Advancements on metal oxide semiconductor photocatalysts in photo-electrochemical conversion of carbon dioxide into fuels and other useful products

Abstract

Due to its fascinating and tunable optoelectronic properties, semiconductor nanomaterials are the best choices for multidisciplinary applications. Particularly, the use of semiconductor photocatalysts is one of the promising ways to harness solar energy for useful applications in the field of energy and environment. In recent years, metal oxide-based tailored semiconductor photocatalysts have extensively been used for photocatalytic conversion of carbon dioxide (CO₂) into fuels and other useful products utilizing solar energy. This is very significant not only from renewable energy consumption but also from reducing global warming point of view. Such current research activities are promising for a better future of society. The present mini-review is focused on recent developments (2–3 years) in metal oxide semiconductor hybrid photocatalysts-based photo-electrochemical conversion of CO₂ into fuels and other useful products. First, general mechanism of photo-electrochemical conversion of CO₂ into fuels or other useful products has been discussed. Then, various metal oxide-based emerging hybrid photocatalysts including tailoring of their morphological, compositional, and optoelectronic properties have been discussed with emphasis on their role in enhancing photo-electrochemical efficienty. Afterwards, mechanism of their photo-electrochemical reactions and applications in CO₂ conversion into fuels/other useful products have been discussed. Finally, challenges and future prospects have been discussed followed by a summary.

Graphical abstract

Keywords

metal-oxide semiconductors / nanohybrid photocatalysts / photo-electrochemical CO₂ conversion / tailoring morphology / fuels / challenges and future prospects

1 Introduction

In the 21st century, two main backbones, one, ever-increasing growth in population and the other, rapid industrialization have enhanced the dependency of mankind on energy resources to fulfill their daily basic needs. This dependency and the great demand for energy have been addressed mainly by the excessive combustion of fossil fuels (oil, coal, and natural gas), leading to other major challenges in society. For example, the extensive combustion of fossil fuels and other carbon-based reagents increases the amount of carbon dioxide (CO₂) emission in the atmosphere [1]. It is also the main component of greenhouse gas causing global warming and has a drastic impact not only on living organisms/humans but also on the global ecological imbalance that includes climate change, rising ocean levels, and environmental pollution. These issues need to be addressed seriously for the betterment of a healthy society and the environment. Similarly, over-consumption of fossil fuels leads to an energy crisis globally as these non-renewable sources of energy are exhaustible and require thousands of years in their formation which itself is a major challenge. The reduction of global warming and the use of renewable energy are two very important research problems of current interest. In this context, the reduction of CO₂ resulting in fuels and other important/useful products using solar light addresses such problems of global warming and energy needs [2–4].

Therefore, it is given much attention to make a carbon-free environment. To achieve this, several efforts are being made. For example, CO₂ has been utilized as a starting material for the production of some value-added chemicals which is more focused on nowadays because it not only helps the environment to be cleaned but also provides something useful to society [5]. Others include carbon capturing, storage technology, the development of alternative renewable energy/fuel sources, and the utilization of clean/greener solar energy for the reduction of CO₂ [6,7]. Similar to the natural photosynthesis process in plants, where CO₂ is used as a raw material which is finally converted into hydrocarbons in the presence of H₂O and solar light [5], researchers have developed photocatalysis processes to reduce CO₂ using some suitable photocatalysts under the effect of light irradiation [8–10]. These promising materials are designed to adsorb the CO₂ on their surfaces and provide surface reactivity to reduce CO₂ into some useful products. Moreover, various CO₂ reduction approaches have been developed including plasma chemical, photocatalytic [11], photochemical, and photo-electrochemical [12] to convert CO₂ into various important hydrocarbons. The photocatalytic processes and products obtained are significant in mitigating energy crises and environmental problems caused by CO₂, and are more efficient, eco-friendly, and cost-effective. However, the photocatalytic CO₂ conversion is not so straightforward and is very challenging because of the stable and inert as well as non-polar nature of CO₂. It needs high dissociation energy input and multi-step electron transfer complex processes to understand, which are still not very clear. Furthermore, the conversion of CO₂ into a variety of valued chemical fuels such as CO, CH₄, HCOOH, HCHO, CH₃OH, C₂H₅OH, C₂H₆, C₂H_4, and other C₁ and {\text{C}}_2^{\text{+}} products, again puts up a barrier in the selectivity.

The use of renewable energy resources for producing useful products is one of the most significant research activities from a large energy consumption point of view in the present society. Recent developments in the field show that solar energy can be used to reduce global warming, resulting in the production of many fuels and useful products. In this context, recent progress in the field of photo-electrochemical conversion of CO₂ into fuels and other useful products using different types of photocatalyst materials has shown promising results as shown in Fig.1. Particularly, the conversion of CO₂ using a photo-electrochemical process is promising given its greener CO₂ utilization method in producing useful products under light irradiation with the application of an external bias potential [13]. It provides the benefits of using both processes in combination with such approaches as electrocatalytic and photocatalytic conversion [13,14]. Owing to its fascinating and tunable optoelectronic properties, semiconductor photocatalyst nanomaterials are the best choice for such applications. In the recent past, metal oxide-based tailored semiconductor photocatalysts [15–17] have extensively been utilized for photocatalytic conversion of CO₂ into fuels and other useful products utilizing solar energy [18–20]. The photo-electrochemical process is useful in promoting the separation efficiency of photogenerated electron-hole pairs attributed to both the photocatalytic and electrocatalytic processes [14].

The present mini-review is focused on advancement in metal oxide semiconductor photocatalysts-based photo-electrochemical conversion of CO₂ into fuels and other useful products conducted in recent past. Encompassing the photo-electrochemical conversion of CO₂ into valuable products i.e., the present review integrates both photocatalysis and electrocatalysis processes offering a synergistic advantage in CO₂ reduction. The various metal oxide-based emerging hybrid photocatalysts with other functional nanomaterials including, tailoring of their morphological, compositional, and optoelectronic properties have been discussed with emphasis on the mechanism of their photo-electrochemical conversion reaction and application in photo-electrochemical conversion of CO₂ into fuels and other useful products utilizing solar energy. Eventually, challenges and future prospects are discussed, followed by a summary.

