Advances in the Preparation of Carrier-Based Composite Photocatalysts and Their Applications

Abstract

Photocatalytic technology offers significant advantages in addressing water pollution and energy regeneration challenges. Notably, photocatalytic CO₂ reduction technology can convert CO₂ into stable, efficient, and clean carbon compounds such as carbon monoxide, methane, ethylene, and other high-value compounds, providing a novel approach to mitigating the global energy crisis and maintaining the carbon balance. However, traditional semiconductor photocatalytic materials face limitations in photocatalytic degradation and reduction due to their low light energy utilization, severe photocorrosion, rapid photogenerated carrier recombination, and slow electron transport rates. Recent studies have shown that introducing various carrier materials can effectively address these issues. Carrier materials, with their unique properties, enhance semiconductor composite photocatalyst systems, promoting photogenerated carrier separation and improving light energy utilization. This review introduces different carrier materials used in photocatalyst fabrication, systematically explains the preparation strategies for carrier-based composite photocatalysts, and summarizes their applications. Finally, future developments in this field are discussed. This review aims to provide diverse strategies for designing carrier-based photocatalysts, leveraging the special effects of carrier materials to control semiconductor composite modes, interface behaviors, and energy band structures.

1. Introduction

Among the most pressing challenges for the sustainable development of global human society, environment and resource issues remain a central focus [1,2,3,4]. As human society becomes increasingly complex, large quantities of pesticides, persistent pollutants, and toxic wastes from industrial pollution are discharged, severely endangering water and soil environments. The global pandemics has further exacerbated this issue, leading to the widespread use of antibiotics, which pose a threat to public health and the environment [5,6,7,8]. The growing global population and increasing carbon consumption have resulted in a per capita carbon footprint exceeding safe limits [9]; the uncontrolled combustion of fossil energy fuels, primarily petroleum and coal, has plunged humanity into a dual crisis of resource depletion and global warming, leading to extreme weather events and ecosystem damage caused by the greenhouse effect [10,11,12,13]. Solar energy-driven green photocatalytic technology not only efficiently degrades organic pollutants in water but also provides a novel approach for reducing and converting CO₂ into hydrocarbon products [14,15,16]. The CO₂ molecule is highly stable (804.4 kJ mol⁻¹), and the CO₂ reduction reaction (CO₂ RR) is non-spontaneous (∆G > 0), making the process challenging. Traditional CO₂ conversion technologies, such as electrocatalysis, are energy-intensive, unstable, complex, and highly polluting [17,18,19]. Photocatalytic CO₂ reduction, however, can effectively utilize solar energy to convert CO₂ into stable, clean carbon-containing compounds such as carbon monoxide (CO), methane (CH₄), methanol (CH₃OH), ethylene (C₂H₄), and other carbonaceous energy sources used in daily life. Thus, photocatalytic technology not only purifies water by degrading organic pollutants but also helps reduce atmospheric CO₂ levels, alleviate the greenhouse effect, and produce clean energy, and it also enables the storage of renewable energy in reduction products for secondary use, helping to mitigate the global energy crisis and maintaining the natural carbon balance [20,21,22]. Therefore, there is an urgent need to develop high-efficiency photocatalyst materials for clean energy storage and conversion to accelerate the industrialization of environmental purification and clean energy substitution processes [23,24].

Constructing a composite photocatalytic material system is an effective way to address the limitations of single semiconductor photocatalysts, such as narrow light absorption ranges, mismatched energy band structures, severe photocorrosion, and rapid photogenerated carrier recombination [25,26,27]. Whether for the photocatalytic degradation of water pollutants or light-driven reduction of atmospheric CO₂, the effective contact area with the target pollutants or carbon source, as well as the adsorption capacity of the composite material, are critical factors influencing photocatalysts’ performance [28,29,30]. The design of loaded composite catalysts has become a hot research topic in photocatalysis, as most carrier materials offer advantages such as large specific surface areas, strong adsorption capacities, and excellent physicochemical properties [31,32,33]. Traditionally, catalyst carriers have been used as skeletons to support and disperse active components, enhancing the material strength. However, common carrier materials such as alumina and silicon oxide and some natural products such as diatomaceous earth, pumice, and activated carbon or molecular sieve carriers are typically coupled with semiconductors through physicochemical methods, which only fix the semiconductor catalysts without strong interfacial interactions; these traditional carriers are also difficult to degrade, and often cause secondary pollution, limiting their ability to enhance a photocatalyst’s activity [34,35,36]. Therefore, developing new semiconductor photocatalyst carrier materials with high specific surface area, adsorption property, and unique material properties is crucial for enhancing the interfacial carrier effect with the active semiconductor components [37,38,39,40].

Herein, this review begins by discussing the classification of photocatalyst carrier materials, followed by an exploration of different preparation strategies for carrier material-based composite photocatalysts and their application in environmental photocatalysis (Scheme 1). Finally, we present remarks and perspectives on the further development of carrier material-based composite photocatalysts. Through this review, we hope to provide useful insights for designing carrier-based composite photocatalysts.

2. Classification of Carrier Materials for Photocatalysts

In recent years, an increasing number of carrier materials suitable for synthesizing compound semiconductor photocatalysts have been used in photocatalytic system design. Different preparation methods achieve various coupling effects, aiming to enhance the catalytic activity and overall performance of compound semiconductor photocatalysts. Generally, factors such as the catalytic efficiency, activity, carrier effect, loading stability, cycle life, cost, and price must be considered when selecting carrier materials for loaded photocatalysts. Based on published studies, carrier materials can be broadly classified into natural mineral carriers [41,42,43], carbon-based carriers [44,45,46], C₃N₄ carrier materials [47,48,49], and other types, including metal oxides [50,51,52], metal sulfides [53,54,55], metal frame organic compounds [56,57,58], and organic polymers [59,60,61]. Numerous studies have demonstrated the crucial role of carrier materials in eco-environmental and energy conversion applications such as degrading organic pollutants in water, photocatalytic hydrogen production, photocatalytic reduction, and the conversion of CO₂ [62,63,64], as well as nitrogen fixation and antimicrobial activity [65,66,67,68].

2.1. Natural Mineral Carriers

Natural minerals, widely distributed in nature, often possess excellent physicochemical properties and are commonly used as catalyst carrier materials. The natural mineral carriers used for synthesizing loaded photocatalytic materials are primarily layered inorganic silicate clay minerals, classified into kaolinite, monazite, vermiculite, hydromica, and fibrous barite based on the layer type, family, and subfamily [69]. These layered clay minerals, composed of aluminum–oxygen octahedral sheets and silica–oxygen tetrahedral sheets connected by shared oxygen, exhibit unique physical and chemical properties such as diverse pore structures, large specific surface areas, hydroxyl-rich inner and outer surfaces, high cation exchange capacities, and negative charges. These properties enable strong adsorption of heavy metals, organic pollutants, and gaseous molecules in water [70,71]. Selecting suitable natural mineral carriers and modifying them through washing, acidification, column support, and calcification can enhance the performance of loaded composite semiconductor photocatalysts, which are widely studied and applied in the photocatalytic degradation of pollutants and wastewater, as well as energy conversion [72,73]. The high-value natural mineral carriers include halloysite nanotubes (HNTs) [74], attapulgite (ATP) [75], montmorillonite (MMT) [76], and hydrotalcite (HT/LDHs) [77].

