The direct synthesis of mesoporous structured MnO(2)/TiO(2) nanocomposite: a novel visible-light active photocatalyst with large pore size

Reverse Micelle Strategy for the Synthesis of MnO x -TiO2 Active Catalysts for NH3-Selective Catalytic Reduction of NO x at Both Low Temperature and Low Mn Content

MnO _x -TiO₂ catalysts (0, 1, 5, and 10 wt % Mn nominal content) for NH₃-SCR (selective catalytic reduction) of NO _x have been synthesized by the reverse micelle-assisted sol-gel procedure, with the aim of improving the dispersion of the active phase, usually poor when obtained by other synthesis methods (e.g., impregnation) and thereby lowering its amount. For comparison, a sample at nominal 10 wt % Mn was obtained by impregnation of the (undoped) TiO₂ sample. The catalysts were characterized by using an integrated multitechnique approach, encompassing X-ray diffraction followed by Rietveld refinement, micro-Raman spectroscopy, N₂ isotherm measurement at -196 °C, energy-dispersive X-ray analysis, diffuse reflectance UV-vis spectroscopy, temperature-programmed reduction technique, and X-ray photoelectron spectroscopy. The obtained results prove that the reverse micelle sol-gel approach allowed for enhancing the catalytic activity, in that the catalysts were active in a broad temperature range at a substantially low Mn loading, as compared to the impregnated catalyst. Particularly, the 5 wt % Mn catalyst showed the best NH₃-SCR activity in terms of both NO _x conversion (ca. 90%) and the amount of produced N₂O (ca. 50 ppm) in the 200-250 °C temperature range.

Construction of TiO2-MnO2 0D-2D nanostructured heterojunction for enhanced photocatalytic hydrogen production

The low transfer efficiency and high recombination loss of photo-induced carriers in TiO2 are significant issues that hinder its photocatalytic activity. Herein, TiO2 nanoparticles (∼5 nm) were loaded on MnO2 nanosheets (40-60 nm) to form TiO2-MnO2 nanostructured heterojunction (0D-2D nanostructure unit), possessing a high specific surface area. The separation/transfer efficiency of photocarriers and the solar absorptivity of TiO2-MnO2 were improved, thus enhancing solar energy conversion efficiency. The enhanced transfer efficiency of carriers is associated with the 2D network of MnO2 and abundant oxygen vacancies serving as media for electron transport. The enhanced visible absorption and reduced recombination should be attributed to the narrowed bandgap and modified energy band structure. The photocurrent of TiO2-MnO2 increased obviously and the H2 production rate increased to 0.38 mmol g-1 h-1, compared with that of pure TiO2 (0.25 mmol g-1 h-1). The enhanced photocatalytic properties are also associated with the excellent water oxidation kinetics caused by MnO2 nanosheets.

Recent developments in MnO2-based photocatalysts for organic dye removal: a review

The textile industry consumes a large volume of organic dyes and water. These organic dyes, which remained in the effluents, are usually persistent and difficult to degrade by conventional wastewater treatment techniques. If the wastewater is not treated properly and is discharged into water system, it will cause environmental pollution and risk to living organisms. To mitigate these impacts, the photo-driven catalysis process using semiconductor materials emerges as a promising approach. The semiconductor photocatalysts are able to remove the organic effluent through their mineralization and decolorization abilities. Besides the commonly used titanium dioxide (TiO₂), manganese dioxide (MnO₂) is a potential photocatalyst for wastewater treatment. MnO₂ has a narrow bandgap energy of 1~2 eV. Thus, it possesses high possibility to be driven by visible light and infrared light for dye degradation. This paper reviews the MnO₂-based photocatalysts in various aspects, including its fundamental and photocatalytic mechanisms, recent progress in the synthesis of MnO₂ nanostructures in particle forms and on supporting systems, and regeneration of photocatalysts for repeated use. In addition, the effect of various factors that could affect the photocatalytic performance of MnO₂ nanostructures are discussed, followed by the future prospects of the development of this semiconductor photocatalysts towards commercialization.

Highly selective silica-based fluorescent nanosensor for ferric ion (Fe3+) detection in aqueous media

A highly selective fluorescence nanosensor was built based on the functionalized SBA-15 mesoporous silica with Dinitrophenylhydrazine (DNPH). The interaction of fabricated nanosensor was studied with metals ions in aqueous media. Under optimum conditions (time 5min, pH7, sensor dose 0.02g dispersed in 1L), SBA-15-DNPH was highly selective toward the Fe³⁺ ion in the presence of the different groups of interfering metal ions. The detection limit and linear concentration range of the nanosensor were 39×10^-6M and 5×10^-5-375×10^-5M; respectively. The efficiency of nanosensor was independent on the pH range studied (6-9), which indicated that SBA-15-DNPH is a promising candidate for sensing and identification of Fe³⁺ ion in the environmental and other real matrix samples. SBA-15-DNPH showed good performance for Fe³⁺ ion detection and quantification in real samples.

