Promising Materials for Photocatalysis‐Self‐Fenton System: Properties, Modifications, and Applications

Photocatalysis‐self‐Fenton system (PSFs), which integrates photocatalysis with Fenton technology, is investigated thoroughly in wastewater treatment due to the virtues of no addition of H2O2 and accelerated circulation of Fe2+/Fe3+. Several kinds of materials that mainly include metal‐free graphite carbon nitride (g‐C3N4), metal oxides (Fe3O4), and metal sulfide (CdS, FeS2) have demonstrated extensive development potential. Nevertheless, most of the developed catalysts are unable to satisfy the requirements of PSFs. Therefore, a comprehensive review committed to the materials and modification methods in PSFs is put forward. First, the concrete mechanism of PSFs from two aspects of photocatalysis and Fenton reaction is discussed. Second, the properties, modifications (morphology modulation, heterojunction construction as well as doping), and applications (contaminants degradation and disinfection) of the promising catalysts are emphasized. Finally, the novel directions for future research in PSFs are looked forward to. This review opens up a new horizon for enhancing the practical application of PSFs in future.


Introduction
Currently, the types and constituents of pollutants in wastewater are getting ever more complicated and general chemical oxidation is formidable to achieve in-depth treatment of organic wastewater, resulting in a long-term potential menace. [1][2][3] Fortunately, profiting from the merits of strong anti-interference, slight secondary pollution, and mild reaction condition, Fenton technology, as a typical kind of advanced oxidation technology, has been effectively applied in the treatment of refractory industrial wastewater, domestic sewage and landfill leachate. [4][5][6][7] Crucially, the advanced treatment of organic wastewater can be achieved by virtue of the numerous •OH radicals without selective oxidation of most refractory pollutants in the reaction process. [8] Nevertheless, the drawbacks of continuous external addition of H 2 O 2 as well as the subsequent activation of H 2 O 2 pave a significant limitation to its practical application, [9,10] urging to devote more efforts in developing other coupling processes to solve the challenges mentioned above. [11] An alternative is to combine Fenton technology with electrocatalytic technology, [12] which has a thorough reaction, less secondary pollution, and a high degree of automation control. [13,14] The process is consuming electrical energy to react on the electrodes to produce H 2 O 2 and subsequent H 2 O 2 activation. [15,16] It is needless to say that the great electrical energy consumption will come at a high cost and still limits the practical application of Fenton technology. [17] In this regard, a technology which is not only green and problem-solving but also low energy cost is exactly what we are looking for. Recently, photocatalytic technology has been widely and deeply explored in oxygen reduction reaction (ORR). H 2 O 2 production and H 2 O 2 activation possess the features of moderate reaction circumstance, clean and efficient, low price, only O 2 , abundant solar energy, and recyclable as raw materials. [18][19][20][21] Herein, from the perspective of H 2 O 2 production [22] and H 2 O 2 activation [23] or economic aspect, [24] photocatalysis technology becomes the first choice to be combined with Fenton technology. Consequently, a photocatalysis-self-Fenton system (PSFs) is established through integrating photocatalytic technology with Fenton technology. [25][26][27] In the PSFs process, the H 2 O 2 can be produced and utilized in situ. [28] In addition, in previous studies, active substances produced in the photocatalytic stage also play an important role in the activation of H 2 O 2 in Fenton stage. [29] It is worth noting that PSFs still faces two crucial technical challenges: 1) the sufficient and consistent production of H 2 O 2 which is a prerequisite for the initiation of PSFs; [30] 2) the efficient activation of H 2 O 2 which is a crucial point for the working of PSFs. [31] In view of the above problems, many DOI: 10.1002/sstr.202200371 Photocatalysis-self-Fenton system (PSFs), which integrates photocatalysis with Fenton technology, is investigated thoroughly in wastewater treatment due to the virtues of no addition of H 2 O 2 and accelerated circulation of Fe 2þ /Fe 3þ . Several kinds of materials that mainly include metal-free graphite carbon nitride (g-C 3 N 4 ), metal oxides (Fe 3 O 4 ), and metal sulfide (CdS, FeS 2 ) have demonstrated extensive development potential. Nevertheless, most of the developed catalysts are unable to satisfy the requirements of PSFs. Therefore, a comprehensive review committed to the materials and modification methods in PSFs is put forward. First, the concrete mechanism of PSFs from two aspects of photocatalysis and Fenton reaction is discussed. Second, the properties, modifications (morphology modulation, heterojunction construction as well as doping), and applications (contaminants degradation and disinfection) of the promising catalysts are emphasized. Finally, the novel directions for future research in PSFs are looked forward to. This review opens up a new horizon for enhancing the practical application of PSFs in future.
efforts have been devoted, mainly focusing on the modification of photocatalyst structure. [25,[32][33][34][35] Photocatalyst is a large family, mainly including metal-free carbon-based materials (g-C 3 N 4 ), [36][37][38] metal oxides (TiO 2 , [39] ZnO, [40] WO 3 , [41] etc.), metal sulfides (CdS, [42] ZnS, [43] MoS 2 , [44] etc.), and other types of materials. [45][46][47] Compared with other promising photocatalysts, these catalysts have emerged great potential in H 2 O 2 production thanks to the merits of low price, easy synthesis, excellent structural characteristics and outstanding photoelectric performance, leading the extensive research in photocatalysis-self-Fenton. Regrettably, pure materials have always been bothered by low absorption and utilization of light, few active sites, and low efficiency of photogenerated carriers separation and migration. [48][49][50] In order to overcome these shortcomings, a series of strategies have been adopted to conquer the defects of low H 2 O 2 production and poor H 2 O 2 activation efficiency, including morphology regulation, [51] heterojunction, [52] defect control, [53] heavy metal loading, element doping, etc. [54,55] For instance, our group designed a garland g-C 3 N 4 (GCN) with carbon defects by morphology regulation and defect control, presenting high degradation performance of 2,4-DCP by increasing H 2 O 2 production and efficient use in situ in the Fenton stage. [56] In addition, Hu et al. prepared a wonderful 2D/1D hierarchical layered ZnIn 2 S 4 / TiO 2 heterojunction via cascade reaction with abundant H 2 O 2 production which was sustainably activated to degrade tetracycline. [57] And yet here some challenges are still existing in the synthesis and regulation of photocatalysts. There are some high-quality publications focusing on summarizing the strategies on photocatalytic H 2 O 2 production and the activation of Fenton technology while scarce systematic summary and induction are centered on the mechanism and performance of PSFs. [58][59][60] In this review, we have systematically summarized the recent advancement of PSFs on H 2 O 2 production and activation. First, the principles and the latest progress of PSFs are reviewed, which is different from the previous researches focusing on single photocatalytic H 2 O 2 production system [61] or Fenton degradation system. [62] Second, types of materials and methods of structure regulation including morphology regulation, heterojunction, defect control, heavy metal loading or element doping, etc. are specifically demonstrated. Finally, the challenges of coupling Fenton technology with other technologies are discussed and the future development of PSFs is prospected.

