Catalytic exploration metallic and nonmetallic nano-catalysts, properties, role in photoelectrochemistry for sustainable applications

2.1 Photochemical reactions and mechanisms

Photochemistry involves the interaction of light with matter to induce chemical transformations. At the nanoscale, light–matter interactions become more pronounced due to increased surface area and confinement effects. Photochemical reactions offer the advantage of providing energy input directly from photons, allowing the activation of high-energy reactions at lower temperatures. Nanoscale catalysts efficiently absorb light due to their small dimensions and tailored electronic structures. This generates photogenerated electron-hole pairs, which can drive redox reactions by transferring charges to the catalyst’s surface. Photocatalysis involves a sequence of steps including light absorption, charge separation, surface reactions, and charge recombination. The nanoscale allows for efficient charge separation, minimizing recombination losses and enabling multiple redox cycles. ³³

2.2 Electrochemical processes and interfaces

Electrochemistry deals with electron transfer reactions occurring at interfaces between electrodes and electrolytes. The nanoscale structure of catalysts significantly influences these interfaces, affecting charge transfer rates and electrocatalytic activity. Nanomaterials offer a high density of electrocatalytic sites due to their large surface area. The electrochemical double layer at the interface plays a critical role in facilitating charge transfer reactions and determining the overpotential required for catalysis. Common electrochemical reaction mechanisms involve steps such as adsorption, charge transfer, and desorption. Nanocatalysts provide abundant sites for these processes, enabling rapid reaction kinetics and lower energy barriers. ³⁴

2.3 Interplay between nanostructures and reactivity

The combined effects of nanoscale size, shape, and composition create intricate nanostructures with diverse catalytic properties. These properties emerge from the synergy between photochemical and electrochemical phenomena. Nanostructures exhibit synergies between photochemical and electrochemical effects. Photoexcited charge carriers can be directed toward electrocatalytic sites, effectively utilizing light-induced energy to drive electrochemical reactions. Some nanomaterials exhibit plasmon resonances, where light interacts with free electrons on the nanoparticle surface. ³⁵ Plasmonic effects can enhance light absorption and provide additional energy pathways for catalytic reactions. ³⁶ In essence, the photo/electrochemistry of catalysts at the nanoscale represents a convergence of principles, where the distinct characteristics of nanomaterials synergize with photochemical and electrochemical processes. This intricate interplay opens up new horizons for tailoring catalysts’ behavior, enabling catalytic reactions with remarkable efficiency and selectivity.

4.1 Surface active sites

The distinctive geometric arrangements of nanocatalysts give rise to expansive specific surface areas, which are directly correlated to highly dense packing and lattice planes. These high-density surface-active sites on the material’s surface serve to enhance catalytic processes. Another avenue to enhance the exposure of surface-active sites involves reducing the lateral dimensions of nanocatalysts. ⁴² For instance, ultrasmall molybdenum disulfide (MoS₂) demonstrates superior performance in hydrogen evolution reactions (HER) compared to bulk MoS₂, ascribed to profusion of HER active sulfur edges. ⁴³

4.2 Carrier mobility

The nanomaterials including black-phosphorus (BP), graphene, and transition metal-dichalcogenides (also called TMDs) exhibit profound electron mobilities or carrier mobilities. ⁴⁴ For example, for the MoS₂ and graphene reported mobilities falls within the range of 102–104 cm² V⁻¹ s⁻¹ and around 101 cm² V⁻¹ s⁻¹ respectively. ⁴¹ Owing to their ultrathin structure characterized by ultra-short transport paths and minimal intrinsic resistance, nanocatalysts facilitate rapid charge migration. Yu et al. observed a distinct decline in HER performance upon attaching an additional atomic layer to MoS₂, which are perhaps due to the electronic mobilities along vertical direction, among material layers. ⁴⁵

4.3 Energy band structures

The manipulation of the layers quantity in the crystal lattice enables the adjustment of the band gap across a range of 2D layered nanomaterials. This tunable band structure significantly influences the photocatalytic capabilities of the materials. MoS₂’s band gap, for example, can be tailored by varying the cumulative layers; the band gaps for single and few layered are 1.85 ± 0.05 and 1 ± 0.20 eV respectively. ⁴⁶ The reported band gap of g-C₃N₄ ranges between 1.6 and 1.1 eV when referenced to a normal hydrogen electrode (NHE). ⁴⁷ By introducing different anions and cations into bismuth-based 2D layered nanomaterials, the band gap can be fine-tuned to span from 0.3 to 3.6 eV, resulting in light response spanning from ultraviolet to near-infrared. ⁴⁸

4.4 Electronic properties

The manipulation of the electrical characteristics can be achieved by adjusting the thickness of nanocatalysts. ⁴⁹ The electronic architectures of 2D nanomaterials play a pivotal role in controlling the binding improve among active sites and reactants, potentially reducing desorption kinetic barriers.

4.5 Mechanical properties

Evident mechanical features characterize nanomaterials imparting high catalyst durability and opening accesses to practical applications that benefit society. Moreover, the durability of nanomaterials paves the way for the conception of hybrid nanocatalysts aimed at catalytic enhancement.

5.1 Classification of nanocatalysts

The acceleration of progress in the realm of ultrathin nanomaterials has been precipitated by the innovation surrounding atomically thin graphene nanomaterials. The majority of these nanomaterials can be broadly classified as layered materials. In these materials, the stacking of layers is driven by van der Waals interactions, while the atom layers within each stratum are frequently robustly interconnected through chemical bonds. ⁵⁴ 2D-materials normally applied for catalysis are graphene, g-CN, MOFs, COFs, Mxenes, LDHs, h-BN etc, where some classes of them are works as effective nanocatalyst. Graphene often conceptualized as an atomic monolayer of graphite. ⁵⁵ Nanomaterials comprised of atomically thin graphene are alluring due to their superior performance in catalytic applications compared to traditional semiconductors. Given that graphene nanomaterials function as zero band-gap, semi-metals, they are generally employed as co-catalysts or as efficient catalyst supports, rather than standalone catalysts. ⁵⁶

The g-C₃N₄ is graphene like 2D-materials ⁵⁷ which possess chemical inertness in harsh acidic or alkaline environments, rendering them potential catalysts in various redox processes.

But still there are certain problems that limit their applicability in catalysis, some of these factors are limited surface area, high electric carrier recombination rates, and inadequate mass transfer. For the purpose of overcoming these issues surface defect engineering and elemental doping strategies has been employed. Transition metal dichalcogenides (TMDs) generally consist of layers of chalcogen atoms separated by layers of transition metal atoms, which can also act as nanocatalyst. ^{58 , 59 , 60 , 61 , 62} The band gap of TMDs can be tailored by adjusting the number of layers in the crystal. MoS₂-based nanomaterials, a prototypical form of TMD, exhibit distinctive lattice vibration properties, notable catalytic activity, cost-effectiveness, and abundance. Owing to these unique qualities, 2D layered MoS₂ nanoparticles have demonstrated considerable potential in a wide array of applications, potentially even replacing graphene nanomaterials. In a notable instance, Zhang et al. showcased impressive catalytic properties of MoS₂ in N₂ reduction, attaining high Faradaic efficiency and NH₃ yield rates. ⁶³ Graphene-like MXenes, including mono- and double-transition metal MXenes, are crafted through the arrangement of stacked scrolls and sheets. Monolayer MXenes, characterized by a significant concentration of electron states proximate to the Fermi level, exhibit metallic traits. By virtue of this electronic configuration, MXenes hold promise as layered materials for catalytic purposes. Due to their excellent electronic conductivity, high elastic moduli, and favorable hydrophilic attributes, MXenes have been employed in different catalytic reaction such as CO-oxidation, ⁶⁴ Water gas shift reaction, ⁶⁵ spanning hybrid electrochemical supercapacitors to Li-ion battery anodes. ⁴¹ Layered double hydroxides (LDHs) are crafted by juxtaposing positively charged host layers and interlayered structural water housing negatively charged anions. ⁶⁶ Numerous reports underscore the catalytic potential of LDHs (Figure 3), especially those integrating transition metals, in various processes related to oxygen and hydrogen production. ⁶⁷

Figure 3:

Water plasma enabled exfoliation for the synthesis of CoFe LDHs ultrathin nanosheet with multiple vacancies (OER electrolysis). The inter layered cations and anions interactions are destroyed which further enhance fast-exfoliation and resulting more vacancies. This route shows excellent kinetic and catalytic performance. The reactor for DBD were designed with plate to plate electrode at 50 V. ⁶⁷

