Transport Of Photoexcited Electrons Research Articles

Visible light-assisted S-scheme p- and n-type semiconductors anchored onto graphene for increased photocatalytic H2 production via water splitting

This work reports on photocatalytic hydrogen (H2) production via water splitting by synthesizing CuO and ZnO semiconductors anchored onto graphene by varying the mass ratios of CuO and ZnO into the graphene matrix. The high H2 production performance for binary nanocomposite (CuO/ZnO) compared to its components confirms the synergistic effect between ZnO and CuO nanoparticles. Introducing 10 wt% graphene into CuO and ZnO displayed the highest H2 production reaching to 37.2 mmol/g.h, which was 3.29 times higher than pristine binary nanocomposite (CuO/ZnO). The graphene played a critical role in enhancing the photocatalytic activity of CuO and ZnO towards H2 production, owing to the accelerated transport of photo-excited electrons between the semiconductors. Under the optimum conditions, H2 production by 10 % of graphene into CuO and ZnO reached a maximum value of 48.6 mmol/g.h when Na2S/Na2SO3 was used as a sacrificial agent. The charge transfer carriers were monitored to explain the photocatalytic H2 production mechanism based on the optical property characterizations and S-scheme heterojunction system. An excellent reusability was achieved after five consecutive cycles of H2 production reaction. The outcome of this research introduces a reusable and effective catalyst with a facile and easy configuration for energy production applications and confirms that the decoration of semiconductors on graphene could be a promising strategy for highly-efficient photocatalytic H2production. Furthermore, by offering a clean alternative to fossil fuels and reducing greenhouse gas emissions—especially when produced from renewable sources—hydrogen can significantly contribute to achieving sustainable development goal 7 (SDG 7), thus advancing toward a more sustainable, equitable future.

Ferroelectric Polarization Enhanced Photodetector Based on Layered NbOCl2

NbOCl2 is an emerging ferroelectric layered material with unique optoelectronic properties, in which the built‐in electric field caused by spontaneous polarization can independently drive the separation and transport of photoexcited electrons and holes. However, the optoelectronic performance of NbOCl2 and its device application have remained elusive. Here, few‐layer NbOCl2 is prepared by the liquid exfoliation method and used to construct photoelectrochemical (PEC)‐type photodetectors. The photodetectors are self‐powered with broadband photoresponse and long‐term cycle stability. Due to the built‐in electric field generated by the spontaneous polarization, the whole system exhibits an open circuit potential of approximately 0.205 V. Interestingly, the open circuit potential can be significantly increased to 0.446 V after poling treatment. The responsivity without external bias is increased by about 2.5 times after 1 V poling and by about 4 times after a poling time of 500 s. Moreover, the tunable ferroelectric polarization shows memory effect and retains about 25% enhancement in photocurrent density even after 60 min. The tuneability of the built‐in electric field in PEC systems based on NbOCl2 offers numerous possibilities for the development of photodetectors and nonvolatile memory devices.

Charge Density Modulation of Pyrene-Related Small Molecules by Nitrogen Heteroatoms Precisely Regulates Photocatalytic Generation of Hydrogen.

Organic semiconductor materials hold promising applications in photocatalytic hydrogen evolution due to their high modifiability and wide range of light absorption capability. In this study, we present an effective strategy for promoting the separation of photoexcited electrons from organic conjugated centers to active sites by modifying different nitrogen-containing groups on pyrene molecules. Building on this foundation, the well-designed catalyst Py-m-2N has demonstrated good performance by achieving a photocatalytic hydrogen evolution rate of 48.86 mmol g-1 h-1, even in the absence of the precious metal platinum cocatalyst. This achievement places the pyrene-based photocatalyst ahead of the majority of its organic counterparts. Our comprehensive characterization and density functional theory calculations reveal that the nitrogen atom not only serves as an active site for hydrogen production but also plays a pivotal role in efficiently accumulating bulk-phase electrons. This electron enrichment process enhances the transport of photoexcited electrons from the light-absorbing pyrene units to the active nitrogen sites. This work provides inspiration for the future design of effective nitrogen-atom-modified organic semiconductor photocatalysts at the molecular level.

