Active photocatalysts for CO2 conversion by severe plastic deformation (SPD)

Excessive CO2 emission from fossil fuel usage has resulted in global warming and environmental crises. To solve this problem, photocatalytic conversion of CO2 to CO or useful components is a new strategy that has received significant attention. The main challenge in this regard is exploring photocatalysts with high activity for CO2 photoreduction. Severe plastic deformation (SPD) through the high-pressure torsion (HPT) process has been effectively used in recent years to develop novel active catalysts for CO2 conversion. These active photocatalysts have been designed based on four main strategies (i) oxygen vacancy and strain engineering, (ii) stabilization of high-pressure phases, (iii) synthesis of defective high-entropy oxides, and (iv) synthesis of low-bandgap high-entropy oxynitrides. These strategies can enhance the photocatalytic efficiency compared to conventional and benchmark photocatalysts by improving CO2 adsorption, increasing light absorbance, aligning the band structure, narrowing the bandgap, accelerating the charge carrier migration, suppressing the recombination rate of electrons and holes, and providing active sites for photocatalytic reactions. This article reviews recent progress in the application of SPD to develop functional ceramics for photocatalytic CO2 conversion.

applied to the material (disc or ring shape) using two anvils that rotate with respect to each other as shown in Fig. 2b [41]. Due to the high processing pressure in HPT, it is applicable to hard and less-ductile materials such as high-melting temperature metals (hafnium [42], molybdenum [43], and tungsten [44]), amorphous glasses [45,46], silicon-based semiconductors [47,48] and even ultrahard diamond [49,50]. Another advantage of HPT is its capacity to induce ultra-SPD (i.e. shear strains over 1,000 for mechanical alloying) [51]. The inducing ultra-SPD [51] together with fast dynamic diffusion [52,53] introduces the HPT method as a unique path to mechanically synthesize new materials even from immiscible systems [54,55]. Due to these unique features of HPT, the method was even used for the process and synthesis of hard and brittle ceramics, but the number of publications on ceramics is quite limited despite the high potential of these materials for various applications [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Published studies regarding the ceramics processed or synthesized by HPT are presented in Table 1, while there are other classic publications on HPT processing of ceramics mainly by geologists [40].
As given in Table 1, the recent usage of HPT to process and synthesize ceramics for photocatalysis has shown a high potential for the enhancement of photocatalytic activity, especially for CO2 conversion, [35][36][37][38]. The HPT method leads to increasing the activity by narrowing the bandgap, increasing the light absorbance, aligning the band structure, introducing the interphases and active sites for chemical adsorption and reaction, and accelerating the charge carrier migration to the active sites [35][36][37][38]. These promising features have been reported in some different photocatalysts by HPT using four main strategies including simultaneous strain and oxygen vacancy engineering, the introduction of high-pressure phases, formation of defective high-entropy phases, and production of low-bandgap high-entropy oxynitride phases. The contribution of each mentioned strategy to improve the photocatalytic CO2 conversion activity is discussed in detail as follows.

