Low-temperature strategy for vapor phase hydrothermal synthesis of C\N\S-doped TiO₂ nanorod arrays with enhanced photoelectrochemical and photocatalytic activity

Abstract

In this study, a material with high photocatalytic activity was synthesized using ternary C/N/S-doped TiO₂ nanorod array (TiO₂); this was done using a practical and straightforward vapor-phase hydrothermal (VPH) method at a low temperature. The effect of C/N/S content on TiO₂ morphology, optical, photocatalytic and photoelectrochemical (PCE) properties of the material was investigated by varying the quality of thiourea. C/N/S-TiO₂ reduced the bonding rate of electron-hole pairs and enhances visible light absorption, photocatalytic, and PCE properties. The C/N/S doping could significantly adjust the absorption cut-off wavelengths (407−602 nm) and shorten the bandgap (3.04−2.18 eV) of TiO₂. Under simulated sunlight, 8-C/N/S-TiO₂ had the highest photocatalytic efficiency of 97.6% for methylene blue (MB) in 150 min with a rate constant of 0.0192 min⁻¹, which is approximately four times that of TiO₂ (0.005 min⁻¹). The 8-C/N/S-TiO₂ photoelectrode had the lowest transfer resistance for interfacial charges and highest transient photocurrent of 33.5 μA/cm², which is five times higher than that of TiO₂ (6.6 μA/cm²). The 8-C/N/S-TiO₂ exhibits the most extensive PCE behavior as a photoelectrode, and has a current density of 38.2 mA/cm² at 2.5V_RHE, which is about two times higher than TiO₂ (19.1 mA/cm²). The favorable sunlight-driven photocatalytic activity is probably due to the synergistic effect of C/N/S-doping, which shifts the valence band maximum of TiO₂ upward. This provides new ideas for future solar cells that can use dye-sensitized TiO₂ nanorod arrays as photoanodes. It is noteworthy that VPH is a very effective strategy for fabricating semiconductors doped with multiple nonmetallic elements.

Introduction

Since it was discovered that semiconductor photocatalysis technology can be used to oxidize and decompose pollutants, research on semiconductor photocatalysis has received increasing attention at home and abroad [1], [2], [3], [4]. It has been confirmed that semiconductor photocatalysis can be used for the degradation of organic wastewater, reduction of heavy metal ions, air purification, and sterilization. Stable, cheap, and high-performance semiconductor catalysts are the foundation of photocatalytic technology. Among the numerous semiconductor photocatalysts, Titanium Dioxide (TiO₂) has become a representative semiconductor photocatalyst and an example of next-generation spintronics with excellent performance. Because sunlight contains a small number of ultraviolet photons (about 5%), photocatalytic materials made with the wide-band gap bare TiO₂ (3.0 eV for rutile and 3.2 eV for anatase) primarily absorb ultraviolet photons (low utilization of sunlight) [3], [5]; thus, efforts have been made to broaden the absorption spectrum of TiO₂. Common methods for optimizing the photocatalytic properties of TiO₂ semiconductors include semiconductor composites, noble metal deposition, semiconductor surface photosensitization, and ion doping. Among them, ion doping optimizes semiconductor performance, which not only causes a blue-shift and a red-shift in the absorption spectra but also significantly improves the conductivity of semiconductors and effectively reduces the recombination of carriers, thereby improving the photocatalytic performance of the semiconductor. The optoelectronic properties of doped semiconductors are much better than those of intrinsic semiconductors. For example, the doping of one impurity atom in 100,000 silicon atoms can increase the electrical conductivity of silicon by approximately 1000 times. Intrinsic semiconductors exhibit semiconductor properties at higher temperatures only, whereas doped semiconductors can have good semiconductor properties at room temperature. In recent years, an increasing number of articles have been published on non-metal doping. various spectroscopic techniques, surface analysis techniques, and theoretical calculations have elucidated the electronic structure, optoelectronic properties, and structural stability of nonmetal -doped TiO₂, which are of key importance in the photocatalytic field [6], [7], [8].

The development of low-cost and high-efficiency photocatalysts for the degradation of water pollution has long been pursued for efficient use of the solar spectrum. According to the literature, TiO₂ materials doped with non-metals (S, N, F, and C) are good semiconductor photocatalytic materials [8], [9], [10], [11]. L. Rizzoa et al. [11] reported in 2014 that N ion-doping through the sol-gel technique can effectively narrow the bandgap of TiO₂ by approximately 0.8 eV effectively. Therefore, the performance of TiO₂ photocatalysts has been extensively improved using visible light. Sakthivel et al. [12] reported that C-TiO₂ degrades 4-chlorophenol four-times more than bare TiO₂ in simulated sunlight. This indicates that the C-doped TiO₂ extended into the visible spectral range. Cations S⁴⁺ and S⁴⁺-doped TiO₂ show minimal optical absorption, and the optical properties are not prominent. However, S²⁻ ion-doping gives TiO₂ a broad absorption spectrum (400−700 nm) [13]. Several studies have reported the doping of TiO₂ with C [14], P [15], or F [16] atoms to improve the photocatalytic activity. In recent years, the co-doping of TiO₂ with two elements, S/N [6], [17], C/N [18], [19], N/P [20] and N/F [21] was found to enable a higher light absorption compared with single ion-doped TiO₂ [6]. For example, Chen et al. reported that C, N ions co-doped TiO₂ prepared via the sol–gel approach exhibited higher photocatalytic activity than N- doped or C- doped TiO₂ in visible light [12]. In another study, Li. et al. [21] prepared N, F co-doped TiO₂ powders, it showed higher photocatalytic degradation of trichloroethy-lene and acetaldehyde compared to N-TiO₂ or F-TiO₂ [22].

N, C, and S doping into TiO₂ materials has been achieved by various methods, such as solid–solid reaction, sol–gel, co-precipitation, solvothermal, chemical, and ion-implantation methods [6], [11], [21], [23]. Ion doping [11] is an effective way to optimize the performance of semiconductors, and doping processes are the main challenges in modern science. However, most of the known processes require excessively high operating temperatures and exhibit insufficient selectivity. Hadis Zangeneh et al. synthetized C, N, and S doping of TiO₂–ZnO which required a temperature of 500 °C [23]. Yan et al. reported that N and S co-doping was achieved by calcining thin films of TiO₂ in a vacuum at 500 °C [17]. Huang et al. [18] synthesized N and C doped TiO₂ materials, and they needed to be heated at 500 °C in an oven and required photocatalytic inactivation of Klebsiella pneumoniae by the visible light response. Rizzo et al. [11] prepared N-doped TiO₂ photocatalysts that required calcination at 450 °C. Zhang et al. synthesized C- and N-doped porous TiO₂ hollow spheres calcined at 650 °C [24]. The disadvantage of the non-metallic doping method at high temperatures is that it is too costly for mass production. Only a few studies have reported the improvement of photocatalysts using the vapor-phase hydrothermal (VPH) method. In this study, a one-step implementation of C/N/S doping into TiO₂ photocatalysts was prepared by the VPH method at 200 °C. Compared with common experimental methods, the VPH process is facile, easier to operate, and doping with non-metallic elements can be achieved at low temperatures. VPH synthesis has been demonstrated to be a versatile method, which can be used for synthesizing materials with non-metallic element doping efficiently by gas at lower temperatures [25]. However, the synthesis of semiconductors using the VPH method has not been extensively studied. To our knowledge, few studies have reported the one-step preparation of C-, N-, and S-doped TiO₂ materials using the VPH process.