Enhanced performance of photocatalytic CO2 reduction via synergistic effect between chitosan and Cu:TiO2

Journal Pre-proof Enhanced Performance of Photocatalytic CO2 Reduction via Synergistic Effect between Chitosan and Cu:TiO2 Houde She (Conceptualization) (Validation) (Resources) (Writing review and editing), Ziwei Zhao (Methodology) (Software) (Formal analysis) (Investigation) (Writing - original draft), Wencai Bai (Investigation) (Resources) (Visualization) (Data curation), Jingwei Huang (Data curation) (Formal analysis), Lei Wang (Writing - review and editing), Qizhao Wang (Supervision) (Project administration)

PII:

S0025-5408(19)32264-0

DOI:

https://doi.org/10.1016/j.materresbull.2019.110758

Reference:

MRB 110758

To appear in:

Materials Research Bulletin

Received Date:

31 August 2019

Revised Date:

26 December 2019

Accepted Date:

26 December 2019

Please cite this article as: She H, Zhao Z, Bai W, Huang J, Wang L, Wang Q, Enhanced Performance of Photocatalytic CO2 Reduction via Synergistic Effect between Chitosan and Cu:TiO2 , Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110758

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Enhanced Performance of Photocatalytic CO2 Reduction via Synergistic Effect between Chitosan and Cu:TiO2 Houde She *, Ziwei Zhao, Wencai Bai, Jingwei Huang, Lei Wang, Qizhao Wang College of Chemistry and Chemical Engineering, Research Center of Gansu Military and Civilian Integration Advanced Structural Materials, Gansu International Scientific and

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Technological Cooperation Base of Water–Retention Chemical Functional Materials, Northwest Normal University, Lanzhou 730070, People’s Republic of China

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*Corresponding author. E-mail: [email protected]

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Graphical abstract

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The Cu:TiO2-CS composite was synthesized by a facile solvothermal process, which

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exhibits excellent photocatalytic CO2 reduction performance.

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Highlights

 The Cu:TiO2-CS composite was successfully synthesized by a solvothermal

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treatment method.

 Cu:TiO2-CS sample exhibited excellent photocatalytic CO2 reduction capability

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than TiO2,TiO2-CS and Cu:TiO2 sample.

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 The synergistic effect between chitosan and Cu:TiO2 in Cu:TiO2-CS composite facilitated the performance of CO2 reduction improvements.

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Abstract Photocatalytic CO2 reduction is considered as a feasible approach for solving greenhouse effect and energy shortage. Promoting electron migration and inhibiting electron-hole recombination are key challenges for efficient photocatalytic CO2 reduction. Here, we successfully synthesized Cu:TiO2-CS composite as photocatalyst by a facile solvothermal method to improve the high-catalytic effect on CO2 reduction. The as-prepared composite exhibits excellent

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photocatalytic CO2 reduction performance under 300 W xenon lamp irradiation, with the yields of

CO and CH4 up to 4.48 μmol/g and 5.34 μmol/g, which is 10 times higher than that of TiO2. This

impressive amelioration can be attributed to the synergy of organic polymer (chitosan) and

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inorganic material (Cu:TiO2). This work may provide a constructing guidance in designing the

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composite photocatalyst concerning CO2 photocatalytic reduction.

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Keywords: A. Composites; A. Semiconductors; B. Solvothermal; D. Catalytic properties

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1 Introduction

The exploration of converting CO2 into valuable products is one of the appropriate methods to

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the global warming problem and ever-increasing energy demand. To solve the problem effectively, using semiconductor material for photocatalytic CO2 reduction has attracted the comprehensive

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attention of many researchers [1]. Consequently, many commonly-seen semiconductor materials such as TiO2 [2-4], g-C3N4 [5-7], ZnO [8], CdS [9, 10] and bismuth-based materials [11-13] have been employed as photocatalysts in CO2 reduction. Owing to stable chemical properties, high catalytic efficiency and strong redox ability, TiO2 has attracted marvelous attention and thorough research. However, limited by its disadvantages such 3

as low hole mobility, short lifetime of photogenerated carriers and high photogenerated charges recombination rate, TiO2 have not been widely put into photocatalysis application [14]. In addition, pristine TiO2 can only be excited by UV irradiation(λ < 387 nm) given that its bandgap is wide (Eg = 3.2 eV) and merely 5% of solar light is able to be utilized during the catalytic process, which severely limits the CO2 photoconversion performance [15]. So far, numerous approaches, like metal ion-doped TiO2 [16, 17], coupling semiconductor material [18], dye-sensitized TiO2 [19],

