Transport Of Photogenerated Charge Carriers Research Articles

A general strategy for enhancing the performance of Ga2O3-based self-powered solar-blind photodetectors through band structure engineering

Abstract The spectral response of Ga2O3 almost perfectly covers the 200~280 nm solar-blind ultraviolet wavelength range, making it an ideal semiconductor material for fabricating solar-blind ultraviolet photodetectors. However, due to the considerably deep valence band energy of Ga2O3 the construction of heterojunctions typically induces a significant valence band offset (EV). Herein, we present a band engineering approach to improve the performance of Ga2O3 bases photodetectors. This pronounced valence band barrier can strongly influence the transport of photo-generated charge carriers, especially the extraction of holes in the depletion region. By introducing nitrogen (N) during the growth process, we elevated the valence band of Ga2O3 by 0.43 eV. The organic high-molecular-weight material of PEDOT:PSS has been utilized in conjunction with Ga2O3 to construct heterojunction photodetectors. The photodetectors exhibit excellent self-powered characteristics, with responsivity, detectivity, and response time being nearly ten times higher than those of Ga2O3 photodetectors before band structure modulation. The investigation into modulating the band structure of Ga2O3 carried out in this study will lay the theoretical foundation and provide technical solutions for developing satisfactory self-powered photodetectors.

High-Performance 1D CdS/2D Te Heterojunction Photodetector

Abstract Van der Waals heterostructures facilitate the versatile combination of two-dimensional (2D) materials with semiconductors that possess distinctive band structures, offering a promising platform for developing photodetectors with extensive spectral range and high sensitivity. This study showcases the flawless integration of one-dimensional (1D) CdS nanowires with 2D Te nanosheets, resulting in the creation of a high-performance, mixed-dimensional heterojunction photodetector. Bandgap engineering at the CdS/Te interface promotes Type II band alignment, markedly improving the separation and transport of photo-generated charge carriers. Additionally, the interaction between materials of different dimensions significantly enhances the device’s photoresponsivity and detectivity. When operated at low bias, the photodetector displays an impressive rectification ratio of up to 8.57×103, along with superior photoelectric performance spanning the ultraviolet to visible light spectrum. Particularly under 450 nm wavelength laser illumination, the device displayed a high photoresponsivity of up to 126 A W−1, a high detectivity of 1.03×1010 Jones, an external quantum efficiency of up to 3.49×102%, and a rapid response time of 35 ms. The photoresponsivity of this device was at least an order of magnitude higher than other reported mixed-dimensional heterojunction photodetectors. The development of the 1D CdS/2D Te heterostructure opens new pathways for high-sensitivity, broad-spectrum photodetection technologies.

Correlated electron-nuclear dynamics of photoinduced water dissociation on rutile TiO2.

Elucidating the mechanism of photoinduced water splitting on TiO2 is important for advancing the understanding of photocatalysis and the ability to control photocatalytic surface reactions. However, incomplete experimental information and complex coupled electron-nuclear motion make the microscopic understanding challenging. Here we analyse the atomic-scale pathways of photogenerated charge carrier transport and photoinduced water dissociation at the prototypical water-rutile TiO2(110) interface using first-principles dynamics simulations. Two distinct mechanisms are observed. Field-initiated electron migration leads to adsorbed water dissociation via proton transfer to a surface bridging oxygen. In the other pathway, adsorbed water dissociation occurs via proton donation to a second-layer water molecule coupled to photoexcited-hole transfer promoted by in-plane surface lattice distortions. Two stages of non-adiabatic in-plane lattice motion-expansion and recovery-are observed, which are closely associated with population changes in Ti3d orbitals. Controlling such highly correlated electron-nuclear dynamics may provide opportunities for boosting the performance of photocatalytic materials.

