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TiO2 nanocomposite based on photoinduced interfacial charge transfer
Applied Surface Science 457 (2018) 764–772
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Full Length Article
Highly efficient visible light photocatalysis of CuC2O4/TiO2 nanocomposite based on photoinduced interfacial charge transfer ⁎
T
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Yuhua Panga, Junke Zhanga, Caixia Fenga, , Yan Wangb, , Ning Sunc, Shanhu Liua, Simeng Wanga, Hongtao Lia, Haiyan Zhaoa, Yanting Dinga, Ling Zhanga, Yanmei Zhoua, Deliang Lia a
Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China Department of Scientific Research, Henan University, Kaifeng 475004, PR China c Lawrence Berkeley National Laboratory, United States b
A R T I C LE I N FO
A B S T R A C T
Keywords: CuC2O4/TiO2 Visible light activity Interfacial charge transfer
A series of CuC2O4/TiO2 heterostructures with different mass ratio have been prepared via a simple precipitation method. Compared to bare P25 TiO2 and pure CuC2O4, the as-prepared CuC2O4/TiO2 exhibited superior activity and stability for the degradation of propylene under visible light. Results of X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy (DRS) indicated that there is an intimate interaction between CuC2O4 and P25 TiO2 nanoparticles by means of the coordination bond of O on the surface of TiO2 with Cu atom in CuC2O4 molecular. The strong activity of CuC2O4/TiO2 heterostructure is due to the interfacial charge transfer (IFCT) from the valence band of the TiO2 to the CuC2O4 nanoparticles. In addition, compared to Cu (II)/TiO2 synthesized using the same amount of CuSO4 solution, CuC2O4/TiO2 exhibited much higher visible light activity for the degradation of propylene because of higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2.
1. Introduction In the last few decades, TiO2 has been studied extensively as an efficient photocatalyst in the fields of organics degradation, photocatalytic conversion of CO2 and water-splitting for H2 production. However, it can only be activated by UV light due to its wide band gap of 3.2 eV (anatase). Thus, numerous studies have been undertaken to expand its spectral absorption into visible light region. The main approaches involve cation doping [1–3], anion doping [4–6], noble metal sensitization [7–13] and combining it with narrow-bandgap semiconductors [14–18]. The visible light activity of doped TiO2 is mainly caused by the photogenerated holes originating from the dopant metal ions or anions in the forbidden band of TiO2, the oxidative ability of such holes decreases upon irradiation. Therefore, the holes having strong oxidative power generated in the valence band of TiO2 cannot be utilized in these doped systems. Recently, the novel visible-light-driven photocatalyst composites related to wide-bandgap semiconductor have been reported based on photoinduced interfacial charge transfer (IFCT). Irie and Hashimoto et al. fabricated efficient photocatalysts sensitive to visible light, Cu(II)grafted TiO2 (Cu(II)/TiO2) and WO3 (Cu(II)/WO3), using CuCl2·2H2O as the source of Cu(II) [19,20]. They reported that Cu(II)/TiO2 and Cu(II)/
⁎
WO3 photocatalysts decomposed 2-propanol under visible light with quantum efficiencies of 8.8% and 17%, respectively, by IFCT from semiconductor to adsorbed Cu(II) ions. Yu and Hashimoto et al. substituted W6+ and Ga3+ ions for Ti4+ sites to narrow the band gap of TiO2 forming Ti1-3xWxGa2xO2 powders and demonstrated that Ti13xWxGa2xO2 can serve as an efficient visible-light-sensitive photocatalyst when its surface is grafted with Cu(II) due to IFCT and narrow band gap [21]. Zhang and Gong et al. designed a novel visible-lightdriven photocatalyst CuS/ZnS porous nanosheet, which can reach a high H2-production rate of 4147 μmol h−1 g−1 at 420 nm based on IFCT from the valence band of ZnS to CuS [22]. Lee and Yong synthesized CuS/ZnO heterostructure nanowires through a simple two-step solution method and found that the synthesized CuS/ZnO heterostructure exhibited superior photocatalytic activity under visible light compared to bare ZnO because of IFCT from the valence band of the ZnO nanowire to the CuS nanoparticles [23]. To the best of our knowledge, however, there is no report regarding CuC2O4/TiO2 nanomaterials as an efficient visible-light-responsive photocatalyst. In this work, for the first time we prepared visible-light-activated CuC2O4/TiO2 series with different ratio via a chemical precipitation (CP) method. Many methods such as successive ion layer adsorption and reaction (SILAR), chemical bath deposition (CBD), microwave
Corresponding authors. E-mail addresses: [email protected] (C. Feng), [email protected] (Y. Wang).
https://doi.org/10.1016/j.apsusc.2018.06.288 Received 4 May 2018; Received in revised form 15 June 2018; Accepted 29 June 2018 Available online 30 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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(MW) and hydrothermal (HT) have been reported to synthesize nanocomposites [24–26]. Chemical precipitation (CP) method was used in this work because it is a simple, low temperature, and inexpensive technique. Moreover, the photocatalytic properties of CuC2O4/TiO2 nanocomposites were investigated under visible light irradiation, and they exhibited high activities and stabilities for the photodegradation of propylene gas. Especially for CuC2O4/TiO2 (0.25:1), with a mass ratio of 0.25:1 of CuC2O4 to TiO2, the highest propylene degradation of 46% in online system was found and the activity was stable in three successive cycling experiments. This strong photocatalytic activity under visible light was attributed to the interfacial charge transfer (IFCT) from the valence band (VB) of the TiO2 to the CuC2O4 nanoparticles. The holes produced in the VB of TiO2 are then capable of decomposing organic substances. By comparison with Cu (II)-grafted TiO2 (denoted as Cu(II)/TiO2) synthesized using the same amount of CuSO4 solution as precursor, CuC2O4/TiO2 nanocomposites displayed superior activity for higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2. CuC2O4/TiO2 photocatalyst is composed of environmental friendly Cu and Ti and without involvement of noble metals [27]. So, it will be utilized extensively in practice application for its nontoxic, cheap and strong oxidative ability.
ultraviolet–visible light diffusion reflectance spectra (DRS) of the photocatalysts were recorded with a CARY5000 spectrometer (BaSO4 was used as a reference). X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Fisher Scientific Escalab 250Xi X-ray photoelectron spectrometer using monochromatized Al-Kα (hν = 1486.6 eV) radiation as excitation source. The binding energies were calibrated with reference to adventitious C 1s line at 284.8 eV. The photocurrent measurements were examined by electrochemical station (CHI 650E Chenhua Instrument Company) in a three-electrode quartz cell with 0.1 M Na2SO4 electrolyte solution. The as prepared samples electrode with an active area of 1.5 cm2 were used as the working electrode. The working electrode was prepared by dip-coating method as follows: 2 mg of sample, 30 μL of water and 5 μL of Nafion were mixed to form homogeneous slurry, and then was dropped onto the precleaned ITO glass and dried at room temperature for 12 h. A platinum wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. A xenon lamp equipped with an ultraviolet cutoff filter (λ > 420 nm) was served as the visible light source and was positioned 10 cm away from the photoelectrochemical cell.
2. Experimental
The visible light photocatalytic activity of P25, Cu2C2O4, Cu (II)/ TiO2 and Cu2C2O4/TiO2 (X:Y) was evaluated by monitoring the oxidation of propylene in an on-line analysis system equipped with a gas chromatograph (GC7900) to monitor the concentration change of C3H6 (C). The photocalyst (ca. 25 mg) was coated on one side of a roughened glass plate (ca. 10 cm2), which was put in a quartz tube reactor surrounded with a cycling water channel to keep the reaction temperature unchanged. The feed gas was mixed of propylene and dry air, and flowed through the reactor at a flow rate of 200 mL/h. A 500 W xenon lamp was used as the visible light source and an ultraviolet (UV) cut 420 filter was inserted between the xenon lamp and reactor to eliminate UV light. Prior to irradiation, the feed gas was allowed to flow through the reactor continuously until the adsorption/desorption equilibrium was established. The concentrations of C3H6 and CO2 production were measured by GC equipped with a flame ionization detector (FID), a GDX-502 column, and a reactor loaded with Ni catalyst for methanization of CO2. The removal rate of C3H6 is calculated as (C0 − C)/ C0 × 100%, where C0 refers to the initial C3H6 concentration with a value of about 500 ppm V.
