KONA Powder and Particle Journal No. 41 (2024) 123–139/https://doi.org/10.14356/kona.2024003

Review Paper

High-Pressure Torsion for Highly-Strained and High-Entropy
Photocatalysts †Saeid Akrami 1, Parisa Edalati 1, Masayoshi Fuji 1,2* and Kaveh Edalati 3,4*
1 Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Japan
2 Advanced Ceramics Research Center, Nagoya Institute of Technology, Japan

3 WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan
4 Mitsui Chemicals, Inc.- Carbon Neutral Research Center (MCI-CNRC), Kyushu University,

Japan

Nowadays, the environmental crisis caused by using fossil fuels and CO2 emissions has
become a universal concern in people’s life. Photocatalysis is a promising clean technology
to produce hydrogen fuel, convert harmful components such as CO2, and degrade pollutants
like dyes in water. There are various strategies to improve the efficiency of photocatalysis sothat it can be used instead of conventional methods, however, the low efficiency of the process
has remained a big drawback. In recent years, high-pressure torsion (HPT), as a severe plastic
deformation (SPD) method, has shown extremely high potential as an effective strategy to
improve the activity of conventional photocatalysts and synthesize new and highly efficientphotocatalysts. This method can successfully improve the activity by increasing the lightabsorbance, narrowing the bandgap, aligning the band structure, decreasing the electron-hole
recombination, and accelerating the electron-hole separation by introducing large latticestrain, oxygen vacancies, nitrogen vacancies, high-pressure phases, heterojunctions, andhigh-entropy ceramics. This study reviews the recent findings on the improvement of theefficiency of photocatalysts by HPT processing and discusses the parameters that lead to these
improvements.
Keywords: photocatalysis, water splitting, oxygen vacancy, nitrogen vacancy, heterojunctions, high-entropy alloys (HEAs),
high-entropy ceramics

1. Introduction

Nowadays, serious environmental crises such as global
warming and the formation of wastewater have forced
humans to find alternatives like using clean fuels withoutcarbon dioxide emissions such as hydrogen, reducing thepollutant gases such as carbon dioxide, and degrading thetoxic pollutants in wastewater through clean strategies(Gaya and Abdullah, 2008). Using photo­catalysts is aneffective strategy to deal with these environmental crises
using renewable sunlight energy (Ma et al., 2014). The
function of photo­catalysts is the acceleration of reduction
and oxidation (redox) reactions under light irradiation.
Reduction and oxidation reactions occur on the surface ofa photo­catalyst using the electrons and holes, respectively(Tong et al., 2012), to promote processes such as dye
degradation (Akpan and Hameed, 2009), water splitting(Maeda, 2011), CO2 conversion (Tu et al., 2014), etc.
In photo­
catalytic dye degradation, photo­excited
Received 9 September 2022; Accepted 19 December 2022
J-STAGE Advance published online 25 March 2023
* Corresponding author: Masayoshi Fuji and Kaveh Edalati;
1,2 Add: 3-101-1 Hon-machi, Tajimi, Gifu 507-0033, Japan
3,4 Add: 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
E-mail: fuji@nitech.ac.jp (M.F.); kaveh.edalati@kyudai.jp (K.E.)
TEL: +81-572-27-6811 (M.F.); +81-92-802-6744 (K.E.)†

electrons take part in the degradation reaction of dyes (e.g.
rhodamine B, methylene orange, acid red B, etc.) which
normally exist in wastewaters produced by factories,
laboratories and different industries (Akpan and Hameed,2009). Photocatalytic water splitting includes the reduction
to hydrogen and oxidation to oxygen (Maeda, 2011). In
excited electrons
photo­catalytic CO2 conversion, photo­contribute to the conversion of CO2 to reactive and usefulcomponents such as carbon monoxide (CO), formic acid(HCOOH), formaldehyde (HCHO), methanol (CH3OH)
and methane (CH4) (Tu et al., 2014). The mechanismof photo­
catalytic dye degradation, water splitting andCO2 conversion is shown in Figs. 1(a), 1(b) and 1(c),respectively.
A photo­catalyst should have some features to support
the photo­catalytic reactions including a narrow bandgap,
appropriate band positions to cover the desired reactions,appropriate light absorbance, potential to absorb andactivate the reactants, easy electron transition, and low
recombination rate of electrons and holes (Akpan and
Hameed, 2009; Maeda, 2011; Tu et al., 2014). Furthermore,high specific surface area, low cost, low toxicity and highstability are the other required properties that should
catalyst (Tong et al.,be considered to select a photo­2012). Challenging in this field is finding a photo­catalyst

Copyright © 2024 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (https://creativecommons.org/licenses/by/4.0/).

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a

b

c

d

Review Paper

Fig. 1 Schematic illustration of (a) photocatalytic dye degradation, (b) photocatalytic water splitting, (c) photocatalytic CO2 conversion, and (d) the
HPT process.

with sufficient properties for considerable photo­catalytic
efficiency compared with the conventional catalytic
processes. TiO2 (Ajmal et al., 2014; Ni et al., 2007; Ola
and Maroto-Valer, 2015), ZnO (Chakrabarti and Dutta,
2004; Lin et al., 2012; Zhao et al., 2019), g-C3N4 (Wenet al., 2017; Ye et al., 2015; Yuan et al., 2016), WO3 (Liu
Y. et al., 2010; Wang et al., 2012; Wang et al., 2019a),
SrTiO3 (Huang et al., 2014a; Iwashina and Kudo, 2011;
Shan et al., 2017), GaN–ZnO (Hagiwara et al., 2017;
Maeda et al., 2005; Ohno et al., 2012) and BiVO4 (Gaoet al., 2017; Sun et al., 2014; Zhou et al., 2010) are someof the most promising photo­catalysts which have beenutilized for photo­catalytic dye degradation, water splitting
and CO2 conversion. The efficiencies of all these photo­catalysts are still low for practical applications, and thus,
various strategies were utilized to enhance the optical andstructural properties and accordingly the photo­catalytic
activity of these materials.
Vacancy introduction (Huang et al., 2014b; Lei et al.,2014; Yu H. et al., 2019), strain engineering (Di J. et al.,2020; Feng et al., 2015; Sa et al., 2014), using mesoporous
structures (Li Y. et al., 2010; Pauporté and Rathouský, 2007;
Puangpetch et al., 2009), formation of hetero­junctions(Cao et al., 2018; Moniz et al., 2015; Uddin et al., 2012),
nanosheet production (Etman et al., 2018; Tu et al., 2012;
Yu J. et al., 2010) and doping with impurities (Kuvarega124