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and engineering the disorder of nanophase TiO2 derived from hydroxylation by hydrogenation

treatment [20], have been made to overcome the above-mentioned problems. For example, Li et al. synthesized Cu/TiO2 photocatalyst for CO2 reduction through a facile strategy, which enhanced

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CO2 photoreduction efficiency attributed to the synergistic effect of the Cu species and the surface

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defect sites like oxygen vacancies [21]. Park et al. investigated hetero-structured CuxO-TiO2

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photocatalyst, which expressed a remarkable methane yield due to the efficacious charge separation between the two semiconductors [22]. Wang et al. prepared dye-sensitized TiO2

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composite and exhibited higher yield of CH4 in photocatalytic reduction of CO2 caused by efficient photosensitizer can greatly broaden the photo-response ability of TiO2 and improve light

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energy utilization [23].

In addition, owing to rich, non-toxic, biocompatible, low cost, and biodegradable, chitosan (CS)

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is proverbially used to adsorb various pollutants such as heavy metals, organic compounds and dyes in wastewater treatment [24]. As a natural polymer compound, CS not only has the characteristics of stable spatial structure, special spatial effects and strong coordination complexing ability, but also can effectively separate photogenerated electron-holes owing to special structures and synergistic effects [25]. Thereby, CS has broad application prospects for 4

improving photocatalytic activity. Nevertheless, in terms of current research status, plenty of research on CS has focused on the fields of medicine and biological storage rather than photocatalysis [26-28]. Herein, the synergistic effect between CS and Cu:TiO2 in Cu:TiO2-CS composite are well-studied. We prepared Cu:TiO2-CS ternary composite photocatalyst by using solvothermal synthesis method and studied its photocatalytic activity in CO2 reduction. The results showed that,

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the as-prepared Cu:TiO2-CS composite manifests an enhanced charge separation efficiency and

broadened light absorption range. Its further application to CO2 reduction illustrated a greatly ameliorated photocatalytic activity. Furthermore, an apropos mechanism of these improved

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performances was methodically speculated.

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2 Experimental

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2.1 Materials

Tetrabutyl titanate (C16H36O4Ti), hydrogen peroxide (30% H2O2), ethylene glycol

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(HOCH2-CH2OH) and ethanol (CH3CH2OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chitosan (CS) was purchased from Aladdin Biochemical Technology Co., Ltd.

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(Shanghai, China). Acetic acid (CH3COOH) was purchased from Kean Longbohua Pharmaceutical Chemical Co., Ltd. (Tianjin, China). Copper acetate ((CH3COO)2Cu) was

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purchased from Shanghai Jianxin Chemical Co., Ltd. All chemicals were of analytical reagent grade and used as received without further purification. 2.2 Synthesis of photocatalysts Cu:TiO2-CS composite photocatalyst was synthesized by a facile solvothermal process：1 g CS was dissolved into a mixed solvent of Tetrabutyl titanate (C16H36O4Ti) (10 mL), H2O (5 mL) and 5

HOCH2-CH2OH (40 mL). Then 0.06 g (CH3COO)2Cu was added and stirred for 2 h to form a homogeneous solution. Subsequently, CH3COOH (1 mL) was added to the above solution and continually stirred for 10 min. The obtained solution was transferred into a Teflon-lined stainless-steel autoclave and kept at 110 °C for 24 h. After that, the cooled mixture was filtered and the sample was then filtered and washed with CH3CH2OH and deionized water, then dried in vacuum at 60 °C for 8 h to acquire targeted product. TiO2-CS, Cu:TiO2 and TiO2 were prepared by

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a similar procedure to that of Cu:TiO2-CS. Only 1g CS or 0.06g (CH3COO)2Cu was added to prepared TiO2-CS or Cu:TiO2 sample, respectively. TiO2 material was synthesized by the above method with neither CS nor (CH3COO)2Cu was added.

