Enhanced electron collection efficiency of nanostructured dye‐sensitized solar cells by incorporating TiO2 cubes

Herein, enhancement of dye‐sensitized solar cell (DSC) performance is reported by combining the merits of the dye loading of TiO₂ nanoparticles and light scattering, straight carrier transport path, and efficient electron collection efficiency of TiO₂ cubes. We fabricate DSC devices with various arrangement styles and compositions of the electrodes in the forms of monolayer and double layer films. For this purpose, the solvothermal synthesized TiO₂ cubic particles (100‐600 nm) are employed as the scattering layer, whereas TiO₂ nanoparticles (15‐30 nm) synthesized via a combination of solvothermal and sol‐gel routes are used as the active layer of devices. We improve the photovoltaic characteristics of DSCs by two mechanisms. First, the light harvesting of DSC devices made of nanoparticles is improved by controlling the thickness of monolayer films, reaching the highest efficiency of 7.0%. Second, the light scattering and electron collection efficiency are enhanced by controlling the composition of double layer films composed of mixtures of TiO₂ nanoparticles and cubes, obtaining the maximum efficiency of 8.21%. The enhancements are attributed to balance between charge transfer resistance and charge recombination of photo‐generated electrons as well as dye loading and light scattering.     

Enhanced photoresponse and photocatalytic activities of graphene quantum dots sensitized Ag/TiO2 thin film

TiO₂ thin films were fabricated through hydrothermal method. Silver nanoparticles were loaded on TiO₂ thin films via photoreduction technique. Subsequently, the graphene quantum dots (GQDs) were spin‐coated on the Ag/TiO₂ nanocomposites thin films. The crystal structure, surface morphology and UV‐vis absorbance were tested by XRD, SEM and ultraviolet‐visible spectrophotometer. These results indicated that Ag nanoparticles and GQDs are anchored on the TiO₂ nanorods. Absorbance of Ag/TiO₂ and GQDs/Ag/TiO₂ nanocomposite thin films have been extended into the visible region. Visible‐light response of the samples were investigated by electrochemical workstation. The photoresponse of the sample can be enhanced by sensitization of the Ag nanoparticles and GQDs. The enhanced visible‐light response may be due to the surface plasmon resonance of silver nanoparticles and visible absorbance of GQDs. The highest photocatalytic activity has been observed in the 9‐GQDs/Ag/TiO₂ composite thin film. The efficient charge separation and transportation can be achieved by introducing the Ag nanoparticles and GQDs in the TiO₂ thin film.     

MoS2 co-catalyst sensitized 3D TiO2/CdS photoanodes with enhanced photoelectrochemical performances

Regulating morphology and constructing heterojunctions to enhance the light absorption and boost the separation of electrons and holes are common and effective means to boost the photoelectrochemical (PEC) performances of TiO₂ photoanodes. In this study, TiO₂ nanoflowers (NFs)/CdS quantum dots (QDs)/MoS₂ nanosheets (NSs) hybrids with two type II band alignments were synthesized by facile hydrothermal, successive ionic layer adsorption and reaction, and dipping methods, respectively. The effects of different amount of MoS₂ co-catalysts on CdS decorated TiO₂ photoanodes were investigated. TiO₂ NFs/CdS QDs/MoS₂ NSs hybrids showed dramatically enhanced PEC performance, especially under visible light illumination. The photocurrent density of TiO₂ NFs/CdS QDs/MoS₂-50 was more than 10 times higher than that of TiO₂ NFs/CdS QDs. This innovative work sheds light on efficiently improving the light absorption by forming heterojunctions and accelerating the electron and hole transfer via specific band engineering design.     

