Titanium Dioxide Nanomaterials for Photovoltaic Applications

Fabrication of metal/semiconductor nanocomposites by selective laser nano-welding

A green and simple method to prepare metal/semiconductor nanocomposites by selective laser nano-welding metal and semiconductor nanoparticles was presented, in which the sizes, phases, and morphologies of the components can be maintained. Many types of nanocomposites (such as Ag/TiO₂, Ag/SnO₂, Ag/ZnO₂, Pt/TiO₂, Pt/SnO₂, and Pt/ZnO) can be prepared by this method and their corresponding performances were enhanced.

Ultrathin TiO2(B) Nanosheets as the Inductive Agent for Transfrering H2O2 into Superoxide Radicals

A reflux method to synthesize ultrathin polycrystalline TiO₂(B) nanosheets (NSs) which are assembled by single crystals, and further stacked into nanoflower structures, is described. On the basis of the theoretical calculations and experiments, H₂O₂ can easily substitute the ethylene glycol adsorbed on the surface of TiO₂(B) NSs, forming H₂O₂-NS due to the lower adsorption energy and the unique structural features of ultrathin TiO₂(B) nanosheets. TiO₂(B) NSs and the H₂O₂ system can be accelerated to generate superoxide radicals under heat or light and thus exhibit a great degradation property on dye molecules; the total organic carbon (TOC) removal rate was 6 times higher than that for H₂O₂ alone. Meanwhile, TiO₂(B) NSs and the H₂O₂ system have a good application on the selective oxidation due to the reactive species of superoxide radicals avoiding overoxidization of benzyl alcohol. The conversion of benzyl alcohol oxidized to benzaldehyde in water solution under low temperature and atmospheric pressure was 51.13%, while the selectivity was close to 100%. We believe that the present findings will provide valuable methods for highly efficient generation of superoxide radicals and broaden their applications in catalysis.

Synthesis, properties, and applications of black titanium dioxide nanomaterials

Photocatalysis has been regarded as one of best solutions to using the sunlight to produce hydrogen from water and to removing organic pollutants from the environment, and titanium dioxide (TiO₂) nanomaterials have been treated as the primary photocatalyst for these purposes. However, their large band gap has largely limited the activity to the UV region of the solar spectrum. The discovery of black TiO₂ in 2011 has triggered world-wide research interests with new hope to overcome this problem. This review briefly summarizes the recent progresses of black TiO₂ nanomaterials, including their synthesis, properties and applications, to provide a timely update and to inspire more ideas in the related research.

Theoretically Manipulating Quantum Dots on Two-Dimensional TiO2 Monolayer for Effective Visible Light Absorption

Low solar energy harvesting and conversion efficiency has become a major problem in solar energy science and engineering owing to the difficulty in capturing solar energy across the wide solar spectrum, especially in the visible light range. Inspired by the extraordinary properties of materials arising from decreased dimensions, in this study, we explore a nanocontact system formed by a two-dimensional (2D) TiO₂ monolayer and II-VI semiconductor (CdX)₁₃ (X = S, Se, and Te) nanocages for engineering the visible light absorption. The nanocontact system, via either Ti-X or Cd-O bond coupling mechanism, forms an ideal type II band alignment, where the stronger donor-acceptor coupling in the Ti-X contact system more efficiently relaxes the coupled geometry and helps it to couple to more electrons, therefore leading to an enhancement of the absorption peaks in the visible frequency range. On changing the element X in (CdX)₁₃ from S to Se then Te, a red shift of the visible light absorption peaks accompanied by stimulating optical response of the whole nanocontact system was observed. Nanocontacting semiconductors comprising low-dimensional (CdX)₁₃ nanocage@TiO₂ monolayer systems, which promote charge separation and optical absorption in the visible range that arise from the effects of adsorbent nature, decreasing size, and efficient interfacial coupling mechanism, are therefore promising photovoltaic and photocatalytic materials.

