Synthesis and Application of Innovative and Environmentally Friendly Photocatalysts: A Review

Modern society faces two major challenges: removing pollutants from water and producing energy from renewable sources. To do this, science proposes innovative, low-cost, and environmentally friendly methods. The heterogeneous photocatalysis process fits perfectly in this scenario. In fact, with photocatalysis, it is possible both to mineralize contaminants that are not easily biodegradable and to produce hydrogen from the water splitting reaction or from the conversion of organic substances present in water. However, the main challenge in the field of heterogeneous photocatalysis is to produce low-cost and efficient photocatalysts active under visible light or sunlight. The objective of this review is to compare the new proposals for the synthesis of innovative photocatalysts that reflect the requirements of green chemistry, applied both in the removal of organic contaminants and in hydrogen production. From this comparison, we want to bring out the strengths and weaknesses of the proposals in the literature, but above all, new ideas to improve the efficiency of heterogeneous photocatalysis guaranteeing the principles of environmental and economic sustainability.

Hydroxyapatite/TiO2 Nanomaterial with Defined Microstructural and Good Antimicrobial Properties

Due to the growing number of people infected with the new coronavirus globally, which weakens immunity, there has been an increase in bacterial infections. Hence, knowledge about simple and low-cost synthesis methods of materials with good structural and antimicrobial properties is of great importance. A material obtained through the combination of a nanoscale hydroxyapatite material (with good biocompatibility) and titanium dioxide (with good degradation properties of organic molecules) can absorb and decompose bacteria. In this investigation, three different synthesis routes used to prepare hydroxyapatite/titanium dioxide nanomaterials are examined. The morphology and semiquantitative chemical composition are characterized by scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX). The obtained materials’ phase and structural characterization are determined using the X-ray powder diffraction method (XRD). The crystallite sizes of the obtained materials are in the range of 8 nm to 15 nm. Based on XRD peak positions, the hexagonal hydroxyapatite phases are formed in all samples along with TiO₂ anatase and rutile phases. According to SEM and TEM analyses, the morphology of the prepared samples differs depending on the synthesis route. The EDX analysis confirmed the presence of Ti, Ca, P, and O in the obtained materials. The IR spectroscopy verified the vibration bands characteristic for HAp and titanium. The investigated materials show excellent antimicrobial and photocatalytic properties.

Promising Cr-Doped ZnO Nanorods for Photocatalytic Degradation Facing Pollution

Chromium (Cr)-doped zinc oxide (ZnO) nanorods with wurtzite hexagonal structure were prepared through a thermal decomposition technique. The concentration effect of the Cr doping on the structural, morphological, and optical properties of the ZnO nanorods was established by correlating various measurements: transmission electron microscopy (TEM), photoluminescence (PL), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and several UV-visible studies. The obtained nanorods were investigated as photocatalysts for the photodegradation process of methyl orange (MO), under UV-vis light illumination. Different weights and time intervals were studied. A 99.8% photodegradation of MO was obtained after 100 min in the presence of 1 wt.% Cr III acetate hydroxide and zinc acetate dehydrate “ZnO-Cr1”. The kinetic rate constant of the reaction was found to be equal to 4.451 × 10−2 min−1 via a pseudo-first order rate model. Scavenger radicals demonstrated the domination of OH• radicals by those of O2•− superoxide species during the photodegradation. The interstitial oxygen site Oi is proposed to play a key role in the generation of holes in the valence band under visible irradiation. The ZnO-Cr1 photocatalyst displayed good cycling stability and reusability.

Construction of magnetically recoverable ZnS-WO3-CoFe2O4 nanohybrid enriched photocatalyst for the degradation of MB dye under visible light irradiation

Easily recyclable photocatalysts have received considerable attention for their practical application, in order to address the wastewater treatments. Here, we report efficient and magnetically recyclable ZnS-WO₃-CoFe₂O₄ nanohybrid prepared through wet impregnation method. The photophysical and optical properties of as-prepared photocatalysts was investigated by different spectroscopic techniques. The photocatalytic activity of as synthesized samples were assessed by the photodegradation of methylene blue (MB) dye under visible light irradiation. Amongst, ZnS-WO₃-CoFe₂O₄ nanohybrid exhibit higher photodegradation activity than the other bare and hybrid samples. The enhanced light absorption and lower emission intensity provide the improved photocatalytic activity of ZnS-WO₃-CoFe₂O₄ nanohybrid. The ZnS-WO₃-CoFe₂O₄ nanohybrid exhibit excellent photostability after four consecutive cycles. The ferromagnetic behavior of the hybrid sample using easily recover from the dye solution using an external bar magnet.

