Energy shortages and global warming have become two major issues closely associated with the tremendous consumption of non-renewable fossil fuels. As a sustainable and economical route, photocatalytic reduction of CO2 conversion, the so-called artificial photosynthesis, provides an alluring strategy to realize the twofold benefits with respect to closing carbon cycle and producing renewable fuels/chemicals, thereby solving the above issues. TiO2 photocatalysts have attracted widespread attention in CO2 reduction reactions owing to their low cost, high stability, and environmental safety. Nevertheless, the limited absorption ability in the visible light range and fast recombination of photogenerated electrons and holes are the two main drawbacks impeding practical applications. This minireview summarizes the fabrication methodologies of nanostructured TiO2 (especially focused on the 1D, 2D, and 3D nanostructures), discusses the fundamentals of photocatalytic CO2 reduction to value-added chemicals, and draws a comparison of photocatalytic performances from modified TiO2 nanostructures. In further contexts, the opportunities and challenges for nanostructured TiO2 based materials on CO2 conversion are proposed.

With the development of industrialization and rapid growth of population, the utilization of fossil fuels is ever increasing, accompanied with the accumulative emissions of greenhouse gas, which intensifies the global energy crisis and anthropogenic climate change.1,2 Developing effective strategies to solve energy shortage and environmental pollution is, therefore, of urgent importance. Photocatalytic CO2 conversion (PCC) into valuable chemicals/fuels (e.g., CO, CH4, C2H6, C2H5OH, and HCOOH)3–8 with the assistance of environmentally friendly solar energy is a promising strategy to satisfy both energy supply and environmental protection. Nevertheless, three hurdles hinder the activation and conversion of CO2: (1) CO2 molecule shows the thermodynamic stability with the dissociation energy of the C=O bond (∼750 kJ mol−1) to be higher than that of C–H (∼430 kJ mol−1) and C–C (∼336 kJ mol−1) bonds,9 (2) CO2 is chemically inert with a low electron affinity and a large energy gap (13.7 eV) between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO),10 and (3) CO2 exhibits a low solubility in water (around 30 mM under room temperature and one atmosphere).11 

To obtain a decent photocatalytic performance for CO2 reduction, photocatalysts need to meet the following requirements: (1) facile transfer of electrons between CO2 and photocatalysts, (2) lower position for conduction band (CB) edges of the photocatalysts than the CO2 reduction potentials, (3) high attraction with the reactants (such as H2O, CO2, or carbonate species) while maintaining rapid desorption of reduced products to avoid photo-oxidation, and (4) sacrificial reagents needed to consume the photogenerated holes in the valence band (VB) of the photocatalysts (e.g., oxidation of H2O to O2). Since Inoue and colleagues12 first reported the photocatalytic CO2 reduction in 1979, photocatalysts with appropriate bandgaps (1.7–3.0 eV), such as TiO2, CeO2, g-C3N4, Bi2WO6, and CdS, have been optimized for the utilization of visible light (∼43% of the solar) and a high potential for PCC.13–17 

As an extensively used semiconductor, TiO2 shows merits in good photostability, high resistance toward corrosion, low cost, environmental friendliness, and easy availability.18–22 However, due to the wide bandgap of 3.2 eV and fast recombination of photogenerated electron–hole pairs, TiO2 still suffers from a low quantum efficiency, slow charge transfer, and poor light trapping (3%–5% of the entire solar) on applications of energy harvesting, photocatalysis, and storage.23–26 To surmount these aforementioned limitations, numerous strategies [e.g., loading metals or doping with non-metal elements, constructing defects with O vacancies or Ti3+ (black TiO2), and designing heterojunctions with low bandgap materials] have been attempted to improve the performance for PCC in the broad visible light region.

Starting with the introduction of synthetic routes for various nanostructured TiO2 with distinct properties including high aspect ratio, directional channels for photogenerated charges, and specific geometry configurations for superb light trapping, this minireview provides a brief overview of recent advances in well-designed nanostructured TiO2-based photocatalysts for the photocatalytic reduction of CO2. Throughout highlighting the fundamental principles of the reaction mechanism over TiO2 in PCC, this minireview then proceeds into summarizing various strategies on architectural engineering and modification of nanostructured TiO2 targeting an enhanced PCC performance. In the final stage, perspectives and challenges for advancing the photocatalytic CO2 conversion over nanostructured TiO2 are covered.