2 Mechanism: photo-electrochemical conversion of CO₂ into fuels and other useful products

The mechanism of photo-electrochemical conversion of CO₂ into valuable fuels and other important products has been discussed in this section.

2.1 CO₂ reduction reactions

Due to the inertness of the CO₂ molecule, CO₂ reduction is thermodynamically and kinetically difficult. It requires a huge amount of energy input. In addition, CO₂ reduction can produce many different reduction products, where different complex reaction paths take place. Equations (1)–(8) list the different reduction reactions from CO₂ conversion in an aqueous solution vs. normal hydrogen electrode (NHE) at pH 7 [21].

(1)\text{C}{\text{O}}_2+{\text{e}}^{-}\to {\text{CO}}_2^{\cdot -}-1.90\mathrm{V}

(2)\text{2C}{\text{O}}_2+12{\text{H}}^{+}+12{\text{e}}^{-}\to {\text{C}}_{\text{2}}{\text{H}}_5\text{OH}+3{\text{H}}_{\text{2}}\text{O}-0.61\mathrm{V}

(3)\text{C}{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}\to \text{CO +}{\text{H}}_{\text{2}}\text{O}-0.53\mathrm{V}

(4)\text{C}{\text{O}}_2+4{\text{H}}^{+}+4{\text{e}}^{-}\to \text{HCHO}+{\text{H}}_{\text{2}}\text{O}-0.48\mathrm{V}

(5)\text{C}{\text{O}}_2+6{\text{H}}^{+}+6{\text{e}}^{-}\to \text{C}{\text{H}}_{\text{3}}\text{OH}+{\text{H}}_{\text{2}}\text{O}-0.38\mathrm{V}

(6)\text{C}{\text{H}}_4\text{C}{\text{O}}_2+8{\text{H}}^{+}+8{\text{e}}^{-}\to \text{C}{\text{H}}_4+2{\text{H}}_2\text{O}-0.24\mathrm{V}

(7)\text{2C}{\text{O}}_2+12{\text{H}}^{+}+12{\text{e}}^{-}\to {\text{C}}_{\text{2}}{\text{H}}_4+4{\text{H}}_{\text{2}}\text{O}-0.34\mathrm{V}

(8)\text{2C}{\text{O}}_2+12{\text{H}}^{+}+12{\text{e}}^{-}\to {\text{C}}_{\text{2}}{\text{H}}_5\text{OH}+3{\text{H}}_{\text{2}}\text{O}-0.33\mathrm{V}

Other than the thermodynamic barrier, CO₂ conversion also has high kinetic barriers. The CO₂ conversion process involves synergistic steps such as adsorption of CO₂ molecules, electron transfer, and proton transfer [22,23]. Thus, CO₂ conversion can only take place when both thermodynamic and kinetic requirements are satisfied, demanding highly efficient catalytic methods and catalysts.

2.2 Photocatalytic CO₂ conversion

Fig.2(a) depicts the fundamentals of the photocatalytic conversion of CO₂ on a semiconductor catalyst. First, being excited by the incident light, the semiconductor catalyst generates photo-generated electrons and holes in the conduction band (CB) and valance band (VB), respectively. Then, the photo-generated electrons and holes transfer to the surface of the semiconductor catalyst. Finally, the catalytic reactions (CO₂ reduction and the oxidation half-reaction) take place, triggered by the photo-generated electrons and holes, respectively [25]. For photocatalytic conversion of CO₂, the CB potential needs to be more negative than the redox potential of the target CO₂ reduction product reactions (Eqs. (1)–(8)) so that the photo-generated electrons in CB can drive the corresponding CO₂ reactions. In the meantime, the VB potential should be more positive than the redox potential of the half-oxidation reaction. Moreover, the final CO₂ reduction products and H₂O oxidation products are mixed in photocatalytic CO₂ conversion, requiring extra steps for product separation [26]. Although eco-friendly and promising, these main difficulties make photocatalytic CO₂ conversion challenging.

2.3 Electrochemical CO₂ conversion

The electrochemical reaction system is compact and can be controllable by electrode potentials. As shown in Fig.2(b), external voltages are applied to the cathode and anode for the CO₂ reduction and oxidation of water, respectively, in a typical electrochemical CO₂ conversion system. A proton exchange membrane separates the compartments of the cathode and anode, individually optimizing the catalytic performance during electrochemical CO₂ conversion for both reduction and oxidation products [27,28]. There are several advantages for the electrochemical CO₂ conversion. For example, renewable energy resources can be utilized to power the electrochemical reaction system. The overall chemical consumption in electrochemical CO₂ conversion can be minimized to only CO₂ and water, as the electrolytes are recyclable. Because of the feasibility of scale-up in industrialization, the electrochemical CO₂ conversion can perform at a much higher reaction rate than that of photocatalytic CO₂ conversion [7]. The better product selectivity and reaction stability of the electrochemical CO₂ conversion are always the goals for future industrial applications [29,30].

2.4 Photo-electrochemical CO₂ conversion

Photo-electrochemical catalysis can be achieved through different approaches, as illustrated in Fig.2(c). One is the photo-electro non-coupled system (or so-called photovoltaic cell-electrochemical cell (PV-EC) system), where solar energy is solely acquired to provide electricity (such as solar cell) for the electrochemical CO₂ conversion system [31,32]. In this case, photochemistry and electrochemistry work separately, which is not of interest in this mini-review. Another one is a photo-electro coupled system, where the cathode and/or anode electrodes are made of semiconductor materials. In this case, the cathode and/or anode can be excited by the incident light during the electrochemical process, combining photocatalysis and electrochemical catalysis. Notably, solar cells can also be used to power the photo-electro coupled photo-electrochemical catalysis system, making the whole device environmentally friendly and sustainable. In this mini-review, the photo-electro coupled system is focused on. The photo-electrochemical system can improve the catalytic performance from both photocatalysis and electrochemical catalysis sides. As aforementioned, semiconductor catalysts suffer from the high recombination rate of the photo-generated electrons and holes in photocatalysis. In the photo-electrochemical system, the external applied voltage directs the electron transfer, which facilitates the separation of photo-generated electrons and holes [33]. Compared to photocatalytic CO₂ conversion, the issue of the gaseous product separation is no longer a problem in the photo-electrochemical system owing to the two-compartment setup design [34]. In addition, benefiting from the highly selective semiconductor catalysts, photo-electrochemical CO₂ conversion also exhibits privilege in product selectivity [35,36]. With renewable energy driving the photo-electrochemical catalysis system, photo-electrochemical CO₂ conversion combines the advantages of photocatalysis and electrochemical catalysis, achieving the green reaction of CO₂ reduction.