2.1.1. Halloysite Nanotubes

Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate minerals with a layered structure, primarily found in the form of nanotubes. Due to differences in geological environments and crystallization conditions, HNTs can form flakes, tubes, and spherical structures. The tubular structure of HNTs consists of aluminum–oxygen octahedra and silica–oxygen tetrahedra, forming the inner and outer layers of the nanotubes [78]. HNTs are hydrated, with a single water molecule present between the layers, resulting in a layer spacing of approximately 10 Å (1 nm). The dimensions and morphologies of HNTs vary, with their inner diameters ranging from 10 to 100 nm, outer diameters from 30 to 190 nm, and lengths from 0.02 to 30 μm. The inner and outer walls of the nanotubes are exposed to Si-O-Si, Al-OH, and Si-OH groups [79].

In addition to their abundance, low cost, and environmental compatibility, HNTs possess a unique mesoporous nanotubular structure, a large specific surface area, and rich surface groups [80,81]. The opposite charges on the inner and outer surfaces enhance their adsorption efficiency, making HNTs suitable for various applications. HNTs have been extensively utilized in the treatment of water pollution, exhibiting remarkable adsorption capabilities for multiple contaminants, including antibiotic wastewater (e.g., hygromycin, tetracycline hydrochloride, and ciprofloxacin), organic dyes (e.g., methylene blue, rhodamine B, and gentian violet), heavy metal ions (e.g., chromium and lead), ammonium ions, and radioactive elements (e.g., uranium). HNTs can reach adsorption equilibrium quickly, making them a cost-effective adsorbent and carrier material with significant potential [82,83]. Modifying HNTs through physical or chemical methods can further enhance their adsorption performance and facilitate the construction of composite photocatalyst systems. Additionally, the natural hollow nanostructure of HNTs is beneficial for gas molecule adsorption and storage, making them suitable for photocatalytic hydrogen production, hydrogen storage, and CO₂ capture and reduction [84,85].

Lin et al. [86] successfully prepared Cd_0.5Zn_0.5S@HNT composites using HNTs as a matrix. They first grafted the hole-trapping agent EDTA onto the surfaces of HNTs and then loaded Cd²⁺/Zn²⁺ and S²⁻ to form Cd_0.5Zn_0.5S nanoparticles through electrostatic assembly. The hydrogen production rate of the EDTA-grafted nanotube composites was significantly higher than those of composites modified with NaCl and CTAB. Under visible light (λ > 420 nm) irradiation, the optimal hydrogen production rate of Cd_0.5Zn_0.5S@HNTs reached 25.67 mmol g⁻¹ h⁻¹, 7.03 times higher than that of pure Cd_0.5Zn_0.5S. The quantum efficiency (AQY) at λ = 420 ± 10 nm was 32.29%. A characterization analysis showed that the good dispersibility of Cd_0.5Zn_0.5S@HNTs and efficient charge separation due to charge-directed migration contributed to the enhanced photocatalytic water splitting performance. Furthermore, the natural hollow nanostructure of HNTs serves as an excellent hydrogen storage container, with the composites capable of storing up to 0.042% hydrogen at ambient temperature (25 °C) and atmospheric pressure (2.65 MPa), achieving integrated hydrogen production and storage.

2.1.2. Attapulgite

Attapulgite (ATP), also known as palygorskite, is a hydrated magnesium–aluminum silicate with a unique fibrous crystal structure. ATP exhibits excellent adsorption, catalysis, and physicochemical stabilization properties [87,88,89]. Its internal and external surfaces contain Si-OH groups, and the substitution of Si⁴⁺ by Al³⁺ results in residual negative charges, enabling the effective adsorption of heavy metal ions, radionuclides, and gaseous substances such as CO₂ [90,91]. As a carrier material for composite photocatalysts, ATP can be functionally activated or modified to impart unique properties to composite materials, expanding their applications in photocatalysis [92,93]. Ma’s team [94] utilized ATP’s characteristics as a high-quality mineral carrier to uniformly deposit Ag₃PO₄ nanoparticles on the ATP’s surface. The ATP-loaded Ag₃PO₄ nanoparticles effectively catalyzed the removal of Orange II from water under sunlight, reducing the Ag content in the composite photocatalyst from 77.3 wt% in pure Ag₃PO₄ to 18.8 wt%, significantly lowering the industrial costs while enhancing the photocatalytic degradation activity and stability. ATP’s excellent CO₂ adsorption and trapping abilities have also been exploited in CO₂ catalytic conversion. Guo et al. [95] introduced ATP as a carrier in CO₂ immobilization reactions, designing and preparing Ag/ATP nanocomposite photocatalysts. These catalysts demonstrated high catalytic activity in the synthesis of asymmetric carbonates, with TOF values of up to 132 h⁻¹. The ATP-loaded composite photocatalysts maintained their activity over 10 consecutive reactions, showcasing their potential for industrial applications. This study not only provides an efficient and stable catalyst for carbon dioxide fixation but it highlights the role of ATP as an excellent mineral carrier and provides new ideas for the development of related multiphase catalysts.

2.1.3. Montmorillonite

Montmorillonite is an excellent photocatalyst carrier due to its regular pore structure, large specific surface area, and superior adsorption properties [96,97,98]. Montmorillonite-supported composite photocatalysts, such as TiO₂/montmorillonite and g-C₃N₄/montmorillonite, combine the adsorption performance of the carrier with the photocatalytic performance of the semiconductor, resulting in higher photodegradation rates and broader application prospects. For example, TiO₂/montmorillonite composite photocatalysts achieve high degradation rates for organic pollutants such as methyl orange under visible light [99]. Similarly, g-C₃N₄/montmorillonite composites exhibit excellent visible light absorption and high antibacterial performance. Montmorillonite-supported composite photocatalysts represent a new type of photocatalytic material with significant potential for environmental treatment and energy conversion. For example, Liu et al. [100] achieved 90% degradation of fluoroquinolone antibiotics by stripping montmorillonite-supported S-scheme BiOBr/Bi₂MO₆ heterojunctions.

2.1.4. Hydrotalcite

Hydrotalcite (LDHs), with its layered structure, provides a large specific surface area, facilitating the uniform dispersion and efficient utilization of photocatalysts. The interlayer ions in hydrotalcite can be exchanged, allowing photocatalytic active substances to be inserted into the interlayer, forming an intercalation structure that enhances the photocatalytic performance. Hydrotalcite carrier composite photocatalysts combine the layered structure and adsorption properties of hydrotalcite with the efficient photocatalytic performance of the photocatalyst, resulting in excellent photocatalytic activity. The common preparation methods for hydrotalcite composite photocatalysts include coprecipitation and hydrothermal methods, which regulate the structure and properties of the composite materials to achieve optimal photocatalytic effects. Hydrotalcite carrier composite photocatalysts have broad application prospects in the photocatalytic degradation of organic pollutants, water splitting for hydrogen production, and photocatalytic CO₂ reduction. Their high photocatalytic efficiency and stable chemical properties make them promising materials for environmental governance and energy conversion [101]. Zheng et al. [102] thermally designed ZnCdS/NiAl hydrotalcite S-type heterojunctions with highly efficient photocatalytic hydrogen precipitation activity levels, resulting in a several-fold improvement in hydrogen precipitation performance, about 7 times that of ZnCdS and 130 times that of NiAl LDH.