The Development Process of Single Fenton, Photo-Fenton, and PSFs
Among the numerous water treatment technologies, Fenton technology was proposed in 1894 due to •OH strong oxidation ability, which can take effect only with the addition of H 2 O 2 and Fe 2þ . [63] However, the practical application is severely limited by the narrow pH response range (about 3) and reunion caused by iron mud from the low pseudo-first-order rates of Fe 3þ to Fe 2þ . [64] Fortunately, compared with single Fenton, photo-Fenton has conquered the abovementioned drawbacks by employing photocatalysts to generating carriers with excellent redox capacity under visible light excitation, which could straightway participate in the activation of H 2 O 2 to produce •OH and promote the circulation of Fe 3þ to Fe 2þ . [65][66][67] However, the remaining key point is how to simultaneously provide the source of H 2 O 2 . Some researches turn their interest to the materials which are provided with eximious H 2 O 2 production, in the meantime triggering the Fenton reaction. Plenty of materials synthesized and modified have been devoted to PSFs. [68] And the following principles of PSFs were put forward in detail.

Principles of PSFs
In the last few years, PSFs has attracted extensive attention and in-depth researches because of its high efficiency in the degradation of different kinds of pollutants such as 2,4dichlorophenol, [69] p-chlorophenol, [70] atrazine, [71] phenol, [72] etc. By definition, PSFs is the coupling of photocatalytic technology and Fenton technology, but not in the simple sense of connection. The catalytic efficiency of the coupled system achieves greater efficiency than the simple addition of two signal systems. The mechanism of PSFs can be divided into two parts: photocatalytic H 2 O 2 production and in situ utilization of H 2 O 2 in Fenton stage.

Photocatalytic H 2 O 2 Production
As a practical application of photocatalytic technology, PSFs is naturally subjected to the principles of photocatalytic reaction, which is mainly consisted of sequential three steps ( Figure 1): 1) inside the photocatalysts, electrons (e À ) migrate from the valence band (VB) to the conduction band (CB) position with absorbing photons whose energy is in parallel with bandgap, leaving holes (h þ ) in the valence band; 2) photoinduced separated e À and h þ rapidly transfer to the surface of the photocatalyst to join the related redox reaction; and 3) some of the carriers react with the absorbed chemicals on the photocatalyst surface to yield H 2 O 2 , meanwhile, the others will combine with each other. [73] Lately, plenty of investigations have revealed that photocatalytic H 2 O 2 production is mainly composed of H 2 O oxidation processes (Equation (1)) [74] and O 2 reduction (Equation (2)-(4)). [75] It is universally acknowledged that H 2 O 2 production is an endothermic reaction and the H 2 O oxidation process requires a large amount of energy, [76] making this process difficult to achieve. Honestly speaking, the research process of H 2 O 2 production mainly focuses on O 2 reduction which would be selectively reduced through two pathways, successive two-step 1-e À reduction reaction (Equation (2), (3)) [77] and direct one-step 2-e À reduction reaction (Equation (4)). [78] The number of electrons transferred during the whole O 2 reduction process can be measured by the rotating disk electrode (RDE). [79] However, except for inherent disadvantages of photocatalysts such as band structure and so on, the weaker O 2 adsorption capacity, inferior selectivity of 2-e À , and other drawbacks strictly render the efficiency of H 2 O 2 production. Therefore, H 2 O 2 yield still fails to reach a high efficiency, and it is crucial to take further actions to boost the yield of H 2 O 2 . Need to add that, in order to enhance the O 2 adsorption capability, several novelty strategies have been explored, such as making more negative charges on the catalysts surface, [80] inducing local charge polarization, [77] and forming the Lewis acidic sites. [81] Some photocatalytic H 2 O 2 production system based on different kinds of photocatalysts are shown in Table 1.

In Situ Utilization of H 2 O 2 in Fenton Degradation
According to the routine, the H 2 O 2 generated above needs to be separated, purified, stored, and transported before putting into use which suffers from huge costs and risks. [82] However, all the confusions originating from the externally added H 2 O 2 can be solved if the H 2 O 2 produced in photocatalytic stage can be used in situ to trigger the Fenton reaction. Therefore, the H 2 O 2 generated in photocatalytic stage can be catalyzed into •OH radicals under the function of external Fe 2þ which is oxidized to Fe 3þ (Equation (5)). [83] And then, the Fe 3þ would react with the remaining H 2 O 2 to form a cyclic Fe 2þ reaction (Equation (6)) ( Figure 1). [84] In addition, photogenerated e À in the photocatalytic stage also plays a significant role in the activation of H 2 O 2 .

pH
The activity of Fenton reaction is highly restricted by the pH of the solution. Normally, Fenton reactions work under acidic conditions in which Fe 2þ reacts with H 2 O 2 to form •OH, but not in neutral or alkaline conditions. Generally, the best pH is around 3-4.

H 2 O 2 Dosage
The concentration of H 2 O 2 is essential to the activity of Fenton reaction. Low H 2 O 2 dosage will restrain the reaction rate while high dosage will result in low utilization and precipitation. When   [175] the concentration is too high, Fe 2þ is oxidized to Fe 3þ at the beginning. The reaction is carried out under the catalysis of Fe 3þ , which inhibits the production of •OH.

Fe 2þ Dosage
When the concentration is too low, the reaction rate is slow; when the concentration is too high, H 2 O 2 will be reduced and itself will be oxidized to Fe 3þ . Excessive consumption of H 2 O 2 will increase the chroma of water and affect the absorption of light.

Other Factors
Reaction temperature; addition method; and dynamic influence.

Metal-Free Graphite Carbon Nitride (g-C 3 N 4 )-Based Photocatalysts
It is universally acknowledged that graphite carbon nitride (g-C 3 N 4 ) is characteristically composed of π-conjugated which is constructed by tri-s-triazine and tertiary amines structures. [85] Therefore, g-C 3 N 4 , as a metal-free material, is equipped with plenty of merits of economic benefits, little pollution, structural stability whatever in slight acidic or alkaline conditions, excellent band structure, etc. [86][87][88] Especially, g-C 3 N 4 , exhibiting excellent performance in yielding H 2 O 2 , has been regarded as one of the most promising photocatalysts to trigger the PSFs for contaminant harnessing. [89] However, several core shortcomings including limited light absorption, confined surface area, faster carriers recombination efficiency, etc. need to be tackled urgently when the g-C 3 N 4 -based photocatalysts are cast into wide range of actually applying in PSFs. [90][91][92] Numerous efforts were devoted to address the aforementioned primary issues, focusing on the strategies of morphology modulation, elemental doping, loading noble metals, defect manufacturing, hybridization, etc.