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2.3 Characterization

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Powder X-ray diffraction (XRD) data patterns were investigated in a Bragg−Brentano Rigaku

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D/MAX-2200/PCX-diffractometer with Cu Kα radiation (40 kV × 20 mA) with scattering angle ranging from 5° to 80°. Surface morphology of the photocatalysts was observed by JSM-6701E

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field emission scanning electron microscope (FE-SEM) and JEOL JEM-2100 transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) measurements were carried

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out using a PHI5702 photoelectron spectrometer. Using Ba2SO4 as the internal reflectance standard, solid-state UV–vis diffuse reflectance spectra were measured by double-beam UV-vis

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spectrophotometer (PuXin, TU-1901) equipped with an integrating sphere attachment. Photoluminescence (PL) emission spectra were recorded on a 325−525 nm fluorescence spectrometer (PE, LS-55) at room temperature. 2.4 Photoelectrochemical measurements All photoelectrochemical performance measurements were performed on a CHI-660D 6

electrochemical workstation (CHI Shanghai) with typical three electrodes cell (including Pt sheet auxiliary electrode, Ag/AgCl reference electrode and as-prepared composite photoanode as the working electrode) under the 300 W xenon lamp (PLS-SXE300C) as light source illumination. The electrolyte was 0.5 M Na2SO4 solution. A solid sample (10 mg) was dispersed in ethanol and sonicated for 30 min, then dropped on FTO conductive glass coated with a Nafion solution. All of the measurements were irradiated from the back side of working electrode with about 1 cm2 areas

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[29]. 2.5 Photocatalytic activity

The as-prepared photocatalysts was carried out in a 50 mL homemade glass reactor system.

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Normally, 2 ml deionized water was filled into a stainless-steel reactor, and then a glass container

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(ɸ 40× 25 mm) loaded with 100 mg catalyst was encased in the reactor during each run. To ensure

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all the impurities and trapped air were completely removed, the reaction setup was purged by pure CO2 and vacuum-treated several times. Afterwards, the photocatalytic reactions were processed by

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irradiation assisted by a light source (300 W xenon lamp, Beijing Au light Co., Ltd. CELHXF300/CEL-HXUV300). The photocatalytic reaction was generally performed for 1 h

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before the objective products (CO/CH4) were collected with extraction syringes and estimated by GC-2080 gas chromatography (Ruipeng, China). Noticeably, flame ionization detector (FID) was

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adopted to respectively quantify the amounts of CO and CH4 rooted in CO2 conversion. 3 Results and discussion 3.1 Structure and morphology analysis The crystalline phases of all the samples were demonstrated by XRD analysis. Fig. 1a exhibits seven sharp peaks at 2θ = 25.3°, 38.6°, 47.8°, 55.1°, 62.6°, 70.2°, 75.1° corresponding to (101), 7

(112), (200), (105), (204), (220), (215) diffraction planes of the anatase phase of TiO2 (JCPDS file no. 21-1272), respectively [30]. No extra peaks are observed in these XRD spectra, implying that adding Cu ions and CS did not affect the crystalline structure of TiO2. At the same time, the similarity of peaks between TiO2, Cu:TiO2, and Cu:TiO2-CS reveals that the all the as-prepared photocatalysts are only traces of anatase without rutile and brookite phase. As shown in Fig. 1b, the surface morphology of Cu:TiO2-CS nanocomposite was examined in the SEM images. It can

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be apparently noticed that the granular Cu:TiO2 grows uniformly on the surface of the spherical

chitosan and has exquisite uniform dispersibility [31]. Simultaneously, TEM analysis found that

the Cu:TiO2 nanoparticles are distributed on the CS surface. As presented in Fig. 1c, TEM image

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indicates TiO2 nanoparticles have a diameter of approximately 10 nm. In more detail, the

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HR-TEM image in Fig. 1d shows that the Cu:TiO2-CS nanocomposite exhibits the characteristic

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spacings of 0.35 and 0.23 nm, which corresponds to the (101) and (112) crystal planes of the

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anatase TiO2 [32]. These results are consistent with the XRD patterns.

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Fig. 1. (a) XRD patterns of TiO2, Cu:TiO2 and Cu:TiO2-CS samples, (b) SEM image, (c) TEM image and (d)

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HR-TEM image of Cu:TiO2-CS.