Many semiconductor materials demonstrate promising photo-electrochemical CO₂ conversion performance [37,38]. Among them, metal oxide semiconductor materials attract much attention due to their unique and tunable properties such as defect engineering [39], surface modification [40], and heterojunction construction [41]. In the following sections, the detailed application of these metal oxide semiconductor catalysts (e.g. TiO₂, ZnO) in photo-electrochemical CO₂ conversion will be systematically discussed.

3 Role of metal oxide-based photocatalysts in photo-electrochemical conversion of CO₂

3.1 TiO₂-based photocatalysts in photo-electrochemical conversion of CO₂

TiO₂-based semiconductor photocatalysts have extensively been investigated for CO₂ photocatalytic reduction. TiO₂ is known to be a promising semiconductor with a band gap of 3.2 eV and UV light activity [17,42,43]. Due to its wide band gap and fast photoexcited electron-hole pair recombination, it has been extensively used in modified form through either surface modifications, doping [44], or formation of nanocomposites/heterojunctions with other functional nanomaterials [45–48]. That provides structural stability and enhances the visible light absorption capability for the photocatalytic activity [49]. The use of such photocatalysts in the photo-electrochemical process for CO₂ photoreduction is synergistically beneficial. For example, Marino-Garcia et al. [14] demonstrated that CO₂ photo-electrochemical reduction using TiO₂ nanoparticle (NPs) prepared in supercritical (SC) medium supported onto carbon paper (SC-TiO₂/Carbon paper) used as photoanode and Cu as a dark cathode showed a better CO₂ conversion efficiency as compared to commercial TiO₂-25 NPs under the UV light irradiation [14]. It was attributed to the superior current density of the SC-TiO₂/carbon paper electrode [14]. The optimized experimental conditions produced methanol and ethylene products with a faradic efficiency of 15.3% and 46.6%, respectively. It was explained that SC-TiO₂ showed a lower band gap energy, which ultimately reduced the external bias requirement, improved crystallinity, and large surface area, resulting in the charge separation and enhanced photocurrent density, leading to excellent results [45]. The use of photo-electrochemical conversion of CO₂ using TiO₂ photocatalyst and photo-electrochemical setup is promising from its electrode design as compared to traditional electrode methodology. Kobayashi et al. [50] demonstrated that photoelectrocatalytic reduction of CO₂ using TiO₂ as photoanode and gas diffusion electrode (GDE) modified with phthalocynanine catalyst as photocathode showed a high Faraday efficiency of CO₂ reduction (98%) attributed to the excellent activity of GDE-modified photocathode. It showed the benefits of both TiO₂ photocatalysts and the catalytic effect of phthalocynanine with various metal cations (Ni, Co, and Sn).

It has been observed that modification of the TiO₂ surface with other functional nanomaterials such as metals, semiconductors, carbon-based or other nanostructures, provides significant ways to improve the performance of TiO₂-based photoanode electrodes in photo-electrochemical CO₂ conversion. Li et al. [51] fabricated a heterostructured Sn/TiO₂/Si photocathode for photo-electrochemical reduction of CO₂, showing an excellent selectivity to HCOOH. It was found that modification of TiO₂ with Sn metal NPs deposited on the Si surface under optimized experimental conditions showed an improved light absorption and a reduced recombination rate of photogenerated electrons and holes, enhancing the performance attributed to the heterojunction composite structure and modulated electronic configuration as shown in Fig.3. The semiconductors TiO₂ and Si, plasmonic Sn NPs, formation of Schottky between Sn and TiO₂, and hot electrons generated by Sn plasmon resonance jointly promoted the photo-electrochemical CO₂ reduction, harvesting a high faradaic efficiency of HCOOH (~69%) and a desirable average current density (−4.72 mA/cm²).

Similarly, de Brito et al. [52] demonstrated the fabrication of a photoanode electrode composed of Ti/TiO₂ nanotube (NT) coated by copper (II) porphyrin (CuP) forming Ti/TiO₂NT-CuP semiconductor for the photo-electrochemical reduction of CO₂. The optimized condition showed a generation of 0.35 and 0.033 mmol/L of methanol and ethanol, respectively. In addition, this system was shown to be better as compared to photocatalysis, electrocatalysis, and other different types of photo-electrochemical systems for obtaining higher yields.

TiO₂ has also been used as a protective layer for Cu₂O which is known to be an excellent semiconductor for photocatalytic CO₂ reduction but its photostability is scarce [53–55]. Interestingly, Gao et al. [55] reported that the photoelectrocatalytic performance of CO₂ reduction to CH₃CH₂OH could be enhanced over Cu₂O/TiO₂ nanoarrays after the addition of ionic liquids ((1-ethyl-3-methyl-imidazolium tetrafluoroborate, [Emim]BF₄) in the electrolyte solution) which not only increased the solubility of CO₂ and but also decreased the activation energy of the CO₂ reduction reaction. Fig.4(a) demonstrates the mechanism of the enhanced photoelectrocatalytic performance on Cu₂O/TiO₂. In another interesting work, Wang et al. [56] reported the various nanoporous CuBiO₄/TiO₂ photocathodes with varied thicknesses of TiO₂ protecting layer for the photo-electrochemical reduction of CO₂. It was shown that TiO₂ coating could influence the alignment of energy levels at the electrode/electrolyte interface enhancing the activity and selectivity for CO₂ reduction as compared to the bare CuBiO₄ coating. It was attributed to the formation of p-n heterojunction, and generation/separation of charge carriers, leading to the enhanced catalytic activity and production of CO and H₂.