Morphology Modulation
The morphology of catalysts has a great relationship with the catalytic activity, affecting the specific surface area, carriers propagation efficiency, and light absorption ability. Universally, g-C 3 N 4 is a 2D photocatalyst whose structure features that tri-s-triazine units connected by carbon atoms with covalent bonds, and the layers are combined by van der Waals force (weak interaction interaction), exhibiting tremendous prospect and potentiality for morphology regulation. [93] Currently, many characteristic works have been devoted to the morphology modification of g-C 3 N 4 , including the development of liquid phase ultrasonic stripping, [94] thermal oxidation etching stripping, [95] chemical oxidation stripping, [96] and gas template stripping, [97] successfully realizing the transformation from bulk structure to ultrathin 2D nanosheets, which makes a significant contribution to the improvement of catalytic performance. For example, the preparation of g-C 3 N 4 nanosheets using isopropanol as liquid phase stripping medium is disclosed by Yang's group and the thickness of nanosheets could be achieved to 2 nm. [98] Soon afterward, Xu et al. have fabricated a graphene-analogue carbon nitride (GA-C 3 N 4 ) with thermal oxidation etching stripping methods with 2D thin-layer structure and a high specific surface area of 30.1 m 2 g À1 . [99] In addition to the investigations of 2D structure to improve the performance, the 3D structure also plays a more significant effect on the catalytic activity. Compared with 2D structure, the 3D structure possesses the merits of larger specific surface area, more reactive active sites, and higher efficiency in facilitating efficient separation and suppressing the recombination of charge carriers. Wang's group synthesized a 3D porous g-C 3 N 4 /C nanosheets composite by facile pyrolysis and carbothermal activation means with common melamine and soybean oil as precursors, displaying high activity in photocatalytic activity (Figure 2a). [100] The phenomenon could be ascribed to the high surface area and large pore volume, which could be seen visually from the scanning electron microscopy (SEM) image ( Figure 2b) and N 2 adsorption-desorption isotherms ( Figure 2c). In addition, Fe-Cu-CNTs and Al 0 -CNTs was successfully constructed by Chen's group, demonstrating effective degradation activity in Sulfamerazine at neutral pH ( Figure 2d). [101] When applied to the visible light irradiation, a PSFs was formed, in which Al 0 -CNTs could generate H 2 O 2 in situ and the H 2 O 2 will be converted to •OH radicals and •O 2 À by Fe-Cu-CNTs composite. Such desired performance partially attributed to the excellent tubular structure that provides the base for metals loading. Based on the above foundation, our group created a PSFs based on g-C 3 N 4 with different morphologies for several pollutants degradation (Figure 2e,f ). [56] The H 2 O 2 production of garland g-C 3 N 4 (GCN) with carbon defects reached up to 507.82 μM h À1 g À1 without any sacrificial agents. Subsequently, the expected apparent rate constant value of pollutants in GCN-PSFs attained to 0.070 min À1 , which is 5.4, 3.3, and 2.6 times than that of bulk g-C 3 N 4 (BCN), BCN-PSFs, and GCN. Such ideal degradation property could mainly be credited to the following aspects: 1) the regulation of GCN morphology provided more reactive active sites; 2) the introduction of carbon defects accelerated the separation efficiency of charge carriers, increased O 2 adsorption capacity, and improved the 2e À ORR of H 2 O 2 production; 3) stable and sufficient production of H 2 O 2 , which would be in situ utilized efficiently to transform to numerous •OH within PSFs ( Table 2).

Heterojunctions Construction
Heterojunctions construction has developed an innovative method to tune the structure of photocatalysts, optimizing the photocatalytic activity. The close contact between the 2D planar structure of g-C 3 N 4 and other admirable materials including metals, metal oxides, inorganic semiconductors, conductive polymers, etc. [102][103][104] can reduce the barrier of e À transport and facilitate the transport of e À and h þ at the material interface. In particular, compared with other common modification methods in modulating the structure of g-C 3 N 4 , establishing phase junction catalysts is more controllable and facile. [105] For instance, a FeOOH/UPCN heterojunction composite was developed through integrating ultrathin porous g-C 3 N 4 (UPCN) nanosheets with amorphous FeOOH quantum dots (QDs) (Figure 3a). [106] From the high resolution transmission electron microscope (HR-TEM) in Figure 3b, it is evident that the FeOOH/UPCN has been successfully synthesized. Under the influence of the synergistic effect of heterojunction structure and ultrathin structure, the high-speed oxytetracycline (OTC) degradation activity (Figure 3d) could be attributed to the formation of photo-Fenton system with desired H 2 O 2 production ( Figure 3c) which could be concluded that the composition of FeOOH/UPCN is better advancing the migration of photogenerated e À from UPCN to FeOOH QDs and then joining the reduction of Fe 3þ to Fe 2þ . Except for the connection with amorphous FeOOH QDs, the g-C 3 N 4 is also often compounded with metal oxides. Geng et al. used ZnO and g-C 3 N 4 together to achieve a ZnO/g-C 3 N 4 heterojunction catalyst by thermal polycondensation applied to H 2 O 2 production and in situ sterilization in natural water under sunlight (Figure 3e,f ). [107] The H 2 O 2 production and sterilization rate were able to reach 23.91 μmol L À1 within 120 min and 97.4% within 60 min, respectively (Figure 3g,h). Recently, Ma's group synthesized a high-efficient material applied in PSFs. The prepared graphitic carbon interface-modified g-C 3 N 4 (CUCN) immensely promoted Fe(III)/Fe(II) cycle, leading to 63.77% mineralization degree for RhB within 3 h. [108] However, due to the existing of electrostatic repulsion, some problems still persist in aforementioned type-II heterojunction including poor redox ability. [109] To overcome this shortcoming, Figure 2. a) Schematic to show the synthesis process of 3D porous g-C 3 N 4 /C nanosheets composite. b) SEM image of the 3D g-C 3 N 4 /C-NS showing the 3D porous nanosheets. c) N 2 adsorption-desorption isotherms. Reproduced with permission. [100] Copyright 2018, Elsevier. d) The reaction mechanism of the Al 0 -CNTs/CNTs-Fe-Cu/O 2 system Reproduced with permission. [101] Copyright 2020, Elsevier. e) Synthesis schematic diagram of g-C 3 N 4 with four different morphologies (BCN, UCN, TCN, and GCN). f ) The photocatalytic decomposition curves of 2,4-DCP with BCN, BCN-PSFs, GCN, and GCN-PSFs under visible light irradiation. Reproduced with permission. [56] Copyright 2022, Elsevier.
Z-scheme heterojunction proposed based on artificial photosynthesis simulated by natural plant photosynthesis not only possesses the advantages of traditional heterojunction but also features the inhibition of carriers recombination, retention of strong oxidation, and expansion of light response range. Within the Z-scheme heterojunction, the conductive medium or internal electric field is constructed to consume part of the photogenerated electrons and holes, so that the electrons and holes with the strongest redox ability can be separated and utilized in the photocatalytic reaction. [110] Recently, Z-scheme heterojunction has been extensively applied in various fields ranging from CO 2 reduction, [111] H 2 production, [112] and environmental remediation. [113] For example, a gas-liquid-solid triphase photocatalytic system (MCC catalyst) was developed based on the Z-scheme heterojunction MIL-101(Fe)/g-C 3 N 4 deposited on the hydrophobic carbon cloth substrate. [114] The MCC catalyst preparation process was displayed, consisting of plasma treatment and deposition of MIL-101(Fe)/g-C 3 N 4 on the hydrophobic carbon cloth ( Figure 4a). As can be seen from Figure 4b,c and the insets, the carbon cloth is hydrophobic but MIL-101(Fe)/g-C 3 N 4 composite was equably distributed on hydrophilic surface of carbon cloth, which also confirmed the successful synthesis of MCC catalyst. Furthermore, according to the reconstructed 3D image (Figure 4d) and the cross-sectional image (Figure 4e), a result could be come to that the gas-liquid-solid interface was constructed completely. The advantages of the system mainly lie in the following issues: 1) the admirable Z-scheme heterojunc- Nonetheless, Z-type heterojunctions are formed by the accession of enthetic redox electron mediator accompanied by some issues including being limited to solution phase, unpredictable side reactions, and sensitivity to pH value. [115] Based on the description, S-scheme heterojunction has been regarded as a versatile substitute where the internal electric field, band bending, and electrostatic interaction not only promote the separation of photogenerated e À and h þ but also maintain a high redox capacity; resultantly, this type of material shows promising prospects in the field of photocatalysis. [116] For instance, Pham et al. demonstrated an admirable S-scheme a-Fe 2 O 3 /g-C 3 N 4 nanocatalyst which has shown high efficiency and stability in antibiotic degradation. [117] Meanwhile, C 3 N 4 -based S-scheme heterojunction photocatalysts are aligned with H 2 O 2 production. [118] Liu et al. found the S-scheme heterojunction endowed ZnO/g-C 3 N 4 with wider range of light absorption, more efficient charge separation, and especially a specific electron-transfer pathway, leading to efficient photocatalytic H 2 O 2 . [119] Thanks to the eminent structural advantages, there is no doubt that S-scheme heterojunction will be one of the most promising materials for photocatalysis-self-Fenton.

