Application of Taguchi method to study morphological evolution of TiO2 nanotubes obtained via anodization process

The growth of titanium dioxide nanotubes (TiO2) via anodization process depends on the controlling parameters such as applied potential, anodization time, and electrolyte composition. In the present work, the Taguchi method was applied to evaluate statistically the influence of the anodization parameters on the morphology of anodized TiO2 films. Mixture of ethylene-glycol and glycerol was used as an electrolyte and the settings of the experimental design were parameterized on the basis of four important anodization factors consisting of chemical pretreatment, amount of fluoride, water content and applied potential. Samples were characterized by XRD and FEG-SEM. Based on 4 variables at 3 different settings, full factorial plan requires 34 = 81 tests. In this work the experiment was designed on the basis of an L9 (34) orthogonal array (4 variables, 3 levels, 9 tests). The optimum conditions were found on the basis of smaller-is-better and larger-is-better analyses. The signal-to-noise ratio was employed to find optimal process parameters levels and to analyze the influence of these parameters on the tubular length, internal and external diameters and formation of nanograss on the film surface. Hence, it is clearly shown that the performance of TiO2 nanotubes can be evaluated by the Taguchi method

Interfacial band alignment and photoelectrochemical properties of all-sputtered BiVO₄/FeNiO<i>_x</i> and BiVO₄/FeMnO<i>_x</i>p–n heterojunctions

BiVO4 is a well-known n-type semiconductor with great potential for photoelectrochemical (PEC) conversion of solar energy into chemical fuels. Nevertheless, photocurrent densities achieved for bare BiVO4 photoanodes are still far from their theoretical maximum due to the sluggish water oxidation kinetics and limitation in electron-hole recombination. In this work, magnetron sputtering deposition was used for depositing FeMOx (M = Ni, Mn) as cocatalyst layers to induce p–n heterojunctions and suppress charge recombination on BiVO4 photoanodes. The all-sputtered p–n heterojunction BiVO4/FeMnOx exhibited the highest photocurrent density (1.25 mA cm−2 at 1.23 V vs. RHE) and excellent chemical stability, indicating that the combination of Mn sites on Fe-based oxides provides promising cocatalytic materials for PEC applications. Experimental and theoretical techniques were used to investigate the interfacial band alignment and charge transport properties of BiVO4/FeMOx (M = Ni, Mn) heterojunctions. Our results show that type II heterojunctions arise in the BiVO4/FeMOx (M = Ni, Mn) interface after equilibrium, thereby providing potential barriers to inhibit electron flow from the BiVO4 to the FeMOx layers. Furthermore, the BiVO4/FeMnOx film showed a larger space charge region (SCR) characterized by a more intense built-in electric field than BiVO4/FeNiOx, explaining its higher PEC performance. In summary, this work provides a viable technique for producing photocatalytic heterojunction systems based on metal oxide semiconductors and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC applications

Photocatalytic performance of Ta2O5/BiVO4 heterojunction for hydrogen production and methylene blue photodegradation

Forming semiconductor heterojunction is promising for improved photocatalytic performance due to synergistic combination of the best properties of each material. The present study reports a simple hydrothermal strategy to form n-n heterojunction of Ta2O5 nanotubes and BiVO4 microstructures. The Ta2O5/BiVO4 heterojunctions were characterized by Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and their photocatalytic activity was evaluated by hydrogen production and photodegradation of methylene blue (MB) dye in aqueous medium under AM 1.5 G (100 mW cm-2 ) condition. The heterojunctions have optical absorption in the visible region (200-500 nm) with crystal structures defined as monoclinic for BiVO4 and orthogonal for Ta2O5. For MB photodegradation, the Ta2O5/BiVO4 obtained via hydrothermal route showed a photodegradation of 72.3%, compared to 28.3% presented by the sample produced through the mechanical mixture, with the maintenance of 86.4% of its photocatalytic performance after 3 cycles of photodegradation. For H2 production, hydrothermally prepared Ta2O5/BiVO4 generated 10.2 μmol g-1 of H2 in 3 h; while Ta2O5 nanotubes and mechanical Ta2O5/BiVO4 mixture shows 6.82 and 2.80 μmol g-1, respectively. The results suggest that Ta2O5/BiVO4 is a promising material for applications in photocatalysis, promoting sustainable energy production through hydrogen and for the treatment of effluents containing cationic dyes

