Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review

Applied Surface Science 392 (2017) 658–686

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Perspective Article

Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2 : a review Jingxiang Low a , Bei Cheng a , Jiaguo Yu a,b,∗ a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China b Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 12 July 2016 Received in revised form 18 September 2016 Accepted 19 September 2016 Available online 20 September 2016 Keywords: TiO2 Surface modification Photocatalytic CO2 reduction Heterojunction construction Alkali modification CO2 reduction selectivity

a b s t r a c t Recently, the excessive consumption of fossil fuels has caused high emissions of the greenhouse gases, CO2 into atmosphere and global energy crisis. Mimicking the natural photosynthesis by using semiconductor materials to achieve photocatalytic CO2 reduction into valuable solar fuels such as CH4 , HCO2 H, CH2 O, and CH3 OH is known as one of the best solutions for addressing the aforementioned issue. Among various proposed photocatalysts, TiO2 has been extensively studied over the past several decades for photocatalytic CO2 reduction because of its cheapness and environmental friendliness. Particularly, surface modification of TiO2 has attracted numerous interests due to its capability of enhancing the light absorption ability, facilitating the electron-hole separation, tuning the CO2 reduction selectivity and increasing the CO2 adsorption and activation ability of TiO2 for photocatalytic CO2 reduction. In this review, recent approaches of the surface modification of TiO2 for photocatalytic CO2 reduction, including impurity doping, metal deposition, alkali modification, heterojunction construction and carbon-based material loading, are presented. The photocatalytic CO2 reduction mechanism and pathways of TiO2 are discussed. The future research direction and perspective of photocatalytic CO2 reduction over surface-modified TiO2 are also presented. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Recently, the increasing CO2 concentration in the atmosphere due to the ever growing consumption of fossil fuels has caused detrimental environmental pollution [1–5]. As shown in Fig. 1, the annual CO2 emission steadily increased in past 35 years due to the tremendous consumption of the fossil fuels [1,6–8]. Since the accumulation of CO2 can trap heat in the atmosphere, it is not surprise that the average global surface temperature on this planet simultaneously increased with the CO2 emission in past 35 years. Therefore, searching for renewable and environmentally friendly energy resources has turned out to be an urgent task for the long-term development of human society [9–15]. Since the solar energy is considered as one of the inexhaustible and green energy sources, much attention has been paid to the conversion of incoming solar energy into valuable solar fuels [16–21]. Among various

∗ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, PR China. E-mail addresses: [email protected], [email protected] (J. Yu). http://dx.doi.org/10.1016/j.apsusc.2016.09.093 0169-4332/© 2016 Elsevier B.V. All rights reserved.

possibilities, photocatalytic CO2 reduction into green solar fuels such as CH4 , HCO2 H, CH2 O, and CH3 OH has been known as one of the most promising technologies because it can simultaneously produce useful solar fuels and imitate the CO2 concentration in the atmosphere [22–27]. Since the pioneering report by Inoue et al. in 1979 [28], various semiconductors such as TiO2 [29–33], CdS [34–36], Fe2 O3 [37,38], g-C3 N4 [39–44], Bi2 WO6 [45,46], Cu2 O [47,48], etc. have been developed as photocatalysts for effective CO2 reduction [49–54]. Attributed to its sufficiently high reduction potential, low cost and high stability, TiO2 has attracted wide attention as one of the most potential photocatalysts for CO2 reduction [55–60]. However, TiO2 suffers from low photoconversion efficiency for practical application of photocatalytic CO2 reduction due to its rapid electron-hole recombination [61–67]. Moreover, attributing to its relatively large bandgap value, merely 5% of incoming solar light can be utilized by the bare TiO2 for photocatalytic reaction [68–71]. Therefore, from practical point of view, it is of great significance to improve the electron-hole separation efficiency and light utilization ability of TiO2 [72–76]. One of the most widely applied approaches to prepare highly efficient TiO2 for photocatalytic CO2 reduction is the surface modification of TiO2 such as impurity doping, metal deposition,

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Fig. 1. The changes of annual CO2 emission and global mean surface temperature relative to 1951–1980 average temperatures from 1980 to 2015 [1].

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Fig. 3. Schematic illustration of photocatalytic CO2 reduction mechanism and pathway on the TiO2 surface, where pathway 1, 2, 3 and 4 indicate CO2 adsorption, electron-hole photogeneration, electron-hole separation (blue line)/recombination (dotted red line) and CO2 reduction, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

alkali modification, heterojunction construction and carbon-based material loading (see Fig. 2) [77–80]. The intention of this paper is to summarize the recent research progress on the surface modification of TiO2 for enhanced photocatalytic CO2 reduction performance. In particular, different surface modification strategies of TiO2 for photocatalytic CO2 reduction are mainly discussed. Furthermore, great emphasis is also devoted to providing better understanding on the fundamental photocatalytic enhancement mechanism of these surface modification strategies. Finally, summary and future perspectives of the surface modification of TiO2 -based photocatalysts are presented and discussed.

key aspects to determine its photocatalytic CO2 reduction activity. Surface modification of TiO2 that can provide it with larger specific surface area and more surface active sites is beneficial for improving the CO2 adsorption ability of TiO2 [93–95]. Moreover, tuning the surface of TiO2 to be alkaline is also a feasible way to improve its CO2 adsorption ability via chemisorption of CO2 [96,97]. Then, electron-hole pairs can be photogenerated when the absorbed photon energy equal to or higher than the bandgap value of TiO2 . Therefore, narrowing the bandgap of TiO2 is also an important strategy to enhance its photocatalytic CO2 reduction performance [98,99]. Surface modification of TiO2 by doping can reduce the bandgap of the TiO2 and thus generate more electron-hole pairs for the photocatalytic reaction. Besides doping, surface loading of plasmonic metal nanoparticles (NPs) can also be employed to enhance the electron-hole pairs’ generation efficiency through the injection of “hot electrons” from plasmonic metal NPs to TiO2 to stimulate photocatalytic reaction [100,101]. In the third stage, these photogenerated electron-hole pairs will either migrate to the surface of TiO2 for redox reaction or recombine for creating useless heat [102,103]. Obviously, it is important to enhance the separation and migration efficiency of electron-hole pairs for achieving enhanced photocatalytic activity [104–106]. The introduction of co-catalysts such as metal or carbon-based materials onto the surface of TiO2 can significantly reduce the electron-hole pairs recombination by accepting the electrons from TiO2 [107,108]. Finally, the electronhole pairs migrating to the surface of TiO2 surface can be utilized for CO2 reduction. However, it should be noted that CO2 molecules are highly stable [109–111]. Therefore, only the electrons with the sufficient reduction potential can be utilized for particular CO2 reduction reactions. Standard redox potentials of different CO2 reduction are listed in Eqs. (1)–(5).

2. Photocatalytic CO2 reduction mechanism

CO2 + 2H+ + 2e− → HCO2 H,E 0 = −0.61 V vs NHE at pH = 7

Fig. 2. Typical surface modification of TiO2 for enhanced photocatalytic CO2 reduction.

Photocatalytic CO2 reduction can mimic the natural photosynthesis system by converting incident solar light energy into valuable solar fuels, without requiring other high-energy input [81–84]. Therefore, it has turned out to be the most studied route to produce oxygenated hydrocarbons. In this section, the basic processes of photocatalytic CO2 reduction will be discussed. Generally, the photocatalytic CO2 reduction reaction can be categorized into 4 main steps (see Fig. 3): (1) CO2 adsorption, (2) electron-hole pair generation by absorbing sufficient incident photon energy, (3) electron-hole pair separation and their migration to the photocatalyst surface and (4) CO2 reduction [85–91]. Typically, the photocatalytic reaction is initiated by the CO2 adsorption step [92]. The adsorption ability of the TiO2 toward CO2 molecule is one of the

+

−

+

−

0

CO2 + 2H + 2e → CO + H2 O,E = −0.53 V vs NHE at pH = 7

(1) (2)

CO2 + 4H + 4e → HCHO + H2 O,E = −0.48 V vs NHE at pH = 7

(3)

CO2 + 6H+ + 6e− → CH3 OH + H2 O,E 0 = −0.38 V vs NHE at pH = 7

(4)

CO2 + 8H+ + 8e− → CH4 + 2H2 O,E 0 = −0.24 V vs NHE at pH = 7

(5)

0

Obviously, TiO2 with a conduction band potential of −0.5 V vs. NHE at pH = 7 can only produce CH4 and CH3 OH during photocatalytic CO2 reduction reaction [112,113]. Meanwhile, it should be noted that multielectron reaction are also required for producing CH3 OH (6 electrons) and CH4 (8 electrons) [114–116]. Therefore, it is highly desired to accumulate electrons on particular reaction sites of TiO2 for achieving multielectron transfer.

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Fig. 4. Schematic illustration for the comparison of band structure of the pure TiO2 , metal doped TiO2 and non-metal doped TiO2 .

3. Modification of TiO2 for photocatalytic CO2 reduction Generally, five surface modification methods have been tested and applied for enhancing photocatalytic CO2 reduction perfor-

mance of TiO2 . In this section, the function and application of different surface modification methods for photocatalytic CO2 reduction are discussed.

Fig. 5. (a,b) Schematic illustration of atomic surface structure of pure TiO2 (a) and La-modified surface of TiO2 (b), (c) UV–vis light absorption spectra of P25 and TiO2 doped with different La amount, where 0 #, 1 #, 2 #, 3 #, 4 # and 5 # refer to the La to Ti molar ratio of 0, 0.01, 0.02, 0.03, 0.04, 0.05, respectively, (d) EPR spectra of TiO2 doped with different La amount, (e) comparison of the photocatalytic CO2 reduction performance of P25 and TiO2 doped with different La amount and (f) schematic illustration of La-TiO2 for photocatalytic CO2 reduction. Reprinted with permission from Ref. [137]. Copyright 2015, Elsevier B.V.

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Fig. 6. (a,b) Scanning electron microscopy (SEM) images of In-TiO2 coated monolith at low resolution (a) and high resolution (b), (c) the nitrogen adsorption-desorption isotherm of TiO2 and In/TiO2 samples with different In-doping concentration and corresponding pore size distribution and (d) comparison of the photocatalytic activity of TiO2 and In/TiO2 samples with different In-doping concentration. Reprinted with permission from Ref. [141]. Copyright 2013, Elsevier B.V.

3.1. Impurity doping As aforementioned, one of the main aspects limiting the practical application of TiO2 is its wide bandgap (3.2 eV), which requires UV light irradiation for activating photocatalytic reaction. In fact, UV light only contributes a small fraction (5%) to the incident solar energy, thereby only small part of incident light can be utilized for photocatalytic reaction by TiO2 [50,117–120]. Therefore, extending the light absorption range of TiO2 into visible light range (account for 45% of incident sunlight energy) is highly desirable [121–123]. Doping is one of the most widely applied techniques for extending the light absorption range of semiconductor because it can effectively reduce the bandgap of semiconductor [124–130]. Basically, there are two proposed ways for doping TiO2 , which are metal doping and non-metal doping (see Fig. 4). In the earlier stage of research, more attentions are put to the metal doping of TiO2 because it can be more easily achieved compared with nonmetal doping [131–134]. Generally, the metal elements such as Cu and In can substitute the titanium atoms in the crystal lattice and thus create an empty energy state below the conduction band of TiO2 (see Fig. 4) [130,135]. The introduction of a new energy level can produce visible light response for TiO2 , thereby improving its overall light utilization ability. For instance, Slemat et al. reported Cu-doped TiO2 (Cu-TiO2 ) for photocatalytic CO2 reduction [136].

Generally, the modified TiO2 was doped by different concentration of Cu through the improved-impregnation method. It was found that the Cu doping can greatly enhance the photocatalytic performance of TiO2 for CO2 reduction. The methanol production of the Cu-TiO2 increased with Cu doping concentration. This is because the light absorption ability of Cu-TiO2 was greatly enhanced by the introduction of an impurity level below the conduction band of TiO2 . However, it should be noted that the overdoping Cu will lead to high defect density in the TiO2 , and consequently reduce its photocatalytic CO2 reduction performance. Besides, Liu et al. reported the La-modified TiO2 (La-TiO2 ) by employing sol-gel method for photocatalytic CO2 reduction [137]. It was found that most of the La3+ was deposited on the surface of TiO2 to form La2 O3 during the synthesis. The presence of La2 O3 could effectively adsorb the CO2 on its surface because of its basic properties. The enhanced CO2 concentration on the surface of TiO2 is beneficial for facilitating the CO2 reduction reaction. Meanwhile, a small portion of La atom was doped into the TiO2 lattice by substituting Ti atoms in the form of Ti-O-La (see Fig. 5a and b). Thanks to La doping, the light absorption edge of the La-TiO2 was red-shifted compared with that of P25 (see Fig. 5c). Moreover, according to the electron paramagnetic resonance (EPR) measurements (see Fig. 5d), the La-modification can lead to the formation of Ti3+ sites on the TiO2 surface, which are beneficial for binding CO2 and separating

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Fig. 7. (a) Schematic illustration for the preparation procedure of ordered mesoporous Co-doped TiO2 , (b) light absorption spectra of the OMT, Co-OMT-1 (Co to Ti molar ratio of 0.002), Co-OMT-2 (Co to Ti molar ratio of 0.005), Co-OMT-3 (Co to Ti molar ratio of 0.01), Co-OMT-4 (Co to Ti molar ratio of 0.025) and Co-OMT-5 (Co to Ti molar ratio of 0.05) and (c) comparison of the photocatalytic CO2 reduction performance of the OMT, Co-OMT-2, Co-OMT-4, Co-OMT-7 (Co to Ti molar ratio of 0.15), Co-OMT-8 (Co to Ti molar ratio of 0.2), Co3 O4 , N-doped TiO2 , C3 N4 , NaOH-WO3 and Au-OMT. Reprinted with permission from Ref. [130]. Copyright 2015, The Royal Society of Chemistry.

the electron and hole pairs [138,139]. Attributed to the advantages of La-doping and La2 O3 deposition, the photocatalytic CO2 reduction activity of La-TiO2 for CH4 production is 13 times higher than that of P25 (see Fig. 5e). It should be noted that the La-TiO2 exhibited significantly enhanced CH4 selectivity in comparison with P25 which produce both CO and CH4 by photocatalytic CO2 reduction. This is because La doping reduces the reduction potential of the conduction band of TiO2 from −0.6 V to ca. −0.52 V vs. NHE that is only suitable for producing CH4 (see Fig. 5f). Recently, Tahir et al. reported indium-doped TiO2 (In-TiO2 ) for photocatalytic CO2 reduction [140]. The In-TiO2 was prepared via a simple sol-gel method by using indium nitrate as the indium source. It was found that the specific surface area of TiO2 was greatly increased after indium doping because the introduction of impurity during the TiO2 synthesis can suppress the further growth of particle and thus limit its size to smaller scale. Moreover, according to the UV–vis absorption spectra, the light absorption edge of TiO2 was red-shifted by the indium doping, indicating the enhanced light absorption ability of the In-TiO2 . This is also attributed to the introduction of the impurity level below the conduction band level of the TiO2 . As a result of high specific surface area and the extended light

absorption range, the photocatalytic CO2 reduction activity of the In-TiO2 was ca. 8 times than that of pure TiO2 for CH4 production. In comparison with the conventional powder photocatalyst, the simple recyclable and reusable photocatalyst are more environmental friendly and suitable for long-term applications. Tahir et al. reported an In-doped TiO2 (In/TiO2 ) microchannel monolith photoreactor for photocatalytic CO2 reduction [141]. The In/TiO2 is intimately attached on the surface of monolith (see Fig. 6a and b). The robust integrity of In/TiO2 with the monolith enables the easy recyclability of the sample. Moreover, the resulting In/TiO2 exhibited an efficient incident light utilization due to the indium doping. In addition, it was found that the specific surface area of the sample increase with doping concentration of the indium on TiO2 (see Fig. 6c). The increased specific surface area of the sample is beneficial for providing more surface active sites for photocatalytic reaction. As a result, the photocatalytic activity of optimal In/TiO2 (10% Indium loaded TiO2 ) microchannel monolith photoreactor is ca. 10 times higher than that of the pure TiO2 photoreactor for photocatalytic CO2 reduction (see Fig. 6d). Moreover, Wang et al. reported ordered mesoporous cobaltdoped TiO2 (Co-OMT) with large specific surface area for visible-light induced photocatalytic CO2 reduction [130]. Co-OMT

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Fig. 8. (a) SEM image of the N-TNT, (b) comparison of the light absorption spectra of pure TiO2 , N-doped TiO2 , N-TNT prepared at 300 ◦ C (N-TNT300), N-TNT prepared at 400 ◦ C (N-TNT400), N-TNT prepared at 500 ◦ C (N-TNT500) and TNT prepared at 400 ◦ C (TNT400) and (c) comparison of the photocatalytic performance of N-TNT500, N-TNT300, TNT500 and TNT300 for CO2 reduction. Reprinted with permission from Ref. [154]. Copyright 2015, Elsevier B.V.

was prepared via a simple self-assembly method. As shown in Fig. 7a, the triblock copolymer F127, phenolic resol, cobalt salt and titanium salt are used as template, binder, precursor for cobalt and precursor for TiO2 , respectively and mixed in ethanolic solution. Then, the mixture was polymerized at 100 ◦ C to form an ordered mesoporous structure. Afterwards, the template was removed by annealing the sample at 350 ◦ C under a N2 atmosphere, thus obtaining a puce polymer-Co-TiO2 . Finally, the Co-OMT can be obtained by removing the binder at 400 ◦ C in air. It was found that the doping of Co into TiO2 can significantly reduce the bandgap of TiO2 , leading to visible-light photocatalytic activity of TiO2 (see Fig. 7b). As a result, the photocatalytic CO2 reduction activity of the OMCo/TiO2 is higher than that of the OM-TiO2 under light irradiation (see Fig. 7c). The optimal molar ratio of Co/TiO2 was determined to be 0.025, which exhibited a CH4 and CO production activity of 0.09 ␮mol g−1 h−1 and 1.94 ␮mol g−1 h−1 , respectively. Although the metal-doping of TiO2 was proved to be effective for enhancing its photocatalytic CO2 reduction performance, the application of metal-doped TiO2 is still limited because it suffers from the photocorrosion problem that can greatly reduce its longterm stability [142–145]. Recently, non-metal doping of TiO2 using iodine, nitrogen, sulfur or carbon elements by replacing the oxygen atoms in the TiO2 lattice has attracted much attention because of its relatively higher photostability in comparison with metal-doped

TiO2 [146–150]. For the non-metal doping of TiO2 , an occupied level is introduced above the valence band of TiO2 , thereby narrowing the bandgap of TiO2 for more efficient utilization of incident light (see Fig. 3b) [55,151–153]. For example, Zhao et al. reported the nitrogen-doped TiO2 nanotube (N-TNT) with enhanced photocatalytic CO2 reduction activity (see Fig. 8a) [154]. The prepared T-TNT exhibited the tubular structure with large surface area, which is beneficial for providing more surface active sites for photocatalytic reaction. Moreover, it was found that the N atoms can substitute the lattice oxygen of TiO2 , thereby reducing the bandgap of TNT and enhancing its absorption ability of visible light. Therefore, the light absorption edge of the N-TNT was red-shifted in comparison with that of the pure TNT (see Fig. 8b). As a result, the photocatalytic CO2 reduction activity of the optimal N-TNT was ca. 4 times higher than that of the pure TNT under visible light irradiation (see Fig. 8c). In other similar works, Zhang et al. studied the influence of different nitrogen sources on the morphology and band-gap engineering of N-doped TiO2 (N-TiO2 ) nanorod arrays [155]. As shown in Fig. 9a–c, it was found that the N-doping can significantly change the morphology of the as-prepared pristine TiO2 nanorod. The pristine TiO2 exhibited cubic column structure with smooth side surface (see Fig. 9a). Meanwhile, both N-doped TiO2 samples showed a cone-like structure with sharp top because the presence of N-dopant can cause the anisotropic growth of TiO2 (see

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Fig. 9. (a-c) Field emission scanning electron microscope (FESEM) images of pristine TiO2 (a), NH3 -TiO2 (b) and N2 H4 -TiO2 (c), (d) light absorption spectra of pristine TiO2 , NH3 -TiO2 and N2 H4 -TiO2, (e,f) photocatalytic CO2 reduction performance of N2 H4 -TiO2 (e) and NH3 -TiO2 (f), and (g) Fourier transform infrared spectroscopy (FTIR) spectra of the pristine TiO2 , NH3 -TiO2 and N2 H4 -TiO2 . Reprinted with permission from Ref. [155]. Copyright 2015, Elsevier B.V.

