Review—Resistive-Type Hydrogen Sensors Based on Zinc Oxide Nanostructures

ZnO is one of the most extensive and intensive researched materials because of their versatile electronic and optoelectronic properties. It is a wide direct bandgap semiconductor (3.37 eV) with a large exciton binding energy (60 meV). Compared to TiO₂, the bulk ZnO and single nanowire ZnO exhibit much higher electronic mobility of 205–300 and 1000 cm² V s⁻¹, respectively.³³ Moreover, ZnO is an environmentally friendly, chemically and thermally stable, and low-cost material, which is widely used in numerous fields, such as field-effect transistors, piezoelectric and varistor devices, solar cells, photocatalysis, UV detectors, light-emitting diodes, acoustic wave devices, biosensors, and gas sensors.^{25,27,33–35}

ZnO has three kinds of crystal structures of hexagonal wurtzite, cubic zinc blende, and NaCl-type rock salt. Among them, hexagonal wurtzite is the most stable and common crystal structure of ZnO in ambient conditions, which is suitable for resistive-type gas sensing application. It is tetrahedrally coordinated with Zn and O atoms stacked alternatively along the c-axis, containing various face terminations—polar Zn-terminated (0001) and O-terminated (0001) facets, and non-polar mix terminated (1010) facets with an equal number of Zn and O atoms.³⁶ The polar (0001) crystal facets have high surface energy and crystal growth rate than non-polar facets, leading to an anisotropic growth. By tuning the growth conditions, extremely abundant ZnO nanostructures, such as nanospheres,³⁷ nanorods (NRs),³⁵ NWs,³⁸ NTs,³⁹ nanosheets,⁴⁰ nanoflake,⁴¹ nanoclips,¹³ and nanoflowers,⁴² have been fabricated successfully by a series of techniques of hydrothermal method, sol-gel process, electrospinning method, electrochemical deposition, vapor-phase transport process, metal-organic chemical vapor deposition (MOCVD), thermal evaporation, RF sputtering, pulsed laser deposition, solution-based polyol process, etc.^{13,27,43–45}

The resistive gas sensor is one of the recognized sensor work principle by measuring the sensor resistance changes while introducing the target gas. The study on sensing mechanism of ZnO-based H₂ gas sensor is essential and useful for the fabrication of outstanding H₂ sensor. In a resistive-type gas sensor, n-type semiconductor ZnO sensing material is deposited on two or more electrodes by various methods. Upon exposure or withdrawal to H₂ gas on the surface of ZnO, the changes in resistance of the sensor are measured during the adsorption and desorption of H₂ gas, as shown in Fig. 1a. At first, oxygen molecules in air with higher electron affinity (0.43 eV) are easily adsorbed on the surface of ZnO according to the following reactions.^21,46,47

$$\begin{eqnarray}&&{{\rm{O}}}_{2}\left({\rm{gas}}\right)\leftrightarrow {{\rm{O}}}_{2}\left({\rm{ads}}\right)\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{2}\left({\rm{ads}}\right)+{{\rm{e}}}^{-}\leftrightarrow {{\rm{O}}}_{2}^{-}\left({\rm{T}}\lt 100\,^\circ {\rm{C}}\right)\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{2}\left({\rm{ads}}\right)+2{{\rm{e}}}^{-}\leftrightarrow 2{{\rm{O}}}^{-}\left(100\,^\circ {\rm{C}}\lt {\rm{T}}\lt 300\,^\circ {\rm{C}}\right)\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{{\rm{O}}}_{2}\left({\rm{ads}}\right)+4{{\rm{e}}}^{-}\leftrightarrow 2{{\rm{O}}}^{2-}\left({\rm{T}}\gt 300\,^\circ {\rm{C}}\right)\end{eqnarray} \tag{ 4 }$$

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The adsorbed oxygen can extract electrons from the conduction band of ZnO and becomes the surface oxygen ions, leading to the decrease of the carrier concentration in ZnO with increased resistance. At different operating temperatures, different charged oxygen ions are formed. While H₂ gas is introduced into the environment, H₂ molecules can easily interact with adsorbed oxygen ions on the surface of ZnO. And the released electrons go back to the conduction band of ZnO, causing a drop in electrical resistance of ZnO, as described by the following equation.^20,48

$$\begin{eqnarray}&&{{\rm{H}}}_{2}\left({\rm{g}}\right)+{{\rm{O}}}^{2-}\left({\rm{ZnO}}\right)\to {{\rm{H}}}_{2}{\rm{O}}\left({\rm{g}}\right)+2{{\rm{e}}}^{-}\end{eqnarray} \tag{ 5 }$$

The gas sensing mechanism of ZnO can be described simply by the following three models along with the actual situations: surface charge layer model,^17,18,47,49 grain boundary barrier model,^20,47,50,51 and valence control model.^{17,48,52–54}

