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Cite this: RSC Adv., 2020, 10, 14630

Porous niobia spheres with large surface area:
alcothermal synthesis and controlling of theircomposition and phase transition behaviour†
Yoshitaka Kumabe,a Hitomi Taga,a Kai Kan,
and Kazuya Kobiro *abcabc

Masataka Ohtani*abc

Submicron-sized niobia (Nb2O5) porous spheres with a high speciﬁc surface area (300 m2 g1) and nanoconcave–convex surfaces were synthesized via a rapid one-pot single-step alcothermal reaction. Prolonged

Received 22nd February 2020
Accepted 3rd April 2020

reaction time or high reaction temperatures resulted in a morphology change of Nb2O5 from amorphous
sphere to rod crystals with hexagonal crystal phase. A similar alcothermal reaction yielded TiO2–Nb2O5
composite porous spheres, whose Ti : Nb molar ratio was controlled by changing the precursor solution
component ratios. A simple thermal treatment of amorphous TiO2–Nb2O5 porous spheres consisting of 1 : 2

DOI: 10.1039/d0ra01704c(molar ratio) Ti : Nb at 600  C for 2 h induced crystal phase transfer from amorphous to a monoclinic crystal

rsc.li/rsc-advances

phase of submicron-sized TiNb2O7 porous spheres with a speciﬁc surface area of 50 m2 g1.

Introduction
Porous materials, including activated carbon, SiO2, metaloxides, zeolites, and metal–organic frameworks (MOFs), arepromising functional materials because of their large surface
area that can be used to adsorb several materials and achievematerial storage-release for material transfer. These uniqueproperties allow for applications such as gas storage and separation, catalysis, and catalyst supports.1 In particular, metaloxides are intriguing because of their intrinsic hightemperature stability and durability.2
To obtain porous materials, several methods, including
solution combustion,3 dealloying,4 electrochemical deposition,5spray pyrolysis,6 sol–gel method,7 as well as hydrothermal andsolvothermal methods,8 have been developed. In particular, thesolvothermal method using high-temperature non-aqueous
solvents as reaction media is a novel approach to prepare size, morphology-, and stoichiometry-controlled metal oxides. In
this context, a unique one-pot and single-step solvothermal
method was developed to aﬀord SiO2, TiO2, ZnO, ZrO2, andCeO2 with porous spherical morphologies, where numerous few
nm-sized primary particles aggregate to yield a sphericalsecondary morphology with a few hundred nm in diameter.9aSchool of Environmental Science and Engineering, Kochi University of Technology,
185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan. E-mail: ohtani.
masataka@kochi-tech.ac.jp; kobiro.kazuya@kochi-tech.ac.jpbLaboratory for Structural Nanochemistry, Kochi University of Technology, 185
Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japanc
Research Center for Material Science and Engineering, Kochi University of Technology,
185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan† Electronic supplementary
10.1039/d0ra01704c

information

14630 | RSC Adv., 2020, 10, 14630–14636(ESI)

available.

See

DOI:

These porous spheres were named meso/macroporously architected roundly integrated metal oxides (MARIMOs). The large
surface areas of MARIMOs with micro- and mesopores andnano concave–convex surface structure impart their uniqueabilities. Both solid and hollow MARIMOs can be obtained with
TiO2. In addition, composite MARIMOs, including CeO2–ZrO2,10
Al2O3–TiO2,11 ZnO–TiO2,11 SiO2–TiO2,12 Co/Mn oxide,13 Ni/Mnoxide,13 and Co/Mn/Fe oxide13 have been prepared via the solvothermal method with tightly controllable stoichiometry.
Thus, solvothermal method is a versatile approach for obtaining metal oxides with a porous spherical structure, large surface
area, and nano concave–convex surface structure.
Nb2O5 has been used as a photocatalyst,14,15 solid acid,16
catalyst support,17,18 and anode material for lithium and sodium
ion batteries.19 Several preparation methods are available forthese materials.20 For example, Li et al. prepared monodisperse
glycolated niobium oxide spheres via a simple antisolvent
precipitation approach. Mesoporous niobium oxide sphereswith a high specic surface area of 312 m2 g1 were obtained byhydrothermal treatment.21 Li et al. synthesized Nb2O5 hollow
nanospheres by heating a water solution of niobium oxalate andsodium dihydrogen phosphate dihydrate at 220  C for 48 h.22
Dai et al. prepared monodispersed Nb2O5 microspheres via
a facile one-pot solvothermal route using an isopropyl alcohol
solution of niobium pentachloride and tetrabutyl ammoniumhydroxide at 200  C for 24 h.23 However, simpler preparation
methods should be developed for further acceleration of theirpractical uses. Thus, niobium oxide and variants of MARIMOs
with large surface area were prepared herein using a facile
synthesis method for Nb2O5, TiO2–Nb2O5 composite, andTiNb2O7 MARIMOs.