Fig. 9b and c). Interestingly, the length of N-TiO2 nanorods changes according to the nitrogen sources. The N-TiO2 prepared by NH3 (NH3 -TiO2 ) and N2 H4 (N2 H4 -TiO2 ) have a length of 1.2 and 1.5 ␮m, respectively. Moreover, N-TiO2 exhibited a significant red-shifted light absorption edge from ca. 375 nm to 420 nm due to the nitrogen doping into the TiO2 lattice (see Fig. 9d). The red-shifted light absorption of N-TiO2 suggests that the bandgap of N-TiO2 is smaller than that of the pristine TiO2 , indicating the better light utilization ability of the N-TiO2 . Notably, the absorbance of N2 H4 -TiO2 is generally higher than that of NH3 -TiO2 . This is because the nitrogen doping concentration of N2 H4 -TiO2 is higher than that of NH3 -TiO2 .

Furthermore, the photocatalytic CO2 reduction activity of the NTiO2 samples is 60–80 times higher than that of the pristine TiO2 under visible light irradiation due to the better light absorption ability of N-TiO2 . In addition, it was found that the N-doping of TiO2 using different nitrogen sources can tune the selectivity of photocatalytic CO2 reduction products (see Fig. 9e and f). In detail, the NH3 -TiO2 and N2 H4 -TiO2 tend to produce CO and CH4 , respectively via photocatalytic CO2 reduction under visible light irradiation. According to the FTIR spectra (see Fig. 9g), this is because the presence of reducing N N groups in the N2 H4 -TiO2 can facilitate the transformation of CO to CH4 . This work concluded that the N-

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into the visible light region by iodine doping. As a result of improved specific surface area and light absorption ability, the photocatalytic CO2 reduction performance of the I-TiO2 is significantly higher than that of the pure TiO2 under UV–vis or visible light irradiation for CO production.

3.2. Metal deposition

Fig. 10. Schematic illustration for the enhancement mechanism of the photocatalytic CO2 reduction activity by the metal loading TiO2 .

doping of TiO2 can not only enhance the light absorption ability of the TiO2 , but can also tune the product selectivity of TiO2 in photocatalytic CO2 reduction. Zhang et al. reported iodine-doped TiO2 (I-TiO2 ) with different doping concentration for enhanced photocatalytic CO2 reduction performance under visible light irradiation [156]. According to the X-ray diffraction (XRD) characterization, the crystal size of the ITiO2 was smaller than that of the pure TiO2 . This is because the iodine doping can greatly inhibit the growth and limit the size of the TiO2 nanocrystals. The smaller particle size of I-TiO2 is beneficial for improving the specific surface area of samples and providing more surface active sites during photocatalytic reaction. Moreover, the light absorption edge of the TiO2 was extended from the UV region

It is well-known that the photocatalytic CO2 reduction efficiency of TiO2 is low even under the UV–vis light irradiation [157–160]. This is because of the rapid electron-hole recombination before photocatalytic CO2 reduction reaction. The most widely applied surface modification method for inhibiting the recombination of photogenerated electron-hole pairs on TiO2 is metal loading [51,161–164]. Different metal NPs including Pt, Au, Ag and Pd have been loaded on TiO2 and proved to be effective for enhancing its photocatalytic performance [164–168]. In detail, the fermi levels of the metal NPs are normally lower than the CB of the TiO2 . By loading of these metal NPs on the TiO2 , Schottky barrier can form at the interface between TiO2 and metal NPs (see Fig. 10) [169–173]. Then, the photogenerated electrons will rapidly migrate from TiO2 to metal NPs through Schottky barrier until their fermi levels are equal [30,173–177]. Meanwhile, the photogenerated holes will remain on TiO2 . Therefore, the spatial separation of photogenerated electron-hole pairs can be achieved. Furthermore, the work function of metal also plays an important role in determining the electron-hole separation efficiency across interface between metal and TiO2 [178–180]. Particularly, the electron accepting ability of the metal improves with increasing work function of metal for enhancing the electron-hole separation. As seen from Table 1 [157], Pt, which exhibits the largest work function, is the most commonly

Fig. 11. (a,b) FESEM image (top-view) (a) and Transmission electron microscopy (TEM) image (side-view) (b) of Pt-TiO2 nanotube arrays and (c) comparison of the photocatalytic CO2 reduction activity of the prepared samples. Reprinted with permission from Ref. [181]. Copyright 2011, The Royal Society of Chemistry.

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Table 1 Work function of the commonly used metals for enhancing photocatalytic activity of TiO2 [157]. Metal

Work function (eV)

Metal

Work function (eV)

Ag Al Au Bi Cd Co Cr Cu Fe Ir Li Mg

4.26–4.74 4.06–4.26 5.10–5.47 4.31 4.08 5.00 4.50 4.53–5.10 4.67–4.81 5.00–5.67 2.90 3.66

Mn Mo Nd Ni Pd Pt Ru Se Si Sn W Zn

4.10 4.36–4.95 3.20 5.04–5.35 5.22–5.60 5.12–5.93 4.71 5.90 4.60–4.85 4.42 4.32–5.22 3.63–4.90

used metal co-catalysts for enhancing photocatalytic CO2 reduction performance of TiO2 . For example, Feng et al. reported the Pt-loaded TiO2 nanotube arrays (NTAs) with enhanced photocatalytic CO2 reduction performance [181]. In detail, the TiO2 NTAs were firstly prepared by an anodic oxidation method. Then, the Pt NPs were deposited on the TiO2 NTAs by a microwave-assisted hydrothermal method. As shown in Fig. 11a and b, the Pt NPs were uniformly deposited on the surface of the TiO2 NTAs, which is beneficial for providing more reaction sites for the photocatalytic reaction. Moreover, TiO2 NTAs/Pt composite has good electron-hole separation efficiency due to the 1D structure of TiO2 NTAs and the electron accepting ability of Pt NPs. Benefitting from good dispersion of the Pt NPs and high electron-hole separation efficiency of the TiO2 /Pt, the photocatalytic CO2 reduction activity of TiO2 /Pt composite is ca. 5 times higher than the pure TiO2 for methane production under AM 1.5G sunlight irradiation (100 mW cm−2 ) for one hour (see Fig. 11c).

Thereafter, Wang et al. systematically investigated the effect of Pt NPs size on the photocatalytic CO2 reduction activity of TiO2 [182]. Particularly, a highly oriented 1D TiO2 (1DTiO2 ) was firstly prepared by an aerosol chemical vapor deposition (ACVD) method. Then, the Pt NPs with different sizes were loaded onto the 1DTiO2 via a unique tilted-target sputtering (TTS) method. As can be seen in Fig. 12a and b, the Pt NPs are uniformly deposited on the surface of 1DTiO2 through the TTS method. Moreover, the diameter of Pt NPs in the range of 0.5–2 nm can be simply controlled by tuning the deposition time from 5 to 60 s. The number of confined electrons on the small Pt NPs with a large fraction of surface atoms is higher than that on its bulk counterpart. Therefore, the Pt NPs with smaller size has a higher energy band separation and work function due to the quantum confinement effect (Fig. 12c). Thus, the photogenerated electrons on the CB of TiO2 can hardly migrate to Pt NPs with very small size. With the increase of Pt NPs size, the work function of the prepared Pt NPs approach that of the bulk Pt, thus allowing rapid migration of photogenerated electrons from TiO2 to Pt NPs (Fig. 12d). Therefore, with the optimization of the Pt NPs size, the photocatalytic CO2 reduction activity of the Pt/1DTiO2 can be far higher than that of P25 and pure 1DTiO2 (Fig. 12e). However, it should be noted that the further increase of the deposition time will cause the decrease of photocatalytic activity. This is because the increase volume of Pt NPs on the 1DTiO2 trims the light absorption ability of TiO2 and reduces its photocatalytic activity. This work accentuates the necessary control of the metal NPs size for optimizing the photocatalytic CO2 reduction activity of TiO2 . Additionally, Sim et al. reported the graphene oxide (GO) and Pt co-modified TiO2 nanotube arrays (GO/Pt-TiO2 NTAs) for enhanced photocatalytic CO2 reduction performance [30]. According to the SEM characterization, the GO and Pt are uniformly deposited on the top surface of TiO2 NTAs, thereby providing more surface active sites for the CO2 reduction reaction (see Fig. 13a and b). Moreover, the GO/Pt-TiO2 NTAs exhibited enhanced light absorption in the

Fig. 12. (a) Elemental mapping image of Pt-1DTiO2 , where green and red respectively denotes Ti and Pt element, (b) TEM images of the prepared Pt-1DTiO2 , (c) schematic illustration of the change of work function with different Pt NPs size, (d) the charge carrier transfer in the Pt-1DTiO2 for enhanced photocatalytic CO2 reduction and (e) comparison of photocatalytic activity for CH4 production between P25 and the prepared Pt-1DTiO2 with different Pt sputtering time. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Reprinted with permission from Ref. [182]. Copyright 2012, American Chemical Society.

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Fig. 13. (a,b) FESEM images of TiO2 NTAs (a) and rGO/Pt-TiO2 NTAs (b), (c) UV–vis absorption spectra of the TiO2 NTAs, Pt-TiO2 NTAs and rGO/Pt-TiO2 NTAs, and (d) comparison of the photocatalytic activity of the TiO2 NTAs, rGO-TiO2 NTAs, Pt-TiO2 NTAs and rGO/Pt-TiO2 NTAs. Reprinted with permission from Ref. [30]. Copyright 2015, Elsevier B.V.

visible light range due to the black color characteristics of the GO and Pt (see Fig. 13c). Although this enhanced light absorption cannot be utilized for the generation of the electron-hole pairs, the photons accumulated on the surface of the Pt and GO can create local heat field around the TiO2 and accelerate the migration of charge carriers on the TiO2 surface. Furthermore, since both GO and Pt has good electron conductivity, the photogenerated electrons can also rapidly migrate from TiO2 to GO or Pt, thus achieving spatial separation of electron-hole pairs. As a result, the photocatalytic activity of the GO/Pt-TiO2 NTAs is higher than that of pure TiO2 NTAs and Pt-TiO2 NTAs (see Fig. 13d). More recently, the loading of plasmonic metals such as Ag and Au, has been extensively explored for harvesting visible-light energy for photocatalytic reaction due to their unique surface plasmon resonance (SPR) effect [183–189]. Based on the SPR effect, a resonance can be formed when the wavelength of the incident photons matches the natural frequency of the oscillation of plasmonic metal surface’s free electrons, causing a strong oscillation of the surface electrons. Then, the SPR-excited electrons (so-called hot electrons) can be created (see Fig. 14) [190–196]. These SPRexcited electrons on the surface of plasmonic metals possess high energy and thus can directly transfer to semiconductor and induce

Fig. 14. Schematic illustration of the SPR effect on TiO2 for photocatalytic CO2 reduction.

photocatalytic reaction [197–200]. Since the plasmonic absorption of Au and Ag are located in the visible light range, these visible light-active plasmonic metals can be utilized as a visiblelight sensitizer for TiO2 with a wide bandgap for achieving visible light photocatalytic CO2 reduction [197,201–205]. In addition, the resonance between the incident photons and frequency of met-

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Fig. 15. (a) TEM image of the Au/TiO2 , (b) comparison of photocatalytic CO2 reduction activity of P25, TiO2 and Au/TiO2 for CH4 and C2 H6 production, (c) photocurrent responses and transient decay lifetime spectra of the TiO2 and Au/TiO2 and (d) schematic of the local EM field enhancement on the Au/TiO2 yolk-shell from the finite difference time domain (FDTD) simulation. Reprinted with permission from Ref. [211]. Copyright 2011, The Royal Society of Chemistry.

als surface free electrons can create a strong and highly localized electromagnetic (EM) fields around TiO2 and boost the electronhole separation efficiency [191,206]. Furthermore, these plasmonic metals exhibit several obvious advantages to be coupled with TiO2 for enhancing photocatalytic CO2 reduction performance, including (a) the light absorption ability of plasmonic metal can be simply controlled by tuning its size and shape, (b) only small amount of plasmonic metals (normally ranging from 0.5–2 wt%) is required for greatly improving the photocatalytic performance of TiO2 and (c) the photostability of these plasmonic metals are high for long-term applications [207–209]. For instance, Hou et al. reported Au loaded TiO2 (TiO2 /Au) for photocatalytic CO2 reduction applications [210]. It was found that pure TiO2 exhibited negligible photocatalytic CO2 reduction activity under visible light irradiation due to its wide bandgap. Surprisingly, the TiO2 /Au composite showed a high photocatalytic CO2 reduction activity for producing CH4 under visible light irradiation, which is 24 times higher than that of pure TiO2 . This is attributed to the superior visible light harvesting ability and the intense local EM fields induced by Au NPs. In detail, the SPR-excited electrons can be transferred from Au to TiO2 for photocatalytic reaction when the incident photon energy is sufficiently high (ca. 532 nm). This work showed that the loading of Au can not only improve the electron-hole separation efficiency of TiO2 , but also enhance its light absorption ability. Apart from the simple coupling of plasmonic metal with the TiO2 NPs, rational design of the plasmonic metal coupled TiO2 is also proved to be effective for further enhancing its photocatalytic CO2 reduction performance. For example, Tu et al. reported Au/TiO2 yolk-shell hollow spheres for enhancing photocatalytic

CO2 reduction by local EM field [211]. It was found that the Au NPs are individually encapsulated within the TiO2 hollow sphere (see Fig. 15a). The yolk-shell structure of Au/TiO2 is beneficial for avoiding the aggregation of Au NPs and detachment of Au from the TiO2 . The photocatalytic CO2 reduction activity of Au/TiO2 is higher than that of pure TiO2 and P25 for CH4 production (see Fig. 15b). According to the photocurrent response test and FDTD simulation (see Fig. 15c and d), this enhanced photocatalytic performance is attributed to the production of SPR-mediated EM field on Au NPs which can boost the migration of electron-hole pairs to the surface of TiO2 . In detail, as shown in Fig. 15c, the loading of Au can greatly promote the generation of charge carriers, which is crucial for enhancing photocatalytic activity. The FDTD simulation indicates that this enhanced charge carrier generation ability of the sample is due to the formation of the EM field between the Au and TiO2 interface (see Fig. 15d). Meanwhile, Jiao et al. prepared Au loaded three-dimensionally ordered macroporous (3DOM) TiO2 using a colloidal crystal template method for photocatalytic CO2 reduction [196]. The prepared sample exhibited a well-defined 3DOM structure, which is highly ordered and interconnected with one another by small pore windows (see Fig. 16a and b). Meanwhile, the Au nanoparticles with the size of ca. 3 nm are highly dispersed on the surface of 3DOM TiO2 (see Fig. 16c). The presence of the SPR effect on the sample can be determined by the UV–vis absorption spectra. As can be seen from Fig. 16d, the 3DOM TiO2 exhibited similar optical properties with commercial P25. By loading Au, a broad absorption peak at ca. 550 nm can then be found. This is due to the collective oscillations of the free electrons on Au NPs through interaction with incident light photons. During photocatalytic reaction, the free electrons

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Fig. 16. (a–c) SEM (a), TEM (b) and high-resolution transmission electron microscopy (HRTEM) (c) images of the Au-3DOM TiO2 , (d) light absorption spectra of P25 (I), 3DOM TiO2 (II), 0.5 wt% loaded 3DOM TiO2 (III), 1 wt% loaded 3DOM TiO2 (IV), 2 wt% loaded 3DOM TiO2 (V), 4 wt% loaded 3DOM TiO2 (VI) and 8 wt% loaded 3DOM TiO2 (VII), and (e) schematic illustration of the enhancement mechanism of Au-3DOM TiO2 for photocatalytic reaction. Reprinted with permission from Ref. [196]. Copyright 2015, Elsevier B.V.

on Au NPs will be photoexcited by incident visible light, creating high-energy “hot electrons” (see Fig. 16e). Then, these hot electrons will migrate to the conduction band of the TiO2 for photocatalytic reaction. Therefore, the photocatalytic CO2 reduction activity of the optimized Au-3DOM TiO2 is higher than that of pure 3DOM TiO2 and P25 for CH4 production under visible light irradiation. More recently, Feng et al. prepared a double-shelled plasmonic Ag-TiO2 hollow-sphere by hydrothermal treatment of colloidal TiO2 and Ag mixed solutions [212]. The prepared samples exhibited double-shelled hollow sphere structure which is beneficial for enhancing the light utilization ability of the samples through multiscattering light within the hollow sphere structure. Moreover, an enhanced absorption centered at ca. 480 nm can be found in the light absorption spectrum of Ag-TiO2 in comparison with that of pure TiO2 , indicating that the Ag NPs in the prepared samples exhibited good SPR properties (see Fig. 17a). Attributed to the good light utilization of hollow sphere structure and enhanced visiblelight absorption by the Ag NPs (see Fig. 17b), the photocatalytic CO2 reduction performance of Ag-TiO2 is around 10 times higher than that of pure TiO2 for CH4 production under 180 min visible light irradiation. 3.3. Alkali modification Enhanced adsorption of CO2 is also an efficient way to improve the photocatalytic CO2 reduction efficiency [213–216]. Since CO2 molecule is an acidic oxide, one of the best techniques for enhancing adsorption of CO2 is to form chemisorption through alkali sorbents. Therefore, it can be expected that the alkali sorbents-loaded TiO2 or alkali-modified TiO2 can have good adsorption ability towards

Fig. 17. (a) Light absorption spectra and (b) comparison of the photocatalytic CO2 reduction activity of pure TiO2 and Ag loaded TiO2 for methanol production. Reprinted with permission from Ref. [212]. Copyright 2015, AIP Publishing LLC.

