A review on emerging homojunction photocatalysts with impressive performances for wastewater detoxification

Abstract Heterogeneous photocatalysis, as an energy-saving technology, is a fascinating strategy to tackle environmental issues and energy shortages. The efficacy of these processes is severely relevant to the suppression of charges recombination, which hinders the widespread utilization of these processes. The segregation of charges from fast recombination can be considerably improved by the formation of heterojunction among the components of photocatalysts. Nevertheless, the formed heterojunction between different semiconductors diminishes the continuity of the charges carriers transfer amongst the semiconductors. Moreover, the construction of heterojunctions significantly relies on matching of band structures of the desired semiconductors. Hopefully, homojunction construction between different exposing facets, morphologies, and crystal phases of a single semiconductor is a feasible strategy for impressively suppression of charges from recombination. In comparison with the heterojunction photocatalysts, the almost identical components in both sides of the junction area provide not only the incredibly matched lattice, but also the integrity of bonding between the counterparts, which facilitates the charge migration within the photocatalyst. Furthermore, the established built-in electric field in the junction regions could enforce the charge carriers for efficiently separation and transfer within these photocatalysts. Herein, we reviewed the latest researches about the emerging homojunction photocatalysts and their application in the degradation of various wastewater pollutants. The present review intends to stimulate the research communities to focus on this kind of photocatalysts, as emerging materials, with the impressive performances in the photocatalytic processes. Graphical Abstract


Introduction
In the recent decades, the environmental crisis has dramatically increased because of rapid industrialization, fast economic development, and the progressively growth of the human population.Emerging pollutants such as organic dyes, pesticides, fertilizers, herbicides, microplastics, heavy metal ions, surfactants, pharmaceutical, and personal care products have been observed in surface and drinking waters, all over the world (Balakrishnan et al., 2023;Mustafa et al., 2022;Naghdi et al., 2023;Rizwan & Bilal, 2022) Besides, harmful gases such as CO 2 , NO x , and SO 2 , and volatile organic compounds are released from vehicles and industries, which cause dramatic deterioration of ecology and threaten humankind᾽s health.Based on the WHO report, air pollution has become one of the main reasons for fatal diseases, which has led to the death of about seven million people every year around the world (Ajma et al., 2022).Moreover, it is estimated that about three million metric tons of petroleum oils are discharged every year into the marine ecosystem.Because of low density, the released hydrocarbons form an oily film on the polluted water, which leads to reduced photosynthesis of aquatic living systems.Moreover, the hydrocarbon layers could decrease the amount of oxygen dissolution in the polluted system, leading to unbalance of marine ecosystems (Samuel et al., 2022).In addition, rapid depletion of fossil fuels and the emission of massive amounts of CO 2 into the air atmosphere are other issues threatening human society᾽s development.Consequently, the replacement of fossil energy with carbon-free renewable sources is a major challenge for modern societies (Sun et al., 2023;Suresh et al., 2023).Moreover, converting solar energy to high-density hydrogen energy and CO 2 conversion to value-added chemicals and renewable fuels can diminish the greenhouse effect through balancing the carbon cycle in our environment (Akhundi et al., 2020;Domínguez-Espíndola et al., 2022;Suresh et al., 2023;Yu et al., 2014).
Fortunately, heterogeneous photocatalysis, as an energy-saving green technology, has promising capabilities to feasibly address many environmental and energy shortage crises, and synthesize chemical compounds, thanks to its environment-friendly and cost-effectiveness (Akhundi et al., 2022;Kumar et al., 2023;Sabri et al., 2023;Zhao et al., 2023).In these processes, solar light, as the sole input, and a proper material, as a catalyst, are utilized, and the reactions take place in ambient pressure and temperature, which are brilliant features of any "green" technology (Gopinath et al., 2020;Jeon et al., 2020).Briefly speaking, in the photocatalytic processes, the photocatalyst produces electron/hole (e − /h + ) pairs under the illumination of an appropriate light source, which usually needs few femtoseconds.In addition, the lifetime of the produced e − /h + is generally several picoseconds.However, the transfer time of the photo-induced charges to the surface of photocatalysts is from the order of nanoseconds, which is more prolonged than that of the bulk phase recombination time.Furthermore, the time needed for the redox reactions on the catalyst surface is significantly longer than several nanoseconds (Bao et al., 2021;Zhang et al., 2022c) (Figure 1).Thereby, in most cases, a large part of generated charges in the conduction (CB) and valence (VB) bands recombine to release the absorbed energy as heat energy.Unfortunately, only a few portions of the photo-induced charges migrate to the photocatalyst surface to participate in various reduction and oxidation reactions, depending on the species present in the reaction system (Figure 2) (Jeon et al., 2020).To date, the charge segregation efficiency of many heterogeneous photocatalysis processes is still lower than the requirement of large-scale application.Consequently, efficacious spatial segregation of charges within the bulk phase of the photocatalysts is of severe importance.Furthermore, photocatalysis technology suffers from some drawbacks, including insufficient absorption of solar light, limited surface area, high resistance for charge migration, and weak adsorption of reactants (Figure 3) (Bao et al., 2021;Kallawar et al., 2021;Velempini et al., 2021;Zhang et al., 2022c).In the present time, the addressing of the mentioned issues in the heterogeneous photocatalytic processes is a research hotspot worldwide.Hopefully, thanks to extensive research activities, there are some enlightenments about designing efficient photocatalysts to raise the efficacy of photocatalytic reactions (Kallawar et al., 2021;Velempini et al., 2021;Xing et al., 2022a;Zhang et al., 2022b).In these regards, various types of photocatalysts have been designed through metal/nonmetal doping, defect engineering, morphology control, band gap engineering, and a combination of various semiconductors and materials.In the proposed strategies to tackle the poor features of heterogeneous photocatalysts, heterojunction construction to obtain heterojunction type II, Z-scheme, and S-scheme heterostructures has attracted many research interests (Bao et al., 2021;Zhang et al., 2022c).
However, the developed interface between different semiconductors could diminish the continuity of the charges carriers transfer within the combined semiconductors.Moreover, the construction of heterojunctions among semiconductors significantly relies on the matching of band structures of the desired semiconductors.Fortunately, homojunction construction between different exposing facets, morphologies, and crystal phases of a single semiconductor is a feasible strategy, which leads to fast transfer of charges in the formed junction interface (Guan et al.,  2022; Phang et al., 2020).Compared with the heterojunction photocatalysts, the almost identical chemical compositions on both sides of the junction area provide not only the highly matched lattice and electronic features, but also the integrity of bonding amongst the counterparts, which facilitates the charge transfer within the photocatalyst.It is worth to mentining that in these photocatalysts, the combined semiconductors do not have precisely identical properties, like Fermi energy, VB and CB energies, and n/p-type characteristics (Guan et al., 2022).In other words, the position of Fermi, VB, and CB energies of semiconductors strictly depends on microstructures, size, morphology, exposed facets, and crystal phases, resulting in developing of a built-in electric field (BEF) in the junction regions.Therefore, the formed BEF could enforce the charges for efficient segregation and migration among the components, which results in promoted photocatalytic performance (Guan et al., 2022;Phang et al., 2020).In addition, effective interfacial contacts are developed between the components of homojunction photocatalysts, because the combined components are prepared from a semiconductor with different characteristics, which is practically attractive from economic aspects, while different precursors and exogenous processes are used to prepare heterojunction photocatalysts (Guan et al., 2022;Phang et al., 2020).
In recent years, extensive researches have been conducted for developing homojunction photocatalysts, and they have shown superior photocatalytic activity in hydrogen production through water splitting, conversion of greenhouse gases into valuable fuels, removal of volatile organic gases, and degradation of organic componds, antibiotics, and removal of inorganic pollutants (Shao et al., 2021;Shen et al., 2021b).This review work provides a comprehensive understanding the methods of designing and preparing homojunction photocatalysts based on g-C 3 N 4 , TiO 2 , and miscellaneous semiconductors, as well as their activity in detoxifying organic, medicinal, and inorganic pollutants.Moreover, this review aims to provide new ideas and stimulate further research activities in the field of homojunction semiconductors.Currently only limited semiconductors have been utilized for fabricationg homojunction photocatalysts; hence, developing homojunction photocatalysts using another semiconductors is highly emphasized to fabricate homojunction photocatalysts with outstanding performances.