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Results & discussion
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Synthesis and morphology changes of Nb2O5 MARIMO
Nb2O5 MARIMO was prepared according to a similar method
reported previously (entry 1 in Table 1).9 Simple heating ofa methanol solution of Nb(OEt)5 and formic acid at 300  C for
10 min aﬀorded Nb2O5 MARIMO in high yield (>90%). The
scanning electron microscopy (SEM) images revealed that theobtained Nb2O5 MARIMO was perfectly monodispersed(Fig. 1a(i)), and their shape was approximately spherical witha diameter of 630  50 nm (Fig. 1a(ii), 1b(i) and S1,† red line).
Also, the obtained Nb2O5 MARIMO was composed of manyamounts of a few nm sized primary particles (Fig. 1b(ii)). The
formation mechanism of Nb2O5 MARIMO was expected to be
similar to a proposed mechanism in our previous report.9 In theinitial steps of the reaction, primitive niobium alkoxide oligomers are formed in high-temperature methanol and interconnected via surface interactions with carboxyl groups. As thefurther heating reaction of precursor mixture proceeds,
niobium alkoxide oligomers react with the water, which
generated by esterication of formic acid and methanol and/or
dehydration condensation of methanol, to form Nb2O5. The
pore structures are formed by gaseous products, such as CO2
and hydrocarbons generated by decomposition of alkoxide andformic acid, trapped inside the sphere. The Nb2O5 MARIMO
obtained without formic acid was polydisperse and obtained in
low yield (<40%; Fig. S2a†). In addition, when the solvent waschanged to water, spherical shapes were not formed (Fig. S2b†).
From the results of the control experiment, the combination of
Nb(OEt)5, formic acid, and methanol is essential for theformation of a spherical morphology and high yield.
The direct evidence of the formation of a pore in Nb2O5 spheres
was conrmed by nitrogen adsorption–desorption measurements(Fig. S3a(i)†). If the obtained Nb2O5 MARIMO has no pores, thespecic surface area should be a few m2 g1 estimated from thediameter (ca. 630 nm) and specic density (d ¼ 4.6 g cm3).
Contrary to this expectation, the obtained Nb2O5 MARIMO showeda high specic surface area (302 m2 g1) and porous structure lessthan 10 nm-sized (Fig. S3a(ii)†). This fact strongly supports thepresence of the penetrated pore inside the spherical particles.
To further characterization of the obtained Nb2O5 MARIMO,
powder X-ray diﬀraction (XRD) and electron microscopic

Table 1

Fig. 1 (a) SEM and (b) TEM images of the prepared Nb2O5 MARIMO at(i) low magniﬁcation and (ii) high magniﬁcation.