2.2. Carbon-Based Carriers

Carbonaceous materials have received wide attention in recent years due to their diversified structure, strong tunability, moderate price, good stability, and excellent conductivity characteristics, which have made carbon-based carriers a hot spot of nanocarrier research both at home and abroad, and they have played a significant role in various fields such as catalysis, energy storage and conversion, and biomedicine [103,104,105]. The use of carbon-based heteroatom coordination has stabilized transition metal single-atom catalysts, which have garnered significant attention in recent years. These catalysts present promising applications in the field of electrocatalysis related to energy conversion, owing to the excellent electrical conductivity of carbon-based carriers, the optimization of single-atom utilization rates, and the unique interactions between metal atoms and carrier materials. Carbon-based carriers have adjustable types and forms of heteroatom doping, which makes it possible to achieve the ideal design of highly active single-atom electrocatalysts by modulating the local atom distribution and coordination environment on the carbon-based carriers. Wu and Mei’s group [106] jointly published a review to classify and summarize the research work on the modulation of the electronic structures of atomically dispersed transition metal sites on carbon-based carriers, introduce the modulation mechanisms involved, and electrocatalyze the oxygen reduction reaction (ORR), hydrogen precipitation reaction (HER), oxygen precipitation reaction (OER), and carbon dioxide reduction reaction (CO₂RR) based on carbon-based monatomic catalysts and the nitrogen reduction reaction (NRR), further describing the related research work on catalytic performance enhancement through local coordination environment modulation. The authors concluded that the development of a more intuitive structure–activity descriptor, as well as the full use of synergistic effects between atomically dispersed polymetallic sites, would help to rationalize the design and further improve the activity of such single-atom catalysts. Meanwhile, the stability of transition metal monatomic systems based on carbon-based carriers is another major challenge for the practical application of such materials, and the search for a balance between the high degree of graphitization of the carbonaceous carriers, the dense distribution of the metal sites, and the optimization of the coordinated atomic environment of the metal monatomic requires further exploration and the refinement of previous research efforts.

Based on the outstanding physicochemical properties of carbon-based materials, numerous carbons have been developed and designed as excellent carrier materials for various applications in photocatalysis research. The preferred carbon-based carrier materials are mainly oxidized and reduced graphene oxide (GO and rGO, respectively), single- and multi-walled carbon nanotubes (SWNTs and MWNTs, respectively), and graphyne developed from fullerene derivation, as well as carbon nanofibers, carbon nanospheres, and other geometrically shaped carbon-based nanocages ranging from 0D randomly filled nanocages (or hollow spheres) to 3D assemblies, which have opened up several new branches of research on carbon-based nanocarrier materials [107]. In addition, biomass carbon, chitosan, activated carbon, carbon black, dicyandiamide, and melamine, among others, can also be prepared as a class of highly promising carbon-based carriers for efficient CRR and HER catalysts [108].

2.2.1. Graphene Carrier Materials

As a single-layer two-dimensional crystalline material with a unique honeycomb structure formed by Sp2 hybridized C atoms, graphene is fully utilized to enhance and improve the weak visible-light responsiveness and high photogenerated electron–hole complexation rates of semiconductor photocatalytic materials due to a series of properties, such as its large specific surface area, excellent electrical conductivity, the half-integer Hall effect, the unique quantum-limited-domain effect, and the bipolar electric field effect [109]. By compounding it with graphene, the forbidden bandwidth can be effectively regulated, and graphene-based carrier materials can not only be used as the carrier and behavioral receptor of semiconductors, precious metals, and other photoexcitation elements but also as highly efficient carriers of the electron transport network, which further creates favorable conditions for the transfer of electrons. Graphene can also be used as a photocatalyst or a co-catalyst to participate in the catalytic reaction, which can help to solve the problems of photocatalytic degradation, cracking, water hydrogen production, and reductive conversion. This provides a feasible way to solve the bottlenecks in photocatalytic degradation, hydrogen production from water, and reductive conversion. Therefore, the development and study of loaded graphene and graphene oxide-modified nanocomposites are of great value in the field of photocatalysis. Zhang’s team at Tianjin Polytechnic University [110] reported a concise and effective synthesis strategy, using GO as a 2D template, firstly doping Co²⁺ ions in GO, then grafting and stabilizing the metal coordination layer of CoMOF on the 2D GO template and preparing Co-MOL with H₂L ligands (Co-MOL@GOs) using the in situ growth method, successfully constructing Co-MOL@GO with three metal coordination layers, while ultrathin 2D MOLs were also successfully constructed. As shown in Figure 1 for the related work and characterization results, it can be seen from the TEM images that small- and medium-sized nanosheets (15–20 nm) of Co-MOL@GO are uniformly distributed on the GO template, and the synergistic interaction between the ultrathin MOLs and the GO electron conductor greatly enhances the intrinsic photocatalytic CO₂ reduction activity, whereas reduced GO can be used as a 2D template to load and stabilize MOLs of uniform thickness (~1.5 nm). In addition, GO can act as an effective electron mediator to bridge the gap between multiphase catalysts and homogeneous antenna molecules, which greatly contributes to the high activity. Thanks to these advantages, Co-MOL@GO exhibits excellent photocatalytic activity and high selectivity in visible-light-driven CO₂ to CO conversion, with an all-time high 12 h total CO yield of 3133 mmolg⁻¹mol⁻¹ among all state-of-the-art MOF and MOL catalysts. This work opens a new path for the low-cost preparation of ultrathin MOLs with excellent performance and demonstrates the key role of carbon-based carrier graphene as an electron mediator in significantly promoting various aspects of photocatalytic performance.

2.2.2. Carbon Nanotubes

The discovery of fullerenes, carbon nanotubes (CNTs), and graphene materials has strongly stimulated active research across multiple disciplines in the fields of nanotechnology, electronics, materials science and engineering, energy, and the environment in past decades, and these low-dimensional carbon nanostructured materials have aroused widespread interest among researchers and have shown great potential for a multitude of applications. Among them, Iijima [111] first reported the properties of carbon nanotubes in detail in 1991. Microscopically, CNTs are helical microtubules with one-dimensional (1D) graphitic carbon, with diameters down to a few nanometers. CNTs can be viewed as single-walled or multiple concentric layers of seamless multi-walled cylindrical tubes formed by convolutions of a honeycomb network of carbon hexagons. The presence of strong covalent C-C bonds along the tube axes gives CNTs ultrahigh stiffness (up to 1 TPa) and tensile strength (theoretically close to 100 GPa), whereas the presence of sp2 hybridization endows CNTs with impressive electrochemical properties (metal or semiconductor electronic behavior depending on the tube diameter and helicity). This unique combination of outstanding thermodynamic, electrical, and mechanical properties, along with a high degree of structural flexibility, distinguishes carbon nanotubes (CNTs) from most traditional bulk or micro- or nanomaterials composed of metals, alloys, polymers, ceramics, or other compounds. More typically, carbon nanotubes are used as ballistic transport quantum wires [112], the finest and toughest fatigue-resistant strings [113,114], and the strongest fiber materials ever made [115], as examples.