2.3. Evaluation of visible light photocatalytic activity
P25-TiO2 crystals were produced from Guangzhou Hualisen Trade Co., Ltd. Other used reagents were of analytical reagent grade and used without further purification. 2.1. Preparation of pure CuC2O4, Cu (II)/TiO2 and CuC2O4/TiO2 composite The nanocrystalline CuC2O4/TiO2 heterostructures with different mass ratio were synthesized via a chemical precipitation method. In brief, 1.0 g of P25-TiO2 was ultrasonic dispersed into 20 mL of distilled water, then a certain volume of Na2C2O4 (0.1 M) aqueous solution was slowly added drop-wise to the above suspension mixture with constant stirring. After 20 min of stirring, a certain volume of CuSO4 solution (0.1 M) was dripped into above suspension to yield CuC2O4/TiO2 precipitate. In the above procedure, the volume of Na2C2O4 was larger than that of CuSO4 to ensure the complete precipitation of Cu2+. Finally, the precipitate was collected, washed with distilled water for several times, and then dried at 60 °C in vacuum oven to get the nanocrystalline CuC2O4/TiO2 hybrid photocatalyst. By changing the volume of Na2C2O4 and CuSO4 solution added, CuC2O4/TiO2 composites with different mass ratio (0.05:1, 0.1:1, 0.15:1, 0.25:1, 0.3:1) have been prepared and labeled as CuC2O4/TiO2 (X:Y), where X:Y means the theoretical mass ratio of CuC2O4 to TiO2 in the composite. To make a comparison, pure CuC2O4 and Cu (II)-grafted TiO2 (Cu (II)/TiO2) were also synthesized. The former was prepared with CuSO4 and Na2C2O4 and the latter was synthesized using CuSO4 and P25-TiO2 without addition of Na2C2O4. For Cu (II)/TiO2, briefly, 1.0 g of P25-TiO2 and certain volume of CuSO4 solution (0.1 M) were ultrasonic mixed into a beaker, and then were stirring for 20 min. The volume of CuSO4 solution is 3.3, 6.6, 10, 16.5 and 20 mL, respectively, which is identical to that for preparing CuC2O4/TiO2 (X:Y). The suspension was filtered and dried at 60 °C in vacuum oven to get Cu (II)/TiO2 series and labeled as Cu (II)/TiO2-I, Cu (II)/TiO2-II, Cu (II)/TiO2-III, Cu (II)/TiO2-IV and Cu (II)/TiO2-V, respectively.
3. Results and discussion 3.1. Characterization of catalysts Fig. 1 displays the powder XRD patterns of pure CuC2O4 and CuC2O4/TiO2 heterostructures with different mass ratio. All characteristic peaks of pure CuC2O4 coincide well with the standard data of orthorhombic CuC2O4 (JCPDS 21-0297) [28,29]. The strong diffraction peaks of CuC2O4 at 2θ angles of 22.902°, 36.251°, 36.946°, 38.835°, 39.063°, 42.443°, 46.865°, 51.469°, and 52.036° are corresponded to the (1 1 0), (1 2 0), (2 1 0), (0 1 1), (1 0 1), (1 1 1), (2 2 0), (1 2 1), and (1 3 0) planes respectively. As for all CuC2O4/TiO2 composites, the major peak of orthorhombic CuC2O4 at 2θ = 22.902° is clearly observed and its intensity increases with increasing the mass ratio of CuC2O4. Moreover, after coupling with CuC2O4, all of the diffraction peaks of anatase (A) and rutile (R) phase still exist without change, suggesting that the heterogeneous process doesn't affect the crystal structure of TiO2. Fig. 2 shows SEM images of CuC2O4 synthesized with CuSO4 and Na2C2O4 at different temperature including 30 °C, 60 °C and 90 °C. It can be seen that all samples contain two shapes of copper oxalate: spherical and fusiform. Fig. S1 shows the proportion of two different shapes in CuC2O4 sample prepared at different temperature. Apparently, the fusiform shape is the main component of CuC2O4 produced at
2.2. Characterization The morphology of as-prepared photocatalysts was studied using a JEOL JSM-7610F scanning electron microscope (SEM) and a JEOL JEM2010 transmission electron microscope (TEM). The crystal phase of the obtained samples was measured with a Bruker D8 X-ray diffractometer (XRD) with Cu Kα radiation at an accelerating voltage of 40 kV. The 765
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CuC2O4/TiO2 (0.05:1) and CuC2O4/TiO2 (0.25:1), CuC2O4 nanoparticles were found with the diameter of several nanometers adhering to the surface of TiO2 substrate, although pure CuC2O4 above mentioned with a diameter of ca. 1 µm. It is well known that there are coordinatively unsaturated surface Ti atoms on the P25-TiO2 particle surface and they are active in solution [30,31]. Therefore, it could be referred that oxalate ion may interact with surface Ti atom (^Ti) as follows: Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.06.288.
(I) When CuSO4 solution was dripped into above mixture, the following reaction occurred: Fig. 1. XRD patterns of pure CuC2O4, P25-TiO2 and CuC2O4/TiO2 composites with different ratio.
Fig. 2. SEM images of CuC2O4 synthesized with CuSO4 and Na2C2O4 at different temperature: (a) 30 °C; (b) 60 °C; (c) 90 °C.
three different temperatures. Moreover, histograms of size distribution of samples are shown in Fig. S2. It can be seen that, with the increase of reaction temperature from 30 °C to 90 °C, the diameter of spherical CuC2O4 and the length of fusiform CuC2O4 increased from 1.4 and 1.2 μm to 2.6 and 2.2 μm, indicating that the reaction temperature affects the size of copper oxalate. Considering that the small size of CuC2O4 is beneficial to the strong interaction between CuC2O4 and TiO2 substrate for CuC2O4/TiO2 heterostructure, 30 °C was chosen to prepare CuC2O4/TiO2 (X:Y) as the most appropriate reaction temperature. Fig. 3 shows SEM images of CuC2O4, TEM images of P25-TiO2 as well as typical CuC2O4/TiO2 (X:Y) samples, CuC2O4/TiO2 (0.05:1) and CuC2O4/ TiO2 (0.25:1), all prepared at 30 °C. As shown in Fig. 3c and d, for
(II) As the chemisorption of oxalate ion on the TiO2 surface is highly dispersive, the diameter of CuC2O4 is small in the range of nanometer. The synthesis process of CuC2O4/TiO2 (X:Y) and interaction of C2O42− and CuC2O4 with TiO2 is illustrated in Fig. 4. Fig. 5 shows the UV–vis diffuse reflectance spectra of P25-TiO2 and composite samples CuC2O4/TiO2 (X:Y). As can be seen from the curve, for P25-TiO2 and CuC2O4/TiO2 (X:Y) composites, the strong absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic 766
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Fig. 3. SEM image of CuC2O4 (a) as well as TEM images of P25-TiO2 (b), CuC2O4/TiO2 (0.05:1) (c) and CuC2O4/TiO2 (0.25:1) (d). CuC2O4 and CuC2O4/TiO2 (X:Y) were both prepared at 30 °C.
Fig. 5. UV–vis-NIR diffuse reflectance spectra of P25-TiO2 (a), pure CuC2O4 (g) and CuC2O4/TiO2 composites with different ratio: (b), CuC2O4/TiO2 (0.05:1); (c), CuC2O4/TiO2(0.10:1); (d), CuC2O4/TiO2 (0.15:1); (e), CuC2O4/TiO2 (0.25:1); (f), CuC2O4/TiO2 (0.30:1). Inset: Picture of CuC2O4/TiO2 (X:Y).
C2O42−
Fig. 4. Schematic illustration of interaction of during the synthesis process of CuC2O4/TiO2 (X:Y).
absorption edges of CuC2O4/TiO2 (0.05:1) and CuC2O4/TiO2 (0.1:1) shift to shorter wavelength while other CuC2O4/TiO2 (X:Y) catalysts showing red-shift. Shifting the absorption edge may be due to interactions between TiO2 substrate and surface CuC2O4 in CuC2O4/TiO2 nanocomposites [32–35]. Mahdiani et al found that, over carbon-based nanocomposites of CuFe12O19, the absorption edge shifts to higher energy resulting the photocatalytic activity was improved [33]. In this work, for CuC2O4/TiO2 (X:Y), the absorption at 600–800 nm is assigned to the d-d transition of Cu (II) [22,23,36–39] and the absorption shoulder from 400 to 500 nm can be ascribed to the photoinduced
and CuC2O4 with TiO2
bandgap absorption of TiO2, and the bandgap energy is estimated to be 3.1 eV according to the equation Eg = 1239.8/λ. As for CuC2O4/TiO2 (X:Y), with increasing the content of CuC2O4 an enhanced absorption in the visible light region is clearly observed and the absorption edges red shifts to longer wavelength increasingly. Compared to pure CuC2O4, the 767
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Fig. 6. Comparison of Cu 2p (a) and O 1s (b) XPS spectra of pure CuC2O4 or TiO2 and CuC2O4/TiO2 (0.25:1) composite.