Fundamentals of homojunction photocatalysts
The mechanism of BEF establishing in homojunction photocatalysts, similar to the heterojunction photocatalysts, is relevant to the differences of VB, CB, and Fermi energies of the integrated components.In general, the key principle in the design of homojunction photocatalysts is combination of two components with various band/Fermi energies originated from a semiconductor.The charges migration between the Fermi levels of the components causes the developing electric field in the junction layer.As seen in Figure 4, the homojunction photocatalysts are classified as a p-n junction, different phase junction, different facet junction, and various morphologies   and crystallinities junctions (Guan et al., 2022).In the p-n homojunction photocatalysts, because of different doping levels of element/s and induction of various elemental vacancies in the desired semiconductor, the electronic characteristics of an identical semiconductor change to n-type or p-type.Thus, after homojunction construction between these semiconductors, the internal electric field is established (Figure 4a) (Liu et al., 2022a;Shan et al., 2022;Wang et al., 2022a).Furthermore, the integration of different phases of an identical semiconductor with some stable phases, such as TiO 2 (with anatase, rutile, and brookite phases) (Ruan et al., 2022;Zhang et al., 2022a), ZnIn 2 S 4 (hexagonal and cubic phases) (Qian et al., 2020), AgVO 3 (with α-AgVO 3 , β-AgVO 3 , and γ-AgVO 3 phases) (Lin et al., 2020), Bi 2 WO 6 (tetragonal and orthorhombic phases) (Chang et al., 2022), Bi 2 O 3 (with α-Bi 2 O 3 , β-Bi 2 O 3 , δ-Bi 2 O 3 , γ-Bi 2 O 3 , and ε-Bi 2 O 3 phases) (Vega-Mendoza et al., 2021), CeVO 4 (tetragonal zircon and monoclinic monazite phases) (Ding et al., 2021), CdS (with zinc-blend and hexagonal wurtzite crystal phases) (Zhang et al., 2022d), could lead to impressive charge separations through formation of type-II homojunction, thanks to the differences of VB and CB energies (Figure 4b).In the crystal-facet-based homojunction photocatalysts, the charge separation enforced through junction between different exposed facets of the defined material (Figure 4c).In this case, the main driving force for hindering rapid recombination of charges is the oriented BEF caused by the differences of surface work function of the integrated crystal facets (Ji et al., 2022;Meng et al., 2023).The last type of homojunction photocatatalysts is constructed by combining different morphologies, sizes, and crystallinities of the desired semiconductor (Figure 4d).The mentioned differences induces changes in the VB, CB, and Fermi energy levels, resulting in the establishment of BEF (Guo et al., 2022b;He et al., 2023;Li et al., 2023b;Ma et al., 2022;Sun et al., 2022;Zhang et al., 2023a).The main advantage of homojunction photocatalysts is related to the highly matched lattice and electronic characteristics, and the continuity of the chemical bonding in their structure, which cause fast migration and transfer of charges (Guan et al., 2022;Phang et al., 2020).While heterojunction photocatalysts are made by combination of different semiconductors, resulting in poor matching of their lattice and discontinuity of bonds (Guan et al., 2022;Phang et al., 2020).The advantages and disadvantages of homojunction and heterojunction photocatalysts are summarized in Table 1.Consequently, the rational design and the fabrication of effective homojunction photocatalysts using various semiconductors have recently attracted growing attention from the research community in the field of heterogeneous photocatalysis, which are described in the section 3.

Preparation methods
As mentiond above, the key principle for rational designing and preparation of homojunction photocatalysts is integration of two components of almost same semiconductor with various band/Fermi energies.Therefore, the methods utilized for fabrication of homojunction photocatalysts are not totally different from the procedures applied for synthesis of heterojunction photocatalysts, and the generally employed methods are hydrothermal, solvothermal, calcination, sol-gel, ultrasonic irradiation, and thermal polymerization procedures.In sections 3.2, 3.3, and 3.4, more explanations are given about the preparation and characterization methods employed for synthesis of homojunction photocatalysts, and they have been summarized in Tables 1S-3S.