Simultaneous strain and oxygen vacancy engineering
Oxygen vacancy engineering is an effective method that has been used to improve photocatalytic CO2 conversion [17,18]. Oxygen vacancies increase the photocatalytic efficiency by increasing the light absorbance, accelerating the charge carrier separation, and enhancing the surface reactions. Oxygen vacancies on the surface of the photocatalysts act as active sites to trap the electrons for various ranges of reduction reactions. It was also observed that oxygen vacancies have a significant role to adsorb and activate the CO2 molecules and increase the local electronic density. Strain engineering via lattice compression/expansion can also decrease the energy barrier for CO2 photoreduction, by improving either the light absorbance, the band structure alignment, or charge carrier separation [18,19].
BiVO4 is one of the common photocatalysts which has been utilized for photocatalytic CO2 conversion, but it suffers from a high recombination rate of electrons and holes and inappropriate conduction band position [36]. Different strategies have been used to solve these problems, but in all of them, impurity atoms or a second phase are added to this material. The HPT method was used to solve the problems of BiVO4 for photocatalytic CO2 conversion by simultaneous engineering of strain and oxygen vacancies without the addition of impurities. BiVO4 was processed by HPT for N = 0.25, 1, and 4 turns to investigate the impact of strain on photocatalytic properties. Increasing the lattice strain and decreasing the crystallite size by increasing the HPT turns is shown in Fig. 2a. The occurrence of lattice strain was also confirmed by Raman peak shift to lower wavenumbers as shown in Fig. 2b. It was also observed that, the concentration of oxygen vacancies increases in BiVO4 by increasing the applied shear strain. Fig. 2c illustrates the oxygen vacancy concentration, calculated by X-ray photoelectron spectroscopy (XPS), against the number of HPT turns, confirming that the concentration of vacancies increases with increasing the applied shear strain. Furthermore, strain and vacancy engineering led to an increase in the light absorbance (Fig. 1d) and a slight narrowing of the bandgap from 2.4 eV for the initial powder to 2.1 eV for the sample proceeded by HPT for N = 4 turns [36].
Simultaneous strain and oxygen vacancy engineering could significantly solve the problem of BiVO4 in terms of the high recombination rate of electrons and holes as shown in Fig. 1e [36]. This figure demonstrates that HPT processing decreases the photoluminescence intensity which is a piece of evidence for the suppression of recombination. Finally, this strategy was successful to improve the photocatalytic activity of BiVO4 as shown in Fig. 1f. The CO production rate from CO2 photoreduction was effectively increased by increasing the number of HPT turns (i.e. by increasing the fraction of oxygen vacancies and lattice strain). This study was the first successful work that used simultaneous strain and oxygen vacancy engineering to improve the photocatalytic activity of BiVO4 without using impurities. In conclusion, SPD can be considered a new path to improve the optical and electronic structure of photocatalysts for CO2 conversion without the addition of impurity atoms [36].
TiO2 with the anatase and rutile crystal structures is one of the most active photocatalysts for CO2 conversion [5][6][7]. As shown in Fig. 2a, in addition to anatase and rutile, TiO2 has a highpressure TiO2-II (columbite) phase with the orthorhombic structure. Despite many studies on TiO2 photocatalysts, there was not any study on photocatalytic CO2 conversion on the TiO2-II phase until 2021 mainly because of the technical limits in the stabilization of this phase. The groups of current authors stabilized the TiO2-II phase by the HPT method and investigated it for photocatalytic CO2 conversion [35]. To decrease the fraction of oxygen vacancies in the bulk, which can act as recombination centers, the HPT-processed sample was further treated by annealing. The formation of high-pressure TiO2-II was proved by X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). Raman spectra along with the appearance of samples are shown in Fig. 2b. New Raman peaks at wavenumbers of 171, 283, 316, 340, 357, 428, 533, and 572 cm -1 correspond to the TiO2-II phase. The changes in the color of the sample from white to dark green after HPT processing and from dark green to white after annealing indicate that large fractions of oxygen vacancies are formed after HPT processing while some of them are annihilated after annealing, a fact that was also proved by various characterization techniques [35].
The light absorbance of the TiO2-II phase produced by HPT processing was higher and it had a narrower optical bandgap of 2.5 eV compared to anatase (3 eV), but the bandgap slightly increased to 2.7 eV after annealing [35]. Introducing the high-pressure TiO2-II phase using HPT suppressed the recombination rate of electrons and holes. It also had a positive impact on photocurrent generation, as shown in Fig. 2c, so that the annealed samples had the highest current density, suggesting the improvement of charge carrier separations by introducing the high-pressure TiO2-II phase. The potential of this new phase for CO2 adsorption was measured by attenuated total reflectance Fourier transforms infrared (ATR-FTIR) spectroscopy. It was observed that the annealed sample has the highest potential for CO2 adsorption which can improve the photocatalytic CO2 conversion. Finally, this new phase showed higher potential for photocatalytic CO production compared to the anatase phase as shown in Fig. 2d. The introduction of the TiO2-II phase with an optimized fraction of oxygen vacancies significantly improved the activity so that the annealed sample had the highest efficiency for CO2 to CO conversion. It was also suggested that the formation of anatase-columbite interphases can also contribute to the high activity of the HPTprocessed and annealed samples by increasing the electron-hole separation and migration. In conclusion, high-pressure phases show great potential to be used as photocatalysts and SPD can be used to stabilize these high-pressure phases under ambient conditions [35].