Fig. 6b, the peak for O 1s located at 529.7 eV is assigned to lattice oxygen in TiO2 crystal, which distinctly shifts to a higher energy in CuC2O4/TiO2, suggesting that the electron density of lattice oxygen on the surface of titanium dioxide becomes lower after combination. Combined with fabrication process and DRS results of CuC2O4/TiO2, it can be concluded that, as shown in Fig. 4, during the synthesis process, the oxygen atoms in C2O42− are in coordination with surface Ti atoms in titanium dioxide before the addition of Cu2+ into the suspension of P25 and C2O42−. Then, after dripping Cu2+ into above suspension, the strong electrostatic attraction enables C2O42− to combine with Cu2+. On the other hand, the coordination of C2O42− with Ti atom is weakened or disappeared, while the oxygen atom on the surface of titanium dioxide provides electron to Cu2+ with unsaturated d 9 configuration to form an intimate action. Such interaction is crucial to the photoinduced interfacial charge transfer from the valence band of TiO2 to CuC2O4.
interfacial charge transfer (IFCT) from the valence band of TiO2 to CuC2O4. The intimate contact between TiO2 and CuC2O4 by the coordination bond is crucial for the IFCT between two components. Jono and co-workers found that the TiO2-TCNQ (7,7,8,8-tetracyanoquinodimethane) surface complex exhibits the interfacial charge transfer (IFCT) absorption band in the visible to near IR and pointed that the IFCT absorption originates from the electron flow from the surface oxygen atom of TiO2 to TCNQ [40]. In order to further confirm the interaction between CuC2O4 nanomaterial and TiO2 support, the comparisons of high resolution XPS spectra of Cu 2p and O 1s between pure CuC2O4 and CuC2O4/TiO2 nanocomposites are displayed in Fig. 6a and b. For pure CuC2O4, XPS results showed the typical Cu2p3/2 and Cu2p1/2 peaks sitting at 934.7 and 954.6 eV, as well as their shake-up satellites, which is indicative of the paramagnetic chemical state of Cu2+ [22,23]. Whereas, the peak position for Cu 2p in the CuC2O4/ TiO2 nanocomposites shifts to a lower binding energy than that in pure CuC2O4, indicating that a higher electron density of Cu atom in CuC2O4/TiO2 composite than that in CuC2O4. In addition, as shown in
Fig. 7. Visible light photoactivity of P25-TiO2, Cu2C2O4 and Cu2C2O4/TiO2 composites evaluated by C3H6 removal (vol%) (a); Change of CO2 production with irradiation time (b); The steady removal rate of propylene over different photocatalysts (c). 768
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3.2. Visible light photocatalytic performance and stability
Table 1 The comparison of atom ratio of Cu to Ti between Cu(II)/TiO2-IV and CuC2O4/ TiO2 (0.25:1) before and after irradiation.
The experiment of propylene degradation was carried out to evaluate the visible light activity of P25-TiO2, pure CuC2O4 and CuC2O4/ TiO2 with different ratios. Before illumination, the feed gas was flowing continuously until the propylene adsorption-desorption equilibrium was established. As shown in Fig. 7a, pure CuC2O4 and P25-TiO2 are nearly inert in oxidizing propylene under visible light illumination. But, CuC2O4/TiO2 composites with different ratios showed apparent visible light activity for the degradation of propylene. With the increase of the ratio of CuC2O4 to TiO2 from 0.05:1 to 0.3:1, the degradation of propylene first increases and then decreases. CuC2O4/TiO2 (0.25:1) exhibits the highest activity with a steady propylene removal rate of 46.5%. For CuC2O4/TiO2 (0.30:1), the removal rate of propylene decreases dramatically. This probably due to the following factors: (1) excessive CuC2O4 may shield part of TiO2 from incident light, and thus decrease the generation of photogenerated electrons. (2) CuC2O4 at high content may act as charge recombination centers, leading to the decrease of the photocatalytic activity. Therefore optimum CuC2O4 content facilitates visible light absorption and high separation efficiency of charge carriers. Moreover, the change of CO2 concentration, mineralization product, as a function of irradiation time during the photocatalytic process has been plotted in Fig. 7b. It can be seen that the highest concentration of CO2 about 400 ppm is obtained over CuC2O4/TiO2 (0.25:1) catalyst, which is well accordant with that result of Fig. 7a. Compared to CuC2O4 and P25-TiO2, The remarkable enhancement of photocatalytic activity of CuC2O4/TiO2 heterostructures is may be caused by the IFCT effect from CuC2O4 nanomaterials to the conduction band of TiO2. Irie and Hashimoto et al. have reported that Cu (II)-grafted TiO2 (Cu(II)/TiO2) and WO3 (Cu(II)/WO3) also showed visible light photocatalytic activity for the decomposition of 2-propanol by IFCT from semiconductor to adsorbed Cu(II) ions [19,20]. In this work, we also synthesized Cu (II)/TiO2 series using CuSO4 solution as the source of Cu(II) by the same method as in reference [19,20]. The propylene remove rate and change of CO2 production with irradiation time over Cu (II)/TiO2 series is shown in Fig. 8a and b, respectively. The highest propylene remove rate obtained over Cu(II)/TiO2-IV is only 15%, which is much lower than that of CuC2O4/TiO2(0.25:1) with 46%. Why is the activity of CuC2O4/TiO2 heterostructures far superior to Cu (II)/TiO2 series? We compared the absorbance and chemical status of two typical catalysts, CuC2O4/TiO2 (0.25:1) and Cu(II)/TiO2-IV. CuC2O4/TiO2 (0.25:1) was prepared using the same amount of P25TiO2 and CuSO4 solution, 1.0 g and 16.5 mL respectively, with that of preparing Cu(II)/TiO2-IV. However, as shown in Table 1, the atom ratio of Cu to Ti with value of 0.0965 in CuC2O4/TiO2 (0.25:1) is obviously larger than that of Cu(II)/TiO2-IV with 0.0725. This indicates that the coordination of C2O42− with surface Ti atom of TiO2 in the initial stage of CuC2O4/TiO2 synthesis is beneficial to the formation of more CuC2O4 on the surface of TiO2. Fig. 9 shows the comparison of absorption
photocatalyst
Atom ratio of Cu to Ti
The amount of using TiO2
The volume of using CuSO4 (0.1 M)
Cu (II)/TiO2-IV CuC2O4/TiO2 (0.25:1)
0.0725 0.0965
1.0 g 1.0 g
16.5 mL 16.5 mL
Cu (II)/TiO2-IV-aft* CuC2O4/TiO2 (0.25:1)-aft*
0.0617 0.0953
1.0 g 1.0 g
16.5 mL 16.5 mL
* Cu(II)/TiO2-IV and CuC2O4/TiO2 (0.25:1) photocatalysts after 4 h visible light irradiation were denoted as Cu(II)/TiO2-IV-aft and CuC2O4/TiO2 (0.25:1)aft, respectively.
Fig. 9. The comparison of absorbance ability between CuC2O4/TiO2 (0.25:1) and Cu (II)/TiO2-IV.
ability between Cu(II)/TiO2-IV and CuC2O4/TiO2 (0.25:1). Apparently, the absorbance of CuC2O4/TiO2 (0.25:1) in the range of 400–800 nm is much stronger than that of Cu(II)/TiO2-IV. This may be due to the higher atom ratio of Cu to Ti in CuC2O4/TiO2 (0.25:1), and strong absorption ability in visible light for CuC2O4/TiO2 (0.25:1) is beneficial to its visible-light photocatalytic activity. To investigate the interaction between Cu2+ with TiO2 in Cu(II)/TiO2-IV, the comparisons of high resolution XPS spectra of Ti 2p and O 1s between pure TiO2 and Cu(II)/ TiO2-IV are displayed in Fig. 10a and b. It can be seen that the peaks for Ti 2p in Cu(II)/TiO2-IV just very slightly shift to a higher binding energy compared to TiO2. Moreover, the situation of peaks for O 1s located at 529.8 eV and 529.7 eV, which is assigned to lattice oxygen of TiO2 [41], in Cu(II)/TiO2-IV and pure TiO2 is nearly unchanged, indicating that the action between Cu2+ and TiO2 in Cu(II)/TiO2-IV is faint. Compared to Cu(II)/TiO2-IV, the comparison of XPS spectra of Cu
Fig. 8. Visible light photoactivity of Cu (II)/TiO2 series evaluated by C3H6 removal (vol%) (a); Change of CO2 production with irradiation time (b). 769
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Fig. 10. Comparison of Ti 2p (a) and O 1s (b) XPS spectra of pure TiO2 and Cu (II)/TiO2-IV catalyst.
