Minimize Charge Recombination Research Articles

Photocorrosion suppression and photoelectrochemical (PEC) enhancement of ZnO via hybridization with graphene nanosheets

Graphene-ZnO nanocomposites with different morphologies (nanoparticles and nanorods) were synthesized by alkali precipitation and solvothermal process. The influence of graphene nanosheets on photocatalytic and photoelectrochemical (PEC) activity of graphene-ZnO nanorods (ZnO/GNR) and graphene/ZnO nanoparticles (ZnO/GNP) was investigated. ZnO/GNR showed greatly increased photocatalytic degradation of methyl orange (MO): 2-, 2.5- and 3.5-fold higher under UV light and 1.6-, 4.7-, and 11-fold higher under visible light than that for ZnO/GNP, ZnO NR and ZnO NP, respectively. In addition, ZnO/GNR showed remarkable photocorrosion suppression (∼0.5%) compared to 10, 35 and 45% for ZnO/GNP, ZnO NR and ZnO NP, respectively. Furthermore, PEC results indicated a remarkable 6-fold enhancement photocurrent density for ZnO/GNR compared to ZnO NR. This enhancement is attributed to the crucial role of graphene in the electron transfer from ZnO NR into graphene sheets increasing the electron lifetime, leading to a minimized charge recombination.

(Invited) Understanding Photoelectrode/Catalyst Interface for Solar Water Splitting

As a potentially low-cost, high-efficiency solar energy storage solution, solar water splitting faces great challenges. One of the issues is the poor catalytic activity and low stability. Applications of co-catalysts have been shown effective to correct the deficiency by promoting desired chemical reactions so as to minimize charge recombination at the surface and to reduce parasitic corrosion reactions. The detailed behaviors of the light absorber/catalyst interface, however, remain poorly understood. Here we present our recent research in this area. We show that the application of the co-catalysts may greatly influence the charge separation behaviors of the photoelectrode. Detailed thermodynamic and kinetic measurements support our understanding. Furthermore, we show that the photoelectrode substrate also exerts great influences on the co-catalyst behaviors. A strong interaction between the photoelectrode and the catalyst can be beneficial. The combined system may also serve as a new platform to understand heterogeneous catalysis such as water oxidation at a level previously inaccessible. The knowledge generated by our work will likely contribute significantly to the development of solar water splitting technology for a future powered by renewable energies.

(Invited) Understanding Photoelectrode/Catalyst Interface for Solar Water Splitting

As a potentially low-cost, high-efficiency solar energy storage solution, solar water splitting faces great challenges. One of the issues is the poor catalytic activity and low stability. Applications of co-catalysts have been shown effective to correct the deficiency by promoting desired chemical reactions so as to minimize charge recombination at the surface and to reduce parasitic corrosion reactions. The detailed behaviors of the light absorber/catalyst interface, however, remain poorly understood. Here we present our recent research in this area. We show that the application of the co-catalysts may greatly influence the charge separation behaviors of the photoelectrode. Detailed thermodynamic and kinetic measurements support our understanding. Furthermore, we show that the photoelectrode substrate also exerts great influences on the co-catalyst behaviors. A strong interaction between the photoelectrode and the catalyst can be beneficial. The combined system may also serve as a new platform to understand heterogeneous catalysis such as water oxidation at a level previously inaccessible. The knowledge generated by our work will likely contribute significantly to the development of solar water splitting technology for a future powered by renewable energies.

Interface Nanojunction Engineering of Electron-Depleted Tungsten Oxide Nanoparticles for High-Performance Ultraviolet Photodetection

This work reports a general and facile route, i.e., thermal decomposition of a precursor followed by ultrafast thermal annealing (TDP-UTA), to the in situ fabrication of a nanojunction-interlinked tungsten oxide nanoparticle (WO3-NP) networks for extraordinary ultraviolet (UV) photodetection. TDP leads a spin-coated ammonium metatungstate thin layer to in situ self-assemble into a highly crystalline WO3-NP mesoporous film on SiO2/Si substrates with prepatterned electrodes. The as-synthesized WO3-NPs have dimensions comparable to the Debye length (≈43 nm), which is critical to the optimal electron-depletion effect for high gain in photodetection. UTA creates the NP–NP interface nanojunctions between neighboring WO3-NPs, which is the key to high-efficiency electron transport with minimized charge recombination in optoelectronic processes. The photodetectors based on such nanojunction-interlinked WO3-NP networks exhibit a photocurrent-to-dark-current ratio of 5600, the highest value for any WOx-based photode...

Conductive FeSe nanorods: A novel and efficientco-catalyst deposited on BiVO4 for enhanced photocatalytic activity under visible light

Developing efficient co-catalyst is an important task in photocatalysis, because co-catalyst can inhibit the recombination of photogenerated charge-carriers in semiconductors and serve as active sites for targeted reactions. Herein we for the first time introduce a superconducting material, β-FeSe, as a noble-metal free co-catalyst, which is deposited onto the surface of BiVO4 for the effective degradation of Rhodamine B (RhB) under visible light irradiation (λ>420nm). Owing to its superior electron conductivity, FeSe nanorods deposited BiVO4with an optimized loading (2wt%) displays 8-time improvement of the RhB degradation rate than BiVO4 alone. The dye molecules can be completely degraded in 20min by adding a low concentration of H2O2. It is confirmed that FeSe nanorods can extend light absorption range, minimize charge recombination rate, and serve as active sites simultaneously, thus improving the photocatalytic performance. This work may provide new insights for developing alternative co-catalysts for photocatalytic applications.