Surface charge layer model

Normally, the conductivity of material is affected by the thickness of depletion layer. After the ZnO nanoparticles (NPs) are exposed to the air, low charge density will exist near the surface and the conductive channel becomes narrow, which leads to the resistance increasing. The region with low electron density nearby the surface is defined as the electron depletion layer (EDL), whose depth is called as Debye length (L_D), as shown in Fig. 1b. The L_D is typically on the order of ∼20–22 nm for ZnO at 250 °C–300 °C based on the following formula,^45,55

$$\begin{eqnarray}&&{{\rm{L}}}_{d}={\left(\displaystyle \frac{k{\varepsilon }_{o}{k}_{B}T}{{q}^{2}{N}_{c}}\right)}^{\displaystyle \frac{1}{2}}\end{eqnarray} \tag{ 6 }$$

where ε_o is the static dielectric constant, k_B is the Boltzmann constant, T is the absolute temperature (523 or 573 K), q is the electrical charge of the carrier, and Nc is the carrier concentration of ZnO (5.1 × 10¹⁶ cm⁻³), and k is relative permittivity of wurtzite structure ZnO (7.8).

When exposed to a reducing gas of H₂, the depletion layer thickness is reduced, causing a measurable change in the electrical conductivity of the material. For single crystal ZnO nanostructures, the conductivity of the material is affected mainly by the thickness of EDL, and generally the increase of EDL thickness generates better H₂ sensing performance.^17,18,47,49

Grain boundary barrier model

Valence control model

The H₂ sensing performance of pristine ZnO materials can be tuned by reducing the size into nanometer level and designing the particular structure/morphology. Some work has confirmed that the gas sensing activity of hexagonal wurtzite ZnO is related to crystal facets.^43,56 Owing to the unsaturated oxygen coordination, the Zn terminated (0001) planes more easily adsorb the oxygen species than the O-terminated (0001) and non-polar (1010) facets, showing a robust chemisorption process. In consequence, the gas sensing activity of the ZnO crystal facets in reducing target gas decreases in the order of (0001) > {1010} > {1011} and (0001).⁵⁶ In addition, ZnO nanomaterials probably have larger specific surface area and lots of nanograins with more grain boundaries and higher barrier height between the grains, which increases the adsorption of H₂ molecules and oxygen species on the surfaces with the improved sensitivity of H₂ sensor. In the recent years, there are many researches on ZnO nanostructures, such as NPs, NWs, NTs, NFs, and nanobelts for H₂ sensing. According to zero-dimension (0D), one-dimension (1D), and two-dimension (2D), the H₂ sensing properties of different pristine ZnO nanostructures-based sensors without any modification are summarized in Table II.

By careful comparing pure ZnO nanostructures-based sensors' results from various dimensions, especially under similar conditions, it is found that in general 1D ZnO nanostructures such as NWs, NTs and NFs exhibit better H₂ sensing performance than 2D ones of TFs and nanobelts and 0D ones of NPs and nanospheres, for example relatively higher sensitivity and slight lower work temperature.

First, the H₂ sensing activity of single ZnO NW-based sensor at room temperature (RT) is introduced.⁶⁰ ZnO NW (diameter of 200 nm) is MOCVD-derived single crystalline without noticeable defects, as seen in Fig. 2a. Pt/ZnO single NW Schottky diodes fabricated by e-beam lithography exhibit good H₂ sensing properties of 1.9^a with a faster response time of 55 s at RT in Fig. 2b. This is attributed to low barrier height of 0.42 V in ZnO/Pt Schottky junction and higher surface-to-volume ratio of ZnO NW surface at RT. Single ZnO NWs allow quick diffusion of H₂ gas into and out the nanostructure. Thus, the rapid reaction rate leads to a higher sensitivity with faster response and recovery.⁶⁰ In addition, multiple ZnO NWs devices are more sensitive than single ZnO NW devices due to the nanojuction effect in the multiple NWs ones as potential barrier for electron flow, however it also cause a significant diffusion resistance in the recovery period because of the lower O₂ diffusivity.⁵⁹

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For ZnO NWs or NRs gas sensors, the diameter (D) of NWs or NRs has a significant impact on H₂ sensing performance. According to the theory of Rothschild et al.,⁶⁹ when D is comparable to 2L_D, the NWs or NRs sensors' response will tend to maximum value. From Table II, it can be observed that with decreasing the D of ZnO NWs or NRs from 100–200 nm to several tens of nanometers, the H₂ sensing response becomes larger on the whole.^57,58,65

Hollow 1D NTs sensor also shows similar tendency, as illustrated in Figs. 2c and 2d. When the wall thickness (t) of ZnO NTs is around 42.7 nm, equal to 2L_D at 250 °C, the highest sensitivity of 139.1^a is achieved.³⁹ Moreover, compared to NWs and NRs, the oxygen species can be adsorbed both inner and outer sides of NTs,⁷⁰ producing stronger response to H₂. However, NTs' hollow structure with high aspect ratio will also affect the gas diffusivity, leading to longer diffusion time, especially for recovery time.^18,39 Porous ZnO NTs sensor presents 2.5 times and 4.2 times improvement in the H₂ sensing activity than nonporous ZnO NTs and porous ZnO thin films sensors, respectively.¹⁸