Formation of defective high-entropy phases
Introducing high-entropy ceramics as new materials with five or more principal elements opened a new path in the field of material science to produce materials with high functionality for various applications [81,82]. High configurational entropy caused by a large number of elements in these materials leads to decreasing the Gibbs free energy and improving the phase stability. High-entropy ceramics have been utilized for various applications, and in many cases, they have shown better efficiencies than conventional materials [81,82]. Li-ion batteries [83], catalysts [84], dielectrics [85], magnetic components [86], thermal barrier coatings [87], etc. are some of the applications of these ceramics. The high potential of high-entropy ceramics for various applications is attributed to their high stability, cocktail effect, lattice distortion, inherent defects, and valence electron distribution [81,82]. Despite promising functionality of these materials, their application for photocatalytic CO2 conversion was not investigated until a study was conducted by the current authors in 2022 [37].
The HPT method followed by oxidation was used to fabricate a defective high-entropy oxide (HEO) with the composition of TiZrNbHfTaO11 and dual crystal structure of monoclinic and orthorhombic [37]. The selection of elements for this high-entropy ceramic was conducted by considering the d 0 electronic structure of cations, an empirical concept that was shown to be effective for the design of high-entropy photocatalysts [29]. The oxidation states of anionic and cationic elements and their uniform distribution were proved by XPS and energy dispersive X-ray spectroscopy (EDS), respectively. The microstructure of the oxide is shown in Fig. 3a using scanning electron microscopy (SEM) and in Fig. 3b using high-resolution TEM. In addition to a nanocrystalline dual-phase structure, the material exhibited the presence of various defects such as vacancies and dislocations, as shown in Fig. 3b. The formation of oxygen vacancies in this material was examined by electron paramagnetic resonance (EPR) spectroscopy. These oxygen vacancies can act as shallow traps between the valence band and the conduction band for easier charge carrier separation as shown in Fig. 3c [37].
TiZrNbHfTaO11 had a higher light absorbance and lower bandgap compared to the binary oxides including TiO2, ZnO, Nb2O5, HfO3, and Ta2O5 [37]. This HEO could successfully generate photocurrent which indicates its potential for easy separation of electrons and holes to improve the photocatalytic activity. TiZrNbHfTaO11 showed higher activity for photocatalytic CO production compared with BiVO4 and TiO2 as two typical photocatalysts (Fig. 3d). Furthermore, as shown in Fig. 3d, it had the same photocatalytic activity as P25 TiO2 as a benchmark photocatalyst. The high activity of this HEO for photocatalytic CO2 conversion was attributed to the presence of defects such as oxygen vacancies and dislocations, interphases, and high light absorbance. This study reported the first application of high-entropy ceramics for photocatalytic CO2 conversion and confirmed the potential of SPD to design and synthesize highly efficient high-entropy photocatalysts [37].

Synthesis of low-bandgap high-entropy oxynitrides
Metal oxides are the most conventional photocatalysts for CO2 conversion but suffer from a large bandgap [2,3]. On the other hand, metal nitrides have a low bandgap but suffer from low stability compared to metal oxides. Metal oxynitrides are rather new materials that can solve the problems of metal oxides and nitrides in terms of large bandgap and low stability, respectively. Although oxynitrides have been widely used for photocatalytic water splitting, their application for photocatalytic CO2 conversion has been limitedly investigated mainly due to their limited chemical stability. The concept of high-entropy materials with high stability is one strategy used to produce high-entropy oxynitrides with low bandgap and high stability for CO2 photoreduction [38].
A high-entropy oxynitride (HEON) with the composition of TiZrNbHfTaO6N3 was fabricated by the HPT method followed by oxidation and nitriding and its photocatalytic performance was compared with corresponding HEO TiZrNbHfTaO6 and P25 TiO2 benchmark photocatalyst [38]. This HEON had dual phases with face-centered cubic (FCC) and monoclinic structures with uniform distribution of elements. The HEON had much higher light absorbance compared to P25 TiO2 and relevant HEO as shown in Fig. 4a. It showed a superior low bandgap of 1.6 eV as one of the lowest bandgaps reported in the literature for oxynitride photocatalysts. The improved electronic band structure of this HEON compared to the HEO and P25 TiO2 is shown in Fig. 4b. The recombination rate of electrons and holes in the HEON was low so that its photoluminescence intensity was negligible compared to the HEO and P25 TiO2 (Fig. 4c). The shape of photocurrent spectra shown in Fig. 4d also confirmed the low recombination rate of electrons and holes in this HEON compared to the HEO and P25 TiO2 catalysts. The potential of this HEON for CO2 adsorption was measured by diffuse reflectance infrared Fourier transform (DRIFT) spectrometry which showed the higher physical adsorption and chemisorption (in the form of carbonate) of CO2 on this HEON compared to the HEO and P25 TiO2 (Fig. 4e). This HEON successfully converted CO2 to CO with extremely high efficiency even compared with P25 TiO2 benchmark photocatalyst as shown in Fig. 4f. In conclusion, the low-bandgap HEON catalysts synthesized by SPD can be considered a new family of highly efficient photocatalysts for CO2 conversion [38]