Optimization of TiO2 paste concentration employed as electron transport layers in fully ambient air processed perovskite solar cells with a low-cost architecture

The optimization of thickness and surface roughness of the TiO₂ layer as an efficient electron transporting layer (ETL) plays a significant role on the performance improvement of perovskite solar cells (PSCs). In the present investigation, TiO₂ pastes synthesized with various concentrations under hydrothermal conditions were utilized to deposit the TiO₂ films of tunable porosities as the ETLs of PSCs. Also, the PSCs were fabricated with a structure of FTO/block-TiO₂ (b-TiO₂)/m-TiO₂/CH₃NH₃PbI₃ (MAPbI₃)/CuInS₂ (CIS)/carbon as a low-cost architecture. Moreover, the effect of the TiO₂ paste concentration was studied on the performances of PSCs under fully ambient conditions. The optimal TiO₂ layer was constructed with 20 wt% TiO₂ paste concentration, which resulted in the formation of a hole‐free, smooth, and compact ETL layer. The champion perovskite solar cell fabricated with the 20 wt% TiO₂ paste concentration showed the highest power conversion efficiency (PCE) of 13.09% (J_SC = 20.80 mA cm^?2, V_OC = 0.98 V and FF = 0.64) but the champion PSC device made with the 10 wt% TiO₂ paste exhibited the lowest PCE = 8.05% (J_SC = 19.83 mA cm^?2, V_OC = 0.91 V and FF = 0.45). These results illustrated that the optimal 20 wt% TiO₂ paste caused ～163% enhancement in the PCE of the device. Consequently, it could be suggested for application in fabrication of cost-effective and large scale PSCs.     

CuGaO2/TiO2 heterostructure nanosheets: Synthesis,enhanced photocatalytic performance,and underlying mechanism

Effective separation and fast transport of photogenerated carriers are vital links determining the photocatalytic performance. Heterostructure constructed by two complementary semiconductors is a feasible strategy to achieve this goal. By one-pot hydrothermal method, 0D-TiO₂ nanoparticles are loaded onto 2D-CuGaO₂ nanosheets, forming a mixed dimension, closely combined heterostructure. The photocurrent density of CuGaO₂/TiO₂ heterostructure is ∼16.6 μA/cm², which is 1.24 times higher than that of pristine CuGaO₂ nanosheets (∼13.4 μA/cm²) and 15 times higher than that of TiO₂ (∼1.1 μA/cm²). In the tetracycline hydrochloride degradation experiment, the degradation efficiency of tetracycline hydrochloride by CuGaO₂/TiO₂ heterostructure reached 99% within 90 min, which was 1.2 times the degradation efficiency of CuGaO₂ nanoparticles (82%) and 20.2 times the degradation rate of TiO₂ (4.9%). A series of experimental characterizations combined with density functional theory calculations revealed that it is the built-in electric field in the CuGaO₂/TiO₂ interface region that drives the photogenerated electron–hole pairs to travel in the opposite direction, thus inhibiting their recombination. Furthermore, the energy band offset of the CuGaO₂/TiO₂ interface makes it easier for the photogenerated holes and electrons to gather onto the valence band of the CuGaO₂ nanosheets and the conduction band of the TiO₂ nanoparticles, respectively. Therefore, appropriate interface lattice matching, suitable configuration of band gap and band edge positions, and strong opposite drive of interface electric field enable CuGaO₂/TiO₂ heterostructure to achieve wide spectral response and effective separation of photogenerated electron–hole pairs at the same time.     

Novel thin‐film reactor for photocatalytic degradation of pesticides in an aqueous solution

A thin‐film reactor was fabricated with immobilised TiO₂ and this reactor was used for photocatalytic mineralisation of common pesticides, 2,4‐dichlorophenoxyacetic acid (DPA) and monocrotophos (MCP), and their commercially formulated products in an aqueous solution. Zeolites HY and Hβ with different physico‐chemical properties were chosen as support material. The supported TiO₂ was used for the degradation and mineralisation studies. TiO₂/zeolite‐supported photocatalyst showed enhanced degradation efficiency compared with bare TiO₂ for both DPA and MCP. Formulated pesticides were mineralised at shorter irradiation times than technical grade pesticides. The results clearly demonstrated that the good adsorption capacity of the support, and the effective light utilisation by TiO₂, improved the photocatalytic activity of supported TiO₂. Reusability studies have also shown the stability of supported photocatalysts. Copyright © 2004 Society of Chemical Industry     

The enhanced performance of Cr(VI) photoreduction and antibiotic removal on 2D/3D TiO2/ZnIn2S4 nanostructures