Alloying Strategy in Cu-In-Ga-Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells

I-III-VI₂ group "green" quantum dots (QDs) are attracting increasing attention in photoelectronic conversion applications. Herein, on the basis of the "simultaneous nucleation and growth" approach, Cu-In-Ga-Se (CIGSe) QDs with light harvesting range of about 1000 nm were synthesized and used as sensitizer to construct quantum dot sensitized solar cells (QDSCs). Inductively coupled plasma atomic emission spectrometry (ICP-AES), wild-angle X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the Ga element was alloyed in the Cu-In-Se (CISe) host. Ultraviolet photoelectron spectroscopy (UPS) and femtosecond (fs) resolution transient absorption (TA) measurement results indicate that the alloying strategy could optimize the electronic structure in the obtained CIGSe QD material, thus matching well with TiO₂ substrate and favoring the photogenerated electron extraction. Open circuit voltage decay (OCVD) and impedance spectroscopy (IS) tests indicate that the intrinsic recombination in CIGSe QDSCs was well suppressed relative to that in CISe QDSCs. As a result, CIGSe based QDSCs with use of titanium mesh supported mesoporous carbon counter electrode exhibited a champion efficiency of 11.49% (J_sc = 25.01 mA/cm², V_oc = 0.740 V, FF = 0.621) under the irradiation of full one sun in comparison with 9.46% for CISe QDSCs.

Functionalized TiO2 nanoparticles by single-step hydrothermal synthesis: the role of the silane coupling agents

A simple, robust and versatile hydrothermal synthesis route to in situ functionalized TiO₂ nanoparticles was developed using titanium(IV) isopropoxide as Ti-precursor and selected silane coupling agents (3-aminopropyltriethoxysilane (APTES), 3-(2-aminoethylamino)propyldimethoxymethylsilane (AEAPS), and n-decyltriethoxysilane (DTES)). Spherical nanoparticles (ca. 9 nm) with narrow size distribution were obtained by using DTES or by synthesis performed without silane coupling agents. Rod-like nanoparticles along with 9 nm spherical nanoparticles were formed using aminosilane coupling agents because of a combination of oriented attachment of nanoparticles and specific adsorption of the aminosilane on crystallographic faces of anatase nanoparticles. The nanoparticles were functionalized in situ and became hydrophobic as silanes reacted to form covalent bonds on the surface of TiO₂. The versatility of the aqueous synthesis route was demonstrated, and by selecting the type of silane coupling agent the surface properties of the TiO₂ nanoparticles could be tailored. This synthesis route has been further developed into a two-step synthesis to TiO₂-SiO₂ core-shell nanoparticles. Combustion of the silane coupling agents up to 700 °C leads to the formation of a nanometric amorphous SiO₂ layer, preventing growth and phase transition of the in situ functionalized nanoparticles.

Insights into the Formation and Properties of Templated Dual Mesoporous Titania with Enhanced Photocatalytic Activity

The one pot synthesis of dual mesoporous titania (2.3 and 7.7 nm) has been achieved from a mixture of fluorinated and Pluronic surfactants. The small and large mesopore networks are templated, respectively, by a fluorinated-rich liquid crystal and a Pluronic-rich liquid crystal, which are in equilibrium. After calcination at 350 °C, the amorphous walls are transformed into semicrystalline anatase preserving the mesoporous structure. Results concerning the photodegradation of methyl orange using the calcined photocatalysts highlight that the kinetic rate constant (k) determined for the dual mesoporous titania is 2.6 times higher than the k value obtained for the monomodal ones.

Microwave-solvothermal synthesis of various TiO2 nano-morphologies with enhanced efficiency by incorporating Ni nanoparticles in an electrolyte for dye-sensitized solar cells

A novel systematic approach is demonstrated to enhance the efficiency of dye-sensitized solar cells by impregnating Ni-nanoparticles into I⁻/I₃⁻ electrolyte with various TiO₂ nanomorphologies-based photo-anodes synthesized via microwave-solvothermal process.

Ag3PO4/CoFe2O4 magnetic nanocomposite: synthesis, characterization and applications in catalytic reduction of nitrophenols and sunlight-assisted photocatalytic degradation of organic dye pollutants

A novel magnetically recyclable Ag₃PO₄/CoFe₂O₄ nanocomposite was synthesized by a facile hydrothermal method and its photocatalytic/catalytic performance for the degradation of organic dyes or reduction of nitro compounds was investigated.

