Abstract
Anodic TiO2 nanotube (NT) array is promising for the flexible and efficient photoanode in photoelectrochemical water splitting (PECWS) cell. However, the photocurrent response of pristine anodic TiO2 NT photoanode in literature has a ca 50 times difference, viz. from 0.05 to 1 mA/cm2. Improvement of the pristine TiO2 NT is the base for achieving a high efficient anode. Here, we examine the size effect in different scale on the PECWS performance with manipulating the macro size of the Ti foil. With decrease of the Ti foil size from 6 to 1 cm2, corresponding to the anodic TiO2 NT growth active area of 3.45 to 0.65 cm2, the photocurrent response increased by 50.6%, achieving 1.13 mA/cm2 at 1.23 VRHE (V versus reversible hydrogen electrode). The Ti foil size also significantly influences the micro sizes of the nanotubes including crystallite size, double wall thickness, inner diameter and tube length, which have profound effects on WS efficiency. The relationships between involved length scales, a span of six orders of magnitude from ten nanometers (10−8 m) to centimeter (10−2 m), and the PECWS efficiency is analyzed and discussed. Transient i-t curves are used to represent the chemical kinetics during the growth of anodic TiO2 NT array. Finally, photon capture scheme is proposed to explain the physics behind the multi length scale effect of the TiO2 NT photoanode. The need of quantitative models during the scale-up of the PECWS process is stressed.
Highlights
As the firstly and mostly reported semiconductor in sunlight-driven water splitting (WS), TiO2 has been intensively employed as the pow der photocatalyst in photocatalytic WS and as the anode semiconductor in photoelectrochemical (PEC) cells. [1] the solar to hydrogen efficiency of TiO2 is limited by its wide bandgap nature
The photocurrent response of the four samples become almost stable after the inflexion points, with the inflexion point of curve a locates at the lowest 0.446 VRHE
The sizes are the numbers read on the Y-axis multiplied by the scale in the parentheses after the term of the factor. Both the crystallite size and the double wall thickness have a scale of 10− 8 m level
Summary
As the firstly and mostly reported semiconductor in sunlight-driven water splitting (WS), TiO2 has been intensively employed as the pow der photocatalyst in photocatalytic WS and as the anode semiconductor in photoelectrochemical (PEC) cells. [1] the solar to hydrogen efficiency of TiO2 is limited by its wide bandgap nature. The reported photocurrent response of the pristine anodic TiO2 NT, viz. Under AM 1.5 G simulated sunlight at 1.6 VRHE, a pristine TiO2 NT array with an average tube length ~8 μm and diameter of 100 nm gave a 0.1 mA/cm photo response in 1 M KOH solution, [18] while another, with a tube length of around 3.6 μm, showed a photocurrent of 0.52 mA/cm in 1 M NaOH solution. Lucas et al [34] coated a SrTiO3 layer on the anodic TiO2 NT surface and further doped the layer with La3+ to form a La:SrTiO3/TiO2 NT junction and obtained a photocurrent response of 0.1442 mA/cm at 1.23 VRHE in a Na2SO3 electrolyte buffered with phosphate at pH = 7.1 under AM 1.5G. The relevance of the sizes in different length scale to the scale up of the anodic TiO2 NT as anode of the PECWS cell is discussed
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