解説

先進セラミックス研究センター年報（2018）. Vol. 7, 31‑38

Effect of High Pressure and Plastic Strain on Optical
Properties of TiO2, ZnO, and Y2O3
Hadi Razavi-Khosroshahi*, Kaveh Edalati**, Zenji Horita**, Masayoshi Fuji**

Advanced Ceramics Research Center, Nagoya Institute of Technology
3-101-1 Honmachi, Tajimi, Gifu 507-0033, JAPAN**Department of Materials Science and Engineering, Faculty of Engineering,
Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, JAPAN

Recently, authors have proposed a unique “non-firing sintering” method for solidifying various ceramics without applying an
external pressure and/or temperature. Surface activation of powders by ball milling plays an important role in this method,
however many aspects of this method regarding its mechanism is not well understood. Since there are many similarities between
ball milling and high-pressure torsion (HPT) method in terms of inducing strain to the surface of powder, in this review somewell-known metal oxides like titania (TiO2), zirconia (ZnO), and yttria (Y2O3) have been subjected to the severe plastic
deformation, and effect of plastic strain on the phase transformations of them is investigated. In addition, the optical/photocatalyic
properties of HPT processed samples is also investigated.
Keywords: Metal Oxides; High-Pressure Torsion (HPT); Titanium Dioxide (Titania); Zinc Oxide (Zirconia): Yttrium Oxide (Yttria)*

Corresponding author: Masayoshi Fuji

E-mail: fuji@nitech.ac.jp, Tel: +81-572-24-8110, Fax: +81-572-24-8109

1. Introduction

A facile “ non-firing sintering ” process has been
proposed by authors recently without heating the sample
and also without applying an external pressure.1-3) The
underlying idea of non-firing sintering is to activate thesurface of particles by ball milling and consolidate theparticles via the joining of activated surfaces. By ball
milling, the surface of particles is rubbed against balls,
and the friction between particle/ball breaks the bonds offunctional groups like ≡ Si–O–Si ≡ on the surface ofparticles. It was shown in previous studies that ball
milling induces a large amount of strain to the surface ofparticles and as the milling time and/or milling energy
increase the surface of particles activates extensively.
The increased amount of water absorption on the surfaceof particles after ball milling clearly proved theactivation process.
The high-pressure torsion (HPT) method, in which thematerial is compressed between two anvils undercontrollable pressures and plastic strain is induced byrotating the two anvils against each other, is an ideal
technique to investigate the effect of plastic strain on
metallic and non-metallic materials. By HPT, the amountof strain can be carefully controlled, in which as thenumber of turns increases the amount of induced strain
increases.
Metal oxide semiconductors which are abundant innature are considered as less expensive, biocompatible,
and stable photocatalysts under various environments.
The application of metal oxide photocatalysts isd e c o m p o s i n g o rg a n i c p o l l u t a n t s i n w a t e r / a i r,
photovoltaics, and splitting water into hydrogen andoxygen. Despite their excellent features, thephotocatalytic activity of most metal oxides is limited tothe ultraviolet (UV) range of sunlight due to their large
band gap4). Since UV accounts for only 5% of the solar
spectrum, designing metal oxide semiconductors withthe capability of absorbing visible light has been widelyattempted. The large band gap of metal oxides can ber e d u c e d b y m e t h o d s l i k e m e t a l l o a d i n g 5), i o n
implantation6), and anion/cation doping7, 8), however,such approaches do not necessarily lead to higher
photocatalytic activity due to the defect-induced
electron/hole recombination losses9, 10).
Narrowing the band gap of pure metal oxide
semiconductors has been extensively investigated inrecent decades11, 12). According to theoretical studies,
high-pressure phases of metal oxides show narrower― 31 ―

