Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2

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Review Article

Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2 Min Cheng a,b,*, Sen Yang a,b, Rong Chen a,b, Xun Zhu a,b, Qiang Liao a,b, Yi Huang a,b a

Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 400030, China b Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China

article info

abstracts

Article history:

As the global warming which is mainly caused by atmospheric CO2 and the depletion of

Received 9 November 2016

fossil fuels becomes more and more serious, the method for reducing CO2 with high effi-

Received in revised form

ciency and low energy consumption is urgently needed. In this letter, an effective photo-

19 January 2017

catalytic reduction of CO2 by using Cu2þeTiO2 nanorod thin films photocatalyst in

Accepted 23 January 2017

optofluidic planar reactors under UV light was studied. Cu2þ -deposited TiO2 nanorod thin

Available online xxx

films were fabricated by using the combination of hydrothermal method and ultrasonicassisted sequential cation adsorption method. The samples were characterized by X-ray

Keywords:

diffraction (XRD), UVevis diffuse reflectance spectra (DRS), scanning electron microscope

TiO2 nanorod

(SEM), energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM).

Photocatalytic CO2 reduction

Their photocatalytic activities were evaluated by reduction of gas-phase CO2, and the main

Cu2þ doping

products were methanol and ethanol. The experiment results showed that when the doped

Methanol and ethanol

concentration of Cu2þ was 0.02 M, the reaction product yield reached the maximum, and

Optofluidic planar reactor

the methanol and ethanol yields were 36.18 mmol/g-cat h and 79.13 mmol/g-cat h at a flow rate of 2 mL/min and under the reaction system temperature of 80  C. The highly efficient photocatalytic activities of Cu2þeTiO2 nanorod thin films in the reduction process of CO2 were attributed to the incorporation of Cu2þ ions and one-dimensional (1D) nanostructure which improved the limitations of photon transfer. In addition, the photocatalytic mechanism was discussed to understand the experimental results over the Cu2þ modified TiO2 nanorod thin films. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 400030, China. Fax: þ86 23 65102474. E-mail address: [email protected] (M. Cheng). http://dx.doi.org/10.1016/j.ijhydene.2017.01.126 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials and catalyst preparation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fabrication of optofluidic planar microreactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization and photocatalytic activity test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics of TiO2 nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of different doping concentration of Cu2þ on the photocatalytic conversion process of CO2 . . . . . . . . . . . . . Effect of different CO2 flow rates on the photocatalytic conversion process of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of different reaction temperatures on the photocatalytic conversion process of CO2 . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction The energy crisis and a series of environmental problems especially the greenhouse effect are getting more and more serious. Carbon dioxide (CO2) is not only the major contribution from fossil fuel consumption, but also the main gas of the global warming effect. Therefore, it is particularly urgent to reduce the concentration of CO2 in the atmosphere. Recently, many efforts have been made to alleviate CO2 emissions through pre-or postcombustion CO2 capture followed by compression and geological sequestration [1]. However, these methods are energy intensive and costly. Fortunately, these drawbacks above can be overcome by the innovation of photocatalytic technology which can transform CO2 to value-added chemicals at relatively low temperature and atmospheric pressure [2,3], and it can solve the energy crisis at the same time. Although the technology of photocatalytic CO2 reduction has many advantages, low efficiency is the biggest obstacle that hinders its industrial application. Photocatalytic reduction of CO2 is limited to the overall process efficiency being largely dependent on two factors-the physicochemical properties of the catalyst and reactor configuration [4]. The photocatalytic reactor can affect the mass transfer process of reactants, light distribution and reaction surface area [5e7]. Conventional reactors such as slurry reactors or fixed reactors [8] have low specific surface area and low light utilization, which seriously affect the catalytic efficiency. Recently, the development of optofluidics [7] which combine the advantages of microfluidics and optics has the abilities of fine flow control, large surface-area-to-volume ratio and short optical path [9,10]. So far optofluidic reactors have been applied in many fields, such as water-splitting [7], water purification [11] and photocatalytic fuel cell [12]. The effect of optofluidic planar reactors (termed as OPMR) is much better than the conventional reactors. It can be inferred that the use of optofluidic planar reactors can greatly improve the efficiency of photocatalytic reduction of CO2.

00 00 00 00 00 00 00 00 00 00 00 00 00 00

The improvement of the catalyst performance which can transfer CO2 into hydrocarbons has always been a focus study [4]. In the study of photocatalytic reduction of CO2, TiO2 has been considered the most appropriate candidate since it has a relative wide band gap and could fulfill the thermodynamic requirements of most photocatalytic reaction researches [13]. Vast majority of the pioneers use the TiO2 nanoparticle powders or TiO2 nanoparticle thin films as catalysts. However, the high recombination rate of photogenerated electroneholes pairs has seriously hindered the photocatalytic reduction of CO2 [13], so the TiO2 nanoparticle powders or TiO2 nanoparticle thin films are difficult to further improve the catalytic efficiency. In order to further improve the photocatalytic performance, the photocatalytic activities of TiO2 can be better enhanced by the incorporation of metal ions [5,14e16] which were used as “charge-carrier traps” and could suppress recombination of photogenereted electronehole pairs [8]. And recently, 1D TiO2 nanostructure array thin films have attracted great attention because of their electronic and optical properties, especially the unique structure of the TiO2 nanorods inhibited the recombination of the electronehole pairs [17e19]. In addition, TiO2 nanostructure array thin films also have the advantages of large surface area, adsorption capacity and surface activity compared with conventional TiO2 nanoparticle films [20,21]. 1D TiO2 nanostructure array thin films have been applied in many fields such as solar cells [19,22], gas sensors [23], semiconductor application [24]. However, there has been little study about photocatalytic reduction of CO2 using the TiO2 nanorod thin films. In conclusion, it can be found that the catalyst used in the photocatalytic reduction of CO2 was mainly concentrated on the TiO2 powders, and CO2 was usually dissolved in saturated water or in the form of bubbles from the analyses of previous studies [13,17,25e27]. However, both the separation of TiO2 powders and the low solubility of CO2 in water have seriously hindered effect on the photocatalytic reduction process of CO2. Fortunately, these drawbacks may be overcome by applying 1D TiO2 nanostructure array thin films and OPMR.