As shown in Fig. 1,27 various crystalline phases of TiO2 have been reported, including rutile, anatase, brookite, columbite-like phase TiO2 (II), baddeleyite-like phase, TiO2 (B), and hollandite-like phase TiO2 (H).28–32 Among them, three distinct polymorphs, namely, anatase, rutile, and brookite, are commonly recognized to exist naturally.33 All the three TiO2 consist of TiO6 octahedral skeletons but with structural distortions. The Ti–O bonds play very important roles affecting the structural and electronic properties of different TiO2 phases.34 Among these phases, rutile is the most thermodynamically stable one, while brookite and anatase are metastable. However, down to nanoscale, the latter two phases show better stability owing to the low surface energy.35 

FIG. 1.

Different crystalline phases of TiO2 in the ball-and-stick models and the polyhedron models: (a) rutile, (b) anatase, (c) brookite, (d) TiO2 (B), (e) hollandite-like TiO2 (H), (f) ramsdellite-like TiO2 (R), (g) columbite-like TiO2 (II), (h) baddeleyite-like phase, (i) OI phase, (j) cotunnite-like OII phase, and (k) fluorite-like cubic phase.27 

FIG. 1.

Different crystalline phases of TiO2 in the ball-and-stick models and the polyhedron models: (a) rutile, (b) anatase, (c) brookite, (d) TiO2 (B), (e) hollandite-like TiO2 (H), (f) ramsdellite-like TiO2 (R), (g) columbite-like TiO2 (II), (h) baddeleyite-like phase, (i) OI phase, (j) cotunnite-like OII phase, and (k) fluorite-like cubic phase.27 

Close modal

The research interests of TiO2 photocatalysis lie on solar energy conversions (i.e., photocatalytic H2O splitting and CO2 reduction), which are mostly implemented with anatase and rutile. In comparison with rutile, anatase exhibits more active sites, owing to the higher surface area and larger amount of oxygen vacancies, which results in an accelerated separation efficiency of the charges.36 Additionally, the larger bandgap of anatase (3.20 eV) leads to a higher redox capability than rutile (3.02 eV).37 Additionally, another unique crystalline phase, TiO2 (B) (space group C2/m), has highly open frameworks. With a lower density and a larger specific capacity than others, TiO2 (B) shows potential in applications of photocatalysis, solar cells, and energy storage.38–40 

The properties of TiO2 based materials are largely dependent on their sizes from quantum to nano and bulk scale.41 Microscale- and nano-particle (0D) TiO2 have been prepared through hydrolysis, sol–gel, solvo/hydrothermal, micro-emulsion or reverse micelles, and chemical vapor deposition (CVD).42–44 However, these structured TiO2 exhibits drawbacks of slow charge transfer, limited light trapping, and difficulty in reuse/recycling, resulting in a poor photocatalytic performance. Thus, we focused on the nanostructured (<100 nm) TiO2 materials, especially 1D (nanotubes, nanowires, and nanorods, etc.), 2D (nanosheets, nanoplates, etc.), and 3D hierarchical structures.45–51 Nanostructured TiO2 materials show merits on high control over the light guiding, reactant diffusion, and electron transfer pathways, which provides significantly specific functionalities for PCC.

1D TiO2 nanostructures (such as nanotubes, nanorods, and nanowires) have given rise to an extensive interest in the application of PCC into hydrocarbon fuels, owing to their large surface areas, fast charge transfer, improved adsorption capacity, and extended light absorption from the light trapping and scattering effect.52 Up to now, moderate amounts of progressive technologies have been reported on the preparation of 1D TiO2 nanostructures, including CVD, hydrothermal method, electrospinning, and electrochemical anodization. Brief introduction of 1D TiO2 nanomaterials with various morphologies is discussed.

From our previous work, highly oriented TiO2 nanotubes (NTs) were vertically arranged along the electrodes with a caliber of around 50–150 nm through a facile, fast, and effective two-step anodization process on Ti sheets [Fig. 2(a)].53 Liu and Aydil54 reported nanostructured nanorods (NRs) via a hydrothermal method on fluorine-doped tin oxide (FTO) conducting glass. Figures 2(b) and 2(c) show the NRs’ morphologies, which can be adjusted by the reaction temperature, growth time, acidity, and precursors. This strategy guides the synthesis of TiO2 NRs on various other substrates, such as carbon fibers, Si wafers, and free standing films. It should be pointed out that those prepared NRs with a large diameter and limited active sites usually delivered a poor solar conversion efficiency. To shorten the width and increase the density of NRs, TiO2 seed layers with 10 nm were applied, which accelerates the nucleation process of rutile NRs.

FIG. 2.

Different morphologies of nanostructured TiO2: (a) 1D nanotubes,53 [(b) and (c)] 1D nanorods,54 (d) 1D nanowires,55 [(e)–(g)] 1D nanowires,56–58 [(h)–(o)] 2D nanosheets,60 [(p)–(s)] 2D nanolayers,62 (t) 3D TiO2 films,63 [(u) and (v)] 3D TiO2 nanoboxes,64 (w) TiO2 spheres,65 and (x) porous 3D TiO2 sphere.66 

FIG. 2.

