Recent Advances in Quantum Dots for Photocatalytic CO2 Reduction: A Mini-Review

Solar energy–driven carbon dioxide (CO2) reduction to valuable solar fuels/chemicals (e.g., methane, ethanol, and carbon monoxide) using particulate photocatalysts is regarded as one of the promising and effective approaches to deal with energy scarcity and global warming. The growth of nanotechnology plays an eminent role in improving CO2 reduction (CO2R) efficiencies by means of offering opportunities to tailor the morphology of photocatalysts at a nanoscale regime to achieve enhanced surface reactivity, solar light absorption, and charge separation, which are decisive factors for high CO2R efficiency. Notably, quantum dots (QDs), tiny pieces of semiconductors with sizes below 20 nm, offering a myriad of advantages including maximum surface atoms, very short charge migration lengths, size-dependent energy band positions, multiple exciton generation effect, and unique optical properties, have recently become a rising star in the CO2R application. In this review, we briefly summarized the progress so far achieved in QD-assisted CO2 photoreduction, highlighting the advantages of QDs prepared with diverse chemical compositions such as metal oxides, metal chalcogenides, carbon, metal halide perovskites, and MXenes.


INTRODUCTION
Carbon dioxide (CO 2 ) is the major constituent of the global warming gases that are destroying the ozone layer of the Earth. Many researchers have been trying to capture and convert greenhouse gases, especially CO 2 , to make it as a pollution-free and recyclable energy source. In the present era, CO 2 has been captured (Ibrahim et al., 2018;Fu et al., 2019;Omodolor et al., 2020;Dhoke et al., 2021;Lau et al., 2021), converted Omodolor et al., 2020) and stored (Lau et al., 2021) by using different technologies. There are many methods and techniques that are studied for the conversion of CO 2 into renewable energy sources; among them, photocatalytic CO 2 reduction (CO 2 R) (Sorcar et al., 2018;Sorcar et al., 2019;Albero et al., 2020;Li et al., 2021), electrochemical CO 2 R Liang et al., 2020), photo-biochemical CO 2 R (Kim et al., 2018), photoelectrochemical CO 2 R (Roy et al., 2016), and thermochemical CO 2 R (Maiti et al., 2018;Pullar et al., 2019) are well-known (He and Janáky, 2020). The photocatalysis process promotes the conversion reactions using clean solar energy, which is an eco-friendly CO 2 conversion technology (shown in Figure 1A).
The photocatalytic CO 2 R reaction comprises three primary steps: 1) the semiconductor photocatalyst absorbs the solar light energy and generates photocharge carriers, 2) photogenerated charge carriers were separated and transported to the surface of the semiconductor photocatalyst, and 3) oxidation and reduction reactions mediated by holes and electrons take place at the active sites on the photocatalyst surface, that is, electrons for CO 2 R and holes for oxidation of sacrificial agent or water, respectively (Xie et al., 2016;Wu et al., 2019) ( Figure 1B). However, to exhibit the CO 2 photoreduction, the photocatalyst should have the ability to adsorb CO 2 and must possess its valence band (VB) at more positive potential than the water oxidation potential and conduction band (CB) at more negative potential than the CO 2 R potential (Xie et al., 2016). It should be noted that CO 2 R into CO 2 .─ radicals through single electrons transfer is unfavorable to occur because of the required high negative potential for the electrons in the CB of photocatalyst (−1.9 V vs NHE) (Shit et al., 2020). However, owing to relatively lower negative potential required for the conversion of CO 2 into hydrocarbons, the proton-assisted multielectron transfer process is more favorable (Shit et al., 2020). Depending on the number of participated electrons, various gas and liquid phase hydrocarbons, such as carbon monoxide (CO), formic acid (CH 2 O 2 ), oxalic acid (C 2 H 2 O 4 ), formaldehyde (CH 2 O), acetaldehyde (C 2 H 4 O), methanol (CH 3 OH), methane (CH 4 ), ethylene (C 2 H 4 ), and ethanol (C 2 H 5 OH), are produced in the CO 2 R reaction (Shit et al., 2020) ( Figure 1B). Major concerns existing for the efficient photocatalytic CO 2 R are 1) photocatalyst materials' limited light absorption ability, 2) quick recombination of photogenerated charge carriers, and 3) poor adsorption of CO 2 molecules on the photocatalyst surface. It was realized that crystalline phases, size, shape, and exposed facets of photocatalyst are crucially influencing the CO 2 R. Maneuvering semiconductors into distinct nanostructures results in significantly altered surfaces and electronic structures, which affect the surface reactivity and positions of energy bands, respectively ( Figure 1C). Furthermore, the morphological features are also pivotal for the transfer of photocharge carriers. For instance, 2D nanosheet morphology having one dimension in the atomic thickness level facilitates shorter charge migration lengths, which is beneficial to avoid the quick recombination kinetics. Furthermore, their flat surface allows the facile heterojunction formation with other 2D, 1D, and 0D nanostructures. However, the quantum confinement in three dimension leaves 0D quantum dots (QDs) with very short charge migration lengths and maximum surface-exposed Frontiers in Chemistry | www.frontiersin.org September 2021 | Volume 9 | Article 734108 atoms ( Figure 1C). The higher sensitivity of energy band positions to the size of QDs allows the precise tailoring of their VB and CB to the required positions in order to initiate the reduction of CO 2 into selective hydrocarbons. Furthermore, the tiny size allows their easy grafting on other 2D and 1D nanostructures to frame the heterojunctions. Hence, QDs of a variety of semiconductors having several advantages garnered significant attention for the photocatalytic CO 2 R applications ( Figure 1D).