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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Their combination may be feasible, industrial scale-up and attractive for use in further investigation of CO2 photoreduction for solving environmental problems. So in this letter, we prepared and used the copper-decorated 1D TiO2 nanostructure array thin films in the OPMR for photocatalytic reduction of CO2 with water vapor. The oriented TiO2 nanorods on transparent conductive fluorine-doped tin oxide (FTO) substrates were synthesized by a hydrothermal method. Cu2þ nanoparticles were then deposited on the TiO2 nanorod thin films by ultrasonic-assisted cation adsorption method. Various techniques such as X-ray diffraction (XRD), UVevis diffuse reflectance spectra (DRS), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM) were employed to characterize the fabricated photocatalysts. The photocatalytic activities were studied in terms of the photocatalytic reduction of CO2 with water vapor under UV irradiation in the OPMR under different variable conditions such as Cu2þ doping concentration, CO2 flow rate and reaction temperature. In addition, the photocatalytic mechanism was analyzed based on the experimental results under different conditions.

Experimental Materials and catalyst preparation method The growth of oriented TiO2 nanorod thin films was prepared using a simple hydrothermal method based on the previously reported literature [19,22]. However, the synthesis parameters were also improved in this letter. Firstly, fluorine-doped tin oxide (FTO) pieces (F:SnO2, 10U/sq, Lixinda Glass China, 25 mm  30 mm  1.1 mm) which were ultrasonically washed for 30 min in a mixed solution of ethanol (Chengdu Kelong, China), acetone (SigmaeAldrich) and 2-propanol (Chengdu Kelong, China) with volume ratios of 1:1:1, and 30 min in deionized water respectively. Then the clean FTO pieces were employed as substrates for the growth of TiO2 nanorods. The substrates were then dried at 100  C at ambient conditions. The reaction solution was prepared by dissolving 40 mL hydrocholoric acid (HCl 36%, Chengdu Kelong, China) in 40 mL deionized water. After 30 min magnetic stirring, 1.33 mL of titanium butoxide (97%, SigmaeAldrich) was added to the solution based on a method published previously [22]. The mixed solution was stirred at ambient conditions until it became transparent. Total reaction solution was taken into a Teflon-lined stainless steel autoclave (100 mL). The FTO substrates were placed at an angle against the wall of the Teflonliner with the conducting side facing down. The hydrothermal synthesis was conducted at 150  C for 6 h in a vacuum drying oven. The FTO substrates were taken out and rinsed extensively with deionized water, and they were allowed to dry in ambient air and followed by annealing in air at 550  C for an additional 3 h with a heating rate of 10  C/min. Thus, the TiO2 nanorod thin films were prepared. Ultrasonic-assisted sequential cation adsorption method was used for modifying the TiO2 nanorod thin films [14]. Five different concentrations (such as 0.01 M, 0.02 M, 0.03 M, 0.04 M, 0.05 M) of aqueous Cu(NO3)2$3H2O (Chengdu Kelong, China) solutions were prepared. TiO2 nanorod thin films were

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immersed into these solutions for 2 min under ultrasonic concussion, and 2 min in deionised water without ultrasonic concussion. This cycle was repeated 10 times for each sample. After drying at 100  C, the samples were calcined at 400  C in the pipe furnace for 1 h with a heating rate of 10  C/min. The TiO2 nanorod thin films were named as 0.01 M Cu2þeTiO2, 0.02 M Cu2þeTiO2, 0.03 M Cu2þeTiO2, 0.04 M Cu2þeTiO2 and 0.05 M Cu2þeTiO2 according to the Cu2þ concentration of the modification solution.

Fabrication of optofluidic planar microreactor The OPMR was fabricated by replica molding method and standard photolithography process [28]. The detailed preparation methods of chamber template are as follows. Firstly, the cover plate with branch shaped microchannels was fabricated on silicon wafer (76 mm  76 mm). Photoresist (SU8, Microchem Corp.) was spin-coated on the silicon wafer in the coater [7]. Before transferring to the photoetching machine, the silicon wafer with photoresist was baked on the heating plate. Then the prepared silicon which was covered with a printed mask was exposed to the UV light followed by a soft-bake process [7]. Finally, the silicon wafer was placed in a beaker which was full of developer solvent, and the unexposed photoresist was cleaned out. After post-baking, the terminal silicon substrate with SU-8 of chamber template (26 mm  30 mm  0.15 mm) was accomplished. The application of Polydimethylsiloxane (PDMS) used as the cover plate material of the OPMR was described in this paper. Because it has the characteristics of chemical inertness, optical transparence and rapid fabrication which meet all the characteristics to fabricate OPMR. The assembly of the reactor included the following steps. At the first step, polymer base (Sylgard 184 A, Dow Coming) and curingagent (Sylgard 184 B, Dow Coming) were mixed and degassed at a ratio of 10:1. Then the mixture was cast onto the patterned wafer and rectangular mold (3.0 cm  2.5 cm  1.1 cm) and all baked at 95  C for 30 min. After that, the PDMS was released from the patterned wafer and rectangular mold. Next, the prepared TiO2 nanorod thin film was placed on the glass substrate and surrounded by the rectangular PDMS mold. At the last step, the PDMS chamber template was covered on the rectangular PDMS mold. In the center of this chamber, the reaction microchamber volume was totally 117 mL. Fig. 1a shows the design of the OPMR. The connections of FTO to glass substrate and syringe needles to the PDMS plates were enhanced by using UV light adhesive curing under 365 nm UV irradiation to avoid leakage. At both the inlet and outlet, four-branch shaped microchannels were adopted to ensure a uniform filling over the entire microreactor. The image of the fabricated OPMR is given in Fig. 1b.

