Metal Oxide-Based Photocatalysts for CO2 Reduction: Comparison
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The photoconversion of CO2 into solar fuels seems to curb greenhouse effect and resolve the energy crisis. One of the challenges in developing practical CO2 photoconversion catalysts is to design materials with a low cost, high activity and good stability. The excellent photocatalysts based on TiO2, WO3, ZnO, Cu2O and CeO2 metal oxides that are cost-effective and long-lasting were discussed. Strategies to improve CO2 photoconversion efficiency are summarized and photocatalysts forms of 0D, 1D, 2D and 3DOM (zero/one/two-dimensional and three-dimensional-ordered macroporous, respectively) are involved, which can inspire the future improvement in photochemistry.

  • photochemistry
  • CO2 reduction
  • metal oxide materials
  • solar fuels

1. Introduction

The transformation of CO2 can be driven by illumination, electricity and heat [3][1]. Solar energy is a safe, clean, renewable and inexhaustible energy source, so it is ingenious to achieve this conversion with sunlight [4][2]. Additionally, the reaction conditions in photocatalytic processes are mild [5][3]; thus, it is easy to conduct experimental tests. A new world opened up since H2 and O2 were obtained after radiating TiO2 with light. Extensive research about photocatalyst were conducted until the year of 2000. Since then, a growing number of materials have been designed and studied to absorb solar energy, including oxide semiconductors, sulfides (ZnS, CdS and MoS2), and polyoxometalates (Bi2WO6, Bi2MoO6 and BiFeO3) [6,7][4][5]. Organics, organometallic complexes, covalent organic polymers and noncovalent self-assembled supramolecular organic matter are also involved. Among them, inorganic metal oxide materials are widely studied for establishing efficient artificial photosystems due to their low cost, facile synthesis, stable crystal structures and environmental friendliness. TiO2, Cu2O, ZnO, WO3 and CeO2 show promising research value as the most common materials.

2. Theoretical Foundation and Strategies of Photocatalytic CO2 Reduction

The photocatalytic process in semiconductors can generally be described as shown in Figure 1. Upon being excited by an incident photon with energy equal to or higher than the bandgap (Eg), charge carriers are generated. Electrons (e−) at the bottom of conduction band (CB) migrate to the surface of the catalyst to initiate reduction reactions with CO2. Holes (h+) at the top of valence band (VB) conduct oxidative reactions.
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Figure 1.
Fundamentals of photocatalytic reduction on a semiconductor catalyst.
The photocatalytic process of CO2 conversion on semiconductors can be divided into five general steps: (1) Formation of electron–hole pairs under light radiation, (2) Separation and migration of the electrons and positive holes, (3) Adsorption and activation of CO2, (4) Redox reactions between surface-adsorbed species and electron–hole pairs, and (5) Desorption of the product. The electrochemical reactions with standard oxidation–reduction potentials (at pH 7 vs. NHE) are as follows:
H
H
2
O + h
+
→ OH• + H
+
 
H
H
+
+ e
− → H•
→ H•  
H• + H• → H
H• + H• → H
2
    
CO
CO
2
+ e
−
→ CO
2−
 
CO
CO
2
+ 2H
+
+ 2e
−
→ HCOOH, E
0 = −0.61 V
= −0.61 V    
CO
CO
2
+ 2H
+
+ 2e
−
→ CO + H
2
O, E
0 = −0.53 V
= −0.53 V   
CO
CO
2
+ 4H
+
+ 4e
−
→ HCHO + H
2
O, E
0 = −0.48 V
= −0.48 V  
CO
CO
2
+ 6H
+
+ 6e
−
→ CH
3
OH + H
2
O, E
0 = −0.38 V
= −0.38 V  
CO
CO
2
+ 8H
+
+ 8e
−
→ CH
4
+ 2H
2
O, E
0 = −0.24 V
= −0.24 V     
To carry out the conversion, it is necessary to comprehend the structure of CO2. Due to the great symmetry and high bond energy of 750 kJ/mol, it is particularly difficult to break the C=O double bond. Therefore, the transmutation between CO2 and bent radical anion of CO2•− on the surface of the catalyst is widely recognized as the first step to activate CO2 for subsequent reaction. Additionally, photoreactions occur favorably only when the CB position of the catalyst presents a more negative potential than the target reduction and the VB position is more positive than the oxidation reaction. Figure 32 summarizes the current strategies that can boost the photocatalytic CO2 reduction pace, with detailed information being presented sequentially below.
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Figure 32.
Strategies to boost the photocatalytic CO
2
reduction.

2.1. Morphological Control

The surface topography of nanocrystals could evidently alter the electronic structure, surface energy and chemical properties of catalysts. Therefore, morphological control is one of the most important issues to researchers in nanoscience, chemistry and physics. Open facets and edges determine the shape of nanocrystals. Thus, the preferential adsorption of additives on certain crystal surfaces provides a good opportunity to tune the surface of nanomaterials.