Apart from this, plasmonic-based nanocomposite materials with better localized surface plasmon resonance (LSPR) properties could be developed with better photocatalytic activity and CO₂ photoreduction capability. Recently, Bharath et. al [57] synthesized the UV-vis active Ag-TiO₂/RGO (reduced graphene oxide) photocathode with mixed hetero-nanostructures, showing superior ability to generate methanol selectively from CO₂ photo-electrochemical reduction with a lower charge resistance value, R_ct of 243 Ω. The surface functionalization of RGO with Ag NP’s and TiO₂-NP’s, with average sizes of 4 and 7 nm respectively enhanced the photoelectrochemical cell (PEC) performance as shown in Fig.4(b)–Fig.4(d). It was attributed to the combination of anatase TiO₂ and spherical shape Ag NPs which generated photoinduced electron-hole pairs that migrated to conductive RGO surface and facilitated the charge transport. The optimized condition showed in UV-vis light illumination, Ag-TiO₂/RGO exhibits a photocurrent density of 23.5 mA/cm² at the cathodic potential of −0.7 V with an optimum reaction time of 150 min in CO₂ saturated 1 mol/L KOH electrolyte solution and provided a methanol yield of 85 µmol/(L·cm²) with a Faradaic efficiency of 61.5% and a quantum efficiency of 20%. Additionally, it was concluded that the metallic Ag NPs suppressed the recombination rate of photoexcited charge carriers by forming a Schottky junction at the TiO₂/Ag interface and prevented the agglomeration of TiO₂ NPs. The combination of plasmonic Ag NPs with TiO₂ and RGO also played a role as an electron scavenger and provided hot electrons due to the surface plasmon resonance (SPR) effect that significantly promoted the electron transfer ability for CO₂ reduction.

Similarly, TiO₂ NTs deposited with graphene nanoribbons containing Cu, Pd, and Pt metal NPs have shown an excellent CO₂ photo-electrochemical reduction [13]. It was found that incorporating different metals on graphene nanoribbon structure affected the overall optical properties of the nanocomposite by enhancing the absorption as compared to only TiO₂. Interestingly, it was demonstrated that 3D shapes i.e., star-like shapes of the metals and their alloys exhibited a greater yield of methanol and ethanol during photo-electrochemical CO₂ reduction. When it was compared with the simple photocatalysis process, it was found that the photo-electrochemical CO₂ reduction process provided greater results around 19 and 44-fold higher for methanol and ethanol respectively. That was attributed to the applied external bias potential which facilitates the electron transfer process on the surface of the catalyst due to electrostatic interaction between the electron and the metal NPs.

TiO₂-based hybrid photocatalysts nanomaterials with other functional nanomaterials attract growing interest in the environmental as well as energy applications owing to their fascinating and tunable characteristitcs [58,59]. Looking at the excellent tunability in surface and structural properties, the tailoring of morphological and improvement in the optoelectronic properties due to the nanocomposite/hybrid formation with functional nanomaterials, it is believed to be one of the promising semiconductor materials for CO₂ conversion using solar light. Its combination with visible light active/2D materials [60,61] may provide more promising results. Future research in this direction would be more interesting to see great achievements in tackling CO₂ related environmental problems.

3.2 ZnO-based photocatalysts in photo-electrochemical conversion of CO₂

ZnO-based semiconductor nanomaterials are known for their untapped potential for addressing environmental challenges and advancing renewable energy technologies. Its unique properties such as ease of synthesis, potential for facilitating CO₂ adsorption, diverse composition, relative abundance in Earth’s crust, high electron mobility, conductivity, cost-effectiveness, biodegradability, less toxicity, and high stability make it a valuable component in various environmental and energy-related applications. Therefore, in recent years, there has been growing research in developing ZnO-based semiconductors to harness its potential for sustainable technology including photo-electrochemical CO₂ reduction. However, despite all these attractive properties as a catalyst in CO₂ reduction, ZnO still faces challenges such as its wide band gap (3.37 eV) and the nature of its reduction products. Hence, researchers are developing ZnO-based nanomaterials to enhance their performance in CO₂ photo-electrochemical reduction through combination with various other functional nanomaterials. For example, Gu et al. [62] modeled a tri-component-based CdS−ZnO−Cu nanocomposite with a 3D branch-like morphology showing a remarkable performance in photo-electrochemical CO₂ reduction and CO and syngas production enhancement. It was observed that when illuminated with light, CO partial current density was enhanced by 6.2 times, attributing to the shifting of d-bands toward a more negative value that decreased the substrate-adsorbate interaction on the catalyst surface and thus enhanced CO₂ reduction product yield. In turn, ZnO−CdS heterojunction showed the ability to decrease the band gap, extending the photo-absorption range to visible light up to 800 nm as well as promoting the fast electron transfer thus minimizing the charge transfer resistance compared to bare ZnO. The nanocomposite allowed for the adjustment of the CO/H₂ ratio in the reaction products, which was found to be advantageous for syngas production. Similarly, Jang et al. [63] reported an Au-coupled ZnTe/ZnO nanowire nanocomposite-based photocathode which showed a selective reduction of CO₂ over H₂O. Au was coupled as a co-catalyst that promoted selectively CO₂ reduction over H₂ by providing reaction centers to CO₂ and exhibited the best photo-electrochemical CO₂ conversion activity at a 0.5 wt.% Au loading. The photocathode was engineered to be more responsive to visible light due to the modification with ZnTe and Au while maintaining the band gap of approximately 2.25 eV. The optimization condition showed incident photon to current conversion efficiency (IPCE) > 97% and photocurrent value (−16 mA/cm²) which is 1.5 times higher than bare ZnTe/ZnO electrode under irradiation at −0.7 V_RHE. The enhanced photo-electrochemical activity was attributed to the Schottky junction formation at the interface of Au and ZnTe material that facilitated the transfer of electrons from the semiconductor (ZnTe) into the electrolyte as well as decreased band gap of ZnTe. Such selectivity is desirable for different applications to produce CO selectively from CO₂ in photo-electrochemical cells. In another interesting work, Chu et al. [64] fabricated a monolithic photocathode made of Cu-ZnO/GaN/n⁺-p Si to get tunable syngas production from CO₂ and H₂O using the photo-electrochemical conversion method. Fig.5(a) shows the schematic of the assembly of the photo-electrochemical cell made of Cu-ZnO/GaN/n⁺-p Si along with the SEM/TEM images (Fig.5(b)–Fig.5(e)) of the formation of different layers and constituents. It was found that such fabricated electrodes exhibited a superior stability for up to 10 h with a low onset potential. ZnO loading selectively enhanced the CO production i.e., 8 times higher as compared to bare GaN/n⁺-p Si coated electrodes that were attributed to the Cu NPs dispersed ZnO nanosheets (as shown in SEM/TEM images of Fig.5(b)–Fig.5(e) exhibiting synergistic co-catalytic effect. This led to a too higher turnover number (TON) of up to 1330 for syngas production after 10 h of the photo-electrochemical conversion process. The n⁺-p junction Si formed provided an excellent light absorption due to its narrow band gap (~1.1 eV) and improved the catalytic efficiency, showing a desirable faradic efficiency of 70% for CO at an underpotential of 180 mV.

