Photocatalytic reduction of carbon dioxide by titanium oxide-based semiconductors to produce fuels

Abstract

To tackle the crisis of global warming, it is imperative to control and mitigate the atmospheric carbon dioxide level. Photocatalytic reduction of carbon dioxide into solar fuels furnishes a gratifying solution to utilize and reduce carbon dioxide emission and simultaneously generate renewable energy to sustain the societies. So far, titanium oxide-based semiconductors have been the most prevalently adopted catalysts in carbon dioxide photoreduction. This mini-review provides a general summary of the recent progresses in titanium oxide-catalyzed photocatalytic reduction of carbon dioxide. It first illustrates the use of structural engineering as a strategy to adjust and improve the catalytic performances. Then, it describes the introduction of one/two exogenous elements to modify the photocatalytic activity and/or selectivity. Lastly, it discusses multi-component hybrid titanium oxide composites.

Keywords

photocatalysis / carbon dioxide reduction / semiconductors / titanium oxide / renewable fuels

Introduction

Global warming caused by the rapid and overwhelming emission of carbon dioxide (CO₂) has become a serious and imperative worldwide issue concerning all human beings [¹–³]. Due to intense anthropogenic activities, especially the massive combustion of fossil fuels, the original balance of nature carbon cycle has been gravely damaged. At present, the atmospheric CO₂ level largely deviates from its reasonable values which have been maintained in the past millions of years and reaches almost the highest level of approximately 410 ppm in history [⁴]. Besides, new records of high concentrations are predicted in the coming years with the current increasing rate. To fight against climate change, scientists have made persistent endeavors to capture, store, and utilize CO₂ in efficient and feasible ways [⁵–⁹]. It is devised that CO₂ can be utilized as an ample, low-cost, and readily available C1 platform resource to produce diverse chemicals and fuels, including formic acid (HCOOH), methanol (CH₃OH), carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), etc [¹⁰–¹²] (see Fig. 1(a)). The C1 chemistry based on CO₂ as a starting material puts forward a new direction to furnish the societies with energy and decrease the reliance on fossil oils, which could potentially close carbon cycle and realize zero-net carbon emission. Nevertheless, one important concern related with CO₂ refinery into fuels is the required energy input to proceed the conversion which may partially or sometimes completely offset the produced energy fuels. As a result, if the used energy is generated from non-renewable fossil fuels, the energy consumption of the reaction systems must be minimized by all kinds. However, to maximize the efficacy of producing fuels from CO₂, another option is to exploit renewable energy for the reduction of CO₂ to generate chemicals and fuels.

Solar energy represents a type of clean, sustainable, and abundant energy source. Based on estimations, the total solar energy reaching the planet per hour upon full utilization could approximately meet the demand of the annual global energy consumption [¹³]. Photocatalytic reduction of CO₂ using semiconductors provides a viable and promising way to harness solar energy and store it in the form of chemical fuels. It resembles the natural photosynthesis in plants transforming CO₂ and water into carbohydrates, whereas in photocatalytic reduction, rationally designed or modified semiconductors are employed as the catalysts to accelerate the reaction rates, adjust the selectivity toward different products and boost the production yields. Titanium oxide (TiO₂)-based materials have been the most widespread semiconductors used for photocatalytic CO₂ reduction since its first application in 1979 [¹⁴], primarily because it is non-toxic, inexpensive, stable, resistant to photo-corrosion, and abundant. The principle of photocatalytic reduction of CO₂ is illustrated in Fig. 1(b). A semiconductor is characterized by its band gap structure. Above the gap lies the conduction band (CB), and below the gap lies the valence band (VB). For TiO₂ materials, the band gap is usually around approximately 3 eV, and thus untreated TiO₂ absorbs energy mainly in the ultraviolet (UV) range. Upon irradiation, an electron can be excited from the VB to the CB, which simultaneously creates an empty state referred to as a hole in the VB. The isolated electron and hole (the charge carrier) may transfer to the surface and serve as active sites to reduce or oxidize adsorbed reactants respectively. Thus, the efficient separation of the electron-hole pairs is fundamentally crucial to improve the photocatalytic efficiency.