Depending on the number of layers of carbon atoms, carbon nanotubes can be classified as single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT), consisting of multilayered graphene sheets, which can be tailored to their specific properties for their application or use. Unlike graphene and many 2D materials that can be fabricated via mechanical or liquid exfoliation, carbon nanotubes are typically synthesized by using high-temperature chemical vapor deposition (CVD) processes to synthesize hydrocarbon and metal catalyst precursors, where problems such as excessive tube curvature are overcome by precipitating carbon from the catalyst nanoparticles to generate the associated strain energy. This has important implications for the macroscopic bottom-up construction of carbon nanotube functional materials (CNTFMs) and the pursuit of practical applications. In a sense, high-temperature environments (700–1100 °C) induce the formation of high-quality single-, double-, or multi-walled carbon nanotubes capable of maintaining excellent mechanical, electrical, and other properties, as well as structural stability in carbon nanotubes. In addition, due to van der Waals interactions between the intertwined high specific surface area carbon nanotubes, the preparatively grown carbon nanotubes are strongly orientated for 1D, 2D, or 3D self-assembly with well-aligned configurations and robust built-in interconnected structures.

Carbon nanotube fibers exhibit excellent thermal, mechanical, and electrical properties. In terms of conductivity, doping can broaden the intertube electron jump channels, potentially exceeding the conductivity limits of metal conductors, making them advantageous for lightweight wires. SWCNTs can grow up to several centimeters in length, exhibiting metallic or semiconducting properties, which have led to significant breakthroughs in integrated circuits and computer development. Moreover, the thermal conductivity of CNT–metal composites can greatly enhance the current-carrying capacity of conductors, potentially replacing traditional metal conductors in future ultrahigh-current applications. The unique assembly properties of CNTs also make thermal radiation on their surfaces particularly significant, which can be optimized by improving the phonon transport between nanotubes to regulate both the apparent and actual thermal conductivity. The electronic properties of SWCNTs, for example, depend on the chiral index (i.e., the way SWCNTs curl along their axes) formed during growth. Controlling the intrinsic factors of SWCNT selective growth contributes to the reproducibility and stability of chiral SWCNT preparation and scale-up applications. Figure 2 shows the work of Christophe Bichara’s team [116] at the University of Aix-Marseille, France, who constructed a thermodynamic model linking the energy and temperature of the tube–catalyst interface to the chirality of carbon nanotubes. This model reveals that the chirality of SWCNTs originates from entropy-driven processes at the nanoscale boundaries, providing theoretical guidance for the selective growth of chiral SWCNTs.

In terms of mechanical properties, significant enhancements in fiber fracture strength and elastic modulus can be achieved through solvent densification, mechanical densification, stepwise drafting, the introduction of polymer network structures within the fibers, and induced covalent linkages between tubes. The rich interfacial structure within the fibers brings about diverse energy dissipation processes, giving carbon nanotube fibers (as well as thin films and composites) dynamic mechanical properties such as damping and creep, which are not found in traditional carbon fibers. This results in a bifunctional combination of rigidity and flexibility. Additionally, the yarn structure and unique flexibility of CNT fibers show unique advantages in areas such as rotary drives and bioelectrodes. Developing composite structural systems that fully utilize the unique properties of carbon nanotube bundle fiber materials is an important challenge for future carbon nanotube engineering.

Carbon nanotube fiber functional materials are an important research area for translating nanoscience and nanotechnology into practical applications, with a wide range of potential impacts in science, technology, and engineering. CNTs with entangled networks, low redox potentials, and high specific surface areas are ideal hosts and carriers for semiconducting composite photocatalyst materials, providing sufficient space for the dispersion and effective migration of photogenerated carriers during photo-redox processes. In particular, semiconducting single-walled carbon nanotubes (s-SWCNTs) with diameters of about 1.0–1.5 nm have band gaps comparable to silicon, and their inner surfaces have suitable curvature and large contact areas, making them attractive for host–guest chemical reactions triggered by electron transfer. This makes them ideal materials for photoelectrocatalytic applications. Therefore, the research is focused on deriving and composing macroscopic composite or layered materials with the different characteristics of carbon nanotube fibers, combined with their compatible structures, properties, and prospective application areas, such as mechanically robust conductive skeletons, high-performance flexible electrodes, advanced energy conversion and storage systems, and environmental applications. The development of high-performance, low-cost carbon-nanotube-based composite carrier catalyst materials with multifunctional catalytic properties will promote the future development of this field. Chen et al. [117] first constructed a novel bifunctional hybrid device comprising CoSnS@CNTs, using carbon nanotubes as a carrier through a simple, surfactant-free one-pot hydrothermal method. As shown in Figure 3, CoSnS@CNTs are composed of ultrathin CoSnS uniformly and tightly immobilized on a highly conductive CNT backbone with a unique hybridized three-dimensional (3D) porous nanostructure, which not only provides abundant catalytic sites for ion diffusion but also accelerates electron mobility. With these advantages, CoSnS@CNTs have become bifunctional catalysts with enhanced electrocatalytic and photocatalytic properties, providing strong oxygen evolution reaction (OER) performance and excellent stability at a low overpotential of 330 mV and a current density of 10 mA/cm², and a photocatalytic degradation rate for rhodamine B dye of up to 91.72%, which is twice that of pure CoSnS. The introduction of carbon nanotube carriers effectively enhanced the photoelectrocatalytic activity of the non-precious metal-based bifunctional catalysts.

2.2.3. Activated Carbon

Activated carbon is widely used in water purification processes due to its low cost, large specific surface area, and numerous micropores. The raw materials for activated carbon can be any carbon-rich organic matter, including coal, asphalt, ore, wood, and fruit shells (such as coconut shells, walnut shells, and apricot shells). These characteristics make activated carbon versatile. Rice-husk-based activated carbon exhibits the capability to adsorb various dyes, including methyl orange, methylene blue, and rhodamine B, as well as heavy metal ions such as chromium (Cr), cadmium (Cd), iron (Fe), and manganese (Mn). Additionally, it has applications in supercapacitors, dye-sensitized batteries, and hydrogen storage systems. Activated carbon from Chinese parasol leaves can adsorb antibiotics, phenols, and other pharmaceuticals, while coconut shell activated carbon (AC5) shows excellent adsorption and desorption properties for carbon dioxide.