High-performance photoelectrochemical (PEC) type self-powered ultraviolet photodetectors (PDs) based on three-dimensional ZnO film/carbon fiber paper

Novel photoelectrochemical ultraviolet detection technologies based on semiconductor/electrolyte heterojunctions have attracted significant attention due to their cost-effective fabrication and self-powered characteristic. In this study, ZnO films were deposited on three-dimensional carbon fiber paper (3D CFP) using a one-step sputtering method, resulting in the preparation of 3D ZnO/CFP composites as photoanodes for photoelectrochemical (PEC) type photodetectors (PDs). The fabricated device demonstrates outstanding photoelectrochemical performance, showcasing a remarkable responsivity of approximately 31.76 mA/W, a detectivity of 3.06 × 1012 jones, and a rapid photoresponse time of 76.51/48.51 ms. Furthermore, the device exhibits exceptional photoresponse stability during cycling tests. The unique structure of 3D CFP, composed of intertwined carbon fiber in three dimensions, offers an enlarged specific surface area and efficient carrier transport pathways for the photoanode in self-powered PEC-PDs. This characteristic facilitates effective electrolyte diffusion throughout the entire film, as well as efficient transport of photo-generated charge carriers, thereby significantly enhancing the overall performance of the PEC-PDs. This study affirms the effectiveness of the synergistic interaction between 3D CFP and ZnO film for advanced UV photodetectors, demonstrating both mass production viability and promise for the future of photoelectrochemical detection technology.

Oxygen-defected WO3-OVs@Bi2MoO6 S-scheme micro flowers heterojunctions with promoted photocatalytic degradation of tetracycline

The synergy between surface oxygen vacancies (OVs) and heterojunction engineering holds promising potential for substantially enhancing photoelectric conversion efficiency through the optimization of photogenerated charge carrier transport. Herein, novel WO3-OVs@Bi2MoO6 S-scheme heterostructures are synthesized via a simple route for degradation of tetracycline hydrochloride (TC) using visible light. The S-scheme charge transfer mode markedly enhances the spatial separation and preservation of high-energy electrons/holes on Bi2MoO6 (reduction) and WO3-OVs (oxidation), consequently providing WO3-OVs@Bi2MoO6 photocatalysts with an improved redox capability. The optimal WO3-OVs@Bi2MoO6 heterojunction demonstrates enhanced visible-light-driven photocatalytic degradation of tetracycline, exhibiting 4.86-fold and 4.17-fold higher efficiency compared to WO3-OVs and Bi2MoO6, respectively, with superior stability and reusability. Confirmation of the existence of the S-scheme charge transfer mechanism and oxygen vacancies (OVs) is achieved through a combination of density functional theory (DFT) calculations and in-situ X-ray photoelectron spectroscopy (XPS), substantiating the enhancement of charge separation and redox capability. Specifically, the presence of oxygen vacancies (OVs) on the surface of WO3 nanofibers results in a reduction of the band gap energy due to the defect levels, consequently promoting increased electron conduction. Furthermore, the abundance of oxygen vacancies (OVs) expands the photo response range and facilitates charge transfer at the interface. This research opens new avenues for the successful photocatalytic degradation of tetracycline (TC) by integrating hierarchical S-scheme heterojunction design and cutting-edge defect engineering in photocatalytic heterojunctions.

Pollution-free interface of 2D-MoS2/1D-CuO vdWs heterojunction for high-performance photodetector

Van der Waals heterostructures provide a new opportunity for constructing new structures and improving the performance of electronic and optoelectronic devices. However, the existing methods of constructing heterojunctions are still faced with problems such as impurity introduction, or complex preparation process and limited scope of application. Herein, a physisorption method is proposed to composite CuO nanorods on the surface of MoS2 nanosheets. CuO nanorods and MoS2 form type-Ⅱ heterojunctions, which promotes the separation and transport of photo-generated charge carriers. More importantly, compared with the transfer and coating methods, the physical adsorption method avoids the introduction of auxiliary materials during the whole process of constructing the heterojunction, and therefore effectively reduces the damage and pollution at the interface. The optimized MoS2/CuO heterojunction photodetector achieves a high photoresponsivity of ∼680.1 A W−1 and a fast response speed of ∼29 μs. The results demonstrate that the physisorption method provides a feasible approach to realize high performance photodetectors with pollution-free interfaces, and it can also be extended to the development of other low-dimensional hybrid heterojunction electronic and optoelectronic devices.

