Effects of Annealing Temperature on the Crystal Structure, Morphology

nanomaterials

Article Eﬀects of Annealing Temperature on the Crystal Structure, Morphology, and Optical Properties of Peroxo-Titanate Nanotubes Prepared by Peroxo-Titanium Complex Ion

Hyunsu Park 1 , Tomoyo Goto 1 , Sunghun Cho 1 , Soo Wohn Lee 2, Masato Kakihana 1,3 and Tohru Sekino 1,*

1 Department of Advanced Hard Materials, The Institute of Scientiﬁc and Industrial Research (ISIR), Osaka University, Osaka 567-0047, Japan; [email protected] (H.P.); [email protected] (T.G.); [email protected] (S.C.); [email protected] (M.K.) 2 Department of Environmental and Bio-chemical Engineering, Sun Moon University, Asan 31460, Korea; [email protected] 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan * Correspondence: [email protected]; Tel.: +81-(0)-6-6879-8435

Received: 13 June 2020; Accepted: 6 July 2020; Published: 8 July 2020

Abstract: This study addresses the effects of annealing temperatures (up to 500 ◦C) on the crystal structure, morphology, and optical properties of peroxo groups (–O–O–) containing titanate nanotubes (PTNTs). PTNTs, which possess a unique tubular morphology of layered-compound-like hydrogen titanate structure (approximately 10 nm in diameter), were synthesized using peroxo-titanium (Ti–O–O) complex ions as a precursor under very mild conditions—temperature of 100 ◦C and alkali concentration of 1.5 M—in the precursor solution. The nanotubular structure was dismantled by annealing and a nanoplate-like structure within the range of 20–50 nm in width and 100–300 nm in length was formed at 500 ◦C via a nanosheet structure by decreasing the specific surface area. Hydrogen titanate-based structures of the as-synthesized PTNTs transformed directly into anatase-type TiO2 at a temperature above 360 ◦C due to dehydration and phase transition. The final product, anatase-based titania nanoplate, was partially hydrogen titanate crystal in nature, in which hydroxyl (–OH) bonds exist in their interlayers. Therefore, the use of Ti–O–O complex ions contributes to the improved thermal stability of hydrogen titanate nanotubes. These results show a simple and environmentally friendly method that is useful for the synthesis of functional nanomaterials for applications in various fields.

Keywords: titanate nanotubes; titanate nanosheets; titanate nanoplates; peroxo-titanium complex ion; annealing

1. Introduction One-dimensional nanostructures, such as nanowires [1], nanobelts [2], nanorods [3], and nanotubes [4] which have received great attention due to their intriguing nanostructure and excellent properties [5]. Titania and titanate materials are nanostructures and they have excellent properties. One-dimensional nanostructured titania and titanate materials have attracted considerable attention and have been widely investigated in various ﬁelds such as environmental puriﬁcation, photocatalysis, self-cleaning coatings, gas-sensor materials, electrode materials for dye-sensitized solar cells, as well as electron-transfer-layer (ETL) for perovskite photovoltaic cells [6–10]. This is due to their high redox potential, chemical stability, inexpensiveness, and non-toxicity [11]. The wide variety

Nanomaterials 2020, 10, 1331; doi:10.3390/nano10071331 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1331 2 of 17 of their functional properties can be improved by the control of the nanostructure of titania and titanate. Nanotube structures [12], in particular, have received a lot of attention due to their large specific surface areas as well as internal and external surfaces, and their high porosity structures [13], which increase the number of potential active sites for a given reaction [14] that improves the catalytic properties and electricity conversion effects [15]. There are many routes for synthesizing titania/titanate nanotubes (TNTs), which include anodizing [16], sol-gel template synthesis [17], hydrothermal [4], and solution chemical synthesis methods [18] that are based on alkali treatment. Among these methods, the chemical synthesis route is relatively simple and facile. Although TNTs with an average diameter of 10 nm and a high specific surface area could be synthesized using this method, this process requires a temperature of 115 ◦C and duration above 24 h, as well as a high alkaline concentration above 10 M NaOH of the synthesis solution. However, the NaOH is classified as a toxic chemical by the Toxic Use Reduction Act. When used in large quantities, it poses significant chemical hazards to the environment or our health. Many groups have tried to modify this process and analyze the structure of TNTs, and they have reported that TNTs are composed of a layered titanate structure. They proposed that synthetic mechanism should be the dissolution/recrystallization process [19]. In many cases, titanate compounds such as porous crystalline titanate [20] or titanium alkoxides were used as starting materials for the synthesis of nanostructured titanates. As mentioned earlier, the use of these raw materials requires severe synthesis conditions and high energy, which is due to the use of a large number of alkaline species for dissolving raw materials containing Ti. From the perspective of green chemistry, the use of titanium chelate complexes have also been reported [21–23]. This study introduces the novel bottom-up synthesis process for tubular structured titanate at a relatively low alkaline concentration of 1.5 M, using peroxo-titanium (Ti–O–O) complex ion as a precursor (named as PTNTs, peroxo-titanate nanotubes). In this way, this PTNTs can be also synthesized at a low temperature of approximately 100 ◦C using a novel bottom-up synthesis method, which is an environmentally-friendly process PTNTs possess a special peroxo-titanium bond (Ti–O–O) in the crystals as well as on the surface, which may enhance light absorption by reducing the optical band gap energy [24,25]. However, to apply this material to various engineering applications, it is very important to evaluate its thermal properties. For example, when applied as electrodes to dye-sensitized solar cells or chemical sensor devices, thermal treatment is required to make strong and stable oxide films/coatings, remove the binder, and for adhesion with substrates. Moreover, many reports demonstrated the TiOx conducting layer annealed at high temperature so the electrons can be transferred through the valence band of the TiOx [26]. However, the heating of nanostructured materials often affects the morphological change and the crystal structure due to sintering and chemical reactions [27,28]. In gas sensor applications as oxide semiconductors, the operating temperature for detecting the target gas is usually high and related to a variety of properties of the material such as the crystal structure, the particle size, and the energy band structure. Therefore, the evaluation of the effect of heating is essential for effectively setting the operating temperature and effective detection [29]. In the case of nanostructured titania/titanate synthesized at low temperature as described above, detailed understanding and clarifications on the relationship between the crystallographic properties induced by varying the operating temperature and the effects on functionality need to be considered. Various studies have reported the structural changes of TNTs effected by thermal treatment [30–34]. Zhang et al. [34] reported the formation of the anatase phase by heating at a temperature above 500 ◦C and the change in nanotubular morphology to a spherical shape. We reported that the synthesis of TNTs following Kasuga’s method preserved the nanotubular morphology and maintained its high surface area, but it was transformed to an anatase-type TiO2 at 450 ◦C, while a sudden decrease in the surface area and morphological change to a spherical shape was confirmed by further heating above 450 ◦C[33]. PTNTs contain peroxo groups (–O–O–) that modify the chemical bonding nature of Ti–O and the resultant crystalline structures. Therefore, PTNTs are expected to show different thermal stability, as well as crystallographic and morphological changes compared to the commonly Nanomaterials 2020, 10, 1331 3 of 17 known titania/titanate nanotubes because of its unique structure with the peroxo group. In addition, understanding the thermal properties of TNTs is an important factor for designing TNTs’ functions and applications. Etacheri et al. [11] reported the high-temperature stability of oxygen-rich titania synthesized by a peroxo-titania complex. However, there are few studies on the effects of heat treatment on the morphological and crystallographic characteristics, as well as physical/chemical properties of peroxo-titanium containing nanostructured titanate, especially nanotubular titanate (thus PTNTs). It is necessary to develop an effective method for the synthesis of titanate nanostructures with good visible light responsibility even after the heating at high temperatures. The purpose of this study is thus to clarify the relationships between the heat treatment of the crystalline structures and the morphology of the titanate nanotubes containing PTNTs that was prepared from peroxo-titanium complex ions at a temperature as low as 100 ◦C. In situ high-temperature X-ray diffraction was used to observe the crystallographic changes induced by the thermal treatment of PTNTs. The effect of annealing temperature on the morphological and optical characteristics of PTNTs is discussed using physicochemical investigations.

