Abstract
Nanoparticle suspensions represent a promising route toward low cost, large area solution deposition of functional thin films for applications in energy conversion, flexible electronics, and sensors. However, parameters such size, stoichiometry, and electronic properties must be controlled to achieve best results for the target application. In this report, we demonstrate that such control can be achieved viain situchemical oxidation ofMoOxnanoparticles in suspensions. Starting from a microwave-synthesized suspension of ultrasmall (d~2 nm)MoOxnanoparticles in n-butanol, we added H2O2at room temperature to chemically oxidize the nanoparticles. We systematically varied H2O2concentration and reaction time and found that they significantly affected oxidation state and work function ofMoOxnanoparticle films. In particular, we achieved a continuous tuning ofMoOxwork function from 4.4 to 5.0 eV, corresponding to oxidation of as-synthesizedMoOxnanoparticle (20% Mo6+) to essentially pure MoO3. This was achieved without significantly modifying nanoparticle size or stability. Such precise control ofMoOxstoichiometry and work function is critical for the optimization ofMoOxnanoparticles for applications in organic optoelectronics. Moreover, the simplicity of the chemical oxidation procedure should be applicable for the development of other transition oxide nanomaterials with tunable composition and properties.
Highlights
Metal oxide nanoparticles represent a large class of materials with applications in areas such as energy conversion and storage, [1, 2] catalysis, [3, 4] sensing, [5] and biomedicine [6]
We found that microwave heating of molybdenum dioxide bis(acetylacetonate) (MoAcAc) resulted in a brown suspension of npMoOx (Figure 1)
The increase in size with longer reaction time is consistent with our previous observation that the average diameter of chemically oxidized npMoOx from small-angle X-ray scattering (SAXS) had increased to 4 nm after ∼15 days of chemical oxidation [18] and indicates that Ostwald ripening for these nanoparticles only occurs with the addition of H2O2
Summary
Metal oxide nanoparticles represent a large class of materials with applications in areas such as energy conversion and storage, [1, 2] catalysis, [3, 4] sensing, [5] and biomedicine [6]. By matching Φ of the HTL to the highest occupied molecular orbital (HOMO) of the organic electron donor material, [7] MoOx films inserted between the active layer and the anode have been shown to improve performance of organic photovoltaic (OPV) devices [8,9,10,11,12,13,14,15]. Most work in this area use thermally evaporated MoOx films [8,9,10,11]. We examined the effect of chemical oxidation conditions on the size and stability of the npMoOx suspension to evaluate
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