MoO3 Powder Research Articles

Synthesis of Mo-Cu nanocomposite powder by hydrogen reduction of copper nitrate coated MoO3 powder mixture

The fabrication of Mo-based nanocomposite powder with homogeneous dispersion of Cu was investigated. The fine MoO3 powders were prepared by high energy ball milling, and Cu particles on the ball-milled powder surfaces were formed by calcination of a precursor solution copper nitrate. In the powder mixture calcined at 300 °C, MoO3 and CuO appeared in the form of aggregates with fine particles. Microstructural analysis showed that the oxide powders were changed to Mo-Cu nanocomposite powders with an average particle size of about 150 nm and had microstructural homogeneity by hydrogen reduction at 800 °C for 1 h. These results are help to optimize the synthesis process of homogeneous Mo-Co nanocomposite powders.

Enhanced Performance of Polymer Solar Cells by Ultraviolet-Ozone Treatment of MoOX Films with non-Thermal Annealing Treatment of MoOX Films

Molybdenum oxide (MoOX) is a commonly used hole extraction material (HEL) in polymer solar cells (PSCs). Here, we perform ultraviolet ozone treatment (UVO, non-thermal annealing treatment) on MoOX film (by spin-coated (NH4)6Mo7O24-4H2O solution on the ITO top) prepared by solution processing to develop hole extraction materials for efficient PSC. After UVO treatment of MoOX films (US-MoOX), the (NH4)6Mo7O24-4H2O molecules decompose to form a denser and higher energy levels MoOX film. Compared with E-MoOX films (vacuum evaporated MoO3 powder), US-MoOX films have similar square resistance and electrical conductivity to E-MoOX films, and the photovoltaic performance of US-MoOX-based PSCs is comparable to that of E-MoOX-based PSCs and higher than that of PEDOT:PSS-based PSCs. Therefore, US-MoOX-based PSCs show the advantages of combining high photovoltaic performance with solution processing. In addition, US-MoOX-based PSCs have the advantage of not requiring thermal annealing treatment. The results indicate that US-MoOX films are promising HEL materials for the practical production of PSCs.

Study on hydrogen reduction mechanism of MoO3 using Ti–Mo core–shell powder

In this study, the hydrogen reduction mechanism of MoO3 was analyzed in detail using Ti–Mo core–shell powder manufactured through a milling and reduction process. MoO3 and Ti powders were ball-milled to refine the MoO3 powder and coat it onto the Ti powder to fabricated core-shell structure. Subsequently, the milled powder was heat-treated at 600°C in a hydrogen atmosphere to reduce MoO3 to nano-sized Mo. In addition, the reduction behavior of MoO3 was analyzed by varing the reduction times to 1, 10, 30 min and 1, 3, 5 h. Phase analysis was conducted using X-ray diffraction, and oxidation state analysis was conducted using X-ray photoelectron spectroscopy. Morphology analysis was performed using scanning electron microscopy, and elemental analysis was conducted using electron probe micro-analysis. It was confirmed that Mo9O26 and Mo4O11 intermediate phases were formed sequentially as MoO3 was reduced to MoO2, and Mo9O26 had a plate shape with a thickness of sub-70 nm, and Mo4O11 exhibited a thickness of sub-300 nm. Especially, the Mo2O3 intermediate phase, previously studied only theoretically, was experimentally confirmed for the first time in the reduction process from MoO2 to Mo. Furthermore, the analysis of Mo-oxide intermediate phases was explained by the structural characteristics of the core–shell. The shell of Mo oxide was thick and dense, making it difficult for the vapor phase necessary for CVT reduction to penetrate and diffuse into the shell. As a result, the rapid CVT reduction occurred outside the shell, while relatively slower reduction occurred inside the shell where intermediate phases were observed.