Enhanced full-spectrum photocatalytic activity of 3D carbon-coated C3N4 nanowires via giant interfacial electric field

Strong interfacial electric field can promote the separation and transfer of photogenerated carriers of photocatalysts. Herein, a three-dimensional (3D) network structure assembled from carbon-coated C3N4 nanowires (denoted as 3D C3N4@C) with a giant interfacial electric field was prepared. The unique 3D structure works with the carbon layer to expand the light absorption range of C3N4 to the full spectrum. Under visible-light irradiation (λ ≥ 420 nm), the degradation rate constants of 3D C3N4@C for bisphenol A and phenol were 41.2 and 5.7 times that of bulk g-C3N4, respectively. Meanwhile, the interfacial electric field of 3D C3N4@C-2 mol/L was 5.14 times that of bulk g-C3N4. The charge transfer process of 3D C3N4@C was confirmed by synchronous illumination X-ray photoelectron spectroscopy and DFT. The significantly improved activity was mainly ascribed to the giant interfacial electric field between g-C3N4 and the carbon layer, which significantly promoted the separation and transportation of photo-generated charge carriers.

Performance Enhancement of Highly Flexible SnS(p)/MoS2(n) Heterostructure based Broadband Photodetector by Piezo-phototronic Effect

The work demonstrates the fabrication of a flexible p-SnS/n-MoS2 heterostructure based piezo-phototronic photodetector and understanding the detailed charge transport mechanism. Electrical measurement of fabricated heterojunction shows excellent rectification behavior with an ideality factor of ∼ 2.06. When the external strain is applied on the device, due to non-centrosymmetric structure of MoS2, piezo-potential gets induced in MoS2, which modulates the band structure and increases the width of the depletion region across the junction. The increase in depletion region results in a strong built-in electric field, which promotes the separation of photogenerated electron-hole (e-h) pairs more effectively and improves photogenerated charge carrier transport across the p-SnS/n-MoS2 interface. The photoresponsivity of the device is increased by ∼ 97 % at ∼ 2% tensile strain by piezo-phototronic effect compared to strain-free conditions. Energy band structure drawn by the values extracted from real-time ultraviolet photoelectron spectroscopy (UPS) measurements are used to explain the physical working mechanism behind the enhancement of photoresponsivity. The fabricated SnS/MoS2 heterostructure showed a higher photoresponsivity in visible region compared to NIR region due to higher absorption in the visible region. This work demonstrates the cost-effective approach for the development high performance photodetectors by using piezotronic interface engineering and paves the way for future flexible and wearable optoelectronics.

The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells.

Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.

Pt clusters embedded in g-C3N4 nanosheets to form Z-scheme heterostructures with enhanced photochemical performance

The development of catalysts with enhanced photocatalytic performance has been receiving growing attention in recent years. The performance of heterostructural catalysts can be dependent on the interface structures of heterostructure. Through a mechanical-chemical pre-reaction, Pt and Au ions are anchored onto red g-C3N4 nanosheets to control the in-situ growth of Pt and Au clusters during thermal polymerization process. Electron spin resonance analysis confirms the formation of Z-scheme heterostructure. The Z-scheme heterostructures consisted of Pt/Au clusters and superior thin red/yellow g-C3N4 nanosheets are created with enhanced photo-chemical activity. The homogeneous distribution of Pt/Au clusters and the formation of Z-scheme heterostructure are the keys in improving the photocatalytic activities of the material. The estimated H2, CO, and CH4 evolution rate of Pt immobilized composites with optimized Pt loading are 5057, 4.82, 1.56 μmol/g/h, respectively. The remarkably enhanced photocatalytic performance of Pt clusters incorporated g-C3N4 samples is due to the formation of Z-scheme heterostructures which improves the photogenerated charge carrier transport. Enhancement in photocatalytic activity is also observed for Au clusters embeded g-C3N4 nanosheets. The results provide important platforms for exploiting highly efficient catalysts.

