Preparation of MnO2-Carbon Materials and Their Applications in Photocatalytic Water Treatment

Water pollution is one of the most important problems in the field of environmental protection in the whole world, and organic pollution is a critical one for wastewater pollution problems. How to solve the problem effectively has triggered a common concern in the area of environmental protection nowadays. Around this problem, scientists have carried out a lot of research; due to the advantages of high efficiency, a lack of secondary pollution, and low cost, photocatalytic technology has attracted more and more attention. In the past, MnO2 was seldom used in the field of water pollution treatment due to its easy agglomeration and low catalytic activity at low temperatures. With the development of carbon materials, it was found that the composite of carbon materials and MnO2 could overcome the above defects, and the composite had good photocatalytic performance, and the research on the photocatalytic performance of MnO2-carbon materials has gradually become a research hotspot in recent years. This review covers recent progress on MnO2-carbon materials for photocatalytic water treatment. We focus on the preparation methods of MnO2 and different kinds of carbon material composites and the application of composite materials in the removal of phenolic compounds, antibiotics, organic dyes, and heavy metal ions in water. Finally, we present our perspective on the challenges and future research directions of MnO2-carbon materials in the field of environmental applications.


Introduction
With the rapid development of modern industry and agriculture and the rapid growth of population, agricultural, industrial and domestic water use has increased tremendously [1,2]. Refractory toxic pollutants such as pesticides [3], antibiotics [4], textile dyes [5], and heavy metals [6,7] are discharged into water bodies, posing a huge threat to aquatic ecosystems and human health. Water pollution has become among the most pressing issues in the whole world [8]. Hence, green, highly efficient, and low-cost water treatment technologies are in urgent demand. Photocatalysis has been recognized as an ideal tool to eliminate recalcitrant contaminants in aqueous environments owing to its high efficiency, energy savings, low cost, environmental friendliness, lack of secondary pollution, and other characteristics [9][10][11].
Photocatalytic materials are the core of photocatalytic technology [2,[12][13][14]. In recent years, semiconductors based on metal oxides are mostly used as photocatalysts for environmental remediation, such as MnO 2 [15], TiO 2 [16], ZnO [17], Fe 2 O 3 [18], SnO 2 [19], etc. Photocatalytic reactions are initiated by absorbing light energy equal to or more than the bandgap of semiconductor photocatalysts, so the bandgap is an important parameter in defining the applicability of semiconductors in specific photocatalytic reactions [20,21]. Narrow bandgap semiconductors can improve the utilization of visible light, which is more beneficial for water purification applications [22]. Therefore, compared to wide-bandgap forms, and morphologies can be obtained by adjusting hydrothermal conditions with h reactivity, controllable conditions, and various synthesis types. In addition, the closed vironment with high temperature and high pressure can effectively enhance the close c tact between MnO2 and carbon materials, improve the transmission speed of electro and improve the photocatalytic activity of the composite materials to some extent [63, The hydrothermal method is widely used, which can prepare different dimensions types of MnO2−carbon composites [65][66][67][68][69][70][71][72][73][74][75].
For example, Chhabra et al. [43] prepared α−MnO2−RGO nanocomposite by a fa hydrothermal method with RGO reduced by chemical reduction (Figure 1a). In the na composite, the one−dimensional (1D) rod−shaped MnO2 increases the flow of electron the longitudinal direction and reduces the possibility of electron−hole pair bonding. the other hand, two−dimensional (2D) RGO nanosheets have a large surface area and p volume, which can prevent charge recombination by aiding in the quick transport of charges. With the introduction of RGO nanosheets, the surface area of the material creased to 87.159 m 2 g −1 , and the composite photocatalyst exhibited efficient adsorp photocatalytic performance. Wang et al. [76] prepared CNT−MnO2 composite film by positing MnO2 nanosheets on CNT film using the hydrothermal method. Under differ hydrothermal times, the coverage of MnO2 on CNTs films will change. The optimi composite film can be folded into different sizes and shapes, exhibiting excellent flexibi and stability. Doping metal or non−metal on carbon materials or MnO2 can incorpo the unique characteristics of different materials to improve performance [77][78][79][80][81][82][83][84][85][86]. Sha al. [87] first prepared K and Na atom doped g−C3N4 via the thermal treatment of thiou and KBr/NaBr, respectively, and then added them into KMnO4 solution for hydrother reaction to synthesize K/Na doped g−C3N4@MnO2 composite. The MnO2 nanosheets w vertically assembled on the surface of g−C3N4 with a stable structure and shortened diffusion path lengths for electrons. Metal atoms intercalated into the g−C3N4 interlay which enhanced the conductivity, served as the charge transfer channel between adjac layers to promote charge transfer and hinder the recombination of photogenerated ca ers. In addition to 2D composites, nano−sized MnO2 can be uniformly incorporated in the porous structure of 3D carbon materials via the hydrothermal method, thus improvi the photocatalytic activity of hybrid catalysts [93][94][95][96][97]. Nui et al. [98] synthesized G phene/nano α−MnO2 hybrid aerogel in an isopropanol−water system via hydroth mal−thermal reduction. The needlelike α−MnO2 nanoparticles are covalently bonded w graphene without damaging the integrity of the graphene structure and are doped in graphene aerogel uniformly. Due to the porous structure of the hybrid aerogel and high dispersibility of the MnO2 on graphene, the as−prepared composite exhibits go catalytic activity. Wan et al. [88] prepared flower−like core−shell MnO2−coated carbon a ogels via the hydrothermal method as a superior photocatalyst to remove organic dy from an aqueous solution (Figure 1b). Dong et al. [99] prepared 3D MnO2/N−doped g phene hybrid aerogel by self−assembly. MnO2 nanosheets and nanotubes were first sy thesized by a double aging method and hydrothermal method, respectively, th N−doped graphene aerogels were created via hydrothermal−freeze drying process usi ethylenediamine as the reductant and nitrogen source. The size and morphology of Mn play an important role in tailoring the structures and properties of 3D graphene aeroge The laminar structure of MnO2 nanosheets with the graphene conductive substrate is b eficial to enhancing the charge transfer, shortening the diffusion pathway of pollutan and affording more active sites. However, excessive MnO2 nanosheets on graphene mig aggregate and inactivate, which adversely affects the overall catalytic activity.