Different morphologies of nanostructured TiO2: (a) 1D nanotubes,53 [(b) and (c)] 1D nanorods,54 (d) 1D nanowires,55 [(e)–(g)] 1D nanowires,56–58 [(h)–(o)] 2D nanosheets,60 [(p)–(s)] 2D nanolayers,62 (t) 3D TiO2 films,63 [(u) and (v)] 3D TiO2 nanoboxes,64 (w) TiO2 spheres,65 and (x) porous 3D TiO2 sphere.66 

Close modal

Despite developing anodization and hydrothermal routes to synthesize 1D nanostructured TiO2, the substantially enhanced electron transfer in these photoelectrodes is rarely researched. Feng and his co-workers successfully obtained single crystal rutile TiO2 nanowires (NWs) on FTO via a ketone–HCl solvothermal process with acetone and n-butanone acted as the reaction media and ketones acted as the solvent. Within the first 15 min, the NWs started to grow, and a length up to 10 µm could be obtained by controlling the growing time. This work revealed that the electron diffusion coefficient of the single crystal TiO2 NWs is 200 times higher than that of rutile for the first time [Fig. 2(d)].55 

Electrospinning is regarded as a powerful and effective route to obtain TiO2 nanofibers (NFs) with nanoscale and large active surface area. Ti(OC3H7)4, poly(vinyl pyrrolidone), acetic acid, and ethanol were successfully used as the precursors for preparing ultralong TiO2 NFs with an average diameter of 80 nm [Figs. 2(e) and 2(f)].56,57 Besides, Yuan et al. synthesized hollow TiO2 NFs by employing activated carbon fiber templates, which were submerged in a mixture of anhydrous ethanol and titanium isopropoxide with a volume ratio of 6:1. After that, the solution was poured into a Teflon reactor and kept at 150 °C for a whole day. Finally, the products were corrected under 600 °C for 5 h to remove the templates [Fig. 2(g)].58 

Fabrication of 2D nanostructured photocatalysts is another attractive and effective strategy to gain improved photocatalytic activity. Similar to 1D nanostructure TiO2, 2D nanostructures also have direct electron pathway together with outstanding electrical and optical properties. Additionally, high specific surface area of 2D nanostructures endows improved surface adsorption ability, more active sites, and band edge alignment leading to product selectivity.

Attracted by the extremely high reactivity of the (001) facet, research studies are concentrated on the development of 2D single crystal TiO2 with high percentage of the reactive (001) facet.59 Wen and co-workers60 obtained highly reactive anatase TiO2 nanosheets (NSs) dominated by (001) and (100) facets with a ratio of 98.7%:1.3% with a large length of 4.1 µm [Figs. 2(h)–2(o)]. Yu et al.61 also synthesized (001) facet dominated anatase TiO2 NSs through a hydrothermal method, showing higher photoelectric conversion efficiency in dye-sensitized solar cells when compared with P25. Reactive (001) facet dominated TiO2 nanoplates (NPs) were systematically investigated by Ma’s group.62 Briefly, 5.0 ml of titanate butoxide was poured into a hydrothermal reactor. 0.6 ml of HF was then stepwisely added with stirring. After that, the autoclave was sealed and reaction was carried out at 180 °C for 30 h. Nanostructured TiO2 NPs ranging from 30 to 50 nm can be obtained through centrifugation and washing [Figs. 2(p)–2(s)].

Combing the preponderances of 1D and 2D nanostructures, hierarchical 3D nanostructures have received wider attention owing to their favorable and exemplary properties, such as porous and interconnected networks, enhanced light harvesting, large surface areas, and increased reactant adsorption, which together ensure improved PCC into valuable chemicals/fuels. Numerous techniques have been conducted for 3D TiO2 architectures with various shapes.