CO2 molecule [217,218]. Moreover, unlike typical physical adsorption, the chemisorption of CO2 based on alkali sorbents normally involves the reaction of CO2 with the reactive groups on the alkali sorbents. The reaction between CO2 with alkali sorbents will produce the intermediate product such as bidentate carbonate species which is beneficial for accelerating the CO2 reduction reaction. Therefore, alkali modification of TiO2 can not only enhance the CO2 adsorption ability of TiO2 , but also activate the CO2 molecule for CO2 reduction reaction. For example, Meng et al. demonstrated that the alkali modification of TiO2 using NaOH can greatly improve the CO2 adsorption and activation ability of TiO2 [219]. In detail, the NaOH-modified TiO2 (NaOH-TiO2 ) was simply prepared by calcination of TiO2 with NaOH in the aqueous solution. The NaOH is mainly loaded onto the surface of TiO2 (see Fig. 18a). It was observed that the photocat-

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Fig. 18. (a) HRTEM image of the NaOH-TiO2 . Comparison of the (b) photocatalytic CO2 reduction activity, (c) CO2 chemisorption ability and (d) FTIR spectra of the TiO2 modified with 0%, 1%, 3%, 5% and 7% NaOH. Reprinted with permission from Ref. [219]. Copyright 2014, The Royal Society of Chemistry.

Fig. 19. (a,b) SEM (a) and HRTEM (b) images of the MgO-TiO2 , (c) the photocatalytic CO production rate with H2 O of TiO2 and MgO-TiO2 by CO2 reduction, (d) schematic illustration of the enhancement mechanism of MgO-TiO2 for photocatalytic CO2 reduction and (e) comparison of the CO2 adsorption ability of TiO2 and MgO-TiO2 . Reprinted with permission from Ref. [220]. Copyright 2014, The Royal Society of Chemistry.

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Fig. 20. (a) Comparison of the CO2 adsorption isotherm of A-TiO2 and TiO2 , (b) FTIR spectra of the (I) A-TiO2 with the presence of CO2 and (II) A-TiO2 , (c) schematic illustration of the enhancement mechanism of A-TiO2 for photocatalytic CO2 reduction and (d) comparison of the photocatalytic performance of TiO2 , A-TiO2 and control experiment for CH4 production. Reprinted with permission from Ref. [223]. Copyright 2014, John Wiley & Sons, Inc.

alytic CO2 reduction activity of the optimal sample calcined with 3 wt% of NaOH is far higher than that of pure TiO2 (see Fig. 18b). This enhanced photocatalytic activity is because of the loading of NaOH that can significantly enhance the chemisorption of TiO2 towards CO2 (see Fig. 18c). Moreover, based on the FTIR characterization (see Fig. 18d), it was found that the introduction of NaOH can also activate the CO2 molecule by promoting the formation of bidentate carbonate species (Vas (CO3 ): 1340 cm−1 ; Vs (CO3 ): 1550 cm−1 ) which are beneficial for accelerating the CO2 reduction into CO or methane. However, it should be taken into account that the introduction of NaOH onto the surface of TiO2 will also cause the aggregation of TiO2 , which can in turn reduce its specific surface area for photocatalytic CO2 reduction. Therefore, the photocatalytic activity of the samples overloaded with NaOH (4–7 w%) is lower than that of TiO2 modified with 3 wt% of NaOH. More recently, Liu et al. reported the earth-abundant alkaline mineral, MgO coupled TiO2 (MgO-TiO2 ) for enhancing photocatalytic CO2 reduction activity [220]. The MgO-TiO2 composite was prepared by an ultrasonic spray pyrolysis method using P25 and

Mg(NO3 )2 as precursors to form a porous microsphere structure (see Fig. 19a and b). The photocatalytic CO2 reduction of MgO-TiO2 was found to be higher than that of pure TiO2 (see Fig. 19c). This is because MgO can significantly enhance the CO2 adsorption ability of TiO2 and also limit the adsorption of H2 O molecule (Fig. 19d). Therefore, the competitive H2 O water splitting reaction on conventional photocatalyst can be also avoided on the MgO-TiO2 . Similar to the NaOH modified TiO2 , this superior CO2 adsorption ability of MgO-TiO2 is attributed to the chemisorption of the CO2 molecule by forming bidentate carbonate which can accelerate the transformation of CO2 into solar fuels. Moreover, the porous microsphere structure of MgO-TiO2 can also further provide more surface active sites for CO2 adsorption and reduction reaction (see Fig. 19e). This work demonstrates that the introduction of alkaline materials onto TiO2 can not only enhance its CO2 adsorption ability but also avoid the competitive H2 O splitting reaction. Introduction of amine functional group have been extensively utilized in industry for capturing CO2 [221,222]. Therefore, introducing amine functional group onto the TiO2 surface is also a

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feasible way to enhance the CO2 adsorption ability of TiO2 for improved photocatalytic CO2 reduction activity. For instance, Liao et al. reported amine-functionalized TiO2 (A-TiO2 ) for CO2 capture and reduction [223]. The A-TiO2 was prepared by a simple hydrothermal method using monoethanolamine (MEA) as the amine source. In detail, the surface hydroxyl (-OH) of MEA can create covalent bonding with TiO2 , while the primary amine (-NH2 ) group on MEA can provide A-TiO2 with an amine-terminated surface. It was found that the CO2 adsorption ability of A-TiO2 is ca. 4 times higher than that of pure TiO2 (see Fig. 20a). According to the FTIR characterization (see Fig. 20b), the A-TiO2 exhibited additional characteristic peaks of carbamate (-OCH2 CH2 NHCOO− ), HCO3 − and NH3 + after exposure to CO2 gas for 1.5 h. The result indicates that the enhanced CO2 adsorption ability of A-TiO2 is attributed to the chemisorption of CO2 by the amine groups on the surface of TiO2 (see Fig. 20c). As a result, the photocatalytic CO2 reduction activity of the A-TiO2 is ca. 8 and 6 times higher than that of pure TiO2 for CH4 and CO generation, respectively (see Fig. 20d). 3.4. Heterojunction construction Combining TiO2 with other semiconductors to build heterojunction possesses several obvious advantages in enhancing the electron-hole pair separation and separating the reduction and oxidation site for improved photocatalytic CO2 reduction performance

Fig. 21. Schematic illustration of the conventional type-II heterojunction photocatalyst.

[224–231]. Generally, the TiO2 -based heterojunction for photocatalytic CO2 reduction can be categorized into 4 different types depending on the charge carrier separation mechanism, which are conventional type-II, p-n, direct Z-scheme and surface heterojunction [88,232–237]. In this section, the basic principles and applications of these heterojunctions for photocatalytic CO2 reduction will be discussed.

Fig. 22. (a,b) Scanning electron microscope images of TZ-MFF at low (a) and high (b) resolution and (c) schematic illustration of the TiO2 /ZnO heterojunction for photocatalytic CO2 reduction. Reprinted with permission from Ref. [249]. Copyright 2011, John Wiley & Sons, Inc.

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Fig. 23. (a) Schematic illustration of the formation mechanism of TiO2 nanosheet by using graphene nanosheet as the template and benzyl alcohol as the cross-linking agent, (b,c) SEM (b) and TEM (c) images of the CdS-TiO2 NS, (d) comparison of the photocatalytic performance of TiO2 NS and CdS-TiO2 NS for CO and CH4 production and (e) schematic illustration of the electron-hole transfer mechanism on CdS-TiO2 NS under light irradiation. Reprinted with permission from Ref. [246]. Copyright 2015, American Chemical Society.

3.4.1. Conventional type-II heterojunction Conventional type-II heterojunction is the most typical heterojunction system for enhancing the photocatalytic CO2 reduction performance of TiO2 [231,238–240]. A conventional type-II heterojunction can form by combining semiconductor I with a higher conduction band with semiconductor II with a lower valence band (see Fig. 21) [241–246]. Under light irradiation, the photogenerated electrons will migrate from semiconductor I to semiconductor II and meanwhile, the photogenerated holes will in turn migrate from semiconductor II to semiconductor I [247,248]. Therefore, the photogenerated electron-hole pairs can be spatially separated. Moreover, since the reduction and oxidation reactions are carried out on different semiconductors, the recombination of electronhole pairs can also be significantly avoided during photocatalytic reaction. For example, Xi et al. reported the TiO2 /ZnO mesoporous “French fries” (TZ-MFF) heterojunction photocatalyst with high specific surface area and large pore volume for photocatalytic CO2 reduction by a facile furfural alcohol-derived polymerization-oxidation route (see Fig. 22a and b) [249]. Benefitting from their unique one-dimensional mesoporous structure, the specific surface of TZMFF (225 m2 g−1 ) was far higher than that of P25 (50 m2 g−1 ). This improved specific surface area of TZ-MFF can not only enhance its adsorption capacity towards the CO2 molecules, but also provide more surface active sites for photocatalytic reaction. Moreover, the type-II heterojunction can be built between TiO2 and ZnO (see Fig. 22c), thereby accelerating the electron-hole separation. As a result of improved specific surface area and high electron-hole separation efficiency, the photocatalytic activity of the TZ-MFF was approximately 6 times higher than that of P25 for CH4 production. Moreover, proper control of the morphology of TiO2 is also important for enhancing the efficiency of TiO2 -based type-II heterojunction photocatalyst for CO2 reduction. For example, Pan and

Xu reported ultralarge binary CdS-TiO2 nanosheets (NS) with high visible light photocatalytic CO2 reduction performance [246]. The TiO2 NS were firstly prepared by using graphene nanosheets as template (see Fig. 23a). In detail, TiO2 was uniformly grafted onto graphene nanosheets through benzyl alcohol. Then, the graphene template was removed by thermal treatment in air. The obtained TiO2 NS exhibited ultralarge 2D surface which is beneficial for providing abundant coupling interface for CdS decoration (see Fig. 23b and c) and thus allowing rapid charge carrier transport across the interfacial domain. The TiO2 NS exhibited a light absorption edge at ca. 380 nm. Meanwhile, by loading CdS, the light absorption edge was extended into the visible light range, suggesting the enhanced light utilization of CdS-TiO2 NS. As shown in Fig. 23d, the photocatalytic CO2 reduction activity of CdS-TiO2 NS is far higher than that of TiO2 NS for CO and CH4 production under visible light irradiation. This is due to the formation of type-II heterojunction and large contact interface between the TiO2 NS and CdS (see Fig. 23e). Furthermore, Zhou et al. prepared the graphitic carbon nitrideN-TiO2 (GCN-N-T) with enhanced photocatalytic CO2 reduction activity [250]. The TiO2 was in-situ doped during the preparation of g-C3 N4 -TiO2 (GCN-T) composite to form GCN-NT. The relatively small bandgap of g-C3 N4 and the reduced bandgap of TiO2 by Ndoping endow the GCN-NT composite with a visible light-induced photocatalytic activity. Meanwhile, the heterojunction formation can also suppress the electron-hole recombination within the GCNNT. Therefore, the photocatalytic CO2 reduction activity of the optimized GCN-NT was much higher than that of pure N-TiO2 under visible-light irradiation. In addition, the combination of two different TiO2 phases can also create type-II heterojunction. Industrially, P25 which consist of 75% anatase and 25% rutile TiO2 has been widely applied in photocatalytic application because of its effectiveness for charge carriers’ separation. In detail, upon light irradiation, the electron and hole

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will respectively accumulate on the CB of rutile TiO2 and VB of anatase TiO2 because the CB and VB of the anatase TiO2 are higher than that of the rutile TiO2 . Moreover, mixed phase TiO2 has been also studied for enhancing the photocatalytic CO2 reduction performance of the TiO2 . For example, Lee et al. reported anatase-rutile TiO2 for photocatalytic CO2 reduction [251]. The photocatalytic CO2 reduction performance of the anatase-rutile TiO2 is higher than that of the pure anatase TiO2 . This is because the formation of the type-II heterojunction between the anatase and rutile can facilitate the electron-hole separation for the photocatalytic reaction. This work demonstrated that the type-II heterojunction can be also built by only single semiconductor with different phase structure for enhancing photocatalytic CO2 reduction efficiency. 3.4.2. P-N junction Since the recombination of electron-hole pairs is an ultra-fast process, the conventional type-II heterojunction is not so effective for separating the photogenerated electron-hole pairs on the TiO2 surface. One concept to accelerate the electron-hole separation in TiO2 is to build an electric field within the photocatalytic systems by forming p-n heterojunctions [252–257]. Generally, a p-n heterojunction can be built between n-type TiO2 and another p-type semiconductor such as Cu2 O (see Fig. 24) [258,259]. Prior to the photocatalytic reaction, the electrons close to the p-n inter-

Fig. 24. Schematic illustration of the TiO2 -based p-n heterojunction photocatalyst for CO2 reduction.

face in TiO2 will migrate to the p-type semiconductor due to the difference of Fermi level between n-type TiO2 and p-type semiconductor. Meanwhile, the holes close to the p-n interface of the p-type semiconductor will migrate to TiO2 to achieve the equilibrium of Fermi level for the system. Therefore, the region close to the p-n interface will be charged and then a space charged region will form (viz. internal electric field). Under light irradiation, both n-type TiO2 and p-type semiconductor will be excited to generate electron-hole pairs. Under the influence of internal electric field of the p-n heterojunction, the photogenerated electrons and holes

Fig. 25. (a,b) SEM (a) and TEM (b) images of the Cu2 O/TiO2 hollow nanospheres, (c) schematic illustration of the charge carrier separation mechanism across the Cu2 O/TiO2 p-n heterojunction for photocatalytic reaction and (d) Comparison of the photocatalytic CH4 production activity of Cu2 O and Cu2 O/TiO2 by CO2 reduction. Reprinted with permission from Ref. [259]. Copyright 2015, Elsevier B.V.

