Maria Jesus Hortigüela

Bismuth ferrite (BiFeO₃) is a promising material for developing the next generation of multifunctional electronic devices. However, the production of high-quality BiFeO₃ thin films is compromised by the tendency for structural and electronic defects to form during synthesis, which degrades their functional properties. In this work, BiFeO₃ thin films were prepared by chemical solution deposition to determine optimal conditions for minimizing oxygen vacancies and to evaluate the impact of these point defects on their physical properties. The films were pyrolyzed at 300 °C for 60 min and 360 °C for 10 min, and crystallized in air and in an O₂ atmosphere, at 600 °C and 640 °C for 40 min. High oxygen vacancies were observed in films prepared at low pyrolysis temperatures and crystallized in air, whereas oxygen vacancies were minimized in the film pyrolyzed and crystallized at high temperatures in an O₂ atmosphere. The oxygen vacancies markedly affected the films’ physical properties, leading to increased dielectric loss, dielectric dispersion, dc conductivity, and leakage current, with consequent degradation of photovoltaic and magnetic performance. These findings highlight the critical importance of controlling synthesis parameters to suppress oxygen vacancy formation and achieve high-quality BiFeO₃ thin films.

Bismuth ferrite (BiFeO₃) is a promising material for developing the next generation of multifunctional electronic devices. However, the production of high-quality BiFeO₃ thin films is compromised by the tendency for structural and electronic defects to form during synthesis, which degrades their functional properties. In this work, BiFeO₃ thin films were prepared by chemical solution deposition to determine optimal conditions for minimizing oxygen vacancies and to evaluate the impact of these point defects on their physical properties. The films were pyrolyzed at 300 °C for 60 min and 360 °C for 10 min, and crystallized in air and in an O₂ atmosphere, at 600 °C and 640 °C for 40 min. High oxygen vacancies were observed in films prepared at low pyrolysis temperatures and crystallized in air, whereas oxygen vacancies were minimized in the film pyrolyzed and crystallized at high temperatures in an O₂ atmosphere. The oxygen vacancies markedly affected the films’ physical properties, leading to increased dielectric loss, dielectric dispersion, dc conductivity, and leakage current, with consequent degradation of photovoltaic and magnetic performance. These findings highlight the critical importance of controlling synthesis parameters to suppress oxygen vacancy formation and achieve high-quality BiFeO₃ thin films.

The field of research in graphene—a single atomic layer of graphitic carbon and other two-dimensional materials has been enjoying extraordinary growth during the past decade because of its extraordinary properties that make it a suitable material for a wide range of technological applications [1–4]. Their excellent charge-carrier mobility quickly grabbed the attention of the scientific community, bringing a hope that one day it will compete with silicon to be the material for the next generations for certain applications in the electronic industries. Furthermore, many extraordinary properties, such as its 2.3% absorption in the white light spectrum, high surface area, high Young’s modulus, and excellent thermal conductivity, have all been reported. Because of its remarkable properties, applications using graphene in a wide range of areas, including high-speed electronic and optical devices, energy generation and storage, hybrid materials, chemical sensors, and even DNA sequencing, have all been explored. Beyond this, very recently, graphene-based field-effect transistors (GFETs) were used, which shows its exciting and bright prospects for sensing applications due to its greatly higher sensitivity and stronger selectivity [5–7]. GFETs play a role for the preparation of ultra-sensitive nanoelectronic biosensors because of its perfect characteristics, including high carrier mobility, strong and flexible structure, and so on. In their work, Seon Joo Park and co-workers were successfully able to make an ultrasensitive and flexible FET olfactory system, which they named ‘Bioelectronic nose’ [8]. There are numerous examples in which graphene was used for biological applications, for example for the detection of living cell and bacteria sensors [9], GFETs functionalized with E.coli antibodies [10], and FETs based on graphene for the detection of malaria-infected red blood cells [11].

This work presents the immobilisation of titanium dioxide (TiO₂) nanoparticles (NPs) and reduced graphene oxide (rGO)-TiO₂ nanocomposite on glass sheets for photocatalytic degradation of methylene blue (MB) under different radiation sources such as ultraviolet and simulated solar radiation. The TiO₂ NPs and rGO-TiO₂ nanocomposite were synthesised through a simple hydrothermal method of titanium isopropoxide precursor followed by calcination treatment. Deposition of prepared photocatalysts was performed by spin-coating method. Additionally, ethylene glycol was mixed with the prepared TiO₂ NPs and rGO-TiO₂ nanocomposite to enhance film adhesion on the glass surface. The photocatalytic activity under ultraviolet and simulated solar irradiation was examined. Further, the influence of different water matrices (milli-Q, river, lake, and seawater) and reactive species (h⁺, ^•OH, and e⁻) on the photocatalytic efficiency of the immobilised rGO/TiO₂ nanocomposite was careful assessed. MB dye photocatalytic degradation was found to increase with increasing irradiation time for both irradiation sources. The immobilisation of prepared photocatalysts is very convenient for environment applications, due to easy separation and reusability, and the investigated rGO/TiO₂-coated glass sheets demonstrated high efficiency in removing MB dye from an aqueous medium during five consecutive cycles.

