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. 2021 Nov 15;6(47):31557–31565. doi: 10.1021/acsomega.1c04003

One-Pot Synthesis of Long Rutile TiO2 Nanorods and Their Photocatalytic Activity for O2 Evolution: Comparison with Near-Spherical Nanoparticles

Suzuko Yamazaki †,*, Masanari Kutoh †, Yukari Yamazaki †, Nanami Yamamoto †, Mamoru Fujitsuka ‡
PMCID: PMC8637597  PMID: 34869981

Abstract

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Rutile TiO2 nanorods with lengths greater than 600 nm and aspect ratios greater than ca. 16 were synthesized through a one-pot hydrothermal method using lactic acid (LA) as a structure-directing agent. Under the hydrothermal treatment at 200 °C, the LA concentration higher than 1.6 mol dm–3 and the hydrothermal time of 72 h were needed to obtain 100% rutile nanorods. The length and the width of the nanorods increased with the increasing LA concentration. The photocatalytic activity of the synthesized nanorods was evaluated for the oxygen evolution in aqueous AgNO3 solutions under ultraviolet irradiation. Calcination of the synthesized nanorods at 400 °C was required to decompose residual organic compounds on the surface and improve the oxygen evolution. The highest oxygen evolution rate was obtained with the nanorods after being calcined at 800 °C. It is worth noting that the nanorods retained their shape (aspect ratio of 8.8) at 800 °C. Selected area electron diffraction patterns indicated that the side or the end surface of the nanorods was attributable to the {110} or {111} facet, respectively. Deposition of Pt or PbO2 on the nanorods revealed that the {110} or {111} facet acted as reductive or oxidative sites. For comparison, near-spherical TiO2 nanoparticles were synthesized by a sol–gel method. Furthermore, using glycolic acid as the structure-directing agent, we synthesized small rutile TiO2 nanorods (aspect ratio of 9) and changed the shape to near-spherical (aspect ratio of 1.3) by calcining at 800 °C. Time-resolved diffuse reflectance spectra were measured to determine the lifetime of the photogenerated electrons. The photocatalytic activity of the nanorods was much lower than that of the near-spherical TiO2 nanoparticles. However, the nanorods synthesized with LA are useful as catalyst support or platforms for various applications because of their unique morphology and high heat resistance.

1. Introduction

Titanium dioxide (TiO2) is one of the most studied semiconductors as a photocatalyst and an anode in photoelectrochemical water splitting. However, TiO2 has an inherent disadvantage such as fast recombination of photogenerated electrons and holes. Their efficient separation, followed by fast transportation toward the TiO2 surface, is a key point to improve the performance of the desired redox reaction.

Recently, one-dimensional structures such as nanorod, nanotube, and nanowire have attracted much attention due to their unique morphology and physical property.1−4 Especially, vertically aligned rutile TiO2 nanorods on fluorine-doped tin oxide glass have been used in various photoelectrochemical applications because of a large contact area with an electrolyte and good separation of the photogenerated charge carriers by applying bias potentials.5−12 In hybrid nanocrystals of semiconductors decorated with metal nanoparticles, rod-shaped semiconductors are utilized because the longitudinal electron delocalization and the size and the number density of the metal nanoparticles play important roles in photocatalytic hydrogen production.13−15 Crystal facet engineering is one of the strategies to improve the photocatalytic activity of semiconductors.16 Different surface energy levels of crystal facets on the rutile TiO2 nanorods facilitate the spatial separation of redox sites, resulting in the improvement of photocatalytic activity. Yang et al. reported that the exposed crystal facets of {110} on the side and those of {111} on both ends of the rutile TiO2 nanorods acted as the reduction and oxidation sites, respectively.17 Djokić et al. reported that an increased {111}/{110} surface ratio of the rutile TiO2 nanorods improved the photocatalytic activity for the degradation of textile dyes.18 Du et al. synthesized the {110}-faceted rutile TiO2 nanocrystals with nanorods and nanoflower morphologies.19 Zhang et al. synthesized mesomorphic ceramic films with uniaxially orientated structures via lyotropic self-assembly of anatase TiO2 nanorods.20 At present, the TiO2 nanorods are used as catalyst support. Abbas et al. synthesized Ni- and Co3O4-supported anatase TiO2 nanorod catalysts for the steam reformation of phenol.21 Sandoval synthesized anatase TiO2 nanotubes deposited with gold particles for CO oxidation.22 Rodríguez-Aguado et al. prepared rutile nanorods and anatase-deformed nanorods by acidic and basic hydrothermal methods, respectively, and demonstrated that the rutile nanorods allowed a regular distribution of gold nanoparticles, resulting in highly selective oxidation of CO to CO2.23