Since UV light only accounts for less than 10% and visible light accounts for about 43% of the full spectra of solar energy, various methods have been explored to prepare visible light responsive TiO₂ materials, most of which can often lead to concurrent improvements in charge separation, CO₂ adsorption, etc. In addition, the suppression of hydrogen (H₂) evolution from water is also targeted in catalyst design to enhance the photocatalytic selectivity toward CO₂ reduction. Generally, these methods involve two major strategies: the nanostructure engineering of TiO₂ or/and the introduction of other components to modify the catalytic system. The nanostructure engineering normally focuses on altering the crystal lattice, crystal phases, the morphology, etc. The introduction of other components includes the incorporation of one or more metal/non-metal elements, and the introduced component is often called the co-catalyst. Besides, multi-component hybrid TiO₂ composites have also been broadly reported to well utilize the synergy in different constitutes. In this mini-review, it is objective to showcase the progresses in photocatalytic reduction of CO₂ using TiO₂-based semiconductors in the past years. First, nanostructure engineering of TiO₂ to improve its photocatalytic activity in related works will be illustrated. Moreover, TiO₂ modifications by incorporating one/two other metal elements will be summarized. Furthermore, TiO₂ modifications with non-metal elements as well as multi-component hybrid TiO₂ composites will also be exemplified. Prior to this review, there have been a plethora of excellent reviews [¹⁵–²²] of the reduction of CO₂ into fuels and/or chemicals with a different emphasis that can be referred to if interested. Note that this mini-review mainly focuses on TiO₂ as the photocatalyst and there have also been outstanding works using other catalysts which will not be covered here [²³–²⁵].

Nanostructure engineering of TiO₂

Crystal lattices and phases

The types and ratios of crystal phases in TiO₂ can considerably affect its photocatalytic activity in CO₂ reduction. Compared to single phase TiO₂ (anatase, rutile or brookite), it was deduced that a mixture of different crystal phases was more advantageous in regards of photocatalytic activity due to the generation of heterojunctions that facilitates charge transfer and separation [²⁶,²⁷]. Kandiel et al. found that rutile TiO₂ nanorods moderately decorated with anatase TiO₂ nanoparticles (NPs) displayed a better activity than pure and overloaded rutile TiO₂ in photocatalytic decomposition of acetaldehyde [²⁸]. Moreover, Tan et al. synthesized oxygen-rich TiO₂ NPs using a facile aqueous peroxo-titania route [²⁹]. The oxygen-enriched synthesis environment stimulated the formation of rutile phase TiO₂ at a relatively low calcination temperature of about 300°C (The temperature commonly required is above 600°C). As a result, the as-obtained TiO₂ NPs contained both anatase and rutile crystal phases, forming a heterojunction to promote the electron-hole separation. Moreover, enriched oxygen has also led to enhanced visible light absorption. Owing to the improved charge separation and light absorption, the dual-phase TiO₂ NPs showed>10 times higher activity than pure anatase TiO₂ when the rutile composition was 17.5%. In addition, the morphology-induced crystal structure change was also disclosed to considerably influence the photocatalytic activity of TiO₂, where the synergistic effects of both morphology and crystalline change contributed. For instance, hierarchical TiO₂ nanofibers constituted by interconnected TiO₂ NPs that formed one-dimension (1D) mesoporous nanofiber were fabricated by using a combined electrospinning and sol-gel method [³⁰]. The 1D mesoporous nanofiber morphology boasted an improved charge transfer rate and a decreased electron-hole recombination rate, compared to the randomly aggregated TiO₂ material. Furthermore, the 1D confinement effect induced partial phase transformation from anatase to rutile, and this heterojunction further promoted the charge separation. Using the 1D TiO₂ nanofibers annealed under static argon (Ar), CO was dominantly produced at a rate of 10.2 mmol/(g∙h) at a pressure of 2 bar at 50°C under UV light irradiation.

Likewise, similar observations were made for crystal lattices. Previously, the (001) facets with high-surface-energy were known to display a better activity than (101) facets, and many techniques were established to upraise the portion of (001) facets in TiO₂ crystals to enhance photocatalytic activity of TiO₂ for CO₂ reduction [³¹–³³]. Nonetheless, the merits of proper blends of these facets were proposed recently. In 2014, Yu et al. put forward a new concept of “surface heterojunction” and emphasized the pivotal role of cooperative effects between (001) and (101) facets [³⁴]. A series of samples with varied facet composition were prepared using hydrofluoric acid (HF) treatment, and investigated by combined experiments and density functional theory (DFT) calculations. With a suitable ratio, the presence of (101) and (001) facets formed a heterojunction similar to type II band alignment in semiconductor-based heterostructures. The heterojunction assisted in the separation of electrons and holes to (101) and (001) facets respectively, and thus resulted in longer charge carrier lifetimes and an inhibited electron-hole recombination rate. With about 0.55 (101) facets, the highest CH₄ yield was realized at 1.35 mmol/(g∙h) under UV light irradiation and atmospheric pressure at room temperature. In line with the work, oxygen-deficient TiO₂ nanocrystals with coexposed (001) and (101) facets were also synthesized to exploit the combined, positive effects of mixed crystal facets and defective sites [³⁵]. TiO₂ nanocrystals with different exposed facets were synthesized for comparison by using a hydrothermal method, whose scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM) characterizations were shown in Fig. 2. The surface and bulk Ti³⁺ /oxygen vacancy defective sites were generated by sodium borohydride (NaBH₄) reduction, and these defects enabled the material to be visible light responsive. The developed catalyst exhibited a much higher activity than unmodified TiO₂ and TiO₂ with merely coexposed facets or merely oxygen defects. Using the synthesized catalyst, CO was formed as the major product in the photoreduction of CO₂ by water vapor, and the yields were about 11.0 and 5.0 mmol/(g∙h) (The quantum yields were 0.3% and 0.1%.) respectively under UV-vis light or visible light irradiation. Based on in situ analysis, coexposed facets presumably rendered a favorable environment for reversible CO₂ adsorption-desorption, while the defective sites offered extra sites for CO₂ adsorption and activation.