2. Materials and Methods

2.1. Synthesis of Materials As a precursor, peroxo-titanium complex ion [35] was synthesized by the following method: Initial 0.63 g of TiH2 (> 99%, Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan) was dissolved in 62.46 mL of mixed solution with a pH of 10 containing H2O2 (30%, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) and NaOH (97%, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) aqueous solution for 2 h at 10 ◦C. The pH of the mixed solution was controlled by gradually adding 10 M of concentrated NaOH solution to the hydrogen peroxide solution. Consequently, the Na concentration in the precursor solution was maintained at 1.5 mol/L (1.5 M) and the last step is to add alkaline (Na) to synthesize nanotubular titanate. It should be noted that the necessary amount of Na required for the synthesis of TNTs in this study was much less compared to the commonly known chemical synthesis process, which is typically 10 M [18]. To synthesize peroxo-titanate nanotubes, the prepared peroxo-titanium complex ion solution was put in a polytetrafluoroethylene bottle equipped with a reflux condenser and it was placed in an oil-bath. It was heated at 100 ◦C for 12 h with and it was stirred at string speed of 200 rpm. Afterward, the precipitates were treated using a 5 M HCl (30%, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) solution until the pH became 5, and it was washed using distilled water and a vacuum pump (MDA-020C, ULVAC, Inc., Kanagawa, Japan) until the ion conductivity of the filtered solution became less than 5 µS/cm. The product was dried using a freeze dryer (EYELA FDU-2200, TOKYO RIKAKIKAI CO, LTD., Tokyo, Japan) and was labeled as PTNTs. Afterward, to investigate the effect of heating on PTNTs, the samples were thermally treated at 200, 300, 400, and 500 ◦C using a furnace. All samples were heated under ambient conditions at 10 ◦C/min and kept at a given temperature for 1 h.

2.2. Characterization The thermal properties of the PTNTs were studied using thermogravimetric differential thermal analysis (TG-DTA, TG8120, RIGAKU, Tokyo, Japan). Approximately 10 mg of the sample was weighed on a Pt pan and the α-alumina powder was used as the reference material. Scans between 25 and 600 ◦C were carried out at a heating rate of 10 ◦C/min under ambient conditions. The temperature of the instrument was calibrated by using purity standard metallic materials (In, Pb and Au). The morphology and particle size were observed using field-emission scanning electron microscopy (FE-SEM, SU-9000, Hitachi High-Technologies Corporation, Tokyo, Japan) with 0.4 nm resolution at 30 kV acceleration voltage. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method based on the adsorption isotherm at a P/P0 range of 0.1–0.3 using an N2 adsorption-desorption instrument (NOVA 4200e, Quantachrome Instruments, Boynton Beach, FL, USA). The crystalline phase was Nanomaterials 2020, 10, 1331 4 of 17 identified using X-ray diffraction (XRD, D8 ADVANCE, Bruker AXS Co. Ltd., Karlsruhe, Germany), and the diffraction patterns of each sample were collected using a vertical goniometer equipped with a high-temperature reaction chamber (XRK-900, Anton Paar, Graz, Austria), operating in the Bragg configuration using Cu Kα radiation (λ = 1.54056 Å) from 5 to 85◦ at a scanning rate of 0.02◦. In-situ XRD studies were recorded during the annealing process, heating occurred from 25 to 560 ◦C at a rate of 10 ◦C/min. From the results of the XRD analysis, the lattice parameters of the samples were evaluated using Equation (1). 1 h2 k2 l2 = + + (1) d2 a2 b2 c2 where, d is the interplanar spacing and h, k, l are Miller indices. a, b, and c are lattice constant of crystal phase. The anatase crystallite size of samples was evaluated using Equation (2).

0.94λ D = (2) βcosθ where D is the crystalline size of materials, λ is the wavelength of X-ray, β is the broadening of the diffraction line measured at half of its maximum intensity in radians, and θ is the angle of diffraction. The shape factor of 0.94 was used by assuming a sphere shape in this study. Transmission electron microscopy (TEM, JEOL-2100, JEOL, Tokyo, Japan) was performed at an acceleration voltage of 200 kV to further characterize individual nanostructures of titanate or titania. The lattice spacing, fast Fourier transform (FFT), and the phase interpretation were investigated using the Gatan Digital Micrograph software [36] (Gatan Inc., Pleasanton, CA, USA). Fourier transform infrared spectrometer 1 (FT-IR, FT/IR4100, JASCO, Tokyo, Japan) spectra were obtained within the range of 4000–500 cm− 1 region at a resolution of 4 cm− . The samples were measured using the KBr pellet method in the transmission mode. Raman spectra were acquired with a Raman micro-spectrometer (HR 800, HORIBA, Kyoto, Japan) using an Ar ion laser (514.5 nm). The spectra were collected in the range of 1000–100 1 1 cm− with a resolution of 1 cm− . The reflectivity of the prepared powder was evaluated using the solid sample measurement mode in the ultraviolet-visible (UV-Vis) machine. Diffuse reflectance spectroscopy (DRS) (V-650, JASCO Co., Tokyo, Japan) and optical band gap energy were measured using the Tauc-plot method using Kubelka–Munk transformation. The powders were uniformly pressed in a powder holder and placed in the sample holder on an integrated sphere for the reflectance measurements. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of the Ti K-edge were measured in an ionization chamber in transmission mode on the Kyushu University Beamline (BL06) of the Kyushu Synchrotron Light Research Centre (SAGA-LS; Tosu, Japan). Pellet samples for Ti K-edge XANES and EXAFS measurements were prepared as mixtures with boron nitride. Anatase (TiO2, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was used as a reference sample. Ti K-edge spectra were collected over a photon energy range of 4635.2–5938.6 eV, and the spectra were analyzed using ATHENA software [37] (Version 0.9.26, Ravel and Newville 2005,).

3. Results and Discussion

3.1. Thermal Properties of Materials The powdered products were successfully synthesized following the chemical processing where peroxo-titanium complex ion solution is used as a precursor for the synthesis of nanostructured titania. The powder was yellow, and the optical bandgap energy was greatly reduced to 2.44 eV when compared with the white color of pure TNTs with an optical band gap of 3.34 eV. The yellow-colored TiO2-based compounds have been synthesized by anion doping such as N [38] and cation doping like Nb [39]. However, the peroxo-titanium complex ion precursor contains only H, O, Na and Ti in solution, and thus the bottom-up process in this study does not include any doping elements like N and Nb that introduce formation of impurity levels among the band gap in TNTs. Therefore, this yellow color is considered to be caused by the reduced band gap of titanate without any doping. In addition, Nanomaterials 2020, 10, 1331 5 of 17 this implies that the present processing method that makes use of precursor complex ions and following Nanomaterialsthe chemical 2020 route, 10, x might FOR PEER be aREVIEW facile but promising method for the production of functionalized oxides5 of 17 with low-dimensional nanostructures. The thermal properties of the prepared PTNTs in the temperature range of 25–600 °C were The thermal properties of the prepared PTNTs in the temperature range of 25–600 ◦C were characterized by TG and DTA, as shown in Figure 1. The total weight loss was approximately 17.2 %, characterized by TG and DTA, as shown in Figure1. The total weight loss was approximately 17.2%, and the significant weight loss was observed in the temperature range of 25–143 °C. Within this range, and the signiﬁcant weight loss was observed in the temperature range of 25–143 ◦C. Within this range, 10.3% of weight loss was estimated to be due to the desorption of molecular H2O such as dissociated 10.3% of weight loss was estimated to be due to the desorption of molecular H2O such as dissociated H2O, physisorbed H2O, and chemisorbed H2O [32]. This weight loss can also be attributed to the H2O, physisorbed H2O, and chemisorbed H2O[32]. This weight loss can also be attributed to the structural water loss of the titanium oxide structure including the Ti–OH bond [40,41]. An structural water loss of the titanium oxide structure including the Ti–OH bond [40,41]. An endothermic endothermic peak was observed at approximately 57 °C in the DTA curve due to water evaporation peak was observed at approximately 57 ◦C in the DTA curve due to water evaporation of physically of physically adsorbed water on the PNTs. Subsequently, a continuous weight loss of 6.9% was adsorbed water on the PNTs. Subsequently, a continuous weight loss of 6.9% was obtained at 600 ◦C, obtained at 600 °C, which may also be assigned to structural water loss, resulting in the which may also be assigned to structural water loss, resulting in the transformation of the crystal transformation of the crystal structure [32]. An exothermic peak is observed in the DTA graph due to structure [32]. An exothermic peak is observed in the DTA graph due to the transformation of a crystal the transformation of a crystal structure at approximately 257 °C. structure at approximately 257 ◦C.