Mamakhel et al. reported on a facile synthesis of rutile TiO2 nanorods by a hydrothermal method using an aqueous solution of glycolic acid (GA).24 We have previously reported the effect of organic additives on the synthesis of TiO2 nanorods by the hydrothermal method and clarified that α-hydroxy acids were good structure-directing agents to synthesize rutile TiO2 nanorods.25 In the hydrothermal treatment at 200 °C for 12 h, the rutile TiO2 nanorods with the length of 297 ± 103 nm and the width of 33 ± 10 nm (aspect ratio of 9) were formed with GA. On the other hand, when lactic acid (LA) was used as the structure-directing agent, the hydrothermal time of 96 h was needed to synthesize the rutile nanorods, and the obtained nanorods had a much longer length (aspect ratio of 12) than that synthesized with GA. The nanorods of different lengths and widths prepared by the facile synthesis will be useful as support or platforms for various applications. Usually, the TiO2 nanorods are calcined at high temperatures to synthesize a composite with other semiconductors and to load metal oxide nanoparticles as a cocatalyst. However, the rutile nanorods synthesized with GA become thick and deform into a round form when calcined at 400 °C. After the thermal treatment at 800 °C, only nanoparticles with a near-spherical shape are obtained.26

In this paper, we synthesized the rutile nanorods with various lengths and widths using LA and examined the effect of the thermal treatment to clarify the conditions under which the rod shape was retained. Furthermore, the photocatalytic activity of the synthesized nanorods was evaluated for water oxidation to evolve oxygen, which is the rate-determining step for the photocatalytic splitting of water into hydrogen and oxygen. As a sacrificial electron acceptor to consume the photogenerated electrons, AgNO3 was used. We compared the photocatalytic activity of the nanorods synthesized with LA and that of the nanoparticles that were obtained by calcining the nanorods synthesized with GA. A schematic representation of the synthetic routes of these samples is shown in Scheme 1. For comparison, the rutile TiO2 nanoparticles were synthesized by a sol–gel method. Our results give the facile method to prepare long rutile TiO2 nanorods and provide insight into the factors affecting the photocatalytic oxygen evolution by the TiO2 photocatalyst.

Scheme 1. Synthetic Routes of TiO2 Used in This Study.