MoSe2-NiSe dual co-catalysts modified g-C3N4 for enhanced photocatalytic H2 generation

Solar energy conversion into hydrogen (H2) energy has attracted much attention. However, the low light utilization rate and fast carrier recombination of photocatalysts extremely limit the practical application of photocatalytic H2 production. In this paper, MoSe2-NiSe with abundant active sites and interfacial electronic structures as dual co-catalysts were assembled on g-C3N4 nanosheets (NSs) vis a solvothermal reaction process. MoSe2-NiSe/g-C3N4 NSs composite exhibited improved light absorption and photoelectrochemical properties. The photocatalytic H2 production rate of MoSe2-NiSe/g-C3N4 composite achieved 2379.04 μmol·h−1·g−1, which is 99.25, 1.44, and 3.67 times those of pure g-C3N4 nanosheets (23.97 μmol·h−1·g−1), MoSe2/C3N4 (1654.57 μmol·h−1·g−1), and NiSe/C3N4 (649.08 μmol·h−1·g−1), respectively. The apparent quantum efficiency (AQE) value of MoSe2-NiSe/g-C3N4 achieved 4.07 % under light at λ = 370 nm. The corresponding characterization and experiments proved that 2D ultrathin g-C3N4 NSs with a large surface area and short charge-transfer distance could facilitate light scattering and the transport of photoexcited electrons. MoSe2-NiSe, as a dual co-catalyst, showed strong electronic synergistic interaction between the interfaces, thus improving the conductivity and promoting the electron transfer process.

Expanding π-bridge and introducing auxiliary acceptor for realizing panchromatic absorption of the phenothiazine dyes in dye-sensitized solar cells

Dyes with panchromatic absorption of the solar radiation is crucial for the photovoltaic performances of dye-sensitized solar cells (DSSCs). In this paper we have constructed four phenothiazine dyes (PS1, PS2, PID1 and PID2), and applied to fabricate the DSSCs. Some important results have been found: (I) PID1 and PID2 with isoindigo as auxiliary acceptor successfully achieve nearly panchromatic absorption. (II) thieno[3,2-b]thiophene is a better π-bridge than 2,2′-bithiophene in terms of facilitating the intramolecular transport of photoexcited electrons, reducing the dye aggregation and improving the photovoltaic properties of DSSCs; (III) it is surprised that PID1 and PID2 exhibit broader absorption spectra, but achieve lower conversion efficiencies than PS2 with thieno[3,2-b]thiophene as π-bridge; (IV) The low device efficiencies of the isoindigo-based dyes might be due to the strong electron-withdrawing ability of isoindigo moiety, affecting the electron transport. These findings suggest that it might be an effective way to realize panchromatic absorption of the organic dyes by introducing isoindigo as an auxiliary acceptor, and a balanced perspective on the molecular design of dye is necessary to improve the performance of DSSCs.

Direct Electrochemical Visualization of the Orthogonal Charge Separation in Anatase Nanotube Photoanodes for Water Splitting

Photoelectrochemical (PEC) water splitting is an important and rapidly developing technology that produces H2 as a renewable resource, but local surface investigations remain a major challenge. Using scanning electrochemical cell microscopy (SECCM), the PEC catalytic effects of anatase TiO2-nanotube arrays grown on Ti felt are explored. The SECCM imaging is performed both perpendicular and parallel to the nanotube growth direction. In contrast to bulk cyclic voltammetry measurements, SECCM measures only the upper region of the nanotubes that remain in contact with the electrolyte, which provides a better understanding of the phenomena connected to the longitudinal charge transport. Despite the presence of regions with higher and lower photocurrent, the PEC reactivities of the nanotube tops and walls are roughly comparable with each other. The data support the model of orthogonal electron–hole separation. This model facilitates the photogenerated hole diffusion over the short distance to the electrolyte interface due to the sufficient transport of photoexcited electrons along the long axial direction of TiO2 nanotubes and is often applied to one-dimensional systems. Observed results were additionally supported by the nanotube decoration with photoelectrochemically deposited PbO2 particles.