Figure 1. Thermogravimetric (TG) and diﬀerential thermal analysis (DTA) curves of as-synthesized Figure 1. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of as-synthesized peroxo-titanate nanotubes (PTNTs) heated at 10 ◦C/min in an air atmosphere. peroxo-titanate nanotubes (PTNTs) heated at 10 °C/min in an air atmosphere. 3.2. Morphology of Materials 3.2. Morphology of Materials The SEM images of prepared samples are shown in Figure2. The as-synthesized samples hadThe tubular SEM structures, images of prepared as shown samples in Figure are2a. shown The averagein Figure diameter 2. The as of-sy tubularnthesized structures samples washad tubularfound tostructures, be approximately as shown in 10–20 Figure nm, 2a and. The after average heating diameter at 200 of ◦tubularC, distorted structures tubular was structures found to bewere approximately often observed 10– (Figure20 nm, 2 andb). At after the heating heating at temperature 200 °C, distorted of 300 tubular◦C, sheet-like structure structuress were often were observedfrequently (Figure seen, 2b as). At shown the heating in Figure temperature2c. The sheet-like of 300 °C, structuressheet-like structures seem to stackwere frequently on each other, seen, asand shown these in stacked Figure 2c sheet-like. The sheet structures-like structures were stillseem observed to stack on in each the sampleother, and that these was stacked heated sheet up to- like400 structures◦C (Figure 2wered). Further still observed annealing in the at 500sample◦C resulted that was in heated the formation up to 400 of °C a plate (Figure or rod 2 d structures,). Further annealingas shown inat 500 Figure °C 2resultede. This was in the observed formation not of only a plate in the or speciﬁc rod structures, part among as shown the agglomerates in Figure 2e in. This the wassamples observed but also not in only the wholein the partspecific of each part sample.among the agglomerates in the samples but also in the whole part of each sample.

Nanomaterials 2020, 10, 1331 6 of 17 Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 17

FigureFigure 2. 2. ScanningScanning electron electron microscopy microscopy (SEM (SEM)) images images of (a) as of-synthesized (a) as-synthesized PTNTs and PTNTs (b–e) andannealed (b–e) annealedPTNTs at PTNTs 200, 300, at 400 200, and 300, 500 400 °C and for 1 500 h, ◦respectively.C for 1 h, respectively.(f) Schematic (descriptionf) Schematic of descriptionthe annealing of thetemperature annealing temperatureeffect on the morphologies eﬀect on the morphologies of PTNTs. of PTNTs.

FigureFigure2f 2f shows shows the the schematic schematic diagram diagram of of the the morphology morphology changechange causedcaused by by thethe increaseincrease inin annealingannealing temperatures, temperatures, which which mainly mainly represents represents the typical the typical structures structures at each at temperature, each temperature, as shown as inshown dotted in boxesdotted in bo eachxes in SEM each imageSEM image (Figure (Figure2a–e). 2a The–e). The inset inset in Figurein Figure2f represents2f represents the the expected expected cross-sectioncross-section of of each each structure. structure. The The as-synthesized as-synthesized PTNTs PTNTs had a tubular structure. This This tub tubularular structurestructure seems seems to to be be “dismantled” “dismantled” above above 200200◦ °CC ofof annealing,annealing, and at approximately 200 °C,◦C, the the end end partpart of of the the tubetube seemsseems to re re-open-open by by forming forming a acleavage cleavage that that seems seems to toextend extend further. further. At 300 At and 300 400 and 400°C,◦ C,“stacked “stacked bamboo bamboo leaf leaf-like-like nanosheets” nanosheets” were were freq frequentlyuently observed observed which which might might be formed be formed by the by there- re-openingopening of of PTNTs. PTNTs. Although Although the the structure, structure, composition, composition, and and formation formation mechanism mechanism of of titania titania nanotubesnanotubes synthesized synthesized by by chemical chemical treatment treatment methods methods are still are under still underreview review [42], a nanotube[42], a nanotube structure originatedstructure originated from the characteristics from the characteristics of the layered of the hydrogen layered hydrogen titanate crystal titanate [13 crystal]. Thus, [13] the. Thus, TNTs the are consideredTNTs are considered to be formed to be by formed the scrolling by the of scrolling titanate of nanosheets titanate nanosheets to make a to multi-wall make a multi titanate-wall nanotube. titanate Consideringnanotube. Considering these facts, thethese formation facts, the of formation stacked leaf-likeof stacked nanosheets leaf-like nanosheets by the re-opening by the re of-opening nanotubes, of asn shownanotubes, in Figureas shown2c,d in as Figure well as 2c the and illustration d as well inas Figure the illustration2e, may be in feasible. Figure 2e From, may these be feasible. perspectives, From thethese phenomenon perspectives, of thethe deformedphenomenon tubular of the structure deformed may tubular imply structure a return may to a sheetimply structure a return asto a sheet result ofstructure an increase as a in result the annealing of an increase temperature. in the annealing The annealing temperature. temperature The annealing at 300 ◦C temperature causes thetubular at 300 structures°C causes to the change tubular into structures stacked to sheet-like change into structures, stacked and sheet the-like sheet structures, structure and was the assumed sheet structure to show unstablewas assumed surface to energy show unstabledue to its surface large surface energy area. due to However, its large the surface layer area. structure However, was gradually the layer distortedstructure due was to the gradually removal distorted of water moleculesdue to the from removal PTNTs of by water the annealing molecules process from as PTNTs shown by by the the TG-DTAannealing results process (Figure as shown1). Therefore, by the TG it- wouldDTA results be built (Figure up together 1). Therefore, to minimize it would its be surface built up energy together at a to minimize its surface energy at a higher temperature (approximately 400 °C). As a result, higher temperature (approximately 400 ◦C). As a result, transformation into a plate-like or rod-like structuretransformation consisting into ofa plate bamboo-like leaf-likeor rod-like nanosheets structure consisting stacks with of a bamboo long aspect leaf-like ratio nanosheets was formed stacks at a with a long aspect ratio was formed at a higher temperature that was up to 500 °C as surrounded by higher temperature that was up to 500 ◦C as surrounded by square dotted frames in Figure2e as well assquare illustrated dotted in frame Figures in2f. Figure The fundamental 2e as well as i mechanismllustrated in ofFigure how 2f the. The heating fundamental or thermal mechanism activation of how the heating or thermal activation would make such a structural variation in the PTNTs has not would make such a structural variation in the PTNTs has not been clariﬁed yet, and thus the further been clarified yet, and thus the further detailed investigations are required through various in situ detailed investigations are required through various in situ analysis using transmission/scanning analysis using transmission/scanning transmission electron microscopy (STEM/TEM), thermal transmission electron microscopy (STEM/TEM), thermal analysis coupled with mass spectroscopy and analysis coupled with mass spectroscopy and Fourier transform infrared (FT-IR) spectroscopy etc. Fourier transform infrared (FT-IR) spectroscopy etc. However, we inferred that the decomposition of However, we inferred that the decomposition of peroxo functional groups (–O–O–) and/or release of peroxo functional groups (–O–O–) and/or release of H2O might contribute to the systematic structure H2O might contribute to the systematic structure degradation of layered titanate nanotubes because degradation of layered titanate nanotubes because such structure degradation has not been reported such structure degradation has not been reported yet for the pure titania/titanate nanotubes. yet for the pure titania/titanate nanotubes. Figure 3 shows the specific surface area (SSA) at various annealing temperatures. SSA decreased linearly (R-squared value = 0.9907) as the annealing temperature increased. In this study, the annealing temperature affected the morphology of the structure as observed in Figure 2. In this study,

Nanomaterials 2020, 10, 1331 7 of 17

Figure3 shows the specific surface area (SSA) at various annealing temperatures. SSA decreased linearly (R-squared value = 0.9907) as the annealing temperature increased. In this study, theNanomaterials annealing 2020 temperature, 10, x FOR PEER aREVIEWffected the morphology of the structure as observed in Figure7 of2 .17 In this study, the surface area decreased, and it is expected that the dehydration of the surface ofthe the surface structures area causeddecreased, agglomeration and it is expected between that each the structural dehydration unit (i.e.,of the individual surface of nanotubes) the structures and thiscaus reducesed agglomeration the specific between surface areaeach becausestructural the unit morphology (i.e., individual and crystal nanotubes) structure and were this reduces affected the at higherspecific temperatures surface area [34 because], which wasthe confirmedmorphology by the and findings crystal from structure the TG-DTA were affected analysis shown at higher in Figuretemperature1. s [34], which was confirmed by the findings from the TG-DTA analysis shown in Figure 1.

