Three-dimensional ZnO@TiO2 core-shell nanostructures decorated with plasmonic Au nanoparticles for promoting photoelectrochemical water splitting

A novel three-dimensional (3D) core-shell nanostructure decorated with plasmonic Au nanoparticles (NPs) was prepared for photoelectrochemical water splitting. In the new nanostructure, ZnO nanorods (NRs) are perpendicular to ZnO nanosheets (NSs), and the ZnO NSs-NRs are coated with a thin TiO₂ shell formed by liquid phase deposition. The plasmonic Au NPs were formed in situ on the surface of ZnO NSs-NRs@TiO₂ by thermal reduction. A thin TiO₂ shell and uniformly distributed Au NPs were successfully obtained. The photoconversion efficiency and photocurrent density of the 3D ZnO NSs-NRs@TiO₂-Au nanostructure respectively reached 0.48% and 1.73 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, 4.80 and 4.33 times higher than those of ZnO NSs, respectively. The thin TiO₂ shell effectively promoted charge separation, while the surface plasmon resonance effects of the Au NPs improved the photocurrent density. The findings suggest that the 3D ZnO NSs-NRs@TiO₂-Au nanostructure is a promising photoanode for photoelectrochemical water splitting.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Chemical processing of double-walled carbon nanotubes for enhanced hydrogen storage

Double-walled carbon nanotubes (DWCNTs) were modified for enhanced hydrogen storage by employing a combination of two techniques: KOH activation for the formation of defects on DWCNT surfaces and loading of the DWCNTs with nanocrystalline Pd. The physical properties of the pristine DWCNTs and chemically modified DWCNTs were systematically characterised by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller (BET) surface area measurements. The amounts of hydrogen storage capacity were measured at ambient temperature and found to be 1.7, 2.0, 3.7, and 2.8 wt% for pristine DWCNTS, 2 wt% Pd DWCNTs, activated DWCNTs, and 2 wt% Pd activated DWCNTs, respectively. Hydrogen molecules could be adsorbed on defect sites created by chemical activation in DWCNTs through van der Waals forces. For Pd nanoparticle loaded DWCNTs, H₂ molecules could be dissociated into atomic hydrogen and adsorbed on defect sites. We found that the hydrogen storage capacity of DWCNTs can be significantly enhanced by chemical activation or loading with Pd nanoparticles.     

Surface ligand effects in MEH-PPV/TiO2 hybrid solar cells

TiO₂ nanorods (NRs) were synthesized by hydrolysis of titanium tetraisopropoxide (TTIP) using oleic acid (99%) as surfactant at low temperatures (80-100 °C) and are modified with different ligands: oleic acid (OLA), n-octyl-phosphonic acid (OPA) and thiophenol (TP) in order to investigate the effect of surface ligand on the excition dissociation and the charge transport in hybrid MEH-PPV/TiO₂ photovoltaic (PV) cells. The morphology and crystalline form of as-prepared TiO₂ NRs are examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and Raman spectrometer (RS). The FTIR analysis confirms all the ligands coordinated with the Ti center of TiO₂ NRs. The optical properties of the modified TiO₂ NRs are characterized by UV-vis absorption spectra and photoluminescence (PL) spectra. Thiophenol modified TiO₂ NRs quench the PL of MEH-PPV more effectively than OLA-TiO₂ NRs and OPA-TiO₂ NRs. The power conversion efficiency of hybrid PV cells from thiophenol modified TiO₂ NRs and MEH-PPV is the highest among the investigated TiO₂ NRs.     

Investigation of hydrogen storage capacity of multi-walled carbon nanotubes deposited with Pd or V

