Design Of Hierarchical Structure Photoanodes And Their Photoelectrochemical Water Oxidation Performance And Mechanism

Using solar photons to drive the chemistry of splitting H2O molecules to generate H2 or reduction CO2 to generate hydrogencarbon fuel is one of the most efficient ways to solve the problem of the growing scarcity of traditional energy and the pollution and hazards to the environment. The H2O oxidation reaction is the bottleneck in the process of water splitting, because it is a 4-protons and 4-electrons transport process. As a result, designing a high efficiency photoanode is very important to the photoelectrochemical water splitting system. However, there are several problems that limit the efficiency of photoanode:the slow kinetics of water oxidation reaction on the surface of photoanode, the surface recombination of holes and electrons, and the low separation efficiency of hole-electron in the bulk of photoanode, etc.This work is aiming at the construction of composite photoanodes (core/shell, heterojunctions) and taking advantages of the double metal hydroxides which has the properties like catalytic and absorbance of visible light. The core content in this article is:1. By using a photo-asistented modification and electro-deposition (PMD) method, the double mental hydroxide/TiO2 nanoarrys core/shell photoanode is successfully constructed and decorated by phosphate species on the surface. This PMD method gives a deposition layer with advances like ultra thin (≈10nm), unifrom, and no effect to the absorption of light. The onset potential of photocurrent of this core/shell photoanode shift about 200 mV cathodically and its oxidation efficiency reaches 100% in the range of 1～1.8 V vs RHE. This enhancement of performance is attributed to the phosphate decorated double mental hydroxide shell increases the water oxidation reaction and can surppress the back reaction on the surface of photoanode simultaneously. Thus, this shell can improve the separation of carieers on the surface of photoanode and lead to the increase of oxidation efficiency.2. The CoAl-LDH@BiVO4 composite photoanode consists of inter-connected CoAl-LDH nanosheets network and BiVO4 photoanode is successfully constructed. The 2D CoAl-LDH nanosheets network grows verticle to the BiVO4 particles and FTO substrate. The onset potential of photocurrent of this composite photoanode gives a cathodic shift as high as 610 mV compared with that of bare BiVO4 photoanode. Meanwhile, the IPCE and oxidation efficiency of the former are also higher than that of the latter. The performance improvement first comes from the CoAl-LDH can increase the kinetics of water oxidation reaction when works as a catalyst. Furthermore, the difference of band gaps of CoAl-LDH and BiVO4 can facilitate the utilization of different wavelengths of light. As a result, this work exhibits a composite photoanode that combines catalyst, photo-sensitizer, and 2D constructure.3. A series of BiVO4 photoanode with different particle sizes and morphologies is sythesized by using Zn2+ cation as an additive. The photoanode with morphology modification has a photocurrent density which is more than 3 times of that of the pristine photoanode at the potential of 1.23 V vs RHE. The increase of photocurrent can be attributed to the modificated photoanode’s higher surface roughness and smaller particle size which leading to a better carriers separation in the bulk of photoanode. The Zn2+ is a nontoxic and low-cost additive without effecting photoanode’s composite. Therefore, this work provides a facile and simple way to control the size and morphology of photoanode.