. In the second step, TBT was chemically modified by OA and OM, followed by hydrolysis and condensation reactions to synthesize TiO2-QDs. As shown in Figure 1b and Figure S2 in the Supporting Information, The synthesized TiO2-QDs with an average size of 4.7 nm were well spaced on the carbon-coated Ni grid and approximately spherical in shape. The HR-TEM image of TiO2-QDs clearly shows lattice fringes, suggesting high crystallinity. The lattice spacings of 0.35 nm and 0.19 nm correspond to the (101) and (200) planes of anatase TiO2, respectively. The SAED pattern corresponds to a set of diffraction rings of TiO2-QDs, which can be clearly assigned to the diffractions of the {101}, {004}, {200}, and {211} planes of anatase TiO2. The TiO2-QDs were monodisperse in the sol and could be kept for a long time without precipitating, because the surfaces of the particles were capped by organics.42,43 Thirdly, the Cu2O-O/TiO2-QD heterojunction composites were synthesized by the precipitation of the TiO2-QD sol on Cu2O-O in the presence of absolute ethanol. When the TiO2-QD sol was introduced into a mixture of Cu2O-O and absolute ethanol, monodisperse TiO2-QDs were precipitated on Cu2O-O, since capping agents were dissolved by the absolute ethanol. As demonstrated in Figure 1c and 1d, the synthesized Cu2O-O/TiO2-QD composite consisted of nearly octadecahedral shaped particles with much rougher surfaces than the Cu2O-O. It was quite intriguing to observe that TiO2-QDs became attached to the faces of Cu2O-O and aggregated to form TiO2 nano-islands with protruding structure. The TiO2 nano-islands were well-dispersed and partially covered the Cu2O-O. No obvious aggregation of TiO2 nano-islands was observed in the composite.


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Figure 1. (a) SEM image of Cu2O-O (inset, enlarged image). (b) TEM image of TiO2-QDs (inset, SAED pattern of TiO2-QDs). (c) and (d) SEM images of Cu2O-O/TiO2-QD.

Figure 2a shows the XRD patterns of Cu2O-O, TiO2-QDs, and Cu2O-O/TiO2-QD composite. It was found that all the diffraction peaks of Cu2O-O can be indexed to face-centered cubic Cu2O phase (JCPDS card No. 05-0667). No diffraction peaks of Cu or CuO could be found. TiO2-QDs were found to solely exhibit the anatase phase (JCPDS card No. 21-1272). The broad diffraction peaks were due to the small crystallite size of the TiO2-QDs. For Cu2O-O/TiO2-QD composite, no characteristic peaks of anatase TiO2 could be detected, due to the low content and high dispersion of TiO2 in this composite photocatalyst system. Closer observations by TEM and HR-


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TEM of the Cu2O-O/TiO2-QD composite further proved the presence of TiO2-QDs. A lower resolution TEM image (inset in Figure 2b) of an individual particle illustrates its polyhedral characteristics. High contrast between the dark center and the gray uneven edge with the protruding structure is clearly observed, indicating the formation of Cu2O-O with a loading of TiO2 nano-islands. Figure 2c and 2d shows representative HR-TEM images of the Cu2O/TiO2 interface area on the Cu2O-O/TiO2-QD composite. The HR-TEM images of the Cu2O-O verify its highly crystallized structure. Two sets of lattice planes can be clearly observed with spacing of 0.3 and 0.25 nm, corresponding to the (110) and (111) planes of the Cu2O phase, respectively. Some of the TiO2-QDs were monodisperse and others aggregated to form TiO2 nano-islands with a size of 15–30 nm. Both the monodisperse TiO2-QDs and the TiO2 nano-islands were firmly anchored on the Cu2O. The HR-TEM images of TiO2 nano-island and monodisperse TiO2-QDs also verify their highly crystallized structure. Two sets of lattice planes can be clearly observed with spacing of 0.35 and 0.19 nm, corresponding to the (101) and (200) planes of anatase TiO2 phase. The TiO2-QDs appeared to have strong interactions with the Cu2O-O, since sonication during the sample preparation did not lead to their dissociation from the Cu2O-O. Furthermore, the interface area between Cu2O and TiO2 could be clearly observed, indicating that the two components were in intimate contact and formed heterojunctions. The good crystallization and strong coupling of Cu2O and TiO2 are favorable to the transfer of photoexcited electrons and holes between them, which is beneficial for improving the photocatalytic performance.


