Conformal TiO2 Research Articles

Protecting hydrogenation-generated oxygen vacancies in BiVO4 photoanode for enhanced water oxidation with conformal ultrathin amorphous TiO2 layer

Introducing appropriate amount of oxygen vacancies by hydrogenation treatment is a simple and efficient way to improve the photoelectrochemical performance of nanostructured oxide photoanodes. However, the hydrogenation effect is often not durable due to the gradual healing of oxygen vacancies at or close to surface of photoanodes. Herein, we tackled the problem by conformal coating the hydrogenated nanoporous BiVO4 (H-BiVO4) photoanode with an ultrathin layer of amorphous TiO2. Photoelectrochemical measurements showed that a 4nm-thick TiO2 layer could significantly improve the stability of H-BiVO4 photoanode for repeated working test, with negligible influence on the initial photocurrent compared to the uncoated one. Mott–Schottky and linear sweep voltammetry measurements showed that donor density and photocurrent density of the H-BiVO4 electrode almost decayed to the values of pristine BiVO4 electrode after 3h test, while the amorphous TiO2-coated electrode only degraded by 6% and 5% of the initial values respectively in the same period. The investigation thus suggested that the amorphous TiO2 layer did protect the oxygen vacancies in H-BiVO4 photoanode by isolating these oxygen vacancies from environmental oxygen, while at the same time not impeding the interfacial charge transfer to water molecules due to its leaky nature.

ALD TiO2-Coated Flower-like MoS2 Nanosheets on Carbon Cloth as Sodium Ion Battery Anode with Enhanced Cycling Stability and Rate Capability

We report the fabrication of 3D flower-like MoS2 nanosheets arrays on carbon cloth as a binder-free anode for sodium ion battery. Ultrathin and conformal TiO2 layers are used to modify the surface of MoS2 by atomic layer deposition. The electrochemical performance measurements demonstrate that the ALD TiO2 layer can improve the cycling stability and rate capability of MoS2. The MoS2 nanosheets with 0.5-nm TiO2 coating electrode show the highest initial discharge capacity of 1392 mA h g-1 at 200 mA g-1, which is increased by 53% compared with that of bare MoS2. After 150 cycles, the capacity retention rate of the TiO2-coated MoS2 achieves 75.8% of its second cycle's capacity at 200 mA h g-1 in contrast to that of 59% of pure MoS2. Furthermore, the mechanism behind the experimental results is revealed by ex situ scanning electron microscope (SEM), X-ray powder diffraction (XRD), and electrochemical impedance spectroscopy (EIS) characterizations, which confirms that the ultrathin TiO2 modifications can prevent the structural degradation and the formation of SEI film of MoS2 electrode.

Stabilized and Improved Photoelectrochemical Responses of Silicon Nanowires Modified with Ag@SiO2 Nanoparticles and Crystallized TiO2 Film.

Semiconductor nanostructure photoelectrochemical (PEC) cells for sunlight conversion have the potential to supply continuous, low-cost, high-efficiency and carbon-free energy; however, in order to provide economically competitive energy, long-time stability and high-efficiency are requisite. In this work, the saturation photocurrent density (Jsat) and stability of the silicon nanowires (SiNWs) in a PEC cell are significantly improved via surface modification with Ag@SiO2 nanoparticles and conformal TiO2 ultrathin film. Compared to the bare SiNWs, a 40% enhancement in Jsat, a cathodic shift of 0.8 V in the applied potential for Jsat, and over 10-h continuous operation with a high photocurrent density without decay are realized by using the properly modified SiNWs. Comparison of minority carrier lifetime indicates that the surface recombination of the modified SiNWs has been effectively suppressed. External quantum efficiency measurements reveal that the superstable and improved PEC responses of the modified SiNWs can be maintained for a broadband spectrum. This work provides an alternative route to the industrial applications for the semiconductor nanostructure PEC cells.

Protected Light-Trapping Silicon by a Simple Structuring Process for Sunlight-Assisted Water Splitting.