The porous anatase films with columnar and rectangular nanostructure can be achieved from the sol of Ti(OC4H9)4 in ethanol/HCl solution after annealing at 500 °C. As displayed in Fig. 2(t), the 3D TiO2 films are 80–140 nm in thickness.63 Wang et al.64 produced particular TiO2 nanoboxes (NBs) via the conversion of TiOF2 (with edge lengths of 300–500 nm) to anatase, which is made up of NRs arranged perpendicularly on the interface with a length of 80 nm and a diameter of 5 nm [Figs. 2(u) and 2(v)]. TiO2 hierarchical spheres were obtained via the solvothermal reactions by Xie’s group.65 The morphologies of the spheres can be facilely controlled by altering the parameters during the reactions [Fig. 2(w)]. Other interesting 3D nanostructures, such as spherical shaped brookite with a size of 200–400 nm, can also be designed [Fig. 2(x)].66 

The conversion of CO2 to chemicals/fuels is thermodynamically unfavorable owing to its chemical stability and low energy grade.67,68Table I lists the involved reactions that might occur in the PCC in H2O.69 The enthalpy changes70 and Gibbs free energy changes71 of all reactions are highly positive, implying that the reactions are endothermic and cannot take place spontaneously. Moreover, it is obvious that the PCC process can store higher energy than water splitting. Therefore, the process of PCC into fuels requires efficient photocatalysts and large energy inputs.

TABLE I.

Possible reactions during the PCC process.

Reaction∆HΘ (KJ mol−1)∆GΘ (KJ mol−1)
H2O (l) → H2 (g) + 1/2O2 (g) 286 237 
CO2 (g) → CO (g) + 1/2O2 (g) 283 257 
CO2 (g) + H2O (l) → HCOOH (l) + 1/2O2 (g) 270 286 
CO2 (g) + H2O (l) → HCHO (l) + O2(g) 563 522 
CO2 (g) + 2H2O (l) → CH3OH (l) + 3/2O2 (g) 727 703 
CO2 (g) + 2H2O (l) → CH4 (g) + 2O2 (g) 890 818 
Reaction∆HΘ (KJ mol−1)∆GΘ (KJ mol−1)
H2O (l) → H2 (g) + 1/2O2 (g) 286 237 
CO2 (g) → CO (g) + 1/2O2 (g) 283 257 
CO2 (g) + H2O (l) → HCOOH (l) + 1/2O2 (g) 270 286 
CO2 (g) + H2O (l) → HCHO (l) + O2(g) 563 522 
CO2 (g) + 2H2O (l) → CH3OH (l) + 3/2O2 (g) 727 703 
CO2 (g) + 2H2O (l) → CH4 (g) + 2O2 (g) 890 818 

The PCC process can output various possible products, such as CO, CH4, and CH3OH.72–75 The thermodynamic potentials of PCC into different products vs normal hydrogen electrode (NHE) at pH = 7 in water at 25 °C and 1 atm are listed in Table II.76 It is unfavorable to realize the reaction of CO2 to CO2 by one electron, owing to the very low thermodynamic potential of −1.9 V. The structural differences between linear CO2 and bent CO2 result in a large kinetic “overvoltage” requirement to trigger the reaction.77 It has been widely accepted that reactions with the multiple proton-coupled electron transfer process are more favorable to take place because of their higher reductive potentials. Converting CO2 into different products displays different redox potentials and requires different numbers of electrons and protons.78 According to the thermodynamic potentials, catalytic reduction of CO2 to CH4 (−0.24 V) is more feasible than that of H+ to H2 (−0.41 V); however, we should also take the kinetic factors into consideration. In term of kinetics, the reduction of H+ to H2 is more favorable, which is a drastic competing reaction with the PCC.79 We often use the selectivity to estimate the efficiency of PCC, and the selectivity of PCC is the result of a compromise between thermodynamics and kinetics.

TABLE II.

Thermodynamic potentials of PCC into different products.

ReactionE (V) vs NHE at pH = 7
CO2 + e → CO2 −1.9 
CO2 + 2H+ + 2e → CO + H2−0.53 
CO2 + 4H+ + 4e → C + 2H2−0.20 
CO2 + 2H+ + 2e → HCOOH −0.61 
CO2 + 4H+ + 4e → HCHO + H2−0.48 
CO2 + 6H+ + 6e → CH3OH + H2−0.38 
CO2 + 8H+ + 8e → CH4 + 2H2−0.24 
2CO2 + 9H+ + 12e → C2H5OH + 3H2−0.33 
2H+ + 2e→ H2 −0.41 
ReactionE (V) vs NHE at pH = 7
CO2 + e → CO2 −1.9 
CO2 + 2H+ + 2e → CO + H2−0.53 
CO2 + 4H+ + 4e → C + 2H2−0.20 
CO2 + 2H+ + 2e → HCOOH −0.61 
CO2 + 4H+ + 4e → HCHO + H2−0.48 
CO2 + 6H+ + 6e → CH3OH + H2−0.38 
CO2 + 8H+ + 8e → CH4 + 2H2−0.24 
2CO2 + 9H+ + 12e → C2H5OH + 3H2−0.33 
2H+ + 2e→ H2 −0.41 

The efficiency of PCC depends highly on the suitable band positions and bandgap energy (Eg) of the photocatalysts.80 To overcome the slow kinetics and to promote the PCC reaching a satisfactory rate, the position of a photocatalyst’s CB should be higher than the redox potential for PCC, whereas the VB should be lower than the oxidation potential of H2O to O2, and the absorbed light energy (hv) should be higher than or equal to the Eg.81,82Figure 3(a) shows the band positions of some typical semiconductors, such as TiO2, Cu2O, ZnO, CdS, and Bi2WO6.83–87 

FIG. 3.

