Dr. Valdes-Morales, Hector

Three-dimensional (3D) bismuth selenide (Bi2Se3) nanostructures were synthesized by microwave synthesis using water as a solvent and hydrazine hydrate as a reducing agent and exfoliated into few layers of Bi2Se3. Bi2Se3- Few Layers (Bi2Se3- FL) exhibited localized surface plasmon resonance and enhanced electrocatalytic behavior. The scanning electron microscope (SEM) and transmission electron microscopy (TEM) characterization indicated the layered structure of Bi2Se3. The electrocatalytic properties of the Bi2Se3-FLmodified GC electrode towards nonenzymatic glucose oxidation were evaluated by cyclic voltammetry (CV) and chronoamperometry. The designed non-enzymatic glucose sensor showed a low detection limit of 6.1 μM, a linear range from 10 μM to 100 μM of glucose concentration and a current sensitivity of 0.112 μAμM 1 .The electrochemical sensor constructed using Bi2Se3-FL attained steady-state level within 3 s upon adding glucose and remained stable even after 19 days with only 17% loss in current signal. The obtained electrodes can be applied for determining glucose in urine samples. The results obtained here are of great significance to use nanostructured Bi2Se3-FL electrode as a potential candidate for non-enzymatic glucose detection.

This study describes theoretical and experimental considerations to optimize the photocatalytic degradation of caffeic acid in water using 3D-BiOBr based materials under visible light irradiation. Three BiOBr materials were synthesized through the solvothermal method using different bromide sources, namely potassium bromide (KBr) and the ionic liquid (IL) 1-butyl-3-methylimidazolium bromide. Morphological and chemical changes were observed in IL based 3D-BiOBr materials. The theoretical optimization of the experimental conditions in heterogeneous photocatalysis tests (pH and dose of catalyst) were simulated using the MODDE 12.0.1 software. A central composite design (CCD) was applied to obtain a response surface to elucidate the optimal conditions. This model predicted that the maximum photocatalytic degradation can be achieved at pH of 6.7 and a photocatalyst dose of 344 mg L−1. The optimal experimental conditions were tested using the three synthesized 3D-BiOBr materials. The results showed that the highest degradation efficiency and mineralization yield were obtained using the BiOBr microspheres synthesized with the IL at 145 °C.

In this work, we prepared bismuth oxide (Bi2O3) nanoparticles with and without fuels (citric acid and urea) using a one-pot solid-state combustion method at 400 °C for visible light photocatalytic degradation of acid blue 25 (AB). The nanoparticle prepared with fuel greatly influences the Bi2O3 properties such as morphology, chemical, structural, and optical properties. Bi2O3 prepared with citric acid as fuel act as an effective photocatalyst for the breakdown of acid blue 25 within 60 min under visible light irradiation. The enhanced photocatalytic property of Bi2O3 is due to the narrow band gap, high crystallinity, flower-like morphology with high active sites, and light stability of the material. Furthermore, an effective photogenerated charge separation, high charge transfer, and lower band gap, improved the absorbing capacity in the visible region of Bi2O3 (1) and enhanced its photocatalytic ability. In the photocatalytic process, the superoxide radicals (O2·) anion played a significant role during the degradation of acid blue 25. The Bi2O3 (1) maintained its effectiveness after three reaction cycles without suffering any appreciable change in structural and functional stability. These findings demonstrated an easy method for treating the hazardous effluents into non-toxic small molecules, which can be potentially applied to purify the various textile effluent.

Cationic doping of TiO2 anatase with sulphur represents a facile method to improve catalytic and photocatalytic activity for hydrogen production and extend the action spectrum of TiO2 into the visible light region. However, there is a lot of misunderstanding when trying to explain the experimental findings and suggest theoretical models. In the present computational research work, novel theoretical models are put forward representing fully hydroxylated small anatase nanoparticles with S(IV) and S(VI) doping in various surface positions and in the bulk. It was found that sulfur in the doped anatase nanoparticles preserves its typical coordination geometries of trigonal pyramid for S(IV) and tetrahedron for S(VI). Doping in the anatase surface is much more energetically favorable compared to doping in the bulk. Doping with S(IV) causes decrease of the band gap from 3.22 to 2.65 eV while S(VI) doping could decrease Eg only to 2.96 eV. Location of photogenerated electrons and holes depends strongly on the position of dopant atoms and their valent state. Contrary to some experimental works, no strong and extended visible light absorption bands could be found with cationic doped hydroxylated anatase nanoparticles. However, improved charges separation is observed indeed and causes improved photocatalytic hydrogen production.

Synthesis and characterization of bismuth oxychloride (BiOCl) has received much attention due to its excellent photocatalytic properties, and low toxicity that allow its potential application for environmental decontamination processes. In this work, experimental conditions (temperature and reaction time) were established to synthesize BiOCl microspheres by a solvothermal method with high photocatalytic efficiency on the degradation of 3,4,5-trihydroxybenzoic acid (gallic acid). BiOCl materials were synthesized according to a design of experiment (DoE) where temperature and reaction time were selected as varying parameters. Obtained BiOCl materials with the highest and lowest degradation toward gallic acid, were characterized using several techniques. Results showed that the applied temperature is the most important parameter during solvothermal synthesis, which influences not only morphology and structure of BiOCl, but also its thermal stability and optical properties. Response surface methodology (RSM) analysis indicated that the highest photocatalytic efficiency of synthesized BiOCl material, is obtained when the temperature and reaction time are fixed at 155 °C and 18 h, respectively. Finally, a reaction mechanism of photocatalytic oxidation of gallic acid was proposed based on an experimental tests by adding different radical’ scavengers.

