Synthetic strategies to nanostructured photocatalysts for CO 2 reduction to solar fuels and chemicals

Arti ﬁ cial photosynthesis represents one of the great scienti ﬁ c challenges of the 21 st century, o ﬀ ering the possibility of clean energy through water photolysis and renewable chemicals through CO 2 utilisation as a sustainable feedstock. Catalysis will undoubtedly play a key role in delivering technologies able to meet these goals, mediating solar energy via excited generate charge carriers to selectively activate molecular bonds under ambient conditions. This review describes recent synthetic approaches adopted to engineer nanostructured photocatalytic materials for e ﬃ cient light harnessing, charge separation and the photoreduction of CO 2 to higher hydrocarbons such as methane, methanol and even ole ﬁ ns.


Introduction
The use of sunlight to split water as a source of electrons that can be stored in chemical bonds for the production of primary energy carriers and molecular building blocks underpins the natural world.Humanity remains highly dependent upon such elegant photosynthetic processes, developed through natural evolutionary selection, for the sustainable growth of biomass, which literally feeds our planet.However, recent technological breakthroughs offer a glimpse of a mid-term future in which anthropogenic systems will enable articial photosynthesis with efficiencies surpassing that of plants, to deliver solar fuels and chemicals. 1,2Arguments for the development and global deployment of articial photosynthesis technology to reduce anthropogenic CO 2 emissions from their historic high, 3,4 increase fuel security, 5 and support a sustainable global economy and ecosystem have been recently expounded, 6,7 and new mechanisms for enhancing the policy and governance prole in the context of energy sustainability sought.
Solar fuels include H 2 (from the reduction of H + derived from water photolysis) or carbon-based fuels (derived from the photoreduction of CO 2 ) such as methane, methanol or CO, and Donna Hui Lin Chen relocated to Sydney, Australia, in 2003 for her BSc degree, majoring in Nanotechnology, at The University of New South Wales.She subsequently moved to the University of Sydney where she commenced a PhD at the School of Chemistry focusing on the synthesis, characterisation, testing and study of potential visible light active catalysts for H 2 generation.In 2012, she took up a Research Assistant position at Cardiff University, where she initiated a CO 2 photoreduction project.She is interested in technologies that minimise the environmental impact of industrial processes.associated higher hydrocarbons via subsequent well-established industrial thermal chemistry.0][11][12] In contrast to a number of competing renewable energy technologies, such as wind, tidal and photovoltaics, wherein electricity is the end product, and hence a dependency exists upon concomitant advances in storage and smart grid capabilities, the trapping of solar energy within chemical bonds provides immediate access to (potentially high) energy density carriers for transportation fuels.Water photooxidation for H 2 production has been reviewed extensively elsewhere; [13][14][15] hence this review focuses on photocatalytic CO 2 reduction.

Xingguang Zhang received his
The potential of CO 2 as a chemical feedstock via chemical and biochemical, 16 electrocatalytic 17,18 and photocatalytic transformations has been widely recognised, with methane 19 and methanol [20][21][22][23] obvious targets to replace fossil fuels in stationary power generation and transportation.5][26] In the context of polymers, the thermochemical incorporation of CO 2 into polycarbonates/carbamates is wellestablished; however, its direct photoreduction to light (C 1 -C 3 ) olens represents a greater challenge.Olens and their polymers are the single largest chemical commodity in the world, with annual global ethene and propene production capacity in 2012 around 156 and 80 Mt respectively. 25,27These olens are currently obtained from non-renewable fossil fuels: 28 commercial ethene and propene manufacture involves steam or catalytic cracking of naphtha, gasoil and condensates to hydrocarbon mixtures followed by distillation.Cracking crude oil to produce ethene or propene is thermodynamically unfavourable (DG z +100 kJ mol À1 ), requiring high temperatures (>600 C) to overcome the huge activation barriers to C-C cleavage (280 kJ mol À1 for kerosene conversion 29 ).Hence steam cracking is the most energy-consuming process in chemistry, accounting for 8% of the sector's primary energy use and annual CO 2 emissions of 180-200 Mt. 28 1.1.Brief history and fundamental principles of photocatalytic materials for CO 2 reduction Renewable solar energy, harnessed via innovative catalysts and reactors, has the potential to photoreduce linear CO 2 molecules by breaking the C]O bonds to form C-H bonds and then to yield products, such as methane, methanol or light olens (e.g.CO 2 + 2H 2 O 4 CH 3 OH + 3/2O 2 ; CO 2 + 2H 2 O 4 CH 4 + 2O 2 ; and 2CO 2 + 2H 2 O 4 C 2 H 4 + 3O 2 ), thereby creating new chemical supply chains free of current dependencies on oil, coal and natural gas and offering an economical and potentially environmentally-benign CO 2 utilisation process (Fig. 1). 30reat challenges lie in the robust chemical state of carbon atoms in CO 2 , which requires not only the participation of incident protons but also effective excited electrons, and thus additional reductive agents can offer better opportunities to facilitate the activation of CO 2 .In this regard, H 2 O is preferred as a reducing agent because it simultaneously takes advantage of water oxidation and CO 2 xation compared with other sacricing reductive agents such as amines, H 2 , and S 2À .Hence, the harvesting of input energy by means of photocatalytic materials from incident light plays a pivotal role in achieving these processes.The rst study on the photocatalytic CO 2 reduction was reported by Halmann in 1978, employing a Hg lamp and single crystal, p-type GaP cathode within an electrochemical cell through which CO 2 was bubbled to yield formic acid, formaldehyde and methanol. 31A year later, Inoue and coworkers reported the photoelectrocatalytic reduction of CO 2 to formic acid, formaldehyde, methanol and methane, over a variery of semiconductors including TiO 2 , ZnO, CdS, GaP, SiC, and WO 3 . 32hese pioneering studies were followed by the development of visible-light driven photocatalysts and production of more functionalised organic moieties, e.g.CO 2 photoreduction via colloidal CdS aqueous solutions to glyoxylic and acetic acids. 33oreover, large-scale photocatalytic CO 2 reduction to solar fuels or chemicals requires intensied processing and carefully engineered robust (and hence likely heterogeneous) catalysts with high accessibility, able to activate small molecules at ambient conditions. 346][37] A practical photocatalytic chemical reactor requires appropriately selected coupled redox reactions, and the ability to efficiently harness light capable of driving the requisite electron-transfer chemistry. 38The former favours water as a green chemical reductant (mimicking natural photosynthesis), while the latter requires active catalysts either able to absorb solar energy directly or coupled to sensitisers which help gather and focus energy onto the active sites.
The optical properties of potential solid photocatalysts are dictated by the rate-limiting oxidation and reduction steps that must occur at the surface of their nanostructures: 39 2H 2 O / O 2 + 4H + +4e À ; CO 2 + e À / CO 2 c À (E o redox ¼ +1.64 V and À1.90 V, respectively, on the basis of NHE (normal hydrogen electrode) at pH ¼ 7).Such values impose a minimum for the photoexcitation energy of electrons needed to reduce CO 2 , whereas few pure materials are photoresponsive to these wavelengths and catalytically active towards CO 2 and water conversion, even though visible light can create sufficiently excited electrons.These insurmountable limitations necessitate the development of new photocatalytic materials particularly with specially-nanostructured antennas to harvest both UV and visible light efficiently and to initiate the chemical reactions effectively.

Strengths and challenges of nanostructured photocatalysts
Nanoscale materials oen exhibit unique physicochemical which diverge widely from their bulk counterparts.Such nanomaterials, commonly synthesised in the form of nanoparticles (NPs), nanobres (NFs) and nanotubes (NTs), in addition to more exotic topologies such as nanobelts and nanoowers, afford high surface areas, tunable architectures and distinct electronic surface states.In particular, photocatalytic nanostructures can offer high densities of photoactive surface sites and thin-walled structures/ultrathin lms able to facilitate rapid interfacial charge carrier transport to adsorbates. 35,40For many years, CO 2 reduction has centred around titania-based photocatalysts, which are only able to utilise a fraction of the solar spectrum ($2%, mainly as UVA), [41][42][43][44][45] and require exploitation of quantum effects or doping to reduce their intrinsic band gaps, 46 sometimes in combination with inorganic or organic supramolecular (biomimetic) sensitisers. 47ignicant advances in articial photosynthesis have been achieved through such photocatalytic nanoarchitectures, 37,48 e.g. the discovery of 'self-repairing' Co x PO 4 catalysts producing O 2 akin to the natural Mn catalyst in photosystem II, 49 corrosion resistant TiO 2 nanocomposites able to split water 50 or reduce CO 2 with H 2 O to yield CH 4 and olens, 40,51 and the use of metal oxynitrides 52 and polymeric nitrides 53 for efficient water splitting under visible light.Solar CH 4 production from CO 2 has seen dramatic improvements in recent years, with TiO 2 NTs delivering 17.5 nanomoles h À1 cm À2 mW À1 of hydrocarbons from 1 bar CO 2 under sunlight, with a quantum efficiency of 0.74%, 40 dened throughout this review as: where M i is the mols of product i, n i is the number of electrons attending the production of product i, and P m is mols of incident photons. 54,55Cu-doped titania coatings deposited on a monolithic substrate have even been reported able to deliver 117 nanomoles h À1 cm À2 m W À1 of methane. 56In addition to maximising the quantum efficiency, productivity and selectivity towards hydrocarbons, the stability of semiconductor photocatalysts under reaction conditions (notably resistance to photocorrosion 57,58 ) and routes to their incorporation into reactor designs which facilitate efficient light absorption and mass transport, [59][60][61] are also key considerations.Signicant developments in molecular nanocatalysts for photochemical CO 2 reduction have been achieved and reviewed; 38,62,63 nevertheless, improvements in the mechanical strength, physicochemical stability and photocatalytic efficiency of such materials remain necessary to meet the engineering requirements for large-scale applications.Molecular band structures control incident light absorption, electron-hole pair generation and charge carrier migration.5][66] A further challenge is the optimisation of CO 2 and H 2 O adsorption over photocatalyst surfaces, which is poorly understood to date, 67,68 and requires a thorough knowledge of the atomic structure of the photocatalyst interface.Advanced inorganic synthetic methods afford precise manipulation of exposed crystal facets, morphologies, structural periodicity, and hierarchical pore networks, [69][70][71] in addition to the co-assembly of sensitisers or co-catalysts and promoters.Meeting these challenges will also necessitate continued advances in our fundamental understanding of photocatalytic reaction and deactivation mechanisms.This review discusses progress in the design of nanostructured solid materials CO 2 to solar fuels and chemicals, with a particular focus on new synthetic routes to inorganic photocatalysts possessing well-dened physichochemical properties and the elucidation of associated structure-function relations.Materials chemistry approaches to porous and layered semiconductor architectures, and hybrid photoactive nanomaterials (e.g.plasmonic nanostructures) are also highlighted, and the most active and selective systems benchmarked.

Mechanistic complications in CO 2 photoreduction
Before considering the vast array of nanomaterials employed in for CO 2 photoreduction, we consider briey some of the particular analytical challenges in evaluating photoactivity and product formation.Carbon-containing precursors and/or solvents are employed in the synthesis of most heterogeneous photocatalysts, and few studies quantify the carbon content of the resulting as-prepared materials.Such carbonaceous residues can either decompose directly upon subsequent irradiation, 68 or react with photoexcited molecular species generated from CO 2 or water, 72 to liberate carbon-containing liquid/gas phase products.Discriminating whether such products originate solely from desired CO 2 conversion and/or carbon residues is not possible by conventional chromatographic methods, and rarely accorded the attention deserved.
Yui and co-workers reported that organic adsorbates such as acetic acid present over commercial, untreated P25 TiO 2 were responsible for CH 4 evolution as a major product during CO 2 photoreduction. 68A photo-Kolbe decaboxylation mechanism was proposed that accounted for the observed methane solely in terms of acetic acid reduction: CH 3 COOH + H + / CH 3 c + CO 2 + H + ; CH 3 c + CH 3 COOH / CH 2 COOHc + CH 4 .Pre-calcination and washing of the TiO 2 suppressed methane production, with CO the dominant products; convincing evidence for the Kolbe mechanism hypothesised.Isotopic-labelling using 13 CO 2 over an organic-free 2 wt% Pd/TiO 2 catalyst revealed the co-production of 13 CH 4 (unequivocally through direct photoreduction) and some 12 CH 4 likely originating from the CO 3 2À species on the parent titania.In a complementary investigation, Yang et al. explored the mechanism of 13 CO 2 photoreduction in H 2 O over Cu(I)/TiO 2 by means of in situ DRIFT spectroscopy. 72 12CO was identied from its 2115 cm À1 Cu(I)-CO signature as the primary product of reaction between photocatalytically activated adsorbed water and carbonaceous residues on the as-prepared photocatalyst surface.The residual surface carbon reects the use of a titanium(IV) butoxide precursor and polyethylene glycol structure-directing agent, with (an unquantied amount of) carbon persisting even aer the resulting sol-gel was calcined at 500 C for 5 h.Prolonged UV irradiation of the fresh catalyst under moist air to remove carbon contaminants signicantly lowered the yield of reactively-formed CO, although some 13 CO was observed, evidencing true CO 2 photoreduction.These studies highlight the importance of eliminating organic (and carbonate) surface residues from photocatalysts prior to CO 2 photoreduction tests, via e.g.high temperature calcination, solvent washing and/or pre-exposure to UV light in the absence of CO 2 .Yang and co-workers also recommended that an additional control experiment be undertaken to ensure no carboncontaining products are formed in the absence of CO  75 the co-evolution of CO, H 2 and O 2 is reported with the predicted 2 : 1 stoichiometry; albeit isotopic labelling experiments with 13 CO 2 also revealed the formation of trace 12 CO over for KTaO 3 due to either 12 CO 2 from reactor purging or organic residues within the photoreactor.Isotope labelling, a full complement of control experiments, and careful analysis of evolved oxygen are thus critical to accurately quantify CO 2 photoreduction and understand underlying reaction pathways.