(a) Band positions of some semiconductors relative to the energy levels of CO2 reduction. (b) Typical process of the photocatalytic reduction of CO2.

FIG. 3.

(a) Band positions of some semiconductors relative to the energy levels of CO2 reduction. (b) Typical process of the photocatalytic reduction of CO2.

Close modal

The typical PCC process with H2O over TiO2 involves the following three main steps [Fig. 3(b)]:87 (1) H2O and CO2 are adsorbed on the surface of TiO2, (2) the electrons in the VB are excited to the CB of TiO2 to produce the photogenerated e–h+ pairs, and (3) the excited electrons then transfer onto the surface of TiO2 and reduce the adsorbed CO2 into solar fuels (e.g., CO, CH4, CH3OH, and HCOOH), while the photogenerated holes oxidize H2O to O2. Thus, to obtain high activity photocatalysts for PCC over TiO2, effective strategies for the modifications of TiO2 should be developed to speed up the charge transfer and prevent the recombination of e/h+ pairs.

Despite the fact that nanostructured TiO2 materials show favorable performances in PCC, their photocatalytic activities are still limited for practical application due to their low utilization of the solar and fast recombination of photogenerated e/h+ pairs. Strategies on the modification of TiO2 are mainly concerned with narrowing the bandgap to improve its photo reactivity, including loading metal species/doping with non-metal elements, constructing defects with O vacancies or Ti3+, and designing heterojunctions with low bandgap candidates.

Doping offers nanostructured TiO2 abundant vacancies and defects, which can change the properties such as the thickness of the space charge layer and the concentration of surface states.88 Loading noble metals (such as Ag, Au, and Pt) on nanostructured TiO2 is an effective way to enhance the efficiency of photocatalytic reactions. Noble metals can act as sinks for photogenerated charge carriers, which promote the delivery of interfacial charges. For instance, Tahir et al.89 reported size-controlled Au nanoparticles loaded 1D TiO2 NWs for PCC into value-added chemicals. Au nanoparticles were deposited on TiO2 NWs via a chemical reduction method [Fig. 4(a)], and the optimized 0.5% Au–TiO2 NWs exhibited a higher production of CO, CH4, and CH3OH (1237 µmol/g h, 13 µmol/g h, and 12.6 µmol/g h, respectively) under visible light irradiation as compared to bare TiO2 NWs (9 µmol/g h, 3 µmol/g h, and 0 µmol/g h, respectively). The enhanced performance can be attributed to the synergetic effect of Au nanoparticles. The smaller nanoparticles act as electron extractors and the larger ones inject hot e into the CB of TiO2 NWs as a result of the localized surface plasmon resonance (LSPR) effect. Figure 4(b) displays the mechanism of the Au–TiO2 NWs for PCC. For a more stable PCC performance, Low et al.90 constructed Ag nanoparticles on the inner side of TiO2 NTs via an electrodeposition method, resulting in an enhanced yield of CH4 with a small amount of CH3OH under light irradiation compared to bare TiO2 NTs.

FIG. 4.

(a) The SEM image of Au–TiO2 NWs.89 (b) Schematic presentation of conversion of CO2 into CO, CH4, and CH3OH by Au–TiO2 NWs. (c) Schemes illustrating various possible changes existing in the bandgap electronic structure of TiO2 when doping with non-metal elements.95 (d) UV−vis absorption spectra of pure TiO2 NSs and N-doped TiO2 NSs with exposed {001} facets.96 (e) Total DOS of doped TiO2 and the projected DOS into the doped anion sites calculated by FLAPW. (f) HRTEM images of the TiO2−x−0.50 sample. (g) The reduction selectivity of PCC for CH4 and CO evolutions of TiO2−x−0 and TiO2−x−0.50.99 (h) Schematic presentation of the impurity band upsweeping due to the fluorination of the MSC. (i) Photocatalytic CH4 production under solar simulated light and in the presence of CO2 and H2O vapor over different samples.101 

FIG. 4.

