**About the Editors**

#### **Tongming Su**

Dr. Tongming Su has been an Associate Professor at the School of Chemistry and Chemical Engineering, Guangxi University, China, since 2022. He obtained his Ph.D. in industrial catalysis from the School of Chemistry and Chemical Engineering, Guangxi University, China, in 2018. From Sept. 2016 to Mar. 2018, he conducted scientific research on nanomaterials at Oak Ridge National Lab (ORNL) and the University of Tennessee, Knoxville, as a Joint PhD student supported by the China Scholarship Council. His research interests focus on the synthesis of nanomaterials and their applications in catalysis, energy conversion, and environmental remediation.

#### **Xingwang Zhu**

Dr. Xingwang Zhu has been an Associate Professor (Contracted) at the College of Environmental Science and Engineering and Technology Research Institute for Carbon Neutrality in Yangzhou University since 2022. He received his BS (2016) in Applied Chemistry from Hainan Normal University. He then earned his PhD degree (2021) at Jiangsu University and studied nanocluster chemistry at the National University of Singapore (2021–2022). His research activities are focused on photocatalytic CO2 reduction, low-dimensional nanomaterials' synthesis, and theoretical calculations.

## *Editorial* **Advanced Photocatalytic Materials for Environmental and Energy Applications**

**Tongming Su 1,\* and Xingwang Zhu 2,\***


With the development of modern society, environmental pollution and energy shortage have become the focus of worldwide attention. A majority of the global energy supplies are generated from fossil fuel, which gives rise to environmental pollution and climate change. As an inexhaustible clean energy source, solar energy has been widely researched and utilized for decades. Photocatalytic technology, which can directly convert solar energy into high value-added chemical energy and chemical materials or degrade a wide range of organic pollutants into easily degradable intermediates or less toxic small molecular substances, is regarded as one of the most crucial ways to solve the global energy shortage and environmental pollution problem.

This Special Issue includes two reviews and nine original research articles prepared by scientists from different countries, and these works focus on advanced photocatalytic materials for the treatment of indoor air, photocatalytic bacterial inactivation, photocatalytic hydrogen evolution, photocatalytic oxygen evolution, photocatalytic CO2 reduction, photocatalytic hazardous pollutant removal, photothermal decomposition of pollutants, and photoelectrochemical water splitting. This Special Issue provided a platform for scientists to present their original research on "Advanced Photocatalytic Materials for Environmental and Energy Applications". The following is a brief summary of each of the papers that we have had the honor of editing to highlight recent advances in the utilization of photocatalytic materials for environmental and energy applications.

Indoor air quality has become a significant public health concern, and photocatalytic technology has been widely studied for environmental remediation, especially for air treatment. Assadi et al. [1] summarized the recent developments on the TiO2-based materials for indoor air treatment and bacterial inactivation. In addition, several strategies, such as doping, heterojunction techniques, and combined catalysts, for enhancing the photocatalytic activity of TiO2-based catalysts was discussed. Moreover, the catalysts used to remove volatile organic compounds and microorganisms was reviewed. Finally, the reaction mechanism of pollutant elimination and microorganism inactivation using photocatalytic technology was also summarized.

The greenhouse effect exerts a great influence on mankind, and the photocatalytic reduction of CO2 into valuable products such as CO and CH4 is considered a promising technology for alleviating the greenhouse effect. Qin et al. [2] synthesized the phosphorusdoped hollow tubular g-C3N4 (x-P-HCN) and used it for photocatalytic CO2 reduction. They found that the phosphorus-doped g-C3N4 can effectively activate the CO2 adsorbed on the surface of photocatalysts, which greatly enhanced the CO production rate of photocatalytic CO2 reduction. Among the x-P-HCN samples, the 1.0-P-HCN sample exhibited the largest CO production rate of 9.00 <sup>μ</sup>mol·g−1·h−1, which was 10.22 times higher than that of bulk g-C3N4. This work provided an in-depth research perspective for accelerating the photocatalytic CO2 reduction rate of g-C3N4.

**Citation:** Su, T.; Zhu, X. Advanced Photocatalytic Materials for Environmental and Energy Applications. *Materials* **2023**, *16*, 7197. https://doi.org/10.3390/ma16227197

Received: 6 November 2023 Accepted: 13 November 2023 Published: 17 November 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Outdoor air pollutants originating from familiar anthropogenic sources, including industry, transportation, heating, and agriculture, exhibit adverse effects on health, the environment, the planet, and climate. In recent years, the dielectric barrier discharge (DBD) reactors have been widely used for hazardous pollutant removal. Khezami et al. [3] investigated the pilot-scale combination of non-thermal plasma and photocatalysis for removing toluene and dimethyl sulfur and revealed the effects of plasma energy and initial pollutant concentration on the performance and by-product formation in both pure compounds and mixtures. They found that coupling DBD plasma with a TiO2 catalyst can achieve a synergetic effect, which resulted in improved toluene and dimethyl sulfur removal. In addition, the ozone can be reduced, and CO2 selectivity was enhanced by combining plasma with photocatalysis.

Phase engineering is an effective strategy for tuning the electronic states and catalytic performance of photocatalysts, such as the light absorption range, charge separation efficiency, and surface redox reactivity. Yi et al. [4] provided a critical insight on the classification and diversified applications of phase-engineered photocatalysts. In this review, the classification of phase engineering for photocatalysis was summarized, and the applications of phase-engineered photocatalysts in hydrogen evolution, oxygen evolution, CO2 reduction, and organic pollutant removal was reviewed. In addition, the synthesis and characterization methodologies for unique phase structures and the correlation between phase structure and photocatalytic performance was introduced. Finally, the current opportunities and challenges of phase engineering for photocatalysis was also discussed.