Characteristics of P-Type and N-Type Photoelectrochemical Biosensors: A Case Study for Esophageal Cancer Detection.

P-type and N-type photoelectrochemical (PEC) biosensors were established in the laboratory to discuss the correlation between characteristic substances and photoactive material properties through the photogenerated charge carrier transport mechanism. Four types of human esophageal cancer cells (ECCs) were analyzed without requiring additional bias voltage. Photoelectrical characteristics were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–vis reflectance spectroscopy, and photocurrent response analyses. Results showed that smaller photocurrent was measured in cases with advanced cancer stages. Glutathione (L-glutathione reduced, GSH) and Glutathione disulfide (GSSG) in cancer cells carry out redox reactions during carrier separation, which changes the photocurrent. The sensor can identify ECC stages with a certain level of photoelectrochemical response. The detection error can be optimized by adjusting the number of cells, and the detection time of about 5 min allowed repeated measurement.

Synergistic effect of CdSe quantum dots (QDs) and PC61BM on ambient-air processed ZnO QDs/PCDTBT: PC61BM:CdSe QDs/MoO3 based ternary organic solar cells

This paper reports the synergistic effect of colloidal CdSe quantum dots (QDs) and PC61BM on the performance of ITO/ZnO QDs/PCDTBT:PC61BM:CdSe QDs/MoO3/Ag based ternary organic solar cells (OSCs). The MoO3 and ZnO QDs (∼2.87 nm) layers work as the transparent hole transport layer (HTL) and electron transport layer (ETL), respectively. The CdSe QDs (∼4.58 nm) are blended with PC61BM:PCDTBT binary solution to improve the optical properties and charge transportation. Significant photoluminescence (PL) quenching is observed when part of the PC61BM is replaced by CdSe QDs with equivalent weight in the PCDTBT. The proposed ternary OSC gives an open-circuit voltage of 854 mV, a short circuit current density of 14 mA cm−2, fill factor of 42% and power conversion efficiency (PCE) of 5.02%. The PCE of the ternary OSC is increased by more than 38% compared to the binary OSC. This significant improvement in the performance parameters is attributed to the enhanced absorption and higher transportation of photo-generated charge carriers, as well as the increased charge dissociation due to the synergistic effect of CdSe QDs and PC61BM. The external quantum efficiency is also enhanced significantly in the ternary OSC due to the better conversion of solar energy into electrical energy.

The photovoltaic conversion enhancement of NiO/Tm:CeO2/SnO2 transparent p-n junction device with dual-functional Tm:CeO2 quantum dots

Abstract The transparent NiO/Tm:CeO2 QDs/SnO2 p-n junction device is prepared via a continuous hydrothermal-chemical-sputtering method. There, the SnO2 nanosheets arrays are grown via the hydrothermal method, then the Tm:CeO2 quantum dots (QDs), prepared via the chemical method, are introduced on the surface of the arrays, subsequently the NiO film is deposited via the sputtering. As demonstrated, the transparent device exhibits highly transmittance of about ~85% in visible light, evident photovoltaic conversion enhancement of about ~104(~9700) folds than the unmodified device, that can be mainly ascribed to the dual-functional Tm:CeO2 QDs transition layer modification, there, the upconversion fluorescence can increase the solar utilization, and the appropriate Fermi level can accelerate the photo-generated charge carrier transport. In addition, the Tm:CeO2 QDs transition layer can improve the p-n junction interface and the SnO2 nanosheets arrays can increase the solar utilization are also important factors.