Crystallinity and the role of engineered surface defects
The catalytic behaviour of photocatalysts is intrinsically linked to their surface electronic/optical properties and adsorption behaviour, all of which can (in principle) be systematically tuned through crystal structure engineering; the nature, dimensions, uniformity and termination of crystalline phases strongly inuence their resultant photocatalytic efficiency. 76For instance, monoclinic sheet-like BiVO 4 signicantly outperformed tetragonal rod-like BiVO 4 counterparts in the rate of CO 2 photoreduction to C 2 H 5 OH (accompanied by O 2 evolution), under both UV and visible light illumination in the presence of water. 77These monoclinic and tetragonal phases were obtained by introducing CTAB (cetyltrimethyl ammonium bromide) or PEG (polyethylene glycol), respectively, during BiVO 4 synthesis.The superior photoactivity of the monoclinic BiVO 4 was attributed to the more asymmetric local environment of the Bi 3+ ions relative to those within the tetragonal phase.Accordingly, the Bi 3+ ions exhibit stronger lone pair character in the monoclinic phase and are hence drive to form Bi-O bonds with CO 2 bound as the carbonate, and consequential transfer of photogenerated electrons from the V 3d bands into the chemisorbed CO 3 2À .It is worth noting that these two phases exhibited different UV-Vis absorbances, with the band gaps of monoclinic and tetragonal BiVO 4 around 2.24 eV and 2.56 eV respectively.More recently, Li et al. demonstrated that cubic NaNbO 3 was more effective than orthorhombic NaNbO 3 for CO 2 photoreduction. 78The cubic NaNbO 3 (c-NaNbO 3 ), which is normally only stable at T > 813 K, was successfully prepared through a furfural alcohol derived polymerisation-oxidation (FAPO) method in the presence of P123 as a surfactant stabiliser.The more common orthorhombic NaNbO 3 (o-NaNbO 3 ) phase was prepared through a polymerised complex (PC) method.While c-NaNbO 3 comprised predominantly cuboid morphologies, a mixture of irregular and cuboid particles was obtained for o-NaNbO  Numerous preparative routes have been developed to nanomaterials with controllable crystal facets such as pH regulation of the synthetic media, or post-synthesis treatments.Truong et al. selectively prepared anatase (pH 2, only 3% rutile), rutile (pH 6, only 6% of anatase) and a bi-crystalline anatase(73%)brookite(27%) composite at pH 10. 79 Under UV (500 W Xe lamp) or visible light irradiation (NaNO 2 solution as a UV cut-off lter), and in the presence of a NaHCO 3 solution, these materials were able to photoreduce CO 2 to CH 3 OH, whereby the photoactivity followed the order anatase-brookite composite > rutile > anatase > commercial P25.Liu et al. also successfully prepared three different nanostructures of TiO 2 polymorphs including anatase (TiA) NPs, rutile (TiR) nanoellipses and brookite (TiB) nanorods for CO 2 photoreduction, 80 by subjecting the as-synthesised titania to heat treatment under a He environment at 220 C for 1.5 h to induce surface defects, such as oxygen vacancies (V O ) and Ti 3+ ions on these TiO 2 polymorphs.Illumination by a 150 W solar simulator in the presence of CO 2 /water vapour revealed the untreated TiO 2 polymorphs as active for CO 2 photoreduction to CO and a small amount of CH 4 under continuous-ow (2.0 mL min À1 ).Product yields decreased in the order TiA > TiB > TiR.As depicted in Fig. 2, He-treated materials TiA(He) and TiB(He) signicantly outperformed their untreated counterparts, with TiB(He) exhibiting the highest photoactivity, followed by TiA(He); such annealing treatments had less impact upon the photoactivity of TiR wherein the activation barrier to surface oxygen vacancy creation was presumably higher.
In situ DRIFTS and DRUVS suggested that the relative V O -: Ti 3+ concentration in TiA(He) was comparable to that of TiB(He), while the surface defect concentration on TiR(He) was negligible.The higher photoactivity of TiB(He) and TiA(He) was thus attributed to surface defects, with the energy of vacancy formation for TiB (He) of 5.52 eV lower than that for anatase (5.58 eV) and rutile (5.82 eV).CO 2 photoreduction over titania is clearly a strong function of the crystal structure and surface defect density; the latter postulated the active catalytic sites.
An increasing volume of research has established the inuence of crystal facets upon photocatalysis; physical and chemical properties of single crystals are highly sensitive to crystallographic termination. 76,81Calculations have established that the average surface energies of low-index anatase facets decrease in the order 0.90 J m À2 (001) > 0.53 J m À2 (100) > 0.44 J m À2 for (101). 82,83The highest energy (001) surface is expected to be the most reactive, and indeed experimental studies conrm that dissociative molecular adsorption of water and methanol occur readily on the (001) surface. 84,858][89][90] However, other studies suggest that photoreduction and photooxidation activities follow the order (001) < ( 101) < (010), 91 with the superior performance of the (010) facet attributed to a synergy between the surface atomic structure (dictating adsorption/reactivity) and electronic band structure (dictating the CB potential).
TiO 2 nanorods preferentially exposing (010) facets were therefore screened in CO 2 photoreduction. 91Such nanorods were synthesised by hydrothermal treatment of a H 0.68 Ti 1.83 O 3 precursor in the presence of Cs 2 CO 3 as a pH mediator, and subsequent promotion by 1 wt% Pt.The resulting Pt-doped TiO 2 nanorods outperformed P25 in CO 2 photoreduction by water vapour to CH 4 under 300 W Xe lamp UV irradiation in a Teonlined steel chamber under 0.06 MPa CO 2 .Enhanced activity was attributed to the unique properties of (010) facets which promote CO 2 adsorption, 92 specically their more negative conduction band potential.The same group prepared hollow anatase TiO 2 single crystals and mesocrystals, wherein both nanostructures were dominated by (101) facets. 93These materials were successfully synthesised by hydrothermal treatment of a Ti(SO 4 ) 2 , Na 3 PO 4 and HF solution.Fig. 3A-D shows the resulting 400 nm diameter octahedral single crystals obtained using 500 mM HF; lowering the HF concentration to 400 mM yielded hollow crystals with less prominent octahedral shapes of $160 nm (Fig. 3B and E).Fig. 3H reveals that these hollow crystals were single crystalline and principally (101) oriented, with a small proportion of higher-index (103) facets.Even lower HF concentrations favoured sphere-like hollow cages 150 nm diameter (Fig. 3C and F), each formed from $35 nm octahedral crystals, which spontaneously self-organised during crystal growth to adopt a common crystallographic orientation, resulting in 'single crystal-like' mesocrystals with well-dened selected area electron diffraction (SAED) patterns.Under UV irradiation (300 W Xe lamp) and in the presence of a RuO 2 co-catalyst and water vapour, the hollow single crystal and mesocrystal materials titanias outperformed a non-porous single crystal by a factor of 4-5 times in photoreducing CO 2 to CH 4 , attributed to their (i) higher surface areas (35 m 2 g À1 and 42 m 2 g À1 respectively) relative to the solid crystal (17 m 2 g À1 ) and (ii) shorter diffusion length of charge carriers and concomitant suppressed bulk recombination.Fig. 3J illustrates the different electronic band structures of the single crystal, hollow single crystal and hollow mesocrystal titanias, and more negative CB potential of the hollow mesocrystal.
Adsorbate-induced restructuring of highly reactive facets remains an intrinsic problem due to their high surface energies.In this regard, reagents such HF and isopropanol have been employed as structure-directing agents to favour exposure of (001) facets over single-crystal anatase nanosheets. 60se of such capping agents is itself problematic since they must be removed by thermochemical treatment prior to photocatalytic application (e.g.solvent extraction or calcination) which in turn may induce surface restructuring/phase changes.Fabrication of stable and uniform nanostructures terminating in specic atomic rearrangements requires continued development.
2.2.Nanostructured photocatalysts 2.2.1.Low dimension nanomaterials.One-dimensional nanomaterials are generally more attractive than their bulk and nanoparticle counterparts in photocatalysis, because they facilitate more efficient electron transport and exhibit higher surface area to bulk volume ratios. 65Recent work has shown that Zn 2 GeO 4 nanoribbons and nanorods photoreduce CO 2 more efficiently than Zn 2 GeO 4 prepared through conventional reaction 96,97 As depicted in Fig. 4a, single crystalline Zn 2 GeO 4 nanoribbons of around 7 nm in thickness, 20-50 nm width and hundreds of microns long, were successfully prepared by Liu et al. via a binary ethylenediamine/water solvent system during solvothermal treatment at 180 C. 94 The solvent system was proposed to regulate the velocity and direction of crystal growth through a solvent-coordination molecular-template mechanism.Nanoribbons were obtained due to preferential crystal growth along the [001] direction.Catalytic screening highlighted the improved photoactivity of the resulting nanoribbons over Zn 2 GeO 4 produced via solid state reaction (SSR) in producing CH 4 under UV illumination (300 W Xe arc lamp) in a CO 2 /H 2 O saturated glass reactor system.The better photocatalytic performance was attributed to (i) the higher surface area of the nanoribbons (28 m 2 g À1 versus 1 m 2 g À1 for the SSR material) (ii) the high quality single crystalline nature of the nanoribbons, which reduced electron-hole pair recombination, and (iii) the improved electron transport properties of the nanoribbons owing to their ultra-longitudinal dimension, and (iv) the geometry of the nanoribbons that permits rapid charge carrier diffusion to the oxide surface for CO 2 reduction.Photoactivity of these Zn 2 GeO 4 nanoribbons could be further promoted by co-catalysts such as Pt, RuO 2 or a combination thereof, and even coupling to metal-organic frameworks such as ZIF-8 to enhance their CO 2 adsorption capacity. 95 similar synthesis was employed to prepare highly crystalline In 2 Ge 2 O 7 (En) hybrid sub-nanowires, as shown in Fig. 4b, with diameters around 2-3 nm. 96Although the nanowires outperformed a SSR prepared sample in CO 2 photoreduction by water vapour under UV illumination (300 W Xe arc lamp) within a gas-tight Pyrex glass reactor, CO was the only photoreduction product.A low temperature solution phase route, which does not require any surfactant, was reported for the preparation of single crystalline, hexagonal Zn 2 GeO 4 nanorods for CO 2 photoreduction. 97A mixed solution of Na 2 GeO 3 and Zn(CH 3 -COO) 2 was heated at 40-100 C to yield Zn 2 GeO 4 nanorods with a regular, hexagonal prism geometry.As shown in Fig. 4c and d, nanorod dimensions were strongly dependent on synthesis temperature: nanorods around 400 nm long and 50 nm wide were obtained at 40 C, while nanorods 250 nm long and 150 nm wide formed at 100 C. Surprisingly, the prepared nanorods contained some blind holes due to the different crystal growth rate along the c-axis, as well as nanosteps in the surface of nanorods prepared at 100 C. The absorption band edge of nanorods was blue-shied (4.68 eV and 4.65 eV for 40 C and 100 C syntheses respectively) relative to that of SSR-prepared material (4.5 eV) due to quantum size effects, restricting their use to hard UV applications.Under illumination, Zn 2 GeO 4 nanorods outperformed those prepared by SSR towards photocatalytic H 2 generation and CO 2 reduction, with nanorods grown at 100 C proving the most active for H 2 evolution owing to their greater crystallinity and the presence of surface nanosteps promoting charge separation.Conversely, the 40 C synthesised nanorods exhibited better CO 2 photoreduction activity than their higher temperature analogues due to their higher surface area and CO 2 adsorption capacity.
][100][101][102][103] The development of TiO 2 NTs arrays, including their synthesis and application, has been thoroughly reviewed by Rani et al. 103 spanning rst generation (0.5 mm in length) to current 4 th generation ($1000 mm) technologies.Electrochemical anodisation is the preferred method for nanotube arrays synthesis since it enables the nanotube dimensions to be precisely tuned through varying synthetic parameters such as electrolyte composition and anodising time.Varghese et al. prepared N-doped TiO 2 NTs arrays with pore size and wall thickness of 95 AE 13 nm and 20 AE 5 nm, respectively by anodising Ti foil in an NH 4 F/water/ethylene glycol electrolyte at 55 V. 40 The resulting TiO 2 NTs arrays were then calcined and sputter coated with large, irregular patches of Pt, Cu or both metals, as co-catalysts, albeit little information was provided on the morphology and dispersion of these promoters, with the nal material containing between 0.4-0.75at% N. Irradiation under natural sunlight and water vapour within a batch reactor, afforded CO 2 photoreduction to mainly H 2 , CH 4 and other alkanes (i.e.C 2 H 6 , C 3 H 8 , C 4 H 10 , C 5 H 12 and C 6 H 14 ), with olens and branched paraffins also formed as minor products.Higher NTs calcination temperatures improved the photocatalytic activity, attributed to their higher crystallinity.Platinum promoted arrays generated more H 2 (almost 200 ppm cm À2 h À1 ) than hydrocarbons, giving a total productivity of 273 ppm cm À2 h À1 , whereas the copper promoted array exhibited superior hydrocarbon production ($104 ppm cm À2 h À1 ).Furthermore, the Cu-TiO 2 NTs arrays evolved ve times more CO than Pt-TiO 2 arrays.Doubly promoted NTs arrays (Fig. 5) afforded a total hydrocarbon productivity of 111 ppm cm À2 h À1 without any detectable CO.It was concluded that Pt was more active for water reduction, while Cu was more efficient at reducing CO 2 , their combination resulting in a strong synergy and maximal CO 2 conversion to hydrocarbons, with one of the highest visible light quantum efficiencies of 0.74%.
The synthesis of two-dimensional WO 3 nanosheets using a solid-liquid, phase arc discharge method for CO 2 photoreduction was recently reported by Chen et al. 104 Crystal growth was proposed to occur via the formation of a large number of 4-5 nm WO 3 seeds.These nanocrystals fused together into twodimensional nanosheets during aging, while maintained a thickness <5 nm, with preferential growth in the [100] and [010] directions.The resulting band gap of these nanosheets (2.79 eV) was larger than that of commercial WO 3 powder (2.63 eV) due to quantum size effects, with their CB potential estimated to be more negative than the reduction potential for CO 2 /CH 4 conversion.Conversely, the CB position of commercial WO 3 appeared more positive than the CO 2 /CH 4 redox potential, which should hence disfavour CO 2 reduction.Indeed, visible light illumination (300 W Xe arc lamp equipped with a 420 nm cut-off lter) of the as-prepared nanosheets showed that they outperformed the commercial WO 3 powder in photoreducing  CO 2 to CH 4 in a gas-tight system.Superior nanosheet photoactivity was therefore attributed to both their ultrathin geometry, expected to promote efficient charge carrier diffusion, and their more negative conduction potential.CO 2 adsorption can be enhanced through the use of alkali or alkaline earth basic oxides.Strontium promoted niobates prepared by conventional SSR have shown promise in CO 2 photoreduction by water vapour, but possess extremely low surface areas and are hence unsuitable for achieving high photoactivity.An alternative hydrothermal approach affords two-dimensional SrNb 2 O 6 nanoplates around 12 nm thick and 5-300 nm across, 105 conferring a 20-fold increase in CO 2 adsorption capacity and quantum efficiency of 0.065% over 10 h, an order of magnitude higher than P25.In contrast to many studies, high selectivity to CO 2 reduction to CO and CH 4 versus competing H 2 O reduction to H 2 was observed, alongside signicant oxygen evolution.Transient photocurrent response measurements suggest that separation of photogenerated electron-hole pairs was ratedetermining, with the nanoplate morphology facilitating charge-carrier diffusion to the surface and subsequent reaction with adsorbed CO 2 and water.
2.2.2.Porous structured photocatalysts.Mesoporous and hierarchical heterogeneous catalysts have proved superior to their non-porous analogues in a number of important chemical transformations, including catalytic cracking, selective oxidation, biomass valorisation and pollution abatement systems.Hence, it may be anticipated that porous semiconductor photocatalysts will likewise outperform their bulk counterparts.0][111] Macropore incorporation into mesoporous architectures can confer additional benets via increased light scattering and faster in-pore mass transport of sterically hindered reagents, nding particular application in the photodegradation of organic dyes.
Crystalline, porous structured Ga 2 O 3 has been successfully synthesised by Park et al. employing a tetradecyl trimethyl ammonium bromide template. 107Field emission scanning electron microscopy (FE-SEM) and transmision electron microscopy (TEM) images (Fig. 6), revealed the co-existence of macropores and mesopores througout the resulting monodispersed material.Subsequent photocatalytic screening showed that CO 2 conversion over this hierarchically porous Ga 2 O 3 was much higher than that of reference Ga 2 O 3 nanoparticles.Further investigations into CO 2 adsorption established that the porous Ga 2 O 3 possessed a vastly superior CO 2 adsorption capacity, 300% higher than that of the bulk reference, accounting for its improved photoreduction of CO 2 to CH 4 .Surfactant templating was also utilised to prepare mesoporous, micrometric ZnO spheres with the Pluronic P123 templating agent through hydrothermal treatment. 112Individual ZnO spheres were composed of stacks of thin ZnO akes, which created interparticle mesoporous voids.This mesoporous architecture gave higher CO 2 conversion than the equivalent non-porous ZnO, and following Cu impregnation the mesoporous ZnO favored the selective production of CH 4 and CH 3 OH.The utility of porous structures for CO 2 photoreduction to CH 4 has also been reported for titania, 79 albeit the surface area of the mesoporous TiO 2 decreased signicantly upon subsequent nitridation to generate visible light active, N-doped TiO 2 .
