Controlling Positive and Negative Photoconductivity in WO3 Nanowires via Water‑Adsorption‑Induced H⁺ Ion Dynamics

Abstract

In Au/WO₃/Au devices, a pronounced negative photoconductivity (NPC) emerges in humid environments, attributable to the surface accumulation of H⁺ ions. When illuminated with 445‑nm violet light, photogenerated holes oxidise adsorbed H₂O, producing H⁺ and O₂. The photo‑excited electrons lack sufficient energy to reduce these H⁺ ions, which then capture mobile electrons, increasing the Schottky barrier and reducing conductance. By tuning relative humidity (RH), light intensity, or bias voltage, the H⁺ concentration and distribution can be modulated, enabling controlled conversion between positive photoconductivity (PPC) and NPC, and optimizing resistive switching behaviour.

Introduction

Tungsten trioxide (WO₃) is renowned for its photo‑electro‑chromic and resistive‑switching capabilities, owing to its unique octahedral WO₆ framework that hosts interstitial species such as H⁺ and alkali ions, thereby tuning its color and conductivity from insulating to metallic regimes. The conduction‑band minimum lies below the H⁺ reduction level, while the valence‑band maximum exceeds the H₂O oxidation level, allowing surface‑adsorbed H₂O to be photo‑oxidised into H⁺ and O₂ by holes, yet precluding electron‑driven reduction of H⁺. This inter‑ion dynamics is central to WO₃ photo‑chromism and resistive switching under ambient conditions.

Single‑crystalline hexagonal WO₃ nanowires (h‑WO₃ NWs) offer an exceptionally large surface area and a well‑defined conductive channel, making them ideal for probing H⁺ ion behaviour. Prior work has demonstrated memristive effects in these nanowires, markedly enhanced by H⁺ ions generated from adsorbed H₂O. Here, we systematically examine the photoconductivity of h‑WO₃ NWs under varying RH, revealing simultaneous PPC and NPC phenomena that can be steered by humidity, illumination, and bias.

Methods

WO₃ Nanowire Synthesis

Hexagonal WO₃ nanowires were synthesized via a hydrothermal route. Briefly, 8.25 g Na₂WO₄·2H₂O was dissolved in 250 mL DI water; the pH was adjusted to 1.2 with 3 M HCl. After filtration and washing with water and ethanol, the precipitate was dispersed in 200 mL 0.1 M citric acid to form a translucent WO₃ sol. 45 mL of this sol, supplemented with 1.3 g K₂SO₄, was sealed in a 50 mL autoclave and heated at 240 °C for 32 h, then cooled, filtered, washed, and dried at 60 °C.

Device Fabrication

Devices were fabricated on heavily n‑doped Si/SiO₂ (100 nm) substrates. Individual h‑WO₃ NWs were positioned between Au electrodes patterned by standard photolithography and 100‑nm Au deposition, followed by lift‑off.

Electrical Measurement

Transport measurements were carried out at room temperature in a custom vacuum chamber (base pressure <10⁻¹ Pa). RH was controlled by humidifying/dehumidifying DI water, with ±1 % sensor accuracy. Current–time (I‑T) and current–voltage (I‑V) data were recorded using a Keithley 2602 system.

Results and Discussion

Figure 1 illustrates typical I‑T curves of an Au/h‑WO₃/Au device under 445‑nm, 500 mW illumination at various RH levels. At 40 % RH (Fig. 1a), illumination induces a modest current increase (PPC), consistent with band‑to‑band transitions. Increasing RH to 50 % (Fig. 1b) introduces a delayed drop in photocurrent—signalling NPC. At 60–70 % RH (Fig. 1c,d), NPC becomes robust and stable. The dark currents remain ~80 nA across RH, indicating that NPC is not a trivial consequence of H₂O desorption but arises from H⁺ ion accumulation.

To probe the conduction mechanism, I‑V sweeps were converted to I‑T plots and analysed via the Schottky law (ln I ∝ V^1/2). Under illumination at 70 % RH, the extracted Schottky barrier height increases relative to dark, confirming that NPC originates from a light‑induced barrier rise.

The underlying mechanism can be summarised as follows: under high RH, continuous photo‑oxidation of adsorbed H₂O generates H⁺ ions that accumulate at the WO₃ surface. These ions capture conduction electrons, forming an electric double layer that depletes carriers and elevates the interface barrier, thus reducing conductance (NPC). At lower RH (<50 %), insufficient H₂O layers limit H⁺ production, so PPC dominates.

Power‑dependent I‑T measurements (Fig. 3) further substantiate this model. At 200 mW, PPC dominates; as power rises to 300–500 mW, NPC appears after a brief PPC phase, with the transition occurring more rapidly at higher intensities. At 600 mW, photocurrent fluctuates due to a competition between H⁺ generation and reduction by hot electrons. These observations illustrate that H⁺ ion production is governed by inter‑band transition efficiency, which scales with light intensity.

Bias‑voltage dependence (Fig. 4) shows that at 50 % RH, NPC is most stable at 2 V but becomes increasingly erratic at higher biases. The band diagram (Fig. 4d) explains this: increased bias lowers the barrier for hot electrons to reduce H⁺ ions near the negatively biased electrode, leading to oscillatory H⁺ accumulation and conductance fluctuations.

Conclusions

We have demonstrated that Au/h‑WO₃/Au nanowire devices exhibit controllable NPC under humid conditions, moderate illumination, and low bias, driven by surface H⁺ ion accumulation that raises the Schottky barrier. By precisely tuning RH, light intensity, and bias, one can modulate H⁺ ion distribution, enabling dynamic switching between PPC and NPC and optimizing resistive‑switching performance. These insights advance the fundamental understanding of ion‑mediated photoconductivity in WO₃ and open pathways for humidity‑sensitive optoelectronic and memory applications.