Impact of Ammonium Hydroxide on the Optical Properties of Hydrothermally Grown ZnO Nanowires: A Photoluminescence Investigation

Abstract

We investigate the role of ammonium hydroxide (NH₄OH) as a growth additive in the hydrothermal synthesis of ZnO nanowires (NWs) on seed‑free Au substrates. By systematically varying NH₄OH concentrations (0–50 mM), we achieve more than a two‑order‑of‑magnitude change in NW density. Photoluminescence (PL) studies at room temperature reveal that higher NH₄OH levels introduce additional surface defects, reducing the UV‑to‑green emission ratio (I_UV/I_G). These findings underscore the critical influence of growth chemistry on the defect‑mediated optical response of ZnO NWs, essential for tailoring materials in next‑generation electronic and optoelectronic devices.

Background

Bottom‑up synthesis has become a cornerstone for producing high‑performance nanomaterials that underpin modern electronic and optoelectronic technologies.1–5 The renewed focus on ZnO follows its demonstrated ability to form single‑crystalline nanostructures—such as nanobelts and nanowires—with superior electronic properties.6–9 These one‑dimensional (1D) nanostructures have been leveraged in field‑effect transistors, nanogenerators, sensors, and display technologies, owing to their unique surface‑to‑volume ratios and defect‑controlled optoelectronic behavior.7–13 Defects are the double‑edged sword in ZnO: they can enhance UV sensor performance by trapping carriers and expanding surface depletion layers, yet excessive defects deteriorate nanogenerator efficiency.18,17,19 Therefore, precise control over stoichiometry and defect populations is paramount.20–29 Hydrothermal growth offers a low‑temperature, scalable route to seed‑free ZnO NWs on diverse substrates, including plastics and textiles.30–31 The process typically yields single‑crystalline wurtzite ZnO, but the defect‑related photoluminescence (PL) bands—spanning blue, green, and orange emissions—remain sensitive to growth conditions and remain a subject of active research.32–34 While ammonium hydroxide (NH₄OH) has been employed to modulate nucleation on Au surfaces, its effect on the optical response of the resulting ZnO NWs has not been thoroughly quantified. In this work, we systematically vary NH₄OH concentrations during hydrothermal growth and employ PL spectroscopy to dissect the influence on defect‑related emissions and the UV band‑edge response.

Methods

ZnO NWs were synthesized on 2 × 2 cm² (100) Si wafers. The wafers underwent a 10 min piranha clean (1:1 H₂SO₄/H₂O₂), a 2 min HF dip (50 %) to remove native oxide, and DI water rinses, followed by N₂ drying and a 200 °C bake. A 200 nm Au film was sputtered at room temperature, with a 100 nm Ti adhesion layer underneath. The hydrothermal precursor consisted of a 1:1 molar ratio of zinc nitrate hexahydrate and hexamethylenetetramine. Substrates were placed face‑down in a Teflon cup sealed within a stainless‑steel autoclave and heated at 85 °C for 15 h. After cooling, the samples were rinsed with DI water and dried in N₂. NH₄OH was added at concentrations ranging from 0 to 50 mM. Morphology was examined by a Hitachi S‑4150 SEM. Photoluminescence was measured at room temperature using a 325 nm He–Cd laser (1.5 mW) modulated at 55 Hz. Detailed PL instrumentation is described in Ref. 33.

Results and Discussions

Figure 1a–f illustrate the evolution of ZnO NW morphology with increasing NH₄OH. A more than two‑order‑of‑magnitude increase in density is achieved as the concentration rises to 40 mM, beyond which saturation occurs. The growth mechanism hinges on NH₄OH’s ability to complex Zn²⁺, modulating supersaturation and nucleation density.15,26,27,28,29 Figure 1g plots density and aspect ratio (L/d) versus NH₄OH. Both parameters rise sharply until ~40 mM, where they plateau. Raman spectroscopy confirms the wurtzite crystal structure across all samples. Figure 2a presents the room‑temperature PL spectra. Two dominant features emerge: a near‑band‑edge (NBE) UV peak at 3.24 eV and a broad green defect band at 2.23 eV. Gaussian deconvolution reveals blue, green, and orange defect contributions; the green component dominates, as shown in Figure 2b for the 40 mM sample. The UV‑to‑green intensity ratio (I_UV/I_G)—a key metric for optical quality—decreases progressively beyond 20 mM NH₄OH, dropping threefold at 40 mM relative to the pristine sample (Figure 2c, Table 2). Two plausible drivers are: (i) the increased aspect ratio, which amplifies surface defect density, and (ii) the heightened basicity of the growth medium, which can etch the NW surface and generate additional point defects. To isolate the effect of basicity, we performed post‑growth NH₄OH treatments on pristine NWs (Table 3). SEM images (Figure 3) show progressively roughened surfaces and eventual NW breakage at ≥100 mM, indicating etching. Corresponding PL spectra (Figure 4a) reveal a steady decline in UV intensity while the defect band remains at the same energy, confirming that no new defect levels are introduced but existing NWs are partially removed. The I_UV/I_G ratio (Figure 4b) decreases sharply with treatment concentration, mirroring the trend observed during growth. These results validate the hypothesis that NH₄OH‑induced basicity enhances point‑defect density and degrades optical performance.

Conclusions

We have demonstrated a scalable, seed‑free hydrothermal method for growing ZnO NWs on Au substrates, with NH₄OH enabling tunable NW density over two orders of magnitude. PL analysis reveals that NH₄OH addition increases surface defect density, markedly reducing the I_UV/I_G ratio beyond 20 mM. Post‑growth basic‑solution treatments further corroborate that elevated basicity etches NWs and raises point‑defect levels. These insights provide a practical pathway for engineering the optical properties of ZnO NWs directly on metallic electrodes, advancing their integration into high‑performance electronic and optoelectronic devices.

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Nanomaterials