2.1.1. Exposed Facet Adjustment

Exploring and figuring out the variation that is connected with exposed surfaces is crucial to elucidate shape-related chemical and physical properties. In semiconductor crystals, different facets have distinct electronic band structures that influence the transport of photoexcited carrier charges. It is wise to expose active facets to tune the CO2 photoreduction efficiency [16][6].

2.1.2. Quantum Dots (QDs)

Zero-dimensional (0D) semiconductor quantum dots (QDs) have many unique properties, such as quantum confinement effect, high extinction coefficient and multiple exciton generation [17][7]. Hence, QDs show much better photoactivity in the visible light region. Unlike bulk materials, surface atoms make up the majority of QD semiconductors. The abundant surface sites enhance the interaction between electron donors and acceptors, thus facilitating the photocatalytic charge transfer.

2.1.3. One-Dimensional and Two-Dimensional Structures

One-dimensional nanostructured catalysts have high aspect ratios, such as nanowires, nanorods and nanotubes. The morphological tuning of the material makes a difference to their thermal, optical, electrical and magnetic properties [18][8]. For instance, TiO2 nanotube can act as a channel for electron transfer and build up the chemical reactions rate. Two-dimensional layered materials can protect a tiny particle component from aggregation. The CO2 adsorption capacity on 2D photocatalysts can be enhanced due to the large specific surface area and bountiful surface defects [19][9]. Two-dimensional lamellar nanosheets are widely used in photocatalysts, such as g-C3N4, MoS2 and WO3.

2.1.4. Macroporous and Three-Dimensional Ordered Macroporous (3DOM) Structures

Macroporous materials are widely used in photocatalytic materials, owing to their excellent properties [20][10]. Unlike dispersed particles, sunlight can penetrate the pore wall easily and scatter widely inside the hollow structure, thus increasing the efficiency of illumination. Subsequently, the slender walls of pore reduce the transfer length of photo-generated charge carriers. Electrons (e−) and holes (h+) are separated more efficiently when heterojunctions are loaded on porous materials. The specific surface it provides is so large that more CO2 molecules have a chance to contact the catalyst for reduction reactions. Growing attention has been paid to hierarchical composite pores, including photonic crystal catalysis and separation of sub-microns. The slow light effect of photons associated with 3DOM materials have been considered to increase solar radiation absorption and enhance photocatalyst performance [21][11]. 3DOM products with periodic macrostructures [22][12], known as inverse opal, have been applied in battery materials, sensors, separation engineering and heterogeneous catalysis.

2.1.5. Preparation of 3DOM Materials

Specifically, 3DOM materials are produced by the colloidal crystal template (CCT) method (Figure 43). Firstly, a uniform and close-connected organic sphere template can be obtained, through three key processes. Then, seeping metallic salt sol into the void of microspheres. After heat treatment, the organic microsphere template fades away and leaves a metal oxide frame. (i) The polymerizable monomer (methyl methacrylate, styrene) and initiator are mixed and heated under the protection of Ar; (ii) the earlier reaction liquid is filtered with microfiltration membrane; (iii) the microsphere mixture is centrifuged at a high speed for a long time, yielding the polymethylmethacrylate (PMMA) or polystyrene (PS) template; (iv) the template is immersed in the precursor solution and (v) is calcinated.
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Figure 43.
The fabrication process of the 3DOM Bi-doped TiO
2 [26].
[13].

2.2. Heterojunction

Photogenerated charge carriers in a single material tend to recombine rapidly due to Coulombic force and they can be separated efficiently in the multi-materials that are in close contact [28][14]. The heterojunction is an interface between various semiconductors with different energy band structures. The heterojunction structure can reduce the probability of charge recombination and is therefore considered an effective method to enhance the photocatalytic activity [29,30][15][16].

2.3. Defects

All semiconductors have surface defects, which are rooted in the absence of host atoms. The amount of oxygen vacancies can be altered by ion doping and nanocrystal modifications. Oxygen defects and other defects [31][17] can facilitate the separation of h+/e−. Over and above, defects can also activate CO2 molecules, reducing the activation energy of the reaction [32][18].

2.4. Ion Doping

Elemental doping is a common strategy to modulate the surface electronic structure. Then, the band gap of the semiconductors make a difference, nonmetal doping (such as N, C and O) mainly alters the VB and metal-doping (e.g., Mo, Co and Ni) influences the CB [33,34][19][20]. For example, Cu-doped TiO2 absorbs more visible light due to the Cu 3d-Ti 3d optical transition [35][21].

2.5. Sensitization

Sensitization means coupling the quantum dots, dyes, etc., with semiconductors to increase the photogenerated carriers and promote the absorption of light by taking advantage of their receptivity in UV, visible or infrared light.