Similarly, the performance of H₂ sensor based on ZnO can be effectively tuned by controlling the grain size (d). When d ≤ 2L_D, the whole nanoparticle experiences electron depletion; When d > 2L_D, EDL only appears at the surface region.⁴⁵ In the former case, the resistance, i.e. the H₂ response, strongly depends on the grain size. The smaller grain size is beneficial for the sensitivity of gas sensors, but excessive drop in d reduces structural stability, producing a deleterious effect on the temporal stability of the sensors.¹¹ In the latter case, the resistance is limited by the Schottky barrier at grain boundaries in independent of the grain size.

Recently, the outstanding H₂ sensing performance of ZnO NFs-based sensors has been reported in much literature.^{20,21,47,48,51} Nanofibers contain numerous nanograins with porous structure and higher specific surface area, so large amounts of H₂ can easily diffuse along the grain boundaries, resulting in the improved H₂ sensing performance. Figure 2e records pristine ZnO NFs' morphologies with d of ∼30 nm and Fig. 2f displays the sensor responses of ZnO and SnO₂ NFs at various H₂ concentrations and work temperatures.⁶⁶ It is easily observed that ZnO NFs exhibit excellent H₂ response of 139^a at 250 °C to 1000 ppm H₂ and better selectivity (see Fig. 6b) than SnO₂ NFs. It is ascribed to the semiconductor-to-metal transition from ZnO to Zn at the surface of nanograins induced by H₂ gas molecules, leading to more effective resistance modulation to ZnO NFs with the exceptional response and selectivity, in comparison with SnO₂ NFs.⁶⁶

Finally, we also show the morphology and H₂ sensitivity of the ZnO TFs by RF sputtering in Figs. 2g and 2h.⁶⁸ The ZnO TFs has strong (002) c-axis orientation with average d of 20.8 nm, very close to L_D, which displays better response of 50^a at 350 °C to 200 ppm H₂. However, by and large, pristine ZnO TFs sensors have worse gas sensitivity owing to smaller specific surface area.^18,62,67

The ZnO nanostructures can be used as sensing layer for H₂ gas sensor. Although pure ZnO NFs-based sensors exhibit admirable H₂ sensing characteristics at higher operation temperature of 350 °C⁶⁶ and single ZnO NW-based nanoscale sensors presents better H₂ sensing activity at RT,^55,60 in most cases, pristine ZnO still has some demerits such as higher operation temperature, worse selectivity, longer response time/recovery time, and relatively lower sensitivity in the H₂ sensing. Hence, great attention has been paid to improve the sensing performance of ZnO based H₂ sensors. Quite a few approaches have been taken to improve the H₂ sensing properties of ZnO, as summarized in Table III. Several kinds of effective strategies, such as surface modification, doping, composite and post-treatment, are elaborated as follows.

Surface modification

In this section, the effect of surface modification of ZnO on the performance of H₂ sensors is discussed. Herein the surface modification main relates to the noble metals, metal oxides and carbon materials. Among them, the incorporation of noble metal NPs on ZnO nanostructures is the most common and valid method for ameliorating the performance of H₂ gas sensor.

The most commonly used noble metals are Pd, Pt and Au,^29,98 which have higher catalytic activity in gas sensing application. Compared to unsensitized pristine ZnO, these noble metals NPs loaded on ZnO surfaces can reduce the adsorption energy of gases, which facilitates the adsorption of more oxygen species and H₂ molecules on the ZnO surface with more surface active sites as a result of the well-known spillover effect (Figs. 3a and 3b).²⁴ Simultaneously, noble metals have higher work function than ZnO, thus the electrons will migrate from ZnO conduction band to metal when in contact with ZnO, which creates the additional surface depletion layer than pristine ZnO, as indicated in Fig. 3c.²⁴ Furthermore, Schottky barrier formation at the interfaces of metal and ZnO can effectively prevent from the recombination of electron-hole pairs.^73,75,99

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Drmosh et al. investigated the effect of Pt, Au and Ag NPs on energy band and H₂ gas response of 40 nm-thick (002)-oriented ZnO TFs, as illustrated in Figs. 3d and 3e.⁷³ It is found that Pt NPs@ZnO with larger work function of 5.6 eV results in an increased depletion layer and an evident improvement of the H₂ sensing activity than Au NPs@ZnO with work function of 5.1 eV at 400 °C, whereas Ag NPs@ZnO with work function of 4.4 eV and heated pristine ZnO TF sensors show similar response. It indicates that the electronic mechanism is dominant in improving the performance of noble metal NPs decorated films due to the difference in work functions of several metals.⁷³