Preparation of MoO3 powder by hydrothermal method

Abstract Based on the raw material of ammonium tetramolybdate in the experiment, molybdenum trioxide was prepared by hydrothermal synthesis, and the effects of different pH, hydrothermal reaction time, filling degree, and calcination temperature on molybdenum trioxide powder were studied. Meanwhile, the molybdenum trioxide powder was characterized through scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and thermal analysis differential scanning calorimetry analysis so as to study the morphology and phase changes during the experiment. As is evident from the research findings, MoO3 powders with uniform and suitable size, smooth and clear surface, good dispersibility, and no adhesion can be obtained at the conditions of pH = 1, 16 h hydrothermal reaction, 90% of filling degree, and 550°C of calcination temperature. In the calcination process, MoO3 crystal undergone crystal transformation and was completely transformed from h-MoO3 to α-MoO3 at the calcination temperature of 350°C.

Production of high purity MoO3 from spent catalyst of formaldehyde synthesis process via a novel two-step leaching-cementation method

This study presents a two-step methodology for producing high-purity MoO3 from spent FORMOX process catalysts. In the first step, a leaching process with a Na2CO3 solution is used to get molybdenum out of formaldehyde synthesis catalyst. The second step involves molybdenum-ion cementation in the presence of metallic zinc powder and plates. The leaching process is thoroughly explored using the design of experiments (DOE) to analyze critical parameters such as temperature, leaching time, Na2CO3/Mo stoichiometric ratio, and catalyst average particle size. A robust quadratic model with an R2 value exceeding 97 % is developed, serving as a predictive tool for molybdenum extraction. Through rigorous numerical optimization, the optimal conditions for 100 % extraction were found to be 74.5 °C, 92.3 min, a stoichiometric ratio of 2.52, and a particle size of 183 μm. However, the use of zinc powder in the cementation stage introduces impurities due to increased solution pH and reduced zinc dissolution. The strategic adoption of zinc plates mitigates these challenges, resulting in a notable enhancement of the final product's purity. Over 91 % of MoO4-2 ions with a purity surpassing 99.8 % are successfully obtained as hexagonal MoO3 powder at 85 °C with an initial solution pH of 1.0 after 15 min of the cementation process.

(Invited) Interface Treatments for High-Efficiency MoOx Based Silicon Heterojunction Solar Cells

Silicon based solar cells dominate the market of photovoltaics, which hold the highest potential for green electricity production. A front/back-contacted architecture combined with the silicon-based heterojunction (SHJ) concept realized a world record efficiency of 26.3%1. However, conventional silicon-based doped layers, which work as carrier selective transport layers, are not optically transparent.Transition metal oxides are promising candidates for SHJ solar cells due to their advantageous opto-electronic properties. Molybdenum oxide (MoOx) with high work function (WF) has achieved highly efficient solar cells with 23.5% efficiency2. However, depositing high quality MoOx thin film on top of a-Si:H is challenging due to highly reactive surfaces3. This interaction may result in interfacial oxidation, lower Mo oxidation states and reduced WF4,5. In turn, electrical performances of solar cells featuring MoOx are poorer3 than standard SHJ devices.Several attempts have been made to prevent the reaction between MoOx and (i)a-Si:H. Pre-annealing4 of a-Si:Hand pre-growth of SiOx layer6 have been proposed as effective ways to prevent the degradation of MoOx. Nevertheless, those approaches require extra process steps that make MoOx integration not immediately viable in industrial environment. In our previous work, we proposed a plasma treatment (PT) at MoOx/(i)a-Si:H interface that demonstrated to mitigate dipole effect and resulted in improved cell performance7. After optimizing the interface treatment, even thinner MoOx layer could be applied.The solar cells were fabricated using 260±20 μm 4-inch double-side textured n-type <100> FZ wafers. (i)a-Si:H was deposited by plasma-enhanced chemical vapor deposition on both sides, while (n)a-Si:H only at the rear side. We carried out two different plasma treatments on the (i)a-Si:H interface prior MoOx deposition: one called PT from a mixture of SiH4, H2 and CO2 and another called PTB from a mixture of SiH4, H2, CO2 and B2H6. One sample without plasma treatment (noPT) was used as reference to evaluate the efficacy of the proposed method. A thickness series from 1 to 4 nm of MoOx was thermally evaporated from MoO3 powder at the pressure of 5×10-6 mbar. After that, optimized 50-nm and 150-nm thick tungsten-doped indium oxide (IWO) layers were sputtered through a hard mask at front and rear side, respectively, defining six 2×2 cm2 solar cells per wafer. As metal contact at the front side, we used room temperature Cu plating8 with a metal coverage fraction of 1.575%; at the rear side, 500-nm thick Ag layer was sputtered on the full device area.We studied the impact of different plasma treatment conditions on MoOx film quality and cells’ performance. MoOx film quality was assessed based on O vacancies inside the film4. The samples with interface treatments contain less O vacancies than the sample without treatment. It is noticeable that the cell precursors endowed with PTB yield higher fill factor (FF) than other types of precursors. Instead, PT and noPT samples benefitted from high short-circuit current density (J SC). Ultimately, PTB and PT samples achieved similar conversion efficiency. For the variation of MoOx thickness, we observed that even a slightly change in its thickness influences the solar cells’ electrical performance drastically. An ultra-thin, ~1.7-nm thick MoOx layer could provide sufficient field passivation (open circuit voltage, V OC = 721.4 mV) and good opto-electrical properties (FF = 82.18% and J SC-illuminated-area = 40.20 mA/cm2). With our approach we demonstrate word record cell with ISFH-certified conversion efficiency of 23.83% 9. 1 LONGi Solar 2021. 2 J. Dréon, et al., Nano Energy, 2020. 3 J. Geissbühler et al., Appl. Phys. Lett. 2015. 4 M.T. Greiner, et al., Adv. Funct. Mater. 2013. 5 M.T. Greiner, et al., Adv. Funct. Mater. 2012. 6 J. Tong, et al. ACS Appl. Mater. Interfaces. 2021. 7 L. Mazzarella, et al., Prog Photovolt Res Appl. 2020. 8 C. Han, et al., Solar RRL 2022. 9 L. Cao, et al., Prog. Photovolt. 2022.