Ionic liquid based dopant-free band edge shift in BiVO4 particles for photocatalysis under simulated sunlight irradiation

Foreign elemental doping is a widely utilized strategy to modify the electronic structure of semiconductors. Herein, we present a dopant-free novel synthesis approach to control the electronic structure of a semiconductor. Utilizing butyl methyl imidazolium ([BMIM]Cl) and methoxyethyl methyl imidazolium ([M(MOE)Im][Cl]) chloride ILs, we prepared four different Bi and V based ILs: 3-butyl-1-methyl-1H-imidazol 3-ium vanadate [BMIm][VO3], 3-(2-methoxyethyl)-1-methyl-1H-imidazol-3-ium vanadate [M(MOE)Im][VO3], 3-butyl-1-methyl-1H-imidazol-3-ium tetrachlorobismate [BMIm][BiCl4] and 3-(2-methoxyethyl)-1-methyl 1H-imidazol-3-ium tetrachlorobismate [M(MOE)Im][BiCl4]. Owing to the bimetallic oxide nature of BiVO4, these gels were mixed either with each other or with Bi/V commercial salts and simply heat-treated to obtain monoclinic BiVO4. Depending on the IL, the bandgap energy of pure BiVO4 will be redshifted (2.44 to 2.25 eV). The IL based synthesis induced oxygen vacancies and uplifted the BiVO4 valence band edge as observed in the X-ray photoelectron spectroscopy (XPS). These effects were profound for IL anchored Bi; however, the side effects of this synthesis were chemisorption of a higher oxygen content and low reactivity of Bi with V to form an additional V2O5 phase. ILs acted as templates to form smooth spherical particles with improved crystallinity. [M(MOE)Im] based synthesis resulted in lower-order crystallinity and a large V–O bonding length of BiVO4 compared to [BMIm] which may be ascribed to its lower-order cationic–anionic electrostatic attraction associated with the presence of oxygen in the ether-group for [M(MOE)Im]. [BMIm] cation-based synthesis suppressed photogenerated charge-recombination and resulted in a five-fold O2 evolution of B30 mmol for 3 h (AM 1.5G illumination) compared to pure BiVO4 which was better compared to the sample prepared by the conventional hydrothermal process. It also improved the photocurrent, and the MS plots have shown that the conduction band was not much affected; however, the defect density was larger for IL based synthesi

Reverse Semi-Combustion Driven by Titanium Dioxide-Ionic Liquid Hybrid Photocatalyst

EP/L015633/1 EP/K005138/1 CAPES (158804/2017/01-and 001 FAPERGS -16/2552-0000 18/2551-0000561-4 88887.195052/2018-00 CNPq :406260/2018-4 169462/2017-0 406750/2016-5 465454/2014-3 grant agreement No 810310Unprecedented metal-free photocatalytic CO2 conversion to CO (up to 228±48 μmol g−1 h−1) was displayed by TiO2@IL hybrid photocatalysts prepared by simple impregnation of commercially available P25-titanium dioxide with imidazolium-based ionic liquids (ILs). The high activity of TiO2@IL hybrid photocatalysts was mainly associated to (i) TiO2@IL red shift compared to the pure TiO2 absorption, and thus a modification of the TiO2 surface electronic structure; (ii) TiO2 with IL bearing imidazolate anions lowered the CO2 activation energy barrier. The reaction mechanism was postulated to occur via CO2 photoreduction to formate species by the imidazole/imidazole radical redox pair, yielding CO and water.publishersversionpublishe

Peering into the formation of template-free hierarchical flowerlike nanostructures of SrTiO3