Effect of High Pressure and Plastic Strain on Optical Properties of TiO2, ZnO, and Y2O3

band gaps in the range of visible light13-17). However,most of these high-pressure phases are not stable underambient pressure and transform to their stabler phases
irreversibly upon decompression. High-pressure torsion(HPT) method18, 19) is known as an effective method forstabilization of high-pressure phases under roompressure. In this technique, high pressure and plasticstrain are simultaneously induced to the material
between two rotating anvils. The large plastic strain andlattice defect formation change the thermodynamic
behavior of phases at ambient pressure. It is reported
that high-pressure phases could be stabilized at ambientpressure in several materials: black phosphorous in P16),
diamond-like phases in Carbon20), ω phase in Zirconium
and Titanium21), cubic phase in barium titanate22). Since
there are few reports on the high-pressure stabilizationof metal oxides, the HPT method is employed forvarious metal oxides like TiO2, ZnO, and Y2O3. In thefollowing, each material will be reviewed in terms ofhigh-pressure phases, phase transformations, optical andphotocatalytic properties.
2. Experimental Methods

In review, anatase TiO2, wurtzite ZnO, and cubic Y2O3
were processed by HPT under hydrostatic pressures of 1
and 6 GPa and the effect of plastic strain and grain sizeon the stability of high-pressure phases at ambientpressure were investigated. Pressures of 1 and 6 GPa
were selected below and above the reported criticalpressure for each metal oxide.
Almost 0.2 g of each powder were placed between
two Bridgman anvils under P = 1 and 6 GPa and shear
strain γ (γ =2πrN/h; r: distance from disc center, N:
number of turns, h: sample thickness [22]) was
introduced by rotating the two anvils with respect toeach other for either 0 (mere compression), 1/16, 1/4, 1
and 4 turns at room temperature. After HPT processing,the material had a disc shape with 10 mm diameter and
0.8 mm thickness. The HPT-processed discs were
polished and examined in terms of (i) X-ray diffraction(XRD) analyses using the CuKα radiation, (ii) Raman
spectroscopy at 3 mm from the center of discs using a
micro-Raman system with a 532 nm solid state laser, (iii)
photoluminescence (PL) spectroscopy using aspectrofluorometer equipped with a Xenon lamp with an
excitation wavelength of 350 nm, and (iv) highresolution transmission electron microscopy (TEM).
Thin foils for TEM were prepared by crushing the edge
part of samples and pouring them on a carbon-type grid.

3. Results and Discussion
3.1. Titanium Dioxide (TiO2)

Titania (TiO2) was the first material described as awater-splitting photocatalyst as described by Fujishima
and Honda in 197123). TiO2 crystallizes in three types ofpolymorphs: Rutile with a band gap of 3.0 eV, Anatasewith a band gap: 3.2 eV, and Brookite24). High-pressure
phases of TiO2 have theoretically been calculated tohave lower band gaps coinciding with the visible light.
For example, columbite TiO2-II with an orthorhombic
structure is shown to have a band gap of 2.59 eV16, 17).
However, TiO2-II is stable at pressures higher than 2
GPa according to the phase diagram.
The starting powder for HPT processing was pureanatase with an average particle size of 150 nm. Plasticstrain was introduced under 1 and 6 GPa by rotating two
anvils for 0, 1/16, 1/4, 1, and 4 turns and discs with 10
mm diameter and 0.8 mm thickness were obtained.
Lattice defects were reduced by annealing the HPTprocessed discs at 500 °C. Phase transformations wereconfirmed by X-ray diffraction (XRD), andmicrostructures were observed by transmission electron
microscopy. Band gap of each sample was calculated by
UV-Vis diffuse reflectance spectroscopy. Photocatalytichydrogen evolution was examined in a solution of 500g
TiO2 sample, 25 cm3 methanol, 225 cm3 distilled waterand 1.325 cm3 H2PtCl6.6H2O under UV light and visible
light.
XRD results for HPT samples under 1 and 6 GPa areshown in Fig. 1(a). A distinct peak broadening is
confirmed for samples after HPT processing, indicationof lattice strain and lattice defects 25). In addition toanatase, TiO2-II is also confirmed for samples after HPT
processing. It is important to notice that TiO2-II phase ispresent below 1 GPa, which is lower than the reportedpressure for TiO 2-II formation. This decrease in thecritical pressure is considered to be due to the plasticstrain effect.
The relation between plastic strain and phasetransformation is shown in XRD results of Figs. 1(b)and (c). The intensity and the fraction of TiO2-II increase
with HPT turns. The relation between phase fraction
increase and strain in Fig. 1(c) is comparable to those of
HPT-processed metallic materials26, 27) and ceramics28-32).