Sintering property of micro/nano core-shell molybdenum powder synthesized by mechanochemical process

In this study, a micro/nano core-shell molybdenum powder was synthesized by mechanochemical process to improve its microstructural uniformity and density by pressureless sintering the core-shell powder. The micro-sized spherical Mo powder, prepared through an electrode induction melting inert gas atomization process, was mixed with MoO3 powder and ball-milled using a three-dimensional shaker mixer. A mechanochemical process involving hydrogen reduction was employed to prepare the core-shell Mo powder. The prepared powders were sintered through the compacting and pressureless sintering. The density was measured using a densitometer to analyze the properties of sintered specimens. Phase analysis was performed using X-ray diffraction. In addition, microstructure analysis was conducted using scanning electron microscopy and electron backscatter diffraction. Comparing the sintered core-shell powder to the sintered conventional mixing powder under the same conditions, we observed that the former exhibited a uniform microstructure and higher density. Furthermore, we experimentally demonstrated the dependence of sintering properties on mixing ratio.

Phenomenological Interactions during CVT Synthesis of Large Scale MoS2

In the Chemical Vapor Transport (CVT), for deposition of MoS2, sulfur and MoO3 powder is evaporated at 200 0C and 650 0C respectively. The Ar flow directs the sulfur atoms to Mo species so as to react chemically as specified in the reactions in Equation-1. During the CVT process there is adsorption of MoS2 on Si/SiO2 substrate associated with the exhuast of SO2 as chemical reaction by-product. The MoS2 is deposited on the substrate by physisorption. Thus, in CVT, the nucleation and growth is a combined effect of chemisorption and physisorption. The three types of nucleation and growth theories for deposition of 2D thin films are presented by a. Volmer–Weber b.Frank–van der Merwe, c. Stranski–Krastanov in Figure 1.Figure 1[Equation 1]The mechanism for MoS2 nucleation and growth during CVT involves the evaporation of powdered precursors at a high temperature which leads to decomposition as well, followed by a chemical reaction. Temperature ramping-up rate, positioning of precursors, type and flow rate of carrier gases, Cooling rate, and pressure simultaneously determine the morphology, thickness, and crystallinity of the physisorbed MoS2. The highly simplified resolution of forces acting during the nucleation process is represented in Figure 2. The σF refers to the forces due to surface tension along the droplet, σS is the force exerted due to the energy associated with the substrate surface, and σFS is the force due to the interface energy between the droplet and the substrate. A little consideration of the comparison of the magnitude of these three forces leads to the determination of Growth Kinetics. When σS is smaller than the sum of σF and σFS, it favors Weber-Volmer theory; when σS is greater than the sum of σF and σFS, it favors Frank van der Merwe theory, and when the balance of this comparison fluctuates or is equal, it favors the Stranski–Krastanov growth model. In practice, the several parameters other than temperature, pressure, flow rate, and precursor flux that influence the nucleation and growth mechanism involve substrate surface texture, functionalization, availability of active cites, steric hindrance, Columbic interactions etc.Figure 2 Interactions in CVT on the basis of Scientific Theories : Considering that CVT involves the gas phase chemical reaction of precursors