Bandgap Shrinkage and Charge Transfer in 2D Layered SnS2 Doped with V for Photocatalytic Efficiency Improvement.

Effects of electronic and atomic structures of V-doped 2D layered SnS2 are studied using X-ray spectroscopy for the development of photocatalytic/photovoltaic applications. Extended X-ray absorption fine structure measurements at V K-edge reveal the presence of VO and VS bonds which form the intercalation of tetrahedral OVS sites in the van der Waals (vdW) gap of SnS2 layers. X-ray absorption near-edge structure (XANES) reveals not only valence state of V dopant in SnS2 is ≈4+ but also the charge transfer (CT) from V to ligands, supported by V Lα,β resonant inelastic X-ray scattering. These results suggest V doping produces extra interlayer covalent interactions and additional conducting channels, which increase the electronic conductivity and CT. This gives rapid transport of photo-excited electrons and effective carrier separation in layered SnS2 . Additionally, valence-band photoemission spectra and S K-edge XANES indicate that the density of states near/at valence-band maximum is shifted to lower binding energy in V-doped SnS2 compare to pristine SnS2 and exhibits band gap shrinkage. These findings support first-principles density functional theory calculations of the interstitially tetrahedral OVS site intercalated in the vdW gap, highlighting the CT from V to ligands in V-doped SnS2 .

Enhanced hydrogen generation at designed heterojunctions of Cu2ZnSnS4-rGO-MoS2 through interface engineering

Here, we are reporting interface engineering to create designed heterojunctions in Cu2ZnSnS4-rGO-MoS2 (CZTS-reduced graphene oxide-MoS2), in which abundant high density of nanoscale interfacial contacts are formed. It has been achieved via two-step optimized electrodeposition approach. Further, as-prepared materials have been characterized by microscopic, spectroscopic and electroanalytical techniques. Finally, the concept of generation of designed heterojunctions and synergetic effect were tested for electrocatalytic and photoelectrocatalytic hydrogen generation. The electron-hole pair recombination has been reduced, since the fast transport of photoexcited electrons of CZTS to MoS2 through reduced graphene oxide interfacial contacts. Further, the synergetic effect of increased charge separation between rGO and more catalytically active sites from MoS2 has been attributed to enhanced hydrogen generation.

Accelerated Electron Transport and Improved Photocatalytic Activity of Ag/AgBr Under Visible Light Irradiation Based on Conductive Carbon Derived Biomass

Ag/AgBr (PLAS) exhibits good photo-oxidative properties for organic pollutants degradation under visible light. However, poor separation of photogenerated charge carriers at the catalyst interface, caused by decelerated photoexcited electron migration results in lower photocatalytic activity and limits the practical application of PLAS. In this work, activated carbon (ACK) from microwave pyrolysis of chemical impregnated biochar (BCR) pinecone is employed as a conductive carbon support for coupling Ag/AgBr (PLAS–ACK) as synthesized through the thermal polyol route. The structural, electrochemical and optical properties of prepared composites are characterized by various techniques. ACK exhibited faster electron transfer compared to BCR and commercial activated carbon (CAC) from the Nyquist EIS plot, due to its abundant sp2 C and interconnected conductive carbon structures. The resulting PLAS–ACK exhibited an improved degradation efficiency of 92% on tetracycline (TC) antibiotic in a neutral solution after 180 min, which is better than PLAS–BCR and PLAS–CAC under visible LED light irradiation. The superior degradation efficiency of PLAS–ACK is ascribed to the accelerated transport of photo-excited electrons, enhancing the separation of photogenerated charge carriers upon introduction of conductive ACK. The reaction mechanism of PLAS–ACK on TC degradation based on a scavenging experiment was also proposed in this work. (a) Electrochemical impedance spectroscopy of different carbons for photo-excited electrons transport and (b) degradation performance of prepared composites on Tetracycline under visible LED light irradiation.