Mesoporous carbon/graphitic carbon nitride spheres for photocatalytic H2 evolution under solar light irradiation

Herein, two different photocatalytic composites based on ordered (OCS) and disordered (DCS) mesoporous hollow carbon spheres and graphitic carbon nitride (gCN) have been successfully fabricated through facile acid treatment. The influence of carbon shell morphology of the spheres on gCN loading and photocatalytic H2 production under simulated solar light irradiation has been revealed. The amount of evolved H2 was ~6.2 (OCS/gCN) and ~5.3 (DCS/gCN) times higher in comparison to pristine gCN. It was found that graphitic carbon nitride was much more homogenously supported onto ordered mesoporous carbon spheres than disordered ones. The deposition of gCN onto ordered carbon spheres was found to be more efficient to increase carrier concentration, enhance photogenerated charge carrier transport and separation. It is assigned to the formation of the graphitic carbon nitride/carbon heterojunction facilitating the contact surface between the two phases of hybrid. Therefore, via tuning of the morphology of carbon shell being a host for gCN it was possible to find more promising candidate as a photocatalyst in H2 production under solar light irradiation.

Achievement of superior efficiency of TiO2 nanorod-nanoparticle composite photoanode in dye sensitized solar cell

TiO2 based nanostructures have been widely explored as photoanodes for dye-sensitized solar cells (DSSCs) because of their favourable band gap and corresponding compatibility with various sensitizers (dye molecules). Various morphologies and textures of titanium oxide have been thoroughly investigated to find the best efficiency in DSSCs. In this work, a detailed study of a nanocomposite of titanium dioxide partly derived from hydrogen titanate precursor has been reported. The current structure is realized in order to achieve optimal photogenerated charge carrier transport and reduced charge recombination. Among many combinations of the composites, the best photoanode showed power conversion efficiency (η) of 8.61% and corresponding open circuit voltage was 0.69 V and short circuit current 17.22 mA cm−2. The efficiency enhancement for the nanorod-nanoparticle composite photoanode is explained by structural, optical, and morphological properties and corroborated with electrochemical impedance spectroscopy.

Plasmonic hole-transport-layer enabled self-powered hybrid perovskite photodetector using a modified perovskite deposition method in ambient air

Herein, we report on an air-processed high performance self-powered hybrid perovskite (Pe) photodetector with plasmonic Silver nanoparticle (Ag NP) embedded hole-transport-layer (HTL), without the use of any electron-transporting layer (ETL). It is demonstrated that in the absence of ETL in the device, the Ag NPs embedded PEDOT:PSS HTL improves the photodetection performance significantly. We used a novel N2 gas assisted fast crystallization method for the deposition of perovskite film in ambient condition to form uniform Pe layer as compared to the nonuniform film obtained in conventional deposition method. The Pe film on Ag NPs embedded PEDOT:PSS layer shows enhanced optical absorption in the UV–visible region due to the plasmonic absorption by the Ag NPs. At zero bias, the ETL-free Ag NPs-Pe hybrid device shows ∼45% enhanced responsivity and ∼3 times faster photoresponse compared to the pristine device. The enhancements in the performance of hybrid photodetector are attributed to plasmon-enhanced optical absorption and hot electron generation, as well as improvement in charge extraction and transport by Ag NPs, which are corroborated by steady-state and time-resolved photoluminescence measurements. Impedance analysis of the devices shows the reduced carrier transfer resistance of the hybrid device, which results in superior transport of photo-generated charge carriers. Direct evidence for the increase in the work function by ∼47 meV for Ag NPs doped PEDOT:PSS film is provided from the Kelvin probe force microscopy analysis. This increase in work function enables favorable band alignment with reduced energy barrier and a superior carrier transport resulting in improved photodetection performance for the hybrid device. Our results are significant for the development of high-performance, low-cost, ETL free plasmonic perovskite photodetectors for futuristic applications.

Fabrication of CdS quantum dots sensitized ZnO nanorods/TiO2 nanosheets hierarchical heterostructure films for enhanced photoelectrochemical performance