et al., 2011; Lee et al., 2016; Liu B. et al., 2018) are someof the reported methods which have been effective toimprove the photo­catalytic activity. Doping with impurities
is the most common method which is used to improve theactivity by narrowing the bandgap, but it usually causes
recombination effects (Kuvarega et al., 2011).
High-pressure torsion (HPT) as one of the severe plastic
deformation (SPD) methods has been introduced as aneffective process to improve the photo­catalytic efficiencyof various materials by introducing vacancies (oxygen
vacancy and nitrogen vacancy), dislocations, nanocrystals,
lattice strain, heterojunctions, high-pressure phases andhigh-entropy ceramics. The HPT method is effective in
reducing the recombination rate of electrons and holes
which are usually observed in doped photo­catalysts. The
HPT method decreases the optical bandgap and enhances
the photo­catalytic activity of various oxides and oxy­nitrides including TiO2 (Razavi-Khosroshahi et al., 2016b),
ZnO (Razavi-Khosroshahi et al., 2017b), Al2O3 (Edalati etal., 2019b), MgO (Fujita et al., 2020a), ZrO2 (Wang et al.,2020b), SiO2 (Wang et al., 2020a), LiTaO3 (Edalati et al.,2020a), CsTaO3 (Edalati et al., 2020a), BiVO4 (Akrami
et al., 2022c), Ga6ZnON6 (Edalati et al., 2020b) for dye
degradation, hydrogen production and CO2 conversion.
Furthermore, HPT was successfully used to synthesize
new high-entropy oxides such as TiHfZrNbTaO11 (Edalati

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et al., 2020c) and high-entropy oxynitrides such as
TiZrHfNbTaO6N3 (Akrami et al., 2022a) for hydrogenproduction, oxygen evolution and CO2 conversion.
Moreover, it was also utilized to form heterojunctions
in binary and high-entropy composites such as TiO2–
ZnO (Hidalgo-Jimenez et al., 2020) and TiZrNbTaWO12(Edalati et al., 2022b) to improve the photo­catalytic
activity for hydrogen and oxygen production.
In this review paper, the influence of the HPT methodon optical properties, electronic structure, electron-hole
separation, migration and recombination rates and photo­catalytic efficiency of various photo­catalysts is discussed.
The improvement of photo­catalytic activity of thesematerials for dye degradation, hydrogen production,oxygen production and CO2 conversion is discussed byconsidering the generation of oxygen vacancies, nitrogenvacancy complexes, high-pressure phases, heterojunctions
and high-entropy ceramics.

2. High-pressure torsion (HPT)The SPD process is typically used to form ultrafinegrained (UFG) and nanostructured materials withenhanced mechanical and functional properties (Estrin and
Vinogradov, 2013; Valiev, 2004). SPD has various methods
including HPT, introduced by Bridgman in 1935 for the
first time (Bridgman, 1935). In this method, torsionalstrain under high pressure is applied to induce large plasticstrain in various ranges of materials. In the HPT process,a disc-shape sample (Bridgman, 1935) that has typicallya 10 mm diameter or a ring sample (Edalati and Horita,2009) is inserted between two anvils under high pressure
and strained by rotation of anvils against each other. A
schematic illustration of HPT is shown in Fig. 1(d). The
applied shear strain (γ) to the sample can be calculated by
the following equation (Edalati and Horita, 2016).

γ

2πrN
h

(1)

where r, N and h are the distance from the center of thedisc or ring, the number of turns and the sample height,(1)
respectively.
In addition to grain refinement, the HPT method is
widely used for hardening of pure metals (Edalati andHorita, 2011; Starink et al., 2013), mechanical alloyingof miscible and immiscible systems (Edalati et al., 2015b;
Edalati, 2019a) and plastic deformation of hard-to-deformmaterials (Edalati et al., 2010a; Edalati et al., 2010b;
Ikoma et al., 2012). Phase transformation, consolidationof powders, the introduction of defects such as oxygen
vacancies and dislocations, narrowing of the bandgap andimprovement of functionalities were frequently reported
after HPT processing (Edalati, 2019d). The HPT method canbe utilized for plastic deformation of oxides and ceramics
which are hard and brittle materials at ambient temperature

Review Paper(Edalati et al., 2022a). The presence of covalent or ionicbonding in ceramics results in their lower grain size after
HPT processing compared to metals. Moreover, suchbonding features result in the formation of large densitiesof defects such as vacancies and dislocations which can
improve the properties and functionality of ceramics.
Despite the high potential of ceramics for various
applications, there are some limited publications on theeffect of HPT on the structure, properties and functionalityof ceramics. A list of ceramics treated by HPT forvarious properties and applications and their relevantpublications are given in Table 1. Table 1 indicatesthat these HPT-processed ceramics, including oxides,
nitrides, oxynitrides and borides, have been investigatedfor photo­catalysis, phase transformation, electrocatalysis,
photo­
current generation, dielectric properties, bandgap
narrowing, optical properties, mechanical properties, Liion batteries and microstructural features. Furthermore,
the HPT method was also utilized to synthesize newceramics for various applications. As can be seen in
Table 1, ceramic photo­catalysts processed or synthesizedby HPT were used for water splitting (CsTaO3, LiTaO3,
ZrO2, GaN–ZnO, TiO2–ZnO, TiO2, TiHfZrNbTaO11,
TiZrNbTaWO12 and TiZrHfNbTaO6N3), dye degradation(ZnO, MgO, γ-Al2O3 and SiO2), CO2 conversion (TiO2,
BiVO4, TiHfZrNbTaO11 and TiZrHfNbTaO6N3) andoxygen production (TiZrNbTaWO12).