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Figure 2. (a) XRD patterns of Cu2O-O, TiO2-QDs and Cu2O-O/TiO2-QD. (b) TEM image of Cu2O-O/TiO2-QD (inset, low magnification image). (c) and (d) HR-TEM images of the Cu2O/TiO2 interface area of Cu2O-O/TiO2-QD.

Information on the components and surface of the Cu2O-O/TiO2-QD composite was provided by XPS. The full survey spectrum clearly indicates the presence of Ti, O, and Cu in the sample (Figure S3a in the Supporting Information). Due to the widespread presence of carbon in the environment, a C 1s peak can also be observed. The binding energies obtained in the XPS spectral analysis were corrected with reference to C 1s (284.6 eV).47 The individual XPS spectra lines for Cu 2p, Ti 2p and O 1s were collected at a high resolution. As shown in Figure 3a, the


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two high peaks at 932.6 and 952.5 eV are respectively assigned to Cu 2p3/2 and Cu 2p1/2 of Cu+, and the appearance of the satellite peak at 945 eV is due to the shake-up process of outer electron in Cu+.48 This observation suggests that the Cu2O-O in the composite possesses excellent stability. The two peaks fitted at 458.5 and 464.2 eV correspond to Ti 2p3/2 and Ti 2p1/2 peaks of TiO2, respectively (Figure S3b in the Supporting Information).39,49 It was reported that the binding energies of O 1s in the crystal lattice and of absorbed oxygen are 528.5–529.7 and 530.54–533.77 eV, respectively.11 As shown in Figure 3b, the peak for O 1s in the composite is broad, and it can be deconvolved into four overlapping peaks. The four peaks presented four chemical states of oxygen. The peaks located at 529.6 and 530.9 eV are attributed to the O in TiO2 and Cu2O, respectively.28,50 The peaks at 531.8 and 533.1 eV are respectively assigned to the O–H bond of the hydroxyl group and to water molecules adsorbed on the surface.11,51 This suggests that the composite can strongly adsorb molecular oxygen and interact with the surrounding environment. Furthermore, the elemental distribution and structure of the Cu2OO/TiO2-QD composite were characterized by EDS. Figure 3c shows the area where the elemental mapping was performed. It contains a BF-STEM image of Cu2O-O decorated with TiO2 nanoislands, which clearly confirms that nano-islands exist on the surface of the Cu2O. The blue, green, and red colors represent the distributions of the elements copper, titanium, and oxygen, respectively. The presence of the three elements in the composites is in agreement with the proposed Cu2O/TiO2 composition. This suggests that Cu element was only present in the large octadecahedron, but not in the nano-island area, while the Ti element is mainly distributed in the nano-island area with a portion on the Cu2O. The distribution of O overlaps well with that of Cu and Ti. These observations demonstrate that TiO2-QDs are well anchored on Cu2O. All of the


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above morphology and structure characterizations indicate that a promising Cu2O-O/TiO2-QD heterojunction composite with well-designed architecture has been constructed.

Figure 3. High resolution XPS spectra of (a) Cu 2p and (b) O 1s. (c) BF-STEM image and the corresponding elemental mapping.

