Hydrogen reduction of molybdenum oxide at room temperature

Abstract

The color changes in chemo- and photochromic MoO3 used in sensors and in organic photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissociated at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a critical challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen reduction of MoO3 thin films using atomic hydrogen in a water free environment. Hydrogen intercalates into MoO3 forming HxMoO3, which slowly decomposes into MoO2 +1/2 H2O as evidenced by the fast reduction of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temperature hydrogen reduction of bulk molybdenum oxide. The specific mechanism is a result of the balance between the reduction by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.

Role of surface microstructure of Mo back contact on alkali atom diffusion and Ga grading in Cu(In,Ga)Se2 thin film solar cells

“…As the deposition Ar pressure increases, the shape of the grains remains needle-like but the size becomes relative larger and the surface becomes more porous. 43 The O 1s peak at 532.1 eV is a strong indication of the existence of OH group. Thus, the more porous the Mo film is, the more air it contains and the smaller the optical constants.…”

“…The optical constants, including both the refractive index (n) and extinction coefficients (k), of Mo films for these three representative samples were also measured by spectral ellipsometry and the results are displayed in Figure 1D and E. From the effective medium approximation (EMA), the optical constants of a porous Mo film can be regarded as a weighted average of those of Mo and air atmosphere. 43,44 All the fitting parameters and the relative amount of each Mo and O species are presented in Table 1. The decreased refractive index and extinction coefficient with increased Ar pressure in the visible wavelength region indeed support the picture that an increasingly porous Mo surface is formed as the Ar pressure increases, in good agreement with the results of AFM images.…”

Friction and Wear Properties of Tetrahedral Si-Containing Hydrogenated Diamond-Like Carbon Coating under Lubricated Condition with Engine-Oil Containing ZnDTP and MoDTC

“…It is interesting to note that intensity variation of MoHO4 − depending on different coating samples almost corresponds to the variation trend of MoO3 − . MoO3 shows catalytic activity and is known to interact with hydrogen, oxygen, For example, it was reported that HxMoO3 can be generated during the reduction process from MoO3 to MoO2 [33]. MoHO4 − was observed at a certain extent even for the DLC coating samples without hydrogen content; therefore, only hydrogen in DLC materials does not necessarily contribute to the MoHO4 − production.…”

Quantum‐Dot‐Mediated Controlled Synthesis of Dual Oxides of Molybdenum from MoS2: Quantification of Supercapacitor Efficacy

“…[37][38][39] These bands support the formation of H 2 MoO 5 ·H 2 OQ Ds. [41] The deconvoluted O 1s peaks (Figure 3e)s uggest three types of bondingf or 530.5, 531.6, and 532.6 eV,c orresponding to Mo=O, MoÀOH, and MoÀperoxol inkages, respectively; [42] this confirms the formation of H 2 MoO 5 ·H 2 OQ Ds. [40] Furthermore, the chemical bonding environment of QDs has been evaluated by XPS.T he XPS survey spectrum of QDs ( ure S2 in the Supporting Information) reveals four distinct peaks at 231.7, 233,2 34.8, and 236.2 eV.T he peaks at 231.7 and 234.8 are assigned to the + 5o xidation state of molybdenum, corresponding to Mo 3d 5/2 and Mo 3d 3/2 electrons,r espectively.S imilarly,t he peaks at 233 and 236.2 eV are allocated to the + 6o xidation state, corresponding to Mo 3d 5/2 and Mo 3d 3/2 electrons, respectively.…”

“…The SAED pattern ( bond vibrations, respectively. [41,42] The peak area ratio indicates the presence of as mall amount of the Mo 5 + (13.6 %) state with the Mo 6 + state;t he + 5o xidation state possibly arises from ah ydrogen insertion effect. The bands at ñ = 1179 and 1044 cm À1 indicate the presence of sulfate ions.…”