Fundamental Pathways for the Adsorption and Transport of Hydrogen

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Fundamental pathways for the adsorption and transport of hydrogen on TiO2 surfaces: origin for effective sensing at about room temperature Zhuo Wang, Xiaohong Xia, Meilan Guo, and Guosheng Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12071 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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ACS Applied Materials & Interfaces

Fundamental Pathways for the Adsorption and Transport of Hydrogen on TiO2 Surfaces: Qrigin for Effective Sensing at about Room Temperature

Zhuo Wang†,‡,§, Xiaohong Xia*,¶, Meilan Guo†,¶, Guosheng Shao*,‡,§ †

‡

Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, China State Centre for International Cooperation on Designer Low-carbon and Environmental

Materials (CDLCEM), Zhengzhou University, Zhengzhou 450001, China §

Institute for Renewable Energy and Environmental Technologies, University of Bolton,

Bolton BL3 5AB, UK ¶

Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, China

Abstract Effective detection of hydrogen at lowered temperature is highly desirable in promoting safety in using this abundant gas as clean energy source. Through first principle calculations in the frame work of density functional theory, we find that the high energy (002) surface for rutile TiO2 is significantly more effective in adsorbing hydrogen atoms through dissociating hydrogen molecules. The pathways for the dissociation of hydrogen molecules and sequential migration of hydrogen atoms are identified through searching along various transitional states. Pathways of low potential barriers indicate promise for hydrogen sensing even close to room temperature. This has been proven through sensors made of thin films of well aligned rutile nanorods, wherein the high energy (002) surface dictates the top surface of the active layer of the sensors.

Keywords: DFT modeling, Mechanism for hydrogen sensing; Room temperature hydrogen sensor, Interaction pathways; Surface energy effect

*Corresponding author email: [email protected], [email protected]

ACS Paragon Plus Environment


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I. Introduction Hydrogen is a completely green renewable fuel with abundant natural resource. A main hindrance to limit its use in large quantity is due to safety concerns associated with possible leakage and sequential hazards such as fire or even explosion. However, as an odorless, colorless and tasteless flammable gas, hydrogen cannot be detected by human senses, and sensors are therefore requested to detect its presence and quantify its concentration. Rapid and accurate measurement of hydrogen gas concentration is essential to alert on the formation of potentially explosive mixtures with air, thus helping preempt potential accidents. Among various hydrogen sensing techniques, resistive sensors based on metal oxides are considered most promising, owing to their low manufacturing cost, long service life and moderate power cost for device heating. Most current commercial hydrogen sensors are based on SnO2 or ZnO, which operates at elevated temperatures as high as 150-200 oC,1-4 a main weakness for applications. Recently, the demand to have low-cost and sensitive sensors with reduced power consumption for operation has been a main driver for world-wide efforts in developing advanced hydrogen sensors based on various semiconducting metal oxides. As an multifunctional transition metal oxide based on cheap and environmentally friendly resources, nano-crystalline TiO2 phases have also been exploited for hydrogen sensing, using films made of either polycrystalline nanotubes of anatase TiO2

5

or, more recently,

self-assembled rutile nanorods.6 Assembling rutile nanorods into dense films 7 led to confined growth along the c-axis of the tetragonal phase of rutile TiO2, thus leading to dominant exposure of the (002) surface. It was postulated 6 that the higher surface energy of the top surface of the self-assembled film was key to the remarkable reduction of operating temperature (from over 225 oC down to below 100 oC) and drastic improvement of sensitivity were achieved at room temperature by introducing a seeding layer to further enhance the [002] orientation in later work.8 An additional advantage in using a dense film over porous nanotubes lies in the simplicity and reliability in making planar devices due to well controlled contact quality.6-8 In spite of wide use of gas sensors based on semiconducting metal oxides, scientific understanding of sensing mechanisms due to change of electrical resistance is rather limited. Fundamentally, one needs to answer the following questions before providing useful guidance


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in achieving reliable sensing of hydrogen, particularly at reduced operational temperature: (a) How will hydrogen molecules interact with the TiO2 surfaces? (b) What are the possible pathways for hydrogen molecules to adsorb onto the oxide surfaces and what are the chemical states of the adsorbed hydrogen species? (c) How the adsorbed hydrogen species migrate over the surface or penetrating into the oxide? (d) How do adsorbed hydrogen species impact on the electronic structure and associated electrical properties of the oxide material? Experimental efforts were made in studying the states of hydrogen adsorbed on the low energy surface of rutile TiO2, (110). Noncontact atomic force microscopy (NC-AFM)