Outlook
The application of SPD to synthesize new photocatalysts for CO2 conversion introduced significant findings from the viewpoints of photocatalysis and SPD. The significance of these two issues and their impact on the future outlook of this research field are discussed here.
Regarding the first issue, Table 2 compares the photocatalytic activity of HEON synthesized by HPT with reported photocatalysts in the literature by normalizing the amount of CO production to the mass and surface area of catalysts [9,32,. Since the photocatalytic reaction occurs on the surface, comparing the results with normalizing to the surface area is more reasonable. According to this table, the amount of CO production for HEON is 4.66 ± 0.3 µmolh -1 m -1 which is higher than the best photocatalysts reported in the literature. This comparison indicates that the contribution of SPD to introducing new families of photocatalysts will receive high appreciation in the future by considering the current demands in finding new catalysts to deal with CO2 emissions. However, the synthesis method and compositions are expected to be modified by experts in the field of photocatalysis. For example, one main disadvantage of SPD for the process and synthesis of catalysts is the low surface area of the synthesized material, while large specific surface areas are desirable in catalysis [74]. Moreover, it is expected that the theoretical studies can clarify the mechanisms underlying the high activity of photocatalysts developed by SPD so that catalysts with even higher activities can be developed in the future.
Concerning the second issue on the significance of these studies in the SPD field, it should be noted that the SPD field experienced significant progress in the past three decades, as discussed in several review papers [121][122][123][124][125][126] and more deeply in a special issue in 2019 [127]. This special issue which gathered overviews on both historical developments [128] and recent advancements [129] clearly shows the state-of-art in the SPD field. A survey of these overviews indicates that despite significant progress on theoretical aspects [130,131], mechanisms [132,133], processing [134][135][136][137][138][139][140][141], microstructure [142][143][144][145][146], and mechanical properties [147][148][149][150][151][152] of metallic materials, there is a recent tendency to apply SPD to a wider range of materials (oxides [153], semiconductors [154], carbon polymorphs [155], glasses [156], and polymers [157]) to control phase transformations [158] and solid-state reaction [159][160][161] for achieving advanced functional properties [162][163][164][165][166][167][168][169]. The CO2 conversion is perhaps the newest application of SPD to functional materials which expanded the synthesis capability of SPD from metallic materials to ceramics [37,38]. Moreover, this application has led to the introduction of new benchmark photocatalysts, which can open new pathways and research directions in corresponding fields. Although the application of SPD for CO2 photoreduction is currently limited to the HPT method, which produces only small amounts of samples, the fundamentals developed by HPT can be used in the future to develop new methods with up-scaled sample sizes and higher potential for industrial applications. This last issue is a general requirement of SPD for future commercialization in almost any application [170].

Conclusions
Global warming has become a significant concern in recent years which seriously threatens the life of creatures. Conversion of CO2 molecules to other components such as CO is a way to stand this event. In this regard, photocatalytic CO2 conversion, which uses solar irradiation as a clean energy source, has been introduced as a new and promising strategy in recent years. Despite the introduction of various materials, which are modified by various strategies, the efficiency of CO2 photoreduction is still low compared to conventional methods for CO2 conversion. The highpressure torsion (HPT) as a severe plastic deformation (SPD) method has been used recently to produce some of the most active photocatalysts for CO2 conversion. The HPT method can increase the CO2 photoreduction efficiency by (i) oxygen vacancy and strain engineering, (ii) stabilization of high-pressure phases, (iii) formation of defective high-entropy phases, and (iv) synthesis of lowbandgap oxynitrides.   Raman spectra, (c) photocurrent spectra, and (c) photocatalytic CO production rate versus time for TiO2 before and after high-pressure torsion processing and after annealing [35].