Ouyang et al. [65] fabricated 1D Bi and α-Fe₂O₃ co-modified ZnO nanorod arrays (Bi@ZFO NTs) as photocathodes as shown in Fig.5(f). From the UV-vis absorbance spectra, it was found that α-Fe₂O₃ co-modified ZnO nanorod arrays (ZFO) and Bi@ZFO NTs showed a stronger absorbance in the range of 360–600 nm, exhibiting that the combination of ZnO and α-Fe₂O₃ could enhance the light absorption ability of ZnO (as shown in Fig.5(g)). This combination showed an excellent photo-electrochemical performance as indicated by the onset potential of −0.53 V (vs. RHE) for photo-electrochemical reduction of CO₂ selectively into formic acid with a small amount of CH₄ and CO. Fig.5(h) and Fig.5(i) show the band gap and Mott−Schottky plots of ZnO and Fe₂O₃ for understanding the n-type semiconducting nature and charge transfer ability as shown in Fig.5(j). Bi was used as a co-catalyst to selectively facilitate CO₂RR over HER by enhancing charge separation and conductivity through the formation of a heterojunction between Bi and ZnO/α-Fe₂O₃ under visible light exposure. The optimized experimental conditions showed a higher Faradaic efficiency of HCOOH (61.2%) at −0.65 V (vs. RHE) and a desirable average current density (3.75 mA/cm²) at −1.2 V that became possible due to modification by Bi and α-Fe₂O₃ on ZFO. Moreover, the energy barrier for the intermediates HCOO* and HCOOH* is found to be decreased formed during the transformation of CO₂ into formic acid due to the strong metal-support interaction. The main reason attributed to enhanced catalytic activity is that the Fe and Zn atoms of Bi@ZFO NTs are intensively bonded and their strong interaction form the bridge for continuous electron transfer. Under the influence of light irradiation of Bi@ZFO NTs, the photogenerated electrons are excited to their CB. The CB of ZnO is found to be lower than that of α-Fe₂O₃, making the electrons on the CB of ZnO tend to flow to the CB of α-Fe₂O₃ and further to the metallic Bi to reduce CO₂ (Fig.5(j)).

Similarly, Zhang et al. [66] reported that the photoelectrocatalytic performance of CO₂ reduction to formate could be enhanced over Bi decorated ZnO/p-Si photocathode (Bi-Bi₂O₃/ZnO/p-Si) that possessed a 3D nanosheet type morphology. It was found that in an optimized experimental condition, an enhancement of 1.8 times was observed in HCOOH concentration with a formation rate of 51.67 µmol/(cm²·h) and high Faradaic efficiency of 84.3% in 0.1 mol/L KHCO₃ at −0.95 V (vs. RHE) respectively as compared to the other photocathodes such as bare ZnO/p-Si and Bi/p-Si. Moreover, the effect of the electrodeposition time of Bi on 3D morphology, light absorption, and potential was studied on the product selectivity and photocurrent density. The enhanced photo-electrochemical performance was attributed to the p-n heterojunction formation between ZnO and p-Si, and the LSPR effect of metallic Bi that improved the light absorption in the wavelength region of 200–950 nm [66].

Cai et al. [67] reported the core/shell porous ZnO@ZnSe nanosheet arrays as photocathode for photo-electrocatalytic CO₂ reduction, showing an onset potential of 0.39 V(vs. RHE) in CO₂-purged 0.5 mol/L NaHCO₃ solution. Fig.6(a)–Fig.6(e) shows the SEM and TEM images along with energy-dispersive X-ray spectroscopy (EDS) mapping, confirming the formation of core-shell structures of ZnO@ZnSe nanosheet arrays. It was found that the coating of ZnSe improved the optical property by enhancing visible light absorption of this photocathode by providing a lower band gap of 2.7 eV (Fig.6(f)–Fig.6(g)). The effect of ZnSe loading, applied voltage, and the effect of height of ZnO nanosheet array grown on fluorine-doped tin oxide (FTO) glass was studied. It was shown that optimizing the applied voltage played a crucial role in enhancing the selectivity for conversion of CO₂ to CO. A Faradaic efficiency of 52.9% was achieved for the reduction of CO₂ to CO over the production of H₂. As shown in schematics of Fig.6(h), under the effect of visible light, the ZnSe shell got excited to produce photogenerated electrons and holes whereas ZnO as core acted as a charge transport medium to inhibit the recombination of photogenerated electrons and holes. The negative enough CB edge of ZnSe provided the large driving force for CO₂ reduction. With the help of the applied negative bias, it was also concluded that ZnO contributed little to the photoelectric performances of composite photocathode but under the photo-electrochemical effect, it showed a better performance due to the synergetic photocatalysis effect of ZnSe.