Studies on the efficiency enhancement of co-sensitized, transparent DSSCs by employment of core-shell-shell gold nanorods

In this article, we present studies concerning the efficiency enhancement of dye-sensitized solar cells (DSSC) containing cocktails of organic donor-π-acceptor (D-π-A) dyes (L0 and L1) and a squaraine sensitizer (SQ2) by means of the localized surface plasmon resonance (LSPR) effect induced by gold nanorods (GNRs) embedded within the semiconductor layer. In view of the potential application of DSSC devices for building integration, dye cocktails were selected to maximize transparency in the 500–600nm region, where human eye sensitivity has its peak. Thus, the chosen organic dyes and the squaraine sensitizer had absorption maxima in the 380–410nm region and above 660nm, respectively. Thanks to their specific asymmetry, GNRs with a 3:1 aspect ratio could enhance organic dye absorption due to their transverse resonance component as well as squaraine absorption in the far-red/near-infrared (NIR) spectral range due to their longitudinal resonance component; furthermore, they were sequentially coated with thin layers of SiO2 and TiO2 in order to make them robust enough to withstand thermal film sintering and minimize charge recombination during the photovoltaic operation. Whereas incorporation of GNRs in the photoanode did not improve the efficiency of cells prepared with the L0/SQ2 dye combination, a significant increase of 23% (from 3.50% to 4.32%), comparable to that observed for Ru-sensitizer N719, was observed in the case of the L1/SQ2 dye cocktail.

Construction of unique two-dimensional MoS2-TiO2 hybrid nanojunctions: MoS2 as a promising cost-effective cocatalyst toward improved photocatalytic reduction of CO2 to methanol.

Two-dimensional MoS2 nanosheets were in situ grown on TiO2 nanosheets to form two-dimensional (2D) hybrid nanojunctions, with which MoS2 nanosheets compactly contact with TiO2 to increase the interfacial area. MoS2 was identified as a promising cost-effective substitute for noble metal cocatalysts such as Pt, Au, and Ag, and shows superior activity and selectivity for reducing CO2 to CH3OH in aqueous solution to these metal cocatalysts under UV-vis light irradiation. The photo-luminescence (PL) spectra and transient time-resolved PL decay measurements reveal that the fast electron transfer from TiO2 to MoS2 can minimize charge recombination losses to improve the conversion efficiency of photoreduction. It reveals that Mo-terminated edges of MoS2 nanosheets possess the metallic character and a high d-electron density, and the Mo cation sites may benefit the stabilization of CHxOy intermediates via electrostatic attraction to enhance the CH3OH formation from the reduction of CO2 in aqueous solution.

Light Manipulation in Organic Photovoltaics.

Organic photovoltaics (OPVs) hold great promise for next‐generation photovoltaics in renewable energy because of the potential to realize low‐cost mass production via large‐area roll‐to‐roll printing technologies on flexible substrates. To achieve high‐efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade‐off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano‐structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry‐induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

Essential role of N and Au on TiO2 as photoanode for efficient dye-sensitized solar cells

We firstly report on the successful application of a gold nanoparticles deposited nitrogen doped-titania (Au/N-TiO2) nanocomposite as an efficient photoanode for highly efficient dye-sensitized solar cells (DSSC) with the standard photosensitizer, N719 dye. The Au/N-TiO2 nanocomposites with different Au contents are prepared using a simple chemical reduction method and characterized using various analytical techniques. The DSSC assembled with the Au/N-TiO2 modified photoanode demonstrated an enhanced solar-to-electrical energy conversion efficiency of 7.90% compared to the photoanode of a DSSC composed of bare TiO2 (2.55%) under full sunlight illumination (100mWcm−2, AM 1.5G). This enhanced efficiency is mainly attributed to the doping of N and deposition of Au NPs on the TiO2 surface as a resultant of reduction in the band-gap energy, plasmonic effect improved interfacial charge transfer process and minimized charge recombination. The influence of Au content on the overall energy conversion efficiency is also investigated, and the optimum Au content for N-TiO2 is found to be 10mM. The enhanced solar energy conversion efficiency demonstrated by the Au/N-TiO2 nanocomposite makes it a promising alternative to conventional photoanode-based DSSCs.