Insights into TiO2 polymorphs: highly selective synthesis, phase transition, and their polymorph-dependent properties

Herein we report the selective synthesis and two kinds of phase transformation of TiO₂ polymorphs under hydrothermal conditions.

Ultrathin TiO2nanosheets synthesized using a high pressure solvothermal method and the enhanced photoresponse performance of CH3NH3PbI3–TiO2composite films

Highly crystalline ultrathin (2–3 nm) TiO₂nanosheets are synthesized using a high pressure solvothermal method. The perovskite–TiO₂films exhibit strikingly enhanced photoresponse performance.

Boosting the photocatalytic H2 evolution activity of Fe2O3 polymorphs (α-, γ- and β-Fe2O3) by fullerene [C60]-modification and dye-sensitization under visible light irradiation

Different morphologies of Fe₂O₃ polymorphs were constructed and modified by fluorescein sensitization and C₆₀ cocatalyst to prepare highly active photocatalysts.

Double-anchoring organic dyes for dye-sensitized solar cells: the opto-electronic property and performance

A series of A–D–π–D–A multi-anchoring organic dyes (MA-201–MA-206) with different core spacers was designed to investigate optoelectronic properties and to develop utility for solar cells.

Bis-tridentate Ru(ii) sensitizers with a spatially encumbered 2,6-dipyrazolylpyridine ancillary ligand for dye-sensitized solar cells

The sensitizer TF-tBu_C₃F₇ has shown the highest overall efficiencies of J_SC = 18.47 mA cm⁻², V_OC = 767 mV, FF = 0.71 and PCE = 10.05% under simulated one sun irradiation, due to the fine balance between dye loading and reduced charge recombination.

Modification of Charge Trapping at Particle/Particle Interfaces by Electrochemical Hydrogen Doping of Nanocrystalline TiO2

Particle/particle interfaces play a crucial role in the functionality and performance of nanocrystalline materials such as mesoporous metal oxide electrodes. Defects at these interfaces are known to impede charge separation via slow-down of transport and increase of charge recombination, but can be passivated via electrochemical doping (i.e., incorporation of electron/proton pairs), leading to transient but large enhancement of photoelectrode performance. Although this process is technologically very relevant, it is still poorly understood. Here we report on the electrochemical characterization and the theoretical modeling of electron traps in nanocrystalline rutile TiO₂ films. Significant changes in the electrochemical response of porous films consisting of a random network of TiO₂ particles are observed upon the electrochemical accumulation of electron/proton pairs. The reversible shift of a capacitive peak in the voltammetric profile of the electrode is assigned to an energetic modification of trap states at particle/particle interfaces. This hypothesis is supported by first-principles theoretical calculations on a TiO₂ grain boundary, providing a simple model for particle/particle interfaces. In particular, it is shown how protons readily segregate to the grain boundary (being up to 0.6 eV more stable than in the TiO₂ bulk), modifying its structure and electron-trapping properties. The presence of hydrogen at the grain boundary increases the average depth of traps while at the same time reducing their number compared to the undoped situation. This provides an explanation for the transient enhancement of the photoelectrocatalytic activity toward methanol photooxidation which is observed following electrochemical hydrogen doping of rutile TiO₂ nanoparticle electrodes.

Review of key factors controlling engineered nanoparticle transport in porous media

Nanotechnology, an emerging technology, has witnessed rapid development in production and application. Engineered nanomaterials revolutionize the industry due to their unique structure and superior performance. The release of engineered nanoparticles (ENPs) into the environment, however, may pose risks to the environment and public health. To advance current understanding of environmental behaviors of ENPs, this work provides an introductory overview of ENP fate and transport in porous media. It systematically reviews the key factors controlling their fate and transport in porous media. It first provides a brief overview of common ENPs in the environment and their sources. The key factors that govern ENP transport in porous media are then categorized into three groups: (1) nature of ENPs affecting their transport in porous media, (2) nature of porous media affecting ENP transport, and (3) nature of flow affecting ENP transport in porous media. In each group, findings in recent literature on the specific governing factors of ENP transport in porous media are discussed in details. Finally, this work concludes with remarks on the importance of ENP transport in porous media and directions for future research.