2.3. C₃N₄ Carrier Material

2.3.1. One-Dimensional Carbon Nitride

One-dimensional (1D) carbon nitride photocatalysts, including nanorods, nanotubes, nanowires, and nanoribbons, have attracted much attention because of their excellent optical and chemical properties and ability for enhance photocatalytic activity by tuning the diameter, length, and aspect ratio. Li et al. [118] synthesized g-C₃N₄ nanorods with an average diameter of 260 nm through the thermal polymerization of nitrile in the nanochannel of an anodized alumina film template. In addition, Li’s team [119] used SBA-15 nanorods as a hard template to fabricate mesoporous g-C₃N₄ nanorods with a length of about 650 nm and a diameter of 100 nm. Zheng et al. [120] synthesized helical g-C₃N₄ nanorods using mesoporous silica as a template.

2.3.2. Two-Dimensional Sheet Carbon Nitride

Bulk-phase g-C₃N₄ is formed by the aggregation of sheet g-C₃N₄, which reduces the specific surface area and carrier migration rate. However, sheet g-C₃N₄ has unique photocatalytic properties, such as a larger surface area, more active sites, longer charge carrier lifetime, and higher photoexcited electron reduction potential. Additionally, flaky g-C₃N₄ can effectively disperse particles, making it a subject of wide attention. Zhang et al. [121] synthesized ultrathin g-C₃N₄ nanosheets using a simple liquid-phase stripping method. This experiment demonstrated that a single g-C₃N₄ nanosheet can remain stably suspended for up to a week under both acidic and alkaline conditions. Lin et al. [122] studied a mixed solvent’s effect on the stripping of bulk-phase g-C₃N₄, obtaining ultrathin g-C₃N₄ nanosheets. Liu et al. [123] treated the precursor dicyandiamide with ammonium chloride, drying the aqueous solution and calcining the mixture at 823 K for 4 h to obtain sheet g-C₃N₄. These methods provide an important basis for the preparation of g-C₃N₄ nanosheets with high specific surface areas, which are widely used.

Graphite-like semiconductor materials with two-dimensional structures present a new opportunity to construct carrier composite photocatalysts, fully utilizing their carrier effects. Two-dimensional layered graphitic carbon nitride (g-C₃N₄) is a high-quality non-metallic semiconductor material that has shown excellent performance in catalytic chemistry, materials chemistry, and photoelectron chemistry [124,125]. Compared to bulk-phase materials and other general carrier materials, the two-dimensional structure of g-C₃N₄ offers advantages such as smaller thickness and larger specific surface area values, providing numerous surface atoms as catalytic active sites to improve catalytic processes and for enhanced catalytic activity. By further reducing the thickness of 2D g-C₃N₄, it is easier to prepare ultrathin 2D g-C₃N₄ materials for surface modification, which can enrich the structure of the carrier catalyst model and optimize the catalytic performance. Ultrathin layered g-C₃N₄, as a representative two-dimensional structure of graphitic-phase carbon and nitrogen materials, has a higher specific surface area compared to bulk-phase g-C₃N₄ due to its low nano-size thickness, two-dimensional anisotropy, and quantum confinement effect, effectively improving the electron mobility efficiency and prolonging the lifetime of photo-excited carriers. The design of composite catalyst systems supported by ultrathin two-dimensional g-C₃N₄ exposes a significant number of coordination-unsaturated surface atoms, thereby enriching the active sites for catalytic reactions. This enhancement makes the material widely applicable in various fields, including photocatalysis, electrocatalysis, photoelectrocatalysis, organocatalysis, and other catalytic applications. Therefore, it is suitable for g-C₃N₄ to act as a carrier and composite with other materials to further enhance its carrier effect, significantly improving the photocatalytic efficiency and increasing the conversion and reduction efficiency of CO₂ [126]. Various types of two-dimensional g-C₃N₄ materials have been successfully prepared via stripping, liquid-phase synthesis, and chemical oxidation. However, pristine g-C₃N₄ still faces challenges due to its small specific surface area, poor long-wavelength light absorption, easy recombination of photogenerated electron–hole pairs, and low quantum efficiency, which limit its photocatalytic performance and application. Utilizing doping modification, the photoelectric properties and band structure of catalytic materials can be optimized, and the chemical composition of catalysts can be adjusted to improve the photocatalytic performance, making it one of the most commonly used modification methods for developing high-efficiency two-dimensional composite g-C₃N₄ photocatalysts. Wang’s group developed a series of metal-doped g-C₃N₄ catalysts, which exhibited photocatalytic performance several times greater than that of pure g-C₃N₄ in the degradation of dye wastewater, such as rhodamine B, and in the decomposition of water for hydrogen production [127]. Ion doping changes the electronic structure of g-C₃N₄ semiconductors, adjusting the semiconductor valence band to a more suitable potential energy position in the reaction direction, enhancing the electronic band structure and redox ability of the catalyst and improving the visible-light absorption and separation of photogenerated electron–hole pairs, resulting in a significant increase in photocatalytic activity [128,129].

However, from the perspective of carrier function, ordinary 2D g-C₃N₄ materials have a relatively single-layer structure, and their interfacial behavior is relatively weak when compounded with other semiconductors or metal or non-metal materials compared to traditional 2D carrier materials. This hinders the research application of 2D ultrathin carbon nitride materials in the field of composite photocatalysts. Therefore, it is necessary to design and develop novel ultrathin 2D g-C₃N₄ materials to fully utilize their 2D structural advantages through surface modification, regulate the electronic structure more precisely, optimize the catalytic performance, and expand the types of composite 2D g-C₃N₄ photocatalytic materials. Among the numerous modification strategies, point defect engineering (i.e., tunable vacancy and dopant introduction) can exploit the excellent structural, optical, and electronic properties of g-C₃N₄ to improve the photocatalytic activity. While hetero-elements are doped, structural defects are inevitably introduced. For example, Zhang et al. [130] found defect energy levels in the energy band structure of P-doped g-C₃N₄ catalysts; some researchers found that O doping of g-C₃N₄ was accompanied by the introduction of new defect energy levels [131], and these defect energy levels play a crucial role in improving the photocatalytic activity. Liu et al. [132] prepared nitrogen-deficient g-C₃N₄, where the introduction of nitrogen defects formed defect energy levels below the conduction band, narrowing the semiconductor’s forbidden bandwidth, effectively broadening the light absorption range of g-C₃N₄, and improving the visible photocatalytic activity for hydrogen production. Surface defect studies provide new research ideas for developing novel loaded ultrathin two-dimensional g-C₃N₄ composite photocatalysts and a solid theoretical basis for further enhancing the catalytic activity of defective two-dimensional g-C₃N₄ materials (Figure 4).