CO2 during recycle experiments may be due to the photo-induced promotion of interaction between CuC2O4 and TiO2. Fig. 13 shows the comparison of Cu 2p spectra on CuC2O4/TiO2 (0.25:1) before and after photocatalytic experiments. After 4 h of visible light irradiation, the satellite is still present but the binding energies of Cu 2p1/2 and Cu 2p3/ 2+ was reduced to 2 shift to lower energy. This indicates that part of Cu + 0 Cu or Cu . Associated with the Auger Cu LMM spectrum of sample CuC2O4/TiO2 (0.25:1) after photocatalytic reaction for 4 h (inset in Fig. 13), the Cu LMM line position is at 569.8 eV, which is the typical value for Cu+ [22,23]. From this analysis, it is assumed that a small amount of Cu2+ in CuC2O4 is reduced to Cu2C2O4 due to the photogenerated electron transfer from the VB of TiO2 to CuC2O4.
2p and O 1s (Fig. 6) between pure CuC2O4 and CuC2O4/TiO2 nanocomposites suggested that the strong interaction between CuC2O4 and TiO2 may be the main reason for its higher activity. To further investigate the charge separation and migration of photocatalysts, the transient photocurrent responses of CuC2O4/TiO2 samples were tested for several cycles of on-off under visible light irradiation. Fig. 11 shows the transient photocurrent responses of P25-TiO2, pure CuC2O4 and CuC2O4/TiO2 composites. P25-TiO2 and pure CuC2O4 electrodes didn’t show any photocurrent response, while CuC2O4/TiO2 electrodes showed apparent response under visible light. This suggests that the intimate interaction formed between CuC2O4 NPs and TiO2 substrate, leading to the effective separation of photogenerated carries. Moreover, as for CuC2O4/TiO2, the photocurrent density was reproducible during the repeated on-off cycles, indicating that photocatalysts were stable under visible light illumination. With the increase of CuC2O4 weight ratio, the photocurrent density initially increases from 0.6 μA cm−2 for CuC2O4/TiO2 (0.05:1) to a maximum value of 1.6 μA cm−2 for CuC2O4/TiO2 (0.25:1), then decreases to 1.0 μA cm−2 for CuC2O4/TiO2 (0.30:1). The highest photocurrent obtained over CuC2O4/TiO2 (0.25:1) could be ascribed to the efficient electron-hole transfer and separation process, which was beneficial to the highest activity for the degradation of propylene. The recycle experiments were carried out to evaluate the stability of CuC2O4/TiO2 (0.25:1) photocatalyst. As illustrated in Fig. 12, after three cycles of photodegradation of propylene, the photocatalytic activity of CuC2O4/TiO2 (0.25:1) slightly increased rather than decline, suggesting that the CuC2O4/TiO2 heterostructure photocatalyst is sufficiently stable and not deactivated during the photodegradation of propylene. This result is further confirmed by the production of CO2 during recycle experiment. The increasing activity and production of
3.3. The possible photocatalytic mechanism On the basis of DRS and XPS results, the visible light photocatalytic mechanism for the degradation of propylene over CuC2O4/TiO2 is proposed as illustrated in Fig. 14. The band gap of P25-TiO2 is estimated to be 3.1 eV according to the equation Eg = 1240/λ, and the band edge positions of conduction and valence band of TiO2 are calculated to be −0.3 and 2.8 eV by a simple approach [42,43], respectively. Bare P25-TiO2 photocatalysts showed negligible photocatalytic activity, which is due to the wide bandgap of TiO2. While CuC2O4/TiO2 composites showed greatly enhanced photocatalytic activity compared to bare TiO2. This may be attributed to the IFCT effect from the valence band of TiO2 to CuC2O4 NP, as showed in Fig. 14. Hashimoto et al found that, in Cu(II)-grafted TiO2 and WO3 systems ((Cu(II)/TiO2 and Cu(II)/ WO3), electrons in the VB of TiO2 can be excited to Cu(II) by visible light, leaving holes in the VB of TiO2 [19]. Thus, Cu(II) are reduced to Cu(I). Yu et al also reported that, in CuS/ZnS heterostructure, visible light irradiation also induces IFCT from the VB of ZnS to CuS clusters [22]. In our CuC2O4/TiO2 composites, we think, IFCT from the VB of TiO2 to CuC2O4 nanoparticles can also be initiated. The evidence is the apparent absorption from 400 to 450 nm due to the IFCT, as shown in Fig. 5. That is, electrons in the VB of TiO2 are transferred to CuC2O4 under visible light irradiation. The transferred electrons can be captured by adsorbed O2 to form active radical %O2– or cause the partial reduction of CuC2O4 to Cu2C2O4. In another way, the photoinduced holes can also oxidize organic pollutants to CO2. The major electron transfer steps in the above photocatalytic mechanism over CuC2O4/ TiO2 composites under visible light irradiation can be described as follows: CuC2O4/TiO2 + hv → CuC2O4 (e−)/TiO2 (h+) CuC2O4 + e−→Cu2C2O4 Cu2C2O4/TiO2 (h+) + O2 → CuC2O4/TiO2 (h+) + %O2–
Fig. 11. Photocurrent response of P25-TiO2, CuC2O4 and CuC2O4/TiO2 electrodes under visible light irradiation (λ > 420 nm).
%
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O2–+propylene → CO2 + H2O
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Fig. 12. Repetitive degradation of propylene and CO2 production over CuC2O4/TiO2 (0.25:1) photocatalyst under visible light illumination.
CuC2O4/TiO2 (h+) + propylene → CO2 + H2O 4. Conclusions In summary, CuC2O4/TiO2 composites with different ratio were synthesized via a facile precipitation method. The prepared CuC2O4/ TiO2 exhibits excellent photocatalytic activity and stability under visible light irradiation. The optimal proportion of CuC2O4 and TiO2 is 0.25:1. It is believed that there is an intimate interaction between CuC2O4 and TiO2 and the IFCT from the VB of TiO2 to CuC2O4 cluster plays an important role in charge separation and reducing recombination for high photocatalytic activity. By comparison with Cu(II)/TiO2, CuC2O4/TiO2 nanocomposites displayed superior activity for higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2. So, CuC2O4/TiO2 composites are promising photocatalysts for practical application to degrade organic pollutants because of their superior photocatalytic activity and stability.
Fig. 13. XPS spectra of Cu 2p of CuC2O4/TiO2 (0.25:1): (a) as-prepared sample and (b) sample treated with 4 h irradiation of visible light. The inset is the Auger Cu LMM line of CuC2O4/TiO2 (0.25:1) sample after 4 h irradiation.
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Fig. 14. Schematic illustration for CuC2O4/TiO2 heterostructure based interfacial charge transfer process from the valence band of TiO2 to CuC2O4 particles under visible light irradiation.
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Full Length Article
Highly efficient visible light photocatalysis of CuC2O4/TiO2 nanocomposite based on photoinduced interfacial charge transfer ⁎
T
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Yuhua Panga, Junke Zhanga, Caixia Fenga, , Yan Wangb, , Ning Sunc, Shanhu Liua, Simeng Wanga, Hongtao Lia, Haiyan Zhaoa, Yanting Dinga, Ling Zhanga, Yanmei Zhoua, Deliang Lia a
Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China Department of Scientific Research, Henan University, Kaifeng 475004, PR China c Lawrence Berkeley National Laboratory, United States b
A R T I C LE I N FO
A B S T R A C T
Keywords: CuC2O4/TiO2 Visible light activity Interfacial charge transfer
A series of CuC2O4/TiO2 heterostructures with different mass ratio have been prepared via a simple precipitation method. Compared to bare P25 TiO2 and pure CuC2O4, the as-prepared CuC2O4/TiO2 exhibited superior activity and stability for the degradation of propylene under visible light. Results of X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy (DRS) indicated that there is an intimate interaction between CuC2O4 and P25 TiO2 nanoparticles by means of the coordination bond of O on the surface of TiO2 with Cu atom in CuC2O4 molecular. The strong activity of CuC2O4/TiO2 heterostructure is due to the interfacial charge transfer (IFCT) from the valence band of the TiO2 to the CuC2O4 nanoparticles. In addition, compared to Cu (II)/TiO2 synthesized using the same amount of CuSO4 solution, CuC2O4/TiO2 exhibited much higher visible light activity for the degradation of propylene because of higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2.
