Role of the growth step on the structural, optical and surface features of TiO2/SnO2 composites

How the Interplay between SnO2 and Zn(II) Porphyrins Impacts on the Electronic Features of Gaseous Acetone Chemiresistors

Herein, the integration of SnO2 nanoparticles with two Zn(II) porphyrins─Zn(II) 5,10,15,20-tetraphenylporphyrin (ZnTPP) and its perfluorinated counterpart, Zn(II) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (ZnTPPF20)─was investigated for the sensing of gaseous acetone at 120 °C, adopting three Zn-porphyrin/SnO2 weight ratios (1:4, 1:32, and 1:64). For the first time, we were able to provide evidence of the correlation between the materials' conductivity and these nanocomposites' sensing performances, obtaining optimal results with a 1:32 ratio for ZnTPPF20/SnO2 and showcasing a remarkable detection limit of 200 ppb together with a boosted sensing signal with respect to bare SnO2. To delve deeper, the combination of experimental data with density functional theory calculations unveiled an electron-donating behavior of both porphyrins when interacting with tin dioxide semiconductor, especially for the nonfluorinated one. The study suggested that the interplay between electrons injected, from the porphyrins' highest occupied molecular orbital to SnO2 conduction band, and the latter's available electronic states has a dramatic impact to boost the chemiresistive sensing. Indeed, we highlighted that the key lies in preventing the full saturation of SnO2 electronic states concomitantly increasing the materials' conductivity: in this respect, the best compromise turned out to be the perfluorinated porphyrin. A further corroboration of our findings was obtained by illuminating the sensors during measurements with light-emitting diode (LED) light. Actually, we demonstrated that it does not have any impact on improving the sensing behavior, most probably due to the electronic oversaturation and scattering caused by LED excitation in porphyrins. Lastly, the most effective hybrids (1:32 ratio) were physicochemically characterized, confirming the physisorption of the macrocycles onto the SnO2 surface. In conclusion, herein, we underscore the feasibility of customizing the porphyrin chemistry and porphyrin-to-SnO2 ratio to enhance the gaseous sensing of bare metal oxides, providing valuable insights for the engineering of highly performing light-free chemiresistors.

Flexible Nano-TiO2 Sheets Exhibiting Excellent Photocatalytic and Photovoltaic Properties by Controlled Silane Functionalization-Exploring the New Prospects of Wastewater Treatment and Flexible DSSCs

TiO₂ nanoparticles surface-modified with silane moieties, which can be directly coated on a flexible substrate without the requirement of any binder materials and postsintering processes, are synthesized and characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, time-correlated single-photon counting, and transmission electron microscopy. The viability of the prepared surface-modified TiO₂ (M-TiO₂) sheets as a catalyst for the photo-induced degradation of a model dye, methylene blue, was checked using UV-visible absorption spectroscopy. The data suggest that, compared to unmodified TiO₂, M-TiO₂ sheets facilitate better dye-degradation, which leads to a remarkable photocatalytic activity that results in more than 95% degradation of the dye in the first 10 min and more than 99% of the degradation in the first 50 min of the photocatalytic experiments. We also demonstrate that M-TiO₂ can be recycled with negligible reduction in photocatalytic activity. Further, the photovoltaic properties of the developed M-TiO₂ sheets were assessed using UV-visible absorption spectroscopy, electrochemical impedance spectroscopy (EIS), and photochronoamperometry. Dye-sensitized solar cells (DSSC) fabricated using M-TiO₂ as the photoanode exhibited a photoconversion efficiency of 4.1% under direct sunlight. These experiments suggested that M-TiO₂ sheets show enhanced photovoltaic properties compared to unmodified TiO₂ sheets, and that, when N-719 dye is incorporated, the dye-TiO₂ interaction is more favorable for M-TiO₂ than bare TiO₂. The simple solution processing method demonstrated in this paper rendered a highly flexible photoanode made of M-TiO₂ with superior charge-separation efficiency to an electrode made of bare TiO₂. We propose that our findings on the photovoltaic properties of M-TiO₂ open up arenas of further improvement and a wide scope for the large-scale production of flexible DSSCs on plastic substrates at room temperature in a cost-effective way.