In material sciences, the change in shape and morphology can usually alter the crystal facets at the same time. Wu et al. conducted a mechanistic study by using DFT calculations and claimed that anatase TiO₂ nanobelts with exposed (101) facets in its two main surfaces (as shown in Fig. 3) displayed superior photocatalytic activity to spherical TiO₂ NPs with the same crystal phase and comparable specific surface area [³⁶]. One important merit of the nanobelt was that the surfaces with (101) facets were highly reactive to interact with molecular oxygen gas (O₂) and favored the formation of active superoxide radicals. The other prominent merit was its much enhanced efficiency in the separation of photogenerated electron and hole pairs. Several reasons were proposed to explain the improved charge separation: the longitudinal dimension of nanobelts led to a better charge mobility; the adsorbed O₂ on the facet acted as electron traps to separate the charge carriers; and the number of localized states near and/or in the band gap were reduced.

In addition, the strategy to achieve an integrated engineering of the multilevel structures was sometimes employed in order to further enhance the photocatalytic activity. For example, hierarchical yolk@shell microspheres with a hollow chamber that self-assembled by TiO₂ nanosheets (2–5 nm thickness) were synthesized, through a solvothermal process of organic amine-mediated alcoholysis of titanium (IV) precursor [³⁷]. During the synthesis, carbon- and nitrogen-containing functional groups were simultaneously grafted onto the surface. With the special morphology, more energetic defective sites were generated on the interfaces to boost the reactivity. Meanwhile, the presence of carbon and nitrogen heteroatoms promoted the interaction and adsorption of CO₂ and narrowed the bandgap to enable visible light absorption. In the synthesis, the solvothermal temperature had an essential influence on the crystallinity and morphology of the catalyst, and no such catalysts could be attained below 160°C, which means the temperature must exceed this threshold to make the successful formation of the desired catalysts. Besides, additional calcination step in air was attempted after the solvothermal process, nevertheless, this extra step reduced the nitrogen content and the surface functional groups of the catalyst which showed inferior photocatalytic activity. The best-performed catalyst was obtained at a solvothermal temperature of 200°C without calcination, and methanol was the major product with a highest yield of 2.1 mmol/(g∙h). The recycling tests were undertaken for this catalyst, which suggested that organic species might adsorb onto the catalyst surface and resulted in unsatisfactory catalytic stability, whereas it could be recovered by simply washing the catalyst after each run.

Black TiO₂

In 2011, a breakthrough was made by Mao’s group in TiO₂-based photocatalysis [³⁸]. In their work, a distinct strategy was adopted to synthesize visible light responsive TiO₂ materials, which introduced disorder in the surface layers of nanophase TiO₂ through hydrogenation. After hydrogenation, the outer layer (about 1 nm thickness) became disordered and the material color turned from white to black with substantially enhanced light harvesting into visible light and infrared range. The bandgap of black TiO₂ NPs prepared by a one-step reduction/crystallization displayed an estimated value of 1.85 eV. The unique crystalline and defective core/disordered shell morphology were the key factors in determining its electronic structure [³⁹].

Black TiO₂ has been widely used in photocatalysis for organic pollutant decomposition, water splitting, etc [⁴⁰–⁴²]. In 2016, platinum (Pt) supported on black TiO₂ catalyst was developed using an impregnation method followed by hydrogenation, and was adopted for visible light photocatalytic reduction of CO₂ by CH₄ (CO₂ reforming of CH₄, abbreviated as CRM reaction) at elevated temperatures [⁴³]. The Pt/black TiO₂ was further loaded onto the light-diffuse reflection surface of a silica dioxide (SiO₂) substrate, in order to significantly intensify light absorption capacity. High reaction temperature was introduced in the system because based on the relationship between band gap values and the redox potentials (see Fig. 4), high temperature altered the CO₂ redox potential to more easily produce CO. Simultaneously, the high temperature also favored the thermocatalytic reaction of CRM. Under the effects of supports and high temperature, exceptional H₂ and CO yields of 120.0 mmol/(g∙h) and 270.0 mmol/(g∙h) were achieved respectively under visible light irradiation at 650°C, with a high apparent quantum yield of 57.8%.