Surface Architectures Built around Perylenediimide Stacks

Simple stacks of perylenediimides (PDIs) grown directly on solid surfaces are an intriguing starting point for the construction of multicomponent architectures because their intrinsic activity is already very high. The ability of PDI stacks to efficiently generate photocurrent originates from the strong absorption of visible light and the efficient transport of both electrons and holes after generation with light. The objective of this study was to explore whether or not the excellent performance of these remarkably simple single-channel photosystems could be further improved in more sophisticated multicomponent architectures. We report that the directional construction of strings of anions or cations along the PDI stacks does not significantly improve their activity; that is, the intrinsic activity of PDI stacks is too high to yield ion-gated photosystems. The directional construction of electron- and hole-transporting stacks of naphthalenediimides (NDIs) and oligothiophenes along the central PDI stack did not improve photocurrent generation under standard conditions either. However, the activity of double-channel photosystems increased with increasing thickness, whereas increasing charge recombination with single-channel PDI stacks resulted in decreasing activity with increasing length. Most efficient long-distance charge transport was found with double-channel photosystems composed of PDIs and NDIs. This finding suggests that over long distances, PDI stacks transport holes better than electrons, at least under the present conditions. Triple-channel photosystems built around PDI stacks with oligothiophenes and triphenylamines were less active, presumably because hole mobility in the added channels was inferior to that in the original PDI stacks, thus promoting charge recombination.

Interfacial design for reducing charge recombination in photovoltaics

Key to high power conversion efficiency of organic solar cells is to minimize charge recombination (CR) at electron donor/acceptor interfaces. Here, nonadiabatic quantum molecular dynamics simulation shows how the interfacial structure can be controlled by molecular design at acene/C60 interfaces to suppress CR. Orders-of-magnitude reduction of the CR rate is achieved through drastic modification of interfacial structure by attaching phenyl groups to tetracene. This finding confirms a molecular design principle for efficient organic photovoltaics underlying a recent experimental study.

Double-channel photosystems with antiparallel redox gradients: templated stack exchange with porphyrins and phthalocyanines

We report the synthesis of multicomponent surface architectures composed of phthalocyanines (Pc), porphyrins (TPP) and naphthalenediimides (NDI). Naphthalenediimide stacks are grown first by self-organizing surface initiated disulfide-exchange polymerization (SOSIP). An oriented redox gradient driving electrons toward the surface is applied by growing electron-richer NDI stacks on top of poorer ones. Lateral stacks of porphyrins and phthalocyanines are then added by templated stack exchange (TSE). A three-component gradient is constructed to drive the holes away from the solid surface. Antiparallel gradients are found to minimize charge recombination during photocurrent generation. Templates used for stack exchange also serve as hole barriers, whereas their size has surprisingly little importance. These results demonstrate the compatibility of SOSIP-TSE technology with porphyrins and phthalocyanines, confirm the importance of oriented antiparallel gradients to minimize charge recombination, and show that electronics rather than the size matter to template stack exchange.

Using Orbital Symmetry to Minimize Charge Recombination in Dye‐Sensitized Solar Cells

Gezielt entworfene symmetrische Farbstoffe können dazu verwendet werden, die Ladungsrekombination in Farbstoffsolarzellen um zwei bis drei Zehnerpotenzen zu verlangsamen. Vermittelt eine konjugierte Brücke die elektronische Kopplung zwischen der Elektrode und dem Farbstoff, so können Systeme entworfen werden, in denen das HOMO nicht an den Halbleiter gekoppelt ist.

Using Orbital Symmetry to Minimize Charge Recombination in Dye‐Sensitized Solar Cells

Suitably designed symmetric dyes can be used in dye-sensitized solar cells to reduce the charge recombination rate by two to three orders of magnitude. If the electron coupling between the electrode and the dye is mediated by a conjugated linker, it is possible to design dyes for which the HOMO of the dye is not coupled to the semiconductor.

A Ga2O3underlayer as an isomorphic template for ultrathin hematite films toward efficient photoelectrochemical water splitting

Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.

Reducing Graphene Oxide on a Visible-Light BiVO4 Photocatalyst for an Enhanced Photoelectrochemical Water Splitting

Bismuth vanadate (BiVO4) is incorporated with reduced graphene oxide (RGO) using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 10-fold enhancement in photoelectrochemical water splitting reaction is observed on BiVO4−RGO composite compared with pure BiVO4 under visible illumination. This improvement is attributed to the longer electron lifetime of excited BiVO4 as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination. Improved contact between BiVO4 particles with transparent conducting electrode using RGO scaffold also contributes to this photoresponse enhancement.

Photoelectronic properties of horseradish peroxidase-functionalized CdSe/silica mesoporous composite and its sensing towards hydrogen peroxide

CdSe nanocrystals were prepared within a template of mesoporous silica (MS) spheres via chemical reaction, and then the as-synthesized CdSe/MS composites were dip coated on an optically transparent electrode. The photoelectric properties of CdSe/MS composite film were examined under ultraviolet (UV) illumination at the excitation wavelength of 365 nm. The CdSe/MS composite can facilitate charge rectification and minimize charge recombination as shown by its higher photocurrent. Most importantly, horseradish peroxidase (HRP)-functionalized CdSe/MS-modified electrode (HRP/CdSe/MS) had a stronger response toward hydrogen peroxide (H2O2) under UV illumination than in the dark. The result demonstrates the potential application of HRP/CdSe/MS composite film as a novel biosensor for monitoring H2O2 under UV illumination.