Bismuth, an environmentally benign metal, boasts a range of intriguing characteristics that render it applicable to diverse processes, including catalysis. The realm of 2D layered nanomaterials founded on bismuth has reported high performance outcomes in storage and energy conversion devices. ⁶⁸ Manipulating the inherent structure by incorporating diverse cations and anions can modulate the band gap, yielding a light responsiveness spanning from ultraviolet to near-infrared wavelengths. This instrumentation also results in enhanced mass and mobility of photoexcited charge carriers, bolstering applications like photochemical catalysis, photodetection, and optoelectronic energy conversion. ⁶⁹ Hexagonal boron nitride (h-BN) mirrors a hexagonal crystal structure reminiscent of graphite’s arrangement. ⁷⁰ Owing to its exceptional resistance to high temperatures, noteworthy thermal conductivity (390 W m⁻¹ K⁻¹), robust chemical stability, resistance to acid corrosion, and superb electrical insulation, it has found utility as a catalyst carrier or catalyst itself. To address its intrinsic low electrical conductivity, efforts have been invested in functionalizing h-BN monolayers through combinations with materials that are electrically conductive in nature for example CNTs, rGO, etc. ⁷¹ MOFs possess single-layered lamellar structures, a mere atom thick, granting them high aspect ratios and the potential for post-synthesis adjustments to create tailored pores for catalysis and selective adsorption, including the incorporation of functional groups. The inclusion of transition metals in MOFs yields wide pore diameters, sufficient surface area, and a diverse spectrum of MOF structures, all advantageous for catalytic applications. ^{72 , 73 , 74 , 75 , 76 , 77 , 78 , 79} The formation of 2D metal nanomaterials, such as nanosheets, nanodisks, nanoplates, nanoribbons, nanorings, and nanobelts, hinges on the utilization of noble metal. ⁸⁰ Owing to their intriguing electrical and structural characteristics, these 2D metal nanoparticles find use in a multitude of catalytic processes. Notably, Huang et al. demonstrated the remarkable catalytic improvement in the electrocatalysis in formic acid oxidation reaction. ⁸¹

5.2 Structures of nanocatalysts

Nanomaterials exhibit distinct crystal phases linked to atomic coordination, atomic arrangement, and layered stacking, ⁸² which can exert substantial control over their properties and catalytic behaviors. ⁸³ Here, we will delve into the structures of the aforementioned nanocatalysts, while Figure 4 showcases a diverse array of catalytic nanomaterials with various structural attributes. Graphene, for instance, demonstrates a hexagonal or honeycomb-like shape owing to each carbon atom’s covalent bonding with the three neighboring atoms. ⁸⁴ In contrast, g-C₃N₄ presents specific disparities in its planar structure compared to graphene. In g-C₃N₄, carbon and nitrogen atoms form N-substituted graphite frameworks in a sp²-hybridized configuration. There are two primary structural types within g-C₃N₄, namely tri-s-triazine units and s-triazine units. ⁸⁵ The monolayer structure of tri-s-triazine-containing g-C₃N₄ becomes disordered at 900 K in a vacuum, disrupting hydrogen bonds among NH/NH₂ groups and causing NH/NH₂ groups to twist outward, as depicted in Figure 4a. ⁸⁶ When exposed to visible light, these amorphous g-C₃N₄ nanomaterials may exhibit enhanced photocatalytic hydrogen production compared to crystalline g-C₃N₄. ⁸⁶ Layered puckered honeycomb structural BP exhibits a Cmca space group and an orthorhombic crystal structure. Each phosphorus atom connects to three adjacent phosphorus atoms; three of these phosphorus atoms lie in the same plane, while the remaining one resides in a different plane. Figure 4b offers a top view of the typical BP structure. ⁸⁷ Generally, van der Waals interactions enable monolayered Transition Metal Dichalcogenides (TMDs) to stack, forming layered TMDs. A single monolayered TMD comprises two chalcogen atomic layers and one interposed transition metal atomic layer. The top and side perspectives of a TMD’s structure are illustrated in Figure 4c, showcasing covalent bonds formed between the metal and chalcogen atoms via trigonal prismatic coordination. ⁸⁸ Periodic porous Covalent Organic Frameworks (COFs) are systematically constructed using organic building blocks with covalent bonds. ^{89 , 90} Figure 4d ⁹¹ provides an overview of a typical COF structure, boasting inherent high porosity, adjustable pore size, excellent conjugation structure, extensive surface area, crystallinity, visible light response within broad range and limited secondary pollution. A type of ternary carbides “M_n+1AX_n, MAX” is used as parent for the designing layered MXene “M_n+1AX_n” via A-group selective etching. Geng et al. successfully catalyzed hydrogen evolution processes using a Mo₂C-on-graphene MXene heterostructure, as illustrated in Figure 4e, ⁹² showcasing the crystal structure of Mo₂C. Furthermore, a highly efficient oxygen evolution electrocatalyst, ultrathin CoFe LDH nanosheets were fabricated through Ar plasma exfoliation, as depicted in Figure 4f. ⁹³ Bismuth-based 2D layered nanomaterials, possess a high-dispersion band, resulting in photogenerated carriers with low effective mass and high mobility. One prominent example is bismuth oxychloride (BiOCl), which exhibits a tetragonal structure with a P4/nmm space group. Figure 4g showcases the layered crystal structure, where the atoms of O and Bi are stacked in the form of sandwich between layers of Cl-atoms. ⁹⁴ H-BN, belonging to the hexagonal crystal system, shares a hierarchical structure reminiscent of graphite, as illustrated in Figure 4h. ⁹⁵ Typically, sp² hybridized B and N atoms form a regular hexagonal ring network between separate layers, creating h-BN nanosheets. ⁹⁵ The B and N atoms are strongly covalently bonded within the layers, akin to other layered nanomaterials. Moreover, weak van der Waals interactions between layers facilitate material exfoliation, yielding ultrathin nanosheets. ⁹⁶ The strong coordination linkage is found among metal clusters (nodes) and organic ligands (linkers) in MOFs. These components self-assemble into compounds with periodic structures. Li and colleagues generated 2D layered MOFs from LDHs via a straightforward ligand-assisted approach, as depicted in Figure 4i. ⁹⁷ Their findings indicated improved performance in water oxidation. However 2D-MOFs still have low electrical conductivity, falling below 10⁻¹⁴ S cm⁻¹, primarily due to internal porosity resulting from the stacking of multiple atomic layers. ⁹⁸

Figure 4:

Some examples of 2D-dimensional nanomaterials. (a) Graphene and graphitic nitride structre. ⁸⁶ (b) Structure of mono-elemental compound family. ⁸⁷ (c) Structure of TMD. ⁸⁸ (d) Structure of COF ⁹¹ (e) structure of MXene. Copyright 2017 reproduced with permission from Wiley–VCH ⁹² (f) structure of LDHs. ⁹³ (g) Structure of BiOX. ⁹⁴ (h) Structure of h-BN. ⁹⁵ (i) Structure of MOFs. ⁹⁷

5.3 Synthesis of nanocatalysts

Top-down and bottom-up approaches are the main techniques for creating layered nanomaterials. Top-down synthesis involves demixing stratified bulk materials, breaking bonds along the layer plane and breaking weak interlayer van der Waals interactions to produce 2D ultrathin nanosheets. ⁹⁹ Techniques include liquid/gas exfoliation, ¹⁰⁰ mechanical cleavage, ¹⁰¹ shaking treatment, ¹⁰² wet ball milling, ¹⁰³ sonication, and chemical etching are used in top down synthesis. Liquid exfoliation introduces weakening of interlayers, resulting in space expansion and debonding. Mechanical cleavage prepares multiple 2D layered nanomaterials, such as graphene layers. The Scotch tape method is credited with discovering graphene and producing functional 2D monolayered nanomaterials.

It is observed that ultrathin layered MOFs with high crystalline nature, lateral area can be produce by wet ball milling grinds materials, with zirconia in proper solvent. The expected thickness will be about 1 nm for synthesized MOFs. However, top-down approaches have drawbacks, such as unstable nanosheet production, unpredictable layer counts, poor homogeneity, restricted use, low product yield, and stripped nanosheets that break apart and reassemble. ³⁰ Further research on top-down synthesis of 2D nanomaterials is crucial, as ongoing work is expected to drive significant improvements. Alternatively, bottom-up synthesis of 2D layered nanomaterials depends on the directional assembly of small molecules, with growth limited to the vertical axis. Methods like chemical vapor deposition, surfactant-assisted synthesis, surfactant self-assembly, template-assisted synthesis, inorganic-organic lamellar, and solvothermal synthesis offer controlled synthesis and large-scale production for bottom up method (see Figure 5). Lang and colleagues used a bottom-up solvothermal approach to create atomic layered binary MOF nanosheets and achieved good oxygen generation. Chemical vapor deposition is a popular method for producing 2D materials on a large scale, offering superior control over material size and thickness. ¹⁰⁴ However, conventional bottom-up synthesis methods often require substrates and surfactants, making it challenging to manufacture dispersed 2D nanomaterials and remove remaining surfactants, potentially limiting their applicability. ⁴¹

Figure 5:

Comparison of top-down and bottom-up approaches in nanomaterial synthesis. Top-down techniques for 2D nanomaterial synthesis.

