Low-temperature liquid precursors of crystalline metal oxides assisted by heterogeneous photocatalysis

Recent Advances in Black Phosphorous-Based Photocatalysts for Degradation of Emerging Contaminants

The recalcitrant nature of emerging contaminants (ECs) in aquatic environments necessitates the development of effective strategies for their remediation, given the considerable impacts they pose on both human health and the delicate balance of the ecosystem. Semiconductor-based photocatalytic technology is recognized for its dual benefits in effectively addressing both ECs and energy-related challenges simultaneously. Among the plethora of photocatalysts, black phosphorus (BP) stands as a promising nonmetallic candidate, offering a host of advantages including its tunable direct band gap, broad-spectrum light absorption capabilities, and exceptional charge mobility. Nevertheless, pristine BP frequently underperforms, primarily due to issues related to its limited ambient stability and the rapid recombination of photogenerated electron-hole pairs. To overcome these challenges, substantial research efforts have been devoted to the creation of BP-based photocatalysts in recent years. However, there is a noticeable absence of reviews regarding the advancement of BP-based materials for the degradation of ECs in aqueous solutions. Therefore, to fill this gap, a comprehensive review is undertaken. In this review, we first present an in-depth examination of the fabrication processes for bulk BP and BP nanosheets (BPNS). The review conducts a thorough analysis and comparison of the merits and limitations inherent in each method, thereby delineating the most auspicious avenues for future research. Then, in line with the pathways followed by photogenerated electron-hole pairs at the interface, BP-based photocatalysts are systematically categorized into heterojunctions (Type I, Type II, Z-scheme, and S-scheme) and hybrids, and their photocatalytic performances against various ECs and the corresponding degradation mechanisms are comprehensively summarized. Finally, this review presents personal insights into the prospective avenues for advancing the field of BP-based photocatalysts for ECs remediation.

Low-Temperature Solution Approaches for the Potential Integration of Ferroelectric Oxide Films in Flexible Electronics

This technical review presents the state of the art in low-temperature chemical solution deposition (CSD) processing of ferroelectric oxide thin films. To achieve the integration of multifunctional crystalline oxides with flexible and semiconductor devices is, today, crucial to meet the demands of coming electronic devices. Hence, amorphous metal-oxide-semiconductors have been recently introduced in thin-film electronics. However, their benefits are limited compared with those of ferroelectric oxides, in which intrinsic multifunctionality would make possible multiple operations in the device. However, ferroelectricity is linked to a noncentrosymmetric crystal structure that is achieved, in general, at high temperatures, over 500 °C. These temperatures exceed the thermal stability of flexible polymer substrates and are not compatible with those permitted in the current fabrication routines of Si-based devices. In addition, the manufacturing of flexible electronic devices not only calls for low-temperature fabrication processes but also for deposition techniques that scale easily to the large areas required in flexible devices. In this regard, CSD processes are the best positioned today to integrate metal oxide thin films with flexible substrates as a large-area, low-cost, high-throughput fabrication technique. Here, we review the progress made in the last years in fabricating at low-temperature crystalline ferroelectric oxide thin films via CSD methods, highlighting the recent work of our group in the preparation of ferroelectric oxide thin films on flexible polyimide substrates.

Photochemistry in the Low-Temperature Processing of Metal Oxide Thin Films by Solution Methods

Photochemistry has emerged in the last few years as a powerful tool for the low-temperature processing of metal oxide thin films prepared by solution methods. Today, its implementation into the fabrication procedure makes possible the integration of amorphous semiconductors or functional crystalline oxides into flexible electronic systems at temperatures below 350 °C. In this review, the effects of UV irradiation at the different stages of the chemical solution deposition of metal oxide thin films are presented. These stages include from the synthesis of the precursor solution to the formation of the amorphous metal-oxygen network in the film and its subsequent crystallization into the oxide phase. Photochemical reactions that can be induced in both the solution deposited layer and the irradiation atmosphere are first described, highlighting the role of the potential reactive chemical species formed in the system under irradiation, such as free radicals or oxidizing compounds. Then, the photochemical effects of continuous UV light on the film are shown, focusing on the decomposition of the metal precursors, the condensation and densification of the metal-oxygen network, and the nucleation and growth of the crystalline oxide. All these processes are demonstrated to advance the formation and crystallization of the metal oxide thin film to an earlier stage, which is ultimately translated into a lower temperature range of fabrication. The reduced energy consumption of the process upon decreasing the processing temperature, and the prospect of using light instead of heat in the synthesis of inorganic materials, make photochemistry as a promising technique for a sustainable future ever more needed in our life.