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Fig. 26. (a) The density of states (DOS) plots of {101} and {001} facets of TiO2 , where O 2p, Ti 2d and TDOS are the partial DOS of O 2P, the partial DOS of Ti 2d and the total DOS, respectively and (b) schematic illustration of the surface heterojunction built between {001} and {101} facets of TiO2 according to the first principles density functional theory calculation. Reprinted with permission from Ref. [9]. Copyright 2014, American Chemical Society.

will respectively transfer to n-type TiO2 and p-type semiconductor, thus achieving spatial separation of electron-hole pairs. Moreover, the electron-hole migration across the p-n heterojunction is also thermodynamically suitable because the CB and VB of n-type TiO2 are normally lower than those of p-type semiconductor to form the type-II heterostructure. Apparently, the p-n heterojunction is more efficient than the conventional type-II heterojunction for enhancing the photocatalytic CO2 reduction performance of TiO2 due to the synergistic effect of internal electric field and band alignment. For example, Bi et al. reported Cu2 O/TiO2 hollow nanospheres composite for photocatalytic CO2 reduction into methane (see Fig. 25a and b) [259]. It was found that the p-n heterojunction can form between p-type Cu2 O and n-type TiO2 . The internal electric field of p-n heterojunction and the band alignment between Cu2 O and TiO2 greatly facilitate the electron-hole separation (see Fig. 25c). Meanwhile, the hollow nanosphere structure of the samples is also beneficial for light utilization because the incident light can be scattered and trapped within such hollow structure. As a result, the photocatalytic CO2 reduction activity of the prepared Cu2 O/TiO2 hollow nanospheres is far higher than that of pure TiO2 and Cu2 O (see Fig. 25d). In short, it is believed that the construction of p-n heterojunction is one of the most promising strategies for enhancing the photocatalytic CO2 reduction performance of the TiO2 due to its effectiveness for spatial separation of photogenerated electronhole pairs through the combination effect of build-in electric field and band alignment between two semiconductors. 3.4.3. Surface heterojunction In 2014, the surface heterojunction of TiO2 was proposed by our group [9]. In fact, the surface heterojunction is the conventional type-II heterojunction that is built between different crystal facets of a single semiconductor [260,261]. Similar to the conventional type-II heterojunction, the photogenerated electrons and holes can be spatially separated to the {101} and {001} facets of TiO2 for reduction and oxidation reaction, respectively (see Fig. 26). Since only one semiconductor is required to build a surface het-

erojunction, the fabrication cost and preparation procedure can be saved. By building such a surface heterojunction, the redox potential loss of the heterojunction system can also be reduced because the potential difference between two different facets of TiO2 is normally very small. For example, our group reported the presence of surface heterojunction between {001} and {101} facets of single anatase TiO2 crystals for enhancing its photocatalytic CO2 reduction activity [9]. In detail, the anatase TiO2 with different ratios of the exposed {001} and {101} facet was prepared by a facile fluorine-assisted hydrothermal method. In general, for anatase TiO2 , the percentage of the exposed {101} facet is much higher than that of {001} facets because of the high reactivity of the latter. However, the introduced fluorine ions can preferably absorb on the {001} facet, thereby reducing its surface reactivity and increasing its exposed percentage in anatase TiO2 . Therefore, the ratio of the exposed {001} and {101} facet can be simply tuned by changing the fluorine concentration (see Fig. 27a–c). It was found that the photocatalytic CO2 reduction activity of the prepared anatase TiO2 with the optimal ratio of exposed {001} to {101} facet is ca. 4 times higher than that of P25 (see Fig. 27d). This is because the formation of surface heterojunction will drive the photogenerated electron and hole respectively to the {101} and {001} facets based on their band alignment. Moreover, it should be noted that the overexposure of either {001} or {101} facets will cause the electron or hole overflow effect (see Fig. 27e), respectively, thus reducing the electron-hole separation efficiency across the surface heterojunction. Consequently, it is important to optimize the exposed ratio of {001} and {101} facet in TiO2 to achieve the optimal photocatalytic CO2 reduction activity. 3.4.4. Direct Z-scheme heterojunction As mentioned above, the photogenerated electrons and holes will respectively migrate to the conduction band with lower reduction potential and the valence band with lower oxidation potential for conventional type-II, p-n or surface heterojunction photocatalyst to achieve spatial separation of electron-hole pairs [262].

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Fig. 27. (a-c) TEM image of HF0 (a) and SEM images of HF4.5 (b) and HF9 (c), (d) photocatalytic CO2 reduction into CH4 under the light irradiation over P25, HF0, HF3, HF4.5, HF6 and HF9 and (e) schematic diagram of TiO2 with overexposed {101} facets, optimal exposed {001} and {101} facets and overexposed {001} facets. Reprinted with permission from Ref. [9]. Copyright 2014, American Chemical Society.

Undoubtedly, both the reduction and oxidation ability of the photocatalyst will be sacrificed [263–265]. Therefore, it is important to overcome this problem in order to prepare highly efficient photocatalyst. In 2013, a direct Z-scheme heterojunction that has similar structure to the conventional type-II heterojunction is reported by our group, aiming at maximizing the redox ability of the photocatalyst [264]. Indeed, the electron-hole migration mechanism of Z-scheme heterojunction photocatalytic system is different from that of conventional heterojunction photocatalytic system [266–268]. Under light irradiation, the photogenerated electrons on semiconductor II will migrate to semiconductor I that has higher reduction potential (see Fig. 28). Thereafter, the photogenerated holes will remain on semiconductor II that has higher oxidation potential, thus resulting in spatial separation of electron-hole pairs. Notably, the electrons and holes respectively accumulate on the semiconductor with higher reduction potential and oxidation potential. Therefore, the redox ability of the Z-scheme heterojunction photocatalyst can be maximized. More importantly, charge separation across the Z-scheme heterojunction is physically more feasible than that across the conventional type-II heterojunction due to the electrostatic force. In the Z-scheme heterojunction, the electrostatic attraction force between the photogenerated elec-

Fig. 28. Schematic illustration for the TiO2 -based Z-scheme heterojunction photocatalyst.

trons on CB of semiconductor II and the photogenerated holes on VB of semiconductor I will facilitate the migration of photogenerated electrons from semiconductor II to semiconductor I. In contrast, in the conventional type-II heterojunction, the electrostatic repulsion force between the photogenerated electrons of semiconductor I and semiconductor II will inhibit the migration of electrons from

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Fig. 29. (a,b) TEM (a) and HRTEM (b) images of ZnFe2 O4 /TiO2 Z-scheme photocatalyst, (c) comparison of the photocatalytic activity of TiO2 , ZnFe2 O4 /TiO2 and ZeFe2 O4 and (d) schematic illustration of the photocatalytic CO2 reduction mechanism of the ZnFe2 O4 /TiO2 Z-scheme photocatalyst. Reprinted with permission from Ref. [270]. Copyright 2015, Elsevier B.V.

semiconductor I to semiconductor II. Therefore, it is obvious that the Z-scheme heterojunction has more potentials for further exploration in achieving more efficient photocatalytic CO2 reduction compared with the conventional type-II heterojunction. For example, Liu et al. reported Si-TiO2 Z-scheme heterojunction for photocatalytic reduction of CO2 [269]. The sample was prepared by a simple hydrothermal method using tetrabutyl titanate and SiO2 as precursors for TiO2 and Si, respectively, followed by the magnesiothermic reduction of SiO2 to Si. According to the photoluminescence spectra, the formation of Z-scheme heterojunction between Si and TiO2 can greatly reduce the electron-hole recombination of Si and TiO2 . Moreover, the specific surface area of Si/TiO2 is also higher than that of pure TiO2 . This is because spherical Si can inhibit the agglomeration of TiO2 nanosheets. As a result, the photocatalytic CO2 reduction activity of the prepared Si/TiO2 is higher than that of the pure Si and TiO2 for CH4 generation. Recently, Song et al. prepared the ZnFe2 O4 /TiO2 Z-scheme photocatalyst through hydrothermal deposition method for photocatalytic CO2 reduction in cyclohexanol under light irradiation [270]. The ZnFe2 O4 NPs were uniformly grown on the surface of TiO2 nanobelts (see Fig. 29a and b), creating large contact interface between TiO2 and ZnFe2 O4 . The large contact interface between TiO2 and ZnFe2 O4 is beneficial for the fast electron-hole migration between TiO2 and ZnFe2 O4 . It was found that the photocatalytic CO2 reduction activity of ZnFe2 O4 /TiO2 is higher than that of pure TiO2 and ZnFe2 O4 due to the formation of the ZnFe2 O4 /TiO2 Z-scheme heterojunction that can greatly reduce electron-hole

recombination (see Fig. 29c). In detail, under light irradiation, the photogenerated electrons on the CB of TiO2 will recombine with the photogenerated holes on the VB of ZnFe2 O4 (see Fig. 29d). Meanwhile, the photogenerated holes on TiO2 and photogenerated electrons on ZnFe2 O4 can be utilized for oxidation and reduction reaction, respectively. Therefore, the spatial separation of the reduction and oxidation site can be achieved in the ZnFe2 O4 /TiO2 Z-scheme photocatalyst.

3.5. Carbon-based material loading As mentioned above, metal loading is one of the most common methods for enhancing the photocatalytic CO2 reduction performance because of its simplicity and effectiveness. However, these metal elements are normally rare and expensive, making them not suitable for wide application [271–274]. Instead, the earth-abundant carbon nanomaterial is known as one of the promising substitutes for these metal elements, since it possesses several extraordinary properties including high electron conductivity, large specific surface area and tunable surface property [275–278]. Moreover, the carbon-based materials are abundant, inexpensive and has good resistance to corrosion for long-term and wide applications [279–282]. Therefore, carbon nanomaterials such as carbon nanotubes (CNTs) and graphene nanosheets have been widely applied for enhancing the photocatalytic CO2 reduction performance of TiO2 .

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Fig. 30. Schematic illustration of the carbon nanotube-modified TiO2 with enhanced photocatalytic activity of CO2 reduction.

Fig. 31. Schematic illustration of the graphene-loaded TiO2 for photocatalytic CO2 reduction.

3.5.1. Carbon nanotube Since the discovery of Carbon nanotube (CNT) by Ijima in 1991 [283], CNTs have attracted wide attention from scientific community due to its wide range of applications in batteries, fuel cells, photocatalysis and so on [284–286]. Moreover, it is well-known for its high electrical conductivity and specific surface area [287]. Therefore, it is one of the most potential candidates for substituting metal elements to enhance the photocatalytic CO2 reduction performance of TiO2 by accepting the photogenerated electrons from TiO2 (see Fig. 30) [288]. For example, Gui et al. reported multi-walled carbon nanotube (MWCNT)-TiO2 composite for photocatalytic CO2 reduction [289]. It was found that the TiO2 NPs can be uniformly coated on the MWCNT to form a core-shell structure which is beneficial to provide more interfacial contact for rapid charge carrier migration. In addition, the prepared MWCNT-TiO2 composite can absorb the whole spectrum of incident light and create heat around TiO2 due to the black color characteristics of MWCNT. This extra heat can boost the electron-hole migration in the composite and thereby enhance the separation of electronhole pairs. As a result, the photocatalytic CO2 reduction activity of MWCNT-TiO2 with the optimal TiO2 : MWCNT ratio of 1: 0.24 is ca. 5 times higher than that of pure anatase TiO2 under power saving light bulb irradiation.

cal conductivity and specific surface area of graphene (see Fig. 31). Notably, the photocatalytic activity of SEG-TiO2 is higher than that of RGO-TiO2 . According to the Raman spectra and HRTEM characterization, this is due to the lower defect density of SEG than that of RGO, which may improve the charge carrier transfer efficiency of SEG-TiO2 . This work shows that reducing the defect density of graphene nanosheets is crucial for optimizing the photocatalytic activity of graphene-TiO2 composite. Tan et al. described the fabrication of graphene oxide-loaded oxygen rich TiO2 (GO-OTiO2 ) through a wet chemical impregnation technique [302]. In detail, the OTiO2 was firstly prepared by a facile aqueous peroxo-titanate route (see Fig. 32a). The prepared OTiO2 exhibited a visible light-induced photocatalytic CO2 reduction activity due to its reduced bandgap via modification of surface oxygen. However, the OTiO2 has very bad photostability, making it not suitable for practical application. Therefore, the GO was loaded onto the TiO2 . After loading GO, the OTiO2 was uniformly dispersed on the surface of GO, allowing rapid migration of charge carriers between GO and TiO2 (see Fig. 32b). More importantly, it was found that the photostability of the OTiO2 was greatly enhanced after the loading of graphene, because the photogenerated electrons can rapidly migrate from OTiO2 to graphene which can prevent the photocorrosion effect of electrons on OTiO2 . As a result, the GOOTiO2 maintained 95.8% reactivity after 6 h of light irradiation (see Fig. 32c). Moreover, the photocatalytic activity of the optimized GO-TiO2 is higher than that of OTiO2 (see Fig. 32d). This work suggests that the loading of GO on TiO2 cannot only improve its photocatalytic activity, but also prevent TiO2 from photocorrosion. Coupling the modified graphene with TiO2 is also an effective strategy for further enhancing the photocatalytic CO2 reduction activity of TiO2 . For example, the coupling of boron-doped graphene (BG) with TiO2 (BG-TiO2 ) was reported by Xing et al. for enhanced photocatalytic CO2 reduction activity [303]. It was revealed that the boron doping can cut graphene nanosheets into smaller size, thereby increasing its specific surface area. Furthermore, the loading of TiO2 onto the BG can further cut BG into highly dispersed nanosheets under ultrasonic condition. Notably, the BG-TiO2 composite has low defect density and excellent conductivity because the ultra-small structure of BG-TiO2 can largely decrease the local density of defects that are generated by substitutional boron doping. Therefore, the high electrical conductivity of graphene can be inherited by the BG-TiO2 composite. Thus, the excited electrons can rapidly migrate from TiO2 to BG, thereby achieving effective electron-hole separation. As a result, the photocatalytic activity of BG-TiO2 is far higher than that of pure TiO2 for CH4 production under solar irradiation. This work shows that the modified graphene has extra advantages for enhancing the photocatalytic CO2 reduction performance of TiO2 . Therefore, the

3.5.2. Graphene Graphene is an exciting 2D-carbon material. Since the pioneering reports of 2D graphene nanosheet by Geim and Novoselov in 2004 [290], graphene has been extensively studied in various applications including solar cell, supercapacitor, fuel cell and photocatalysis, due to its ultra-large theoretical specific surface area, high electrical conductivity and good resistance to corrosion [291–295]. Therefore, it is not surprising that graphene has also been applied in photocatalytic CO2 reduction. Generally, the loading of graphene can enhance the photocatalytic CO2 reduction performance of TiO2 from 5 aspects, which are (1) accelerating the electron-hole separation, (2) improving the specific surface area, (3) enhancing the CO2 adsorption through ␲-␲ conjugation between CO2 molecules and graphene, (4) activating the CO2 molecules for reduction reaction and (5) enhancing the light utilization ability [296–299]. For example, Liang et al. conceptually reported the graphene-coupled TiO2 for photocatalytic CO2 reduction [300,301]. It was found that the surface defects of graphene have great effect on its photocatalytic activity. Particularly, two different methods were carried out to prepare the graphene-TiO2 composite, which are solvent-exfoliated graphene (SEG) coupled TiO2 (SEG-TiO2 ) and reduced graphene oxide (RGO) coupled TiO2 (RGO-TiO2 ). Both the SEG-TiO2 and RGO-TiO2 exhibited enhanced photocatalytic CO2 reduction activity towards CH4 production, due to the high electri-

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Fig. 32. (a) Schematic illustration of the synthesis procedure of OTiO2 , (b) SEM image of the GO-OTiO2 , (c) photostability of the prepared samples for photocatalytic CO2 reduction and (d) comparison of the photocatalytic CO2 reduction performance of GO-OTiO2 with different weight percentage of GO. Reprinted with permission from Ref. [302]. Copyright 2015, Elsevier B.V.

modification of graphene/TiO2 deserves more attention from scientific community in the future. On the whole, the above mentioned works have demonstrated the remarkable prospect of graphene in enhancing the photocatalytic CO2 reduction performance of TiO2 . However, the study of graphene in the field of photocatalytic CO2 reduction is still at the primary stage. Therefore, more studies in this field is urgently needed to fully explore the potential of graphene for enhancing the photocatalytic CO2 reduction performance of TiO2 .

Apparently, the modification of TiO2 has been rapidly growing with recent combining efforts of both chemical and material scientists. However, enormous challenges remain to be solved before TiO2 can be practically used as an efficient photocatalyst to utilize sunlight and CO2 for energy production. More interesting and breakthrough discoveries are highly necessary for this prospective technology. In order to achieve the benchmark photoconversion efficiency of 10% set by the US Department of Energy for commercialization [304], future efforts of modifying TiO2 for more efficient photocatalytic CO2 reduction can be focused on the following aspects:

4. Future perspectives and summary In this review, recent progress of TiO2 modification for photocatalytic CO2 reduction has been systematically summarized.

(1) There are both advantages and disadvantages for each of these TiO2 modification techniques (see Table 2). Fortunately, the disadvantages of these modification methods can be well over-

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Table 2 Comparison of the advantages and disadvantages of the different modification methods for TiO2 .

Impurity doping Metal deposition Alkali modification Heterojunction construction − Type-II heterojunction − P-N junction − Surface heterojunction − Z-scheme heterojunction

Carbon-based material loading

Advantages

Disadvantages

− Reduce bandgap − Reduce particle size − Enhance electron-hole separation − Enhance CO2 adsorption

− Introduce defects − Expensive − Forbid the light absorption of the TiO2

− Enhance electron-hole separation − Spatially separate the reduction and oxidation site

− Reduce the reduction and oxidation potential of photogenerated electrons and holes

− Enhance electron-hole separation − Spatially separate the reduction and oxidation site − Optimize the reduction and oxidation potential of photogenerated electrons and holes − Enhance electron-hole separation − Enhance the concentration of electron on the TiO2 surface − Enhance CO2 adsorption through ␲-␲ conjugation

− Controversy on the actual charge carrier migration pathway

come or inhibited by more ingenious design and engineering based on TiO2 . Combining different modification methods can be expected to produce more efficient TiO2 -based photocatalysts. Further research into the combination of these different TiO2 modification techniques can lead to a deeper understanding of the function of these methods, which can in turn have great impact on the development of advanced photocatalysts for CO2 reduction. (2) Up to now, the investigations of photocatalytic CO2 reduction focus mainly on the overall efficiency of photocatalytic reaction. Detailed study on the photocatalytic CO2 reduction mechanism on the TiO2 surface remains scarce and should be paid more attention for further exploitation of TiO2 modification. Combining the time-resolved and space-resolved spectroscopy can be an ideal strategy for investigating the generation, trapping and migration of charge carriers. Moreover, materials simulation by first-principle density functional theory is also an effective way to determine the mechanism of photocatalytic CO2 reduction. Equipped with such a comprehensive knowledge of photocatalytic CO2 reduction mechanism, the fabrication of more efficient TiO2 -based photocatalyst for CO2 reduction can be achieved. (3) The recyclability of the surface-modified TiO2 for photocatalytic CO2 reduction is also crucial for its wide-range practical application. A photocatalyst exhibiting very high photoconversion performance will still be considered impractical if its longterm stability is too low. Therefore, in order to achieve practical application of TiO2 in photocatalytic CO2 reduction, there is an urgent need to develop a TiO2 -based photocatalyst with both high efficiency and stability via surface modification. (4) It is well known that multiple products can be obtained by photocatalytic CO2 reduction. Particularly, a good selectivity of photocatalytic CO2 reduction towards specific hydrocarbon fuel formation is highly preferred for facilitating the subsequent separation process. Since both the redox potential and the surface electron density play the critical roles in determining the final product of photocatalytic CO2 reduction, tuning the band structure or accumulating the photogenerated electrons through TiO2 modification can be a potential method to control its selectivity of photocatalytic CO2 reduction. In summary, despite the previous reports, the possibility of TiO2 modification for more efficient photocatalytic CO2 reduction apparently has not been fully explored. It should be realized that both opportunities and challenges are present in future development of this promising technology. Hopefully, this review will stimulate further development of TiO2 modification for achieving its