3. Fundamentals and mechanism of photo­catalysis

In photo­catalysis, which is also called artificial photo­
synthesis, electrons in the valence band absorb the light
photons, separate from the holes and transfer to the
conduction band of the photo­catalyst to form the electronhole charge carriers. The charge carriers then migrate to
the surface of the photo­catalyst and finally take part invarious chemical reactions. In this process, both reduction
and oxidation reactions occur on the surface of the photo­catalyst, provided that the thermodynamic and kinetic
conditions are satisfied. From the thermodynamic pointof view, a photo­catalytic process will perform when the
reduction and oxidation reaction potentials are between thevalence band and the conduction band of a photo­catalyst(Tong et al., 2012). For the reduction reactions, potentials
lower than the valence band top are desirable, while for theoxidation reactions, potentials higher than the conduction
band bottom are desirable. From the kinetic point of view,the electron-hole separation should be fast, and theirrecombination should be slow. Here, the fundamentals of
three main photo­catalytic reactions are mentioned.

3.1 Photocatalytic dye degradation

In photo­catalytic dye degradation, there are two direct
and indirect pathways. In the direct pathway, the dye

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Review Paper

Table 1 Summary of major publications about ceramics treated by HPT and their major applications or features.
Materials

Investigated properties and application

Reference

TiHfZrNbTaO11

Photocatalytic activity for CO2 conversion(Akrami et al., 2022b)

TiZrHfNbTaO6N3

Photocatalytic activity for CO2 conversion(Akrami et al., 2022a)

BiVO4

Photocatalytic activity for CO2 conversion(Akrami et al., 2022c)

TiO2-II

Photocatalytic activity for CO2 conversion(Akrami et al., 2021b)

MgO

Photocatalytic activity for dye degradation(Fujita et al., 2020a)

SiO2

Photocatalytic activity for dye degradation(Wang et al., 2020a)

γ-Al2O3Photocatalytic activity for dye degradation(Edalati et al., 2019b)

ZnO

Photocatalytic activity for dye degradation(Razavi-Khosroshahi et al., 2017b)

TiZrHfNbTaO6N3

Photocatalytic activity for hydrogen production(Edalati et al., 2021)

ZrO2

Photocatalytic activity for hydrogen production(Wang et al., 2020b)

CsTaO3, LiTaO3

Photocatalytic activity for hydrogen production(Edalati et al., 2020a)

GaN–ZnO

Photocatalytic activity for hydrogen production(Edalati et al., 2020b)

TiHfZrNbTaO11

Photocatalytic activity for hydrogen production(Edalati et al., 2020c)

TiO2–ZnO

Photocatalytic activity for hydrogen production(Hidalgo-Jimenez et al., 2020)

TiO2-II

Photocatalytic activity for hydrogen production(Razavi-Khosroshahi et al., 2016b)

TiZrNbTaWO12Photocatalytic activity for oxygen production(Edalati et al., 2022b)

Review on HPT of oxides(Edalati, 2019b)

Review on HPT(Edalati and Horita, 2016)

SiO2, VO2

Phase transformation(Edalati et al., 2019a)

TiO2

Grain coarsening and phase transformation(Edalati et al., 2019c)

ZrO2

Phase transformation by ball milling and HPT(Delogu, 2012)

ZrO2

Allotropic phase transformations(Edalati et al., 2011)

Photocatalysis

Reviews
OxidesPhase transformation

TiO2

Plastic strain and phase transformation(Razavi-Khosroshahi et al., 2016a)

BN

FEM modeling of plastic flow and strain-induced phase transformation(Feng et al., 2019)BN

Coupled elastoplasticity and plastic strain-induced phase transformation(Feng and Levitas, 2017)

Electrocatalysis for hydrogen generation(Edalati et al., 2019c)

Bi2O3

Enhanced photocurrent generation(Fujita et al., 2020b)

TiO2-II

Visible light photocurrent generation(Wang et al., 2020c)

Electrocatalysis
TiO2-II
Photocurrent

Dielectric propertiesBaTiO3

Optical and dielectric properties(Edalati et al., 2015a)

CuO

Dielectric properties(Makhnev et al., 2011)

Bandgap narrowing(Qi et al., 2021)

Y2O3

Optical properties(Razavi-Khosroshahi et al., 2017a)

CuO, Y3Fe5O12, FeBO3

Optical properties and electronic structure(Gizhevskii et al., 2011)

Cu2O, CuO

Middle infrared absorption and X-ray absorption(Mostovshchikova et al., 2012)

CuO, Y3Fe5O12, FeBO3

Optical properties(Telegin et al., 2012)

α-Al2O3

Microstructure and mechanical properties(Edalati and Horita, 2010)

Fe53.3Ni26.5B20.2, Co28.2Fe38.9Cr15.4Si0.3B17.2

Microstructure and mechanical properties(Permyakova and Glezer, 2020)

Lithium-ion batteries(Qian et al., 2018)

Bandgap investigation
ZnO
Optical properties

Mechanical properties

Lithium-ion batteries
Fe3O4
Microstructural features
ZnO

Plastic flow and microstructural instabilities(Qi et al., 2018)

YBa2Cu3Oy

Microstructural investigation(Kuznetsova et al., 2017)

Fe71.2Cr22.7Mn1.3N4.8

Microstructural features(Shabashov et al., 2018)126

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Review Paper(Lu et al., 2021), as shown in Fig. 2. These CO2 adsorption
modes determine the reaction pathway for photo­catalytic
CO2 conversion. For instance, the oxygen coordinationmode, with the bidentate bonding of oxygen atoms withthe surface of the photo­catalyst, leads to the fabricationof formate anions as an intermediate product and formic
acid as the final product. In the carbon coordination mode,with the monodentate binding of carbon and photo­catalyst
surface, the reaction results in carboxyl radical production.
After the adsorption step of CO2 to the surface of thephoto­catalyst, the conversion reactions occur via different
pathways.
It was reported that there are three main pathways forCO2 photo­reduction: (i) carbene pathway (ii) formaldehyde
pathway and (iii) glyoxal pathway (Habisreutinger et al.,2013; Maeda and Domen, 2007; Wang et al., 2021). The
reactions for these three pathways are presented in Table 2.
In all these pathways, the CO •2 radical is the main inter­mediate product which is formed by the reaction of

degrades directly by the excited electrons and holes in
the conduction band and the valence band, respectively,
as shown in Fig. 1(a). In the indirect pathway, which ismore conventional than the direct pathway, the electrons
in the conduction band react with the O2 to produce theO •2 radicals. The O •2 radicals then react with the dye toform the reduction products (Li X. et al., 2018). In thevalence band, holes react with H2O to produce •OH, andthe •OH radicals react(11)(11)with the dye to produce the oxidation
products as shown in Fig. 1(a).

