Experimental and computational study of metal oxide nanoparticles for the photocatalytic degradation of organic pollutants: a review

Photocatalysis is a more proficient technique that involves the breakdown or decomposition of different organic contaminants, various dyes, and harmful viruses and fungi using UV or visible light solar spectrum. Metal oxides are considered promising candidate photocatalysts owing to their low cost, efficiency, simple fabricating method, sufficient availability, and environment-friendliness for photocatalytic applications. Among metal oxides, TiO2 is the most studied photocatalyst and is highly applied in wastewater treatment and hydrogen production. However, TiO2 is relatively active only under ultraviolet light due to its wide bandgap, which limits its applicability because the production of ultraviolet is expensive. At present, the discovery of a photocatalyst of suitable bandgap with visible light or modification of the existing photocatalyst is becoming very attractive for photocatalysis technology. However, the major drawbacks of photocatalysts are the high recombination rate of photogenerated electron-hole pairs, the ultraviolet light activity limitations, and low surface coverage. In this review, the most commonly used synthesis method for metal oxide nanoparticles, photocatalytic applications of metal oxides, and applications and toxicity of different dyes are comprehensively highlighted. In addition, the challenges in the photocatalytic applications of metal oxides, strategies to suppress these challenges, and metal oxide studied by density functional theory for photocatalytic applications are described in detail.

Ceria-doped SnO2 nanocubes for solar light-driven photocatalytic hydrogen production

The photocatalytic generation of hydrogen via solar energy using metal oxide semiconductor catalysts is a clean and renewable process which has the potential of solving the current energy nexus. SnO₂ is one such well-studied and established photocatalyst currently in practice but is only ultraviolet-light active which accounts for only 4% of the total incoming solar energy. The current study focuses on bringing this SnO₂ into the visible range using ceria as a dopant. Sol-gel and combustion methods were employed for synthesis and the as-synthesized catalysts were characterized using XRD, BET, UV diffuse reflectance spectra, PL spectra, and SEM micrographs. A unique cuboid type morphology was observed in 6% ceria-doped SnO₂ which provided more active sites for light absorption and thus reported a remarkable hydrogen production rate of 1.978 mmol/h under sunlight which was almost 346 times that of pure SnO₂ (5.71 µmol/h). Photoluminescence spectra of ceria-doped SnO₂ showed lower peak positions as compared to the pure SnO₂ indicating a reduction in charge recombination and an increase in the life time of the active species which explains the enhanced hydrogen production rates. The recyclability study of the catalysts showed that the hydrogen amount produced in the fifth recycle was nearly 80% as the first cycle showing that the catalyst can be used very effectively for more than five cycles without compromising on the yield.

The strategy for organic dye and antibiotic photocatalytic removal for water remediation in an example of Co-SnO2 nanoparticles

A challenging problem to create an efficient photocatalyst suitable for industrial water remediation, aiming to remove cyclic organic compounds attracts increasing attention. The current study aimed to clarify a few "dark spots" in the field, namely to find out if it is possible to make an efficient photocatalyst activated with visible light by using a simple and cheap strategy and what are the key factor impacting its efficiency. In this work, a new procedure to obtain spherical nanoparticles with the same average size but different amounts of oxygen vacancies and defects and dopant concentrations was developed. The approach based on hydrothermal treatment was suggested to obtain rod-shaped nanoparticles. The systematic study of photocatalytic behavior on the example of oxytetracycline and methylene blue degradation under visible light of widely available LED lamp was performed. Based on chemical and computational experiments the main factor affecting the process efficiency was determined.

Al-Doped SnO₂ as Transparent Heater: Influence of Deposition Time

The research on transparent heater (Thf) films rapidly increases due to their unique photoelectric properties, leading to new generation of optoelectronic device. Here, we report a simple method to fabricate transparent heater based on Al-doped SnO2 (ASO) thin films. ASO films with 5 wt% Al as dopant were synthesized with various deposition times, namely, 5, 10 and 15 minutes using ultrasonic spray pyrolysis method. The correlation of deposition time on their structural characteristic, optical, electrical and thermal properties has been investigated. X-ray diffraction studies found that all samples exhibit tetragonal structure with preferred orientation along (110) plane. Meanwhile, the UV-Vis transmittance indicated that the sample having good optical transparency in visible light spectrum with the average transmittance up to 89.7%. The sheet resistance of ASO thin films was found to decrease as the deposition time increases to 10 minutes. Furthermore, Al-doped SnO2 based transparent heater prepared with 10 minutes deposition time presents the excellent thermal temperature up to 76.3 °C at the applied voltage of 20 volt. The above findings reveal that Al-doped SnO2 can be used as an alternative compound to substitute higher cost indium tin oxide as transparent heater. Keywords: aluminium, composite, spray pyrolysis, SnO2, transparent heater