measurements were performed. No peaks were observed in thecorresponding XRD pattern (Fig. 2c, red line), and no fringe
structure in the Nb2O5 MARIMO primary particle was observedin the high-resolution transmission electron microscopy (HRTEM) image (Fig. 1b(ii)). This indicated that the obtained
Nb2O5 MARIMO was not crystalline but amorphous.
To date, almost 15 polymorphic forms of Nb2O5 have beenreported; however, hexagonal, orthorhombic, and monoclinic
phases are relatively common.20 The crystal phases signicantly
aﬀect the activity of the photocatalyst24 and solid acid.25 Thus, it
is important to control of the crystal phase formation in theobtained Nb2O5 MARIMO. The easiest method to improve thecrystallinity is further heating. When the heating time of the
precursor solution was prolonged to 60 min, the crystal phasedramatically changed from amorphous to the hexagonal phaseof Nb2O5 (Fig. 2c). This was concomitant with a drastic
morphology change from spherical to rod-shaped (Fig. 2a(ii)).
The corresponding HR-TEM image (Fig. 2b) showed that the
crystal lattice distances were 0.39 nm. In addition, the fast
Fourier transform (FFT) pattern in the HR-TEM image indicatedMolar ratios of Ti and Nb, speciﬁc surface area, pore size, and TiO2 crystalline size of the prepared MARIMOs
Molar ratio of Ti and Nb

Entry Sample name of MARIMOa Precursor solution Productb
1
2
3
4
5

Nb2O5
TiO2–Nb2O5-75
TiO2–Nb2O5-50
TiO2–Nb2O5-25
TiO2

0 : 100
25 : 75
50 : 50
75 : 25
100 : 0—21.7 : 78.3
50.1 : 49.9
74.3 : 25.7—

Specic surface areac [m2 g1] Pore sized [nm] TiO2 crystalline sizee [nm]
302
274
200
250
234

3.3
5.9
8.9
3.4
3.6

—
34.7
13.6
10.0
9.6

a
Denotes the atomic% of Nb to total number of Nb and Ti atoms. b Evaluated by STEM/EDX analysis. c BET method. d BJH plot. e Estimated using
the Scherrer equation.

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Fig. 2 (a) SEM images of the Nb2O5 obtained at 300  C with diﬀerent
solvothermal-reaction times (i) 30 min and (ii) 60 min. (b) HR-TEM
image of the rod-like Nb2O5; corresponding FFT image is inset. (c) XRD
patterns of the powdery products obtained using diﬀerent solvothermal-reaction times (blue, 60 min; green, 30 min; red, 10 min).
Reference peaks of hexagonal Nb2O5 (JCPDS 00-007-0061) arerepresented by the black line.

that the direction of rod growth was along the [001] facet. Whena 30 min heating procedure was used, spheres and rods coexisted. Compared to 10 min heating, a volume reduction of
approximately 40% was observed for the 30 min heatingprocess, where the diameter of the obtained Nb2O5 MARIMO
decreased to 530  50 nm (Fig. 2a(i) and S1,† blue line). Ostwald
ripening was likely responsible for this shrinkage.26,27 Similarly,
higher reaction temperatures (325 and 350  C) with a xed
reaction time (10 min) resulted in rod-like hexagonal phasenanocrystals (Fig. S4†). Although the crystallinity was successfully improved by extending the reaction time or using a highreaction temperature, the porosity was lost due to themorphology change, and the specic surface area was greatly
reduced.
Calcination of the as-synthesized amorphous Nb2O5 MARIMO was used as an alternative method to change the crystal
phase. Generally, amorphous Nb2O5 crystallizes at 500  C into
hexagonal or orthorhombic phase, tetragonal phase at 800  C,
and monoclinic phase at >1000  C.28 An exothermic phenomenon was observed at approximately 240 and 560  C in thethermogravimetric-diﬀerential thermal analysis (TG-DTA,
Fig. 3a). The exothermic peak at approximately 240  C originated from the combustion of organic residues included in
MARIMO. In addition, the exothermic peak with negligible
weight loss at approximately 560  C was assigned to a crystalline
phase transfer.29 Calcination of the as-synthesized amorphous
Nb2O5 MARIMO yielded a hexagonal phase at 500  C and an
orthorhombic phase at >600  C, as determined from their XRD
patterns (Fig. 3b). The spherical morphology was maintained at#600  C, but collapsed at 700  C (Fig. 3c(i) and S5†). Thus,
Nb2O5 MARIMO with improved crystallinity was successfully
obtained via calcination at 500 or 600  C for 2 h (Fig. 3b, c andS5a†). The specic surface area of Nb2O5 MARIMO calcined at