― 32 ―

Hadi Razavi-Khosroshahi, Kaveh Edalati, Zenji Horita, Masayoshi Fuji

QFig. 2. (a) UV-Vis profiles, and (b) band gap calculation of

TiO231).

Fig. 1. (a) XRD profiles for raw powder and HPT processed

samples at 1 and 6 GPa for 4 turns, (b) XRD profiles after HPT
under 6 GPa for different turns, and (c) Rietveld analysis
results23).

Fig. 2a shows the UV-Vis spectra of samples before
HPT, after HPT and after annealing at 500 °C33). While
the absorption edge of starting powders is 400 nm, it
shifts to the visible light region, 470 nm, after HPT. A
blue shift of absorption edge after annealing is due toslight phase transformation of TiO2-II to anatase.
The Band gaps of all samples are calculated by the
Kubelka-Munk equation as shown in Fig. 2b, being 3.1,
2.4, and 2.7 eV for samples before HPT, after HPT andafter annealing, respectively. Comparing the band gapsof starting powder and that of the annealed sample
shows that the formation of columbite TiO 2 -II
effectively narrows down the band gap of TiO2.

Finally, the rate of photocatalytic hydrogen generation
by water splitting was examined. Photocatalytichydrogen generation against reaction time under UV andvisible lights are shown in Figures 3a and 3b,
respectively. It is obvious that hydrogen production rate
under is slower for the HPT sample under UV comparing
with that for the starting sample. However, the water
split rate is faster for the HPT sample under visible light.
Water split rate both under UV and visible lights
significantly enhances after annealing. It can beconcluded that HPT process successfully stabilized thehigh-pressure TiO2-II phase, and as a consequence band
gap was reduced. This led to an efficient water split
reaction under visible light.
Another feature that should be considered is grain size
effect. Authors by examining 80 grains by highresolution TEM noticed that nanograins with sizes lessthan 15 nm had a TiO2-II structure, while larger grains
had an anatase structure.

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Effect of High Pressure and Plastic Strain on Optical Properties of TiO2, ZnO, and Y2O3

of ZnO at the ambient pressure, while the rocksalt phasecan only exist at pressures higher than 6-10 GPa.
Recently, authors have successfully stabilized rocksalt
phase with a large fraction of oxygen vacancies by HPT
method, as schematically shown in Fig. 4b. The
photocatalytic activity of the rocksalt phase is also
examined for the first time39).

J

Fig. 3. Photocatalytic water splitting test of TiO2 under (a) UVlight and (b) visible light31).

3.2. Zinc Oxide (ZnO)

Zinc oxide (ZnO) has a wurtzite crystal structure atambient pressure and temperature with the space groupof P63mc. Wurtzite ZnO is known as a stable, non-toxic
and less expensive semiconductor with excellent
photocatalytic features 34), finding its application as
antibacterials, biochemical sensors, and photovoltaic
cells. However, like TiO2, a large band gap of 3.4 eVlimits the photocatalytic application of pure ZnO to the
UV range of sunlight. Although doping ZnO withcations or anions can reduce the band gap of ZnO, there
are also some limitations in this regard 35). In ZnO,
dopants tend to precipitate and change the chemicalcomposition of the host. Another limitation is thesolubility of dopants in ZnO, which are usually very
low. Since high temperatures are usually required forenhancing the crystallinity of ZnO, undesired expulsionof dopant can occur easily. Also, under rigorous pHchanges dopants usually leach from the host ZnO.
Wurtzite ZnO can transform to high-pressure rocksalt
phase under high pressures as shown in Fig. 4a 36).
Theoretical studies on ZnO37, 38) have proved that theband gap of the rocksalt phase is in the range of 1.2-2.6
eV. However, the wurtzite phase is the only stable phaseFig. 4. (a) Phase diagram of ZnO, (b) HPT device, (c) XRD

profiles after HPT37).