TiO2 with controllable oxygen vacancies for efficient isopropanol degradation: photoactivity and reaction mechanism

Flame-synthesized TiO_2−x with controllable defects exhibits a remarkable photooxidation efficiency of gaseous isopropanol with the reaction mechanism investigated.

Enhanced Photocatalytic and Antibacterial Ability of Cu-Doped Anatase TiO2 Thin Films: Theory and Experiment

Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol-assisted chemical vapor deposition to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase and showed excellent antibacterial (vs Staphylococcus aureus and Escherichia coli) ability. Using a combination of transient absorption spectroscopy, photoluminescence measurements, and hybrid density functional theory calculations, we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our analysis has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes observed in the doped samples and the enhanced UV photoactivities observed.

Nanocasting synthesis of chromium doped mesoporous CeO2 with enhanced visible-light photocatalytic CO2 reduction performance

Chromium doped mesoporous CeO₂ catalysts were synthesized via a simple nanocasting route by using silica SBA-15 as the template and metal nitrates as precursors. The effect of Cr doping concentration (5%, 10%, 15% and 20% of the initial Cr/(Cr+Ce) molar percentage) on the structures of these catalysts and their photocatalytic performances in reduction of CO₂ with H₂O were investigated. The results indicated that the introduction of Cr species could effectively extend the spectral response range from UV to visible light region (400-700nm) and improve the electronic conductivity for the mesoporous CeO₂ catalysts which exhibited an enhanced photocatalytic activity in the reduction of CO₂ with H₂O when compared with the non-doped counterpart. The highest CO and CH₄ yield of 16.2μmol/g-cat. and 10.1μmol/g-cat., respectively, were acquired on the optimal chromium doped CeO₂ catalyst with the initial Cr(Cr+Ce) molar percentage of 15% under 8h visible-light irradiation, which were more than twice as high as that of bare CeO₂. The remarkably increased photocatalytic performance should be attributed to the advantageous structural and compositional features of the chromium doped mesoporous CeO₂.

pH-Controlled photocatalytic abatement of RhB by an FeWO4/BiPO4 p–n heterojunction under visible light irradiation

The novel FeWO₄/BiPO₄ heterojunction generates an inner electric field to promote electron–hole separation efficiency and is a proficient photocatalyst.

Mesoporous TiO2 Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency

Mixed metal oxide nanoparticles have interesting physical and chemical properties, but synthesizing them with colloidal methods is still challenging and often results in very heterogeneous structures. Here, we describe a simple method to synthesize mesoporous titania nanoparticles implanted with a uniform distribution of copper oxide nanocrystals (CuO@MTs). By calcining a titanium-based metal-organic framework (MIL-125) in the presence of Cu ions, we can trap the Cu in the TiO₂ matrix. Removal of the organic ligand creates mesoporosity and limits phase separation so that tiny CuO nanocrystals form in the interstices of the TiO₂. The CuO@MTs exhibits superior performance for photocatalytic hydrogen evolution (4760 μmol h^-1) that is >90 times larger than pristine titania.

Antimicrobial Properties of Copper-Doped ZnO Coatings under Darkness and White Light Illumination

We report the first antimicrobial study of transparent and robust Cu-doped ZnO coatings that displayed potent antimicrobial activity that resulted in bacterial (Escherichia coli) reduction below detection limits within 6 h of illumination via a white light source that is found in hospital environments. The same bacterial reduction rate was observed even under darkness for 4.0 atom % Cu-doped ZnO films thus providing an efficient 24 h disinfection. All films were produced via a novel, inexpensive, and easily scalable route and were also thoroughly analyzed for their material properties.