High Efficient and Cost Effective Titanium Doped Tin Dioxide Based Photocatalysts Synthesized via Co-precipitation Approach

High efficient and large surface area of titanium doped tin dioxide (SnO2) based photocatalysts with various titanium doping contents varying from 0 to 4 mol% have been successfully prepared via a facile, low cost and eco-friendly co-precipitation method. Structural, morphological, textural, microstructural and optical properties of the prepared Ti-SnO2 nanoparticles (NPs) have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), the Brunauer–Emmett-Teller (BET) method, Raman spectroscopy, Fourier transform infrared (FTIR), UV-Vis spectroscopy and photoluminescence (PL) spectroscopy. It was found that both undoped and Ti doped SnO2 NPs were crystallized in tetragonal structure and the crystallite sizes have been reduced from 19.9 nm for undoped SnO2 NPs to 13.1 nm for SnO2: Ti 4%. As compared to pure SnO2, a decrease in size and a uniform distribution of spherical aggregates for 4% Ti doped SnO2 sample have been noticed. Nitrogen (N2) adsorption-desorption isotherms of all synthesized NPs indicate that each nanopowder showed a IV type- isotherm with a hysteresis loop resulted in a typical porous materials containing macropores and mesopores. The raman spectra was marked with the appearance of three well resolved peaks including one intense peak centered at 633 cm−1 and two other peaks at about 475 cm−1 and 772 cm−1 which might be ascribed to the characteristic modes of of the SnO2 rutile-type. FTIR spectra of Ti doped SnO2 NPs show a broad band situated in the region from 630 cm−1 to 625 cm−1 for all Ti–SnO2 samples which could be assigned to the stretching vibrations of Sn–O–Sn. Optical studies revealed that the absorption edge of SnO2: Ti NPs showed a redshift with rising titanium concentration. This redshift resulted in a decrease in the optical band gap from 3.31 eV for pure SnO2 to 2.87 eV for 4% Ti doped SnO2 nanoparticles respectively. Rhodamine B dye (RhB) has been adopted to study the photocatalytic degradation of all synthesized Ti–SnO2 NPs. Pure SnO2 NPs has an intrinsic large band gap and it was sensitive to UV light. Thus, pure SnO2 NPs display higher UV photocatalytic performance for decomposing the RhB. Titanium incorporation into SnO2 has widely improved its photocatalytic performance towards RhB photodegradation under UV and Visible light irradiations. Precisely, the 4% Ti–SnO2 based photocatalyst display the highest photacatalytic activity and can degrade both of 95% and 52% of RhB dye within 120 min respectively under UV and visible light irradiations. The enhanced photocatalytic activity of the 4% doped SnO2 photocatalyst was further proved with the minimum PL intensity. The homogeneous incorporation of low Ti contents into the SnO2 matrix allow to a significant reduce in the band gap leading to an efficient separation of photogenerated electron-hole pairs and consequently improves the absorption capability in the visible light.

Zn- and Ti-Doped SnO2 for Enhanced Electroreduction of Carbon Dioxide

The electrocatalytic reduction of CO₂ into useful fuels, exploiting rationally designed, inexpensive, active, and selective catalysts, produced through easy, quick, and scalable routes, represents a promising approach to face today’s climate challenges and energy crisis. This work presents a facile strategy for the preparation of doped SnO₂ as an efficient electrocatalyst for the CO₂ reduction reaction to formic acid and carbon monoxide. Zn or Ti doping was introduced into a mesoporous SnO₂ matrix via wet impregnation and atomic layer deposition. It was found that doping of SnO₂ generates an increased amount of oxygen vacancies, which are believed to contribute to the CO₂ conversion efficiency, and among others, Zn wet impregnation resulted the most efficient process, as confirmed by X-ray photoelectron spectroscopy analysis. Electrochemical characterization and active surface area evaluation show an increase of availability of surface active sites. In particular, the introduction of Zn elemental doping results in enhanced performance for formic acid formation, in comparison to un-doped SnO₂ and other doped SnO₂ catalysts. At −0.99 V versus reversible hydrogen electrode, the total faradaic efficiency for CO₂ conversion reaches 80%, while the partial current density is 10.3 mA cm⁻². These represent a 10% and a threefold increases for faradaic efficiency and current density, respectively, with respect to the reference un-doped sample. The enhancement of these characteristics relates to the improved charge transfer and conductivity with respect to bare SnO₂.