TiO₂ modifications by incorporating other elements

The incorporation can use a single metal element or multiple metal elements as the co-catalysts. Table 1 summarizes the type of co-catalyst employed and their performances in CO₂ photoreduction.

Incorporation of a single metal element

The introduction of another element often imposes effects on the electron transfer and/or electron-hole separation, which modifies the reaction process. Noble metals were frequently used as the incorporating element to improve the performance of the commercialized TiO₂ (P25). In the following paragraphs, the TiO₂ referred to the P25 material if not specifically described. Yu et al. fabricated silver (Ag) NPs loaded TiO₂ by a simple silver mirror method with enhanced photocatalytic activity due to the surface plasmonic resonance (SPR) and electron sink effect of the Ag component [⁴⁴]. The SPR promoted visible light absorption and generated hot electrons on Ag, while the photoexcited electrons transferred from the surface of TiO₂ to Ag concurrently because of the lower Fermi level. Hence, the electrons enriched on Ag surface to enhance the reduction capacity and retard the recombination of photogenerated electron-hole pairs. Since particle size influences the SPR, Ag NPs with a smaller size of 2–3 nm performed better than the larger NPs, reaching a CH₄ yield of 1.4 mmol/(g∙h) under UV-vis light irradiation in the gas phase. When the photoreduction was conducted in aqueous phase, the major product was methanol with a yield of about 4.3 mmol/(g∙h). Possible explanations (see Fig. 5) were given for the altered reaction pathway that CO₂ might undergo a chain of fast deoxygenation reactions in gas phase, whereas a chain of fast hydrogenation reactions occurred in aqueous phase because water existed in dominantly excessive amount.

Aside from noble metals, non-noble metals were also reported in TiO₂ modifications. Tahir and Amin fabricated indium (In)-incorporated TiO₂ NPs by a controlled sol-gel method [⁴⁵]. The In mainly located on the surface of the NPs in its metallic state, which facilitated the formation of mesoporous anatase phase TiO₂ NPs with smaller crystalline sizes, increased surface areas, and enlarged bandgaps. The incorporation of In also enhanced charge separation and transfer, leading to a higher yield of CH₄ product in CO₂ photoreduction with water vapor. Corma’s group reported copper (Cu)-modified TiO₂ NPs prepared by a solvothermal method [⁴⁶]. The Cu²⁺ was incorporated into the crystal matrix of TiO₂, leading to a narrowed bandgap and an improved photocatalytic activity. Under UV-rich light, with Na₂S as the sacrificial electron donor, the Cu-doped catalyst promoted the reduction of CO₂ into formic acid with 25.7 mmol/(g∙h) yield in aqueous solution at room temperature after 15 h reaction time. In another work, Cu-modified TiO₂ hollow microspheres were synthesized by a one-pot template-free method (see Fig. 6) [⁴⁷]. The Cu element existed as copper oxide (CuO) and dominantly dispersed on the surface of the shell which did not insert into the crystal matrix of the anatase TiO₂. The large surface area and hierarchical porous structure of the hollow microspheres enhanced the light harvesting of the catalyst, and the introduction of CuO improved the electron trapping ability. The 3 wt% CuO doped TiO₂ hollow microsphere catalyst afforded a higher CO yield of 14.5 mmol/(g∙h), which was 5.8 times than that of using un-doped counterpart catalyst under UV light irradiation. Besides, Cu⁰ doped TiO₂ hollow microsphere catalyst was obtained by hydrogenating the CuO-doped TiO₂ catalyst, which promoted selective reduction of CO₂ into CH₄ with a yield of 30.1 mmol/(g∙h), because Cu⁰ favored the capture of holes and the H₂ generation from water for CH₄ formation.