Figure 3. Variation of the specific surface areas of PTNTs annealed at 200, 300, 400, and 500 ◦C for 1 h, respectively. Figure 3. Variation of the specific surface areas of PTNTs annealed at 200, 300, 400, and 500 °C for 1 h, 3.3. Crystallographicrespectively. Characteristics of Materials The crystalline phase variation of the PTNTs caused by heating was analyzed by in situ 3.3. Crystallographic Characteristics of Materials high-temperature X-ray diffraction, and the results are shown in Figure4a. The as-synthesized PTNTsThe show crystalline a typical phase hydrogen variation titanate of phasethe PTNTs [43] (Powdercaused by Di ffheatingraction was File (PDF)analyzed Card# by 00-047-0124)in situ high- withtemperature an orthorhombic X-ray diffraction, structure. and Titanate the results nanotube are shown (H Ti Oin FigureH O) is4a considered. The as-synthesized to be formed PTNTs by 2 2 5· 2 scrollingshow a typical layers hydrogen of titanate titanate nanosheets phase and [43] thus (Powder forming Diffraction a layered-structure File (PDF) Card# that corresponds 00-047-0124) to with the 200an orthorhombic reflections of TNTs.structure. The T diitanateffraction nanotube peaks related (H2Ti2O to5·H the2O)c-axis is considered (e.g., 501, to 002) be wereformed much by scrolling weaker thanlayers the of peaks titanate at 200, nanosheets 110, 310, 020 and [ 44 thus]. As form theing annealing a layered temperature-structure increases, that correspond the crystals to phase the 200 is convertedreflections from of TNTs. hydrogen The diffraction titanate to anpeaks anatase-type related to (PDF the c Card#-axis (e.g. 00-021-1272), 501, 002) with were tetragonal much weaker structure. than Thethe lattice peaks constants at 200, 110, of reported 310, 020 H[44]Ti. O As H theO[ annealing43] were: temperaturea = 18.03 Å, increases,b = 3.784 theÅ, and crystalc = phase2.998 Å is 2 2 5· 2 0 0 0 (PDFconverted Card# from 00-047-0124), hydrogen and titanate the lattice to an constants anatase-type of the (PDF reported Card# anatase 00-021-1272) were: witha0 = tetragonal3.785 Å, bstructure.0 = 3.785 Å,The and latticec0 = 9.514constants Å (PDF of reported Card# 00-021-1272). H2Ti2O5·H2O The [43] lattice were: constants a0 = 18.03 of Å anatase, b0 = 3.784 obtained Å , and after c0 = heating2.998 Å to (PDF 500 ◦Card#C were: 00a-0047= 3.764-0124) Å,, andb0 = the3.764 lattice Å, and constantsc0 = 9.483 of the Å. The reported lattice anatase constants were: were a0 calculated = 3.785 Å , usingb0 = 3.785 Equation Å , and (1) c0 and= 9.514 the Å values (PDF ofCard# the as-synthesized00-021-1272). The PTNTs lattice were constantsa0 = 19.27 of anatase Å, b0 = obtained3.743 Å andafter cheating0 = 2.976 to Å. 500 Among °C were: the a0 3 = parameters, 3.764 Å , b0 = the3.764b0 Åand, andc0 cvalues0 = 9.483 agreed Å . The with lattice those constants of the basewere hydrogencalculated titanateusing Eq structure.uation ( However,1) and the the valuesa0 value, of the which as- relatessynthesized to the interlayerPTNTs were spacing a0=19.27 of the Å titanate, b0=3.743 structure Å and increasedc0=2.976 Å by. Among 6.9%, which the 3 is parameters more than that, the for b0 theand common c0 values hydrogen agreed with titanate. those The of large the base increase hydrogen in the atitanate-axis may structure. be due to However the enlargement, the a0 ofvalue, interlayer which distance relates at to the the 200 interlayer plane. Sugita spacing et al. of [45 the] reported titanate anstructure increase increased in the interplanar by 6.9 % spacing, which is of more titanate than when that the for radius the common of the Li hydrogen+ ions was titanate. greater thanThe large that ofincrease H+ ions in located the a-axis between may be the due hydrogen to the enlargement titanate layers. of Konginterlayer et al. distance [24] obtained at the similar 200 plane. results Sugita when et aal. peroxo-titanium [45] reported an bond increase (Ti–O–O–H) in the interplanar was present spacing between of titanate the layers when ofhydrogen the radius titanate of the Li crystals.+ ions was greater than that of H+ ions located between the hydrogen titanate layers. Kong et al. [24] obtained similar results when a peroxo-titanium bond (Ti–O–O–H) was present between the layers of hydrogen titanate crystals. The variations of each peak position (2), and the intensity assigned to 110 diffraction peaks of PTNTs (hydrogen titanate), the 101 diffraction peak of anatase phases, and the crystallite size with the annealing temperature of samples are displayed in Figure 4b. The 2 angle of the diffraction peak assigned to 110 of hydrogen titanate crystal was maintained at 25.00° ± 0.12° till the heating

Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 17 temperature was up to 350 °C. Then, the peak position quickly shifted to a higher angle at the subsequent temperature range, and the 2 value was 25.28° ± 0.12° which is the similar peak position of 101 in anatase TiO2 phase (calculated as 25.281° from the d-value). At the same time, as the temperature increased, the intensity of peak maintained a low value till the temperature was 350 °C and it linearly increased to 460 °C from the subsequent annealing temperature where it maintained its maximum value. The crystallite size of the anatase phase was calculated at 350 °C to be Nanomaterials 2020, 10, 1331 8 of 17 approximately 13 nm, which then increased to 19 nm as the annealing temperature increased up to 460 °C were maintained its highest size.

FigureFigure 4. 4. (a(a) )In In situ situ X X-ray-ray diffraction diffraction (XRD (XRD)) patterns patterns of of the the PTNTs PTNTs during during an an annealing annealing temperature. temperature. (b(b) ) Annealing Annealing temperaturetemperature e ff effectect on on the the variations variations of 2 θ ofand 2 Peakand intensity Peak intensity assigned assigned to the 110 to hydrogen the 110 hydrogentitanate and titanate 101 anatase and 101 reflections, anatase reflections, and the crystal and sizethe crystal of the anatasesize of the phase. anatase (c) Annealing phase. (c) temperature Annealing temperatureeffect on the effect variations on the of variations 2θ and peak of 2 intensity and peak assigned intensity to the assigned 200 hydrogen to the 200 titanate hydrogen reflection titanate and reflectiond-value derived and d-value from derived 2θ.(d) Illustrationfrom 2. (d) for Illustration variation for of interlayervariation of distance interlayer by annealing.distance by annealing.

TheThe change variations in ofthe each peak peak was position also observed (2θ), and at the the intensity diffraction assigned peak to according 110 diffraction to the peaks 200 of of hydrogenPTNTs (hydrogen titanate. Figure titanate), 4c theshows 101 the diff variationsraction peak in the of anatasepeak position phases, (2 and, intensity, the crystallite and d- sizevalue with of thethe 200 annealing plane depending temperature on ofthe samples annealing are temperature. displayed in No Figure noticeable4b. The change 2 θ angle was ofobserved the diff ractionat the peak assigned to 110 of hydrogen titanate crystal was maintained at 25.00 0.12 till the heating diffraction peak of 200 until a temperature change of 100 °C was attained.◦ ± As◦ the annealing temperaturetemperature increases, was up tothe 350 peak◦C. positio Then,n the shifted peak to position a higher quickly 2 angle shifted with toa adecrease higher in angle the atpeak the subsequent temperature range, and the 2θ value was 25.28 0.12 which is the similar peak position of intensity. As earlier mentioned, this 200 peak was assigned◦ ±to the◦ interlayer of hydrogen titanate, for which101 in the anatase interlayer TiO2 phase distance (calculated of PTNTs as could 25.281 be◦ from said theto decreased-value). with At the the same heating, time, as as shown the temperature in Figure 4dincreased,. From the the TG intensity-DTA analysis of peak in maintained Figure 1, the a low heating value at till low the-temperature temperature up was to 350 approximately◦C and it linearly 120 °C,increased in which to the 460 major◦C from phenomenon the subsequent was the annealing removal temperatureof physically where-adsorbed it maintained water from its the maximum surface, didvalue. not Theaffect crystallite the structural size of characteristics the anatase phase of PTNTs. was calculated Furthermore, at 350 subsequent◦C to be approximately water loss could 13 nm,be attributedwhich then to increasedthe interlayer to 19 water nm as loss the as annealing the peak temperature shifted to a high increased angle, up which to 460 indicates◦C were a maintained reduction inits the highest interlayer size. distance, as observed in XRD results. Our results show that the interlayer distance of approximatelyThe change in0.9 the nm peak in the was as also-synthesized observed PTNTs at the di wasffraction maintained peak according at 120 °C, to followed the 200 of by hydrogen a rapid decreasetitanate. Figure in the4 c distance shows the within variations a temperature in the peak range position of (2 upθ , tointensity, 200 °C. and Then,d-value as of the the annealing 200 plane temperaturedepending on further the annealing increases, temperature. the interplanar No distance noticeable decreases change linearly was observed which atresults the di inff ractiona reduction peak ofof d 200 = 0.7 until nm aat temperature 560 °C. change of 100 ◦C was attained. As the annealing temperature increases, the peak position shifted to a higher 2θ angle with a decrease in the peak intensity. As earlier mentioned, 3.4.this Transmission 200 peak was Electron assigned Microscopy to the interlayer of hydrogen titanate, for which the interlayer distance of PTNTs could be said to decrease with the heating, as shown in Figure4d. From the TG-DTA analysis in Figure1, the heating at low-temperature up to approximately 120 ◦C, in which the major phenomenon was the removal of physically-adsorbed water from the surface, did not affect the structural characteristics of PTNTs. Furthermore, subsequent water loss could be attributed to the interlayer water loss as the peak shifted to a high angle, which indicates a reduction in the interlayer distance, as observed in XRD results. Our results show that the interlayer distance of approximately Nanomaterials 2020, 10, 1331 9 of 17

0.9 nm in the as-synthesized PTNTs was maintained at 120 ◦C, followed by a rapid decrease in the distance within a temperature range of up to 200 ◦C. Then, as the annealing temperature further increases, the interplanar distance decreases linearly which results in a reduction of d = 0.7 nm at 560 ◦C.