In this study, 2D/3D TiO₂/ZnIn₂S₄ nanostructures with TiO₂ sheet cluster embedded into ZnIn₂S₄ micro flowers were fabricated via hydrothermal method. The matched band structure, the enlarged surface area and the efficient photo-induced charge transfer offered by effective heterostructures formed between the two components endowed the TiO₂/ZnIn₂S₄ nanoarchitecture with excellent photocatalytic Cr(VI) reduction and tetracycline hydrochloride (TC) degradation performance. Especially, the Cr(VI) photoreduction efficiency of 50% TiO₂/ZnIn₂S₄ was 8.8 times higher compared to pure ZnIn₂S₄. The enhanced separation efficiency of photo-excited charge carriers was induced by the matched band structure, the enlarged surface area and the strong interaction between TiO₂ and ZnIn₂S₄. The key roles of ·O₂^? was confirmed via trapping experiments. Otherwise, the pathway of TC degradation was investigated. The proposed mechanism during photocatalysis process was also discussed according to the photocatalytic and characterization results.     

Does photoelectrocatalysis by TiO2 work?

BACKGROUND: This paper examines TiO₂ photoelectrocatalysis (PEC), a process that increases the efficiency of TiO₂ photocatalysis (PC) by applying a potential to separate the UV‐generated charge carriers whose recombination typically limits photonic efficiencies of conventional photocatalysis. RESULTS: Four representative photoelectrocatalytic reactions, nitrophenol oxidation, oxalate degradation, E. coli inactivation and dye decolouration were considered. For all four, a small applied potential raised the rate of pollutant removal by TiO₂ electrodes. Because the improvements were probably insufficient to make PEC technologically viable except in niche applications, rates of pollutant removal by PEC and by PC using TiO₂ particle dispersions were directly compared. PEC rates were not significantly larger than rates of PC by dispersions. CONCLUSION: Discussions of the implications of these conclusions focus on whether PEC is currently limited by reactor design (irradiation geometry, or mass transfer) or by electrode materials. It is inferred that the performance of present electrodes is not limited significantly by mass transfer constraints. Since the choice of electrode materials (sol–gel or thermal electrodes) has been shown to influence PEC efficiency, recent results on titania nanotubes (TNT) are reviewed. It is concluded that the enhancement factors—the PEC:PC ratio—of TNT electrodes are no higher than those of conventional materials. Copyright © 2011 Society of Chemical Industry     

Preparation and photocatalytic performance of TiO2/PbTiO3 fiber composite enhanced by external force induced piezoelectric field

In this paper, anatase TiO₂ nanoparticles (14 nm) were coated on the surface of piezoelectric PbTiO₃ single crystal fibers by a secondary hydrothermal method, thus a p-n junction was formed at the interface between TiO₂ and PbTiO₃. The PbTiO₃ fibers were exposed to an external force (by ultrasound) to generate a dynamic piezoelectric electric field in the crystal, which incessantly separates the carriers and improves the photocatalytic efficiency of TiO₂. When the ultrasonic frequency is 40 kHz, the TiO₂/PbTiO₃ fibers have the highest photocatalytic degradation efficiency (96%) for methylene orange (MO), which is up to 60% improvement compared to TiO₂ nanoparticles.     

Construction of Au/TiO2 Heterojunction with high photocatalytic performances under UVA illumination

Anatase TiO₂ nanoparticles (NPs) were successfully prepared through a hydrothermal approach, and Au NPs at various Au (0.1–2 wt%) contents were photodeposited onto the TiO₂ NPs surface. The photocatalytic efficiency for the Au/TiO₂ NPs for resorcinol photodegradation throughout UVA illumination was assessed. The TEM images and XPS findings indicated that the Au NPs are highly distributed onto TiO₂ surface in the metallic state. The 0.1%Au/TiO₂ NPs exhibited the highest photocatalytic efficiency of about 95.34%; however, 72.36% is given by pure TiO₂ NPs. It was found that the photodegradation rate of 0.1% Au/TiO₂ NPs exhibited 1.5 times of magnitude higher than pure TiO₂ NPs. 0.1%Au/TiO₂ NPs was considered to be the outstanding photoactive due to the ultimate efficient charge-carriers separation through charge transfer between Au and TiO₂ NPs. The Au NPs sizes, its dispersity on TiO₂ surface and surface plasmon resonance (SPR) were believed the critical factors for the higher photocatalytic performance of 0.1% Au/TiO₂ NPs. The prepared photocatalysts are found to be the promising materials for toxic organic compounds remediation and solar conversion.     

