Work Function Tuning in Hydrothermally Synthesized Vanadium-Doped MoO3 and Co3O4 Mesostructures for Energy Conversion Devices

Abstract

The wide interest in developing green energy technologies stimulates the scientific community to seek, for devices, new substitute material platforms with a low environmental impact, ease of production and processing and long-term stability. The synthesis of metal oxide (MO) semiconductors fulfils these requirements and efforts are addressed towards optimizing their functional properties through the improvement of charge mobility or energy level alignment. Two MOs have rising perspectives for application in light harvesting devices, mainly for the role of charge selective layers but also as light absorbers, namely MoO3 (an electron blocking layer) and Co3O4 (a small band gap semiconductor). The need to achieve better charge transport has prompted us to explore strategies for the doping of MoO3 and Co3O4 with vanadium (V) ions that, when combined with oxygen in V2O5, produce a high work function MO. We report on subcritical hydrothermal synthesis of V-doped mesostructures of MoO3 and of Co3O4, in which a tight control of the doping is exerted by tuning the relative amounts of reactants. We accomplished a full analytical characterization of these V-doped MOs that unambiguously demonstrates the incorporation of the vanadium ions in the host material, as well as the effects on the optical properties and work function. We foresee a promising future use of these materials as charge selective materials in energy devices based on multilayer structures.

Design Principles and Insights into the Liquid-Phase Exfoliation of Alpha-MoO3 for the Production of Colloidal 2D Nano-inks in Green Solvents

“…To further investigate the variation in the electronic properties of α-MoO 3 following LPE, KPFM was performed to probe the work function (WF) of the material. The KPFM data on the samples with the largest optical band gap (colloids in H 2 O and IPA/H 2 O) show an average value for the WF of 4.56 ± 0.56 eV, which appears decreased as compared to the values obtained from the bulk material of 5.11 ± 0.50 eV, indicating a shift of the Fermi level toward the vacuum level, as expected following quantum confinement toward the 2D form …”

“…UV–vis absorption spectra were acquired for all the LPE α-MoO 3 colloidal inks, as summarized in Figure , to verify whether a change in the energy of the optical band gap ( E gap ) of α-MoO 3 can be detected following exfoliation, applying the Tauc method , to calculate its value at the absorption onset (all E gap values are reported in Table ).…”

“…In addition to varying the liquid medium, the type of instrument (either SM or TS) and the exfoliation time (2, 4, 8 h) were also tuned to perform a systematic study and identify the optimal conditions for the LPE of α-MoO 3 . Figure 1 summarizes the overall process and different conditions applied: powder bulk α-MoO 3 produced via hydrothermal synthesis 46 dispersed in the selected liquid media undergoes LPE in the different established conditions, and the final mixture after exfoliation is subjected to multiple steps of liquid cascade centrifugation at an increasing speed to selectively precipitate the non-exfoliated products (pellets) before obtaining a final colloidal ink of the most exfoliated α-MoO 3 material, used for further characterizations (see the experimental details of this process in the Materials and Methods section).…”

“…All chemicals were purchased from Sigma-Aldrich and used without any further purification. α-MoO 3 was synthesized using an already reported procedure developed by some of us . Technical grade isopropanol (IPA) was used for LPE.…”

“…α-MoO 3 was synthesized using an already reported procedure developed by some of us. 46 Technical grade isopropanol (IPA) was used for LPE. UV−visible (UV−vis) absorption spectra of the colloidal inks were recorded on a Goebel Uvikon spectrometer using a quartz cuvette of 1 cm optical length.…”