3.4. Transmission Electron Microscopy Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 17 To conﬁrm the interlayer distance and distribution of the crystallites of PTNTs, high-resolution (HR)To TEM confirm observation the interlayer was performed distance and for thedistribution samples obtainedof the crystallites at diﬀerent of annealingPTNTs, high temperatures,-resolution (HR)as shown TEM observation in Figure5. Allwas theperformed samples for show the samples a periodic obtained lattice at image different that annealing corresponds temperatures, to the 200 asplane shown of layeredin Figure hydrogen 5. All the titanate,samples whichshow a was periodic also observedlattice image from that XRD corresponds analysis results to the (Figure200 plane4). ofThe layered interlayer hydrogen distance titanate, was calculated which was to be also 0.99 observed nm for thefrom as-synthesized XRD analysis PTNTs results (Figure (Figure5a), 4). and The it interlayer distance was calculated to be 0.99 nm for the as-synthesized PTNTs (Figure 5a), and it clearly decreased with an increase in annealing temperature to 0.73 nm for PTNTs annealed at 500 ◦C clearly(Figure decreased5e). The interlayer with an increase spacing obtainedin annealing is summarized temperature in to Figure 0.73 nm5f, and for PTNT the observeds annealed results at 500 were °C (Figureconsistent 5e). with The thatinterla calculatedyer spacing from obtained the XRD is analysissummarized (d-spacing in Figure in Figure 5f, and4c). the Their observed results results were werecompared consistent and replottedwith that as calculated Figure S2, from which the clearly XRD exhibitedanalysis (d similar-spacing tendency. in Figure 4c). Their results were compared and replotted as Figure S2, which clearly exhibited similar tendency.

Figure 5. Transmission electron microscopy (TEM) images of (a) as-synthesized PTNTs and (b–e)

Figureannealed 5. PTNTsTransmission at 200, 300, electron 400, and microscopy 500 ◦C for (TEM 1 h,) respectively. images of ( (af)) asVariation-synthesized in the PTNTsinterlayer and distance (b–e) annealedfor annealing PTNTs temperatures at 200, 300, 400, taken and from 500 30 °C zones, for 1 h, respectively. respectively. (f) Variation in the interlayer distance for annealing temperatures taken from 30 zones, respectively. Figure6 shows the HR-TEM images and the FFT di ffraction pattern of the annealed PTNTs at 400 andFigure 500 ◦C. 6 The showsd-values the HR of- theTEM lattice images fringes and originated the FFT diffraction from diffraction pattern spots of the which annealed were calculatedPTNTs at 400from and the 500 FFT °C. analysis The d of-values the TEM of the images; lattice they fringes were originated compared from with diffraction the PDF card spots information which were for calculatedhydrogen titanate from the (# 00-047-0124) FFT analysis and ofanatase the TEM (# 00-021-1272). images; they In werethe case compared of the PTNTs with sample the PDF annealed card informationat 400 ◦C, as for shown hydrogen in Figure titanate6a, the (# calculated 00-047-0124) di ff andraction anatase spots (# for 00 one-021 region-1272). was In the estimated case of to the be PTNTthe 200s sample plane ofannealed the hydrogen at 400 titanate°C, as shown with din-value Figure of 6a 0.7, nm.the calculated On the other diffraction hand, aspots lattice for fringe one regionwith a wasd-value estimated of 0.37 to nm be wasthe 200 also plane observed of the in hydrogen the same titanate sample with (enlarged d-value image of 0.7 and nm. an On FFT the pattern other hand,in Figure a lattice6a), whichfringe with corresponds a d-value to of the0.37 101 nm plane was also of theobserved anatase, in the in accordancesame sample with (enlarged XRD results, image andas shown an FFT in pattern Figure 4in. AsFigure was 6a evident), which from corresponds the XRD analysis to the 101 results, plane the of the annealed anatase, PTNTs in accordance at 400 ◦C withpossess XRD low results, crystallinity; as shown the in lattice Figure fringes 4. As was and evident diffraction from spots the assignedXRD analysis to only results, the main the annealed 101 plane PTNTsof anatase at 400 were °C expectedpossess low to be crystallinity observable,; the while lattice other fringes spots (e.g.,and diffraction corresponding spots to assigned 011, 112) to are only not theexpected main 101 to be plane observed. of anatase were expected to be observable, while other spots (e.g., corresponding to 011, 112) are not expected to be observed. At an annealing temperature of 500 °C, as shown in Figure 6b, two kinds of crystalline phases were also identified. The FFT diffraction spots were estimated as (011), (101), and (112) planes of the anatase as well as (200) plane of the hydrogen titanate. The enlarged image clearly shows the lattice fringes of 0.65, 0.37, 0.37, and 0.24 nm, which are assigned to 200 of hydrogen titanate and 101, 011, and 112 of anatase, respectively. These results are consistent with the results of the in situ XRD as mentioned earlier in Figure 4. As earlier discussed, the sample annealed at 500 °C exhibited a rod- like morphology with a width of 20–50 nm (see square dotted frames in Figure 2e). This rod-like PTNTs contained the two crystalline structures of hydrogen titanate and anatase. This analysis reveals that the 101 plane of anatase is parallel to the 200 plane of hydrogen titanate, and thus it is believed that the crystallites of anatase were converted directly from the peroxo-modified titanate nanotubes by the dehydration and morphological development as mentioned earlier and shown in Figure 2f.

Nanomaterials 2020, 10, 1331 10 of 17 Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 17

FigureFigure 6. 6. TEMTEM images images and and fast fast Fourier Fourier transform transform ( (FFT)FFT) diffraction diﬀraction patterns (T = Titanate,Titanate, A == Anatase)Anatase) ofof annealed annealed PTNT PTNT at at ( (aa)) 400 400 and and ( (bb)) 500 500 °C◦C for for 1 1 h, h, respectively. respectively.