Electron Transport in Quasi-Two-Dimensional Porous Network of Titania Nanoparticles, Incorporating Electrical and Optical Advantages in Dye-Sensitized Solar Cells

The integration of fast electron transport and large effective surface area is critical to attaining higher gains in the nanostructured photovoltaic devices. Here, we report facilitated electron transport in the quasi-two-dimensional (Q2D) porous TiO₂ . Liquid electrolyte dye-sensitized solar cells were prepared by utilizing photoanodes based on the Q2D porous substructures. Due to electron confinement in a microscale porous medium, directional diffusion toward collecting electrode is induced into the electron transport. Our measurements based on the photocurrent and photovoltage time-of-flight transients show that at higher Fermi levels, the electron diffusion coefficient in the Q2D porous TiO₂ is about one order of magnitude higher when compared with the conventional layer of porous TiO₂ . The results show that microstructuring of the porous TiO₂ leads to an approximately threefold improvement in the electron diffusion length. Such a modification may considerably affects the electrical functionality of moderate or low performance dye-sensitized solar cells for which the internal gain or collection efficiency is typically low.

Self-assembly graphitic carbon nitride quantum dots anchored on TiO2 nanotube arrays: An efficient heterojunction for pollutants degradation under solar light

In this study, an efficient heterojunction was constructed by anchoring graphitic carbon nitride quantum dots onto TiO2 nanotube arrays through hydrothermal reaction strategy. The prepared graphitic carbon nitride quantum dots, which were prepared by solid-thermal reaction and sequential dialysis process, act as a sensitizer to enhance light absorption. Furthermore, it was demonstrated that the charge transfer and separation in the formed heterojunction were significantly improved compared with pristine TiO2. The prepared heterojunction was used as a photoanode, exhibiting much improved photoelectrochemical capability and excellent photo-stability under solar light illumination. The photoelectrocatalytic activities of prepared heterojunction were demonstrated by degradation of RhB and phenol in aqueous solution. The kinetic constants of RhB and phenol degradation using prepared photoelectrode are 2.4 times and 4.9 times higher than those of pristine TiO2, respectively. Moreover, hydroxyl radicals are demonstrated to be dominant active radicals during the pollutants degradation.

Effects of Organic Molecules with Different Structures and Absorption Bandwidth on Modulating Photoresponse of MoS2 Photodetector

Organic dye molecules possessing modulated optical absorption bandwidth and molecular structures can be utilized as sensitizing species for the enhancement of photodetector performance of semiconductor via photoinduced charge transfer mechanism. MoS2 photodetector were modified by drop-casting of methyl orange (MO), rhodamine 6G (R6G), and methylene blue (MB) with different molecular structures and extinction coefficients, and enhanced photodetector performance in terms of photocurrent, photoresponsity, photodetectivity, and external quantum efficiency were obtained after modification of MO, R6G, and MB, respectively. Furthermore, dyes showed different modulating abilities for photodetector performance after combination with MoS2, mainly due to the variation of molecular structures and optical absorption bandwidth. Among tested dyes, deposition of MB onto monolayer MoS2 grown by CVD resulted in photocurrent ∼20 times as high as pristine MoS2 due to favorable photoinduced charge transfer of photoexcited electrons from flat MB molecules to the MoS2 layer. Meanwhile, the corresponding photoresponsivity, photodetectivity, and an external quantum efficiency are 9.09 A W(1-), 2.2 × 10(11) Jones, 1729% at 610 nm, respectively. Photoinduced electron-transfer measurements of the pristine MoS2 and dye-modified MoS2 indicated the n-doping effect of dye molecules on the MoS2. Additionally, surface-enhanced Raman measurements also confirmed the direct correlation with charge transfer between organic dyes and MoS2 taking into account the chemically enhanced Raman scattering mechanism. Present work provides a new clue for the manipulation of high-performance of two-dimensional layered semiconductor-based photodetector via the combination of organic dyes.

