TiO2 Inverse Opal Research Articles

Inverse opal photoelectrode of Nb-doped TiO2 nanoparticles for dye-sensitized solar cell

In this study, Nb-doped TiO2 nanoparticles were prepared by solution synthesis, and utilized as photoelectrode for dye-sensitized solar cell aiming to obtain better photon-to-current conversion efficiency. Nb doping was carried out using two different Nb precursors, and subsequent hydrothermal treatment resulted in the size-controlled (~10 nm) Nb-doped TiO2 nanoparticles with typical ellipsoidal shapes and anatase morphologies. It was observed that NbCl2 precursor resulted in more uniform particle size, while Nb oxalate resulted in relatively broader particle size distributions and higher aspect ratio ellipsoid. Using these nanoparticles, 10 μm-thick porous inverse opal (IO) photoelectrodes were fabricated by coating the mixed aqueous dispersions of Nb-TiO2 nanoparticles and polystyrene particles of 270 nm diameter on conductive glass followed by thermal sintering. DSSC single cell performance tests revealed that a photoelectrode from 2 mol% Nb- TiO2 obtained from NbCl2 exhibited 20 % enhancement for solar cell efficiency compared to bare TiO2 IO.

The Role of Order in the Amplification of Light-Energy Conversion in a Dye-Sensitized Solar Cell Coupled to a Photonic Crystal.

We investigate the cause of amplification of light-energy conversion when coupling a nc-TiO2 film to a TiO2 inverse opal by comparing it to an inverse TiO2 glass (i-TiO2 -g) fabricated with the exact monodisperse air-hole size as an inverse opal with a stop band at 600 nm (600-i-TiO2 -o). A significant twofold average gain in the photon-to-current conversion efficiency is measured to the red of the stop band at the 600-i-TiO2 -o/nc-TiO2 bilayer under front-wall and back-wall illumination, greater than the gain within the stop band. A smaller amplification is measured under front-wall illumination-and no gain is measured under back-wall illumination-for i-TiO2 -g/nc-TiO2 at these energies. The photonic crystal therefore causes trapping of light through the bilayer, not only within the gap but also to the red, at frequencies within its dielectric band. This light-trapping effect is found to be dependent on structural order, as a highly disordered inverse glass film with the same air-hole size and thickness does not yield the same gain. A drop in the transmission of light is measured within the same frequencies to the red of the stop band upon adding nc-TiO2 to 600-i-TiO2 -o, consistent with light trapping in the bilayer.

Guided mode extraction in monolayer colloidal crystals based on the phase variation of reflection and transmission coefficients

An accurate and fast method for guided modes extraction in monolayer colloidal crystals and their inverse replicas is presented. These three-dimensional structures are composed of a monolayer of spherical particles that can easily and simply be prepared by self-assembly method in close packed hexagonal lattices. In this work, we describe how the guided modes, even or odd modes and light cone boundary can be easily determined using phase variations of reflection and transmission coefficients. These coefficients are quickly calculated by Fourier modal method. The band structures are obtained for a monolayer of polystyrene particles and two-dimensional TiO2 inverse opal by this proposed method.

Fabrication of inverse opal TiO2-supported Au@CdS core–shell nanoparticles for efficient photocatalytic CO2 conversion

The photocatalytic conversion of CO2 with H2O into renewable hydrocarbon fuels by solar energy is of significance in solving both energy and environmental problems. Herein, we firstly report a practical fabrication of all-solid-state three-component Z-scheme system with the core–shell structured Au@CdS core–shell nanoparticles on inverse opal TiO2 (Au@CdS/IO-TiO2) via the gas bubbling-assisted membrane reduction-precipitation (GBMR/P) method. All the catalysts possess well-defined inverse opal structure with the interconnected networks of spherical voids, and the Au@CdS core–shell nanoparticles with different molar ratios of Cd/Au are well dispersed and supported on the inner wall of uniform macropore. The slow photon effect of inverse opal structure with moderate macropore sizes can enhance the light-harvesting efficiency. And the all-solid-state Z-scheme system with CdS(shell)-Au(core)-TiO2(support) nanojunction is favourable for the separation of photogenerated electrons and holes due to the vectorial electron transfer of TiO2→Au→CdS. The Au@CdS/IO-TiO2 catalysts exhibit super photocatalytic performance for CO2 reduction to CH4 under the simulated solar irradiation. Among the as-prepared catalysts, Au@CdS/IO-TiO2-1 catalyst with the moderate thickness of CdS nanolayer shell shows the highest photocatalytic activity and selectivity for CO2 reduction, e.g., its formation rate of CH4 is 41.6μmolg−1h−1 and its selectivity to CH4 production by CO2 reduction is 98.6%. The design and versatile synthetic approach of all-solid-state Z-scheme system on the surface of inverse opal oxides are expected to throw new light on the fabrication of highly efficient photocatalyst for CO2 reduction to hydrocarbon.

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.

Light harvesting in photonic crystals revisited: why do slow photons at the blue edge enhance absorption?