A review on TiO2/SnO2 heterostructures as a photocatalyst for the degradation of dyes and organic pollutants

Industrialization, civilization and human activities have all grown steadily in recent years. As a result, small and large industries discharge many organic pollutants into the environment and contribute to environmental pollution. These compounds are quite stable and challenging to break down over time, posing a long-term risk. The heterogeneous advanced oxidation processes technology has gained tremendous attention. It depends on the light-induced formation of e^-/h⁺ pairs, which combine with water and aqueous oxygen to generate highly reactive hydroxyl radicals that degrade the organic pollutants in a solution and convert them ultimately into non-toxic products. In this paper, the synergetic impact of TiO₂-SnO₂ coupling with other semiconductor materials and their photodegradation performance on toxic contaminants in an aqueous medium has been reviewed. In addition, multiple approaches for the synthesis of TiO₂-SnO₂ photocatalysts have been discussed. Among them, hydrothermal, sol-gel, electrospinning, precipitation and even their combination are extensively used to synthesize various forms of nanostructures. These techniques demonstrate better tunability for visible absorption, suppression of e^-/h⁺ pair recombination and enhanced e^-/h⁺ separation to improve photocatalytic performance. This paper also summarises the role of different operating factors such as catalyst loading, pH, pollutants variation concentration, various light sources and oxidizing agents on the photodegradation of organic pollutants.

Semiconductor Gas Sensors: Materials, Technology, Design, and Application

This paper presents an overview of semiconductor materials used in gas sensors, their technology, design, and application. Semiconductor materials include metal oxides, conducting polymers, carbon nanotubes, and 2D materials. Metal oxides are most often the first choice due to their ease of fabrication, low cost, high sensitivity, and stability. Some of their disadvantages are low selectivity and high operating temperature. Conducting polymers have the advantage of a low operating temperature and can detect many organic vapors. They are flexible but affected by humidity. Carbon nanotubes are chemically and mechanically stable and are sensitive towards NO and NH3, but need dopants or modifications to sense other gases. Graphene, transition metal chalcogenides, boron nitride, transition metal carbides/nitrides, metal organic frameworks, and metal oxide nanosheets as 2D materials represent gas-sensing materials of the future, especially in medical devices, such as breath sensing. This overview covers the most used semiconducting materials in gas sensing, their synthesis methods and morphology, especially oxide nanostructures, heterostructures, and 2D materials, as well as sensor technology and design, application in advance electronic circuits and systems, and research challenges from the perspective of emerging technologies.

Role of Synthetic Parameters on the Structural and Optical Properties of N,Sn-Copromoted Nanostructured TiO2: A Combined Ti K-Edge and Sn L2,3-Edges X-ray Absorption Investigation

Sn-modification of TiO2 photocatalysts has been recently proposed as a suitable strategy to improve pollutant degradation as well as hydrogen production. In particular, visible light activity could be promoted by doping with Sn2+ species, which are, however, thermally unstable. Co-promotion with N and Sn has been shown to lead to synergistic effects in terms of visible light activity, but the underlying mechanism has, so far, been poorly understood due to the system complexity. Here, the structural, optical, and electronic properties of N,Sn-copromoted, nanostructured TiO2 from sol-gel synthesis were investigated: the Sn/Ti molar content was varied in the 0-20% range and different post-treatments (calcination and low temperature hydrothermal treatment) were adopted in order to promote the sample crystallinity. Depending on the adopted post-treatment, the optical properties present notable differences, which supports a combined role of Sn dopants and N-induced defects in visible light absorption. X-ray absorption spectroscopy at the Ti K-edge and Sn L2,3-edges shed light onto the electronic properties and structure of both Ti and Sn species, evidencing a marked difference at the Sn L2,3-edges between the samples with 20% and 5% Sn/Ti ratio, showing, in the latter case, the presence of tin in a partially reduced state.

