An Ultrasensitive Room-Temperature H2 Sensor Based on a TiO2 Rutile–Anatase Homojunction

Metal oxide semiconductor hetero- and homojunctions are commonly constructed to improve the performance of hydrogen sensors at room temperature. In this study, a simple two-step hydrothermal method was employed to prepare TiO2 films with homojunctions of rutile and anatase phases (denoted as TiO2-R/A). Then, the microstructure of anatase-phase TiO2 was altered by controlling the amount of hydrochloric acid to realize a more favorable porous structure for charge transport and a larger surface area for contact with H2. The sensor used a Pt interdigital electrode. At an optimal HCl dosage (25 mL), anatase-phase TiO2 uniformly covered rutile-phase TiO2 nanorods, resulting in a greater response to H2 at 2500 ppm compared with that of a rutile TiO2 nanorod sensor by a factor of 1153. The response time was 21 s, mainly because the homojunction formed by the TiO2 rutile and anatase phases increased the synergistic effect of the charge transfer and potential barrier between the two phases, resulting in the formation of more superoxide (O2−) free radicals on the surface. Furthermore, the porous structure increased the surface area for H2 adsorption. The TiO2-R/A-based sensor exhibited high selectivity, long-term stability, and a fast response. This study provides new insights into the design of commercially competitive hydrogen sensors.


Introduction
As a renewable source of energy, hydrogen is widely used in chemical and metallurgical industries owing to its relatively clean and pollution-free nature and its high calorific value.It can also be stored on a large scale [1][2][3][4].However, gaseous hydrogen is prone to leaking and is extremely flammable when it contacts oxygen in the atmosphere.Moreover, its tendency to explode upon encountering open flame poses a grave threat to human life and infrastructure.Therefore, detecting hydrogen gas in real-time at low concentrations is crucial to ensure the safe usage of hydrogen as an energy source [5,6].
Among the various gas detection methods, gas chromatographs use chromatographic columns to separate the individual gas components in a mixture to identify each component.Mass spectrometers identify gas molecules on the basis of their characteristic deflections from a magnetic field.Optical sensors utilize the change of optical properties of certain materials when they interact with H 2 to detect H 2 .These methods typically require complex instrumentation and relatively large, expensive, and high maintenance [7][8][9], and electrochemical sensors generally have poor selectivity and long-term stability and are subject to interference from reducing gases [10].Therefore, hydrogen sensors based on chemical resistance have been widely explored owing to their low cost, good stability, and excellent application prospects [11].Among the numerous sensing materials, metal oxide semiconductors (MOSs) (e.g., TiO 2 , SnO 2 , and ZnO) are popular owing to their low cost and good sensitivity [12,13].Among these, TiO 2 , a typical n-type semiconductor, is non-toxic, harmless, and inexpensive.Moreover, its unique performance in gas sensing is well-documented [14,15].However, the response of TiO 2 to hydrogen is limited by the rapid recombination of electron-hole pairs.Therefore, to improve the response of TiO 2 to hydrogen, it is generally necessary to regulate its band structure and suppress the recombination rate of electrons and holes to ensure that more electrons combine with O 2 to generate O 2 − .One effective method to suppress the recombination rate is to construct heterojunctions of TiO 2 such as TiO 2 /MoS 2 [16], Bi 2 MoO 6 /TiO 2 [17], TiO 2 /Cu 2 O [18], and TiO 2 /SrTiO 3 [19].Owing to the advantages of the two types of metal oxide semiconductors, heterojunction composites exhibit significantly different physical and chemical properties and sensitivity to gases than their corresponding single-component counterparts.However, heterojunction composites contain many defects, which are not conducive to the performance of the heterojunction in terms of gas sensitivity [20,21].
A homojunction is formed between different crystal phases of the same semiconductor with different band structures.When carriers are transferred across a homojunction, their recombination rate would be much smaller than that at the heterojunction interface [22,23].TiO 2 has two stable phases, viz., rutile and anatase, with bandgaps of approximately 3.0 and 3.2 eV, respectively [21,24,25].Therefore, in this study, we prepared composite materials with rutile-anatase TiO 2 homojunctions for gas sensing.Biphasic TiO 2 with homojunctions has a uniform composition and near-perfect lattice matching, which can ensure a reduced contact barrier, the regulation of the band structure, and effective charge transfer at the interface of the two phases (homojunction) [26,27].
Table 1 compares the performance of some previously reported MOS H 2 sensors.It is seen that most MOS H 2 sensors need to operate at high temperatures (>150 • C) [28][29][30][31][32][33].Some reports indicate that modifying MOS sensors with precious metals can lower the operating temperature to room temperature [30].However, there are few reports on MOS sensors that have a high response, a low detection limit, and a fast response time and operate at room temperature simultaneously.In this work, we designed and developed a growth-oriented material with rutile-anatase TiO 2 homojunctions (denoted as TiO 2 -R/A) using a simple two-step hydrothermal method for H 2 detection.In the second step of the synthesis, we controlled the amount of hydrochloric acid (HCl) to adjust the morphology of anatase-phase TiO 2 , promote charge transfer across the junction [34], and increase the contact area of the material surface with H 2 .Our results showed that the response of the bilayered mixed-phase TiO 2 films with homojunctions to picomolar hydrogen was much greater than that of single-layer rutile-phase TiO 2 .The best performance in H 2 sensing was achieved when anatase TiO 2 was uniformly formed on the surface of the rutile TiO 2 nanorods, at a hydrogen concentration of 2500 ppm, the response reached 1661.The characterization of the mixed-phase TiO 2 material using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), potentiometry, and X-ray photoelectron spectroscopy (XPS) is indicated, and the sensing mechanism is analyzed.First, a self-assembled rutile TiO 2 nanorod array was grown on an FTO substrate, as reported previously [35].Briefly, 28 mL of deionized water, 2 mL of ethanol, 30 mL of hydrochloric acid, and 1 mL of tetrabutyl titanate (TBOT) were mixed in a beaker and stirred to obtain a homogeneous precursor solution.Then, the precursor solution and FTO substrates, which were cleaned sequentially with acetone, ethanol, and deionized water, were placed in a 200 mL reactor and reacted at 150 • C for 8 h.After the reaction, the reactor was cooled to room temperature, and the FTO substrate was soaked in deionized water for 3 h.The rutile-phase TiO 2 nanorod array was annealed in a 400 • C tube furnace for 20 min, and the prepared sample is referred to as NR-TiO 2 .