Light harvesting enhancement by slow photons in photonic crystal catalysts or dye-sensitized solar cells is a promising approach for increasing the efficiency of photoreactions. This structural effect is exploited in inverse opal TiO2 photocatalysts by tuning the red edge of the photonic band gap to the TiO2 electronic excitation band edge. In spite of many experimental demonstrations, the slow photon effect is not fully understood yet. In particular, observed enhancement by tuning the blue edge has remained unexplained. Based on rigorous couple wave analysis simulations, we quantify light harvesting enhancement in terms of absorption increase at a specific wavelength (monochromatic UV illumination) or photocurrent increase (solar light illumination), with respect to homogeneous flat slab of equivalent material thickness. We show that the commonly accepted explanation relying on light intensity confinement in high (low) dielectric constant regions at the red (blue) edge is challenged in the case of TiO2 inverse opals because of the sub-wavelength size of the material skeleton. The reason why slow photons at the blue edge are also able to enhance light harvesting is the loose confinement of the field, which leads to significant resonantly enhanced field intensity overlap with the skeleton in both red and blue edge tuning cases, yet with different intensity patterns.

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.

Nitrogen-fluorine co-doped titania inverse opals for enhanced solar light driven photocatalysis.

Three dimensionally ordered nitrogen-fluorine (N-F) co-doped TiO2 inverse opals (IOs) were fabricated by templating with polystyrene (PS) colloidal photonic crystals (CPCs) by infiltration. During preparation, the TiO2 precursor was treated with a mixture of nitric acid and trifluoroacetic acid to facilitate N-F co-doping into the TiO2 lattice. Enhanced solar light absorption was observed in the samples as a consequence of the red shift in the electronic band gap of TiO2 due to N-F co-doping. The photonic band gap (PBG) of these TiO2 IO films was tuned by varying the sphere size of the PS CPC templates. The as-prepared N-F co-doped TiO2 IO films were used as photocatalysts for the degradation of Rhodamine B (RhB) dye under solar light irradiation. A significant enhancement in the photocatalytic activity was observed in N-F co-doped TiO2 IO films prepared using PS spheres of 215 nm as a template, with the red edge of the PBG closer to the electronic band gap (EBG) of TiO2. 100% of the dye molecules were degraded within 2 minutes under direct solar irradiation, which is one of the fastest reaction times ever reported for RhB degradation in the presence of TiO2 photocatalysts. The N-F co-doped TiO2 IO film prepared using PS of 460 nm with its PBG centered at 695 nm also showed good photocatalytic activity. It was found that the IO films displayed improved photocatalytic activity in comparison to ordinary nanocrystalline (nc)-TiO2 films. The enhancement could be attributed to the bandgap scattering effect and the slow photon effect, leading to a significant improvement in solar light harvesting.

Research on the Depositon of CdS Quantum Dots on TiO<sub>2</sub> Inverse Opal

As the third generation solar cells, quantum dot sensitized solar cells (QDSSC) has received wide attention from researchers because of its high theoretical energy conversion efficiency and low production costs. The preparation of CdS quantum dots for TiO2 inverse opal based QDSSC by successive ionic layer absorption and reaction (SILAR) was investigated. The optimized number of SILAR cycle and ionic reaction time was obtained by the analysis of absorption spectra.

Ordered Macroporous CdS-sensitized N-doped TiO2 Inverse Opals Films with Enhanced Photoelectrochemical Performance

The CdS sensitized N-TiO2 (CdS-N-TiO2) inverse opals films were prepared by a sol-gel method integrated with successive ionic layer adsorption and reaction (SILAR). In order to harvest the visible sunlight and enhance the photocurrent, photosensitization of CdS (narrow-bandgap semiconductor) and nitrogen doping are used to couple with TiO2 inverse opals. The good visible light absorption ability and significant increase of photoresponse of CdS-N-TiO2 inverse opals films have been observed compared to TiO2 or N-TiO2 inverse opals films. A promising photocurrent density of 0.83mA/cm2 has been achieved for the CdS-N-TiO2 inverse opal as photoanode at 1.23mV versus reversible hydrogen electrode (RHE) bias under simulated solar-light illumination. Because the synergistic effect of CdS sensitization and N-doping with periodically ordered inverse opal nanostructures, the photocurrent density is enhanced in comparison to the pristine TiO2 inverse opals films.

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.

Highly improved photocurrents of dye-sensitized solar cells containing ultrathin 3D inverse opal electrodes sensitized with a dithienothiophene-based organic dye

Ultrathin but highly efficient dye-sensitized solar cells (DSCs) may be beneficial for making flexible devices and lowering production costs. Here, a uniform, ultrathin inverse opal (IO) electrode sensitized with a dithienothiophene (DTT)-based sensitizer (Carbz-PAHTDTT) was successfully prepared and demonstrated as a high-efficiency DSC electrode. The ultrathin IO was fabricated using a polystyrene opal template subsequently coated with TiO2 via a chemical vapor deposition approach. The Carbz-PAHTDTT-sensitized IO film was compared to an IO film sensitized by a conventional N719 dye. The charge collection efficiency of the Carbz-PAHTDTT-sensitized TiO2 IO electrode was comparable to the N719-sensitized electrode. However, the Carbz-PAHTDTT TiO2 IO electrode DSCs exhibited remarkably high light-harvesting efficiency due to high adsorption density and visible light absorption features of the Carbz-PAHTDTT-sensitized electrode. The Carbz-PAHTDTT DSCs containing a 3.5 μm thick IO electrode displayed a photocurrent density of 11.23 mA cm−2, which was 1.5 times higher compared to the N719-sensitized DSCs.