Given the booming development in this field, it is timely to review and study the recent advances in g-C₃N₄ point defect engineering, particularly the rational utilization of two-dimensional g-C₃N₄, an ideal artificial photosynthesis non-metallic photocatalyst, for photocatalytic energy storage and conversion. To address the inherent drawbacks of g-C₃N₄, Yu et al. [133] detailed the role, synthesis, characterization, and systematic control of point defects, as well as the wide range of applications of defective g-C₃N₄-based nanomaterials for photocatalytic water decomposition, carbon dioxide reduction, and nitrogen fixation. A systematic review of g-C₃N₄ point defect engineering is presented, including the central role of point defects in g-C₃N₄, synthesis strategies, characterization, and the application of defective g-C₃N₄-based nanomaterials in many photo-redox energy processes. It is evident from the large number of research activities that the introduction of point defects, either as cavities or dopants, plays a powerful role in aiding the surface modification and optical and electrical properties of g-C₃N₄, thereby improving its photocatalytic performance in water decomposition, carbon dioxide reduction, and nitrogen fixation. Overall, g-C₃N₄ nanomaterials undergo remarkable changes upon the introduction of point defects, such as tunable band gaps, defect-induced mid-gap states, increased surface areas, the inhibition of photogenerated electron–hole recombination, and the improved adsorption and activation of reactant molecules. The work on point defect engineering in g-C₃N₄ is still underdeveloped, and there is a lack of reliable methods to accurately and uniformly engineer defects. Notably, a common synthetic problem with g-C₃N₄ is its tendency to aggregate, which requires a layering process to achieve the uniform introduction of defects. Additionally, the synthesis methods should be low-cost and scalable to allow the controlled large-scale production of defective catalysts, which remains a significant challenge. Another obstacle is the limitations of the existing material characterization methods in accurately identifying and quantifying g-C₃N₄ defects. Revealing the stability of defects could play an important role in elucidating the effective mechanism of defects on catalytic activity. The relationship between defects and the effects of different morphologies of g-C₃N₄ on photocatalytic activity require further exploration, both experimentally and computationally, in terms of their analysis. Further studies of g-C₃N₄ defects could be focused on the design of defect-integrated composite structures. In addition, the combination of photodriving with other electrochemical or biocatalytic reactions is an innovative and advanced concept (Figure 5).

Photocatalytic total water splitting can be achieved using Z-type heterojunction catalysts by connecting different semiconductors in series to mimic natural photosynthesis. However, it is still challenging to prepare catalyst materials that catalyze the production of both H₂ and O₂ efficiently. Shen et al. [134] addressed this issue by designing boron-doped, nitrogen-deficient two-dimensional (2D) C₃N₄ nanosheets based on boron-doped, nitrogen-deficient C₃N₄ nanosheets for the photocatalytic total water splitting study. As shown in Figure 6, the authors prepared ultrathin carbon nitride nanosheets with varying degrees of boron doping and nitrogen defects, allowing them to act as photocatalysts for H₂ or O₂ generation. Using an electrostatic self-assembly strategy, the nanosheets were coupled to obtain 2D/2D polymer heterostructures. Due to their ultrathin nanostructures, strong interfacial interactions, and staggered energy band arrangement, an efficient Z-type heterojunction pathway for charge–carrier separation and transfer can be achieved. In the presence of Pt and Co(OH)₂ co-catalysts, the resulting heterostructures achieve stoichiometric H₂ and O₂ release. The hydrogen production efficiency reached 1.16% under solar illumination.

2.3.3. Three-Dimensional Carbon Nitride

The development of ordered and macro-sized 3D g-C₃N₄ components is of great significance for the practical application of photocatalysis. Liang et al. [135] demonstrated a template method by using a melamine sponge as a support frame, filling the sponge voids with urea, and then obtaining a 3D porous g-C₃N₄ material through thermal polymerization. Wang et al. [136] reported the synthesis of porous g-C₃N₄ using calcium carbonate (CaCO₃) particles as a hard template, providing a green pathway instead of using toxic and expensive fluorine-containing etchers. Additionally, Wang et al. [137] synthesized mesoporous g-C₃N₄ through the thermal polymerization of dicyandiamide with various soft templates, such as non-ionic surfactants, amphiphilic block polymers, and ionic surfactants.

2.4. Other Carrier Materials

In addition, there are many other common inorganic semiconductor carriers with photocatalytic activity such as metal oxides, metal sulfides, and metal frame organic compounds, while semiconductor photocatalyst carriers also include alumina, silica gel, ceramics, and polymers. Alumina is one of the most widely used catalyst carriers. Alumina, with its large specific surface area. has a developed pore structure, which can make the active components of the supported catalyst highly dispersed into particles. Silica gel is a porous material and is one of the commonly used catalyst carriers. Ceramic is also a kind of porous material, which shows good adhesion to photocatalyst particles, good acid and alkali resistance, and high temperature resistance, and can also be used as a catalyst carrier. Polymers, such as saturated carbon chain polymer and fluoropolymer, have strong antioxidant capacity rates, so they can also be used in the study of supported photocatalysts. However, due to the strong oxidation of ·OH⁻ and ·O₂⁻, these polymer carriers can only be used in the short term. Glass carriers such as glass sheets, glass fiber nets (cloth), and hollow glass beads, due to their strong chemical stability and good light transmission, can be processed into various shapes of reactors according to one’s needs, are also widely used as carriers of semiconductor photocatalysts.

When investigating how natural mineral carrier materials (e.g., hydrotalcite, bumpy clay, montmorillonite), carbon-based materials (e.g., carbon nanotubes, graphene), and carbon-nitride materials can specifically enhance the photocatalytic activity at the electronic level, the charge transfer kinetics, modulation of the energy–band structure, and interfacial effects have to be considered together. These materials work synergistically through different mechanisms to enhance the overall performance; natural mineral materials optimize the charge separation and transport pathways by modulating the metal composition of the laminate or via intercalation modification (e.g., hydrotalcite improves the charge separation efficiency by adjusting the Mg/Al ratio, while bumpy clay and montmorillonite improve the charge transfer capability via surface modification or as composites with other materials). Carbon-based materials significantly enhance the charge separation and transfer efficiency by virtue of their excellent electrical conductivity and tunable energy band structure. Graphene and carbon nanotubes further optimize the charge separation through doping or functionalization and form heterojunctions with semiconductor materials to promote charge transfer at the interface. Carbon nitride materials (g-C₃N₄) modulate the energy band structure by introducing defect states or compositing with other materials to extend their light absorption range and improve their charge separation efficiency. In addition, the interfacial interactions between these materials form an effective heterojunction, which not only improves the charge separation efficiency but also reduces the chance of electron–hole complexation. Taken together, the overall performance of the photocatalysts can be significantly enhanced by rationally designing the material compositions, structures, and interfacial interactions, thereby causing efficient and stable photocatalytic reactions. This multidimensional optimization strategy provides a solid theoretical foundation and technical guidance for the development of high-performance photocatalysts (

Table 1. Key parameters of different photocatalysts.

Material	Efficiency (Relative)	Light Absorption Range (nm)	Charge Separation Capability	Practical Application
Hydrotalcite	Moderate	UV	Ordinary	Environmental purification
Attapulgite	Slightly below	UV	Ordinary	Absorbent
Montmorillonite	Slightly below	UV	Ordinary	Absorbent
Carbon nanotubes	High	visible area	Large	Composite photocatalysts
Graphene carrier materials	High	Visible region and near infrared	Large	Composite photocatalysts
g-C3N4	Medium–high	visible area	Slightly higher	Environmental purification

).

