2023 Volume 64 Issue 7 Pages 1497-1503
The electronic structure of the band gap determines the amount of light and its wavelength that can be absorbed by a semiconductor. Most photocatalysts are semiconductor materials, therefore, the state-of-art band gap engineering plays an important role in the efficiency of the photocatalytic reactions. Metal oxides are the most abundant semiconductors in the Earth’s crust, most of which possess large band gaps. In order for oxides to be able to absorb solar energy, the band gap must be reduced. In this review, band gap of high-pressure phases of some well-known metal oxides like TiO2, ZnO, and Y2O3 are studied, which are known to be unstable at ambient pressure while having the advantage of narrow band gaps. High-pressure torsion (HPT) is introduced as an effective method for stabilization of high-pressure phases, and these phases show good activity under visible light for water splitting hydrogen or oxygen production, and/or CO2 reduction reactions. High-entropy oxides and oxynitrides are another group of materials that will be introduced for effective photocatalytic properties, synthesized by the HPT method.
Fig. 1 (a) UV-Vis profiles, and (b) band gap calculation of TiO2.22)
In one hour, the earth receives 173,000 TWh of energy from the sun. During the year 2021 humanity consumed 176,000 TWh of energy, which means the sun generates enough energy of our annual need in about one hour. Sunlight-driven photocatalysis commercialization have attracted much attention during the last decade due to the accelerated need for solving environmental issues and worldwide energy demand. However, the commercial application of photocatalysis reactions is still a challenge, mainly due to the low photocatalytic efficiency of them under sunlight. The first step in the development of a high-performance photocatalyst is to design a material which can absorb the visible light because the fraction of visible light in the solar spectrum is about 50%, while the fraction of ultraviolet (UV) is only 5%.
Metal oxides are earth abundant materials which are promising for photocatalytic and photovoltaic applications thanks to their very stable characteristics under various environments. The low activity of metal oxides under sunlight stems mainly from their large band gap, limiting their application to photo-excitation under UV light. Several methods such as doping with cations or anions,1–4) defect engineering like introducing oxygen vacancy5) or nitrogen vacancy,6) sensitizing,7) surface engineering,8) morphology control,9,10) and heterojunctions11,12) are shown to be effective for reducing the large band gap of metal oxides. Although methods like doping lead to the successful band gap reduction, they do not necessarily result in photocatalytic activity under visible light due to the defect-induced recombination losses. In the literature, there are many excellent reviews13–15) on the band gap reduction and enhanced photocatalytic performance of various materials including oxides. In this review, we will focus on the effect of severe plastic deformation, in particular high-pressure torsion (HPT) method, on the band gap and photocatalytic properties of semiconductors and ceramics, and discuss future outlook of this new method for designing the visible-light active photocatalysts.
The seminal work of Honda and Fujishima in 1972 introduced titanium dioxide (TiO2) or titania as a first water-splitting photocatalyst.16) Three polymorphs of TiO2 at ambient pressure are: rutile with a band gap of 3.0 eV, anatase with a band gap of 3.2 eV, and brookite. TiO2 has other metastable polymorphs at elevated pressures in the range of 0–100 GPa, including TiO2(B), hollandite (OI), fluorite (Pca21), ramsdellite, columbite (TiO2-II), and cotunnite.17–19) These high-pressure phases of TiO2 have theoretically been calculated to have better light absorbance and narrower band gaps which coincide with the visible light.20) For example, columbite TiO2-II with a orthorhombic structure is calculated to have a band gap of 2.59 eV.21) As is evident from the phase diagram,22) the high-pressure phase TiO2-II is stable only at pressures higher than 2 GPa. Authors for the first time, introduced HPT method for stabilizing the TiO2-II phase and it was shown that the high-pressure phase was stable for at least 6 months.22) The concept of HPT method was first proposed by Bridgman in 1946. In this method, the sample is pressurized between two anvils at pressures in the range of several gigapascals, while simultaneously rotating the anvils against each other. This unique method enables us to induce high pressures and huge amount of strain to the sample at the same time. In a typical process, pure anatase powder with an average particle size of 150 nm was simultaneously pressurized and strained under 1 and 6 GPa by rotating two anvils for 0, 1/16, 1/4, 1, and 4 turns. Discs with 10 mm diameter and 0.8 mm thickness were obtained and lattice defects were reduced by annealing the HPT-processed discs at 500°C for one hour.