In order to further improve the properties of MnO2−carbon materials, many resear ers also add green and economical polymers, oxides, or other carbon materials to MnO2−carbon materials to prepare ternary composites, and the hydrothermal method the most common preparation scheme [100][101][102]. For example, Iqbal et al. [103] prepar In addition to 2D composites, nano-sized MnO 2 can be uniformly incorporated into the porous structure of 3D carbon materials via the hydrothermal method, thus improving the photocatalytic activity of hybrid catalysts [93][94][95][96][97]. Nui et al. [98] synthesized Graphene/nano α-MnO 2 hybrid aerogel in an isopropanol-water system via hydrothermalthermal reduction. The needlelike α-MnO 2 nanoparticles are covalently bonded with graphene without damaging the integrity of the graphene structure and are doped in the graphene aerogel uniformly. Due to the porous structure of the hybrid aerogel and the high dispersibility of the MnO 2 on graphene, the as-prepared composite exhibits good catalytic activity. Wan et al. [88] prepared flower-like core-shell MnO 2 -coated carbon aerogels via the hydrothermal method as a superior photocatalyst to remove organic dyes from an aqueous solution (Figure 1b). Dong et al. [99] prepared 3D MnO 2 /N-doped graphene hybrid aerogel by self-assembly. MnO 2 nanosheets and nanotubes were first synthesized by a double aging method and hydrothermal method, respectively, then N-doped graphene aerogels were created via hydrothermal-freeze drying process using ethylenediamine as the reductant and nitrogen source. The size and morphology of MnO 2 play an important role in tailoring the structures and properties of 3D graphene aerogels. The laminar structure of MnO 2 nanosheets with the graphene conductive substrate is beneficial to enhancing the charge transfer, shortening the diffusion pathway of pollutants, and affording more active sites. However, excessive MnO 2 nanosheets on graphene might aggregate and inactivate, which adversely affects the overall catalytic activity.
In order to further improve the properties of MnO 2 -carbon materials, many researchers also add green and economical polymers, oxides, or other carbon materials to the MnO 2carbon materials to prepare ternary composites, and the hydrothermal method is the most common preparation scheme [100][101][102]. For example, Iqbal et al. [103] prepared PANI@CNT/MnO 2 ternary composite with rough interwoven fibrous and porous structure by the combination of hydrothermal methodology and in situ oxidative polymerization of aniline. The synergistic effect of the three enhances the specific surface area, thermal and electrical conductivity, and provides channels for the transport of charge carriers, thus enhancing the performance of the material. Wang et al. [89] also utilized the α-Fe 2 O 3 core/shell configuration to modify g-C 3 N 4 , and prepared a dual Z-scheme α-Fe 2 O 3 @MnO 2 /g-C 3 N 4 ternary composite by two-step hydrothermal method (Figure 1c). The Fe 2 O 3 @MnO 2 core/shell promoter modulates the electronic structure through the dual Z-scheme heterojunction, thus improving the separation efficiency of photo-generated electron-hole pairs. Due to its narrow bandgap, the composite material has a broad absorption in the visible light region, low cost and excellent performance, which is more conducive to practical application. Xu et al. [104] selected CC as the substrate for the growth of MnO 2 , coated RGO on the surface of CC, and synthesized CC/RGO/MnO 2 composites by dipping method and hydrothermal method. CC/RGO can provide a large specific surface area as the skeleton, and the good conductivity of carbon materials can accelerate electron transfer, the resulting composite shows good photoelectrochemical activity. Li et al. [105] synthesized CNT/rGO@MnO 2 particles through a hydrothermal reaction and then obtained a sandwich-like film with a 3D multilevel porous conductive structure via vacuum filtration and freeze-drying treatment. Nano-sized pores increase the specific surface area and provide a large number of active sites. MnO 2 grows in situ on the carbon skeleton, and the two are tightly connected, which facilitates electron transportation and enhances structural stability.
Solvothermal and microwave irradiation are improved methods of hydrothermal synthesis [61]. Among them, the solvothermal method is based on the same principle as the hydrothermal method. As water-sensitive compounds cannot be synthesized by the hydrothermal method, water can be replaced with an organic solvent to carry out the reaction [106][107][108][109]. For example, Asif et al. [90] synthesized an urchin-like morphology Ni-doped MnO 2 /CNT nanocomposites by a one-step solvothermal reaction (Figure 1d). Ni doping enhances the conductivity of MnO 2 and increases the surface area and cycle stability of the composite. Microwave irradiation heating reduces energy consumption compared to hydrothermal reactions, effectively shortens the synthesis time of complexes, and improves product homogeneity [110][111][112]. For example, Sivaraj et al. [110] reported a microwaveassisted process to synthesize the hybrid CNTs-MnO 2 nanocomposite. The dispersed MnO 2 nanospheres are uniformly attached to the CNTs' side walls, and a synergistic effect increases the light absorption range, promotes charge separation, and enhances stability.