As described previously, ZnO 1D nanostructures such as NWs and NFs usually have better H₂ sensing properties than ZnO TFs. Figures 3f and 3g record the response of the bare and Pd-functionalized ZnO NW sensor exposed to various gases such as H₂, C₆H₆, C₇H₈ at 350 °C, respectively.¹⁷ The modification of Pd NPs on ZnO NW sensors not only improves the sensitivity but also the selectivity (see Fig. 7f) due to the formation of Pd hydride (PdH_x) and H₂-induced Zn metallization of ZnO surface.^17,48,54

Recently, bimetallic NPs decorated ZnO NRs has been explored to enhance the H₂ gas sensors.^78,79 Figure 3h shows the HRTEM images of the Pt-Au alloy NPs loaded on ZnO NRs with the average size of 2.95 nm and the Pt: Au atomic ratio of 0.97. Pt-Au alloy NPs/ZnO sensors exhibit highest sensing response of 157.4^a at 130 °C to 250 ppm H₂ in Fig. 3i, which is 157, 47, 9.6, and 2.7 fold of that of pure ZnO, Pt NPs/ZnO, Au NPs/ZnO, and (Pt+Au) NPs/ZnO, respectively.⁷⁸ Even at RT, it also achieves the high sensitivity of 25^a to 250 ppm H₂. The outstanding performance of Pt-Au NPs-loaded ZnO is ascribed to the combination of synergistic and electronic effects of bimetallic alloy, notably improving the H₂ adsorption and catalytic activity on ZnO.

In addition, Pt/Pd bimetallic core–shell (3 nm/3 nm) NPs decorated ZnO NRs sensors show a reasonably high response of 2.4^a to 10000 ppm H₂, an extreme fast response time of 5 s, and a wide detection range from 0.2 to 40,000 ppm at 100 °C. It also has a negligible humidity effect over the entire detection range and a good selectivity when other gases such as CO₂, CO, N₂, NO₂, and O₂ exist.⁷⁹ The better sensing features are related to the enhancement of H₂-induced changes in the work function of the quantum-size Pt/Pd-ZnO NRs. The atomic arrangements and chemical potentials of the core–shell interfacial region also play important role in facilitating the H₂ sensing capability.⁷⁹

It is worth pointing out that at present Pd is the most suitable noble metal for the detection of H₂ due to the high costs of Pt and Au and better catalytic effect of Pd than that of Ag.

Apart from noble metal modifications, the efficiency of H₂ sensor can be also enhanced by the metal oxide NPs loaded on ZnO nanostructures, such as WO₃ and PdO.^16,81 The former enhanced response of 4.3 times is ascribed to the increased depletion layer width due to the n-type WO₃ NPs loaded on the surface of ZnO NWs,¹⁶ and the latter improvement in sensitivity and selectivity against ethanol and ammonia gases results from the spillover effect of H₂ in contact with the PdO particles and surface metallization of ZnO NRs.⁸¹

Finally, the carbon modification of ZnO nanostructures also exhibits promising H₂ sensing performance in Table II, such as 0.2 wt% nanoporous carbon NFs (CNFs) loaded on ZnO nanostructures derived by RF sputtering and 0.75 wt% reduced graphene oxide (rGO) loaded on 4 at% Ni-doped ZnO nanostructures (See Figs. 4k and 4l).^72,82 It is related to the formation of p-n heterojunction at the interface of CNF/rGO and ZnO. The presence of oxygen functional groups on CNF/rGO surfaces also contributes to the enhancement of the sensing performance.

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Doping

Composite

The H₂ sensing performance of ZnO can be enhanced by formation of composites including NCPs such as ZnO–SnO₂ composite NFs⁵¹ and graphene/ZnO NCP,⁸⁷ and core-shell (C–S) composites such as ZnO–SnO₂ C–S NWs⁶⁵ and ZnO@ZIF-8 C–S NRs.⁶⁴ This can be attributed to the increased potential barrier height and surface depletion layer width, causing the significant resistance change after the introduction of H₂ on the composite.^51,102

An additional depletion layer is formed due to the difference in work function of two materials.¹⁰³ In 0.9 SnO₂-0.1 ZnO composite NFs, when nanograins of SnO₂ contact with ZnO, electrons will be transferred from SnO₂ to ZnO through the heterostructure until reaching the equilibrium of Fermi levels with reduced resistance of composite.⁵¹ In Fig. 5a, the high-resolution lattice fringes of several SnO₂ and ZnO nanograins can be clearly observed in the SnO₂–ZnO composite NFs, forming homo-interfaces and hetero-interfaces. Usually the grain size will affect the gas sensing ability. Grain size tuned H₂ response characteristics are depicted in Fig. 5b. The smaller the grain sizes of SnO₂–ZnO composite NFs such as 12 nm, the more the number of heterojunctions and homojuntions, resulting in the superior sensitivity of 3.6^a to 30 ppb H₂ at 350 °C (Fig. 5c).⁵¹