Hierarchically-Structured TiO2/MnO2 Hollow Spheres Exhibiting the Complete Mineralization of Phenol

Although TiO2 or MnO2-based materials have been widely used for the degradation of phenolic compounds, complete mineralization is still a challenge, especially for TiO2-based materials. Here, we devise a hierarchically-structured TiO2/MnO2 (HTM) hollow sphere, in which hollow TiO2 acts as a skeleton for the deposition of MnO2 in order to prevent the aggregation of MnO2 nanoparticles and to maintain its hollow structure. During the oxidation reaction, the as-synthesized HTM can fully exert their respective advantages of the TiO2 and MnO2 species to realize the first stage of the rapid oxidation degradation of phenol and the second stage of the complete photo-mineralization of residual phenol and its intermediates, which efficiently overcomes the incomplete mineralization of phenolic compounds. The degradation mechanism and pathway of phenol are also proposed according to the analysis of Mass Spectrometry (MS). Therefore, this work provides a new insight for exploring hierarchically-structured materials with two or more species.

Nano-MnO₂ Decoration of TiO₂ Microparticles to Promote Gaseous Ethanol Visible Photoremoval

TiO₂-based photocatalysis under visible light is an attractive way to abate air pollutants. Moreover, developing photocatalytic materials on a large-scale requires safe and low-cost precursors. Both high-performance TiO₂ nanopowders and visible-light active noble metals do not match these requirements. Here, we report the design of novel Mn-decorated micrometric TiO₂ particles. Pigmentary TiO₂ replaced unsafe nano-TiO₂ and firmly supported MnO_x particles. Mn replaced noble metals such as Au or Ag, opening the way for the development of lower cost catalysts. Varying Mn loading or pH during the impregnation affected the final activity, thus giving important information to optimize the synthesis. Photocatalytic activity screening occurred on the gas-phase degradation of ethanol as a reference molecule, both under ultraviolet (UV) (6 h) and Light Emitting Diode (LED) (24 h) irradiation. Mn-doped TiO₂ reached a maximum ethanol degradation of 35% under visible light after 24 h for the sample containing 20% of Mn. Also, we found that an acidic pH increased both ethanol degradation and mineralization to CO₂, while an alkaline pH drastically slowed down the reaction. A strict correlation between photocatalytic results and physico-chemical characterizations of the synthesized powders were drawn.

Self-Organized TiO₂-MnO₂ Nanotube Arrays for Efficient Photocatalytic Degradation of Toluene

Vertically oriented, self-organized TiO₂-MnO₂ nanotube arrays were successfully obtained by one-step anodic oxidation of Ti-Mn alloys in an ethylene glycol-based electrolyte. The as-prepared samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis absorption, photoluminescence spectroscopy, X-ray diffraction (XRD), and micro-Raman spectroscopy. The effect of the applied potential (30-50 V), manganese content in the alloy (5-15 wt. %) and water content in the electrolyte (2-10 vol. %) on the morphology and photocatalytic properties was investigated for the first time. The photoactivity was assessed in the toluene removal reaction under visible light, using low-powered LEDs as an irradiation source (λ_max = 465 nm). Morphology analysis showed that samples consisted of auto-aligned nanotubes over the surface of the alloy, their dimensions were: diameter = 76-118 nm, length = 1.0-3.4 μm and wall thickness = 8-11 nm. It was found that the increase in the applied potential led to increase the dimensions while the increase in the content of manganese in the alloy brought to shorter nanotubes. Notably, all samples were photoactive under the influence of visible light and the highest degradation achieved after 60 min of irradiation was 43%. The excitation mechanism of TiO₂-MnO₂ NTs under visible light was presented, pointing out the importance of MnO₂ species for the generation of e^- and h⁺.

Synthesis, characterization and photocatalytic activity of mesoporous Na-doped TiO2 nano-powder prepared via a solvent-controlled non-aqueous sol–gel route

The superior photo-catalytic activity of mesoporous Na doped TiO₂ attributed to the combined effect of electron–hole recombination rate, increased surface area and enhanced crystallinity.

Complex doping chemistry owing to Mn incorporation in nanocrystalline anatase TiO2 powders

Mn-doped TiO2 powders with a wide range of nominal doping levels were fabricated using a one-step hydrothermal method followed by 400 °C annealing. Anatase powders with a uniform size distribution below 10 nm were obtained. The maximum solubility of Mn in the TiO2 lattice was around 30%, beyond which the Mn3O4 compound appeared as a secondary phase. The optical absorption edges for Mn-doped anatase TiO2 were red-shifted effectively through increasing Mn content. Alloying chemistry and associated elemental valences were elaborated through combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and theoretical simulation in the framework of density functional theory (DFT). The results showed that the Mn species exhibited mixed valence states of 3+ and 4+ in anatase TiO2, with the latter being the key to remarkable photocatalytic performance.