Scheme 1

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2. Results and Discussion

2.1. Synthesis of Nanorods and Photocatalytic Oxygen Evolution

The effect of the LA concentration on the morphology was examined under the hydrothermal time of 72 h. The X-ray diffraction (XRD) patterns indicated that only in the case of 1.2 mol dm–3 LA, small peaks were observed at 2θ = 25.3 and 30.8°, which were attributable to anatase (101) and brookite (121), respectively (Figure S1). The Rietveld analysis indicated that TiO2 synthesized with 1.2 mol dm–3 LA contained 4.7% anatase, 8.7% brookite, and 86.6% rutile. All other TiO2 synthesized with more than 1.6 mol dm–3 LA were 100% rutile nanorods. Their transmission electron microscopy (TEM) images are shown in Figure S2. Table 1 lists the average length, average width, aspect ratio, and Brunauer–Emmett–Teller (BET) specific surface area (SSA) of the synthesized nanorods. With an increase in the LA concentration from 1.6 to 3.0 mol dm–3, both length and width of the rutile nanorods tend to increase, although no significant difference is observed in the lengths of the samples prepared with 2.4 and 3.0 mol dm–3. This fact suggests that a higher concentration of LA is more beneficial to the growth of the rutile nanorods. Therefore, we tried to reduce the time of the hydrothermal treatment using 3.0 mol dm–3 LA. As shown in the XRD patterns (Figure 1a) and the composition of the crystal phase (Tables 2), 79.2% anatase and 20.8% rutile coexist at the hydrothermal time of 12 h. No brookite was detected. The anatase fraction decreases with an increase in the hydrothermal time, and 100% rutile is obtained at the hydrothermal time of 72 h. The SSA decreased with an increase in the rutile fraction (Table S1). The average crystallite size of anatase and rutile, which are estimated by analyzing the XRD peaks attributable to anatase (101) and rutile (110) using the Scherrer equation, are also listed in Table 2. Figure 1b,c shows the TEM images of TiO2 synthesized at the hydrothermal time of 12 and 72 h, respectively. In the former case, many nanoparticles less than 10 nm were observed around the rod and might be anatase because the crystallite sizes of anatase or rutile were 6 and 27 nm, respectively. By increasing the hydrothermal time, the nanoparticles on the nanorods might disappear via the Ostwald ripening mechanism where small particles are dissolved and redeposited on the growing rod.24,27 Our data suggest that the hydrothermal time of 72 h is required to obtain the 100% rutile nanorods even at the LA concentration of 3.0 mol dm–3.

Table 1. Average Length, Width, and Aspect Ratio of the Synthesized Nanorods.

[LA] (mol dm–3) length (nm) width (nm) aspect ratio SSA (m2 g–1)
1.6 628 ± 214 36 ± 14 17.3 20.4
2.0 667 ± 222 40 ± 15 16.9 23.6
2.4 854 ± 316 44 ± 16 19.3 21.4
3.0 843 ± 315 52 ± 22 16.2 20.8
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Figure 1.

Figure 1

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(a) XRD patterns of TiO2 synthesized with 3.0 mol dm–3 LA and at various hydrothermal times. TEM images of TiO2 synthesized at the hydrothermal times of 12 h (b) and 72 h (c).

Table 2. Composition of Crystal Phase and Crystallite Size.

hydrothermal time (h) anatase (%) rutile (%) crystallite size of anatase (nm) crystallite size of rutile (nm)
12 79.2 20.8 6 27
24 36.3 63.7 7 32
48 6.6 93.4 9 44
72 0 100   38
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The photocatalytic O2 evolution was conducted on the rutile nanorods to examine the effect of the LA concentration. As shown in Figure 2a, the amount of O2 evolved increases almost linearly with an increase in the irradiation time after an induction period of 17.6–30 min. Thermogravimetric and differential thermal analysis (TG-DTA) curves of the rutile nanorods synthesized with 3.0 mol dm–3 LA indicated that the weight decreased gradually with exothermic peaks at 220 and 380 °C (Figure S3). These results suggest that some organic compounds originating from LA remain on the surface of the nanorods. Chen et al. reported that LA adsorbed on the TiO2 powder (Degussa P25) as lactate and 2-oxy-propionic acid and decomposed to propionate and acetate at temperatures higher than ca. 250 °C.28Figure 2b shows that after calcination at 400 °C, no induction period is observed for the O2 evolution. The O2 evolution rate was estimated from the initial slope in Figure 2b where the effect of the Ag deposition was negligible and plotted against the LA concentration in Figure 2c, indicating that the nanorods synthesized with 3.0 mol dm–3 LA exhibit a higher O2 evolution rate by a factor of 1.2–1.4 than other LA concentrations.

Figure 2.

Figure 2

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Oxygen evolution on the rutile nanorods synthesized with various LA concentrations: (a) as-prepared sample and (b) sample calcined at 400 °C. (c) Effect of the LA concentration on the O2 evolution rate.