In Situ Redox Deposition
Although hydrothermal self-assembly is economical and environmentally friendly, and widely used, it requires high temperature and pressure and a long reaction time, which is not suitable for large-scale production and applications [113]. In situ redox deposition is mild, simple, and suitable for the compounding of a wide range of metal oxides with carbon materials, which is another important method for the preparation of MnO 2 -carbon composites. It uses the carbon material as a substrate and involves the in situ deposition of MnO 2 nanostructures onto the surface of carbon materials through a redox reaction to form a nanocomposite, using the carbon material as a substrate, where the MnO 2 has uniformly and tightly adhered to the surface of the carbon material [114][115][116][117][118][119][120][121][122][123][124][125].
For example, Qu et al. [91] adopted the modified Hummers method and prepared a pristine GO/MnSO 4 suspension, then the pristine suspension of GO/MnSO 4 was in situ transformed into GO/MnO 2 composites in combination with KMnO 4 , and finally further into RGO/MnO 2 composites by means of glucose-reduction ( Figure 1e). Singu et al. [126] synthesized CNTs-MnO 2 nanocomposites through the in situ reduction of KMnO 4 using MWCNTs as the reducing agent and supporting substrate. During the preparation process, the loading of MnO 2 can be adjusted by varying the amount of KMnO 4 , thereby optimizing the performance of the composite material. Wang et al. [127] adsorbed Mn 2+ on the surface of g-C 3 N 4 through the NH 2 groups in g-C 3 N 4 for the first time, underwent a redox reaction with KMnO 4 , and synthesized a novel 2D MnO 2 /g-C 3 N 4 heterojunction composite by in situ deposition of δ-MnO 2 . The bandgaps of MnO 2 and g-C 3 N 4 synthesized have a wide Nanomaterials 2023, 13, 541 6 of 27 visible light response and light absorption range, which are 1.56 eV and 2.69 eV, respectively. At the same time, the matched band structures and the heterojunctions with solid (C-O) bonding between them interface promoted the transfer/separation of photogenerated charge carriers, enhanced the light-harvesting ability, thus the photocatalytic activity can be greatly enhanced. Peng et al. [128] also synthesized N-doped CNT (NCNT) by chemical vapor deposition, and deposited MnO 2 onto the NCNT surface using in situ oxidation to prepare MnO 2 /NCNT composites. The synergistic effect of MnO 2 and NCNT obviously improved interfacial electron transfer, which can replace noble metals for the catalytic oxidation of organics.
In carbon materials, the existence of π-π interactions and van der Waals forces between the graphene nanosheets make it easy to aggregate and stack during processing, resulting in reduced surface areas and hidden active sites [129,130]. The quantitative loss of nanoscale materials during the recycling process may influence the fate of adsorbed contaminants, thus causing potential environmental risks [131]. The porous structure of composite films and aerogels can prevent the aggregation of nanosheets and afford more active sites for pollutant diffusion and oxidation. The structure is stable and easily recycled for reuse, which is a superior support for MnO 2 in the treatment of water, while in situ deposition of MnO 2 can also improve the mechanical and electron transport properties of carbon materials [132][133][134][135][136][137]. For example, Lv et al. [92] used 3D CQDs/graphene composite aerogels formed by the hydrothermal method as the reducing agent, which reacted with KMnO 4 to synthesize stable MnO 2 /CQDs/graphene composite aerogel ( Figure 1f). The 3D network structure avoided the reunion of the graphene nanosheets and the MnO 2 nanoparticles, and the CQDs served as a bridge for connecting MnO 2 and graphene, which effectively improved the conductivity and stability of the composite. Jyothibasu et al. [138] prepared cellulose/f-CNT/MnO 2 composite films via the direct redox deposition method to uniformly grow MnO 2 nanostructures on cellulose/functionalized CNT (f-CNT) conductive substrates. The synthetic procedure is simple, inexpensive, environmentally friendly, and can be synthesized in large-scale batches. The synthesized materials have unique porous structures, large specific surface areas, and excellent conductivities.