Macroporous layers are grown onto n-type silicon by successive photoelectrochemical etching in HF-containing solution and chemical etching in KOH. This specific latter treatment gives highly antireflective properties of the Si surface. The duration of the chemical etching is optimized to render the surface as absorbent as possible, and the morphology of the as-grown layer is characterized by scanning electron microscopy. Further functionalization of such structured Si surface is carried out by atomic layer deposition of a thin conformal and homogeneous TiO2 layer that is crystallized by an annealing at 450 °C. This process allows using such surfaces as photoanodes for water oxidation. The 40 nm thick TiO2 film acts indeed as an efficient protective layer against the photocorrosion of the porous Si in KOH, enhances its wettability, and improves the light absorption of the photoelectrode. The macroporous dual-absorber TiO2/Si has a beneficial effect on water oxidation in 1 M KOH and leads to a considerable negative shift of the onset potential of ∼400 mV as well as a 50% increase in photocurrent at 1 V vs SCE.

Impact of sol aging on TiO2 compact layer and photovoltaic performance of perovskite solar cell

Perovskite solar cells are known to have a power conversion efficiency dependent on subtle variation in chemical composition and crystal and microstructures of materials, processing conditions, and device fabrication procedures and conditions. The present work demonstrates such strong dependence of power conversion efficiency on a TiO2 film made of the same sol with various aging time. A dense and conformal TiO2 film was prepared by sol-gel method, and the influences of its surface morphology and thickness on performance of perovskite solar cells have been investigated. The surface morphology and thickness of the TiO2 film were tuned by adjusting the aging time of sol, resulting in enhanced short-circuit current density and fill factor of the perovskite solar cells due to increased coverage and roughness of perovskite films, light refraction, and effective charge recombination blocking effect, which were verified by means of the light absorption spectra, photoluminescence of perovskite films with and without hole transport layer, cyclic voltammogram, and electrochemical impedance spectra. The cells with a dense and conformal TiO2 compact layer derived from the sol aged for 4 h exhibit a power conversion efficiency of 15.7%, 50% higher than the efficiency based on TiO2 layer derived from 0 h aging sol and 3 times of the efficiency with TiO2 layer made from 8 h aged sol.

Improving the photovoltaic performance and flexibility of fiber-shaped dye-sensitized solar cells with atomic layer deposition

A conformal and thin TiO2 film fabricated with atomic layer deposition (ALD) improves the performance of fiber-shaped dye-sensitized solar cells (FDSSCs). Electrical contact at Ti/TiO2 is improved through insertion of a uniform and pinhole-free TiO2 film. The film thickness varies from 5nm to 40nm, and the high power conversion efficiency of 7.41% is achieved from a typical device with a 15nm TiO2 film and a 10μm TiO2 nanoparticles layer. The compact and high-quality TiO2 film improves Ti/TiO2 interfacial property. Surface modification hastens electron transport, which thus improves the charge collection efficiency. Both photocurrent density and bending performance are significantly enhanced compared with those of the corresponding FDSSCs without ALD treatment.

Superior Photostability and Photocatalytic Activity of ZnO Nanoparticles Coated with Ultrathin TiO2 Layers through Atomic-Layer Deposition.

Atomic-layer deposition (ALD) is a thin-film growth technology that allows for conformal growth of thin films with atomic-level control over their thickness. Although ALD is successful in the semiconductor manufacturing industry, its feasibility for nanoparticle coating has been less explored. Herein, the ALD coating of TiO2 layers on ZnO nanoparticles by employing a specialized rotary reactor is demonstrated. The photocatalytic activity and photostability of ZnO nanoparticles coated with TiO2 layers by ALD and chemical methods were examined by the photodegradation of Rhodamine B dye under UV irradiation. Even though the photocatalytic activity of the presynthesized ZnO nanoparticles is higher than that of commercial P25 TiO2 nanoparticles, their activity tends to decline due to severe photocorrosion. The chemically synthesized TiO2 coating layer on ZnO resulted in severely declined photoactivity despite the improved photostability. However, ultrathin and conformal ALD TiO2 coatings (≈ 0.75-1.5 nm) on ZnO improved its photostability without degradation of photocatalytic activity. Surprisingly, the photostability is comparable to that of pure TiO2, and the photocatalytic activity to that of pure ZnO.

