Metal–Organic Framework Derived Co3O4/TiO2/Si Heterostructured Nanorod Array Photoanodes for Efficient Photoelectrochemical Water Oxidation

A novel hierarchical structured photoanode based on metal–organic frameworks (MOFs)‐derived porous Co₃O₄‐modified TiO₂ nanorod array grown on Si (MOFs‐derived Co₃O₄/TiO₂/Si) is developed as photoanode for efficiently photoelectrochemical (PEC) water oxidation. The ternary Co₃O₄/TiO₂/Si heterojunction displays enhanced carrier separation performance and electron injection efficiency. In the ternary system, an abnormal type‐II heterojunction between TiO₂ and Si is introduced, because the conduction band and valence band position of Si are higher than those of TiO₂, the photogenerated electrons from TiO₂ will rapidly recombine with the photogenerated holes from Si, thus leading to an efficient separation of photogenerated electrons from Si/holes from TiO₂ at the TiO₂/Si interface, greatly improving the separation efficiency of photogenerated hole within TiO₂ and enhances the photogenerated electron injection efficiency in Si. While the MOFs‐derived Co₃O₄ obviously improves the optical‐response performance and surface water oxidation kinetics due to the large specific surface area and porous channel structure. Compared with MOFs‐derived Co₃O₄/TiO₂/FTO photoanode, the synergistic function in the MOFs‐derived Co₃O₄/TiO₂/Si NR photoanode brings greatly enhanced photoconversion efficiency of 0.54% (1.04 V vs reversible hydrogen electrode) and photocurrent density of 2.71 mA cm^?2 in alkaline electrolyte. This work provides promising methods for constructing high‐performance PEC water splitting photoanode based on MOFs‐derived materials.     

Nickel Phosphide Clusters Sensitized TiO2 Nanotube Arrays as Highly Efficient Photoanode for Photoelectrocatalytic Urea Oxidation

The photoelectrocatalytic urea oxidation reaction (PEC-UOR) holds a great promise for the wastewater remediation and energy production. However, the low efficiency of semiconductor/cocatalysts type photoanodes for UOR restricts their applications in photoelectrocatalytic system. Herein, a new semiconductor/cocatalyst, Ni₂P clusters sensitized TiO₂ nanotube arrays photoanode (Ni₂P/TiO₂-NTAs) for PEC-UOR with high efficiency are developed. The 1D TiO₂-NTAs structure accelerates urea molecules diffusion and promotes CO₂ gas release at the electrode interface. Meanwhile, Ni₂P is also beneficial to urea molecule absorption and CO₂ desorption and enable to lower the energy barrier for amine (N H) dehydrogenation. Furthermore, the robust interfacial charge transfer pathway between Ni₂P and TiO₂ interface promotes the separation of photogenerated electrons and holes and the transfer of photogenerated electrons from Ni₂P to TiO₂. Therefore, this photoanode shows excellent PEC-UOR performance with the potential of 1.43 V versus reversible hydrogen electrode (RHE) when the current density reaches 10 mA cm⁻², which is much lower than that of 2.24 V versus RHE and 1.58 V versus RHE for TiO₂-NTAs and Ni(OH)₂/TiO₂-NTAs, respectively.     

Dye‐sensitized TiO2 solar cells based on nanocomposite photoanode containing plasma‐modified multi‐walled carbon nanotubes

A series of anatase TiO₂‐based nanocomposite incorporated with plasma‐modified multi‐walled carbon nanotubes (MWNTs) was prepared by physical blending and shows its capability for efficient electron transport when used as photoanode in dye‐sensitized solar cells (DSSCs). These MWNTs characterized with good dispersal performance were obtained by functionalization technique via in situ plasma treatment and subsequent grafting with maleic anhydride (MA) onto the external walls reported previously. Compared with the conventional DSSCs, the TiO₂ film with 1D carbon nanotubes possesses more outstanding ability to transport electrons injected from the excited dye within the device under illumination. As a result, at an optimum addition of 0.3 wt% MWNTs‐MA in TiO₂ matrix, the photocurrent–voltage (J–V) characteristics showed a significant increase in the short‐circuit photocurrent (J_sc) of 50%, leading to an increase in overall solar conversion efficiency by a factor of 1.5. Electrochemical impedance spectroscopy analyses reveal that the MWNTs‐MA/TiO₂ incur smaller resistances at the photoanode in assembled DSSCs when compared with those in the anatase titania DSSCs. These features suggest that the conducting properties of the MWNTs‐MA within the anodes are crucial for achieving a higher transport rate for photo‐induced electrons in TiO₂ layer by exhibiting lower resistance in the porous network and hence retard charge recombination that could result in poor conversion efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