N-Doped TiO2 Nanobelts with Coexposed (001) and (101) Facets and Their Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production

To narrow the band gap (3.2 eV) of TiO2 and extend its practical applicability under sunlight, the doping with nonmetal elements has been used to tune TiO2 electronic structure. However, the doping also brings new recombination centers among the photoinduced charge carriers, which results in a quantum efficiency loss accordingly. It has been proved that the {101} facets of anatase TiO2 are beneficial to generating and transmitting more reductive electrons to promote the H2-evolution in the photoreduction reaction, and the {001} facets exhibit a higher photoreactivity to accelerate the reaction involved of photogenerated hole. Thus, it was considered by us that using the surface heterojunction composed of both {001} and {101} facets may depress the disadvantage of N doping. Fortunately, we successfully synthesized anatase N-doped TiO2 nanobelts with a surface heterojunction of coexposed (101) and (001) facets. As expected, it realized the charge pairs' spatial separation and showed higher photocatalytic activity under a visible-light ray: a hydrogen generation rate of 670 μmol h(-1) g(-1) (much higher than others reported previously in literature of N-doped TiO2 nanobelts).

Significant Influences of Elaborately Modulating Electron Donors on Light Absorption and Multichannel Charge-Transfer Dynamics for 4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic Acid Dyes

4-(Benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA) as a promising electron acceptor has been used in the highly efficient organic dye-sensitized solar cells (DSCs) recently. Because of its strong electron-deficient character, BTEBA could bring forth a remarkable decline in the energy level of the lowest unoccupied molecular orbital (LUMO) and further reduce the energy gap of dye molecules significantly. In this contribution, two metal-free organic dyes WEF1 and WEF2 were synthesized by simply combining BTEBA with two slightly tailored electron-releasing moieties: 4-hexylphenyl substituted indaceno[1,2-b:5,6-b']dithiophene (IDT) and cyclopenta[1,2-b:5,4-b']dithiophene[2',1':4,5]thieno[2,3-d]thiophene (CPDTDT), which were screened rationally from an electron-donor pool via computational simulation. With respect to those of WEF1, WEF2-sensitized solar cells demonstrate a far better short-circuit photocurrent density (JSC) and open-circuit photovoltage (VOC), resulting in a ∼50% improved power conversion efficiency of 10.0% under irradiance of 100 mW cm(-2) AM1.5G sunlight. We resorted to theoretical calculations, electrical measurements, steady-state, and time-resolved spectroscopic methods to shed light on the fatal influences of elaborately modulating electron donors on light absorption, interfacial energetics, and multichannel charge-transfer dynamics.

Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-Sensitized H2 Evolution

Graphene is an advanced carbon energy material due to its excellent properties. Reduction of graphene oxide (GO) is the most promising mass production route of graphene/reduced graphene oxide (rGO). To maintain graphene's properties and avoid restacking of rGO sheets in bulk, the preparation of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy. This article presents a facile route to synthesize graphene sponge by thermal treating GO powder at low temperature of 250 °C under N2 atmosphere. The sponge possesses macroporous structure (5-200 nm in size) with BET specific surface area of 404 m(2) g(-1) and high conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer Eosin Y (EY) and cocatalyst Pt was investigated. The rGO sponge shows highly efficient dye-sensitized photocatalytic H2 evolution compared to that obtained via a chemical reduction method. The maximum apparent quantum yield (AQY) reaches up to 75.0% at 420 nm. The possible mechanisms are discussed. The synthesis method can be expanded to prepare other graphene-based materials.

Simultaneous Spectroscopic and Topographic Imaging of Single-Molecule Interfacial Electron-Transfer Reactivity and Local Nanoscale Environment

The fundamental information related to the energy flow between molecules and substrate surfaces as a function of surface site geometry and molecular structure is critical for understanding interfacial electron-transfer (ET) dynamics. The inhomogeneous nanoscale molecule-surface and molecule-molecule interactions are presumably the origins of the complexity in interfacial ET dynamics; thus, identifying the environment of molecules at nanoscale is crucial. We have developed atomic force microscopy (AFM) correlated single-molecule fluorescence intensity/lifetime imaging microscopy (AFM-SMFLIM) capable of identifying and characterizing individual molecules distributed across the heterogeneous surface at the nanometer length scale. Using the novel AFM-SMFLIM imaging, we are able to obtain nanoscale morphology and interfacial ET dynamics at a single-molecule level. Moreover, the observed blinking behavior and lifetime of each molecule in combination with the topography of the environment at nanoscale provide the location of each molecule on the surface (TiO2 vs cover glass) at nanoscale and the coupling strength of each molecule with TiO2 nanoparticles.