Enhancement of photocatalytic and photoelectrochemical properties of BiOI nanosheets and silver quantum dots co-modified TiO2 nanorod arrays

In this study, TiO₂ nanorod arrays (TNR), Ag quantum dots (QDs) sensitized with TNR TiO₂/Ag, bismuth oxyhalide (BiOI) nanosheets, and Ag QDs co-modified with TNR and TiO₂/BiOI/Ag (TBA) were prepared by a stepwise process. The morphological, structural, compositional, optical, photocatalytic (PC), and photoelectrochemical (PEC) properties of the samples were investigated. The TBA-2 sample exhibited the highest photocurrent density (281.8 μA/cm²) and photodegradation efficiency (93.3%), with values 9.7 times and 2.25 times higher than those for TNR, respectively. The improvement in sample performance can be attributed to the formation of a heterojunction between BiOI and TiO₂, thereby enhancing the absorption of visible light and improving the charge separation efficiency; Ag QDs limit interfacial electron-hole pair recombination. The experimental results show that TBA can effectively promote light-induced carrier transport and visible light absorption, while inhibiting the recombination rate of the electron-hole pairs, PEC, and PC.     

Photovoltaic performance improvement of dye-sensitized solar cells based on tantalum-doped TiO2 thin films

Dye-sensitized solar cells based on a tantalum (Ta)-doped TiO₂ thin film prepared by the hydrothermal method show a photovoltaic efficiency of 8.18%, which is higher than that of the undoped TiO₂ thin film (7.40%). The Mott-Schottky plot indicates that the Ta-doped TiO₂ photoanode shifts the flat band potential positively and increases the electron density. The positive shift of the flat band potential improves the driving force of injected electrons from the LUMO of the dye to the conduction band of TiO₂. Furthermore, the increased electron density caused by the Ta-doped TiO₂ improves the fill factor of the solar cell. The increased electron density accelerates the transfer rate of electrons in the Ta-doped TiO₂ thin films by comparison to undoped films, which is confirmed by intensity-modulated photocurrent spectroscopy measurements.     

Enhanced electron harvesting in next generation solar cells by employing TiO2 nanoparticles prepared through hydrolysis catalytic process

Titanium dioxide (TiO₂) nanoparticles (NPs) were synthesized through solvothermal route by changing the rate of hydrolysis in the catalytic process. In order to change the hydrolysis rate, the concentration of acetic acid, as additive, was varied as 2 M, 3 M and 4 M. The synthesized NPs were examined by various physico-chemical characterization techniques. The powder X-ray diffraction (PXRD) analysis of the NPs reveals only the anatase phase of TiO_2. The spherical shaped morphology of the NPs was observed in the high-resolution transmission electron microscopy (HR-TEM) analysis. The optical behaviour such as absorption, bandgap, diffuse reflectance and photoluminescence (PL) emission of the NPs were studied. The material's nature and behaviors were scrutinized and they were employed as photoanode in dye sensitized solar cell (DSSC) and as electron transport layer (ETL) in carbon-based perovskite solar cell (C-PSC). The charge transfer at the interface of the devices was studied with electrochemical impedance spectroscopy (EIS). The fabricated DSSC and C-PSC show highest power conversion efficiency (PCE) of 6.1% and 10.6%, respectively. The highest current collection was detected in C-PSC and the results are discussed in detail.     