A New Material for High‐Temperature Lead‐Free Actuators

Unimorph cantilevers are made from 0.5BaTiO₃‐0.5Sm₂O₃ (BTO‐SmO) self‐assembled vertical heteroepitaxial nanocomposite thin films, grown by PLD on (001) SrTiO₃ single crystal substrates. The films remain piezoelectric up to at least 250 °C without losing any actuation. The longitudinal piezoelectric coefficient, d₃₃, is ≈45 to 50 pm V^?1 measured from room temperature to 250 °C. The transverse piezoelectric coefficient, d₃₁, a key parameter of actuator performance, exceeds PZT (Pb_1–xZr_xTiO₃) films at >200 pm V^?1. Since the d₃₁ coefficient was found to be positive, this opens up exciting new applications opportunities. The possible reasons for d₃₁ > 0 are discussed in the light of 3D strain control in the nanocomposites.     

Evaluating the Critical Thickness of TiO2 Layer on Insulating Mesoporous Templates for Efficient Current Collection in Dye‐Sensitized Solar Cells

In this paper, a way of utilizing thin and conformal overlayer of titanium dioxide on an insulating mesoporous template as a photoanode for dye‐sensitized solar cells is presented. Different thicknesses of TiO₂ ranging from 1 to 15 nm are deposited on the surface of the template by atomic layer deposition. This systematic study helps unraveling the minimum critical thickness of the TiO₂ overlayer required to transport the photogenerated electrons efficiently. A merely 6‐nm‐thick TiO₂ film on a 3‐μm mesoporous insulating substrate is shown to transport 8 mA/cm² of photocurrent density along with ≈900 mV of open‐circuit potential when using our standard donor‐π‐acceptor sensitizer and Co(bipyridine) redox mediator.     

An Effective Approach for High‐Efficiency Photoelectrochemical Solar Cells by Using Bifunctional DNA Molecules Modified Photoanode

This paper firstly reports the effect of deoxyribonucleic acid (DNA) molecules extracted from chickpea and wheat plants on the injection/recombination of photogenerated electrons and sensitizing ability of dye‐sensitized solar cells (DSSCs). These high‐yield DNA molecules are applied as both linker bridging unit as well as thin tunneling barrier (TTB) at titanium dioxide (TiO₂ )/dye interface, to build up high‐efficient DSSCs. With its favorable energy levels, effective linker bridging role, and double helix structure, bifunctional DNA modifier shows an efficient electron injection, suppressed charge recombination, longer electron lifetime, and higher light harvesting efficiency, which leads to higher photovoltaic performance. In particular, a photoconversion efficiency (PCE) of 9.23% is achieved by the binary chickpea and wheat DNA‐modified TiO₂ (CW@TiO₂) photoanode. Furthermore, time‐resolved fluorescence spectroscopy measurements confirm a better electron transfer kinetics for DNA‐modified TiO₂ photoanodes, implying a higher electron transfer rate (k_ET). This work highlights a great contribution for the photoanodes that are linked with DNA molecule, which act as both bridging unit and TTB to control the charge recombination and injection dynamics, and hence, boost the photovoltaic performance in the DSSCs.     