G-C 3 N 4 -based homojunction photocatalysts
Graphite carbon nitride (g-C 3 N 4 ) is a semiconductor with a polymeric structure.In the recent decade, this semiconductor has been extensively utilized in various photocalalysis processes, including water decontamination, hydrogen generation through water splitting, photofixation of nitrogen, photoreduction of CO 2 , disinfection of microorganisms, and production of high-value chemicals, and it also has been subject of many review papers (Akhundi et al., 2020(Akhundi et al., , 2022;;Sun et al., 2023;Xing et al., 2022a).The widespread utilization of this metal-free semiconductor is related to its unique properties, including innoxious characteristics, earth-abundant resources, high stability, easy synthesis, liable surface modification, and tunable energy band gap (Balakrishnan et al., 2023;Jiang et al., 2022;Li et al., 2021;Liao et al., 2021;Praus, 2023;Sayed et al., 2022;Xue et al., 2022;Zou et al., 2023).Along with the design and fabrication of novel heterojunction photocatalysts, many attempts have been devoted to the synthesis of homojunction photocatalysts based on g-C 3 N 4 through: (i) integration of modified g-C 3 N 4 (containing carbon or nitrogen vacancies) with pristine one, (ii) combination of g-C 3 N 4 with various dimensions such as 0D with 1D, 0D with 2D, 1D with 2D, 2D with 2D, and 3D with 2D, (iii) synthesis of homojunctions between triazine and tri-s-traizine g-C 3 N 4 , (iv) combination of nonmetallic elemental-doped g-C 3 N 4 with pristine g-C 3 N 4 , and (v) combination of high-crystalline with low-crystalline/amorphous g-C 3 N 4 , which are described as follows.
Liu et al. synthesized g-C 3 N 4 /g-C 3 N 4 homojunction photocatalyst by calcination the supramolecule prepared from melamine and dicyandiamide using a hydrothermal procedure at 180 °C for 10 h (Liu et al., 2017).The homojunction photocatalyst had a nanomesh morphology, extended surface area, and promoted charge segregation characteristics.The promoted interface contect between the components led to 4.3-folds of visible-light performance for rhodamine B degradation relative to the conventional g-C 3 N 4 /g-C 3 N 4 composite.This work presented an effective method for the preparation of g-C 3 N 4 homojunction through a simple supramolecular pre-organization route with highly efficient photocatalytic ability with numerous interfacial regions.In 2017, 2D/2D homojunction g-C 3 N 4 photocatalysts were fabricated using liquid exfoliation and chemical blowing approaches (Ye et al., 2017).In this paper, two types of g-C 3 N 4 nanosheets with different band energies were combined and an enhanced activity of 12.8-folds relative to the pristine g-C 3 N 4 was observed.The improved activity of homojunction photocatalyst was assigned to the impressive charge migration within the interface of two phases with high matched band structures.This method, by adjusting the composition and modifying the surface of the same substrate, is a general strategy to prepare the homojunction photocatalysts without metal elements and compatible with the environment, which had high stability.
Figure 5. suggested mechanism for the enhanced photocatalytic activity in the fabricated 2d g-C 3 n 4 /g-C 3 n 4 nanocomposite; reproduced with permission from (Qiao et al., 2018).
In the study conducted by Tan et al., mesoporous g-C 3 N 4 /g-C 3 N 4 nanosheets (Meso-g-C 3 N 4 /g-C 3 N 4 ) photocataltsts were synthesized by template-calcination strategy (Tan et al., 2017).The Meso-g-C 3 N 4 /g-C 3 N 4 photocatalyst had a high surface area and large pore size.The visible-light-induced performance for methyl orange degradation and hydrogen production of the homojunction photocatalyst was almost 12.5 and 6.5-folds as large as the pristine g-C 3 N 4 , respectively (Figs.S1 (a, b)).The impressive enhancement was attributed to the segregation of photogenerated charge via the mechanism, presented in Fig. S1c, and the facilitating diffusion of species in the mesoporous structure.
Liu et al. synthesized a homojunction photocatalyst comprised of crystalline g-C 3 N 4 (CGCN) and amorphous g-C 3 N 4 (AGCN) (Liu et al., 2018).The difference between VB and CB of CGCN and AGCN components were 0.50 and 0.33 eV, respectively.Hence, the CGCN/AGCN homojunction photocatalyst acted like a type-II heterojunction photocatalyst to effectively separate charge carriers from recombination, promoting activity in the degradation of organic pollutants and H 2 generation reaction.Qiao et al. fabricated 2D g-C 3 N 4 /g-C 3 N 4 homojunction photocatalyst through heating the formed supramolecular precursors by self-assembly of cyanuric acid (C), melamine (M), and thiourea (T) (Qiao et al., 2018).The resultant 2D homojunction photocatalysts presented severely enhanced performance for RhB degradation upon visible light, which was 3.9-folds as large as the conventional g-C 3 N 4 /g-C 3 N 4 homojunctions fabricated by thermal polycondensation of M and T compounds.Figure 5 displayed the Z-scheme junction established between g-C 3 N 4 components, in which the photoinduced electrons moved from the CB of CN-MC to the VB of CN-T component.Therefore, the spatially separated CB electrons of CN-T and the VB holes of CN-MC started to degrade the pollutant molecules effectively.As a result, this work provided an effective and easy approach through supramolecular aggregation to fabricate Z-scheme homojunction photocatalysts.
In 2019, a 2D/2D homojunction photocatalyst was prepared through a combination of pristine g-C 3 N 4 with acid-treated white g-C 3 N 4 (Wg-C 3 N 4 ) (Vidyasagar et al., 2019).The Wg-C 3 N 4 /g-C 3 N 4 photocatalyst exhibited significant adsorption and impressive activity for the degradation of anionic acid violet 7 (AV-7) dye.The textural analyses confirmed that Wg-C 3 N 4 provided numerous active sites for the adsoprtion of the dye molecule and transferred it to the surface of g-C 3 N 4 , as photoactive material, within many reactive species have been produced.The resultant 2D/2D homojunction photocatalyst based on g-C 3 N 4 presented a rational, low-cost, and robust design for the rapid treatment of industrial wastewaters.
G-C 3 N 4 -based p-n homojunction photocatalysts were prepared through introducing nitrogen vacancies within an n-type g-C 3 N 4 by secondary calcination of pristine g-C 3 N 4 in the atmosphere of hydrogen (Cao et al., 2019).The RhB degradation constant over the optimum homojunction was about 8.3-folds as large as bulk g-C 3 N 4 .The outcomes of PL, photocurrent, and EIS characterizations presented that the admirable activity was related to the satisfactory segregation and transfer of charges, due to the established BEF in the junction area between the p-type and n-type g-C 3 N 4 .In this research, the combined g-C 3 N 4 had different electronic characteristics, which confirmed by the Mott-Schottky plot with a "V" shape scheme.
Ba et al. synthesized O-modified g-C 3 N 4 (MCN) composed of massive mesopores and homojunctions by calcining cyanuric acid-loaded melamine (Ba et al., 2019).In the case of MCN-x homojunction photocatalysts (x denoted weight of trichloroisocyanuric acid in g), the intensity of g-C 3 N 4 (CN) diffraction peaks at 13.0° and 27.5° diminished, presenting formation of mesoporous structures (Figure 6a).The Mott-Schottky and valance-band XPS analyses displayed that the energy bands and Fermi energy of the resultant photocatalysts were different (Figure 6(b,  c)).Consequently, the developed BEF between the components led to more segregation and transfer of charges, as affirmed by PL and photocurrent analyses (Figure 6(d, e)).Hence, the photocatalytic performance of the optimum photocatalysts was enhanced three times in comparison with the pure CN.This work employed a procedure to prepare the hierarchical structure of MA (melamine), the simultaneous formation of mesopores and homojunctions in CN, which can guide the preparation of similar photocatalysts.
Zhang et al. combined Mo-doped g-C 3 N 4 (Mo-CN) with g-C 3 N 4 (CN) to fabricate Mo-CN/ CN homojunction photocatalysts using Mo-CN and urea as the precursors (Zhang et al., 2020).The homojunction materials exhibited impressive photocatalytic performance in NO oxidation to NO 2 under visble light.As proved by PL analyses, the construction of homojunction between Mo-CN and CN counterparts had a vital role in charge segregation within CN, which resulted in improved activity.The presence of Mo atoms on the Mo-CN/g-CN surface provided abundant electrons to the adsorbed O 2 to increase the production of • O 2 -species, leading to increased NO 2 production.
Crystalline g-C 3 N 4 /amorphous g-C 3 N 4 (denoted as H-LCN) homojunction photocatalysts were fabricated using a one-step thermal polymerization route through adjusting synthesis parameters, and utilized for RhB degradation under visible light (Song et al., 2020).The performance of H-LCN homojunction photocatalyst was superior and reached to 98% in 10 min with a degradation constant of 0.363 min −1 .Owing to more negative CB of amorphous carbon nitride in comparison with the crystalline material, the photoinduced electrons transferred from the amorphous phase to the crystalline structure.In the contrary, the holes migrated from the crystalline phase to the amorphous one, resulting in improved separation of charges proved by PL analysis.
Wei et al. prepared a cyano group modulated porous g-C 3 N 4 composed of p-type cyano modulated g-C 3 N 4 /n-type g-C 3 N 4 homojunction photocatalyst anchored by silver nanowire (Wei et al., 2021).In the Mott-Schottky curve, there was p-n homojunction between the modulated g-C 3 N 4 and pristine g-C 3 N 4 , which was confirmed by a "Λ-shaped" curve.The prepared ternary Ag-CN/ KOH nanocomposites exhibited impressive activity in the removal of aqueous Cr (VI) upon visible light.The photocatalytic removal constant over the Ag-CN/KOH-2, as optimum photocatalyst, was about 13.8 folds larger than the bulk g-C 3 N 4 .When the ternary system photoexcited under visible light, e − /h + pairs were produced over both the modulated g-C 3 N 4 and g-C 3 N 4 .Then, the electrons migrated from the CB of the modulated g-C 3 N 4 to that of g-C 3 N 4 , while the holes transferred in the contrary direction, resulting in impressively separation of charges, as affirmed by PL and EIS analyses.This work provided a new insight for the fabrication of plasmonic p-n homojunction photocatalyst with high stability for photocatalytic applications.
In another research, a g-C 3 N 4 homojunction photocatalyst was prepared by calcination a molecular mixture of urea and thiourea and the fabricated homojunction photocatalysts were demonstrated admirable activity against RhB degradation under visible light (Phang et al., 2021a).The impressive performance was devoted to the impressively retardation of charge from recombination, because of fast interfacial charge migration between the counterparts of the homojunction photocatalyst.Consequently, the preparation of metal-free g-C 3 N 4 /g-C 3 N 4 homojunction photocatalyst provided an excellent opportunity for industrial-scale wastewater treatment using sunlight, earth abundant precursors, and safe for the environment.
In the research conducted by Zheng et al., 0D/1D g-C 3 N 4 homojunction photocatalyst (MUCN) was fabricated by an in-situ thermal polymerization route using the supramolecular of melamine-cyanuric acid (MCS) and urea as co-precursor (Zheng et al., 2021).The homojunction photocatalyst exhibited admirable activity in RhB degradation, which was 37, 7, and 2 times of BCN, UCN, and MCN, respectively.The MCN and UCN components of the photocatalyst formed type-II junction photocatalyst, in which the VB and CB of UCN were more negative than those of MCN.Upon visible-light radiation, both components produced charge carrier, due to their small energy gap.The electrons migrated from the CB UCN to the CB MCN , and the holes in the contrary direction, resulting in generation of more charge carriers to participate in the degradation process.The authors showed that the MUCN photocatalyst had not only a controllable and uniform morphology, but also outstanding photocatalytic activity in the degradation of RhB and the production of H 2 O 2 .
Guo et al. homojunction photocatalysts composed of g-C 3 N 4 (CN) integrated with three crystallization levels using a melt-polymerization of the mixture of urea, thiourea, and melamine at 600 °C for 3 h (Guo et al., 2022a).The CN components with different crystallinity had various VB and CB energies and the CB position decreased with increase of crystallinity.Hence, after junction construction between the components with different crystallinity, electrons transferred from the low crystalline CN to the high crystalline one, resulting in establishing BEF.After band bending, the electrons transferred from the CB of high crystalline CN to that of low crystalline component, which led to more separation of charges.The authors confirmed the proposed mechanism for charge separation using EIS, PL, and photocurrent analyses.That is why, the photocatalytic ability of homojunction photocatalyst in degradations of RhB, levofloxacin, tetracycline, ciprofloxacin, and vancomycin was impressively promoted, which was 56, 429, 29, 72, and 56-folds as much as high-crystallinity CN.Therefore, with the rational design of homojunction photocatalyst through crystallinity difference, a photocatalyst with high compatibility with the environment was developed, which could be beneficial for the industrial scale.
The K + ˗decorated 1D/2D g-C 3 N 4 /g-C 3 N 4 photocatalyst was prepared by calcination a mixture of K 2 CO 3 and melamine at 550 °C (He et al., 2022).The synergistic impact of K + and CO 3 2-ions led to the fabrication of the 1D/2D homojunction photocatalyst.The photocatalytic abilities were studied against RhB and ciprofloxacin degradations and a feasible mechanism for the admirable activity was proposed.When the photocatalyst illuminated by visible light, the photoexcited charges produced within the 1D and 2D components.Due to the more negative potential of 1D CN (−0.95 V) relative to the 2D CN (−0.90 V), the electrons rapidly transferred from the ID to 2D CN.In the contrary, the holes transferred from the 2D to 1D CN, because of more positive potential of the VB 2D in comparison with the VB 1D .Consequently, the charge carriers impressively separated and transferred, which led to enhanced photocatalytic activity.After four cycles of using the photocatalyst, it showed significant stability, which makes it possible to use this ion-doping homojunction and metal-free photocatalyst, for large-scale applications.
Xing et al. prepared 2D S-doped g-C 3 N 4 nanosheets/3D g-C 3 N 4 flower-like (2D/3D CNSCN) homojunction photocatalysts using a polymerization method (Xing et al., 2022b).The 2D/3D CNSCN homojunction systems displayed impressive activity for RhB degradation upon visible light.Activity of the optimum photocatalyst was 4.8 and 5.7-folds of CNS and CN components, respectively.In the PL and photocurrent outcomes, the outstanding activity was devoted to the effectively segregation/transfer of the charges.In the mechanism for enhanced H 2 evolution and RhB degradation, the 2D CNS and 3D CN components formed type-II junction, in which the photogenerated electrons transferred from the 3D CN to 2D CNS, while the holes transferred in the contrary direction, resulting in the promoted activity.This study provided a new way to design 2D/3D homojunction photocatalyst with excellent stability and durability for application in energy production and environmental protection.
Recently, g-C 3 N 4 quantum dots (QDs) were anchored on O-doped g-C 3 N 4 by a facile route to fabricate g-C 3 N 4 QDs/O-doped g-C 3 N 4 homojunction photocatalyst and applied for efficiently degradation of tetracycline (Liu et al., 2022b).The enhanced activity of homojunction photocatalyst was 2.43, 3.06, and 4.29-folds of g-C 3 N 4 /g-C 3 N 4 QDs, g-C 3 N 4 /g-C 3 N 4 non-QDs, and g-C 3 N 4 , respectively.After homojunction establishing between g-C 3 N 4 QDs and O-doped g-C 3 N 4 components, electrons transferred from the CB of O-doped g-C 3 N 4 to the VB of g-C 3 N 4 QDs, leaving the CB electrons and VB holes retained within the g-C 3 N 4 QDs and O-doped g-C 3 N 4 components, respectively.Therefore, the charges effectively separated and migrated to the surface of the homojunction photocatalyst to participate in the degradation reaction, as proved by PL, EIS, and photocurrent characterizations.
A mechanical mixture of polymerized melamine (M) and 3-amino-1,2,4-triazole (T) was calcined at 550 °C air to prepare homojunction photocatalysts, denoted as MTCN (Yang et al., 2022).The synthesized photocatalysts presented an admirable performance in the degradation of tetracycline.As seen in Figs.S2(a-c), the MTCN homojunction photocatalysts had significant success in the suppressing charges from recombination and facile migration.Figure S2d illustrates the energy band structure of the binary photocatalysts comprised of pristine MCNS and TCNS components.Due to more negative Fermi energy of and VB of the TCNS relative to the MCNS, an electric filed was established between these counterparts, after equilibrating the Fermi levels.Under the enforcement of BEF, the CB electrons transferred from MCNS to TCNS, while the holes migrated in the contrary direction, leading to a significant photocatalytic performance in pollutant degradation and CO 2 reduction.In addition to being simple and effective, this strategy provides more active sites for the adsorption of CO 2 molecules for reducing them to valuable fuels.
In a work conducted by Wu et al., supramolecular melamine-cyanuric acid, oxalic acid, and urea were used for the preparation of O-doped g-C 3 N 4 homojunction photocatalysts (Wu et al., 2022a).The oxygen doping regulated the band structure of g-C 3 N 4 through the substitution of the N atoms, which led to the broadening of the light-absorption range.Furthermore, the introduction of oxygen atoms changed the charge arrangement and band levels of g-C 3 N 4 , which improved the separation efficacy of charges.Also, by shortening the transfer distance of charge carriers by the g-C 3 N 4 nanosheets, the recombination of charges decreased.Consequently, the fabricated photocatalysts exhibited superior performance against 2,4-dichlorophenol degradation.This work presents a new approach with multi-regulation within g-C 3 N 4 -based photocatalyst for environmental protection and remediation.
Ternary boron carbon nitride hollow nanotubes (BCN-HNTs) were prepared by calcination a molecular mixture of urea, boric acid, and polyethylene glycol at 900 °C for 4 h under Ar atmosphere (Chen et al., 2022b).The Mott-Schottky analyses presented that the fabricated homojunction photocatalyst composed of p/n-types domains.The p-n BCN-HNTs homojunction photocatalyst exhibited a significant activity in reduction of U (VI) upon visible light.The photoreduction rate constant of the optimum photocatalyst enhanced 6.18-folds in comparison with the pure boron nitride nanosheets (hBNNSs).An internal electric field was established between the p-type and n-type domains, resulting in the efficaciously separation of charges which is beneficial for promoted activity.
Very recently, Li et al. utilized a two-step polymerization method to fabricate homojunction photocatalysts comprised of thin g-C 3 N 4 nanosheets with modulated energy gap using melamine and dicyandiamide (Li et al., 2023a).The optimum homojunction system degraded 10 mg/L of RhB within 15 min, which was 2.3 and 1.2 times superior than its counterparts.The impressive activity was attributed to the established type-II junction between the components, which led to more separation of charges.The prepared homojunction photocatalyst showed significant stability after 12 h of exposure to visible light, which can be very beneficial for practical applications.
Table S1 provides a summary of the items discussed about g-C 3 N 4 -based homojunction photocatalysts in terms of the method of preparation, the dosage of utilized photocatalyst, the light source, the duration of light radiation, the type of pollutant, and the degradation percentages of pollutants.

