Ultra‑Fast, High‑Sensitivity Self‑Powered UV Detector Using SnO₂‑TiO₂ Nanomace Arrays

Background

Ultraviolet photodetectors find widespread use in fields ranging from remote control and chemical analysis to water purification, flame detection, missile plume monitoring, and secure space‑to‑space communication [1]. To eliminate costly UV pass filters and achieve visible‑blind operation, wide‑bandgap semiconductors—particularly nanostructured variants such as nanorods, nanowires, nanotubes, and nanobranches—have been extensively studied due to their high surface‑to‑volume ratios and tunable morphologies [2–13]. Photoelectrochemical (PEC) photodetectors fabricated with these nanostructures exhibit high responsivity and rapid transient response compared to conventional photoconductive thin‑film devices. Moreover, PEC architectures bypass the need for complex epitaxial growth and expensive single‑crystal substrates, thereby aligning with the demand for affordable optoelectronic solutions. Consequently, self‑powered UVPDs based on PEC designs have attracted intense attention.

Early self‑powered UVPDs employed the I⁻/I₃⁻ redox couple in liquid electrolyte [14–18] and were often coupled with TiO₂ nanocrystalline films or multilayer TiO₂ nanorod arrays [15, 21]. Although impressive performance metrics were reported, the I⁻/I₃⁻ electrolyte is corrosive, volatile, and photoreactive, limiting long‑term stability. Water‑based electrolytes offer a safer, more stable, and environmentally friendly alternative. For instance, ZnO/CuO heterojunctions combined with Na₂SO₄ aqueous solution demonstrated strong UV‑visible photodetection capability [19], and TiO₂ films interfaced with water achieved high photosensitivity and fast response [20]. Nonetheless, water‑based UVPDs generally exhibit lower responsivity than their I⁻/I₃⁻ counterparts, largely due to TiO₂’s modest electron mobility and the probability of electron–hole recombination in the electrolyte. In contrast, SnO₂ boasts high electron mobility, facilitating rapid electron transport to the transparent conductive oxide (TCO) collector. Recent advances have yielded high‑quality TiO₂/SnO₂ heterojunction nanostructures for optoelectronic applications [17, 22], achieving remarkable performance in UVPDs that employ TiO₂/SnO₂ branched heterojunctions and SnO₂ mesoporous spheres @ TiO₂ electrodes [16, 17]. However, these devices typically rely on disordered nanostructures. Ordered SnO₂‑TiO₂ nanomace arrays with efficient electron transport pathways are expected to further elevate photodetection performance.

In this study, we synthesized ordered SnO₂‑TiO₂ nanomace arrays (STNMAs) via a soft‑chemical route and assembled a self‑powered UVPD employing these arrays as the photoanode and water as the electrolyte. Figure 1 illustrates the schematic of the STNMAs/H₂O UVPD. Vertically oriented STNMAs on fluorine‑doped tin oxide (FTO) glass served as the active electrode. Compared to bare SnO₂ nanotube devices, the STNMA‑based detector delivered higher photocurrent density under UV irradiation. We evaluated spectral photosensitivity, response time, and investigated the influence of TiO₂ branch growth duration on device performance. Optimized STNMAs yielded a responsivity of 0.145 A W⁻¹, a rise time of 0.037 s, a decay time of 0.015 s, and excellent spectral selectivity—all while utilizing a cost‑effective, stable, and eco‑friendly water electrolyte.

The schematic structure of the nanostructured SnO₂‑TiO₂/H₂O solid‑liquid heterojunction‑based UV detector

Methods

Synthesis of SnO₂ Nanotube Arrays

FTO glass (2 cm×2 cm) was ultrasonically cleaned in ethanol and deionized water for 15 min each, then air‑dried. A 10‑nm Sn film was deposited by thermal evaporation and annealed in air at 550 °C for 1 h to form a dense SnO₂ layer. High‑quality ZnO nanorod arrays were fabricated on the SnO₂‑buffered FTO via a two‑step hydrothermal method, as described previously [23]. SnO₂ shell layers were grown on the ZnO nanorods by liquid‑phase deposition: the FTO/ZnO sample was immersed in 0.2 M Na₂SnO₃ aqueous solution at 60 °C for 1 h, then etched in 0.01 M HCl to remove the ZnO template, yielding uniform SnO₂ nanotube arrays (SNAs).

Synthesis of SnO₂‑TiO₂ Nanomace Arrays

TiO₂ nanobranches were grown on the SnO₂ nanotube trunks by a simple aqueous chemical growth method. The SnO₂ nanotube arrays on FTO were immersed in 0.2 M TiCl₄ solution at room temperature for 6, 12, 18, or 24 h to achieve varying branch lengths. The resulting STNMAs were rinsed with deionized water and annealed at 450 °C for 30 min.

Assembly of the UV Detector

The PEC‑type photodetector was assembled analogously to a dye‑sensitized solar cell [24]. The STNMAs on FTO functioned as the active electrode, while a 20‑nm Pt film sputtered onto FTO served as the counter electrode. The two electrodes were face‑to‑face bonded with a 60‑µm sealing layer (SX‑1170‑60, Solaronix SA). Deionized water was injected into the inter‑electrode gap, defining an effective detector area of ~0.2 cm².