In recent years, a new synthetic protocol has been developed to create multicomponent, mesoporous photocatalysts for CO 2 reduction.Here an inorganic precursor is used to construct more thermally robust mesostructures. 113The synthetic route of Yan et al. to prepare mesoporous ZnGa 2 O 4 is illustrated in Fig. 7A.A powdered NaGaO 2 starting material was synthesised through a solid-state route, which formed colloidal particles upon dispersion in water.Weak surface repulsion between the resulting nanoparticles enabled their subsequent occulation to form a mesoporous framework, followed by Na + and Zn 2+ ionexchange to yield a mesoporous ZnGa 2 O 4 (visualised in Fig. 7B).The nal material remained crystalline aer the ion-exchange, obviating the need for hydrothermal treatment or high temperature calcination.The nal ZnGa 2 O 4 exhibited a mean pore diameter of 3.5 nm and surface area around 110 m 2 g À1 .Catalytic screening demonstrated that the mesoporous ZnGa 2 O 4 was superior to ZnGa 2 O 4 prepared through solid-state synthesis, with photogenerated CH 4 prevalent when a RuO 2 cocatalyst was employed.In common with many such studies there was no justication fo the choice of co-catalyst, nor its mode of operation, which was simply presumed to improve separation of photogenerated electron-hole pairs.This synthetic procedure was further adapted to prepare a ZnAl 2 O 4 -modied, mesoporous ZnGaNO visible light photocatalyst 111 from a NaGa 1Àx Al x O 2 precursor to form an amorphous mesoporous colloid template by occulation.Following ion-exchange with Zn 2+ , the resultant Zn(Ga 1Àx Al x ) 2 O 4 was hydrothermally treated to improve its crystallinity prior to nitridation; Al was claimed to enhance the thermal stability of the mesostructure.same method also evidenced superior CO 2 photoconversion compared to their solid state synthesised counterparts. 106,114egular mesopore voids can also be created through the selfassembly of nanoparticles wherein the mesopores are formed at the junction between nanoparticles.This approach was successfully employed by Wang et al. to prepare sub-micron TiO 2 -SiO 2 mesoporous composites. 108These materials were obtained by atomising TiO 2 and SiO 2 nanocolloidal solutions of 20-45 nm particles into micron-sized droplets at elevated temperature.Solvent evaporation drove colloidal self-assembly to yield the mesoporous TiO 2 -SiO 2 composite imaged by FE-SEM image in Fig. 8a.Cu doped variants were also produced by introducing Cu(NO 3 ) 2 into the colloidal precursor solution.Higher synthesis temperatures promoted TiO 2 nanocrystal segregation at the composite surface (Fig. 8b).The porous SiO 2 matrix ensured that TiO 2 dispersed across the composite surfaces as $20 nm crystals, rather than larger agglomerates.TiO 2 -SiO 2 composites exhibited superior photoactivity to pure TiO 2 for CO 2 photoreduction to CO.Activity increased with processing temperature up to 1000 C due to an increasing density of TiO 2 nanocrystals decorating the mesoporous framework, with a composite comprising 2 mol% TiO 2 and 0.01 mol% Cu giving the highest rate of CO 2 photoreduction.Mesoporous In(OH) 3 has also been prepared through a templatefree hydrothermal treatment 110 resulting in the self-assembly of crystalline In(OH) 3 nanocubes.The introduction of mesoporosity enhanced CO 2 photoreduction to CH 4 by 20-fold relative to conventional In(OH) 3 crystallites attributed to the higher surface area and CO 2 adsorption affinity of the mesoporous variant.
6][117][118] Switching the precursor from melamine to melamine chloride followed by rapid heating afforded porous C 3 N 4 : 119 Fig. 9 compares the platelet-like structure of melamine-derived C 3 N 4 (g-C 3 N 4 ) with the porous platelets obtained from the melamine chloride analogue, (p-g-C 3 N 4 ) whose surface area was signicantly higher (69 m 2 g À1 versus 1.7 m 2 g À1 for g-C 3 N 4 ).Despite these textural differences the non-porous g-C 3 N 4 outperformed the porous pg-C 3 N 4 during CO 2 the photoreduction to CO under visible light.This surprising result was attributed to the slightly broadened band gap and poorer crystallinity of p-g-C 3 N 4 , which may arise from a higher density of defects acting as electron-hole recombination centres.Porous C 3 N 4 was also prepared by Mao et al. (denoted u-g-C 3 N 4 ) from a urea precursor, which displayed superior CO 2 photoreduction activity relative to melaminederived C 3 N 4 (m-g-C 3 N 4 ). 120Akin to the ndings of Dong and coworkers, porous u-g-C 3 N 4 was less crystalline than the melamine analogue, with an order of magnitude higher surface area and slightly wider band gap.However, in this instance u-g-C 3 N 4 exhibited superior photoactivity, presumably due to enhanced reactant adsorption and more efficient charge carrier separation.Under visible light, and in the presence of 1 M NaOH, u-g-C 3 N 4 photoreduced CO 2 to a mixture of CH 3 OH and C 2 H 5 OH, while m-g-C 3 N 4 favoured C 2 H 5 OH with only trace methanol.Selective photocatalytic CO 2 reduction is hence achievable through tuning of material crystallinity and microstructure.
Table 1 shows that most mesoporous semiconductors possess modest surface area, however there is scope for enhancing these through their immobilisation onto higher area supports such as mesoporous silicas. 121,122Such high area templates, albeit inert, facilitate dispersion of photoactive phases, thereby increasing the surface active site density and hence photoconversion efficiency.Yang et al. reported that TiO 2 supported on SBA-15 exhibited superior photoactivity to P25 during CO 2 photoreduction to CH 3 OH. 122The crystallite size of the dispersed TiO 2 increased with loading but remained smaller than that of commercial P25, however it is unclear how much titania was incorporated with the SBA-15 pore network versus external surface, and hence performance may have been suboptimal.Further studies revealed that higher CH 3 OH yields were achievable by Cu doping these TiO 2 /SBA-15 catalysts.Li      Three-dimensional, hierarchical photocatalysts with novel properties arising from their unique morphology have also been exploited to further optimise photocatalytic performance.Many studies have shown that such complex architectures, typically possessing high surface areas and diverse pore-interconnectivities, offer enhanced catalytic activity relative to their bulk, nanoparticulate or monomodal nanoporous counterparts. 1379][140][141][142][143] To date, semiconductors with diverse three-dimensional, hierarchical structures have been developed for harvesting solar energy, including ZnO nanowire forests, 143  and NiO-CdS architectures, 151 CdS@ZnO and CdS@Al 2 O 3 urchin-like heteroarrays, 91 N-doped (BiO) 2 CO 3 microspheres, 141 Fe 3 O 4 /WO 3 core-shell structures, 152 WO 3 hollow dendrites, spheres and dumbbells, 153 porous TiO 2 hollow microspheres, 138,154 and g-TaON urchin-like hollow spheres. 1395][146][147][148][149][150][151][152] In some instances, pore networks created within hierarchical networks not only increase the total surface area, but also serve to accelerate reactant/product diffusion to/from active centres. 141,154While relatively few studies have targeted the development of hierarchical structures for CO 2 photoreduction, a number of promising three-dimensional nanostructures have been created for solar fuels production.
Highly ordered, sheaf-like, hyperbranched Zn 2 GeO 4 superstructures were prepared in the presence of ethylenediamine through a solvothermal method by Liu et al. 155 The FE-SEM image in Fig. 12a highlights 3-4 mm sheaf-like features which assembled from closely packed, well-aligned nanowires with fantails oriented toward the center and projecting radially outwards.This unique morphology was proposed to arise through a crystal-splitting growth mechanism, with the degree of splitting a function of reaction time, precursor concentration and solvent.In order to improve the visible light performance of the normally large band gap Zn 2 GeO 4 , these as-prepared fusiform bundles were subject to nitridation at 700 C, resulting in a yellowish zinc germanium oxynitride (Zn 1.7 GeN 1.8 O) photocatalyst.While the bundle morphology of the parent Zn 1.7 -GeN 1.8 O precursor was retained aer this high temperature treatment, an added benet was a concomitant signicant increase in surface roughness (Fig. 12b), which doubled the Zn 1.7 GeN 1.8 O surface area relative to the Zn 2 GeO 4 parent.Under visible light illumination (300 W Xe arc lamp equipped with a 420 nm cut-off lter), and only upon promotion with Pt or RuO 2 co-catalysts, this hierarchically structured Zn 1.7 GeN 1.8 O material showed good ambient pressure CO 2 photoreduction to CH 4 , albeit with an apparent quantum efficiency still only approaching 0.024%.It is noteworthy that the CH 4 yield reached saturated around 6 h, possibly due to reversible adsorption of reactive intermediates.
Hexagonal nanoplate-textured micro-octahedral Zn 2 SnO 4 were synthesised through a hydrothermal treatment in the presence of L-tryptophan. 156The resulting material comprised ordered and uniform micro-octahedra with edges of approximately 3 mm (Fig. 13a).At higher magnication, it is apparent that each micro-octahedron is densely packed with $50 nm thin, hexagonal nanoplates Fig. 13b.These nanoplates are vertically arranged within octahedra such that they lie parallel to neighbouring edges.The octahedral geometry was proposed a consequence of L-tryptophan impeding reaction along the [111] direction, favouring faster nucleation and growth in the [100] direction.The resulting hierarchical nanoplate/microoctahedron morphology blue-shied the Zn 2 SnO 4 band gap from the bulk value of 3.67 eV to 3.87 eV, yet delivered exclusively CH 4 production at 20 ppm g À1 from CO 2 under UV irradiation within a gas-tight Pyrex glass cell, albeit for only 1 h in the absence of promoters.This far exceeded the photoactivity of bulk Zn 2 SnO 4 (1.5 ppm g À1 ), smooth Zn 2 SnO 4 micro-octahedra (4.7 ppm g À1 ) or Zn 2 SnO 4 atactic particles (2.4 ppm g À1 ) prepared without L-tryptophan.This rate enhancement was ascribed to such superstructures acting as light-scattering centres promoting photon harvesting, in combination with the nanoplate morphology promoting electron transport along individual plates, and the open nanoplate-/micro-octahedron architecture which facilitated rapid molecular diffusion.
Hierarchical, hollow Bi 2 WO 6 microspheres, which were constructed from crossed nanosheets prepared through an anion exchange method by Cheng et al., successfully photoreduced CO 2 to CH 3 OH under visible light irradiation. 157As depicted in Fig. 14, the synthesis involved anion exchange of BiOBr solid microspheres, prepared from Bi(NO 3 ) 3 and a [C 12 Mim]Br ionic liquid, with Na 2 WO 6 under hydrothermal conditions at 160 C. BiOBr and Bi 2 WO 6 comprise crystalline layered structures consisting of alternating (Bi 2 O 2 ) 2+ slabs separated by intercalating anion layers.Since the solubility of Bi 2 WO 6 was lower than that of BiOBr, thin layers of Bi 2 WO 6 formed at the BiOBr À surfaces during the exchange process.Owing to the slower diffusion of WO 4 2À versus Br À , a fast, continuous outow of Br À through the Bi 2 WO 6 shell resulted in the formation of hollow cores over a prolonged period.These 2-5 mm diameter, hollow microspheres were themselves mesoporous, with a surface area far higher than that of Bi 2 WO 6 prepared by a solid state route (Bi 2 WO 6 SSR).Hollow Bi 2 WO 6 microspheres generated around 25 times more CH 3 OH from CO 2 photoreduction by water than Bi 2 WO 6 SSR at under visible light (300 W Xe arc lamp equipped with a 420 nm cut-off lter).This performance was notably achieved without the aid of noble metal co-catalysts.As in the previous examples, this enhanced photocatalysis was attributed to the higher surface area and CO 2 adsorption affinity (eight times that of Bi 2 WO 6 SSR) of the hierarchical structure.
An exciting biomimetic approach was recently adopted by Zhou et al. to prepare hierarchical, three-dimensional perovskite titanates, ATiO 3 (A ¼ Sr, Ca, Pb), for CO 2 photoreduction. 158In an effort to move one-step closer to articial photosynthesis, fresh Cherry Blossom green leaves were employed as a structure-directing agent to prepare inorganic photocatalysts via a modied sol-gel protocol; natural leaves possess high porosity, connectivity and surface areas, tailored to facilitate efficient gas diffusion, water transport and the harnessing of solar energy.The success of this synthesis was evidenced by the FE-SEM images in Fig. 15a and b, wherein the resulting SrTiO 3 exhibited a macroporous venation network characteristic of the parent leaves.Higher magnication TEM imaging revealed the network walls consisted of a  mesoporous array of crystallites (Fig. 15c and d).Such biomimetic SrTiO 3 and CaTiO 3 architectures outperformed corresponding reference titanates in CO 2 photoreduction to CO (major product) and CH 4 (secondary product) during UV illumination (300 W Xe arc lamp) under water vapour in a gas closed circulation system and 80 kPa of CO 2 .Incorporation of Au, Ag and Cu into these hierarchical titanates enhanced their photoactivity, and also led to the evolution of trace C 2 H 4 and C 2 H 6 , although the latter were proposed to arise via photocatalytic non-oxidative coupling of CH 4 rather than direct CO 2 photoreduction. 159he excellent performance of these leaf-architectured titanates presumably reects their highly interconnected macroporousmesoporous structure, which promotes gas diffusion in addition to increasing surface active sites.This sophisticated architecture may also enhance light absorption in a manner similar to that of natural leaves, by which incident light is scattered and transmitted into deeper layers of the network, thereby increasing the quantum efficiency and hence photocatalytic activity.
2.2.5.Hybrid structured photocatalysts.As touched upon in the preceding sections, semiconductor photocatalysts can be coupled with other materials which may comprise efficient charge carrier components (electron donor or acceptor), such as semiconductors, metal oxides or zeolites.Such combinations offer hybrid photocatalyst with superior physicochemical or optical properties to the parent active phase, e.g. a higher dispersion of active centres, enhanced accessibility of catalytic centres (cf.hierarchical architectures) or novel band structures.For instance, two semiconductors (SC) with different band gaps and valence/conduction band energy levels can be combined to create a staggered type II band structure as depicted in Fig. 16. 160When light is absorbed by a small (visible) band gap semiconductor (SC 1 ), interparticle electron transfer can occur, in which photogenerated electrons are injected into the CB of a large band gap (SC 2 ) semiconductor, while photogenerated holes remain in the valence band (VB) of SC 1 , thereby effecting charge separation through spatial localisation, and hence maximising the lifetime of photoinduced charge carriers and photocatalytic efficiency.Direct particle-to-particle contact is generally held to be crucial for efficient operation of such hybrid systems. 1612.5.1.Inorganic hybrid photocatalysts.A number of such SC 1 /SC 2 hybrid systems have been developed for CO 2 photoreduction.Wang et al. prepared CdSe/Pt/TiO 2 heterostructures using commercially available CdSe quantum dots (QDs) of two different sizes (2.5 nm and 6 nm) and commercial P25 titania.162 The resulting heterostructures, containing 0.5 at% Pt and 1 at%   Cd, were active for the photoreduction of CO 2 to CH 4 and CH 3 OH (respective major and minor products) under visible light irradiation, with trace H 2 and CO also reported; neither Pt/ TiO 2 or CdSe components were individually active under identical reaction conditions.Consequently, the observed visible light photoactivity of the semiconductor heterostructures must arise from a synergy between CdSe and TiO 2 .It was proposed that photoexcited electrons promoted into the CB of CdSe under visible light irradiation, were subsequently injected into the CB of TiO 2 , increasing their liime and probability of transfer to CO 2 .However, these composites deactivated aer 4-6 h irradiation, most likely due to oxidation of the QDs by the accumulated photoinduced holes in the VB of CdSe.
In other work, coupling of AgBr with P25 was also undertaken to form nanocomposites active for reducing CO 2 under visible light, 163 wherein the indirect band gap of AgBr, which spans wavelengths between 450-700 nm, was held responsible for the visible light activity.This hybrid was synthesised utilising a cetyltrimethylammonium bromide (CTAB) surfactant stabiliser to create well-dispersed AgBr NPs (of around 5 nm) decorating the surface of P25.The nanocomposites exhibited excellent visible light photoactivity (150 W Xe lamp with a 420 nm cut-off lter) and photostability in the presence of aqueous KHCO 3 , relative to pure AgBr, with CH 4 as the major product together with CH 3 OH, CH 3 CH 2 OH and CO under high CO 2 pressure (7.5 MPa).A composite containing 23.2 wt% of AgBr exhibited optimum performance, with the photocatalytic mechanism again proposed to involve visible light excitation of electrons into the AgBr conduction band and subsequent transfer into the TiO 2 CB with attendant charge separation.In addition to increasing charge carrier lifetime, these hybrid structures also impeded the formation of metallic Ag through photolysis, as was observed for pure AgBr.Qin et al. followed a similar route employing CTAB to produce 14 nm CuO/TiO 2 composite nanostructures, 164 although here UV irradiation in the presence of methanol (rather than water) resulted in CO 2 photoreduction to methyl formate within a slurry reactor.A 1 wt% CuO/TiO 2 composite calcined at 450 C exhibited the highest photoactivity, outperforming both pure TiO 2 and composites synthesised in the absence of CTAB, suggesting a role for both charge separation and particle size effects.Liu et al. also explored the synergy between copper and titania heterojunctions, employing a solgel route to prepare Cu 2 O/TiO 2 composites which demonstrated superior photoactivity over pure TiO 2 in photoreducing CO 2 to CH 4 in a stirred batch, annular quartz reactor under 1 bar of CO 2 and low power (32 W) UVA illumination. 165XPS and pulse chemisorption measurements identied the presence of a twodimensional Cu 2 O species decorating the surface of titania at extremely low doping levels (0.03 wt% Cu).Higher copper loadings resulted in a structural transformation to threedimensional Cu 2 O crystallites, which reduced the efficiency of photoinduced electron transfer from TiO 2 to Cu 2 O, and hence lowered their activity.TiO 2 has also been coupled with iron to form FeTiO 3 /TiO 2 hybrids able to photoreduce CO 2 to CH 3 OH under high power UV (500 W Xe lamp) and visible light irradiation under basic conditions. 166Materials synthesised with 20 mol% Fe gave the highest rate of CO 2 reduction.Under UV illumination, photoinduced electrons were proposed to transfer from the (higher energy) CB of TiO 2 to that of FeTiO 3 , with photoinduced holes returned from the VB of FeTiO 3 to that of TiO 2 , charge separation again inferred as improving the overall photoactivity.In contrast, under visible light irradiation, electrons were photoexcited into the FeTiO 3 CB, resulting in a partially vacant FeTiO 3 VB, which was charge compensated for by the consequential movement of electrons from the almost isoenergetic TiO 2 VB, leaving residual holes in the TiO 2 VB, and net electron-hole charge separation across the two semiconductors.Efforts have been made to couple TiO 2 with CdS and Bi 2 S 3 in order to photoreduce CO 2 to CH 3 OH. 165Li et al. precipitated CdS or Bi 2 S 3 onto TiO 2 NTs to yield visible light active CdS/TiO 2 and Bi 2 S 3 /TiO 2 heterostructures, thereby delivering improved photoactivity relative to P25 or bare TiO 2 NTs (although activities were not normalised to take into account the two-fold higher surface areas of the nanorod morphologies).Photoinduced electrons from the CB of CdS or Bi 2 S 3 could be injected into that of TiO 2 to achieve efficient charge separation.Nevertheless, these complex heterojunction nanorods failed to generate more CH 3 OH than their respective pure CdS and Bi 2 S 3 bulk analogues.