Metal Oxide Quantum Dots
Metal oxide semiconductor materials such as MgO (Kohno et al., 2001), ZrO 2 (Hengne et al., 2018;Miao et al., 2019), ZnO (Gokon et al., 2003), WO 3 (Jin et al., 2015), and TiO 2 (Kočí et al., 2009;Yu et al., 2014) have been studied as catalysts and co-catalysts for the photocatalytic reduction of CO 2 . In order to enhance the photocatalytic reduction of CO 2 , 0D metal oxide QDs (MOQDs) have been studied apart from their bulk counterparts due to their advantages like economical, eco-friendly, high surface area, good dispersibility, and well-maintained light absorption ability. For instance, the activity of CuO QDs is in good compatibility with Ti in the metal organic framework (MOF) MIL-125 coupled with g-C 3 N 4 toward the efficient photocatalytic CO 2 R to form CO, CH 3 OH, CH 3 CHO, and C 2 H 5 OH (Li et al., 2020). The good compatibility between CuO QDs and active sites of Ti in MIL-125 the electrons generated by photocatalytic activity will easily transfer to CuO from MIL-125/g-C 3 N 4 . The combination of g-C 3 N 4 /CuO on MIL-125 has drastically improved the yield of CO, CH 3 OH, CH 3 CHO, and C 2 H 5 OH in the presence of water. However, most of the MOQDs have some unresolved technical issues such as the low yield of available electrons and large intrinsic bandgaps that are restricting the wide range applicability under visible light irradiation (Heng et al., 2021). Introduction of defects, doping, and heterojunction formation are the commonly practicing strategies to improve the CO 2 R efficiency of MOQDs.

Transition Metal Chalcogenide Quantum Dots
Transition metal chalcogenide (TMC) materials are formed by the combination of IV-VII transition metal elements (Mo, W, V, Nb, Ta, Ti, Zr, Hf, Tc, or Re) and chalcogens (S, Se, or Te). By the controlled synthesis of the TMCs from bulk to 2D nanosheets or 0D QDs, the bandgaps in TMCs can be tuned with respect to size and shape (Yao et al., 2019;Pandey et al., 2020). There are more than 40 kinds of TMCs available till date, which can be synthesized in large quantities by using synthesis techniques such as the CVD method (Bosi, 2015;Severs Millard et al., 2020), hydrothermal method (Chen and Fan, 2001), and Langmuir-Schaefer deposition method (Kalosi et al., 2019). The TMCQDs and their composites such as CdS (Kuehnel et al., 2017), CdS/Ni (Wang et al., 2010), CdSe/TiO 2 (Sarkar et al., 2016), PbS (Wang et al., 2011), ZnS/CuInS 2 (Lian et al., 2018), and Mn:CdS/CdSeTe/TiO 2 (Nie et al., 2018) have proven to be effective performing photocatalysts. Wang et al. reported the heterostructured catalyst CdSe/Pt/TiO 2 for the photoreduction of CO 2 under visible light in the presence of water (Wang et al., 2010). CdSe QD-sensitized TiO 2 heterostructure materials are capable of catalyzing CO 2 R under visible light illumination (λ > 420 nm). The CdSe QD's surface was modified by removing surfactant caps through annealing and using a hydrazine reducing agent, which enhanced the direct contact between CdSe QDs and TiO 2 . Although TMCQDs show good performance, they slowly become inactive after continuous exposure to the visible light illumination, which is a commonly observed issue in TMC(QD)s due to gradual oxidation of TMCs. The surface stoichiometry of the TMCQDs influences the exciton kinetics such as in CdSe QDs, the presence of a higher surface ratio of Se increases the possibility of electron-hole recombination at trap sites. The surface stoichiometry manipulation drives effective ways to improve the photocatalytic performance of TMCQDs.