Electrochemical Deposition
Electrochemical deposition is an effective strategy for the synthesis of nanoscale materials and functionalized composites [106] and has been widely used in synthesizing carbon materials such as MnO 2 -modified carbon cloth and graphene [139][140][141][142][143][144][145][146]. Zhang et al. [141] synthesized hierarchical MnO 2 nanostructures on activated carbon cloth via a high-voltage anodic electro-deposition process, and the activated carbon cloth substrate enhanced the conductivity and hydrophilicity of the material. Zhu et al. [147] synthesized PANI@γ-MnO 2 /CC ternary hybrid material via hydrothermal and in situ electrochemical polymerization ( Figure 2a). The coating PANI layer with a 3D hierarchical structure provides a high specific surface area (96.3389 m 2 g −1 ), which is higher than that PANI@γ-MnO 2 (41.8632 m 2 g −1 ) and CC(21.1902 m 2 g −1 ) and accelerates the ion diffusion and electron transfer. Li et al. [148] synthesized mesoporous MnO 2 with high density pores on carbon aerogels substrate by electrochemical deposition. Mesoporous materials can increase the active sites and enhance the electric conductivity, which is more conducive to the transport of electrons and ions. In photocatalytic applications, they can effectively prevent the recombination of photogenerated electrons and holes to improve photocatalytic activity. At the same time, the obtained MnO 2 /carbon aerogel composites are green, low-cost and good in cycle stability, which have potential research and application value.

Co−Precipitating Method
The chemical co−precipitation method is a simple process, with low calcination te peratures and good homogeneity of the prepared complexes, and is one of the comm methods for the preparation of carbon composites at low temperatures [64,[152][153][154]. Ze et al. [155] synthesized 1D α−MnO2 nanowires and 2D GO nanowires to prepa α−MnO2/GO nanohybrids by mechanical grinding and co−precipitating method. T sub−micron GO sheets can occupy the interspace of the interconnected network α−MnO2 nanowires so that the two can be better combined. By comparing the materi prepared by the two methods, it can be found that the co−precipitating method is mo conducive to the tight binding of MnO2 and GO and facilitates heat and electron trans between these two materials. However, mechanical grinding may destroy the layer structure of GO and produce more defects, which is not conducive to photon absorpti and electron transfer. Liu et al. [149] first synthesized α−MnO2 nanofibres/carbon nan tubes hierarchically assembled microspheres (α−MnO2/CNT HMs) via a facile chemi precipitation/spray−granulation combined methodology (Figure 2b). The α−MnO2 N were homogeneously anchored on a highly conductive CNTs framework, forming close−packed network structure, which remarkably improved the electron−transfer cap bility. The composite material has excellent stability and cycling durability, low cost, a wide application prospects. Kumar et al. [156] prepared an Ag−doped MnO2−CNT nan composite using a co−precipitation route. The spheroidal−shaped Ag nanoparticles co

Co-Precipitating Method
The chemical co-precipitation method is a simple process, with low calcination temperatures and good homogeneity of the prepared complexes, and is one of the common methods for the preparation of carbon composites at low temperatures [64,[152][153][154]. Zeng et al. [155] synthesized 1D α-MnO 2 nanowires and 2D GO nanowires to prepare α-MnO 2 /GO nanohybrids by mechanical grinding and co-precipitating method. The sub-micron GO sheets can occupy the interspace of the interconnected network of α-MnO 2 nanowires so that the two can be better combined. By comparing the materials prepared by the two methods, it can be found that the co-precipitating method is more conducive to the tight binding of MnO 2 and GO and facilitates heat and electron transfer between these two materials. However, mechanical grinding may destroy the layered structure of GO and produce more defects, which is not conducive to photon absorption and electron transfer. Liu et al. [149] first synthesized α-MnO 2 nanofibres/carbon nanotubes hierarchically assembled microspheres (α-MnO 2 /CNT HMs) via a facile chemical precipitation/spray-granulation combined methodology (Figure 2b). The α-MnO 2 NFs were homogeneously anchored on a highly conductive CNTs framework, forming a closepacked network structure, which remarkably improved the electron-transfer capability. The composite material has excellent stability and cycling durability, low cost, and wide application prospects. Kumar et al. [156] prepared an Ag-doped MnO 2 -CNT nanocomposite using a co-precipitation route. The spheroidal-shaped Ag nanoparticles covered the CNT surface, and its high surface area to volume ratio provides a large number of active sites, showing excellent adsorption performance. Xia et al. [157] grew MnO 2 nanosheets in situ on the surface of exfoliated g-C 3 N 4 nanosheets by a wet-chemical method, forming a 2D/2D g-C 3 N 4 /MnO 2 heterojunction. The photoinduced electrons in MnO 2 can combine with the holes in g-C 3 N 4 to enhance the extraction and utilization of photo-generated carriers and improve the degradation rate of pollutants.