(a) The SEM image of Au–TiO2 NWs.89 (b) Schematic presentation of conversion of CO2 into CO, CH4, and CH3OH by Au–TiO2 NWs. (c) Schemes illustrating various possible changes existing in the bandgap electronic structure of TiO2 when doping with non-metal elements.95 (d) UV−vis absorption spectra of pure TiO2 NSs and N-doped TiO2 NSs with exposed {001} facets.96 (e) Total DOS of doped TiO2 and the projected DOS into the doped anion sites calculated by FLAPW. (f) HRTEM images of the TiO2−x−0.50 sample. (g) The reduction selectivity of PCC for CH4 and CO evolutions of TiO2−x−0 and TiO2−x−0.50.99 (h) Schematic presentation of the impurity band upsweeping due to the fluorination of the MSC. (i) Photocatalytic CH4 production under solar simulated light and in the presence of CO2 and H2O vapor over different samples.101 

Close modal

Doping TiO2 with non-metal elements, such as N, C, and S, has also been widely reported to enhance the photocatalytic performance by realizing the absorption of longer wavelengths and fast charge separation since 1980s.91–94 As displayed in Fig. 4(c),95 the predicted effects of doping on the band structure of TiO2 could give rise to the red shift of the TiO2 absorption edge by creating localized states and narrowing the bandgap. Liu and colleagues96 synthesized N-doped TiO2 NSs with exposed {001} facets through a hydrothermal route. The as-doped N existed as O–Ti–N bonds or interstitial N, which obviously enhanced the absorption of visible light [Fig. 4(d)]. As shown in Fig. 4(e),97 N acts as the best dopant with the mixed p states with O2p, causing a bandgap narrowing. Similarly, iodine doping could also be an appropriate route. He’s research group98 reported the synthesis of iodine-doped fluorinated TiO2, which offers visible-light-driven activity for PCC. Differing from the replacement of O2− by N, the iodine doping could partially replace Ti4+ to form the I–O–Ti and I–O–I bonds, which introduced states slightly above the VB and below the CB in TiO2, respectively. Meanwhile, iodine doping and surface fluorination may enhance the generation of Ti3+, which can boost the formation of CO2 and prolong the lifetime of the photogenerated charges.

Recently, the reactivity of O vacancies and Ti3+ species has been connected to the processes beyond H2 generation, which demand more energy, such as NH3 synthesis and CO2 reduction. The O vacancy is a significant defect, which directly affects the surface property of TiO2. It is a scientifically interesting way to introduce O vacancies and the corresponding Ti3+ species into TiO2, which could improve the PCC performance. Fang’s research group designed a unique nanostructured TiO2 with an increased amount of Ti3+ species. A layer of amorphous surface with a thickness of 1–2 nm can protect the Ti3+ species far away from oxidation in air and water [Fig. 4(c)].99 The exposed (001) and (101) planes function as the collectors for the holes and electrons, respectively, therefore impeding their recombination. The adjustable surface morphology exhibits good selectivity for the conversion of CO2 to CO and methane [Fig. 4(g)]. Under the guidance of DFT calculations, Rodriguez et al.100 presented that the brookite (210) surface had negligible charge transfer to the CO2. Nevertheless, the O vacancies on brookite (210) can boost the charge transfer and expedite the interaction of CO2 with TiO2. Another work relating the effect of fluorinated TiO2-x on the selective PCC toward methane was presented by Xing’s group.101 DFT calculations revealed that the F atoms substituted O vacancies in the lattice interval of TiO2−x, which attracted the electrons from the Ti3+ species, regenerating Ti4+ owing to the strong electro-negativity of F atoms. As a result, an internal strong electrical field appeared, leading to the upsweep of the Ti3+ impurity level and to a stronger reduction potential of the material [Fig. 4(h)]. The higher selectivity of the fluorinated TiO2−x was explained in terms of better kinetics, which allowed a fast reduction of the intermediate CO to methane [Fig. 4(i)].

A well-designed heterojunction of nanostructured TiO2 with other low bandgap materials can also effectively improve the photocatalytic performance for PCC. Generally, it can speed up the charge separation and suppress the recombination of eh+ pairs. In this section, typical heterostructures based nanostructured TiO2 will be introduced and discussed in the application of CO2 photocatalytic reduction.

Constructing surface heterojunctions of TiO2 crystals is an efficient method for PCC due to the separation of photogenerated e–h+ pairs on different facets. Yu et al.102 reported that anatase TiO2 with {001} and {101} facets was approximately ten times higher than that with the single {101} facet for the yield of CH4. Similarly, Ohno and colleagues103 acquired enhanced PCC performance from brookite with {210} and {212} facets over the commercial brookite. It was found that larger ratios of exposed {210}/{212} facets yield higher amount of methanol, which could be attributed to the combination of reduction sites ({210}) and oxidation sites ({212}).