Characterization and photocatalytic activity test The schematic diagram of the photocatalytic reduction process of CO2 and water vapor in the OPMR is shown in Fig. 2. The photocatalytic activity was evaluated under UV-light irradiation of a UV-LED with the wavelength of 365 nm (Shanghai Maixin, China). The UV light intensity was

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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Fig. 1 e (a) Design and (b) Image of the optofluidic planar microreactor.

controlled by adjusting the distance between the light source and catalyst surface, and the UV light intensity was measured by a UV radiometer (UV-A, Photoelectric Instrument Factory of Beijing Normal University, China). The whole reaction equipment was wrapped by screen paper to prevent the interference of the outside light. Compressed CO2 (99.999%) which was controlled by a mass flow controller (Omega, America) was passed through a boiling flask-3-neck to generate the mixture of CO2 and water vapor. The ratio of H2O/CO2 was adjusted by controlling the temperature in the boiling flask-3neck which was equipped with armored thermocouple A. The temperature of the gaseous mixture in the boiling flask-3-neck was controlled by the oilbath device, and the temperature was kept at 60  C in this paper, hence the ratio of H2O/CO2 was kept at 1:10. The gaseous mixture was then introduced to the OPMR and operated in a continuous-flow mold. Thereinto, the flow rate of the gaseous mixture was achieved by controlling the flow of CO2. The gaseous mixture was flowed through the OPMR for at least 1 h before illumination to ensure that there was only water vapor and CO2 in the OPMR. The function of the heating tape was to ensure that water vapor did not appear condensation in the process from the boiling flask-3neck to OPMR as shown in Fig. 2. The temperature in the

OPMR was controlled by the constant temperature heating system. Each experiment was carried out for 6 h. The collection vessel was equipped with 1 mL deionized water, which acted as solvent to collect the products from the OPMR. Then the methanol concentration and ethanol concentration at the collection vessel were measured by gas chromatograph (GC2010 Plus, Shimadzu, Japan). The gas chromatograph was equipped with a SH-Rtx-Wax, 30 m length, 0.25 mm inside diameter and a 0.25 mm film capillary column. All experiments were repeated three times.

Results and discussion Characteristics of TiO2 nanorods The crystalline phase of TiO2 nanorod samples were characterized by X-ray diffraction (XRD) using Ultima IV diffractometer (Glancing incidence) with CuKa radiation operated at 40 kV, 190 mA. Samples were scanned from 20 to 80 at a rate of 2 /min (in 2q). As shown in Fig. 3, the XRD patterns of unmodified TiO2 nanorod sample and modified TiO2 nanorod samples with different initial concentrations of Cu2þ aqueous

Fig. 2 e Schematic diagram of photocatalytic reduction system. A, B: Armored thermocouple; C: K type thermocouple. Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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Fig. 3 e XRD pattern of Cu2þ modified TiO2 nanorod thin film samples.

solutions are listed successively. All the samples showed one main peak at 36.2 which corresponded to the crystal plane of [101] of rutile phase structure (JCPDS No: 34-0180). The result indicates that the ultrasonic-assisted sequential cation adsorption method did not affect the phase of TiO2 nanorods. No diffraction peaks of Cu2þ elements were detected because of low concentration in the photocatalysts, and it also indicates that small Cu2þ clusters were highly dispersed on the surface of TiO2 nanorods. Scanning electron microscope (SEM) analysis was conducted on a JSM-7800F electron microscope (JEOL, Japan)

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equipped for energy dispersive spectroscopy (EDS) measurement. As shown in Fig. 4a and b, uniform nanorods at two different magnifications were found to be grown on the FTO substrates. The cross section shape of TiO2 nanorods was about rectangle. The nanorods were 4.0 mm in length and 120 nm in side length on average. However, the deposition of Cu2þ did not affect the crystalline structure of TiO2 nanorods. EDS analysis showed that the deposition of Cu2þ in the samples were 1.2wt.%, 1.5wt.%, 1.8wt.%, 2.0wt.% and 2.2 wt.% with doped Cu2þ concentration of 0.01 M, 0.02 M, 0.03 M, 0.04 M and 0.05 M, respectively. The EDS results are in agreement with XRD results that the diffraction peaks of Cu2þ element did not appear when the amount of Cu2þ loading was less than 3 wt.% [25,29,30]. To further characterize the microstructure of the obtained TiO2 nanorods with different growth conditions, transmission electron microscope (TEM) and high resolution electron microscope (HR-TEM) analyses which were conducted on a JEM 1200EX electron microscope (JEOL, Japan) are shown in Fig. 5. The average side length of a single TiO2 nanorod which is shown in Fig. 5a and b was about 120 nm. It agrees with the observed view by SEM in Fig. 4b. And each nanorod consisted of a bundle of smaller nanorods which was marked by the red box in Fig. 5a. The High-resolution TEM (HR-TEM) image of a single small nanorod is clearly shown in Fig. 5c. The lattice fringers with a spacing of 0.29 nm and 0.32 nm were found, which are in agreement with the spacing of [001] plane and [110] plane of the rutile phase [19,22], respectively. TEM images of 0.02 M Cu2þeTiO2 nanorods are clearly observed in Fig. 5d and e. Cu2þ modified TiO2 nanorods were in the form of clusters that were mainly covered the surface of the nanorods. However, the Cu2þ clusters did not affect the shape of nanorods which are consistent with the SEM results as shown in