The optical properties of the samples were investigated by UV–vis absorbance spectroscopy and PL spectroscopy. Fig. 4a shows the UV–vis absorbance spectra of commercially available Degussa P25 TiO2 nanoparticles, the TiO2-QDs, the Cu2O-O, and the Cu2O-O/TiO2-QD


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composite. The Degussa P25 nanoparticles exhibited no absorption in the visible region of 400– 800 nm, while the TiO2-QDs exhibited slight absorption in the visible region, and the absorption intensity decreased with increasing wavelength. The Cu2O-O showed a broad and strong absorbance from about 640 nm to the UV region. After being partially covered with TiO2-QDs, there was no significant change in the absorption region or intensity of the Cu2O-O. This indicates that the Cu2O-O retained the strong light absorption in the visible region after the introduction of TiO2-QDs. The band gap energies of the samples could be calculated by the formula:52,53 αhν = A(hν-Eg)n/2, where α, h, ν, A, and Eg are the absorption coefficient, Planck constant, light frequency, a constant, and the band gap, respectively. The values of n are 1 and 4 for the direct transition of Cu2O and the indirect transition of TiO2. Therefore, the Eg values of the TiO2-QDs and Cu2O-O can be estimated from the plots of (αhν)1/2 versus hν and (αhν)2 versus hν, respectively. The Eg values of the P25 and the TiO2-QDs were found to be 3.2 eV, and the Eg values of the Cu2O-O and the Cu2O-O/TiO2-QD were evaluated to be 2.1 eV (Figure S4 in the Supporting Information). This suggests that there was no change in Eg after the deposition of TiO2-QDs on Cu2O-O, indicating that the light absorption properties of the Cu2O-O and the Cu2O-O/TiO2-QD are rather similar. Furthermore, the TiO2-QDs showed stronger absorption intensity than P25 in both the ultraviolet and the visible light region. This gives TiO2-QDs a significant advantage over P25 in photocatalytic applications of Cu2O/TiO2 heterojunction composite. Nevertheless, strong light absorption by a photocatalyst does not necessarily imply that the photocatalytic activity is high because of the recombination of photoexcited electron−hole pairs.39 It is well known that the PL spectrum is an effective way to investigate the transfer and trapping of charge carriers, and to understand the fate of photoexcited electrons and holes in semiconductor particles.36,54 Figure 4b shows the PL spectra of Cu2O-O and Cu2O-


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O/TiO2-QD. Compared with the spectrum of Cu2O-O, the PL intensity of Cu2O-O/TiO2-QD revealed a significant decrease. The observed PL spectra in semiconductors can be attributed to the radiative recombination process of self-trapped excitations, so the decrease in PL intensity in the composite indicates decreased recombination of the photoexcited electron−hole pairs.55,56 Since the conduction and valence bands of TiO2 are lower than those of Cu2O, this difference thermodynamically drives the transfer of photoexcited electrons from the conduction band (CB) of Cu2O to the CB of TiO2 and of photoexcited holes from the valence band (VB) of TiO2 to the VB of Cu2O.25,26 Therefore, the high separation efficiency of photoexcited electron−hole pairs can be attributed to the matched energy levels, the good crystallization, and the large interfaces between Cu2O-O and TiO2-QDs, which means that charge carriers can easily transfer between them. The Cu2O-O/TiO2-QD heterojunctions with {110} facets exposed exhibited high utilization of light and effective separation of photoexcited electron−hole pairs, which might greatly enhance photocatalytic activity.

Figure 4. (a) UV–vis absorbance spectra of P25, TiO2-QDs, Cu2O-O, and Cu2O-O/TiO2-QD. (b) PL spectra of Cu2O-O and Cu2O-O/TiO2-QD.