Nonmetal Element and Molecule Doping
Heterojunctions construction is a close-knit and excellent contact through two or more materials, which is an adjustment for the interface characteristics between materials. Nevertheless, doping is introducing new structural units into the catalyst to achieve the adjustment of the physical and chemical properties, further improving the photocatalytic performance. [91] Namely, doping (nonmetal element doping, mental doping, and molecule doping) paves an effective method to tuning the bandgap structure, light-harvesting capability, and conductivity. For g-C 3 N 4, the most commonly used modification method is nonmetal element doping and molecule doping. Ma and co-workers fabricated a P-doped g-C 3 N 4 (P-g-C 3 N 4 ) by a PSFs to efficiently degrade and mineralize organic pollutants with the H 2 O 2 generated in situ and FeSO 4 added externally (Figure 5a). [120] From the characterizations of P-g-C 3 N 4 , terminal carbon in triazine rings was partially replaced by P in the P-g-C 3 N 4 , which resulted in the π delocalization of CN, further enhancing the capability of absorbing visible light and oxidizing. Therefore, the P-g-C 3 N 4 was eligible for the outstanding H 2 O 2 generation. Besides, the density of doped element has a vital influence on the activity of photocatalytic materials due to the existing of recombination centers by excessive heteroatom doping. As can be seen from Figure 5b, the H 2 O 2 yielding of P-g-C 3 N 4 -10 was superior to other carbon nitride containing P ratios. With the desired H 2 O 2 yielding, a PSFs was constructed based on the mechanism that the H 2 O 2 was activated by the Fe 2þ to produce numerous •OH radicals with strong oxidizing property. Compared with whatever photocatalysis and traditional Fenton technology, the degradation and mineralization capacity was boosted significantly. The PSFs based on the P-g-C 3 N 4 -10 reached to 0.82 h À1 in 2,4-DCP and mineralization ability was 3.8 and 22.5 times higher than Fenton technology and bulk-g-C 3 N 4 photocatalysis, respectively (Figure 5c).
In addition to introducing atoms into g-C 3 N 4 molecules, organic molecules can also be introduced by doping. Meanwhile, a great deal of scientific researches have proved that the incorporation of organic molecules with oxygen functional groups displayed great potential in H 2 O 2 production. On the one hand, the incorporation of oxygen-containing groups administers to form the intermediates during H 2 O 2 production. [121]  Reproduced with permission. [106] Copyright 2022, Elsevier. e) Synthetic process of ZnO/g-C 3 N 4 heterojunction photocatalyst. f ) HR-TEM image of 10% ZnO/g-C 3 N 4 . g) H 2 O 2 productivity under simulated sunlight over various catalysts (optimal conditions: 500 W Xe lamp (320 nm < λ < 780 nm), 0.05 g of the catalyst, 50 mL of the reaction solution, 10 vol% sacrificial agent, pH = 3). h) Sterilization efficiency for bacteria over different photocatalysts (optimal conditions: 500 W Xe lamp (320 nm < λ < 780 nm), 0.05 g of the catalyst, 50 mL of the reaction solution, 10 vol% sacrificial agent, pH = 3). Reproduced with permission. [107] Copyright 2021, Elsevier. On the other hand, the creation of donor-acceptor (D-A) improves the efficiency of exciton dissociation, selectivity of 2-e À O 2 reduction, and visible light absorption. [122] As an example, the 4-phenoxyphenol (PP) moieties were grafted onto the polymeric alkalinized CN framework via water bath, oil bath, and calcination. Under irradiation, sequential 3-e À O 2 reduction reaction occupied a dominant position while H 2 O 2 produced in the photocatalytic stage was mainly reduced in the dark condition.
The selective reaction at different stages led to the high efficiency of pollutant degradation [123] The high-efficient and stable H 2 O 2 production performance provides the prerequisite for the PSFs. In other work, Wang's group prepared a pyromellitic acid diimine (PDI)-modified g-C 3 N 4 (g-C 3 N 4 /PDI) by incorporating the pyromellitic acid diimine on the g-C 3 N 4 molecules for the bacterial inactivation in ballast water (Figure 5d). [124] In Figure 5e, the H 2 O 2 production remain went up to 199.9 μmol L À1 even in seawater. In addition, when the Fe 2þ was added to the system, %6.5 log of Vibrio alginolyticus was almost inactivated within 60 min (Figure 5f ). Obviously, Figure 5g-i reveals that the cells were perfect in 0 min; the cell membranes were wrinkled and pored in 20 min and mostly concave in 30 min. Therefore, we could draw the conclusion that the introduction of pyromellitic acid diimine molecules improved the photocatalytic property significantly and the H 2 O 2 has been fully utilized in in situ photo-Fenton system for sterilization. From the abovementioned literatures, different engineering strategies are responsible for mending the corresponding deficiencies in physical and chemical properties of catalysts. For instance, morphology modulation can solve the drawbacks of lower specific surface area, slower carriers propagation efficiency, and limited light absorption ability while doping is of great benefit in addressing the light-harvesting capability. Overall, a separate modification strategy has certain limitations from the aspect of optimizing the properties. Integrating different modification strategies to make them work together and maximizing the synergistic effect is an effective way to enhance the PSFs performance. Theoretically, the most feasible method is to combine the morphology control with other modification i) The enlarged image shows sequential photocatalytic reactions, including photocatalytic in situ generation and activation of H 2 O 2 . The hydrophobic surface of MIL-101 (Fe)/g-C 3 N 4 / carbon cloth can capture O 2 in the air and transfer it directly to the surface of the hydrophilic catalyst. Hydrophilic water concentrates on organic pollutants. Photo-Fenton reaction occurs at the gas-liquid-solid interface, generating reactive oxygen species and degrading pollutants. Reproduced with permission. [114] Copyright 2022, Elsevier. methods, such as introducing defect for more reactive active sites, heterojunction construction for electrons transfer, and doping for stronger conductivity. For instance, Zhu's group synthesized a self-assembled tetra(4-carboxylphenyl)porphyrin (SA-TCPP)/oxidized g-C 3 N 4 (O-CN) heterojunction photocatalyst realized by an in situ electrostatic method. [125] g-C 3 N 4 (O-CN) heterojunction photocatalyst which was realized by the was first doped with oxygen and then formed a heterojunction with SA-TCPP due to the π-π interaction between the SA-TCPP and O-CN (Figure 6a,b). The catalyst SA-TCPP/O-CN was equipped with fine light-harvesting ability and eminent interfacial charge transfer ability from the built-in electric field and electron delocalization effect. The above advantages endowed the catalyst with outstanding oxygen evolution, pollutant degradation, and disinfection activity. In view of the above experimental effects, the group further applied this method to the PSFs and successfully synthesized the catalyst with excellent H 2 O 2 and pollutant degradation activity. [126] From Figure 6c, it could be obviously noticed that the high-efficiency degradation performance was ascribed to the faster carrier transfer, plentiful reactive sites from the doping and morphological modifications, the quicker cycle of Fe 3þ and Fe 2þ , and high-efficiency utilization of H 2 O 2 . As expected, the system contains a large number of •OH by electron spin resonance (ESR) detection (Figure 6d). In this part, in order to enhance the photocatalytic activity of g-C 3 N 4 and better applied to the PSFs, many approaches were put forward to including morphology modulation, heterojunction construction, and doping (nonmetal element and molecule doping). In detail, morphology modulation mainly affects the specific surface area, carriers propagation efficiency and the light absorption ability. As for heterojunctions construction, it mainly focus on the tuning the structure of photocatalysts, optimizing the transformation of e À and h þ and the effective of heterojunction acts as a coupling of various material properties. Compared with heterojunction, doping is a strategy which is a regulation within a single material, further changing the physical and c) The comparison of k value and TOC removal rate between photocatalysis-self-Fenton and photocatalysis. Reproduced with permission. [120] Copyright 2020, Elsevier. d) Schematic illustration of in situ photo-Fenton system inactivated mechanism proposed in the g-C 3 N 4 /PDI composite system. e) Comparison of photocatalytic H 2 O 2 production in deionized water (DI) and seawater (SW) using g-C 3 N 4 and g-C 3 N 4 /PDI as the photocatalysts (or with the addition of 1.8 mM Fe 2þ ). f ) In situ photo-Fenton inactivation of V. alginolyticus in g-C 3 N 4 , g-C 3 N 4 /PDI, and g-C 3 N 4 /PDI/Fe 2þ systems. g-i) SEM images of V. alginolyticus cells treated by the g-C 3 N 4 /PDI/Fe 2þ system with different VL irradiation times: 0, 20, and 30 min. Experimental conditions: cell density of 7 log cfu mL À1 , 1.0 g L À1 catalyst, [Fe 2þ ] = 1.8 mM, λ ≥ 420 nm, T = 30°C, [pH] 0 = 8.1. Reproduced with permission. [124] Copyright 2021, American Chemical Society. chemical properties of catalysts. Both nonmetal element and molecule doping have a great influence in the photocatalytic activity. However, there are still some problems which is that the photocatalytic performance is always limited by the types, concentration, and state of the doped elements and molecules. We believe that the aforementioned methods make a great influence in g-C 3 N 4 performance and PSFs and more approaches will be come up with efforts of all. Furthermore, by means of the abovementioned modifications, the yield of H 2 O 2 over g-C 3 N 4 was significantly improved, but it is well known that the Fenton reaction also requires the participation of metal ions (Fe 2þ / Fe 3þ ) while the g-C 3 N 4 fails to meet the requirement. Therefore, a catalyst which is not only equipped with outstanding photocatalytic performance of H 2 O 2 production but also could supply the iron ions is urgently needed.