CuO-Cu 2 O thin lm nanorod arrays prepared by Chadimkhani et al. were recently shown to photoreduce CO 2 into CH 3 OH under simulated solar irradiation (AM 1.5) without a cocatalyst via photoelectrocatalysis. 167Coupling of such small band gap semiconductors is advantageous in exploiting a larger fraction of the visible light region of the solar spectrum than achievable by either alone.CuO nanorods were rst thermochemically grown over a Cu substrate, following which Cu 2 O crystallites were electrodeposited on the nanorod surfaces to generate a CuO core/Cu 2 O shell hybrid thin lm which outperformed a compact Cu 2 O electrodeposited lm.This enhancement may reect more efficient transfer of photoinduced electrons from the CB of the Cu 2 O shell to that of the CuO core and consequent increased probability of photoinduced electrons reacting with CO 2 .Such a model necessitates that the tips of CuO nanorods are exposed to a CO 2 saturated aqueous environment.The CuO-Cu 2 O thin lm was reported to generate CH 3 OH at a potential of only À0.2 V vs. SHE, lower than the standard redox potential for CO 2 /CH 3 OH (À0.38 V vs. SHE), a feature characteristic of p-type semiconductors (such as CuO and Cu 2 O).
Copper oxide has also been employed by In et al. to form hollow CuO-TiO 2Àx N x hybrid nanocubes that are effective for photoreducing CO 2 to CH 4 . 168As shown in Fig. 17a, Cu 3 N nanocubes, synthesised through the thermal decomposition of Cu(NO 3 ) 2 in octadecylamine at 240 C, were employed as the templating agent for subsequent TiO 2 deposition on their external surfaces through slow hydrolysis of titanium n-butoxide.The initial Cu 3 N nanocubes were uniform in shape and around 27 nm across (Fig. 17b), and underwent signicant roughening following TiO 2 coating (TiO 2 @Cu 3 N) as apparent in Fig. 17c.Subsequent heat treatment of the TiO 2 @Cu 3 N powder at 450 C under air transformed these phase-separated structures into hollow CuO-TiO 2Àx N x hybrid nanocubes (Fig. 17d-f).
The nal material was composed of hollow, truncated cuboidal CuO hollow cubes of $57 nm diameter, decorated by small TiO 2Àx N x particles.FE-SEM imaging conrmed the cubic morphology and hollow core of this hybrid material (Fig. 17g-f).The authors proposed that TiO 2 crystallised onto the surfaces of Cu 3 N during thermal processing, but did not fully encaspulate the underlying nanocubes, permitting O 2 to diffuse into, and oxidise, the Cu 3 N surface.Assuming that the inward rate of atomic oxygen diffusion is slower than the outward diffusion of N 2 , high temperature treatment would drive the formation of hollow cores through the Kirkendall effect. 169As nitrogen atoms diffuse outward, they are able to react with TiO 2 and hence form an oxynitride TiO 2Àx N x phase.Under simulated solar illumination and water vapour, this particular hybrid composite photoreduced CO 2 to CH 4 , and C 2 H 6 , C 3 H 8 and C 4 H 10 as minor products.The hollow CuO-TiO 2Àx N x hybrid nanocubes outperformed P25, CuO (annealed Cu 3 N), CuO@TiO 2 , Cu 3 N and TiO 2 @Cu 3 N in the photocatalytic generation of CH 3 OH, likely due to their improved visible light absorption characteristics and the synergy between CuO and TiO 2Àx N x .
2.2.5.2.Organic hybrid photocatalysts.A hybrid catalyst composed of a Naon (peruorinated polymer with sulfonate groups, Nf) overlayer on TiO 2 with Pd co-catalyst was prepared by Kim et al. for CO 2 photoreduction 170 as illustrated in Fig. 18a.Pd was rst impregnated onto P25, and subsequently coated with Naon.The rationale for integrating Naon with a photoactive semiconductor was that the superacidic polymer, which exhibits excellent proton conducting behaviour, would facilitate proton-coupled multielectron transfer (PCET) pathways during product (hydrocarbon) formation, thereby improving net CO 2 reduction.It was also believed that Naon might inhibit the reoxidation of photoreduction products, and as a uorinated polymer, be stable towards photocorrosion and redox reactions and thus a good host support.Aer catalyst precleaning and UV irradiation the hybrid Nf/Pd-TiO 2 facilitated CO 2 photoreduction to CH 4 , C 2 H 6 and H 2 (and trace C 3 H 8 ) in a sodium carbonate aqueous solution.As shown in Fig. 18b, CO 2 reduction was highly sensitive to reaction pH, with photoactivity increasing at lower pH, the higher H + concentration aiding product generation.In the presence of Naon, considerably more CH 4 was generated at pH 3 and 11 but the effect was less prominent at pH 1.However, Naon signicantly enhanced ethane production at pH 3 and 1, with propane only observed over the Nf/Pd-TiO 2 hybrid photocatalyst.The observation that Naon favours longer chain hydrocarbons suggest that it may stabilise CO 2 reduction intermediates in a manner that promotes sequential multi-electron transfer.Additional experiments with Nf/Pd-TiO 2 under natural sunlight only resulted in CH 4 , C 2 H 6 and C 3 H 8 formation.
2][173] Yuan et al. recently demonstrated the utility of a Cu[(bpy) 2 ] + dye-sensitised TiO 2 hybrid system for CO 2 photoreduction. 174To obtain an air stable Cu complex, the 6and 6 0 -position of the 2,2 0 -bipyridine (bpy) ligand were modied with a methyl group to protect the surroundings of the labile Cu(I) centre.This ligand was also designed to terminate in cyanoacetic acid moieties, thus enabling efficient anchoring of the metal complex onto the semiconductor surface.The Cu(I) complex modied P25 hybrid successfully photoreduced CO 2 with water vapour into CH 4 in a gas-solid reaction system under visible light illumination.It is likely that the Cu(I) complex was responsible for harnessing photons from visible light, with the resulting photoexcited electrons rapidly injected into the conduction band of the visible-inactive TiO 2 , thereby prolonging the lifetime of photoinduced electrons to drive CO 2 reduction.
A visible light hybrid photocatalyst system, which utilised a naturally occurring enzyme CO dehydrogenase (CODH 1) to reduce CO 2 , was developed by Woolerton et al. 175,176 This enzyme was homodimeric, such that each sub-unit contained a buried  [Ni4Fe-4S] active site, wired to the protein surface through a series of [4Fe-4S] clusters that were also responsible for relaying electrons in and out of CODH 1. Fig. 19 illustrates the complete photosystem comprising CODH 1 modied, and Ru dye (RuP) sensitised, P25 NPs.During visible light irradiation, photoexcited electrons generated through metal-to-ligand chargetransfer on the RuP dye could be injected into the CB of TiO 2 and transferred through the [4Fe-4S] clusters to the CODH 1 active sites, with the resulting photo-oxidised RuP regenerated via 2-(N-morpholino) ethanesulfonic acid (MES) as a sacricial electron donor.It was proposed that the reduction process was initiated through CO 2 binding as a bridging ligand between the Ni and dangling Fe atom of the enzyme.Under visible light illumination, TiO 2 thus functioned as a medium for relaying electrons between the dye and enzyme.It is noteworthy that CODH 1 usually oxidises CO to CO 2 , but that the enzyme can be driven to catalyse the reverse reaction by applying a small overpotential. 177Further investigations revealed P25 as the preferred semiconductor with respect to ZnO, SrTiO 3 , rutile and anatase TiO 2 . 175Photoactivity was further enhanced through the addition of another sacricial electron donor, ethylenediaminetetraacetic acid (EDTA).To improve CODH 1 attachment to titania, P25 was modied with various molecules, including polymyxin B sulphate and glutamic acid, albeit to no effect.On the other hand, functionalisation with o-phosphorylethanol led to a signicant decrease in activity.Although this particular system was capable of photoreducing CO 2 to CO, the photoconversion efficiency was lower than expected, and it was sensitive to O 2 passivation. 175,176Photoactivity decreased aer 4 h on-stream, potentially due to the difficulty in regenerating oxidised RuP, or the ineffectiveness of CODH 1 in harnessing electrons for activating CO 2 .
2.2.5.3.Carbonaceous media hybrid photocatalysts.Carbon nanomaterials, such as activated carbons, carbon monoliths, carbon NTs, graphene and graphites 178,179 are promising candidates for catalyst supports, offering low cost, robust mechanical strength, high surface area, excellent porosity, chemical inertness and high electron conductivity.Moreover, carbons are amenable to the efficient recovery of precious metal promoters via combustion, and their surfaces may be readily functionalised by physical or chemical treatments, thereby modifying the resulting performance of hybrid catalysts.Signicant work has been undertaken to integrate photoactive materials with carbonaceous media such as carbon NPs and graphene to deliver improve photocatalysts.
][182][183] Because of these unique electronic properties, surface-passivated carbon QDs are a promising visible light harvesting medium.Surface-functionalised carbon NPs prepared by Cao et al. were active in photoreducing CO 2 under visible light illumination. 184Sub-10 nm carbon NPs were functionalised with oligomeric poly(ethylene glycol) diamine (PEG 1500N ), rendering the resulting nanoparticles photoactive and water soluble.PEGfunctionalised carbon QDs readily generated surface conned electrons and holes under irradiation that recombined radiatively (Fig. 20).This recombination process can be quenched by either electron donors or acceptors.Efficient charge separation and supressed radiative recombination was achieved through coated these surface-functionalised carbon NPs with Au or Pt through solution-phase photolysis; subsequent exposure to a light source would channel photogenerated electrons into the resulting metal nanoparticles, thereby prolonging the lifetime of charge carriers required for subsequent CO 2 reduction.Under visible light irradiation (425-720 nm), and in the presence of water, metal decorated and surface-functionalised carbon NPs proved active for photoreducing CO 2 to formic acid, although product analysis relied upon solution phase 1 H and 13 C NMR spectroscopy, and hence was unable to quantify the  presence of potential gas phase reduction products such as light alkanes and alkenes.
As a conductive electron reservoir, carbon is also effective in channelling photoinduced electrons away from a photoactive host, thereby increasing the probability that photoexcited electrons reduce CO 2 rather than recombining with holes.Ong et al. reported a hybrid catalyst, composed of CNTs grown on a Ni impregnated TiO 2 composite, for CO 2 photoreduction to CH 4 under visible light. 185The authors rst deposited 10 wt% Ni onto commercial anatase via precipitation in the presence of glycerol.Carbon NTs were subsequently grown over the calcined Ni/TiO 2 through chemical vapour deposition.The band gap of the resulting hybrid material was 2.22 eV, signicantly smaller than that of anatase TiO 2 (3.32 eV).Hence CNT incorporation served to narrow the band gap and improve visible light absorption in the hybrid catalyst.Visible light irradiation under water vapour revealed the CNT/Ni/TiO 2 hybrids were superior to either Ni/TiO 2 or pure anatase TiO 2 , although progressive deactivation occurred aer 4.5 h irradiation at which point CO 2 photoreduction activity was maximal.Photoactivity of the CNT/ Ni/TiO 2 hybrid catalyst originated from improved visible light absorption and the excellent electron storage and transfer properties of the CNTs which helped to suppress electron-hole recombination.
Graphene, a two-dimensional planar structure of sp2 hybridised carbon, has received much attention in regard to photocatalytic applications for H 2 evolution, 58,163,[186][187][188][189][190][191][192][193][194] photocurrent generation, 190,[194][195][196][197][198] and dye degradation, 90,144,145,163,189,[199][200][201] due to its outstanding electronic properties. 144,1976][207] However, less work has been conducted into the integration of graphene with photoactive materials for CO 2 photoreduction.Nevertheless, it is known that graphene oxide (GO), an unreduced form of graphene, despite being a poor electrical conductor, 195 is capable of photoreducing CO 2 to CH 3 OH. 208Liang et al. demonstrated that the photocatalytic activity of commercial P25 is greatly enhanced for CH 4 formation under both UV and visible light irradiation when used in conjunction with a low defect density, solventexfoliated graphene (SEG), compared to that achievable over defective, solvent-reduced graphene oxide (SGRO). 206The photoactivities of these SEG-P25 and SRGO-P25 composite lms, which were synthesised by blade-coating a physical mixture of different amounts of graphene and P25 ink and subsequent annealing, are shown in Fig. 21a.Note that under UV illumination the presence of SGRO exerted little impact upon the photoactivity of TiO 2 , whereas graphene possessing a low defect density exhibited better electrical mobility and concomitant superior photocatalytic efficiency under either UV or visible light.
Despite the popularity of graphene-derived materials, a direct comparison between graphene and its carbon nanotube (CNT) allotrope has not been well-explored.Further work by Liang et al. extended their studies on CO 2 photoreduction catalysed by blade-coated lms to examine the integration of titania nanosheets (TiNS) with SEG or single-walled carbon nanotubes (SWCNT) to form either a two-dimensional graphene-titania nanosheet composite, or a one/two-dimensional carbon nanotube-titania nanosheet composite, respectively. 209Fig. 21b shows that the SEG-TiNS composites outperformed SWCNT-TiNS in photoreducing CO 2 to CH 4 under UV irradiation.Conversely, composites of SWCNT-TiNS demonstrated better photoactivity under visible light illumination.It was proposed that the two-dimensional interface between SEG and TiNS facilitated intimate electronic and physical coupling relative to the one-/two-dimensional interface within the SWCNT-TiNS composite.Consequently, interfacial electron transfer from TiNS to SEG was greatly improved, inhibiting the recombination of photogenerated electron-hole pairs.In contrast, the better visible light photoactivity from SWCNT-TiNS was proposed to originate from the increased optical absorption of the SWCNT component at longer wavelengths relative to that of SEG.Accordingly, it was proposed that under visible light irradiation, photoinduced electrons generated at the SWCNT  surface were transferred to the TiNS substrate for subsequent CO 2 photoreduction.
A series of graphene-Ta 2 O 5 composites, with different loadings of the carbon, were prepared by Lv et al. through a hydrothermal route. 207Impregnation with a Ni/NiO co-catalyst activated these composites for CO 2 photoreduction to CH 3 OH and H 2 under UV-Vis illumination under aqueous conditions.Composites with 1 wt% graphene exhibited the highest photoactivity; in CO 2 -aerated water the composite produced 3.4 times and 2.3 times the amount of CH 3 OH and H 2 respectively than achieved over pure Ta 2 O 5 .This enhanced photocatalysis was attributed to the benets of the graphene support in promoting effective interfacial charge transfer, as illustrated in Fig. 22. Photoinduced electrons generated at the Ta 2 O 5 surface could be effectively transported to the surfaces of either graphene or the Ni/NiO dopant for subsequent transfer and CO 2 reduction.
Unlike the preceding materials, wherein as-synthesised graphene nanosheets were integrated with semiconductor particles through a simple impregnation/adsorption process, Tu et al. followed an alternative approach to prepare hollow spheres of alternating titania and graphene nanosheets for CO 2 photoreduction. 210Fig. 23 shows how the surface of a poly(methyl methacrylate) (PMMA) template was rst modied with a cationic polyelectrolyte, e.g.polyethylenimine (PEI) which electrostatically attracted negatively charged Ti 0.91 O 2 nanosheets onto their surface.Subsequent surface modication of these titania-coated PMMA beads with PEI facilitated coating by negatively charged graphene oxide (GO) nanosheets.This process was repeated multiple times to thereby prepare a PMMA core composite with ve alternating layers of (PEI/Ti 0.91 O 2 /PEI/ GO) 5 .Subsequent microwave treatment under an Ar environment decomposed the PMMA template, removed the PEI moiety, and reduced the GO to graphene (G).TEM (Fig. 23b and  c) revealed that these Ti 0.91 O 2 and graphene nanosheets alternated in a parallel, ordered lamellar arrangement, with a repeat distance estimated to be around 1.15 nm.Catalytic screening under UV-Vis illumination showed that the (G/Ti 0.91 O 2 ) 5 hollow spheres were superior for CO 2 photoconversion versus (Ti 0.91 O 2 ) 5 hollow spheres (synthesised without GO) and P25 titania in a glass reactor lled with CO 2 at ambient pressure, albeit the principal photolysis product was CO not CH 4 .A number of explanations for the excellent behaviour of these (G/ Ti 0.91 O 2 ) 5 hollow composites were advanced.The ultrathin nature of the Ti 0.91 O 2 nanosheets likely drove rapid transfer of charge carriers to active sites, increasing the probability of the photogenerated electrons participating in subsequent photoreduction chemistry; the compact stacking of Ti 0.91 O 2 and graphene nanosheets would allow efficient interfacial electron transfer from the Ti 0.91 O 2 to graphene layers, this spatial separation increasing the lifetime of photoexcited electrons.Hollow composite cores may also trap and multiply scatter incident photons, thereby improving light absorption and quantum efficiency.
Reduced graphene oxide (RGO) has also been employed within unique heterostructured optoelectronic materials, in which "tree"-like three-dimensional hierarchical titania nanorods, 211 or one dimensional ZnO nanorods 212 (as wide-band-gap UV absorbing semiconductors) are contacted with CdS visible light sensitising nanoparticles via a suspended, RGO lm.The resulting architectures ensure precise spatial localisation of each component and tunable charge-transport properties, facilitating a 100-150 % increase in the associated photocurrent density relative to the metal oxide/chalcogenide system alone, and 25% increase in methylene blue decomposition (ZnO/RGO/ CdS), and 240% increase in the applied bias to photoelectrochemical hydrogen generation efficiency for hydrogen production via water-splitting (TiO 2 /RGO/CdS).