Investigation of nanostructured TiO2 surface and interface electric fields with photoreflectance spectroscopy

The electrical properties of buried solid–solid interfaces are essential to the optimization of devices such as dye‐sensitized solar cells and photocatalysts. The degree of fixed charge buildup at these interfaces can be sample‐dependent, influenced by only a small fraction of total surface sites, and challenging to quantify. This work describes the applicability of photoreflectance spectroscopy (PR) to the characterization of thin film nanostructured TiO₂. The approach involves the synthesis of polycrystalline anatase TiO₂ on quartz and Si(100) by atomic layer deposition with Ti(OCH(CH₃)₂)₄ and H₂O as precursors. PR reveals negligible band bending at the TiO₂ free surface. A distinct spectral feature at 299.0 ± 0.3 kJ/mol (3.10 ± 0.0031 eV) is attributed to electronic states at the TiO₂‐Si interface. Temporal variations in the magnitude of this feature are discussed in the context of bulk carrier concentration, solid–solid interface chemical reactions, and charge exchange between interface and grain boundary states and the bulk bands. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1049–1055, 2013     

Photocatalytic performance of Al and Nb-doped TiO2 nanoparticles prepared by microwave-assisted hydrothermal method

Microwave-assisted hydrothermal synthesis of aluminum (Al)-doped TiO₂ (ALT) and niobium (Nb)-doped TiO₂ (NBT) nanoparticles were carried out. Investigations were performed to examine the crystallinity, vibrational modes, surface morphology, and composition using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), respectively. The XRD study indicated the higher crystallinity of the ALT particles compared to the Al and Nb-doped TiO₂ (ALNBT) particles, and only the presence of the anatase phase was observed in all samples. As a result of doping, the Raman mode at ∼147 cm⁻¹ is found to be shifted toward a higher wavenumber for both samples. SEM analysis showed the spherical morphology of ALT and NBT nanoparticles. The elemental composition peaks of Al, titanium, Nb, and oxygen were noticed by EDS measurements. Furthermore, both prepared nanoparticles were used as photocatalytic materials. The Nb and Al-doped samples showed an improvement in the photocatalysis response in relation to the pure TiO₂ sample, in which Al-doped sample was able to decolorize 100% of rhodamine B in 75 min of analysis.     

Conversion enhancement of flexible dye-sensitized solar cells based on TiO2 nanotube arrays with TiO2 nanoparticles by electrophoretic deposition

We prepared highly ordered titanium dioxide nanotube arrays (TNAs) by anodizing Ti foils in F⁻ containing electrolyte. The thickness and dye loading amount of TNAs were 26 μm and 1.06 × 10⁻⁷ mol cm⁻², respectively. TiO₂ nanoparticles (TNPs) were electrophoretically deposited on the inner wall of nanotube to produce coated nanotube arrays (TNAP). The dye loading was increased to 1.56 × 10⁻⁷ mol cm⁻², and the electron transport rate improved. TNAs and TNAP were sensitized with ruthenium dye N3 to yield dye-sensitized TiO₂ nanotube solar cells. The power conversion efficiency of TNA-based dye-sensitized solar cells (DSSCs) was 4.28%, whereas the efficiency of TNAP-based DSSCs increased to 6.28% when illuminated from the counter electrode. The increase of power conversion efficiency of TNAP-based DSSCs is ascribed to the increased surface area of TNAs and the faster electron transport rate.     

Phenol photodegradation over titanium dioxide

The heterogeneous photocatalytic oxidation of phenol in aqueous diluted solutions over TiO₂ particles under UV-illumination has been investigated. It has been observed that the yield of the phenol photooxidation depends strongly on the pH of the solution. Maximum yields are obtained at pH 8 and also in very alkaline media. These results are explained considering the processes that take place at the surface of the semiconductor, in which OH radicals have an important role. With respect to the initial charge transfer steps a kinetic analysis has been performed. From this analysis it was deduced that the constant rate of formation of OH radicals is about 4.0 × 10⁴ times greater than the constant rate of the direct reaction between phenol and photogenerated holes in the TiO₂ particles. Finally, from HPLC analysis, hydroquinone, paraquinone and 1,2.4-benzenetriol have been detected as intermediate products prior to the total phenol mineralization.     