Incorporation of dual metal co-catalysts

Wang’s group reported the binary co-catalysts Cu₂O/Pt/TiO₂ system and MgO (magnesium oxide)-Pt/TiO₂ system for selective photoreduction of CO₂ into CO and CH₄ with water (at 50°C, 2 bar CO₂ pressure) under UV-vis light irradiation [⁴⁸,⁴⁹]. The catalysts were synthesized by a stepwise photodeposition technique with Pt NPs/TiO₂ formed first. Then, Cu₂O was preferentially deposited on the Pt NPs forming a core-shell structure, while MgO was deposited onto the TiO₂ as amorphous layers. The major role of Pt NPs as a co-catalyst was to capture and enrich the photogenerated electrons as well as to delay the electron-hole recombination. Nevertheless, Pt NPs promoted H₂ formation significantly and led to low selectivity toward CO₂ reduction. Incorporating Cu₂O or MgO remarkably curbed the competing reaction of H₂ generation and facilitated the chemisorption and activation of CO₂. Hence, Cu₂O/Pt/TiO₂ and MgO-Pt/TiO₂ catalysts achieved a high selectivity of 85% and 83% respectively, which were 2-fold of that using pristine TiO₂ or Pt/TiO₂. The highest yield of CH₄ was obtained at about 33.0 mmol/(g∙h) with Cu₂O/Pt/TiO₂. Moreover, the reaction mode was proved to considerably affect the product yield, and higher CH₄ yield was achieved in gas phase reaction mode presumably because of the low solubility of CO₂ in water and the more severe side reaction in liquid water (see Fig. 7). Meanwhile, the synthesis method played a role in the catalytic performance that the catalysts obtained by the photodeposition method exhibited a better activity than those prepared by the impregnation H₂-reduction method and the hydrazine-reduction method.

Garcia’s group developed the gold-copper (Au-Cu)/TiO₂ catalyst for photoreduction of CO₂ by water under simulated concentrate sunlight [⁵⁰]. The Au-Cu bimetallic alloy NPs were fabricated by a stepwise deposition-precipitation method. The catalyst realized 97% of selectivity toward CH₄, and a high production rate of about 2000 mmol/(g∙h) that was about 10 times of those using Au/TiO₂ or Cu/TiO₂ counterparts. Surface species were probed by using FTIR and chemical analysis, including CO₂^{•-, Cu-CO, and elemental carbon. As proposed, the role of Au was to induce the visible light responses, and the Cu adsorbed CO as an intermediate and governed the reduction pathway. The type of excitation lights played a crucial role in reaction pathways that UV light irradiation promoted the H₂ evolution reaction whereas visible light illumination led to the generation of CH₄ product (see Fig. 8). Afterwards, Cu-Pt bimetallic alloy NPs supported on TiO₂ was developed by an impregnation-calcination method, and the size effect of the Cu-Pt nanoalloys were studied [51]. As the particle size decreased, the production yield of CH₄ correspondingly increased. The highest yield of 11.4 mmol/(g∙h) was obtained when the size of Cu-Pt NPs was 1.2 nm under UV-vis light irradiation at 40°C. Several reasons were proposed for the size-dependent photoreduction of CO₂. First, catalysts with smaller size might possess stronger reductive powder due to more quantized energy level, and enhanced binding capacity of CO₂ with increased low-coordinated sites. Moreover, Cu-Pt NPs with a smaller size probably had reinforced interaction with the TiO₂ support, which possibly enabled the co-adsorption of intermediates with –OH groups on the support. Besides, the Pt atoms of smaller Cu-Pt NPs were observed to show a stronger adsorption of protons for hydrogenation of intermediates.}

Cu₂O NPs and Pt NPs decorated on TiO₂ nanocrystals with coexposed (101) and (001) facets were synthesized [⁵²]. The facet-engineered TiO₂ support was obtained by a solvothermal method and the NPs were prepared by a sodium borohydride (NaBH₄) reduction method. The catalyst system significantly suppressed H₂ production, improving the selectivity toward CH₄ to 97% under UV light irradiation. The Cu₂O NPs showed a poor adsorption toward water and thus inhibited the competing reaction, whereas they displayed enhanced chemisorption of CO₂ by Cu₂O NPs as verified by temperature programmed desorption (TPD)-CO₂. Meanwhile, Pt NPs were found to not only extract the photogenerated electrons but also increase the electron density on Cu₂O for selective eight-electron reduction of CO₂ into CH₄. Control experiments suggested that CH₄ was primarily produced via the direct photocatalytic reduction of CO₂, instead of the hydrogenation of CO/CO₂. Likewise, special supports such as amorphous SrTiO₃/TiO₂ coaxial nanotube arrays were employed by Kang et al. with Au-Cu bimetallic alloys as the co-catalysts [⁵³]. The support material could enhance the gas diffusion with its porosity and high surface area and promoted the photogenerated charge separation thanks to its heterostructures. Besides, hydrous hydrazine was used as the reducing agent which exhibited better reductive capacity and offered a protective atmosphere for the Au-Cu NPs. Under UV-vis light irradiation, the highest CH₄ yield was obtained at 421.2 mmol/(g∙h) from diluted CO₂ stream with Au₃Cu co-catalysts. It was believed that the Au atom facilitated the desorption of CO and boosted the formation of hydrocarbons on the Cu atom, and the proposed mechanism mainly followed the glyoxal pathway.