5.4 Catalytic applications of nanomaterials

Research shows the importance of nanomaterials, predominantly for, applications in biochemical and environmental technologies, including cancer treatment, toxicant removal, dye degradation, OER, and CO₂RR. Table 1 also provides a comprehensive list of nanomaterial-based electrocatalysts utilized in practical applications. It includes information on synthesis procedures, environmental factors, unique electrocatalytic performance, and the fundamental catalysis mechanisms. ¹⁰⁵ Consequently, there is an imperative need to exp cutting-edge technologies capable of effectively eliminating organic molecules from aquatic environments. In this context, the emerging catalytic applications of 2D nanomaterials for biochemical and environmental remediation were investigated.

5.5 Dye degradation

It is noteworthy that, nanomaterials are exceptionally compatible for photodegradation oxidative reactions of dyes, due to their optimal thickness and limited kinetic barriers. For instance, Zhang and collaborators employed a hydrothermal technique to synthesize BiOCl, which is a single-layered, highly crystallized material. ¹¹² Figure 6a illustrates the process of photodegradation excitation of BiOCl nanosheet, along with the direct effect of semiconductor degradation efficiencies of BiOCl-001 and BiOCl-010 about 99 % and 59 % respectively. Intriguingly, the photocatalytic activity of direct semiconductors surpassed that of indirect semiconductors, and the photoexcitation performance of BiOCl nanomaterials was more pronounced under UV light than visible light. Additionally, as demonstrated in Figure 6b and c, the photocatalysts BiOCl and BiOCl–OH were developed for photocatalysis and degradation of dye named Rhodamine-B in wastewater, upon exposure to UV-radiation with wavelength equal to 365 nm. ¹¹³ The increase in oxygen vacancies induced by UV light was found to enhance the photocatalytic activity of BiOCl–OH in comparison to pure BiOCl. The OH-functional group in Rhodamine B dye, play a sufficient role in the photocatalytic degradation, as revealed by FT-IR analysis. ¹¹³ In the pursuit of effective photocatalysis for dye degradation and the generation of highly reactive oxygen species like H₂O₂, a heterostructure BP/CN nonmaterial (black porous: graphitic carbon nitride) was designed by Zheng et al. ¹¹⁴

Figure 6:

Applications of nanomaterials in terms of catalytic degradation. (a) A type of degradation of pollutant by UV radiation by direct semiconductor photoexcitation in single crystalline nanosheets of BiOCl. ¹¹⁵ (b) BiOCl, and (c) BiOCl–OH as photocatalyst for dye degradation under UV light. ¹¹³

5.6 Elimination of toxicants

Phenolic compounds constitute highly detrimental organic pollutants in water, stemming mainly from activities in the oil refining, printing, pesticide, and pharmaceutical industries. The presence of phenols in industrial wastewater poses a significant risk to surface water contamination. Consequently, the global pursuit of efficient and environmentally safe degradation methods is of utmost importance. ⁴⁰ For instance, in a groundbreaking discovery, Liu and colleagues found that g-C₃N₄/Bi₂WO₆/rGO hetero-junctioned photocatalysts, containing 3 wt% of rGO, achieved an impressive 86 % reduction in ibuprofen through photocatalytic degradation under visible light and a remarkable 98 % reduction under solar light irradiation. ¹¹⁶ Considering the potential accumulation of antibiotics in the human body, posing a risk of irreversible harm, photocatalytic oxidation has emerged as a highly effective approach for removing antibiotics from wastewater. Despite potentially lower photodegradation rates in winter, Norvill and colleagues demonstrated a 93 % reduction of the antibiotic tetracycline under summer-like conditions, along with chemical oxygen demand and sufficient biomass. The studies highlight the superior removal capability of algal wastewater treatment compared to traditional biological wastewater treatment and mark a significant achievement in tetracycline removal from outdoor wastewater environments.

5.7 Hydrogen evolution reaction (HER)

In the light of escalating environmental pollution and the global energy crisis, the search for alternative energy sources has become imperative. ¹¹⁷ Hydrogen energy, a promising and clean energy source, offers the advantage of high energy density and minimal environmental impact. ¹¹⁸ The Hydrogen Evolution Reaction (HER), often referred to as the second half of water splitting, involves the cathodic reaction represented by the formula 2H⁺ + 2e^{− ---} > H₂. ¹¹⁹

The HER comprises two primary processes: proton adsorption and hydrogen desorption, which can be elaborated further in acidic solutions. These processes are rooted in the fundamental mechanisms known as the Tafel–mechanism, Heyrovsky–mechanism, and Volmer–mechanism. The sites of adsorptions of electrocatalysts, play an essential role in governing the HER. Extensive research is being conducted to harness the appealing physicochemical characteristics of 2D materials as potential catalysts for highly efficient HER activity. ¹²⁰ For instance, Ma and colleagues utilized bulk black phosphorous to create ice-assisted exfoliated BP/g-C3N4 nanosheets, characterized by exceptional product quality, minimal structural flaws, and substantial lateral dimensions. Figure 7 illustrates the pertinent characteristics of the BP/g-C₃N₄ nanosheets. Upon taking absorption spectra of g-C₃N₄, BP, and g-C₃N₄/BP nanosheets, it was revealed that BP nanosheets exhibit a broad absorption band spanning the UV, visible, and NIR regions. In contrast, g-C₃N₄ and BP/g-C₃N₄ nanosheets possess absorption edges at 466 and 474 nm, respectively. Furthermore, various component ratios for BP/g-C₃N₄ nanosheets were examined, demonstrating their superior performance compared to individual BP and g-C₃N₄ catalysts. The presence of BP broadens the g-C₃N₄/BP absorption band, and the inclusion of g-C₃N₄ not only protects BP from oxidation but also creates a shallow interface with trapped charge sites, enhancing the separation of electric carriers in composite photocatalysts. This results in reduced limitations on fast carrier recombination in g-C₃N₄ or BP nanosheets.

Figure 7:

Nanomaterial sheets of BP/C₃N₄ for high catalytic production of hydrogen with properties i.e. larger lateral size, high quality and lower anomalous structural defects. A nanosheet of BP/g-C₃N₄ for efficient catalytic hydrogen production due to properties like larger lateral size and limited structure defects. ¹²¹

5.8 Oxygen evolution reaction (OER)

The OER, which makes up the second half of water splitting, can be thought of as an oxidative process that needs four electrons and proton transfer. This reaction also has a high overpotential need and a slow kinetic response. Due to their huge specific area, high surface atom density, and atomic level thickness, 2D LDH nanosheets are able to significantly improve catalytic performance. ¹²² In order to improve catalytic OER, Song et al. ¹²³ used layered LDH nanosheets that were subjected to liquid phase exfoliation. As observed in Figure 8, the exfoliated single layer LDH nanosheets perform better in the OER test than the bulk-layered LDHs. Additionally, Qin et al. created a 2D CoCo-LDH nanomesh as an OER electrocatalyst, which had a mesoporous structure and abundant high activity atoms with low ligancy, improving the diffusion of reactants and products as shown in Figure 8. ¹¹¹ The CoCo-LDH nanomesh’s onset overpotential and overpotential (10) were reduced to 220 mV and 319 mV, respectively.

Figure 8:

Catalytic enhancement of oxygen evaluation on CoCo-LDH nanomesh. ¹¹¹

5.9 Carbon dioxide reduction reaction (CO₂RR)

The CO₂ content in the atmosphere has now surpassed the previous threshold of 23 million years and is rising at an unprecedented rate. One of the most powerful greenhouse gases is CO₂, and an increase in CO₂ levels is directly linked to climate change. An urgent worldwide issue is the capture and effective use of CO₂. ¹²⁴ In order to convert CO₂ into non-toxic organics, 2D-layered nanomaterials have gained attention for applications in photocatalysis and electrocatalysis. ¹²⁵ According to Figure 9a and b, ¹²⁵ Ye and colleagues successfully developed a CO₂RR for homogeneous Zn-MOF nanomaterials with a 4.7 nm layer thickness that has a 103.8 cm³ g⁻¹ CO₂ adsorption capacity. The synergistic effect of the increased lifespan of photogenerated electric carriers makes it possible to use 2D layered MOF nanosheets with optimal catalytic CO₂RR activity in contrast to bulk MOFs with low efficiency. According to Zhao’s group, 2D ZnO was used for photocatalytic CO₂RR, as depicted in Figure 9c–e. ¹²⁶ The 2D ZnO nanosheets have larger surface catalytic active sites for CO₂RR, a suitable bandgap, and optical absorbance when compared to their bulk equivalent.