Heterojunctions based on nanostructured TiO2 and p-type semiconductors can also provide a desirable PCC performance. As the typical p-type semiconductor with a narrow bandgap (−2.0 eV), Cu2O was successfully introduced to construct a porous Cu2O/TiO2 heterojunction with a high surface area by Xu and co-workers.104 The Cu2O/TiO2 heterojunction displayed better CO2 adsorption and higher CH4 production than P25 and porous TiO2. As shown in Fig. 5(a), e could transfer from the CB of Cu2O to the CB of TiO2 under the light irradiation, whereas h+ could transfer from the VB of TiO2 to the VB of Cu2O, resulting in a fast separation of photogenerated e and h+. Wei et al.105 developed a core–shell structured Au@CdS/TiO2 ternary heterojunction [Figs. 5(b)–5(d)], showing a high formation rate of CH4 (41.6 μmol g−1 h−1) with good stability and selectivity.

FIG. 5.

(a) Schematic of the charge transfer process on the surface of the Cu2O/TiO2 heterojunction.104 (b) SEM, TEM, and HRTEM images of the Au@CdS/TiO2 heterojunction. (c) Stability of the formation rate of H2, CH4, and CO over the Au@CdS/TiO2 heterojunction. (d) Mechanism for PCC over the Au@CdS/TiO2 heterojunction.105 (e) TEM image, (f) UV–vis DRS spectra, and [(g) and (h)] PCC activity of the N–TiO2/GR heterojunction.106 

FIG. 5.

(a) Schematic of the charge transfer process on the surface of the Cu2O/TiO2 heterojunction.104 (b) SEM, TEM, and HRTEM images of the Au@CdS/TiO2 heterojunction. (c) Stability of the formation rate of H2, CH4, and CO over the Au@CdS/TiO2 heterojunction. (d) Mechanism for PCC over the Au@CdS/TiO2 heterojunction.105 (e) TEM image, (f) UV–vis DRS spectra, and [(g) and (h)] PCC activity of the N–TiO2/GR heterojunction.106 

Close modal

Graphene is a kind of 2D carbonaceous material with high thermal conductivity, large specific surface areas, and unexceptionable charge mobility. Combining TiO2 with graphene to form a G-TiO2 heterojunction can also improve the efficiency of PCC. The large surface area offers a close contact between TiO2 and graphene for the formation of the G-TiO2 heterojunction, which can promote the transfer of photogenerated e from the CB of TiO2 to graphene, resulting in a fast separation of the photogenerated e–h+ pairs. Ong’s research group106 successfully prepared graphene-supported N-doped TiO2 (N–TiO2/GR), which displayed 11-fold higher yield of CH4 than the pure TiO2 under irradiation [Figs. 5(e)–5(h)]. The enhanced performance can be ascribed to the formed Ti–O–C bond and N doping, which enhanced the visible light absorption. Table III draws a comparison of performances for the photocatalytic reduction of CO2 between various nanostructured TiO2 based photocatalysts.

TABLE III.

Summary of nanostructured TiO2 based photocatalysts for CO2 reduction.