3.5. BondingAt an annealing Characteristics temperature of 500 ◦C, as shown in Figure6b, two kinds of crystalline phases were also identified. The FFT diffraction spots were estimated as (011), (101), and (112) planes of the Raman spectroscopy and FT-IR analysis were carried out to investigate the effect of annealing anatase as well as (200) plane of the hydrogen titanate. The enlarged image clearly shows the lattice temperature on the structures. Figure 7a shows the Raman spectra of structures annealed at various fringes of 0.65, 0.37, 0.37, and 0.24 nm, which are assigned to 200 of hydrogen titanate and 101, 011, temperatures from 200 to 500 °C for 1 h. The Raman pattern of as-synthesized PTNTs showed a and 112 of anatase, respectively. These results are consistent with the results of the in situ XRD as hydrogen titanate structure [46]. The peaks assigned to Ti–O, Ti–O–Ti, and Ti–O–H were observed at mentioned earlier in Figure4. As earlier discussed, the sample annealed at 500 ◦C exhibited a rod-like 276, 444, and 700 cm−1, respectively. After annealing at 500 °C, the intensity of peaks assigned to Ti– morphology with a width of 20–50 nm (see square dotted frames in Figure2e). This rod-like PTNTs O and Ti–O–Ti of hydrogen titanate structure decreased, and several new peaks surfaced at 135, 395, contained the two crystalline structures of hydrogen titanate and anatase. This analysis reveals that 510, and 650 cm−1. These peaks were assigned to Eg, B1g, A1g, and Eg of anatase. The Raman spectrum the 101 plane of anatase is parallel to the 200 plane of hydrogen titanate, and thus it is believed that the of the PTNTs annealed at 500 °C mainly consisted of the pattern of anatase, while weak peaks of crystallites of anatase were converted directly from the peroxo-modified titanate nanotubes by the hydrogen titanate structures were also confirmed. dehydration and morphological development as mentioned earlier and shown in Figure2f. The annealing effect on the PTNTs was further examined using a local structure-determining probe,3.5. Bonding XAFS, Characteristics as shown in Figure S1. Figure S1a shows the Ti K-edge XANES spectra of the as- synthesized PTNTs and annealed PTNTs at various temperatures with anatase-type TiO2 as the Raman spectroscopy and FT-IR analysis were carried out to investigate the effect of annealing reference sample, which demonstrate structural differences in the local structure and the electronic temperature on the structures. Figure7a shows the Raman spectra of structures annealed at various state of Ti4+ ions. Figure 7b shows the FT-IR spectra of samples obtained at the various annealing temperatures from 200 to 500 C for 1 h. The Raman pattern of as-synthesized PTNTs showed a temperatures. These spectra were◦ quite similar and were characterized by a broad and strong band hydrogen titanate structure [46]. The peaks assigned to Ti–O, Ti–O–Ti, and Ti–O–H were observed at 3400 cm−1, which can be assigned to OH groups [47] as well as to the H2O molecule. The existence at 276, 444, and 700 cm 1, respectively. After annealing at 500 C, the intensity of peaks assigned to of this peak indicates the− hydroxyl group and water molecules◦ existed in the structure or in the Ti–O and Ti–O–Ti of hydrogen titanate structure decreased, and several new peaks surfaced at 135, interlayer space between the crystal layers. The peak intensity decreased with an increase in the 395, 510, and 650 cm 1. These peaks were assigned to E ,B ,A , and E of anatase. The Raman annealing temperature,− which led to the evaporation ofg water1g molecules1g g and the elimination of spectrum of the PTNTs annealed at 500 C mainly consisted of the pattern of anatase, while weak hydroxyl groups by a dehydration reaction.◦ This is also supported by the findings from the TG-DTA peaks of hydrogen titanate structures were also confirmed. analysis shown in Figure 1. The presence of water molecules in the sample was also identified by the peak located at 1630 cm−1, which is assigned to the H–O–H deformation mode (H–O–H). Additionally, in all samples, a weak peak assigned to peroxo bond (–O–O–) was observed at approximately 915 cm−1. The intensity of the peak was slightly reduced by the heat treatment but the peak was still present even for the sample annealed at 500 °C. This sample mainly consists of anatase crystal as mentioned earlier; however, it still contains hydrogen titanate. Thus it could be inferred that the

Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 17 peroxo bond found in the FT-IR spectrum might still exist in the PTNTs samples annealed at 500 °C. In addition, it can also be confirmed by the X-ray photoelectron spectroscopy (XPS) analysis. The O1s XPS spectrum of as-synthesized PTNTs had Ti–O–O (peroxo-) bond at 533.1 eV [24]. As can be seen inNanomaterials Figure S3,2020 this, 10 peroxo, 1331 -bond still existed at 533.4 eV in the PTNTs samples annealed at 500 °C11 with of 17 slightly decreased intensity, implying the thermal stability of the peroxo-bond in the present PTNTs.

Figure 7. a b Figure 7. ((a)) Raman spectra and and ( (b) Fourier t transformransform i infrarednfrared (FT (FT-IR)-IR) spectra of the as-synthesizedas-synthesized PTNTs and annealed PTNTs at different temperatures. PTNTs and annealed PTNTs at different temperatures. The annealing effect on the PTNTs was further examined using a local structure-determining probe, 3.6. Optical Properties of Materials XAFS, as shown in Figure S1. Figure S1a shows the Ti K-edge XANES spectra of the as-synthesized PTNTsThe and effects annealed of annealing PTNTs at temperature various temperatures on the optical with properties anatase-type and TiO electrical2 as the referenceband gap sample, of the sampleswhich demonstrate were studied structural using UV diff-Viserences spectroscopy in the local anal structureysis. Figure and the 8 shows electronic the reflectivity state of Ti4 + ofions. the samplesFigure7b annealed shows theat various FT-IR spectratemperatures. of samples The UV obtained-Vis spectrum at the of various the as-synthesized annealing temperatures.PTNTs clearly 1 indicatesThese spectra that the were absorption quite similar edgeand located were in characterized the visible light by regions a broad is and largely strong red band-shifted at 3400compared cm− , withwhich comm can beon assignedanatase TiO to2 OH. This groups corresponds [47] as to well the as fact to that the Hthe2O as molecule.-synthesized The PTNT existences are yellow. of this Aspeak the indicates annealing the temperature hydroxyl group increases, and waterthe reflectivity molecules of existed the incident in the light structure in the or wavelength in the interlayer range ofspace 350 to between 500 nm theincreases. crystal An layers. increase The in peak reflectivity intensity is expect decreaseded to withreduce an the increase absorption in the of annealing light due totemperature, an increase whichin bandgap led to energy. the evaporation The inset of in water Figure molecules 8 show the and Tauc the plot elimination for determining of hydroxyl the optical groups bandby a dehydration gap of samples reaction. calculated This is also by the supported Kubelka by–Munk the findings model. from The the bandgap TG-DTA energy analysis of shown the as in- synthesizedFigure1. The PTNTs presence was of 2.5 water5 eV, moleculesand the value in the increased sample wasto 3.09 also eV identified at an annealing by the peaktemperature located atof 1 5001630 °C. cm −The, which variation is assigned of the bandgap to the H–O–H energy deformation was related mode to the (δH–O–H presence). Additionally, of the peroxo in all bond samples, and 1 crystala weak transformation. peak assigned to Recently, peroxo bond several (–O–O–) research was groups observed [11,24] at approximately have shown that 915 the cm ban− . Thedgap intensity energy of theTiO peak2 and was related slightly compounds reduced by can the be heat reduced treatment due butto the the peakpresence was stillof the present peroxo even bond for thein titanate sample structuresannealed at that 500 ◦ increaseC. This sample the valance mainly band consists level. of anatase However, crystal the asbandgap mentioned energy earlier; was however, increased it still by annealing,contains hydrogen as shown titanate. in Figure Thus 8, it couldwhich be might inferred be thatdue the to the peroxo decomposition bond found in of thethe FT-IR peroxo spectrum bond. Savinkinamight still et exist al. [48] in the reported PTNTs samplesthat the peroxo annealed bond at 500 is stable◦C. In at addition, room temperature it can also bein confirmedthe air but byit can the beX-ray destroyed photoelectron by annealing spectroscopy above of (XPS) 200 analysis.°C, and this The phenomenon O1s XPS spectrum was also of as-synthesized observed in this PTNTs study had as mentionedTi–O–O (peroxo-) above. bond at 533.1 eV [24]. As can be seen in Figure S3, this peroxo-bond still existed at 533.4However, eV in the forPTNTs the 200 samples °C annealed annealed sample at 500 in the◦C withpresent slightly study, decreased two kinds intensity, of optical implying absorption the edgesthermal can stability be seen of in the the peroxo-bond reflectance curve in the around present 360 PTNTs. nm and 420 nm. These can also be found as the two linear regions in the Tauc plot, and correspond to the band gap energy of 2.92 eV and 2.60 eV, respectively3.6. Optical Properties. The latter of value Materials is closed to that of as-synthesized PTNT (2.55 eV) that contains –O–O– bondingThe, ewhileffects the of annealingformer one temperature is more likely on to the those optical of annealed properties at higher and electrical temperature band (above gap of 300 the °C).samples By considering were studied these using facts, UV-Vis we speculated spectroscopy that the analysis. sample Figure annealed8 shows at 200 the °C reflectivity may contain of two the kindssamples of phases annealed; one at is various similar temperatures. to as-synthesized The PTNT UV-Vis and spectrum the other of was the as-synthesizedmore likely to anatase PTNTs phase clearly. Therindicatesefore, that it is the considered absorption that edge the locatedcrystalline in the phase visible transformation, light regions isin largelyanother red-shifted words nucleation compared of anatase-based crystal, might have started around 200 °C, and thus two kinds of optical nature might with common anatase TiO2. This corresponds to the fact that the as-synthesized PTNTs are yellow. beAs seen the annealing in the sample temperature annealed increases, at this temperature the reflectivity. of the incident light in the wavelength range of 350 to 500 nm increases. An increase in reflectivity is expected to reduce the absorption of light due to an increase in bandgap energy. The inset in Figure8 show the Tauc plot for determining the optical band gap of samples calculated by the Kubelka–Munk model. The bandgap energy of the as-synthesized PTNTs was 2.55 eV, and the value increased to 3.09 eV at an annealing temperature of Nanomaterials 2020, 10, x FOR PEER REVIEW 12 of 17

