Catalyst-Free Growth of ZnO Nanowires Based on Topographical Confinement and Preferential Chemisorption and Their Use for Room Temperature CO Detection

Abstract

Two novel approaches for patterned growth of ZnO nanowires (NWs) based on the selective deposition of zinc acetate (ZA) solution precursors are presented and compared. The first is using the topographical confinement within a photoresist pattern on Si/SiO 2 substrates (type I), and the second is using preferential chemisorption on self-assembled monolayer modified Au electrodes on Si/SiO 2 substrates (type II). In both approaches, the ZnO seeds from the ZA solution form crystallites without severe defects over a large area via annealing at 350°C. These seed layers were used to grow ZnO NWs via a catalyst-free vapor-phase deposition method using temperatures of up to 900°C. The presented method is effective for realizing NW growth on both Si and Au electrodes by preserving the electrode configuration even at such high temperatures used for NW growth, which is a novelty and crucial for future sensor applications. As a result NWs connected through metal contact pads or electrodes have been realized in very simple and effective way. As a test of principle the resulting configurations were used to demonstrate a highly sensitive room temperature CO sensor. A CO concentration as low as 120 ppb was detected using both types of sensors. The type II sensor exhibited enhanced sensing properties compared to that of type I.

Oxidation behaviour of carbon monoxide at the photostimulated surface of ZnO nanowires

“…Furthermore, due to the increased surface area of such arrays of ZnO nanowires, the gas sensing properties are expected to be enhanced. The structural properties of the as grown ZnO nanowire arrays have been reported elsewhere [17]. .29 eV, which is in agreement with the reported value in literature [18].…”

“…The self-assembled monolayers (SAMs) of MPA, possessing two functional groups, namely, thiol and carboxylic acid, were selectively formed on the gold lines and used to discriminate between Au and Si surface as well as to interact with cationic Zn species in the solution phase. The growth mechanism has been explained in our previous work [14]. The distance between two Au lines was 2 µm.…”

“…The ZnO nanowires were grown using the vapor-solid (VS) method [14] and characterized by highresolution scanning electron microscopy (SEM, Nova NanoSEM, FEI). First, the Si substrate was patterned with Au electrodes by conventional photolithography, physical vapor deposition (Au: 100 nm, Cr: 5 nm) and lift off processing, followed by surface modification of Au by immersing the substrates into a 1 mM ethanol solution of 2-mercaptopropionic acid, MPA (Sigma Aldrich, 99.8%) for 5 h and subsequent washing with ethanol to remove excess MPA.…”

“…In our previous work, we enabled the surface reaction of CO at room temperature by photostimulation [14]. Here, the high energetic photon replaces the high temperature process normally used in high-temperature gas sensors.…”

“…Comment #2: "… why is there no particle seed formation on the bare Si in between the Au lines" Answer #2: The growth mechanism has been shortly explained in the text and a literature has been given: "The self-assembled monolayers (SAMs) of MPA, possessing two functional groups, namely, thiol and carboxylic acid, were selectively formed on the gold lines and used to discriminate between Au and Si surface as well as to interact with cationic Zn species in the solution phase. The growth mechanism has been explained in our previous work [14]." Comment #3: "Why is there no Au-supported growth on the metal lines without the particle layer at the high temperatures used.…”