Template Method
The template method is mostly used for the preparation of 3D composites [158,159]. Le et al. [160] used diatomite as a template for the massive production of 3D porous graphene by the chemical vapor deposition method. After removing the template, the 3D graphene was N-doped by a hydrothermal reaction, and then the N-doped 3D porous graphene@MnO 2 hybrid structure was obtained by deposition of MnO 2 nanosheets. The MnO 2 nanosheets with a brushy structure were uniformly deposited on the surface of porous graphene, and the synergistic interactions between them enhanced the stability of the composite. After removing the diatomite, the composite retained the 3D structure and surface features of the diatomite template. Moreover, the abundant edges and defects formed during the template removal process and defects caused by nitrogen doping improve the conductivity and charge transfer rate of the composite. The MnO 2 nanosheets with a brushy structure were uniformly deposited on the surface of porous graphene, and the synergistic interactions between them enhanced the stability of the composite. Wang et al. [150] fabricated 3D CNT@NCNT@ MnO 2 composites with unique tube-in-tube nanostructures through the sacrificial template method (Figure 2c). The composite has an N-doped 3D double-carbon layers hollow structure and attaches tightly with MnO 2 nanoflowers grown on its surface, exhibiting large pores, high conductivity, large specific surface areas, and fast diffusion of electrons. Shan et al. [161] also prepared C-doped g-C 3 N 4 (CCN) using polyporous melamine foam (MRF) as a template and then exploited the synergistic advantages of 2D architectures, coupled CCN with MnO 2 nanosheets by a hydrothermal method to prepare efficient CCN@MnO 2 composite. The doping of carbon promoted electron transfer, and the MRF template can prevent the aggregation of sulfourea crystals, thereby reducing the thickness of CCN nanosheets and increasing the specific surface area (40.2 m 2 g −1 ).

Ultrasonic-Assisted and Sonochemical Methods
The Sonochemical-assisted uses sound energy to agitate the composite solution, causing it to undergo a physical or chemical transformation [162]. It can prevent material stacking, enlarge the interlayer spacing of carbon materials such as graphene, facilitate uniform loading of MnO 2 and enhance the photocatalytic properties of the synthesized semiconductors [64,106]. As a result, it is widely used to prepare various MnO 2 -carbon nanocomposites [163][164][165][166][167][168][169]. Chai et al. [170] synthesized S,O co-doped graphite, carbonitride quantum dots (S, O-CNQDs) by a solid-state reaction method, and in situ synthesized MnO 2 nanosheets in S,O-CNQDs dispersion solution to prepare MnO 2 -S,O-CNQDs nanocomposite with the ultrasonic-assisted. The as-prepared composite material has uniform size and good dispersion, which is a promising nanomaterial. Xu et al. [151] synthesized the CQDs/MnO 2 nanoflowers through the sonochemical method (Figure 2d), which has a high specific surface area (168.8 m 2 g −1 ) and excellent cycle stability. CQDs were uniformly distributed on the transparent petals of δ-MnO 2 , which improved the conductivity of MnO 2 nanoflowers and provided a large number of functional groups and active sites.

Other Methods
Many other novel options are also used to prepare MnO 2 -carbon materials [171][172][173]. For example, Jia et al. [174] prepared CNTs/MnO 2 composites by in situ synthesis of CNTs on MnO 2 nanosheets using the hydrothermal method and the chemical vapor deposition method. The vertically aligned MnO 2 nanosheets shortened the ion diffusion path, the in situ formed CNTs improved the electrical conductivity and structural stability, and the hierarchical porous structure increased the specific surface area (20.4 m 2 g −1 to 38.2 m 2 g −1 ) and active sites. Abdullah et al. [175] used polyacrylonitrile (PAN) as a carbon precursor to prepare nanofibers (NFs) by an electrospinning process and incorporated MnO 2 nanoparticles into ACNFs to prepare composite activated carbon nanofibers (ACNFs/MnO 2 ) by carbonization and activation. The incorporation of MnO 2 increased the specific surface area (478.2 to 599.4 m 2 g −1 ), pore size (0.285 cm 3 g −1 ), and total pore volume (0.299 cm 3 g −1 ) of the composite material. Wei et al. [176] prepared MnO 2 /3D graphene composites by the reverse microemulsion method. In this reaction, the graphene substrate was used as a sacrificial reductant to undergo a redox reaction with KMnO 4 to grow MnO 2 in situ on 3D graphene, and the MnO 2 mass loading of the composite was controlled by changing the ultrasonication time in the in situ growth process. Nanoscale MnO 2 layers were uniformly coated on the internal surface of 3D graphene, and the continuous 3D interpenetrating microstructures prevented the restacking of graphene sheets. Zhu et al. [177] prepared free-standing 3D graphene/MnO 2 hybrids by depositing MnO 2 nanosheets onto a 3D graphene framework through a solution-phase assembly process. Unlike 1D MnO 2 , the flower-like architecture of deposited MnO 2 nanosheets have a larger specific surface area and are uniformly anchored on a 3D graphene framework with strong adhesion, there is a strong interaction between them, so the prepared hybrids showed good mechanical properties. Pang et al. [178] proposed a simple room-temperature water bath method to deposit crystalline MnO 2 on CNTs to prepare CNT-MnO 2 nanocomposites. This scheme can control the phases and morphologies of the composite products by changing the pH of the reaction solution. Wang et al. [179] assembled GO, MnOx, and polymer carbon nitride (CN) into free-standing GO/MnO x /CN ternary composite film by employing the vacuum filtration method. The prepared composite film has good stability, mechanical property, and recyclability and is more suitable for the practical application of photocatalysis.
To sum up, MnO 2 -carbon composites can be prepared and modified in various ways, and the finally obtained multifunctional materials have great application potential in water treatment. Each preparation method has its own unique advantages, and the fabrication of specific nanocomposites can be improved by selecting the most suitable preparation method, which can be used to treat various types of sewage treatment to different pollutants (Table 1).