Preparation of Rutile-and Anatase-Phase TiO 2 Homojunctions
A known volume of hydrochloric acid (X = 10, 15, 20, 25, or 30 mL), 1 mL of concentrated sulfuric acid, and 1 mL of TBOT were added to a solution of ethanol (2 mL) and deionized water (28 mL) in a beaker and mixed evenly by stirring.Then, the NR-TiO 2 sample was placed in a 200 mL reactor and reacted with the acidified TBOT solution at 150 • C for 8 h.After the reaction, the sample was rinsed several times with deionized water and then soaked in deionized water for 3 h.Finally, the sample was annealed in a tube furnace at 400 • C for 20 min.The samples thus prepared were denoted as TiO 2 (R/A-X), where R and A represent rutile and anatase phases and X represents the volume of HCl (10,15,20,25, or 30 mL).

Characterization
XRD (D8-Advance, Bruker, Cu Kα line as the X-ray source), field-emission SEM (Sigma 500, Zeiss, Oberkochen, Germany), Raman spectrometry (CLY19 Lab RAM HR Evolution), and XPS were performed to characterize the microstructure, morphology, and chemical composition of the samples.
Platinum interdigitated electrodes were deposited onto thin films of TiO 2 (R/A-X) through DC magnetron sputtering.The standard test gas, 75% of H 2 mixed with 25% of Ar or 0.75% of NH 3 , SO 2 , NO, C 2 H 6 , or NO 2 dry gas mixed with 99.25% of Ar (Wuhan Xiangyun Industry and Trade Co., Ltd., Wuhan, China) was introduced into a 15 L sealed testing chamber via a computer-controlled mass flow controller (D07-7C, Beijing Seven Star Huachuang Flow Co., Ltd., Beijing, China).The sensing evaluation of the target gas was conducted at ambient temperature (25 • C) and environmental humidity (44%) unless specified otherwise.A Keithley 2400 multimeter was used to measure the fluctuation in sensor resistance while maintaining a constant voltage of 1 V across the sensor.Equation (1) can be used to ascertain the response of the sensor to H 2 (S).
In Formulas (1) and ( 2), the initial resistance of the sensor when exposed to air is denoted as R air , while the initial resistance in a H 2 atmosphere is denoted as R gas .∆R represents the recorded difference in resistance between the sensor in air and in an H 2 atmosphere.The response time refers to the duration required for the sensor's resistance to a decrease by 90% of ∆R from R air , whereas recovery time refers to the duration required for the sensor's resistance to increase by 90% of ∆R from R gas .

Characterization of the TiO 2 (R/A-X) Samples
Figure 1a presents the XRD patterns of the FTO substrate and the NR-TiO 2 and TiO 2 (R/A-X) samples grown on the FTO substrates.The main sample peaks are consistent with those of rutile and anatase TiO 2 (JCPDS No. 21-1276 and No. 21-1272).The diffraction peaks observed at 2θ = 36.1 • and 62.7 • correspond to the (101) and (002) crystallographic planes of rutile TiO 2 (JCPDS 21-1276), and the peak at 2θ = 37.82 • occurred due to the (004) plane of the anatase phase.A comparison with the standard patterns showed that the strongest peaks of the rutile and anatase phases did not appear in the XRD patterns of the prepared materials.The anatase (004) peak replaced the (101) peak as the strongest diffraction peak of the film.This indicates a tendency for the preferential growth of the nanorod in the (001) direction.The peak of 2θ = 62.7 • in Figure 1a corresponds to the (002) crystal plane of rutile TiO 2 , which is consistent with previously reported results for rutile nanorod arrays, which tend to grow in the direction of (001) [35].Therefore, the nanorods with rutile and anatase phases in the synthesized TiO 2 (A/R-X) thin films tended to grow in the (001) direction.As the main peak of the FTO substrate (PDF# 46-1088) at 2θ = 37.76 • is very close to the (004) peak of the anatase phase, this region of Figure 1a was magnified.The locally magnified pattern revealed that the peaks observed near 2θ = 37.76 • for the NR-TiO 2 sample arose owing to the underlying FTO substrate.A comparison of the XRD patterns revealed that the diffraction peak of rutile (101) became progressively weaker with a gradual reduction in the amount of hydrochloric acid used in the second step of the synthesis.Further, the (004) diffraction peak of the anatase phase gradually increased, which was accompanied by a proportional decrease in the peak intensity of the rutile phase.This occurred because the inhibitory effect of Cl − on anatase-phase TiO 2 was weakened with the decrease in the amount of hydrochloric acid, which is consistent with previous reports [36,37].The above phenomena were further confirmed by Raman spectroscopy.

