Hydrogen, as an efficient, non-polluting, sustainable, and abundant energy source, plays an important role in addressing the pressing global energy crisis. However, the risk of hydrogen leakage and its flammable and explosive characteristics pose a threat to the safety of life and property. Consequently, developing and utilizing reliable hydrogen sensing technology is of utmost necessity. Fiber optic hydrogen sensors based on metal-oxide semiconductors (MOS) have become a research hotspot in this field due to their inherent safety, compact size, resistance to electromagnetic interference, and suitability for explosive environments. The preparation method, material selection, and micro-nanostructure of the sensing layer of fiber optic gas sensors are key factors affecting the performance of the sensors. Traditional methods, such as dip coating, drop casting, and sputtering, are difficult to use to form a sensing layer on the surface of optical fibers that has strong bonding and good stability. In addition, the sensing layer, whether it is a coating or a film, typically has a dense structure with a low specific surface area, limited gas adsorption active sites, and insufficient internal gas transmission channels, which leads to poor gas-sensitive performance. Furthermore, gas sensors based on pure MOS, such as zinc oxide (ZnO), face challenges such as high operating temperatures, low sensitivity, and poor selectivity. Incorporating noble metal-modified MOS can significantly enhance their sensing performance, but this improvement comes at a substantial cost increase. Research indicates that transition metal nickel (Ni)-doped ZnO (Ni∶ZnO), characterized by high activity and low cost, can significantly enhance gas sensing performance. In this study, we design and fabricate a fiber optic hydrogen sensor based on Ni∶ZnO nanorod arrays, which demonstrates significant advantages in terms of the fabrication process, cost, and response.

We design a fiber optic hydrogen sensor based on ZnO nanorod arrays. Firstly, we theoretically analyze the advantages of the nanorod array structure in fiber optic sensors. These advantages not only provide a high specific surface area and gas transmission channels but also change the light transmission mode and the effective transmission path. As a result, the output light intensity increases monotonically with the increase in conductivity, thus enabling effective gas detection (Fig. 2). Then, we prepare Ni∶ZnO nanorod arrays on cladding-etched surfaces using a two-step method: impregnating the ZnO seed layer followed by growing the Ni∶ZnO nanorod arrays in a water bath. The as-prepared samples, with Ni/Zn molar ratios of 1%, 1.5%, 2%, 2.5%, and 3%, are numbered Ni∶ZnO-1, Ni∶ZnO-2, Ni∶ZnO-3, Ni∶ZnO-4, and Ni∶ZnO-5, respectively. Next, the morphology and chemical composition of the nanorod arrays are characterized by scanning electron microscopy and X-ray diffractometry, respectively. ZnO nanorod array-based fiber optic hydrogen sensors with different Ni doping concentrations are then prepared to experimentally investigate the optimal Ni doping concentration. Finally, we test the ZnO nanorod array sensors with the optimal Ni doping concentration to evaluate their hydrogen-sensitive performance.

The ZnO nanorod arrays grown on the fiber surface are hexagonal prismatic, uniformly aligned, and well-dispersed, with nanorod diameters ranging from 50 to 100 nm and an array thickness of approximately 5.5 μm (Fig. 4). The Ni element is successfully incorporated into the ZnO lattice in the form of interstitial doping, which increases the surface defects of ZnO (Figs. 4 and 5). The responsivity of the Ni∶ZnO-based fiber optic sensor firstly increases and then decreases with increasing Ni doping concentration. The response of pure ZnO is only 1.13%, while the Ni∶ZnO-4 sensor exhibits the best response at 8.44%, an enhancement of about 7.5 times, and has the fastest response time (75 s). In contrast, the response of the Ni∶ZnO-5 sensor decreases to 3.33%. Moreover, the Ni∶ZnO-4 sensor shows good linearity between the response and hydrogen volume fraction (1×10?⁵?1×10?³), with a sensitivity of 76.8% and its response to 1×10^-5 hydrogen still being 0.42% (Fig. 6). The sensors exhibit excellent stability and repeatability. In two sets of 12 consecutive cyclic tests, the average response decrease is only 0.3% and 0.33%, and during four weeks of regular monitoring, the response of the sensors decreases by less than 0.25%. Additionally, the sensor exhibits good gas selectivity (Fig. 7). Compared to other fiber optic hydrogen sensors, the designed Ni-doped ZnO nanorod array sensors exhibit significant advantages in terms of hydrogen sensing performance and cost-effectiveness (Table 1), which makes them a very promising option for hydrogen detection.

In this paper, we propose a fiber optic hydrogen sensor based on ZnO nanorod arrays. The special micro-nanostructure of ZnO nanorod arrays, serving as the sensing layer material for fiber optic sensors, not only alters the light transmission mode and path within optical fibers but also provides a large specific surface area, abundant active sites, and gas transmission channels for gas-sensitive detection, which enhances the sensor’s performance. The sensor is fabricated by growing Ni-doped ZnO nanorod arrays on the fiber surface, using a ZnO seed layer followed by hydrothermal synthesis within a water bath. The Ni∶ZnO-4-based sensor, which exhibited the best gas response, achieves a response of 8.44% to a hydrogen volume fraction of 1×10^-3, with a sensitivity of 76.8 /%, a fast response time (75 s), and a lower detection limit of 1×10^-5. Additionally, the sensor maintains good repeatability, stability, and selectivity for hydrogen. Compared with similar fiber optic hydrogen sensors, this sensor offers significant advantages in terms of the fabrication process, cost, and response. In conclusion, the fiber optic hydrogen sensor based on Ni-doped ZnO has potential applications in the field of hydrogen safety monitoring.