The crystal structure of wurtzite starting powder and
HPT processed samples under 3 and 6 GPa are examined
by XRD, as shown in Fig. 4(c). In addition to wurtzite
peaks in the powder and in the samples after HPT, adistinct peak broadening after HPT processing was
confirmed due to the lattice strain and grain size
reduction with an average grain size of 11 nm. Also,
rocksalt phase with a fraction of 50 wt.% could be
confirmed for the HPT sample under 6 GPa. The
rocksalt phase formation, suggested that simultaneous
introduction of pressure and strain is vital forstabilization of the rocksalt phase under ambientpressure. In this study, a correlation between grain sizeand crystal structure could not be confirmed for ZnO
due to the unstable feature of the rocksalt phase underelectron beam irradiation. In addition, the color ofsamples changed from white to dark yellow after HPT
processing due to the formation of oxygen vacancies.
Fig. 5a shows the UV-Vis profile of the starting ZnO
powder, HPT sample under 3 GPa, and 6 GPa. ZnO
powder showed an absorption edge at 400 nm, while it― 34 ―

Hadi Razavi-Khosroshahi, Kaveh Edalati, Zenji Horita, Masayoshi Fuji

measuring the degradation rate of Rhodamine B (RhB)
under visible light and results are shown in Fig. 5c. On
one hand, wurtzite powder shows little RhB degradation,and on the other hand, the sample processed at 3 GPa
exhibits higher photocatalytic activity due to theformation of oxygen vacancies. The HPT sample at 6
GPa shows even better activity due to the rocksalt phaseformation with a band gap of 1.8 eV. It is clear fromthese results that HPT could successfully stabilize therocksalt phase of ZnO with a band gap as narrow as 1.8
eV, which was able to absorb the visible light
extensively.
3.3. Yttrium Oxide (Y2O3)

Fig. 5. (a) UV-Vis profiles of ZnO before and after HPT,(b) band gap calculation, (c) photocatalytic activity undervisible light37).

redshifted to the visible light area with absorption edgeof 460 and 650 nm for the samples processed at 3 and 6
GPa, respectively. Band gaps were calculated from the
UV-Vis as shown in Fig. 5b, being 3.1, 2.8, and 1.8 eV
for the starting powder and for the samples processed at
3 and 6 GPa. Band gap narrowing of sample processedat 3 GPa is due to the introduction of oxygen vacancies.
Oxygen vacancies are known to create energy states
above the valance band, leading to band gap reductionof ZnO. However, a large band gap reduction of sample
at 6 GPa is anomalous and cannot be explained only by
the presence of oxygen vacancies. This band gap
reduction is attributed to the formation of rocksalt phase.
The photocatalytic activity of samples was examined by

In this part, high-pressure phase stabilization and band
gap reducing of Y2O3, as well as its optical properties,
will be introduced. Y2O3 is an insulator ceramic with thehigh melting point, superior mechanical properties, andgood chemical stability40). Y2O3 doped with rare earth
elements is widely used as phosphor materials, lasers,
bioimaging, fluorescent lamps, and field emission
displays thanks to its optical properties.
According to the pressure-temperature phase diagram,
Y 2O 3 has a cubic structure at ambient pressure andtransforms into the monoclinic and hexagonal phases aspressure increases41). However, since these high-pressure
phases are unstable at ambient pressure upon
decompression, the HPT method may stabilize thesephases, inspired by previous works on TiO2 and ZnO. It
is reported that monoclinic Y 2 O 3 , show better PL
intensities than that of the cubic phase, therefore its
stabilization at ambient pressure is extremely
meaningful.
XRD profiles of samples processed by HPT at 6 GPa
are shown in Fig. 6(a)42). Again, a peak broadening due
to lattice strain and grain size reduction is confirmed
during the process. Peak broadening was not distinct bypure compression (N = 0), while became more distinct
by rotating the anvils, revealing that the plastic strain
could be successfully introduced to the hard and brittle
Y2O3. Also, mere compression only leads to cubic phase,
while the monoclinic phase appears after HPT
processing. In addition, the fraction of monoclinic phaseincreased with increasing the plastic strain and reached90%. Phase fraction results estimated by Rietveld
analysis are shown in Fig. 6(b). The color of the samples
also changed from white to dark pink by HPT processing
not due to the monoclinic phase formation, but due tothe particle size reduction of Y2O3 nanometer level.

