Facile hydrothermal synthesis of α-MnO2 nanorods for low-cost, scalable and stable photoresponsive devices

Abstract

Morphologically well-defined α-MnO2 nanorods were synthesized through a facile one-pot hydrothermal method. The structural, morphological and electronic characteristics were systematically investigated using XRD, XPS, FESEM, FTIR, UV-vis and ultraviolet photoelectron spectroscopy. Analysis confirmed the formation of pure α-phase MnO2, exhibiting n-type semiconducting behavior with an indirect bandgap of ∼1.24 eV (from Tauc analysis) and a work function of 4.53 eV, as determined from UPS. XPS confirms the coexistence of Mn3+, along with Mn4+, in the synthesized MnO2. To evaluate the optoelectronic properties, a simple photoresponsive device was fabricated with an in-plane geometry, where the ITO substrate was patterned via a straightforward etching process to define lateral electrodes, followed by drop-casting α-MnO2 nanorods across the active region. The device exhibited a distinct photoresponse under varying illumination conditions (dark, red, and green lasers at different intensities). Under 532 nm green laser excitation, the photocurrent increased by ∼34%, attributed to enhanced charge carrier separation and electron–hole recombination. The fabricated device demonstrated robust stability over repeated measurement cycles, with response and recovery times of 76.5 s and 77.5 s, respectively, at room temperature. A maximum responsivity of 8.66 mA W−1 at 4 V bias was achieved under 17.2 mW cm−2 green laser illumination, along with an external quantum efficiency (ηEQE) of 2.018% at room temperature. The device shows superior performance at elevated temperatures, demonstrating a response time of ∼12 seconds. At 160 °C temperature, the device shows a responsivity of 240.29 mA W−1 at 4 V bias and an ηEQE of 56.2%, highlighting its applications in next-generation optoelectronic devices. Temperature-dependent measurements confirmed the role of thermally activated carrier transport, revealing enhanced photocurrent at elevated temperatures due to increased carrier mobility. This study establishes α-MnO2 nanorods as a promising platform for cost-effective and stable photoresponsive devices, highlighting the role of band alignment and temperature-activated dynamics in advancing next-generation MnO2-based photodetectors and energy-harvesting systems.