A maskless synthesis of TiO2-nanofiber-based hierarchical structures for solid-state dye-sensitized solar cells with improved performance

TiO₂ hierarchical nanostructures with secondary growth have been successfully synthesized on electrospun nanofibers via surfactant-free hydrothermal route. The effect of hydrothermal reaction time on the secondary nanostructures has been studied. The synthesized nanostructures comprise electrospun nanofibers which are polycrystalline with anatase phase and have single crystalline, rutile TiO₂ nanorod-like structures growing on them. These secondary nanostructures have a preferential growth direction [110]. UV–vis spectroscopy measurements point to better dye loading capability and incident photon to current conversion efficiency spectra show enhanced light harvesting of the synthesized hierarchical structures. Concomitantly, the dye molecules act as spacers between the conduction band electrons of TiO₂ and holes in the hole transporting medium, i.e., spiro-OMeTAD and thus enhance open circuit voltage. The charge transport and recombination effects are characterized by electrochemical impedance spectroscopy measurements. As a result of improved light harvesting, dye loading, and reduced recombination losses, the hierarchical nanofibers yield 2.14% electrochemical conversion efficiency which is 50% higher than the efficiency obtained by plain nanofibers.     

Optimization of the dye-sensitized solar cell performance by mechanical compression

In this study, the P25 titanium dioxide (TiO₂) nanoparticle (NP) thin film was coated on the fluorine-doped tin oxide (FTO) glass substrate by a doctor blade method. The film then compressed mechanically to be the photoanode of dye-sensitized solar cells (DSSCs). Various compression pressures on TiO₂ NP film were tested to optimize the performance of DSSCs. The mechanical compression reduces TiO₂ inter-particle distance improving the electron transport efficiency. The UV–vis spectrophotometer and electrochemical impedance spectroscopy (EIS) were employed to quantify the light-harvesting efficiency and the charge transport impedance at various interfaces in DSSC, respectively. The incident photon-to-current conversion efficiency was also monitored. The results show that when the DSSC fabricated by the TiO₂ NP thin film compressed at pressure of 279 kg/cm², the minimum resistance of 9.38 Ω at dye/TiO₂ NP/electrolyte interfaces, the maximum short-circuit photocurrent density of 15.11 mA/cm², and the photoelectric conversion efficiency of 5.94% were observed. Compared to the DSSC fabricated by the non-compression of TiO₂ NP thin film, the overall conversion efficiency is improved over 19.5%. The study proves that under suitable compression pressure the performance of DSSC can be optimized.     

Inverted pyramid Er3+ and Yb3+ Co-doped TiO2 nanorod arrays based perovskite solar cell: Infrared response and improved current density

In this study, a Yb³⁺, Er³⁺ co-doped TiO₂ inverted pyramid nanorod (NR) array and a compact TiO₂ film are simultaneously fabricated as the mesoporous support layer and electron-blocking layer, respectively, by a one-pot hydrothermal method. The scanning electron microscopy results show that the incorporation of Er³⁺ and Yb³⁺ causes changes not only in the growth rate of the NRs, but also in the TiO₂ NR morphology. The Er³⁺, Yb³⁺ co-doped TiO₂ NRs exhibit an inverted pyramidal morphology, which is beneficial for perovskite permeation and light utilization. Notably, the Er³⁺, Yb³⁺ co-doping causes changes in the band gap of TiO₂ and leads to 25% increase in the current density. The electrochemical impedance spectroscopy results show that the device based on the doped TiO₂ NRs has a higher recombination resistance and a lower transfer resistance than those of the undoped device, and thereby, the doped device exhibits a lower electron recombination rate. In addition, the upconversion Er and Yb co-doped device exhibits 25% higher current density and 17% higher photon-to-electron conversion efficiency, as revealed by the J-V test results. Moreover, the optimized efficiency of the TiO₂ NR array-based perovskite solar cell is determined to be 10.02%. Furthermore, the Er and Yb co-doped device exhibits a near-infrared response, an efficiency of 0.1% is achieved under infrared light (800–1100 nm) irradiation. This upconversion material can widen the photovoltaic responses of solar cells into the near-infrared region and improve the utilization of sunlight.