Boron Induced c-Axis Growth and Ammonia Sensing Signatures of Spray Pyrolysis Deposited ZnO Thin Films: Relation between Crystallinity and Sensing

Research summary

Zinc oxide (ZnO) thin films are widely studied for gas sensing, and doping them with other elements is a common way to tune their properties. Boron is an attractive dopant because of its strong metal-oxide bonding and oxygen-vacancy behavior, but in earlier work — including films made by spray pyrolysis, the same deposition method used here — adding boron typically distorted the ZnO crystal lattice and reduced crystallinity. This study set out to find deposition conditions where boron doping could instead be used constructively, and to see whether the resulting films could sense ammonia gas at room temperature.

What the study examined

• ZnO thin films doped with 2, 4, 6, 8 and 10 mol% boron, deposited on glass by spray pyrolysis under optimized (slower spray rate, lower substrate temperature) conditions
• Crystal structure and orientation via X-ray diffraction, and film morphology via electron microscopy
• Optical absorbance and band gap across the doping range
• Room-temperature sensing response and selectivity toward ammonia and other volatile organic compounds (formaldehyde, acetaldehyde, ethanol, acetone)

Main findings

Under the optimized deposition conditions, increasing boron content from 2 to 10 mol% progressively strengthened the film’s preferred growth along the (002) crystal plane (the c-axis), rather than degrading crystallinity as boron doping usually does. At the lowest boron level (2 mol%), boron atoms appeared to occupy interstitial sites in the lattice, increasing crystallite size; at higher concentrations, boron increasingly substituted into zinc lattice sites, producing tensile stress along the c-axis and a corresponding narrowing (red shift) of the optical band gap.

Gas-sensing tests showed that all the films responded to some degree to the tested gases, but the 10 mol% boron-doped film gave by far the strongest and most selective response — a sensing response of about 1620 toward 100 ppm ammonia at room temperature, well ahead of plain ZnO and the other doping levels. The authors attribute this selectivity to how well the adsorption energy of the (002) plane matches the N-H bond in ammonia, compared with the O-H and C-H bonds involved in adsorbing the other test gases.

Why it matters

The results show that boron doping does not have to come at the cost of crystallinity in spray-pyrolysis ZnO films — with the right deposition conditions, it can instead be used to engineer a more oriented crystal structure that is specifically favorable for ammonia detection. Because the sensing occurs at room temperature rather than the elevated temperatures many metal oxide gas sensors require, the approach is relevant to low-power, low-cost ammonia sensors for applications such as air-quality or industrial safety monitoring.