Photoelectrochemical H₂ Generation from Suboxide TiO₂ Nanotubes: Visible‐Light Absorption versus Conductivity

Abstract

In the present work we report on the key factors dictating the photoelectrochemical (PEC) performance of suboxide titania (TiO ) nanotubes. TiO nanotubes were produced by a systematic variation of reduction heat treatments of TiO in Ar/H . The properties of the TiO tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity, reflectivity measurements, photocurrent spectroscopy, and photoelectrochemical hydrogen evolution. In line with earlier literature, these suboxide tubes show a drastically improved photoelectrochemical water-splitting performance compared to non-reduced anatase TiO tubes. In this work we show that the key improvement in water-splitting performance is due to the strongly improved conductivity of TiO semimetalic tubes, reaching 13.5 KΩ per tube compared to 70 MΩ (for non-reduced anatase), and is not due to the enhanced visible-light absorbance.

Visible‐Light Direct Conversion of Ethanol to 1,1‐Diethoxyethane and Hydrogen over a Non‐Precious Metal Photocatalyst

“…Converting water into renewable hydrogen (H 2 ) fuel by photocatalytic water splitting, inspired by natural photosynthesis, has been considered as a promising avenue to settle the energy and environmental issues, attracting enthusiastic attention in recent decades . However, a sacrificial agent (e.g., methanol and ethanol) needs to be added to improve the efficiency of solar‐to‐H 2 conversion in most reaction systems . The main function of these sacrificial agents is capturing photo‐generated holes to accelerate water oxidation kinetics.…”

“…[11][12][13][14][15] However,asacrificial agent (e.g.,m ethanol and ethanol) needs to be added to improvet he efficiency of solar-to-H 2 conversion in most reactions ystems. [16][17][18][19][20][21][22] The main function of theses acrificial agents is capturing photogenerated holes to accelerate water oxidationk inetics. Unfortunately,m osto ft he sacrificial agent is oxidized to CO or CO 2 ,w hich is harmful to the environment.…”

Electrochemically Faceted Bamboo‐type TiO2 Nanotubes Provide Enhanced Open‐Circuit Hydrogen Evolution

“…The secondary but apparent resonance line at g ≈1.984 can be attributed to Ti 3+ dislocated from the regular position (interstitial) ,,. Liu et al .…”

“…The former resonance signal is ascribed to Ti 3+ centers located in regular lattice positions. These Ti 3+ states generally form electronic states within the band gap and are responsible for visible‐light absorption . Obviously, it can be seen that the intensity of this Ti 3+ signal ( g ≈1.998) of FR‐TiO 2 is higher than that of PR‐20 TiO 2 , which indicates FR‐TiO 2 NTAs prepared by 3000 s reduction carries a higher amount of Ti 3+ defects in the bulk of the TiO 2 .…”

“…The secondary but apparent resonance line at g % 1.984 can be attributed to Ti 3 + dislocated from the regular position (interstitial). [18,41,42] Liu et al [18] and Zhou et al [42] found that such off-regular position Ti 3 + defects act as intrinsic co-catalytic centers that enable photocatalytic hydrogen generation activity under open-circuit conditions. Therefore, the broad EPR signal is considered to originate from a similar type of (catalytically active) Ti 3 + defect.…”