Characterization of the TiO2(R/A-X) Samples
Figure 1a presents the XRD patterns of the FTO substrate and the NR-TiO2 and TiO2(R/A-X) samples grown on the FTO substrates.The main sample peaks are consistent with those of rutile and anatase TiO2 (JCPDS No. 21-1276 and No. 21-1272).The diffraction peaks observed at 2θ = 36.1°and 62.7° correspond to the (101) and (002) crystallographic planes of rutile TiO2 (JCPDS 21-1276), and the peak at 2θ = 37.82° occurred due to the (004) plane of the anatase phase.A comparison with the standard patterns showed that the strongest peaks of the rutile and anatase phases did not appear in the XRD patterns of the prepared materials.The anatase (004) peak replaced the (101) peak as the strongest diffraction peak of the film.This indicates a tendency for the preferential growth of the nanorod in the (001) direction.The peak of 2θ = 62.7° in Figure 1a corresponds to the (002) crystal plane of rutile TiO2, which is consistent with previously reported results for rutile nanorod arrays, which tend to grow in the direction of (001) [35].Therefore, the nanorods with rutile and anatase phases in the synthesized TiO2(A/R-X) thin films tended to grow in the (001) direction.As the main peak of the FTO substrate (PDF# 46-1088) at 2θ = 37.76° is very close to the (004) peak of the anatase phase, this region of Figure 1a was magnified.The locally magnified pattern revealed that the peaks observed near 2θ = 37.76° for the NR-TiO2 sample arose owing to the underlying FTO substrate.A comparison of the XRD patterns revealed that the diffraction peak of rutile (101) became progressively weaker with a gradual reduction in the amount of hydrochloric acid used in the second step of the synthesis.Further, the (004) diffraction peak of the anatase phase gradually increased, which was accompanied by a proportional decrease in the peak intensity of the rutile phase.This occurred because the inhibitory effect of Cl − on anatase-phase TiO2 was weakened with the decrease in the amount of hydrochloric acid, which is consistent with previous reports [36,37].The above phenomena were further confirmed by Raman spectroscopy.Figure 1b shows the Raman spectra of the TiO2(R/A-X) samples.This is consistent with previous reports [38], which show a Raman band at 144 cm −1 corresponding to the Eg vibration mode of the anatase phase.The peaks at 448 and 608 cm −1 correspond to the Eg and A1g vibrational modes of the rutile phase of TiO2.A comparison of the spectra revealed that as the amount of hydrochloric acid used in the second step increased, the main peak of the anatase-phase TiO2 (144 cm −1 ) increased gradually.Notably, the Raman spectrum of the TiO2(R/A-30 mL) sample did not show the anatase peak at 144 cm −1 because the content of the anatase-phase TiO2 in this sample was too low to be detected.Thus, the XRD and Raman spectroscopy results confirm the successful preparation of gas-sensitive Figure 1b shows the Raman spectra of the TiO 2 (R/A-X) samples.This is consistent with previous reports [38], which show a Raman band at 144 cm −1 corresponding to the E g vibration mode of the anatase phase.The peaks at 448 and 608 cm −1 correspond to the E g and A 1g vibrational modes of the rutile phase of TiO 2 .A comparison of the spectra revealed that as the amount of hydrochloric acid used in the second step increased, the main peak of the anatase-phase TiO 2 (144 cm −1 ) increased gradually.Notably, the Raman spectrum of the TiO 2 (R/A-30 mL) sample did not show the anatase peak at 144 cm −1 because the content of the anatase-phase TiO 2 in this sample was too low to be detected.Thus, the XRD and Raman spectroscopy results confirm the successful preparation of gas-sensitive films Sensors 2024, 24, 978 5 of 16 composed of anatase-phase TiO 2 supported on the rutile-phase TiO 2 nanorod array using a two-step hydrothermal method.
Figure 2 shows the surface morphologies of the NR-TiO 2 and TiO 2 (R/A-X) samples.The surface of the NR-TiO 2 sample shows the cauliflower-like morphology of the nanorod tips (Figure 2a) with diameters of approximately 100-200 nm.In this study, the formation of anatase-phase TiO 2 on NR-TiO 2 was controlled by varying the ratio of Cl − to SO 4 2− .Figure 2 shows that as the amount of hydrochloric acid was decreased, a larger amount of anatase-phase TiO 2 was generated in the reaction.When the amount of hydrochloric acid was decreased from X = 30 mL to X = 25 mL, the gaps between the nanorods became smaller.When the amount of hydrochloric acid was decreased further, below X = 25 mL, the gaps between the nanorods were gradually filled with anatase-phase TiO 2 .
Sensors 2024, 24, x FOR PEER REVIEW 5 o films composed of anatase-phase TiO2 supported on the rutile-phase TiO2 nanorod ar using a two-step hydrothermal method.
Figure 2 shows the surface morphologies of the NR-TiO2 and TiO2(R/A-X) samp The surface of the NR-TiO2 sample shows the cauliflower-like morphology of the nano tips (Figure 2a) with diameters of approximately 100-200 nm.In this study, the format of anatase-phase TiO2 on NR-TiO2 was controlled by varying the ratio of Cl − to SO4 2− .F ure 2 shows that as the amount of hydrochloric acid was decreased, a larger amoun anatase-phase TiO2 was generated in the reaction.When the amount of hydrochloric a was decreased from X = 30 mL to X = 25 mL, the gaps between the nanorods beca smaller.When the amount of hydrochloric acid was decreased further, below X = 25 m the gaps between the nanorods were gradually filled with anatase-phase TiO2. Figure 3 shows the cross-sectional morphologies of the NR-TiO2 and TiO2(R/A samples.The cross-sectional SEM image of the NR-TiO2 sample exhibited smooth na rods (Figure 3a) with a height of approximately 2.3 µm.Figure 3b-f show the cross-s tional images of the TiO2(R/A-X) samples, which reveal that the morphology of the na rods changed owing to modification with anatase-phase TiO2.As the amount of hyd chloric acid was decreased from X = 30 mL to X = 25 mL, the spaces between the nanor gradually decreased.When X = 20 mL, anatase-phase TiO2 was enriched at the top of nanorod, resulting in a distinct double-layer structure.With the reduction in the amo of hydrochloric acid, the anatase-phase-TiO2-enriched layer gradually became thicker, hibiting an increase in thickness from 1.1 to 2.63 µm, as shown in Figure 3d-f.To comp the effects of the hydrochloric acid dosage (X = 25 mL and X = 30 mL) on anatase-ph TiO2 more clearly, we compared the micromorphologies of the NR-TiO2, TiO2(R/A-25 m and TiO2(R/A-30 mL) samples at a higher magnification (50 kx) (Figure 3g-i).The surf of the nanorods became rough due to modification with anatase-phase TiO2, and the s face roughness of the nanorods gradually increased with the decrease in hydrochloric a dosage.Thus, SEM analyses confirmed that hydrochloric acid could significantly inh the growth of anatase-phase TiO2, which is consistent with the XRD and Raman res shown in Figure 1. Figure 3 shows the cross-sectional morphologies of the NR-TiO 2 and TiO 2 (R/A-X) samples.The cross-sectional SEM image of the NR-TiO 2 sample exhibited smooth nanorods (Figure 3a) with a height of approximately 2.3 µm.Figure 3b-f show the cross-sectional images of the TiO 2 (R/A-X) samples, which reveal that the morphology of the nanorods changed owing to modification with anatase-phase TiO 2 .As the amount of hydrochloric acid was decreased from X = 30 mL to X = 25 mL, the spaces between the nanorods gradually decreased.When X = 20 mL, anatase-phase TiO 2 was enriched at the top of the nanorod, resulting in a distinct double-layer structure.With the reduction in the amount of hydrochloric acid, the anatase-phase-TiO 2 -enriched layer gradually became thicker, exhibiting an increase in thickness from 1.1 to 2.63 µm, as shown in Figure 3d-f.To compare the effects of the hydrochloric acid dosage (X = 25 mL and X = 30 mL) on anatase-phase TiO 2 more clearly, we compared the micromorphologies of the NR-TiO 2 , TiO 2 (R/A-25 mL), and TiO 2 (R/A-30 mL) samples at a higher magnification (50 kx) (Figure 3g-i).The surface of the nanorods became rough due to modification with anatase-phase TiO 2 , and the surface roughness of the nanorods gradually increased with the decrease in hydrochloric acid dosage.Thus, SEM analyses confirmed that hydrochloric acid could significantly inhibit the growth of anatase-phase TiO 2 , which is consistent with the XRD and Raman results shown in Figure 1.