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The photocatalytic properties of Cu2O-O/TiO2-QD were evaluated by the degradation of MO under visible light illumination. Figure 5a shows curves of the MO concentration ratio (the ratio of the residue concentration to the original concentration) versus degradation time under different conditions. The MO solution without photocatalyst was stable under visible light illumination, indicating that the self-degradation of MO solution was negligible. All the photocatalysts showed low adsorption capabilities for MO. The absorption capability of Cu2O-O was slightly enhanced by the coupling with TiO2-QDs. This can be ascribed to the fact that TiO2QDs have higher adsorption capability than Cu2O-O. Single TiO2-QDs exhibited low degradation capability under 60 min visible light illumination, with residual MO concentration of ~82%. This could be ascribed to the weak light absorption in the visible region of the TiO2-QDs. On the other hand, the Cu2O-O sample exhibited much better photocatalytic activity (with residual MO concentration of ~55%) than the TiO2-QDs. After the TiO2-QDs were deposited on the Cu2O-O, the Cu2O-O/TiO2-QD showed the highest photocatalytic activity, with the MO concentration reduced by as much as 50% in 15 min and 97% in 60 min. When there was no visible light illumination on the sample of Cu2O-O/TiO2-QD, the MO concentration showed no obvious reduction, which indicates that the degradation of MO was mainly controlled by the photocatalytic effect. In addition, the Cu2O-O@TiO2-QD sample prepared by physical mixture exhibited a similar photocatalytic performance to that of the Cu2O-O alone. Furthermore, the photocatalytic efficiency enhancement could be investigated quantitatively by kinetic analysis. When the original MO concentration is very small, the degradation process follows approximately a pseudo-first-order kinetics equation:57–59 ln(c0/ct) = kt, where k is the apparent rate constant of the degradation, which is known to reflect the photocatalytic activity. c0 and ct are the initial concentration and the concentration at time t of the substance to be degraded.


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Figure 5b shows kinetic plots of the degradation processes under visible light with various photocatalysts, based on the data in Figure 5a. The k value of the Cu2O-O/TiO2-QD (0.05531 min-1) was 5.5 times higher than that of the Cu2O-O (0.01008 min-1). The results clearly demonstrate that the coupling of TiO2-QDs on Cu2O-O is an effective method for improving the photocatalytic properties of Cu2O.

Figure 5. (a) Curves of the MO concentration ratio versus treatment time under different conditions. (b) Kinetic plots of the degradation process under visible light with various photocatalysts, based on the data in (a). (c) N2 adsorption-desorption isotherms of Cu2O-O, Cu2O-O/TiO2-QD, and Cu2O-O@TiO2-QD. (d) TEM image and schematic illustration of Cu2OO/TiO2-QD (synthesized with 1.8 mL TiO2-QDs sol).


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A nitrogen adsorption-desorption method was used to evaluate the specific surface areas of samples. The specific surface areas obtained from the isotherms were determined to be 5.5 m2 g-1 for Cu2O-O, 7.1 m2 g-1 for Cu2O-O/TiO2-QD, and 11.2 m2 g-1 for Cu2O-O@TiO2-QD. Although the specific surface area of Cu2O-O@TiO2-QD was larger than that of Cu2O-O/TiO2-QD, the Cu2O-O@TiO2-QD sample showed much lower activity than that of Cu2O-O/TiO2-QD. This was because the two components in the physical mixture could not achieve good contact with each other to form heterojunctions, as shown in Figure S5 in the Supporting Information. Thus the separation of photoexcited electron−hole pairs was not greatly improved in Cu2O-O@TiO2-QD. Therefore, the strong interface adhesion between the two semiconductors plays a crucial role in the excellent photocatalytic performance of Cu2O-O/TiO2-QD. When an excess amount of TiO2QDs sol (1.8 mL) was added in preparation of Cu2O-O/TiO2-QD, TiO2-QDs would accumulate on Cu2O surface, resulting in a core-shell structure (Figure S6a in the Supporting Information). The contact area between Cu2O and TiO2 was much enlarged and the separation efficiency of photoexcited electron−hole pairs was improved. However, it resulted in an obvious decrease in photocatalytic activity (Figure S6b in the Supporting Information). Figure 5d suggests that TiO2QDs agglomerated together to form integument when an excess amount of TiO2-QDs sol was used. Since the surface of Cu2O was thickly covered with TiO2, the contact area between Cu2O and the solution was reduced. Cu2O could not effectively contact with the aqueous solution. As a result, holes in Cu2O could not react quickly with adsorbed H2O or OH− species to produce highly reactive •OH radicals, and thus could not accomplish degradation.60,61 Instead, Cu2O was rapidly oxidized by holes. This would not only decrease significantly the photocatalytic activity but also cause photocorrosion of photocatalysts. Therefore, for a good degradation effect, the contact area between Cu2O and TiO2 should not be too large. Consequently, the enhanced visible