XRD results showed that TiO2-II phase was formed for samples processed at 1 and 6 GPa.22,23) It was noteworthy that TiO2-II phase was present at 1 GPa, which was lower than the reported pressure for TiO2-II formation. This showed that HPT method was effective for reducing the critical pressure and also for stabilizing the high-pressure phase at ambient pressure. Theoretical studies are also interesting in this regard, where the plastic straining is effective for reducing the transformation pressure and retaining the high-pressure phases at ambient pressure.24–26) Figure 1(a) shows the UV-Vis light absorbance spectra of samples before HPT, after HPT and after annealing at 500°C. The absorption edge of the starting powder was 400 nm, but it shifted to 470 nm, after HPT. The Band gap of all samples were calculated by the Kubelka-Munk equation as shown in Fig. 1(b), being 3.1, 2.4, and 2.7 eV for samples before HPT, after HPT and after annealing, respectively. Comparing the band gaps of samples before HPT and that of the HPT processed samples showed that the formation of columbite TiO2-II effectively narrowed down the band gap of TiO2.
(a) UV-Vis profiles, and (b) band gap calculation of TiO2.22)
Photocatalytic hydrogen generation under UV and visible lights are shown in Figs. 2(a) and 2(b), respectively. It was shown that water split rate was faster for the HPT processed samples under visible light. It was concluded that HPT processing successfully stabilized the high-pressure TiO2-II phase, and as a consequence the band gap was reduced and the photocatalyst was able to split the water under visible light. Interestingly, the same trend is confirmed for the photocatalytic CO2 conversion.27) It is shown that high-pressure TiO2-II phase is very effective for the generation of CO from CO2 under visible light. The color of samples changed from white to green due to the formation of high-pressure phase, and oxygen vacancies.
Photocatalytic water splitting test of TiO2 under (a) UV light and (b) visible light.22)
Zinc oxide (ZnO) with a wurtzite crystal structure is known as a stable, non-toxic and cheap semiconductor with excellent photocatalytic properties under UV. It is widely used for application like antibacterials, biochemical sensors, and photovoltaic cells. However, similar to TiO2, the large band gap of 3.4 eV limits the photocatalytic application of pure ZnO to the UV range of sunlight. Doping is an effective method for reducing the band gap of ZnO, however dopants usually tend to precipitate and change the chemical composition of the host. Another limitation is the solubility of dopants in ZnO, which are usually very low. Since high temperatures are usually required for enhancing the crystallinity of ZnO, undesired expulsion of dopant can also occur easily. In our study, we focused on the high-pressure phase of the ZnO. Wurtzite ZnO can transform to high-pressure rocksalt phase under high pressures.28) Theoretical studies on ZnO have proved that the band gap of the rocksalt phase is in the range of 1.2–2.6 eV.29,30) However, the wurtzite phase is the only stable phase of ZnO at the ambient pressure, while the rocksalt phase can only exist at pressures higher than 6–10 GPa. HPT method is shown to be effective for stabilizing the rocksalt phase with a large fraction of oxygen vacancies.
Figure 3(a) shows the UV-Vis profile of the starting ZnO powder, HPT samples under 3 GPa, and 6 GPa. ZnO powder showed an absorption edge at 400 nm, while it redshifted to the visible light area with absorption edges of 460 and 650 nm for the samples processed at 3 and 6 GPa, respectively. Band gaps were calculated to be 3.1, 2.8, and 1.8 eV, as shown in Fig. 3(b).31) Band gap narrowing of sample processed at 3 GPa was due to the introduction of oxygen vacancies. Oxygen vacancies are known to create energy states above the valance band, leading to band gap reduction of ZnO. However, a large band gap reduction of the sample at 6 GPa was anomalous and difficult to explain only by the presence of oxygen vacancies. This band gap reduction was attributed to the formation of rocksalt phase. The photocatalytic activity of samples was examined by measuring the degradation rate of Rhodamine B (RhB) dye under visible light and results are shown in Fig. 3(c). Starting powder shows little RhB degradation, while the sample processed at 3 GPa exhibited higher photocatalytic activity due to the formation of oxygen vacancies. The HPT sample at 6 GPa showed even better activity due to the rocksalt phase formation with a band gap of 1.8 eV. It was evident that HPT successfully stabilized the rocksalt phase of ZnO with a band gap as narrow as 1.8 eV, which was able to absorb the visible light extensively. The color of samples changed from white to yellow by HPT processing due to the formation of high-pressure rocksalt phase and oxygen vacancies.
(a) UV-Vis profiles of ZnO before and after HPT, (b) band gap calculation, (c) photocatalytic activity under visible light.31)
Yttrium oxide (Y2O3) is an insulator ceramic with the high melting point, reliable mechanical properties, and superior chemical stability.32) The optical properties of Y2O3 doped with rare earth elements are interesting with its application as phosphors, lasers, bioimaging, fluorescent lamps, and field emission displays. Y2O3 has a cubic crystal structure at ambient pressure and transforms to the monoclinic and hexagonal phases at higher pressures. However, since these high-pressure phases are unstable at ambient pressure, the HPT method was tried to stabilize high-pressure phases, in a same manner as done for TiO2 and ZnO. It is reported that monoclinic Y2O3 shows better photoluminescence (PL) intensities than the cubic phase.