Sensing Properties
The hydrogen-sensing curves of NR-TiO2 and TiO2(R/A-X) samples are shown in F ure 4a-f.All samples exhibited a consistent n-type response to hydrogen.Owing to reducing nature of hydrogen, the resistance of the sensor decreased when hydrogen tered the ventilation chamber and increased to the initial value when hydrogen exited ventilation chamber.As shown in Figure 4a, the response of NR-TiO2 to hydrogen at ro temperature was relatively poor, and the response values at 2500 and 20,000 ppm w 1.44 and 2.7, respectively (Figure S1).As shown in Figure 4b-f, the resistance of TiO2(R/A-X) sample first increased and then decreased with the reduction in the amo of hydrochloric acid (X) used in the second step of the synthesis.Notably, in the sam prepared with X = 30 mL, only a very small number of anatase particles adhered to sides of the rutile nanorods because the excessive chloride ions inhibited the growth the anatase phase.In this case, the homojunction area formed between the rutile and a tase phases was relatively small, and the gap between the nanorods was large, result in relatively low resistance.

Sensing Properties
The hydrogen-sensing curves of NR-TiO 2 and TiO 2 (R/A-X) samples are shown in Figure 4a-f.All samples exhibited a consistent n-type response to hydrogen.Owing to the reducing nature of hydrogen, the resistance of the sensor decreased when hydrogen entered the ventilation chamber and increased to the initial value when hydrogen exited the ventilation chamber.As shown in Figure 4a, the response of NR-TiO 2 to hydrogen at room temperature was relatively poor, and the response values at 2500 and 20,000 ppm were 1.44 and 2.7, respectively (Figure S1).As shown in Figure 4b-f, the resistance of the TiO 2 (R/A-X) sample first increased and then decreased with the reduction in the amount of hydrochloric acid (X) used in the second step of the synthesis.Notably, in the sample prepared with X = 30 mL, only a very small number of anatase particles adhered to the sides of the rutile nanorods because the excessive chloride ions inhibited the growth of the anatase phase.In this case, the homojunction area formed between the rutile and anatase phases was relatively small, and the gap between the nanorods was large, resulting in relatively low resistance.The response values of the NR-TiO2 and TiO2(R/A-X) samples during hydrogen se ing are shown in Figure 5a.TiO2(R/A-X) samples with homojunctions exhibited sign cantly higher response values than NR-TiO2.In particular, the highest response (1661) 2500 ppm hydrogen was observed for the TiO2(R/A-25 mL) sample prepared using 25 m of HCl.Further, with a further increase in hydrogen concentration, the response value this sensor continued to show an increasing trend.At 20,000 ppm hydrogen, the respon value of the TiO2(R/A-25 mL) sensor reached 76,702, which is 28,408 times that of NR-Ti It is worth noting that for the sample prepared with X = 30 mL, the response to hydrog was only slightly improved compared with that of NR-TiO2 because only a very sm amount of anatase TiO2 was formed on the rutile nanorod surface.According to the resu shown in Figure 5, when the dosage of hydrochloric acid did not exceed 25 mL, TiO2(R/A-X) sample showed a highly sensitive response to hydrogen at ro The response values of the NR-TiO 2 and TiO 2 (R/A-X) samples during hydrogen sensing are shown in Figure 5a.TiO 2 (R/A-X) samples with homojunctions exhibited significantly higher response values than NR-TiO 2 .In particular, the highest response (1661) of 2500 ppm hydrogen was observed for the TiO 2 (R/A-25 mL) sample prepared using 25 mL of HCl.Further, with a further increase in hydrogen concentration, the response value of this sensor continued to show an increasing trend.At 20,000 ppm hydrogen, the response value of the TiO 2 (R/A-25 mL) sensor reached 76,702, which is 28,408 times that of NR-TiO 2 .It is worth noting that for the sample prepared with X = 30 mL, the response to hydrogen was only slightly improved compared with that of NR-TiO 2 because only a very small amount of anatase TiO 2 was formed on the rutile nanorod surface.According to the results shown in Figure 5, when the dosage of hydrochloric acid did not exceed 25 mL, the TiO 2 (R/A-X) sample showed a highly sensitive response to hydrogen at room temperature.In order to further evaluate the detection limit (LOD) of TiO 2 (R/A-25 mL), the response curve at a low concentration of 25-125 ppm H 2 was measured, and the LOD was theoretically evaluated using Equation (3) [39,40], as shown in the Figure 5b.
where R noise is the measured noise of the sensor and L slope is the slope of the linear fitting curve.The R noise as 0.01 is calculated using the equation [39,40] below: where R i is the experimental data (i.e., various responses of H 2 concentration) and R is the fitting value based on Figure 5b.Calculation reveals that the LOD of the TiO 2 (R/A-25 mL) sensor is ~6.3 ppm.

