TiO2 Inverse Opal Films Research Articles

Steaming‐Assisted Conversion: A New Strategy for the Synthesis of Anatase TiO2, Nb, and W‐doped Anatase TiO2 2D Inverse Opal Films

AbstractSteaming‐assisted conversion route, a new strategy, is first adapted for the synthesis of highly crystallized anatase TiO2 2D inverse opal (IO) monolayer films, and then to Nb‐doped TiO2 and W‐doped TiO2 2D IO monolayer films. Pure water, ammonia, or HCl solutions are used as a source of steaming vapor to convert dry films of amorphous TiO2 IO, NbCl5/TiO2, and WCl6/TiO2 composite IOs into anatase TiO2, Nb‐doped TiO2, and W‐doped TiO2 IO films. This new strategy renders possible the doping of metal ions within the framework of the anatase TiO2 IO films under low temperature and liquid‐free conditions. Further, the ordered array structure of the IO films is also effectively retained. The low steaming conversion temperature allows high dopant rates of homogeneously distributed heteroatoms, resulting in Nb doping as high as ≈34%. The thus prepared TiO2, Nb‐doped TiO2, and W‐doped TiO2 anatase IO films are successfully used as active electrodes in the fabrication of electrochromic devices.

Graphene Quantum Dot-TiO2 Photonic Crystal Films for Photocatalytic Applications.

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO2 photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO2 interfacial electron transfer. TiO2 inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO2 photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO2 films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films.

Fabrication and photoelectrochemical activity of hierarchically Porous TiO2–ZnO heterojunction film

For the first time, we report the fabrication of hierarchically (macro with nested meso) porous nanocrystalline TiO2–ZnO heterojunction film onto fluorine-doped tin oxide-coated glass substrate by colloidal crystal templating technique using poly(methyl methacrylate) (PMMA) spheres as template. Accordingly, the precursor solutions of titanium isopropoxide and zinc acetate dihydrate in the presence of Pluronic P123 were used to impregnate the individual solution into the template. Initially, nanocrystalline TiO2 inverse opal mesoporous film was deposited using the titanium precursor. The film was cured at 450 °C in an air atmosphere. A similar process was adopted to deposit nanocrystalline ZnO inverse opal mesoporous film onto the TiO2 to obtain hierarchically porous TiO2–ZnO heterojunction nanocrystalline film. Morphology of the fabricated films showed a periodic arrangement of macropores, whereas the microstructural analysis confirmed the presence of nested mesopores in the film network. Chemical interaction existed between TiO2 with ZnO forming the heterojunction film was ascertained by X-ray photoelectron spectroscopy. Light harvesting efficiency of the samples was studied, and the photoelectrochemical (PEC) performance of the hierarchically porous heterojunction films as photoanode showed about 5 times enhancement in photocurrent density compared to the pristine metal–oxide–semiconductor film under visible light exposure. The porous nanocrystalline hierarchically porous inverse opal heterojunction film could be used as an efficient photoanode in PEC cell.

New One Step Self-assembly Strategy of Large-Area Highly Ordered, Crack-Free 2D Inverse Opal Films of Transition Metal Oxides and Its Application to Fabrication of Bilayer Inverse Opal Films

A new one step method for self-assembly of sacrificial polystyrene (PS) spheres colloidal crystal with a transition metal oxide (TMO) precursor matrix material is developed to fabricate highly ordered, crack-free two-dimensional (2D) TiO2 and 2D WO3 inverse opal (IO) films over a centimeter scale. Opal composite monolayer PS/WO3 and PS/TiO2 films have been successfully generated via simultaneous assembly of polymeric colloidal spheres floating on aqueous TiO2 and WO3 precursor solutions, thereby avoiding the infiltration step of the TMO precursor solution into a preassembled opal template. Large-area crack-free 2D WO3 and 2D TiO2 IO films were subsequently obtained after the removal of the PS opal template by chemical method. Such new strategy avoids the need for liquid infiltration into a preassembled PS opal template and minimizes the associated large cracks and inhomogeneity that often occur during the fabrication of IO films. The obtained PS/WO3 and PS/TiO2 opal composite monolayer films were also used as building blocks for the fabrication of highly ordered bilayer (3D) IO films with homo- and hetero-structure via a bottom-up, layer by layer route, through repeated operations of this new one step self-assembly method. Hence, four types of bilayer IO films were synthesized: homo-structural TiO2 and WO3, hetero-structural TiO2 (bottom layer)/WO3 (top layer) and WO3 (bottom layer)/TiO2 (top layer). The underlying mechanism of multilayer assembly that may account for the formation of large area crack-free multilayer films, is discussed. The electrochromic behavior of these 2D and 3D IO samples was investigated.