3.2 Photocatalytic water splitting

In photo­
catalytic water splitting, electrons in the
conduction band take part in the reduction reaction to
produce H2, while holes in the valence band take part in theoxidation reaction to produce O2 (Lu et al., 2021), as shownin Fig. 1(b). In most photo­catalysts, overall water splitting
does not occur, and the process is limited to either reduction
or oxidation reactions thus a sacrificial agent or scavenger
is usually required to produce electrons or holes. Methanol
and AgNO3 are popular sacrificial agents for photo­catalytichydrogen and oxygen production, respectively.

. (12).

δ-C

In photo­
catalytic CO2 conversion, electrons in the
conduction band along with protons (H+) produced fromwater oxidation in the valence band, take part in various
reduction reactions leading to the formation of CO, CH4
and some other hydrocarbons as shown in Fig. 1(c).
The first step in photo­catalytic CO2 conversion is theadsorption of CO2 molecules on the surface of the photo­catalyst which occurs in three modes including oxygen
coordination, carbon coordination and mixed coordination

δ-

O

O

3.3 Photocatalytic CO2 conversion

δ-

C

δ-

Oxygen CoordinationO
C

Carbon Coordination

O

δ-

O

O
C

O

O

δ-

δ-

O

C

δ-

O

Mixed Coordination
Fig. 2CO2 adsorption modes on the surface of photocatalysts.

Table 2 CO2 photoreduction pathways (Habisreutinger et al., 2013).

Carbene Pathway

Formaldehyde Pathway

Glyoxal Pathway

(1) CO2 + e– → CO •2(1) CO2 + e– → CO 2

(1) CO2 + e– → CO •2(2) CO •2 + e– + H+ → CO + OH–(2) CO •2 + H+ → •COOH(2) CO •2 + e– + H+ → CHOO–•–(3) CO + e– → CO[21](3) •COOH + e– [21]+ H+ → HCOOH(3) CHOO– + H+[21]→ HCOOH(4) [21]CO•– + e– + H+ → C + OH–(4) [21]
HCOOH + e– + H+ → H3OOC•(4) [21]
HCOOH + e– → HOC•

(5) C + e– + H+ → CH•(5) HCOOH •2 + e– + H+ → HCOH + H2O(5) HOC• + OH– → C2H2O2(6) CH• + e– + H+ → CH2(6) HCOH + e– → H2C•O–(6) C2H2O2 + e– + H+ → H3O2C2•(7) CH2 + e– + H+ → CH •3(7) [24]
H2C•O– + H+ → H2OHC•(7) H3O2C2• + e– + H+ → C2H4O2(8) CH •3 + e– + H+ → CH4(8) H2OHC• + e– + H+ → CH3OH(8) C2H4O2 + e– + H+ → H3OC2• + H2O(9) CH •3 + OH– → CH
3OH
[23]

(9) CH3OH + e– + H+ → •CH3 + H2O(9) H3OC2• + e– + H+ → C2H4O(10) •CH3 + e– + H+ → CH4(10) C2H4O + h+ → H3OC2• + H+

[23]
[23]

•

(11) H3OC2• → CH •3 + CO(12) CH •3 + e– + H+ → CH4

[23]
[23]
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adsorbed CO2 and an electron. In the carbene pathway,
CO •2 converts to CO• and leads to the production of CO,
CH2, CH4 and CH3OH. In this pathway, carbon coordinationis the main binding mode. In the formaldehyde pathway,
CO •2 converts to •COOH to produce HCOOH, CH3OH and(12)

CH4. In this pathway, CO •2 attaches to the catalyst bymonodentate or bidentate binding. In monodentate binding,
either one oxygen atom in CO •2 or one carbon atom in(12)
CO •2 are bonded to the surface(12) of the photo­catalyst. In thebidentate mode, both oxygen atoms in CO •2 are bonded tothe photo­catalyst surface. (12)In the glyoxal pathway, C2
components are formed by some complicated reactions,(12)though there are limitations to producing
C2 due to the low(12)
selectivity of the photo­catalyst. In this pathway, CO •2converts to the formyl radical HOC• to form C2H2O2,
C2H4O2, C2H4O, CO and CH4. In all these pathways, CO2adsorption and activation are key parameters to adjust
the(12)
selectivity of photo­catalysts and this should be empoweredby some strategies such as oxygen vacancy generation andstrain engineering (Fu et al., 2020).

4. Impact of HPT on photo­catalysis

As given in Table 1, numerous studies in recent years
reported that the HPT method can be used to improve thephoto­
catalytic activity of ceramics. Bandgap narrowing
for the easier transition of photo­excited electrons from thevalence band to the conduction band, increasing the lightabsorbance in the visible region of light, electronic band
structure alignment, decreasing the recombination rateof electrons and holes and accelerating the electron-hole
separation and migration are some phenomena responsible
for the high activity of photo­catalysts processed orsynthesized by HPT. The occurrence of these phenomena
was attributed to the formation of oxygen vacancies,
nitrogen-vacancy complexes, high-pressure phases,
heterojunctions and high-entropy phases which will be
discussed in detail as follows.