Co-catalysts and plasmonic photocatalysts
4][215][216] Noble metals such as Au and Ag have lower Fermi levels than their partnered semiconductors, and hence can act as electron sinks that accept photoexcited electrons, 217,218 thereby retarding electron-hole recombination and  increasing the availability of electrons for CO 2 photoreduction.It is generally held that, when co-catalysts are employed, photoconversion efficiency varies with the co-catalyst particle size, in turn usually governed by the method of their incorporation, 75,157,219 albeit the nature of this relationship (proportional or inversely proportional), magnitude and origin remains poorly studied and understood.
Recent work by Iizuka et al. has shown that the photoactivity of plate-like ALa 4 Ti 4 O 15 (A ¼ Ca, Sr, and Ba) layered perovskites impregnated with a Ag co-catalyst varies with the method of cocatalyst loading. 75Silver modication of ALa 4 Ti 4 O 15 with a AgNO 3 precursor was achieved via: (i) wet impregnation followed by H 2 reduction; (ii) in situ photodeposition; or (iii) liquid phase reduction using NaPH 2 O 2 .Under UV illumination and water, all these Ag-loaded ALa 4 Ti 4 O 15 materials were active in photoreducing CO 2 into CO predominantly, and a small amount of HCOOH, accompanied by H 2 and O 2 evolution, under a constant CO 2 ow rate of 15 mL min À1 .Ag loaded by liquid phase reduction demonstrated the highest photoactivity, followed by that through impregnation/H 2 reduction, ahead of those photodeposited.When photodeposition was employed, Ag NPs of around 30-40 nm populated edges of the as-prepared BaLa 4 Ti 4 O 15 , while liquid phase reduction resulted in a homogeneous dispersion of <10 nm nanoparticles over the layered perovskite.Materials prepared via impregnation/H 2 reduction produced large aggregates of $50 nm Ag particles, although some 10-20 nm Ag particles were observed at the edges of impregnated/reduced samples aer photolysis.The basal plane and edges of plate-like BaLa 4 Ti 4 O 15 were suggested as the respective active sites for photooxidation and photoreduction.Accordingly, Ag deposited at the basal plane may undergo photodissolution and redeposition at the semiconductor edges during photocatalysis, with the particle size of such redeposited Ag dependent upon the concentration of dissolved silver ions.Consequently, in situ photolysis modied the Ag particle size to increase in the order: liquid phase reduction < impregnation/H 2 reduction < photodeposition.It was concluded that the photoactivity of Ag/BaLa 4 Ti 4 O 15 varied inversely with Ag particle size, a function of the co-catalyst incorporation route.
TiO 2 NTs arrays promoted by ultrane Pt NPs, uniformly deposited via a rapid, microwave-assisted solvothermal approach, were investigated by Feng and co-workers. 220In this method, the crystalline nanotube arrays were rst loaded in a Teon reaction chamber containing a 0.15 M H 2 PtCl 6 /CH 3 OH solution.Aer an overnight immersion process, the reaction chamber was rapidly heated to 120 C and held for 20 min before cooling.The resulting Pt/TiO 2 NTs arrays were isolated, washed and dried prior to catalytic testing.Dark eld (S)TEM shown in Fig. 24a highlights the uniform distribution of Pt NPs (with an average diameter of 3.4 nm) across the TiO 2 NTs arrays.While such wet impregnation is simple to conduct and yields small Pt NPs, they exhibited a broad size distribution spanning 1.8-4.9nm.Nevertheless, under irradiation by a solar simulator and water vapour, these Pt/TiO 2 NTs arrays were superior to unpromoted TiO 2 NTs arrays in photoreducing CO 2 to CH 4 and trace C 2 H 6 at 45 C. The excellent photoactivity of Pt/TiO 2 NTs arrays was attributed to the high Pt nanoparticle dispersion (and hence density of surface active sites), coupled with efficient electron diffusion properties through the nanotube arrays.
In contrast, Wang et al. utilised a unique gas phase technique, tilted-target sputtering (TTS), to deposit size-controlled ultrane Pt NPs onto a single crystalline titania thin lm comprised of 1-D TiO 2 columns. 51TTS enables precise control over both the ux of sputtered metal atoms and their radial distribution.Locating the TiO 2 lm in the outer region of the deposition ux zone, where low energy atoms participate in nanoparticle formation, permitted the selective growth of ultrane Pt NPs on the semiconductor surface as shown by TEM in Fig. 24b; Pt NPs with an average particle size of 1.04 AE 0.08 nm were uniformly coated on anatase TiO 2 .It was claimed that such Pt NPs were preferentially deposited at surface defects on the titania substrate.While this gas-phase method necessitates high vacuum instrumentation, and hence is not readily scalable, it does offer Pt NPs possessing a narrow size distribution and tunable diameter (through varying the deposition time).The smallest and largest nanoparticles reported via this approach exhibited mean particle diameters of 0.63 AE 0.06 nm and 1.87 AE 0.31 nm, respectively (albeit a narrow range spanned).Subsequent exposure of Pt/TiO 2 catalyst lms to water vapour and UV irradiation evidenced improved photoactivity compared with P25 and an analogous bare TiO 2 lm for CO 2 photoreduction, forming predominantly CH 4 with trace CO under a slow CO 2 stream in a continuous ow reactor.Optimal photoactivity was achieved for a 1.04 AE 0.8 nm Pt/TiO 2 catalyst lm, which delivered a remarkable combined quantum efficiency to CO and CH 4 of 2.41%.The observed photoactivity enhancement reected a combination of the high surface area and single crystalline nature of the TiO 2 thin lm, and excellent charge carrier separation characteristics conferred by the ultrane Pt NPs.However, photocatalysis deteriorated aer 5 h of irradiation, attributed to saturation of reactive sites by strongly-bound intermediate products, and consequent siteblocking.
2][223][224][225] This latter hot topic requires an understanding of the basic principles underlying the plasmonic phenomenon.The LSPR is a collective oscillation of free electrons within metal structures, established when the natural oscillation frequency of surface electrons (induced by charge density redistribution), resonates with the frequency of incident photons.6][227] Such intraband LSPR absorption (e.g. by Au 6sp electrons) induced by visible light is fundamentally different from the interband absorption caused by ultraviolet light (Au electrons excited from the 5d to 6sp bands). 228he strength and frequency of plasmon resonances depends on the size, shape, composition (for alloys) and local dielectric environment of the associated nanoparticles (see Fig. 26).These variations in LSPR peak positions and strengths strongly inuence the photocatalytic capacities of such plasmonic nanomaterials. 215,230n the basis of the literature, at least three mechanisms have been proposed to account for the enhanced visible light photoactivity of noble metal/semiconductor plasmonic photocatalyts.First, it is hypothesised that visible light photoexcited electrons at the surface of the plasmonic metal can migrate across the metal-semiconductor interface into the conduction band of the associated semiconductor, or transferred directly to adsorbates, providing a source of localised, energetic reductants. 231Second, the polarised electric eld induced by the LSPR of metal NPs could enhance reactant activation over neighbouring supports by the interplay between the resonating electron cloud of the metal NPs and nearby adsorbates (polarisable or possessing polar functional groups).Finally, photoexcited electrons may not actually transfer from plasmonic metal NPs to adjacent semiconductors/reactants nor interact with them indirectly via eld effects, but may rather liberate heat during relaxation of photoelectrons to their groundstate, thereby promoting thermal catalysis, although it has proven extremely difficult to qualitatively or quantitatively determine any such heating effect. 232All these models account for an increase in the photoexcited electrons available for chemical reduction. 227lasmon-enhanced CO 2 photoconversion has been recently reported, 233,234 exemplied by AgX : Ag (X ¼ Cl or Br) plasmonic NPs synthesised through a glycerol-mediated precipitation route in the presence of polyvinylpyrolidone. 235 Despite their similar synthetic protocols, AgCl : Ag and AgBr : Ag exhibited cube-tetrapod-like and hexagonal nanoplates respectively.XPS and XRD conrmed the presence of metallic Ag on the asprepared AgX : Ag NPs, with UV-Vis revealing signicantly different band gaps of 3.95 (AgCl) and 3.45 eV (AgBr).When dispersed in NaHCO 3 aqueous medium under visible light, the AgX : Ag plasmonic photocatalysts were active for photoreducing CO 2 into CH 3 OH, attributed to the metallic Ag LSPR inuencing the contacted AgX : Ag NPs.Recycling experiments indicated that these plasmonic catalysts were stable under the screening conditions, although in situ Ag + reduction to additional silver metal is a plausible deactivation pathway following prolonged irradiation.
Hou et al. deposited islands of plasmonic Au NPs onto a solgel prepared TiO 2 thin lm via electron beam evaporation. 148nder visible light illumination and water vapour the resulting Au-promoted TiO 2 thin lm exhibited a 24-fold activity enhancement relative to a bare TiO 2 thin lm for CO 2 photoreduction to CH 4 in a stainless steel reactor at 75 C.The low, but nite, photoactivity of the parent TiO 2 lm was attributed to defect-induced, sub-band gap photoexcitation.Complementary calculations suggested that direct electron transfer from Au NPs to TiO 2 was not feasible under plasmonic excitation, and hence the superior photoactivity was ascribed to an increased subband gap charge carrier formation rate, indirectly induced by the LSPR of gold.Methane was the only reductant formed over the bare TiO 2 thin lm under UV irradiation, however a range of products were formed over the Au/TiO 2 thin lm wherein the production rate followed CH 4 > C 2 H 6 > HCHO > CH 3 OH.Surprisingly, the pure Au NPs were also reported to generate these aforementioned organic products, although the overall productivity was lower than that of the Au/TiO 2 composite.The photocatalytic activity of Au NPs was proposed due to interband electronic transitions (and not the LSPR) wherein electrons were photoexcited to a potential more negative than the TiO 2 conduction band and redox potential required to form desired reduction products.The synergy between Au NPs and the TiO 2 lm under UV irradiation was attributed to improved charge carrier separation.
Small Cu and Ag NPs which exhibit plasmonic effects have also been successfully loaded onto TiO 2 nanorod lms by an electrochemical method. 219,236Tan et al. loaded Cu NPs onto a TiO 2 lm via a 0.01 mM CuSO 4 and 0.1 M K 2 SO 4 mixed electrolyte at a pulse potential of À0.8 V for between 0-300 s. 236 Under UV illumination and water, the resulting composite lm (prepared at the optimum 100 s deposition time) demonstrated superior activity to a bare TiO 2 or P25 lm in photoreducing CO 2 into CH 4 .The copper co-catalyst benets were again ascribed to efficient charge separation due to plasmonic effects.However, it was reported that surface oxidation of Cu NPs to copper oxide occurred during photocatalyic screening.An electrochemical deposition method known as the doublepotentiostatic method was also used to deposit Ag NPs onto a TiO 2 nanorod lm. 219This method favoured the formation of uniform and dense metallic nanoparticles due to quick nucleation and slow nanoparticle growth at high potential and low precursor concentration.Accordingly, Kong et al. loaded silver onto a TiO 2 lm employing a 0.1 M KNO 3 /0.2mM sodium citrate/0.05mM AgNO 3 electrolyte at nucleation potentials from À1.4 V to À0.8 V for 100 s, followed by a growth potential of À0.2 V for 2400 s.TiO 2 /Ag lms prepared at the optimum nucleation potential of À1.0 V exhibited improved photoactivity relative to a bare TiO 2 lm in photoreducing CO 2 into CH 4 under UV illumination and water vapour.Photocatalytic performance was again postulated to arise from efficient charge separation due to silver plasmonic effects.Moreover, alloy or bimetallic plasmonic nanostructures, such as Au-Pt (as shown in Fig. 27a) 237 and Ag-Pt (as shown in Fig. 27b), 233 have also been discovered to enhance CO 2 the photoreduction by titania.In both cases, the introduction of plasmonic Au or Ag particles increased photocatalytic efficiency in terms of product selectivity and/or yield, ascribed to the LSPR effect of Au (Ag) NPs signicantly increasing light absorption and charge separation of the photoexcited titania.Co-deposition of Au/Pt on TiO 2 NFs considerably enhanced photocatalytic activities for both H 2 generation and CO 2 reduction compared to singly decorated (Pt/TiO 2 or Au/TiO 2 ) bres.Bijith and co-workers' study demonstrates that Ag, Pt and bimetallic Ag-Pt NPs enable CH 4 and CO product selectivity to be tuned: 233 product yields were enhanced more than seven-fold compared with native TiO 2 , with a high CH 4 selectivity.This was attributed to the enhanced availability of photoexcited electrons via improved electron-hole pair generation, and surface trapping of photoexcited electrons within the titania lattice, enabling the necessary accumulation of charge to drive the eight-electron CH 4 forming process from adsorbed CO 2 .