Effects of material structure on device efficiency of III-nitride intersubband photodetectors

<sec> The photodetection potential applications of III-nitride semiconductor are not only restricted in the ultraviolet range, but also extended to the infrared and even terahertz wave range through intersubband transition (ISBT) process between quantum-confined electronic states. The large conduction band offset (~1.75 eV for GaN/AlN heterostructures) and the strong electron-phonon interaction in these materials enable the III-nitride ISBT photodetectors to operate with ultrahigh speed for near-infrared telecommunication applications. Furthermore, due to the large energy of longitudinal optical phonons of nitride materials (~92 meV for GaN), the corresponding ISBT photodetectors are demonstrated as a promising candidate for operating in a specific terahertz (THz) range (5－12 THz) at high temperature, which is fundamentally in accessible to As-based devices. One of the major difficulties in realizing high performance III-nitride ISBT photodetectors is the existence of undesired polarization field inherited from conventional c-axis nitrides, which greatly makes the design complicated. In this paper, we theoretically study the influences of critical material structure parameters on the band structure, electron distribution, polarization field intensity, and tunneling effect based on near-infrared photovoltaic photodetectors and terahertz double-step photodetectors. </sec> <sec> III-nitride ISBT photodetectors operating in near-infrared are designed to be able to suppress the internal electric field in the active region, so that as many quantum wells as possible can be populated by electrons to enhance the absorption efficiency. It is found that with the increase of well periods and well doping concentration, a weaker average polarization field is observed, and as a result more wells will be populated with electrons. In addition, keeping the lattice constant of the contact layer material the same as the average lattice constant of the active region is beneficial to making more quantum wells effectively populated with electrons. On the other hand, the devices operating in a terahertz range require a more accurate design of energy band than the near-infrared ones due to its rather small energy interval. The design difficulties lie in how to ensure both the efficient absorption of photons and the following transportation of photo-excited electrons in the presence of polarization-induced field. The simulation results indicate that the polarization field in an active region is sensitive to the aluminum mole composition of barrier layer, the thickness of step well and step barrier layers, while even if the thickness of well and barrier layers change, the polarization field in barrier layer will stay the same. Moreover, reducing the aluminum mole composition of step barrier layer or appropriately increasing its thickness will be helpful in enhancing the photo-excited electrons tunneling. The above knowledge is beneficial to the optimal design of III-nitride ISBT photodetectors with high efficiency. </sec>

The High Performance of Quantum Dot Sensitized Solar Cells Co-Sensitized with Mixed-Joint CdS and ZnS Quantum Dots

We propose a facile way to prepare CdS and ZnS quantum dots (QDs) in photoanode oxide films in quantum dot sensitized solar cells (QDSCs). CdS QDs as sunlight harvester and ZnS as passivation layer are deposited in mesoporous RGO/TiO2 (reduced graphene oxide and TiO2 composite) films alternately through successive ionic layer adsorption and reaction (SILAR) to engineer the mixed-joint CdS/ZnS QDs. For comparison, another CdS and ZnS QDs are prepared with depositing all CdS firstly and then ZnS QDs in mesoporous RGO/TiO2 films. The power conversion efficiency (PCE) of mixed-joint CdS/ZnS based cell samples improve greatly, up to 6.37%, almost two times higher than that of bare CdS based quantum dot sensitized solar cells (2.54%), and also higher than that of CdS/ZnS based QDSCs (4.12%) obviously. Since the conduction band of ZnS (bandgap, 3.6ev) is higher than that of CdS (bandgap, 2.25ev), ZnS quantum dots can suppress reversed transport of photo-excited electrons and improve the efficiency of electron collection and meanwhile, they also impede hole transfer from the valance band of CdS to electrolytes. It seems to be an optimal way to solve this contradiction when mixed-joint CdS/ZnS are used to cosensitize QDSCs due to their characteristic architecture. The mechanism of effect of ZnS QDs on the photovoltaic performance of QDSCs is investigated combined sensitized photoanode microstructural characterization and electrochemical impedance spectroscopy (EIS) and J-V curve analysis of cell samples.