Titania photonic crystal photocatalysts functionalized by graphene oxide nanocolloids

Photonic crystal-assisted semiconductor photocatalysis has been attracting significant attention as an advanced photon management approach that combines light harvesting with the macro/mesoporous structured materials properties permitting enhanced mass transport and high adsorption. In this work, surface functionalization of well-ordered photonic band gap engineered TiO2 inverse opal films fabricated by the convective evaporation-induced co-assembly method was performed by graphene oxide nanocolloids (nanoGO). The loading of GO nanosheets was determined by the films’ macropore size, with minimal effects on their long range periodicity and photonic properties. While nanoGO deposition reduced mesoporosity of the nanocrystalline titania walls, their surface functionality was greatly improved by the abundant oxygen groups of the GO nanosheets leading to increased pollutant adsorption. Slow photon amplification in the aqueous phase methylene blue photodegradation was identified for the unmodified TiO2 photonic films under both UV–vis and Vis illumination upon spectral overlap of the low energy edge of the inverse opal stop band (in water) with the dye electronic absorption, due to (red) slow photons localized in the titania skeleton that distinctly accelerated dye photodegradation kinetics. The photocatalytic efficiency was further improved for the nanoGO functionalized TiO2 inverse opal films via the synergetic action of interfacial electron transfer from TiO2 to the GO nanosheets. Under UV–vis light, the functionalized photonic films outperformed benchmark mesoporous Aeroxide® P25 TiO2 films where nanoGO modification, despite the enhanced dye adsorption, resulted in adverse effects in photocatalytic degradation due to pore clogging. Combination of the exceptional structural and photonic properties of TiO2 inverse opals with the high adsorption capacity and charge separation afforded by GO nanocolloids is proposed as a promising modification route for the development of efficient photocatalytic films.

Ordered CdSe-sensitized TiO2 inverse opal film as multifunctional surface-enhanced Raman scattering substrate

The surface-enhanced Raman scattering (SERS) study on semiconductor material has attracted much attention, but the traditional semiconductor materials possess low SERS sensitivity and single function, which cannot meet the need of people very well. Here, the ordered three-dimensional CdSe-sensitized TiO2 (CdSe-TiO2) composite film with inverse opal structure (IOS) and multifunction was synthesized successfully. The as-prepared CdSe-TiO2 IOS film was first used as the excellent SERS substrate and photoelectrochemical material. The synergistic effect of slow photons, multiple scattering and CdSe sensitization make the SERS substrate more sensitive. The detection limit of the methylene blue (MB) on CdSe-TiO2 IOS film can be as low as 7 × 10−9 M. In addition, CdSe-TiO2 IOS exhibits excellent photoelectric converting performance, the photocurrent density of CdSe-TiO2 IOS can achieve 0.924 mA/cm2 under chopped light illumination at a fixed bias of 0 V versus Ag/AgCl. The above performance enhancement of CdSe-TiO2 IOS film makes it becoming a promising multifunctional material to be used in SERS detection and photoelectrochemical field.

Fabrication of titania inverse opals by multi-cycle dip-infiltration for optical sensing

We have demonstrated a low-cost method to fabricate TiO2 inverse opal photonic crystals with high refractive index skeleton. The TiO2 inverse opal films were fabricated from a polystyrene opal template by multi-cycle dip-infiltration-coating method. The properties of the TiO2 inverse opal films were characterized by scanning electron microscopy and Bragg reflection spectroscopy. The reflection spectroscopic measurements of the TiO2 inverse opal films were compared with theories of photonic band calculations and Bragg law. The agreement between experiment and theory indicates that we can precisely predict the refractive index of the infiltrated liquid sample in the TiO2 inverse opal films from the measurement results. The red-shift of the peak wavelength in the Bragg reflection spectra for both alcohol mixtures and aqueous sucrose solutions of increasing refractive index was observed and respective refractive index sensitivities of 296 and 286nm/RIU (refractive index unit) were achieved. As the fabrication of the TiO2 inverse opal films and reflection spectroscopic measurement are fairly easy, the TiO2 inverse opal films have potential applications in optical sensing.