3. Photocatalysts with Different Basis Matrices

3.1. TiO

2

-Based Photocatalysts

Titanium dioxide (TiO2) is notable in photochemistry, with advantages such as non-toxic, cheapness, corrosion resistant, good physical and chemical stability. However, owing to the wide Eg of 3.0–3.2 eV, TiO2 only absorbs energy in the ultraviolet region (3–5% of the solar energy), and photoexcited charge pairs are easy to combine, resulting in quantum inefficiency. Usually, TiO2 is divided into the rutile, anatase and brookite on the basis of atomic arrangement modes. Rutile TiO2 is thermodynamically stable and does not distort or decompose at high temperatures. It has a narrower energy gap (3.0 eV) and a wider spectral response than the anatase phase (3.2 eV). Rutile TiO2 seeds generally grow larger in size and tend to form an agglomerated structure. Smaller anatase TiO2 particles have a wider lattice gap and abundant surface oxygen defects, which make it favorable for ion doping and photoreactions.

3.1.1. Morphological Control

In hollow nanotube-shaped catalysts, the transport speed of CO2 and photoproducts can be facilitated. Ru phase is inclined to form methane in CO2 hydrogenation process [36][22] and Yang et al. [37][23] entrapped Ru nanoparticles in TiO2 nanotubes. Restricted Ru nanoparticles were resistant to sintering and leaching in the Ru-in/TNT catalyst channel (Figure 54). Electrons tend to gather in the tubes because of a confinement effect, which leads to an abundant, accessible metallic phase. It is easier for high-priced Ru species to combine with free electrons and then exhibit a superb CH4 and CH3OH yield.
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Figure 54.
The reaction pathway for the Ru entrapped in the TiO
2
nanotubes catalyzes CO
2 methanation [37].
methanation [23].
On the contrary, Kar et al. [38][24] loaded metal nanoparticles onto vertically oriented 1D TiO2 nanotube arrays (TNAs) platforms via the graft method. In the synthesis process, Au, Ru and ZnPd NPs grow anodically on transparent glass substrates. There is no band bending phenomenon in Au NP-grafted TiO2, which can be observed from ultraviolet photoelectron spectroscopy (UPS). TPD experiments proved that all NP-grafted samples absorb more CO2 than TNAs. It is worth noting that, in nanoparticle-grafted TNAs, blue photons close to and below the TiO2 band edge were excited to drive CO2 photoreduction process. Ru-TNAs, ZnPd-TNAs and Au-TNAs had the CH4 formation rates of 26, 27 and 58 µmol·g–1·h–1, respectively. 
Metal–organic frameworks (MOFs), also known as porous coordination polymers, composed of organic linkers and metal nodes (metal ions or clusters), are a kind of porous crystalline inorganic–organic hybrid materials. The unique advantages of MOFs, such as extremely high surface area, uniform adjustable porous structure and high density coordination unsaturated metal sites, have been extensively researched.

Exposed Facet Adjustment

A Pt-TiO2 single atomic site catalyst (PtSA/Def-s-TiO2) was prepared [41][25] by the “thermal solvent-argon treatment and hydrogen reduction” method. In order to construct Ti–Pt–Ti structures, TiO2 nanosheets with oxygen deficient sites were used to anchor monatomic Pt particles, which can retain the stability of isolated single atomic Pt and improve photocatalytic performance. The exposed (101) and (001) crystalline of TiO2 nanosheets were determined by transmission electron microscopy, and a thickness of 6.9 nm was observed through atomic force microscope. The EPR spectra of the samples confirm that the rich oxygen defect structure can be obtained by heating TiO2 nanosheets in argon atmosphere.

3DOM Structure Ti-Based Materials

A ternary 3DOM Bi-doped TiO2 photocatalyst decorated with carbon dots (CDs) was obtained, whose pore engineering of the 3DOM skeleton greatly promoted the response in the whole solar spectrum range [26][13]. It exhibits enhanced photocatalytic performance because of its excellent exquisite structure and high charge transfer efficiency. Similarly, a BiVO4/3DOM TiO2 nanocomposite [43][26] was synthesized as a highly efficient photocatalytic catalyst for the degradation of dye pollutants.

3.1.2. Heterojunction

p–n heterojunction: A p–n heterojunction is formed by combining p-type and n-type semiconductors. Even without light irradiation, electrons can diffuse from an n-type semiconductor to a nearby p-type one, in the case of the combination of two materials. Correspondingly, the holes on the surface of a p-type semiconductor are transferred to the n-type one, which results in an efficient separation of charge carriers. A ZnFe2O4-modified TiO2 was synthesized by the hydrothermal method [51][27], and the p-n heterojunction system could reduce CO2 to methanol at a yield of 75.34 μmol g−1 h−1. rGO composite: graphene materials have been widely used because of their large specific surface area, unique thermal stability and excellent electrical conductivity. Graphite nanomaterials are visible-light-responsive materials with appropriate band gaps, and the energy levels of CB and VB are in optimal positions relative to ordinary hydrogen electrodes. These unique photocatalytic properties have made them prime candidates for photocatalytic CO2 reduction. Fortunately, tightly contacted ultra-thin graphene layers and TiO2 compounds and can be prepared with some additives [52][28].