Photothermal catalysis, which combines photochemical and thermochemical contributions of sunlight, has attracted a great amount of attention in the field of pollutant decomposition. Tryba et al. [5] prepared the Ni-TiO2 photocatalyst with nickel foam as the support and used it for the photothermal decomposition of acetaldehyde. They found that with the nickel foam as the support, the photocatalytic acetaldehyde decomposition performance over TiO2 can be enhanced from 31% to 52% under room temperature, and from 40% to 85% at 100 ◦C. In addition, the mineralization of acetaldehyde into CO2 doubled in the presence of nickel foam. This enhanced performance was attributed to the synergistic effect between nickel foam and TiO2, which can enhance the separation of free carriers and provide more space for the interaction between the reactant and the photocatalyst.

In recent years, the photoelectrochemical process was considered as an effective way to generated clean, green hydrogen (H2) directly from water using solar energy. Arifin et al. [6] successfully decorated Mo2C on TiO2 nanotube arrays (NTs) and used them for photoelectrochemical water splitting. They found that the photocurrent density was greatly increased from 0.21 mA cm−<sup>2</sup> to 1.4 mA cm−<sup>2</sup> when TiO2 NTs were decorated with Mo2C. This enhanced photocurrent density may have been due to the fact that the Mo2C cocatalyst greatly improved the photocatalytic characteristics of the TiO2 NTs. This work provides an effective method for developing the photoelectrochemical water splitting devices.

As already well known, process optimization is a feasible strategy to enhance the efficiency of polycrystalline silicon solar cells in the photovoltaic industry. To improve the efficiency of polycrystalline silicon solar cells, Wang et al. [7] developed a "low-highlow" temperature step of the POCl3 diffusion process. Compared with the online lowtemperature diffusion process, the open-circuit voltage and fill factor of solar cells increased up to 1 mV and 0.30%, respectively, in the optimization diffusion process. In addition, the efficiency of solar cells and the power of PV cells were also increased by 0.1% and 1 W, respectively. These enhancements can be ascribed to the low surface concentration of P doping and the strong impurity absorption effect of Si wafers obtained from the low-high-low temperature diffusion process.

Harnessing solar energy has been revealed to be a green strategy for solving the environmental pollution and energy crises. To make full use of solar energy, visible lightresponsive photocatalysts have attracted a wide range of attention. Hu et al. [8] synthesized a novel carbon quantum dots (CQDs)-modified PbBiO2I photocatalyst and used it for

the photocatalytic degradation of organic contaminants under visible/near-infrared light irradiation. After PbBiO2I was modified with CQDs, the photocatalytic performance for the degradation of rhodamine B and ciprofloxacin was significantly enhanced under visible/near-infrared light irradiation. The enhanced photocatalytic performance can be ascribed to the following reasons: CQDs can absorb light in the near-infrared region, and the modification of CQDs enhanced the separation of photogenerated electrons and holes and increased the contact between the catalyst and the organic molecules. This work expanded the application of CQDs in the fields of photocatalysis and solar energy conversion.

Environmental issues, such as air pollution and water pollution, directly affect our lives. The photocatalytic degradation of organic pollutants in water is an effective method to deal with water pollution. Dai et al. [9] prepared a novel Co3O4/g-C3N4 composite photocatalyst and used it for the photocatalytic degradation of rhodamine B in water. They found that the type II heterojunction was formed between the Co3O4 and g-C3N4, and that the type II heterojunction significantly enhanced the transfer and separation of photogenerated electrons and holes in the Co3O4/g-C3N4 photocatalyst. Therefore, the photocatalytic performance for the degradation of rhodamine B over optimized 5% Co3O4/g-C3N4 photocatalyst was 5.8 times higher than that of g-C3N4. In addition, they found that the main active species for the degradation of rhodamine B are •O2<sup>−</sup> and h+.

In order to promote the photocatalytic degradation of methylene blue in water, Tao et al. [10] successfully synthesized the MoS2/SnS2 composite photocatalysts via a facile hydrothermal method. They found that the SnS2 nanosheets were grown on MoS2 nanoparticles under a smaller size, and that a heterojunction was formed between MoS2 and SnS2. The optimal MoS2/SnS2 composite exhibited a methylene blue degradation efficiency of 83.0%, which was 8.3 times and 16.6 times higher than that of MoS2 and SnS2, respectively. The enhanced photocatalytic performance of the MoS2/SnS2 composite can be attributed to the improved visible light absorption, more active sites at the exposed edges of MoS2, and the enhanced separation of photogenerated electrons and holes.

Mizael et al. [11] synthesized ZnO nanoparticles with *Prosopis laevigata* extract as a stabilizing agent, the prepared ZnO nanoparticles were used for the photocatalytic degradation of methylene blue, and the degradation kinetics were investigated using the LHHW model. They found that the LHHW model sufficiently fits the experimental data, and that the size of the ZnO nanoparticles can be controlled using different concentrations of *Prosopis laevigata* as a reducing and stabilizing agent. In addition, the number of active sites increased with the decreasing nanoparticle size, which was demonstrated using the LHHW model. This work indicated that the extract of *Prosopis laevigata* can be used as a reducing and stabilizing agent to control the size and the photocatalytic performance of ZnO nanoparticles.

**Funding:** This Special Issue was founded by the National Natural Science Foundation of China (22208065, 22308300), Guangxi Natural Science Foundation (2022GXNSFBA035483), Natural Science Foundation of Jiangsu Province (BK20220598), Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province (KFKT2022001), Special Funding for 'Guangxi Bagui Scholars'.

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

#### **References**


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## *Review* **Recent Advances in Phase-Engineered Photocatalysts: Classification and Diversified Applications**

**Jianjian Yi 1,2,\*, Guoxiang Zhang 1, Yunzhe Wang 1, Wanyue Qian <sup>1</sup> and Xiaozhi Wang 1,\***


**Abstract:** Phase engineering is an emerging strategy for tuning the electronic states and catalytic functions of nanomaterials. Great interest has recently been captured by phase-engineered photocatalysts, including the unconventional phase, amorphous phase, and heterophase. Phase engineering of photocatalytic materials (including semiconductors and cocatalysts) can effectively affect the light absorption range, charge separation efficiency, or surface redox reactivity, resulting in different catalytic behavior. The applications for phase-engineered photocatalysts are widely reported, for example, hydrogen evolution, oxygen evolution, CO2 reduction, and organic pollutant removal. This review will firstly provide a critical insight into the classification of phase engineering for photocatalysis. Then, the state-of-the-art development of phase engineering toward photocatalytic reactions will be presented, focusing on the synthesis and characterization methodologies for unique phase structure and the correlation between phase structure and photocatalytic performance. Finally, personal understanding of the current opportunities and challenges of phase engineering for photocatalysis will also be provided.