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In addition, p-type NiO incorporated with n-type ZnO NFs forms the P-N heterojunction, which is more sensitive towards H₂ sensing.²¹ Besides, it is noteworthy that higher amounts of oxygen ions could be easily adsorbed on the surface of the NiO, because Ni²⁺ ions are easily oxidized into Ni³⁺.²¹

Graphene offers an extensive 3D network to enhance the interconnectivity with ZnO, hence the optimal sensor performance towards H₂ at lower temperature of 150 °C can be achieved using 1.2 wt% rGO/ZnO NCP, as shown in Fig. 5d.⁸⁷ Due to the large specific surface area, high electronic mobility and low electrical noise, the defects and functional groups on the rGO surfaces act as high energy adsorption sites for the H₂ molecules.⁸⁷

As discribed previously, Pd NPs loaded on ZnO nanostructures significantly facilitate the H₂ sensing capability of gas sensors.¹⁷ Herein, the Pd NPs-functionalized 0.1In₂O₃-0.9ZnO NFs were prepared by electrospinning and UV irradiation method.⁸⁰ HRTEM image shows the microstructure of Pd NPs-functionalized 0.1In₂O₃-0.9ZnO NFs with clear lattice fringes from ZnO, In₂O₃, and Pd nanograins, as seen in Fig. 5e. The Pd NPs-functionalized 0.1In₂O₃-0.9ZnO NFs also present a strong response of 172^a to 50 ppb H₂ at 350 °C, much higher than 72^a from 0.1In₂O₃-0.9ZnO NFs without Pd NPs' modification in Fig. 5f. The improved H₂ response is attributed to the outstanding catalytic ability of Pd to H₂, the formation of PdH_x, and more junctions of Pd/ZnO, In₂O₃/ZnO, and ZnO/ZnO.⁸⁰

In core–shell composite structure, it is found that the gas sensing capability has been improved significantly.^54,64,90,92 Based on Kim's research group's review report on core–shell composite NWs or NRs gas sensors, gernerally when the shell thickness is comparable to the L_D of the shell, the maximum response can be obtained due to a fully electron-depleted shell, leading to an enhanced gas sensing response.⁴⁵ However, sometimes the optimal shell thickness is observed to be equal to 2L_D, producing the highest gas response to reducing gas.⁴⁵ For example, Park et al. reported that the H₂ sensing properties of Nb₂O₅–ZnO C–S NW sensors is strongly dependent on the shell thickness (0–63 nm) of ZnO prepared by atomic layer deposition (ALD).⁹² The best response to 1% H₂ at 300 °C is confirmed in Nb₂O₅–ZnO C–S NR sensors with a shell thickness of 46 nm, close to 2L_D of ZnO. The reason lies in that apart from the surface depletion layer at outer surface of the shell, depletion layers also form at core–shell the interface, affecting the optimal shell thickness. So, the transfer of electrons though interface barrier is restrained, which increases the width of depletion layer and leads to an increased change in resistance and the enhanced sensitivity of H₂ sensor.

Finally, a selective H₂ gas sensor based on novel MOF ZIF-8 coated Pd NPs/ZnO NWs has been developed, as indicated in Fig. 5h.⁵⁴ The GIXRD pattern in Fig. 5g and corresponding HRTEM (not shown here) has demonstrated that an ultrathin 2−3 nm thick ZIF-8 layer has covered homogeneously on the Pd NPs-decorated ZnO NW surface. The Pd NPs is beneficial to achieve maximal H₂ signal responses, whereas the ZIF-8 coating enables for an excellent selectivity. As though the ZIF-8/Pd/ZnO NWs composite H₂ sensor has a slight decreased sensitivity of 3.24^a to low 10 ppm H₂ at 200 °C than Pd NPs-decorated ZnO NWs with 4.26^a, it displays enhanced response than pure ZnO NWs with 1.35^a, as seen in Fig. 5i. More importantly, the introduction of ZIF-8 nanomembrance as a molecular sieve in Pd/ZnO NWs composite gets a superior selective H₂ sensor, which will be discussed later.

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Post-treatment

Gas selectivity has always been a major issue for the resistive-type ZnO-based H₂ sensors. From the view of the practical application, a sensor should present a rather high selectivity in the presence of other interfering gases such as ethanol (C₂H₅OH), CO, CH₄, benzene (C₆H₆) or NO₂. Different approaches have been employed to solve this problem, such as noble metal functionalization or sensitization, metal doping, nanocomposite or core–shell structure, molecular sieving effect, metallization effect of ZnO surface, irradiation, and synergistic effect of bimetallic alloy. Table IV summarizes recent progress on selectivity of ZnO nanosturctures-based H₂ sensors.