3.1.3. Ion Doping

In recent years, elements such as B, N, Co and Bi have been widely applied in TiO2 doping. A carbon-based hybrid nanocomposite reduced graphene oxide (rGO), belonging to the narrow band gap, with oxygen-containing functional groups on the surface that can be enhanced by π interactions [57][29]. Laminar graphene carriers not only prevent TiO2 repolymerization, but also hybridize the function of the catalytic system. Co-doped TiO2 was loaded on the rGO [58][30], and the Co peak in EDX spectra and C-O peak in FT-IR spectra confirmed the successful doping and the presence of graphene support, respectively. The size of TiO2 particles decreased from 48–80 nm to 23–28 nm, which is consistent with earlier reports of changes in titanium doping with transition metal ions.

3.1.4. Sensitization

A growing number of semiconductor materials are used to modify TiO2, but randomly mixed catalysts are not stable enough to achieve reproducibility. Therefore, Lee [59][31] grew well dispersed p-type NiS nanoparticles on the surface of a highly aligned n-type TiO2 film to obtain the NiS-sensitized TiO2 films. The band gaps of two components were estimated by wavelength relation. Some inferences can be drawn when considering the results of both the ultraviolet and visible spectra. It indicates that more electrons are subpoenaed from the short-Eg NiS and transferred to TiO2 conduction band. The spectra results reconfirmed the electron contribution of the NiS and the design of a catalyst that produced 3.77-fold CH4 compared to the TiO2 film.

3.2. WO

3

-Based Photocatalysts

Tungsten-based oxides (WO3) have been extensively studied in recent decades and various morphologies have been presented. In the WO3 structure, the crystal in the stoichiometric ratio is connected with a twisted WO6 octahedra to form a perovskite crystal structure. It has monoclinic, orthorhombic and hexagonal crystal forms. At the same time, the oxygen lattice can be lost easily, resulting in oxygen vacancies and coordinatively unsaturated W atoms. Therefore, tungsten oxide has many non-stoichiometric compounds, such as WO2.72, WO2.8, WO2.83 and WO2.9.

3.2.1 Morphological Control

Bi2WO6 is one of the tungsten-based materials that belongs to Aurivillius crystal oxides. Its crystal has an orthorhombic system, and its narrow band gap (2.7–2.9 eV) structure allows it to meet the response absorption of visible light. Moreover, its stable structure and eco-friendly properties have attracted many scientific researchers to study it. Since the valence band of Bi2WO6 is composed of O2p and Bi6p, and the conduction band is composed of W5d-assisted Bi6p orbitals, the VB energy levels can be dispersed broadly. By employing the Kirkendall effect in ion exchange and BiOBr precursor, Huang et al. [65][32] prepared a bowl-shaped Bi2WO6 HMS material. Based on the large specific surface area of the material, its adsorption capacity for CO2 reaches 12.7 mg g−1 at room temperature and pressure. The material adsorbs a large number of HCO3− and CO32− species on the surface during the reaction, which makes the catalytic reaction easier. The Bi2WO6 HMS thus has a high catalytic activity, and the methanol yield is 25 times higher than that of the Bi2WO6 SSR. Iron phthalocyanine FePc is neatly assembled on porous WO3 under induction and coupled with surface atoms by H-bonding [66][33]. The optimized FePc/porous WO3 nanocomposites exhibit enhanced CO2 photoreduction activity, which is attributed to the synergistic effects of a high specific surface area, a better charge separation and proper central metal cation. A series of mesoporous WO3 with interconnected networks were synthesized by the silica KIT-6 hard template method [67][34], which became oxygen-deficient after hydrogenation treatment. Both the ordered porous structure and oxygen vacancies contributed to the increased yield of CH4 and CH3OH. WO3 with a hollow nest morphology with hierarchical micro/nanostructures (HNWMs) was synthesized [68][35] by the one-step hydrothermal method (Figure 105), with a particle diameter of about 2.5 μm. The 2D nanosheets, which have an average thickness of 30–40 nm, were assembled to build a distinctive hollow nest structure with a good stability and reusability under visible light. Hao et al. [69][36] prepared core–shell heterojunctions of two-dimensional lamellar WO3/CuWO4 by the in situ method. After the modification of amorphous Co-Pi co-catalyst, the photoanode of ternary homogeneous core–shell structure exhibited a high photocurrent of 1.4 mA/cm2 at 1.23 V/RHE, which was 6.67 and 1.75 times higher than that of the pristine WO3 and 2D homogeneous heterojunction.
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Figure 105. Schematic diagram of the HNWM formation process [68].
Schematic diagram of the HNWM formation process [35].
Ti atoms in ultrathin Ti-doped WO3 nanosheets promoted the charge transfer [71][37], as they accelerate the generation of key intermediates COOH*, which was revealed by in situ characterization. Furthermore, Gibbs free energy calculations were calculated to verify that ion doping can reduce the CO2 activation energy barrier and CH3OH desorption energy barrier by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH3OH. The ultrathin Ti-WO3 nanosheets showed an excellent CH3OH yield of 16.8 μmol g −1 h −1. Two-dimensional bilayered WO3@CoWO4 were prepared [72][38] via a facile interface-induced synthesis method. The optical energy conversion efficiency can be improved by both p–n heterojunctions and interfacial oxygen vacancies. The narrow band gap of the WO3@CoWO4 heterojunction was proved by DFT calculations and some characterizations, which allows a better visible light absorption. A tree-like WO3 film was prepared [73][39] by the hydrothermal process, which has a large specific surface area. The WO3 product was a unity of hexagonal/monoclinic crystals, which contained W5+ defects and oxygen vacancies. The products were further subjected to a mild reduction solution at the lower temperature of 333 K to introduce more defects.