− Forbid the light absorption of TiO2

wide-range practical application of high efficient photocatalytic CO2 reduction. Acknowledgments This work was partially supported by the 973 program (2013CB632402), NSFC (51320105001, 51372190, 21573170, 51272199 and 21433007), NSFHB (No. 2015CFA001), Selfdetermined and Innovative Research Funds of SKLWUT (2015ZD-1), Research Foundation of SKLWUT (2015-KF-16) and Fundamental Research Funds for the Central Universities (WUT: 2015-III-034). References [1] National Aeronautics and Space Administration (NASA) USA, Climate Change: Vital Signs of the Planet: Carbon Dioxide, 2016, http://climate.nasa. gov/vital-signs/carbon-dioxide/ (accessed 13.09.16). [2] H. Zhou, P. Li, J. Liu, Z. Chen, L. Liu, D. Dontsova, R. Yan, T. Fan, D. Zhang, J. Ye, Biomimetic polymeric semiconductor based hybrid nanosystems for artificial photosynthesis towards solar fuels generation via CO2 reduction, Nano Energy 25 (2016) 128–135. [3] L.-L. Tan, W.-J. Ong, S.-P. Chai, A.R. Mohamed, Visible-light-activated oxygen-rich TiO2 as next generation photocatalyst: Importance of annealing temperature on the photoactivity toward reduction of carbon dioxide, Chem. Eng. J. 283 (2016) 1254–1263. [4] H.G. Baldoví, S.t. Neat¸u, A. Khan, A.M. Asiri, S.A. Kosa, H. Garcia, Understanding the origin of the photocatalytic CO2 reduction by Au-and Cu-loaded TiO2 : a microsecond transient absorption spectroscopy study, J. Phys. Chem. C 119 (2015) 6819–6827. [5] S. Qamar, F. Lei, L. Liang, S. Gao, K. Liu, Y. Sun, W. Ni, Y. Xie, Ultrathin TiO2 flakes optimizing solar light driven CO2 reduction, Nano Energy 26 (2016) 692–698. [6] T.-M. Su, Z.-Z. Qin, H.-B. Ji, Y.-X. Jiang, G. Huang, Recent advances in the photocatalytic reduction of carbon dioxide, Environ. Chem. Lett. 14 (2016) 99–112. [7] B. Han, W. Wei, L. Chang, P. Cheng, Y.H. Hu, Efficient visible light photocatalytic CO2 reforming of CH4 , ACS Catal. 6 (2015) 494–497. [8] S. Bai, W. Yin, L. Wang, Z. Li, Y. Xiong, Surface and interface design of cocatalysts toward photocatalytic water splitting and CO2 reduction, RSC Adv. 6 (2016) 57446–57463. [9] J.G. Yu, J.X. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2 -reduction activity of anatase TiO2 by coexposed {001} and {101} facets, J. Am. Chem. Soc. 136 (2014) 8839–8842. [10] Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han, C. Li, Titanium dioxide-based nanomaterials for photocatalytic fuel generations, Chem. Rev. 114 (2014) 9987–10043. [11] F. Fresno, R. Portela, S. Suárez, J.M. Coronado, Photocatalytic materials: recent achievements and near future trends, J. Mater. Chem. A 2 (2014) 2863–2884. [12] I. Ganesh, Conversion of carbon dioxide into methanol-a potential liquid fuel: fundamental challenges and opportunities (a review), Renewable Sustainable Energy Rev. 31 (2014) 221–257. [13] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Photocatalytic reduction of CO2 on TiO2 and other semiconductors, Angew. Chem. Int. Ed. 52 (2013) 7372–7408.

J. Low et al. / Applied Surface Science 392 (2017) 658–686 [14] F. Chen, Y. Li, Z. Liu, P. Fang, Facile synthesis of TiO2 /trititanate heterostructure with enhanced photoelectric efficiency for an improved photocatalysis, Appl. Surf. Sci. 341 (2015) 55–60. [15] L. Yuan, C. Han, M. Pagliaro, Y.-J. Xu, Origin of enhancing the photocatalytic performance of TiO2 for artificial photoreduction of CO2 through a SiO2 coating strategy, J. Phys. Chem. C 120 (2015) 265–273. [16] Y. Izumi, Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond, Coord. Chem. Rev. 257 (2013) 171–186. [17] S. Ye, R. Wang, M.-Z. Wu, Y.-P. Yuan, A review on g-C3 N4 for photocatalytic water splitting and CO2 reduction, Appl. Surf. Sci. 358 (2015) 15–27. [18] J. Wang, C. Huang, X. Chen, H. Zhang, Z. Li, Z. Zou, Photocatalytic CO2 reduction of BaCeO3 with 4f configuration electrons, Appl. Surf. Sci. 358 (2015) 463–467. [19] M. Marszewski, S.W. Cao, J.G. Yu, M. Jaroniec, Semiconductor-based photocatalytic CO2 conversion, Mater. Horiz. 2 (2015) 261–278. [20] S. Protti, A. Albini, N. Serpone, Photocatalytic generation of solar fuels from the reduction of H2 O and CO2 : a look at the patent literature, Phys. Chem. Chem. Phys. 16 (2014) 19790–19827. [21] H. Peng, J. Lu, C. Wu, Z. Yang, H. Chen, W. Song, P. Li, H. Yin, Co-doped MoS2 NPs with matched energy band and low overpotential high efficiently convert CO2 to methanol, Appl. Surf. Sci. 353 (2015) 1003–1012. [22] E.V. Kondratenko, G. Mul, J. Baltrusaitis, G.O. Larrazábal, J. Pérez-Ramírez, Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes, Energy Environ. Sci. 6 (2013) 3112–3135. [23] A. Dhakshinamoorthy, S. Navalon, A. Corma, H. Garcia, Photocatalytic CO2 reduction by TiO2 and related titanium containing solids, Energy Environ. Sci. 5 (2012) 9217–9233. [24] L. Yuan, Y.-J. Xu, Photocatalytic conversion of CO2 into value-added and renewable fuels, Appl. Surf. Sci. 342 (2015) 154–167. [25] W. Dai, H. Xu, J. Yu, X. Hu, X. Luo, X. Tu, L. Yang, Photocatalytic reduction of CO2 into methanol and ethanol over conducting polymers modified Bi2 WO6 microspheres under visible light, Appl. Surf. Sci. 356 (2015) 173–180. [26] B.S. Kwak, M. Kang, Photocatalytic reduction of CO2 with H2 O using perovskite Cax Tiy O3 , Appl. Surf. Sci. 337 (2015) 138–144. [27] S.W. Jo, B.S. Kwak, K.M. Kim, J.Y. Do, N.-K. Park, S.O. Ryu, H.-J. Ryu, J.-I. Baek, M. Kang, Effectively CO2 photoreduction to CH4 by the synergistic effects of Ca and Ti on Ca-loaded TiSiMCM-41 mesoporous photocatalytic systems, Appl. Surf. Sci. 355 (2015) 891–901. [28] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Photoelectrocatalytic reduction of carbon dioxide in aqueous suspensions of semiconductor powders, Nature 277 (1979) 637–638. [29] Q.L. Xu, J.G. Yu, J. Zhang, J.F. Zhang, G. Liu, Cubic anatase TiO2 nanocrystals with enhanced photocatalytic CO2 reduction activity, Chem. Commun. 51 (2015) 7950–7953. [30] L.C. Sim, K.H. Leong, P. Saravanan, S. Ibrahim, Rapid thermal reduced graphene oxide/Pt-TiO2 nanotube arrays for enhanced visible-light-driven photocatalytic reduction of CO2 , Appl. Surf. Sci. 358 (2015) 122–129. [31] Y. Li, W.-N. Wang, Z. Zhan, M.-H. Woo, C.-Y. Wu, P. Biswas, Photocatalytic reduction of CO2 with H2 O on mesoporous silica supported Cu/TiO2 catalysts, Appl. Catal. B 100 (2010) 386–392. [32] T. Wang, X. Meng, P. Li, S. Ouyang, K. Chang, G. Liu, Z. Mei, J. Ye, Photoreduction of CO2 over the well-crystallized ordered mesoporous TiO2 with the confined space effect, Nano Energy 9 (2014) 50–60. [33] L. Wang, T. Sasaki, Titanium oxide nanosheets: graphene analogues with versatile functionalities, Chem. Rev. 114 (2014) 9455–9486. [34] J.G. Yu, J. Jin, B. Cheng, M. Jaroniec, A noble metal-free reduced graphene oxide-CdS nanorod composite for the enhanced visible-light photocatalytic reduction of CO2 to solar fuel, J. Mater. Chem. A 2 (2014) 3407–3416. [35] J. Jin, J.G. Yu, D.P. Guo, C. Cui, W.K. Ho, A hierarchical Z-scheme CdS-WO3 photocatalyst with enhanced CO2 reduction activity, Small 11 (2015) 5262–5271. [36] S. Ijaz, M.F. Ehsan, M.N. Ashiq, N. Karamat, T. He, Preparation of CdS@CeO2 core/shell composite for photocatalytic reduction of CO2 under visible-light irradiation, Appl. Surf. Sci. 390 (2016) 550–559. [37] J. Wang, L. Zhang, W. Fang, J. Ren, Y. Li, H. Yao, J. Wang, Z. Li, Enhanced photoreduction CO2 activity over direct Z-scheme ␣-Fe2 O3 /Cu2 O heterostructures under visible light irradiation, ACS Appl. Mater. Interfaces 7 (2015) 8631–8639. [38] P. Li, H. Jing, J. Xu, C. Wu, H. Peng, J. Lu, F. Lu, High-efficiency synergistic conversion of CO2 to methanol using Fe2 O3 nanotubes modified with double-layer Cu2 O spheres, Nanoscale 6 (2014) 11380–11386. [39] J.G. Yu, K. Wang, W. Xiao, B. Cheng, Photocatalytic reduction of CO2 into hydrocarbon solar fuels over g-C3 N4 -Pt nanocomposite photocatalysts, Phys. Chem. Chem. Phys. 16 (2014) 11492–11501. [40] K. Wang, Q. Li, B.S. Liu, B. Cheng, W.K. Ho, J.G. Yu, Sulfur-doped g-C3 N4 with enhanced photocatalytic CO2 -reduction performance, Appl. Catal. B 176 (2015) 44–52. [41] W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, Heterojunction engineering of graphitic carbon nitride (g-C3 N4 ) via Pt loading with improved daylight-induced photocatalytic reduction of carbon dioxide to methane, Dalton Trans. 44 (2015) 1249–1257. [42] W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, A.R. Mohamed, Surface charge modification via protonation of graphitic carbon nitride (gC3 N4 ) for electrostatic self-assembly construction of 2D/2D reduced graphene oxide

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59] [60]

[61]

[62]

[63]

[64] [65]

[66]

[67]

[68]

681

(rGO)/gC3 N4 nanostructures toward enhanced photocatalytic reduction of carbon dioxide to methane, Nano Energy 13 (2015) 757–770. W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Graphitic carbon nitride (g-C3 N4 )-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159–7329. W.-J. Ong, L.K. Putri, L.-L. Tan, S.-P. Chai, S.-T. Yong, Heterostructured AgX/g-C3 N4 (X = Cl and Br) nanocomposites via a sonication-assisted deposition-precipitation approach: emerging role of halide ions in the synergistic photocatalytic reduction of carbon dioxide, Appl. Catal. B 180 (2016) 530–543. S. Murcia-López, V. Vaiano, M. Hidalgo, J. Navío, D. Sannino, Photocatalytic reduction of CO2 over platinised Bi2 WO6 -based materials, Photochem. Photobiol. Sci. 14 (2015) 678–685. M. Li, L. Zhang, X. Fan, Y. Zhou, M. Wu, J. Shi, Highly selective CO2 photoreduction to CO over g-C3 N4 /Bi2 WO6 composites under visible light, J. Mater. Chem. A 3 (2015) 5189–5196. F. Li, L. Zhang, J. Tong, Y. Liu, S. Xu, Y. Cao, S. Cao, Photocatalytic CO2 conversion to methanol by Cu2 O/graphene/TNA heterostructure catalyst in a visible-light-driven dual-chamber reactor, Nano Energy 27 (2016) 320–329. H. Xu, S. Ouyang, L. Liu, D. Wang, T. Kako, J. Ye, Porous-structured Cu2 O/TiO2 nanojunction material toward efficient CO2 photoreduction, Nanotechnology 25 (2014) 165402–165409. W. Jiang, X. Yin, F. Xin, Y. Bi, Y. Liu, X. Li, Preparation of CdIn2 S4 microspheres and application for photocatalytic reduction of carbon dioxide, Appl. Surf. Sci. 288 (2014) 138–142. Z. Sun, Z. Yang, H. Liu, H. Wang, Z. Wu, Visible-light CO2 photocatalytic reduction performance of ball-flower-like Bi2 WO6 synthesized without organic precursor: effect of post-calcination and water vapor, Appl. Surf. Sci. 315 (2014) 360–367. Y.-X. Pan, Z.-Q. Sun, H.-P. Cong, Y.-L. Men, S. Xin, J. Song, S.-H. Yu, Photocatalytic CO2 reduction highly enhanced by oxygen vacancies on Pt-nanoparticle-dispersed gallium oxide, Nano Res. 9 (2016) 1689–1700. P. Li, H. Xu, L. Liu, T. Kako, N. Umezawa, H. Abe, J. Ye, Constructing cubic-orthorhombic surface-phase junctions of NaNbO3 towards significant enhancement of CO2 photoreduction, J. Mater. Chem. A 2 (2014) 5606–5609. L. Zhang, W. Wang, D. Jiang, E. Gao, S. Sun, Photoreduction of CO2 on BiOCl nanoplates with the assistance of photoinduced oxygen vacancies, Nano Res. 8 (2015) 821–831. B. AlOtaibi, S. Fan, D. Wang, J. Ye, Z. Mi, Wafer-level artificial photosynthesis for CO2 reduction into CH4 and CO using GaN nanowires, ACS Catal. 5 (2015) 5342–5348. M.S. Akple, J.X. Low, Z.Y. Qin, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, S.W. Liu, Nitrogen-doped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction, Chin. J. Catal. 36 (2015) 2127–2134. J.W. Fu, S.W. Cao, J.G. Yu, J.X. Low, Y.P. Lei, Enhanced photocatalytic CO2 -reduction activity of electrospun mesoporous TiO2 nanofibers by solvothermal treatment, Dalton Trans. 43 (2014) 9158–9165. M. Tahir, B. Tahir, N.A.S. Amin, Gold-nanoparticle-modified TiO2 nanowires for plasmon-enhanced photocatalytic CO2 reduction with H2 under visible light irradiation, Appl. Surf. Sci. 356 (2015) 1289–1299. M. Tahir, B. Tahir, Dynamic photocatalytic reduction of CO2 to CO in a honeycomb monolith reactor loaded with Cu and N doped TiO2 nanocatalysts, Appl. Surf. Sci. 377 (2016) 244–252. P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. J.B. Joo, Q. Zhang, M. Dahl, I. Lee, J. Goebl, F. Zaera, Y. Yin, Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity, Energy Environ. Sci. 5 (2012) 6321–6327. S. Rani, N. Bao, S.C. Roy, Solar spectrum photocatalytic conversion of CO2 and water vapor into hydrocarbons using TiO2 nanoparticle membranes, Appl. Surf. Sci. 289 (2014) 203–208. D. Kong, J.Z.Y. Tan, F. Yang, J. Zeng, X. Zhang, Electrodeposited Ag nanoparticles on TiO2 nanorods for enhanced UV visible light photoreduction CO2 to CH4 , Appl. Surf. Sci. 277 (2013) 105–110. Y.-F. Li, Z.-P. Liu, Particle size, shape and activity for photocatalysis on titania anatase nanoparticles in aqueous surroundings, J. Am. Chem. Soc. 133 (2011) 15743–15752. N. Serpone, A. Emeline, Semiconductor photocatalysis past present, and future outlook, J. Phys. Chem. Lett. 3 (2012) 673–677. J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chin. J. Catal. 36 (2015) 2049–2070. W.J. Ong, L.L. Tan, S.P. Chai, S.T. Yong, A.R. Mohamed, Facet-dependent photocatalytic properties of TiO2 -based composites for energy conversion and environmental remediation, ChemSusChem 7 (2014) 690–719. I. Levchuk, M. Sillanpää, C. Guillard, D. Gregori, D. Chateau, S. Parola, TiO2 /SiO2 porous composite thin films: role of TiO2 areal loading and modification with gold nanospheres on the photocatalytic activity, Appl. Surf. Sci. 383 (2016) 367–374. M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S. Dunlop, J.W. Hamilton, J.A. Byrne, K. O’shea, A review on the visible light active titanium dioxide photocatalysts for environmental applications, Appl. Catal. B 125 (2012) 331–349.