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light photocatalytic activity in the regular composite was attributed to the effective separation of photoexcited electron−hole pairs and the instant scavenging of charge carriers in the unique heterojunction composite. Stability is particularly important for Cu2O-based photocatalysts, since they easily suffer from photocorrosion in degradation process and oxidation during storage. Therefore, recycling experiments on the degradation of MO by Cu2O-O/TiO2-QD were carried out to evaluate the degradation stability. As shown in Figure 6a, Cu2O-O/TiO2-QD generally demonstrated similar degradation behavior under visible light illumination for seven cycles. 97.1% of MO could be degraded within 60 min for the first cycle, while 91.6% of MO could still be degraded within 60 min by Cu2O-O/TiO2-QD for the seventh cycle. This indicates that photocorrosion could be inhibited by scavenging photoexcited holes instantly. This is because the Cu2O-O was partially covered by TiO2-QDs and thus could ensure good contact with solution. The structural stability of the catalysts is an important photocatalytic property which determines the quality of catalysts. In order to confirm the structural stability of the Cu2O-O/TiO2-QD, the recycled sample was characterized by SEM after photocatalytic tests. As seen in Fig. S7, the TiO2 particles were firmly anchored on Cu2O, and the heterojunction structure did not change in morphology during the degradation process. The XRD pattern shown in Figure 6b indicates that the Cu2O-O/TiO2QD composite keeps its crystal structure even after photocatalytic degradation for seven cycles. No diffraction peaks of CuO could be found. Thus, the Cu2O-O/TiO2-QD heterojunction is an effective and stable photocatalyst in service. Furthermore, the storage stability of Cu2O-O/TiO2QD in atmosphere was investigated. Figure 6c shows the effect of storage time on the photocatalytic properties of the composite. All the samples with different storage times exhibit a downward trend with increasing irradiation time. Notably, the composite after 30 days (30 D) of


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storage still had high photocatalytic efficiency with 79.4% degradation of MO within 60 min, which is slightly lower than for the 0 D sample. The reason for the slight decrease is that a thin layer of CuO was formed on the surface of the Cu2O during storage under ambient conditions. The small amount of CuO can be detected by XPS. The two main peaks at 932.6 and 952.5 eV shown in Figure 6d were respectively assigned to Cu 2p3/2 and Cu 2p1/2 of Cu+, while the slight peak at 942.9 eV indicates the existence of Cu2+ on the sample surface.42 The thin CuO layer prevented further oxidation of Cu2O and thus ensured the stability of the composite. The results suggest that the Cu2O-O/TiO2-QD composite with surprising photocatalytic activity exhibited high stability in the degradation and storage processes.

Figure 6. (a) Seven cycle experiment on the degradation of MO by Cu2O-O/TiO2-QD under visible light illumination. (b) XRD patterns of Cu2O-O/TiO2-QD before and after photocatalytic


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degradation for seven cycles. (c) Curves of MO concentration ratio versus irradiation time for Cu2O-O/TiO2-QD with different storage times. (d) High resolution Cu 2p XPS spectrum of Cu2O-O/TiO2-QD after storage for 30 days.

According to the above characterizations and experiments, a stable Cu2O-O/TiO2-QD heterojunctions composite with high photocatalytic activity was successfully synthesized. Since {110} high energy facet was quite active and the surface energy levels difference between {110} faces and {100} faces promoted the separation of electrons and holes, the Cu2O-O microsized single crystals exhibited high photocatalytic activity and good dispersibility. Thus the Cu2O-O was an ideal structure to load the highly dispersed TiO2 to improve its photocatalytic applications. When TiO2-QDs were coupled onto the Cu2O-O surface to form numerous p-n heterojunctions, the migration of electrons from TiO2 to Cu2O, as well as the migration of holes in the opposite direction, occurred at the interface, which was due to the existence of a charge carrier concentration gradient. The migration of charge carriers promoted stability of the composite and resulted in an inner electrostatic field.28,39 It is well known that the band gaps of TiO2 and Cu2O are about 3.2 and 2.1 eV, with the CB at -0.2 and -1.4 eV, and the VB at 3 and 0.7 eV, respectively, compared with normal hydrogen electrode (NHE).62,63 When Cu2O-O/TiO2-QD heterojunctions were applied to the degradation of MO under visible light, it produced electron−hole pairs. Since the conduction and valence bands of TiO2 are lower than those of Cu2O, electrons photoexcited from Cu2O transfer from the CB of Cu2O to that of TiO2, and holes photoexcited from TiO2 transfer from the VB of TiO2 to that of Cu2O. Therefore, both the band potential difference and the electrostatic field in the heterojunction composite drive the transfer of charge carriers. The good crystallization of the synthesized Cu2O-O and TiO2-QDs