Figure 9:

Photocatalytic conversion of CO₂ by 2D-nanomaterials. (a & b) Photoreduction of carbon dioxide into carbon monoxide by ultrathin 2D-nanosheets of Zn-MOF. ¹²⁵ (c, d & e) selective photoreduction of CO₂ by 2D-Zno nanomaterials. ¹²⁶

7.1 Electrochemical processes and interfaces in nanocatalysis

Electrochemical processes and interfaces play a crucial role in nanocatalysis, where the combination of nanomaterials and electrochemistry leads to unique and efficient catalytic reactions. The fundamental principles of electrochemical processes in nanocatalysis, focusing on the electrocatalytic interfaces that govern charge transfer reactions are explored here. The interface between the nanocatalyst and the electrode, known as the electrocatalytic interface, is where charge transfer reactions take place. ¹⁴¹ The nanoscale dimensions of catalysts bring about changes in the electrocatalytic interface that influence reaction kinetics and efficiency. The electrocatalytic interface includes the electrical double layer, consisting of charged species near the electrode surface. The high surface area of nanocatalysts leads to an increased number of ions in the double layer, affecting charge transfer kinetics. Nanocatalysts offer a greater number of active sites due to their large surface area. These sites are crucial for adsorption of reactants and facilitating charge transfer processes. ¹⁴²

Electrocatalysis involves charge transfer reactions at the electrocatalytic interface. Electrons are exchanged between the electrode and the nanocatalyst, leading to redox reactions that drive catalysis. In redox reactions, electrons are transferred between the nanocatalyst and the electrode. Nanocatalysts accelerate these electron transfer processes due to their high surface area and increased availability of active sites. Tafel slopes describe the relationship between current density and overpotential in electrocatalytic reactions. Nanocatalysts can exhibit different Tafel behavior compared to bulk materials, influencing reaction kinetics. ¹⁴³ Nanoscale dimensions and composition significantly impact electrocatalytic behavior. These effects arise from the unique electronic and surface properties of nanomaterials. Nanocatalysts may exhibit lower overpotentials compared to bulk materials due to quantum size effects. ¹⁴⁴ This reduces the energy required to initiate catalytic reactions. Quantum size effects and surface modifications influence the electronic structure of nanocatalysts. This affects charge transfer rates and the reactivity of catalytic sites. Nanocatalysts offer multiple pathways for charge transfer reactions, enhancing catalytic efficiency and providing opportunities for intricate reaction mechanisms. The presence of different sites and orientations on nanocatalysts allows for parallel reaction pathways, accommodating various reaction intermediates and enhancing overall catalytic performance. ¹⁴⁵ Nanocatalysts may follow different reaction mechanisms compared to bulk materials due to the unique coordination environments of surface atoms. This can lead to the formation of unique reaction products. ¹⁴⁶

7.2 Interplay between nanostructures and reactivity in nanocatalysis

The behavior of nanocatalysts is profoundly influenced by their nanostructures, which encompass size, shape, composition, and surface properties. This interplay between nanostructures and reactivity dictates catalytic performance, selectivity, and mechanistic pathways. Understanding and manipulating this relationship are paramount in tailoring nanocatalysts for specific applications. ¹⁴⁷ This section explores into the complex influences between nanostructures and reactivity in nanocatalysis. Nanoparticles’ size dictates the distribution of surface atoms and quantum size effects, leading to distinctive reactivity profiles. Different sizes expose diverse facets and coordination environments, leading to size-specific active sites. These sites can exhibit varying binding affinities for reactants and intermediates. Quantum confinement modifies electronic energy levels, altering adsorption energies and reactivity. This size-dependent reactivity influences reaction kinetics and selectivity. ¹⁴⁸ Nanoparticle shape profoundly influences catalytic behavior due to variations in exposed crystal facets and surface curvature. Different facets possess varying surface energies and arrangements of atoms. These facets can act as active sites for specific reactions, impacting catalytic activity. Curved surfaces on nanoparticles lead to higher surface atom density, potentially enhancing catalytic activity by providing more active sites. ¹⁴⁹

8.1 Homogeneous catalysis with nanocatalysts

Homogeneous catalysis involves catalysts and reactants in the same phase. Nanocatalysts offer advantages in this context due to their increased surface area and unique reactivity. Homogeneously dispersed nanocatalysts possess active sites throughout the reaction mixture, enhancing catalytic efficiency. Nanocatalysts in homogeneous systems enable rapid and efficient interactions between reactants and catalysts, resulting in higher reaction rates. Homogeneous catalysis with nanocatalysts finds applications in organic synthesis, polymerization, and other fine chemical processes. ¹⁵³

Homogeneous catalysis involves catalysts and reactants in the same phase, often dissolved in a solvent. The integration of nanocatalysts into homogeneous catalytic systems has opened new avenues for efficient and selective reactions. This section explores the principles, advantages, and applications of homogeneous catalysis using nanocatalysts. Nanocatalysts, due to their high surface area and unique reactivity, exhibit distinct behavior in homogeneous catalytic systems. The increased surface area of nanocatalysts provides more active sites for reactant adsorption and reaction initiation, leading to higher catalytic activity. Nanoparticle size and shape influence catalytic behavior by modulating the distribution of active sites and quantum size effects. The choice of solvent in homogeneous catalysis with nanocatalysts can influence reaction rates, solubility, and coordination to the catalyst surface. ¹⁵⁴

8.2 Heterogeneous catalysis on nanostructured surfaces

Heterogeneous catalysis involves catalysts in a different phase from the reactants. Nanostructured surfaces provide exceptional platforms for heterogeneous catalysis due to their high surface area and tunable properties. ^{157 , 158 , 159} Nanostructured surfaces offer increased surface area for reactant adsorption and catalytic reactions, leading to improved activity. ^{160 , 161 , 162 , 163 , 164 , 165 , 166 , 167} The specific arrangement of atoms on nanostructured surfaces can selectively expose certain active sites, enabling control over reaction pathways. Nanostructured surfaces allow easy separation of catalysts from the reaction mixture, enabling their reuse and contributing to sustainability. ^{168 , 169} Nanostructured surfaces introduce new dimensions to heterogeneous catalysis by offering a range of active sites and enhanced surface interactions. Nanostructured surfaces provide larger surface areas for reactant adsorption, leading to increased catalyst-substrate interactions for heterogenous catalysis. The specific arrangement of atoms on nanostructured allows for the exposure of different crystal facets and active sites, influencing catalytic surfaces activity. these surfaces exhibit size-dependent properties, where nanoscale features affect surface energy and reactivity which are very valuable in term of heterogenous catalysis. There are so many benefits of heterogeneous catalysis on nanostructured surfaces. The integration of nanostructured surfaces into heterogeneous catalytic systems offers numerous advantages including high activity, selective catalysis, recycling etc. ¹⁷⁰ Nanostructured surfaces enhance catalytic activity by exposing more active sites, enabling efficient reactions with lower catalyst loadings. The controlled arrangement of atoms on nanostructured surfaces enables selectivity by favoring specific reaction pathways. Separation and reuse of nanostructured catalysts are facilitated due to their enhanced surface interactions and ease of recovery. The nanostructured surfaces drive reactions such as hydrogenation, oxidation, and hydrogenolysis, playing a pivotal role in industrial catalysis. These catalysts also contribute to environmental remediation by degrading pollutants, converting harmful gases, and purifying water. Energy-related catalytic processes like fuel cells, where they facilitate efficient energy conversion are also consider as their efficient applications. ⁸⁵

Despite its advantages, heterogeneous catalysis on nanostructured surfaces poses challenges that require addressing. The exposure of high-energy facets on nanostructured surfaces can lead to catalyst degradation under harsh reaction conditions. Translating lab-scale synthesis of nanostructured catalysts to industrial-scale production involves challenges related to reproducibility and cost-effectiveness. Understanding the structure and dynamics of nanostructured surfaces during catalysis requires advanced characterization techniques. ¹⁷¹ Heterogeneous catalysis on nanostructured surfaces represents a frontier in catalytic science, blending the benefits of traditional heterogeneous catalysis with the unique properties of nanomaterials. The increased surface area, tailored reactivity, and efficient reactant interactions make nanostructured surfaces powerful tools for catalytic transformations across industries. As research advances and challenges are addressed, nanostructured surfaces will continue to redefine and innovate the field of heterogeneous catalysis.