Photonic Crystal-Based Sensors for Detecting Alcohol Concentration

Polystyrene (PS) opal and titania (TiO2) inverse opal films were fabricated by the self-assembly colloidal crystal template technique. Based on Bragg’s law, these sensors were used to detect the different concentrations of ethanol solution. The results indicated that TiO2 inverse opal films were advantageous over PS opal film for detecting the ethanol concentration. TiO2 inverse opal films sintered at 600 °C retained the highest sensitivity for ethanol concentration identification, since the anatase phase was transformed into the rutile phase, which resulted in an enhancement of the refractive index, i.e., an increase in the amount of the red shift.

Silver nanoparticles deposited inverse opal film as a highly active and uniform SERS substrate

Ag-decorated TiO2 inverse opal films (ATIO) with high surface-enhanced Raman scattering (SERS) enhancement were prepared using an electroless deposition process. The Ag nanoparticles (NPs) are well-dispersed and deposited on the edge of macroporous walls. The structure and optical properties of the sample ATIO have been characterized. The Ag-loading cycles and pore sizes of TiO2 inverse opal are the key factors determining the magnitude of SERS signal enhancement. The optimized ATIO samples exhibit high SERS signal enhancement ability and reproducibility. The enhancement factor about 104 and the detection limit of 10−10M for R6G were achieved. The further application in detecting malachite green is demonstrated. A limit of detection approximately 10−9M was achieved. The results show that the ATIO nanostructure is promising for using as sensor substrates in SERS applications.

Uniform Decoration of CdS Nanoparticles on TiO2 Inverse Opals for Visible Light Photoelectrochemical Cell

We present a CdS nanoparticle-decorated TiO2 inverse opal (IO) for use as an electrode for visible-light photoelectrochemical cells. A facile, low-temperature deposition of CdS nanoparticles is achieved via a sonochemical method. The use of TiO2 IO structures enable the uniform deposition of CdS nanoparticles throughout the film. The quantity of deposited CdS nanoparticles is controlled by the concentration of the CdS precursor solution. The deposited layer of CdS nanoparticles sensitizes the TiO2 surface to visible light; CdS-sensitized TiO2 IO films demonstrate absorption of up to approximately 550nm light, with enhanced absorption for increased CdS deposits. The CdS-sensitized TiO2 IO film is employed as a photoanode for the PEC cell. We obtain a maximum photocurrent of 7.29mA/cm2 at the optimum CdS deposition of 39.33ms%. The optimum amount of deposited CdS is defined by balancing the light harvesting rate against the charge recombination rate, both of which increase with increasing CdS deposition. The CdS-sensitized TiO2 IO electrodes are compared with mesoporous TiO2 nanoparticulate electrodes; higher photocurrent is obtained with the IO, due to the uniform pores of the IO structures.

A Nitrogen-Doped Carbon Dot-Sensitized TiO2 Inverse Opal Film: Preparation, Enhanced Photoelectrochemical and Photocatalytic Performance

In this study, a facile strategy for preparing nitrogen-doped carbon dots (N-CDs) was developed. A novel N-CD-sensitized, three-dimensionally ordered, porous TiO2 inverse opal structure (IOS) using a polystyrene (PS) opal as the template was constructed for the first time. The as-prepared N-CDs/TiO2 IOS shows enhanced photoelectrochemical performance and photocatalytic activity. The photo-response range of the composite film is expanded to the visible region due to the addition of N-CDs and the highly ordered void structure. The photocurrent density of the N-CDs/TiO2 IOS is approximately 6 times higher than that of TiO2 IOS and 18 times that of TiO2 nanoparticles (NPs). The photocurrent density is also enhanced 2-fold through sensitization of the N-CDs compared with that of CDs. In addition, the N-CDs/TiO2 IOS shows better photocatalytic activity in degrading organic dyes than CDs/TiO2 IOS, TiO2 IOS and TiO2 NPs. The good photoelectrochemical and photocatalytic performance makes N-CDs/TiO2 IOS a promising material in clean energy, water treatment and other applications.