TiO2-based homojunction photocatalysts
TiO 2 , as a widely utilized photocatalyst, has many appealing characteristics, including environmentally green, high stability, earth abundance, and excellent photooxidation capability (Domínguez-Espíndola et al., 2022;Jeon et al., 2020).Nevertheless, the wide energy gap of about 3.2 eV and rapid charges recombination lead to its poor activity under solar light.To date, many strategies, such as elemental doping, introducing oxygen vacancies, morphology controlling, and heterojunction construction with various small-energy-gap semiconductors have been introduced to improve its photocatalytic performance (Rostami et al., 2022;Zeshan et al., 2022).Based on the potential of TiO 2 for designing homojunction photocatalysts, many research activities have been carried out about the photocatalytic performances of TiO 2 in environmental and energy issues, because of its capability to fabricate various combinations of crystal phases, dimensions, and crystal facets, which are described as follows.
In 2010, Shifu et al. prepared Fe-TiO 2 /TiO 2 homojunction photocatalyst using a ball-milling method, and they applied it for the removal of methyl orange and Cr (VI) under UV light (Shifu et al., 2010).For the Fe-TiO 2 (5 wt %)/TiO 2 photocatalyst, the Cr (VI) photoreduction and methyl orange photo-oxidation efficiencies were reached 78.3 and 51.2%, respectively.The PL analysis affirmed the efficacious separation of charges, which was relevant to the developed BEF between Fe-TiO 2 and TiO 2 components.
A microporous TiO 2 layer was grown over anatase TiO 2 by a low-temperature hydrothermal procedure (Lyu et al., 2017).The microporous TiO 2 /anatase TiO 2 homojunction photocatalyst presented enhanced adsorption and degradation of toluene molecules under UV light.The improved activity was attributed to the synergistic impact of microporous TiO 2 and anatase TiO 2 in BEF construction, which led to electron migration from the CB of anatase TiO 2 to the that of microporous TiO 2 , and on the contrary, the holes from the VB of microporous TiO 2 to that of anatase TiO 2 .Consequently, the charges are efficaciously suppressed from fast recombination, which improved the photocatalytic performance.The increase of the surface area through the growth of microporous TiO 2 on the surface of anatase TiO 2 led to an increase in the adsorption of toluene molecules.Also, the developed homojunction at the interface of microporous TiO 2 and anatase TiO 2 improved the separation of charge carriers.
By Duan et al., high energy TiO2 nanostructures with exposed (001) facets (HE-TiO2-NCs) were decorated over TiO 2 nanofibers (TiO 2 -NFs) by an electrospinning method preceded a calcination step (Duan et al., 2018).The resultant HE-TiO2-NCs/TiO2-NFs homojunction photocatalysts exhibited promoted activity in the oxidation of acetone and NO air pollutants under UV light.The impressive activity of the homojunction photocatalyst was ascribed to the transfer of electrons from the CB of TiO 2 -NFs to the VB of HE-TiO 2 -NCs through a Z-scheme mechanism.Therefore, the segregated holes on VB of TiO 2 -NFs utilized as oxidation species, while the electrons on CB of HE-TiO 2 -NCs acted as reduction species.
Lee et al. utilized aluminum acetylacetonate (Al(acac) 3 ) as a catalyst for the synthesis of Ti 3+doped TiO 2 (Ti 3+ -TiO 2 ) using a sol-gel process (Lee et al., 2018).In the proposed route, the Ti 3+ -TiO 2 was formed over bulk TiO 2 and Al(acac) 3 was removed by an annealing process.The surface Ti 3+ -TiO 2 had a thickness of about 2-10 nm, and bulk TiO 2 was relevant to the (101) plane of anatase TiO 2 with a lattice spacing of 0.345 nm.The resultant Ti 3+ -TiO 2 /TiO 2 photocatalyst presented an enhanced performance in the degradation of methylene blue under simulated solar light.The electrochemical analyses confirmed that the formed junction between Ti 3+ -TiO 2 and TiO 2 components had vital role in the migration of electrons from the CB of Ti 3+ -TiO 2 to that of TiO 2 , and holes from the VB of TiO 2 to Ti 3+ -TiO 2 , similar to a type-II heterojunction photocatalyst.
The TiO 2 quantum dots (QDs)/TiO 2 nanosheet homojunction was prepared by Wang et al., using a facile grinding method (Wang et al., 2018b).The optimum TiO 2 QDs/TiO 2 -40 photocatalyst, in which 40 was the volume of added TiO 2 QDs, displayed superior performance in the degradation of RhB under UV light.The CB and VB energies of TiO 2 QDs were respectively more negative and less positive than those of the TiO 2 nanosheets.Thereby, photoinduced electrons moved from the CB of TiO 2 QDs to that of TiO 2 nanosheets; while, the holes in the VB of TiO 2 nanosheets were migrated to that of TiO 2 QDs, which led to more pronounced activity.Regarding the high stability achieved during five cycles and the utilized easy and pollution-free preparation procedure can meet the demands of practical applications.
Miao et al. utilized a facile hydrothermal method to combine white TiO 2 (W-TiO 2 ) with black TiO 2 (B-TiO 2 ) to fabricate a W-TiO 2 /B-TiO 2 homojunction photocatalyst (Miao et al., 2019).The activity of W-TiO 2 /B-TiO 2 photocatalyst was nearly 3.9 and 5.2-times as high as B-TiO 2 and W-TiO 2 photocatalysts, respectively.Upon visible-light illumination, the CB electrons of W-TiO 2 rapidly transferred to the CB of B-TiO 2 , and the photoinduced holes transferred from the VB of B-TiO 2 to that of W-TiO 2 , which led to impressive activity of W-TiO 2 /B-TiO 2 homojunction photocatalyst.This study provides an interesting idea for the preparation of homojunction photocatalysts based on TiO 2 for environmental protection, thanks to the promoted separation of charge carriers and high stability after five cycles.
An efficient rutile TiO 2 /brookite TiO 2 homojunction photocatalyst was synthesized through crystal phase control using a facile hydrothermal method at 150 °C for 24 h (Chen et al., 2019).The resultant photocatalyst showed excellent performance in hydrogen generation and photodegradation of RhB upon visible light.The VB XPS studies presented that the VB and CB energies of brookite TiO 2 were more positive than those of the rutile TiO 2 , which resulted in the construction of a type-II junction between the counterparts.Hence, the electrons moved from the CB of brookite TiO 2 to that of rutile TiO 2 ; meanwhile, the holes transferred in the reverse direction.Thus, due to the well-matched band alignment in the rutile TiO 2 /brookite TiO 2 homojunction photocatalyst, the charge carriers suppressed from the fast recombination, which are utilized for production of more reactive species.
In 2021, Mirzaei et al. applied a pulsed laser deposition route for nitrogen incorporation in the anatase TiO 2 to synthesize substitutional N-doped TiO 2 with various nitrogen content (Mirzaei et al., 2021).Then, a six-layer homojunction was prepared using gradient N-doped TiO 2 (g-N-TiO 2 ).The activity of fabricated materials was evaluated in terms of sulfamethoxazole degradation upon simulated solar light.The degradation constant of the mentioned antibiotic over the optimum homojunction photocatalyst was 0.0061 min −1 , which was 10-folds higher than TiO 2 .The impressive activity of g-N-TiO 2 photocatalyst was related to the promoted charge separation by the established BEF within the junction regions.
Yuan et al. supported the homojunction of oxygen vacancy-rich TiO 2-x with TiO 2 (TO V -T) over natural diatomite (D) to fabricate ternary TO V -TD photocatalysts, and they utilized them for indoor photocatalytic removal of gaseous formaldehyde (Yuan et al., 2022).Under visible-light illumination, the TO V -TD-1:3 composite was the most effective material in removing this toxic gas.The mineralization rate constant over the composite was nearly 4.42 folds of TO V and 1.74 folds of TO V -T homojunction, respectively.The support of TO V -T homojunction photocatalyst over diatomite considerably increased the oxygen vacancy concentration and provided extended adsorption centers for formaldehyde gas.As seen in Figure 7a, in the first step, formaldehyde molecules were adsorbed over the composite.Then, under the light exposure, the photoinduced holes transferred from the TiO 2-x to TiO 2 , because of less positive potential for VB of TiO 2 (+2.95V/NHE) in comparison with TiO 2-x (+3.15 V/NHE).The efficiently separated hole over the VB of TiO 2 produced hydroxyl radicals through oxidation of water molecules (+2.38 V/NHE).The produced radicals and holes started to oxidize adsorbed formaldehyde molecules in the last step.This work presents a new approach to prepare a diatomite-based homojunction photocatalyst for photocatalytic removal of gaseous formaldehyde.
Very recently, Kang et al. prepared oxygen vacancy-rich anatase TiO 2 /rutile TiO 2 (A-Vo/R) homojunction photocatalysts using a two-step hydrolysis and calcination procedure, and they monitored the production of reactive species over the fabricated materials under visible light (Kang et al., 2023).The photocatalytic studies exhibited that surface and interface oxygen vacancies had a synergistic impact on the generation of reactive species.The surface oxygen vacancies enforced O 2 adsorption, and transmitted the photoinduced electrons to these molecules on the surface of A-Vo to rapidly produce • O 2 − and 1 O 2 species.The interfacial oxygen vacancies not only facilitated the electron migration between the biphasic TiO 2 , but also changed the electron transfer path from type-I junction to Z-scheme junction, leaving the holes and electrons on the rutile and anatase phases, respectively, which had high potential for production of the mentioned reactive species (Figure 7b).
Table S2 provides a summary of the items discussed about TiO 2 -based homojunction photocatalysts in terms of the method of preparation, the dosage of utilized photocatalyst, the light source, the duration of light radiation, the type of pollutant, and the degradation percentages of pollutants.