LOD =
where  is the measured noise of the sensor and  is the slope of the linear ting curve.The  as 0.01 is calculated using the equation [39,40] below: where  is the experimental data (i.e., various responses of H2 concentration) and R the fitting value based on Figure 5b.Calculation reveals that the LOD of the TiO2(R/A mL) sensor is ~6.3 ppm.As shown in Figure 5c,d, the TiO2(R/A-X) samples exhibited a slightly longer sponse time and a longer recovery time than NR-TiO2 (see Figure S2).This was the ca because the increased response causes the resistance to fluctuate over a wider range, a the resistance of the system takes more time to reach the equilibrium value, especially environments with high hydrogen concentrations, where this phenomenon is much mo evident.In summary, the response values of NR-TiO2 at all tested hydrogen concent tions ranged from 1 to 2, whereas the response values of the TiO2(R/A-X) samples at t same tested hydrogen concentrations were significantly better.
After the surface of the rutile TiO2 nanorods was coated with anatase TiO2, TiO2(R/ 25 mL) exhibited exceptional sensing properties.It is crucial to comprehend its key det minants.The sensing performance of this sample can be attributed to the presence of ty II alternating band alignment of ~0.4 eV between anatase and rutile phases and the high As shown in Figure 5c,d, the TiO 2 (R/A-X) samples exhibited a slightly longer response time and a longer recovery time than NR-TiO 2 (see Figure S2).This was the case because the increased response causes the resistance to fluctuate over a wider range, and the resistance of the system takes more time to reach the equilibrium value, especially in environments with high hydrogen concentrations, where this phenomenon is much more evident.In summary, the response values of NR-TiO 2 at all tested hydrogen concentrations ranged from 1 to 2, whereas the response values of the TiO 2 (R/A-X) samples at the same tested hydrogen concentrations were significantly better.
After the surface of the rutile TiO 2 nanorods was coated with anatase TiO 2 , TiO 2 (R/A-25 mL) exhibited exceptional sensing properties.It is crucial to comprehend its key determinants.The sensing performance of this sample can be attributed to the presence of type II alternating band alignment of ~0.4 eV between anatase and rutile phases and the higher electron affinity of anatase-phase TiO 2 .Therefore, electron flow occurs from the rutile phase to the surface anatase phase, as shown in Figure 6a,b.Because the rutile and anatase phases of TiO 2 have different band gaps (E g ) and different valence band and conductive band energies (E V and E C , respectively), when the rutile-phase TiO 2 nanorod contacts the anatase-phase TiO 2 layer, an internal potential is established at the junction of the two phases.Owing to the continuous flow of electrons from the rutile to the anatase phase of TiO 2 , an electron depletion layer is formed at the rutile phase TiO 2 nanorods, whereas an electron accumulation layer is formed at the anatase layer, as illustrated in Figure 6b.phases of TiO2 have different band gaps (Eg) and different valence band and conducti band energies (EV and EC, respectively), when the rutile-phase TiO2 nanorod contacts t anatase-phase TiO2 layer, an internal potential is established at the junction of the tw phases.Owing to the continuous flow of electrons from the rutile to the anatase phase TiO2, an electron depletion layer is formed at the rutile phase TiO2 nanorods, whereas electron accumulation layer is formed at the anatase layer, as illustrated in Figure 6b.To investigate this, XPS analyses were conducted on samples of NRs-TiO2, anata phase TiO2, and TiO2(R/A-X), as depicted in Figure 6c,d.The O1s spectrum of the NR To investigate this, XPS analyses were conducted on samples of NRs-TiO 2 , anatase phase TiO 2 , and TiO 2 (R/A-X), as depicted in Figure 6c,d.The O1s spectrum of the NRs-TiO 2 sample (Figure 6c) exhibited two prominent peaks at 529.71 eV and 530.91 eV, corresponding to lattice oxygen (Ti-O-Ti) and hydroxide (Ti-OH), respectively.In the Ti2p spectrum (Figure 6d), two distinct nuclear level signals may be observed at 464.30 eV and 458.54 eV, which can be attributed to the Ti2p3/2 and Ti2p1/2 levels of Ti 4+ , respectively.
Consistent with previously reported findings [41], the O1s spectra of anatase-phase TiO 2 exhibited higher binding energy peaks for lattice oxygen (Ti-O-Ti) and hydroxyl groups (Ti-OH).For both the sensor TiO 2 (R/A-10 mL) and anatase-phase TiO 2 samples, no significant change in peak values was observed; this can be explained by the aforementioned microscopic morphology, in which a rutile-phase nanorod was present on the top surface covered with anatase-phase TiO 2 with a thickness of approximately 2.6 µm.Consequently, the XPS analysis only detected signals from the anatase phase of TiO 2 .
