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.
Biofilms causing medical conditions
or interfering with technical
applications can prove undesirably resistant to silver nanoparticle
(AgNP)-based antimicrobial treatment, whereas beneficial biofilms
may be adversely affected by the released silver nanoparticles. Isolated
biofilm matrices can induce reduction of silver ions and stabilization
of the formed nanosilver, thus altering the exposure conditions. We
thus study the reduction of silver nitrate solution in model experiments
under chemically defined conditions as well as in stream biofilms.
Formed silver nanoparticles are characterized by state-of-the art
methods. We find that isolated biopolymer fractions of biofilm organic
matrix are capable of reducing ionic Ag, whereas other isolated fractions
are not, meaning that biopolymer fractions contain both reducing agent
and nucleation seed sites. In all of the investigated systems, we
find that silver nanoparticle–biopolymer interface is dominated
by carboxylate functional groups. This suggests that the mechanism
of nanoparticle formation is of general nature. Moreover, we find
that glucose concentration within the biofilm organic matrix correlates
strongly with the nanoparticle formation rate. We propose a simple
mechanistic explanation based on earlier literature and the experimental
findings. The observed generality of the extracellular polymeric substance/AgNP
system could be used to improve the understanding of impact of Ag+ on aqueous ecosystems, and consequently, to develop biofilm-specific
medicines and bio-inspired water decontaminants.
Although of pivotal importance in heterogeneous hydrogenation reactions, the amount of hydrogen on catalysts during reaction is seldom known. We demonstrate the use of neutron imaging to follow and quantify...
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