Fig. 4 e SEM images of TiO2 nanorod array (a, b) and 0.02 M Cu2þeTiO2 nanorod array (c, d). Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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Fig. 5 e TEM images of a single TiO2 nanorod (a, b) and 0.02 M Cu2þeTiO2 nanorods (d, e), HR-TEM images of small TiO2 nanorods (c) and 0.02 M Cu2þeTiO2 nanorods (f).

Fig. 4d. The High-resolution TEM (HR-TEM) image of the 0.02 M Cu2þeTiO2 nanorod is clearly shown in Fig. 5f. The d spacing was measured to be 0.25 nm which correspond to [002] plane of the monoclinic CuO [31,32]. The optical properties of all the TiO2 nanorod thin films were measured by using UVevis diffuse reflectance spectroscopy (Shimadzu, UV-2100, UVeVis spectrophotometer) with BaSO4 as the reference standard. As depicted in Fig. 6, the UVevis spectra of the pure TiO2 nanorod thin film showed the lowest absorption among the thin films, The absorption spectrum of pure TiO2 nanorod thin film appeared around 411 nm, which had red shift compared with intrinsic anatase TiO2 (387 nm). Moreover, visible light absorption has increased with the increase of concentration of Cu(NO3)2$3H2O solution. Red shift observed for the increasing amount of Cu2þ of the thin film samples shows that the band gap energy of the samples is decreased gradually. The direct band gap energy of the samples is calculated by using equation shown in Eq. (1) [33]. n=2

ahv ¼ Aðhv  EgÞ

allowed and indirect forbidden transitions, respectively [36]. In this experiment, n ¼ 1/2 namely the direct allowed transition was used for the samples [37]. The Eg values for all samples were calculated from a plot of (f(R)hv)1/2 versus hv using direct method as shown in Fig. 7. The Eg estimated from the intercept of the tangents to the plots were 3.01ev, 2.99ev, 2.97ev, 2.95ev, 2.85ev and 2.75ev for TiO2, 0.01 M Cu2þeTiO2,

(1)

where a, h, n, hv, Eg and A are the absorption coefficient, Planck's constant, frequency of vibration, photon energy, band gap energy and a proporyional constant, respectively. The absorbance or KubelkaeMunk function of the reflectance f(R) is proportional to the absorption coefficient a [34,35]. The coefficient ‘n’ characterizes the electronic transition during absorption process. The ‘n’ value is determined by the type of optical transition of a semiconductor especially having values 1/2, 3/2, 2, and 3 for direct allowed, direct forbidden, indirect

Fig. 6 e UVevis absorption spectra of the photocatalyst film samples.

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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CO2 þ e /CO 2 ðadÞ

(3)

H2 O þ hþ /OH þ Hþ

(4)

Hþ þ e /H

(5)

  CO 2 ðadÞ þ H/CO þ OH



(6)



 H

CO þ e /CO ! C þ OH 

H



H

(7) 

H

C þ H/CH! CH2 ! CH3 !CH4

(8)

CH3 þ OH/CH3 OH

(9)





þ  2CO 2 ðadÞ þ 12 H þ 2h /C2 H5 OH þ 3H2 O

Fig. 7 e Band gap energy calculation graphs of the photocatalyst film samples.

0.02 M Cu2þeTiO2, 0.03 M Cu2þeTiO2, 0.04 M Cu2þeTiO2 and 0.05 M Cu2þeTiO2, respectively. As we can see from the above description, the TiO2 nanorods were orderly arranged on the FTO surface, and each nanorod was composed of a lot of nano clusters. When the TiO2 nanorods were doped by Cu2þ, Cu2þ clusters were uniformly covered on the surface of the nanorods. The band gap energy of the TiO2 nanorods decreased with the increase of the concentration of doped Cu2þ. It means that the absorption spectra of the doped TiO2 nanorods were extended to the visible light.

Photocatalytic reduction of CO2 Before the photocatalytic reduction of CO2 experiments, a series of blank tests were carried out in the absence of catalyst and the gaseous mixture under UV irradiation for 6 h at 60  C in the following experiments: (1) without catalyst, (2) with water vapor and helium, (3) only CO2. In these circumstances, methanol and ethanol were not detected. In addition, blank tests were also conducted using CO2 and water vapors and without illumination. No organic compounds were found. These results confirmed that catalyst, feed (CO2 and water vapor) and light source were essential in the photocatalytic reduction experiment.