Solar Cells by Design: Photoelectrochemistry of TiO2 Nanorod Arrays Decorated with CdSe

One‐dimensional (1D) nanostructures of TiO₂ are grown directly on transparent, conductive glass substrate using hydrothermal/solvothermal methods. When employed as a photoanode in photoelectrochemical cells, the vertically aligned TiO₂ nanorod array exhibits slower charge recombination at electrolyte interface as compared to mesoscopic TiO₂ particulate film. Electrochemical deposition of CdSe onto TiO₂ nanorod array is carried out to extend absorption into visible light region. The role of CdSe‐sensitized, 1D rutile TiO₂ architecture in the solar cell design is discussed.     

A Cocatalytic Electron‐Transfer Cascade Site‐Selectively Placed on TiO2 Nanotubes Yields Enhanced Photocatalytic H2 Evolution

Separation and transfer of photogenerated charge carriers are key elements in designing photocatalysts. TiO₂ in numerous geometries has been for many years the most studied photocatalyst. To overcome kinetic limitations and achieve swift charge transfer, TiO₂ has been widely investigated with cocatalysts that are commonly randomly placed nanoparticles on a TiO₂ surface. The poor control over cocatalyst placement in powder technology approaches can drastically hamper the photocatalytic efficiencies. Here in contrast it is shown that the site‐selective placement of suitable charge‐separation and charge‐transfer cocatalysts on a defined TiO₂ nanotube morphology can provide an enhancement of the photocatalytic reactivity. A TiO₂–WO₃–Au electron‐transfer cascade photocatalyst is designed with nanoscale precision for H₂ production on TiO₂ nanotube arrays. Key aspects in the construction are the placement of the WO₃/Au element at the nanotube top by site‐selective deposition and self‐ordered thermal dewetting of Au. In the ideal configuration, WO₃ acts as a buffer layer for TiO₂ conduction band electrons, allowing for their efficient transfer to the Au nanoparticles and then to a suitable environment for H₂ generation, while TiO₂ holes due to intrinsic upward band bending in the nanotube walls and short diffusion length undergo a facilitated transfer to the electrolyte where oxidation of hole‐scavenger molecules takes place. These photocatalytic structures can achieve H₂ generation rates significantly higher than any individual cocatalyst–TiO₂ combination, including a classic noble metal–TiO₂ configuration.     

Highly Conductive Cable‐Like Bicomponent Titania Photoanode Approaching Limitation of Electron and Hole Collection

TiO₂‐based materials are cheap and stable choices for photoelectrochemical devices. However, the activity is still limited by the inefficient charge extraction. Here a highly conductive cable‐like bicomponent titania photoanode, consisting of reduced anatase‐coated TiO₂‐B nanowires, is proposed to simultaneously establish effective electron and hole transport channels separately, which meets the requirements of electronic dynamics for efficient water splitting. A synergistic effect of charge separation from the built‐in electric field is demonstrated with this 1D TiO₂‐B/anatase heterojunction, in which a high electron collection efficiency of up to 97.1% at 0.6 V versus reversible hydrogen electrode is achieved. The efficient electron collection approaching the limitation is also attributed to the large electron conducting region in the photoanode. Moreover, the O‐deficient amorphous layer is found to be more catalytic toward the oxygen evolution reaction through quantifying rate constants for charge recombination and charge transfer. It can reduce onset potential and suppress charge‐carrier recombination simultaneously, prompting surface hole collection efficiency up to 95% at 0.6 V versus reversible hydrogen electrode.     

An efficient titanium‐based photoanode for dye‐sensitized solar cell under back‐side illumination

Pretreatment of H₂O₂ is performed on titanium (Ti) foil as an efficient photoanode substrate for dye‐sensitized solar cell (DSSC). The H₂O₂‐treated Ti shows high surface area because of the formation of networked TiO₂ nanosheets, which enhances electrical contact between screen‐printed TiO₂ nanoparticles and Ti foil. Electron transfer on the photoanode is improved, as identified by reduced charge transfer resistance and improved electron transport properties. Compared with DSSC based on non‐treated Ti photoanode, DSSC with this H₂O₂‐treated Ti photoanode exhibits remarkable increases in short‐circuit current density (from 8.55 to 14.38 mA/cm²) and energy conversion efficiency (from 4.68 to 7.10%) under AM1.5 back‐side illumination. Copyright © 2011 John Wiley & Sons, Ltd.     