Photocatalytic Method for the Simultaneous Synthesis and Immobilization of Ag Nanoparticles onto Solid Substrates

A photocatalytic method based on layer-by-layer photocatalyst films made of titanium dioxide nanoparticles and graphene oxide, TiO2NPs/GO, is proposed for performing the synthesis and immobilization of spherical AgNPs simultaneously using diluted AgNO3 solutions and UV irradiation (254 nm). The novelty provided by this method is that the amount and aggregation extent of AgNPs can be controlled by the number of TiO2NP/GO bilayers and the composition of the outermost layer in the photocatalyst film. When the outermost layer is made of GO, more AgNPs are deposited because GO serves as an anchoring harbor for Ag+ ions and a venue to the transportation of photoexcited electrons for subsequent photo reduction to Ag0. The photodeposition follows a first-order kinetics, whereas in the equilibrium it can be fitted by the Langmuir isotherm model for which surface coverage as high as 60% is attained after 70 min of UV irradiation. The TiO2NP/GO/AgNP substrates show surface-enhanced resonant Raman scattering (SERRS) ...

Investigation on the Thickness Effect of TiO<sub>2 </sub>Photo- Anode on Dye-Sensitized Solar Cell Performance

Dye-sensitized solar cell (DSSC) has attracted immense interest over the past decade due to its low cost and simplicity as a renewable energy source. In DSSC, the photo-anode that governs the collection and transportation of photo-excited electrons plays a critical role in determining the cell performance. In particular, the thickness of the photo-anode is known to be one of the decisive factors in dictating cell efficiency. In this paper, the thickness effect of TiO2 film as photo-anode on the photovoltaic performance of DSSC is studied. The TiO2 films were characterized by UV-visible absorbance spectroscopy to correlate the film thickness with the light harvesting efficiency of DSSC. The effects of film thickness on the photocurrent density-voltage characteristics of DSSCs were also investigated. The optimized thickness (4m) of the TiO2 film was discovered to be the best compromise between the amount of dye loading and the electron transport resistance.

Improved efficiency of ZnO hierarchical particle based dye sensitized solar cell by incorporating thin passivation layer in photo-anode

Present article describes the DSSC performances of photo-anodes prepared using hydrothermal route derived ZnO particles having dissimilar morphologies i.e. simple micro-rod and nano-tips decorated micro-rod. The surface of nano-tips decorated micro-rod is uneven and patterned which facilitate more dye adsorption and better scattering of the incident light resulting superior photo-conversion efficiency (PCE) (η ~ 1.09%) than micro-rod ZnO (η ~ 0.86%). To further improve the efficiency of nano-tips decorated micro-rod ZnO based DSSC, thin passivation layer of ZnO is introduced in the corresponding photo-anode and a higher PCE (η ~ 1.29%) is achieved. The compact thin passivation layer here expedites the transportation of photo-excited electrons, restricts the undesired recombination reactions and prevents the direct contact of electrolyte with conducting substrates. Attempt is made to understand the effect of passivation layer on the transportation kinetics of photo-excited electrons by analyzing the electrochemical impedance spectra of the developed cells.