XRD results showed that only compression without straining led to cubic phase, while the monoclinic phase appeared after HPT processing.33) In addition, the fraction of monoclinic phase increased with increasing the plastic strain and reached 90%. Band gaps 5.82, 5.77, 5.68, and 5.69 eV for the starting powder and for the samples processed by HPT for N = 1/8, 1 and 5 turns were calculated based on the UV-Vis diffuse reflection results. It was evident that as the monoclinic phase fraction increased with increasing the plastic strain, the band gap was reduced.
Strain-induced oxygen vacancy was shown to be effective for reducing the band gap of bismuth vanadate (BiVO4) semiconductor for photocatalytic properties. BiVO4 is a promising semiconductor for the photo-oxidation of water under visible light due to its narrow band gap. However, its conduction band edge is not suitable for photo-reduction of the water and hydrogen production. In this study, the effect of strain on the band gap reduction and photocatalytic CO2 conversion was investigated. The color change of samples, x-ray photoelectron (XPS) spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy all conformed the formation of oxygen vacancies in the HPT processed BiVO4.34) Authors showed that the simultaneous introduction of strain and oxygen vacancies into BiVO4 could lead to improving the light absorbance, narrowing the bandgap, modifying the electronic band structure, decreasing the recombination rate of electrons and holes and as a consequence enhancing the photocatalytic activity for CO2 conversion. A band gap reduction from 2.4 eV to 2.1 eV was reported due to the formation of lattice defects after HPT processing, however this study did not mentioned about any high-pressure phase formation by the HPT process. The photocatalytic conversion of CO2 into fuels is important both economically, and also from the point of view of the environmental issues.
The concept of high-entropy alloys was first introduced by Yeh et al.35) and Cantor et al.,36) in 2004. The high-entropy alloys are defined as multi-principal elements which are composed of five or more elements with equal atomic fractions. These alloys exhibit a unique combination of composition, microstructure and properties. In recent years, high-entropy ceramics are defined as the solid solution of five or more cations or anions sublattices with a high configuration entropy.37) Cations are usually metals and anions are mainly oxygen, nitrogen, sulfur, phosphorous, and/or carbon. The high-entropy ceramics now include a wide range of materials including high-entropy oxides, nitrides, carbides, borides, hydrides, sulfides, fluorides, phosphides, oxynitrides, and etc. High-entropy oxides are probably the most studied high-entropy ceramics in recent years.38–44) They have been studied for various properties and applications including thermomechanical properties, catalytic properties, ionic conductivity for batteries, magnetic properties, electrical properties and optical properties.
The synthesis methods for high-entropy ceramics can be classified into three main groups: solid-state methods (e.g., ball milling, high-pressure torsion, sintering, thermal reduction, and self-propagating high-temperature synthesis), liquid-phase methods (e.g., sol-gel, solution combustion, solvothermal/hydrothermal synthesis, co-precipitation and hydrothermal treatment, sonochemical-based synthesis, facile electrochemical synthesis, polymeric steric entrapment, two-step soft urea method, polymeric synthesis, dealloying, nebulized spray pyrolysis, flame spray pyrolysis and droplet-to-particle aerosol spray synthesis) and gas-phase methods (e.g., sputtering deposition, and pulsed laser deposition).