PhotocatalystReaction conditionsYields of productsReferences
Au/TiO2 HID 35 W Car lamp CO: 1237 µmol g−1 h−1 89  
  CH4: 13 µmol g−1 h−1  
  CH3OH: 13.65 µmol g−1 h−1  
Flame annealed TiO2 100 W Xenon solar simulator CH4: 156 µmol g−1 h−1 107  
Graphene/TiO2 300 W Xe lamp CO: 75.8 µmol g−1 h−1 108  
  CH4: 12.3 µmol g−1 h−1  
Ag/TiO2 35 W HID Car lamp CO: 983 µmol g−1 h−1 109  
  CH4: 9.73 µmol g−1 h−1  
  CH3OH: 13 µmol g−1 h−1  
CdS QDs-Cu2+-TiO2 300 W Solar simulated Xenon lamp C2H5OH: 109.12 µmol g−1 h−1 110  
TiO2 300 W Xe lamp, gas phase, 100 mg catalyst CH4: 1.35 µmol g−1 h−1 102  
I/TiO2 500W Xe lamp, gas phase, 0.3 g catalyst CH4: 9.09 µmol g−1 h−1 98  
  CO: 3.43 µmol g−1 h−1  
N–TiO2/graphene 15 W Xe lamp, gas phase, 1 g catalyst CH4: 0.37 µmol g−1 h−1 106  
g-C3N4/TiO2 300 W Xe lamp, gas phase, 60 mg catalyst CO: 0.84 µmol g−1 h−1 111  
  CH4: 5.21 µmol g−1 h−1  
MoS3/TiO2 500 W Xe lamp, liquid phase, 0.15 g catalyst CO: 0.53 µmol g−1 h−1 112  
  H2: 361.1 µmol g−1 h−1  
  CH4: 0.03 µmol g−1 h−1  
Cu2O/TiO2 300 W Xe lamp CH4: 0.0284 µmol g−1 h−1 104  
CuO–TiO2–xNx AM 1.5 CH4: 41.3 µmol g−1 h−1 113  
CdS–TiO2 Hg lamp CO: 2 µmol g−1 h−1 114  
  CH4: 0375 µmol g−1 h−1  
FeTiO3/TiO2 300 W Xe lamp CH3OH: 0.462 µmol g−1 h−1 115  
CdS/rGO/TiO2 Xe lamp CH4: 0.12 µmol g−1 h−1 116  
PhotocatalystReaction conditionsYields of productsReferences
Au/TiO2 HID 35 W Car lamp CO: 1237 µmol g−1 h−1 89  
  CH4: 13 µmol g−1 h−1  
  CH3OH: 13.65 µmol g−1 h−1  
Flame annealed TiO2 100 W Xenon solar simulator CH4: 156 µmol g−1 h−1 107  
Graphene/TiO2 300 W Xe lamp CO: 75.8 µmol g−1 h−1 108  
  CH4: 12.3 µmol g−1 h−1  
Ag/TiO2 35 W HID Car lamp CO: 983 µmol g−1 h−1 109  
  CH4: 9.73 µmol g−1 h−1  
  CH3OH: 13 µmol g−1 h−1  
CdS QDs-Cu2+-TiO2 300 W Solar simulated Xenon lamp C2H5OH: 109.12 µmol g−1 h−1 110  
TiO2 300 W Xe lamp, gas phase, 100 mg catalyst CH4: 1.35 µmol g−1 h−1 102  
I/TiO2 500W Xe lamp, gas phase, 0.3 g catalyst CH4: 9.09 µmol g−1 h−1 98  
  CO: 3.43 µmol g−1 h−1  
N–TiO2/graphene 15 W Xe lamp, gas phase, 1 g catalyst CH4: 0.37 µmol g−1 h−1 106  
g-C3N4/TiO2 300 W Xe lamp, gas phase, 60 mg catalyst CO: 0.84 µmol g−1 h−1 111  
  CH4: 5.21 µmol g−1 h−1  
MoS3/TiO2 500 W Xe lamp, liquid phase, 0.15 g catalyst CO: 0.53 µmol g−1 h−1 112  
  H2: 361.1 µmol g−1 h−1  
  CH4: 0.03 µmol g−1 h−1  
Cu2O/TiO2 300 W Xe lamp CH4: 0.0284 µmol g−1 h−1 104  
CuO–TiO2–xNx AM 1.5 CH4: 41.3 µmol g−1 h−1 113  
CdS–TiO2 Hg lamp CO: 2 µmol g−1 h−1 114  
  CH4: 0375 µmol g−1 h−1  
FeTiO3/TiO2 300 W Xe lamp CH3OH: 0.462 µmol g−1 h−1 115  
CdS/rGO/TiO2 Xe lamp CH4: 0.12 µmol g−1 h−1 116  

The photocatalytic CO2 reduction technology is an economical and sustainable solution to alleviate the concentration of atmospheric CO2 by which CO2 as a feedstock can be converted into value-added chemicals/fuels. In this minireview, recent advances of nanostructured TiO2 materials for the photocatalytic CO2 reduction are systematically summarized. The fabrication strategies of 1D, 2D, and 3D nanostructured TiO2 are reviewed, along with presenting relating bottlenecks. In turn, extensive efforts on the modification of nanostructured TiO2 for the better performance of PCC are further covered.

Nevertheless, there is still a long way to realize the industrial application of PCC to value-added fuels. On the one hand, the energy efficiency for the conversion of photon energy into chemical energy is unsatisfactory. Additionally, the reduced hydrocarbons may be instantaneously oxidized and retained on the surface of the photocatalysts during the reactions, which would passivate the photocatalytic activity. These influences should be mitigated as much as possible. On the other hand, the product selectivity is still low due to the accompanied HER reaction during the CO2 reduction. To tackle this concern, photocatalysts should be precisely tailored to possess superb adsorption and activation ability of CO2 through increasing the surface area, constructing the surface defects, and designing the heterojunctions. Moreover, close attention should also be paid to the development of emerging and high-efficiency photoreactors for practical PCC.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51702225 and 21825501) and the Jiangsu Youth Science Foundation (Grant No. BK20170336). J. Cai, F. Shen, Z. Shi, and J. Sun acknowledge the support from the Suzhou Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Suzhou, China.

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