As seen from Table IV, most pristine ZnO nanostructures-based sensors, such as NPs, NWs, NRs, NTs, and TFs, have poor selectivity for H₂ in the presence of interfering gases such as C₂H₅OH, acetone, i-butane, CH₄, C₂H₂, CO_2, CO, H₂S, and NO₂. However, very few exceptions can be observed in single ZnO NW-based nanoscale sensor and electrospinning-derived ZnO NFs H₂ sensor.^55,66

The effect of diameter of single crystal ZnO NW sensor on the H₂ response and selectivity at RT is shown in Fig. 7a and Table IV.⁵⁵ It can be seen that the nanoscale sensor based on individual ZnO NW with diameter of 100 nm have better selectivity for H₂ when reducing gases of NH₃, i-butane, and CH₄ exist. As the diameter of ZnO NWs increases from 100 nm to 200 nm and 600 nm, the H₂ response evidently decreases and the H₂ selectivity becomes worse. For example, the corresponding value of S_H2/S_i-butane reduces from 6.8 (D = 100 nm) to 1.8 (D = 200 nm) and 0.44 (D = 600 nm). Lupan et al. attributed this to the diameter of ZnO NW on the influence of surface to volume ratio and surface defect concentration.⁵⁵

Katoch et al.⁶⁶ made a comparative study on the selectivity of SnO₂ and ZnO NFs to H₂, CO, C₆H₆, and toluene (C₇H₈), as illustrated in Fig. 7b. Compared to SnO₂ NFs, ZnO NFs have unmistakable H₂ selectivity due to the Zn metallization effect on the surface of ZnO nanograins in NFs. Although SnO₂ NFs consist of 30 nm nanograins similar to ZnO ones, it lacks the reaction from SnO₂ to Sn on the nanograin surface induced by H₂. As a result, the value of S_H2/S_CO of SnO₂ NFs is only 1.55, however that of ZnO is up to 4.52.

Generally speaking, the selectivity to H₂ of pristine ZnO-based gas sensors can be improved by some measures such as doping and surface modification. As mentioned earlier in Figs. 4h–4i, the 4 at% Ni-doped ZnO nanoplates and 0.75 wt% rGO loaded 4 at% Ni doped ZnO nanoplates show enhanced H₂ sensitivity than without Ni-doped and rGO-decorated ZnO samples.^72,84 Figures 7c and 7d illustrate the selectivity of 3 kinds of samples.^72,84 It can be seen that undoped ZnO TFs has worse selectivity to H₂, CH₄, CO_2, and H₂S. The 4 at% Ni-doping and 0.75 wt% rGO surface decoration improves the selectivity to H₂ over other reducing gases. For instance, the S_H2/S_CH4 of bare ZnO films is merely 1.12. After the Ni-doping, it becomes 2.41. Similar result occurs in rGO loaded doped ZnO sample with higher sensitivity, revealing the helpful role of Ni-doping and rGO surface modification on the selectivity of H₂ sensors.

Apart from rGO decoration, modification of noble metals is one of the most effective methods to reduce the energy required for the gas adsorption/disorption reaction. Fan et al. investigated the selective properties to H₂ of Pt, Au, Pt+Au and Pt-Au alloys NPs loaded on ZnO NRs in presentence of C₂H₂, CO, and CH₄.⁷⁸ The values of S_H2/S_CO are 60.07, 27.49, 8.23, 1.98, and 1 for the Pt-Au/ZnO, (Pt+Au)/ZnO, Au/ZnO, Pt/ZnO, and bare ZnO NRs calculated from the data in Fig. 7e, respectively. Individual Pt and Au NPs play a valid role in increasing the H₂ selectivity of ZnO NRs. Furthermore, the mixture of Pt and Au NPs produces much better H₂ selectivity than single Pt or Au NPs. However, the Pt-Au bimetallic NPs show the best H₂ selectivity over C₂H₂, CO, and CH₄ because the adsorption of H₂ onto the Pt-Au NPs is more favorable than that on monometallic.

In addition, Pd functionalized ZnO nanostructures are highly selective for H₂ detection due to the formation of Pd hydride (PdH_x),^17,48,54 resulting in the great improvement in the selectivity of the H₂ sensors. In Fig. 7f, the values of S_H2/S_C6H6 are 1.68 and 47.38 before and after Pd functionalized ZnO NWs.¹⁷ Similarly, individual Au NPs decorated ZnO NW also exhibits stronger H₂ selectivity with negligible response to other interfering gases such as acetone, C₂H₅OH, CH₃OH, 2-propanol, n-butanol.⁷⁵

Another very effective method for improving the selectivity of H₂ sensor can be achieved by choosing metal organic frameworks (MOFs) with different pore sizes as a molecular sieve.⁶⁴ Weber et al. reported that the selectivity of Pd-decorated ZnO NWs can be notably improved by the introduction of ZIF-8 MOF nanomembranes.⁵⁴ The values of S_H2/Sacetone are increased from 1.44 for bare ZnO NWs to 1.80 for Pd NPs/ZnO NWs and 6.6 for ZIF-8/Pd NPs/ZnO NWs by adding Pd and ZIF-8, respectively, as shown in Fig. 7g. The related mechanism is illustrated in Fig. 7h. The ZIF-8 membranes with micropores size of 3.4 Å only allow smaller H₂ molecules of 2.9 Å to pass through the membranes layer instead of other larger gas molecules such as acetone, C₆H₆, C₇H₈, and C₂H₅OH.⁵⁴ Thus, the ZIF-8/Pd/ZnO NWs composite H₂ sensor displays admirable selectivity than pure ZnO NWs and Pd NPs-decorated ZnO NWs.⁵⁴