Targeting and Photodynamic Killing of Cancer Cell by Nitrogen-Doped Titanium Dioxide Coupled with Folic Acid

Titanium dioxide (TiO₂) has attracted wide attention as a potential photosensitizer (PS) in photodynamic therapy (PDT). However, bare TiO₂ can only be excited by ultraviolet illumination, and it lacks specific targeting ligands, which largely impede its application. In our study, we produced nitrogen-doped TiO₂ and linked it with an effective cancer cell targeting agent, folic acid (FA), to obtain N-TiO₂-FA nanoconjugates. Characterization of N-TiO₂-FA included Zeta potential, absorption spectra and thermogravimetric analysis. The results showed that N-TiO₂-FA was successfully produced and it possessed better dispersibility in aqueous solution than unmodified TiO₂. The N-TiO₂-FA was incubated with human nasopharyngeal carcinoma (KB) and human pulmonary adenocarcinoma (A549) cells. The KB cells that overexpress folate receptors (FR) on cell membranes were used as FR-positive cancer cells, while A549 cells were used as FR-negative cells. Laser scanning confocal microscopy results showed that KB cells had a higher uptake efficiency of N-TiO₂-FA, which was about twice that of A549 cells. Finally, N-TiO₂-FA is of no cytotoxicity, and has a better photokilling effect on KB cells under visible light irradiation. In conclusion, N-TiO₂-FA can be as high-value as a PS in cancer targeting PDT.

Study on a series of novel self-assembly supramolecular solar cells based on a double-layer structured chromophore of Zn-porphyrins

We prepared in this work an anchoring porphyrin and a series of hat-porphyrins. The zinc atom of the hat-porphyrins can be coordinated axially with the pyridine moiety of the anchoring porphyrin which is anchored on the titania surface by a carboxyl group. The structures of the assemblies were confirmed using computational calculations, transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). Solar cell devices of the monomer anchoring porphyrin and its assemblies were fabricated and the photovoltaic performances were measured under standard AM 1.5 sunlight irradiance. We found that the assembly devices showed higher JSC and lower VOC than that of the monomer anchoring porphyrin device. However, the comprehensive influence of JSC and VOC led to an enhancement in the solar-to-electric power-conversion efficiency (PCE) of the assemblies. We also studied the variation of JSC and VOC using electronic absorption and emission spectroscopy, charge extraction measurements, transient photovoltage decay measurements and electrochemical impedance spectroscopy.

A Systematic Study on the Influence of Electron-Acceptors in Phenanthrocarbazole Dye-Sensitized Solar Cells

In this work, by conjugating 2-cyanoacrylic acid (CA), 4-(benzo[c][1,2,5]thiadiazol-7-yl)benzoic acid (BTBA), 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (EBTBA), and 4-((7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)ethynyl)benzoic acid (EBTEBA) to a binary electron-donor diphenylamine-phenanthrocarbazole (DPA-PC), we systematically investigate the impacts of electron-acceptors upon energy level, energy gap, light-harvesting ability, photovoltaic parameter, and cell stability of donor-acceptor dyes in photoelectrochemical cells. In conjunction with an ionic liquid composite electrolyte, the DPA-PC dye with EBTEBA as electron-acceptor yields a high power conversion efficiency of 8% and an outstanding stability after a 1000 h aging test under the soaking of full sunlight at 60 °C in a dye-sensitized solar cell. Femtosecond fluorescence up-conversion measurements have suggested that energy relaxation and electron injection both occur to dye molecules in the nonequilibrium excited states. Moreover, the time constants of injecting electrons from dye molecules in the excited states to titania are very dispersive for over 1 order of magnitude, mainly owing to the broad energy distribution of excited states.