Block Copolymer-Templated, Single-Step Synthesis of Transition Metal Oxide Nanostructures for Sensing Applications

Abstract

The synthesis of transition metal oxide nanostructures, thanks to their high surface-to-volume ratio and the resulting large fraction of surface atoms with high catalytic activity, is of prime importance for the development of new sensors and catalytic materials. Here, we report an economical, time-efficient, and easily scalable method of fabricating nanowires composed of vanadium, chromium, manganese, iron, and cobalt oxides by employing simultaneous block copolymer (BCP) self-assembly and selective sequestration of metal-organic acetylacetonate complexes within one of the BCP blocks. We discuss the mechanism and the primary factors that are responsible for the sequestration and conformal replication of the BCP template by the inorganic material that is obtained after the polymer template is removed. X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (PXRD) studies indicate that the metal oxidation state in the nanowires produced by plasma ashing the BCP template closely matches that of the precursor complex and that their structure is amorphous, thus requiring high-temperature annealing in order to sinter them into a crystalline form. Finally, we demonstrate how the developed nanowire array fabrication scheme can be used to rapidly pattern a multilayered iron oxide nanomesh, which we then used to construct a prototype volatile organic compound sensor.

Keywords: block copolymers; directed self-assembly; gas sensors; solvent evaporation annealing; transition metal oxide nanowires.

Figure 1

Schematic of the single-step rapid fabrication of metal oxide nanostructures using BCP templating. (a) The composition of the casting solution, (b) simultaneous spin-casting and ordering, and (c) an SEM image of the iron oxide nanowires that were obtained by casting Fe(acac)₃/C116 with a 1:2 Fe:VP ratio from the 20% DMOT–toluene mixture followed by O₂ plasma ashing.

Figure 2

SEM morphologies of transition metal oxide nanostructures derived from cylindrical PS-b-P2VP of 275 kg mol^–1 (C275) blended with (a) iron(III), (b) manganese(III), (c) vanadium(III), (d) chromium(III), and (e) cobalt(III) acetylacetonates at various Me:VP stoichiometric ratios. All of the samples were prepared by spin-casting from a 1.5% BCP/10% TMOT–toluene mixture at room temperature. The insets span 400 × 400 nm.

Figure 3

The morphologies of the cobalt oxide nanostructures obtained from cylindrical PS-b-P2VP C116 at different metal loadings (indicated as Co:VP ratios in the gray tabs), cast from a 1% BCP/10% TMOT–toluene mixture at elevated temperatures: (a) 40 °C, (b) 60 °C, and (c) 80 °C.

Figure 4

The evolution of the iron oxide replica with the thickness of the as-cast films of C116 at (a) 1:2 and (b) 1:1 Fe:VP metal loadings, along with the thickness measurement standard deviation values. The films yielding a horizontal monolayer of nanowires are outlined in green.

Figure 5

X-ray photoelectron spectra of the Me(acac)₃-infused BCP templates at a 1:2 Me:VP metal loading ratio (black curves) and the inorganic nanowire replicas that were obtained after plasma ashing the organic material (green curves). Spectra of freshly cleaned metallic foils (orange curves) and oxidized metal surfaces (red curves) are shown as references. The vertical dashed lines mark the binding energy of the reference materials reported in the literature. Me = (a) Fe, (b) Co, (c) Mn, (d) Cr, and (e) V. The intensities of the XP spectra were normalized.

Figure 6

XRD patterns of the Fe(acac)₃-infused C116 sample (Fe:VP of 1:1) cast on a silicon wafer from 10% TMOT in toluene after plasma ashing (black curve) and after being further annealed in air in a stepwise manner with 1 h dwells at 250, 400, and 550 °C (orange, blue, and green curves, respectively), with diffractograms registered at the end of each step with identified indices of the reflexes. The simulated diffraction pattern based on the α-Fe₂O₃ oxide CIF entry, along with the JCPDS card number, is shown at the bottom of the graph in red. The asterisks and triangles mark the reflexes originating from the silicon substrate and the silver heater, respectively.

Figure 7

The C116-templated metal oxide nanowire gas sensor. (a) A false-color SEM image of the testing device having Au contact electrodes deposited on top of the porous (100 nm thick) metal oxide nanomesh. (b) A magnified SEM image that shows the boundary between the Au electrode and α-Fe₂O₃, which was annealed at 550 °C for 1 h. (c) The electrical response of the unannealed sensor to ethanol vapor, which was delivered in a stream of dry nitrogen at 10, 20, 30, and 50 ppm at 300 °C. (d) A comparison of the response (R₀/R) of the unannealed sensor material (black points) and the sensor material after 1 h of annealing at 550 °C (red points) at different operating temperatures, measured 120 s after introducing the vapor into the test chamber. The reference ethanol concentrations that are indicated in (c) are specified within a ±15% confidence band due to the accuracy of the external sensor being subject to additional ±5% readout fluctuations. The error bars represent the StD of the signal readout within ±30 s from the central point value.