4.1 Generation of oxygen vacancies

Oxygen vacancies on the surface have a significant
effect on the photo­catalytic activity of materials, since they
can act as active sites to adsorb and activate the reactants
and as trap sites for photo­excited electrons. In contrast,
oxygen vacancies in the bulk of photo­catalysts may act
as recombination centers and lead to decreasing photo­catalytic efficiency. Oxygen vacancies can also reduce theoptical bandgap by changing the electronic structure ofmaterials via the formation of defect states between thevalence band and the conduction band. The formation ofoxygen vacancies after HPT processing has been evidenced
by several analysis methods such as electron paramagnetic
resonance (EPR), X-ray photo­electron spectroscopy (XPS),
Raman spectroscopy, differential scanning calorimetry(DSC), and X-ray diffraction (XRD) (Fujita et al., 2020a;

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Razavi-Khosroshahi et al., 2016b; Wang et al., 2020a).
Furthermore, changing the color of catalysts to the darker
ones, which are frequently observed after HPT processing,indicates the light absorbance in the visible region due tothe formation of oxygen vacancies as color centers (Fujita
et al., 2020b; Razavi-Khosroshahi et al., 2017b; Wang etal., 2020b). Among the photo­catalysts treated by HPT,
Al2O3 (Edalati et al., 2019b), MgO (Fujita et al., 2020a),
ZrO2 (Wang et al., 2020b), SiO2 (Wang et al., 2020a),
LiTaO3 (Edalati et al., 2020a), CsTaO3 (Edalati et al.,2020a) and BiVO4 (Akrami et al., 2022c) are oxides which
exhibited enhanced photo­catalytic activity by oxygen
vacancy formation.
Al2O3: Al2O3 is an insulator with a large bandgap(~9 eV) and does not show photo­catalytic activity. Firstprinciples calculations suggested that the formation ofoxygen vacancies can reduce the optical bandgap of Al2O3
even to the visible light region (Edalati et al., 2019b). The
application of HPT to Al2O3 resulted in the formationof oxygen vacancies, the reduction of optical bandgap
and its photo­catalytic activity. The formation of oxygen
vacancies after HPT is shown in Fig. 3(a) using DSC
analysis by the appearance of a peak at 530 K. After the
HPT process, a narrow bandgap of 2.5 eV was achieved, ingood agreement with the first-principles calculations, andthe material showed photo­catalytic activity for rhodamine
B dye degradation under UV irradiation with an activity
comparable to TiO2 (Fig. 3(b)).
MgO: MgO, as another ceramic insulator, also exhibitedhigh light absorbance and bandgap narrowing (from 7.8 eV
to 3.9 eV) after HPT processing (Fujita et al., 2020a).
The HPT-processed MgO showed photo­catalytic activity
for methylene blue dye degradation under UV irradiation(55 % decomposition after 3 h). The formation of oxygen
vacancies in this material was evidenced by XPS analysisof Mg 2p, which showed a shift to lower energy, and sucha shift became more significant by increasing the applied
shear strain (i.e. by increasing the number of HPT turns).
ZrO2: ZrO2 is another ceramic that was processed by
HPT to introduce a high concentration of oxygen vacancies(Wang et al., 2020b). The optical bandgap of this material
decreased from 5.1 to 4.0 eV due to the formation of oxygen
vacancies after HPT. The formation of oxygen vacancies
after HPT processing at various temperatures is shownin Fig. 3(c) using the EPR analysis. The appearance of apair peak with a g factor of ~2 indicates the formation ofoxygen vacancies, while the formation of oxygen vacancies
is enhanced by increasing the HPT processing temperature.
Improvement of photo­catalytic activity of ZrO2 using HPT
for hydrogen evolution is shown in Fig. 3(d), indicatingthat the photo­catalytic activity is improved by increasing
the concentration of oxygen vacancies.
SiO2: SiO2 quartz sand also was examined for photo­catalytic activity after processing using the HPT method

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Fig. 3 Formation of oxygen vacancies achieved by HPT processing and improvement of photocatalytic activity. (a) DSC analysis and (b) photocatalytic rhodamine B degradation for γ-Al2O3 before and after HPT processing compared with TiO2 (Edalati et al., 2019b). (c) EPR spectra and (d)corresponding photocatalytic hydrogen production for ZrO2–3 wt% Y2O3 before and after HPT processing at 300, 437, 573 and 673 K (Wang et al.,2020b). (e) Oxygen vacancy concentration and (f) photocatalytic CO2 to CO conversion for BiVO4 before and after HPT processing for N = 0.25, 1 and4 turns (Akrami et al., 2022c). Copyright: (2019) Elsevier, (2020) Royal Society of Chemistry, (2022) Elsevier.

(Wang et al., 2020a). This material with the initiallyreported bandgap of ~9 eV and negligible light absorbance
exhibited a bandgap narrowing to 2.8 eV, with a small
activity for rhodamine B dye degradation under UV lightirradiation.
LiTaO3 and CsTaO3: LiTaO3 and CsTaO3 are twotantalate perovskites that show activity for photo­catalytichydrogen production. After oxygen vacancy generation
by HPT processing, both materials exhibited bandgap
narrowing (4.7 to 4.2 eV for LiTaO3 and 4.7 to 3.6 eV for
CsTaO3) and enhanced photo­catalytic activity for hydrogengeneration (Edalati et al., 2020a).
BiVO4: BiVO4 is a popular photo­catalyst for CO2
conversion due to its low bandgap, but it suffers from ahigh recombination rate of electrons and holes and a low
position of the conduction band. HPT could successfully
solve these two problems by simultaneously introducing
the oxygen vacancies and lattice strain (Akrami et al.,2022c). Increasing the oxygen vacancy concentration byincreasing the number of HPT turns, calculated from XPS,is shown in Fig. 3(e). These structural modifications by
HPT processing resulted in significant improvement ofphoto­catalytic CO2 conversion on BiVO4, as shown in
Fig. 3(f).

4.2 Generation of nitrogen-vacancy complexes

Oxynitrides are promising photo­catalysts due to their
low bandgap compared with oxide photo­catalysts. It
was reported that nitrogen vacancies, which are formed
during the synthesis of these materials, can act asrecombination centers, and reduce photo­catalytic activity(Maeda and Domen, 2007). However, it was observedthat HPT processing of Ga6ZnON6 led to the formation of
nitrogen-based vacancies, decreasing the electron and hole
recombination and enhancement of photo­catalytic activity(Edalati et al., 2020b). Fig. 4(a) shows the EPR spectra of129

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Fig. 4 Formation of nitrogen-based vacancy complexes and suppression of recombination in oxynitride processed by HPT. (a) EPR spectraand (b) photoluminescence spectra for Ga6ZnON6 before and after
HPT processing (r: radial distance from center of HPT-processed disc)(Edalati et al., 2020b). Copyright: (2020) Elsevier.