Miscellaneous photocatalysts
9][240][241][242] Richardson et al. codoped TiO 2 with Cu and Ga via a sol-gel route to form composites that were active in CO 2 photoreduction at longer wavelengths than possible utilising P25. 139The absorption band edge wavelength of doped photocatalysts increased in the order P25 < Ga/TiO 2 < Cu/TiO 2 < Cu, Ga/doped TiO 2 , with a corresponding decrease in the band of the doped-materials relative to P25.It should be noted that the absorption band edges of doped materials still resides in the UV region despite their modied optical properties.    resulting in the absorption band edge shiing to longer wavelengths in comparison to pure InTaO 4 .Results were attributed to hybridisation between the N 2p and O 2p orbitals, which shis the valence band upwards, thereby narrowing the InTaO 4 band gap.Under visible light illumination (390 to 770 nm) and in the presence of water, the prepared N-doped InTaO 4 outperformed pure InTaO 4 in photoreducing CO 2 into CH 3 OH in a continuous ow reactor.XANES analysis indicated that N doping narrowed the band gap, and induced oxygen vacancies, the latter possibly contributing to photocatalytic enhancement.The photoactivity of the N-doped InTaO 4 was further improved by the addition of a Ni@NiO core-shell cocatalyst, incorporated through a sequence of wet impregnation, chemical reduction and nal calcination.Promotion by the Ni@NiO core-shell co-catalyst was ascribed to enhanced charge separation and light harvesting.
5][246][247] In general, the component semiconductors must possess the same crystal structure and similar lattice parameters.Liu et al. recently reported the application of Cu x -Ag y In z Zn k S m solid solutions as visible light photocatalysts for CO 2 photoreduction. 248The band gap of this composite was a strong function of composition, and spanned 1.33-1.44eV.Visible light irradiation (1000 W Xenon lamp at l > 400 nm) in a NaHCO 3 aqueous medium of these solid solution photocatalysts facilitated CO 2 photoreduction to CH 3 OH in a closed glass gas circulation system.Systematic relations could not be derived between the catalyst composition and photoactivity, although photoactivity was composition dependent.A range of co-catalysts were investigated, including RuO 2 and Rh 1.32 Cr 0.66 O 3 , although these did not promote CO 2 conversion.Methanol productivity was enhanced under a H 2 environment, wherein a higher density of protons was formed.