Tetrapod CdSe-sensitized macroporous inverse opal electrodes for photo-electrochemical applications

Quantum dots (QDs) possess promising characteristics that are important to light harvesting, but their mesoscale size limits their application in the direct sensitization of TiO2 porous films for photo-electrochemical cells (PECs). Here, inverse opal (IO) TiO2 structures were sensitized by tetrapod-CdSe (tp-CdSe) QDs, which were used in visible-light PECs. Because of the interconnected macropores in the IO structure, tp-CdSe penetrated the entire film and deposited on its surface. In contrast, infiltration was limited to the surface of the conventional mesoporous TiO2 film. The amount of tp-CdSe deposited was dependent on the immersion time of the film in the tp-CdSe dispersion. Optimum deposition of tp-CdSe was observed at the highest photocurrent density. Light harvesting, thus the photocurrent, increased with increasing amounts of deposit, but there was a corresponding decrease in electron lifetime. The maximum photocurrent density per Cd mass was 0.474 mA cm−2, which is greater than previous results from experiments using QD-sensitized PECs. We thus believe that a combination of tetrapod QDs and the TiO2 IO film may provide a new platform for PEC electrodes.

Optically pumped distributed feedback dye lasing with slide-coated TiO₂ inverse-opal slab as Bragg reflector.

We demonstrate an optical amplification of organic dye within a TiO2 inverse-opal (IO) distributed feedback (DFB) reflector prepared by a slide-coating method. Highly reflective TiO2 IO film was fabricated by slide coating the binary aqueous dispersions of polystyrene microspheres and charge-stabilized TiO2 nanoparticles on a glass slide and subsequently removing the polymer-opal template. TiO2 IO film was infiltrated, in turn, with the solutions of DCM, a fluorescent dye in various solvents with different indices of refraction. Optical pumping by frequency-doubled Nd:YAG laser resulted in amplified spontaneous emission in each dye solution. In accordance with the semi-empirical simulation by the FDTD method, DCM in ethanol showed the best emission/stopband matching for the TiO2 IO film used in this study. Therefore, photo excitation of a DCM/ethanol cavity showed a single-mode DFB lasing at 640 nm wavelength at moderate pump energy.

TiO2 inverse opal based CdS/CdSe quantum dot co-sensitized solar cells

CdS and CdSe quantum dots were introduced as co-sensitizers into TiO2 inverse opal quantum dot sensitized solar cells. Herein, the three-dimensionally ordered porous TiO2 inverse opal film leads to a better infiltration of both sensitizers and hole transporting material, and the smaller surface area of TiO2 inverse opal film is effectively offset by the incorporating of co-sensitization. It was found that the presence of CdS/CdSe co-sensitizers provides enhanced light absorption, and leads to a lower recombination rate of the electrons due to the stepwise structure of band edge in TiO2/CdS/CdSe, which resulted in the observed enhanced photocurrent and energy conversion efficiency of the solar cells. A cell efficiency of 1.01 % has been attained.

Crack-free 2D-inverse opal anatase TiO2 films on rigid and flexible transparent conducting substrates: low temperature large area fabrication and electrochromic properties

A ‘dynamic-hard-template’ strategy is developed for large-area crack-free synthesis of 2D anatase TiO2 inverse opal film on rigid and flexible ITO substrates by using PS templates.

Probing significant light absorption enhancement of titania inverse opal films for highly exalted photocatalytic degradation of dye pollutants

The continuous titania inverse opal (TiO2-IO) films have been prepared by sol–gel infiltration method and calcined at different temperatures. The morphologies of the TiO2 inverse opal films remain unchanged under high temperature treatment. XRD patterns reveal an anatase crystalline phase between 550 and 900°C and a mixture of anatase and rutile phase at 1000°C. Comparing with the mesoporous TiO2 films obtained under the same conditions, all the TiO2 inverse opal films demonstrate a highly enhanced photocatalytic activity in photodegradation of rhodamine B (RhB) as dye pollutant model in aqueous solution. In spite of the fact that the TiO2 inverse opal films with open macroporous structures and possible light scattering effect of the wavelengths can result in the higher photocatalytic activity in the degradation of the dye pollutant, the phenomenon of the slow photon occurring in the TiO2 inverse opal photonic crystals can explain the extraordinary enhancement of the photocatalytic activity. In consequence, the TiO2-IO-700, TiO2-IO-550 and TiO2-IO-800 films show the best photocatalytic performance mainly due to the slow photon effect at the light incident angle at 0°, 20° and 45°, respectively, a direct proof of light absorption enhancement due to the slow photon effect. The slow photon effect in TiO2 inverse opals to enhance light absorption and further to enhance photocatalysis is very important for further potential applications in solar cells and other processes linked to the light absorption.