Tuning the magnetic properties of Sn1-x-y Ce4+x Ce3+y O2 nanoparticles: an experimental and theoretical approach

During the last decade, there was a substantial increase in the research on metal-doped oxide semiconductor nanoparticles due to advances in the engineering of nanomaterials and their potential application in spintronics, biomedicine and photocatalysis fields. In this regard, doping a nanomaterial is a powerful tool to tune its physicochemical properties. The aim of this work is to shine a new light on the role of the neighbouring elements on the oxidation state of the Ce-impurity, from both experimental and theoretical points of view. Herein, we present an accurate study of the mechanisms involved in the oxidation states of the Ce-ions during the doping process of SnO2 nanoparticles (NPs) prepared by the polymeric precursor method. X-ray diffraction measurements have displayed the tetragonal rutile-type SnO2 phase in all samples. However, the Bragg's peak (111) and (220) located at 2θ ∼29° and ∼47° evidence the formation of a secondary CeO2 phase for samples with Ce content up to 10%. X-ray absorption near-edge structure (XANES) measurements, at Ce L3 edge, were performed on the NPs as a function of Ce content. The results show, on one side, the coexistence of Ce3+ and Ce4+ states in all samples; and on the other side, a clear reduction in the Ce3+ population driven by the increase of Ce content. It is shown that this is induced by the neighboring cation, and confirmed by magnetic measurements. The monotonic damping of the Ce3+/Ce4+ ratio experimentally, was connected with theoretical calculations via density functional theory by simulating a variety of point defects composed of Ce impurities and surrounding oxygen vacancies. We found that the number of oxygen vacancies around the Ce-ions is the main ingredient to change the Ce oxidation state, and hence the magnetic properties of Ce-doped SnO2 NPs. The presented results pave the way for handling the magnetic properties of oxides through the control of the oxidation state of impurities.

Synthesis, structural evolution, and optical properties of SnO2 hollow microspheres with manageable shell thickness

SnO2 hollow (H-SnO2) microspheres were successfully prepared via a facile one-step synthesis using SiO2 microspheres as templates and NaOH as a reactive etching agent.

Optical and photocatalytic properties of a ZnO@C core/shell sphere with rich oxygen vacancies

We develop a novel method to prepare an oxygen-deficient ZnO@C core/shell based on a traditional hydrothermal reaction, and investigate the role of the carbon core and oxygen vacancies in the photocatalytic activity and photoluminescence processes.

Effects of dopants on the adhesion and electronic structure of a SnO2/Cu interface: a first-principles study

Doping has been adopted as a versatile approach for tuning the adhesion of metal oxide/metal interfaces. Understanding the mechanism of doping at the interface adhesion on the atomic and electronic scale is crucial for the rational design and optimization of metal oxide/metal composites. In this work, we have investigated the effects of dopants on the adhesion of SnO2/Cu interfaces through first-principles calculations. Firstly, O-terminated a SnO2(110)/Cu(111) interface (denoted as I) was considered and the work of separation values of the interfaces with various dopants (Mo, Sb, Ti, Zn and Cu) were calculated to evaluate the interface adhesion strength. It was demonstrated that low-valence dopants (Zn2+ and Cu2+) enhance the adhesion strength of interface I, while high-valence dopants (Mo6+ and Sb5+) play the opposite role. Secondly, the strengthening effects of low-valence dopants were further verified in four candidate interfacial models with different atomic structures (denoted as II-V). The work of separation values indicated that the adhesion of all of the interfaces involved could be enhanced by low-valence doping. The electronic structure of the interface was demonstrated through density of states, charge density and charge density difference analyses. The results revealed that upon low-valence doping, the holes facilitate charge transfer between Cu and SnO2, which generates strong covalent bonds across the interface and thus significantly enhances the interface adhesion. This work not only provides insight into rational doping to enhance the adhesion of SnO2/Cu composites but can also be expanded upon for the design of other metal oxide/metal composites with strong interface adhesion.

Facile hydrothermal synthesis and improved photocatalytic activities of Zn2+ doped Bi2MoO6 nanosheets

A novel sheet-like Zn²⁺ doped Bi₂MoO₆ photocatalyst with excellent UV-visible-light induced photocatalytic ability was successfully fabricated through a facile hydrothermal synthesis strategy.