The development of efficient advanced functional materials is highly dependent on properties such as morphology, crystallinity, and surface functionality. In this work, hierarchical flowerlike nanostructures of SrTiO3 have been synthesized by a simple template-free solvothermal method involving poly(vinylpyrrolidone) (PVP). Molecular dynamics simulations supported by structural characterization have shown that PVP preferentially adsorbs on {110} facets, thereby promoting the {100} facet growth. This interaction results in the formation of hierarchical flowerlike nanostructures with assembled nanosheets. The petal morphology is strongly dependent on the presence of PVP, and the piling up of nanosheets, leading to nanocubes, is observed when PVP is removed at high temperatures. This work contributes to a better understanding of how to control the morphological properties of SrTiO3, which is fundamental to the synthesis of perovskite-type materials with tailored properties

Polypyrrole/ionic liquid/Au nanoparticle counter-electrodes for dye-sensitized solar cells : improving charge-transfer resistance at the CE/electrolyte interface

To provide a viable alternative for counter electrodes used in dye sensitized solar cells, polypyrrole (PPy) based films have been synthesized via electrochemical deposition in the presence of the ionic liquid 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imidate (NTf2) and incorporated with gold nanoparticles (Aunanop). The films were analyzed by SEM, UV-Vis-NIR, Raman, Electrochemical impedance spectroscopy, Cyclic voltammetry and Conductivity measurements. The presence of the ionic liquid is found to result in a more conductive film, to improve catalytic reduction of I3 − and the electrochemical reversibility of the electrode. In addition to increase conductivity, impedance spectroscopy has shown that incorporating Aunanop in the PPy/NTf2 film helps improving the interfacial charge transportation, the electrocatalytic properties and solar energy conversion efficiency. DSSCs assembled with PPy based CE presented nearly the same J-V characteristic parameters as observed from conventional Pt based devic

Experimental and DFT study of GO-decorated CaO quantum dots for catalytic dye degradation and bactericidal potential

This research lays the groundwork for preparing graphene oxide (GO)-doped CaO nanocomposites for efficient antibacterial potential and dye degradation. The study aimed to reduce the recombination rate of the electron hole (e−/h+) of CaO and improve charge transfer. This issue can be minimized by doping high-surface area GO into CaO quantum dots (QDs). Herein, the one-pot co-precipitation technique has prepared various concentrations (1, 3, and 5 wt%) of GO-doped CaO. Characterization techniques were used to investigate optical, elemental analysis, microstructural, functional, and morphological properties. The addition of GO into QDs showed excellent catalytic activity (CA) to control sample CaO against methylene blue (MB) in basic and acidic media compared to the neutral media. The synergistic effect of morphological alternation attributed to an increase in the mechanism of CA upon doping. Various concentrations of GO to QDs promised remarkable bactericidal potency against Escherichia coli

Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO2; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na2S/Na2SO3). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti2N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO2, and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO2 valence band maximum. In addition, due to O vacancies and Ti3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO2, the PEC performance of the samples only surpasses pristine TiO2 at relatively high biasing (>0.3 V vs. Ag/AgCl)