9

suggested that hydrogen molecules are dissociated into atoms to adsorb on the (110) surface of rutile, which prefer to settle on the bridge sites of oxygen, O(2). High-resolution electron energy loss spectroscopy (HREELS) revealed an intense energy loss peak at 456 meV,10 which was attributed to the O-H stretching mode. Scanning tunneling microscopy (STM) showed such O-H stretching was associated with H atoms adsorbed on the O bridge sites.10 This indicated significant coverage of OH species on the TiO2 (110) surface. Also, no evidence for the presence of Ti-H stretching mode was observed in the HREEL spectrum, and the researchers thus concluded that H atoms did not adsorb on the Ti atoms over the surface at room temperature. Diffusional behavior of atomic H was studied in order to elaborate potential pathways between different transition states on the rutile TiO2 (011) surface.11 According to STM imaging, there was a high OH concentration on the surface at room temperature. When the temperature increased to 500 K, only very few hydroxyls were observed in the same sample, indicating atomic H was formed through dissociation of hydroxyls on the oxide surface. When the system was cooled back down to room temperature again, the hydroxyls reappeared on the surface. This phenomenon of atomic H migrating out on to the surface was attributed to atomic H adsorbed on sub layers. More importantly, this new finding in experiments suggested that the atomic H diffusion channels existed in the rutile phase. Single atomic H diffusion behavior between surface and sub layers was demonstrated on the (110) slab by some first-principles calculations12 -14. In order to assist discussion about the literature findings, here in Fig. 1(a) we show the lattice planes and H sites for the rutile (110) surface. When H moved from a O(2) site (O(2)-H) directly to a subsurface site O_sub2 , a big



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potential barrier of about 2.4 eV had to be overcome in the process.12 On the other hand, considering the O(3) site as an intermediate state O(3)-H, the barrier from O(2)-H to O(3)-H was as low as 0.63 eV.13 Through the assistance of this intermediate state, it would be much easier for an atomic H to move from the surface O(3)-H to a sub layer with a much smaller reaction barrier of about 0.93 eV.14 To date, all previous efforts were based on the atomic H moving among different O adsorption sites in a (110) or (011) slab, but there was no work about the interaction of hydrogen with the high energy surface for the rutile phase, (002), which would be expected to be more active in adsorbing hydrogen.6,44 The process of hydrogen adsorption on the rutile surfaces and subsequent migration pathways are of essential importance for hydrogen sensing, through their impact on the surface or bulk electronic structures and associated effect on junction characteristics with the metal contacts15-16 and overall response to electrical conductivity. When atomic H diffuses from surface to the sub-layers, the final configuration is expected to be consistent with interstitial H doping in rutile TiO2.17,18 The electronic properties of rutile TiO2 with nonmetal dopants (such as N, C, N+C and N+H) as electron donor or acceptor, were investigated using DFT simulation with Hubbard U correction to the exchange-correlation functional for transition metals.17-19 These typical nonmetal dopants had great effects on both the band structure and the position of the Fermi level. It is expected H atoms on the surface would behave differently from those within the host compound phase. Such difference would thus lead to different response to hydrogen sensing. In the case of sensors based on rutile TiO2, little effort has yet been attempted in theoretically studying the fundamental pathways that control the interaction between hydrogen and the oxide surfaces, which is critical for hydrogen response of the material owing to related impact on electronic structures and possible Schottky barrier between metal contacts and the TiO2 film. Here in this work we aim to elaborate the processes for hydrogen adsorption and desorption, through interaction with both low- and high-energy surfaces of the rutile oxide. The work covers interaction of hydrogen molecules with the rutile surfaces of critical interest to hydrogen sensing, and identification of potential pathways for hydrogen transport. It has been discovered, that comparing to the low energy surface (110), the high energy surface (002) is significantly more effective in adsorbing hydrogen atoms via


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dissociation of hydrogen molecules. Such proactive interaction between hydrogen species and their subsequent migration is consistent with experimental observation in remarkable sensing performance in self-assembled rutile nanorods.6,8 The current theoretical finding also indicated potential sensing ability of hydrogen at the room temperature, which has since been proven by experimental work using dense films of rutile made of well aligned nanorods to expose the high energy surface of (002).8

II. Method Theoretical calculations are performed using the Vienna Abinitio Simulation Package (VASP),20-21 with the ionic potentials including the effect of core electrons being described by the

projector

augmented

wave

(PAW)

method.22-23

In

this

work,

the

Perdew−Burke−Ernzerhof (PBE) GGA exchange−correlation functionals25-26 are used to relax the configurations of the hydrogen species adsorbed on a rutile (002) surface. Although standard DFT calculations (GGA) are sometimes inadequate to describe the localization of excited electrons, particularly in correlated metal oxides, they are largely suitable for the simulation of structural-energetic properties at the ground state, with the pronounced advantage of low computing cost.