1. Introduction In the last few decades, TiO2 has been studied extensively as an efficient photocatalyst in the fields of organics degradation, photocatalytic conversion of CO2 and water-splitting for H2 production. However, it can only be activated by UV light due to its wide band gap of 3.2 eV (anatase). Thus, numerous studies have been undertaken to expand its spectral absorption into visible light region. The main approaches involve cation doping [1–3], anion doping [4–6], noble metal sensitization [7–13] and combining it with narrow-bandgap semiconductors [14–18]. The visible light activity of doped TiO2 is mainly caused by the photogenerated holes originating from the dopant metal ions or anions in the forbidden band of TiO2, the oxidative ability of such holes decreases upon irradiation. Therefore, the holes having strong oxidative power generated in the valence band of TiO2 cannot be utilized in these doped systems. Recently, the novel visible-light-driven photocatalyst composites related to wide-bandgap semiconductor have been reported based on photoinduced interfacial charge transfer (IFCT). Irie and Hashimoto et al. fabricated efficient photocatalysts sensitive to visible light, Cu(II)grafted TiO2 (Cu(II)/TiO2) and WO3 (Cu(II)/WO3), using CuCl2·2H2O as the source of Cu(II) [19,20]. They reported that Cu(II)/TiO2 and Cu(II)/
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WO3 photocatalysts decomposed 2-propanol under visible light with quantum efficiencies of 8.8% and 17%, respectively, by IFCT from semiconductor to adsorbed Cu(II) ions. Yu and Hashimoto et al. substituted W6+ and Ga3+ ions for Ti4+ sites to narrow the band gap of TiO2 forming Ti1-3xWxGa2xO2 powders and demonstrated that Ti13xWxGa2xO2 can serve as an efficient visible-light-sensitive photocatalyst when its surface is grafted with Cu(II) due to IFCT and narrow band gap [21]. Zhang and Gong et al. designed a novel visible-lightdriven photocatalyst CuS/ZnS porous nanosheet, which can reach a high H2-production rate of 4147 μmol h−1 g−1 at 420 nm based on IFCT from the valence band of ZnS to CuS [22]. Lee and Yong synthesized CuS/ZnO heterostructure nanowires through a simple two-step solution method and found that the synthesized CuS/ZnO heterostructure exhibited superior photocatalytic activity under visible light compared to bare ZnO because of IFCT from the valence band of the ZnO nanowire to the CuS nanoparticles [23]. To the best of our knowledge, however, there is no report regarding CuC2O4/TiO2 nanomaterials as an efficient visible-light-responsive photocatalyst. In this work, for the first time we prepared visible-light-activated CuC2O4/TiO2 series with different ratio via a chemical precipitation (CP) method. Many methods such as successive ion layer adsorption and reaction (SILAR), chemical bath deposition (CBD), microwave
Corresponding authors. E-mail addresses: [email protected] (C. Feng), [email protected] (Y. Wang).
https://doi.org/10.1016/j.apsusc.2018.06.288 Received 4 May 2018; Received in revised form 15 June 2018; Accepted 29 June 2018 Available online 30 June 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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(MW) and hydrothermal (HT) have been reported to synthesize nanocomposites [24–26]. Chemical precipitation (CP) method was used in this work because it is a simple, low temperature, and inexpensive technique. Moreover, the photocatalytic properties of CuC2O4/TiO2 nanocomposites were investigated under visible light irradiation, and they exhibited high activities and stabilities for the photodegradation of propylene gas. Especially for CuC2O4/TiO2 (0.25:1), with a mass ratio of 0.25:1 of CuC2O4 to TiO2, the highest propylene degradation of 46% in online system was found and the activity was stable in three successive cycling experiments. This strong photocatalytic activity under visible light was attributed to the interfacial charge transfer (IFCT) from the valence band (VB) of the TiO2 to the CuC2O4 nanoparticles. The holes produced in the VB of TiO2 are then capable of decomposing organic substances. By comparison with Cu (II)-grafted TiO2 (denoted as Cu(II)/TiO2) synthesized using the same amount of CuSO4 solution as precursor, CuC2O4/TiO2 nanocomposites displayed superior activity for higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2. CuC2O4/TiO2 photocatalyst is composed of environmental friendly Cu and Ti and without involvement of noble metals [27]. So, it will be utilized extensively in practice application for its nontoxic, cheap and strong oxidative ability.
ultraviolet–visible light diffusion reflectance spectra (DRS) of the photocatalysts were recorded with a CARY5000 spectrometer (BaSO4 was used as a reference). X-ray photoelectron spectroscopy (XPS) data were obtained using a Thermo Fisher Scientific Escalab 250Xi X-ray photoelectron spectrometer using monochromatized Al-Kα (hν = 1486.6 eV) radiation as excitation source. The binding energies were calibrated with reference to adventitious C 1s line at 284.8 eV. The photocurrent measurements were examined by electrochemical station (CHI 650E Chenhua Instrument Company) in a three-electrode quartz cell with 0.1 M Na2SO4 electrolyte solution. The as prepared samples electrode with an active area of 1.5 cm2 were used as the working electrode. The working electrode was prepared by dip-coating method as follows: 2 mg of sample, 30 μL of water and 5 μL of Nafion were mixed to form homogeneous slurry, and then was dropped onto the precleaned ITO glass and dried at room temperature for 12 h. A platinum wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. A xenon lamp equipped with an ultraviolet cutoff filter (λ > 420 nm) was served as the visible light source and was positioned 10 cm away from the photoelectrochemical cell.
2. Experimental
The visible light photocatalytic activity of P25, Cu2C2O4, Cu (II)/ TiO2 and Cu2C2O4/TiO2 (X:Y) was evaluated by monitoring the oxidation of propylene in an on-line analysis system equipped with a gas chromatograph (GC7900) to monitor the concentration change of C3H6 (C). The photocalyst (ca. 25 mg) was coated on one side of a roughened glass plate (ca. 10 cm2), which was put in a quartz tube reactor surrounded with a cycling water channel to keep the reaction temperature unchanged. The feed gas was mixed of propylene and dry air, and flowed through the reactor at a flow rate of 200 mL/h. A 500 W xenon lamp was used as the visible light source and an ultraviolet (UV) cut 420 filter was inserted between the xenon lamp and reactor to eliminate UV light. Prior to irradiation, the feed gas was allowed to flow through the reactor continuously until the adsorption/desorption equilibrium was established. The concentrations of C3H6 and CO2 production were measured by GC equipped with a flame ionization detector (FID), a GDX-502 column, and a reactor loaded with Ni catalyst for methanization of CO2. The removal rate of C3H6 is calculated as (C0 − C)/ C0 × 100%, where C0 refers to the initial C3H6 concentration with a value of about 500 ppm V.
2.3. Evaluation of visible light photocatalytic activity
P25-TiO2 crystals were produced from Guangzhou Hualisen Trade Co., Ltd. Other used reagents were of analytical reagent grade and used without further purification. 2.1. Preparation of pure CuC2O4, Cu (II)/TiO2 and CuC2O4/TiO2 composite The nanocrystalline CuC2O4/TiO2 heterostructures with different mass ratio were synthesized via a chemical precipitation method. In brief, 1.0 g of P25-TiO2 was ultrasonic dispersed into 20 mL of distilled water, then a certain volume of Na2C2O4 (0.1 M) aqueous solution was slowly added drop-wise to the above suspension mixture with constant stirring. After 20 min of stirring, a certain volume of CuSO4 solution (0.1 M) was dripped into above suspension to yield CuC2O4/TiO2 precipitate. In the above procedure, the volume of Na2C2O4 was larger than that of CuSO4 to ensure the complete precipitation of Cu2+. Finally, the precipitate was collected, washed with distilled water for several times, and then dried at 60 °C in vacuum oven to get the nanocrystalline CuC2O4/TiO2 hybrid photocatalyst. By changing the volume of Na2C2O4 and CuSO4 solution added, CuC2O4/TiO2 composites with different mass ratio (0.05:1, 0.1:1, 0.15:1, 0.25:1, 0.3:1) have been prepared and labeled as CuC2O4/TiO2 (X:Y), where X:Y means the theoretical mass ratio of CuC2O4 to TiO2 in the composite. To make a comparison, pure CuC2O4 and Cu (II)-grafted TiO2 (Cu (II)/TiO2) were also synthesized. The former was prepared with CuSO4 and Na2C2O4 and the latter was synthesized using CuSO4 and P25-TiO2 without addition of Na2C2O4. For Cu (II)/TiO2, briefly, 1.0 g of P25-TiO2 and certain volume of CuSO4 solution (0.1 M) were ultrasonic mixed into a beaker, and then were stirring for 20 min. The volume of CuSO4 solution is 3.3, 6.6, 10, 16.5 and 20 mL, respectively, which is identical to that for preparing CuC2O4/TiO2 (X:Y). The suspension was filtered and dried at 60 °C in vacuum oven to get Cu (II)/TiO2 series and labeled as Cu (II)/TiO2-I, Cu (II)/TiO2-II, Cu (II)/TiO2-III, Cu (II)/TiO2-IV and Cu (II)/TiO2-V, respectively.