To explain the response mechanism of the sensor to H 2 more clearly, one usually needs to calculate the Schottky barrier formed at the two-phase contact surface.Therefore, we tested the I-V change curve of TiO 2 (R/A-X) at room temperature to estimate the difference in barrier height between in air (Figure 7a) and H 2 (2500 ppm) atmosphere (Figure 7b).The I-V curves of the entire series of TiO 2 (R/A-X) samples changed significantly before and after exposure to H 2 atmosphere.By observing the phenomenon of the nonlinear relationship between I-V, we can consider that there was a potential barrier [42].The effective height of the barrier can be calculated by Equation ( 5) [43,44].In the formula given above, q represents the amount of charge carried by the ele and n represents the ideal factor.There are two main factors that affect the height o effective barrier in this series of sensors.First, H2 reacts with the topmost Pt electro form PtHx, which decreases the height of the Schottky barrier [44][45][46].Second, the ch in the charge concentration in the anatase-phase TiO2 layer also changes the barrier he Therefore, the potential barrier of the TiO2(R/A-X) sample in air showed a trend of increasing and then decreasing with increasing anatase TiO2 content.The highest p tial barrier was observed for TiO2(R/A-25 mL).The increase in the barrier heigh TiO2(R/A-30 mL) and TiO2(R/A-25 mL) was due to the gradual increase in the gener of anatase TiO2 and the resultant gradual increase in the interface area between the and anatase phases.As the generation of anatase TiO2 was increased further by redu the amount of HCl, lateral current leakage occurred due to the accumulation of an TiO2 on the surface of the rutile-phase TiO2 nanorods, which decreased the barrier he Figure 7c,d show the calculated effective barrier heights of different samples and 2500 ppm H2 environments and the correlation between the barrier heights o samples and the corresponding response of the sensor.Under 2500 ppm H2, the ba heights of all samples were less than those in air, and the barrier difference was consi with the curve of the sensing response.The largest change in the barrier height of 0.3 was observed for the TiO2(R/A-25 mL) sample, which exhibited the highest respon H2.
Figure 8a,b show the changes in the morphology of the anatase-phase TiO2 layer where k represents the Boltzmann constant, T represents the test temperature, A represents the area where the diode is constructed, I s represents the saturation current, and A* represents the Richardson constant of rutile-phase TiO 2 .In this series of sensors, the value of A was equal to 2.5 cm × 2.5 cm = 6.25 cm 2 , T at room temperature usually takes a value of 300 K, and A* = (m*/m) A/cm 2 K, for rutile-phase TiO 2 m*/m = 20.The saturation current can be calculated by the following formulas ( 6) and ( 7): ln In the formula given above, q represents the amount of charge carried by the electron and n represents the ideal factor.There are two main factors that affect the height of the effective barrier in this series of sensors.First, H 2 reacts with the topmost Pt electrode to form PtHx, which decreases the height of the Schottky barrier [44][45][46].Second, the change in the charge concentration in the anatase-phase TiO 2 layer also changes the barrier height.Therefore, the potential barrier of the TiO 2 (R/A-X) sample in air showed a trend of first increasing and then decreasing with increasing anatase TiO 2 content.The highest potential barrier was observed for TiO 2 (R/A-25 mL).The increase in the barrier heights of TiO 2 (R/A-30 mL) and TiO 2 (R/A-25 mL) was due to the gradual increase in the generation of anatase TiO 2 and the resultant gradual increase in the interface area between the rutile and anatase phases.As the generation of anatase TiO 2 was increased further by reducing the amount of HCl, lateral current leakage occurred due to the accumulation of anatase TiO 2 on the surface of the rutile-phase TiO 2 nanorods, which decreased the barrier height.
Figure 7c,d show the calculated effective barrier heights of different samples in air and 2500 ppm H 2 environments and the correlation between the barrier heights of the samples and the corresponding response of the sensor.Under 2500 ppm H 2 , the barrier heights of all samples were less than those in air, and the barrier difference was consistent with the curve of the sensing response.The largest change in the barrier height of 0.30 eV was observed for the TiO 2 (R/A-25 mL) sample, which exhibited the highest response to H 2 .
Figure 8a,b show the changes in the morphology of the anatase-phase TiO 2 layer with the reduction in the amount of hydrochloric acid used in the synthesis.As the amount of hydrochloric acid was gradually decreased, anatase-phase TiO 2 was first coated on the surface of rutile nanorods (Figure 8a), As the amount of hydrochloric acid continued to decrease, the deposition of anatase-phase TiO 2 on the surface of the rutile nanorods gradually increased, resulting in a double-layer structure (Figure 8b).In the schematic shown in Figure 8a, anatase-phase TiO 2 completely covers the surface of rutile-phase TiO 2 nanorods to form a porous structure.In this situation, only the longitudinal current I 1 along the nanorods is present (Figure 8a).As the amount of hydrochloric acid was decreased, anatase-phase TiO 2 