Miscellaneous homojunction photocatalysts
In 2010, Bi 2 WO 6 nanostructures with Bi 2 WO 6 quantum dots (QDs), dispersed on single crystalline Bi 2 WO 6 nanosheets, were prepared using a solvothermal procedure in ethylene glycol at 180 °C for 15 h (Cui et al., 2010).The photocatalytic activity of homojunction photocatalyst in degradation of RhB was improved under visible-light exposure, because of the formed junction between Bi 2 WO 6 QDs and Bi 2 WO 6 nanosheets, which was beneficial for the charges segregation.
The α/γ-Bi 2 O 3 homojunction photocatalysts were successfully synthesized using a hydrothermal method by Sun et al. in 2012(Sun et al., 2012).In the homojunction photocatalysts, γ-Bi 2 O 3 nanoparticles deposited over the surface of α-Bi 2 O 3 flakes.The visible-light-triggered activity of the homojunction photocatalyst in RhB degradation was dramatically higher than those of the α-Bi 2 O 3 and γ-Bi 2 O 3 components.The authors assigned the promoted activity to the charge transfer from α-Bi 2 O 3 phase to γ-Bi 2 O 3 phase and the large surface area, which makes it a promising photocatalyst with excellent stability for environmental treatment.
Rajbongshi and Samdarshi fabricated Co-doped biphasic ZnO comprised of zinc-blende and wurtzite phases (Rajbongshi & Samdarshi, 2014).The activity of synthesized materials were studied by removal of methylene blue and phenol under visible light, which showed almost 3 and 1.5 folds more activity than the wurtzite undoped ZnO and Co-doped ZnO photocatalysts, respectively.The enhanced activity of biphasic photocatalyst was devoted to the homojunction construction between the zinc-blende and wurtzite phases of ZnO, and the enhancing oxygen vacancies at the surface, ensuring more separation of photogenerated carriers.This work is an effective one-step and low-energy method for the synthesis of homojunction photocatalysts using ZnO.
In a research conducted by Kong et al., the hexagonal and cubic phases of tungsten oxide were combined via a hydrothermal method through adjusting solution pH (Kong et al., 2015).In the fabricated photocatalyst, the rod-like hexagonal structure, with the (001) growth direction, integrated with block cubic phase, in the optimum pH.The established BEF between the integrated phases led to transfer of electrons from the hexagonal phase to the cubic phase; meanwhile, the holes in the inverse direction, which resulted in improved performance for RhB degradation.The authors attributed the significant increase in photocatalytic activity to the matching of the energy levels, efficient electron-hole transfer, and improved absorption of visible light.
Bi 5+ -doped Bi 4 V 2 O 11 /Bi 4 V 2 O 11 (Bi 5+ -BVO) homojunction photocatalyst was fabricated via a facile oxygen-induced procedure (Lv et al., 2017).Interestingly, the Mott-Schottky plot for Bi 5+ -BVO photocatalys displayed simultaneously positive and negative slopes, confirming the presence of both n-type and p-type semiconductors in this photocatalyst.On the cotrary, the BVO exhibited n-type characteristic.The developed p-n homojunction introduced a BEF at the interface, due to the differences of their Fermi levels.In the developed p-n homojunction photocatalyst, the charge carriers considerably retarded from recombination, because of facile charge transfer between the components, as affirmed by PL and EIS studies.That is why, the Bi 5+ -BVO photocatalyst exhibited superior performance in the photoreduction of Cr (VI) relative to the pristine Bi 4 V 2 O 11 .
In another attempt, the submicron spherical particles (SSA) and rhombic dodecahedron (RDA) particles of Ag 3 PO 4 were combined to fabricate homojunction photocatalyst using a facile physical combination route (Xie et al., 2018).The photocatalysis studies presented that the photocatalyst synthesized by 0.6% wt. of RDA and 99.4% wt. of SSA exhibited the best activity in the degradation of RhB upon visible light.The VB XPS analyses conducted to calculate the VB of SSA and RDA, which were 3.18 and 2.89 eV, respectively (Figure 8a).Using the energy gaps (Figure 8b), the CB of RDA and SSA were determined to be 0.80 and 1.03 eV, respectively.Consequently, the morphology differences of Ag 3 PO 4 semiconductor led to differences in the VB and CB energy levels, as presented in Figure 8c.Thereby, the joined RDA and SSA components established a type II junction structure, in which the CB electrons transferred from RDA to that of SSA, while the holes transferred in the contrary direction, resulting in the suppressed charge carriers.Therefore, the impressively suppressed charges effectively participated in the degradation reactions.This work presented a facile methodology for synthesis homojunction photocatalysts based on Ag 3 PO 4 through controlling the morphology and size.
In a research conducted by Wang et al., homojunction photocatalysts composed of α-In 2 Se 3 nanosheets and γ-In 2 Se 3 nanoparticles were prepared by a facile solvothermal procedure in triethylene glycol using InCl 3 and Se as precursors (Wang et al., 2018a).The homojunction photocatalysts exhibited improved performance in the removal of Cr (VI) from aqueous solutions.In the TEM image, the γ-In 2 Se 3 nanoparticles, with distorted hexagonal morphology, were anchored on the surface of α-In 2 Se 3 nanosheets.In the HRTEM image presented the lattice fringes related to the (111) plane of γ-In 2 Se 3 and ( 104) plane of α-In 2 Se 3 .Upon visible-light exposure, the photoinduced electrons moved from the CB of γ-In 2 Se 3 to that of α-In 2 Se 3 , while the holes transferred from α-In 2 Se 3 to γ-In 2 Se 3 , which resulted in impressive segregation of electron/hole pairs.Consequently, the separated charges effectively participated in the removal of Cr (VI), leading to promoted activity.
Ai et al. prepared S defect-rich Cl-doped Bi 2 S 3 homojunction photocatalysts composed of (1-12) and ( 001) exposed facets by polyvinyl pyrrolidone-assisted hydrothermal procedure, and their performances were assessed against removal of Cr (VI) upon visible light (Ai et al., 2020).The homojunction photocatalyst displayed 45 and 19-folds enhanced photoreduction activity than those of BiOCl and Bi 2 S 3 photocatalysts, respectively, because of the collaboration of the developed facets in the promoted charge segregation.This work is an effective method for the reduction of Cr (VI) in the aqueous phase using defect-rich Cl-doped Bi 2 S 3 nanorods, with high stability.
The W 18 O 49 nanofibers were anchored onto WO 3 microrods to fabricate WO 3 /W 18 O 49 homojunction photocatalysts using a solvothermal method at 160 °C for 12 h (Feng et al., 2020).The binary homojunction photocatalyst exhibited more impressive performance in the degradation of 2,4-dichlorophenol in comparison with the WO 3 , and W 18 O 49 counterparts.The significantly improved activity was related to more harvesting of visible light due to produced hot electrons in W 18 O 49 and more suppressing of charges from recombination, because of charge transfer between WO 3 and W 18 O 49 counterparts via established type-II junction.
Bi 2 MoO 6 /Bi 2 MoO 6-x homojunction photocatalysts were rationally fabricated by a simple solvothermal-calcination procedure, as presented in Figure 9a (Shen et al., 2021a).The optimum photocatalyst presented impressive activity in the simultaneous removal of phenol and Cr (VI) upon visible light, which was 4 and 8-folds as high as Bi 2 MoO 6 , respectively.The PL, EIS, and photocurrent analyses revealed that in the homojunction photocatalyst e − /h + pairs more suppressed, and rapidly transferred to the surface of photocatalyst to participate in the removal reactions efficaciously.As presented in Figure 9b, the CB and VB of Bi 2 MoO 6-x are more negative than those of Bi 2 MoO 6 .Thereby, the electrons migrated from the CB of Bi 2 MoO 6-x to that of Bi 2 MoO 6 , while the holes transferred from the VB of Bi 2 MoO 6 to that of Bi 2 MoO 6-x , resulting in more suppression of the charges from recombination.
By Ding et al., tetragonal zircon CeVO 4 (t-CeVO 4 ) was combined with monoclinic monazite CeVO 4 (m-CeVO 4 ) to fabricate t-CeVO 4 /m-CeVO 4 homojunction photocatalysts by partial phase transition through P-doping method (Ding et al., 2021).Among the photocatalysts, the 7 P-CeVO 4 (7% P-doped CeVO 4 nanobelts) photocatalyst presented superior activity in levofloxacin degradation upon sunlight irradiation.As depicted in Figure 9c, the established homojunction between t-CeVO 4 and m-CeVO 4 components played a vital role in the separation of photoinduced charges because of the developed type-II junction between t-CeVO 4 and m-CeVO 4 components, as confirmed by PL, EIS, and photocurrent analyses.This work provides an excellent approach for environmental remediation through the preparation of t-CeVO 4 /m-CeVO 4 homojunction photocatalyst, which has narrow energy gap and surface oxygen vacancy.
BiOBr nanosheets/BiOBr microplates photocatalysts were synthesized through the growth of BiOBr nanosheets over the BiOBr microplates (Guo et al., 2022b).The homojunction photocatalysts displayed promoted activity for the removal of aqueous bisphenol A and NO from air, compared with the pristine BiOBr.The improved performance was attributed to the constructed junction between BiOBr nanosheets and BiOBr microplates, resulting in diminished charge recombination.Thanks to the facile and economical procedure utilized for fabrication of this photocatalyst, it can be considered for large scale applications to remove pollutions from the environment and solar energy harvesting applications.
Recently, the utilization of MOFs in the field of photocatalysis has attracted widespread attention (Rojas & Horcajada, 2020).Despite crystalline MOFs, MOF gel states (MOGs) have composed of uncontinuous crystalline nanoparticles, which combined by weak Van der Waals forces (Hou et al., 2020).The MOFs xerogels (denoted as MOXs) are prepared after removing solvent from MOGs, leading to formation of micro/meso pores.The established larger pores enables MOXs for rapid migration of larger molecules within pores (Hou et al., 2020).
Recently, tetragonal Bi 2 WO 6 (t-Bi 2 WO 6 ) and orthorhombic (o-Bi 2 WO 6 ) semiconductors were integrated to fabricate t/o-Bi 2 WO 6 homojunction photocatalysts by Bi-induced phase transformation, and utilized for rhodamine B degradation (Chang et al., 2022).The Fermi energy of o-Bi 2 WO 6 is more negative than that of t-Bi 2 WO 6 .Hence, after the n-n homojunction between o-Bi 2 WO 6 and t-Bi 2 WO 6 phases, the electrons transferred from o-Bi 2 WO 6 to t-Bi 2 WO 6 , which resulted in establishing BEF in the junction area.Therefore, the photoinduced electrons in the CB of t-Bi 2 WO 6 rapidly transferred to o-Bi 2 WO 6 and holes in the opposite direction, leading to improved photocatalytic activity.This work is a promising strategy to reduce environmental pollution over highly stable homojunction photocatalysts developed through combination of Bi 2 WO 6 containing different crystal phases.
Very recently, Liu et al. prepared Ni-doped FeS 2 /FeS 2 homojunction photocatalyst by growth of Ni-doped FeS 2 nanoparticles (denoted as Ni-FeS 2 -P) on FeS 2 nanobelts (marked as FeS 2 -B) and applied them for the photocatalytic removal of Cr (VI) (Liu et al., 2023).The activity of optimum Ni-doped FeS 2 /FeS 2 photocatalyst in the removal reaction was 21.8 and 4.54-folds higher than those of Ni-FeS 2 -P and FeS 2 -B photocatalysts, respectively (Figs.S3(a, b)).As presented in Fig. S3(c, d), the electron/hole pairs impressively separated and migrated in the homojunction photocatalyst in comparison with the counterparts, resulting in admirable photocatalytic activity.As observed in Fig. S3e, due to the difference between the Fermi energies of FeS 2 -B and 2Ni-FeS 2 -P components, the electrons transferred from 2Ni-FeS 2 -P to FeS 2 -B until reaching the equilibrium state, which led to the establishing BEF in the junction area.Upon the light irradiation, the electrons transferred from the CB of FeS 2 -B to 2Ni-FeS 2 -P, because of the established internal electric field.Therefore, the accumulated electrons on the CB of 2Ni-FeS 2 -P reduced toxic Cr (VI) to Cr (III) anions.
Table S3 provides a summary of the items discussed about homojunction photocatalysts prepared by various semiconductors in terms of the method of preparation, the dosage of utilized photocatalyst, the light source, the duration of light radiation, the type of pollutant, and the degradation percentages of pollutants.