3. Results and discussion 3.1. Characterization of catalysts Fig. 1 displays the powder XRD patterns of pure CuC2O4 and CuC2O4/TiO2 heterostructures with different mass ratio. All characteristic peaks of pure CuC2O4 coincide well with the standard data of orthorhombic CuC2O4 (JCPDS 21-0297) [28,29]. The strong diffraction peaks of CuC2O4 at 2θ angles of 22.902°, 36.251°, 36.946°, 38.835°, 39.063°, 42.443°, 46.865°, 51.469°, and 52.036° are corresponded to the (1 1 0), (1 2 0), (2 1 0), (0 1 1), (1 0 1), (1 1 1), (2 2 0), (1 2 1), and (1 3 0) planes respectively. As for all CuC2O4/TiO2 composites, the major peak of orthorhombic CuC2O4 at 2θ = 22.902° is clearly observed and its intensity increases with increasing the mass ratio of CuC2O4. Moreover, after coupling with CuC2O4, all of the diffraction peaks of anatase (A) and rutile (R) phase still exist without change, suggesting that the heterogeneous process doesn't affect the crystal structure of TiO2. Fig. 2 shows SEM images of CuC2O4 synthesized with CuSO4 and Na2C2O4 at different temperature including 30 °C, 60 °C and 90 °C. It can be seen that all samples contain two shapes of copper oxalate: spherical and fusiform. Fig. S1 shows the proportion of two different shapes in CuC2O4 sample prepared at different temperature. Apparently, the fusiform shape is the main component of CuC2O4 produced at
2.2. Characterization The morphology of as-prepared photocatalysts was studied using a JEOL JSM-7610F scanning electron microscope (SEM) and a JEOL JEM2010 transmission electron microscope (TEM). The crystal phase of the obtained samples was measured with a Bruker D8 X-ray diffractometer (XRD) with Cu Kα radiation at an accelerating voltage of 40 kV. The 765
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CuC2O4/TiO2 (0.05:1) and CuC2O4/TiO2 (0.25:1), CuC2O4 nanoparticles were found with the diameter of several nanometers adhering to the surface of TiO2 substrate, although pure CuC2O4 above mentioned with a diameter of ca. 1 µm. It is well known that there are coordinatively unsaturated surface Ti atoms on the P25-TiO2 particle surface and they are active in solution [30,31]. Therefore, it could be referred that oxalate ion may interact with surface Ti atom (^Ti) as follows: Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.06.288.
(I) When CuSO4 solution was dripped into above mixture, the following reaction occurred: Fig. 1. XRD patterns of pure CuC2O4, P25-TiO2 and CuC2O4/TiO2 composites with different ratio.
Fig. 2. SEM images of CuC2O4 synthesized with CuSO4 and Na2C2O4 at different temperature: (a) 30 °C; (b) 60 °C; (c) 90 °C.
three different temperatures. Moreover, histograms of size distribution of samples are shown in Fig. S2. It can be seen that, with the increase of reaction temperature from 30 °C to 90 °C, the diameter of spherical CuC2O4 and the length of fusiform CuC2O4 increased from 1.4 and 1.2 μm to 2.6 and 2.2 μm, indicating that the reaction temperature affects the size of copper oxalate. Considering that the small size of CuC2O4 is beneficial to the strong interaction between CuC2O4 and TiO2 substrate for CuC2O4/TiO2 heterostructure, 30 °C was chosen to prepare CuC2O4/TiO2 (X:Y) as the most appropriate reaction temperature. Fig. 3 shows SEM images of CuC2O4, TEM images of P25-TiO2 as well as typical CuC2O4/TiO2 (X:Y) samples, CuC2O4/TiO2 (0.05:1) and CuC2O4/ TiO2 (0.25:1), all prepared at 30 °C. As shown in Fig. 3c and d, for
(II) As the chemisorption of oxalate ion on the TiO2 surface is highly dispersive, the diameter of CuC2O4 is small in the range of nanometer. The synthesis process of CuC2O4/TiO2 (X:Y) and interaction of C2O42− and CuC2O4 with TiO2 is illustrated in Fig. 4. Fig. 5 shows the UV–vis diffuse reflectance spectra of P25-TiO2 and composite samples CuC2O4/TiO2 (X:Y). As can be seen from the curve, for P25-TiO2 and CuC2O4/TiO2 (X:Y) composites, the strong absorption at wavelengths shorter than 400 nm can be assigned to the intrinsic 766
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Fig. 3. SEM image of CuC2O4 (a) as well as TEM images of P25-TiO2 (b), CuC2O4/TiO2 (0.05:1) (c) and CuC2O4/TiO2 (0.25:1) (d). CuC2O4 and CuC2O4/TiO2 (X:Y) were both prepared at 30 °C.
Fig. 5. UV–vis-NIR diffuse reflectance spectra of P25-TiO2 (a), pure CuC2O4 (g) and CuC2O4/TiO2 composites with different ratio: (b), CuC2O4/TiO2 (0.05:1); (c), CuC2O4/TiO2(0.10:1); (d), CuC2O4/TiO2 (0.15:1); (e), CuC2O4/TiO2 (0.25:1); (f), CuC2O4/TiO2 (0.30:1). Inset: Picture of CuC2O4/TiO2 (X:Y).
C2O42−
Fig. 4. Schematic illustration of interaction of during the synthesis process of CuC2O4/TiO2 (X:Y).
absorption edges of CuC2O4/TiO2 (0.05:1) and CuC2O4/TiO2 (0.1:1) shift to shorter wavelength while other CuC2O4/TiO2 (X:Y) catalysts showing red-shift. Shifting the absorption edge may be due to interactions between TiO2 substrate and surface CuC2O4 in CuC2O4/TiO2 nanocomposites [32–35]. Mahdiani et al found that, over carbon-based nanocomposites of CuFe12O19, the absorption edge shifts to higher energy resulting the photocatalytic activity was improved [33]. In this work, for CuC2O4/TiO2 (X:Y), the absorption at 600–800 nm is assigned to the d-d transition of Cu (II) [22,23,36–39] and the absorption shoulder from 400 to 500 nm can be ascribed to the photoinduced
and CuC2O4 with TiO2
bandgap absorption of TiO2, and the bandgap energy is estimated to be 3.1 eV according to the equation Eg = 1239.8/λ. As for CuC2O4/TiO2 (X:Y), with increasing the content of CuC2O4 an enhanced absorption in the visible light region is clearly observed and the absorption edges red shifts to longer wavelength increasingly. Compared to pure CuC2O4, the 767
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Fig. 6. Comparison of Cu 2p (a) and O 1s (b) XPS spectra of pure CuC2O4 or TiO2 and CuC2O4/TiO2 (0.25:1) composite.