and physisorption for the MoS2 adsorption on the Si/SiO2 substrate, the modified ideal gas law can be applied more realistically with the introduction of the van der Waals parameters for intermolecular interaction forces ‘a’ and molecular size ‘b’. The modified gas law is stated by van der Waals equation as,[Equation 2]a – Constant for Intermolecular forces and b – Constant for Molecular size for the specified gas under consideration. Thus, if studies related to variation in temperature are carried out, Volume being physically constant and significantly large space in case of CVT may be considered as independent parameter. Thus, from relation 2, pressure is directly affected with the change in temperature.Further, the pressure in accordance with Amonton’s law affects the adsorption and mean free path of the Mo and S atoms that determines the reaction between the chemical species [3].The mean free path for the atomized precursor is expressed as follows.[Equation 3]The dependence of the mean free path λ on T – temperature and P – the pressure is conclusive for the parametric interactions resulting in accordance with the Amonton’s law for constant volume affect the CVT process. In the equation, is the Boltzman’s constant.As referred to in the beginning, physisorption is the mechanism for depositing 2D thin films in CVT. Physisorption is caused by adsorption and thus, the variation in process parameters will have impact on Langmuir’s adsorption [4]given by[Equation 4]Langmuir’s adsorption isotherm explains the interfacial interaction of the gas phase molecules with the substrate. The Figure 3. depicts the variation in corresponding to the KP at a constant temperature. As is evident the Langmuir’s adsorption is derived at isothermal conditions for variation in pressure. Thus, it is implied that during the CVT synthesis of MoS2 if the variation in temperature is studied, it will inherently affect the adsorption.Arrhenius equation [5] governs the reaction rate based on A- pre-exponent rate factor, Ea – activation energy, R – gas constant and temperature.[Equation 5]Considering the van der Waals equation and Amontons law, the temperature is going to affect the pressure. The rate constant will be affected by Ar flow rate. These phenomological interactions occur and influence the nucleation and large scale growth of MoS2 during CVT synthesis. Figure 1

Effects of Sintering Processes on Microstructure Evolution, Crystallite, and Grain Growth of MoO2 Powder

MoO2 micro-powders with a mean pore size of 3.4 nm and specific surface area of 2.5 g/cm3 were compacted by dry pressing, then pressureless sintered at a temperature of 1000–1150 °C for 2 h or for a sintering time of 0.5–12 h at 1050 °C in an N2 atmosphere. Then, their microstructure evolution for morphology, crystallite, and grain growth were investigated. By sintering at a certain temperature and times, the irregular shape of the MoO2 powders transformed into an equiaxed structure, owing to the surface energy, which contributed to faster grain growth at the initial stage of sintering. The crystallite and grain sizes exponentially increased with the sintering time, and the growth exponent, n, was approximately 2.8 and 4, respectively. This indicates that the crystallite growth is governed by dislocation-mediated lattice diffusion, and the grain growth is determined by surface diffusion-controlled pore mobility. The increase in sintering temperature increased both crystallite and grain size, which obeyed the Arrhenius equation, and the activation energies were determined to be 95.65 and 76.95 kJmol−1 for crystallite and grain growths, respectively.