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X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical compositions and oxidation states of samples. The survey spectra of the Cu:TiO2-CS

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nanocomposite were emerged in Fig. 2a. It can be illustrated that the sample is composed of O, C, N, Cu and Ti elements [30]. Fig. 2b displays the O 1 s XPS spectrum, with three different peaks

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centered at 530.1, 531.1 and 532.0 eV. The peak at 530.1 eV is assigned to the lattice oxygen of

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TiO2. The peak observed at 531.1 eV which corresponds to the surface hydroxyl group of TiO2. It is worth emphasizing that the peak at 532.0 eV, which is ascribed to the presence of the peroxo group introduced into the TiO2 lattice [33]. As presented in Fig. 2c, the XPS spectrum of C 1s reveals four peaks. Remarkably, two peaks correspond to the binding energy located at 285.5 and 286.7 eV were obviously observed, which are assigned to the C-NH2 and C=O bond, respectively. The peaks at 284.7 eV can be assigned to the C-C and C-H bonds in the CS backbone. 9

Simultaneously, the peak at 288.5 eV can be attributed to the chemical combination of O-C-O and N-C=O [34]. Fig. 2d shows the high-resolution XPS spectrum of the N 1s exhibits three peaks: a weak peak at 401.7 eV which can be attributed to C-N+, and two strong peaks at 399.2 and 400.1 eV correspond to C-NH2 and C-NH-C=O. Fig. 2e exhibits the peaks at 933.3 eV (2p3/2) and 952.8 eV (2p1/2) in Cu 2p XPS spectrum, which is a convincible evidence to confirm the oxidation state of the Cu2+, and the existence of Cu ions in the TiO2 lattice [35]. In the XPS spectrum of Ti 2p, a

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Ti 2p3/2 peak and a small Ti 2p1/2 peak were detected, and they correspond to binding energies of 458.6 and 464.3 eV as presented in Fig. 2f. The existence of two Ti 2p peaks indicates that a

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normal matrix of TiO2 was obtained [21].

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Fig. 2. (a) Wide survey XPS spectrum, and the high-resolution XPS spectra of (b) O 1s, (c) C 1s, (d) N 1s, (e) Cu

2p and (f) Ti 2p of the Cu:TiO2-CS sample.

The absorption properties of synthetic photocatalysts were assessed by UV-vis diffuse reflectance spectroscopy (DRS) analysis. Fig. 3a shows the UV-vis diffuse reflectance spectra of TiO2, Cu:TiO2 and Cu:TiO2-CS. The absorption band edge of TiO2 was located at 380 nm, which is consistent with the standard anatase TiO2 [36]. The spectrum of Cu:TiO2 photocatalyst is

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red-shifted to 398 nm when copper ions were doped into the TiO2 lattice. Furthermore, the

absorption band edge of Cu:TiO2-CS is located at 438 nm, which indicates an obvious red-shift compared to the TiO2 and Cu:TiO2 photocatalysts. This may be due to the synergistic effect

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between chitosan and Cu:TiO2 in Cu:TiO2-CS composite [37]. Concurrently, the values of the

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optical band gap (Eg) for single and composites could be found by the Tauc equation [38]. As

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presented in Fig. 3b, the band gaps of TiO2, Cu:TiO2 and Cu:TiO2-CS were calculated to be 3.01, 2.95 and 2.48 eV, respectively. The decrease of Eg can widen the absorption spectrum range,

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which is beneficial to the enhancement of photoelectrochemical (PEC) property.

Fig. 3. (a) UV–vis diffuse reflectance, and (b) band gap energy calculations from absorption spectra of TiO2,

Cu:TiO2 and Cu:TiO2-CS samples.