2.2. Effect of Thermal Treatment

The rutile nanorods synthesized with 3.0 mol dm–3 LA were calcined at 400–800 °C. The intensity of the XRD peaks increased with the increase in the calcination temperature, suggesting the growth of rutile crystallites (Figure S4). On the other hand, the BET specific surface area decreased, i.e., 20.8, 18.8, 16.4, 13.3, and 8.5 m2 g–1 after calcination at 400, 500, 600, 700, and 800 °C, respectively. The X-ray photoelectron spectroscopy (XPS) survey spectra revealed that the nanorods were composed of Ti, O, and a small amount of carbon (Figure 3a,b). The presence of carbon originated from the ambient atmosphere when the pellets of the nanorods were prepared for the measurements. The XPS spectra of the as-prepared nanorods indicate that the Ti 2p3/2 and Ti 2p1/2 peaks were located at 458.7 and 464.3 eV, respectively, which were ascribed to Ti4+ of TiO2, and the O 1s peak appeared at 529.9 eV, which was attributed to lattice oxygen.29,30 It is worth noting that the spectral shape and the peak positions hardly changed due to calcination at 400–800 °C (Figure 3c,d). The effect of calcination temperature on the O2 evolution rate was examined (Figure S5). Figure 4 indicates that the O2 evolution rate is the lowest for the sample prepared at 600 °C and the highest for the sample prepared at 800 °C. The TEM images show that these samples retained their rod shape even after calcination at 800 °C, although the surface seemed to be rounded at 700 and 800 °C (Figure S6 and Table 3).

Figure 3.

Figure 3

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XPS survey spectra of the nanorods (a) before and (b) after calcination at 800 °C. XPS spectra of (c) Ti 2p and (d) O 1s for the nanorods before and after calcination at 400, 600, and 800 °C.

Figure 4.

Figure 4

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Dependence of the O2 evolution rate of the rutile nanorods on the calcination temperature.

Table 3. Average Length, Width, and Aspect Ratio of the Rutile Nanorods after Calcination.

temperature (°C) length (nm) width (nm) aspect ratio
400 794 ± 258 53 ± 23 15.0
500 566 ± 239 43 ± 22 13.2
600 623 ± 233 46 ± 21 13.5
700 666 ± 259 63 ± 28 10.6
800 703 ± 343 80 ± 26 8.8
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To understand the dependence of the O2 evolution rate on the calcination temperature, time-resolved diffuse reflectance (TDR) spectra of the rutile nanorods after calcination at 400–800 °C are measured. Figure 5a shows the typical TDR spectra, and a decay profile is obtained using the average values of %Abs in the wavelength region of 900–1000 nm. The %Abs increases just after irradiation by a UV laser pulse and then decreases. Figure 5b indicates that the samples calcined at 400–600 °C exhibit almost identical decay curves, whereas those at 700 and 800 °C decay more slowly, and the latter decays the slowest. These findings are coincident with the results that the O2 evolution rate is the highest on the rutile nanorods calcined at 800 °C (LAR-800) and decreases in the following order: 800 °C > 700 °C > 600 °C (Figure 4). On the other hand, the O2 evolution rate of the nanorods sintered at 600 °C is the lowest in spite of the similar decay curves as 400 and 500 °C. Figure 6 shows the HRTEM and field-emission scanning electron microscopy (FESEM) images of the as-prepared rutile nanorods before and after calcination at 600 or 800 °C. Other FESEM images are shown in Figure S7. The as-prepared rutile nanorods are quadrangular prism with pyramidal ends. The selected area electron diffraction pattern indicates that the crystal growth occurs in the [001] direction with the side or the end surface, which is parallel to {110} or {111}, respectively, and the observed lattice fringes are shown in Figure S8. When samples are calcined at 600 °C, some small pieces are also observed and the shape of the end surface of the nanorods is different from those of other samples. Some exhibit a truncated structure and others show the collapse of the pyramidal shape. However, when the samples are calcined at 700 or 800 °C, the shape of the end surface becomes pyramidal (Figure S7), suggesting that the {111} facet might grow again at the end. The rutile {110} and {111} facets are generally considered as the reductive and oxidative sites, respectively.17,31−34 Therefore, the slowest O2 evolution rate observed with the nanorods sintered at 600 °C might be due to the disappearance of the {111} facet available for water oxidation. However, it remains unclear why the end surface of the nanorods is collapsed by calcination at 600 °C.