Centimeter-level MoS2 films with controllable number of layers by face-to-face chemical vapor deposition strategy

Two-dimensional layered transition metal dichalcogenides (TMDCs) are highly promising in field-effect-transistors (EFTs), photodetectors, flexible devices, but the limitation of uncontrollable size and number of layers is a huge challenge for the development of these devices. Although some breakthroughs like wafer-level MoS2 have been achieved on specific substrates such as copper foil or sapphire using advanced chemical vapor deposition, electrical and optoelectronic testing requires the transfer of material from these substrates to SiO2/Si substrates. In this paper, we developed a face-to-face confined space CVD scheme for the preparation of centimeter-level MoS2 thin films on amorphous SiO2/Si substrate. In contrast to traditional confined-space CVD scheme where the MoO3 powder is often piled like a small hill in the small quartz boat, The MoO3 powder is evenly spread in the small quartz boat, facing directly up to the growing substrate with a “face-to-face” growth mode. Therefore, the nucleation centers are uniformly distributed on the growing substrate, finally resulting in a uniform and clean continuous film originating from the coalescence of numerous MoS2 domains grown up from these nucleation centers. The layer number can be well controlled by adjusting growing parameters like growing temperature, gas flow rate and growth time. Our facile but robust face-to-face growth strategy may be a great supplementary to the wafer-level growth of 2D TMDCs.

Valence modulation and morphological engineering of MoO3 as high-performance cathode for aqueous zinc ion batteries

Molybdenum trioxide (MoO3) has become the preferred choice of cathode materials for aqueous zinc ion batteries (AZIBs) because of their multivalency, high theoretical capacity, and unique layered structure. However, MoO3 suffers from low conductivity and poor cycling stability, which severely limits its practical application for commercialization. Here, we develop a morphological engineering couple valence modulation to remarkably enhance the Zn2+ storage capability of commercial MoO3 powder. Uniform MoO3 with low-valence-state Mo (denoted as MoO3−x) nanoparticles are readily manufactured through a simple solvothermal method with ethylene glycol as a reductant. Taking the merits of the increased electrical conductivity, Zn2+ diffusion rate and active sites, the Zn//MoO3−x battery achieves a boosted capacity of 144 mAh g−1 at 2 A g−1, substantially superior to the Zn//MoO3 one (only 17 mAh g−1). In addition, the Zn//MoO3−x battery exhibits an excellent energy density of 146 Wh kg−1 and good cycle stability with a capacity loss of only 0.07% per cycle over 500 cycles. This work provides an attractive approach to enhance the electrochemical properties of molybdenum-based materials and may open new idea to design advanced cathode material for AZIBs.

Effect of MoO3 and TiO2 Powder Particle Sizes on the Phase Composition and Density of Dysprosium Titanate Pellets

Effect of MoO3 and TiO2 Powder Particle Sizes on the Phase Composition and Density of Dysprosium Titanate Pellets

Growth of 2D MoS2 and MoSe2 layers for photodetector application

We have grown a few layers MoS2 and MoSe2 on Si (100) substrates using a chemical vapor deposition (CVD) technique at different approaches and growth temperatures. Structural, surface morphology and electronic properties were investigated by Raman spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The MoS2 thin films were grown using three-zone CVD system by employing the MoO3 and sulfur powder precursors. We have obtained characteristics E12g and A1g Raman modes of MoS2 peaks and the separation between these two modes revealed the formation of a 2-monolayer of MoS2 on Si (100) substrate. The triangular morphology of MoS2 layer with a thickness of ∼1.4 nm i.e. nearly 2-monolayer was deduced by atomic force microscopy. Similarly, a few layers of MoSe2 were grown on Si (100) by selenization of Mo-coated Si (100) and 2H-MoSe2 characteristics Raman peaks were obtained. The XPS analysis revealed the formation of pure MoS2 and MoSe2 thin films on Si (100) substrates. The two-dimensional materials on Si (100) can be used for broadband photodetector applications.