This work presents the synthesis and characterization of multi-walled carbon nanotubes (multi-walled CNTs) deposited with Pd or V and their hydrogen storage capacity measured by Sievert's volumetric apparatus. The CNTs were grown by the CVD method using LPG and LaNi₅ as the carbon source and catalyst, respectively. Pd was impregnated on the CNTs by the reflux method with hydrogen gas as a reducing agent, while V was embedded on the CNTs by the vapor deposition method. The average metal particle size deposited on the CNTs was around 5.8 nm for Pd and 3.6 nm for V. Hydrogen adsorption experiments were performed at room temperature and at −196 °C under a hydrogen pressure of 65 bar. At −196 °C, the treated CNTs had a maximum hydrogen uptake of 1.21 wt%, while the CNTs deposited with Pd (Pd-CNTs) and CNTs deposited with V (V-CNTs) possessed lower surface areas, inducing lower hydrogen adsorption capacities of 0.37 and 0.4 wt%, respectively. For hydrogen sorption at room temperature, the CNTs decorated with the metal nanoparticles had a higher hydrogen uptake compared to the treated CNTs. Hydrogen adsorption capacity was 0.125 and 0.1 wt% for the Pd-CNTs and V-CNTs, respectively, while the hydrogen uptake of the treated CNTs was <0.01 wt%. For the second cycle, only half of the first hydrogen uptake was obtained, and this was attributed to the re-crystallization of the defect sites on the carbon substrate after the first hydrogen desorption.     

Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor

Highly-ordered, vertically oriented TiO₂ nanotubes are synthesized, and their hydrogen sensing properties are investigated. Self-organized TiO₂ nanotube arrays are grown by anodic oxidation of a titanium foil in an aqueous solution that contains 1 wt% hydrofluoric acid at 20 °C. We use a potential ramp at a rate of 100 mV s⁻¹, increasing from the initial open-circuit potential (OCP) to 20 V, and this final potential of 20 V is then held constant during the anodization process. The fabricated TiO₂ nanotubes are approximately 1 μm in length and 90 nm in diameter. For the sensor measurements, two platinum pads are used as electrodes on the TiO₂ nanotube arrays. The hydrogen sensing characteristics of the sensor are analyzed by measuring the sensor responses ((I − I₀)/I₀) in the temperature interval of 20–150 °C. We find that the sensitivity of the sensor is approximately 20 for 1000 ppm H₂ exposure at room temperature, and increases with increasing temperature. The sensing mechanism of the TiO₂ nanotube sensor could be explained with chemisorption of H₂ on the highly active nanotube surface.     

La-modified TiO2/carbon nanotubes assembly nanocomposite for efficient photocatalytic hydrogen evolution from glycerol-water mixture

Lanthanum (La) modified TiO₂ embedded over carbon nanotubes (CNTs) to develop nanocomposite for enhanced photocatalytic hydrogen evolution from glycerol-water mixture has been investigated. The samples, synthesized by sol-gel assisted hydrothermal method, were characterized by XRD, FESEM, HRTEM, FTIR, BET, UV–Visible, Raman and PL spectroscopy. La/TiO₂ anchored CNTs has larger surface area with uniform distribution of La³⁺ ions. The performance of hybrid assembly was evaluated in a continuous flow slurry photoreactor system under UV–visible light irradiations. The highest H₂ evolution rate of 17265 ppm g-cat⁻¹ h⁻¹ was obtained over 5% La-5% CNTs/TiO₂ NRs, 1.14 folds the amount produced over 5% La-5% CNTs/TiO₂ NPs, 1.53 time than 5% La/TiO₂ and 4.10 folds than using pure TiO₂ NPs. This significant improvement in the photo-activity was obviously due to synergistic effect between La and CNTs, larger BET surface area, cleavage of glycerol by La, appropriate band structure, and hindered charges recombination rate. Among the sacrificial reagents, highest H₂ evolution was obtained using glycerol due to the presence of α-hydrogen atoms attached to carbon atoms. A correlation between BET surface area and photonic flux in terms of photocatalytic H₂ production rate was established to understand the performance of hybrid nanostructures. It was observed that BET surface area and photo-flux utilization for the generation of charge carriers with their efficient separation has significant contribution towards improved H₂ production rate. More interestingly, catalyst persisted prolonged stability with a steady H₂ production in cyclic runs. This study provides promising pathway for constructing composite of La/TiO₂ coupled CNTs for efficient H₂ production under visible light.     

