Highlights
- Varying the zinc acetate to HMTA molar ratio produced three distinct ZnO morphologies: nanoprisms, nanoparticle-decorated nanoprisms, and flat-topped nanodisks.
- HMTA was shown to play a non-mutually-exclusive dual role, capping both lateral and vertical ZnO crystal growth via steric hindrance.
- ZnO nanoparticle-decorated nanoprisms gave the best selectivity toward acetaldehyde among the tested vapors.
- That morphology reached a response of 9.5 toward 100 ppm acetaldehyde with 13 s and 9 s response and recovery times.
Abstract
This study examines how hexamethylenetetramine (HMTA), a common additive in hydrothermal ZnO growth, plays two simultaneous roles in shaping ZnO nanostructures on glass substrates. Varying the molar ratio of zinc acetate to HMTA produced ZnO nanoprisms, ZnO-nanoparticle-decorated nanoprisms, and flat-topped hexagonal nanodisks, showing that protonated HMTA acts as a capping agent that steers both lateral and vertical crystal growth through steric hindrance. Among the morphologies, the nanoparticle-decorated nanoprisms gave the best room-temperature selectivity toward acetaldehyde over ethanol, ammonia, and acetone, with a response of 9.5 toward 100 ppm and response and recovery times of 13 and 9 seconds, attributed to their higher density of structural junctions.
Research summary
Hexamethylenetetramine (HMTA) is a common growth-control additive in hydrothermal synthesis of ZnO nanostructures, but exactly how it shapes crystal growth is not always clear. This study investigates HMTA’s role by growing ZnO nanostructures on glass substrates (topped with a sputtered ZnO seed layer) via a hydrothermal method, while systematically varying the molar ratio of zinc acetate to HMTA, and then testing the resulting structures as room-temperature gas sensors.
What the study examined
- ZnO nanostructures grown hydrothermally under different zinc acetate to HMTA molar ratios
- Morphology of the resulting structures, using electron microscopy
- The mechanism by which HMTA influences lateral versus vertical crystal growth
- Room-temperature sensing response and selectivity of each morphology toward acetaldehyde, ethanol, ammonia, and acetone
Main findings
Varying the HMTA concentration produced three distinct ZnO morphologies: plain nanoprisms, nanoparticle-decorated nanoprisms, and flat-topped hexagonal nanodisks. The results show that protonated HMTA acts as a capping agent with a steric hindrance effect that influences both lateral and vertical growth of ZnO nuclei simultaneously – a non-mutually-exclusive dual role, rather than HMTA simply favoring one growth direction over another.
Among the three morphologies, the nanoparticle-decorated nanoprisms gave the best gas-sensing performance, with good selectivity toward acetaldehyde over ethanol, ammonia, and acetone. This structure reached a response of 9.5 toward 100 ppm acetaldehyde, with response and recovery times of 13 and 9 seconds. The authors attribute this to the higher density and variety of structural junctions where nanoparticles meet the nanoprism surface, which create additional depletion regions and potential barriers that enhance charge-transfer sensitivity to surface gas reactions.
Why it matters
By pinning down HMTA’s dual growth-control mechanism, the study gives researchers a clearer handle on how to engineer ZnO nanostructure morphology deliberately rather than empirically. It also identifies nanoparticle-decorated nanoprisms as a structurally favorable design for selective, room-temperature acetaldehyde sensors, a useful reference point for future ZnO-based gas-sensor development.
Citation
H. Avireddy, H. Kannan, Prabakaran Shankar, G. K. Mani, A. J. Kulandaisamy, J. B. B. Rayappan. Non-Mutually Exclusive Dual Role of Hexamethylenetetramine on the Growth of ZnO Nanostructures and Their Sensing Footprints. Materials Chemistry and Physics 212 (2018) 394-402.