An Interface Engineered Multicolor Photodetector Based on n‐Si(111)/TiO2 Nanorod Array Heterojunction

A multicolor photodetector based on the heterojunction of n‐Si(111)/TiO₂ nanorod arrays responding to both ultraviolet (UV) and visible light is developed by utilizing interface engineering. The photodetector is fabricated via a consecutive process including chemical etching, magnetron sputtering, hydrothermal growth, and assembling. Under a small reverse bias (from 0 to ≈?2 V), only the photogenerated electrons in TiO₂ are possible to tunnel through the low barrier of ΔE_C, and the device only responses to UV light; as the reverse bias increases, the photogenerated holes in Si also begin to tunnel through the high barrier of ΔE_V. As a result, the device is demonstrated to have the capacity to detect both UV and visible lights, which is useful in the fields of rapid detection and multicolor imaging. It has been also observed that the crystal orientation of Si affects the characteristics of bias‐controlled spectral response of the n‐Si/TiO₂ heterojunctions.     

An In Situ Simultaneous Reduction‐Hydrolysis Technique for Fabrication of TiO2‐Graphene 2D Sandwich‐Like Hybrid Nanosheets: Graphene‐Promoted Selectivity of Photocatalytic‐Driven Hydrogenation and Coupling of CO2 into Methane and Ethane

A novel, in situ simultaneous reduction‐hydrolysis technique (SRH) is developed for fabrication of TiO₂‐‐graphene hybrid nanosheets in a binary ethylenediamine (En)/H₂O solvent. The SRH technique is based on the mechanism of the simultaneous reduction of graphene oxide (GO) into graphene by En and the formation of TiO₂ nanoparticles through hydrolysis of titanium (IV) (ammonium lactato) dihydroxybis, subsequently in situ loading onto graphene through chemical bonds (Ti–O–C bond) to form 2D sandwich‐like nanostructure. The dispersion of TiO₂ hinders the collapse and restacking of exfoliated sheets of graphene during reduction process. In contrast with prevenient G‐TiO₂ nanocomposites, abundant Ti³⁺ is detected on the surface of TiO₂ of the present hybrid, caused by reducing agent En. The Ti³⁺ sites on the surface can serve as sites for trapping photogenerated electrons to prevent recombination of electron–hole pairs. The high photocatalytic activity of G‐TiO₂ hybrid is confirmed by photocatalytic conversion of CO₂ to valuable hydrocarbons (CH₄ and C₂H₆) in the presence of water vapor. The synergistic effect of the surface‐Ti³⁺ sites and graphene favors the generation of C₂H₆, and the yield of the C₂H₆ increases with the content of incorporated graphene. The work may open a new doorway for new significant application of graphene for selectively catalytic C–C coupling reaction     

Self‐Assembling TiO2 Nanorods on Large Graphene Oxide Sheets at a Two‐Phase Interface and Their Anti‐Recombination in Photocatalytic Applications

TiO₂ nanorods are self‐assembled on the graphene oxide (GO) sheets at the water/toluene interface. The self‐assembled GO–TiO₂ nanorod composites (GO–TiO₂ NRCs) can be dispersed in water. The effective anchoring of TiO₂ nanorods on the whole GO sheets is confirmed by transmission electron microscopy (TEM), X‐ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR), and thermogravimetric analysis (TGA). The significant increase of photocatalytic activity is confirmed by the degradation of methylene blue (MB) under UV light irridiation. The large enhancement of photocatalytic activity is caused by the effective charge anti‐recombination and the effective absorption of MB on GO. The effective charge transfer from TiO₂ to GO sheets is confirmed by the significant photoluminescence quenching of TiO₂ nanorods, which can effectively prevent the charge recombination during photocatalytic process. The effective absorption of MB on GO is confirmed by the UV‐vis spectra. The degradation rate of MB in the second cycle is faster than that in the first cycle because of the reduction of GO under UV light irradiation.     