8.3 Enzymatic catalysis and nanozymes

Enzymatic catalysis, using natural or engineered enzymes, has found synergy with nanomaterials, leading to the concept of nanozymes. Nanozymes mimic enzymatic activity and offer stability and reusability advantages over natural enzymes. Nanozymes exhibit high catalytic efficiency and can operate under a wide range of conditions, making them versatile tools for catalysis. Nanozymes find applications in biomedicine, including drug delivery, imaging, and therapeutic treatments, due to their catalytic and biocompatible properties. ¹⁷²

Enzymatic catalysis, a biological process facilitated by enzymes, has been augmented through the development of nanozymes nanomaterials with enzyme-like catalytic properties. The synergy between enzymatic catalysis and nanozymes offers new avenues for versatile and efficient catalytic transformations. ¹⁷³ Enzymes are biocatalysts that accelerate chemical reactions in living organisms. Enzymatic catalysis relies on the precise arrangement of active sites, substrates, and cofactors. Enzymes exhibit high substrate specificity, allowing for selective reactions in complex chemical mixtures. Enzymatic reactions often occur under mild conditions, making them suitable for biologically relevant transformations. Enzymes often demonstrate exquisite regio- and stereoselectivity, enabling the synthesis of complex molecules with high precision. ¹⁷⁴ Nanozymes are nanomaterials that mimic enzyme-like catalytic activity, offering advantages such as stability, reusability, and ease of synthesis. Nanozymes can mimic a wide range of enzymatic reactions, from oxidations and reductions to hydrolysis and more. Nanozymes are often more stable than natural enzymes and can withstand a broader range of reaction conditions. Some nanozymes are biocompatible and can be used in biomedical applications, including drug delivery and diagnostics. The integration of nanozymes with enzymatic catalysis combines the strengths of both approaches. Nanozymes can match or even exceed the catalytic efficiency of natural enzymes, making them potent catalysts. Nanozymes are more robust and can be easily recovered and reused, reducing the need for enzyme regeneration. Nanozymes expand the scope of enzymatic reactions to non-biological substrates, broadening their synthetic applications. ¹⁷⁵

The synergy between enzymatic catalysis and nanozymes has diverse applications. For example, the natural enzymes and nanozymes find applications in green chemistry, pharmaceutical synthesis, and the production of biofuels. Nanozymes are employed in biosensing, disease diagnosis, and drug delivery due to their biocompatibility and catalytic activity. Enzymes and nanozymes are used to degrade pollutants, detoxify environmental contaminants, and purify water. ¹⁷⁶ Despite their potential, enzymatic catalysis and nanozymes face certain challenges. Expanding the substrate scope of nanozymes to match that of natural enzymes remains a challenge. Ensuring the biocompatibility of nanozymes for biomedical applications requires careful design and testing. The regulatory approval of nanozyme-based products requires comprehensive safety assessment and standardized testing. ¹⁷⁷ Enzymatic catalysis and nanozymes present a harmonious blend of biological and nanotechnological approaches to catalysis. The combination of enzymatic selectivity with the robustness and versatility of nanozymes holds immense potential for revolutionizing catalytic transformations in both synthetic chemistry and biotechnology. As research advances and challenges are overcome, the collaboration between enzymatic catalysis and nanozymes will continue to shape the landscape of modern catalysis.

8.4 Photocatalysis and photoelectrocatalysis

Photocatalysis utilizes light energy to drive catalytic reactions, while photoelectrocatalysis combines light-induced charge transfer with electrocatalysis. Photocatalysis utilizes sunlight to initiate reactions that were previously energetically unfavorable, such as water splitting or pollutant degradation. Photoelectrocatalysis combines the advantages of both photochemistry and electrochemistry, allowing for efficient charge separation and reaction initiation. Photocatalysis and photoelectrocatalysis hold potential in renewable energy conversion processes, like hydrogen generation from water or carbon dioxide reduction. ¹⁷⁸ The integration of nano photo/electrochemistry with different types of catalysis opens up a realm of possibilities in various fields, from energy to environmental remediation and beyond. Each type of catalysis leverages the unique properties of nanomaterials and the principles of photochemistry and electrochemistry, offering unprecedented control over reaction outcomes and catalytic efficiency. The synergy between these approaches is poised to redefine how we harness light and electron transfer processes for a sustainable and innovative future.

10.1 Functionalized noble metal nanocrystals for electrocatalysis

The excessive utilization of fossil fuels leads to energy depletion, environmental degradation, and climate change, posing significant threats to both human security and the development of sustainable energy sources. ²¹¹ The foremost priority of nations worldwide is to accelerate the establishment of a dependable, enduring, and state-of-the-art energy supply system. Electrochemical technologies play a pivotal role in generating, converting, and storing clean, renewable energy, exemplified by technologies such as fuel cells and water electrolysis. These methods offer efficient solutions to reduce our dependence on fossil fuels and address the challenges of global energy scarcity and environmental pollution. ²¹² Nevertheless, there remains a critical need for highly efficient and dependable electrocatalysts to facilitate the sluggish cathode and anode reactions essential for fuel cell and water electrolysis applications. ²¹³

Noble metal nanocrystals exhibit a unique electronic structure, characterized by incompletely filled d orbitals, and possess a high degree of chemical inertness. These attributes contribute to their remarkable electrocatalytic activity and stability. The electrocatalytic performance of noble metal nanocrystals is intricately linked to surface and interface parameters, in addition to their shape, composition, and crystal facets. In theory, the chemical functionalization of the electrocatalyst surface holds great significance as it can alter the structure of the electrode-electrolyte interface, resulting enhancing activity. ²¹³ Various techniques, including electrochemical reduction, atmospheric heat treatment, and wet chemical procedures, have been developed to produce noble metal nanocrystals with controlled morphology and structure. Among these, the wet chemical approach stands out due to its distinct advantages, offering a homogeneous environment and greater tunability in the liquid phase. During wet chemical synthesis, surfactants play a dual role, acting as stabilizers to reduce nanoparticles to nanoscale sizes and preventing agglomeration through electrostatic stabilization and steric hindrance mechanisms. ^{213 , 214}

Furthermore, surfactants exhibit varying adsorption states on different metal crystal planes, consequential in steady growth rates on several crystal planes, thereby foremost the favored alignment development of noble metal-nanocrystals. ²¹⁵ Polyethyleneimine (PEI, Figure 11(a)) and polyallylamine (also known as polyallylamine hydrochloride, PA/PAA/PAH, Figure 11(b)) are two typical polymeric amine surfactants (PAM) frequently employed in our research. Their excellent hydrophilicity, strong coordination ability, and inherent electrostatic repulsion make them ideal surfactants and complexing agents. ²¹⁶ In general, it is widely accepted that producing uniformly shaped noble metal nanocrystals in an aqueous solution presents greater challenges compared to oil phase synthesis. ²¹³

Figure 11:

Noble metal as nanococatalyst. Molecular structure of (a) = PEI and (b) PA/PAAPAH. (c) UV visible spectra of PAH, RhCl₃ and their mixture. (d) LSV or linear sweep voltammetry curve of chemical reactions of RhCl₃ with KCl and KCl + PAH at glassy carbon electrode. (e) Noble metal as nanococatalyst via PAM-assisted growth mechanism. ²¹⁷

The PAMs can make coordination complexes with metals like Rh(III), Pt (II), Pd(II), and Ag(I). This is due to the existence of lone pairs on N-atom within NH₂ and –HN– groups , ²¹⁷ which help in nanocatalyst formulation. Consequently, anisotropic nanostructures such as nanocubes, nanowires, nanosheets, and nanonetworks are produced under varying reaction conditions, as nanospheres with low surface free energy are no longer favored in the final shape of noble metal nanocrystals. The UV–vis absorption peaks of a combined solution of PAH and Rh(III) differ from those of RhCl₃ (Figure 11(c)), providing evidence of the formation of a RhIII-PAH complex. ²¹⁷ Electrochemical measurements reveal that the formation of the RhIII-PAH complex substantially lowers the reduction potential of RhIII precursors, enabling kinetic control over the reduction and crystallization process (Figure 11(d)). ²¹⁷ Initially, Rh(III)-PAH complexes are reduced to nearly spherical Rh seeds at the start of the reaction. The high adsorption of PAH prevents the formation of Rh (111) planes, ultimately resulting in ultrathin nanosheets exposing the Rh (111) plane. ²¹⁷ Subsequent epitaxial growth of these nanosheets gives rise to three-dimensional Rh nanosheet nanoassemblies. Variations in atomic deposition and diffusion rates can significantly influence the ultimate morphology of the nanocrystals. While the rate of deposition can be controlled by varying the reduction rate of the metal precursor, the rate of diffusion is closely linked to nanocrystal surface characteristics. Hence, modifications in chemical kinetics parameters like temperature, reaction time, pH, reducing agent, metal ion precursor, PAM molecular weight, etc., can dramatically alter the rate of metal precursor reduction, thereby influencing the final nanocrystal morphology. ²¹³ In recent years, an array of noble metal nanostructures has been synthesized by investigating the PAM-assisted synthesis mechanism of noble metal nanocrystals. Examples include Pt nanocubes, Pd icosahedrons, Pt tripods, and Pt/Pd nanodendrites ^{213 , 218} (Figure 11(e)). Importantly, these anisotropic noble metal nanocrystals have exhibited significantly improved catalytic activity across various electrochemical reactions.