Preferentially Exposed Facets

According to studies, infrared (IR) light makes up nearly 50% of solar energy, and it is challenging to make use of the majority of the light. Liang et al. [74][40] fabricated 2D ultrathin WO3 with an intermediate band gap. They achieved the first complete decomposition of CO2 driven by infrared light without the addition of sacrificial agents. Theoretical calculations indicated that the generation of the intermediate energy band resulted from the critical density of the generated oxygen vacancies, which has also been verified by synchrotron valence band spectroscopy, photoluminescence spectroscopy, ultraviolet-visible-near-infrared spectroscopy and synchrotron infrared reflection spectroscopy.

3DOM Structure W-Based Materials

Unexpectedly, it was found that the resistance of 3DOM-WO3(270) and the Ag3PO4 electron absorption band were comparable. By depositing Ag3PO4 nanoparticles in the micropores of 3DOM-WO3, Chang et al. [79][41] achieved a higher photocatalytic activity and more efficient light harvesting at the wavelengths of 460–550 nm. A Z-scheme g-C3N4/3DOM-WO3 catalyst designed by Tang et al. [80][42] also has a high CO2 photoreduction activity.

3.2.2. Heterojunction

Quantum dot composite: CuO quantum dots (QDs) were combined with WO3 nanosheets by a self-assembly method and the diameter of 6%CuO QDs/WO3 NSs was mainly located at 1.6 nm [81][43]. The bandgap energy of CuO/WO3 fell in 2.28 eV and the complex catalyst possessed a lower resistance for charge carrier transfer that showed in UV-vis DRS and EIS analysis. Due to the low CB position, CO cannot be obtained when using pure WO3. However, the photogenerated electrons gathered in the WO3 CB position was able to reach the CuO VB position when the Z-scheme (Figure 116) was formed by intimate heterojunctions. At the same time, the reduction reaction that transformed CO2 into CO occurred at the CuO CB position.
/media/item_content/202303/641902b0b7a66molecules-28-01653-g011.png
Figure 116.
The proposed charge transfer mechanisms: (
A
) Ⅱ-scheme and (
B
) Z-scheme for CuO/WO
3 [81].

3.2.3. Ion Doping

Molybdenum with a similar ionic size was chosen to dope the WO
[43].

3.2.3. Ion Doping

Molybdenum with a similar ionic size was chosen to dope the WO
3
as a low-valence metal species. The W
5+
/W
6+
ratio of the catalyst was increased, making it easier to exchange electrons with reactants. The conductivity of protons was enhanced by the presence of hydrogen bronze, which originated from a chemical reaction between WO
3
and Brønsted protons and excess electrons in their lattices.

3.3. ZnO-Based Photocatalysts

ZnO, a common metal oxide, is a n-type semiconductor with an Eg value of 3.37 eV. It is a kind of amphoteric oxide that has the advantages of nontoxic harmlessness, low cost, abundant reserves, convenient preparation, low dielectric constant and low optical coupling rate. ZnO has three main lattice structures: wurtzite structure, zinc-blended structure and tetragonal rock salt structure. The wurtzite structure is considered the most stable and common structure in nature. It is a kind of hexagonal crystal, in which the O and Zn atoms are aligned with the hexagonal density stacking.