Efficiency Enhancement of Nanotextured Black Silicon Solar Cells Using Al2O3/TiO2 Dual-Layer Passivation Stack Prepared by Atomic Layer Deposition.

In this study, efficient nanotextured black silicon (NBSi) solar cells composed of silicon nanowire arrays and an Al2O3/TiO2 dual-layer passivation stack on the n(+) emitter were fabricated. The highly conformal Al2O3 and TiO2 surface passivation layers were deposited on the high-aspect-ratio surface of the NBSi wafers using atomic layer deposition. Instead of the single Al2O3 passivation layer with a negative oxide charge density, the Al2O3/TiO2 dual-layer passivation stack treated with forming gas annealing provides a high positive oxide charge density and a low interfacial state density, which are essential for the effective field-effect and chemical passivation of the n(+) emitter. In addition, the Al2O3/TiO2 dual-layer passivation stack suppresses the total reflectance over a broad range of wavelengths (400-1000 nm). Therefore, with the Al2O3/TiO2 dual-layer passivation stack, the short-circuit current density and efficiency of the NBSi solar cell were increased by 11% and 20%, respectively. In conclusion, a high efficiency of 18.5% was achieved with the NBSi solar cells by using the n(+)-emitter/p-base structure passivated with the Al2O3/TiO2 stack.

Highly Controllable Surface Plasmon Resonance Property by Heights of Ordered Nanoparticle Arrays Fabricated via a Nonlithographic Route.

Perfectly ordered nanoparticle arrays are fabricated on large-area substrates (>cm(2)) via a cost-effective nonlithographic route. Different surface plasmon resonance (SPR) modes focus consequently on their own positions due to the identical shape and uniform size and distance of these plasmonic metallic nanoparticles (Ag and Au). On the basis of this and FDTD (finite-difference time-domain) simulation, this work reveals the variation of all SPR parameters (position, intensity, width, and mode) with nanoparticle heights, which demonstrates that the effect of heights are different in various stages. On increasing the heights, the major dipole SPR mode precisely blue-shifts from the near-infrared to the visible region with intensity strengthening, a peak narrowing effect, and multipole modes excitation in the UV-vis range. The intensity of multipole modes can be manipulated to be equal to or even greater than the major dipole SPR mode. After coating conformal TiO2 shells on these nanoparticle arrays by atomic layer deposition, the strengthening of the SPR modes with increasing the heights results in the multiplying of the photocurrent (from ∼2.5 to a maximum 90 μA cm(-2)) in this plasmonic-metal-semiconductor-incorporated system. This simple but effective adjustment for all SPR parameters provides guidance for the future design of plasmonic metallic nanostructures, which is significant for SPR applications.

Enhanced photoelectrochemical water oxidation via atomic layer deposition of TiO2 on fluorine-doped tin oxide nanoparticle films.

TiO2 is an exemplary semiconductor anode material for photoelectrochemical (PEC) water-splitting electrodes due to its functionality, long-term stability in corrosive environments, nontoxicity, and low cost. In this study, TiO2 photoanodes with enhanced photocurrent density were synthesized by atomic layer deposition (ALD) of TiO2 onto a porous, transparent, and conductive fluorine-doped tin oxide nanoparticle (nanoFTO) scaffold fabricated by solution processing. The simplicity and disordered nature of the nanoFTO nanostructure combined with the ultrathin conformal ALD TiO2 coatings offers advantages including decoupling charge carrier diffusion length from optical penetration depth, increased photon absorption probability through scattering, complimentary photon absorption, and favorable interfaces for charge separation and transfer across the various junctions. We examine the effects of porosity of the nanoFTO scaffold and thickness of the TiO2 coating on PEC performance and achieve an optimal photocurrent of 0.7 mA cm(-2) at 0 V vs. Ag/AgCl under 100 mW cm(-2) AM 1.5 G irradiation in a 1 M KOH aqueous electrolyte. Furthermore, the fundamental mechanisms behind the improvements are characterized via cyclic voltammetry, incident photon-to-current efficiency, transient photocurrent spectroscopy, and electrochemical impedance spectroscopy and are contrasted with those of single crystal rutile TiO2 nanowires. The strategies employed in this work highlight the opportunities inherent to these types of heteronanostructures, where the lessons may be applied to improve the PEC conversion efficiencies of other promising semiconductors, such as hematite (α-Fe2O3) and other materials more sensitive to visible light.