S-scheme homojunction photocatalysts
Recently, emerging s-scheme heterojunction photocatalysts have been developed due to their appealing characteristics in effective charges transfer among the integrated semiconductors, compared with the traditional type-II and Z-scheme structures (Zhang et al., 2022c).The s-scheme photocatalysts have composed of an oxidation photocatalyst and a reduction photocatalyst with a staggered band structure.In these photocatalysts, the potential of the semiconductor on which the reduction reaction takes place must have a higher CB and Fermi level than the oxidation semiconductor.Although the configuration of s-scheme photocatalysts are similar to type-II heterojunction photocatalysts, the path of charges transfer in the reduction and oxidation semiconductors are totally different, and they are favorable from the thermodynamic and dynamic view points (Manna et al., 2023;Xu et al., 2020b).In these photocatalysts, band bendings take place, resulting in easy and fast transfer of electrons at the interface driven by the developed BEF.Consequently, the charge carriers with poor redox potentials are recombined, while the charges with high redox potentials are maintained, resulting in severely boosted activity (Wang et al., 2022b;Zhang et al., 2022c).Recently, Chen et al. fabricated MOF/MOX (Metal-organic framework/Metal-organic xerogel) homojunction photocatalysts using MIL-100 (Fe) with improved photocatalytic activity for degradations of benzene, toluene, xylenes, and styrene (Chen et al., 2022a).The established BEF between the crystalline MIL-100 (Fe) and MIL-100 (Fe) xerogel through the S-scheme mechanism led to a more pronounced separation of charges, leading to improved activity (Figure 10).Due to the promising characteristics of S-scheme photocatalysts, and rarely developed homojunction photocatalysts with this structure, more research attempts are needed to fabricate homojunction photocatalysts with this pattern for efficacious charges transfer among the components.