This study presents a novel method for the development of TiO₂/reduced graphene oxide (rGO) nanocomposites for photocatalytic degradation of dyes in an aqueous solution. The synergistic integration of rGO and TiO₂, through the formation of Ti–O–C bonds, offers an interesting opportunity to design photocatalyst nanocomposite materials with the maximum absorption shift to the visible region of the spectra, where photodegradation can be activated not only with UV but also with the visible part of natural solar irradiation. TiO₂@rGO nanocomposites with different content of rGO have been self-assembled by the hydrothermal method followed by calcination treatment. The morphological and structural analysis of the synthesized photocatalysts was performed by FTIR, XRD, XPS, UV-Vis DRS, SEM/EDX, and Raman spectroscopy. The effectiveness of the synthesized nanocomposites as photocatalysts was examined through the photodegradation of methylene blue (MB) and rhodamine B (RhB) dye under artificial solar-like radiation. The influence of rGO concentration (5 and 15 wt.%) on TiO₂ performance for photodegradation of the different dyes was monitored by UV-Vis spectroscopy. The obtained results showed that the synthesized TiO₂@rGO nanocomposites significantly increased the decomposition of RhB and MB compared to the synthesized TiO₂ photocatalyst. Furthermore, TiO₂@rGO nanocomposite with high contents of rGO (15 wt.%) presented an improved performance in photodegradation of MB (98.1%) and RhB (99.8%) after 120 min of exposition to solar-like radiation. These results could be mainly attributed to the decrease of the bandgap of synthesized TiO₂@rGO nanocomposites with the increased contents of rGO. Energy gap (E_g) values of nanocomposites are 2.71 eV and 3.03 eV, when pure TiO₂ particles have 3.15 eV. These results show the potential of graphene-based TiO₂ nanocomposite to be explored as a highly efficient solar light-driven photocatalyst for water purification.

Reduced graphene oxide (rGO) is a promising graphene-based material, with transversal applicability to a wide range of technological fields. Nevertheless, the common use of efficient—but hazardous to environment and toxic—reducing agents prevents its application in biological and other fields. Consequently, the development of green reducing strategies is a requirement to overcome this issue. Herein, a green, simple, and cost-effective one-step reduction methodology is presented. Graphene oxide (GO) was hydrothermally reduced in the presence of caffeic acid (CA), a natural occurring phenolic compound. The improvement of the hydrothermal reduction through the presence of CA is confirmed by XRD, Raman, XPS and TGA analysis. Moreover, CA polymerizes under hydrothermal conditions with the formation of spherical and non-spherical carbon particles, which can be useful for further rGO functionalization. FTIR and XPS confirm the oxygen removal in the reduced samples. The high-resolution scanning transmission electron microscopy (HRSTEM) images also support the reduction, showing rGO samples with an ordered graphitic layered structure. The promising rGO synthesized by this eco-friendly methodology can be explored for many applications.

Graphene oxide (GO) assists a diverse set of promising routes to build bioactive neural microenvironments by easily interacting with other biomaterials to enhance their bulk features or, alternatively, self-assembling towards the construction of biocompatible systems with specific 3D geometries. Herein, we firstly modulate both size and available oxygen groups in GO nanosheets in order to adjust the physicochemical and biological properties of polycaprolactone-gelatin electrospun nanofibrous systems. Results show that the incorporation of customized GO nanosheets modulates the properties of the nanofibres and, subsequently, markedly influence the viability of neural progenitor cell cultures. Interestingly, the partially reduced GO (rGO) nanosheets with larger dimensions trigger the best cell response, while the rGO nanosheets with smaller size provoke an accentuated decrease in the cytocompatibility of the resulting electrospun meshes. Then, the most auspicious nanofibres are synergistically accommodated onto the surface of 3D rGO heterogeneous porous networks, giving rise to fibrous-porous combinatorial architectures suitable for enhancing adhesion and differentiation of neural cells. By varying the chemical composition of the nanofibres, it is possible to adapt their performance as physical crosslinkers for the rGO sheets, leading to the modulation of both pore size and structural/mechanical integrity of the scaffold. Importantly, the biocompatibility of the resultant fibrous-porous systems is not compromised after 14 days of cell culture, including standard differentiation patterns of neural progenitor cells. Overall, in the light of these in vitro results, the reported scaffolding approach presents not only an indisputable capacity to support highly viable and interconnected neural circuits, but also the potential to unlock novel strategies for neural tissue engineering applications.