Effect of different doping concentration of Cu2þ on the photocatalytic conversion process of CO2 This part mainly studied the photocatalytic mechanism and the performance of TiO2 nanorod thin films with different doped concentration of Cu2þ. The main reaction products of our experiments were methanol and ethanol. The possible reaction mechanism of photocatalytic reduction of CO2 with water vapor on the catalysts can be proposed in the following ways (Eqs. (2e10)). hv

TiO2 !e þ hþ

(2)

7

(10)

In the complex process of photocatalytic reduction of CO2, electronehole pairs are generated firstly under illumination when the absorbed light energy is greater than or equal to the band gap energy of the semiconductor. The electronehole pairs then are transferred to the surface of the reactant to carry out the redox reaction. Subsequently the electrons (e ) are transferred from the TiO2 conduction band for reacting þ with CO2 to form,CO 2 . Meanwhile, holes (h ) react with absorbed H2O molecules on the surface of catalysts to form hydroxyal radicals (OH) and hydrogen ions (Hþ ) and further oxidize with the excited electrons led to,H radicals [36]. It has been found thatCO 2 is metastable and could change the electron affinity of CO2 [38]. In this case, the CO 2 anion radicals are further formed in water with high dielectric constant and can be greatly stabilized by the solvent that results in weakening the interaction of the radical with the catalyst surface [39]. Then the CO 2 radicals react with,H radicals to generate CO. At the same time, carbon radicals C come into being by consecutive reactions, and then,CH3 radicals are formed. The CH3 radicals react withOH to produce methanol, andCO 2 radicals are further oxidized to ethanol under the action of holes (hþ ). Fig. 8 shows the methanol and ethanol yields of photocatalytic reduction of CO2 with different amounts of doped Cu2þ in TiO2 nanorod thin films. The average methanol and ethanol yields were 6.69 mmol/g-cat h and 6.31 mmol/g-cat h respectively without doped Cu2þ. The methanol and ethanol yields increased with Cu2þ loading, but then decreased when the Cu2þ concentration exceeded 0.02 M, namely Cu2þ loading exceeded 1.5 wt.%. Meanwhile, the average yields of methanol and ethanol reached the highest values of 24.42 mmol/g-cat h and 47.25 mmol/g-cat h at the flow rate of 2 mL/min when the doping concentration of Cu2þ was 0.02 M. On the one hand, doped Cu2þ can effectively increase spectral response of TiO2 nanorods by narrowing electronic properties and increasing the amount of active sites. The addition of CuO whose band gap is about 1.4 eV has changed the whole catalytic mechanism. The CuO is more easily excited under visible irradiation, and the conduction band of CuO which is located at c.a. 1.03 eV versus NHE is more cathodic than the conduction band of TiO2 (0.5 eV) [40]. Therefore, the electrons generated on the CuO conduction band can injected into the inactivated TiO2 easily. On the other hand, Cu2þ which acts as free ions

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Fig. 8 e Effect of doped Cu2þ concentration in Cu2þeTiO2 catalysts for photocatalytic reduction of CO2. CO2 flow rate: 2 mL/min, reaction temperature: 60  C.

Fig. 9 e Effect of the CO2 flow rate on the methanol and ethanol yields. 0.02 M Cu2þeTiO2 nanorod thin films, reaction temperature: 60  C.

can also serve as electron traps to reduce the recombination rate of electronehole pairs during photo excitation of photocatalyst [7]. Surface hydroxyls (,OH) of TiO2 can also promote the adsorption of the reactant as shown in the Formula (5), so that the photo reaction is further enhanced. However, excess Cu2þ clusters covered the surface of TiO2 resulting in less light that reached to catalyst, and it blocked the sensitization of TiO2 nanorods. So there exists an optimum amount of Cu2þ loading around 1.5 wt.% under the experimental conditions of this work.

47.25 mmol/g-cat h at the flow rate of 2 mL/min in our work. In summary, too slow or too fast flow velocity of the gaseous mixture is not favorable for the increase of product yield. There exists an optimal flow rate of reactants for the photocatalytic reduction process of CO2.

Effect of different CO2 flow rates on the photocatalytic conversion process of CO2 The flow rate can influence the transmission process of reactive material, and further affect the reaction efficiency. So this part mainly studied the effect of different flow rates on the photocatalytic reduction of CO2. The flow rate of CO2 ranged from 1 mL/min to 5 mL/min, and the 0.02 M Cu2þeTiO2 nanorod thin films were used as the research objects in this part experiments. As shown in Fig. 9, both methanol and ethanol yields were increased first and then decreased with the increase of the flow rate of CO2, and the optimal yield was appeared at 2 mL/min. On one hand, it is easy to understand that the driving force of reactants which makes the gaseous mixture diffuse to reaction interface is small at a relative low flow rate. So the reactants diffusion process consume much more time which led to reduce the update speed of the gaseous mixture on the surface of catalyst layer. Therefore, the reaction rate is relatively slow at a lower flow rate of CO2. On the other hand, the residence time of reactants in the reactor is short at relatively higher flow rate. The reactants are driven out of the reactor chamber before they diffuse to the surface of catalyst layer. Therefore, the reactants on the surface of catalyst layer can not be updated duly. Similarly, it is not conducive to the catalytic reaction when flow rate is too high. Consequently, the average yields of methanol and ethanol reached the highest values of 24.42 mmol/g-cat h and

Effect of different reaction temperatures on the photocatalytic conversion process of CO2 Fig. 10 shows the relationships between the methanol and ethanol yields under different reaction temperatures. It is obvious that the average methanol and ethanol yields were enhanced basically as the increase of the reaction temperature. Higher temperature gives positive effects on photocatalytic reactions for following reasons. Firstly, Photocatalytic reduction of CO2 and water vapor is an

Fig. 10 e Methanol and ethanol yields of the photocatalytic reduction of CO2 of 0.02 M Cu2þeTiO2 nanorod thin films at different reaction temperatures. Flow rate: 2 mL/min.