14632 | RSC Adv., 2020, 10, 14630–14636

Fig. 3 (a) TG-DTA proﬁles of the as-synthesized Nb2O5 MARIMO
measured in air at a heating rate of 2  C min1. (b) XRD patterns ofcalcined Nb2O5 MARIMO at 400–700  C (blue, 700  C; green, 600  C;
red, 500  C; orange, 400  C). Reference peaks of orthorhombic Nb2O5(JCPDS 01-071-0336) and hexagonal Nb2O5 (JCPDS 00-007-0061)
are represented by black lines. (c) SEM (i) and HR-TEM (ii) images of the
Nb2O5 MARIMO calcined at 500  C for 2 h. The crystal lattice distanceswere 0.31 nm.

500  C was 88 m2 g1 and average pore size expanded from 3.3
to 6.4 nm upon calcination (Fig. S3b†).
Synthesis of TiO2–Nb2O5 composite MARIMO
Ti–Nb binary oxides have attracted signicant attention since
their discovery as they exhibit reduced electrical resistance,30
high photocatalytic activity under visible light irradiation,31 andcontrolled Brønsted/Lewis acidity.32 In particular, using Ti–Nb
binary oxides in lithium and sodium ion batteries19,33,34 has
attracted great attention because of its improved electronic
conductivity,35 ion diﬀusion path expansion,35 and high theoretical capacity.36 As mentioned in the introduction, composite
MARIMOs consisting of plural metal oxides have beensuccessfully synthesized.10–13 Herein, a one-pot and single-step
technique was used to obtain TiO2–Nb2O5 composite
MARIMOs.
A methanol solution of Ti(OiPr)4 and Nb(OEt)5 with formic
acid was heated at 300  C for 10 min. The molar ratio of Ti to Nb

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in the product MARIMO was easily controlled by changing theratio in the corresponding precursor solution (entries 2–4 inTable 1). Nearly equivalent Ti/Nb ratios between the precursor
solutions and products were obtained. As shown in Fig. 4a(i–iii),
Ti and Nb atoms were equally distributed at the nano-level in
the TiO2–Nb2O5-75, TiO2–Nb2O5-50, and TiO2–Nb2O5-25
samples, where the numbers denote the atomic percentage of
Nb among the total number of Nb and Ti atoms. In addition, theTiO2–Nb2O5 composite MARIMOs were highly porous withspecic surface areas of >200 m2 g1 (Table 1 and Fig. S6†).
Only the tetragonal (anatase-type) TiO2 peaks were observedin the XRD proles when Nb atoms were included in the
product MARIMOs (Fig. 4b(i)). Lower angle shis of thediﬀraction peaks were also observed in the XRD proles(Fig. 4b(ii)), indicating that the composite MARIMOs consistedof solid solutions and the crystal lattice was enlarged by the
doped Nb atoms with larger ionic radii (Nb5+ ¼ 0.64 A, Ti4+ ¼
0.61 
A).37 Additionally, precursor solutions containing larger
amounts of Nb(OEt)5 resulted in the larger primary particle
sizes of TiO2 in the resulting MARIMOs (Table 1). However, thiscrystalline growth of TiO2 was not observed for the Al2O3–TiO2,

Fig. 4 (a) STEM/EDX analysis of the prepared TiO2–Nb2O5 composite
MARIMOs; (i) TiO2–Nb2O5-75, (ii) TiO2–Nb2O5-50, (iii) TiO2–Nb2O525, and (iv) TiO2. (b) XRD patterns of the obtained powdery products
prepared with diﬀerent molar ratios of Ti/Nb (blue, TiO2–Nb2O5-75;
green, TiO2–Nb2O5-50; red, TiO2–Nb2O5-25; orange, TiO2). Reference peaks corresponding to tetragonal TiO2 (JCPDS 00-021-1272)
are represented by the black line. The 2q range: (i) 20–70 , (ii) 22–28 .