generated by the reaction was not only deposited on the nanorod surface but also filled the gaps between the nanorods.This structure not only has longitudinal current I 1 along the nanorods but also generates transverse current I 2 (Figure 8b).This model explains the initial increase followed by the decrease in the resistance of the TiO 2 (R/A-X) samples, as shown in Figure 4.The schematic in Figure 8c,d shows the response mechanism of the sensor to H 2 .Electrons flow from rutile-phase TiO 2 nanorods to the surface anatase-phase TiO 2 layer and react with O 2 adsorbed to the surface to generate a large amount of O 2 − species in air.At this stage, a large depletion layer is formed on the sides of the rutile-phase nanorods (Figure 8c), and the resistance of the system increases.When H 2 enters the test chamber, H 2 reacts with O 2 − on the surface of the sensor so that the released electrons return to the rutile-phase TiO 2 nanorods, the depletion layer becomes smaller, and the resistance of the system decreases (Figure 8d).The associated reaction can be represented as follows [44][45][46].
According to the micromorphology of the TiO 2 (R/A-25 mL) sample in Figures 2 and 3, rutile-phase TiO 2 nanorods are tightly wrapped by anatase-phase TiO 2 , which has a porous structure that is more conducive to contact with H 2 .According to the micromorphology of the TiO2(R/A-25 mL) sample in Figures 2 and  3, rutile-phase TiO2 nanorods are tightly wrapped by anatase-phase TiO2, which has a porous structure that is more conducive to contact with H2.
The selectivity of the TiO2(R/A-25 mL) sample was tested at 2500 ppm concentrations of H2, NO, SO2, C2H6, NH3, and NO2.The dynamic curve of the resistance is shown in Figure 9a, and the response values of the sensor corresponding to different gases are shown in Figure 9b.The sensor exhibited an N-type response to reducing gases, and TiO2, The selectivity of the TiO 2 (R/A-25 mL) sample was tested at 2500 ppm concentrations of H 2 , NO, SO 2 , C 2 H 6 , NH 3 , and NO 2 .The dynamic curve of the resistance is shown in Figure 9a, and the response values of the sensor corresponding to different gases are shown in Figure 9b.The sensor exhibited an N-type response to reducing gases, and TiO 2 , as a typical N-type semiconductor, exhibited this phenomenon in line with the oxygen adsorption theory [44].However, NO 2 , an oxidizing gas, also exhibited an N-type response, which contradicts the theory of oxygen adsorption.This phenomenon was a result of the catalytic action of the Pt electrode.The highly oxidizing NO 2 gas reacts preferentially with Pt, and NO 2 is broken down into NOx and O, thereby transferring electrons to the surface of TiO 2 and reducing the resistance of the film.When the sensor was exposed to NH 3 and C 2 H 6 atmospheres, the initial resistance of the sensor did not change, reflecting that the sensor was not responsive to these two gases.Because these two gas molecules show high stability at room temperature, they are less likely to break the chemical bond and cause a change in the resistance of the TiO 2 [45].As shown in Figure 9b, at the same concentration of 2500 ppm, the response of the TiO 2 (R/A-25 mL) sensor to hydrogen was 109 times greater than that of NO, 230 times more than that of SO 2 , and 503 times greater than that of NO 2 , indicating that the TiO 2 (R/A-25 mL) sensor has excellent selectivity to H 2 .
as a typical N-type semiconductor, exhibited this phenomenon in line with the oxyg adsorption theory [44].However, NO2, an oxidizing gas, also exhibited an N-type sponse, which contradicts the theory of oxygen adsorption.This phenomenon was a res of the catalytic action of the Pt electrode.The highly oxidizing NO2 gas reacts prefer tially with Pt, and NO2 is broken down into NOx and O, thereby transferring electrons the surface of TiO2 and reducing the resistance of the film.When the sensor was expo to NH3 and C2H6 atmospheres, the initial resistance of the sensor did not change, reflect that the sensor was not responsive to these two gases.Because these two gas molecu show high stability at room temperature, they are less likely to break the chemical bo and cause a change in the resistance of the TiO2 [45].As shown in Figure 9b, at the sa concentration of 2500 ppm, the response of the TiO2(R/A-25 mL) sensor to hydrogen w 109 times greater than that of NO, 230 times more than that of SO2, and 503 times grea than that of NO2, indicating that the TiO2(R/A-25 mL) sensor has excellent selectivity H2.  Figure 9c shows the results of the cyclic testing of the TiO 2 (R/A-25 mL) sample with 25, 250, 1250, 2500, and 5000 ppm of H 2 .The samples were exposed to room-temperature air for 170 d and tested in four cycles.The response value was then compared with the initial value to calculate the attenuation in the response (Figure 9d).As shown in Figure 9d, when the hydrogen concentrations were 25, 250, 1250, 2500, and 5000 ppm, the decay rates of the homojunction were 7, 8.5, 11, 11.4, and 20%, respectively, indicating that the homojunction is very stable at room temperature.Figure 9e shows the response of the