3.3.1. Morphological Control

The 3nm Pt particles were uniformly dispersed over ZnS@ZnO with a mesoporous heterostructure [90][44] and more CH3OH was obtained. Reactant charge carriers entered the pore channels of the porous heterozygous layer, thus reducing the likelihood of flow resistance and electron–hole recombination. The S-scheme photocatalyst delivered a high CH3OH formation rate of 81.1 μmol g−1 h−1, which is roughly 40 and 20 times larger than that of bare ZnO (3.72 μmol g−1 h−1) and ZnO–ZnS (4.15 μmol g−1 h−1). On the other hand, a porous ZnO@ZnSe core/shell nanosheet array material (Figure 127A) was prepared in a controlled manner [91][45]. The final n-type semiconductor composites had a proper negative CB band edge. In comparison to ZnO or ZnSe, more pairs of electron–holes were formed under visible light. Electrons tend to land on ZnO, which is aimed at methanol production. Mei et al. prepared a ZnO microsphere with different numbers of shells [92][46] and the photoelectric performance of ZnO was optimal when the number of shells reached three.
/media/item_content/202303/641902cf75e0fmolecules-28-01653-g012.png
Figure 127. (A) Schematic illustration of the fabrication process of ZnO@ZnSe photocathode [88][47]. (B) ZnO/ZnS (C) ZnO/ZnS/g-C3N4 SEM images [91][45].
ZnO/ZnS nanoflowers (Figure 127B) were combined with g-C3N4 nanosheets (Figure 127C) to construct a double Z-scheme structure [94][48]. ZnO/ZnS nanoflowers provide a large specific surface area and g-C3N4 helps to absorb more photons under solar light irradiation. Optimized interfacial charge transfer dynamics in ternary heterostructure can be characterized by photocurrent measurements. As a result, the formation rate of H2 product over the novel double Z-scheme mixture increases to 301 μmol g−1 h−1 on water splitting.

3DOM Structure Zn-Based Materials

Wang et al. [99][49] published metal-organic-framework-derived 3DOM N-C doped ZnO (Figure 138) for efficient CO2 reduction. The ultra-tiny CoOx clusters were anchored on the surface of catalyst and no Co-Co peak was found in CoOx/N-C-ZnO. The charge transfer rate was jacked up by ion doping and the recombination of electron-hole pairs was tamed because of the CoOx clusters. Furthermore, CoOx on the orderly connected channels can act as an electron trap to capture electrons, which makes a contribution to photoreaction efficiency. The density theory calculations (DFT) was also used to detect the CO2 adsorption ability, and CoOx/N-C-ZnO exhibited the most negative CO2 binding energy due to improved electron structure of adsorption site.
/media/item_content/202303/641902e7355d5molecules-28-01653-g013.png
Figure 138.
HAADF-STEM image of CoO
x/N-C-ZnO and related elemental mapping images [99].
/N-C-ZnO and related elemental mapping images [49].

3.3.2. Heterojunction

Recently, zeolitic framework (ZF) composite fabricated by the microwave-hydrothermal synthesis method (MWH) has attracted attention, which can provide a fast heating-speed and produce morphologically uniform samples. With biodegradable template, the zeolitic framework (ZF) was synthesized via MWH method from volcanic ashes. The NaAlSiO4 (NAS) framework was composed of 50 nm circular channels and has a large surface area.  The synergistic effect among ZnO, CuO and ZF support accelerated the photocatalytic process of CO2 reduction, which offers higher HCOOH evolution rate.

3.4. Cu

2

O-Based Photocatalysts

Cuprous oxide (Cu2O) is a potential p-type semiconductor with a wide visible-light response range and high photo-electric conversion efficiency (18%) [106][50], and it displays attractive prospects in solar energy conversion and heterogeneous photocatalysis. Although Cu2O possesses many excellent properties, photocorrosion and the rapid recombination of e−/h+ pairs affect its activity and limit its application. The photocorrosion is believed to occur in two ways: (1) self-reduction caused by generated electrons and (2) self-oxidation caused by the generated holes.
Self-reducing photocorrosion: Cu
Cu
2
O + H
2
O + 2e
−
→ 2Cu + 2OH
−
    
Self-oxidative photocorrosion: Cu
Cu
2
O + 2OH
−
+ 2h
+
→ 2CuO + H
2O
O   
Therefore, developing Cu-based catalysts with excellent activity, selectivity and stability has become the research hotspot in the area of the photocatalytic reduction of CO2. Many successful attempts have been made to improve the photostability and photocatalytic performance of Cu2O. In general, most studies focus on enhancing the charge transfer from Cu2O to reactants or cocatalysts to prevent charges from accumulating within the particles.