A novel CdS/ZnO/TiO2 hierarchical heterostructure for photoelectrochemical (PEC) application was fabricated by growing ZnO nanorods (NRs) onto TiO2 nanosheets (NSs) with dominant high-energy {001} facets via hydrothermal method, subsequently coated CdS quantum-dots (QDs) sensitized layer via successive ionic layer adsorption and reaction (SILAR) technique. The as-prepared TiO2, ZnO/TiO2 and CdS/ZnO/TiO2 films were characterized by XRD, SEM, TEM, UV–vis absorption spectrum and EIS techniques to investigate the effects of ZnO NRs and CdS QDs on the structure, morphology, optical property and charge carrier transmission characteristics. The results demonstrated that the growth of ZnO NRs obviously enlarges the specific surface area, while the deposition of CdS QDs significantly enhances the light harvesting ability. And the heterojunctions formed among TiO2 NSs, ZnO NRs and CdS QDs effectively promote the photogenerated charge carrier transport and separation, simultaneously inhibit the photogenerated electron-hole recombination. Moreover, it is notable that ZnO NRs trend to grow on the low-energy {101} facets of TiO2 NSs due to the impact of crystal facet controlling agent, which can effectively shorten the electron conduction path-ways and facilitate the photogenerated electron-hole separation due to the lower band edge position of {101} facets than that of {001} facets. As a result, the photocurrent density of the optimized CdS/ZnO/TiO2 composite film enhances to an excellent value of 12.1 mA/cm2 at a potential of 0 V vs SCE, which is more than 27 times higher than that of bare TiO2 NSs films. In addition, the corresponding optimal condition and physical mechanism were also discussed in the paper.

Solution processed Mo doped SnO2 as an effective ETL in the fabrication of low temperature planer perovskite solar cell under ambient conditions

Abstract Metal oxide electron transport layers play a crucial role on the extraction and transportation of charge carriers in organic-inorganic hybrid perovskite solar cells (PSCs). However, generally, fabrication of conductive metal oxide thin films requires high temperature annealing which is not compatible with roll to roll fabrication. It has been observed that a low temperature fabricated pristine metal oxide film exhibited low electrical conductivity, which has still prohibited to achieve high performance of device. Therefore, a low amount of metal doping within the pristine metal oxide films followed by low temperature thermal annealing is an effective approach to improve the electrical properties of the metal oxide thin films, and, thus, leading the higher performance of PSCs. Herein, we have developed a low temperature (180 °C) solution processed Molybdenum (Mo) doped SnO2 (SnO2:Mo) as an efficient electron transport layer (ETL) for the fabrication of planer PSCs. The electrical conductivity of pristine film was significantly enhanced after the incorporation of a small amount of Mo dopant within the neat SnO2 film, which could facilitate fast transportation of photo-generated charge carriers. Moreover, due to n-type Mo doping, Fermi level of resulting film was slightly shifted upwards, which could diminish charge recombination and, hence, improved the photovoltaic performance. The power conversion efficiency (PCE) for pristine SnO2 and SnO2:Mo based PSCs was found to be 8.47% and 10.52%, respectively, when ETL and perovskite absorber films were processed in ambient conditions with the relative humidity range of 65%–75%. Compared to pristine SnO2, SnO2:Mo based PSC exhibited higher performance, which was attributed to the enhanced electrical properties of the resulting doped ETL thin film. Our results indicated that low temperature (180 °C) solution processed SnO2:Mo is a promising ETL for cost effective roll to roll fabrication of PSC.

Three-Dimensional Tin Oxide Nanohelix Structures with Thin Iron Oxide Layer for Efficient Visible Light Driven Water Splitting