Carbon Quantum Dots
Carbon QDs (CQDs), with their sizes in the range of 20 nm, have attracted much attention for their photoluminescence properties and co-catalyst role in different photocatalytic reactions. They exhibit low toxicity, good chemical stability, and exceptional water solubility compared to widely used semiconductor photocatalysts (CdS, TiO 2 , etc) . Importantly, CQDs possess upconversion photoluminescence property that allows the utilization of NIR light. All the aforementioned features and the high CO 2 adsorption characteristics make CQDs an auspicious candidate for the photocatalytic CO 2 R application. Furthermore, surface functionalization with different organic functional groups tailors the semiconducting property and bandgap of CQDs to make them most suitable for CO 2 R. The functionalization of CQDs with 1,1′-bi(2-naphthylamine) enables the formation of intramolecular Z-scheme with a narrow bandgap for the efficient CO 2 R under visible light . Combining CQDs with other semiconductors is reported to enhance the CO 2 R efficiency by utilizing broad range of solar energy, where CQDs absorb visible light that enables the transfer of photogenerated charge carriers through the interface for efficient charge separation and improved CO 2 R. Specifically, heteroatom (N, B, S, Cl, etc.)doped CQDs are more suitable to form the heterojunction owing to their enhanced light absorption, electron transport, chemical activity, and specific surface area properties. For instance, the N-rich CQDs/TiO 2 composite showed an enhanced performance for the CO 2 R with CH 4 and CO yield of 7.79 and 7.61 times higher than that of pristine TiO 2 .

Perovskite Quantum Dots
Metal halide perovskites are a class of semiconductors having ABX 3 chemical stoichiometry, where A represents the alkali (e.g., Cs) or organic (e.g., formamidinium or methylammonium) cation; B denotes the divalent metal cation such as Pb, Bi, or Sn; and X stands for halide anions such as Cl, Br, or I. QDs of these materials are familiar for their excellent optical and electrical properties including strong light absorption, charge carrier's high mobility and long diffusion lengths, and prolonged charge carrier lifetimes. The tuning of the cation/ anion composition could facilitate the tailoring of the perovskite Frontiers in Chemistry | www.frontiersin.org September 2021 | Volume 9 | Article 734108 QD (PQD) absorption from UV to the NIR region (Shyamal and Pradhan, 2020). Furthermore, the favorable VB and CB positions of these CQDs enable the utilization of photogenerated charge carriers for CO 2 R prior to their recombination. However, selection of appropriate solvent for the photocatalytic CO 2 R over PQDs is a difficult task due to their instability upon exposure to polar solvents. Solvents like ethyl acetate have been selected because their mild polarity protects PQDs and CO 2 is highly soluble in them (Xu et al., 2017). The addition of water to this solvent has been demonstrated to increase the selectivity of CO 2 R by minimizing H 2 production (Shyamal and Pradhan, 2020). But an excessive amount of water addition will have negative impact on the stability of PQDs (Hou et al., 2017). However, the careful surface protection of cobalt-doped CsPbBr 3 /Cs 4 PbBr 6 QDs with hexafluorobutyl methacrylate enabled the use of aqueous medium for the CO 2 R (Mu et al., 2019). Furthermore, to protect PQDs from contamination and to hinder their erosion by organic solvents, PQDs were encapsulated with metal oxides and MOFs while applying for CO 2 R Xu et al., 2018). The size optimization of PQDs is significant to accomplish the enhanced photocatalytic CO 2 R. The large size of PQDs decreases the surface area, while the smaller size leads to the aggregation, which will affect the optical absorption and charge carrier's separation and transport properties. The CO 2 R performance of four different size (3.8, 6.1, 8.5, and 11.6 nm) CsPbBr 3 PQDs in ethyl acetate/water medium under the solar illumination for 8 h concluded that PQDs with 8.5 nm size yielded more CH 4 , CO, and H 2 products (Hou et al., 2017) (Figures 2A-F). The crystalline phase of PQDs influence the CO 2 R performance such as CsPbBr 3 PQDs with the cubic phase are more active than the orthorhombic phase counterparts . The sluggish catalytic reaction dynamics of PQDs are dealt by employing a conducting material with high electron extraction efficiency (Xu et al., 2017;Pan et al., 2019).