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ZnO–TiO2, and SiO2–TiO2 composites.11,12 Thus, the observed
crystalline growth of TiO2 can be ascribed to the catalytic eﬀectof solid Nb2O5 as a solid acid that promoted crystalline growth.
Interestingly, hollow MARIMO were formed at a 50 : 50 Ti/Nb
molar ratio (Fig. 4a(ii) and S7†). However, solid TiO2–Nb2O5-50
MARIMO was obtained (Fig. S8†) when the reaction was
immediately quenched at 0 min (the reaction temperature was300  C). From the XRD patterns, the longer reaction time yielded better crystallinity (Fig. S9†). Ostwald ripening was likely
the cause of the hollow MARIMO formation.38 These uniquehollow structures imparted large surface area, low density, andhigh loading capacity. Therefore, the prepared materials oﬀer
promising potential for use in advanced micro-/nanoreactors,
catalysis, energy storage/conversion, biomedicine, sensors,and environmental remediation applications.39–41

Formation of TiNb2O7 MARIMO by crystallization of
amorphous TiO2–Nb2O5 composite MARIMO
TiNb2O7 as an anode material for lithium ion batteries has
a high theoretical capacity of 387.6 mA h g1 derived from its 5electron transfer mechanism (Ti4+/Ti3+, Nb5+/Nb4+, and Nb4+/
Nb3+). This value is approximately two times higher than that of
the commonly used Li4Ti5O12 (175 mA h g1).36 Thus, TiNb2O7
has attracted signicant attention is promising for applicationin electric vehicles.19,33,34 However, TiNb2O7 suﬀers from low
ionic and electric conductivity. Tailoring special nanostructuresof TiNb2O7 can be used to harness the advantages of nanomaterials, including high electrode/electrolyte contact area andshort ion diﬀusion distances. In particular, microscale spherical materials with hierarchical nanoscale structure are promising for solving the drawbacks of pure nanoscale porous
materials with low tap densities and poor coulombic eﬃciency.
Although, the syntheses of TiNb2O7 porous spheres have beenpreviously reported, those with larger surface areas remain
elusive (Table S1†). Thus, to obtain TiNb2O7 porous sphereswith large surface area, the synthetic conditions of the
precursor composite TiO2–Nb2O5 as well as the calcination
conditions were closely examined.
First, a similar alcothermal treatment of a 1 : 2 (mol ratio)
Ti(OiPr)4 and Nb(OEt)5 mixed methanol solution including
formic acid at 300  C aﬀorded a precursor composite MARIMO
with tetragonal (anatase-type) TiO2 and amorphous Nb2O5 asdetermined from its XRD pattern (Fig. S10,† blue line). The
corresponding SEM images revealed characteristically large
primary particles on the MARIMO surface (Fig. S11a(i)†). The
second step involved calcination of the precursor MARIMO at
700  C for 2 h and yielded monoclinic TiNb2O7 MARIMO(Fig. S11a(ii) and S12a†, blue line). However, the XRD patterns
showed that the calcined MARIMO contained mainly monoclinic TiNb2O7 with a small amount of tetragonal (anatase-type)TiO2 and orthorhombic Nb2O5 (Fig. S12b†, blue line). This
indicates that a pure TiNb2O7 MARIMO was not aﬀorded by thecalcination of the as-synthesized TiO2–Nb2O5 MARIMO obtained at 300  C.
Therefore, amorphous TiO2–Nb2O5 composite MARIMO wasselected as an alternative precursor (Fig. S10, red line and