12.1 Metal free carbon-based catalysts

Pure carbon compounds, due to their electroneutral nature with low CO₂ adsorption capacity and high energy barriers for CO₂ activation, are not considered effective for CO₂ reduction. ²³³ To enhance a carbon framework’s ability to activate CO₂, the incorporation of heteroatoms is a promising approach. The introduction of heteroatoms disrupts the electroneutrality of pure carbon structures, leading to charge redistribution and the creation of reactive sites for CO₂ adsorption and activation. Heteroatoms like boron (B), nitrogen (N), and sulfur (S) exhibit distinct electronegativities compared to carbon atoms, influencing their reactivity. ²³⁵ In addition to heteroatom incorporation, the crystalline structures of carbon materials, characterized by unique binding energies of CO₂-related intermediates, play a significant role in determining selectivity and activity in CO₂ reduction. ²³⁴ Various carbon materials represent different dimensional structures, including OD, 1D, 2D, and 3D porous carbons (Figure 13). ^{233 , 234 , 236} Metal-free carbon materials can be modified to reduce CO₂ into C²⁺ oxygenates through controlled heteroatom doping and structural engineering. The following section introduces several metal-free carbon materials suitable for converting CO₂ into C²⁺ oxygenates. ²³⁴

Figure 13:

Various carbon architectures, OD, 1D, 2D and 3D carbon base materials. ²³⁴

12.2 Carbon nanotubes

Carbon nanotubes (CNTs) consist of a graphitic honeycomb structure composed of sp2-hybridized carbon atoms, forming a cylindrical structure akin to a rolled-up graphite sheet. CNTs feature a unique architecture with layered inner and outer wall structures. ²³⁷ The introduction of oxygen through acid treatment can lead to the formation of oxygenated carbon nanotubes (O-CNTs), generating a variety of oxygen-containing groups. O-CNTs demonstrate the ability to convert CO₂ into formic acid (HCOOH) and acetic acid (CH₃COOH) at rates of 0.51 mol/h and 0.32 mol/h, respectively. CH₃COOH production exhibits significantly higher Faradaic selectivity (71.5 %) compared to HCOOH (28.5 %). Oxygen functional groups on O-CNTs provide electron trapping sites, aiding in the reduction of adsorbed intermediates. However, the precise mechanism of CO₂ reduction leading to CH₃COOH production on O-CNTs remains uncertain. ²³⁸ CNTs, with their abundant functionalization sites, hold promise as electrocatalysts for CO₂ reduction. ²³⁴

12.3 Graphene

Graphene, a two-dimensional (2D) material composed of sp²-hybridized carbon atoms arranged in a hexagonal lattice, boasts a high surface area, exceptional chemical stability, and excellent electrical conductivity. ²³⁹ Pristine graphene is typically inert towards CO₂ reduction due to its ineffective CO₂ activation, and ability to adsorb certain intermediates like COOH and CO. The undoped zigzag edge of graphene presents a significant energy barrier of approximately 1.3 eV for CO₂ adsorption. Consequently, surface modification and heteroatom doping are employed to alter graphene’s electronic structure, enhancing its CO₂ electroreduction activity. Nitrogen (N) doping is a common and effective method to modify graphene’s electronic structure and boost CO₂ reduction activity for C²⁺ product synthesis. ⁵⁸ Nitrogen-doped graphene quantum dots (NGQDs), featuring nanoscale dimensions, have been developed for the electrocatalytic conversion of CO₂ into C²⁺ products (Figure 14). ²⁴⁰ NGQDs yield primarily formic acid (HCOOH) and carbon monoxide (CO) at an applied potential of 0.26 V versus reversible hydrogen electrode (RHE). However, when the applied potential shifts negatively, CO₂ conversion extends to include oxygenates like CH₃COO, C₂H₅OH, nC₃H₇OH, as well as hydrocarbons like CH₄ and C₂H₄. The highest faradaic efficiency (FE) for C²⁺ oxygenates reaches 26 % at an applied potential of 0.78 V, with C₂H₅OH accounting for 16 % of the total. ^{22 , 234}

Figure 14:

NGQDs for electrocatalysis and their characterization. (a) Mechanism of electrocatalytic conversion of CO₂ on nitrogen doped graphene quantum dots. ²⁴¹ (b) TEM images of NGQDs. (c) The faradaic efficiency of reduced product on the surface of NGQDs. (d) The current densities of reduced products (C₂H₄, CH₄, C₂H₅OH, CO, CH₃COO⁻, C₃H₇OH) on the surface of NGQDs. ²⁴⁰

12.4 Nanodiamond

Diamond nanocrystals exhibit tetrahedral bonding units with sp³-hybridized carbon atoms. ²⁴² When heteroatoms are introduced as dopants into diamonds, they experience a significant improvement in their electrical conductivity compared to their unaltered state. Heteroatom-doped diamonds offer several advantages, including a wide electrochemical potential window, a strong H₂ evolution overpotential, and exceptional long-term stability, making them effective catalysts for electrochemical reactions. The unique sp3 bonding in diamonds plays a crucial role in facilitating C–C coupling and demonstrating remarkable selectivity in generating C²⁺ oxygenates through CO₂ reduction. ^{234 , 243} Nanodiamond structures can enhance their electrochemical activity for CO₂ by incorporating nitrogen (N). Our team achieved this by depositing N-doped nanodiamond (NDD/Si RA) on a Si rod array substrate using microwave plasma-enhanced chemical vapor deposition, as shown in Figure 15a–c. ²⁴³ NDD/Si RA exhibits an onset potential of 0.36 V versus reversible hydrogen electrode (RHE) for reducing CO₂ to CH₃COOH. At the potential range of 0.8–1.0 V, the total faradaic efficiency (FE) for CO₂ reduction is impressively high, ranging from 91.2 % to 91.8 %, effectively suppressing the H₂ evolution reaction. Specifically, at 0.8 V, the FE for CH₃COOH production reaches approximately 77.3–77.6 %, surpassing the FE of HCOOH (13.6–14.6 %) under similar conditions. Notably, the CH₃COOH generation rate peaks at 96.1 mg L⁻¹ h⁻¹ at a potential of 1.0 V. These findings underscore the advantages of NDD/Si RA in achieving C–C coupling and CH₃COOH production, outweighing the challenges associated with low C₂ selectivity in CO₂ reduction. Our analysis suggests that CC coupling occurs when two CO₂ radicals dimerize, forming OOCCOO, which subsequently undergoes hydrogenation to yield CH₃COOH. The N-sp³-C species on NDD/Si RA serve as reactive sites for CO₂ reduction and are positively correlated with CH₃COOH formation. ²³⁴

Figure 15:

Doped/undoped nanodiamonds (NDDs) and their role in catalytic reduction reactions. (a) NDD SEM images. (b) The rate of production of reduced products, on the surface of NDD. (c) The faradaic efficiencies (for formate and acetate) in the production on the surface of NDD. ²⁴³ (d) SEM images of boron and nitrogen doped nanodiamonds (BND). (e) The rate of production of reduced products, on the surface of BND. The faradaic efficiencies (for formate and acetate) in the production on the surface of BND. ²⁴⁴

Additionally, we employed a hot filament chemical vapor deposition method to create boron and nitrogen co-doped nanodiamond (BND) on a Si substrate, as depicted in Figure 15d–f. BND exhibits the ability to transform CO₂ into HCOOH, HCHO, CH₃OH, CH₃COOH, and C₂H₅OH, with C₂H₅OH being the predominant product, boasting a maximum FE of 93.2 % at 1.0 V versus RHE. The increased presence of N in BND enables the synthesis of C₂H₅OH. Density functional theory (DFT) calculations indicate that the synergistic effect between B and N dopants significantly contributes to the enhanced capacity for C₂H₅OH production on BND. The connection of an oxygen (O) atom to a boron (B) atom enhances CO₂ adsorption, while nitrogen (N) atoms serve as active sites for H transfer, expediting hydrogenation during CO₂ reduction.

12.5 Porous carbon

In addition to traditional graphite and diamond structures, porous carbon materials featuring hierarchically structured 3D pores present an enticing option as electrocatalysts for CO₂ reduction. These materials offer extra active sites for CO₂ capture and reduction due to their large specific surface areas and diverse porous topologies. Porous carbon can be categorized based on pore size, such as microporous (pores smaller than 2 nm), mesoporous (pores between 2 and 50 nm), or macroporous (pores larger than 50 nm). The pore size, distribution, and hierarchically porous structure influence the accessibility of CO₂ to reactive sites and the dynamics of electrolyte diffusion. Thus, tailoring the porous structure, in conjunction with heteroatom doping, plays a pivotal role in enhancing CO₂ reduction capacity. ^{234 , 245} By manipulating the macro-, meso-, and microporous 3D carbon structures, we can create distinctive surfaces and electronic properties. Additionally, porous carbon typically contains both sp² and sp³ hybridized carbon, benefiting from the unique characteristics of both carbon atom types. Therefore, by optimizing the concentrations of sp²/sp³ and porous distribution, the CO₂ reduction activity of porous carbon can be finely tuned. The customization of porous structures, alongside heteroatom doping, represents a critical element in enhancing CO₂ reduction capability. Song and colleagues, for instance, developed metal-free nitrogen-doped mesoporous carbon with an ordered cylindrical channel structure, as shown in Figure 16a–c. ²⁴⁶ The resulting catalysts efficiently and selectively convert CO₂ into C₂H₅OH, with a FE of 77 % at 0.56 V versus RHE. The uniformly distributed pyridinic-N and pyrrolic-N in the inner layer of the cylindrical channel play a pivotal role in facilitating electron transfer for the dimerization of CO intermediates and the subsequent C–C coupling in electrochemical CO₂ conversion. Furthermore, the cylindrical structural configuration of nitrogen-doped mesoporous carbon proves essential for electrochemical CO₂ conversion. However, the rate of C₂H₅OH synthesis in N-doped mesoporous carbon remains limited due to significant kinetic barriers in C–C bond formation. ²³⁴ To alleviate these limitations and enhance important intermediate coupling, Song and colleagues introduced catalysts with carefully tailored designs, resulting in the creation of nitrogen-doped ordered mesoporous carbon (Figure 16d–f). ²⁴⁴ This modification led to an increased C₂H₅OH generation rate of up to 2.3 mmol g⁻¹ h⁻¹. The medium microporous structure, combined with high electron density in pyrrolic and pyridinic N, simplifies CO₂ activation and C–C bond formation. The presence of medium micropores in the channel wall, along with the cylindrical structural configuration, enables the accumulation of electrolyte ions, resulting in high local electrical potentials that drive desolvation during electrochemical CO₂ conversion. ²⁴⁶