MXene Quantum Dots
MXenes, a set of 2D materials represented by a general formula of M n+1 X n T x (n 1-4; X C, N, and C/N; T x -O, -F, -OH, etc.), have exhibited a great potential in various applications owing to their exceptional electrical conductivity, metal-terminated surfaces, and hydrophilic characteristics (Lim et al., 2020;Tang et al., 2021). DFT calculations predicted that the chemisorption of CO 2 is favorable compared to water on the MXene surface and higher electrical conductivity of MXene could cause the photocatalytic CO 2 R (Tahir et al., 2021). MXenes can be synthesized by the selective chemical etching of "A" layers from their sandwich-like parent MAX phase precursors, consisting of a stacked MXene nanosheets separated by the layers of A group elements.

CONCLUSIONS AND PERSPECTIVES
Photochemical CO 2 R is one of the efficient methods for the conversion of solar to fuel without releasing any toxic wastes into the environment. An ideal photocatalyst should have the qualities like a high surface area, more active sites, long-term stability, low cost, and easy to produce in industrial scale to commercialize. Several kinds of QDs such as MOQDs, TMCQDs, CQDs, PQDs, and MQDs have been studied so far for the photocatalytic CO 2 R. Overall, the research on QDs for CO 2 R is still in its infancy, and following aspects need to be addressed to reach further growth for the ease in applicability. The size control of most QDs involves complicated synthesis procedures. Developing a simple, cost-effective, size-controlled, and highly efficient synthesis approaches will lead to wide utilization of QDs for CO 2 R. Most of the TMCQDs and PQDs for CO 2 R are based on Cd-and Pb-containing compositions, respectively, which are not ideal in the perspective of safety and eco-friendliness. Hence, more research is needed for improving the stability and CO 2 R efficiency of Cd-and Pb-free QDs (such as InP, ZnSe, and ZnS) . The poor oxidation stability of the TMCQDs, PQDs, and MQDs in the presence of water and light are the major challenging aspects to be addressed immediately. The QD-based hybrids are mostly achieved by simple blending of QDs with other semiconductors, which does not generate a strong chemical interaction at the interfaces for efficient charge transfer process. Hence, in situ growth methodologies and/or external functionalization with different functional groups/molecules should be adopted to fully exploit the advantages of QDs. The full spectrum of solar light utilization by QDs for CO 2 R is not yet accomplished. More innovative technologies like making QDs comprising upconversion material as core should be investigated. At present, formic acid and CO are the main products of CO 2 R via two electron reduction. Hence, CO 2 R via four-, six-, and eight-electron reduction needs significant attention. Especially, methane production by eight-electron reduction can make a vital change in the application of QDs for CO 2 R. The H 2 production via proton reduction that reduces the CO 2 R efficiency is another critical concern for the QD-based systems. Although the efficient properties of CQDs depends on the size, shape, surface defects, and heteroatom doping concentrations, a wellestablished method(s) for precise tuning is needed. Furthermore, the functional groups of CQDs are known to be reduced with the prolonged exposure of light during the photocatalytic reactions, which may influence their CO 2 R activity. Hence, improving the stability is a bottleneck concern in CQD-based CO 2 R research. The advancement in QDs stability under light, highly interactive interface with other materials, morphological control, and quick adoptability to the reaction environment will make them as futuristic materials for not only in CO 2 R but also in other interdisciplinary fields.

AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct, and intellectual contribution to the manuscript writing, editing, and reviewing work, and approved it for publication.

FUNDING
The authors acknowledge the support of the National research foundation of Korea (NRF) (2017R1E1A1A01074890, 2018R1A6A1A03023788, 2019R1A2C1010692 (MSIT), 2021R1I1A1A01055790, 2021R1A2C2009459, and 5199991614244 (BK21 FOUR)) and a Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean government (MOTIE) (P00008500, the Competency Development Program for Industry Specialist).