Role of Ni in hetero-architectured NiO/Ni composites for enhanced catalytic performance

The effect of Ni in NiO/Ni nanocomposites and their contribution in photoelectrooxidation of dyes and substituted phenols were studied.

Enhanced photocatalytic inactivation of bacteria on Fe-containing TiO2 nanoparticles under fluorescent light

In this paper, the photocatalytic activity of Fe-TiO2 nanoparticles (NPs) under fluorescent light was studied using Escherichia coli and Staphylococcus aureus. Fe-TiO2 NPs were synthesized using a sol-gel method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (UV-vis DRS) and transmission electron microscopy. The efficiency of photocatalytic inactivation towards E. coli was studied under different physicochemical parameters. The photocatalytic inactivation rate increased with increasing Fe content in TiO2 NPs and the highest inactivation was achieved for 3.0 mol% Fe-TiO2 NPs under fluorescent light. These results demonstrate that the presence of an optimum concentration of Fe in TiO2 matrix enhances the photocatalytic inactivation of TiO2 NPs under fluorescent light.

A Highly Efficient UV-Vis-NIR Active Ln(3+)-Doped BiPO4/BiVO4 Nanocomposite for Photocatalysis Application

In this Article, we report the synthesis of Ln(3+) (Yb(3+), Tm(3+))-doped BiPO4/BiVO4 nanocomposite photocatalyst that shows efficient photocatalytic activity under UV-visible-near-infrared (UV-vis-NIR) illumination. Incorporation of upconverting Ln(3+) ion pairs in BiPO4 nanocrystals resulted in strong emission in the visible region upon excitation with a NIR laser (980 nm). A composite of BiPO4 nanocrystals and vanadate was prepared by the addition of vanadate source to BiPO4 nanocrystals. In the nanocomposite, the strong blue emission from Tm(3+) ions via upconversion is nonradiatively transferred to BiVO4, resulting in the production of excitons. This in turn generates reactive oxygen species and efficiently degrades methylene blue dye in aqueous medium. The nanocomposite also shows high photocatalytic activity both under the visible region (0.010 min(-1)) and under the full solar spectrum (0.047 min(-1)). The results suggest that the photocatalytic activity of the nanocomposite under both NIR as well as full solar irradiation is better compared to other reported nanocomposite photocatalysts. The choice of BiPO4 as the matrix for Ln(3+) ions has been discussed in detail, as it plays an important role in the superior NIR photocatalytic activity of the nanocomposite photocatalyst.

Energy revamping of solar panels through titania nanocomposite coatings; influence of aqueous silica precursor

A novel, multifunctional, titania–silica–lanthanum phosphate (TSL) nanocomposite coating on solar panel modules was found to significantly increase the output voltage to ~15% under outdoor exposure leading to the energy revamping of solar panels.

Shape-selective oriented cerium oxide nanocrystals permit assessment of the effect of the exposed facets on catalytic activity and oxygen storage capacity

The catalytic performance of a range of nanocrystalline CeO2 samples, prepared to have different morphologies, was measured using two accepted indicators; oxygen storage and diesel soot combustion. The same powders were characterized in detail by HR-TEM, XRD, XPS, and Raman methods. The study demonstrates that activity is determined by the relative fraction of the active crystallographic planes, not by the specific surface area of the powders. The physical study is a step toward quantitative evaluation of the relative contribution to activity of the different facets. The synthetic protocol permits fabrication of CeO2 nanostructures with preferentially grown active planes, and therefore has potential in developing catalytic applications and in nanocompositing.

Generation of oxygen vacancies in visible light activated one-dimensional iodine TiO2 photocatalysts

Oxygen vacancies induced by multi-valences of iodine in two-step hydrothermal synthesized I/TiO₂ with enhanced visible photoactivity.

N-doped TiO2/C nanocomposites and N-doped TiO2 synthesised at different thermal treatment temperatures with the same hydrothermal precursor

A hydrothermal precursor method for the synthesis of coloured titania and its enhanced visible light photocatalytic properties.