682

J. Low et al. / Applied Surface Science 392 (2017) 658–686

[69] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241. [70] C.u. Gomes Silva, R. Juárez, T. Marino, R. Molinari, H. Garci´ıa, Influence of excitation wavelength (UV or visible light) on the photocatalytic activity of titania containing gold nanoparticles for the generation of hydrogen or oxygen from water, J. Am. Chem. Soc 133 (2010) 595–602. [71] S. Bingham, W.A. Daoud, Recent advances in making nano-sized TiO2 visible-light active through rare-earth metal doping, J. Mater. Chem. 21 (2011) 2041–2050. [72] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986. [73] X. Pan, M.-Q. Yang, X. Fu, N. Zhang, Y.-J. Xu, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013) 3601–3614. [74] A. Di Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211 (2012) 3–29. [75] M. Kapilashrami, Y. Zhang, Y.-S. Liu, A. Hagfeldt, J. Guo, Probing the optical property and electronic structure of TiO2 nanomaterials for renewable energy applications, Chem. Rev. 114 (2014) 9662–9707. [76] Z. He, J. Tang, J. Shen, J. Chen, S. Song, Enhancement of photocatalytic reduction of CO2 to CH4 over TiO2 nanosheets by modifying with sulfuric acid, Appl. Surf. Sci. 364 (2016) 416–427. [77] G. Liu, N. Hoivik, K. Wang, H. Jakobsen, Engineering TiO2 nanomaterials for CO2 conversion/solar fuels, Sol. Energy Mater. Sol. Cells 105 (2012) 53–68. [78] H. Park, Y. Park, W. Kim, W. Choi, Surface modification of TiO2 photocatalyst for environmental applications, J. Photochem. Photobiol. C 15 (2013) 1–20. [79] W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, A.R. Mohamed, Highly reactive {001} facets of TiO2 -based composites: synthesis, formation mechanism and characterization, Nanoscale 6 (2014) 1946–2008. [80] H. Xu, S. Ouyang, L. Liu, P. Reunchan, N. Umezawa, J. Ye, Recent advances in TiO2 -based photocatalysis, J. Mater. Chem. A 2 (2014) 12642–12661. [81] S. Navalón, A. Dhakshinamoorthy, M. Álvaro, H. Garcia, Photocatalytic CO2 reduction using non-titanium metal oxides and sulfides, ChemSusChem 6 (2013) 562–577. ´ Photocatalytic reduction of CO2 over TiO2 based [82] K. Koˇcí, L. Obalova, Z. Lacny, catalysts, Chem. Pap. 62 (2008) 1–9. [83] A. Corma, H. Garcia, Photocatalytic reduction of CO2 for fuel production: possibilities and challenges, J. Catal. 308 (2013) 168–175. [84] B. Yu, Y. Zhou, P. Li, W. Tu, P. Li, L. Tang, J. Ye, Z. Zou, Photocatalytic reduction of CO2 over Ag/TiO2 nanocomposites prepared with a simple and rapid silver mirror method, Nanoscale 8 (2016) 11870–11874. [85] G. Dey, Chemical reduction of CO2 to different products during photo catalytic reaction on TiO2 under diverse conditions: an overview, J. Nat. Gas Chem. 16 (2007) 217–226. [86] K. Li, X. An, K.H. Park, M. Khraisheh, J. Tang, A critical review of CO2 photoconversion: catalysts and reactors, Catal. Today 224 (2014) 3–12. [87] J. Mao, K. Li, T. Peng, Recent advances in the photocatalytic CO2 reduction over semiconductors, Catal. Sci. Tech. 3 (2013) 2481–2498. [88] A. Sarkar, E. Gracia-Espino, T. Wågberg, A. Shchukarev, M. Mohl, A.-R. Rautio, O. Pitkänen, T. Sharifi, K. Kordas, J.-P. Mikkola, Photocatalytic reduction of CO2 with H2 O over modified TiO2 nanofibers: understanding the reduction pathway, Nano Res. 9 (2016) 1–13. [89] J. Ângelo, P. Magalhães, L. Andrade, A. Mendes, Characterization of TiO2 -based semiconductors for photocatalysis by electrochemical impedance spectroscopy, Appl. Surf. Sci. 387 (2016) 183–189. [90] J. Wu, H. Lu, X. Zhang, F. Raziq, Y. Qu, L. Jing, Enhanced charge separation of rutile TiO2 nanorods by trapping holes and transferring electrons for efficient cocatalyst-free photocatalytic conversion of CO2 to fuels, Chem. Commun. 52 (2016) 5027–5029. [91] B.-Y. Lan, H.-F. Shi, Review of systems for photocatalytic conversion of CO2 to hydrocarbon fuels, Acta Phys. Chim. Sin. 30 (2014) 2177–2196. [92] L. Liu, C. Zhao, J. Xu, Y. Li, Integrated CO2 capture and photocatalytic conversion by a hybrid adsorbent/photocatalyst material, Appl. Catal. B 179 (2015) 489–499. ˇ ´ J. Jirkovsky, ´ O. Solcová, [93] K. Koˇcí, L. Obalová, L. Matˇejová, D. Plachá, Z. Lacny, Effect of TiO2 particle size on the photocatalytic reduction of CO2 , Appl. Catal. B 89 (2009) 494–502. [94] X. Li, H. Liu, D. Luo, J. Li, Y. Huang, H. Li, Y. Fang, Y. Xu, L. Zhu, Adsorption of CO2 on heterostructure CdS (Bi2 S3 )/TiO2 nanotube photocatalysts and their photocatalytic activities in the reduction of CO2 to methanol under visible light irradiation, Chem. Eng. J. 180 (2012) 151–158. [95] L. Mino, G. Spoto, A.M. Ferrari, CO2 capture by TiO2 anatase surfaces: a combined DFT and FTIR study, J. Phys. Chem. C 118 (2014) 25016–25026. [96] S. Krischok, O. Höfft, V. Kempter, The chemisorption of H2 O and CO2 on TiO2 surfaces: studies with MIES and UPS (HeI/II), Surf. Sci. 507 (2002) 69–73. [97] J. Kapica-Kozar, E. Kusiak-Nejman, A. Wanag, Ł. Kowalczyk, R.J. Wrobel, S. Mozia, A.W. Morawski, Alkali-treated titanium dioxide as adsorbent for CO2 capture from air, Microporous Mesoporous Mat. 202 (2015) 241–249. [98] W. Tu, Y. Zhou, Z. Zou, Photocatalytic conversion of CO2 into renewable hydrocarbon duels: state-of-the-art accomplishment, challenges, and prospects, Adv. Mater. 26 (2014) 4607–4626.

[99] M. Tahir, N.S. Amin, Advances in visible light responsive titanium oxide-based photocatalysts for CO2 conversion to hydrocarbon fuels, Energy Convers. Manage. 76 (2013) 194–214. [100] J.Z. Tan, Y. Fernández, D. Liu, M. Maroto-Valer, J. Bian, X. Zhang, Photoreduction of CO2 using copper-decorated TiO2 nanorod films with localized surface plasmon behavior, Chem. Phys. Lett. 531 (2012) 149–154. [101] B.D. Mankidy, B. Joseph, V.K. Gupta, Photo-conversion of CO2 using titanium dioxide: enhancements by plasmonic and co-catalytic nanoparticles, Nanotechnology 24 (2013) 405402–405409. [102] Y.-P. Yuan, L.-W. Ruan, J. Barber, S.C.J. Loo, C. Xue, Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion, Energy Environ. Sci. 7 (2014) 3934–3951. [103] S. Farsinezhad, H. Sharma, K. Shankar, Interfacial band alignment for photocatalytic charge separation in TiO2 nanotube arrays coated with CuPt nanoparticles, Phys. Chem. Chem. Phys. 17 (2015) 29723–29733. [104] R. Marschall, Semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity, Adv. Funct. Mater. 24 (2014) 2421–2440. [105] C.C. Mercado, F.J. Knorr, J.L. McHale, S.M. Usmani, A.S. Ichimura, L.V. Saraf, Location of hole and electron traps on nanocrystalline anatase TiO2 , J. Phys. Chem. C 116 (2012) 10796–10804. [106] H.H. Mohamed, D.W. Bahnemann, The role of electron transfer in photocatalysis: fact and fictions, Appl. Catal. B 128 (2012) 91–104. [107] J.X. Low, J.G. Yu, W.K. Ho, Graphene-based photocatalysts for CO2 reduction to solar fuel, J. Phys. Chem. Lett. 6 (2015) 4244–4251. [108] J.X. Low, B. Cheng, J.G. Yu, M. Jaroniec, Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels, Energy Storage Mater. 3 (2015) 24–35. [109] X. Li, J.Q. Wen, J.X. Low, Y.P. Fang, J.G. Yu, Design and fabrication of semiconductor photocatalyst for photocatalytic reduction of CO2 to solar fuel, Sci. China Mater. 57 (2014) 70–100. [110] C.D. Windle, R.N. Perutz, Advances in molecular photocatalytic and electrocatalytic CO2 reduction, Coord. Chem. Rev. 256 (2012) 2562–2570. [111] L. Liu, Y. Li, Understanding the reaction mechanism of photocatalytic reduction of CO2 with H2 O on TiO2 -based photocatalysts: a review, Aerosol Air Qual. Res. 14 (2014) 453–469. [112] T. Hirakawa, P.V. Kamat, Charge separation and catalytic activity of Ag@TiO2 core-shell composite clusters under UV-irradiation, J. Am. Chem. Soc. 127 (2005) 3928–3934. [113] A. Cybula, M. Klein, A. Zaleska, Methane formation over TiO2 -based photocatalysts: reaction pathways, Appl. Catal. B 164 (2015) 433–442. [114] C. Wang, X. Zhang, Y. Liu, Promotion of multi-electron transfer for enhanced photocatalysis: a review focused on oxygen reduction reaction, Appl. Surf. Sci. 358 (2015) 28–45. [115] A.J. Cowan, J.R. Durrant, Long-lived charge separated states in nanostructured semiconductor photoelectrodes for the production of solar fuels, Chem. Soc. Rev. 42 (2013) 2281–2293. [116] V. Artero, M. Fontecave, Solar fuels generation and molecular systems: is it homogeneous or heterogeneous catalysis? Chem. Soc. Rev. 42 (2013) 2338–2356. [117] M.M. Khan, S.A. Ansari, D. Pradhan, M.O. Ansari, J. Lee, M.H. Cho, Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies, J. Mater. Chem. A 2 (2014) 637–644. [118] Z.H. Xu, J.G. Yu, Visible-light-induced photoelectrochemical behaviors of Fe-modified TiO2 nanotube arrays, Nanoscale 3 (2011) 3138–3144. [119] Q. Deng, X. Xia, M. Guo, Y. Gao, G. Shao, Mn-doped TiO2 nanopowders with remarkable visible light photocatalytic activity, Mater. Lett. 65 (2011) 2051–2054. [120] Y. Deng, L. Tang, G. Zeng, H. Dong, M. Yan, J. Wang, W. Hu, J. Wang, Y. Zhou, J. Tang, Enhanced visible light photocatalytic performance of polyaniline modified mesoporous single crystal TiO2 microsphere, Appl. Surf. Sci. 387 (2016) 882–893. [121] E. Shin, S. Jin, J. Kim, S.-J. Chang, B.-H. Jun, K.-W. Park, J. Hong, Preparation of K-doped TiO2 nanostructures by wet corrosion and their sunlight-driven photocatalytic performance, Appl. Surf. Sci. 379 (2016) 33–38. [122] E.M. Samsudin, S.B.A. Hamid, J.C. Juan, W.J. Basirun, G. Centi, Synergetic effects in novel hydrogenated F-doped TiO2 photocatalysts, Appl. Surf. Sci. 370 (2016) 380–393. [123] M.H.N. Assadi, D.A. Hanaor, The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO2 : A theoretical study, Appl. Surf. Sci. 387 (2016) 682–689. [124] S. Das, W.W. Daud, A review on advances in photocatalysts towards CO2 conversion, RSC Adv. 4 (2014) 20856–20893. [125] S. Das, W.W. Daud, Photocatalytic CO2 transformation into fuel: a review on advances in photocatalyst and photoreactor, Renewable Sustainable Energy Rev. 39 (2014) 765–805. [126] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M. Chi, Y. Yin, A highly active titanium dioxide based visible-light photocatalyst with nonmetal doping and plasmonic metal decoration, Angew. Chem. Int. Ed. 123 (2011) 7226–7230. [127] C. Han, M. Pelaez, V. Likodimos, A.G. Kontos, P. Falaras, K. O’Shea, D.D. Dionysiou, Innovative visible light-activated sulfur doped TiO2 films for water treatment, Appl. Catal. B 107 (2011) 77–87. [128] M. Khan, J. Xu, N. Chen, W. Cao, First principle calculations of the electronic and optical properties of pure and (Mo, N) co-doped anatase TiO2 , J. Alloys Compd. 513 (2012) 539–545.

J. Low et al. / Applied Surface Science 392 (2017) 658–686 [129] V. Chandraboss, J. Kamalakkannan, S. Senthilvelan, Synthesis of activated charcoal supported Bi-doped TiO2 nanocomposite under solar light irradiation for enhanced photocatalytic activity, Appl. Surf. Sci. 387 (2016) 944–956. [130] T. Wang, X. Meng, G. Liu, K. Chang, P. Li, Q. Kang, L. Liu, M. Li, S. Ouyang, J. Ye, In situ synthesis of ordered mesoporous Co-doped TiO2 and its enhanced photocatalytic activity and selectivity for the reduction of CO2 , J. Mater. Chem. A 3 (2015) 9491–9501. [131] Y. Gong, C. Fu, L. Ting, J. Chenu, Q. Zhao, C. Li, Exploring the effect of boron and tantalum codoping on the enhanced photocatalytic activity of TiO2 , Appl. Surf. Sci. 351 (2015) 746–752. [132] X. Lei, X. Xue, H. Yang, C. Chen, X. Li, M. Niu, X. Gao, Y. Yang, Effect of calcination temperature on the structure and visible-light photocatalytic activities of (N, S and C) co-doped TiO2 nano-materials, Appl. Surf. Sci. 332 (2015) 172–180. [133] L. Liu, F. Gao, H. Zhao, Y. Li, Tailoring Cu valence and oxygen vacancy in Cu/TiO2 catalysts for enhanced CO2 photoreduction efficiency, Appl. Catal. B 134 (2013) 349–358. [134] Q. Zhang, T. Gao, J.M. Andino, Y. Li, Copper and iodine co-modified TiO2 nanoparticles for improved activity of CO2 photoreduction with water vapor, Appl. Catal. B 123 (2012) 257–264. [135] C. Li, F. Xue, E. Ding, X. He, Glutaraldehyde assisted synthesis of collagen derivative modified Fe3+ /TiO2 nanocomposite and their enhanced photocatalytic activity, Appl. Surf. Sci. 356 (2015) 852–861. [136] Slamet, H.W. Nasution, E. Purnama, S. Kosela, J. Gunlazuardi, Photocatalytic reduction of CO2 on copper-doped titania catalysts prepared by improved-impregnation method, Catal. Commun. 6 (2005) 313–319. [137] Y. Liu, S. Zhou, J. Li, Y. Wang, G. Jiang, Z. Zhao, B. Liu, X. Gong, A. Duan, J. Liu, Photocatalytic reduction of CO2 with water vapor on surface La-modified TiO2 nanoparticles with enhanced CH4 selectivity, Appl. Catal. B 168 (2015) 125–131. [138] J. Wang, P. Yang, B. Huang, Self-doped TiO2-x nanowires with enhanced photocatalytic activity: facile synthesis and effects of the Ti3+ , Appl. Surf. Sci. 356 (2015) 391–398. [139] X. Xin, T. Xu, L. Wang, C. Wang, Ti3+ -self doped brookite TiO2 single-crystalline nanosheets with high solar absorption and excellent photocatalytic CO2 reduction, Sci. Rep. 6 (2016) 23684–23691. [140] M. Tahir, N.S. Amin, Indium-doped TiO2 nanoparticles for photocatalytic CO2 reduction with H2 O vapors to CH4 , Appl. Catal. B 162 (2015) 98–109. [141] M. Tahir, N.S. Amin, Photocatalytic CO2 reduction and kinetic study over In/TiO2 nanoparticles supported microchannel monolith photoreactor, Appl. Catal. A 467 (2013) 483–496. [142] D. Li, F. Chen, D. Jiang, W. Shi, W. Zheng, Enhanced photocatalytic activity of N-doped TiO2 nanocrystals with exposed {001} facets, Appl. Surf. Sci. 390 (2016) 689–695. [143] S.A. Bakar, C. Ribeiro, Low temperature synthesis of N-doped TiO 2 with rice-like morphology through peroxo assisted hydrothermal route: materials characterization and photocatalytic properties, Appl. Surf. Sci. 377 (2016) 121–133. [144] H. Li, Y. Hao, H. Lu, L. Liang, Y. Wang, J. Qiu, X. Shi, Y. Wang, J. Yao, A systematic study on visible-light N-doped TiO2 photocatalyst obtained from ethylenediamine by sol-gel method, Appl. Surf. Sci. 344 (2015) 112–118. [145] C.L. Muhich, J.Y. Westcott IV, T. Fuerst, A.W. Weimer, C.B. Musgrave, Increasing the photocatalytic activity of anatase TiO2 through B C, and N doping, J. Phys. Chem. C 118 (2014) 27415–27427. [146] R. Asahi, T. Morikawa, H. Irie, T. Ohwaki, Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects, Chem. Rev. 114 (2014) 9824–9852. [147] N.T. Nolan, D.W. Synnott, M.K. Seery, S.J. Hinder, A. Van Wassenhoven, S.C. Pillai, Effect of N-doping on the photocatalytic activity of sol-gel TiO2 , J. Hazard. Mater. 211 (2012) 88–94. [148] J.G. Yu, P. Zhou, Q. Li, New insight into the enhanced visible-light photocatalytic activities of B-, C-and B/C-doped anatase TiO2 by first-principles, Phys. Chem. Chem. Phys. 15 (2013) 12040–12047. [149] M. Behpour, V. Atouf, Study of the photocatalytic activity of nanocrystalline S, N-codoped TiO2 thin films and powders under visible and sun light irradiation, Appl. Surf. Sci. 258 (2012) 6595–6601. [150] O. Ola, M.M. Maroto-Valer, Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction, J. Photochem. Photobiol. C 24 (2015) 16–42. [151] W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, A.R. Mohamed, Self-assembly of nitrogen-doped TiO2 with exposed {001} facets on a graphene scaffold as photo-active hybrid nanostructures for reduction of carbon dioxide to methane, Nano Res. 7 (2014) 1528–1547. [152] C. Di Valentin, G. Pacchioni, Trends in non-metal doping of anatase TiO2 : B, C, N and F, Catal. Today 206 (2013) 12–18. [153] W. Zhou, H. Fu, Mesoporous TiO2 : preparation, doping, and as a composite for photocatalysis, ChemCatChem 5 (2013) 885–894. [154] Z. Zhao, J. Fan, J. Wang, R. Li, Effect of heating temperature on photocatalytic reduction of CO2 by N-TiO2 nanotube catalyst, Catal. Commun. 21 (2012) 32–37. [155] Z. Zhang, Z. Huang, X. Cheng, Q. Wang, Y. Chen, P. Dong, X. Zhang, Product selectivity of visible-light photocatalytic reduction of carbon dioxide using titanium dioxide doped by different nitrogen-sources, Appl. Surf. Sci. 355 (2015) 45–51.