Figure 16:

Synthetic route and properties of N-doped mesopore/micropore porous carbon mateirals for catalytic CO₂ reduction, resulting catalysts efficiently and selectively convert CO₂ into C₂H₅OH (V versus RHE). (a) General scheme of mesoporous carbon and exchange of materials. (b) Various section TEM images of N-doped mesoporous carbon. (c) Faradaic efficiencies of CO₂ reduction reactions on the surface of N-doped mesoporous carbon. ²⁴⁶ (d) General scheme of mesoporous, narrowpore, narrow micropore, madium micropore carbon. (e) TEM images of N-doped microporous carbon. (f) Faradaic efficiencies of CO₂ reduction reactions on the surface of N-doped microporous carbon. ²⁴⁷

12.6 Metal and carbon composite catalysts

The development of metal and carbon composite electrocatalysts represents another design strategy for achieving CO₂ reduction to C²⁺ oxygenates. These composite materials, comprising carbon and metal components (like Cu, Fe, Ni, Pt), that have ability to reduce CO₂ to C²⁺ oxygenates at lower overpotentials compared to metal-free carbon catalysts and to CH₃COOH, C₂H₅OH, CH₃COCH₃, and C₃H₇OH. The synergistic effects between metals and carbon materials appear to be responsible for the high catalytic activity and selectivity observed in these composite catalysts. ²⁴⁸ The activation and adsorption of CO₂ are facilitated by metal particles, given their high CO binding energies and low H₂ evolution overpotentials. The high surface area and flexible porous structure of carbon materials provide an extensive electrochemically active area for CO₂ reduction, reducing resistance to reactant and product diffusion and, thereby, facilitating CO₂ reduction.

14.1 Strategies for improving photocatalysis

Utilizing an innovative substance known as a nanocomposite, specifically designed to address certain shortcomings within the field of photocatalysis, offers an efficient solution. This approach is particularly promising for overcoming limitations associated with various substrates such as carbon-containing materials, ²⁵⁸ silica, ²⁵⁹ and primarily assisted polymers, ²⁶⁰ which have been previously reported as suitable for attaching nanoparticles. Among these substrates, polymer hosts stand out as a compelling choice for long-term applications due to their capacity to anchor nanoparticles, their virtually limitless architectural versatility, sophisticated and multifunctional surface chemistry, and robust mechanical strength, all of which are conducive to practical applications. The key driving force behind the growing interest in inorganic functional polymer composites, which combine novel optical, electrical, and catalytic capabilities of nanoparticles with the flexibility and transparency of the polymer matrix, lies in their ability to offer unique environmental processes, chemical inertness, excellent durability, and long-lasting stability. Additionally, these polymers facilitate the attachment of target organic molecules to their surfaces, leading to an overall enhancement in photocatalytic efficiency. ²⁶¹ Photocatalysis, as a versatile technique, can facilitate various reactions, ranging from the mineralization of organic contaminants to complex organic processes. Its fundamental advantage lies in the conversion of light energy into chemical energy, thereby reducing energy consumption and providing a sustainable and cost-effective solution to pollution. Consequently, photocatalysis aligns with the principles of sustainable chemistry and green organic synthesis. However, it is worth noting that current photocatalytic materials predominantly rely on semiconductors, primarily TiO₂. ²⁶²

14.2 Doping process

Doping is an important technique used to enlarge the photoactive zone, which prevents recombining of the electrons and holes produced during photogeneration, in addition to broadening the response spectrum range of the photocatalyst. ²⁶³ It is common practice to modify the photocatalyst by doping in order to improve its photocatalytic performance. An example of this is the use of nitrogen doping to alter and improve the performance of photocatalyst oxides. ²⁶⁴ Different from cation doping, which produced more unfavorable recombination locations for the electron-hole, is anion doping. It has been discovered that nitrogen doping has a greater visible light response and is more significant than carbon and sulfur doping. Due to its potent capacity to oxidize and significant reactivity to visible light. Since 2001, interest in the nitrogen-doped TiO₂ photocatalyst has increased. In addition, vanadium and chromium doping of TiO₂ lengthens its wavelength in the region of visible light. The photocatalytic effectiveness is nonetheless impacted by the recombination of the electrons and holes that occurs during the fusing of metal ions. ²⁶³ Doping, however, considerably lowers the bandgap, improving the photocatalytic characteristics. For instance, only absorption in the ultraviolet (UV) range (which includes 5 % of the solar spectrum) can result in the creation of electron-hole pairs. The observed band gap varies with the TiO₂ polymorphs and is greater than 3.0 eV for rutile, 3.2 eV for anatase, and 3.13 eV for brookite, ²⁶⁵ indicating that the majority of solar energy cannot be converted into a reaction that uses photocatalysis. This restriction has been studied by narrowing the bandgap by doping pure TiO₂ with an impurity element as N, S, or Fe. ²⁶⁶ TiO₂ activity can be increased by visible light by absorbing more energy through doping. While the electron-hole recombination site may be trapped by intermediate energy states created by newly added atoms and defects, which reduces the effectiveness of the photocatalytic process. ²⁶⁷ Additionally, it has been noted that elemental doping of ZnO can compensate for the limited surface area and insufficient optical absorption of ZnO. ²⁶⁸

The bandgap energy of TiO₂ can be reduced by synergistic co-doping, resulting functional visible light photocatalyst. Similarly, the co-doping of Fe³⁺ and graphene can also boost the photo-efficiency of semiconductors. ²⁶⁹ Therefore, according to Zhang et al. , ²⁷⁰ the co-doping of Ni²⁺ and Ti³⁺ caused a notable drop in the TiO₂ anatase band, as demonstrated in Figure 17. ²⁷⁰ It is possible to increase photocatalytic activity in the visible light area by using a number of nonmetal atoms with high energies for ionization and electronegativity, such as I, Br, Cl, F, S, O, and N. ²⁷¹ This is accomplished by narrowing the bandgap and shifting edge absorption. These heteroatoms with doped carbon skeletons have recently been employed to improve the catalytic process that led to the electron-induced regulation of carbon atoms. ²⁷² Co-doping will undoubtedly increase the photocatalytic effectiveness by two or more atoms. By lowering the development of recombination centers and inactivating the impurity bands, it will increase the solubility of dopants.

Figure 17:

General scheme of photocatalytic reaction for Ti³⁺ and Ni²⁺ co-doped TiO₂, where organic contaminations can converted into CO₂ and H₂O. ²⁷⁰

14.3 Semiconductors immobilization on the surfaces of polymers

Due to the advantages of immobilizing TiO₂, researchers worldwide are focusing on efficient methods to anchor TiO₂ onto suitable substrates. The ideal polymer matrix should meet several criteria, including strong catalyst-substrate affinity, compatibility with the chosen linkage method, precise surface properties, good contaminant adsorption capacity, and prevention of photocatalyst leaching. One notable example is the use of 3,5-dinitrosalicylic acid/chitosan/MnFe₂O₄ (DNSA/Cs/MnFe₂O₄) nanocatalyst, which significantly enhances adsorption and photodecomposition. The addition of DNSA/Cs as an interface connector between MnFe₂O₄ and methylene blue (MB) accelerates photodegradation and enhances catalytic efficiency, providing a promising method for water purification as show in Figure 18. ²⁷³

Figure 18:

The proposed mechanism of MB decomposition a Fento-like activation, onto DNSA@Cs@MnFe₂O₄ magnetic as a special type of nanocatalyst. ²⁷³