**Keywords:** phase engineering; photocatalysis; water splitting; CO2 reduction; pollutant degradation

#### **1. Introduction**

The crystal phase is an emerging structural parameter of solid materials which holds the key to affecting the functionalities and properties of solid catalytic materials [1–3]. With phase transition, different atom arrangements over bulk and surface will lead to the change of physical and chemical properties. As a result, properties such as optical adsorption range, electrochemical conductivity, and molecular adsorption ability can be adjusted, which allows the modulation of function-oriented behaviors for various catalytic applications [4–6]. Phase engineering can therefore be defined as a strategy that constructs a specific phase for typical investigative purposes. Many catalytic materials exist in more than one crystal phase, thus providing the possibility of optimizing the performance or broadening the scope of applications. After decades of rapid progress in this research area, great achievements have been made in the phase engineering of nanomaterials, including synthetic methods to realize controllable synthesis of desired phase-engineered materials, fine characterization techniques to clearly analyze the phase structures, and development in various applications [7–10].

Photocatalysis is considered as an emerging concept of catalysis that can realize the conversion of solar energy to chemical energy [11–14]. As a green sustainable catalytic technology, the main working mechanism of photocatalysis is exciting semiconductors to generate electron-hole pairs, which can participate in different redox reactions, for example, water splitting [15], CO2 reduction [16], and organic pollutant degradation [17]. Nevertheless, the development of photocatalysis up to practical applications still confronts a big challenge, since the performance of the traditional photocatalytic systems is far

**Citation:** Yi, J.; Zhang, G.; Wang, Y.; Qian, W.; Wang, X. Recent Advances in Phase-Engineered Photocatalysts: Classification and Diversified Applications. *Materials* **2023**, *16*, 3980. https://doi.org/10.3390/ma16113980

Academic Editor: Fotios Katsaros

Received: 20 April 2023 Revised: 17 May 2023 Accepted: 24 May 2023 Published: 26 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

from satisfactory. Generally, the total photocatalytic process can be divided into six steps, (i) photon absorption, (ii) exciton separation, (iii) carrier diffusion, (iv) carrier transport, (v) catalytic conversion (redox), and (vi) mass transfer of reactants. Additionally, the total conversion efficiency in photocatalytic processes could be generally determined by the efficiency of three main steps, i.e., light absorption efficiency, charge separation efficiency, and surface redox reactivity [18]. The phase engineering of photocatalytic nanomaterials has recently captured great interest in photocatalysis since there are many unique superiorities of phase engineering for tuning photocatalytic behavior regarding the three main steps during photocatalysis. Firstly, phase engineering of semiconducting photocatalysts can provide possibility to adjust the electronic band structures for tunable light absorption ranges. Secondly, phase engineering can change the atomic arrangement in bulk and surface of materials, resulting in the optimization of built-in electric fields. As a result, the charge separation behavior can further be affected. Thirdly, the surface redox reactivity strongly depends on the adsorption/activation ability of reactant molecules, which can be finely tuned by phase-engineering-induced surface atomic reconfiguration [19]. In recent years, various investigations have resulted in improved photocatalysis by rational phase engineering, not limited to phase engineering for pristine semiconductors [20,21] but also for cocatalysts [22–24]. Moreover, phase engineering of nanomaterials is not just limited to phase transition from one phase to another. The formations of the amorphous phase [25] and heterophase (e.g., phase junction) [26,27] have also been widely studied.

In this review, we focus on presenting recent advances in phase engineering for photocatalytic reactions. The classification of phase engineering toward photocatalysis is firstly clarified. Then, an overview of state-of-the-art developments in photocatalytic applications based on phase engineering is presented, including but not limited to photocatalytic water splitting, CO2 reduction, and pollutant removal. As a focus, the synthesis and characterization methods for desired phase structures and the correlation between phase structure and photocatalytic performance are emphasized. Finally, the current challenges and further opportunities of phase engineering for photocatalysis are envisioned. We try to summarize phase engineering for photocatalytic materials to tackle limited catalytic efficiency and highlight the importance of phase engineering for photocatalysis. The description of representative samples in each catalytic reaction would be sufficient for a general review for researchers who are not familiar with phase-engineered photocatalysts.

#### **2. Classification of Phase-Engineered Nanostructures**

Considering that the emerging phase engineering strategies are complicated and diversified, the classification of phase engineering is important for the discussion about phase engineering for photocatalytic applications. As illustrated in Figure 1, we classify the phase-engineered nanostructures into the following three types: the unconventional phase, amorphous phase, as well as the heterophase.

The definition of the unconventional phase is a relative concept to the conventional phase. In general, nanomaterials exist in the form of thermodynamically stable phases in bulky components, which can be defined as the conventional phase. However, nanomaterials with different phases can be obtained by adjusting reaction kinetics and surface energy under certain experimental conditions. The obtained phases with different atom arrangements compared to the conventional phases can be denoted as the unconventional phase. Taking metal as an example, Au is usually crystallized in the conventional facecentered cubic (fcc) phase, but Zhang et al. demonstrated that Au can also be crystallized into 2H and 4H phases by controlling the synthetic parameters [28,29]. In this case, the fcc phase is the conventional phase for Au, while the 2H and 4H phases are unconventional phases. Until now, unconventional phases have been found in a variety of nanomaterials, including metals, metal oxides, and transition metal chalcogenides. The obtained nanomaterials with unconventional phases show unique and enhanced performance effects in many applications.