Recent advances in photo-anode for dye-sensitized solar cells: a review

Dye-sensitized solar cell (DSSC) attracts immense interest in the last few decades due to its various attractive features such as low production cost, ease of fabrication and relatively high conversion efficiency, which make it a strong competitor to the conventional silicon-based solar cell. In DSSC, photo-anode performs two important functions, viz. governs the collection and transportation of photo-excited electrons from dye to external circuit as well as acts as a scaffold layer for dye adsorption. The photo-anode usually consists of wide band gap semiconducting metal oxides such as titanium dioxide (TiO2) and zinc oxide (ZnO) deposited on the transparent conducting oxide substrates. The morphology and composition of the semiconductor oxides have significant impact on the DSSC photovoltaic performance. Therefore, enormous research efforts have been undertaken to investigate the influences of photo-anode modifications on DSSC performance. The modifications can be classified into three categories, namely interfacial modification through the introduction of blocking and scattering layer, doping with non-metallic anions and metallic cations and replacing the conventional mesoporous semiconducting metal oxide films with one-dimensional or two-dimensional nanostructures. In the present review, the previously mentioned modifications on photo-anode are summarized based on the recent findings, with particular emphasis given to published works for the past 5 years. Copyright © 2017 John Wiley & Sons, Ltd.

The nature of free-carrier transport in organometal halide perovskites.

Organometal halide perovskites are attracting great attention as promising material for solar cells because of their high power conversion efficiency. The high performance has been attributed to the existence of free charge carriers and their large diffusion lengths, but the nature of carrier transport at the atomistic level remains elusive. Here, nonadiabatic quantum molecular dynamics simulations elucidate the mechanisms underlying the excellent free-carrier transport in CH3NH3PbI3. Pb and I sublattices act as disjunct pathways for rapid and balanced transport of photoexcited electrons and holes, respectively, while minimizing efficiency-degrading charge recombination. On the other hand, CH3NH3 sublattice quickly screens out electrostatic electron-hole attraction to generate free carriers within 1 ps. Together this nano-architecture lets photoexcited electrons and holes dissociate instantaneously and travel far away to be harvested before dissipated as heat. This work provides much needed structure-property relationships and time-resolved information that potentially lead to rational design of efficient solar cells.

A ZnO/ZnO:Cr isostructural nanojunction electrode for photoelectrochemical water splitting

The fabrication and photoelectrochemical characterization of a ZnO/ZnO:Cr isostructural nanojunction electrode is presented. When compared to its constituent components the isostructural nanojunction showed superior performance for water splitting under simulated solar light and visible light (λ>510nm) illumination. In the engineered structure, the presence of intra-bandgap states associated with Cr impurities increases optical absorption, and the nanorod morphology provides a direct pathway for transport of photo-excited electrons to the back contact. The overall photoelectrochemical performance of the ZnO/ZnO:Cr engineered structure is relatively low, however the concept may prove to be applicable to more optimized structures or material systems.

Numerical calculation model for spin-dependent transport of photoexcited electrons across Fe/GaAs(0 0 1) interfaces

Spin-dependent transport of photogenerated electrons across Fe/GaAs(0 0 1) interfaces is calculated using a one-dimensional electron transport model. Creation of spin-polarized electrons by photoexcitation in this model is defined by the band structure of GaAs along the [0 0 1] direction and the optical selection rules. The tunnel probability across the interface is obtained from Chang's model and first principles calculations are employed to calculate the spin polarization of Fe for electrons propagating along the [0 0 1] direction. By combining the above ingredients, the spin-filtering current, ISF, was calculated for different parameter values, including Schottky barrier height and photon energy. The model is used to fit with experimental results of the photoexcitation technique, yielding qualitative agreement especially for the observed sign switching of the spin-filtering current.

Ultrafast terahertz photoconductivity in nanocrystalline mesoporous TiO2 films

Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO2 films prepared by the recently proposed “brick and mortar” technology [Szeifert et al.Chem. Mater. 21, 1260 (2009)] with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar.” Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.

Generation and transport of photoexcited electrons in single-crystal diamond

We report time-dependent photocurrent and transport measurements of sub-bandgap photoexcited carriers in nitrogen-rich (type Ib), single-crystal diamond. Transient carrier dynamics are characteristic of trapping conduction with long charge storage lifetimes of ∼3 hours. By measuring the photoexcited Hall effect, we confirm that the charge carriers are electrons and by varying the excitation energy we observe a strong turn-on in the photoconduction at ∼1.9 eV. These findings shed light on sub-bandgap states in nitrogen-doped single-crystal diamond.