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contributed to a rapid transfer of photoexcited charge carriers between them. This enhanced the separation efficiency of electron−hole pairs. Owing to the unique heterojunction structure of Cu2O-O partially covered by TiO2-QDs, Cu2O and TiO2 could effectively come into contact with the aqueous solution. The separated electrons in the CB of TiO2-QDs could be easily trapped by the adsorbed O2 to form superoxide radicals (•O2−), and holes in the VB of Cu2O could react with adsorbed H2O or OH− species to produce highly reactive •OH radicals in aqueous solution.48,60,61 These reactive oxidizing groups would react with organic pollutants to accomplish degradation. The instant scavenging of holes protected the Cu2O from photocorrosion. A schematic illustration of effective separation and instant scavenging of photoexcited electron−hole pairs in the unique heterojunction structure is presented in Figure 7. Since Cu2O was the major constituent of the heterojunction and showed strong absorbance under visible light, the Cu2OO/TiO2-QD heterojunctions composite with {110} facets exposed achieved high utilization of light. The heterojunctions composite with excellent photocatalytic activity exhibited good stability in the degradation and storage processes. Furthermore, this Cu2O-O/TiO2-QD composite was easy to recycle and exhibited good dispersibility. Therefore, these unique advantages of Cu2O-O/TiO2-QD heterojunctions make the composite suitable for practical applications.


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Figure 7. Schematic illustration of effective separation and instant scavenging of photoexcited electron−hole pairs in the unique heterojunction structure.

4. CONCLUSION In summary, a Cu2O-O/TiO2-QD heterojunction composite was designed and synthesized by a novel method. It was found that highly dispersed TiO2-QDs were firmly anchored on Cu2O-O single crystals with (110) and (100) facets exposed. The heterojunctions had high utilization of light and effective separation of photoexcited electron−hole pairs. The heterojunctions exhibited high visible light photocatalytic activity and good stability during degradation and storage process in atmosphere. These excellent performances were attributed to high utilization of light, effective separation of photoexcited electron−hole pairs, and instant scavenging of holes in the unique heterojunction structure. With its higher photocatalytic activity, good stability, and easy recycling, the Cu2O-O/TiO2-QD heterojunction composite is an excellent candidate material for practical applications.


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ASSOCIATED CONTENT Supporting Information Additional data, including size distribution histograms of Cu2O-O and TiO2-QDs, the SAED pattern of Cu2O-O viewed along the [100] direction, a HR-TEM image of TiO2-QDs, the full survey XPS spectrum of Cu2O-O/TiO2-QD, XPS spectrum of individual line of Ti 2p measured at a high resolution, plots of (αhν)1/2 versus hν of P25 and TiO2-QDs, plots of (αhν)2 versus hν of Cu2O-O and Cu2O-O/TiO2-QD, a SEM image of Cu2O-O@TiO2-QD, a SEM image and the MO degradation curve of Cu2O-O/TiO2-QD (synthesized with 1.8 mL TiO2-QDs sol), and SEM images of Cu2O-O/TiO2-QD after photocatalytic tests. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author ∗

Corresponding author Tel: +86-022-27402494 E-mail: [email protected] (Y. Q. Liang)

∗

Corresponding author Tel: +86-0731-88822137 E-mail: [email protected] (J. M. Ma)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports by National Natural Science Foundation of China (51402211) and Natural Science Foundation of Tianjin (15JCQNJC03600) are gratefully acknowledged.


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TiO2 Quantum Dot

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