The following table shows the photocatalytic efficiency of some different carrier materials. For natural mineral materials, when used as photocatalysts alone, they often face problems such as low utilization rates of visible light and easy compounding of the electron–hole pairs. They usually have to be combined with other semiconductors, and the photocatalytic efficiency is low. Carbon-based materials such as graphene and carbon nanotubes themselves have excellent conductive properties and are easy to modify for various operations. They have excellent mechanical properties and their charge separation ability is excellent, and often carbon-based materials will show good photocatalytic efficiency. Regardless of the dimensionality of C₃N₄ materials, they show unique advantages in the field of photocatalysis. One-dimensional materials excel in directional transport, two-dimensional materials possess excellent optical and electrical properties, and three-dimensional materials stand out with their porous structure and good stability. Therefore, the photocatalytic efficiency is also high.

3. Preparation of Carrier-Based Composite Photocatalytic Materials

3.1. Solvent (Water) Thermal Method

The hydrothermal method involves placing reactants in a sealed reaction vessel with an aqueous solution or other solvents as the reaction medium. By heating the reaction vessel (autoclave), the reactants are subjected to high temperature and pressure levels, which typically results in the dissolution of insoluble reactants and crystal growth. The hydrothermal method offers several advantages: (1) The high-temperature and high-pressure environment accelerates the thermodynamic reaction rate compared to room-temperature conditions [138]. (2) By coordinating reaction conditions such as the pH, reaction time, temperature, concentration, and reactant ratios, products with different structures, shapes, and particle sizes can be obtained [139]. (3) It enables the production of highly homogeneous nanocrystalline particles [140]. (4) With advancements in technology, the hydrothermal method can be combined with other techniques, such as microwave hydrothermal and microemulsion-assisted hydrothermal methods, expanding its application scope [141]. For carrier-based composite photocatalysts, the hydrothermal method not only enhances the stability of the composite but also optimizes and improves carrier materials by leveraging the high-temperature and high-pressure synthesis environment.

3.2. Precipitation Method

The precipitation method is a widely used technique for synthesizing nanomaterials. It involves adding a precipitating agent to a mixed solution to precipitate nanoparticles of different components, followed by heating, ultrasound, or other processing methods to obtain the desired product. Common precipitating agents include alkaline substances such as ammonia, urea, sodium bicarbonate, sodium carbonate, and sodium hydroxide. This method allows control over the crystalline phase, shape, and size of the resulting product by adjusting the amount of precipitating agent, the rate of precipitation, and temperature. It also includes efficient variations such as co-precipitation. The nano-precipitation method, used in biomedicine, catalysis, and electrochemistry, offers simplicity, uniform dispersion, and stable properties, making it particularly suitable for preparing loaded composites.

3.3. Sol–Gel Method

The sol–gel method involves using chemically active compounds as precursors. These precursors are uniformly mixed in a liquid phase, where hydrolysis and condensation reactions occur, forming a stable transparent sol. As the sol ages, gel particles slowly polymerize, creating a three-dimensional network structure filled with solvent. After drying, the solvent is removed, leaving a porous dry gel, which is then sintered to produce molecular or nanostructured materials. Sol–gel technology is a mature process with several advantages, including simplicity, low equipment costs, energy efficiency, and the ability to dope a wide range of materials. It allows for homogeneous mixing at the atomic and molecular levels, making it ideal for producing uniform materials and doping. This method is widely used in anti-corrosion, biomedical applications, and electronic devices. For carrier photocatalysts, the sol–gel method offers unique benefits. The precursor is dispersed in a solvent to form a low-viscosity solution, enabling molecular-level homogeneity in a short time. During gel formation, the reactants can be uniformly mixed with the carrier at the molecular level, resulting in more homogeneous loading and higher material purity. Additionally, the sol–gel process operates at lower temperatures, allowing better control over the material composition, especially for multi-component materials. It also enables control over the porosity of the carrier, enhancing the performance and stability of composite photocatalytic materials.

3.4. Electrospinning Technology

Electrospinning technology produces nanofibers with a large specific surface area, high porosity level, diverse composition, and uniform diameter distribution. These nanofibers can be easily functionalized (e.g., surface coating or modification) and have significant applications in biomedicine, environmental engineering, and textiles. For instance, electrospun nanofibers are used in optical and resistance sensors, water treatment, protective clothing, masks, catalysts, wound dressings, tissue engineering scaffolds, and drug carriers. For photocatalyst carrier materials, electrospinning is an excellent method for preparing composites. It effectively distributes nanofibers on the carrier surface, allows flexible adjustment of the nanomaterial size for different photocatalytic applications, and facilitates functional processing. By combining electrospinning with photocatalytic technology, the unique advantages of both methods can be leveraged to maximize their applications in environmental energy and other fields.

3.5. Vapor Deposition

Chemical vapor deposition (CVD) involves gaseous or vapor substances reacting in the gas phase or at the gas–solid interface to form solid deposits. Two or more gas sources are introduced into a reaction chamber, where they react to produce new materials, such as synthetic coatings or nanomaterials, which are deposited on a substrate. CVD is widely used in the semiconductor industry and includes reactions such as thermal decomposition, chemical synthesis, and chemical transport. CVD offers several advantages, including the ability to coat complex shapes and deep holes uniformly; deposit a wide range of materials (metallic, non-metallic, and multi-component alloys; ceramics; and chemical compounds); and produce films with high purity, good densification, low residual stress, and complete crystallization. By adjusting the deposition parameters, the chemical composition, morphology, crystal structure, and grain size of the coating can be controlled. CVD is also used in the preparation and optimization of high-performance graphene films and carbon nanotubes, making it valuable in designing carrier-based semiconductor photocatalytic systems.

3.6. Surface Modification

The surface modification of carrier materials can significantly enhance the catalytic efficiency of composite photocatalysts. For semiconductor carriers such as graphitic carbon nitride (g-C₃N₄), surface modification leverages the interfacial coordination effect between quantum dots and the carrier matrix. Carbon dot (CD)-modified multiphase nanocomposites offer broad spectral absorption, suitable electronic band structures, fast carrier mobility, abundant reserves, excellent chemical stability, and easy synthesis, making them promising for photocatalytic solar fuel production and pollutant decomposition. The process of Cr(VI) removal from an aqueous solution using hydrophobic Schiff base modified TiO₂ nanoparticles (HSB@TiO₂) under metal halide lamp irradiation was carried out as in the study by Mohamed et al. [142]. HSB@TiO₂ composites were synthesized, which greatly improved the degradation rate and mitigated the Cr(VI) contamination in light water sources.