Apart from this, newer modified photocathode could be synthesized, leading to a better selectivity for C₂ product-based chemical fuels. In this regard, recently, Cao et al. [68] reported the NiMoO₄/ZnO-based 3D core-shell nanostructure photocathode with a flower-like nanosheet morphology of NiMoO₄ on ZnO/C dodecahedron surface (Fig.6(i)–Fig.6(k)). It showed enough ability to reduce CO₂ by providing a flat band potential of −1.2 V (vs. NHE) and a low onset potential. The optimized experimental conditions showed a better current density of 7 mA/cm² at −0.8 V (vs. SHE) and C₂ products (ethylene glycol and ethanol) with a selectivity of 72.6% at a rate of 29.2 µmol/L/(cm²·h), which is found to be 5 times greater than NiMoO₄. The main reason was attributed to the presence of Ni–Mo bicenter in the (111) plane of lattice, both of which act as the adsorption sites for CO₂ and produce the intermediate *OCH₂ that is at a proper distance for showing C−C coupling. Along with it, the formation of NiMoO₄/ZnO heterojunction improved the charge transportation and enhanced the production rate. It was found that this photocatalyst showed a low PL intensity, and a good stability due to the incorporation of carbon in ZnO, and the hierarchical structure resulting from NiMoO₄ provided a large surface area and thus fast electron transfer kinetics for this catalyst. Interestingly, it was found that the performance of NiMoO₄/ZnO core-shell 3D morphology under photo-electrochemical conditions was excellent (i.e., around 3 times more C₂ products are formed) as compared to the performance under electrochemical or photocatalytic conditions, as depicted in Fig.6(l).

The above discussion indicates that ZnO-based hybrid nanomaterials could be promising materials in CO₂ conversion under the influence of solar light. There is a great scope in tailoring of its photo-electrochemical properties thorugh hybrid formation with emerging nanomaterials including 2D photocatalysts nanomaterials [69,70]. This needs to be investigated in more details in order to enhance the CO₂ conversion efficiency and other environmental applications.

3.3 Cu₂O-based photocatalysts in photo-electrochemical conversion of CO₂

As mentioned above, Cu₂O is an excellent semiconductor for photocatalytic CO₂ reduction but its photostability is scarce. Its significant application is possible through some surface modification to avoid its photodegradation. For example, it was shown that surface modification of Cu₂O with TiO₂ for producing Type II heterostructure photocathode electrode and avoiding Cu₂O photo corrosion resulted in a 4-times enhancement CO₂ photoreduction rate under UV-vis light as compared to pure Cu₂O or TiO₂ [53]. Similarly, Akbar et al. [54] studied the effect of TiO₂ layer thickness on a Cu₂O-based photocathode at varying temperatures for photo-electrochemical reduction of CO₂. It was found that a photocathode composed of Cu₂O covered with 2 nm TiO₂ film showed an excellent output in terms of photo-electrochemical reduction of CO₂ into various products like CO, HCOOH, and CH₃COOH. Cu₂O with ZnO-based heterostructures as photoanode have also shown better results as Guo et al. [71] demonstrated a high-performance chlorine-doped Cu₂O/ZFO as photocathode achieving an impressive Faradaic efficiency of 88.6% for photo-electrochemical conversion of CO₂ to CH₄.

Guo et al. [72] reported a photocathode that was composed of Cu₂O/Cu₂S heterostructures on TiO₂ substrate for the photo-electrochemical reduction of CO₂. The optimized condition indicated that the Cu₂O/Cu₂S/TiO₂ hybrid system prepared at 300 °C showed a better absorption in the 480–500 nm range and produced a maximum current density of 10.7 mA/cm² at an overpotential of −0.26 V, a Faradaic efficiency of CO more than 81%, and an increment in current density of more than 50% upon illumination of light due to copper vacancies in the lattice that results in LSPR as well as a lowest carrier transferring barrier and a higher surface activity. They studied the current density that is affected by the amount of Cu₂S loading in Cu₂O/Cu₂S heterostructures and found that product formation (CO and H₂) varies with changing potential.

Cu₂O-doped with various metals was used to enhanced the photo-electrochemical activity toward CO₂ reduction. For example, Guo et al. [73] illustrated the Ga-doped Cu₂O (Ga/Cu₂O) semiconductor for photoelectrocatalytic reduction of CO₂ yielding C₂₊, CH₃CH₂OH and CH₃CH₂CH₂OH, respectively. The doping of Ga into pure Cu₂O improved band gap from 1.89 to 1.86 eV, promoted electron-hole separation, brought changes in the electronic structure of catalyst surface, and created oxygen vacancies through partial substitution in the Cu₂O lattice which enhanced electrochemical active surface area, thereby increasing CO₂ adsorption and activation. Under optimized conditions, Ga/Cu₂O-3 (with a Cu:Ga ratio of 3:1) exhibited a maximum Faradaic efficiency of 20% for C₂₊, 6.50% for ethanol and 6.64% for propanol at −1.8 V (vs. RHE). Computational studies also confirmed that two adjacent *CO intermediates which stabilized at the active sites of the catalyst promoted the direct C−C dimerization, forming a *COCOH intermediate that subsequently lead to the formation of C₂₊ products. Similarly, Wang et al. [74] synthesized the core-shell In/Cu₂O nanowires possessing a band gap of 2.0 eV for photo-electrochemical reduction of CO₂ to CO. It is found that the incorporation of In in Cu₂O resulted in the formation of Schottky junction at metal-semiconductor interface, enhancing CO₂ adsorption and facilitating the formation of *COOH intermediate under visible light irradiation. This, in turn, led to the selective production of CO with a Faradaic efficiency of 82% over 12 h of illumination. The study revealed that In NPs played an important role in charge transfer efficiency. It also acted as a cocatalyst serving as electron trapping sites as well as a protective layer, preventing direct contact between Cu₂O and the electrolyte as shown in Fig.7(a).

Zhang et al. [75] reported Cu₂O/Sn photocathode with a bilayer surfactant dihexadecyldimethylammonium bromide (DHAB) as the photocathode for CO₂ photo-electrochemical reduction in aqueous electrolyte. Sn was used as a cocatalyst and DHAB as a surfactant that mimics the role of the cell membrane and enhances the selectivity for CO₂ reduction over H₂. The optimized conditions showed that the photoelectrode produced formic acid at a rate of 1.32 mmol⁻¹·cm⁻²·h⁻¹·L⁻¹ at −0.7 V (vs. RHE) and its Faradaic efficiency reached up to 83.3% that is 5 times higher than bare Cu₂O which exhibits a Faradaic efficiency of only 30.1%. The results could be explained based on the double layer structure of the surfactant DHAB on the photoelectrode surface which facilitated the diffusion of CO₂ on the electrode surface and inhibited H₂ because of the repulsive interaction between cationic surfactant and H⁺ ion. It was also mentioned that Sn and DHAB jointly enhanced the stability of Cu₂O/Sn/DHAB photoelectrode during 10 h of photo-electrochemical reduction process. Sn and DHAB also promoted charge transfer ability and energetically favored the formation of *OCHO intermediate, leading to the HCOOH selectivity. Similarly, Yuan et al. [76] synthesized the photocathode decorated with CuFeO₂ NPs on CuInS₂ thin film surface (CFO/CIS) for photoelectrocatalytic reduction of CO₂ into ethanol and methanol.