Cu-Pt NPs supported on TiO₂ nanotube arrays were prepared, with an emphasis on the influence of synthesis methods [⁵⁴]. The transparent 1D TiO₂ nanotube arrays as support provided more reaction sites and co-catalyst loading sites due to the high surface area. The photodeposited Cu-Pt NPs have an average size of about 1 nm, and a Schottky barrier was formed at the co-catalyst-TiO₂ interface with an obvious band bending. Nevertheless, sputtered and thermally dewetted Cu-Pt NPs had larger particle sizes and the band bending was not observed. Using the photodeposition method, a higher CH₄ yield of about 0.6 mmol/(cm²∙h) could be achieved which was nearly 20-fold than that when sputtered Cu-Pt NPs catalysts were used. In a following work, the group also synthesized Au, ruthenium (Ru), and zinc-palladium (Zn-Pd) NPs loaded TiO₂ nanotube arrays and examined their photocatalytic activity for CO₂ reduction [⁵⁵]. Au NPs on the support generated CH₄ at the highest yield of 58.5 mmol/(g∙h) possibly because of its smallest particle size. The metal NPs loaded TiO₂ nanotube arrays could not only utilize UV protons but also the blue photons. Tan et al. prepared Ag-Pd bimetallic alloys on nitrogen-doped (N-doped) TiO₂ nanosheets by a hydrazine-reduction method [⁵⁶]. The doping of nitrogen into the nanosheets could favor charge separation and boost visible light absorption. By means of combined analytical methods, the well-dispersed Ag-Pd NPs (having a diameter of about 8.5 nm) facilitated visible-light response, charge transfer and separation, as well as the activation of CO₂. The ratio of Ag and Pd was optimized and a linear growth of the product was found with the prolonged reaction time (see Figs. 9(a) and 9(b)). When simulating sunlight irradiation, the highest CH₄ yield of 79.0 mmol/(g∙h) was obtained by using Ag-Pd/TiO₂ nanosheets in CO₂-saturated aqueous solution at room temperature and ambient pressure, which was 3-fold higher than using monometallic counterparts, and recycling tests were conducted to show the stability of the catalysts (see Figs. 9(c) and 9(d)).

Amin’s group developed a series of dual components-incorporated TiO₂ catalysts (Cu-In, nickel (Ni)-In, Au-In) using the sol-gel method for the photocatalytic reduction of CO₂ into CO with H₂ reductant in a monolith reactor [⁵⁷–⁵⁹]. The dual elements displayed synergistic effects and significantly inhibited the recombination of photogenerated electron-hole pairs. In these catalysts, the elements located at the surface and existed as Cu⁺ and In³⁺ (Cu-In co-doped), Ni²⁺ and In³⁺ (Ni-In co-doped), and Au⁰ and In³⁺ (Au-In co-doped). The selectivity toward the CO product for these three co-doped catalysts were 99.3%, 99.7%, and 99.0% respectively, with the highest yields of 6.5, 12.0 and 9.0 mmol/(g∙h). In a following work, In and Cu-decorated TiO₂ NPs with surface dispersed In³⁺ and Cu²⁺ were obtained by using the modified sol-gel method [⁶⁰]. After the introduction of the two elements, the catalyst boasted smaller crystalline size, reduced bandgap, and enhanced charge separation, which resulted in a better CO₂ photoreduction activity. Under optimal conditions, CH₄ was formed at the highest yield of 181.0 mmol/(g∙h) with water as the reductant, and methanol was formed at the highest yield of 68.0 mmol/(g∙h) with water and H₂ as the reductants. On the other hand, the introduction of a metal element and a non-metal element was exploited. Cu-vanadium (V) modified TiO₂ was synthesized and supported on porous polyurethane (PU) with a honeycomb structure [⁶¹]. The use of support was to ease the recycling and increase the adsorption ability. The Cu element presented as Cu⁺ and Cu²⁺ species either on the surface or in the matrix. The V element existed as V⁴⁺ and V⁵⁺ species, with V⁴⁺ species incorporating into the lattice while V⁵⁺ species distributing on the surface. The Cu and V elements have induced the generation of Ti³⁺ and oxygen vacancies in the lattice, leading to an enhanced charge separation and CO₂ adsorption. Under visible light irradiation, CO₂ was efficiently reduced with water vapor into CH₄ and CO with the production rates of 933.0 mmol/(g∙h) and 588.0 mmol/(g∙h).