Metal Oxides (Fe 3 O 4 )
Metal ions (Fe 2þ /Fe 3þ ) also need to be supplied externally except for H 2 O 2 in the traditional Fenton process which has given rise to many problems that need to be solved urgently. Owning to the easy formation of iron mud and uneasy reuse, the secondary pollution and high cost severely limit the practical application of Fenton technology. Therefore, an outstanding class of materials are needed that not only provide a source of Fe 2þ but also can be recycled very well. Besides, the circulation of Fe 2þ /Fe 3þ pairs also needs to be noticed. Iron-based compounds have come into view because they are magnetic and can be easily separated from solution.  [127] Compared with other common iron oxides, Fe 3 O 4 possesses the virtues of easy separation, good thermal stability, and easy particle size control, making it the most active and widely studied catalyst. [128] More importantly, considering that the octahedral structure of Fe 3 O 4 could contain Fe 2þ and Fe 3þ , it is possible to trigger the reversible redox in the same position. [129] Following problem is that too small Fe 3 O 4 particle size will lead to agglomeration. [130] Many efforts have been performed to overcome the issue such as morphological modification, [131] wrapping eminent materials on the surface, [132] and loading on materials with large surface areas. [133] For example, Zhang et al. constructed the 1D Fe 3 O 4 nanotubes (NTs) with vortexdomain structure through the combination of electrospinning and calcination process, showing high energy conversion efficiency. [134] Moreover, Fe 3 O 4 @SiO 2 @PMMA core-shell-shell magnetic microspheres were successfully synthesized which possessed not only fine dispersion but also excellent magnetism. [135] Loading on surface of other materials is a promising way to modify the property of catalysts, not only enhancing the specific surface area but also optimizing the photoelectric performance. Zhu's group obtained a Fe 3 O 4 @tourmaline (Tml) catalyst by depositing the Fe 3 O 4 on the surface of Tml. [136] From Figure 7a  Reproduced with permission. [126] Copyright 2022, Elsevier. To reveal the mechanism of e À behavior in Fe 2þ /Fe 3þ circulation, the computational calculation was performed shown in Figure 8. Based on the above characterizations, we have already speculated that the e À came from the decomposition of water. According to the calculation results in Figure 8a, the adsorption energy of the -OH was the lowest at the Fe adsorption site, indicating that the Fe was mainly used as the -OH adsorption site. Homoplastically, it was evident that the Fe 3þ owned the lowest adsorption energy in B atom, indicating that B atom was the main active site of Fe 3þ (Figure 8b). Hence, two pathways were put out. One is that the -OH adsorbed on the positive surface of the Tml is oxidized to release e À , and the other one is that Fe 3þ adsorbed on the negative of the Tml got e À from the Tml. Furthermore, the former pathway owned the lower free energy in the whole reaction, demonstrating the source of e À of Fe 2þ / Fe 3þ circulation is the oxidation of -OH (Figure 8c,d).
Combined the superior property of Fe 3 O 4 and heterojunction modification, Liu and co-workers built a ZnO@Fe 3 O 4 composite by loading Fe 3 O 4 nanoparticles on ZnO microflower to oxide nitrophenol (Figure 9a). [137] In this system, the construction of ZnO@Fe 3 O 4 heterojunction has improved the specific surface area (Figure 9b) and light absorption for in situ H 2 O 2 production as well as provided numerous Fe 2þ /Fe 3þ . The most novel aspects in this system are that the H 2 O 2 generated could be activated to create reactive radicals mainly containing •OH radicals to efficiently degrading the p-nitrophenol and finally the ZnO@Fe 3 O 4 would be separated from the system due to the magnetic effect of iron Fe 3 O 4 (Figure 9c-e). Beyond that, combined doping and heterojunction, the Ti-Mn 3 O 4 /Fe 3 O 4 was fabricated with oxygen vacancies by Pan et al., which distinctly improved the electrical conductivity owing to the formed electron delocalization impacts, contributing to the high H 2 O 2 yield and tiamulin removal efficiency. [138] In sum, the Fe 3 O 4 , one of the metal oxides, was primarily described in PSFs application. The physical and chemical properties were optimized by constructing heterojunctions with other materials, including surface area, light absorption range, and photoelectric properties. Most importantly, the participation of Fe 3 O 4 not only overcame the problem of Fe 2þ /Fe 3þ source, but also made the catalyst easily separated from the reaction solution quickly due to the magnetic property of Fe 3 O 4 . However, from the above experimental results, it is obviously noticed that pH value strictly limits the proceeding of photocatalysisself-Fenton. The pH value of all Fe 3 O 4 system is adjusted to about 3 which will lead to the subsequent economic cost and environmental issues.