Robust temperature–strain coupling in phase and shape evolution of MoTe2 nanosheets

Molybdenum ditelluride (MoTe2) has a stable semiconducting hexagonal (2H) phase and a metastable metallic distorted octahedral (1T′) phase at the same time, which attracts much attention due to its attractive properties. However, the mechanism of phase and shape evolution in the preparation of MoTe2 is still unclear, which limits the controllable preparation and the wider device application of MoTe2. Here, we prepare few-layer MoTe2 with controllable phase and shape by using MoO3 and Te powders as precursors. With this method, triangle and hexagon 2H MoTe2 can be prepared, and long-strip and irregular 1T′ MoTe2 can be obtained. The phase and shape of as-prepared MoTe2 are determined by the coupling effect in the growth temperature and the lattice strain between 2H and 1T′ MoTe2. Low growth temperature combined with low Te concentration could induce small growth strain potential, leading to the growth of triangle and hexagon 2H MoTe2. While high growth temperature combined with high Te concentration could induce large strain potential, which is conducive to the preparation of long-strip and irregular 1T′ MoTe2. This study deeply investigates the evolution mechanism of phase and shape in MoTe2 growth, which has important guiding significance for the controllable preparation of phase and shape of other two-dimensional materials.

Mechanism and kinetics of decomposition reaction of ultrafine ammonium molybdate for preparing ultrafine MoO3 powder

The decomposition reaction of ammonium molybdate to prepare molybdenum trioxide (MoO3) is a crucial step for the production of molybdenum powder. In this study, ultrafine MoO3 was prepared by calcining ultrafine ammonium molybdate in air atmosphere. The decomposition of ultrafine ammonium molybdate to generate MoO3 was proved to be a single-step reaction without intermediates in air atmosphere. Furthermore, the effects of temperature and reaction time on the phase composition, morphology and particle size distribution of products were also studied. The results showed that the temperature (in the range of 350 °C–450 °C) had a significant influence on the reaction rate, and the time required for completing the decomposition reaction was shortened as the temperature increased. However, the particle size of produced MoO3 was almost unchanged. It was also found that the phase compositions of products at different material layers (up, middle and bottom layers) were different, because the mass transfer of gaseous product in material layers affected the decomposition reaction. Additionally, a kinetic model considering the mass transfer of gaseous product in the material layer as well as the interface chemical reaction was proposed. Application of the developed model validated that the model could well describe the decomposition kinetics of ultrafine ammonium molybdate.

Кинетика растворения триоксида молибдена в щелочной среде

The paper presents experimental study results of the MoO3 powder samples dissolution kinetics in the aqueous ammonia solution at various pH concentrations and values. Molybdenum ions concentration in the filtrate samples was determined spectrophotometrically. Kinetic characteristics were obtained, and kinetic parameters (specific dissolution rate, reaction order with respect to the H+ ion) were calculated. It was established that the dissolution rate was increasing in the ammonia solution concentration range of 0.02--1.26 mol/l, and with the growing pH it passed through the maximum. Taking into account the acid-base equilibria constants, the dissolution process was simulated, and its stage-by-stage nature was established. Fractional reaction order with respect to the H+ ions calculated from the curves plotted in the α--t/t0.5 coordinates (affine transformation method) indicated the adsorption mechanism of dissolution. It was shown that the MoO3 dissolution proceeded with formation of the intermediate adsorption complexes. Due to the HMoO4- low concentration in the MoO3 concentration within the studied pH range by the surface-active particle, on which groups of ions were adsorbed, the MoO4- could be considered. The results obtained are an addition to the data possessed on the molybdenum oxide phase and other transitional metals behavior. They could be applied in the practical applications associated with dissolving molybdenum in the alkaline electrolytes

N-plasma modulation for boosting electromagnetic wave absorption behavior of MoO3 ceramic based on non-metallic doping