3.7. Ball Milling

The mechanochemical method utilizes the strong impact, shear, and friction effects of the grinding media (usually hard spheres) on the raw materials in a high-speed rotating ball mill, which leads to particle refinement, lattice defect formation, and local temperature increases, conditions that can promote chemical reactions. Compared to traditional methods, mechanochemical methods do not require large amounts of organic solvents, are environmentally friendly, involve low energy consumption rates, and are easy to operate, making them suitable for large-scale production. However, it is sometimes difficult to precisely control the particle size distribution of the products, which may affect the consistency and stability of the photocatalytic performance (

Table 2. Advantages and disadvantages of different synthesis methods.

Synthesis Method	Advantages	Disadvantages
Solvent (water) thermal method	High crystallinity and good particle size control	Higher energy consumption
Precipitation method	Higher energy consumption	Low product purity
Sol–gel method	Uniformity of composition and variety of forms	High cost and complex drying process
Electrospinning technology	Controlled fiber morphology	Limited production
Vapor deposition	Continuous production with high purity	Process is complex
Surface modification	Flexible and long life	Flexible and long life
Ball milling	Environmentally friendly, good nano-sizing effect	Many crystal defects

).

4. Application of Carrier-Based Composite Photocatalytic Materials in Environmental Photocatalysis

4.1. Photocatalytic Degradation

The morphology and dimensions of carrier materials directly affect their specific surface area, which influences the interfacial loading behavior of composites and their contact area with reactants or pollutants. The characteristics and adsorption capacity of the carrier are crucial for the performance of composite photocatalysts in liquid environments. Whether using natural clay mineral carriers, carbon-based carriers, or biomass carriers, different carrier materials exhibit unique features in the photocatalytic degradation of organic pollutants.

A key challenge is identifying suitable carrier materials and modifying them to enhance the performance of composite photocatalysts. For example, the presence of antibiotics in water bodies has become a major environmental concern due to their toxic effects and the risk of spreading drug-resistant genes. The use of photocatalytic technology, capable of generating highly oxidative radicals, is recognized as an efficient and sustainable method for degrading such pollutants. Qu et al. [143] optimized the degradation of lincomycin using a g-C₃N₄-based, metal-free composite photocatalytic system. Through doping with oxygen and modification with carbon quantum dots (CDs) loaded on reduced graphene oxide (rGO), they achieved a 10-fold increase in degradation compared to intrinsic g-C₃N₄. The study also elucidated the catalytic mechanism, highlighting the roles of intermediate H₂O₂, rGO as an active site, and the peroxidase-like activity of CDs.

4.2. Photocatalytic Reduction of CO₂

The search for efficient and stable photocatalytic materials for CO₂ reduction has led to the development of noble metal-free molecular catalysts hybridized with carbon-based light-absorbing carriers. For example, Song et al. [144] constructed a gold-modified CN-based donor-acceptor system coupled with a dual photothermal effect for the effective photoreduction of carbon dioxide.

Although challenges remain, such as understanding the electronic coupling between the complex and the carrier, this hybrid material represents an ideal system for studying catalytic reactions, combining the stability of solid materials with the high activity and selectivity of molecular catalysts.

4.3. Photocatalytic H₂(O₂) Products

Photocatalytic water splitting is a promising method for producing renewable hydrogen. Carrier-based composite photocatalysts are widely studied for this purpose. For instance, Professor Zhu Yongfa’s team [145] constructed a g-C₃N₄/rGO/PDIP Z-scheme heterojunction with high charge transfer efficiency and photocatalytic water decomposition activity levels. The composite exhibited hydrogen and oxygen evolution rates 12.1 times higher than pure g-C₃N₄, demonstrating the potential of carrier materials for enhancing the photocatalytic performance.

4.4. Nitrogen Fixation

Ammonia (NH₃) is a vital industrial chemical but its conventional production via the Haber–Bosch process is energy-intensive and polluting. Photocatalytic nitrogen fixation offers a sustainable alternative under mild conditions. However, developing high-performance catalysts remains challenging.

Efficient photocatalytic nitrogen fixation using boron-doped graphene quantum dot photocatalysts was achieved by Liu et al. [146]. This catalyst exhibited ultrahigh catalytic activity for NH₃ synthesis, with excellent stability and selectivity. The study demonstrated the potential of GDY-based catalysts in photocatalytic nitrogen fixation, offering a new approach to sustainable ammonia production.

4.5. Photocatalytic Antibacterial Agents

Microbial contamination, including from drug-resistant strains, is a growing concern. Silver-based nanomaterials are effective antibacterial agents due to their cytotoxicity and broad-spectrum activity. For example, the bioactive carbon-modified magnesium oxide particles developed by Muhammad et al. [147] have excellent photocatalytic antimicrobial properties. Similarly, the surface plasmon resonance technique designed by Wang et al. [148] produces excellent antimicrobial properties for enhancing graphene heterojunction materials.

5. Conclusions and Prospects

This paper reviews the construction of supported photocatalysts with different types of carrier materials, systematically introducing a series of composite photocatalyst materials with different functional carrier effects and their applications in the photocatalytic degradation of organic pollutants, photoreduction of CO₂, photocatalytic production of hydrogen and oxygen, nitrogen fixation, and enhancement of antibacterial activity. However, there are still shortcomings in the construction of carrier composite photocatalytic material systems, material characterization, the application of carrier effects, and the practical application of photocatalysis. Further suggestions for constructing composite photocatalytic materials via the functional carrier effect are as follows:

(1) In terms of the coupling between carrier materials and other materials in the composite material system, the advantages of carrier materials should be further utilized for the compositing of different semiconductors or other modified materials, reducing rejection phenomena and enhancing the degree of compositing and synergistic effect between carrier materials and other materials, achieving the accurate optimization of the internal balance of the carrier migration in different types of carrier-based composite photocatalytic material systems.

(2) Most composite photocatalyst materials are multi-dimensional composite material systems, conducive to directional and accelerated charge transfer, although the synthesis process and preparation methods of materials in various dimensions, especially the combination of three-dimensional and low-dimensional materials, need further optimization. Ensuring the integrity of the dominant morphology, structure, and properties of monomer materials in the composite system is crucial for improving the performance and stability of carrier-based composite photocatalysts.

(3) Accurate monitoring for material band structure modulation and the intuitive characterization of surface defect states are relatively lacking, and the specific interface behavior of each material in the composite photocatalytic system is relatively vague. The carrier migration and transport behavior processes need further exploration.

Carrier-based photocatalysts show great potential for application in the fields of environmental purification and energy conversion but still face many challenges in their practical applications. Catalysts may be affected by chemical changes in specific environments (e.g., strong acids and bases), so we should select materials with high chemical stability. In photodegradation, the generation of by-products may cause secondary pollution, and parameters such as the temperature and pH should be optimized. As the activity of photocatalysts tends to decrease with time, we should study effective regeneration methods, such as high-temperature calcination and chemical cleaning. Small batches of high-performance photocatalysts prepared under laboratory conditions are often difficult to be scaled up directly to industrial-scale production, facing problems related to cost control and technology transfer. We could explore synthesis processes suitable for large-scale production, such as continuous flow reactor and spray drying methods, to ensure product quality. There is a need to establish a sound quality inspection and control system to monitor key indicators in real time and ensure batch-to-batch consistency.