Characterization

Crystal structures were examined by X‑ray diffraction (XRD; XD‑3, PG Instruments Ltd.) using Cu Kα radiation (λ = 0.154 nm). Surface morphologies were characterized by field‑emission scanning electron microscopy (FESEM; Hitachi S‑4800) and transmission electron microscopy (TEM; F‑20, FEI). Optical transmittance was measured with a UV‑visible dual‑beam spectrophotometer (TU‑1900, PG Instruments). Spectral response was recorded using a 500‑W Xenon lamp (7ILX500) with a monochromator (7ISW30). Responsivity measurements employed a programmable sourcemeter (2400, Keithley). Photoresponse switching was monitored on an electrochemical workstation (RST5200, Zhengzhou Shirusi).

Results and Discussion

FESEM imaging revealed uniformly grown, vertically oriented SNAs with diameters of 50–80 nm and wall thicknesses <10 nm, yielding a density of ~30 nanotubes µm⁻². When immersed in TiCl₄, the SnO₂ trunks were decorated with TiO₂ nanobranches, forming a nanomace morphology. The branch density and length increased with growth time, reaching an optimal configuration at 18 h. Beyond 24 h, the branches became continuous, diminishing the TiO₂/electrolyte interface and reducing performance.

XRD patterns (Fig. 2f) confirmed the presence of rutile SnO₂ and TiO₂ phases without secondary impurities. Optical transmittance (Fig. 3a) showed a sharp absorption edge near 320 nm for FTO, with red‑shifted edges for 12–24 h STNMAs, indicating enhanced TiO₂ absorption. Transmittance below 305 nm dropped to zero, defining the UV response limit. TiO₂ branches increased light scattering, lowering transmittance in the 400–550 nm range.

Responsivity spectra (Fig. 3b) displayed a peak of 0.145 A W⁻¹ at 365 nm for 18‑h STNMAs, corresponding to an IPCE of 49.2 %. This value surpasses water‑based PEC detectors reported in the literature [20, 23, 24]. The high IPCE stems from enlarged TiO₂ surface area, efficient photon harvesting, rapid hole transport to the TiO₂/water interface, and swift electron collection through high‑mobility SnO₂. Devices with 24‑h branches exhibited reduced responsivity due to the interconnected network limiting TiO₂/electrolyte contact.

Time‑resolved photocurrent measurements (Fig. 4) under 365 nm illumination (129 µW cm⁻²) demonstrated stable ON/OFF switching across five cycles. The rise and decay times—0.037 s (90 % of steady state) and 0.015 s (1/e decay)—were markedly faster than many reported self‑powered UVPDs, underscoring the rapid charge‑transfer dynamics afforded by the core‑shell architecture.

Energy band alignment (Fig. 5) elucidates the device operation: photoexcited electrons in TiO₂ are driven into SnO₂, where their high mobility facilitates transport to the FTO collector. Holes migrate to the TiO₂ surface, reacting with OH⁻ in water to form OH· radicals, while the Pt counter electrode catalyzes the reduction of oxidized species, completing the circuit and enabling self‑powered operation.

SEM and TEM images and XRD patterns of SnO₂ nanotube arrays and SnO₂‑TiO₂ nanomace arrays. a High‑magnification top‑view SEM image of SnO₂ nanotube arrays. b SEM image of 6‑h‑grown STNMAs. c SEM image of 12‑h‑grown STNMAs. d SEM image of 18‑h‑grown STNMAs. e SEM image of 24‑h‑grown STNMAs. f X‑ray diffraction patterns of the substrate, SnO₂ nanotube arrays, and STNMAs. g TEM image of bare SNA. h TEM image of 18‑h‑grown STNMAs

UV‑visible transmittance spectra and responsivity spectrum of photodetectors. a Spectrum of transmittance for FTO glass substrate, SNAs, and STNMAs with different growth time. b Responsivity spectrum of photodetectors based on SNAs and STNMAs

Time response of the STNMAs/water UV detector. a Photocurrent response under on/off radiation of 129 µW cm⁻² UV light illumination. b Enlarged rising and c decaying edge of the photocurrent response

Schematic energy band diagram and the electron‑transfer processes for the STNMAs/H₂O heterojunction

Conclusions

We successfully fabricated SnO₂‑TiO₂ nanomace arrays via a soft‑chemical route and integrated them into a self‑powered UV detector with water as the electrolyte. The core–shell design accelerates electron–hole separation, expands TiO₂ surface area, and leverages SnO₂’s high electron mobility, culminating in an IPCE of 49.2 % at 365 nm—over tenfold higher than that of bare SnO₂ nanotube devices. The detector also features a rapid rise time (0.037 s) and excellent spectral selectivity. These attributes position the STNMA‑based photodetector as a promising platform for high‑performance, environmentally friendly UV sensing, and the underlying architecture may be extended to other photoelectrochemical technologies such as dye‑sensitized solar cells and photoelectrochemical hydrogen production.

Abbreviations

FTO

Fluorine‑doped tin oxide

IPCE

Incident photon‑to‑current conversion efficiency

PEC

Photoelectrochemical cell

SEM

Scanning electron microscope

SNAs

SnO₂ nanotube arrays

STNMAs

SnO₂‑TiO₂ nanomace arrays

TEM

Transmission electron microscope

UV

Ultraviolet

UVPDs

Ultraviolet photodetectors

XRD

X‑ray diffraction