Dominant Role of Heterojunctions in Gas Sensing with Composite Materials

Abstract

It is well known that composite materials, consisting of at least two metal oxides, show qualities and sensing behavior very different from the single components. Recently, the preparation of core–shell nanomaterials for gas sensors has become extremely popular. Specifically, these materials have been found to show desirable sensor responses. The preparation of core–shell nanomaterials is, however, complex, limiting the commercial applicability. Composite materials can be more easily attained simply through the mechanical mixing of the various components. Although some studies exist that attempt to compare mechanically mixed composites to those prepared via a synthetic route, these examinations are often flawed, as due to varying preparation methods, the basic characteristics of the materials are not the same. Here, it was possible to separate the role of the contacts between the materials from that of the secondary core–shell structure, by using a soft method to mechanically break apart the structure. This ensures that the difference in morphology is the only change in the material characteristics. It was verified that the composite materials show a different sensing behavior from that of the pure materials. It was also found that regardless of the secondary structure, the composite materials showed very similar sensor responses. By examining materials containing different ratios of Cr2O3 to SnO2, it was possible to attribute the sensor behavior changes to the contacts between the different metal oxides. It was shown that by varying the concentration of each oxide it is possible to attain either an n- or p-type response and at a certain concentration even no response. This work is significant because it identifies that the contact between the materials plays the dominant role in the sensor response and it shows the viability of mechanical mixing for composite sample preparation.

On the Role of Heterojunctions of Core-Shell Heterostructures in Gas Sensing

“…The broad diffraction peaks show the nanocrystalline nature of the NiO deposited film. [4] This is in-line to our already reported results for the ALD-NiO on different substrates. As expected for this very thin film of NiO with 200 ALD cycles (ca.…”

“…[1g,3a,b,d,4-5] Indeed, the sensing responses of heterostructured materials discussed in the literature are very disparate and not always related to the p-n semiconductor heterojunction, [1e,h,3b,4] but could be limited to the core or the shell material. [4,6] For example, it was demonstrated that even by depositing a second material conformally onto already contacted (core-core junctions) 1D SnO 2 nanowires (SnO 2 -core) or carbon nanotubes (CNTs-core) modifies both the width and the thickness of the depletion region at the interface, but the sensing responses were exclusively attributed to the core materials. [1j,5b,7] In that case, the shell-layers modified the surface of the already fabricated devices without dramatically modifying the underlying transduction mechanism.…”

Room Temperature Syntheses of ZnO and Their Structures

“…Metal and mixed metal and semiconductor oxides are widely applied in several fields, as supercapacitors [1][2][3], gas sensors based on chemoresistivity [4][5][6][7][8] or dielectric excitation [9,10], heterojunctions [11,12], energy storage in batteries [13][14][15], and electrochemistry [16][17][18]. Their properties can be varied in different ways, by doping [19,20], capping, and controlling the structure, shape, and size with bottom-up and top-down approaches [21][22][23][24][25][26][27][28].…”

Heteronanostructural metal oxide-based gas microsensors

“…When the size and shape of the grains are uniform in terms of donor density and semiconductor type, the response results should remain the same if the contact geometry is unchanged 110 . Staerz et al 111 reported that the sensing performance of SnO 2 -Cr 2 O 3 core-shell nanofibers (CSNs) and crushed SnO 2 -Cr 2 O 3 CSNs are practically identical, indicating that the nanofiber morphology shows no advantages if the characteristics of the samples remain unchanged.…”