One-step fabrication of N-doped TiO2 inverse opal films with visible light photocatalytic activity

Abstract N-doped TiO2 inverse opal films were fabricated by a novel method through one-step coassembly of polymer colloidal spheres and titania precursor. This coassembly approach removed the need of preassembled colloidal crystal for precursor solution infiltration and respective chemicals for titania formation and nitrogen doping. Less crack films with well ordered inverse opal structure could be produced by adding appropriate amount of TiBALDH (titanium(IV)-bis-lactato-bis-ammonium dihydroxide) into the precursor solution. The films prepared at the optimized condition showed enhanced visible light photocatalytic activity, which could be attributed to both the N doping effect and their unique inverse opal structure.

High photocatalytic activity enhancement of titania inverse opal films by slow photon effect induced strong light absorption

The photonic effect on the photocatalytic activity of the continuous titania inverse opal (TiO2-IO) films differing only by the air sphere size (185 and 165 nm) and prepared by a colloid crystal template approach and annealing at different temperatures (700 and 800 °C) has been investigated by aqueous solution degradation of dye pollutants using mesoporous TiO2 thin films as the reference. The high quality of TiO2 inverse opal films has been confirmed by a blue shift of the Photonic Band Gap (PBG) with increasing light incident angle. Excellent agreement was found between theoretical and experimental reflectance spectra, confirming the photonic crystal structure of the samples. The slow photon light absorption enhancement effect inducing highly improved photocatalytic degradation of dye pollutants has been revealed in an aqueous reaction system. When compared with the mesoporous (m-TiO2) films obtained under the same conditions, all the TiO2-IO films demonstrate a much higher photocatalytic activity. At a light incident angle of 0°, the TiO2-IO-700 film (air sphere size: 185 nm) showed a better photocatalytic activity than that of TIO2-IO-800 (air sphere size: 165 nm). Most importantly, with increasing light incident angle, the photocatalytic activity of the TiO2-IO-700 film decreases whilst that of the TiO2-IO-800 film increases sharply due to the enhancement of light absorption related to a slow photon effect, generating more electron-holes. The present work revealed that photocatalytic activity can be dramatically enhanced by utilizing slow photons located at the PBG edges with energies close to the electronic bandgap of the semiconductor. The study using the slow-photon effect on the basis of photonic crystals to improve the photocatalytic activity by enhancing the light absorption could be an important future research direction. The slow-photon effect can open a new exciting avenue to all the fields related to light absorption including solar cells, optical telecommunications and optical computing.

Facile synthesis of Ag nanoparticles supported on TiO2 inverse opal with enhanced visible-light photocatalytic activity

TiO2 inverse opal films loaded with silver nanoparticles (ATIO) were synthesized on glass substrates. TiO2 inverse opal (TIO) films were prepared via a sol–gel process using self-assembly of SiO2 colloidal crystal template and a facile wet chemical route featuring an AgNO3 precursor solution to fabricate silver nanoparticles on the TIO films. The inverse opal structure and Ag deposition physically and chemically modify titania, respectively. The catalysts were characterized by Raman spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), UV–vis absorption spectra, X-ray photoelectron spectroscopy and photoluminescence spectroscopy. The HRTEM results show that Ag nanoparticles measuring 5–10nm were evenly distributed on TIO. Both the UV- and visible-light photocatalytic activities of the samples were evaluated by analyzing the degradation of methylene blue (MB) in aqueous solution. The results reveal that the apparent reaction rate constant (kapp) of MB degradation of the sample ATIO under UV-light irradiation is approximately 1.5 times that of the conventional Ag-loaded TiO2 film (ATF) without an ordered porous structure at an AgNO3 concentration of 5mM in the precursor solution. At an AgNO3 concentration of 10mM, the sample exhibits a kapp value approximately 4.2 times that of ATF under visible-light irradiation. This enhanced visible-light photocatalytic performance can be attributed to the synergistic effect of optimized Ag nanoparticle deposition and an ordered macroporous TIO structure. Repeated cycling tests revealed that the samples showed stable photocatalytic activity, even after six repeated cycles.

Photoelectrodes: Rapid Fabrication of an Inverse Opal TiO2 Photoelectrode for DSSC Using a Binary Mixture of TiO2 Nanoparticles and Polymer Microspheres (Adv. Funct. Mater. 16/2011)

Highly reflective TiO2 inverse opal films are presented by Hyunjung Lee, Wonmok Lee, and co-workers on page 3094. A binary composite film of polystyrene/TiO2 is directly fabricated on a fluorine-doped tin oxide glass substrate, which shows a highly ordered opalline structure of PS embedded into the TiO2 matrix. The TiO2 inverse opal film was tested as a dye-sensitized solar cell photoelectrode and showed a relatively high photon-to-current conversion efficiency.