Pulsed laser deposited porous nano-carpets of indium tin oxide and their use as charge collectors in core-shell structures for dye sensitized solar cells.

Porous In2O3:Sn (ITO) films resembling from brush carpets to open moss-like discrete nanostructures were grown by pulsed laser deposition under low to high background gas pressures, respectively. The charge transport properties of these mesoporous substrates were probed by pulsed laser photo-current and -voltage transient measurements in N719 dye sensitized devices. Although the cyclic voltammetry and dye adsorption measurements suggest a lower proportion of electro-active dye molecules for films deposited at the high-end background gas pressures, the transient measurements indicate similar electron transport rates within the films. Solar cell operation was achieved by the deposition of a conformal TiO2 shell layer by atomic layer deposition (ALD). Much of the device improvement was shown to be due to the TiO2 shell blocking the recombination of photoelectrons with the electrolyte as recombination lifetimes increased drastically from a few seconds in uncoated ITO to over 50 minutes in the ITO with a TiO2 shell layer. Additionally, an order of magnitude increase in the electron transport rate in ITO/TiO2 (core/shell) films was observed, giving the core-shell structure a superior ratio of recombination/transport times.

Structural optimization of 3D porous electrodes for high-rate performance lithium ion batteries.

Much progress has recently been made in the development of active materials, electrode morphologies and electrolytes for lithium ion batteries. Well-defined studies on size effects of the three-dimensional (3D) electrode architecture, however, remain to be rare due to the lack of suitable material platforms where the critical length scales (such as pore size and thickness of the active material) can be freely and deterministically adjusted over a wide range without affecting the overall 3D morphology of the electrode. Here, we report on a systematic study on length scale effects on the electrochemical performance of model 3D np-Au/TiO2 core/shell electrodes. Bulk nanoporous gold provides deterministic control over the pore size and is used as a monolithic metallic scaffold and current collector. Extremely uniform and conformal TiO2 films of controlled thickness were deposited on the current collector by employing atomic layer deposition (ALD). Our experiments demonstrate profound performance improvements by matching the Li(+) diffusivity in the electrolyte and the solid state through adjusting pore size and thickness of the active coating which, for 200 μm thick porous electrodes, requires the presence of 100 nm pores. Decreasing the thickness of the TiO2 coating generally improves the power performance of the electrode by reducing the Li(+) diffusion pathway, enhancing the Li(+) solid solubility, and minimizing the voltage drop across the electrode/electrolyte interface. With the use of the optimized electrode morphology, supercapacitor-like power performance with lithium-ion-battery energy densities was realized. Our results provide the much-needed fundamental insight for the rational design of the 3D architecture of lithium ion battery electrodes with improved power performance.