683

[156] Q. Zhang, Y. Li, E.A. Ackerman, M. Gajdardziska-Josifovska, H. Li, Visible light responsive iodine-doped TiO2 for photocatalytic reduction of CO2 to fuels, Appl. Catal. A 400 (2011) 195–202. [157] D.R. Lide, CRC Handbook of Chemistry and Physics, CRC Press, 2010. [158] P. Pathak, M.J. Meziani, L. Castillo, Y.-P. Sun, Metal-coated nanoscale TiO2 catalysts for enhanced CO2 photoreduction, Green Chem. 7 (2005) 667–670. [159] B. Fang, A. Bonakdarpour, K. Reilly, Y. Xing, F. Taghipour, D.P. Wilkinson, Large-scale synthesis of TiO2 microspheres with hierarchical nanostructure for highly efficient photodriven reduction of CO2 to CH4 , ACS Appl. Mater. Interfaces 6 (2014) 15488–15498. [160] C. Feng, Y. Pang, Y. Wang, M. Sun, C. Zhang, L. Zhang, Y. Zhou, D. Li, Opposite effect of photocorrosion on photocatalytic performance among various Agx My Oz /TiO2 (M = C, P) photocatalysts: a novel effective method for preparing Ag/TiO2 composite, Appl. Surf. Sci. 376 (2016) 188–198. ˇ [161] L. Matˇejová, K. Koˇcí, M. Reli, L. Capek, A. Hospodková, P. Peikertová, Z. Matˇej, L. Obalová, A. Wach, P. Ku´strowski, Preparation, characterization and photocatalytic properties of cerium doped TiO2 : on the effect of Ce loading on the photocatalytic reduction of carbon dioxide, Appl. Catal. B 152 (2014) 172–183. [162] A. Bazzo, A. Urakawa, Origin of photocatalytic activity in continuous gas phase CO2 reduction over Pt/TiO2 , ChemSusChem 6 (2013) 2095–2102. [163] J.H. Lee, H. Lee, M. Kang, Remarkable photoconversion of carbon dioxide into methane using Bi-doped TiO2 nanoparticles prepared by a conventional sol-gel method, Mater. Lett. 178 (2016) 316–319. [164] A.R. Koirala, S. Docao, S.B. Lee, K.B. Yoon, Fate of methanol under one-pot artificial photosynthesis condition with metal-loaded TiO2 as photocatalysts, Catal. Today 243 (2015) 235–250. [165] I.-H. Tseng, J.C.-S. Wu, Chemical states of metal-loaded titania in the photoreduction of CO2 , Catal. Today 97 (2004) 113–119. [166] J. Mao, L. Ye, K. Li, X. Zhang, J. Liu, T. Peng, L. Zan, Pt-loading reverses the photocatalytic activity order of anatase TiO2 {001} and {010} facets for photoreduction of CO2 to CH4 , Appl. Catal. B 144 (2014) 855–862. [167] N. Zhang, S. Liu, Y.-J. Xu, Recent progress on metal core@ semiconductor shell nanocomposites as a promising type of photocatalyst, Nanoscale 4 (2012) 2227–2238. [168] S.t. Neat¸u, J.A. Macia´ı-Agulló, P. Concepcio´ın, H. Garcia, Gold-copper nanoalloys supported on TiO2 as photocatalysts for CO2 reduction by water, J. Am. Chem. Soc. 136 (2014) 15969–15976. [169] N. Murakami, D. Saruwatari, T. Tsubota, T. Ohno, Photocatalytic reduction of carbon dioxide over shape-controlled titanium (IV) oxide nanoparticles with co-catalyst loading, Curr. Org. Chem. 17 (2013) 2449–2453. [170] H. Chen, S. Chen, X. Quan, H. Yu, H. Zhao, Y. Zhang, Fabrication of TiO2 -Pt coaxial nanotube array schottky structures for enhanced photocatalytic degradation of phenol in aqueous solution, J. Phys. Chem. C 112 (2008) 9285–9290. [171] C. Wang, L. Yin, L. Zhang, N. Liu, N. Lun, Y. Qi, Platinum-nanoparticle-modified TiO2 nanowires with enhanced photocatalytic property, ACS Appl. Mater. Interfaces 2 (2010) 3373–3377. [172] S. Bera, J.E. Lee, S.B. Rawal, W.I. Lee, Size-dependent plasmonic effects of Au and Au@ SiO2 nanoparticles in photocatalytic CO2 conversion reaction of Pt/TiO2 , Appl. Catal. B 199 (2016) 55–63. [173] B.-R. Chen, V.-H. Nguyen, J.C. Wu, R. Martin, K. Koˇcí, Production of renewable fuels by the photohydrogenation of CO2 : effect of the Cu species loaded onto TiO2 photocatalysts, Phys. Chem. Chem. Phys. 18 (2016) 4942–4951. ˛ [174] M. Klein, J. Nadolna, A. Gołabiewska, P. Mazierski, T. Klimczuk, H. Remita, A. Zaleska-Medynska, The effect of metal cluster deposition route on structure and photocatalytic activity of mono-and bimetallic nanoparticles supported on TiO2 by radiolytic method, Appl. Surf. Sci. 378 (2016) 37–48. [175] J. Ma, S. Guo, X. Guo, H. Ge, A mild synthetic route to Fe3 O4 @ TiO2 -Au composites: preparation, characterization and photocatalytic activity, Appl. Surf. Sci. 353 (2015) 1117–1125. [176] J. Lv, H. Gao, H. Wang, X. Lu, G. Xu, D. Wang, Z. Chen, X. Zhang, Z. Zheng, Y. Wu, Controlled deposition and enhanced visible light photocatalytic performance of Pt-modified TiO2 nanotube arrays, Appl. Surf. Sci. 351 (2015) 225–231. [177] N. Singhal, A. Ali, A. Vorontsov, C. Pendem, U. Kumar, Efficient approach for simultaneous CO and H2 production via photoreduction of CO2 with water over copper nanoparticles loaded TiO2 , Appl. Catal. A 523 (2016) 107–117. [178] R.J. Tayade, R.G. Kulkarni, R.V. Jasra, Transition metal ion impregnated mesoporous TiO2 for photocatalytic degradation of organic contaminants in water, Ind. Eng. Chem. Res. 45 (2006) 5231–5238. [179] M. Kitano, K. Tsujimaru, M. Anpo, Decomposition of water in the separate evolution of hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: effect of the work function of the substrates on the yield of the reaction, Appl. Catal. A 314 (2006) 179–183. [180] K. Hiehata, A. Sasahara, H. Onishi, Local work function analysis of Pt/TiO2 photocatalyst by a Kelvin probe force microscope, Nanotechnology 18 (2007) 084007–084012. [181] X. Feng, J.D. Sloppy, T.J. LaTempa, M. Paulose, S. Komarneni, N. Bao, C.A. Grimes, Synthesis and deposition of ultrafine Pt nanoparticles within high aspect ratio TiO2 nanotube arrays: application to the photocatalytic reduction of carbon dioxide, J. Mater. Chem. 21 (2011) 13429–13433. [182] W.-N. Wang, W.-J. An, B. Ramalingam, S. Mukherjee, D.M. Niedzwiedzki, S. Gangopadhyay, P. Biswas, Size and structure matter: enhanced CO2 photoreduction efficiency by size-resolved ultrafine Pt nanoparticles on TiO2 single crystals, J. Am. Chem. Soc. 134 (2012) 11276–11281.

684

J. Low et al. / Applied Surface Science 392 (2017) 658–686

[183] E. Liu, L. Kang, F. Wu, T. Sun, X. Hu, Y. Yang, H. Liu, J. Fan, Photocatalytic reduction of CO2 into methanol over Ag/TiO2 nanocomposites enhanced by surface plasmon resonance, Plasmonics 9 (2014) 61–70. [184] X. Zhang, Y.L. Chen, R.-S. Liu, D.P. Tsai, Plasmonic photocatalysis, Rep. Prog. Phys. 76 (2013) 046401–046441. [185] 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. [186] P. Wang, B. Huang, X. Zhang, X. Qin, H. Jin, Y. Dai, Z. Wang, J. Wei, J. Zhan, S. Wang, Highly efficient visible-light plasmonic photocatalyst Ag@AgBr, Chem. Eur. J. 15 (2009) 1821–1824. [187] K. Kimura, S.-I. Naya, Y. Jin-Nouchi, H. Tada, TiO2 crystal form-dependence of the Au/TiO2 plasmon photocatalyst’s activity, J. Phys. Chem. C 116 (2012) 7111–7117. [188] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia, Controlling the synthesis and assembly of silver nanostructures for plasmonic applications, Chem. Rev. 111 (2011) 3669–3712. [189] C. Clavero, Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices, Nat. Photonics 8 (2014) 95–103. [190] P. Wang, B. Huang, Y. Dai, M.-H. Whangbo, Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles, Phys. Chem. Chem. Phys. 14 (2012) 9813–9825. [191] S.K. Cushing, J. Li, F. Meng, T.R. Senty, S. Suri, M. Zhi, M. Li, A.D. Bristow, N. Wu, Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor, J. Am. Chem. Soc. 134 (2012) 15033–15041. [192] K. Saha, S.S. Agasti, C. Kim, X. Li, V.M. Rotello, Gold nanoparticles in chemical and biological sensing, Chem. Rev. 112 (2012) 2739–2779. [193] S. Mubeen, J. Lee, N. Singh, S. Krämer, G.D. Stucky, M. Moskovits, An autonomous photosynthetic device in which all charge carriers derive from surface plasmons, Nat. Nanotechnol. 8 (2013) 247–251. [194] Z. Zhan, R. Xu, Y. Mi, H. Zhao, Y. Lei, Highly controllable surface plasmon resonance property by heights of ordered nanoparticle arrays fabricated via a nonlithographic route, ACS nano 9 (2015) 4583–4590. [195] Y. Wei, J. Jiao, Z. Zhao, J. Liu, J. Li, G. Jiang, Y. Wang, A. Duan, Fabrication of inverse opal TiO2 -supported Au@ CdS core-shell nanoparticles for efficient photocatalytic CO2 conversion, Appl. Catal. B 179 (2015) 422–432. [196] J. Jiao, Y. Wei, Z. Zhao, W. Zhong, J. Liu, J. Li, A. Duan, G. Jiang, Synthesis of 3D ordered macroporous TiO2 -supported Au nanoparticle photocatalysts and their photocatalytic performances for the reduction of CO2 to methane, Catal. Today 258 (2015) 319–326. [197] Z. Bian, T. Tachikawa, P. Zhang, M. Fujitsuka, T. Majima, Au/TiO2 superstructure-based plasmonic photocatalysts exhibiting efficient charge separation and unprecedented activity, J. Am. Chem. Soc. 136 (2013) 458–465. [198] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911–921. [199] Z. Zhan, F. Grote, Z. Wang, R. Xu, Y. Lei, Degenerating plasmonic modes to enhance the performance of surface plasmon resonance for application in solar energy conversion, Adv. Energy Mater. 5 (2015) 1501654–1501660. [200] Y.-C. Yao, X.-R. Dai, X.-Y. Hu, S.-Z. Huang, Z. Jin, Synthesis of Ag-decorated porous TiO2 nanowires through a sunlight induced reduction method and its enhanced photocatalytic activity, Appl. Surf. Sci. 387 (2016) 469–476. [201] F.-X. Xiao, Z. Zeng, B. Liu, Bridging the gap: electron relay and plasmonic sensitization of metal nanocrystals for metal clusters, J. Am. Chem. Soc. 137 (2015) 10735–10744. [202] F. Pincella, K. Isozaki, K. Miki, A visible light-driven plasmonic photocatalyst, Light Sci. Appl. 3 (2014) e133–e138. [203] X. Fu, L. Pan, S. Li, Q. Wang, J. Qin, Y. Huang, Controlled preparation of Ag nanoparticle films by a modified photocatalytic method on TiO2 films with Ag seeds for surface-enhanced Raman scattering, Appl. Surf. Sci. 363 (2016) 412–420. [204] L.G. Devi, R. Kavitha, A review on plasmonic metal-TiO2 composite for generation trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system, Appl. Surf. Sci. 360 (2016) 601–622. [205] B. Yu, Y. Zhou, P. Li, W. Tu, P. Li, L. Tang, J. Ye, Z. Zou, Photocatalytic reduction of CO2 over Ag/TiO2 nanocomposites prepared with a simple and rapid silver mirror method, Nanoscale (2016). [206] D.B. Ingram, S. Linic, Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface, J. Am. Chem. Soc. 133 (2011) 5202–5205. [207] I. Thomann, B.A. Pinaud, Z. Chen, B.M. Clemens, T.F. Jaramillo, M.L. Brongersma, Plasmon enhanced solar-to-fuel energy conversion, Nano Lett. 11 (2011) 3440–3446. [208] S. Mubeen, G. Hernandez-Sosa, D. Moses, J. Lee, M. Moskovits, Plasmonic photosensitization of a wide band gap semiconductor: converting plasmons to charge carriers, Nano Lett. 11 (2011) 5548–5552. [209] R. Jiang, B. Li, C. Fang, J. Wang, Metal/semiconductor hybrid nanostructures for plasmon-enhanced applications, Adv. Mater. 26 (2014) 5274–5309. [210] W. Hou, W.H. Hung, P. Pavaskar, A. Goeppert, M. Aykol, S.B. Cronin, Photocatalytic conversion of CO2 to hydrocarbon fuels via

[211]

[212]

[213]

[214]

[215]

[216]

[217]

[218]

[219]

[220]

[221]

[222]

[223]

[224] [225]

[226]

[227]

[228]

[229]

[230]

[231]

[232]

[233]

[234]

[235]

[236]

[237]

[238]

plasmon-enhanced absorption and metallic interband transitions, ACS Catal. 1 (2011) 929–936. W. Tu, Y. Zhou, H. Li, P. Li, Z. Zou, Au@TiO2 yolk-shell hollow spheres for plasmon-induced photocatalytic reduction of CO2 to solar fuel via a local electromagnetic field, Nanoscale 7 (2015) 14232–14236. S. Feng, M. Wang, Y. Zhou, P. Li, W. Tu, Z. Zou, Double-shelled plasmonic Ag-TiO2 hollow spheres toward visible light-active photocatalytic conversion of CO2 into solar fuel, APL Mater. 3 (2015) 104416–104423. W. Pipornpong, R. Wanbayor, V. Ruangpornvisuti, Adsorption CO2 on the perfect and oxygen vacancy defect surfaces of anatase TiO2 and its photocatalytic mechanism of conversion to CO, Appl. Surf. Sci. 257 (2011) 10322–10328. X. Chang, T. Wang, J. Gong, CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts, Energy Environ. Sci. 9 (2016) 2177–2196. N. Linares, A.M. Silvestre-Albero, E. Serrano, J. Silvestre-Albero, J. García-Martínez, Mesoporous materials for clean energy technologies, Chem. Soc. Rev. 43 (2014) 7681–7717. F. Wang, Y. Zhou, P. Li, L. Kuai, Z. Zou, Synthesis of bionic-macro/microporous MgO-modified TiO2 for enhanced CO2 photoreduction into hydrocarbon fuels, Chin. J. Catal. 37 (2016) 863–868. S.C. Lee, B.Y. Choi, T.J. Lee, C.K. Ryu, Y.S. Ahn, J.C. Kim, CO2 absorption and regeneration of alkali metal-based solid sorbents, Catal. Today 111 (2006) 385–390. H. Li, X. Wu, J. Wang, Y. Gao, L. Li, K. Shih, Enhanced activity of Ag MgO TiO2 catalyst for photocatalytic conversion of CO2 and H2 O into CH4 , Int. J. Hydrogen Energy 41 (2016) 8479–8488. X. Meng, S. Ouyang, T. Kako, P. Li, Q. Yu, T. Wang, J. Ye, Photocatalytic CO2 conversion over alkali modified TiO2 without loading noble metal cocatalyst, Chem. Commun. 50 (2014) 11517–11519. L. Liu, C. Zhao, H. Zhao, D. Pitts, Y. Li, Porous microspheres of MgO-patched TiO2 for CO2 photoreduction with H2 O vapor: temperature-dependent activity and stability, Chem. Commun. 49 (2013) 3664–3666. Q. Huang, J.G. Yu, S.W. Cao, C. Cui, B. Cheng, Efficient photocatalytic reduction of CO2 by amine-functionalized g-C3 N4 , Appl. Surf. Sci. 358 (2015) 350–355. A.D. Ebner, M. Gray, N. Chisholm, Q. Black, D. Mumford, M.A. Nicholson, J.A. Ritter, Suitability of a solid amine sorbent for CO2 capture by pressure swing adsorption, Ind. Eng. Chem. Res. 50 (2011) 5634–5641. Y. Liao, S.W. Cao, Y. Yuan, Q. Gu, Z. Zhang, C. Xue, Efficient CO2 capture and photoreduction by amine-functionalized TiO2 , Chem. Eur. J. 20 (2014) 10220–10222. M. Dahl, Y. Liu, Y. Yin, Composite titanium dioxide nanomaterials, Chem. Rev. 114 (2014) 9853–9889. W. Zhang, X. Xiao, Y. Li, X. Zeng, L. Zheng, C. Wan, Liquid-exfoliation of layered MoS2 for enhancing photocatalytic activity of TiO2 /g-C3 N4 photocatalyst and DFT study, Appl. Surf. Sci. 389 (2016) 496–506. L. Yang, M. Gao, B. Dai, X. Guo, Z. Liu, B. Peng, Synthesis of spindle-shaped AgI/TiO2 nanoparticles with enhanced photocatalytic performance, Appl. Surf. Sci. 386 (2016) 337–344. S. Demirci, T. Dikici, M. Yurddaskal, S. Gultekin, M. Toparli, E. Celik, Synthesis and characterization of Ag doped TiO2 heterojunction films and their photocatalytic performances, Appl. Surf. Sci. 390 (2016) 591–601. M. Mazur, D. Wojcieszak, D. Kaczmarek, J. Domaradzki, S. Song, D. Gibson, F. Placido, P. Mazur, M. Kalisz, A. Poniedzialek, Functional photocatalytically active and scratch resistant antireflective coating based on TiO2 and SiO2 , Appl. Surf. Sci. 380 (2016) 165–171. D. Wang, Y. Xu, F. Sun, Q. Zhang, P. Wang, X. Wang, Enhanced photocatalytic activity of TiO2 under sunlight by MoS2 nanodots modification, Appl. Surf. Sci. 377 (2016) 221–227. Y. Lu, Y. Zhang, J. Zhang, Y. Shi, Z. Li, Z. Feng, C. Li, In situ loading of CuS nanoflowers on rutile TiO2 surface and their improved photocatalytic performance, Appl. Surf. Sci. 370 (2016) 312–319. G. Song, F. Xin, J. Chen, X. Yin, Photocatalytic reduction of CO2 in cyclohexanol on CdS-TiO2 heterostructured photocatalyst, Appl. Catal. A 473 (2014) 90–95. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244. S.J. Moniz, S.A. Shevlin, D.J. Martin, Z.-X. Guo, J. Tang, Visible-light driven heterojunction photocatalysts for water splitting-a critical review, Energy Environ. Sci. 8 (2015) 731–759. L. Wu, Y. Zhou, W. Nie, L. Song, P. Chen, Synthesis of highly monodispersed teardrop-shaped core-shell SiO2 /TiO2 nanoparticles and their photocatalytic activities, Appl. Surf. Sci. 351 (2015) 320–326. D. Xiong, H. Chang, Q. Zhang, S. Tian, B. Liu, X. Zhao, Preparation and characterization of CuCrO2 /TiO2 heterostructure photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 347 (2015) 747–754. M. Tahir, B. Tahir, N.A.S. Amin, H. Alias, Selective photocatalytic reduction of CO2 by H2 O/H2 to CH4 and CH3 OH over Cu-promoted In2 O3 /TiO2 nanocatalyst, Appl. Surf. Sci. 389 (2016) 46–55. Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong, Y. Yu, Octahedral Cu2 O-modified TiO2 nanotube arrays for efficient photocatalytic reduction of CO2 , Chin. J. Catal. 36 (2015) 2229–2236. J. Zhou, L. Yin, K. Zha, H. Li, Z. Liu, J. Wang, K. Duan, B. Feng, Hierarchical fabrication of heterojunctioned SrTiO3 /TiO2 nanotubes on 3D microporous