DFT calculations in homojunction photocatalysts
Density functional theory (DFT) calculations are generally conducted to determine the band gap and electronic density of states (DOS) of photocatalysts to obtain more insights about the mechanism of photocatalytic reactions (Zhu et al., 2019).In 2022, Yang et al. used DFT calculations to calculate the energy band structures of the prepared photocatalysts.The energy gap obtained from theoretical calculations for MTCN, MCNS, and TCNS photocatalysts was 2.55, 2.95, and 2.85 eV, respectively, which was almost equal to the results obtained from the Tauc's plots (Yang et al., 2022).In addition, according to the DOS diagrams, in these photocatalysts, the VB and CB are composed of 2p orbitals of N elements, and 2p orbitals of C and N elements, respectively.Moreover, according to frontier molecular orbital distribution maps, HOMO and LUMO are all distributed on tri-s-triazine structure.
Additionally, DFT calculations in the research conducted by Liu et al. confirmed that Ni doping was effective in creating sulfur vacancies, and also modified the energy gap and band structure of the FeS 2 semiconductor (Liu et al., 2023).The work function and Fermi level for FeS 2 nanobelts were 5.64 and −0.03 eV, respectively, and the work function and Fermi level for 2Ni-FeS 2 nanoparticles were 5.31 and −0.12 eV, respectively.Considering that the work function value for FeS 2 nanobelts was larger than that of 2Ni-FeS 2 nanoparticles, the electrons were spontaneously transferred from FeS 2 nanobelts to 2Ni-FeS 2 nanoparticles at the 2Ni-FeS 2 /FeS 2 homojunction to develop internal electric field.

Emerging homojunction-heterojunction photocatalysts
To utilize the synergistic benefits of heterojunction and homojunction photocatalysts, the combination of these photocatalysts is also another interesting strategy in designing novel photocatalysts with impressive performances.In this regard, BiVO 4 with the ( 040) and ( 110) facets were combined, and subsequently integrated with Bi 2 WO 6 nanoparticles to fabricate BiVO 4 /Bi 2 WO 6 nanocomposite via a hydrothermal route at 100°C for 12 h (Li et al., 2017).The BiVO 4 /Bi 2 WO 6 nanocomposite displayed significant activity in methyl orange degradation, which was 2.7 and 2.0 times as high as Bi 2 WO 6 and BiVO 4 photocatalysts, respectively.The admirable activity of the designed homo/hetero junction photocatalyst was mainly assigned to the adequate segregation of the photoinduced charges at the interface of the junction areas, as affirmed by EIS, photocurrent density, and PL analyses.
In 2018, Meng et al. combined Bi 2 MoO 6 QDs/Bi 2 MoO 6 homojunction photocatalyst with boron carbon nitride (BCN) to fabricate a Bi 2 MoO 6 QDs/Bi 2 MoO 6 /BCN homo/hetero junction photocatalyst by a hydrothermal procedure at 180 °C (Meng et al., 2018).Interestingly, the optimum ternary system showed impressive performance for Cr (VI) photoreduction under visible light, which was nearly 7.14, 2.28, and 1.66-folds larger than those of BCN, Bi 2 MoO 6 , and Bi 2 MoO 6 QDs, respectively.The PL, photocurrent, and EIS studies presented that in the ternary system, the photoinduced charges were efficaciously segregated, because of faster transfer of holes from Bi 2 MoO 6 QDs to Bi 2 MoO 6 , and subsequently to BCN, resulting in enhanced performance.The preparation of this nanocomposite is a facile route to improve the photocatalytic performance through the integration of homojunction and heterojunction photocatalysts, resulting in preparation of effective photocatalyst with significant stability after five runs.
Guo et al. combined monazite monoclinic BiPO4 and monoclinic BiPO4 to fabricate twinned BiPO 4 photocatalyst, and then integrated with BiOCl to synthesize BiOCl/BiPO 4 (BOC/BP) photocatalysts (Guo et al., 2018).The photocatalytic activity of optimum BOC/BP nanocomposite significantly enhanced in methyl orange degradation under UV light, which was 35.7-folds of twinned BiPO4.The optimum BOC/BP photocatalyst had the most significant photocurrent density, the smallest semicircle in the EIS plots, and the lowest intensity in the PL spectra, which affirmed the suppressed charges separations and rapidly transfer of segregated e − /h + pairs.The BiOCl and twined BiPO 4 components with hetero/homo junctions collaborated in the segregation of charges, in which electrons transferred from the CB of BiOCl to that of BiPO 4 and holes from the VB of BiPO 4 to that of BiOCl, resulting in the enhanced activity.
To utilize the benefits of homo/hetero junction constructions, QDs self-modified Bi 2 MoO 6 / Bi 4 Ti 3 O 12 (BMO(Q)/BTO) materials were rationally synthesized by a solvothermal method at ethylene glycol (Qin et al., 2019).The BMO(Q)/BTO photocatalysts presented excellent activity toward photocatalytic removal of rhodamine B, Cr (VI), and 2-mercaptobenzothiazole under visible light.The EIS and photocurrent analyses revealed impressive separation of e − /h + pairs, due to established homojunction/heterojunction interfaces among the components in charges separations.This work shows a new insight for the fabrication of bismuth-based hom/hetero junction photocatalyst with considerable stability and reusability in detoxification of polluted water.
Lin et al. combined α-AgVO 3 and β-AgVO 3 phases (denoted as AV) with WO 3 (denoted as W) to fabricate Ag/AgVO 3 /WO 3 (AVW) nanocomposites via a facile method (Lin et al., 2020).The photocatalytic performance of materials were explored against RhB degradation and the optimum AVW-10 nanocomposite exhibited promoted activity, which was 3.11 and 57.2-folds as high as AV-50 and WO 3 photocatalysts, respectively.The enhanced activity of AVW-10 was assigned to extended visible-light absorption, as well as improved e − /h + segregation and transportation.The photocatalyst is a combination of type-I and Z-scheme structures through the integration of α and β phases of AgVO 3 with WO 3 .After charge migration among the components, the separated holes in the VB of WO 3 and electrons in the CB of β-AgVO 3 participated in the degradation reaction, leading to remarkably promoted activity.
In 2020, Zhou et al. combined 3D hollow spheres of TiO 2 with ZnO QDs and ZrO 2 to fabricate homojunction between anatase and rutile phases of TiO 2 , and the heterojunction between TiO 2 and ZrO 2 counterparts (Zhou et al., 2020).Under the light irradiation, the degradation of Congo red by 0.5 ZnO QDs@ZrO 2 -TiO 2 nanocomposite reached 94.62%, which was higher than the counterparts.The textural analyses exhibited that the loading of ZnO QDs enhanced the surface area of ZrO 2 -TiO 2 nanocomposite.The PL and photocurrent analyses confirmed the lowest recombination of the charges, and more effective transfer of them in the nanocomposite, which was assigned to the well-matched band levels of ZrO 2 , and anatase and rutile phases of TiO 2 .This research provides the general construction of a 3D regular hollow sphere array structure and stable homo/hetero junction photocatalyst.
Cheng et al. decorated CdS nanoparticles over the nanobelts of biphasic TiO 2 composed of anatase and monoclinic phase of TiO 2 (abbreviated as TiO 2 (B)) by hydrothermal route (Cheng et al., 2020).In the TEM and HRTEM images, CdS nanoparticles have adhered on the nanobelts of TiO 2 .Moreover, the XRD patterns of the fabricated composites, containing the diffraction patterns of CdS, anatase TiO 2 , and TiO 2 (B) counterparts.The PL, photocurrent, and EIS data confirmed more pronounced separation transfer of the charges within the ternary system, which led to significant photocatalytic ability in degradation of RhB under visible light.Under UV-vis light, the CdS/anatase-TiO 2 (B) nanocomposites produced charges in the components.The electrons on the CBs of biphasic TiO 2 migrated to the VBs of adhered CdS nanoparticles through the established double-Z-scheme heterojunction.Consequently, the more redox ability electrons on the CB of CdS and holes on the VB of biphasic TiO 2 utilized to generate more reactive species.
In 2020, Xu et al. combined anatase (A)/rutile (R) homojunction photocatalyst with BiOI to fabricate A/R TiO 2 /BiOI p-n heterojunction photocatalys by a solvothermal procedure at 140 °C for 8 h (Xu et al., 2020a).The photocatalytic performances were studied against simultaneous degradation of ceftriaxone sodium and removal of Cr (VI) upon visible light.The activity of optimum photocatalyst was 1.72 and 25.38-times as high as the A/R TiO 2 and BiOI for degradation of ceftriaxone sodium, and 3.47 and 25.61-folds higher for Cr (VI) reduction, respectively.As seen in Figure 11a, p-n heterojunction construction between BiOI and biphasic A/R TiO 2 led to BEF between the components.The established hom/hetero junctions between A/R TiO 2 and BiOI components, led to impressive activity of the ternary system, which were confirmed by PL, EIS, and photocurent analyses.Hence, the construction of a multi-junction structure leads to the efficacious transfer and separation of charges, resulting in accelerated removal of organic pollutants and heavy metals.
Bi 4 O 5 I 2 -loaded biphasic anatase-TiO 2 (B) nanowires were prepared using an in-situ calcination route, and they applied for degradation of acetaminophen (Mu et al., 2020).As depicted in Figures 11(b, c), the biphase TiO 2 had wire-like morphology with an width of 50-100 nm, while Bi 4 O 5 I 2 composed of highly aggregated particles.As observed in the TEM image of the ternary nanocomposite, Bi 4 O 5 I 2 particles adhered to the biphasic TiO 2 nanowires (Figure 11d).The HRTEM image (Figure 11e) presented lattice spacing of 0.302, 0.342, and 0.375 nm, which were relevant to Bi 4 O 5 I 2 , anatase, and TiO 2 (B) components, respectively.The photoactivity analyses exhibited that the composite with 67% Bi 4 O 5 I 2 had superior performance in the degradation of acetaminophen, which was 10 and 25-folds of Bi 4 O 5 I 2 and TiO 2 biphase nanowires, respectively.As illustrated in Figure 11f, the electrons on the CB of Bi 4 O 5 I 2 were migrated to the CBs of biphasic TiO 2 .In the countrary, the photo-generated holes transferred from the VB of biphasic TiO 2 to the VB of Bi 4 O 5 I 2 .These charge transfer contributed to a considerably enhanced segregation of charges, which had a key role in promoting the photocatalytic activity.
Recently, Hu et al. anchored anatase/rutile homojunction QDs onto g-C 3 N 4 nanosheets, and the prepared anatase/rutile/g-C 3 N 4 (TCN) nanocomposite was utilized for oxytetracycline degradation under visible light (Hu et al., 2022).The photo-induced electrons transferred from the CB of anatase TiO 2 to that of rutile TiO 2 .The accumulated electrons on the CB of rutile TiO 2 reacted with the holes on the VB of g-C 3 N 4 , leaving more reactive holes and electrons over the VB of anatase TiO 2 and CB of g-C 3 N 4 , respectively.Then, the established homojunction and heterojunction among the anatase TiO 2 , rutile TiO 2 , and g-C 3 N 4 components enforced effective segregation of the charge carriers, which was clearly confirmed by PL, photocurrent, and EIS analyses.As a result, this study provides a suitable strategy for the degradation of antibiotics through the synergistic role of anatase and rutile phases of TiO 2 decorated over the g-C 3 N 4 nanosheets.
To utilize the synergistic impact of homojunction and type-II heterojunction constructions, Xiang et al. combined anatase TiO 2 with two different sizes having different band energies with MIL-101(Cr), as a metal-organic framework (MOF) (Xiang et al., 2022).Due to the synergistic effect of the established homojunction and heterojunction structures, more suppression of charges from recombination was observed, which led to superior activity for toluene photodegradation by oxidation of 93.7% after 6 h of visible-light radiation.It is worth mentioning that this homo/ hetero junction composite maintained its stability after being used for four cycles.
By Xu et al., γ/γ′-Bi 2 MoO 6 /N-doped carbon (Bi 2 MoO 6 /N-C) homo/hetero junction membrane photocatalyst was fabricated, and utilized for degradation of tetracycline upon visible light (Xu et al., 2023).The Bi 2 MoO 6 /N-C photocatalyst presented strong visible-light absorption and more segregation for the charge carriers due to the collaboration of γ and γ′ phases of Bi 2 MoO 6 in charges separations, which led to more efficacy in tetracycline degradation.Moreover, the prepared photocatalyst showed good biocompatibility for the treated solution through the growth and germination of Vigna radiata seeds.
Phang et al. hybridized B-doped CQDs with an n/n-type g-C 3 N 4 homojunction system to fabricate BCQD/CN photocatalysts by a facile hydrothermal procedure (Phang et al., 2022).The hybridized BCQD/CN photocatalysts showed considerable photocatalytic performance in the degradation of RhB upon visible-light supplied by 18 W LED lamp.The promoted activity of the hybridized photocatalysts was ascribed to the synergistic effects of the established homojunction/heterojunctins interfaces on the charges suppression from recombination.The utilization of a low-power light source is appealing advantage of this homojunction photocatalyst, which is attractive from the economical view point.
In 2023, Zhang et al. rationally designed and fabricated a P-doped tubular g-C 3 N 4 /g-C 3 N 4 / TiN composite (FCN/CN/TiN) through thermal polycondensation followed by molecular assembly strategy (Zhang et al., 2023b).The photocatalytic activities were studied toward degradation of recalcitrant fungicide carbendazim (CBZ) upon visible light.As seen in Figure 12a, the FCN photocatalyst was composed of tubes with different diameters and they had smooth surfaces (Figure 12a).In the synthesized FCN/CN homojunction photocatalyst, the thin CN nanosheets adhered over the tubes of FCN (Figure 12b).In the FCN/CN/TiN composite, CN and TiN components anchored over the tubes of FCN (Figure 12c).The XRD of FCN/CN/TiN composite well matched with the diffraction peaks of g-C 3 N 4 and TiN components, which confirmed integration of TiN with FCN/CN (Figure 12d).As noticed in the UV-vis DR spectra, the light-absorption range of the FCN/CN/TiN was significantly enhanced in visible region, due to the strong plasmonic resonance of TiN nanoparticles (Figure 12e).The degradation constant of CBZ on the FCN/CN/TiN photocatalyst was 0.0205 min −1 , which was 3.60 and 5.26 times as high as of FCN and CN photocatalysts, respectively.According to the proposed mechanism (Figure 12f), the hot electrons produced within TiN transferred to the CB of FCN.Due to more negative CB of FCN than CN, the electrons subsequently migrated to the CB of CN to produce superoxide anion radicals.Meanwhile, the produced holes transferred from the VB of CN to that of FCN to participate in the degradation reaction of CBZ molecules.Hence, the plasmonic resonance effect and homojunction construction synergistically collaborated in the suppression of charge carriers produced by TiN nanoparticles and small energy gap FCN and CN counterparts, resulting in promoted activity.