Photocatalytic hydrogen production on Pt-loaded TiO2 inverse opals

TiO2 inverse opals present increased photocatalytic production of H2. TiO2 inverse opals with different pore size and TiO2 macroporous structures with disordered arrangement of the pores have been tested in the photocatalytic production of hydrogen in aqueous solution with a formate buffer as hole scavenger. TiO2 inverse opals belong to the family of metamaterials and exhibit unique catalytic properties arising from their peculiar interaction with light. To discriminate the effects of slow photons the hydrogen photoproduction experiments were carried out at two different wavelengths, at 365nm where the effect of slow photons is maximized, and at 254nm where it is negligible. The resulting hydrogen production rates suggest a strong effect of the slow light and of the polymer template used in the synthesis of the TiO2 powders. The chemical properties of the polymeric sacrificial template determine the crystalline phase of the sample and as a consequence affect the catalytic performances of the resulting TiO2 structures.

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.

Enhanced photocatalytic performance of TiO2 based on synergistic effect of Ti3+ self-doping and slow light effect

A range of TiO2 inverse opals with tunable macroporous size were synthesized using different sized PS (polystyrene sphere) arrays as hard templates. After a simple heating treatment in vacuum, Ti3+ doped TiO2 inverse opals were obtained. The materials were characterized and verified by XRD, XPS, UV–vis DRS, EPR, and SEM. The result indicated that the optical responses of TiO2 and Ti3+ doped TiO2 inverse opals could be enhanced by choosing PS arrays with appropriate size as hard templates due to the slow light effect of inverse opal structure. The photocatalytic efficiency was evaluated by the photo degradation of AO7, and it is proved that the cooperation of slow light effect and Ti3+ doping is an effective way to improve visible-light driven photocatalytic performance of TiO2 photocatalyst.

Design and fabrication of carbon quantum dots/TiO2 photonic crystal complex with enhanced photocatalytic activity.

TiO2 photonic crystal photocatalyst with inverse opal structure were first prepared from self-assembled polystyrene spheres template, and then carbon quantum dots (CQDs) was coupled with TiO2 inverse opal through a facile electrodeposition method. The obtained CQDs/TiO2 complex photocatalysts exhibit enhanced photocatalytic activity compared to pure TiO2 inverse opal, especially under the irradiation of visible light. Our results provide a promising methodology for designing high performance photocatalysts based on photonic crystal and CQDs, which is benefit for catalytic and new energy applications.

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.

Photonic bandgap extension of surface-disordered 3D photonic crystals based on the TiO2 inverse opal architecture.

A photonic bandgap (PBG) extension of surface-disordered 3D photonic crystals (PCs) based on the TiO2 inverse opal (TiO2-IO) architecture has been demonstrated. By using a liquid phase deposition (LPD) process based on the controlled hydrolysis of ammonium hexafluorotitanate and boric acid, an extra layer of TiO2 nanoparticles were deposited onto the internal surface of the air voids in the TiO2-IOs to increase their surface roughness, thereby introducing surface disorder in the 3D order structures. The PBG relative width of surface-disordered TiO2-IOs has been broadened significantly, and, compared to the original TiO2-IO, its largest rate of increase (27%) has been obtained. It was found that the PBG relative width increased rapidly at first and then to a much slower rate of change with increase of the duration of the LPD time. A possible cause for this finding is discussed in this Letter.

The Synthesis and Properties of the Inverse Opals Structure Used in Dye-Sensitized Solar Cells

In this paper, the inverse opal structures used in dye-sensitized solar cells were prepared by Sol-Gel method. The TiO2inverse opal photonic crystal was formed by coating glass slices using dip-coating process. The structure, surface morphology and the optical properties were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet spectrograph (UV-is). The structural characteristics of the films were investigated by the SEM showed that inverse opal titanium dioxide could trap electrons, thus enhance the photoelectric current in the dye-sensitized solar cells. By use of the inverse opals into TiO2photo anodes with the screen printing method, I-V testing measurement showed that the highest photoelectric conversion efficiency is 1.2%. And the best preparation process of inverse opal titanium dioxide is using Ti (OC4H9)4as precursor, ethanol as the solvent, nitric acid as inhibitor, and dipping-lift number for 3 times. The result of the test showed that this inverse opals structure has the most appropriate thickness and the optimal performance. Based on the research, the results indicate that the TiO2inverse opals structure could apply in dye-sensitized solar cells.