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

2011

Ours

CO2-saturated H2O solution CO2-bubbled KHCO3 solution containing Na2SO3 CO2 and H2O vapor

2015 2016

[16] [46]

[45]

2005 2005 2007

442.5 mmol/g-cat h CH3OH 0.415 mmol/g-cat h CH3OH 11.74 mmol/g-cat h CH4 18.67 mmol/g-cat h HCOOH 29.88 mmol/g-cat h C2H5OH 25.72 mmol/g-cat h CH4 15.57 mmol/g-cat h C2H5OH 1.8 mmol cm2 h1 2.1 mmol/g-cat h CH3OH 4.5 mmol/g-cat h C2H5OH 36.18 mmol/g-cat h CH3OH 79.13 mmol/g-cat h C2H5OH CO2 and H2O vapor

[25] [26] [41]

1994 1998 2001 2002

[42] [43] [44] [27]

Year Products

2.1 mmol/g-cat h HCOOH 7.5 mmol/g-cat h CH4 3 mmol/g-cat h CH3OH 20 mmol/g-cat h CH4 5.25 mmol/g-cat h CH3OH 19.75 mmol/g-cat h CH3OH

Reductant

CO2-saturated H2O solution CO2 and H2O vapor CO2 and H2O vapor CO2-bubbled 0.2 N NaOH aqueous solution CO2-bubbled 1 M KHCO3 solution CO2 and H2O vapor CO2-saturated H2O solution

9

endothermic process [13], higher temperature can accelerate the reaction rate naturally. Secondly, the increase of temperature is beneficial to the desorption process of products from the catalyst surface. Thirdly, the photocatalytic reduction of CO2 and water vapor occurs on the surface of the catalyst layer, reaction rates are usually determined by the value of reactant concentration on the catalyst surface. Increasing the temperature celebrates the diffusion velocity of CO2 to the surface of the catalyst layer, and it increases the number of catalytic sites which results in higher reaction rates. Finally, the increase of temperature can decrease the activation energy of the photocatalyst so as to enormously promote the reaction rate [36]. As shown in Fig. 10, the average methanol and ethanol yields reached 36.18 mmol/g-cat h and 79.13 mmol/ g-cat h under the reaction temperature of 80  C respectively. In order to further clarify the excellent characteristics of 1D nanostructure with doped Cu2þ in the photocatalytic reduction of CO2, we also compared our experimental results with the previous experimental results [16,25e27,41e46]. As shown in Table 1, the catalytic performances of the doped catalysts are obviously improved compared with the undoped catalysts. At the same time, the doped catalysts were beneficial to the production of the multi carbon compounds. In comparison, our results are much better in both liquid phase or gas phase reactions. In addition to the doping effect of Cu2þ, the application of the 1D TiO2 nanorod thin films which increased the catalytic areas and catalytic active sites promoted the catalytic efficiency. Moreover, the TiO2 nanorod thin film in the microreactor fabricated in this work is a continuous catalytic reaction process, which can be widely used in industrial production in the future.

Cu and Pt

0.5 wt.% Cu 3 wt.% CuO

1.5 wt.% CuO

N-doped TiO2 nanotube arrays

TiO2 nano-flower films TiO2 nanoparticles

TiO2 nanorods

Solar simulating light (AM 1.5) UV Hg lamp and Xenon lamp UV Hg lamp

UV light (365 nm)

3 wt.% CuO 1.2 wt.% Cu e TiO2 (P25) Cu/TiO2-coated fibers MWCNTs supported TiO2 UV lamp (2450 mW/cm2) UV Hg lamp (365 nm) 15 W UV lamp (365 nm)

e 1 wt.% Pt e 2 wt.% CuO

Co-catalyst Catalyst

TiO2 Ti-MCM-41 or Ti-MCM-48 zeolites Ti-b (F) or Ti-b (OH) zeolites TiO2 particles High pressure Xe lamp UV Hg lamp UV Hg lamp UV Hg lamp (254 nm)

Conclusion

Light source

Table 1 e Comparison of the methanol and ethanol yields with existing works.

References

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1

In this study, the 1D TiO2 nanostructure array thin films with doped Cu2þ in OPMR were used for photocatalytic reduction of CO2 with water vapor. The photocatalytic activities of TiO2 nanorod thin films photocatalyst were evaluated by the reduction yield in the presence of CO2 and water vapor under UV irradiation. Methanol and ethanol were found to be the major products. The maximum methanol and ethanol yields were 36.18 mmol/g-cat h and 79.13 mmol/g-cat h at a flow rate of 2 mL/min which was under the reaction temperature of 80  C and under the light intensity of 15 mW/cm2 by using the 0.02 M Cu2þeTiO2 nanorod thin films. Effects of different Cu2þ doping concentrations, CO2 flow rates and reaction temperatures were investigated. The optimal Cu2þ loading on the TiO2 nanorods was found to be 1.5 wt.% in our experiments. It was conceived that Cu2þ ions were served as active sites of electron traps and could suppress the electronehole recombination. At the same time, the band gap energy of the TiO2 nanorods decreased with the increase of the concentration of doped Cu2þ. The absorption spectra of the doped TiO2 nanorods were extended to the visible light. The reaction could not only promote the diffusion of reactants to the catalyst surface, but also promote the desorption of the reaction products at the optimal flow rate. And the increasing of temperature could promote the adsorption of CO2 and accelerate photogenerated species. Experiments results showed that the yields of

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

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methanol and ethanol in this study were much higher than the previous studies. The reason of this significantly enhanced CO2 photoreduction rates was due to the synergistic combination of Cu2þ deposition, high surface area of TiO2 nanorods and enhanced mass transfer of OPMR. Combination of 1D TiO2 nanostructure array thin films and OPMR is found to be feasible, industrial scale-up and attractive for use in further investigation of CO2 photoreduction for solving environmental problems.