Constructing novel Ag nanoparticles anchored on MnO2 nanowires as an efficient visible light driven photocatalyst

We report the first construction of novel Ag nanoparticles anchored on MnO₂ nanowires as an efficient visible light driven photocatalyst.

Performance modulation of α-MnO₂ nanowires by crystal facet engineering

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO₂ nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO₂ (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

Synthesis of mesoporous Mn/TiO2 nanocomposites and investigating the photocatalytic properties in aqueous systems

Mesoporous 20 wt% Mn/TiO2 nanocomposites were synthesized adopting modified sol-gel method at different pH (pH = 2, 7 and 11) conditions and calcined at 400 °C. Based on the characteristics of the 20 wt% Mn/TiO2 nanocomposites synthesized at pH 11, same procedure was adopted for the synthesis of different wt% Mn/TiO2. The nanocomposite samples and their surface properties were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), mapping, inductively coupled plasma optical emission spectrometry (ICP-OES), Fourier transform infrared (FTIR), and fluorescence spectrometry. The nanocomposites existed in the anatase phase of TiO2 with no peak assigned to Mn on the diffractogram. The photocatalytic activities of the materials were evaluated by monitoring degradation of a model dye (methylene blue (MB)) in presence of visible light and ozone. The nanocomposite synthesized under neutral condition (pH = 7) exhibited the best photocatalytic activity resulting from its relatively smaller crystal size (5.98 nm) and larger pore volume (0.30 cm(3)/g). One percentage of weight Mn/TiO2 showed 100% decolouration of MB in the presence of O3 after 100 min.

Preparation of a novel flower-like MnO2/BiOI composite with highly enhanced adsorption and photocatalytic activity

A novel flower-like MnO₂/BiOI composite has been fabricated by a simple and cost-effective approach.

Modelling and quantification of intergrowth in γ-MnO2by laboratory pair distribution function analysis

γ-MnO2is a material formed by random intergrowth of two phases, β-MnO2andR-MnO2. It is demonstrated here on seven γ-MnO2samples that pair distribution function analysis using a conventional X-ray diffraction setup (Bragg–Brentano geometry with a molybdenum anode) allows the quantification of this intergrowth simplyviaa simulation of the actual material by a mixture of β-MnO2andR-MnO2phases. Although this method does not take into account specifically the relaxed distances in the vicinity of the intergrowth zone, it is found to be very robust, accurate and in full agreement with the widely used quantification based on the empirical approach of Chabre & Pannetier [Prog. Solid State Chem.(1995),23, 1–130].

First-principles study on transition metal-doped anatase TiO2

The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.

Developing high-efficiency π conjugated polymer semiconductor for photocatalytic degradation of dyes under visible light irradiation

π-conjugated polymer with a good coplanar backbone conformation exhibits high activity for degradation of pollutant under visible irradiation.

Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination

The photocatalytic degradation of organic dyes such as methylene blue and methyl orange in the presence of various percentages of composite catalyst under visible light irradiation was carried out. The catalyst ZnO nanorods and ZnO/CuO nanocomposites of different weight ratios were prepared by new thermal decomposition method, which is simple and cost effective. The prepared catalysts were characterized by different techniques such as X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, Fourier transform infrared spectroscopy and UV-visible absorption spectroscopy. Further, the most photocatalytically active composite material was used for degradation of real textile waste water under visible light illumination. The irradiated samples were analysed by total organic carbon and chemical oxygen demand. The efficiency of the catalyst and their photocatalytic mechanism has been discussed in detail.

Synthesis of Mn and Se-Doping TiO2 Mesoporous Materials and their Antibacterial Efficacy under Visible Light Irradiation

Two kinds of visible light driven mesoporous structured Mn and Se-doping TiO2 nanocrystal photocatalysts have been synthesized through a modified sol-gel method, and N2 adsorption-desorption isotherm confirms that the mesoporous materials possess large pore size and narrow pore size distribution. X-Ray powder Diffraction (XRD) analyses reveal that the doping of Mn and Se inhibits the growth of TiO2 anatase nanocrystal and the Mn species are higher dispersed on the surface of TiO2. Ultraviolet (UV)-vis spectrum demonstrates the excellent adsorption property of Mn doping TiO2 for visible light region, which enables this novel photocatalysis material to possess remarkable activity in photocatalytic antibacterial for E. Coli. bacteria under visible lights radiation.