Applications of Photocatalytic Technology in Water Treatment
With the growth of population and continuous development of industry and agriculture, the problem of water pollution has become increasingly prominent [180][181][182]. Various organic and inorganic pollutants have been detected in surface, ground, sewage, and drinking waters [183,184]. Among them, the pollutants (phenols, antibiotics, organic dyes, heavy metal, etc.) produced by agriculture [185], aquaculture [186,187], carbon aerogels, textiles [188,189] and other industries are highly toxic and difficult to biodegrade [180]. The pollution of water bodies will not only destroy the ecosystem but also seriously threaten human health [190,191]. These stubborn compounds have become important contaminants in water that need to be removed urgently. As common green materials, MnO 2 and carbon materials can use solar energy to degrade many types of pollutants, and the photocatalytic process is economical and environmentally friendly [192]. Therefore, the combination of the two has momentous research potential and application prospects in the field of photocatalytic water treatment, and the photocatalytic degradation of various pollutants by MnO 2 -carbon materials has also been widely studied [193].

Phenolic Wastewater
Phenolic compounds are typical aromatic organic compounds that exist in sewage discharged from petroleum refineries, manufacturing of paints, pulp and paper manufacturing plants, and other industries [194][195][196]. At the same time, they are also a kind of important organic raw materials in the field of agricultural production and are widely used in the manufacture of pesticides, insecticides, and herbicides [197][198][199]. Their wide use in industry and agriculture makes them a large number of residues in the environment and a common organic pollutant in water, which have potential carcinogenicity, teratogenicity, and mutagenicity, with wide source, great harm and refractory degradation [200][201][202]. Among them, phenol and its derivatives (such as bisphenol A, chlorophenol, nitrophenol, etc.) are common phenolic pollutants in the water environment, which are highly toxic and cause serious pollution even at low concentrations [194,[203][204][205][206]. Compared with other organic substances, they have a great impact on the environment.
Phenolic compounds usually have one or more hydroxyl groups attached to the aromatic ring [194]. In the photocatalytic process, hydroxyl radicals attack the cyclic carbon to produce various oxidation intermediates (such as hydroquinone, catechol, p-benzoquinone, etc.) [195]. These organic compounds are less harmful than the parent compounds and will eventually be photomineralized to carbon dioxide (CO 2 ), so as to achieve the purpose of degradation [207]. Table 2 summarizes the progress in the photocatalytic degradation of phenolic compounds by various MnO 2 -carbon materials. For example, Mehta et al. [207] prepared MnO 2 @CQDs nanocomposites with a bandgap of 1.3 eV by a one-step hydrothermal method, which was used to degrade phenol under visible light. The spherical CQDs were deposited on the surface of MnO 2 nanorods, and the nanocomposite had a high specific surface area (95.3 m 2 g −1 ). The optimal operating parameters were obtained after optimization under different reaction conditions, and after 50 min of visible light irradiation, the degradation rate of phenol reached 90%. The degradation rate was basically unchanged after three consecutive cycles, and the degradation rate can still reach 80% after five cycles, which the stability is good. Xia et al. [157] synthesized g-C 3 N 4 /MnO 2 heterostructured photocatalyst via in situ growth of MnO 2 nanosheets on the surface of exfoliated g-C 3 N 4 nanosheets using a wet-chemical method. MnO 2 nanosheets and the g-C 3 N 4 layers are closely combined, and the 2D layered structure can provide abundant active sites and shorten the transport distance of photogenerated charge carriers. Under the irradiation of xenon lamps, the ability of the composite material to degrade phenol is significantly increased, and it has good durability. Preparing the photocatalyst of the MnO 2 -carbon composites with low bandgap can make full use of solar energy and provide a sustainable green approach for photocatalytic water treatment, and its application potential needs to be further developed [208][209][210][211][212].