Low-Temperature Solution Crystallization of Nanostructured Oxides and Thin Films

As an introduction to this themed issue, a critically selected overview of recent progress on the topic of solution methods for the low-temperature crystallization of nanoscale oxide materials is presented. It is focused on the low-temperature solution processing of oxide nanostructures and thin films. Benefits derived from these methods span from minimizing the environmental impact to reducing the fabrication costs. In addition, this topic is regarded as a key objective in the area because it offers a unique opportunity for the use of these materials in areas like flexible electronics, energy conversion and storage, environmental sciences, catalysis, or biomedicine.

Aqueous Chemical Solution Deposition of Functional Double Perovskite Epitaxial Thin Films

Double perovskite structure (A₂ BB'O₆ ) oxides exhibit a breadth of multifunctional properties with a huge potential range of applications in fields as diverse as spintronics, magneto-optic devices, or catalysis, and most of these applications require the use of thin films and heterostructures. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films combining high performance with high throughput and low cost. In addition, the physical properties of these materials are strongly dependent on the ordered arrangement of cations in the double perovskite structure. Thus, promoting spontaneous cationic ordering has become a relevant issue. In this work, our recent achievements by using polymer-assisted deposition (PAD) of environmentally friendly, water-based solutions for the growth of epitaxial ferromagnetic insulating double perovskite La₂ CoMnO₆ and La₂ NiMnO₆ thin films on SrTiO₃ and LaAlO₃ single-crystal substrates are presented. It is shown that the particular crystallization and growth process conditions of PAD (very slow rate, close to thermodynamic equilibrium conditions) promote high crystallinity and quality of the films, as well as favors spontaneous B-site cationic ordering.

Direct and converse piezoelectric responses at the nanoscale from epitaxial BiFeO3 thin films grown by polymer assisted deposition

We use an original water-based chemical method to grow pure epitaxial BiFeO3 (BFO) ultra-thin films with excellent piezoelectric properties. Particularly, we show that this novel chemical route produces higher natural ferroelectric domain size distribution and coercive field compared to similar BFO films grown by physical methods. Moreover, we measured the d33 piezoelectric coefficient of 60 nm thick BFO films by a direct approach, using Direct Piezoelectric Force Microscopy (DPFM). As a result, first piezo-generated charge maps of a very thin BFO layer were obtained applying this novel technology. We also performed a comparative study of the d33 coefficients between standard PFM analysis and DPFM microscopy showing similar values i.e. 17 pm V-1 and 22 pC N-1, respectively. Finally, we proved that the directionality of the piezoelectric effect in BFO ferroelectric thin films is preserved at low thickness dimensions demonstrating the potential of chemical processes for the development of low cost functional ferroelectric and piezoelectric devices.

Low-temperature crystallization of solution-derived metal oxide thin films assisted by chemical processes

Low-temperature chemical solution methods to prepare crystalline metal oxide thin films and to integrate them with flexible substrates are shown.