Ga6ZnON6 before and after HPT processing. A g factor of
1.958 for powder is a piece of evidence for the formation of
nitrogen monovacancies during the synthesis of oxynitrides,
while a g factor of 2.006 indicates the presence of nitrogenbased vacancy complexes by HPT processing. Introducing
the nitrogen-based vacancy complexes to this material by
HPT, led to reducing the bandgap from 2.7 eV to 2.4 eV,enhancing the light absorbance and improvement of photo­catalytic hydrogen production under UV irradiation. This
study showed that in addition to the location of vacancies(surface or bulk), their arrangement (monovacancy orvacancy complex) can affect photo­catalytic activity. These
vacancy complexes can act as active sites for reactions
rather than the recombination centers for electrons andholes. Decreasing the recombination of electrons and holes
after HPT processing for this oxynitride is presented inFig. 4(b), in which the photo­
luminescence intensity issuppressed effectively after the HPT process.

4.3 Formation of high-pressure phases

Due to high pressure and high strain in HPT, the method

130

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can lead to the formation of metastable and high-pressure
phases. These phases usually form with fast kinetics atlower pressures during HPT processing compared to thetransition pressures under static conditions due to thepresence of high concentrations of nanograin boundaries
and defects. It was suggested that the formation ofthese defects and their interaction increase localized
pressure and result in the formation and stability of highpressure phases (Edalati, 2019b). BaTiO3 (tetragonal to
cubic) (Makhnev et al., 2011), TiO2 (anatase-tetragonal
to orthorhombic TiO2-II) (Razavi-Khosroshahi et al.,2016b), Y2O3 (cubic to monoclinic) (Razavi-Khosroshahi
et al., 2017a), ZnO (wurtzite-hexagonal to rocksalt-cubic)(Razavi-Khosroshahi et al., 2017b) and SiO2 (quartz
to coesite) (Wang et al., 2020a) are some oxides whichshowed transition to high-pressure phases after HPT
processing. Among these materials, the formation of highpressure phases in TiO2 and ZnO was reported to lead tothe improve­ment of photo­catalytic activity for hydrogenproduction, CO2 conversion and dye degradation. Figs. 5(a)and 5(b) illustrate the pressure-temperature phase diagramsof TiO2 and ZnO, respectively; Figs. 5(c) and 5(d) show
the formation of TiO2-II and rocksalt-ZnO high-pressure
phases after HPT processing, respectively; and Figs. 5(c)and 5(d) show the influence of HPT processing on theenhancement of the photo­catalytic activity of TiO2 andZnO, respectively. Since TiO2 and ZnO are the mostcommon photo­catalysts, the reports on the applicationof HPT to these two photocatalysts are of significance as
they suggested new solutions to enhance the activity bynanostructure and polymorphic control and without dopant
or impurity addition (Razavi-Khosroshahi et al., 2016b;
Razavi-Khosroshahi et al., 2017b).
TiO2: TiO2 light absorbance improved significantly after
the formation of the high-pressure TiO2-II (columbite)
phase by HPT processing (Akrami et al., 2021b; RazaviKhosroshahi et al., 2016b). The color of TiO2 changed
from white to gray or green and its bandgap decreased
from 3.1 eV to 2.5 eV, which resulted in photo­catalytic
activity of the material under visible light. To solve therecombination issue due to the presence of bulk defects
after HPT processing, the HPT-processed sample wasprocessed by annealing which resulted in a further increasein the photo­catalytic activity and photo­current generation.
It was shown that the formation of TiO2-II results in photo­catalytic activity for hydrogen production under visible
light and CO2 conversion under UV light (Fig. 5(e)),
although the high activity is influenced by other factors
such as the presence of oxygen vacancies and interphaseboundaries.
ZnO: HPT processing of ZnO led to the formationof a high-pressure rocksalt phase and accordingly to theimprovement of light absorbance in the visible region
with a bandgap narrowing from 3.0 eV to 1.8 eV (Razavi-

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Fig. 5 Improved photocatalytic activity by introducing high-pressure phases. (a) Phase diagram of TiO2 (Akrami et al., 2021b), (b) phase diagramof ZnO (Razavi-Khosroshahi et al., 2017b), (c) high-resolution lattice image of TiO2 including TiO2-II high-pressure phase (Akrami et al., 2021b), (d)XRD profile of ZnO including rocksalt high-pressure phase (Razavi-Khosroshahi et al., 2017b), (e) photocatalytic CO2 conversion on TiO2 before andafter HPT processing and after annealing under UV light (Akrami et al., 2021b), and (f) photocatalytic rhodamine B dye degradation for ZnO before
and after HPT processing under visible light (Razavi-Khosroshahi et al., 2017b). Copyright: (2021) Elsevier, (2017) Royal Society of Chemistry.

Khosroshahi et al., 2017b). This bandgap narrowing
was also predicted by first-principles calculations. The
HPT-processed ZnO, containing a high-pressure phasewith a large fraction of oxygen vacancies and interphaseboundaries, showed photo­catalytic rhodamine B dye
degradation under visible light, while its activity improved
with increasing the processing pressure (Fig. 5(f)).

4.4 Formation of heterojunctions

Heterojunctions are the space-charge regions that areformed by the combination of two different phases ordifferent semiconductors. Charge carriers can penetrateheterojunctions which results in the formation of anelectronic field in this region (Moniz et al., 2015).
Heterojunctions accelerate the electron and hole separationand improve their migration to the surface of the photo­catalyst for the reactions. Furthermore, they significantly
suppress recombination (Cao et al., 2018; Uddin etal., 2012). As mentioned earlier, among HPT-processedmaterials, some of them exhibit phase transformation
resulting in the formation of heterojunctions between
two or more phases such as cubic-rhombohedral phases
in Al2O3 (Edalati et al., 2019b), monoclinic-tetragonalphases in ZrO2 (Wang et al., 2020b) and coesite-quartz