Antibiotic Wastewater
Antibiotics can prevent and treat a variety of bacterial infections in humans and animals and are widely used for human beings, animal husbandry, and aquaculture industries [213][214][215]. However, the overuse of antibiotics has imposed severe water environment problems [215]. According to statistics, approximately 60-90% of antibiotics cannot be completely metabolized by the human or animal body and will be excreted through feces [216][217][218][219]. These wastes may be dumped directly into wastewater or enter farmland as fertilizer and enter nearby water bodies through rainfall and irrigation [220,221]. Due to the poor biodegradability of most antibiotics, the sustained use of antibiotics makes them stay in the water for a long time, which may generate antibiotic-resistant genes (ARGs) and antibiotic-resistant bacteria (ARBs), resulting in increased microbial resistance, which poses a potential threat to human health and ecological systems [222][223][224]. Therefore, the degradation of antibiotics in water is an important and urgent task.
It has been reported that sunlight-driven photocatalytic technology can effectively remove antibiotics from water, among which the visible light-responsive MnO 2 -carbon composite photocatalyst has great practical application potential [225,226]. We selected and listed the photocatalytic degradation rates of different types of antibiotics using MnO 2carbon as a photocatalyst (Table 3). Du et al. [227] synthesized g-C 3 N 4 /MnO 2 /GO heterojunction photocatalyst by wet-chemical method (Figure 3a). Composites with different ratios of g-C 3 N 4 , MnO 2, and GO have different catalytic activities, and the composites, after optimization, can degrade 91.4% of TC at most after 60 min of visible light irradiation. The TC removal rate only decreases by 10% after four cycles, and the sample structure has no change (Figure 3b-d). Excellent stability is more conducive to the practical application of photocatalysis. Liu et al. [228] synthesized the pumice-supported reduced graphene oxide and MnO 2 (PS@rGO@MnO 2 ) as a solid photocatalyst by a two-step hydrothermal method, which can effectively degrade 80% ciprofloxacin within 6 h under simulated sunlight, and the performance was not obviously decreased after three cycles, and all characteristic peaks remained intact, which proved its excellent reusability. In addition, the catalytic performance of PS@rGO@MnO 2 solid photocatalyst under actual sunlight is comparable to that under simulated sunlight, it has good removal performance for ciprofloxacin in actual natural water, and it can also degrade other antibiotics in water, which has great potential in the treatment of drinking water and surface water.   [210] and listed the photocatalytic degradation rates of different types of antibiotics usi MnO2−carbon as a photocatalyst (Table 3). Du et al. [227] synthesized g−C3N4/MnO2/G heterojunction photocatalyst by wet−chemical method (Figure 3a). Composites with d ferent ratios of g−C3N4, MnO2, and GO have different catalytic activities, and the comp sites, after optimization, can degrade 91.4% of TC at most after 60 min of visible lig irradiation. The TC removal rate only decreases by 10% after four cycles, and the samp structure has no change (Figure 3b-d). Excellent stability is more conducive to the prac cal application of photocatalysis. Liu et al. [228] synthesized the pumice−supported duced graphene oxide and MnO2 (PS@rGO@MnO2) as a solid photocatalyst by a two−st hydrothermal method, which can effectively degrade 80% ciprofloxacin within 6 h und simulated sunlight, and the performance was not obviously decreased after three cycl and all characteristic peaks remained intact, which proved its excellent reusability. In a dition, the catalytic performance of PS@rGO@MnO2 solid photocatalyst under actual su light is comparable to that under simulated sunlight, it has good removal performance ciprofloxacin in actual natural water, and it can also degrade other antibiotics in wat which has great potential in the treatment of drinking water and surface water.

Dye Wastewater
Dyes can impart or alter the color of a substance, which are widely used in a wide variety of industries, including textile, printing, leather, agriculture, pharmaceutical, and food industries [229][230][231][232]. According to statistics, more than 7 × 10 5 tons of dyes are produced annually worldwide, and about 15% of dyes will enter the environment with the loss of wastewater during manufacturing and application processes [233,234]. The dyes have a complex structure, high biological toxicity, and are easily soluble in water but have poor biodegradability, which may accumulate in the water environment [132,235]. The colored dyes in the water will affect the transparency of water bodies, absorb and reflect sunlight entering the water, hinder the photosynthesis of aquatic plants and abolish the ecological balance of the water body [230,236]. In addition, its potential carcinogenicity, teratogenicity, and mutagenicity will also cause negative effects on human health [237,238].
Photocatalytic technology has the remarkable ability to degrade and decolorize organic dyes. In environment-cleaning applications, different kinds of semiconductor compounds play an important role in the photocatalytic removal of dyes [229]. In recent years, MnO 2carbon materials have shown excellent performances in the photocatalytic degradation of organic dyes, which has attracted extensive research by researchers [239][240][241][242][243][244][245][246][247][248][249] (Table 4). Park et al. [250] synthesized PANI-rGO-MnO 2 ternary composites by polymerizing aniline with rGO and incorporating MnO 2 . PANI can act as an excellent electron donor and hole conductor, as channels for electron transport and storage, and is a suitable substrate for visible light-responsive photocatalysts. The ternary heterostructure reduced the recombination of photogenerated electron-hole pairs and extended the light absorption range. The composites showed excellent photocatalytic activity, and 90% of methylene blue (MB) could be degraded under visible light irradiation within 2 h (Figure 4). Panimalar et al. [251] constructed the MnO 2 /g-C 3 N 4 heterostructure, which showed higher photocatalytic activity than pristine MnO 2 and g-C 3 N 4 after 100 min of visible light irradiation. The degradation rate of MO could reach 92%. After five cycles, the composite photocatalyst was not obviously inactivated, showing high stability. This sort of material could be used as a photocatalytic practical device for wastewater treatment.