[13]

[14]

[15]

[16]

Acknowledgment The authors gratefully acknowledge the financial support of the National Science Foundation for Young Scientists of China (No. 51606019), the National Natural Science Funds for Distinguished Young Scholar (No. 51325602), and the National Natural Science Foundation of China (No. 51576021).

references

[1] White CM, Strazisar BR, Granite EJ, Hoffman JS, Pennline HW. Separation and capture of CO2 from large stationary sources and sequestration in geological formations-coalbeds and deep saline aquifers. J Air Waste Manage 2003;53(6):645e715. [2] Teramura K, Okuoka SI, Tsuneoka H, Shishido T, Tanaka T. Photocatalytic reduction of CO2 using H2 as reductant over ATaO3 photocatalysts (A¼Li, Na, K). Appl Catal B-Environ 2010;96(3e4):565e8. [3] Mori K, Yamashita H, Anpo M. Photocatalytic reduction of CO2 with H2O on various titanium oxide photocatalysts. RSC Adv 2012;2(8):3165e72. [4] Ola O, Maroto-Valer MM. Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction. J Photoch Photobio C 2015;24:16e42. [5] Khemthong P, Photai P, Grisdanurak N. Structural properties of CuO/TiO2 nanorod in relation to their catalytic activity for simultaneous hydrogen production under solar light. Int J Hydrogen Energy 2013;38(36):15992e6001. [6] Cheng X, Chen R, Zhu X, Liao Q, He XF, Li SZ, et al. Optofluidic membrane microreactor for photocatalytic reduction of CO2. Int J Hydrogen Energy 2016;41(4):2457e65. [7] Li L, Chen R, Liao Q, Zhu X, Wang GY, Wang DY. High surface area optofluidic microreactor for redox mediated photocatalytic water splitting. Int J Hydrogen Energy 2014;39(33):19270e6. [8] Li Y, Wang WN, Zhan ZL, Woo MH, Wu CY, Biswas P. Photocatalytic reduction of CO2 with H2O on mesoporous silica supported Cu/TiO2 catalysts. Appl Catal B-Environ 2010;100(1):386e92. [9] Erickson D, Sinton D, Psaltis D. Optofluidics for energy applications. Nat Photonics 2011;5(10):583e90. [10] Fainman Y, Lee L, Psaltis D, Yang CH. Optofluidics: fundamentals, devices, and applications. McGraw-Hill Inc; 2009. [11] Wang N, Zhang XM, Chen BL, Song WZ, Chan NY, Chan HLW. Microfluidic photoelectrocatalytic reactors for water purification with an integrated visible-light source. Lab Chip 2012;12(20):3983e90. [12] Li L, Wang GY, Chen R, Zhu X, Wang H, Liao Q, et al. Optofluidics based micro-photocatalytic fuel cell for efficient

[17]

[18]

[19]

[20] [21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

wastewater treatment and electricity generation. Lab Chip 2014;14(17):3368e75. Zhang QH, Han WD, Hong YJ, Yu JG. Photocatalytic reduction of CO2 with H2O on Pt-loaded TiO2 catalyst. Catal Today 2009;148(3):335e40. € Boz I. _ Synergistic effect of Pt0 and M2þ Kerkez O, (Cu2þ,Ni2þ,Co2þ) on photo(electro) catalytic activity of TiO2 nanorod array thin films. J Photoch Photobio A 2015;301:32e9.  ndez Y, Liu D, Maroto-Valer M, Bian J, Tan JZY, Ferna Zhang XW. Photoreduction of CO2 using copper-decorated TiO2 nanorod films with localized surface plasmon behavior. J Photoch Photobio A 2012;531(13):149e54. Liu EZ, Qi LL, Bian JJ, Chen YH, Hu XY, Fan J, et al. A facile strategy to fabricate plasmonic Cu modified TiO2 nanoflower films for photocatalytic reduction of CO2 to methanol. Mater Res Bull 2015;68:203e9. Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, Gates B, et al. One-dimensional nanostructures: synthesis, characterization, and applications. Adv Mater 2003;15(5):353e89. Zhang H, Liu P, Liu X, Zhang S, Yao X, An T, et al. Fabrication of highly ordered TiO2 nanorod/nanotube adjacent arrays for photoelectrochemical applications. Langmuir 2010;26(13):11226e32. Kumar A, Madaria AR, Zhou C. Growth of aligned singlecrystalline rutile TiO2 nanowires on arbitrary substrates and their application in dye-sensitized solar cells. J Phys Chem C 2010;114(17):7787e92. Law M, Greene LE, Johnson JC, Saykally R, Yang PD. Nanowire dye-sensitized solar cells. Nat Mater 2005;4(6):455e9. Liu K, Zhao N, Kumacheva E. Self-assembly of inorganic nanorods. Chem Soc Rev 2011;40(2):656e71. Liu B, Aydil ES. Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dyesensitized solar cells. J Am Chem Soc 2009;131(11):3985e90. Paulose M, Varghese OK, Mor GK, Grimes CA, Ong KG. Unprecedented ultra-high hydrogen gas sensitivity in undoped titania nanotubes. Nanotechnology 2006;17(2):398e402. Hsu YY, Hsiung TL, Wang HP, Fukushima Y, Wei YL, Chang JE. Photocatalytic degradation of spill oils on TiO(2) nanotube thin films. Mar Pollut Bull 2008;57(6e12):873e6. Slamet, Nasution HW, Purnama E, Kosela S, Gunlazuardi J. Photocatalytic reduction of CO2 on copper-doped Titania catalysts prepared by improved-impregnation method. Catal Commun 2005;6(5):313e9. Wu JCS, Lin HM, Lai CL. Photo reduction of CO2 to methanol using optical-fiber photoreactor. Appl Catal A-Gen 2005;296(2):194e200. Tseng IH, Chang WC, Wu JCS. Photoreduction of CO2 using sol-gel derived titania and titania-supported copper catalysts. Appl Catal B-Environ 2002;37(1):37e48. Hong JW, Fujii T, Seki M, Yamamoto T, Endo I. Integration of gene amplification and capillary gel electrophoresis on a polydimethylsiloxane-glass hybrid microchip. Electrophoresis 2001;22(2):328e33. Xu SP, Ng JW, Zhang XW, Bai HW, Sun DD. Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water. Int J Hydrogen Energy 2010;35:5254e61. Choi HJ, Kang M. Hydrogen production from methanol/water decomposition in a liquid photosystem using the anatase structure of Cu loaded. Int J Hydrogen Energy 2007;32:3841e8. Yang MQ, He JH. Fine tuning of the morphology of copper oxide nanostructures and their application in ambient degradation of methylene blue. J Colloid Interf Sci 2011;355(1):15e22.