Single crystalline TiO2 nanorods with enhanced visible light activity for solar hydrogen generation

Single crystalline TiO₂ nanorods and polycrystalline nanotubes were fabricated with same length to investigate the effects of their nanostructures on photocatalytic properties for splitting water. In order to enhance the visible light absorbance, TiO₂ nanorods and nanotubes were sensitized with semiconductor nanoparticles such as CdS, CdSe, and CdS/CdSe, and compared in viewpoint of solar hydrogen generation. It was observed that single-crystalline nanorods showed superior photocatalytic properties to polycrystalline nanotubes, and also the potential level of the nanorods with rutile phase was measured as lower than that of the nanotubes with mixture of anatase and rutile. Further improvement of photo-conversion efficiency was obtained by subsequent heat treatments of the sensitized photoelectrodes. It turns out that the improvement is attributed to the improved crystallinity and the increased size of the nanoparticles during the post-annealing treatments. It was demonstrated that TiO₂ nanorods with lower potential level and a single crystalline phase on FTO glass were advantageous for effective charge injection from the sensitized nanoparticles and transport without recombination lost at grain boundaries.     

Modification of the photocatalytic properties of self doped TiO2 nanoparticles for hydrogen generation using sunlight type radiation

Synthetic approaches/methodologies can change the properties of nanoparticles significantly. In this study, the photocatalytic property of self (Ti³⁺) doped TiO₂ nanoparticles was modified by synthesizing through different routes. Solvothermal (T-Sol), sonochemical (T-Son) and polyol (T-Pol) methods were employed to prepare TiO₂ nanoparticles and the photocatalytic activities of these samples were compared with that of the sample prepared by precipitation using ammonia solution (T-Ppt). All samples had particle size below 30 nm except T-Son, where small nanoparticles existed as large spherical agglomerates with size around 500 nm. Surface area and porosity measurements of these different TiO₂ samples showed a significant dependency on the synthesis method. UV–Visible absorption spectra showed the onset of absorption at ∼440 nm for all samples due to the presence of defect levels originating from anion vacancies. Photocatalytic activity for hydrogen generation decreased in the order T-Sol > T-Son > T-Pol > T-Ppt and the observed activity is correlated with their physical properties such as surface area and crystallinity. The hydrogen yield was highly enhanced by the addition of Pd metal as co-catalyst on the surface of TiO₂ photocatalysts. Present experiments clearly demonstrate the importance of synthesis route to improve the photocatalytic activity of TiO₂.     

Cooling of minichannel heat sink using nanofluids

Nanofluids contain a small fraction of solid nanoparticles in base fluids. Nanofluids cooled small channel heat sinks, have been anticipated to be an excellent heat dissipation method for the next generation electronic devices. In this study, nanofluids are used with different volume fractions of nanoparticles as a coolant for the minichannel. Al₂O₃–water nanofluid and TiO₂–water nanofluid were tested for the copper minichannel heat sink, with the bottom of 20 × 20 mm laminar flow as a coolant, through hydraulic diameters. The result showed that adding Al₂O₃ nanoparticles to water at 4% of volume fractions, enhanced the thermal conductivity by 11.98% and by dispersing TiO₂ to the base fluid, was 9.97%. It was found that using nanofluid such as Al₂O₃–water instead of water, improved the cooling by 2.95% to 17.32% and by using TiO₂–water, 1.88% to 16.53% was achieved. The highest pumping power by using Al₂O₃–water and TiO₂–water at 4 vol.% and 0.1 m/s was 0.000552 W and at 4 vol.% and 1.5 m/s was 0.12437 W.     

Enhanced electrochemical hydrogen storage capacity of multi-walled carbon nanotubes by TiO2 decoration

Electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) decorated by TiO₂ nanoparticles (NPs) has been studied by the galvanostatic charge and discharge method. The TiO₂ NPs are deposited on the surface of MWCNTs by sol-gel method. Structural and morphological characterizations have been carried out using XRD, SEM and TEM, respectively. TiO₂ NPs can significantly enhance the discharge capacity of MWCNTs. The cyclic voltammograms analysis indicates that the electrical double layer contributes little to the discharge capacity of TiO₂-decorated MWCNTs. The MWCNTs modified with a certain amount of TiO₂ NPs have a discharge capacity of 540 mAh/g, corresponding to an electrochemical hydrogen storage capacity of about 2.02 wt%, which is quite interesting for the battery applications. The enhancement effect of TiO₂ NPs on the discharge capacity of MWCNTs could be related to the increased effective area for the adsorption of hydrogen atoms in the presence of TiO₂ NPs on MWCNTs and the preferable redox ability of TiO₂ NPs.     