Figure 5.

Figure 5

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(a) Typical TDR spectra of the rutile nanorods after calcination at 500 °C. From top: 2, 4, 9, 19, 49, 104, 204, 304, 404, and 504 ps. (b) Decay profiles of the rutile nanorods calcined at 400–800 °C.

Figure 6.

Figure 6

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HRTEM images with the selected area electron diffraction patterns (a–c) and FESEM images (d–f) of the as-prepared sample (a, d) and samples calcined at 600 °C (b, e) and 800 °C (c, f).

2.3. Comparison of Nanorods with Spherical Nanoparticles

As previously reported, the rutile nanorods prepared hydrothermally with GA (hereafter denoted as GARod) changed their morphology by calcination.26 After the thermal treatment at 800 °C, nanoparticles with a near-spherical shape were obtained (GAS-800; Figure 7c). The O2 evolution and the decay curves of the photogenerated electrons of the following samples are compared: Sol–gelSphere, which was synthesized by the sol–gel method, followed by the calcination at 800 °C; GAS-800; LAR-800; and GARod (Figure 7a,b). The initial O2 evolution rates for these samples were estimated to be 77.2, 75.9, 35.9, and 28.0 μmol h–1, respectively. There is no significant difference between Sol–gelSphere and GAS-800, and their O2 evolution rates are almost twice high as that of LAR-800. The O2 evolution rate of GARod is the lowest. On the other hand, the longest lifetime of the photogenerated electron, i.e., the slowest decay curve, is obtained for Sol–gelSphere and the decay curves of GAS-800 and LAR-800 are almost identical. The slowest decay profile is obtained for GARod. The decay profile of Sol–gelSphere can be analyzed by a single exponential curve fitting, and the determined lifetime of the photogenerated electron is 609 ps. Bian et al. reported that the concentration of the photogenerated electrons in TiO2 decreased in a multiexponential fashion.35,36 This inconsistency might be due to our short measurement time of 1.0–500 ps. The decay profiles of three other samples can be fitted by two-exponential functions as follows: % Abs(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2) + constant, where τ1 and τ2 are the lifetimes and A1 and A2 are pre-exponential parameters. There might be slower decay components with lifetimes on the order of nanoseconds. The obtained lifetimes are summarized in Table 4. Yamakata et al. reported that the electron traps in rutile TiO2 powder are much deeper than those in anatase, and most of the free electrons are quickly trapped within a few picoseconds.37,38 Therefore, the slow decay observed with Sol–gelSphere (τ = 609 ps) might be due to the recombination between the trapped electrons and the photogenerated holes. Because two-component decay curves were obtained for LAR-800, GAS-800, and GARod, the electron trap sites in these samples might be different from those in Sol–gelSphere. Both τ1 and τ2 values are in the following order: GAS-800 > LAR-800 > GARod. In TiO2 photocatalysis, the deposition of Pt or PbO2 gives information on whether the exposed crystal facets act as reductive or oxidative sites.39−41 Photogenerated electrons and holes are transferred to reductive and oxidative sites, respectively, due to a built-in electric field to suppress the recombination. Previously, we demonstrated that the crystal facets for oxidation and reduction were spatially separated on Sol–gelSphere and GAS-800 but Pt and PbO2 were distributed homogeneously over the entire surface of GARod.26 Therefore, the short lifetimes of GARod (26, 271 ps) are understandable because the recombination of the photogenerated electrons and holes occurs easily. In contrast to GARod with a rod shape, TEM mapping of Pt and PbO2 deposited on LAR-800 indicates that they are distributed separately, i.e., Pt and PbO2 are detected, respectively, on the side (the {110} facet) and the end (the {111} facet) of the nanorods (Figure 8). This might be the reason why the lifetimes of the photogenerated electrons in LAR-800 are longer than those in GARod. On the other hand, the O2 evolution rate of the LAR-800 nanorods is much lower than that of the near-spherical nanoparticles, Sol–gelSphere or GAS-800. This finding suggests that the number of photogenerated holes that reach the {111} facet of LAR-800 to oxidize water molecules is small. As described above, the length and the width of LAR-800 are estimated to be 703 ± 343 and 80 ± 26 nm, respectively. In contrast to the photogenerated electrons that are transferred to the exposed {110} facet, the photogenerated holes in LAR-800 should move a longer distance to reach the {111} facet at both ends. In addition, the surface area of the exposed {111} facet is relatively small and thus the consecutive four-electron transfer, which is needed to evolve O2 from water molecules, may not occur or do so with difficulty. As another possible explanation for the low O2 evolution, the energy level of the electron trap sites in LAR-800 might be different from that in the near-spherical nanoparticles.