Size and concentration effects of gold nanoparticles on optical and electrical properties of plasmonic dye sensitized solar cells

Gold nanoparticles (GNPs) of various sizes (range of 5–85nm) were synthesized and various concentrations (range of 0.1–0.7wt%) were blended with TiO2 nanopowder for fabricating conformal TiO2–Au nanocomposite (NC) films. In optical and electrical studies, we have observed that GNPs of sizes in the range of 15–40nm, and concentrations in the range of 0.1–0.25wt% offer the maximum enhancement in dye-sensitized solar cell (DSSC) performance due to the enhanced near-field excitation of dye molecules along with incident light far-field. The best plasmonic DSSC performance was observed with 0.24wt% of ∼36nm GNPs with an enhancement of 18.44% in photocurrent. Despite the strong absorptance with ∼5nm GNPs, only a modest improvement in photovoltaic behavior was observed due to plasmonic heating effects of strongly localized near-fields instead of dye molecules excitation. With ∼85nm GNPs, we have observed minimal enhancement in device performance due to large scattering cross-sections, which result in the incident energy to be sent back to the far-field after interacting with GNPs instead of localizing around them. The optimized size and concentration of GNPs were also used for fabricating high efficiency DSSCs using commercial TiO2 paste and two different dyes (N719 and N749) in order to study the effects of apparent extinction coefficients of the dyes as well as device thickness on photocurrent and energy conversion efficiency enhancements of DSSCs.

Effects of TiO₂ interfacial atomic layers on device performances and exciton dynamics in ZnO nanorod polymer solar cells.

The performances of organic electronic and/or photonic devices rely heavily on the nature of the inorganic/organic interface. Control over such hybrid interface properties has been an important issue for optimizing the performances of polymer solar cells bearing metal-oxide conducting channels. In this work, we studied the effects of an interfacial atomic layer in an inverted polymer solar cell based on a ZnO nanorod array on the device performance as well as the dynamics of the photoexcited carriers. We adopted highly conformal TiO2 interfacial layer using plasma enhanced atomic layer deposition (PEALD) to improve the compatibility between the solution-prepared active layer and the ZnO nanorod array. The TiO2 interfacial layer facilitated exciton separation and subsequent charge transfer into the nanorod channel, and it suppressed recombination of photogenerated carriers at the interface. The presence of even 1 PEALD cycle of TiO2 coating substantially improved the short-circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF), leading to more than 2-fold enhancement in the power conversion efficiency (PCE). The dynamics of the photoexcited carriers in our devices were studied using transient absorption (TA) spectroscopy. The TA results clearly revealed that the TiO2 coating played a key role as an efficient quencher of photogenerated excitons, thereby reducing the exciton lifetime. The electrochemical impedance spectra (EIS) provided further evidence that the TiO2 atomic interfacial layer promoted the charge transfer at the interface by suppressing recombination loss.

Atomic layer deposition of TiO2 on mesoporous nanoITO: conductive core-shell photoanodes for dye-sensitized solar cells.

Core-shell structures consisting of thin shells of conformal TiO2 deposited on high surface area, conductive Sn-doped In2O3 nanoparticle. Mesoscopic films were synthesized by atomic layer deposition and studied for application in dye-sensitized solar cells. Results obtained with the N719 dye show that short-circuit current densities, open-circuit voltages, and back electron transfer lifetimes all increased with increasing TiO2 shell thickness up to 1.8-2.4 nm and then decline as the thickness was increased further. At higher shell thicknesses, back electron transfer to -Ru(III) is increasingly competitive with transport to the nanoITO core resulting in decreased device efficiencies.

Toward Higher Photovoltage: Effect of Blocking Layer on Cobalt Bipyridine Pyrazole Complexes as Redox Shuttle for Dye-Sensitized Solar Cells

Fluorine-doped tin-oxide (FTO) transparent conducting substrates, for use in cobalt bis(bipyridine pyrazole) complex based dye-sensitized solar cells (DSCs), were compared using the conventional TiCl4 treatment with conformal TiO2 blocking layers formed by atomic layer deposition (ALD). The conformal blocking layer prepared by ALD promotes a decrease in the dark current, owing to retarded recombination between the FTO and the cobalt electrolyte. The thickness for the conformal blocking layer was optimized to attain the best photovoltaic performance. Optimized photovoltaic devices employing a double layer provided the best results, and a peak power conversion efficiency of 10.6% was achieved under full sun light intensity.