Conclusions and future directions
A wide variety of approaches have been developed for the synthesis of photocatalytically active materials for CO 2 reduction to solar fuels and chemicals.To date, there is no standard protocol for evaluating photocatalytic performance, or single parameter that enables quantitative benchmarking of CO 2 conversion efficiencies to specic carbon-containing solar fuels or chemicals.In a rst step towards addressing these deciencies, Table 2 compares a range of the nanomaterials discussed in this review, with a view to informing on which show the most promise with respect to productivity per unit catalyst mass, and thus provide a strong rationale for future development.Of course, many additional factors such as earth abundance, cost, toxicity, and scalability will also inuence which materials are ultimately best suited to commercialisation.However, a step-change in photoactivity, and comprehensive mechanistic insight into the catalytic cycle and nature of the surface sites participating in CO 2 adsorption and reduction, is required before such considerations should inuence ongoing fundamental research efforts.
The optoelectronic properties of crystalline materials, and hence their photoactivity, are a strong function of the crystal structure, with different semiconductor polymorphs exhibiting disparate photocatalytic behaviour reecting their surface atom arrangement and attendant electronic band structure.Although surface defects may serve as electron-hole recombination centres detrimental to the achievement of high quantum efficiencies, such sites can also promote visible light absorption and hence photocatalysis.Band gap engineering of earth abundant, large band gap semiconductor to facilitate visible light photocatalysis remains an important barrier to visible light absorption properties, although transition metal and pblock dopants provide a simple route to band-gap narrowing.Integrated dyes or small band gap semiconductors may also be employed as visible light sensitisors of conventional semiconductor photocatalysts, with the resulting hybrid, composite materials facilitating both UV and visible light capture and efficient charge carrier separation via preferential transfer of photoexcited electrons into one material and of holes into the other.In the latter regard, one-and two-dimensional semiconductor nanostructures, exhibiting high surface to volume ratios, can promote efficient transport of charge carriers to adsorbates during photocatalysis.
Integration of photoactive semiconductors with electrically conductive materials such as graphene offers new routes to maximising charge carrier lifetimes and consequent electron transfer to CO 2 .High surface area and porous materials increase the density of surface active sites available to chemically bind and activate CO 2 , its reactive intermediates, and water.Such nanostrutures can be created through the synthesis of semiconductors with intrinsic porosity on the micro-, mesoor macroscale, or via immobilisation (embedding) of photocatalysts onto (within) an inert, high area architecture.Threedimensional hierarchical materials epitomise catalyst engineering, affording rapid molecular transport to/from active sites, enhanced light harvesting, and superior charge carrier transport and recombination characteristics.It seems extremely unlikely that any single semiconductor composition or structure can deliver the requisite optical, material and catalytic properties necessary to harness sunlight under ambient conditions for the rapid and selective photoreduction of CO 2 into the key chemical targets of methane, methanol, formaldehyde or ethene, hence the majority of promising systems employ co-catalysts whose contributions depend strongly on their method of incorporation.
Noble metal co-catalysts in particular offer exciting possibilities for visible light absorption through plasmonic effects, and indeed have even proven efficient photocatalysts in their own right for the selective oxidation of CO, ethene and ammonia 251 and reduction of nitroaromatics. 252Photoreactor design will play an increasingly important role in ensuring optimum light (and catalyst) utilisation for CO 2 reduction, and enable quantitative benchmarking of different photocatalysts in terms of their absolute productivity, selectivity and quantum efficiency.Innovation is required to facilitate CO 2 separation/ recycle, on-stream gas/liquid product separation, and optical materials to avoid losses in transmission to the internal reactor volume.
Ultimately, the nal selection and commercialisation of the most promising (i.e.highly active, selective and stable) photocatalysts will be dictated by process economics, since even the highest value oxygenate or olen products directly derivable from CO 2 will continue to face stiff competition with low cost fossil fuel sources for the foreseeable future.Indicative 2015 prices per metric ton are: methanol $420; formaldehyde $420; formic acid $500, and ethylene/propylene $800-1100.Overall process costs will be a strong function of the choice of chemical precursors, type and recyclability of solvents, (re)use of textural templates, mechanothermal processing, photocatalyst forming, and reactor operating conditions (temperature, pressure, separation and recycle).Many of the most promising photocatalysts described in this review utilise noble metal co-catalysts, 40,51,94,123,161,213,220,234,249 and with common H 2 PtCl 6 and HAuCl 4 prices in excess of $10 000 per kg, high turnover numbers per metal atom (>3000) would be essential to even recoup the precursor costs.An obvious target for synthetic improvement is thus maximising noble metal efficiency through e.g.4][255][256] So and hard templating methods are widely adopted in heterogeneous catalysis to introduce porosity, and hence enhance the surface density and accessibility of active sites. 137A number of photocatalysts for CO 2 reduction employ such templates to similar effect, typically surfactants (e.g.TTAB, 107 CTAB 77,163,164 and P123 (ref.78 and 112)) or polymers. 210While their costs are comparatively low ($150-350 per kg), in the majority of cases such surfactants are removed through combustion and hence are not recoverable, and even solvent-extraction protocols rarely recover sufficiently pure and chemically unaltered templates amenable to re-use, though ultrasonic template removal shows promise as an energy and atom efficient solution. 257The majority of synthetic strategies for nanostructure photocatalysts employ solvothermal processing, and hence comparatively low (<200 C) temperatures (in the absence of a template calcination step) offering little scope for cost savings via energy input reduction.As with all syntheses, Green Chemistry principles provide the best general guidelines to reduce the number of reaction steps and minimise associated waste generation and the use of auxiliaries, 258 in concert with lower production costs.
Bachelor of Engineering in Chemical Engineering and Technology from Guizhou University (Guiyang, China) in 2007, Master of Engineering in Industrial Catalysis under the supervison of Professor Jun Wang in Nanjing Tech University (Nanjing, China) in 2010, and PhD under the supervision of Dr Xuebin Ke and Professor Huaiyong Zhu at Queensland University of Technology (Brisbane, Australia) in 2014.He is currently a research associate working with Professor Adam Lee and Professor Karen Wilson to develop supported metal catalysts and solid acid catalysts for clean chemicals synthesis within Aston University and UK Catalysis Hub.