16.1 Advanced catalysts for photocatalysis

Photocatalysis, the use of light energy to drive chemical reactions, holds significant promise for sustainable energy production, environmental remediation, and synthesis of valuable compounds. Advanced catalysts play a crucial role in enhancing the efficiency and selectivity of photocatalytic processes. ²⁷⁷ Semiconductor photocatalysts, such as titanium dioxide (TiO₂) and metal oxides, harness light energy to generate electron-hole pairs, initiating redox reactions. Advances in semiconductor design, including bandgap engineering and surface modifications, enhance their photocatalytic activity. ²⁷⁸ Plasmonic materials, like gold and silver nanoparticles, exhibit surface plasmon resonance, enabling localized electromagnetic fields that enhance light absorption and catalytic activity. Plasmonic nanoparticles can facilitate reactions that are not easily achievable with traditional catalysts. ²⁷⁹ Similarly the two-dimensional materials, such as graphene, transition metal dichalcogenides (TMDs), and black phosphorus, offer high surface area and tunable electronic properties. They can be used as photocatalysts themselves or as co-catalysts to enhance activity and selectivity. MOFs and COFs exhibit high porosity and tunable structures, making them ideal candidates for photocatalysis. Their pore structures can facilitate reactant adsorption, while metal nodes can act as active sites for redox reactions. ²⁸⁰ Integrating homogeneous catalysts into heterogeneous photocatalytic systems enables the exploitation of both light-induced charge carriers and solution-phase catalytic intermediates. This synergy enhances reaction efficiency and selectivity. ²⁸¹ likewise the introduction of co-catalysts, such as noble metal nanoparticles or co-catalytic species, onto photocatalyst surfaces can also facilitate charge separation and enhance reaction kinetics. Rational design and precise loading are crucial for optimizing their effects. Hybrid systems can tailor light absorption and charge transfer processes. Using light-harvesting molecules, quantum dots, or dye-sensitized systems can expand the light absorption range of photocatalysts. These systems transfer energy to the catalyst, initiating photochemical reactions. ²⁸² Developing catalysts that enable multi-electron transfer processes and cascade reactions can facilitate complex transformations, making photocatalysis a versatile tool for organic synthesis. ²⁸³ Beyond water splitting, advanced photocatalysts are being explored for carbon dioxide reduction, pollutant degradation, and fine chemical synthesis. Tailoring catalyst properties for specific reactions is a key focus. Advanced catalysts are pivotal in advancing the field of photocatalysis, unlocking its potential for sustainable energy conversion and environmental remediation. From innovative materials to synergistic co-catalysts and tailored structures, these catalysts enable more efficient light-driven reactions with applications ranging from hydrogen generation to pollutant removal and organic synthesis. As research continues, the development of advanced photocatalysts will contribute to addressing pressing global challenges and creating a greener and more sustainable future.

16.2 Semiconductor nanomaterials for photocatalysis

Semiconductor nanomaterials have revolutionized the field of photocatalysis by enabling the conversion of light energy into chemical reactions. These materials, through their unique electronic properties and tunable surfaces, have found applications in various domains, from environmental remediation to energy production. The significance of semiconductor nanomaterials in photocatalysis, their mechanisms, and their diverse applications are very valuable. ²⁸⁴

16.3 Photogenerated charge carriers and redox reactions in photocatalysis

Photogenerated charge carriers serve as the driving force for redox reactions in photocatalysis, enabling a wide range of applications from water splitting to environmental remediation. Understanding and optimizing charge carrier dynamics and their interaction with the catalyst’s surface are key factors in enhancing the efficiency and selectivity of photocatalytic processes. As researchers continue to unravel these intricate mechanisms, the field of photocatalysis holds promise for contributing to clean energy production and sustainable chemical transformations. The efficient utilization of photogenerated charge carriers is a fundamental aspect of photocatalysis, enabling redox reactions that drive various chemical transformations. This interplay between light-induced charge separation and subsequent redox processes is essential for the success of photocatalytic reactions. ²⁸⁵

16.4 Photogenerated charge carriers

When semiconductor nanomaterials absorb photons with energy greater than their bandgap, electron-hole pairs (e-h pairs) are generated. Upon promotion of electron from the valence band (VB) to the conduction band (CB), mobile charge carrier is generated, which result the corresponding positive charge, or holes, left in the VB (as shown in Figure 17). The spatial separation of these charge carriers is crucial for initiating redox reactions. The efficient charge carrier dynamics involve in the charge separation, transport and charge recombination. In the charge separation the photo-generated e-h pairs should be separated quickly, typically within femtoseconds to picoseconds, to prevent recombination. Then the separated carriers move towards the material surface or interface where redox reactions occur and lastly the undesirable e-h recombination can lead to energy loss and reduced photocatalytic activity. ²⁸⁶

16.5 Redox reactions in photocatalysis

The photogenerated charge carriers participate in redox reactions at the catalyst’s surface. In the oxidation reactions, the photogenerated holes (h⁺) can oxidize molecules adsorbed on the catalyst surface, leading to the removal of electrons and the generation of radical cations. While the photogenerated electrons (e−) reduce adsorbed species, leading to the formation of radical anions. But sometimes the radicals generated during oxidation and reduction can further react to form intermediate species crucial for the final products. The applicability photogenerated charge carrier is more prominent in water splitting, degradation of pollutants, reduction of carbon dioxide etc, In water splitting, photogenerated holes (h⁺) are involved in water oxidation, while photogenerated electrons (e−) reduce protons to form hydrogen. Charge carriers participate in the oxidation of organic pollutants adsorbed on the catalyst’s surface which help in the degradation of pollutants. Photogenerated charge carriers also reduce carbon dioxide to valuable chemicals such as hydrocarbons and alcohols. In some cases, the efficiency of charge carrier utilization is not very good, so we need to improve via various available techniques for example using cocatalysts, surface modifications and tandem catalysis. Co-catalysts can trap and transfer charge carriers more efficiently, preventing recombination and enhancing redox reactions. Tailoring the catalyst’s surface properties can also improve charge separation and favor certain redox reactions. In the same way combining multiple catalysts can facilitate charge carrier transfer between different materials, enhancing overall efficiency. ²⁸⁷

16.6 Cocatalysts and surface modifications in photocatalysis

Cocatalysts and surface modifications play a crucial role in enhancing the efficiency, selectivity, and overall performance of photocatalytic reactions. These strategies involve tailoring the surface properties of semiconductor nanomaterials to improve charge separation, redox reactions, and reactant adsorption. Cocatalysts are secondary materials integrated with semiconductor photocatalysts to facilitate charge transfer, prevent recombination, and enhance catalytic activity. Common types of cocatalysts include noble metal nanoparticles, co-catalytic metal oxides, and organic molecules. Various mode of mechanisms is available in literature including charge carrier transfer, surface reaction sites, and energy level alignment. Cocatalysts can act as electron or hole acceptors, preventing recombination and prolonging the lifetime of photogenerated charge carriers. Cocatalysts also provide additional active sites for reactant adsorption and redox reactions, accelerating overall reaction rates. When the energy levels of cocatalysts match with those of the semiconductor, it promote efficient charge transfer across the interface. ²⁸⁸

Cocatalysts like platinum (Pt) or nickel oxide (NiO_x) enhance the efficiency of hydrogen evolution and oxygen evolution reactions in water splitting. They improve the degradation of organic pollutants by facilitating radical generation and surface reaction sites. Also they aid in the reduction of carbon dioxide to valuable chemicals by promoting specific reaction pathways. ²⁸⁹ Surface modifications involve altering the surface properties of semiconductor nanomaterials to enhance photocatalytic performance. Common surface modifications include doping, functionalization, and heterostructure. Surface modifications can alter the band structure of semiconductors, leading to improved light absorption and charge separation. Functional groups introduced through surface modifications can enhance the adsorption of reactants, facilitating redox reactions. Surface defects introduced by modifications can act as charge traps, reducing charge carrier recombination. ²⁹⁰ Surface-modified photocatalysts can efficiently produce hydrogen or other solar fuels by enhancing reaction rates and charge carrier dynamics. Modified photocatalysts can effectively remove pollutants from water and air by enhancing their adsorption and degradation. Tailored surface properties enable the synthesis of complex molecules, mimicking natural photosynthesis. Still the optimization of cocatalyst loading to achieve the desired effect without blocking active sites is challenging. Ensuring the stability of cocatalysts and surface modifications under harsh reaction conditions is crucial for long-term performance. Developing cocatalysts and surface modifications specific to each photocatalytic reaction requires a deep understanding of reaction mechanisms. Cocatalysts and surface modifications serve as effective strategies to enhance the performance of semiconductor photocatalysts. By improving charge transfer, reaction kinetics, and surface properties, these approaches unlock the full potential of photocatalysis for various applications. As researchers continue to innovate in cocatalyst design and surface modification techniques, the field of photocatalysis is poised to contribute significantly to sustainable energy production, environmental protection, and green chemistry. ²⁹¹ A short comparison between photocatalysis and electrocatalysis is given in Table 2.