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phases in SiO2. (Wang et al., 2020a). There is also anapplication of HPT to TiO2–ZnO composite to generateheterojunctions for enhanced photo­catalytic hydrogenproduction (Hidalgo-Jimenez et al., 2020). There is anotherattempt to generate 10 different heterojunctions in ahigh-entropy oxide TiZrNbTaWO12 for visible-light-driven
photo­catalytic oxygen production (Edalati et al., 2022b). In
all these cases, the interphase boundaries formed by HPT
can act as sites to accelerate the electron-hole separation toeffectively improve the photo­catalytic activity, as will be
discussed below for the TiO2–ZnO composite.
TiO2–ZnO: A recent study generated heterojunctions

Review Paper

using HPT by making a composite of TiO2 and ZnO as two
typical catalysts for photo­catalytic hydrogen production(Hidalgo-Jimenez et al., 2020). This work was performed
to ease electron-hole separation as shown in Fig. 6(a). In
addition to TiO2–ZnO heterojunctions, HPT processing led
to the formation of high-pressure phases and formation ofvarious kinds of heterojunctions. XRD profiles of TiO2–
ZnO composite show the presence of five different phasesof TiO2 and ZnO in Fig. 6(b). After HPT processing, thelight absorbance of the composite significantly increasedin the visible light region and a bandgap narrowing wasobserved from 3.2 eV to 2.6 eV and 1.6 eV for N = 3
and 15 HPT turns, respectively. The recombination rateof electrons and holes was also suppressed effectively
after HPT processing. Although electronic structure, lightabsorbance and recombination suppression improved byincreasing the number of HPT turns to N = 15, photo­catalytic hydrogen production for N = 15 was lower
compared to the powder mixture and sample processed by
N = 3 due to the formation of very small crystals and poor
crystallinity (Fig. 6(c)). The composite processed with
N = 3 had a hydrogen production amount of 2.5 time better
than initial powders mixture under UV irradiation.

4.5 Production of high-entropy ceramics

Fig. 6 Enhanced photocatalytic hydrogen production by heterojunction formation. (a) Schematic illustration of electronic band structureof TiO2–ZnO composite including heterojunctions. (b) XRD profile and(c) photocatalytic hydrogen production on TiO2–ZnO composite before
and after HPT processing for N = 3 and 15 turns (Hidalgo-Jimenez et al.,2020). Copyright: (2020) Elsevier.

132

High-entropy ceramics are materials with at least
five principal cations and a mixing entropy higher than
1.5R (R: gas constant) (Akrami et al., 2021a; Oses et al.,2020). These materials have been employed for various
applications due to their high stability, lattice strain,
inherent defects and high entropy of mixing (Zhang andReece, 2019; Xiang et al., 2021). They have potential to
be used as Li-ion batteries (Lun et al., 2021; Nguyen etal., 2020; Wang et al., 2019b), catalysts (Albedwawi et al.,2021; Qiao et al., 2021; Xu et al., 2020), photo­catalysts(Akrami et al., 2022a; Akrami et al., 2022b; Edalati et al.,2020c), supercapacitors (Jin et al., 2018; Liang et al., 2020;
Talluri et al., 2021), dielectrics (Bérardan et al., 2016; Zhou
et al., 2020; Zhou et al., 2022), thermal barrier coatings(Li F. et al., 2019; Ren et al., 2020; Zhang et al., 2022),
etc. Utilizing the high-entropy ceramics as photo­catalysts,
which was performed for the first time by the authors of thecurrent work, is a new functional aspect of these materials
which is in its initial research stage. The high stability ofhigh-entropy ceramics and the formation of defects andstrain in these materials were the main motivations to
use them as photo­catalysts. As will be discussed below,
the oxides TiZrHfNbTaO11 and TiZrNbTaWO12 and theoxynitride TiZrHfNbTaO6N3 are high-entropy ceramics
synthesized by HPT and used for photo­catalytic hydrogenproduction, CO2 conversion and O2 evolution. Since oxides
and oxynitrides with d0 and d10 electronic configurations
show good photo­catalytic activity, five d0 cations wereselected to design these photo­catalysts.

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Fig. 7 High light absorbance and significant photocatalytic activity of high-entropy ceramics. (a) UV-vis spectra of TiZrHf NbTaO11,
TiZrHf NbTaO6N3 and relevant binary oxides including TiO2, ZrO2, HfO2, Nb2O5 and Ta2O5 (Edalati et al., 2021), (b) diffuse reflectance infrared
Fourier transform spectra for TiZrHf NbTaO11 and TiZrHf NbTaO3N6 compared with P25 TiO2 in which the peaks at 665 cm –1 and 2350 cm –1 represent
chemisorption and physisorption of CO2 on surface (Akrami et al., 2022a), (c) photocatalytic hydrogen production on TiZrHf NbTaO6N3 comparedwith Ga6ZnON6 (Edalati et al., 2021), and (d) photocatalytic CO2 to CO conversion on TiZrHf NbTaO11 and TiZrHf NbTaO6N3 compared with P25 TiO2
(Akrami et al., 2022a). Copyright: (2021) Royal Society of Chemistry, (2022) Elsevier.

TiZrHfNbTaO11: TiZrHfNbTaO11 high-entropy oxide(HEO) with two monoclinic and orthorhombic phases
which was synthesized by the HPT method and hightemperature oxidation, showed better light absorbancecompared to relevant binary oxides including TiO2, ZrO2,
HfO2, Nb2O5 and Ta2O5 (Fig. 7(a)) (Edalati et al., 2020c).
This material contained defects such as oxygen vacancies
and dislocations as shown in Fig. 8(a). Furthermore, it
demonstrated lower photo­luminescence intensity compared
to anatase TiO2 and BiVO4 as two typical photo­catalysts,
indicating the low recombination rate of electrons andholes in this HEO (Akrami et al., 2022b). Moreover,this material showed the potential to generate photo­current, produce hydrogen and convert CO2 to CO under
UV light. The rate of hydrogen and CO production for
TiZrHfNbTaO11 was higher than anatase TiO2 and BiVO4
and almost the same as P25 TiO2 as a benchmark photo­catalyst. This high photo­catalytic activity was attributed
to the formation of defects, interphases and high-entropy
phases which led to the acceleration of electron-hole
separation and diminishing their recombination.