Fig. 6b, the peak for O 1s located at 529.7 eV is assigned to lattice oxygen in TiO2 crystal, which distinctly shifts to a higher energy in CuC2O4/TiO2, suggesting that the electron density of lattice oxygen on the surface of titanium dioxide becomes lower after combination. Combined with fabrication process and DRS results of CuC2O4/TiO2, it can be concluded that, as shown in Fig. 4, during the synthesis process, the oxygen atoms in C2O42− are in coordination with surface Ti atoms in titanium dioxide before the addition of Cu2+ into the suspension of P25 and C2O42−. Then, after dripping Cu2+ into above suspension, the strong electrostatic attraction enables C2O42− to combine with Cu2+. On the other hand, the coordination of C2O42− with Ti atom is weakened or disappeared, while the oxygen atom on the surface of titanium dioxide provides electron to Cu2+ with unsaturated d 9 configuration to form an intimate action. Such interaction is crucial to the photoinduced interfacial charge transfer from the valence band of TiO2 to CuC2O4.
interfacial charge transfer (IFCT) from the valence band of TiO2 to CuC2O4. The intimate contact between TiO2 and CuC2O4 by the coordination bond is crucial for the IFCT between two components. Jono and co-workers found that the TiO2-TCNQ (7,7,8,8-tetracyanoquinodimethane) surface complex exhibits the interfacial charge transfer (IFCT) absorption band in the visible to near IR and pointed that the IFCT absorption originates from the electron flow from the surface oxygen atom of TiO2 to TCNQ [40]. In order to further confirm the interaction between CuC2O4 nanomaterial and TiO2 support, the comparisons of high resolution XPS spectra of Cu 2p and O 1s between pure CuC2O4 and CuC2O4/TiO2 nanocomposites are displayed in Fig. 6a and b. For pure CuC2O4, XPS results showed the typical Cu2p3/2 and Cu2p1/2 peaks sitting at 934.7 and 954.6 eV, as well as their shake-up satellites, which is indicative of the paramagnetic chemical state of Cu2+ [22,23]. Whereas, the peak position for Cu 2p in the CuC2O4/ TiO2 nanocomposites shifts to a lower binding energy than that in pure CuC2O4, indicating that a higher electron density of Cu atom in CuC2O4/TiO2 composite than that in CuC2O4. In addition, as shown in
Fig. 7. Visible light photoactivity of P25-TiO2, Cu2C2O4 and Cu2C2O4/TiO2 composites evaluated by C3H6 removal (vol%) (a); Change of CO2 production with irradiation time (b); The steady removal rate of propylene over different photocatalysts (c). 768
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3.2. Visible light photocatalytic performance and stability
Table 1 The comparison of atom ratio of Cu to Ti between Cu(II)/TiO2-IV and CuC2O4/ TiO2 (0.25:1) before and after irradiation.
The experiment of propylene degradation was carried out to evaluate the visible light activity of P25-TiO2, pure CuC2O4 and CuC2O4/ TiO2 with different ratios. Before illumination, the feed gas was flowing continuously until the propylene adsorption-desorption equilibrium was established. As shown in Fig. 7a, pure CuC2O4 and P25-TiO2 are nearly inert in oxidizing propylene under visible light illumination. But, CuC2O4/TiO2 composites with different ratios showed apparent visible light activity for the degradation of propylene. With the increase of the ratio of CuC2O4 to TiO2 from 0.05:1 to 0.3:1, the degradation of propylene first increases and then decreases. CuC2O4/TiO2 (0.25:1) exhibits the highest activity with a steady propylene removal rate of 46.5%. For CuC2O4/TiO2 (0.30:1), the removal rate of propylene decreases dramatically. This probably due to the following factors: (1) excessive CuC2O4 may shield part of TiO2 from incident light, and thus decrease the generation of photogenerated electrons. (2) CuC2O4 at high content may act as charge recombination centers, leading to the decrease of the photocatalytic activity. Therefore optimum CuC2O4 content facilitates visible light absorption and high separation efficiency of charge carriers. Moreover, the change of CO2 concentration, mineralization product, as a function of irradiation time during the photocatalytic process has been plotted in Fig. 7b. It can be seen that the highest concentration of CO2 about 400 ppm is obtained over CuC2O4/TiO2 (0.25:1) catalyst, which is well accordant with that result of Fig. 7a. Compared to CuC2O4 and P25-TiO2, The remarkable enhancement of photocatalytic activity of CuC2O4/TiO2 heterostructures is may be caused by the IFCT effect from CuC2O4 nanomaterials to the conduction band of TiO2. Irie and Hashimoto et al. have reported that Cu (II)-grafted TiO2 (Cu(II)/TiO2) and WO3 (Cu(II)/WO3) also showed visible light photocatalytic activity for the decomposition of 2-propanol by IFCT from semiconductor to adsorbed Cu(II) ions [19,20]. In this work, we also synthesized Cu (II)/TiO2 series using CuSO4 solution as the source of Cu(II) by the same method as in reference [19,20]. The propylene remove rate and change of CO2 production with irradiation time over Cu (II)/TiO2 series is shown in Fig. 8a and b, respectively. The highest propylene remove rate obtained over Cu(II)/TiO2-IV is only 15%, which is much lower than that of CuC2O4/TiO2(0.25:1) with 46%. Why is the activity of CuC2O4/TiO2 heterostructures far superior to Cu (II)/TiO2 series? We compared the absorbance and chemical status of two typical catalysts, CuC2O4/TiO2 (0.25:1) and Cu(II)/TiO2-IV. CuC2O4/TiO2 (0.25:1) was prepared using the same amount of P25TiO2 and CuSO4 solution, 1.0 g and 16.5 mL respectively, with that of preparing Cu(II)/TiO2-IV. However, as shown in Table 1, the atom ratio of Cu to Ti with value of 0.0965 in CuC2O4/TiO2 (0.25:1) is obviously larger than that of Cu(II)/TiO2-IV with 0.0725. This indicates that the coordination of C2O42− with surface Ti atom of TiO2 in the initial stage of CuC2O4/TiO2 synthesis is beneficial to the formation of more CuC2O4 on the surface of TiO2. Fig. 9 shows the comparison of absorption
photocatalyst
Atom ratio of Cu to Ti
The amount of using TiO2
The volume of using CuSO4 (0.1 M)
Cu (II)/TiO2-IV CuC2O4/TiO2 (0.25:1)
0.0725 0.0965
1.0 g 1.0 g
16.5 mL 16.5 mL
Cu (II)/TiO2-IV-aft* CuC2O4/TiO2 (0.25:1)-aft*
0.0617 0.0953
1.0 g 1.0 g
16.5 mL 16.5 mL
* Cu(II)/TiO2-IV and CuC2O4/TiO2 (0.25:1) photocatalysts after 4 h visible light irradiation were denoted as Cu(II)/TiO2-IV-aft and CuC2O4/TiO2 (0.25:1)aft, respectively.
Fig. 9. The comparison of absorbance ability between CuC2O4/TiO2 (0.25:1) and Cu (II)/TiO2-IV.
ability between Cu(II)/TiO2-IV and CuC2O4/TiO2 (0.25:1). Apparently, the absorbance of CuC2O4/TiO2 (0.25:1) in the range of 400–800 nm is much stronger than that of Cu(II)/TiO2-IV. This may be due to the higher atom ratio of Cu to Ti in CuC2O4/TiO2 (0.25:1), and strong absorption ability in visible light for CuC2O4/TiO2 (0.25:1) is beneficial to its visible-light photocatalytic activity. To investigate the interaction between Cu2+ with TiO2 in Cu(II)/TiO2-IV, the comparisons of high resolution XPS spectra of Ti 2p and O 1s between pure TiO2 and Cu(II)/ TiO2-IV are displayed in Fig. 10a and b. It can be seen that the peaks for Ti 2p in Cu(II)/TiO2-IV just very slightly shift to a higher binding energy compared to TiO2. Moreover, the situation of peaks for O 1s located at 529.8 eV and 529.7 eV, which is assigned to lattice oxygen of TiO2 [41], in Cu(II)/TiO2-IV and pure TiO2 is nearly unchanged, indicating that the action between Cu2+ and TiO2 in Cu(II)/TiO2-IV is faint. Compared to Cu(II)/TiO2-IV, the comparison of XPS spectra of Cu
Fig. 8. Visible light photoactivity of Cu (II)/TiO2 series evaluated by C3H6 removal (vol%) (a); Change of CO2 production with irradiation time (b). 769
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Fig. 10. Comparison of Ti 2p (a) and O 1s (b) XPS spectra of pure TiO2 and Cu (II)/TiO2-IV catalyst.
CO2 during recycle experiments may be due to the photo-induced promotion of interaction between CuC2O4 and TiO2. Fig. 13 shows the comparison of Cu 2p spectra on CuC2O4/TiO2 (0.25:1) before and after photocatalytic experiments. After 4 h of visible light irradiation, the satellite is still present but the binding energies of Cu 2p1/2 and Cu 2p3/ 2+ was reduced to 2 shift to lower energy. This indicates that part of Cu + 0 Cu or Cu . Associated with the Auger Cu LMM spectrum of sample CuC2O4/TiO2 (0.25:1) after photocatalytic reaction for 4 h (inset in Fig. 13), the Cu LMM line position is at 569.8 eV, which is the typical value for Cu+ [22,23]. From this analysis, it is assumed that a small amount of Cu2+ in CuC2O4 is reduced to Cu2C2O4 due to the photogenerated electron transfer from the VB of TiO2 to CuC2O4.