3.2 Evaluation of PEC performance 11

To explore the influence of the as-prepared photocatalysts on the transport behavior of charge carries and response to light on photoelectrodes, photocurrents of TiO2, Cu:TiO2 and Cu:TiO2-CS were dripped onto FTO electrodes are examined, as presented in Fig. 4a. During the ON and OFF cycle of illumination, the steady and prompt photocurrent response was observed for all samples. It can be seen that the photocurrent boosted rapidly when the light was turned on and finally remained a relative constant value. Conversely, when the light was turned off, photocurrent

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declined to zero instantaneously [39]. It can be observed clearly that the photocurrents of TiO2 and Cu:TiO2 are relatively small, and the photocurrent of Cu:TiO2-CS reaches to 1.6 μA/cm2, which is 2 times TiO2 photocurrent (0.8 μA/cm2). This phenomenon indicates the charge separation

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efficiency of TiO2 can be effectively promoted. Moreover, as shown in Fig. 4b, the

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Electrochemical impedance spectroscopy (EIS) of the as-prepared materials were further

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investigated transfer properties of charge carries and enhanced photoelectrochemical (PEC) performance. The Nyquist plots exhibits the photoelectron-hole separation efficiency of

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Cu:TiO2-CS are better than that of the TiO2 and Cu:TiO2, which mainly reflected by the radius of the arc [40]. It can be illustrated that Cu:TiO2-CS embraces a higher efficiency of electrons and

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holes transport in the interface between semiconductor and electrolyte [41]. Photoluminescence (PL) spectra analysis is an effective approach to measure separation and recombination

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performances of the photo-excited electron-hole pairs of the obtained samples. Theoretically, the intensity of photoluminescence would have an effect on the photocatalytic activity. It can be seen in Fig. 4c, the PL spectrum of Cu:TiO2 shows lower intensity than that of TiO2, which can be attributed to the charge transfer between TiO2 and Cu2+. The conduction band (CB) of CuO (Cu2+) (ECB = 0.46 eV) is less cathodic than that of TiO2 (ECB = -0.29 eV) [42]. Hence the photogenerated 12

electron in the CB of TiO2 may slip to the CB of CuO (Cu2+). Similarly, the valence band (VB) of TiO2 (EVB = 2.91 eV) is more anodic than that of CuO (Cu2+) (EVB = 2.16 eV). This may also lead to hole transfer from the VB of TiO2 to the VB of CuO (Cu2+). In the case of Cu:TiO2-CS, the addition of CS into Cu:TiO2 could further decrease the PL intensity, due to the effect of CS can inhibit recombination of photoinduced electrons and holes by capturing the electrons[40, 43]. Thereby, according to this phenomenon, the Cu:TiO2-CS nanocomposite exhibits the ability of

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effectively decrease the photoexcited electron-hole recombination rate [44]. This is consistent with the results of photocurrent and EIS measurements, which provides convincing evidence for the

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photocatalytic CO2 reduction mechanism.

Fig. 4. (a) Transient photocurrent response of TiO2, Cu:TiO2 and Cu:TiO2-CS, (b) Nyquist plots of EIS

measurements on the TiO2, Cu:TiO2 and Cu:TiO2-CS electrodes under the visible light irradiation in 0.5 M Na2SO4 (pH ∼ 7.35), and (c) PL emission spectra of TiO2, Cu:TiO2 and Cu:TiO2-CS catalysts. 13

3.3 Photocatalytic CO2 reduction performance Fig. 5a exhibits the effects of all the as-prepared samples for photocatalytic CO2 reduction were measured under irradiation with a 300 W xenon lamp for 1 h. The photocatalytic CO2 reduction performance is roughly evaluated by the yield and product selectivity of CO and CH4. It can be clearly observed that the yields of CO and CH4 of pure TiO2 are 0.32 μmol/g and 0.45 μmol/g, respectively. Similarly, TiO2-CS produced 0.41 μmol/g CO and 0.24 μmol/g CH4, while Cu:TiO2

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produced 0.99 μmol/g CO and 1.06 μmol/g CH4. It is worth noticing that the yields of CO and

CH4 of composite Cu:TiO2-CS can attain to 4.48 μmol/g and 5.34 μmol/g. Thereby, the

performance of Cu:TiO2-CS composite for photocatalytic CO2 reduction is better than TiO2,

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TiO2-CS and Cu:TiO2, which can be increased by up to ten times. The performance of CO2

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reduction improvements could attributed to the synergistic effect between chitosan and Cu:TiO2.