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S11b(i)†). The material was prepared using a lower temperature(200  C) alcothermal reaction. As shown in Fig. 5a, calcinationof the amorphous TiO2–Nb2O5 composite MARIMO at >600  C
for 2 h produced the desired TiNb2O7 MARIMO with a pure
monoclinic phase. In addition, tetragonal (anatase-type) TiO2
and orthorhombic Nb2O5 peaks were not observed even in caseof the obtained sample at 700  C (Fig. S12b†, red line). Fringe
structures were observed in the HR-TEM image (Fig. 5b(i)) of the
primary particles, clearly showing that the MARIMO wascomposed of single crystals of TiNb2O7. The TiNb2O7 porous
morphology was clearly demonstrated by high angle annulardark-eld scanning transmission electron microscopy (HAADFSTEM; Fig. 5b(ii)). The specic surface area of the obtained
monoclinic TiNb2O7 MARIMO was smaller (50 m2 g1) than that
before calcination (Fig. S13†, 497 m2 g1), but 50 m2 g1 is thehighest value reported for this type of material to the best of ourknowledge (Fig. 5c(i) and Table S1†). As expected, the averagepore size was enlarged from 2.5 to 8.8 nm (Fig. 5c(ii)), which isfavourable for guest materials to access deep into the porous

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spheres. In addition, the pore space between the TiNb2O7
MARIMO can buﬀer volume changes derived from the phasetransitions occurring during lithiation/delithiation, which may
result in good cycling performance via enhanced structural
stability.42 Thus, TiNb2O7 porous spheres were successfully
prepared with a monoclinic crystal phase and wide pores by
calcination of the amorphous TiO2–Nb2O5 composite MARIMO
precursor.

Conclusions
Monodispersed Nb2O5 porous spheres (Nb2O5 MARIMO) with
630 nm in diameter were prepared via the simple alcothermal
reaction of a methanol solution of Nb(OEt)5 with formic acid.
The crystallinity and pore size of the prepared Nb2O5 MARIMO
were improved by prolonging the reaction time or simple
calcination of the product. TiO2–Nb2O5 composite MARIMO
was obtained using a similar alcothermal reaction of Ti(OiPr)4,Nb(OEt)5, and formic acid in methanol, with a product Ti : Nbatomic ratio that was controlled by changing molar ratio ofTi(OiPr)4 and Nb(OEt)5 in the precursor solution. A two-step
synthetic approach for the generation of TiNb2O7 porous
spheres was developed via an alcothermal treatment of a 1 : 2(molar ratio) mixed solution of Ti(OiPr)4 and Nb(OEt)5 inmethanol followed by simple calcination. Considering the factthat the obtained submicron-sized porous spheres of niobia
and their composites have a large surface area, high performance can be expected in the eld of catalysts and batteries. To
evaluate catalyst and battery performance, the research fora practical application is undergoing.

Experimental section
Materials
Methanol, formic acid, niobium(V) ethoxide [Nb(OEt)5], andtitanium tetraisopropoxide [Ti(OiPr)4] were obtained from
FUJIFILM Wako Pure Chemical Corporation. All reagents wereused as received unless otherwise noted.

Synthetic procedure(a) XRD patterns of the calcined amorphous TiO2–Nb2O5
composite MARIMO at 500–700  C (blue, 700  C; green, 600  C; red,
500  C). Reference peaks of monoclinic TiNb2O7 (JCPDS 00-0391407) are indicated by the black line. (b) HR-TEM (i) and HAADF-STEM(ii) images of the amorphous TiO2–Nb2O5 composite MARIMO
calcined at 600  C. (c) Nitrogen adsorption–desorption isotherms (i)and pore size distributions (ii) of the amorphous TiO2–Nb2O5
composite MARIMO calcined at 600  C.
Fig. 514634 | RSC Adv., 2020, 10, 14630–14636