**Figure 1.** Schematic illustration of different types of phase engineering and the applications in photocatalysis.

The amorphous phase is a relative concept to the crystalline phase, with unique structural characteristics. The key feature of the amorphous phase is the short-range order but long-range disorder atomic structure. In contrast, the crystalline phase generally is in the form of short-range orders and long-range orders [30,31]. Due to the disordered arrangement of atoms and high entropy resulting from unsaturated bonds, amorphous materials are usually metastable and readily change to crystalline states under external heat or pressure. The synthesis of amorphous phase nanomaterials can be realized through direct preparation by finely controlling the crystallization process or indirect preparation by breaking the long-range order structure of the crystalline phase [32].

The heterophase structure is different from the mentioned unconventional phase and amorphous phase. It refers to the multi-phase structure composed of two or more crystalline phases of the same material.

The most classical example for the heterophase structure is the commercial TiO2 materials, namely P25, which is composed of mixed anatase and rutile phases [33]. Compared to the pristine anatase phase and rutile phase, P25 can exhibit improved photocatalytic activity in many reactions due to the improved charge separation efficiency with the formation of a space-charge layer. In addition to the phase I/phase II heterophase structure, crystalline/amorphous structures can also be defined as heterophase structures [34,35].

With the rational design of material structural engineering based on constructing different phase structures, the electronic structure and catalytic functions can be finely tuned for improved photocatalysis. Different components in a typical photocatalytic material, including light-harvesting semiconductors and surface active cocatalysts, can be tuned based on phase engineering to meet different requirements in different reactions. With the development of phase engineering, phase-dependent properties and photocatalytic applications (e.g., water splitting, CO2 reduction and pollutant removal) have been witnessed. Detailed discussion in terms of recent advances in phase engineering for photocatalysis will be provided in the following section.

#### **3. Phase Engineering for Photocatalytic Applications**

Phase-engineered nanomaterials have endowed them with unique electronic structures and catalytic properties for various applications such as hydrogen evolution, oxygen evolution, CO2 reduction, and pollutant removal. Phase engineering can enhance the

catalytic performance of typical reactions by broadening the light absorption or steering charge transfer kinetic or by maneuvering the surface redox reaction. In addition, photocatalysts or cocatalysts with different phase structures can also affect the reaction selectivity and stability. In this section, an overview of some recent advances in phase engineering for photocatalytic reactions will be provided.

#### *3.1. Hydrogen Evolution*

Developing clean and renewable fuels with high energy density is the common pursuit of the academic community. Hydrogen energy is one of the candidates to meet the requirements mentioned above. Photocatalytic water splitting to produce hydrogen is the "holy grail" in solar energy conversion, which is still restricted by the activity, stability, and economic cost [13,36,37]. It has been widely found that phase engineering can significantly boost the photocatalytic hydrogen evolution performance.

Recently, phase-engineered semiconducting photocatalysts have been widely designed to modulate their photocatalytic hydrogen evolution performance. Some important electronic structures such as light absorption edges and strength of built-in electric fields can be tuned by phase engineering. Taking the most classic photocatalyst, TiO2, as an example, it was reported that the photocatalytic hydrogen evolution performance of brookite phase TiO2 was significantly higher than that of anatase phase TiO2 (Figure 2a) [38]. The author explained that the conduction band (CB) edge of brookite phase TiO2 was more negative than that of anatase TiO2 supported by experimental characterizations. Electrons excited at the CB of brookite phase TiO2 with higher reduction ability can more effectively reduce H+ to H2, leading to higher hydrogen evolution activity. Similar cases can also be found in sulfide photocatalysts such as ZnS. Feng et al. reported that the phase structure of ZnS can be regulated by ambient S annealing [39]. The photocatalytic hydrogen evolution measurement showed that wurtzite phase ZnS showed better hydrogen evolution activity than sphalerite phase ZnS. The reason for the phase dependence was owing to the strengthened inter-polar electric field of wurtzite phase ZnS, which could promote the electron-hole separation.

Apart from the formation of a typical phase, the construction of hybrid photocatalysts with heterophase (e.g., phase junction) structure is considered as another effective phase engineering strategy for improved hydrogen evolution catalysis. For instance, CdS, a photocatalyst with desirable bandgap and availability limited by photo-corrosion and inefficiency, was reported to have photocatalytic hydrogen evolution performance that can be optimized by constructing phase junctions [20,40]. Experimental results revealed that the phase junction composed of cubic and hexagonal phase CdS (denoted as c-CdS/h-CdS) showed a high hydrogen evolution rate (4.9 mmol h−<sup>1</sup> g<sup>−</sup>1) and external quantum efficiency (EQE) of 41.5% at 420 nm (Figure 2b) [40]. The hydrogen evolution rate was 60 times higher than those of c-CdS and h-CdS. Notably, photo-corrosion can also be inhibited over the phase junction. The origin of improved activity and stability was owing to the greatly enhanced charge separation by the regulation of bonding region between cubic and hexagonal phases. Recently, Yu et al. demonstrated an efficient heterophase red P photocatalyst with a hydrogen evolution rate over 1280 μmol h−<sup>1</sup> g−<sup>1</sup> (Figure 2c) [41]. The formation of red P heterophases consisting of fibrous and Hittorf's phases can be realized by Bi-mediated catalytic synthetic method. From the fact that each phase red P possessed different band alignments, the intimate heterophase junction afforded an effective built-in driving force for efficient charge transport, thus achieving high catalytic performance. Similar case studies were also demonstrated over TiO2 [34], phosphorus [42], Cd1−xZnxS [43], ln2O3 [44], and ZnIn2S4 [45], highlighting the advance of heterophase structures.