Enhancement of photoelectrochemical response by Au modified in TiO2 nanorods

We report on the design and synthesis of a novel Au/TiO₂/Au heterostructure and its implementation as a photoanode for photoelectrochemical (PEC) application. The Au/TiO₂/Au heterostructure was produced by assembling Au nanoparticles and TiO₂ nanorods (NRs) onto FTO substrate, followed by electrodepositing Au nanoparticles on the TiO₂NRs. Field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize the prepared photoanodes. Compared to the system involving Au nanoparticles directly linked to TiO₂, this Au/TiO₂/Au heterostructure exhibits significant improved photoresponse as a photoanode, as demonstrated good performance in PECs. This study illustrates the importance of pre-deposited Au underlayers in influencing PEC properties of hybrid assembled nanostructures. As the Au/TiO₂NRs/Au photoanodes are easily fabricated and highly stable, Au/TiO₂NRs/Au can serve as a good substitution for TiO₂ in a variety of solar energy driven applications including PEC water splitting, photocatalysis, and solar cells.     

Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles

Photo-induced reforming of methanol, ethanol, glycerol and phenol at room temperature for hydrogen production was investigated with the use of ultra-small Pt nanoparticles (NPs) loaded on TiO₂ nanotubes (NTs). The Pt NPs with diameters between 1.1 and 1.3 nm were deposited on TiO₂ NTs by DC-magnetron sputtering (DC-MS) technique. The photocatalytic hydrogen rate achieved an optimum value for a loading of about 1 wt% of Pt. Apparent quantum yield for hydrogen generation was measured for methanol and ethanol water solutions reaching a maximum of 16% under irradiation with a wavelength of 313 nm in methanol/water solution (1/8 v/v). Pt NPs loaded on TiO₂ NTs represented also a true water splitting catalyst under UV irradiation and pure distilled water. DC-MS method appears to be a technologically simple, ecologically benign and potentially low-cost process for production of an efficient photocatalyst loaded with ultra-small NPs with precise size control.     

Multi-walled carbon nanotubes functionalized by carboxylic groups: Activation of TiO2 (anatase) and phosphate olivines (LiMnPO4; LiFePO4) for electrochemical Li-storage

Multi-walled carbon nanotubes functionalized by carboxylic groups, exhibit better affinity towards TiO₂ (P90, Degussa) as compared to that of pristine nanotubes. Also the electrochemical performance of TiO₂ is improved by nanotube networking, but the Li-storage capacity of TiO₂ is unchanged. Whereas the composite of TiO₂ with non-functionalized nanotubes demonstrates simple superposition of the behavior of pure components, the composite with functionalized nanotubes shows unique faradaic pseudocapacitance which is specific for this composite only. The surface functionalization of nanotubes enhances charge storage capacity and reversibility of a composite with LiMnPO₄ (olivine), but mediates also the electrolyte breakdown at potentials >4.2 V. Whereas the electrochemical activation of LiMnPO₄ (olivine) by functionalized nanotubes is quite modest, excellent performance was found for LiFePO₄ (olivine) in composite materials containing only 2 wt% of functionalized nanotubes.     

Improved hydrogen storage performance of defected carbon nanotubes with Pd spillover catalysts dispersed using supercritical CO2 fluid

Hydrogen storage properties of carbon nanotubes (CNTs) modified by oxidative etching and decoration of Pd spillover catalysts are investigated. A mixed H₂SO₄/H₂O₂ solution containing ferrous ions (Fe²⁺) is useful to open the caps, to shorten the length, and to generate defects on CNTs. The Pd catalysts are deposited on the CNTs with the aid of supercritical carbon dioxide (scCO₂); as a result, a highly dispersed Pd nanoparticles and an intimate connection between Pd and carbon surface can be obtained. Combination of the two approaches can optimize a hydrogen spillover reaction on CNTs, resulting in a superior hydrogen storage capacity of 1.54 wt% (at 25 °C and 6.89 MPa), which corresponds to an enhancement factor of ∼4.5 as compared to that of pristine CNTs.     

