The development of photoelectrochemical processes for pollutants elimination and hydrogen production via water splitting have attracted considerable attention in order to meet the global challenges of environmental pollution and energy demand. The efficiency of photoelectrochemical devices essentially relies on the semiconductor-based photoelectrode design with respect to solar light harvest and conversion. Among various architectures, highly-ordered and vertically-oriented TiO
2 nanotube arrays (NTAs) and TiO
2 nanowire arrays (NWAs) are extensively studied. However, the practical applications of TiO
2 NTAs and TiO
2 NWAs are significantly hindered by their wide band gap (3.0-3.2 eV) and fast charge recombination. Coupling TiO
2 with semiconductors of narrower band gap can effectively expand light absorption range and improve carrier transport in the formed hybrid.
Novel phosphorus-doped graphitic carbon nitride (P-C
3N
4) was first developed to modify TiO
2 NTAs successfully by a facile polymerization in the present study. The prepared P-C
3N
4/TiO
2 heterojunction exhibits enhanced light-absorption characteristics and improved
charge transport ability, thus resulting in a 3-fold photocurrent (1.98 mA cm
-2 at 1.0 V vs reversible hydrogen electrode (RHE)) increase from that of pristine TiO
2 NTAs, giving a H
2 evolution rate of 36.6 μmol cm
-2 h
-1 at 1.0 V vs RHE under solar light illumination.
A structure composed of TiO
2 nanowire arrays (NWAs) was further designed by decoration with graphene-linked graphitic carbon nitride (GCN) layers, serving as a robust photoanode for solar-driven water splitting. Under solar light, the optimal GCN/TiO
2 NWAs photoelectrode produces a photocurrent density of 1.7 mA cm
-2 at 1.23 V vs RHE, around
2.6 times higher than that of pristine TiO
2. This promoted activity can be mainly attributed to the improved charge transport within the heterojunction and enhanced light absorption. Based on the above design, hydrogen-terminated graphene-linked carbon nitride (H-GCN) is presented as an efficient photo-sensitizer to modify TiO
2 nanowire arrays (NWAs). The composite photoanode shows much improved photo-response under solar light, nearly 3.4 times higher (2.4 mA cm
-2 at 1.23 V vs RHE) compared with pristine TiO
2 NWAs under the same condition. The maximum photo-conversion efficiency for the prepared heterojunction reaches up to 1.36%.
In addition, a facile chemical method was explored to optimize the surface reactivity of the as-prepared GCN/TiO
2 NWAs to create oxygen vacancies on the surface of TiO
2 and hydroxyl groups on GCN. The optimized sample yields a photocurrent density of 2.35 mA cm
-2 at 1.23 V vs RHE and a maximum photo-conversion efficiency of 1.25% at a rather low bias of 0.40 V vs RHE under solar light illumination, because of the extended the light absorption region and enhanced charge transport.
To exploit the full potential of visible light, a three-dimensional integrated heterojunction with highly interconnected porous structure, comprising of GCN nanosheets and bismuth vanadate (BiVO
4) nanowall arrays, was successfully synthesized. After deposition of cobalt phosphate as water-oxidation electrocatalyst, a plateau photocurrent of 3.40 mA cm
-2 (1.23 V vs RHE) with an optimized photo-conversion efficiency of 1.28% was achieved.
Finally, an integrated system was carefully structured with nitrogen-doped carbon protected ultrathin Cu
2O (NC/Cu
2O) NWAs as the photocathode and branched TiO
2 (Bh-TiO
2) NWAs as the photoanode to realize unassisted solar driven pollutants removal and water splitting. A remarkable short circuit photocurrent of 0.6 mA cm
-2 for H
2 production was achieved. Also, 2.2 times and 3.2 times enhanced kinetic rates for simultaneous Cr(VI) reduction and RhB oxidation was obtained in this integrated system, showing a great potential of using unassisted system for artificial photosynthesis. These rational designs based on nanostructured photoelectrodes and promising findings offer new opportunities towards practical photoelectrochemical applications.
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