In the last decades, air pollution control has received much attention due to the increase of environmental and health problems. The design of new materials with potential applications in air pollution control systems is a challenge nowadays. In this work, BiOI nanostructured materials were synthesized and used for photocatalytic oxidation of nitric oxide (NO). A microwave-assisted solvothermal method was successfully applied for BiOI synthesis at 126 °C, using ethylene glycol (EG) as a solvent. Several samples were prepared by varying the microwave irradiation time between 5 and 120 min. Resulting materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, diffuse reflectance spectroscopy (DRS) and photoluminescence measurements (PL). The photocatalytic activity of BiOI samples was evaluated in the photo-oxidation reaction of nitric oxide (NO) in gas phase under visible light irradiation. BiOI sample synthesized after 15 min of microwave exposition shows the highest photocatalytic activity, even greater than that obtained when TiO2 Evonik P-25 is used. This nanostructured material was applied into the formulation of two types of materials (ceramic paint and stucco) for its potential use in the construction industry. Preliminary results show that the application of nanostructured BiOI into stucco formulation has a great potential to develop commercial products to remove NO from air.

The influence of seven commercial activated carbons (ACs) to promote hydrogen peroxide decomposition and radical generation is assessed during four operating cycles. The amount of generated hydroxyl radicals is estimated from quenching experiments using methanol as a radical scavenger. The change in chemical surface composition of ACs upon contact with hydrogen peroxide after each operating cycle is measured by Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and by following the change in the value of the pH of the point of zero charge (pHPZC). Results reveal that when ACs are exposed to hydrogen peroxide for extended periods, their chemical surface composition is modified, reducing the capacity of these materials to promote hydrogen peroxide decomposition, and in turn decreasing the generation of hydroxyl radicals. Moreover, DRIFTS analyses show that ACs with an appreciable content of basic surface functionalities, such as chromene-type structures, would guarantee a continuous radical generation, reducing the loss of catalytic activity.

A facile combined method, namely sonothermal-hydrothermal, was adopted to assemble titanium dioxide (TiO2) nanoparticles on the surface of reduced graphene oxide (RGO) to form nanocomposites. Characterization techniques confirm that RGO-TiO2 composite is well constituted. Enhanced photocatalytic CO2 reduction to methanol by the composites under UVA and visible irradiation suggests the modification in the band gap of the composite and promotion of the separation of photogenerated carriers, yielding methanol production rate of 2.33 mmol g−1 h−1. Theoretical investigation demonstrated that combining RGO with TiO2 resulted in an upward shift of TiO2 bands by 0.2 V due to the contribution of RGO electrons. Relatively strong adsorption of RGO over the (101) anatase surface with the binding energy of approximately 0.4 kcal mol−1 per carbon atom was observed. Consideration of orbitals of TiO2, RGO and RGO-TiO2 composite led to a conclusion that UVA photoreaction proceeds via the traditional mechanism of photogenerated electron transfer to RGO while visible light CO2 reduction proceeds as a result of charge transfer photoexcitation that directly produces electrons in RGO and holes in TiO2. Superior photocatalytic activity of RGO-TiO2 composite in the present study is attributed to the formation of tight contact between its constituents, which is required for efficient electron and charge transfer.

The ZnTiO3 material was synthesized by sol–gel method with the assistance of ethanol as solvent. The oxidative polymerization method was used to synthesize polycarbazole (PCz). The ball milling technique was employed to synthesize the mechanically composited nanoparticles—ZnTiO3/PCz nanocomposite. The synthesized composites were analysed using powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectrum (XPS), UV–Vis absorption spectrum (UV–Vis), scanning electron microscope (SEM), and high-resolution transmission electron microscope (HRTEM). The degradation of crystal violet (CV) in water under visible-light irradiation was used to assess the photocatalytic behaviour of the synthesized catalyst. The result shows that the ZnTiO3/PCz composites exhibit greater photocatalytic activity than other materials. Polycarbazole in composite material acts as an electron reservoir, actively trapping the photogenerated electrons which considerably lowers the probability of recombination and increases the degrading effect of the catalyst.

Gold nanocrystals (AuNCs) were synthesized by economical and green strategy in aqueous medium by using N[3(trimethoxysilyl)propyl]ethylenediamine (TMSPED) as both a reducing and stabilizing mediator to avoid the aggregation of gold nanocrystals. Then, the AuNCs were capped with graphene quantum dots (GQDs) using an ultrasonic method. The resulting nanocomposites of GQD-TMSPED-AuNCs were characterized by X-ray photoelectron, X-ray diffraction, Raman, UV-vis and FTIR spectroscopies. The size and shape of the nanocomposites were confirmed by using transmission electron microscopy and atomic force microscopy. The GQD-TMSPED-AuNCs placed on a glassy carbon electrode enable simultaneous determination of dopamine (DA) and epinephrine (EP) with peak potentials at 0.21 and 0.30 V (vs. Ag/AgCl). The response is linear in the 5 nM – 2.1 μM (DA) and 10 nM – 4.0 μM (EP) concentration ranges, with detection limits of 5 and 10 nM, respectively. The sensor shows good selectivity toward DP and EP in the presence of other molecules, facilitating its rapid detection in practical applications.