Figure 7.

Figure 7

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Comparison of LAR-800 with GARod, Sol–gelSphere, and GAS-800. (a) O2 evolution, (b) decay profiles, and (c) TEM images of GAS-800.

Table 4. Lifetimes of the Photogenerated Electrons.

  τ1/ps (A1) τ2/ps (A2)
Sol–gelSphere 609 (100%)
LAR-800 91 (11%) 488 (89%)
GAS-800 125 (22%) 586 (78%)
GARod 26 (53%) 271 (47%)
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Figure 8.

Figure 8

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TEM images of LAR-800 deposited with (a) only Pt and (b) Pt, followed by the deposition of PbO2. (c) Enlarged view of the Pt deposition in the circled area in (a). Element mappings of Ti (d) and Pb (e) in the squared area in (b).

In this paper, the photogenerated electrons are consumed for the reduction of Ag+ ions. Since O2 evolution is the rate-determining step in splitting of water to generate hydrogen and oxygen simultaneously, the O2 evolution should be enhanced to improve the overall efficiency. Our data indicate that near-spherical TiO2 particles such as Sol–gelSphere and GAS-800 (aspect ratio of 1.3) exhibit higher activity than the nanorods LAR-800 (aspect ratio of 8.8) and GARod (aspect ratio of 9).

Similarly, Fu et al. reported that the overall photocatalytic water splitting activity increased with the decrease in the aspect ratio of rutile TiO2 nanorods on which 1 wt % Pt was deposited.42 Therefore, larger {111} facets might be required for the photocatalytic application where the oxidation by the photogenerated holes is the rate-determining step.

3. Conclusions

The rutile TiO2 nanorods with lengths greater than 600 nm were synthesized through a one-pot hydrothermal method. The length and width were varied by changing the LA concentration during the hydrothermal treatment at 200 °C for 72 h. During the calcination process, the aspect ratio of the synthesized nanorods decreased from 15.0 at 400 °C to 8.8 at 800 °C. Even at 800 °C, the rod shape was retained. Therefore, the synthesized rutile nanorods are beneficial for developing the composite with other semiconductors or metals and for the application as catalyst support, which requires thermal treatment at high temperatures. On the other hand, the photocatalytic activity of the nanorods was lower than that of the near-spherical TiO2 nanoparticles for the O2 evolution from water. This might be ascribed to the fact that the exposed {111} facets acting as the oxidative sites are small. The synthesized nanorods might be promising for the reduction reaction because lateral {110} facets acting as the reductive sites are large. We are currently conducting research to estimate the photocatalytic activity of the synthesized nanorods for the reductive deposition of metal ions.

4. Experimental Section

4.1. Synthesis

The rutile TiO2 nanorods were synthesized by the hydrothermal method in the presence of LA (HO(CH3)CHCO2H). Briefly, approximately 5.0 mL of titanium tetraisopropoxide (Ti(OC3H7)4: TTIP) and 5.0 mL of 2-propanol were poured into 50 mL of aqueous LA solution. The resulting suspension was heated under vigorous stirring till a homogeneous solution was formed. The obtained solution was sealed in a Teflon-lined stainless steel autoclave (100 mL) and heated at 200 °C for 12–72 h. After the solution was cooled to room temperature, TiO2 powders were separated in a centrifuge tube. The powders were washed with ethanol three times and collected by filtration, followed by drying at room temperature. The final sintering process was conducted by heating at 400–800 °C for 3 h with a ramping rate of 3 °C min–1. We also synthesized the rutile nanorods using GA as the structure-directing agent26 and then calcined at 800 °C to prepare the near-spherical nanoparticles (Scheme 1). For comparison, the rutile TiO2 nanoparticles were prepared by calcining the TiO2 powder at 800 °C, which was synthesized by the sol–gel method as reported previously.39