Metal Sulfide (CdS, FeS 2 )
Apart from being limited by the addition of H 2 O 2 and Fe 2þ , the Fenton reaction also faces the challenge of acidic pH in the reaction. And only in the pH range from 2 to 4, the Fenton reaction could produce the maximum amount of active substances to achieve the optimal catalytic performance. [139] Because acidic condition is not only conducive to the production of H 2 O 2 but also the efficient releasing of Fe 2þ without marked precipitating. [140,141] However, the ultimate solution must be  neutralized before discharging the system which greatly increases the burden of manpower and cost. Accordingly, an innovative system is urgently established to decompose the pollutants in natural pH. Metal sulfide, as a kind of photocatalytic material, has been extensively studied in pollutants degradation, CO 2 reduction, and water splitting especially in the H 2 O 2 production in recent years due to its excellent bandgap structure and photoelectric properties. Most importantly, the abundant iron content in FeS 2 makes it a promising candidate in photocatalysis-self-Fenton.

Morphology Modulation
CdS, a transition metal sulfide, gradually comes into view due to the excellent visible light reaction and high catalytic efficiency. And the bandgap of it is 2.4 V, which is able to meet thermodynamic requirements of many photocatalytic redox reactions. [142] As is well known, the morphology of catalyst has a great influence on the catalytic activity and the morphology of CdS includes 1D/2D nanosheet and 3D arborization. Especially, the hollow structure of CdS owns strong light absorption ability, abundant redox active sites, and short migration distance of photogenerated e À and h þ . [143] Therefore, a PSFs could be preliminary formed based on CdS. As shown in Figure 10a,b, a pine-leaf-like shape with main trunks CdS nanorods was self-assembled via facile hydrothermal method, which presented significant degradation efficiency for sulfamethazine under a wide range of pH from 2.8 to 6.8. [144] The TEM images of CdS nanorods further revealed the main trunks were almost 10 μm with many branches and confirmed the successful preparation (Figure 10c,d). Based on the outstanding production of H 2 O 2 (153.13 μmol L À1 within 90 min), the experiments to degrade sulfamethazine were performed on a large pH scale from 2.8 to 6.8. Surprisingly, no matter in mild acidic or alkaline environment, the degradation performance of the CdS nanorods can always maintain a high efficiency and even reach to 0.064 min À1 in almost neutral condition (pH = 6.4) (Figure 10e). The variation trend of total iron ions and ferrous ions was also traced. As displayed in Figure 10f, at the beginning of the reaction, due to the formation of Fe(OH) 3 precipitation and the transformation of Fe 2þ to Fe 3þ , the total iron and Fe 2þ showed a downward trend. Subsequently, with the production of H 2 O 2 in the system, the transformation of Fe 2þ and f Fe 3þ reached a balance. This phenomenon can be attributed to the accessible e À generated from the PSFs based on CdS nanorods and this is also the reason why the high degradation activity can maintain on a large pH scale.