The aforementioned pure ZnO NFs exhibit better selectivity to 10 ppm H₂ at higher work temperature of 300 °C. Based on this foundation, Kim's research group has performed a series of in-depth improvement studies on the H₂ selectivity of ZnO-based nanocomposite NFs using SnO₂,^20,51 NiO,²¹ Co₃O₄,⁸⁹ and Pd NPs decorated In₂O₃,⁸⁰ as indicated in Figs. 8a–8e. Using CO as an interfering gas, the value of S_H2/S_CO for pristine ZnO NFs (300 °C, 10 ppm H₂), 0.9SnO₂-0.1ZnO NFs (350 °C, 0.1 ppm H₂), 0.1SnO₂-0.9ZnO NFs (300 °C, 0.05 ppm H₂), 0.05Co₃O₄-0.95ZnO NFs (300 °C, 10 ppm H₂), 0.05NiO-0.95ZnO NFs (200 °C, 10 ppm H₂), and Pd NPs/0.1In₂O₃–0.9ZnO NFs (350 °C, 0.05 ppm H₂) is 4.52, 6.39, 31.85, 13.64,17.65, and 11.28, respectively. The experimental results reveal that the ZnO nanocomposite NFs exhibit evidently enhanced H₂ selectivity over other interfering gases, especially for 0.1SnO₂-0.9ZnO NFs sample with an outstanding 50 ppb-level H₂ selectivity at 300 °C owing to the formation of heterojunctions between ZnO and SnO₂ grains and the metallization effect of ZnO nanograins.²⁰ Moreover, Pd-functionalized 0.1In₂O₃–0.9ZnO NFs also display 50 ppb-level H₂ selectivity with superior sensitivity up to 172^a at 350 °C, which is attributed to the synergetic effect of the Pd catalytical activity, large amount of junctions in composite NFs, and Zn metallization effect.⁸⁰

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In addition, it is found that the EB and UV irradiation can also enhance the H₂ selectivity of ZnO-based has sensors, as seen in Figs. 8f–8h.^47,93,94 After 150 kGy EB irradiation, the value of S_H2/S_CO of ZnO NFs can be remarkably increased to 106.38 at 10 ppm H₂ and 350 °C, which is related to enhanced oxygen vacancies and BET specific surface area produced by EB-irradiation and the transition from ZnO to Zn.⁴⁷ For Au NPs-decorated ZnO TFs, UV illumination at 365 nm may slightly ameliorate the H₂ selectivity to reducing gases of SO₂, CO, CO₂, C₂H₅OH, CH₄, and H₂S, and oxidizing of NO₂ at 1000 ppm H₂ and 250 °C, but not very evident in Fig. 8g.⁹⁴ For Au decorated rGO/ZnO NCPs, UV irradiation shows superior H₂ selectivity to C₄H₁₀, CH₄, CO₂, NH₃, and O₂ at RT, as depicted in Fig. 8h.⁹³ The UV light enhancement of gas-sensing performance can be ascribed to the drop of activation energy between the surface of the sensor and H₂ gas.

Above all, ZnO is a suitable and excellent sensing material for the detection of H₂ gas. However, the pristine ZnO suffers from a limited selectivity owing to the good responses to other interfering gases. Some effective and viable strategies such as the Pd-functionalization,¹⁷ the formation of metal oxide-ZnO nanocomposite NFs,²⁰ combination of several approaches, e.g. Pt-Au alloys NPs modified ZnO NRs,⁷⁸ Pd-functionalized 0.1In₂O₃-0.9ZnO NFs,⁸⁰ ZIF-8 encapsulated Pd/ZnO NWs,⁵⁴ and UV-activated Au decorated rGO/ZnO NCP,⁹³ have been explored to circumvent this problem. A few gas sensors with admirable H₂ selectivity have been achieved. However, continuous efforts are needed to obtain selective H₂ sensor with superior selectivity and extremely low detection limit operated at RT.

With an aim to improve the performance of H₂ sensor based on ZnO nanostructures, this review paper compares the sensing properties and related mechanism of various kinds of pristine ZnO nanostructures for H₂ detection. Furthermore, some improvement strategies of H₂ sensing performance based on ZnO nanostructures sensors are focused and summarized. Especially, the selectivity of ZnO nanosturctures-based H₂ sensors in the presence of interfering gases is strengthened and discussed.