Synthesis of Nb2O5 MARIMO. First, Nb(OEt)5 (0.35 mmol)
was added to a solution of formic acid (1.75 mmol) in methanol(3.5 mL) under vigorous stirring. The resultant precursor solution was transferred to an SUS-316 stainless steel tubular
reactor with a 10 mL inner volume and the reactor was thensealed with an SUS-316 screw cap. The sealed reactor was heatedto 300  C at a rate of 5.4  C min1 and the temperature wasmaintained at 300  C for 10 min. The reaction was quenched byplacing the reactor in an ice-water bath. The obtained precipitates were centrifuged, washed several times with methanol,and dried under vacuum to aﬀord a powdery product.
Synthesis of TiO2–Nb2O5 composite MARIMOs. Aliquots ofTi(OiPr)4 and Nb(OEt)5 were added to a solution of formic acid
(1.75 mmol) in methanol (3.5 mL) under vigorous stirring.
Similar treatments of the solution as the above mentioned

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Nb2O5 MARIMO synthesis yielded the TiO2–Nb2O5 composite
MARIMO powdery products.
Formation of TiNb2O7 MARIMO via crystallization of theamorphous TiO2–Nb2O5 composite MARIMO. First, Ti(OiPr)4(0.58 mmol) and Nb(OEt)5 (1.17 mmol) was added to a solutionof formic acid (8.75 mmol) in methanol (17.5 mL) undervigorous stirring. The resultant precursor solution was sealed ina Teon-lined autoclave with a 50 mL inner volume. The sealed
autoclave was heated to 200  C at a rate of 5.4  C min1 and thetemperature was maintained at 200  C for 3 h. Aer cooling atroom temperature, the precipitates were centrifuged, washedseveral times with methanol, and dried under vacuum to aﬀord
a powdery amorphous TiO2–Nb2O5 composite MARIMO. The
obtained product was calcined in air at 600  C for 2 h at
2  C min1 heating rate to yield the nal product.

Characterization methods
Transmission electron microscopy. The transmission electron microscopy (TEM) images were obtained using a JEOL JEM2100F microscope. The specimens were prepared as follows:
samples were dispersed in methanol; 10 mL of the resultant
dispersion was drop-casted onto a carbon-coated copper micro
grid (Okenshoji, Japan); and the solvent was evaporated underatmospheric pressure. Energy-dispersive X-ray (EDX) mapping
and line-scan plots for the STEM images were obtained using an
Oxford INCA X-max 80 EDX spectrometer.
Scanning electron microscopy. Scanning electron microscopy (SEM) was performed using a Hitachi SU8020 FE-SEM
microscope. The specimens were prepared by drop-casting thesample dispersion on a silicon wafer.
X-ray diﬀraction measurements. X-ray diﬀraction (XRD)
patterns were recorded using a Rigaku SmartLab diﬀractometerwith graphite-monochromatized Cu Ka radiation (X-ray wavelength:1.5418 A) in steps of 0.02 over the 2q range of 10–80 . The powdery
samples were placed on a non-refractive silicon holder (Overseas Xray Service, Japan). The average TiO2 crystalline sizes were estimatedfrom the obtained (101) peaks using the Scherrer equation.
Thermogravimetric-diﬀerential
thermalanalysis.
Thermogravimetric-diﬀerential thermal analysis (TG-DTA) was
performed using a HITACHI STA7200RV instrument. The
samples were placed on an open platinum sample pan and theexperiments were conducted in air at a 2  C min1 heating rate.
Nitrogen adsorption–desorption isotherm measurements.
Nitrogen adsorption–desorption isotherms were obtained usinga Belsorp Mini II instrument (MicrotracBEL Corp.). The specicsurface areas were determined using the Brunauer–Emmett–Teller(BET) multipoint method with BELMaster7 soware. The averagepore size distributions were calculated using the Barrette–Joyner–
Halenda (BJH) method from the nitrogen adsorption isotherm data.

Conﬂicts of interest
There are no conicts to declare.

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Acknowledgements
We acknowledge the nancial support of the Creation of New
Business and Industry Program through the Kochi Prefectural
Industry-Academia-Government Collaboration Research Promotion Operation and funding from JSPS KAKENHI Grant Number
15K06560, 17K14858, and 19K05143. The authors also wish to
thank the Research Center for Nanotechnology in Kochi Universityof Technology for performing TEM and SEM analyses.

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