TiO₂ modifications by non-metal elements

Apart from metal elements, non-metal elements were also used to modify the structure of TiO₂ to increase its visible light responses and CO₂ adsorption ability. For example, N-doped TiO₂ materials were widely used as photocatalysts [⁶²]. Liu’s group synthesized N-doped anatase TiO₂ microsheets with exposed (001) and (101) facets by using the hydrothermal method with HF and hydrochloric acid (HCl) using titanium nitride (TiN) as the precursor. The N-doped microsheets exhibited a much enhanced activity than undoped TiO₂ for CO₂ photoreduction into CH₄ [⁶³]. Zhang et al. fabricated 1D N-doped TiO₂ nanorod arrays by a hydrothermal treatment using hydrazine or ammonia as the nitrogen source, and pointed out that the use of different nitrogen source had an impact on the selectivity of CO₂ photoreduction [⁶⁴]. The major product was CH₄ and CO respectively when using hydrazine and ammonia as the source, probably because of the highly reductive N-N bonds introduced by hydrazine. In addition, hierarchical phosphate-doped (P-doped) TiO₂ nanotubes were prepared and supported on Ti plates by simple pyrolytic phosphating and electrochemical anodic oxidation [⁶⁵]. Hierarchical nanostructures would enhance the mass transfer and light harvesting. The incorporated P⁵⁺ fractionally substituted Ti⁴⁺ atoms in the crystal lattice to form Ti-O-P linkages, suppressing recombination rate of photogenerated electron-hole pairs. Besides, the P-doped material boasted a reduced bandgap, a higher activity, and a better selectivity. Under visible light irradiation, the maximal methanol yield was obtained at 286.8 mmol/(g∙h) by using water vapor as the reducing agent.

Sulfur-doped (S-doped) TiO₂ anatase was synthesized by using the sonothermal method, and S atoms were predominantly incorporated as S⁴⁺ in the crystal lattice [⁶⁶]. By DFT calculations, S-doping induced the generation of additional states in the bandgap closer to the VB, and presumably enhanced the surface conductivity, the charge transfer efficiency, and the photocatalytic activity. Under visible light irradiation, a production of 35.3 mmol/(g∙h) of CH₄ and 167.9 mmol/(g∙h) of methanol was achieved in basic acetonitrile-water solution (CO₂-saturated) with triethanolamine (TEOA) as sacrificial donor. Moreover, iodine-doped (I-doped) TiO₂ nanosheets with high exposed (001) facets were developed by a two-step hydrothermal treatment followed by calcinations [⁶⁷]. After doping, Ti⁴⁺ replacement by I⁵⁺ was observed with the formation of I-O-Ti and I-O-I bonds, leading to increased visible light responses. Meanwhile, the high exposed (001) facets favored water oxidation and the increased unsaturated Ti atoms curbed the recombination rate of electron-hole pairs. In gas phase, CH₄ and CO were obtained at the yields of 9.1 mmol/(g∙h) and 3.4 mmol/(g∙h) respectively under visible light irradiation.

Multi-component hybrid TiO₂ nanocomposites

Structurally blended TiO₂ with carbon-based materials along with the addition of co-catalysts and/or other elements was employed in some studies to fabricate a hybrid nanocomposite in an effort to further utilize the synergistic effects among multiple components and improve the photocatalytic activity and/or stability. TiO₂-graphene hybrid nanosheets were established by an in situ reduction-hydrolysis technique in ethylenediamine/water solvent [⁶⁸]. The TiO₂ NPs were chemically loaded onto graphene sheets via Ti-O-C linkages and generated a two-dimension (2D) sandwich-like nanostructure. The loaded TiO₂ NPs stabilized the graphene nanosheets preventing them from restacking or collapsing. Abundant Ti³⁺ sites were identified at the surface of TiO₂ NP in hybrid composites which could trap electrons and suppressed photogenerated electron-hole recombination. The hybrid composites selectively promoted the formation of C₂H₆ product, which was distinct from the single component counterparts. Besides, the yield of C₂H₆ correspondingly grew with the increased ratio of graphene in the composite, indicating that the construction of a hybrid composite might provide a facile way to adjust the reaction selectivity.

A hybrid RuRe/TiO₂/graphic carbon nitride (g-C₃N₄) nanosheet was contrived by first anchoring a supramolecular Ru(II)-Re(I) binuclear complex onto rutile TiO₂ NPs which were then coated on g-C₃N₄ nanosheets [⁶⁹]. The band structure of the hybrid catalyst is shown in Fig. 10. The hybrid composites showed superior photocatalytic activity to the analog without TiO₂ in CO₂ reduction to produce CO, which demonstrated about 4-fold production rate and turnover number (TON). The improved photocatalytic activity was ascribed to the enhanced charge separation efficiency at the interface of TiO₂ and g-C₃N₄, as indicated by the results from transient absorption spectroscopy. Moreover, the TiO₂ NPs offered stronger sites to anchor the RuRe complexes than the carbon materials, leading to a better hybrid structure. These two factors in combination determined the improved performances of the hybrid composites.

