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

Abstract

Rutile TiO 2 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 structuredirecting 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 AgNO 3 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 PbO 2 on the nanorods revealed that the {110} or {111} facet acted as reductive or oxidative sites. For comparison, near-spherical TiO 2 nanoparticles were synthesized by a sol−gel method. Furthermore, using glycolic acid as the structure-directing agent, we synthesized small rutile TiO 2 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 TiO 2 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.

Photochromism of TiO2 Nanoparticles Doped with Molybdenum Ions: Neutral Color Change from Colorless to Black via Gray

“…To enhance the reduction of Mo 6+ to Mo 5+ to increase the number of the color centers, the photogenerated holes should be consumed effectively to suppress the recombination with the photogenerated electrons. The oxidation of water to evolve oxygen proceeds slowly and is known as the rate-determining step for water splitting on TiO 2 photocatalysts. , Therefore, we added EG as a sacrificial donor to consume the photogenerated holes. TiO 2 ( h + + e − ) + EG → k 3 TiO 2 ( e − ) + EG oxidized TiO 2 ( e − ) + Mo 6 + → k 4 Mo 5 + …”

“…The oxidation of water to evolve oxygen proceeds slowly and is known as the rate-determining step for water splitting on TiO 2 photocatalysts. 35,36 Therefore, we added EG as a sacrificial donor to consume the photogenerated holes.…”

Cocatalyst Modification of Rutile TiO2 Nanorods for Photocatalytic Water Oxidation

“…For comparison, NR(LA) was synthesized using 3.0 mol/L LA instead of 1.6 mol/L GA and heated at 200 °C for 76 h to synthesize 100% rutile. 14 Scheme S1 shows the preparation method of TiO 2 loaded with cocatalysts. The NR(GA) modified with 1.0 wt % Pt (denoted as 1.0Pt-NR(GA)) was synthesized by photoreductive deposition.…”

“…The rutile TiO 2 nanorods synthesized with LA (NR(LA)) possess the length of 843 ± 315 nm and the width of 52 ± 22 nm (r AR = 16.2) 14 whereas NR(GA) shows the length of 297 ± 103 nm and the width of 33 ± 10 nm (r AR = 9). 13 However, their photocatalytic activities for the water oxidation to evolve O 2 were lower than those of rutile TiO 2 nanoparticles synthesized by the sol−gel method, 14 indicating no advantage using the nanorod shape. In this paper, we examined the effect of cocatalysts to improve the photocatalytic activities of NR(GA) and NR(LA).…”

Microflowery, Microspherical, and Fan-Shaped TiO2 Crystals via Hierarchical Self-Assembly of Nanorods with Exposed Specific Crystal Facets and Enhanced Photocatalytic Performance

“…Since Fujishima and Honda first discovered the photocatalytic water-splitting reaction in 1972, TiO 2 has become the most suitable and promising semiconductor material in the application of water splitting, dye-sensitized solar cells, Li-ion batteries, gas sensors, etc., because of its advantages of high physical and chemical stability, non-toxic, harmlessness, environmental friendliness, and low price [1][2][3][4]. The design and synthesis of TiO 2 with different polymorphs, morphologies, and exposed facets are key to effectively improving their practical application in the field of photocatalysis, as its photocatalytic activities depend critically on the crystal phase, morphology, size, surface area, heterojunction structure, and exposed facets of TiO 2 [5][6][7][8]. Among the four polymorphs of TiO 2 (i.e., anatase, rutile, brookite, and TiO 2 (B)) that mainly exist in nature [9][10][11], anatase usually exhibits the highest photocatalytic activity due to the most increased electron mobility and the lowest photogenerated electron-hole recombination in anatase and the rapid interaction between many organic molecules and anatase surfaces [2].…”