Heterojunction Regulation
Additionally, Jiang et al. constructed a CdS/rGO heterojunction via encasing CdS nanoparticles sulfide in rGO. [145] The forms of CdS and rGO are spherical nanoparticles and 2D sheets are shown in Figure 11a,b. Generally speaking, granular catalysts have always suffered from the defect of easy agglomeration and CdS performance is unstable, easy to decompose under the influence of the external pH. Thus, encasing CdS nanoparticles sulfide in rGO have prevented the CdS nanoparticles from agglomerating and spread evenly on the rGO sheets (Figure 11c). Compared with pure CdS and rGO, CdS/rGO was endowed with H 2 O 2 production of 580.68 μmol L À1 within 2 h (Figure 11d) and complete removal of phenol in 1 h (Figure 11e). And more remarkably, the degradation activity of CdS/rGO heterojunction exhibited a high degree of stability over a wide range of pH due to the external rGO, which contained abundant high functional groups of GO (Figure 11f ). In order to reveal the active substances in the PSFs, the PL spectra were performed to detect the concentration of •OH radicals. It could be obviously observed that the PL intensity was apparently higher than that of CdS and rGO, implying that the system contained a large number of •OH radicals (Figure 11g). Finally, a mechanism could be proposed displayed in Figure 11h, based on the admirable structural properties of heterojunction, abundant H 2 O 2 was generated through O 2 reduction and H 2 O oxidation, and then the H 2 O 2 was productively utilized by the additional of Fe 2þ to produce numerous •OH radicals with strong oxidability. As discussed above, CdS, a typical metal sulfide, presents various merits in photocatalysis. However, there are still some safety problems in the practical application. For example, CdS will undergo a photocorrosion reaction under the light. [146] Namely, it is unstable under light and photodissolution occurs at the same time as photocatalysis when some of the toxic metal ions will dissolve out which is toxicity to organisms and harmful to the environment. [147] Collectively, it is necessary to find a  [137] Copyright 2020, Elsevier. catalyst with excellent photocatalysis as well as environmentally friendly characteristic. Consequently, FeS 2 has been regarded as a promising alternative due to the low cost, abundance on earth, and numerous Fe 2þ . [148] All of these advantages lay a good foundation for the formation of PSFs. Li and co-workers conducted FeS 2 with resorcinol-formaldehyde resins to create a heterojunction (FeS 2 -RFR) photocatalyst (Figure 12a). [149] It can be seen from Figure 12b that the FeS 2 -RFR exhibited a high H 2 O 2 concentration under the light radiation. This phenomenon revealed that the FeS 2 -RFR heterojunction photocatalyst possessed great ability for O 2 reduction. Meanwhile, as the Fe 2þ was ionized from FeS 2 -RFR catalyst under the excitation of light, joining the activation of H 2 O 2 in situ generated, a PSFs was provoked to effectively degrade roxarsone (Figure 12c). In addition, this high degradation efficiency can be maintained over a wide range of pH from 2.8 to 6.8 (Figure 12d). Many researches have proved that the RFR is a photocatalyst with great capability in H 2 O 2 production. [150,151] In Figure 12e, compared with pure RFR, although the position of conduction band (CB) and valence band (VB) in FeS 2 -RFR was changed, the FeS 2 -RFR still possessed outstanding ability in O 2 reduction and H 2 O oxidation to generate H 2 O 2 . Finally, the complete mechanism of degradation in PSFs is illustrated in Figure 12f. In this part, to overcome the drawbacks of the strict acidic condition and the external addition of Fe 2þ in traditional Fenton technology, the metal sulfide is introduced in detail, mainly focusing on CdS and FeS 2 . Combining morphology modulation and heterojunction regulation can further improve the photocatalytic performance whatever in H 2 O 2 production or in the degradation of pollutants. On the one hand, the ionization of FeS 2 satisfies the meet of abundant Fe 2þ . One the other hand, the morphology modulation and heterojunction regulation in CdS and FeS 2 not only modulate the physical and chemical structure but also promote the circulation of Fe 2þ and f Fe 3þ . This lays the foundation for the expanding application of the traditional Fenton reaction in practice. These catalysts containing metal ions play a high catalytic performance, at the same time leaching ions or nanoparticles will enter environmental waters, cause pollution, or be bioenriched, causing harm to human health. It is crucial to inhibit or reduce the leaching rate of metal ions in practical application.