According to the above mentioned discussion, it has been established that annealing caused the Nanomaterialsdecomposition2020, 10of, 1331peroxo bond from PTNTs and it is expected to result in a decrease in the valance12 of 17 band level due to the increase in the titanium electron density. The XRD results showed that the hydrogen titanate crystal of PTNTs transforms into an anatase crystal structure by annealing at a 500 ◦C. The variation of the bandgap energy was related to the presence of the peroxo bond and crystal temperature of 500 °C (Figure 4). At the same time, the TEM observation and Raman spectrum transformation. Recently, several research groups [11,24] have shown that the bandgap energy of TiO2 indicated that an anatase phase in a sample was formed at 500 °C with partial hydrogen titanate and related compounds can be reduced due to the presence of the peroxo bond in titanate structures structures (Figure 6 and Figure 7). The reflectance spectrum of the sample annealed at 500 °C showed that increase the valance band level. However, the bandgap energy was increased by annealing, as a lower reflectivity in the visible light range compared to pure anatase. Furthermore, the bandgap shown in Figure8, which might be due to the decomposition of the peroxo bond. Savinkina et al. [ 48] was calculated to be 3.09 eV which is lower than that of anatase (3.2 eV). By combining the reflectance reported that the peroxo bond is stable at room temperature in the air but it can be destroyed by spectrum with FT-IR analysis (Figure 7), although the peroxo bond was affected by annealing, the annealing above of 200 ◦C, and this phenomenon was also observed in this study as mentioned above. peroxo bond that exists in the crystal of PTNTs had high stability.

Figure 8. Reflectance spectra of PTNTs annealed at different temperatures. (Inset) Plots of (αhv)1/2 vs. 1/2 energyFigure for8. Reflectance the annealed spectra PTNTs of atPTNTs different annealed temperatures. at different temperatures. (Inset) Plots of (hv) vs. energy for the annealed PTNTs at different temperatures. However, for the 200 ◦C annealed sample in the present study, two kinds of optical absorption edges3.7. Formation can be seen Mechanism in the reflectanceof Peroxo-Modified curve aroundAnatase 360Crystal nm and 420 nm. These can also be found as the two linear regions in the Tauc plot, and correspond to the band gap energy of 2.92 eV and 2.60 eV, When the crystallographic structure is considered, the H2Ti2O5·H2O (or expressed also as respectively. The latter value is closed to that of as-synthesized PTNT (2.55 eV) that contains –O–O– H2Ti2O4(OH)2) structure, which is hydrogen di-titanate, has been suggested to comprise of two- bonding, while the former one is more likely to those of annealed at higher temperature (above 300 C). dimensional layers in which TiO6 octahedra are combined through edge sharing [34], as shown◦ in By considering these facts, we speculated that the sample annealed at 200 C may contain two kinds Figure 9. The lattice is orthorhombic, and the TiO6 layers were laminated◦ with an alternating ofinterlayer phases; cation one is (H similar+) along to as-synthesizedthe [100] direction PTNT [49] and. In the the case other of was the morenanotube likely structure to anatase of layered phase. Therefore,hydrogen titanate, it is considered Zhang et that al. the[50] crystalline reported that phase these transformation, layers scroll the in another[010] direction words nucleationwith the tube of anatase-basedaxis pointing towards crystal, mightthe [001] have direction. started aroundThe configuration 200 ◦C, and of thus the twotitanate kinds layer, of optical as shown nature in Figure might be9, with seen inprojection the sample along annealed the [001] at thisdirection temperature. of the titanate, is reported to be similar to the principal unit According layer of the to the anatase above projected mentioned along discussion, the [101] it has direction been established [49]. It is that hypothesized annealing caused that, upon the decompositionannealing, the titanate of peroxo layers bond shrink from locally, PTNTs by and reducing it is expected the interlayer to result distance in a decrease and transforming in the valance into bandthe anatase level due crystal to the [47,49] increase. Our in re thesults titanium as well electron as those density. of others The [47,49] XRD results have shown showed that that local the hydrogenshrinkage titanateof the titanate crystal layers of PTNTs on the transforms hydrogen titanate into an anatasestructure crystal to form structure anatase crystal by annealing structure at is a temperaturepossible. of 500 ◦C (Figure4). At the same time, the TEM observation and Raman spectrum indicated that an anatase phase in a sample was formed at 500 ◦C with partial hydrogen titanate structures (Figures6 and7 ). The reflectance spectrum of the sample annealed at 500 ◦C showed a lower reflectivity in the visible light range compared to pure anatase. Furthermore, the bandgap was calculated to be 3.09 eV which is lower than that of anatase (3.2 eV). By combining the reflectance spectrum with FT-IR analysis (Figure7), although the peroxo bond was a ffected by annealing, the peroxo bond that exists in the crystal of PTNTs had high stability. Nanomaterials 2020, 10, 1331 13 of 17

3.7. Formation Mechanism of Peroxo-Modiﬁed Anatase Crystal When the crystallographic structure is considered, the H Ti O H O (or expressed also as 2 2 5· 2 H2Ti2O4(OH)2) structure, which is hydrogen di-titanate, has been suggested to comprise of two-dimensional layers in which TiO6 octahedra are combined through edge sharing [34], as shown in Figure9. The lattice is orthorhombic, and the TiO 6 layers were laminated with an alternating interlayer cation (H+) along the [100] direction [49]. In the case of the nanotube structure of layered hydrogen titanate, Zhang et al. [50] reported that these layers scroll the [010] direction with the tube axis pointing towards the [001] direction. The conﬁguration of the titanate layer, as shown in Figure9, with projection along the [001] direction of the titanate, is reported to be similar to the principal unit layer of the anatase projected along the [101] direction [49]. It is hypothesized that, upon annealing, the titanate layers shrink locally, by reducing the interlayer distance and transforming into the anatase crystal [47,49]. Our results as well as those of others [47,49] have shown that local shrinkage of the titanate layers on Nanomaterialsthe hydrogen 2020 titanate, 10, x FOR structure PEER REVIEW to form anatase crystal structure is possible. 13 of 17

Figure 9. Hypothetical schematic view of a hydrogen titanate crystal. Figure 9. Hypothetical schematic view of a hydrogen titanate crystal. Figure 10 illustrates the hypothesis system for the transformation from titanate nanotubes to titaniaFigure nanoplates, 10 illustrates mimicking the hypothesis the lattice fringessystem offor titanate the transformation or titania shown from in titanate the above nanotubes TEM results. to titaniaWe observed nanoplates, thatwater, mimicking hydroxyl the lattice (–OH), fringes and/or of peroxo titanate (–O–O–) or titania groups shown were in removedthe above by TEM dehydration results. Wereaction, observed which that resulted water, from hydroxyl annealing (–OH that), occurred and/or peroxo in the interlayer (–O–O–) of groups hydrogen were titanate removed structure, by dehydrationthereby reducing reaction the, distancewhich resulted between from that annealing titanate layers that occurred (also see inFigure the 9 interlayer). During of this hydrogen process, titanatetubular structure, structures thereby with hydrogen reducing titanate the distance were transformed between that into titanate sheet-like layers structures (also see by Figure annealing 9). Duringbecause this of the process, removal tubular of water structures from the with interlayer. hydrogen As discussed titanate were in the transformed previous section into and sheet shown-like structuresin Figure 2 by, this annealing process maybecause govern of the the removal morphological of water change from theby the interlayer. formation As of discussed cleavage in in the the previoustubular titanatesection alongand shown the tube in axisFigure (crystallographic 2, this process variation may govern is illustrated the morphological in Figure 10 change), which by results the formationin the formation of cleavage of stacked in the titanate tubular nanosheets titanate along during the heat tube treatment. axis (crystallographic Anatase crystal variation was then is illustratedformed simultaneously in Figure 10), alongwhich the results layered in the hydrogen formation titanate of stacked by annealing, titanate therebynanosheets titania during nanoplate heat treatment.or rod structure Anatase that crystal partially was then contained formed hydrogen simultaneously titanate along crystal the was layered formed. hydrogen Several titanate research by annealing,groups [30 thereby–32,34] have titania reported nanoplate the annealing or rod structure effects on that TNTs, partially in which contained almost hydrogenall TNTs transform titanate crystal was formed. Several research groups [30–32,34] have reported the annealing effects on TNTs, into a spherical morphology with a clear anatase structure after annealing at 500 ◦C. in whichHowever, almost theseall TNTs previous transform reports into are a spherical quite diff morphologyerent from our with present a clear study. anatase The structure difference after in annealingmorphological at 500 change°C. by annealing might be attributed to the peroxo groups in the interlayer of PTNTs. Our bottom-up process using the peroxo-titanium complex ion precursor was different from traditional methods [4,16–18] for the synthesis of nanotubular titania. During this process, the special peroxo-titanium bonds (Ti–O–O–) can be formed directly within the interlayers of the hydrogen titanate crystal as well as on the surface of materials during crystal formation without any chemical treatment [24]. Usually, the peroxo groups of TNTs are expected to be expelled by annealing. However, peroxo titanium bonds in the crystal structure of PTNTs could be restricted by the crystal transformation

Figure 10. Hypothetical scheme for the transformation of titanate nanotubes to titania nanoplates. The structure models presented are the projections along the nanotube. The transformed structure shown is the anatase phase.