2p and O 1s (Fig. 6) between pure CuC2O4 and CuC2O4/TiO2 nanocomposites suggested that the strong interaction between CuC2O4 and TiO2 may be the main reason for its higher activity. To further investigate the charge separation and migration of photocatalysts, the transient photocurrent responses of CuC2O4/TiO2 samples were tested for several cycles of on-off under visible light irradiation. Fig. 11 shows the transient photocurrent responses of P25-TiO2, pure CuC2O4 and CuC2O4/TiO2 composites. P25-TiO2 and pure CuC2O4 electrodes didn’t show any photocurrent response, while CuC2O4/TiO2 electrodes showed apparent response under visible light. This suggests that the intimate interaction formed between CuC2O4 NPs and TiO2 substrate, leading to the effective separation of photogenerated carries. Moreover, as for CuC2O4/TiO2, the photocurrent density was reproducible during the repeated on-off cycles, indicating that photocatalysts were stable under visible light illumination. With the increase of CuC2O4 weight ratio, the photocurrent density initially increases from 0.6 μA cm−2 for CuC2O4/TiO2 (0.05:1) to a maximum value of 1.6 μA cm−2 for CuC2O4/TiO2 (0.25:1), then decreases to 1.0 μA cm−2 for CuC2O4/TiO2 (0.30:1). The highest photocurrent obtained over CuC2O4/TiO2 (0.25:1) could be ascribed to the efficient electron-hole transfer and separation process, which was beneficial to the highest activity for the degradation of propylene. The recycle experiments were carried out to evaluate the stability of CuC2O4/TiO2 (0.25:1) photocatalyst. As illustrated in Fig. 12, after three cycles of photodegradation of propylene, the photocatalytic activity of CuC2O4/TiO2 (0.25:1) slightly increased rather than decline, suggesting that the CuC2O4/TiO2 heterostructure photocatalyst is sufficiently stable and not deactivated during the photodegradation of propylene. This result is further confirmed by the production of CO2 during recycle experiment. The increasing activity and production of
3.3. The possible photocatalytic mechanism On the basis of DRS and XPS results, the visible light photocatalytic mechanism for the degradation of propylene over CuC2O4/TiO2 is proposed as illustrated in Fig. 14. The band gap of P25-TiO2 is estimated to be 3.1 eV according to the equation Eg = 1240/λ, and the band edge positions of conduction and valence band of TiO2 are calculated to be −0.3 and 2.8 eV by a simple approach [42,43], respectively. Bare P25-TiO2 photocatalysts showed negligible photocatalytic activity, which is due to the wide bandgap of TiO2. While CuC2O4/TiO2 composites showed greatly enhanced photocatalytic activity compared to bare TiO2. This may be attributed to the IFCT effect from the valence band of TiO2 to CuC2O4 NP, as showed in Fig. 14. Hashimoto et al found that, in Cu(II)-grafted TiO2 and WO3 systems ((Cu(II)/TiO2 and Cu(II)/ WO3), electrons in the VB of TiO2 can be excited to Cu(II) by visible light, leaving holes in the VB of TiO2 [19]. Thus, Cu(II) are reduced to Cu(I). Yu et al also reported that, in CuS/ZnS heterostructure, visible light irradiation also induces IFCT from the VB of ZnS to CuS clusters [22]. In our CuC2O4/TiO2 composites, we think, IFCT from the VB of TiO2 to CuC2O4 nanoparticles can also be initiated. The evidence is the apparent absorption from 400 to 450 nm due to the IFCT, as shown in Fig. 5. That is, electrons in the VB of TiO2 are transferred to CuC2O4 under visible light irradiation. The transferred electrons can be captured by adsorbed O2 to form active radical %O2– or cause the partial reduction of CuC2O4 to Cu2C2O4. In another way, the photoinduced holes can also oxidize organic pollutants to CO2. The major electron transfer steps in the above photocatalytic mechanism over CuC2O4/ TiO2 composites under visible light irradiation can be described as follows: CuC2O4/TiO2 + hv → CuC2O4 (e−)/TiO2 (h+) CuC2O4 + e−→Cu2C2O4 Cu2C2O4/TiO2 (h+) + O2 → CuC2O4/TiO2 (h+) + %O2–
Fig. 11. Photocurrent response of P25-TiO2, CuC2O4 and CuC2O4/TiO2 electrodes under visible light irradiation (λ > 420 nm).
%
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Fig. 12. Repetitive degradation of propylene and CO2 production over CuC2O4/TiO2 (0.25:1) photocatalyst under visible light illumination.
CuC2O4/TiO2 (h+) + propylene → CO2 + H2O 4. Conclusions In summary, CuC2O4/TiO2 composites with different ratio were synthesized via a facile precipitation method. The prepared CuC2O4/ TiO2 exhibits excellent photocatalytic activity and stability under visible light irradiation. The optimal proportion of CuC2O4 and TiO2 is 0.25:1. It is believed that there is an intimate interaction between CuC2O4 and TiO2 and the IFCT from the VB of TiO2 to CuC2O4 cluster plays an important role in charge separation and reducing recombination for high photocatalytic activity. By comparison with Cu(II)/TiO2, CuC2O4/TiO2 nanocomposites displayed superior activity for higher atom ratio of Cu to Ti, more visible light absorption and stronger action between CuC2O4 and TiO2. So, CuC2O4/TiO2 composites are promising photocatalysts for practical application to degrade organic pollutants because of their superior photocatalytic activity and stability.
Fig. 13. XPS spectra of Cu 2p of CuC2O4/TiO2 (0.25:1): (a) as-prepared sample and (b) sample treated with 4 h irradiation of visible light. The inset is the Auger Cu LMM line of CuC2O4/TiO2 (0.25:1) sample after 4 h irradiation.
Acknowledgment The authors acknowledge the financial support provided by the Science & Technology Research Project of Henan province (Grant nos. 18A150022, 17B610003 and 182102410090) and Provincial Training Programs of Innovation and Entrepreneurship for Undergraduates (no. 201710475047). References [1] T.H. Ji, F. Yang, Y.Y. Lv, J.Y. Zhou, J.Y. Sun, Synthesis and visible-light photocatalytic activity of Bi-doped TiO2 nanobelts, Mater. Lett. 63 (2009) 2044–2046. [2] T. Ohno, N. Murakami, Development of visible-light active S cation-doped TiO2 photocatalyst, Curr. Org. Chem. 14 (2010) 699–708. [3] V.B.R. Boppana, R.F. Lobo, Photocatalytic degradation of organic molecules on mesoporous visible-light-active Sn(II)-doped titania, J. Catal. 281 (2011) 156–168. [4] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science 293 (2001) 269–271. [5] X.J. Wang, J.K. Song, J.Y. Huang, J. Zhang, X. Wang, R.R. Ma, J.Y. Wang, J.F. Zhao, Activated carbon-based magnetic TiO2 photocatalyst codoped with iodine and nitrogen for organic pollution degradation, Appl. Surf. Sci. 390 (2016) 190–201. [6] S.A. Bakar, C. Ribeiro, Rapid and morphology controlled synthesis of anionic Sdoped TiO2 photocatalysts for the visible-light-driven photodegradation of organic pollutants, RSC Adv. 6 (2016) 36516–36527. [7] Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota, A. Fujishima, Multicolour photochromism of TiO2 films loaded with silver nanoparticles, Nat. Mater. 2 (2003) 29–31. [8] K. Awazu, M. Fujimaki, C. Rockstuhl, J. Tominaga, H. Murakami, Y. Ohki, N. Yoshida, T. Watanabe, A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide, J. Am. Chem. Soc. 130 (2008) 1676–1680. [9] Y. Tian, T. Tatsuma, Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles, J. Am. Chem. Soc. 127 (2005) 7632–7637. [10] F. Ansari, P. Nazari, M. Payandeh, F.M. Asl, B. Abdollahi-Nejand, V. Ahmadi, J. Taghiloo, M. Salavati-Niasari, Novel nanostructured electron transport compact layer for efficient and large-area perovskite solar cells using acidic treatment of titanium layer, Nanotechnology 29 (2018) 10.
Fig. 14. Schematic illustration for CuC2O4/TiO2 heterostructure based interfacial charge transfer process from the valence band of TiO2 to CuC2O4 particles under visible light irradiation.
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