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In detail, the synergistic effect origins from the following three factors. First, the spherical CS nanoparticles can support the uniform growth of Cu:TiO2 according to the SEM and TEM images,

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which is beneficial to expose more surface area (active sites) of Cu:TiO2, and enhance the coupling between Cu:TiO2 and chitosan. Second, the existence of CS can enhance CO2 adsorption

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of the composite photocatalyst owing to the special structure of terminal amine groups in CS. It has been reported that CO2 adsorption can take place on the free amine groups of the

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d-glucosamine units by the cooperative adsorption of one CO2 molecule with two adjacent amine groups [45]. Last but most importantly, copper ions and CS can inhibit recombination of photoinduced electrons and holes by capturing the electrons on the basis of the photoluminescence spectra [40]. To determine the effect of the Cu:TiO2-CS photocatalyst itself decomposing under light 14

irradiation, photocatalytic reduction experiments were carried out without CO2 [46]. It can be clearly seen in Fig. 5b that the Cu:TiO2-CS ternary composite only produced 0.27 μmol/g CO. Meanwhile, CH4 was not obtained during this process. The amount of both products of the reactions was significant decreased compared with the sample in CO2 atmosphere, which indicates the self-decomposition of Cu:TiO2-CS under light irradiation has no significant effect on the

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photocatalytic CO2 reduction.

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Fig. 5. Total CO/CH4 evolution amount of TiO2, TiO2-CS, Cu:TiO2 and Cu:TiO2-CS with 300 W xenon lamp

within 1 h, (b) the catalytic activity of Cu:TiO2-CS in the presence or absence of CO2 atmosphere.

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3.4 Mechanism of Photocatalytic CO2 reduction

According to the above results, a possible mechanism for photocatalytic reduction of CO2 was

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advanced in Fig. 6. After absorbing energy equivalent to or greater than the band gap of TiO2, the TiO2 nanoparticles are activated to generate holes (h+) in the valence band (VB) and electrons (e-)

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in the conduction band (CB). Consequently, the doped Cu2+ in TiO2 effectively suppress the recombination of photogenerated electrons and holes. Meanwhile, CS has a good CO2 adsorption effect, which obviously promotes the photocatalytic CO2 reduction reaction to some extent. Under light irradiation, the resultant holes form Cu:TiO2-CS can oxidize the chemisorbed water molecules on the surface to generate O2 and protons. At the same time, the photo-induced 15

electrons on the surface could be trapped by the CO2 to generate CO and CH4 under the assistance of the protons, as explained by the following equations [46-48].

Cu : TiO2  CS  hv  Cu : TiO2  CS e , h 

(1)

2H 2O  4h  O2  4H 

(2)

CO2  2H   2e  CO  H 2O

(3)

CO2  8H   8e  CH 4  2H 2O

(4)

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To conclude, in terms of photocatalytic CO2 reduction, the introduction of Cu:TiO2 and the adsorption of CS generate interesting synergistic effect, which can better promote the migration of

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electrons and inhibit the electron-hole recombination.

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Fig. 6. Possible mechanism of convert CO2 into CH4/CO photocatalyzed by Cu:TiO2-CS composite.

4 Conclusions

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In summary, we have successfully synthesized a series of photocatalysts by solvothermal

treatment. The experimental results exhibited that the obtained Cu:TiO2-CS composite possess exceptional photocatalytic activity for CO2 reduction. Remarkably, it can be attributed to the synergistic effect between organic polymer (chitosan) and inorganic material (Cu:TiO2). Based on the analysis of relevant tentative results, the possible mechanism of Cu:TiO2-CS activator for 16

photocatalytic CO2 reduction was proposed. Hence, this study perhaps is a referential cogitation for exploring fabulous photocatalysts, which are suitable for other catalytic systems established to enhance photocatalytic CO2 conversion performance.

Conflicts of interest

Author statement

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The authors declare no conflict financial interest.

Houde She: Conceptualization, Validation, Resources, Writing - Review & Editing.

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Ziwei Zhao: Methodology, Software, Formal analysis, Investigation, Writing -

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Original Draft.

Wencai Bai: Investigation, Resources, Visualization, Data Curation.

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Jingwei Huang: Data Curation, Formal analysis.

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Lei Wang: Writing - Review & Editing.

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Qizhao Wang: Supervision, Project administration.

Acknowledgements

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This work was financially supported by the National Natural Science Foundation of China

(21663027, 21808189, 21962018), and the Science and Technology Support Project of Gansu Province (1504GKCA027).

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