A series of researches indicate that the H₂ sensing activity of ZnO can be improved by increasing the active sites, widening the depletion region, and enhancing grain boundary barrier. In terms of nanostructures, the grain sizes in NPs, NFs and TFs, the diameter of NWs or NRs, the shell thickness of NTs have significant effect on the sensing capability of H₂ sensor based on pure ZnO nanostructures according to surface charge layer model related to L_D. For polycrystalline ZnO material, grain boundary barrier model has to be considered. Different from other metal oxide, e.g. SnO₂, the H₂-induced metallization effect of ZnO at high temperature plays a crucial role in improving the sensitivity and selectivity of ZnO-based H₂ sensors along with valence control model.

At present, the H₂ sensing activity of individual single-crystal ZnO NW-based sensor at RT has been confirmed by using Pt/ZnO Schottky diode structure.⁵⁵ It is attributed to low barrier height of 0.42 V in ZnO/Pt Schottky junction and higher surface-to-volume ratio of ZnO NW surface. ZnO NTs has more adsorption sites at inner and outer sides of hollow NTs, leading to stronger response to H₂. But hollow structure with high aspect ratio will also affect the gas diffusivity with longer recovery time.³⁹ Recently, the superior H₂ sensing characteristics of pure ZnO NFs sensors have been reported at 10 ppm H₂ and 350 °C,⁶⁶ which is related to the existence of numerous nanograins and porous structure in NFs. However, the higher work temperature impeding the practical application due to the power consumption issue.

Therefore, although ZnO is an outstanding H₂ sensing material with high response, good stability, low cost, and extremely abundant nanostructures, pristine ZnO suffers from drawbacks such as higher operation temperature, worse selectivity, longer response/recovery time, and higher limit of detection. Quite a few approaches have been explored to enhance the performance of ZnO-based H₂ sensors such as surface modification, doping, composite, and post-treatment. Among these strategies, noble metal modification and nanocomposite fabrication are two kind of effective and simple method for improving the H₂ sensing ability, especially Pd NPs decoration^17,29 and ZnO composite NFs' formation.^21,51,80 In addition, bimetallic NPs such as Pt-Au alloy exhibit better H₂ sensing responses compare to individual metal ones owing to the synergistic effect.⁷⁸ Some studies have combined two or three improvement routes to achieve excellent performance of ZnO based H₂ sensors, e.g. Pd NPs-functionalized 0.1In₂O₃-0.9ZnO NFs for ultra low detection of 50-ppb H₂, ZIF-8 coated Pd NPs decorated ZnO NWs for superior selectivity to H₂, Pt/Pd core–shell NPs decorated ZnO NRs for ultrafast response of 5 s at 100 °C,⁷⁹ and Au loaded 1.2 wt% rGO/ZnO NCP with UV irradiation for higher sensitivity and selectivity at RT.^54,79,80,93

Above all, by extensive and intensive researches, some significant progress has been achieved for resistive-type ZnO nanostructures-based H₂ sensors such as higher response, lower work temperature even at RT, faster response time, lower detection concentration, longer durability, and better selectivity. However, until now, the development of ZnO-based H₂ sensor with excellent comprehensive performance, which can simultaneously meet with outstanding sensitivity, superior selectivity and extremely low detection limit operated at RT, is still a tough challenge. Sometimes, we have to reach certain trade-off between sensitivity, selectivity, and work temperature.

At present, the sensing activity of ZnO based H₂ sensor at RT is still evidently worse. They usually need to work at high temperature, which causes the higher power consumption with inconvenience. Therefore, it is necessary and urgent to improve the H₂ sensing performance at RT. Recently, H₂ gas can be detected at RT by using two dimension electron gas in the interface of Al₂O₃/TiO₂ ultra-thin film derived by novel atomic layer deposition,¹⁰⁹ which is an enlightenment of ZnO based H₂ sensor. Atomic layer deposition (ALD) is a novel materials fabrication method compatible with the semiconductor technology. The unique self-limiting and self-saturation reaction mechanism enables the deposited nanomaterials with large area uniformity, simple and precise thickness control, excellent three-dimensional conformality and low deposition temperature.¹¹⁰ It may be a suitable method for the preparation of H₂ sensing materials, such as core–shell structures, nanotubes, nanolaminate, and metal nanocrystals, with controlled thickness even at sub-nanometer. In future, the highly sensitive, flexible and wearable H₂ sensor will be fabricated by using ZnO based gas sensor combined with the advanced micro-electro-mechanical system (MEMS) and other preparation techniques such as ALD.

In summary, at the tipping point the development of ZnO nanostructures-based H₂ sensors is facing tremendous opportunities and challenges ahead. It is expected that ZnO nanostructures-based H₂ sensors have promising futures and prospects in the Internet of Things, energy fields, environmental protection and safety through persistent efforts of academia and industry.

This work was supported in by the National Science Foundation of China (grant No. 51571111, 51721001, 51802150) and Jiangsu Province (BK20161397 and BK20170645), and a grant from the State Key Program for Basic Research of China (2015CB921203).