[Avance del boletín diario]: 1943 Diciembre 14

In this work thin films and nanotubes (NTs) of Ta3N5 have been synthesized by thermal nitridation of amorphous Ta2O5 starting materials. Ta2O5 thin films were prepared by radio frequency magnetron sputtering; whereas Ta2O5 NTs were prepared by electrochemical anodization. With the aim to investigate electronic, optical structural, surface, and particularly photoelectrochemical properties; the Ta3N5 samples were studied employing thorough characterization techniques. X-ray diffraction and high resolution electron microscopy analyses have shown that Ta3N5 thin films exhibit monoclinic phase whereas Ta3N5 NTs present orthorhombic crystalline structure of Ta3N5. Utilizing Ta3N5 thin films the optical constants were obtained by spectroscopic ellipsometry. The obtained dielectric constant of Ta3N5 thin film was in the range of 7–9 in the visible spectral region (3.1–1.7 eV). After studying Ta3N5 thin films the investigations were focused on the NTs. To preserve the tubular morphology of Ta3N5 NTs at higher nitridation temperatures the anodization was optimized by fine-tuning the adherence and the wall thickness of Ta2O5 NTs. The Rietveld refinement has confirmed that in addition to oxygen substitutional defects the nitridation process results in Schottky defects of nitrogen and tantalum within the crystalline structure. Utilizing cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy it was observed for the first time that lower photoelectrochemical performance of pristine T3N5 NTs is attributed to the presence of trapping states associated with T3N5 NTs–electrolyte interface in standard electrolyte. Even highly crystalline pristine Ta3N5 NTs could not cope with these trapping states. These states devastate the performance of the photoanode and present the necessity of applying higher biasing (> 1.23 V vs RHE); which is a major drawback of using pristine Ta3N5 NTs for water splitting. These were not observed in the electrolyte containing sacrificial reagent due to its efficient hole scavenging ability. Electrochemical Impedance spectroscopy has shown that the charge transportation at the Semiconductor–Electrolyte interface is highly influenced by the nitridation conditions; however, the flat band of pristine Ta3N5 NTs remained unchanged. It was found that for improved photoelectrochemical performance of Ta3N5 NTs the nitridation temperature should be high enough to improve crystallinity but the time should be short enough to preserve the tubular morphology. The improved photoelectrochemical performance was related to low oxygen content, high crystallinity, low defects formation and low interfacial charge transfer at Semiconductor–Electrolyte interface, obtained at optimum anodization and nitridation conditions.Neste trabalho, filmes finos e nanotubos (NTs) de Ta3N5 foram sintetizados por termo-nitretação a partir de Ta2O5 amorfo. Filmes finos de Ta2O5 foram preparados por rádio frequência magnetron sputtering e os nanotubos foram preparados por anodização electroquímica. Foram investigadas as propriedades eletrônicas, ópticas, estruturais, superfíciais e, particularmente, as propriedades fotoeletroquímicas dos amostras de Ta3N5. Difração de raios-X e análises de microscopia eletrônica de alta resolução mostraram que filmes finos de Ta3N5 apresentam fase monoclínica, enquanto nanotubos de Ta3N5 estrutura cristalina ortorrômbica. Para os filmes finos de Ta3N5 foram obtidos constantes ópticas por elipsometria espectroscópica. O valor obtido para a constante dielétrica foi de 7–9 na região espectral visível (3,1-1,7 eV). Após o estudo dos filmes finos de Ta3N5 as investigações centraram-se nos nanotubos. Para preservar a morfologia tubular em altas temperaturas de nitretação, o processo de anodização foi otimizado para aumentar a aderência e a espessura da parede dos nanotubos de Ta2O5.O refinamento Rietveld mostrou que o processo de nitretação resulta em defeitos Schottky de nitrogênio e tântalo, além de isso, a amostra apresenta defeitos substitucionais do oxigênio. Utilizando voltametria cíclica, cronoamperometria e espectroscopia de impedância electroquímica foi observado que o desempenho fotoeletroquímico inferior dos nanotubos puros de Ta3N5 é atribuído à presença de estados aprisionados associados a interface dos nanotubos de Ta3N5–electrólito com electrólito padrão. Ainda, mesmo altamente cristalinos os nanotubos puros de Ta3N5 não podem suportar os estados de aprisionamento mencionados, que prejudicam o desempenho do fotoanodo e, assim, necessitam a aplicação de maior polarização externa (> 1,23 V vs RHE). Estes resultados não foram observados no electrólito contendo reagente de sacrifício. A espectroscopia de impedância electroquímica mostrou que o transporte de carga na interface de semicondutores–eletrólito é altamente influenciada pelas condições de nitretação. No entanto, a banda plana de nanotubos de Ta3N5 puro mantem-se inalterada. Verificou-se que para um melhor desempenho fotoelectroquímico das nanotubos de Ta3N5 a temperatura de nitretação deve ser suficientemente elevada para melhorar a cristalinidade, mas o tempo deve ser curto o suficiente para preservar a morfologia tubular. A melhora do desempenho fotoeletroquímico foi relacionada com baixo teor de oxigênio, alta cristalinidade, baixa formação de defeitos e baixa transferência de carga na interface do semicondutor com o eletrólito, obtidos em condições de anodização e nitretação ideais