Exploring SnxTi1-xO2 Solid Solutions Grown onto Graphene Oxide (GO) as Selective Toluene Gas Sensors

The major drawback of oxide-based sensors is the lack of selectivity. In this context, Sn_xTi_1-xO₂/graphene oxide (GO)-based materials were synthesized via a simple hydrothermal route, varying the titanium content in the tin dioxide matrix. Then, toluene and acetone gas sensing performances of the as-prepared sensors were systematically investigated. Specifically, by using 32:1 SnO₂/GO and 32:1 TiO₂/GO, a greater selectivity towards acetone analyte, also at room temperature, was obtained even at ppb level. However, solid solutions possessing a higher content of tin relative to titanium (as 32:1 Sn_0.55Ti_0.45O₂/GO) exhibited higher selectivity towards bigger and non-polar molecules (such as toluene) at 350 °C, rather than acetone. A deep experimental investigation of structural (XRPD and Raman), morphological (SEM, TEM, BET surface area and pores volume) and surface (XPS analyses) properties allowed us to give a feasible explanation of the different selectivity. Moreover, by exploiting the UV light, the lowest operating temperature to obtain a significant and reliable signal was 250 °C, keeping the greater selectivity to the toluene analyte. Hence, the feasibility of tuning the chemical selectivity by engineering the relative amount of SnO₂ and TiO₂ is a promising feature that may guide the future development of miniaturized chemoresistors.

Visible range optical absorption, Urbach energy estimation and paramagnetic response in Cr-doped TiO2 nanocrystals derived by a sol-gel method

We have carried out a detailed study of the morphological, structural, optical and magnetic properties of Cr doped TiO₂ nanocrystals with doping concentrations varying from 3 to 12 atomic weight%. The results obtained from transmission electron microscopy analysis, size-strain plots of all the Cr-doped samples and crystallite size estimation reveal the particle size of the prepared nanocrystals to be well below 10 nm, which is observed to exhibit a decreasing trend with an increase in the Cr dopant concentration. All the samples crystallize in the anatase tetragonal phase of TiO₂, which is confirmed from the Rietveld refinement of the X-ray diffraction patterns and the different modes present in the Raman spectra. The E_g(1) mode shows a clear red shift and broadening with increase in the Cr concentration, which indicates the replacement of Ti ions with Cr ions in the TiO₂ lattice. The possibility of the presence of different functional groups present is verified by Fourier transform infra-red spectroscopy. The presence of Cr³⁺ and Ti⁴⁺ is confirmed from the X-ray photoelectron spectroscopy (XPS) results suggesting the formation of oxygen vacancies to compensate for the charge neutrality. The XPS results validate the Cr³⁺ existence in the Cr:TiO₂ system and corroborate with a slight peak shift towards lower diffraction angle and further confirm the substitutional doping in the present case. Enhanced visible range optical absorption and a clear red shift associated with the absorption edge also suggest the incorporation of Cr³⁺ ions into the host system. The estimated band-gap of Cr-doped TiO₂ nanocrystals reveals a decreasing trend with increasing Cr concentration. The Urbach energy associated with all the Cr-doped samples signifies the presence of oxygen vacancy related defects in the present system, which is further verified using photoluminescence (PL) spectra, and the deconvolution of the PL spectra provides an insight into the oxygen vacancy defects associated with the system. Paramagnetic (PM) behaviour is observed with an increase in the PM moment, suggesting the increase in isolated Cr ions with increase in the Cr concentration, which is further explained using a bound magnetic polaron (BMP) model. Isolated BMP formation could be the reason for the observed PM behavior of the present system, where the trapping of 3d electrons associated with Cr³⁺ in the vacancy sites could ultimately lead to fewer overlapped BMPs, yielding a net PM moment. The present Cr:TiO₂ system could be modified with tailored optical and magnetic properties for functional applications such as magneto-optics and optoelectronic devices.