**Figure 2.** (**a1**) Proposed routes for the formation of anatase and brookite phase TiO2. (**a2**) HRTEM image of TiO2 with mixed anatase and brookite phase. (**a3**) Photocatalytic hydrogen evolution activity of TiO2 with different phase structures using Pt as cocatalyst; -, •, , and indicate anatase phase, anatase/brookite mixed phase, brookite phase, and P25, respectively (copyright from Ref. [38], 2010, American Chemical Society). (**b1**,**b2**) TEM and HRTEM images of CdS with mixed hexagonal and cubic phases. (**b3**) Photocatalytic hydrogen evolution activity of the catalysts (copyright from Ref. [40], 2018, Elesvier). (**c1**) Hydrogen product yield of red P samples with different phase structures under visible light irradiation. (**c2**,**c3**) Schematic representation of proposed charge dynamics in (**c2**) red P nanowire/nanorod and (**c3**) heterophase red P with fibrous and Hittorf's phases (copyright from Ref. [41], 2022, Elesvier).

Transitional metal dichalcogenides (TMDs) represented by MoS2 show potential to improve photocatalytic hydrogen evolution performance as cocatalysts on semiconductor surfaces. TMD nanomaterials show various phases owing to the different electronic structures of transition metal atoms with different d orbital filling states [22,46,47]. For example, 2H-MoS2 and 2H-WS2 are semiconducting with band gaps, whereas 1T-MoS2 and 1T-WS2 are metallic with good conductivity for hydrogen evolution reactions. Our group found that 1T-MoS2/O-g-C3N4 demonstrated a greatly higher photocatalytic hydrogen evolution performance compared to 2H-MoS2/O-g-C3N4 (Figure 3a) [48]. The optimal 1T-MoS2/O-g-C3N4 sample showed hydrogen evolution rate over 1800 μmol/g/h with external quantum efficiency of 7.11% at 420 nm. The origin for the high catalytic activity of 1T-MoS2 can be ascribed to the metal-like conductivity and the active edge and basal

sites for hydrogen evolution. By contrast, low conductivity and limited active sites at edge sites leads to poor performance. Recent studies also reported that 1T-MoS2, 1T-WS2, and 1T-MoSe2 can improve the catalytic activity of hosting semiconductors not limited to g-C3N4 but also other semiconductors such as TiO2 and CdS [23,24,49–51].

In addition to MoS2 with layered structure, TMD materials with non-layered structures such as CoSe2 and NiSe2 also show phase-dependent catalytic hydrogen evolution performance. Our group demonstrated the phase-dependent photocatalytic hydrogen evolution catalysis of CoSe2. In a practical photocatalytic process, it was observed that CoSe2 with orthorhombic phase (o-CoSe2) can better improve the hydrogen evolution rate of g-C3N4 semiconductors than CoSe2 with cubic phase (c-CoSe2) (Figure 3b) [52]. It was revealed by density functional theory (DFT) calculations that the Co site on o-CoSe2 surface showed more appropriate hydrogen adsorption Gibbs free energy (ΔGH\* = 0.27 eV) than the c-CoSe2 surface, resulting in improved catalysis. Despite c-CoSe2 possessing better conductivity than o-CoSe2, the charge separation efficiency may not be the rate-determining step in this case. Interestingly, unlike the higher catalytic performance of o-CoSe2/g-C3N4 compared to c-CoSe2/g-C3N4, we recently found that the hydrogen evolution of o-CoSe2/TiO2 (2.601 μmol/h) was lower than that of c-CoSe2/TiO2 (12.001 μmol/h) [53]. We propose the reason for this phenomenon would be that the interfacial charge transfer between TiO2 and c-CoSe2 is the dominating factor but not the surface redox reactivity. Impressively, phase engineering of TMD-based cocatalysts show not only phase-dependent activity but also stability. Very recently, we reported that the phase structure of NiSe2 played important role in determining the photocatalytic hydrogen evolution stability instead of stability (Figure 3c) [54]. Upon light irradiation on m-NiSe2/CN and p-NiSe2/CN in TEOA/H2O, comparable photocatalytic hydrogen evolution rates of 3.26 μmol h−<sup>1</sup> and 3.75 μmol h−<sup>1</sup> can be observed. Importantly, we found that NiSe2 exhibited phase-dependent stability, i.e., m-NiSe2 can evolve H2 steadily, but p-NiSe2 showed a ~57.1% rate decrease after 25 h of reaction. After fine characterization, we proposed the origin of phase-dependent stability. The chemical structure of m-NiSe2 can be well preserved in a catalytic process, but partial p-NiSe2 tends to be converted to NiOOH, resulting in different catalytic stability.

#### *3.2. Oxygen Evolution*

Photocatalytic water splitting to produce hydrogen or CO2 reduction to obtain high value-added chemical products are ideal pathways to achieve solar energy storage and conversion. In the overall reactions, water oxidation is the most important half reaction of these two energy photocatalytic reactions [55–57]. In water splitting and CO2 reduction reactions, water oxidation provides protons and electrons, which is the premise of the reduction reaction. It should be noticed that the water oxidation reaction is a four-electron reaction, and the overpotential of this reaction is very high. It is thus considered as the ratedetermining step of the overall reactions, which holds the key to the proceeding of hydrogen evolution from water or CO2 reduction [58,59]. Phase engineering of nanomaterials could improve photocatalytic oxygen evolution performance by enhancing charge separation, or decreasing the reaction energy barrier.

Phase-dependent photocatalytic performance was reported over a classical oxygen evolution semiconducting photocatalyst, i.e., BiVO4. Kudo et al. synthesized tetragonal phase and monoclinic phase BiVO4 successfully, characterized the phase and optical structures, and measured the photocatalytic O2 evolution performance in AgNO3 solution (Figure 4a) [60]. Given the similar light harvesting capacity of tetragonal BiVO4 and monoclinic BiVO4, it was interesting that negligible O2 gas product can be detected over tetragonal BiVO4, but monoclinic BiVO4 exhibited high O2 evolution activity (over 120 μmol/h under visible light and over 70 μmol/h under UV light). Mechanism analysis revealed that distortion of a Bi-O polyhedron by a 6s<sup>2</sup> lone pair of Bi3+ in monoclinic BiVO4 was beneficial for the surface conversion from H2O to O2, leading to high photocatalytic activity. In another research work conducted by Amal et al., the authors investigated the amorphous and crystalline evolution of BiVO4 during the synthesis by flame spray pyrolysis. In terms of the photocatalytic test, the first finding was that amorphous BiVO4 cannot produce O2 by photocatalysis. For crystalline BiVO4, the photocatalytic oxygen evolution rate increased with the increased content of monoclinic phase in BiVO4, highlighting the important role of monoclinic phase [61].