Recently, various other emerging functional nanomaterials have been used to enhance the photo-electrochemical CO₂ redcution efficiency of Cu₂O-based heterostructures such as GO [77] and MOFs [78]. In case of MOF funcitonalization, remarkable a Faradaic efficiency of 95% was achieved for CO₂ reduction to CO and doubled the yield compared to bare Cu₂O. It means Cu₂O-based nanocomposites could also be better photocatalyst materials for photo-electrochemical CO₂ reduction through suitable modifications.

3.4 WO₃ and other metal oxide-based photocatalysts in photo-electrochemical conversion of CO₂

WO₃ is a wide band gap semiconductor with a band gap of 2.6–3.0 eV and an excellent conductivity, and a high electron Hall mobility. It is known to be a good material for photocatalysis and optolectronic applications [79]. Regarding photo-electrochemical conversion of CO₂, WO₃-based heterostructures have shown promising results. For example, Paul et al. [80] demonstrated the fabrication of Ag/WO₃ nanocomposite-based heterostructure for photo-electrochemical reduction of CO₂ which showed an excellent selectivity for formic acid. It was shown that 1.5 wt.% of Ag loading on WO₃ nanorods broadened the light absorption to visible range due to the SPR effect of Ag NPs and thus promoted the photo-electrochemical CO₂ reduction, harvesting a desirable current density (0.4 mA/cm²) and generating selectivily formate (> 87%) with a rate of 31.7 mmol/h. This is attributed to the increasing size and concentration of defects in interface of Ag NP and WO₃ nanorods. In another experiment, Lu et al. [81] developed a WO₃/BiVO₄ heterostructure photoanode with Ag nanocube-based membrane cathode assembly for photoelectrocatalytic CO₂ conversion. The membrane exhibited a remarkable selectivity in favour of CO generation while effectively suppressing the competitive reduction of H₂O to H₂ attributed to two key factors: First, Ag has a high selectivity for producing CO from CO₂ and secondly, the facile adsorption of CO₂ contributed to an increased concentration of CO₂ in proximity to the (100) plane of Ag nanocube, facilitating the CO₂ reduction while impeding the coverage of H species. The results show that WO₃ could be promising photocatalyst materials for photo-electrochemcial conversion of CO₂. However, it needs to be researched more with other functional nanomaterials to enhance its photo-electrochemical activities.

In recent years, several other functional metal oxide photocatalyst nanomaterials have been developed with better optical and electrical properties and have shown promising results given their applications in photo-electrochemical conversion of CO₂. Recently, Gao et al. [82] demonstrated that Cu-doped bismuth vanadate (Cu-doped BiVO₄) which acted as an electrocathode catalyst for the efficient reduction of CO₂ to formate (HCOO⁻) when assisted with WO₃ photoanode in the photo-electrochemical CO₂ reduction process, exhibited a higher stability for 24 h. The optimized Cu doping (8%) in BiVO₄ produced the best Faradaic efficiency of 87.15% of formate which is 5.6 times higher than that of pristine BiVO₄ and 3.3 times higher than that of the electrocatalytic system as shown in Fig.7(b) and Fig.7(c). The Cu doping modified the electronic structure of BiVO₄, creating active sites which enhanced the activity for the reduction of CO₂ to formate. In addition, Cu doping also improved the electron transfer due to the mismatch of oxidation states between Cu²⁺ and Bi³⁺ ions, leading to the decreased interfacial charge transfer.

Wang et al. [83] synthesized 1D α-Fe₂O₃ nanowire arrays modified with plasmonic Ag NPs (as photocathode with lotus-leaf structure (Fig.7(d)) for photo-electrochemical CO₂ reduction. It was found that coupling 1D α-Fe₂O₃ nanowire with Ag NPs had promising effects on the photo-electrochemical CO₂ reduction performance, providing enhanced visible light absorption capability, and photocatalytic efficiency by promoting charge generation, separation, and transportation over α-Fe₂O₃. The optimized condition showed a higher Faradaic efficiency of 51% and selective reduction of CO₂ at −0.5 V (vs. RHE), providing methanol at a rate of 11.72 mmol/(cm²·h) which was found to be much higher as compared to bare α-Fe₂O₃. Fig.7(e) shows the Faradaic efficiency of different products at various potentials for (A) α-Fe₂O₃ and (B) Ag/α-Fe₂O₃. It was demonstrated that as compared to dark conditions, Ag/α-Fe₂O₃ exhibited a high CH₃OH yield under the AM 1.5G light irradiation for 150 min as shown in Fig.7(f). It all attributed to the LSPR effect of Ag NPs, and the high conductivity of Ag which promoted the interfacial charge transfer and also acted as an electron scavenger that reduced the recombination rate of photogenerated electrons and holes. The mechanism is demonstrated in Fig.7(g). Sha et al. [84] illustrated the photo-electrochemical reduction of CO₂ to CH₄ and CO by employing a CuO−MgO nanohybrid. It was observed that the CuO component significantly contributed to generating a high photocurrent, while the presence of MgO improves stability allowed for the retention of current over an extended duration. In addition, this mixed metal oxide not only amplifies catalytic active sites for CO₂ adsorption, but also enhances conductivity, reduces diffusion length, and induces changes in the electronic structure.

In recent years, there are several emerging metal oxide nanomaterias except the traditions metals oxides such as TiO₂, and ZnO, and their hybrid nanocomposites as discussed above have been developed with enhanced optoelectronic properties and promising results in CO₂ conversion. Such emerging metal oxide hybrid nanomaterials have a great tendency to be tailored and modified for the enhanced CO₂ conversion efficiency. More research in this field is required.