Other Admirable Photocatalysts
Apart from the common catalysts mentioned above, some other catalysts have also been investigated to some extent due to their excellent potential for H 2 O 2 production in O 2 reduction. Except for the fine stability, low price, and easy to synthesize, the resorcinol formaldehyde (RF) resin is considered as a suitable material for photocaytalysis by virtue of the wide light absorption range and superior light energy conversion efficiency. [152] To mainly overcome the drawback of low mineralization capacity, Zhang and co-workers formed a PSFs based on RF, attaining a desired in degrading and mineralizing pollutants due to the rapid consumption of photogenerated e À and prolonged h þ lifetime. [153] The whole condition of photogenerated e À and h þ transfer is depicted in Figure 13a. All the time, the lower reaction rate of Fe 3þ to Fe 2þ has been confusing us by limiting the subsequent H 2 O 2 production reactions, which was described in the second Figure 10. a,b) Field emission scanning electron microscope (FESEM) images of as-prepared CdS nanorods. c,d) TEM image and the corresponding HR-TEM image of self-assembled CdS nanorods. e) Initial pH and the corresponding pseudo-first-order kinetic curves (the inset). f ) The changes in the concentrations of total iron ions and ferrous ions for the photo-Fenton degradation of SMT by CdS nanorods. Reproduced with permission. [144] Copyright 2022, Elsevier.
www.advancedsciencenews.com www.small-structures.com pathway of Figure 13b. In addition, from Figure 13c, the more positive VB endowed h þ of RF with even stronger oxidation property compared to P-g-C 3 N 4 , CdS/rGO. And meanwhile it can be seen in Figure 13d that the reaction rate between Fe 3þ and e À was higher than the rate between Fe 2þ with H 2 O 2 , which was on account of the rapid consumption of photogenerated e À .
In conclusion, the intensive mineralization of PSFs over RF could be attributed to the synergistic effect of h þ with superior oxidability and •OH radicals, which are derived from the efficient utilization of H 2 O 2 with Fe 2þ from the conversion of Fe 3þ by photogenerated e À . To enhance the practical application of RF PSFs, a system device simulating the actual dynamic wastewater treatment process was obtained, presenting distinguished and long-term degradation efficiency for RhB (Figure 13e,f ).
Carbon nanodots (CDots), a special 0D material, reveals a broad prospect in merging with other materials for photocatalysis. [154,155] Zhang et al. fabricated a FeOCl/CDots photocatalyst via embedding CDots onto the surface of FeOCl by chemical bonds applied in PSFs. [29] Compared with the above RF, the FeOCl/CDots in PSFs avoided externally adding the iron ions owing to the existence of FeOCl. From the HR-TEM image and the clear lattice fringes in Figure 14a, the CDots were successfully embedded on the FeOCl and distributed identically. Then, the bonding mode between the CDots and FeOCl was determined through a series of characteristics, and the structure is shown in Figure 14b. Consequently, the FeOCl/CDots was provided with ideal H 2 O 2 yielding (%337.2 μmol L À1 ) and high-efficient degradation activity in 3 h, almost 14 and 187 times Figure 11. a-c) The TEM images of rGO, CdS, and CdS/rGO. d) In situ generation of H 2 O 2 by using rGO, CdS, and CdS/rGO. e) Photo-Fenton degradation of phenol with in situ-generated H 2 O 2 by using rGO/Fe 2þ , CdS/Fe 2þ , and CdS/rGO/Fe 2þ . f ) The influence of different pH for the phenol degradation by using in situ-generated H 2 O 2 . g) PL spectra of rGO/Fe 2þ , CdS/Fe 2þ , and CdS/rGO/Fe 2þ system under visible light irradiation in O 2 -saturated solution. h) A plausible mechanism proposed for the photo-Fenton degradation of phenol with in situ-generated H 2 O 2 . Reproduced with permission. [145] Copyright 2019, Elsevier. higher than signal FeOCl and CDots, respectively (Figure 14c,d).
To clarify the reduction process of O 2 , the rotating disk electrode was performed and the n values by fitting the Koutecky-Levuch plots ( Figure 14e) were 2.4-2.7, implying that oxygen reduction is dominated by the one-step 2-electron reduction reaction. [156] Besides, the density functional theory (DFT) calculation results further confirmed the availability of one-step 2-electron reduction reaction during the process of H 2 O 2 production ( Figure 14f ). As a result, the FeOCl/CDots displayed efficient visible-light-driven degradation of 4-CP over a wide pH scale (2.9-8.7) (Figure 14g). All these could be ascribed the improvement of light absorption and photocarrier transfer from the tight bond between FeOCl and CDots. Traditionally, the mass transfer efficiency of H 2 O 2 from the outside or production sites to activation sites has been puzzling us. However, the production and activation reactions of H 2 O 2 all took place on the surface of FeOCl/CDots, which greatly boosted the degradation efficiency of pollutants (Figure 14h). Some other materials have also been innovatively explored. Hu (Figure 15a). [157] Similarly, Guo and co-workers constructed a green PSFs based on the natural pyrite and organic acid. [158] In this system, the presence of organic acids sped the forming of H 2 O 2 and Fe(II) species (Figure 15b). In short, the various merits of unusual materials including RF, FeOCl/CDots, and Ti 3 C 2 MXene/MIL-100(Fe) hybrid have been synthetically discussed and they all showed high degradation   Figure 16. [159][160][161][162][163][164][165][166][167] 6. Summary and Future Outlook In short, from this perspective, this article summarizes the catalysts which are conducive to the photocatalysis and of great potential in PSFs as well as the corresponding structure modulations. In particular, the meticulous roundup in regulation methods composed of morphology modulation, heterojunctions construction as well as doping is provided. In addition, we also explored profoundly how modification methods could enhance catalytic performance via improving the internal vital properties of the catalysts including specific surface area, light absorption ability, electron transport, and H 2 O 2 activation. Besides this, on the basis of current elegant documents, different pollutants and different catalytic environment are applied to the catalyst to study the performance and mechanism in specific PSFs. Among various catalysts, metal-free graphite carbon nitride (g-C 3 N 4 ), metal oxides (Fe 3 O 4 ), and metal sulfides (CdS, FeS 2 ) have displayed a significant outstanding prospect in PSFs. Morphology modulation is universally approved as the most maneuverable manner to optimize the specific surface area, carriers propagation efficiency, and light absorption ability in improving photocatalysis activity. As for heterojunctions construction, it can expedite the transport of e À and h þ at the material interface through the intimate combination of materials. Furthermore, the introduction of other materials can also provide a source of Fe 2þ /Fe 3þ and accelerate the circulation of Fe 2þ /Fe 3þ avoiding the drawbacks of additional Fe 2þ /Fe 3þ and iron mud precipitate. Recently, more efforts have been concentrating on the design of Z-scheme heterojunction owing to the extra benefits of retaining e À and h þ with stronger oxidation reduction inside the material compared with traditional heterojunction. In addition, doping (nonmetal element and molecule doping) is of great benefit in light-harvesting capability and conductivity which plays an important role in circulating of Fe 2þ / Fe 3þ . What is encouraging is that great progress has been made in some excellent catalysts applied in the advancement of PSFs under the continuous efforts of everyone. But there are still some challenges that need to be faced in the future from the in-depth mechanism exploration, more optimized structural adjustment, development of new catalysts, and high-efficient practical application. In order to better optimize the PSFs performance, some insights and suggestions are as follows: 1) At present, the H 2 O 2 ultimate yielding in situ determined by both H 2 O 2 generation and decomposition is a crucial factor in PSFs which is still subjected to a low output. It is vital to improve the H 2 O 2 production when restraining the H 2 O 2 decomposition. For this reason, direct one-step 2-electron reduction reaction should be promoted to accelerate the H 2 O 2 production route, and the degradation of H 2 O 2 can be suppressed by loading some cocatalysts or tuning the energy band. Additionally, although many works have been  devoted to optimizing the structure of catalysts, light utilization efficiency, the separation and migration of carrier and reaction on the catalyst surface are still the primary respects limiting the performance of PSFs. It is necessary to develop new catalysts with excellent performance, solving the problems fundamentally.
2) As we all know, sacrificial reagents have a great significance in the process of photocatalytic H 2 O 2 production which mainly play the role of capturing photogenerated h þ or e À which will Reproduced with permission. [29] Copyright 2020, Elsevier. Figure 15. a) Proposed reaction mechanism for photo-Fenton degradation of TCL over MXFAA/MIL(Fe)-POR. Reproduced with permission. [157] Copyright 2022, Elsevier. b) Proposed degradation mechanism scheme of CBZ in the presence of pyrite and organic acids under simulated solar light irradiation. Reproduced with permission. [158] Copyright 2021, Elsevier. participate in the degradation of pollutants. In some PSFs, h þ can be used directly to oxidize and degrade pollutants and e À can participate in the circulation of Fe 2þ /Fe 3þ . In the PSFs mentioned in the article, some of them do not contain sacrificial agents while the other systems are with sacrificial agents. Therefore, we need to further study whether sacrificial agents need to be added and the added dosage, which will have a bearing on the subsequent degradation part. 3) From the perspective of environment friendliness, it is dramatically urgent to pay attention to the recycle of catalysts and leaching of toxic metal ions, and namely the potential hazards of the catalyst itself. For the recycle of catalysts, some studies on hydrogels and thin films have made great progress, providing a new idea for catalyst recovery. Concretely, the catalyst powder can be loaded in sodium alginate and then prepared into hydrogels and photocatalytic films applied to the degradation system. After the reaction, the hydrogels and films can be easily separated from the system and it has been proved that some hydrogels and films supported by catalysts can be exerted for many times with fine catalytic performance. For the leaching of toxic metal ions, inserting a single atom on the catalyst frame can not only expand the utilization efficiency of metal atoms, but also enhance the stability of the catalyst to prevent the leaching of metal. 4) Generally speaking, the understanding of the mechanism of PSFs is still in an immature stage and there are still many problems to be explored in more depth. Typically, electron transfer quantity is obtained by the photocurrent response (PCR) or the linear sweep voltammetry (LSV) but the electron transfer routes are difficult to trace. In particular, capture experiments and ESR are often used to qualitatively detect the active substances in reaction system, but whether they participate in redox reactions and how much they contribute is difficult to quantify. Sophisticated in situ and real-time characteristics could be used to detect variation in active substances. Advanced DFT calculation including electron band structure, adsorption energy, and reactive active sites should be used to provide the theoretical basis for further probing the internal mechanism.
All theoretical experiments are ultimately designed to be applied to practical application to bring convenience to people. 5) Although PSFs has been used to some extent in practice, including the treatment of organic wastewater and disinfection, many obstacles must be got over to achieve large-scale application industrially. We can focus on two aspects including coupling with other techniques and applying the technology to advanced processing equipment. On the one hand, combining PSFs with other technologies such as piezoelectric technology can further treat the wastewater on the basis of the PSFs. On the other hand, PSFs can be embedded into a safe and efficient water treatment plant (tower reactors and fluidized beds, etc.) to give full play to its degradation performance.
Above all, PSFs provides us with a new option for efficient treatment of organic wastewater and a wide space for whether selection or modification of catalysts. Raw safe and efficient catalysts as well as excellent modification methods need to be further explored. We are looking forward to that PSFs will develop its ability to the full under massive efforts in the future.