TiZrHfNbTaO6N3:
Low-bandgap
high-entropy
oxynitride (HEON) TiZrHfNbTaO6N3 was produced by
HPT processing followed by oxidation and nitriding(Edalati et al., 2021). This HEON showed significantly
higher light absorbance compared to corresponding highentropy and binary oxides (Fig. 7(a)) with a narrow
bandgap of 1.6 eV, which is lower than the bandgap of
most reported oxynitride photo­catalysts in the literature.
This HOEN could successfully generate photo­current witha significantly low recombination rate of electrons andholes compared to HEO and P25 TiO2 as a benchmark
photo­
catalyst (Akrami et al., 2022a). Furthermore, it
showed a high potential to adsorb the CO2 molecules on
the surface as shown in Fig. 7(b). This material producedhydrogen under UV irradiation with better activity than
Ga6ZnON6 as one of the most popular oxynitride photo­catalysts (Fig. 7(c)). It also showed a higher photo­catalytic
activity per surface area for CO2 conversion compared
to HEO and P25 TiO2 (Fig. 7(d)) and all reported photo­catalysts in the literature. Improved photo­catalytic activity
on this HEON was attributed to the high light absorbance133

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Fig. 8 Formation of nanograins, grain boundaries, interphases, and dislocation defects in high-entropy ceramics synthesized by HPT. Highresolution lattice images of (a) TiZrHf NbTaO11 including dislocations (Akrami et al., 2022b) and (b) TiZrHf NbTaO6N3 including monoclinic and FCC
phases and their heterojunctions (Edalati et al., 2021). Copyright: (2022) Elsevier, (2021) Royal Society of Chemistry.

and easy electron-hole separation, low recombination rate,and high surface activity for adsorption and conversion ofwater and CO2. The presence of two phases in this HEON
and the formation of interphases, as shown in Fig. 8(b),was also believed to contribute to its high activity.
TiZrNbTaWO12: The HEO, TiZrNbTaWO12, wasproduced by HPT and consequent high-temperatureoxidation (Edalati et al., 2022b). It had five different
phases including one orthorhombic, two monoclinic
and two tetragonal phases and ten heterojunctions. This
material showed a low bandgap of 2.3–2.8 eV and alight absorbance higher than relevant binary oxides.
Introducing the heterojunctions in this material resulted insuccessful photo­catalytic oxygen production under viable
light irradiation.

5. Concluding remark and outlook

Photocatalytic production of hydrogen from water,conversion of CO2 to value-added compounds anddegradation of toxic materials in wastewater undersunlight, are clean processes that have high potential
to be used instead of conventional chemical methods.
Photocatalysts used for these applications should have anarrow bandgap, appropriate band positions to support134

the reactions, low recombination rate of electrons andholes and high surface activity. In recent years, the highpressure torsion (HPT) method was used to generate
active photo­catalysts for various applications. This method
applies very high strain and pressure to material andsubsequently introduces nanocrystals, lattice defects,
lattice strain and hetero­junctions. Furthermore, HPT can
synthesize high-pressure and high-entropy phases. One
main reason for the successful application of HPT todevelop highly active photo­catalysts is that the method cansimultaneously introduce a few structural/microstructuralfeatures as effective strategies to enhance the activity. It
was shown that the HPT method improves photo­catalytic
activity by increasing light absorbance, decreasing theoptical bandgap, optimizing the electronic band structure,accelerating the electron-hole separation and migration andreducing the recombination rate of electrons and holes.
The HPT method also contributed to the introduction ofhigh-entropy photo­catalysts which show high activity
for hydrogen production, oxygen evolution and CO2
conversion.
Despite the high activity of HPT-processed photo­catalysts, the mechanism for their high efficiency needs to beinvestigated further by employing theoretical calculations.

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Moreover, the catalysts produced by HPT have usually low
surface area and new methods are expected to be developed
to enhance the surface area of these catalysts. Although the
HPT method has opened a new path to employing highpressure and high-entropy phases as new photo­catalysts,
other synthesis methods should be used in the future forthe large-scale production of these new catalysts. It is
expected that HPT will continue its contribution to the fieldof photo­catalysis, particularly due to the significance of
establishing a carbon-neutral society in recent years.

Acknowledgments

The authors S.A. and P.E. thank Hosokawa Powder
Technology Foundation, Japan for funding HPTF21511
and HPTF21512. The author K.E. was supported in partby the MEXT, Japan through Grants-in-Aid for Scientific
Research (JP19H05176 & JP21H00150, JP22K18737).

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Authors’ Short Biographies
Saeid Akrami
Saeid Akrami obtained a M.Sc. degree in Chemical Engineering, and he is currently a Ph.D.
candidate in Applied Chemistry and Life Sciences at Nagoya Institute of Technology, Japan. His
research interest is the development of highly-strained and high-entropy ceramics for CO2 conversion
and he is the author of several papers in this field.

Parisa Edalati
Parisa Edalati is currently a Ph.D. candidate in the Advanced Ceramics Research Center, Nagoya
Institute of Technology, Japan. She was a visiting researcher at Kyushu University in 2019 before
starting her Ph.D. study. She works on the design and synthesis of functional high-entropy alloys
and ceramics for various applications including photocatalysis, biomedical application, hydrogenstorage, and batteries.

Masayoshi Fuji
Masayoshi Fuji received his Dr. Eng. in 1999 and joined the Tokyo Metropolitan University, asa research associate in 1991. He was a visiting researcher at the University of Florida, USA, from2000 to 2001. He joined the Nagoya Institute of Technology as an Associate Professor in 2002 andbecame a Professor in 2007. He is the editor-in-chief of Advanced Power Technology since 2019.
His representative awards include the award from the Minister of Education, Culture, Sports, Scienceand Technology Japan in 2013, CerSJ award for academic achievements in ceramic science andtechnology in 2014, and the science award of the Society of Inorganic Materials, Japan in 2017.

Kaveh Edalati
Kaveh Edalati received his Ph.D. degree in Materials Physics and Chemistry from Kyushu
University, Japan, in 2010. He currently serves as an Associate Professor at the International
Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka, Japan. His interests are
the development of functional materials for carbon-neutral energy and hydrogen-related applications
such as hydrogen storage and photocatalytic hydrogen generation and CO2 conversion. He particularly
utilizes the high-pressure torsion method to develop new functional materials. He is the author ofover 150 journal papers and serves as the editor of several journals.

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