J. Low et al. / Applied Surface Science 392 (2017) 658–686

[239]

[240] [241]

[242] [243]

[244]

[245]

[246]

[247]

[248]

[249]

[250]

[251]

[252] [253] [254]

[255]

[256]

[257] [258]

[259]

[260]

[261]

[262]

[263] [264]

[265]

[266]

[267]

Ti substrate with enhanced photocatalytic activity and adhesive strength, Appl. Surf. Sci. 367 (2016) 118–125. X. Zou, Y. Dong, X. Zhang, Y. Cui, Synthesize and characterize of Ag3 VO4 /TiO2 nanorods photocatalysts and its photocatalytic activity under visible light irradiation, Appl. Surf. Sci. 366 (2016) 173–180. X. Li, J.G. Yu, M. Jaroniec, Hierarchical photocatalysts, Chem. Soc. Rev. 45 (2016) 2603–2636. C.L. Yu, W.Q. Zhou, J.C. Yu, H. Liu, L.F. Wei, Design and fabrication of heterojunction photocatalysts for energy conversion and pollutant degradation, Chin. J. Catal. 35 (2014) 1609–1618. A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar, J. He, Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review, Nanoscale 5 (2013) 8326–8339. H. Sun, B. Dong, G. Su, R. Gao, W. Liu, L. Song, L. Cao, Modification of TiO2 nanotubes by WO3 species for improving their photocatalytic activity, Appl. Surf. Sci. 343 (2015) 181–187. J. Tao, Z. Gong, G. Yao, Y. Cheng, M. Zhang, J. Lv, S. Shi, G. He, X. Jiang, X. Chen, Enhanced optical and photocatalytic properties of Ag quantum dots-sensitized nanostructured TiO2 /ZnO heterojunctions, J.Alloys Compd. 688 (2016) 605–612. X. Pan, Y.-J. Xu, Graphene-templated bottom-up fabrication of ultralarge binary CdS-TiO2 nanosheets for photocatalytic selective reduction, J. Phys. Chem. C 119 (2015) 7184–7194. A.A. Beigi, S. Fatemi, Z. Salehi, Synthesis of nanocomposite CdS/TiO2 and investigation of its photocatalytic activity for CO2 reduction to CO and CH4 under visible light irradiation, J. CO2 Util. 7 (2014) 23–29. C. Dong, M. Xing, J. Zhang, Economic hydrophobicity triggering of CO2 photoreduction for selective CH4 generation on noble-metal-free TiO2 -SiO2 , J. Phys. Chem. Lett. 7 (2016) 2962–2966. G. Xi, S. Ouyang, J. Ye, General synthesis of hybrid TiO2 mesoporous french fries toward improved photocatalytic conversion of CO2 into hydrocarbon fuel: a case of TiO2 /ZnO, Chem. Eur. J. 17 (2011) 9057–9061. C. Tang, W. Hou, E. Liu, X. Hu, J. Fan, CeF3 /TiO2 composite as a novel visible-light-driven photocatalyst based on upconversion emission and its application for photocatalytic reduction of CO2 , J. Lumin. 154 (2014) 305–309. K.-Y. Lee, K. Sato, A.R. Mohamed, Facile synthesis of anatase-rutile TiO2 composites with enhanced CO2 photoreduction activity and the effect of Pt loading on product selectivity, Mater. Lett. 163 (2016) 240–243. L. Li, P.A. Salvador, G.S. Rohrer, Photocatalysts with internal electric fields, Nanoscale 6 (2014) 24–42. L. Zhang, Y. Su, W.Z. Wang, Internal electric fields within the photocatalysts, Prog. Chem. 28 (2016) 415–427. J. Lin, J. Shen, R. Wang, J. Cui, W. Zhou, P. Hu, D. Liu, H. Liu, J. Wang, R.I. Boughton, Nano-p-n junctions on surface-coarsened TiO2 nanobelts with enhanced photocatalytic activity, J. Mater. Chem. 21 (2011) 5106–5113. Y. Zhao, X. Huang, X. Tan, T. Yu, X. Li, L. Yang, S. Wang, Fabrication of BiOBr nanosheets@TiO2 nanobelts p-n junction photocatalysts for enhanced visible-light activity, Appl. Surf. Sci. 365 (2016) 209–217. B. Sun, G. Zhou, T. Gao, H. Zhang, H. Yu, NiO nanosheet/TiO2 nanorod-constructed p-n heterostructures for improved photocatalytic activity, Appl. Surf. Sci. 364 (2016) 322–331. H. Okumura, S. Endo, S. Joonwichien, E. Yamasue, K. Ishihara, Magnetic field effect on heterogeneous photocatalysis, Catal. Today 258 (2015) 634–647. T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520–7535. F. Bi, M.F. Ehsan, W. Liu, T. He, Visible-light photocatalytic conversion of carbon dioxide into methane using Cu2 O/TiO2 hollow nanospheres, Chin. J. Chem. 33 (2015) 112–118. C.P. Sajan, S. Wageh, A.A. Al-Ghamdi, J.G. Yu, S.W. Cao, TiO2 nanosheets with exposed {001} facets for photocatalytic applications, Nano Res. 9 (2015) 1–25. S. Gao, W. Wang, Y. Ni, C. Lu, Z. Xu, Facet-dependent photocatalytic mechanisms of anatase TiO2 : A new sight on the self-adjusted surface heterojunction, J. Alloys Compd. 647 (2015) 981–988. W. Yu, D. Xu, T. Peng, Enhanced photocatalytic activity of g-C3 N4 for selective CO2 reduction to CH3 OH via facile coupling of ZnO: a direct Z-scheme mechanism, J. Mater. Chem. A 3 (2015) 19936–19947. P. Zhou, J.G. Yu, M. Jaroniec, All-solid-state Z-scheme photocatalytic systems, Adv. Mater. 26 (2014) 4920–4935. J.G. Yu, S.H. Wang, J.X. Low, W. Xiao, Enhanced photocatalytic performance of direct Z-scheme g-C3 N4 -TiO2 photocatalysts for the decomposition of formaldehyde in air, Phys. Chem. Chem. Phys. 15 (2013) 16883–16890. L. Kuai, Y. Zhou, W. Tu, P. Li, H. Li, Q. Xu, L. Tang, X. Wang, M. Xiao, Z. Zou, Rational construction of a CdS/reduced graphene oxide/TiO2 core-shell nanostructure as an all-solid-state Z-scheme system for CO2 photoreduction into solar fuels, RSC Adv. 5 (2015) 88409–88413. G. Liu, L. Wang, H.G. Yang, H.-M. Cheng, G.Q.M. Lu, Titania-based photocatalysts-crystal growth, doping and heterostructuring, J. Mater. Chem. 20 (2010) 831–843. J. Zhang, Y. Hu, X. Jiang, S. Chen, S. Meng, X. Fu, Design of a direct Z-scheme photocatalyst: preparation and characterization of Bi2 O3 /g-C3 N4 with high visible light activity, J. Hazard. Mater. 280 (2014) 713–722.

685

[268] S. Chen, Y. Hu, L. Ji, X. Jiang, X. Fu, Preparation and characterization of direct Z-scheme photocatalyst Bi2 O3 /NaNbO3 and its reaction mechanism, Appl. Surf. Sci. 292 (2014) 357–366. [269] Y. Liu, G. Ji, M.A. Dastageer, L. Zhu, J. Wang, B. Zhang, X. Chang, M.A. Gondal, Highly-active direct Z-scheme Si/TiO2 photocatalyst for boosted CO2 reduction into value-added methanol, RSC Adv. 4 (2014) 56961–56969. [270] G. Song, F. Xin, X. Yin, Photocatalytic reduction of carbon dioxide over ZnFe2 O4 /TiO2 nanobelts heterostructure in cyclohexanol, J. Colloid Interface Sci. 442 (2015) 60–66. [271] A. Qu, H. Xie, X. Xu, Y. Zhang, S. Wen, Y. Cui, High quantum yield graphene quantum dots decorated TiO2 nanotubes for enhancing photocatalytic activity, Appl. Surf. Sci. 375 (2016) 230–241. [272] V.Z. Baldissarelli, T. de Souza, L. Andrade, L.F.C. de Oliveira, H.J. José, R.F.P.M. d. Moreira, Preparation and photocatalytic activity of TiO2 -exfoliated graphite oxide composite using an ecofriendly graphite oxidation method, Appl. Surf. Sci. 359 (2015) 868–874. [273] E.-C. Cho, J.-H. Ciou, J.-H. Zheng, J. Pan, Y.-S. Hsiao, K.-C. Lee, J.-H. Huang, Fullerene C70 decorated TiO2 nanowires for visible-light-responsive photocatalyst, Appl. Surf. Sci. 355 (2015) 536–546. [274] E. Vasilaki, I. Georgaki, D. Vernardou, M. Vamvakaki, N. Katsarakis, Ag-loaded TiO2 /reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity, Appl. Surf. Sci. 353 (2015) 865–872. [275] J.R. Ran, J. Zhang, J.G. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev. 43 (2014) 7787–7812. [276] R. Leary, A. Westwood, Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis, Carbon 49 (2011) 741–772. [277] M.-Q. Yang, N. Zhang, Y.-J. Xu, Synthesis of fullerene-, carbon nanotube-, and graphene-TiO2 nanocomposite photocatalysts for selective oxidation: a comparative study, ACS Appl. Mater. Interfaces 5 (2013) 1156–1164. [278] M. Inagaki, Carbon coating for enhancing the functionalities of materials, Carbon 50 (2012) 3247–3266. [279] H. Tang, C.M. Hessel, J. Wang, N. Yang, R. Yu, H. Zhao, D. Wang, Two-dimensional carbon leading to new photoconversion processes, Chem. Soc. Rev. 43 (2014) 4281–4299. [280] C. Lin, Y. Song, L. Cao, S. Chen, Effective photocatalysis of functional nanocomposites based on carbon and TiO2 nanoparticles, Nanoscale 5 (2013) 4986–4992. [281] K. Qi, R. Selvaraj, T. Al Fahdi, S. Al-Kindy, Y. Kim, G.-C. Wang, C.-W. Tai, M. Sillanpää, Enhanced photocatalytic activity of anatase-TiO2 nanoparticles by fullerene modification: a theoretical and experimental study, Appl. Surf. Sci. 387 (2016) 750–758. [282] C. Lai, M.-M. Wang, G.-M. Zeng, Y.-G. Liu, D.-L. Huang, C. Zhang, R.-Z. Wang, P. Xu, M. Cheng, C. Huang, Synthesis of surface molecular imprinted TiO2 /graphene photocatalyst and its highly efficient photocatalytic degradation of target pollutant under visible light irradiation, Appl. Surf. Sci. 390 (2016) 368–376. [283] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [284] Y. Yao, G. Li, S. Ciston, R.M. Lueptow, K.A. Gray, Photoreactive TiO2 /carbon nanotube composites: synthesis and reactivity, Environ. Sci. Technol. 42 (2008) 4952–4957. [285] Y. Yu, J.C. Yu, J.G. Yu, Y.C. Kwok, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal. A 289 (2005) 186–196. [286] X. Liu, M. Wang, S. Zhang, B. Pan, Application potential of carbon nanotubes in water treatment: a review, J. Environ. Sci. 25 (2013) 1263–1280. [287] M.J. Sampaio, C.G. Silva, R.R. Marques, A.M. Silva, J.L. Faria, Carbon nanotube-TiO2 thin films for photocatalytic applications, Catal. Today 161 (2011) 91–96. [288] K. Woan, G. Pyrgiotakis, W. Sigmund, Photocatalytic carbon-nanotube-TiO2 composites, Adv. Mater. 21 (2009) 2233–2239. [289] M.M. Gui, S.P. Chai, B.Q. Xu, A.R. Mohamed, Enhanced visible light responsive MWCNT/TiO2 core-shell nanocomposites as the potential photocatalyst for reduction of CO2 into methane, Sol. Energy Mater. Sol. Cells 122 (2014) 183–189. [290] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, Y. Zhang, S.a. Dubonos, I. Grigorieva, A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [291] H. Zhang, X. Lv, Y. Li, Y. Wang, J. Li, P25-graphene composite as a high performance photocatalyst, ACS Nano 4 (2009) 380–386. [292] Q.J. Xiang, J.G. Yu, M. Jaroniec, Graphene-based semiconductor photocatalysts, Chem. Soc. Rev. 41 (2012) 782–796. [293] N. Zhang, Y. Zhang, Y.-J. Xu, Recent progress on graphene-based photocatalysts: current status and future perspectives, Nanoscale 4 (2012) 5792–5813. [294] I. Shown, H.-C. Hsu, Y.-C. Chang, C.-H. Lin, P.K. Roy, A. Ganguly, C.-H. Wang, J.-K. Chang, C.-I. Wu, L.-C. Chen, Highly efficient visible light photocatalytic reduction of CO2 to hydrocarbon fuels by Cu-nanoparticle decorated graphene oxide, Nano Lett. 14 (2014) 6097–6103. [295] L.K. Putri, W.-J. Ong, W.S. Chang, S.-P. Chai, Heteroatom doped graphene in photocatalysis: a review, Appl. Surf. Sci. 358 (2015) 2–14. [296] S.C. Cui, X.Z. Sun, J.G. Liu, Photo-reduction of CO2 using a Rhenium complex covalentlys supported on a graphene/TiO2 composite, ChemSusChem 9 (2016) 1698–1703. [297] Z. Xiong, Y. Luo, Y. Zhao, J. Zhang, C. Zheng, J.C. Wu, Synthesis, characterization and enhanced photocatalytic CO2 reduction activity of

686

[298]

[299] [300]

[301]

J. Low et al. / Applied Surface Science 392 (2017) 658–686 graphene supported TiO2 nanocrystals with coexposed {001} and {101} facets, Phys. Chem. Chem. Phys. 18 (2016) 13186–13195. K.M. Cho, K.H. Kim, H.O. Choi, H.-T. Jung, A highly photoactive, visible-light-driven graphene/2D mesoporous TiO2 photocatalyst, Green Chem. 17 (2015) 3972–3978. Q.J. Xiang, B. Cheng, J.G. Yu, Graphene-based photocatalysts for solar-fuel generation, Angew. Chem. Int. Ed. 54 (2015) 11350–11366. Y.T. Liang, B.K. Vijayan, K.A. Gray, M.C. Hersam, Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production, Nano Lett. 11 (2011) 2865–2870. P.V. Kamat, Graphene-based nanoassemblies for energy conversion, J. Phys. Chem. Lett. 2 (2011) 242–251.

[302] L.-L. Tan, W.-J. Ong, S.-P. Chai, B.T. Goh, A.R. Mohamed, Visible-light-active oxygen-rich TiO2 decorated 2D graphene oxide with enhanced photocatalytic activity toward carbon dioxide reduction, Appl. Catal. B 179 (2015) 160–170. [303] M. Xing, F. Shen, B. Qiu, J. Zhang, Highly-dispersed boron-doped graphene nanosheets loaded with TiO2 nanoparticles for enhancing CO2 photoreduction, Sci. Rep. 4 (2014) 6341–6347. [304] G. Ozin, Real or Artifact: CO, 2014, http://materialsviews.com/real-orartifact-co2-photo-catalysis-versus-carbon-contamination/(accessed 13.09.16).