4 Graphene oxide-based photocatalysts in photo-electrochemical conversion of CO₂

GO has been an emerging semiconducting material and recently has been used for several photocatalytic applications including CO₂ reduction [61]. It is used in the form of both GO and RGO. Liu et al. [85] prepared the solid planar PEC device i.e., indium tin oxide (ITO)/RGO/ITO carrying two strips which function as anode and cathode respectively for performing gas solid photoelectrocatalytic CO₂ reduction. RGO was prepared at varying temperatures and morpohology dependent studies were conducted for CO₂ reduction. It was found that RGO-prepared at 130 °C exhibited the best photoelectrocatalytic efficiency for CO₂ reduction i.e., 2.2 times higher than PC and exhibiting exfoliated 2D flake structures providing it with a large specific surface area. This enhancement was attributed to the more reactive sites, large number of persistent free radicals, defect density, higher specific surface area, and narrower band gap capable of absorbing visible light. For photo-electrochemical CO₂ reduction, GO-based nanocomposites have been developed with better photocatalytic and electrocatalytic properties, leading to the better conversion efficiencies. A photocathode engineered from GO with CuFe₂O₄ composites in cubic shape NPs in photoelectrocatalytic CO₂ reduction exhibited a Faradaic efficiency of 87% with a highest methanol yield, reaching 28.8 µmol/(cm²·L) for methanol [86]. It was attributed to the synergistic effects of both photocatalysis and electrocatalysis due to the integration of GO which served as an active site for CO₂ reduction and facilitated the proton-coupled multielectron transfer in CO₂ reduction. Nandal et. al [87] reported a molecular hybrid photoelectrode with a band gap of 1.5 eV composed of cobalt phthalocyanine tetrasolfonamide (CoPcS) chemically attached to carboxylated G (GO-COOH) for photo-electrochemical CO₂ reduction. It led to a Faradaic efficiency of around 84% for formate at the rate of 2.35 mmol/(cm²·h), harvesting a current density of −1.5 mA/cm² at a voltage of −1.0 V (vs. Ag/AgCl) under visible light illumination. Due to the strong intermolecular interaction between GO-COOH and CoPcS and formation of Type-II band alignment, it facilitated the fast electron transfer and reduced e⁻/h recombination rate, leading to the enhanced charge separation as also supported by density functional theory (DFT) calculations. Similarly, GO/RGO coupled with various oxide materials such as BiVO₄ [88], Cu_xO/MOF [89], and 3D Co-Pi/BiVO₄/SnO₂ [90] with an excellent photoelectrocatalytic CO₂ reduction has been reported in recent years. Due to the excellent photocatalytic as well as electrocatalytic activities of GO/RGO along with light weight, these carbon-based nanomaterials could be efficient co-catalysts for photoelectrocatalytic CO₂ reduction. In brief, most recent metal oxide-based photocatalysts and their nanocomposites with other functional nanomaterials, their preparation—methods, morphologies, selectivities, and Faradaic efficiencies have been summerized in Tab.1.

5 Summary, challenges, and future prospects

To sum up, various metal oxide-based semiconductor materials have been demonstrating the potential for photo-electrochemial CO₂ reduction with proper catalyst designs, including the modifications on morphological, compositional, and optoelectronic properties. TiO₂ and ZnO are the most popular ones because of their photocatalytic activities which have been discussed in this review along with GO/RGO and other metal oixdes. However, the wide band gap and fast photo-generated carriers recombination rate of these oxide materials hinder further application. With surface modifications such as doping and heterojunction construction (i.e., with other metal oxides or pure metals), the modified nanocomposite materials could exhibit a narrowed band gap for visible light absorption and good photo-generated carriers separation rate to boost the catalytic reactions in both anode and cathode for photo-electrochemical CO₂ reduction which have been discussed. Morphological constructions such as 2D, 3D, and hierarchical structures can efficiently promote the separation of the photo-generated carriers which have been emphasized throughout the review. Metal oxides of other transition-metal elements contribute to photo-electrochemical CO₂ reduction in various aspects. For example, Fe₂O₃ can broaden the light absorption ability of the photocatalyst. As for the diversity of the reduction products, Bi-based semiconductors can produce HCOOH while Cu-based semiconductors can produce {\text{C}}_{2^{+}} products in photo-electrochemical CO₂ reduction. Other than the introduction of Cu in the catalyst system, the precise catalyst design to realize the tandem reaction can achieve {\text{C}}_{2^{+}} production as well. To allow the industrialization of photo-electrochemical CO₂ reduction, there are still some important obstacles to be overcome:

1) The rational cell design to maximize the production rate of photo-electrochemical CO₂ reduction. Examples from electrochemical CO₂ reduction (such as the application of flow cells) demonstrate that the current density of CO₂ reduction can reach an ampere level, which is close to industrial requirement. The state-of-the-art PEC design should maintain the high current density production while being able to absorb sunlight to the maximum to further boost the production rate and product selectivity.

2) The stability of the catalytic system. For now, the stability of the photo-electrochemical CO₂ reduction system usually lasts for dozens of hours at the laboratory level, which is far less qualified for industrialization. The better catalyst design along with the proper cell design need to be synchronized to realize the long duration of the photo-electrochemical CO₂ reaction.

3) The advanced characterization techniques. To fabricate the next-generation catalyst design, the mechanism of the catalytic reaction on certain catalyst models needs to be fully understood. Advanced characterization techniques such as X-ray absorption spectrometry, in situ measurements, and DFT are required to demystify the underlying mechanism to better explore the reaction pathway and the catalyst structure-catalytic performance relationship.

4) The carbon source of the produced products from CO₂ reduction is of importance, which typically should be solidly confirmed by suitable ¹³CO₂ isotope experiments [68].

5) Direct air capture from the atmosphere to reduce the CO₂ concentration in the air is not so straightforward, however, efforts are being made [91–93] and scientific community should work more seriously on this. Alternatively green CO₂ conversion methods should be investigated [6].

This short paper reviews the recent progress of metal oxide semiconductor-based hybrid photocatalyst in photo-electrochemical CO₂ conversion into useful fuels and other products. It is believed that further research in this field would provide excellent database and knowledge for better understanding and real practical applications.

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