Rational design and fabrication of MoO3 ceramic with controllable electronic structure is expected to deliver favorable magnetic moments, leading to excellent electromagnetic behavior. However, achieving large-scale precise modulation of the MoO3 electronic structure is still a challenge. Here, a low-temperature, high-efficiency nitrogen radio frequency (RF) plasma treatment method was adopted to modify the surface of MoO3 with N doping and investigate its electromagnetic wave absorption (EMA) mechanism in depth. After MoO3 powder treating with N2 plasma for 30 s, at the doping ratio is 70%, the minimum reflection loss (RLmin) is –43.35 dB and the maximum effective absorption bandwidth (EAB) could reach 6.16 GHz at a low thickness (2.1 mm). The significant improvements in dielectric loss and permeability loss of N-doped MoO3 are because the doping of N atoms reduces the bandgap of MoO3, promotes electron jumping, dramatically improves electrical conductivity of semiconductors, and introduces interfacial polarization and dipole polarization effects. The presence of magnetic moments renders the N-doped MoO3 with weak magnetic properties and significant eddy current losses. This study provides an explicit EMA mechanism for non-metal doped transition metal oxidation.

Fluorane sensitive supercapacitive microcrystalline MoO3: dual application in energy storage and HF detection

Microcrystalline MoO3 powder, possessing supercapacitive energy storage capability, shows interesting HF sensing performance with a high sensitivity of 6656 mF mM−1 g−1 and a low limit of detection of 1.2 μM.

Edge-Active Enriched MoS2 Porous Nanosheets as Efficient Pt-Free Catalysts toward the Triiodide Reduction Reaction

Developing high-efficiency counter electrode (CE) catalysts for the triiodide (I3–) reduction reaction (IRR) is of great significance for the development of cost-effective dye-sensitized solar cells (DSSCs). From the perspective of further enhancing the catalytic activity of CE catalysts by exposing more active sites, edge-active enriched MoS2 porous nanosheets are elaborately constructed using the as-prepared one-dimensional MoO3 rods and poly(vinylpyrrolidone) (PVP) as a template-directing agent and surfactant based on the dissolution–recrystallization strategy (MoS2-P), respectively, while the MoS2 quasi-particles (MoS2-C) and irregular MoS2 aggregations are also synthesized using cetyltrimethyl ammonium bromide (CTAB) as a surfactant and commercial MoO3 powder as a molybdenum source under similar reaction processes. Benefiting from the high specific surface area, exposure of edge active sites, and effective ion diffusion-favored structure, MoS2-P delivers good photovoltaic performance (PCE = 8.16%, Voc = 0.782 V, Jsc = 16.77 mA/cm2, FF = 0.62) when used as a CE catalyst for DSSCs, superior to that of MoS2-C (PCE = 6.87%, Voc = 0.741 V, Jsc = 15.91 mA/cm2, FF = 0.58), which is comparable with that of the Pt-based device (PCE = 8.33%, Voc = 0.775 V, Jsc = 16.85 mA/cm2, FF = 0.64). A series of electrochemical results further reveal that the obtained MoS2-P has good catalytic activity for the IRR and electrochemical stability in iodine-based electrolytes. Hence, our work may provide a promising catalyst for the energy conversion field.

In situ powder X-ray diffraction during hydrogen reduction of MoO3 to MoO2

The hydrogen reduction of molybdenum trioxide to molybdenum dioxide is not yet fully understood as evident by continuous scientific interest. Especially the effect of the potassium content on the reduction process has not yet been considered. We prepared several samples of molybdenum trioxide containing varying amounts of potassium by addition of potassium molybdate (K2MoO4). In situ powder X-ray diffraction experiments were then conducted to study the hydrogen reduction of these samples. We especially focused on the influence of the alkali content and on gaining insight into the importance of the intermediary product γ-Mo4O11. During the reduction process, MoO2 is formed from the reduction of MoO3, which then reacts with the starting material to form γ-Mo4O11. With increasing potassium content, the reduction rate is decreased and the fractional content of γ-Mo4O11 built up during the reduction process is increased. As evident from bulk sample reduction, this results in a significant increase in the grain size visualized via scanning electron microscopy. Our investigations once again underline the importance of γ-Mo4O11 on the morphology of the resulting MoO2 powder.