Heavy Metal Wastewater
The high solubility, bioaccumulation, and non-biodegradability of heavy metals make them easily accumulate in living beings through the food chain and drinking water [252][253][254]. The heavy metals entering the organism are easy to bind with essential cellular components such as proteins, nucleic acids, and enzymes, destroying organic cells in the body and endangering the health of organisms and human bodies [195,255]. However, the toxicity, mutagenicity, and carcinogenicity of heavy metals are strongly dependent on the oxidation state [256]. Reducing a high-valence state and highly toxic heavy metal ion into a low-valence state and low-toxic or non-toxic heavy metal ion is an effective way to mitigate the potential hazards of heavy metals [257][258][259].
It has been reported that MnO 2 -carbon materials can be used as photocatalysts to reduce toxic heavy metals to non-toxic metals using light energy. Padhi et al. [260] reported a highly efficient hydrothermal method to fabricate an RGO/α-MnO 2 nanorod composite, which showed outstanding photoreduction ability. A 97% reduction in Cr(VI) under visible light irradiation for 2 h and no significant loss of photoreduction ability up to the third cycle. Wang et al. [261] prepared MnO 2 @g-C 3 N 4 composite photocatalyst by compounding MnO 2 on g-C 3 N 4 via the hydrothermal method for the treatment of uraniumcontaining wastewater. Under optimal conditions, the photocatalytic reduction rate of U(VI) reached 96.3% under visible light irradiation for 120 min. There is little research on the photoreduction of heavy metals by MnO 2 -carbon materials, and related applications still need further exploration.

Conclusions and Outlook
Photocatalytic technology has attracted extensive attention from researchers because of its green, energy-saving, and high efficiency. It is significant to develop low-cost and non-toxic, environmentally friendly photocatalysts. MnO 2 and carbon materials are commonly green and low-cost materials, the composite methods are simple and diverse, and different methods can synthesize photocatalytic materials of various dimensions and sizes. Compared with a single photocatalyst, the photocatalytic activity of MnO 2 -carbon composites is significantly improved, and a variety of pollutants can be removed efficiently. At present, many synthetic methods have been developed to prepare MnO 2 -carbon materials for degrading various pollutants, but the practical application is still in the early stage, and no major breakthrough has been made. The transition from the laboratory to the actual water body is still facing great challenges. In future research, the following aspects need further exploration and development.
(1) The performance optimization of MnO 2 -carbon materials. The photocatalytic efficiency of MnO 2 -carbon composites is mostly around 80% or 90%, and the photocatalytic activity needs to be improved further. Therefore, the improvement of photocatalytic performance of MnO 2 -carbon materials is the core problem of photocatalytic technology improvement, and the proportion and preparation process of MnO 2 -carbon composite material fundamentally determine its photocatalytic performance. On the one hand, the properties of MnO 2 -carbon materials can be optimized by adjusting the ratio of MnO 2 and carbon materials. On the other hand, it can be improved by doping metal or nonmetal, adding polymers, oxides, or other carbon materials.
(2) The separation and recovery of MnO 2 -carbon materials. Powdered MnO 2 -carbon materials not only have the disadvantages that cannot be dispersed evenly and recovered difficulty, but the quantitative loss during the recycling process may influence the fate of adsorbed contaminants, thus causing potential environmental risks. Therefore, it is necessary to explore effective methods to prepare high-dimensional materials that are more conducive to recycling, such as hydrogels, aerogels, and flexible films. Compared with low-dimensional materials, high-dimensional materials have broader prospects in practical applications.
(3) The large-scale application of photocatalytic technology. The application of photocatalytic treatment of MnO 2 -carbon materials mostly stays in the laboratory stage, and it is difficult to use it on a large scale. To achieve large-scale utilization, we need to consider the cost, stability, and quantifiable productivity of photocatalysts. Therefore, it is necessary to explore 3D MnO 2 -carbon materials with better stability, enlarge the size of materials in equal proportion and test their properties, improve the reuse rate of the materials, and reduce the material costs. The stability of MnO 2 -carbon materials and the amplified photocatalytic performance are crucial issues to be solved to realize the large-scale application of photocatalytic technology.
(4) The research on MnO 2 -carbon materials in actual water treatment. Most of the MnO 2 -carbon materials are studied for single pollutants, but the pollutants in actual water bodies have complex components, various kinds, and different concentrations, which are far more complicated than the laboratory simulation. Therefore, we need to evaluate the ability of MnO 2 -carbon materials as a photocatalyst to treat multiple pollutants simultaneously, explore the potential adverse effects of multiple pollutants, develop different sizes and types of MnO 2 -carbon materials, select the study area, collect wastewater samples from actual water bodies, and study the photocatalytic performance of MnO 2 -carbon materials for actual wastewater treatment. The use of MnO 2 -carbon materials for photocatalytic degradation of various organic pollutants in water bodies, from laboratory study to practical water application, is a major challenge and a key research direction for the future.

Conflicts of Interest:
The authors declare no conflict of interest.