Solar-powered photoelectrochemical (PEC) water splitting has been a promising candidate for producing hydrogen in a clean and renewable way. Photoelectrodes are key components in PEC cells for efficient and stable hydrogen generation because they play crucial roles in absorption of photons, the separation and transportation of photo-generated charge carriers, as well as the chemical reactions with water. A variety of metal oxides for efficient photoelectrode have been intensively explored, but it is still challenging to find desirable materials to satisfy lots of requirements for PEC water splitting. Iron oxide (hematite, Fe2O3) has recently attracted much attention due to its earth abundance, low cost as well as desirable material properties for PEC water oxidation including narrow band gap energy of 2.0~2.2eV for visible light absorption and proper energy band alignment, etc. However, Fe2O3 has very short hole diffusion length and low carrier mobility, which causes considerable recombination of photo-generated electrons and holes. A lot of approaches such as nanostructures, heterojunction with other materials, surface modification, etc. have been reported to improve electrical properties of Fe2O3 which unfortunately cause poor light absorption. In this study, three-dimensional tin oxide (SnO2) nanohelix (NH) structures were fabricated as scaffolds for a thin Fe2O3 layer on fluorine-doped SnO2 glass substrate by oblique angle deposition method using electron beam evaporator. SnO2 NH structures based photoelectrode can simultaneously enhance light absorption and charge separation and transport for efficient PEC water splitting by trapping the incident light through strong light scattering effect as well as by providing pathways for charge separation and transport and large surface area. In addition, SnO2 NH structures are easy to be hybridized with Fe2O3 through only simple solution-based spin-coating method due to its high porosity, aspect ratio and large surface area. Thin Fe2O3 layer was coated along the surface of SnO2 NH structures forming a type-II band hetero-junction and these Fe2O3 coated SnO2 NH are significantly desirable for efficient visible light driven water splitting by simultaneously improving light absorption, charge separation and transport. Highly efficient PEC performance of our hybridized photoelectrode (including more than 5.0mAcm-2 photocurrent density at 1.23VRHE under AM1.5G solar spectrum and 1 sun illumination which is a significantly excellent performance for Fe2O3 based photoelectode) will be investigated and the advantages of three-dimensional SnO2 NH structures as scaffolds for very thin Fe2O3 layer as well as its potential for practical application will be discussed in detail based on electrochemical analysis, FEM simulation, angular-dependent reflectance, PEC analysis at different intensity of incident light, etc.

Defect-Pairs of Titanium and Carbon in Iron Oxide Film for Efficient Visible Light Driven Water Splitting

Solar-powered photoelectrochemical (PEC) water splitting has been a promising candidate for producing hydrogen in a clean and renewable way. Photoelectrodes are key components in PEC cells for efficient and stable hydrogen generation because they play crucial roles in absorption of photons, the separation and transportation of photo-generated charge carriers, as well as the chemical reactions with water. A variety of metal oxides for efficient photoelectrode have been intensively explored, but it is still challenging to find desirable materials to satisfy lots of requirements for PEC water splitting. Iron oxide (hematite, Fe2O3) has recently attracted much attention due to its earth abundance, low cost as well as desirable material properties for PEC water oxidation including narrow band gap energy of 2.0~2.2eV for visible light absorption and proper energy band alignment, etc. However, Fe2O3 has very short hole diffusion length and low carrier mobility, which causes considerable recombination of photo-generated electrons and holes. A lot of approaches such as nanostructures, heterojunction with other materials, surface modification, etc. have been reported to prevent the recombination of charge carriers and improve electrical properties of Fe2O3; however, these require complex manufacturing processes. In the present work, we found a much simpler way to improve the electrical properties of Fe2O3 film, namely defect-pairs due to co-doping. Titanium (Ti) and carbon (C) co-doped thin Fe2O3 film (i.e. (Ti,C)-Fe2O3) has been realized via a combination of simple solution-based spin-coating and tube furnace annealing process. This film turns out to lead significantly enhanced PEC performance when used as a photoanode: an impressively high photocurrent density of more than 4.5mAcm-2 was achieved at 1.23VRHE under AM1.5G solar spectrum and 1 sun illumination. This is compared to the value of Ti-doped Fe2O3 film, which is only about 2.6mAcm-2 photocurrent density at 1.23VRHE even though the optical properties of each film are similar. The origin for such substantial enhancement was revealed using a series of experimental and computational spectroscopies. X-ray absorption spectroscopy, electrochemical impedance measurements and density-functional-theory calculations both indicate that C atoms can be more deeply and heavily doped under the existence of Ti dopants in Fe2O3 film and then the defect-pairs of Ti and C increase not only charge carrier density but also electron’s mobility. An emphasis should be placed on the fact that this achievement was not assisted by co-catalysts and complex nanostructuring methods; hence even higher performance is expected when the film is further treated with extra-cares.