Figure 5 .
Figure 5. Hydrogen-sensing performances of the TiO2(R/A-X) sensors.(a) Response values at diff ent hydrogen concentration from 25 to 30,000 ppm and (b) LOD of TiO2(R/A-25 mL) at low H2 c centration.(c,d) Response and recovery times of the sensors at different hydrogen concentrat from 25 to 30,000 ppm.

Figure 5 .
Figure 5. Hydrogen-sensing performances of the TiO 2 (R/A-X) sensors.(a) Response values at different hydrogen concentration from 25 to 30,000 ppm and (b) LOD of TiO 2 (R/A-25 mL) at low H 2 concentration.(c,d) Response and recovery times of the sensors at different hydrogen concentration from 25 to 30,000 ppm.

Figure 6 .
Figure 6.(a,b) Bandgaps (Eg) and the energies of the valence band (Ev) and conduction band (Ec) the rutile and anatase phases of titania.(c) O1s and (d) Ti2p XPS profiles of the NR-TiO2 a TiO2(R/A-25 mL) samples.

Figure 6 .
Figure 6.(a,b) Bandgaps (E g ) and the energies of the valence band (E v ) and conduction band (E c ) for the rutile and anatase phases of titania.(c) O1s and (d) Ti2p XPS profiles of the NR-TiO 2 and TiO 2 (R/A-25 mL) samples.

Sensors 2024 , 11 Figure 7 .
Figure 7. (a,b) Representation of I-V characteristics of the TiO2(R/A-X) sensor in air and H2 ppm), respectively.(c) Changes in the Schottky barrier height of the TiO2(R/A-X) sensor befor after exposure to H2 at 2500 ppm.(d) Relationship between the barrier height of the TiO2(R sensor and response value.

Figure 7 .
Figure 7. (a,b) Representation of I-V characteristics of the TiO 2 (R/A-X) sensor in air and H 2 (2500 ppm), respectively.(c) Changes in the Schottky barrier height of the TiO 2 (R/A-X) sensor before and after exposure to H 2 at 2500 ppm.(d) Relationship between the barrier height of the TiO 2 (R/A-X) sensor and response value.

Figure 8 .
Figure 8. (a,b) Schematic showing the change in the morphology of TiO2 nanorods during the second step of the reaction.(c,d) Schematic illustration of the mechanism of the sensor response.

Figure 8 .
Figure 8. (a,b) Schematic showing the change in the morphology of TiO 2 nanorods during the second step of the reaction.(c,d) Schematic illustration of the mechanism of the sensor response.

Figure 9 .
Figure 9. Sensing performance of TiO2(R/A-25 mL) for H2, NO, SO2, NH3, C2H6, and NO2 at 2 ppm: (a) resistance change with time and (b) sensor response to different gases.(c) Cycling stabi of TiO2(R/A-25 mL) at 25, 250, 1250, 2500, and 5000 ppm hydrogen after being placed in air for d.(d) Comparison of the sensor response after 170 d with the initial response and percentage dec in the response.(e) Resistance changes of the TiO2(R/A-25 mL) sensor in response to H2 under ferent humidities and (f) responses of the sample under different humidities.

Figure 9 .
Figure 9. Sensing performance of TiO 2 (R/A-25 mL) for H 2 , NO, SO 2 , NH 3 , C 2 H 6 , and NO 2 at 2500 ppm: (a) resistance change with time and (b) sensor response to different gases.(c) Cycling stability of TiO 2 (R/A-25 mL) at 25, 250, 1250, 2500, and 5000 ppm hydrogen after being placed in air for 170 d.(d) Comparison of the sensor response after 170 d with the initial response and percentage decline in the response.(e) Resistance changes of the TiO 2 (R/A-25 mL) sensor in response to H 2 under different humidities and (f) responses of the sample under different humidities.

Table 1 .
Comparison of sensing performances of H 2 sensors.