**Figure 3.** (**a1**) TEM image of 1T-MoS2/O-g-C3N4. (**a2**) Fitted EXAFS results of prepared 1T-MoS2 sample and commercial 2H-MoS2. (**a3**) Time-dependent hydrogen evolution rate of the samples (copyright from Ref. [48], 2018, Elesvier). (**b1**) A comparison of light-driven hydrogen evolution rate of CoSe2-based catalysts. (**b2**) Charge behavior of o-CoSe2/CN and c-CoSe2/CN (copyright from Ref. [52], 2020, Elesvier). (**c1**) A comparison of light-driven hydrogen evolution rate of NiSe2-based catalysts. (**c2**) HRTEM images of NiSe2/CN samples before and after photocatalysis. (**c3**) Mechanism illustration of phase-dependent stability of NiSe2 (copyright from Ref. [54], 2023, Elesvier).

Except for phase engineering of semiconductors, phase-dependent oxygen evolution photocatalysis was also found in surface active cocatalysts. For example, our research group demonstrated that the phase structure of CoSe2 cocatalysts plays an important role in determining the oxygen evolution performance of Fe2O3 semiconductors (Figure 4b) [62]. Experimental results found that orthorhombic phase CoSe2 (o-CoSe2) showed better potential than cubic phase CoSe2 (c-CoSe2) in enhancing photocatalytic oxygen evolution performance of Fe2O3. o-CoSe2/Fe2O3 can realize the qualitative changes of oxygen evolution rate from "0" to "1" under visible light irradiation, using AgNO3 as sacrificial agent

and La2O3 as pH balance agent. However, c-CoSe2/Fe2O3 cannot work in photocatalytic oxygen evolution processes under the same conditions. Combined with photoelectrochemical characterization and theoretical simulations, we proposed that the key factor for the superior activity of o-CoSe2 was the decreased activation barrier of H2O on its surface.

**Figure 4.** (**a1**) XRD patterns of BiVO4 with different phases, and • indicate tetragonal and monoclinic phase respectively. (**a2**,**a3**) Photocatalytic oxygen evolution performance of BiVO4 samples under visible light irradiation (λ > 420 nm) and under ultraviolet light (300 nm < λ < 380 nm). Inset: (**a**)—tetragonal phase, (**b**)—monoclinic phase (copyright from Ref. [60], 2001, American Chemical Society). (**b1**) Schematic of oxygen evolution process over CoSe2 with different phase structures. (**b2**) Photocatalytic oxygen evolution performance of the catalysts. (**b3**) Diagram of the calculated free energy change in each reaction step over o-CoSe2 and c-CoSe2 (copyright from Ref. [62], 2022, Elesvier).

#### *3.3. CO2 Reduction*

Converting CO2 into valuable chemical feedstocks or liquid fuels under mild conditions is of great significance for reducing the greenhouse effect and achieving carbon neutrality [63–65]. Among them, photocatalysis is considered a green technology that converts CO2 from solar energy [66,67]. The key steps of photocatalytic CO2 reduction include the generation of electron-hole (e-h) pairs, electron transfer to catalytic active sites, and catalytic CO2 reduction. In order to meet these requirements, the rational structural design of catalysts is required [68]. Due to the ability to optimize the multiple electronic structures of catalysts, especially the regulation of CO2 molecular adsorption activation behavior, phase engineering in the field of CO2 photoreduction has also attracted extensive attention [69].

Phase engineering of pristine semiconductor has shown potential in tuning the photocatalytic CO2 reduction performance. Li et al. reported the phase-controlled synthesis of three types of TiO2 (anatase, rutile, and brookite), and the evaluation of photocatalytic CO2 reduction performance with water vapor (Figure 5a) [70]. Experimental results found that the production of CO and CH4 using anatase and brookite phase TiO2 were enhanced by 10-fold in contrast to rutile phase TiO2. The new finding in this study is that brookite phase TiO2, with less attention among the three TiO2 polymorphs, exhibited the highest CO and CH4 production rates. Detailed mechanism studies revealed that the superior activity of brookite phase TiO2 may be owing to the existence of surface oxygen vacancies,

faster reaction rate of CO2 − with surface adsorbed H2O or OH groups, and a new reaction pathway involving an HCOOH intermediate.

**Figure 5.** (**a1**) UV-visible diffuse reflectance spectra of unpretreated and He-pretreated TiO2 (anatase, rutile, and brookite). (**a2**) XRD patterns of He-pretreated TiO2 (anatase, rutile, and brookite). (**a3**) Photocatalytic CO2 reduction performance of TiO2 with different phases (copyright from ref. [70], 2012, American Chemical Society). (**b1**,**b2**) TEM and HRTEM images of ln2O3−X(OH)y composed of rhombohedral and cubic phase. (**b3**) XRD patterns illustrating the phase transition from rhombohedral to cubic phase with increased temperature. (**b4**,**b5**) CH3OH and CO evolution rates over the catalysts with different phase structures. (**b6**) Schematic illustration of charge carrier separation and recombination pathways in polymorphic heterostructures of rh/c-In2O3−x(OH)y (copyright from ref. [71], 2020, Royal Society of Chemistry).