Jin et al. utilized a Cu-TiO₂/g-C₃N₄ ternary photocatalyst for CO₂ reduction to CH₄ with water vapor under visible light irradiation [⁷⁰]. The interface heterojunctions of the ternary system could efficiently boost the charge separation. Cu-loaded TiO₂ NPs were first prepared by a NaBH₄ reduction process, and then the composites were dispersed on g-C₃N₄ by stirring and annealing. The small Cu NPs were in metallic state and well-dispersed on the surface of TiO₂ NPs, which promoted light absorption. Cu loading was identified as a critical factor to influence the photocatalytic activity with an optimal value of 2.5 wt%, which achieved CH₄ formation at a rate of 23.9 mmol/(g∙h). The reaction mechanism was proposed that the photoexcited electrons were initially injected from the VB to CB of g-C₃N₄, and then migrated to the CB of TiO₂ which eventually accumulated at Cu sites for efficient CO₂ reduction.

Similarly, a Ag-TiO₂/g-C₃N₄ catalyst was developed by adopting simple solvent evaporation-calcination processes [⁷¹]. The remarkable synergy among the components led to increased light harvesting and charge separation to more effectively reduce CO₂ into CH₄ and CO at simulated sunlight illumination. Photogenerated electrons were presumably transferred in this way g-C₃N₄→ TiO₂→ Ag, and the collected electrons were further reinforced by the SPR effect. The multicomponent catalyst of Pt/CoO_x/TiO₂-SiO₂ were synthesized [⁷²] to demonstrate the spatial influence of the catalyst. Pt NPs and CoO_x NPs were anchored at separated spatial locations on hierarchically ordered TiO₂-SiO₂ (HTSO) support which advanced the separation of charge carriers. The HTSO was prepared by the two template evaporate induced self-assemble (EISA) method, and CoO_x NPs were in situ grown and embedded in the skeleton of the support during its formation. Afterwards, Pt NPs were decorated onto the outer surface of HTSO. The Pt NPs acted as electron reservoirs while CoO_x NPs were effective hole collectors, and the physical isolation of the two co-catalysts notably retarded the recombination rate of photoexcited electron-hole pairs because the possibility of an electron recombined with a hole were reduced by the prolonged distance. Compared to randomly dispersed Pt-CoO_x with contacts, spatially separated Pt NPs and CoO_x NPs on HTSO exhibited a higher activity and selectivity to produce CH₄ from CO₂ at simulated solar light illumination. In addition, the composition, porous structure and specific surface area of the support are influential to the photocatalytic performance by mediating the light reflection, mass transfer, electron-hole recombination rate, etc [⁵³–⁵⁶].

Conclusions and future remarks

Photocatalytic reduction of CO₂ has attracted growing attention recently because it exploits clean and renewable solar energy to create fuels, which decreases the dependence on depleting fossil oils and points out a potentially low-carbon future for the sustainable development. TiO₂-based materials are economically viable as photocatalysts because of its cheap price and high stability. The key factors affecting the performances of CO₂ photoreduction mainly include the surface affinity, charge separation efficiency, and the light absorption ability of the catalyst employed. Surface affinity determines the adsorption of CO₂ on the surface of the catalyst, while a strong affinity can help improve the selectivity toward CO₂ conversion and suppress the side reaction, which will influence both the selectivity and yield. A common strategy to improve the affinity is to incorporate basic sites onto the surface. The charge separation efficiency and the light absorption ability are crucially associated with the quantum yields and product yields.

Although various methods have been used to enhance the performance of TiO₂ catalysts, the production rate of photocatalytic reduction of CO₂ is relatively low compared to conventional chemical transformations. Major challenges in CO₂ photoreduction include the further enhancement in product yields, the efficiency of electron-hole separation, and the light harvesting efficiency. Therefore, in the future, different effective and cooperative strategies are anticipated to be developed to considerably increase the CO₂ reduction efficiency and the production rate. For instance, the combined uses of photocatalysis and thermal catalysis may lead to much enhanced results [⁷³]. In addition, the manipulations on reactors and reaction modes can be further studied and optimized in detail to adjust the process conditions. Besides, more rational and elaborate catalyst design is always essential to achieve highly selective reduction of CO₂ with solar lights.

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