However, these previous reports are quite different from our present study. The difference in morphological change by annealing might be attributed to the peroxo groups in the interlayer of PTNTs. Our bottom-up process using the peroxo-titanium complex ion precursor was different from traditional methods [4,16–18] for the synthesis of nanotubular titania. During this process, the special peroxo-titanium bonds (Ti–O–O–) can be formed directly within the interlayers of the hydrogen titanate crystal as well as on the surface of materials during crystal formation without any chemical treatment [24]. Usually, the peroxo groups of TNTs are expected to be expelled by annealing.

Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 17

Nanomaterials 2020, 10, 1331 14 of 17 Figure 9. Hypothetical schematic view of a hydrogen titanate crystal. to anataseFigure because 10 illustrates this bond the withinhypothesis the interlayer system for is harderthe transformation to remove than from that titanate on the nanotubes surface due to totitania the di nanoplates,fference in mimicking the heat-transfer the lattice distance fringes from of titanate the surface or titania to the shown interior in the [51 above]. Peroxo TEM bonding results. wasWe confirmed observed that using water, the FT-IR hydroxyl of the ( annealed–OH), and/or PTNTs. peroxo As a( result,–O–O– the) groups peroxo were group removed is thought by todehydration suppress the reaction shrinkage, which that resulted would fromhave occurredannealing between that occurred layers in as the a result interlayer of heat of treatment. hydrogen Thetitanate titania structure, nanoplates thereby as the reducing final products the distance were considered between that to be titanate composed layers of anatase(also see crystal Figure and 9). layeredDuring hydrogen this process, titanate tubular structure structures with peroxowith hydrogen bond in their titanate layers were (Figure transformed 10). Etacheri into sheetet al.- [like11] reportedstructures that by oxygen annealing vacancy because accelerates of the removal the Ti–O of breaking water from and the phase interlayer. transition As of discussed titania during in the annealing.previous section Although and ashown detailed in Figure analysis 2, isthis required process in may future govern studies, the morphological the peroxo groups change in PTNTs by the areformation expected of to cleavage inhibit the in thegeneration tubular of titanate oxygen along vacancy the due tube to axis the release (crystallographic of oxygen variationby thermal is decomposition.illustrated in Figure Therefore, 10), which PTNTs results synthesized in the from formation Ti–O–O of complex stacked ionstitanate are alsonanosheets expected during to partially heat remaintreatment. in the Anatase structures crystal due was to their then high formed thermal simultaneously stability. These along results the layered indicated hydrogen that the bottom-uptitanate by processannealing, using thereby ion-complex titania materials, nanoplate which or rod is an structure environmentally that partially friendly contained method, is hydrogen a unique method titanate forcrystal constructing was formed. crystalline Several structure research andgroups morphology [30–32,34] tunable have reported nanostructured the annealing titanate effects and iton has TNTs, the potentialin which toalmost enhance all TNTs the thermal transform stability into a of spherical the nanomaterials. morphology with a clear anatase structure after annealing at 500 °C.

FigureFigure 10.10. HypotheticalHypothetical scheme scheme for for the the transformation transformation of oftitanate titanate nanotubes nanotubes to titania to titania nanoplates. nanoplates. The Thestructure structure mod modelsels presented presented are the are projection the projectionss along along the nanotube. the nanotube. The transformed The transformed structure structure shown shownis the anatase is the anatase phase. phase. 4. Conclusions However, these previous reports are quite different from our present study. The difference in morphologicalIn this study, change the thermal by annealing stability might of peroxo be attributed titanate to nanotubes the peroxo (PTNTs) groups synthesizedin the interlayer by the of peroxoPTNTs. titaniumOur bottom complex-up process ion was using evaluated. the peroxo The-titanium as-synthesized complex structure ion precursor was a was tubular different structure from oftraditional hydrogen methods titanate [4,16 H Ti–18]O HforO the that synthesis contained of nanotubular peroxo groups titania. within During the interlayers this process, of the crystalspecial 2 2 5· 2 structure.peroxo-titani Dehydrationum bonds (Ti and–O thermal–O–) can decomposition be formed directly by annealing within the were interlayers carried out of the in two hydrogen steps: (I)titanate Dehydration crystal as reaction well as of on a waterthe surface molecule of materials on the surface during at crystal an annealing formation temperature without any range chemical below 143treatment◦C, (II) [24] Dehydration. Usually, of the water peroxo and groups –OH groups of TNTs in the are crystal expected and to transition be expelled into byanatase annealing. at an annealing temperature range above 143 ◦C. In step I, the water loss occurred by evaporation, and in step II, dehydration reaction resulted in interstructural agglomeration and shrinkage. In addition, the transition of the interlayer of the crystal from hydrogen titanate to anatase occurred, and at the same time, the tubular structure was dismantled due to the formation of a crystal structure. The as-synthesized PTNTs had the highest surface area and it continued to decrease as the annealing temperature increased. Moreover, as the annealing temperature increased to 360 ◦C, anatase crystal began to form on the structure and the morphology evolved from nanotube to nanoplate-like structure along with the formation of a long axis of the nanotube structure through the transient morphology with stacked leaf-like nanosheets at an intermediate temperature that ranged from 200 ◦C to approximately 400 ◦C. The crystal structure of the nanoplate heated at 500 ◦C was a mixture of hydrogen titanate and anatase. Optical studies showed the extension in bandgap energies upon annealing. In addition, the results from our experiment showed that the presence of peroxo bonds Nanomaterials 2020, 10, 1331 15 of 17

within the interlayer space, which is conﬁrmed to remain even after heat treatment at 500 ◦C in the mixed titanate crystal, was favors its thermal stability more than when the bond exists on the surface. Although the crystallographic properties changed with an increase in temperature above 143 ◦C, the relationship between the structure, morphology, optical properties of peroxo-titanate nanotubes, and the annealing temperature became clear from this study. Therefore, our ﬁndings are quite helpful for the structural design of titanate nanomaterials for various applications based on the synergy of their unique low-dimensional nanostructures and physical-chemical properties. Moreover, our visible light-activated titanate nanostructures with the high thermal stability of peroxo bond can be considered as promising material for photocatalytic and/or photovoltaic applications.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/7/1331/s1: Figure S1: X-ray absorption fine structure (XAFS) of anatase, as-synthesized PTNTs and annealed PTNTs at 200, 300, 400, and 500 ◦C for 1 h, respectively: (a) X-ray absorption near edge structure (XANES) spectra, (b) enlarged pre-edge regions, (c) enlarged white line regions, and (d) Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra. The radial distribution functions were not corrected for phase shift. Author Contributions: Conceptualization, H.P. and T.S.; Formal analysis, H.P., T.G. and S.C.; Funding acquisition, T.S.; Investigation, H.P., T.G. and S.C.; Methodology, H.P. and T.S.; Project administration, T.S.; Resources, S.W.L.; Supervision, T.S.; Writing – original draft, H.P.; Writing – review and editing, T.G., M.K. and T.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Japan Society for the Promotion of Science (JSPS) under the Grants-in-Aid for Scientific Research (S) (grant number 15H05715), and supported by the “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” in “Network Joint Research Center for Materials and Devices” (MEXT, Japan). Acknowledgments: TG-DTA and FT-IR analysis were performed at the Comprehensive Analysis Center, ISIR, Osaka University, Japan. The authors are grateful to T. Takehara (Osaka Univ., Japan) for his support in TG-DTA analysis. Raman investigation of samples was performed at the Sunmoon University, Republic of Korea. The XAFS experiments were performed at Kyushu University Beamline (SAGA-LS/BL06) with the proposal of No. 2019IIK010. The authors are grateful to T. Sugiyama (Kyushu Univ., Japan) and T. Ishioka (Kyushu Univ., Japan) for their support in XAFS measurement. The authors are grateful to Prof. S. Seino for his support with TEM observations. Conflicts of Interest: The authors declare no conflict of interest.

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