3.4.1. Morphological Control

A branch-like CdxZn1-xSe nanostructure was obtained [107][51] by the cation-exchange method, which was then mixed with Cu2O@Cu to form heterojunctions. Selenium (Se) vacancies were created during the ion exchange process and the crystal growth was limited due to the additive diethylenetriamine (DETA), leading to insufficient coordination of the surface atoms, which then become active adsorption sites. Highly hierarchical branching-like structures assembled by one-dimensional structural materials not only facilitate electron accumulation at their tips but also increase the light-accepting area, and characterization results show that branching structures can effectively absorb visible light. Cd0.7Zn0.3Se/Cu2O@Cu step-scheme heterojunction exhibited a CO release yield of 50.5 μmol g−1 h−1. Ultrafine cuprous oxide U-Cu2O (<3 nm) was grown on the polymeric carbon nitride (PCN) (Figure 159) by the in situ method [108][52]. PCN has a narrow band gap of 2.7 eV that can capture visible light. Both ultrafine nanoclusters and Z-scheme heterojunction can protect U-Cu2O from degradation. The photocatalyst U-Cu2O-LTH@PCN has high stability, maintaining more than 95% activity after five cycles of testing, while bare Cu2O grades completely within three cycles. A large number of heterojunctions were formed by U-Cu2O particles and lamellar PCN, expediting the electron transfer efficiency. The product can convert CO2 to methanol with water vapor under light irradiation at the high yield of 73.46 μmol g−1 h−1.
/media/item_content/202303/6419031a9adfamolecules-28-01653-g015.png
Figure 159.
Ultrafine U-Cu
2O nanoclusters anchored on the photosensitizing PCN support [108].
O nanoclusters anchored on the photosensitizing PCN support [52].

Preferentially Exposed Facets

Cu2O is an ideal compound to study the influence of electron-related effects. The rare occurrence of the O-Cu-O 180o linear coordination of Cu2O makes its (111), (100) and (110) facets chemically active. Zhang et al. [111][53] successfully achieved the morphology control of Cu2O nanocrystals by utilizing the selective surface stabilization of PVP on the (111) plane of Cu2O. With different amounts of PVP, the surface area ratio of (111) to (100) was subtly tuned, which resulted in the shape evolution of the system and various Cu2O structures (Figure 160). The detailed modification mechanism was elucidated from the structural and kinetic perspectives.
/media/item_content/202303/641903303c2f9molecules-28-01653-g016.png
Figure 160.
FESEM images of the Cu
2O polyhedrons with different volume ratios of (100) to (111) [111].
3DOM Structure Cu-Based Materials
The 3DOM Cu
O polyhedrons with different volume ratios of (100) to (111) [53].

3DOM Structure Cu-Based Materials

The 3DOM Cu
2O structure was luckily obtained [115] via polystyrene crystal templates. Under the contrived “sunlight” irradiation, incident light was reflected and absorbed around and around again. In the ultra-visible absorption spectra (350 to 800 nm), Cu
O structure was luckily obtained [54] via polystyrene crystal templates. Under the contrived “sunlight” irradiation, incident light was reflected and absorbed around and around again. In the ultra-visible absorption spectra (350 to 800 nm), Cu
2
O with large orifices absorbs more photons than bulk samples, making it more advisable for solar applications. 3DOM Cu
2O was prepared [116] by the electrochemical method to reduce CO
O was prepared [55] by the electrochemical method to reduce CO
2
, and its Faraday efficiency was five times higher than that of Cu film. The CO
2
·
−
intermediate in 3DOM channels is more stable and leads to the possibility of forming CO and HCOOH products (
Figure 17).
1).
/media/item_content/202303/64190349466demolecules-28-01653-g017.png
Figure 117.
Proposed mechanism for CO
2
reduction to CO and HCOOH on Cu
2O-derived Cu-IOs (The symbol “*” represents the surface.) [116].
O-derived Cu-IOs (The symbol “*” represents the surface.) [55].

3.4.2. Heterojunction

Liu et al. [117][56] reported a facile solution and chemistry route to synthesize rGO-incorporated crystal Cu2O with various facets as visible-light-active photocatalysts for CO2 reduction. The enhanced activity was attributed to the formation of the heterojunction and the existence of rGO as the electron transport mediator. M. Flores et al. [118][57] adopted the microwave-hydrothermal method to couple the powders of Mg(OH)2, CuO and Cu2O. The synthesis method allowed a sufficient interaction between Mg(OH)2/CuO and Cu2O without inhibiting the gas adsorption capacity of Mg(OH)2. They found that the presence of Cu2O favored the selectivity towards CH3OH production because a higher Cu+ concentration led to better selectivity.

3.5. CeO

2

-Based Photocatalysts

Cerium oxide (CeO2) has an octahedral face-centered cubic fluorite structure, in which the coordination numbers of Ce and O are 8 and 4. When reduced at a high temperature, it can be converted to nonstoichiometric CeO2−x (0 < x < 0.5). Notably, CeO2−x maintains a fluorite crystal structure and forms oxygen vacancies after losing a certain amount of oxygen. CeO2−X materials with different Ce/O ratios were also obtained in different conditions and it could be reconverted to CeO2 again if it returned to an oxidizing environment. Because of the unique electrical structure, cerium oxide (CeO2) is famous for the conversion sates between Ce4+ and Ce3+, which have been studied as oxygen storage catalytic materials and solid oxide full cells by many scholars [124,125][58][59]. In summary, CeO2 is a rare-earth metal oxide with a good photochemical stability, low cost and environment friendly characteristics.