The HPT method was recently used to synthesize various high-entropy oxide materials for functional applications.45–47) The high-entropy oxide with the composition of TiHfZrNbTaO11 was first introduced as a photocatalyst.45) The material had a band gap of 2.9 eV in the visible-light region with desirable band structure and good photocatalytic activity for water splitting. The TiZrHfNbTaO11 oxide was synthesized by a two-step process, where in the first step the high-purity powders of Ti, Zr, Hf, Nb, and Ta with equal molar ratios were mixed in acetone. Then, the powder mixture was mechanically alloyed using the HPT method for 200 turns under a pressure of 6 GPa. In the second step, the mechanically-alloyed samples were oxidized in air at 1443 K for 24 h which resulted in a two-phase oxide with an overall composition of TiZrHfNbTaO11. The material absorbed a significant amount of visible light with two distinct absorbance edges at 430 nm and 670 nm, as shown in Fig. 4(a). The band gap was calculated by the Kubelka–Munk equation, as shown in Fig. 4(b), confirming the band gap of the material as 2.9 eV. The photocatalytic water splitting test of TiZrHfNbTaO11 showed that amount of produced hydrogen increased linearly with increasing the exposure time. However, no oxygen production was confirmed during photocatalytic test, indicating that the oxide did not show overall water splitting and thus it required a scavenger for long-term hydrogen production. The material was shown to also be effective for the photocatalytic CO2 conversion. The presence of defects like oxygen vacancies or color centers was attributed to the color change, which were also confirmed by the low energy shoulders in XPS spectra of cations.46)
(a) UV-Vis absorbance spectra, and (b) Kubelka–Munk plot for band gap estimation of TiZrHfNbTaO11 high-entropy oxide.46)
Another high-entropy oxide material introduced for photocatalytic properties was a high-entropy oxide in the Ti–Zr–Nb–Ta–W–O system with multiple heterojunctions.47) To design the material, authors selected elements with the d0 oxidation states. The selected combination led to the formation of a dual-phase high-entropy oxide with a rather narrow band gap which was active in the UV and visible range of light. Authors applied a three-step synthesis in this study. First, small pieces of high purity Ti, Zr, Nb, Ta, and W in stoichiometric molar ratios were arc-melted under Ar atmosphere to produce a metallic ingot. After arc melting, the metallic discs were further homogenized by the HPT method. The band structure of oxide was investigated by UV-Vis showed that the material absorbed light in both UV and visible regions. Band gap estimation showed the presence of two band gaps with energy levels of 2.8 and 2.3 eV. Although it was hard to differentiate which band gap belongs to which phase(s), it was evident that both band gaps were in the visible-light region of solar spectrum. It is noteworthy that the initial binary oxides of TiO2, ZrO2, Nb2O3, Ta2O5, and WO2 all possess wide band gaps in the UV region, while the mixture of them which led to the high-entropy oxide had a band gap in the visible light region. The synthesized oxide successfully produced oxygen from water under visible light without using a co-catalyst, generating 1.4 µmol of oxygen after 300 min. It is not clear why despite the suitable band structure of the oxide for water reduction reaction, no hydrogen production was observed under visible light.
Nitride semiconductors usually show narrower band gaps as compared to their oxide counterparts because of the higher energy of 2p orbitals in nitrogen than that of oxygen 2p. However, nitrides are usually less stable than oxides. Metal oxynitrides have both features of narrower band gap of nitrides and higher stability of oxides, and thus, they can be a potential solution for the photocatalytic reactions under the sunlight. Some famous oxynitrides are already introduced in the literature such as TaON, LaZrO2N, LaTaON2, PrTaON2, LaW(O,N) CaTaO2N, SrTaO2N, and BaNbO2N. First high-entropy oxynitride was proposed recently with the composition of TiZrHfNbTaO6N3.48) All elements were chosen so that they had an electronic structure of d0. A three-step synthesis method was employed to produce the high-entropy oxynitride. In the first step, same molar ratio of Ti, Zr, Hf, Nb, and Ta were mixed in acetone and dried in air. The dried mixture was then treated by the HPT method to make a high-entropy alloy. The resultant crystal structure was cubic. In the second step, the alloy was crushed and oxidized in air at 1373 K for 24 h to produce a two-phase high-entropy TiZrHfNbTaO11. In the third step, the high-entropy oxide was subjected to nitriding in ammonia for 7 h at 1373 K. Upon nitridation the color changed from orange to black, revealing a light absorbance in the visible light range. UV-Vis absorbance spectra also showed that high-entropy oxynitrides were able to absorb visible light more efficiently. The band gap after nitridation was 1.6 eV, while it was 2.9 eV before nitridation, as shown in Fig. 5. Authors also attributed the black color of the sample to the oxygen and nitrogen vacancies, as well the band gap reduction by nitridation. Photocatalytic water splitting test showed that the material was able to split water into hydrogen in the presence of the sacrificial agent.
(a) UV-Vis absorbance spectra, and (b) Kubelka–Munk plot for band gap estimation of TiZrHfNbTaO11 high-entropy oxide.48)
The material is also reported to be effective for photocatalytic CO2 to CO conversion. The reaction rate of photoreduction was better for the oxynitride than its oxide counterpart. The oxynitride here again showed better efficiency than the oxide high-entropy material. They also compared both reactions with P25 photocatalyst, and showed that high-entropy oxides and high-entropy oxynitrides had better photocatalytic properties than P25 both for water splitting reaction and CO2 conversion reaction, as shown in Fig. 6.49)
(a) Rate of (a) CO2 to CO photoreduction, and (b) hydrogen generation versus UV irradiation time for high-entropy oxynitride compared to corresponding high-entropy oxide and P25 TiO2.49)
Table 1 is a summary of publications in the literature which studied the effect of high pressure torsion method on the photocatalytic properties and the band gap of various semiconductors and ceramics.22,23,31,33,34,45–55) Also, excellent review articles are published regarding the history of the severe plastic deformation and its effects on nanomaterials with advanced functionalities.56,57)