4.2. Characterization

The XRD analysis was carried out using MiniFlex600 (Rigaku, Japan) with Cu Kα radiation. The crystalline phase compositions of TiO2 were evaluated by Rietveld analysis with PDXL software (Rigaku, Japan) and the Scherrer equation was used to estimate the crystallite size. The TEM images were obtained using JEM-2100 (JEOL, Japan) at an acceleration voltage of 200 kV. The average length and width of the synthesized nanorods were estimated from the TEM images of 50 randomly selected samples. The FESEM images were observed using JSM-7600F (JEOL, Japan). Thermogravimetric and differential thermal analysis was performed on TG-DTA8122 (Rigaku, Japan) from room temperature to 990 °C at a heating rate of 20 °C min–1. The specific surface areas of the synthesized nanorods were determined using a TriStar II 3020 analyzer (Micromeritics) and the BET equation. XPS (Thermo Scientific K-α) was used to analyze the valence states of the synthesized nanorods. The nanorods powder was placed in Clear Disk (JASCO, CD-05) and pressed into a pellet with 5 mm diameter using a hand press. The binding energies were calibrated with reference to the C 1s peak (284.8 eV) originating from the surface impurity carbons. The TDR spectra were measured using a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz) pumped by a Nd:YLF laser (Spectra-Physics, Empower 15). The seed pulse was generated by a titanium sapphire laser (Spectra-Physics, Mai Tai VFSJW, FWHM 80 fs). The output of the optical parametric amplifier (Spectra-Physics, OPA-800CF) was used as the excitation pulse (365 nm, 4 μJ pulse–1). A white light continuum pulse, which was generated by focusing the residue of the fundamental light on a sapphire crystal, was used as the probe light. Details of the TDR measurement have been reported previously.25,35,36 The synthesized sample was dispersed in ethanol by ultrasonication and then spread on a glass coverslip. The probe and reference lights were both directed at the glass coverslip coated with the sample and the reflected lights were detected by a linear InGaAs array detector with a polychromator (Solar, MS3504). The %absorption (%Abs) is defined as (R0 – R)/R0 × 100 where R0 and R indicate the intensities of the diffuse reflected monitor light with and without excitation, respectively.

4.3. Evaluation of Photocatalytic Activity

After argon gas was purged for 1 h through aqueous AgNO3 solutions (0.05 mol dm–3, 150 mL) containing the synthesized TiO2 samples (0.3 g), a 250 W super-high-pressure Hg lamp (Ushio Inc., Japan) was irradiated through a U330 bandpass filter (HOYA CANDEO OPTRONICS, Japan). The light intensity was measured using UIT201 (Ushio Inc., Japan) of 17 mW cm–2 at 365 nm. The reaction temperature was maintained at 30 °C and vigorous stirring with a magnetic stirrer was conducted throughout the experiments. The amount of evolved O2 was determined by gas chromatography with a thermal conductivity detector (Shimadzu, Japan, GC-8A with MS-5A column).

Acknowledgments

This work was partially supported by JSPS KAKENHI Grant No. 18K05298 and performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. We thank Yamaguchi University Science Research Center and Innovation Center for the SEM, TEM, and XRD measurements. We appreciate Mr. T. Tonosaki for the technical support on the XPS measurements at Collaborative Center for Engineering Research Equipment, Faculty of Engineering, Yamaguchi University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04003.

  • SSA of the samples synthesized at various hydrothermal times; XRD patterns and TEM images of the nanorods synthesized with various LA concentrations; TG-DTA curves; O2 evolution on the nanorods calcined at various temperatures; XRD patterns; TEM images and FESEM images of the nanorods calcined at various temperatures; and HRTEM images with lattice fringes (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04003_si_001.pdf (795.8KB, pdf)

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