Enhanced Photocatalytic Activities of g-C3N4 via Hybridization with a Bi-Fe-Nb-Containing Ferroelectric Pyrochlore

Ferroelectricity may promote photocatalytic performance because the carrier-separation efficiency can be effectively improved by the internal electrostatic field caused by spontaneous polarization. Heterostructures that combine ferroelectric materials with other semiconductor materials can be further advantageous to the photocatalysis process. In this work, Bi_1.65Fe_1.16Nb_1.12O₇ was hybridized with g-C₃N₄ via a facile low-temperature method. The results of high-resolution transmission electron microscopy confirmed that a tight interface was formed between g-C₃N₄ and Bi_1.65Fe_1.16Nb_1.12O₇, which gave the (g-C₃N₄)-(Bi_1.65Fe_1.16Nb_1.12O₇) heterojunction a more superior visible light photocatalytic performance. The degradation of rhodamine B by optimized (g-C₃N₄)_0.5-(Bi_1.65Fe_1.16Nb_1.12O₇)_0.5 under visible light was almost 3.3 times higher than that by monomer Bi_1.65Fe_1.16Nb_1.12O₇ and 7.4 times higher than that by g-C₃N₄. The (g-C₃N₄)_0.5-(Bi_1.65Fe_1.16Nb_1.12O₇)_0.5 sample also showed the highest photocurrent in the photoelectrochemical tests. To further verify the benefit of the built-in electric field in terms of the photocatalytic performance, Bi₂FeNbO₇, with a higher spontaneous polarization, was also synthesized and hybridized with g-C₃N₄. Both Bi₂FeNbO₇ and (g-C₃N₄)_0.5-(Bi₂FeNbO₇)_0.5 exhibited better photocatalytic activities than those of Bi_1.65Fe_1.16Nb_1.12O₇ and (g-C₃N₄)_0.5-(Bi_1.65Fe_1.16Nb_1.12O₇)_0.5, although the latter ones had a stronger visible-light absorbance. This implies the very promising prospects of applying ferroelectric materials for solar energy harvest.

Photochemical solution processing of films of metastable phases for flexible devices: the β-Bi2O3 polymorph

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the β-Bi₂O₃ polymorph. The pure β-Bi₂O₃ phase is thermodynamically stable at high temperature (450-667 °C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a -Bi-O-Bi- continuous network in the deposited layers and the further conversion into the β-Bi₂O₃ polymorph at a temperature as low as 250 °C. The stabilization of the β-Bi₂O₃ films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of β-Bi₂O₃ films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases.

Active layers of high-performance lead zirconate titanate at temperatures compatible with silicon nano- and microeletronic [corrected] devices

Applications of ferroelectric materials in modern microelectronics will be greatly encouraged if the thermal incompatibility between inorganic ferroelectrics and semiconductor devices is overcome. Here, solution-processable layers of the most commercial ferroelectric compound ─ morphotrophic phase boundary lead zirconate titanate, namely Pb(Zr_0.52Ti_0.48)O₃ (PZT) ─ are grown on silicon substrates at temperatures well below the standard CMOS process of semiconductor technology. The method, potentially transferable to a broader range of Zr:Ti ratios, is based on the addition of crystalline nanoseeds to photosensitive solutions of PZT resulting in perovskite crystallization from only 350 °C after the enhanced decomposition of metal precursors in the films by UV irradiation. A remanent polarization of 10.0 μC cm⁻² is obtained for these films that is in the order of the switching charge densities demanded for FeRAM devices. Also, a dielectric constant of ~90 is measured at zero voltage which exceeds that of current single-oxide candidates for capacitance applications. The multifunctionality of the films is additionally demonstrated by their pyroelectric and piezoelectric performance. The potential integration of PZT layers at such low fabrication temperatures may redefine the concept design of classical microelectronic devices, besides allowing inorganic ferroelectrics to enter the scene of the emerging large-area, flexible electronics.

A photoelectrochemical methanol fuel cell based on aligned TiO2 nanorods decorated graphene photoanode

We report the photoelectrochemical (PEC) oxidation of methanol on a rationally designed graphene-TiO₂ nanorod array (G-TNR) photoanode.

Bandgap Tunability in Sb-Alloyed BiVO₄ Quaternary Oxides as Visible Light Absorbers for Solar Fuel Applications

The challenge of fine compositional tuning and microstructure control in complex oxides is overcome by developing a general two-step synthetic approach. Antimony-alloyed bismuth vanadate, which is identified as a novel light absorber for solar fuel applications, is prepared in a wide compositional range. The bandgap of this quaternary oxide linearly decreases with the Sb content, in agreement with first-principles calculations.