Metal–Organic Framework Derived Narrow Bandgap Cobalt Carbide Sensitized Titanium Dioxide Nanocage for Superior Photo‐Electrochemical Water Oxidation Performance

Despite recent progress in photo‐electrochemical (PEC) water oxidation systems for TiO₂‐based photoanodes, PEC performance improvement is still seriously hampered due to poor carrier transport efficiency and sluggish surface water oxidation kinetics of pristine TiO₂. Herein, for the first time a brand new metal–organic framework (MOF)‐derived Co₃C nanosheet with narrow bandgap energy is demonstrated, to effectively sensitize TiO₂ hollow cages as a heterostructure photoanode for PEC water oxidation. It is found that MOF‐derived Co₃C nanosheet with narrow bandgap characteristic can simultaneously accelerate the surface water oxidation kinetics and extend the light harvesting range of pristine TiO₂. Meanwhile, a uniquely matched type‐II heterojunction constructed between MOF‐derived Co₃C and TiO₂ results in an evidently spontaneous e^?/h⁺ separation. MOF‐derived Co₃C/TiO₂ heterostructure photoanodes bring about drastically improved PEC water oxidation performance. Specifically, MOF‐derived Co₃C‐3/TiO₂ photoanode with an optimized content of Co₃C achieves the highest photocurrent density and charge separation efficiency of 2.6 mA cm^?2 and 92.6% at 1.23 V versus reversible hydrogen electrode, corresponding to 201% and 152% improvement compared with pristine TiO₂ nanocages. The ingeniously prepared MOF‐derived Co₃C carbide with narrow bandgap energy as a cocatalyst paves new way to construct potentially high performance solar‐energy conversion system.     

Enhanced photoelectrocatalytic activity of Cr-doped TiO2 nanotubes modified with polyaniline

A facile method for preparation of Cr-doped TiO₂ nanotubes (Cr–TiO₂ NTs) modified with polyaniline (PANI) was developed. The obtained materials were analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The photoelectrochemical property of PANI/Cr–TiO₂ nanotubes was studied by voltammetry, photocurrent and electrochemical impedance spectroscopy (EIS). Using PANI/Cr–TiO₂ NTs as photoanode, the removal of p-nitrophenol (PNP) by photoelectrocatalytic oxidation technique was investigated. Compared with Cr–TiO₂ NTs, PANI/Cr–TiO₂ NTs showed an increased efficiency in the photoelectrocatalytic degradation of PNP. Moreover, photoelectrocatalysis was more efficient for PNP degradation than electrochemical oxidation, direct photolysis, and photocatalysis. The influences of applied bias potential, initial concentration of PNP and solution pH on the photoelectrocatalytic degradation of PNP were investigated. Under optimized conditions, almost all PNP could be degraded on PANI/Cr–TiO₂ NTs after 2-h photoelectrocatalytic treatment.     

Honeycomb‐Like Organized TiO2 Photoanodes with Dual Pores for Solid‐State Dye‐Sensitized Solar Cells

A solid‐state dye‐sensitized solar cell (ssDSSC) with 7.4% efficiency at 100 mW/cm² is reported. This efficiency is one of the highest observed for N719 dye. High performance is achieved via a honeycomb‐like, organized mesoporous TiO₂ photoanode with dual pores, high porosity, good interconnectivity, and excellent light scattering properties. The TiO₂ photoanodes are prepared without any TiCl₄ treatment via a one‐step, direct self‐assembly of hydrophilically preformed TiO₂ nanocrystals and poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer as a titania source and a structure‐directing agent, respectively. Upon controlling the secondary forces between the polymer/TiO₂ hybrid and the solvent by varying the amounts of HCl/H₂O mixture or toluene, honeycomb‐like structures are generated to improve light scattering properties. Such multifunctional nanostructures with dual pores provide good pore‐filling of solid polymer electrolyte with large volume, enhanced light harvesting and reduced charge recombination, as confirmed by reflectance spectroscopy, incident photon‐to‐electron conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) analysis.     

