No abstract

The electronic band structure of a semiconductor photocatalyst intrinsically controls its level of conduction band (CB) and valence band (VB) and, thus, influences its activity for different photocatalytic reactions. Here, we report a simple bottom-up strategy to rationally tune the band structure of graphitic carbon nitride (g-C3N4). By incorporating electron-deficient pyromellitic dianhydride (PMDA) monomer into the network of g-C3N4, the VB position can be largely decreased and, thus, gives a strong photooxidation capability. Consequently, the modified photocatalyst shows preferential activity for water oxidation over water reduction in comparison with g-C3N4. More strikingly, the active species involved in the photodegradation of methyl orange switches from photogenerated electrons to holes after band structure engineering. This work may provide guidance on designing efficient polymer photocatalysts with the desirable electronic structure for specific photoreactions.

Despite the fact that Ta3N5 absorbs a major fraction of the visible spectrum, the rapid decrease of photocurrent encountered in water photoelectrolysis over time remains a serious hurdle for the practical application of Ta3N5 photoelectrodes. Here, by employing a Co3O4 nanoparticle water oxidation catalyst (WOC) as well as an alkaline electrolyte, the photostability of Ta3N5 electrode is significantly improved. Co3O4/Ta3N5 photoanode exhibits the best durability against photocorrosion to date, when compared with Co(OH)x/Ta3N5 and IrO2/Ta3N5 photoanodes. Specifically, about 75% of the initial stable photocurrent remains after 2 h irradiation at 1.2 V vs. RHE (reversible hydrogen electrode). Meanwhile, a photocurrent density of 3.1 mA cm−2 has been achieved on Co3O4/Ta3N5 photoanode at 1.2 V vs. RHE with backside illumination under 1 sun AM 1.5 G simulated sunlight. The reason for the relatively high stability is discussed on the basis of electron microscopic observations and photoelectrochemical measurements, and the surface nitrogen content is monitored by X‐ray photoelectron spectroscopic analysis.

No abstract

3535 www.MaterialsViews.com wileyonlinelibrary.comhave attracted considerable interest because they meet several criteria for efficient PEC water splitting, such as high theoretical solar-to-hydrogen effi ciency (over 15%) and suitable band edge positions for unassisted water splitting.Ta 3 N 5 , [ 8c ] LaTiO 2 N, [ 8a ] and SrNbO 2 N [ 8b ] photoelectrodes prepared by electrophoretic deposition (EPD) methods function as photoanodes for water oxidation. However, considerable water splitting photocurrents have only been achieved with Ta 3 N 5 photoanodes, regardless of the fact that LaTiO 2 N and SrNbO 2 N materials can absorb comparable wavelengths of sunlight and possess similar band edge positions as Ta 3 N 5 . The easier reduction of Ti 4+ and Nb 5+ ions in comparison with Ta 5+ during the high temperature ammonolysis process is considered to produce defects in the bulk and therefore result in less active LaTiO 2 N and SrNbO 2 N powders. [ 9 ] Optimizing the structure and crystallinity of LaTiO 2 N and SrNbO 2 N photoelectrodes may afford desirable improvements in PEC performance, considering that charge carrier transport among the fi lm particles (or layers) depends largely on these parameters. [ 7c ] Increasing photoelectrode crystallinity has been demonstrated to be effective in improving PEC performance by reducing the charge carrier recombination rate. [ 10 ] Effi cient electron and hole transport rely on long-range fi lm continuity and reduced hole transport distance, respectively, which require different fi lm morphological characteristics. [ 11 ] It is desirable but always challenging to obtain highly crystalline interconnected particle fi lm with small particle size. [ 3b ] Despite the above rules to follow and previous work on TaON and Ta 3 N 5 photoanodes to refer to, obtaining highly photo-active LaTiO 2 N photoelectrodes (and more broadly, all the potentially available (oxy)nitride photoelectrodes) is still a great challenge, suggesting that charge transport limitations remain. The goal of this study is to defi ne the limitations present in these (oxy) nitride photoelectrodes, using LaTiO 2 N as an example. Finally, we reveal that the poor electron transport associated with insuffi cient inter-particle connection contributes to the suppressed PEC activities of LaTiO 2 N photoanodes, which cannot be eliminated through necking treatment alone. [ 8c ] Here, the With a theoretical maximum photocurrent of ca. 12.5 mA cm −2 under AM 1.5 G 100 mW cm −2 irradiation, the presently achieved plateau photocurrent of about 0.6 mA cm −2 on bare LaTiO 2 N photoanodes indicates the presence of serious charge transport limitations. Only recently, a plateau photocurrent of about 4 mA cm −2 was achieved on precious IrO 2 modifi ed LaTiO 2 N photoanodes prepared by a rather expensive and complex method, without discussing the intrinsic mechanism of improved photocurrents. In this study, by establishing highly crystalline porous LaTiO 2 N particles and superior interparticle connectivity with reduced density of...

photovoltaic (PV) cell (using, for example, silicon or gallium arsenide), though capable of achieving a high conversion effi ciency, faces the main obstacle of high cost for largescale application because of the requirement of high purity and low defect density.In the past few decades, some novel cells of low cost have been explored to convert solar energy into electrical or chemical energy, such as organic photovoltaic cells (OPV), photoelectrochemical solar cells, and photoelectrochemical solar water splitting cells. [1][2][3][4][5] In these devices, a key limit in a planar structure cell for high effi ciency is the incompatibility between a short diffusion length of minority carriers and a long optical absorption length. To overcome the limit, bulk heterojunction (BHJ) concept is fi rst introduced in an OPV cell, [ 6 ] which consists of an interpenetrating network of donor and acceptor materials. [ 7,8 ] The performance of the BHJ solar cell depends on the morphology very sensitively. A large interface area is usually necessary for high separation effi ciency. In a photoelectrochemical solar cell or water splitting cell, one-dimensional nanostructure (nanowires, nanorods, or nanotubes), [9][10][11][12] branched nanostructure, [ 13,14 ] and porous structures, [15][16][17][18][19] are used to increase light capture and decrease diffusion distance of minority carriers to the interface. In a nanostructured photoelectrochemical cell, electrolyte can penetrate into the bulk of a solid photoelectrode. Therefore, analogically, a nanostructured photoelectrochemical cell can be considered as a solid-electrolyte bulk heterojunction.Generally, higher light absorption and/or charge separation effi ciency are considered as the main reasons for improved performance in a nanostructured cell than a planar structure. However, quantitative analysis and defi nite experimental evidence remain elusive. It is diffi cult to directly compare a nanostructured cell with a planar cell because the controlled samples are usually prepared by different methods or different conditions. Especially, in an OPV, it is diffi cult to investigate the details once the cell is assembled and a solid-solid interface forms. Therefore, it is impossible to identify quantitative contribution of each factor. Only some qualitative conclusions are given in previous studies.In the past few decades, some novel low-cost nanostructured devices have been explored for converting solar energy into electrical or chemical energy, such as organic photovoltaic cells, photoelectrochemical solar cells, and solar water splitting cells. Generally, higher light absorption and/or charge separation effi ciency are considered as the main reasons for improved performance in a nanostructured device versus a planar structure. However, quantitative analysis and defi nite experimental evidence remain elusive. Here, using BiVO 4 as an example, comparable samples with porous and dense structures have been prepared by a simple method. The porous and dense fi lms are assembled into a sol...

No abstract

No abstract