Deposition and electrochemical activity of Pt-based bimetallic nanocatalysts on carbon nanotube electrodes

This article reports an approach to prepare bimetallic Pt–M (M = Fe, Co, and Ni) nanoparticles as electrocatalysts and examines their electrochemical activities in 1 M sulfuric acid. The approach consists of chemical oxidation of carbon nanotubes (CNTs), two-step refluxing, and subsequent thermal reduction in hydrogen atmosphere. Three bimetallic pairs of Pt–M catalysts are found to deposit well onto CNT surface, forming Pt–M/CNT composites. The electrochemical behavior of Pt–M/CNT electrodes was investigated in 1 M H₂SO₄ using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The active surface coverage (=electrochemical surface area/geometric surface area) of Pt–M catalysts is significantly enhanced, i.e., Pt–Co (85.1%) > Pt–Ni (80.4%) > Pt–Fe (76.2%) > Pt (26.3%). This enhancement of electrochemical activity can be attributed to the fact that the introduction of Co and Ni may reduce the required potential for water electrolysis and thus the associated carbon oxidation, thereby contributing to hydrogen adsorption. Equivalent circuit analysis indicates that charge transfer resistance accounts for (i) the major proportion of the equivalent serial resistance of Pt–M/CNT electrodes, and (ii) Pt–Co and Pt–Ni catalysts not only improves the electrochemical capacitance but also lowers the equivalent serial resistance. The results shed some light on how use of Pt–M/CNT composite would be a promising electrocatalyst for high-performance fuel cell applications.     

Preparation and electroactivity of polymer-functionalized graphene oxide-supported platinum nanoparticles catalysts

Polymer-functionalized graphene oxide or pristine graphene oxide supported platinum nanoparticles (Pt NPs) was prepared to study the surface modification effects. The catalysts were characterized by transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction and thermogravimetric analysis. The electrochemical activities of Pt NPs were measured by cyclic voltammograms. The poly(diallyldimethylammonium chloride) (PDDA) was used as a modifier agent which formed a functionalized layer on graphene oxide (GO) sheets. As a result, the electrochemical active surface area (ESA) of PDDA functionalized GO supported Pt (Pt/PDDA–graphene) was shown to 66 m²/g that indicated higher hydrogen adsorption amount than 55 m²/g of the pristine Pt/graphene. In addition, an average particle size of Pt/PDDA–graphene NPs was measured to 1.8 nm slightly smaller than 2.0 nm of pristine Pt/graphene NPs.     

Electroless deposition of Ni nanoparticles on carbon nanotubes with the aid of supercritical CO2 fluid and a synergistic hydrogen storage property of the composite

A hybrid synthesis protocol that combines electroless plating and the supercritical CO₂ (scCO₂) technique is developed for the first time to decorate multi-walled carbon nanotubes (CNTs) with Ni nanoparticles. The scCO₂ fluid, which is immiscible with aqueous plating solution, renders a heterogeneous Ni deposition reaction and suppresses the lateral growth of Ni, which leads to the formation of nanoparticles. A uniform dispersion of tightly anchored particles, a few nanometers in diameter, on CNTs can be achieved. Since the electroless deposition process can be easily manipulated, large-scale production should be realizable. The constructed CNT/Ni nano-composite exhibits a synergistic property in hydrogen storage performance, which is evaluated using a high-pressure microbalance. The deposited nanoparticles enhance the hydrogen spillover reaction on CNTs, tripling the hydrogen storage amount at room temperature as compared to pristine CNTs.     

Electrospun nickel doped titanium dioxide nanofibers as an effective photocatalyst for the hydrolytic dehydrogenation of ammonia borane

In this study, Ni-doped titanium dioxide (TiO₂) electrospun nanofibers are introduced as novel material for dehydrogenation of ammonia borane (AB) complex. Hydrolysis experiments with introduced catalytic nanofibers are prevailed to rapidly release hydrogen from AB complex. Typically, Ni nanoparticles (NPs) behave as a catalyst, meanwhile the incorporation of nickel NPs lead to decrease in the electrons/holes recombination rate in TiO₂ which resulted in the increase of active ions in the solution to a rapid evolution of hydrogen gas at room temperature. The utilized physiochemical analyses indicate that the introduced Ni-doped TiO₂ nanofibers have a smooth surface and uniform diameters along their lengths. Under sunlight irradiation, the hydrogen production rate in case of utilizing Ni-doped TiO₂ nanofibers is rapidly increased compared to the pristine TiO₂ nanofibers, the maximum hydrogen equivalent in case of the doped nanofibers is 2.6 while the pristine one is 1.4. Both formulations exhibit almost equal low activity in daylight as the observed hydrogen equivalent is 0.4. Overall, this study proposes cheap, stable and effective material for AB dehydrogenation at room temperature.     

Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.