Conclusions and future outlook
In summary, we reviewed the utilization of homojunction photocatalysts for impressive promotion of photocatalytic activity toward the detoxification of water, mainly through improving segregation and transfer of charge carriers.It was noted that homojunction photocatalysts, along with heterojunction photocatalysts, have a high potential to address environmental issues.As noticed, the fast transfer of charges in the junction area of homojunction photocatalysts was related to the establishing an electric field through a combination of two phases, different morphologies and crystallinities, various dopant concentrations, and defect engineering on an almost identical semiconductor.Hence, precisely controlling the preparation condition to build close contact and enhance junction regions amongst the components and controlling grain boundaries in the junction areas are highly important.Consequently, the properties of homojunction photocatalysts can be substantially manipulated by changing the reaction conditions, including temperature, reaction time, solution pH, and precursors.Moreover, adjusting the inherent characteristics of semiconductors through defect engineering and morphology modification is a facile strategy for promoting the catalytic performances of homojunction photocatalysts.Despite extensive developments in the field of homojunction photocatalysis, it is far from industrial utilization requirements.Therefore, in order to efficiently tackle the energy and environmental issues, the rational designing and preparation of more efficient homojunction photocatalysts are highly desired.Furthermore, for the complete harvesting of solar energy in the photocatalytic processes, band gap engineering of these photocatalysts will be highly attractive.Currently only limited semiconductors have been utilized for fabricationg homojunction photocatalysts; hence, developing homojunction photocatalysts using another semiconductors is highly emphasized to fabricate homojunction photocatalysts with ousstanding performances.In order to utilize the synergistic benefits of heterojunction and homojunction photocatalysts, the combination of these photocatalysts is also another interesting strategy in designing novel photocatalysts with impressive performances.Furthermore, to save additional costs, energy and time, magnetically retrievable homojunction photocatalysts are suitable for quick collection of photocatalysts from the reactors for repeated utilization, or immobilizing homojunction photocatalysts on a materials to get ride of separation process.In addition, in most of the published works, high-power light sources have been used, while for practical applications, the replacement of them with energy-saving sources is demanded.For the last speech, although many research activities have been conducted about synthesis of various homojunction photocatalysts, this field of study is in the infancy stage, and there are many research vacancies to be answered.

Figure 1 .
Figure 1.heterogeneous photocatalysis mechanism within a particle of a photocatalyst.

Figure 2 .
Figure 2. various heterogeneous photocatalytic reactions over a photocatalyst depending the presented species in the reaction media.
semiconductors with different compositions effective interfacial contact due to lattice matching and chemical bond continuity in the junction area Poor interfacial contact due to lattice mismatching and discontinuity in the junction area Facile charges transfer within the interface slow charges transfer within the interface high stability due to strong connection between the components low stability due to weak connection between the combined semiconductors simple catalyst structure with limited elements Complicated catalyst structure due to the presence of various elements

Figure 7 .
Figure 7. (a) the photocatalytic efficiency enhancement mechanism for t ov -td; reproduced with permission from (yuan et al., 2022), and (b) schematic diagram of band bending of oxygen vacancy anatase tio 2 /rutile tio 2 ; reproduced with permission from (Kang et al., 2023).

Figure 9 .
Figure 9. (a) synthesis procedure, (b) the suggested mechanism for improved activity of Bi 2 moo 6 @Bi 2 moo 6-x homojunction photocatalysts for removal of phenol and Cr (vi); reproduced with permission from (shen et al., 2021a), and (c) the scheme for impressive activity of Cevo 4 homojunction photocatalysts; reproduced with permission from (ding et al., 2021).

Table 1 .
advantages and disadvantages of homojunction and heterojunction photocatalysts.