Plasma activation and atomic layer deposition of TiO2 on polypropylene membranes for improved performances of lithium-ion batteries

Atomic layer deposition (ALD) of TiO2 was applied on porous polypropylene (PP) membranes which were used as separators in lithium-ion batteries (LIBs) composed of Li4Ti5O12 (LTO) anode/Li cathode. Without plasma activation on the bare PP membrane, the initial deposition of TiO2 was based on the subsurface nucleation mechanism, which prevented the formation of a conformal hydrophilic TiO2 layer at low ALD cycles. The improvement of wettability of the PP membrane to the electrolyte could only be achieved at high ALD cycles up to 500. However, the severe narrowing of membrane pores counterbalanced the wetting enhancement, which hardly improved the performance of the LIBs. Plasma pretreatment was efficient to generate active groups on the highly chemically inert surface of polypropylene membranes, thus ultrathin TiO2 films could be conformally deposited by ALD on the membrane surface based on the layer-by-layer mechanism at cycles as low as 20. Such a conformal ultrathin layer of TiO2 was confirmed to concurrently overcome both the thermal shrinkage and poor wettability of the PP membranes. Beneficial from the improved wettability at no expense of pore size, the electrochemical performances of LIBs such as specific discharge capacities at different discharge rates were upgraded.

Conformal coating of TiO2 nanorods on a 3-D CNT scaffold by using a CNT film as a nanoreactor: a free-standing and binder-free Li-ion anode

A free-standing and binder-free electrode, which consists of a 3-D conductive scaffold and conformal TiO2 nanorods, is synthesized by using an interconnected CNT film as a nanoreactor. Such a conformal 3-D nanostructure can not only provide the electrodes with fast charge transportation and long-term structural integrity, but also eliminate the heavy metal current collector, polymer binder and conductive additive. The conformal CNT–TiO2 anode demonstrates exceptional electrochemical properties with high capacity, high rate capability and excellent cycling stability. It shows a charge capacity of 129 mA h g−1 after 1000 cycles at 10 C with a low capacity fade of 0.007% per cycle in a half cell. Meanwhile, the CNT–TiO2/LiFePO4 full cell exhibits superior capacity retention with a high Coulombic efficiency of approximately 100% during 400 cycles at 2 C. Such a 3-D conformal nanostructure is promising for application in high-performance energy storage.

Pseudocapacitance of Amorphous TiO2 Thin Films Anchored to Graphene and Carbon Nanotubes Using Atomic Layer Deposition

Amorphous TiO2 thin films were conformally coated onto the surface of both graphene (G) and multiwalled carbon nanotube (CNT) samples using atomic layer deposition (ALD). An ultrathin Al2O3 adhesion layer was employed to obtain the conformal TiO2 ALD films. Using 1 M KOH as the electrolyte, the electrochemical characteristics of TiO2 ALD films grown using 25 and 50 TiO2 ALD cycles were then determined using cyclic voltammetry, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy. Because the TiO2 ALD films were ultrathin, the poor electrical conductivity and low ionic diffusivity of TiO2 did not limit the ability of the TiO2 ALD films to display high specific capacitance. The specific capacitances of the TiO2 ALD-coated G and CNT samples after 50 TiO2 ALD cycles were 97.5 and 135 F/g, respectively, at 1 A/g. The pseudocapacitance of the TiO2 ALD films greatly exceeded the electric double layer capacitance of the uncoated G and CNT samples. The galvanostatic charge/discharge ex...

High Surface Capacity Li-Ion All Solid State 3D Microbattery Based on Anatase TiO2 Deposited by ALD on Silicon Microstructures

The proposed study deals with the design, the fabrication and the characterization of 3D silicon microstructures decorated with step conformal and pin hole free anatase TiO2 negative electrode deposited by atomic layer deposition. A high aspect ratio and dense array of 3D silicon micropillars or microtubes are fabricated on silicon wafer. The obtained surface capacity of the TiO2 layer (150nm) deposited on 3D microtubes (area enlargement factor =25) reaches 0.15 mAh/cm2 at C/4. The reported 3D topologies enhance the capacity of standard TiO2 2D thin film electrode.