3.5.1. Morphology Control

Yb-, Er-doped CeO2 hollow nanotubes were synthesized [127][60] using silver nanowires coated with silica, and the products had a narrower band gap of 2.8 eV. The core–shell structured CeO2 was converted into mesoporous hollow spheres by the Ostwald ripening method in the presence of urea and hydrogen peroxide [128][61]. CeO2 nanocages can be fabricated by mixing (NH4)2Ce(NO3)2 with templates of Cu2O nanocubes [129][62], in which Cu2O is finally sacrificed. The photocatalytic results [130][63] indicated that CeO2 nanocages exhibit higher activity than hollow spheres.

Preferentially Exposed Facets

It was found that molecular CO2 can be distorted and participate in reactions at a low energy on the CeO2 surface [131][64]. A p-type NiO material was designed to modify the rod-like CeO2 nanostructure [132][65], allowing electrons and holes to migrate to opposite directions. They then operated the Mott–Schottky test, which showed a typical p–n junction. The presence of hexagon-shaped NiO plates broadened the range of light responses, which can be verified in the UV-Vis absorption spectra. Graphene oxide (rGO) was introduced as a “network” of for photoreduction electron transportation (Figure 192A–C). The impedance can be seen in the EIS Nyquist plot, which shows that the NiO/CeO2/rGO achieved the minimum value. The HCHO production rate of the ultimate catalyst was 421 μmol g−1h−1 with the synergy of several favorable factors. It is worth mentioning that a range of in situ techniques have been used to detect oxygen vacancies, structural changes, free radicals and formate on the surface of CeO2.
/media/item_content/202303/6419036813450molecules-28-01653-g019.png
Figure 192. The structural diagrams of (A) n-CeO2 nanorods, (B) p-NiO/CeO2 composite and (C) NiO/CeO2/rGO hybrid composite [132][65].

Macroporous

Mesoporous N-doped CeO2(NMCe), a relatively ordered intermediate structure with enhanced CO2-capturing capability, was prepared without any convoluted procedures or expensive equipment [124][58]. In the Roman spectrum, the bands from 550 to 650 cm−1, which are closely related to oxygen vacancy, were more salient than the MCe band. In addition, N-doped porous CeO2 has a higher CO2 absorption capacity than porous CeO2.

3DOM Structure Ce-Based Materials

Under the protection of poly alcohol, Zhang et al. [134][66] synthesized 3DOM CeO2 that was loaded with Au–Pd alloys. 3DOM CeO2 photocatalytic materials are expected to emerge in the field of CO2 emission reduction, which could open up more possibilities for the development of super-catalysts.

3.5.2. Heterojunction

Researchers tried to combine CeO2 with g-C3N4, which is popular for its energy bands and chemical stability. Through hard work, a three-dimensional porous g-C3N4(3DCN) was achieved, with the advantages of multi-channel structure. To accommodate more electrons and heighten the density of photoelectric currents, Zhao et al. [125][59] loaded Pt nanoparticles (5–6 nm) on CeO2/3DCN using photodeposition techniques that require UV lamp radiation. The photoreduction rate gradually increased as the CeO2 amount rose in the range of 15~45% and the yield rates of 4.69 and 3.03 μmol·h−1·g−1 for CO and CH4 were achieved, respectively, after decorating with Pt crystalline grains.

4. Other 3DOM Materials

In order to introduce advanced porous structures to slow the self-aggregation of quantum dots, Wang et al. [143][67] devised 3DOM N-doped carbon (NC) to support CdS and ZnO QDs (Figure 2013). They filled the interspace in an ordered PS microsphere template and then employed a pyrolytic treatment and in situ growth methods. Compared to bulky CdS, the 3DOM compounds have a larger cathodic current density and enhanced light harvesting, bearing a satisfactory carbon monoxide yield of 5210 μmol g−1 h−1. A three-dimensional SnO2 inverse opal structure was synthesized as gas sensors, soot oxidation catalyst and photoanode. The 3DOM BiVO4/SnO2 heterostructure was obtained by adding a BiVO4 precursor to fill the space between SnO2 skeleton and periodic PS template. The compatibility of energy states with SnO2 significantly reduced their photoluminescence intensity. Meanwhile, Au nanoparticles enhanced the slow photon effect, which in turn increased the incident light utilization efficiency.
/media/item_content/202303/64190381d2344molecules-28-01653-g020.png
Figure 2013. Proposed mechanism for the photocatalytic CO2 reduction on 3DOM CdS QD/NC. Schematic illustration of the photocatalytic CO2 reduction coupled with selective arylamine oxidation reaction system [143][67].

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