Tailoring ZnO thin films via ALD: Impact of cycle number on structural, optical, wettability, photocatalytic and morphological properties

Zinc oxide (ZnO) thin films were deposited on glass substrates by atomic layer deposition (ALD), with the number of cycles varied from 50 to 500 to control film growth and properties. The thickness increased from 12.5 to 100.75 nm with increasing cycle number. X-ray diffraction and Raman spectroscopy revealed a structural evolution from amorphous to polycrystalline wurtzite ZnO, accompanied by enhanced crystallinity at higher cycles. Atomic force microscopy showed a non-linear variation in surface roughness, with a maximum at 100 cycles and a minimum at 500 cycles, along with an increased fractal dimension (FD = 2.753). Optical transmittance in the visible region decreased from over 91% to 78.6%, while the optical band gap widened from 3.23 to 3.42 eV. The refractive index increased with deposition cycles, indicating improved film density. Contact angle measurements revealed a transition from hydrophilic to hydrophobic behavior, attributed to smoother surfaces and reduced surface energy. Photocatalytic degradation of methylene blue under UV illumination reached a maximum efficiency of 33% at 250 cycles, due to an optimal balance between grain size, surface roughness, and defect density. Kinetic analysis followed a pseudo-second-order model (R 2 ≈ 1), suggesting surface-controlled reaction kinetics. These results highlight the effectiveness of ALD cycle tuning for optimizing ZnO thin films for photocatalytic and optoelectronic applications. • ZnO film thickness increases from 12.5 nm (50 cycles) to 100.7 nm (500 cycles) with ALD cycle number, enabling precise control of crystallinity and morphology. • XRD and Raman confirm the transition from amorphous to polycrystalline wurtzite structure, with enhanced ordering at ≥250 cycles. • Optical transmittance decreases from >91% to 78.6%, while the bandgap widens from 3.23 eV to 3.42 eV and refractive index increases with thickness. • Contact angle increasess with cycle number, indicating a shift from hydrophilic to hydrophobic behavior due to grain compaction. • Photocatalytic efficiency peaks at 250 cycles (33% MB degradation), then declines at 500 cycles due to dense, low-porosity surface. • Optimal performance is achieved by balancing crystallinity, defect levels, Eg tuning, and surface wettability.