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Effect of High Pressure and Plastic Strain on Optical Properties of TiO2, ZnO, and Y2O3

better crystallinity and larger particles show higher PL
intensity, while nanomaterials show poor PL features
because quench sites for photo-excited electrons areabundant on the free surfaces and grain boundaries.

Fig. 7 . PL spectra profiles for Y 2O 3 before and after HPT

process40).

Finally, the effect of grain size on the phasetransformation was studied by examining of 150 grainsusing TEM. Results showed that grains smaller than 22
nm had a monoclinic structure, while larger grains had a
cubic structure, indicating the important role of grain
size on the phase formation and stability.
Fig. 6. (a) XRD results for Y2O3 before and after HPT, (b)Rietveld analysis results40).

4. Conclusions

Fig. 7 shows the photoluminescence (PL) profile ofstarting powder and samples processed by HPT under 6
GPa with different turn numbers. In Y 2O 3 with Eu
impurities, four PL transitions are allowed: 5D0→7FJ (J = 1,
2, 3, 4)43). The 5D0→7F1 transition is centrosymmetric,
while three other transitions are non-centrosymmetric. It
is shown that as the turn numbers of HPT increase, theintensity of 5D0→7F2 decreases, and the 5D0→7F3 intensity
increases. Since the 5D0→7F2 transition has the characterof electric dipole, and the 5D0→7F3 transition is mainly
magnetic dipole, it can be concluded that the PL
characteristic changes from the electric dipole tomagnetic dipole by the formation of monoclinic phase.
In addition, 4f electrons are known to be shielded byouter electrons like 5s 2 and 5p 6, they are extremely
sensitive to the surrounding lattice structure44). To thisreason, the cubic-to-monoclinic transition changes theY2O3 electric field strength and a new peak at 629 nm
forms. It is important to note that, although the grain
size and crystallinity of Y2O3 are significantly reducedby HPT, PL intensity is almost the same. Materials withIn this review, the effect of plastic strain on the phasetransformation and defect engineering of well-known
m e t a l o x i d e s l i k e Ti O 2 , Z n O , a n d Y 2 O 3 w e r esummarized. Severe plastic deformation was applied to
all materials by a high-pressure torsion (HPT) method,and high-pressure phases were stabilized at ambientpressure. Furthermore, the effect of phase transformation
on photocatalytic and optical properties were
investigated.
I) Anatase TiO 2 transformed to columbite TiO 2-IIphase, and the band gap reduced from 3.1 eV to 2.4
eV. Photocatalytic water split test under visible lightshowed that the high-pressure phase stabilized by
HPT was able to split water successfully, whileanatase did not show any activity.
II) Wurtzite ZnO transformed to rocksalt ZnO by HPT
and remained stable at ambient pressure. The band
gap reduced from 3.1 eV to 1.8 eV andphotocatalytic activity under visible light improved
extensively.
III) Cubic Y 2O 3 transformed to monoclinic Y 2O 3 by
HPT and remained stable at room pressure.

― 36 ―

Hadi Razavi-Khosroshahi, Kaveh Edalati, Zenji Horita, Masayoshi Fuji

Comparing PL profile of samples before and after
HPT showed that a new peak at 629 nm formed
after HPT processing, indicating the change of PL
characteristic from electric dipole to magnetic
dipole.
IV) While a critical grain size for phase transformation
was confirmed for TiO2 and Y2O3, this correlation
was not confirmed for ZnO.

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Acknowledgements

This study was supported in part by the Japan Scienceand Technology Adaptable and Seamless Technology
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