A Band‐Edge Potential Gradient Heterostructure to Enhance Electron Extraction Efficiency of the Electron Transport Layer in High‐Performance Perovskite Solar Cells

As the key component in efficient perovskite solar cells, the electron transport layer (ETL) can selectively collect photogenerated charge carriers produced in perovskite absorbers and prevent the recombination of carriers at interfaces, thus ensuring a high power conversion efficiency. Compared with the conventional single‐ or dual‐layered ETLs, a gradient heterojunction (GHJ) strategy is more attractive to facilitate charge separation because the potential gradient created at an appropriately structured heterojunction can act as a driving force to regulate the electron transport toward a desired direction. Here, a SnO₂/TiO₂ GHJ interlayer configuration inside the ETL is reported to simultaneously achieve effective extraction and efficient transport of photoelectrons. With such an interlayer configuration, the GHJs formed at the perovskite/ETL interface act collectively to extract photogenerated electrons from the perovskite layer, while GHJs formed at the boundaries of the interconnected SnO₂ and TiO₂ networks throughout the entire ETL layer can extract electron from the slow electron mobility TiO₂ network to the high electron mobility SnO₂ network. Devices based on GHJ ETL exhibit a champion power conversion efficiency of 18.08%, which is significantly higher than that obtained from the compact TiO₂ ETL constructed under the comparable conditions.     

Boosting the Efficiency of Perovskite Solar Cells with CsBr‐Modified Mesoporous TiO2 Beads as Electron‐Selective Contact

Rapid extraction of photogenerated charge carriers is essential to achieve high efficiencies with perovskite solar cells (PSCs). Here, a new mesoscopic architecture as electron‐selective contact for PSCs featuring 40 nm sized TiO₂ beads endowed with mesopores of a few nanometer diameters is introduced. The bimodal pore distribution inherent to these films produces a very large contact area of 200 m² g^?1 whose access by the perovskite light absorber is facilitated by the interstitial voids between the particles. Modification of the TiO₂ surface by CsBr further strengthens its interaction with the perovskite. As a result, photogenerated electrons are extracted rapidly producing a very high fill factor of close to 80% a V_OC of 1.14 V and a PCE up to 21% with negligible hysteresis.     

Room‐Temperature Synthesis of Porous Nanoparticulate TiO2 Films for Flexible Dye‐Sensitized Solar Cells

A novel room‐temperature method for the preparation of porous TiO₂ films with high performance in dye‐sensitized solar cells (DSSCs) has been developed. In this method a small amount of Ti^IV tetraisopropoxide (TTIP) is added to an ethanolic paste of TiO₂ nanoparticles, where it hydrolyzes in situ and connects the TiO₂ particles to form a homogenous and mechanically stable film of up to 10 μm thickness without crack formation. Residual organics originating from the TTIP were removed by UV–ozone treatment of the films, leading to a remarkable improvement of the cell efficiency. Intensity‐modulated photocurrent/voltage spectroscopy (IMPS/IMVS) showed that the main effect of the UV–ozone treatment is to suppress the recombination of photogenerated electrons, thereby extending their lifetime. The efficiency was further increased by preheating the TiO₂ nanoparticles before the paste preparation to remove contaminants originating from the preparation process of the particles. Solar‐to‐electric energy conversion efficiencies of 4.00 and 3.27 % have been achieved for cells with conductive glass and plastic film substrates, respectively, under illumination with AM 1.5 (100 mW cm^–2) simulated sunlight.     

Improved‐Performance Dye‐Sensitized Solar Cells Using Nb‐Doped TiO2 Electrodes: Efficient Electron Injection and Transfer

Well‐crystallized Nb‐doped anatase TiO₂ nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye‐sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO₂ lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line‐scanning analyses. After Nb doping, the conductivity of the TiO₂ powder increases, and its flat‐band potential (V_fb) has a positive shift. The energy‐conversion efficiency of a cell based on 5.0 mol% Nb‐doped TiO₂ is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO₂. The as‐prepared Nb‐doped TiO₂ material is proven in detail to be a better photoanode material than pure TiO₂, and this new synthetic approach using a water‐soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.