In addition to the complete phase transition in semiconductor photocatalyst, the construction of heterophase photocatalyst recently attracts intensive attention in CO2 reduction. Yan et al. reported a heterophase photocatalyst based on ln2O3−X(OH)y to optimize the CO2 reduction performance (Figure 5b) [71]. The continuous transition from cubic to rhombohedral can be realized by temperature variety so that the heterophase structure can be obtained by partial phase transition at an appropriate temperature. The charge separation efficiency can be greatly improved due to the formation of the cubic/rhombohedral interface. As a result, the optimized cubic/rhombohedral ln2O3−X(OH)y exhibited improved CO2 reduction activity compared to that of the pure cubic or rhombohedral phase, with a CH3OH evolution rate of 92 μmol/g/h and CO evolution rate of 1120 μmol/g/h. This strategy can also be extended to other material systems with similar working mechanisms for CO2 reduction, for example, CulnS2 [72] and CdS [73].

In photocatalytic hybrid structures, cocatalysts are widely used to improving the activity and selectivity of CO2 reduction by promoting the electron-hole separation and providing highly active catalytic sites for CO2 catalytic conversion. Phase structure engineered

metal cocatalysts are recently known as efficient cocatalysts for CO2 reduction. For instance, Bai et al. reported that the Ru cocatalyst with hexagonal close-packed (hcp) phase (hcp Ru) can more effectively boost CO2 reduction efficiency of C3N4 semiconductor in contrast to face-centered cubic (fcc) phase Ru (fcc Ru) (Figure 6a) [74]. Thanks to phase engineering of Ru, not only CO and CH4 evolution rates but also selectivity for carbon-based products can be improved over C3N4-hcp Ru. To uncover the mechanism for phase-dependent activity and selectivity, the authors explained this phenomenon supported by electrochemical characterizations and theoretical calculations. The improved performance was not owing to the altered interfacial electron transfer from C3N4 to Ru, but the higher CO2 adsorption

energy on (10<sup>−</sup> 11) face of hcp Ru was higher than that on the (111) face of fcc Ru. Except for the design of an unconventional phase, amorphous phase cocatalysts can also work in CO2 reduction reaction with optimized performance. In another case, also reported by Bai et al., the effect of crystallinity on photocatalytic CO2 reduction performance was systematically studied by using Pd nanosheets as model cocatalysts (Figure 6b) [75]. When Pd nanosheets were assembled with CdS quantum dots (QDs), it was found that Pd with high crystallity and good lattice periodicity was more conducive to electron transfer from CdS to Pd, which more effectively inhibited H2 production on the surface of CdS. In contrast, low-crystallinity Pd provided a large number of surface unsaturated atoms and defects as highly active centers for efficient CO2-CO/CH4 conversion. The formation rates of CO (23.93 μmol/g/h) and CH4 (0.35 μmol/g/h) in CdS-Pd-48 with low-crystallinity Pd were 10.3 and 5.9 times of those of the pristine CdS QDs, respectively. This result highlighted that amorphous-phase Pd contributed to efficient CO2 adsorption and activation, and crystalline Pd favored charge migration. Taken together, amorphous Pd was a better cocatalyst for CO2 reduction in terms of the yield and selectivity of carbon products.

**Figure 6.** (**a1**,**a2**) TEM and HRTEM images of C3N4-fcc Ru. (**a3**,**a4**) TEM and HRTEM images of C3N4-hcp Ru. (**a5**) Photocatalytic CO2 reduction activity and selectivity (copyright from ref. [74], 2018, Elesvier). (**b1**) XRD patterns of CdS-Pd samples. (**b2**) Photocatalytic CO2 reduction activity and selectivity (copyright from ref. [75], 2020, Royal Society of Chemistry).

#### *3.4. Pollutant Removal*

Semiconductor photocatalysis has been widely applied in environmental remediation. Upon light irradiation, photocatalysts can harvest solar energy to generate electron-hole (e-h) pairs for pollutant removal. Various reactive oxygen species, including O2 −, OH, and 1O2, can be generated by different redox reactions. In addition, the photogenerated holes can also directly oxidize organic pollutants. All the mentioned reactive species generated by photocatalysis can contribute to efficient pollutant removal [18,76,77]. Phase engineering of photocatalysts can improve the pollutant removal performance by several working mechanisms, for example, decreasing the formation energies of oxygen species, improving the light absorption range, promoting charge separation, improving reduction and oxidation potentials of electrons or holes, and so on.

Phase-controlled synthesis of TiO2 nanorods can be realized by a hydrothermal method using peroxide titanic acid solution with different pH values, forming rutile, anatase, and brookite phase TiO2, respectively (Figure 7a) [78]. The phase-dependent photocatalytic activities of the samples were evaluated by reduction of Cr (VI) and degradation of methylene blue (MB). Experimental results indicated that rutile TiO2 possessed best photo-degradation performance of MB, while brookite TiO2 showed best activity for Cr (VI) photo-reduction. The improved photo-oxidation performance can be explained that rutile TiO2 can expose more {111} facets with high surface energy, which can boost the oxidation reaction more easily. For photo-reduction reaction, the more negative CB potential of brookite TiO2 contributed to more effective Cr (IV) reduction. This case study highlights the importance of phase design for different environmental photocatalytic reactions.

In many recent studies, it has been proven that amorphization of catalysts is an effective method to tune the physical/chemical properties and thus modulate their photocatalytic degradation performance [79,80]. For example, Mao et al. reported a classical study about increasing light absorption of TiO2 by amorphization, forming the so-called black TiO2 (Figure 7b) [81]. The researchers demonstrated that the color of white TiO2 changed to black for surface amorphization by hydrogenation treatment, leading to light absorption broadening from ultraviolet region to near-infrared region. In a practical photocatalytic degradation of methylene blue (MB) solution, black TiO2 with crystalline core and amorphous surface showed significantly improved degradation rate with high stability compared to that of white TiO2. The origin for the phase-dependent performance was mainly owing to the change of electronic and optical properties of black TiO2, especially the greatly reduced band gap promoted by the formation of midgap states. Similarly, highly efficient photocatalytic degradation can also be demonstrated over other semiconductors though phase transition to amorphous phase by virtue of the high surface energy and outstanding adsorption/desorption properties. Hu et al. reported the synthesis of a series of phase-engineered Sb2S3 photocatalysts with different degrees of amorphization by adjusting the concentration of hydrochloric acid in hydrothermal processes [82]. The amorphous Sb2S3 exhibited the best photocatalytic methyl orange degradation activity, which was 13 times higher than that of crystalline Sb2S3.