Fig. 1
Fig. 1 Schematic of a carbon zero route to solar fuels and renewable olefins through the photocatalytic reduction of CO 2 .

Fig. 2
Fig. 2 CH 4 and CO productivity from untreated and He-treated TiO 2 polymorphs during 6 h illumination under 150 W solar simulator in the presence of CO 2 /H 2 O vapour, in a continuous-flow mode photoreactor operating at a flow rate of 2 mL min À1 .Reprinted with permission from ref. 80.Copyright 2012 American Chemical Society.

Fig. 3
Fig. 3 SEM images (A)-(C), TEM images (D)-(F), HRTEM images (G)-(I) and (J) electronic band structures based on UV-Vis and valence band XPS spectra of single crystal solid anatase TiO 2 , single crystal hollow anatase TiO 2 , and hollow anatase TiO 2 mesocystals.Insets in (B) and (C) highlight hollow cores in the single and mesocrystal particles, insets in (G)-(I) show SAED patterns for the solid single crystal, hollow single crystal and mesocrystal anatase TiO 2 .Reprinted with permission from ref. 93.Copyright 2012 American Chemical Society.

Fig. 4
Fig. 4 TEM images of (a) Zn 2 GeO 4 nanoribbons 94 and (b) In 2 Ge 2 O 7 sub-nanowires (inset: corresponding SAED pattern) 96 prepared by solvothermal treatment using a binary ethylenediamine/water solvent system, and Zn 2 GeO 4 sub-nanowires prepared through solution phase route (c) at 40 C and (d) 100 C. Reproduced from ref. 97 with permission from The Royal Society of Chemistry.

Fig. 5
Fig. 5 Product formation of N-doped TiO 2 NTs arrays, calcined and sputtered coated with Pt and Cu.CO 2 photoreduction was conducted in a batch reactor under natural sunlight illumination in the presence of water vapour at 44 C. Reprinted with permission from ref. 40.Copyright 2009 American Chemical Society.
CO 2 photoreduction was negligible over the parent ZnGaNO (with or without ZnAl 2 O 4 modication) without Pt doping to promote electron-hole separation.The superior performance of the ZnAl 2 O 4 modied material was attributed to improved mass transport, increased basicity and hence CO 2 chemisorption, and narrowing of the band gap.Micro/mesoporous Zn 2 GeO 4 and mesoporous zinc germanium oxynitride synthesised via the

Fig. 6
Fig. 6 FE-SEM images of (a) porous Ga 2 O 3 , (b) vertical cross-section of porous Ga 2 O 3 fractured by ion beam and (c) TEM image of the porous Ga 2 O 3 .Reproduced from ref. 107 with permission from The Royal Society of Chemistry.

Fig. 7
Fig. 7 (A) Schematic diagram showing the synthesis steps of preparing mesoporous ZnGa 2 O 4 through mesoporous colloidal template and ion-exchange reaction and (B) TEM image of the resulting mesoporous ZnGa 2 O 4 (inset: HR-TEM image of the crystal fringes).Reproduced with permission from ref. 113.Copyright 2010 Wiley.

Fig. 8
Fig. 8 (a) FE-SEM and (b) TEM and HR-TEM images of mesoporous TiO 2 -SiO 2 composites prepared at 800 C. Reproduced from ref. 108 with permission from The Royal Society of Chemistry.

Fig. 9
Fig. 9 TEM images of (a) non-porous g-C 3 N 4 , and (b) porous p-g-C 3 N 4 .Reproduced from ref. 119 with permission from The Royal Society of Chemistry.

Fig. 12
Fig. 12 FE-SEM images of (a and b) sheaf-like Zn 2 GeO 4 superstructure solvothermally treated for 12 h, and (c) Zn 1.7 GeN 1.8 O prepared through high temperature nitridation of Zn 2 GeO 4 fusiform bundles.Reproduced from ref. 155 with permission from The Royal Society of Chemistry.

Fig. 13
Fig. 13 FE-SEM images of the nanoplate/micro-octahedron Zn 2 SnO 4 at (a) lower magnification and (b) at higher magnification.The inset of (b) shows an isolated hexagonal nanoplate.Reprinted with permission from ref. 156.Copyright 2012 American Chemical Society.

Fig. 14
Fig. 14 Schematic of hollow Bi 2 WO 6 microspheres formation through anion exchange of solid BiOBr microspheres.Reproduced from ref. 157 with permission from The Royal Society of Chemistry.

Fig. 15 FE
Fig. 15 FE-SEM images showing (a) the cross section of the porous venation architecture of SrTiO 3 (scale bar, 1 mm) (b) cross section of the porous network axially (scale bar, 10 mm), (c) magnified image of the wall of the network (scale bar, 1 mm) and (d) TEM image showing the mesoporous structure of SrTiO 3 (scale bar, 2 nm).Reprinted by permission from Macmillan Publishers Ltd ref. 158, copyright 2013.

Fig. 16
Fig. 16 Electron transfer pathway in a SC/SC hybrid system.Adapted from ref. 160, Copyright 1995, with permission from Elsevier.

Fig. 18
Fig. 18 (a) Schematic of CO 2 photoreduction on Nf/Pd-TiO 2 nanoparticles and (b) CH 4 (at 5 h) and C 2 H 6 (at 1 h) production at different pH ranges on Pd-TiO 2 and Nf/Pd-TiO 2 NPs under UV illumination (l > 300 nm) in the presence of 0.2 M Na 2 CO 3 aqueous solution.Catalyst concentration was fixed at 1.5 g L À1 (with 1.0 wt% Pd and 0.83 wt% Nafion).Reproduced from ref. 170 with permission from The Royal Society of Chemistry.

Fig. 21
Fig. 21 Methane formation rate of (a) SEG-P25 and SRGO-P25 films under UV and visible light illumination, and (b) SEG-TiNS and SWCNT-TiNS films under UV (100 W Hg vapour lamp, 365 nm) and visible light irradiation (natural daylight bulb, $400 to 850 nm).Reprinted with permission from ref. 206 and 209.Copyright 2011 and 2012 American Chemical Society.

Fig. 22
Fig. 22 Schematic of potential pathways for the transport of photoinduced electrons from a Ta 2 O 5 semiconductor nanoparticle to active CO 2 photoreduction sites mediated by graphene.Reproduced from ref. 207 with permission from The Royal Society of Chemistry.

Fig. 23 (
Fig. 23 (A) Stepwise preparation of alternating TiO 2 and graphene nanosheet hollow sphere assemblies, (B) TEM and (C) and HR-TEM images of (G-Ti 0 O 2 ) 5 hollow spheres.Scale bar of the inset in (c): 100 nm.Inset in (C) is the fast Fourier Transform pattern of the ordered lamellar structures exhibited by the (G-Ti 0.91 O 2 ) 5 hollow spheres.Reproduced with permission from ref. 210.Copyright 2012 Wiley.

Fig. 24 (
Fig. 24 (a) Dark field (S)TEM image of ultrafine Pt NPs deposited TiO 2 nanotube arrays, 220 and (b) TEM image of size resolved ultrafine Pt NPs on TiO 2 film.Inset: HRTEM image showing the lattice fringes corresponding to anatase TiO 2 (112) and Pt (111) NPs.Reproduced from ref. 157 with permission from The Royal Society of Chemistry.

Fig. 25
Fig. 25 Electromagnetic field of visible light interacting with plasmonic metal NPs: a dipole oscillator forms and resonates with the electromagnetic field of the incident visible light.Reproduced with permission from ref. 229.

Fig. 26
Fig. 26 The LSPR bands of plasmonic metal nanostructures with different shapes, sizes or compositions (for alloys): (a) nanoparticles of Au, Ag, and Cu; (b) Ag nanostructures with various shapes; (c) nanocubes of Ag with different cubic sizes; and (d) alloy of Au-Ag with different Au : Ag ratios.Reprinted by permission from Macmillan Publishers Ltd.Ref. 227, copyright 2011.Reproduced with permission from ref. 230.Copyright 2011 Wiley.
Fig. 27 (a) Illustration of the photocatalytic process to produce H 2 and to reduce CO 2 over Au/Pt/TiO 2 NFs.Reprinted with permission from ref. 237.Copyright 2010 American Chemical Society.(b) Illustration of different photoexcited charge pathways following electron-hole generation assisted by plasmonic nanoparticles over core-shell Ag@SiO 2 : transfer to the [a] semiconductor surface, [b] Pt NP surface, [c] Ag NP surface, and [d] bimetallic Ag-Pt surface.Reproduced with permission from ref. 233.Copyright 2013 IOP Publishing Ltd.
/ 2H 2 + O 2 and CO 2 + H 2 O / CO + H 2 + O 2 ), for which the molar ratio of (H 2 + CO) : O 2 should be 2 : 1.However, very few studies report such oxygen evolution, let alone endeavour to quantify it and thereby demonstrate that water is the sole electron donor.In the case of ZrO 2 in aqueous NaHCO 3 , 73 KTaO 3 (ref.74) and Ag doped ALa 4 Ti 4 O 15 (A ¼ Ca, Sr, and Ba) perovskites, Accordingly, electron excitation and transfer was more efficient over the high symmetry c-NaNbO 3 phase than o-NaNbO 3 .In addition to CO (0.082 mmol h À1 ) and H 2 (0.71 mmol h À1 ), trace amounts of C 2 H 4 , C 2 H 6 and C 3 H 8 were detected during CO 2 photocatalysis over the c-NaNbO 3 .
4(0.486 mmol h À1 ) than that obtained from o-NaNbO 3 (0.245 mmol h À1 ) in a gaseous, closed circulation system pressurised with 80 kPa CO 2 .Density functional theory (DFT) calculations ascribed this variation in photocatalytic performance to differences in conduction band (CB) energies, reecting different octahedral ligand elds.

Table 1
Textural properties of porous, structured photocatalysts and associated products of CO 2 photoreduction 3 2À ions originating within the parent LDH, although this could not account for the total product yield.Replacing Ga 3+ with Al 3+ to create a [Zn 3 Al(OH) 8 ] 2 + (CO 3 ) 2À $mH 2 O LDH enhanced overall photoconversion efficiency, but favoured CO formation (even aer Cu 2+ incorporation).These layered catalysts were reported active for 20 h continuous CO 2 photoreduction, although methanol productivity declined over time.In an effort to boost selectivity towards methanol, the interlayer CO 3 2À anions were exchanged for [Cu(OH) 4 ] 2À anions, thereby creating a [Zn 1.5 -Cu 1.5 Ga(OH) 8 ] 2 + (CO 3 ) 2À $mH 2 O material 136 with a methanol yield almost three times that achievable over the carbonate counterparts.Copper was thus hypothesised as the active photocatalytic site, with the localisation of Cu sites within the interlayers facilitating their accessibility to CO 2 , while the replacement of CO 3 2À with [Cu(OH) 4 ] 2À counterions signicantly decreased the band gap of the resulting LDH while expanding the interlayer spacing, both factors likely contributing towards the enhanced photoactivity of the copper anion doped variant.2.2.4.Hierarchical ordered photocatalysts.

Table 2
Photocatalytic reaction conditions and CO 2 reduction performance of exemplar nanostructured materials 4 ), 15 (CO), 45 (H 2 ), 69% cubic 5.86 (CH 4 ), 16 (CO), 42 (H 2 ), 98% cubic 4.91 (CH 4 ), 12 (CO), 25 (H Cu and Ga co-doped TiO 2 photocatalysts were superior to P25 and the monometal-doped titanias in photoreducing CO 2 to HCOOH and other carbonic products (determined by TOC) in a continuous ow reactor at room temperature.Highest photoactivity was observed for the Cu 0.78 -Ga 0.22 /TiO 2 catalyst, with Cu the proposed CO 2 adsorption and reduction site for HCOOH and other organic products which include trace CH 3 OH and HCHO.Anion doping by Tsai et al. improved the visible light absorption properties of InTaO 4 .Doping was performed by nitridation of SSR prepared InTaO 4 under owing NH 3 at 800 C,