Please cite this article in press as: Cheng M, et al., Copper-decorated TiO2 nanorod thin films in optofluidic planar reactors for efficient photocatalytic reduction of CO2, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.126

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1

[32] Yu ZM, Meng JL, Li Y, Li YD. Efficient photocatalytic hydrogen production from water over a CuO and carbon fiber comodified TiO2 nanocomposite photocatalyst. Int J Hydrogen Energy 2013;38(36):16649e55. [33] Praus P, Kozak O, Koci K, Panacek A, Dvorsky R. CdS nanoparticles deposited on montmorillonite: preparation, characterization and application for photoreduction of carbon dioxide. J Colloid Interf Sci 2011;360(2):574e9. [34] Kislov N, Srinivasan SS, Emirov Y, Stefanakos EK. Optical absorption red and blue shifts in ZnFe2O4 nanoparticles. Mat Sci Eng B 2008;153(1e3):70e7. [35] Yakuphanoglu F. Electrical characterization and device characterization of ZnO microring shaped films by sol-gel method. J Alloy Comp 2010;507(1):184e9. [36] Tahir M, Amin NAS. Photocatalytic reduction of carbon dioxide with water vapors over montmorillonite modified TiO2 nanocomposites. Appl Catal B-Environ 2013;142e143(5):512e22. [37] Wang GM, Wang HY, Ling YC, Tang YC, Yang XY, Fitzmorris RC, et al. Hydrogen-Treated TiO2 nanowire arrays for photoelectrochemical water splitting. Nano Lett 2011;11(7):3026e33. [38] Indrakanti VP, Schobert HH, Kubicki JD. Quantum mechanical modeling of CO2 interactions with irradiated stoichiometric and oxygen-deficient anatase TiO2 surfaces: implications for the photocatalytic reduction of CO2. Energ Fuel 2009;23(10):5247e56. [39] Sasirekha N, Basha SJS, Shanthi K. Photocatalytic performance of Ru doped anatase mounted on silica for

[40]

[41]

[42] [43]

[44]

[45]

[46]

11

reduction of carbon dioxide. Appl Catal B-Environ 2006;62(1e2):169e80. Grigorieva AV, Goodilin EA, Dubova KL, Anufrieva TA, Derlyukova LE, Vaayacheslavov AS, et al. Titania nanotubes, nanorods and nanopowder in the carbon monoxide oxidation process. Solid State Ionics 2010;12(6):1024e8. Xia XH, Jia ZJ, Yu Y, Liang Y, Wang Z, Ma L. Preparation of multi-walled carbon nanotube supported TiO2 and its photocatalytic activity in the reduction of CO2 with H2O. Carbon 2007;45(4):717e21.  cz I. Photocatalytic reaction of H2OþCO2 Solymosi F, Tomba over pure and doped Rh/TiO2. Catal Lett 1994;27(1):61e5. Anpo M, Yamashitaa H, Ikeuea K, Fujii Y, Zhang SG, Ichihashia Y, et al. Photocatalytic reduction of CO2 with H2O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts. Catal Today 1998;44:327e32. Ikeue K, Hiromi Yamashita A, Anpo M, Takewaki T. Photocatalytic reduction of CO2 with H2O on Ti-b zeolite photocatalysts:effect of the hydrophobic and hydrophilic properties. J Phys Chem B 2001;105(35):8350e5. Asi MA, He C, Su MH, Xia DH, Lin L, Deng HQ, et al. Photocatalytic reduction of CO2 to hydrocarbons using AgBr/ TiO2 nanocomposites under visible light. Catal Today 2011;175:256e63. Li HH, Li CX, Han LJ, Li CS, Zhang SJ. Photocatalytic reduction of CO2 with H2O on CuO/TiO2 catalysts. Energ Source Part A 2016;38(3):420e6.

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