Except for the direct photocatalytic degradation of pollutants, phase engineering in photo-assisted environmental catalysis has recently attracted wide attention [83–85]. For instance, the Qu research group reported the construction of heterophase MoS2, i.e., 2H/1T MoS2, and its application in photo-assisted permonosulfate (PMS) activation for water pollutant degradation (Figure 8) [86]. Although MoS2 has been proven as an efficient activator of PMS, it is still restricted by the loss of low-valence Mo during the catalytic process. The authors found that the integration of semiconducting 2H phase MoS2 and 1T phase MoS2 forming 2H/1T MoS2 can favor the catalytic reaction, by introducing photogenerated electrons of 2H MoS2 under light irradiation to trigger the formation of low-valence Mo. In this case, the key of this reaction is the regeneration of low valence Mo in 1T MoS2. 2H MoS2 with semiconductor characteristic in heterophase MoS2 can transfer electrons to 1T MoS2 under light irradiation, leading to the reduction of high-valence Mo to low-valence Mo (active center in PMS activation). As a result, 2H/1T phase MoS2 showed efficient and continuous degradation of organic pollutants in the existence of PMS and light. The discovery in this work highlighted the merits of constructing heterophase structure in heterogeneous photocatalytic degradation reactions.

**Figure 7.** (**a1**) Schematic illustration of synthetic process of TiO2 with different phases. (**a2**) Photocatalytic performance of MB degradation. (**a3**) Photocatalytic performance of Cr (VI) reduction (copyright from ref. [78], 2015, American Chemical Society). (**b1**) Schematic illustration of the structure and electronic DOS of a semiconductor in the form of a disorder engineered nanocrystal with dopant incorporation. (**b2**) Digital graphs of unmodified white TiO2 and black TiO2 with amorphous surface layer. (**b3**,**b4**) HRTEM images of unmodified white TiO2 and black TiO2 with amorphous surface layer. (**b5**) Photocatalytic MB degradation performance. (**b6**) Stability tests (copyright from ref. [81], 2011, American Association for the Advancement of Science).

**Figure 8.** (**a**) Removal efficiency of 2,4-D in different reaction systems. (**b**) Rate constants of 2,4-D degradation using different MoS2 samples with (red dashed line) and without (gray dashed line) light irradiation. (**c**) Proposed mechanism for the photoinduced PMS activation on the surface of multiphase MoS2 (copyright from ref. [86], 2019, American Chemical Society).

#### **4. Conclusions and Perspectives**

In summary, this review summarizes the advances in phase engineering for photocatalytic applications, aiming to outline the route for designing efficient photocatalysts based on phase engineering. Table 1 summarizes some representative nanomaterials with phase engineering and their applications in photocatalysis. Phase-engineered materials can exist in different forms, including the unconventional phase, amorphous phase, and heterophase. Phase engineering can also be performed on both light-harvesting semiconductor and surface active cocatalyst. With optimized electronic structures and physico-chemical properties, the light absorption, charge separation, or surface redox reaction behavior can be tuned, providing the possibility for improved photocatalysis. Phase engineering has gained huge success in the photocatalysis research community, owing to the positive role in various energy and environmental applications.

**Table 1.** Summary of some representative phase-engineered photocatalysts and their applications.


Although considerable success has been witnessed, there are still some challenges in the research field. Firstly, phase engineering has made surprising progress in enhancing catalyst efficiency, including activity, selectivity, and stability, but record-breaking highperformance photocatalytic efficiency is rarely reported. For example, 1T-MoS2 has shown great potential in hydrogen evolution, but the performance is still far from noble metal catalysts such as Pt. Secondly, the phase purity is a general concern in this research field. The incomplete phase transition or unsatisfied synthetic methods are still limiting the phase purity of phase-engineered nanostructures. For instance, 1T-MoS2 with pure 1T phase is hard to obtain. Most of the reported studies are 1T/2H mixed phase. Additionally, the phase stability should also be focused. Thirdly, the delicate control over other structural parameters in addition to phase is still challenging. Different morphology, size, or other structural parameters of phase-engineered materials make it difficult to reveal the mechanism. For example, TiO2 with different phase structures but also different sizes would make it difficult to determine whether the increase in catalytic activity is due to the crystal phase structure or size.

Despite the presented challenges, there are plenty of opportunities in phase engineering for photocatalytic applications. Firstly, based on the fast development of material science, synthesizing novel nanomaterials such as MOFs and COFs with different phase structures is promising. Secondly, given that phase-engineered photocatalysts are now mainly used in limited applications, exploring new energy and environmental applications such as organic synthesis, H2O2 production, and N2/NO3 reduction may broaden the functions. Thirdly, the integration of phase engineering to other structural engineering strategies may make it possible to construct record-breaking high performance photocatalytic systems, and fourth, the development of facile and scale-up synthetic methods

is of great significance in future research, with aims to realizing large scale synthesis of phase-engineered materials with high pure purity and stability. Last but not least, from the mechanism point of view, using advanced characterization technologies such as in situ XRD, in-situ Raman spectroscopy, and in situ TEM to uncover the phase evolution mechanism is still under development.

**Author Contributions:** J.Y.: investigation, data curation, writing—original draft preparation, funding acquisition; G.Z.: writing—review and editing; W.Q.: writing—review and editing; Y.W.: writing—review and editing; X.W.: project administration, supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors appreciate for the financial support by Natural Science Foundation of Jiangsu Province (BK20210827) and China Postdoctoral Science Foundation (2021M700117).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Raw data are available upon request.

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

#### **References**


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