For the calculation of band structures, we use the

GGA+U approach of Dudarev et al.27-28 to treat the 3d electrons of Ti with the effective Hubbard on-site Coulomb interaction parameter (U ᇱ = U − J). We choose U ᇱ = 7 according to the proposed value from recent work.17 For the geometric relaxation of the surface structures, summation over the Brillouin Zone (BZ) is performed with a 5 × 5 × 1 Monkhorst−Pack k-point mesh. A plane-wave energy cutoff of 400 eV is used in all calculations. All structures are geometrically relaxed until the total force on each ion was reduced below 0.02 eV/Å. The climb image nudged elastic band (CI-NEB) method with the Limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) optimizer29-31 is used to search the paths for the dissociation of hydrogen molecules, the diffusion of atomic H, and the saddle points along these paths. The initial (reactant) and final (product) configurations are obtained from fully converged relaxation. The number of inserted images used in the CI-NEB calculations depends on the reaction coordinates between the reactant and product. The searching of



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reaction path is based on: (1) The energy difference between the reactant and product determines the direction of the reaction. Significant difference means that the reaction path is not reversible. (2) In order to save computing cost, the smaller spatial distance between the coordinates of the reactant and product are preferred, which needs fewer images to avoid angle between vectors of nearest images close to 90°. (3) The chemical reaction prefers to take place along the path with a smaller reaction barrier. The surface free energy for a slab between vacuum is defined as follows:

Esurf =

Eslab − ETiO2 NTiO2

(1)

2S

where Eslab is the total energy of the supercell, ETiO2 is the reference energy for a TiO2 unit in bulk phase, NTiO2 is the number of TiO2 units in the supercell, and S is the surface area of one side of the slab.

III. Results and discussion 1. Symmetric slab models for rutile TiO2 We adopt the symmetric slab model for the simulation of hydrogen adsorption on the (002) surface of the rutile TiO2 phase, as shown in Fig. 2(a). The slab is cleaved from relaxed bulk rutile TiO2 along surface vectors of u and v with a mesh length of 6.497Å. Tests are carried out to make sure that the surface area of the supercell is adequate to avoid interaction between images of absorbed hydrogen species, while maintaining manageable computational cost. Also, a vacuum layer with an adequate thickness of 20 Å is coupled with the TiO2 slab, so as to eliminate interaction between the top and bottom surfaces. For structural relaxation, three middle layers of the TiO2 slab are fixed to represent the atomic configuration in the bulk lattice, while the top and bottom layers are totally free to form the buffer and surface regions. After sufficient relaxation, the average inter-planar distance along the [002] direction equals to 2.874 Å, which is very close to the experimentally reported value ranging from 2.8 Å

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to 3.0 Å

33

. Convergence

tests of surface energy have been carried out with layers from 3 to 19 in the symmetric (002) slab, with the results shown in Fig.2 (b). Through calculation using equation (1), we can find that the surface energy difference between 9 layers and 19 layers is as small as 0.0034eV/ Å2, as indicated


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by the dotted blue line in Fig.2 (b). This indicates that it is sufficiently accurate to use a symmetric (002) slab containing nine layers of atoms as a basic model for energetic calculations. The rutile TiO2 (110) slab model is set up in a similar way.

2. Hydrogen adsorption on the (002) and (110) surfaces On the basis of the relaxed symmetric slab models, we proceed to identify the energetically preferred adsorbent configurations on the (002) surface, through detailed consideration of the structure-energy characteristics associated with exposed O and Ti atoms on the (002) surface. Referring to Fig. 1, exposed sites on the (002) surface such as Ti(6), O(2) and Ti(4) are indicated in Fig. 1(b). Since O(2) and Ti(4) are the atoms exposed on the (002) surface, possible configurations for hydrogen adsorption on the surface are therefore limited to H2-O(2), H2-Ti(4), and H-O(2)-Ti(4)-H. Here, the integers in the brackets represent the number of bond around the specific atom, e.g. each Ti atom is bonded to six O atoms for Ti(6) for Ti atoms in the bulk material. Three typical oxygen atoms in the sublayers are also marked as sub1O, sub2O and sub3O, for the first, second and third layers below the surface correspondingly. The low energy configurations of two H atoms in the sublayers are identified to be “H-O(2) diaO(2)-H” and “H-O(2) sub2O-H”. All together six configurations of the hydrogen molecule and atomic H adsorbed in the rutile TiO2 (002) slab, which covers all the most likely configurations of hydrogen on the specific atoms, are considered. The details of the configurations are shown in Fig.3, with the corresponding total energies for the relaxed configurations at the ground state being calculated. In Fig. 3 (a), a free hydrogen molecule with H-H bond length of 0.75Å is 5.0Å away from the surface, which is defined as the H2+slab. While the interaction between H2 and the O site on (002) surface is repelling, H2 can be attracted by the dangling bonds on the Ti(4) atom to form a stable state of H2-Ti(4), with a bond length of 2.5Å between H and Ti, as shown in Fig.3(b). It is worth emphasizing that the formation of H2-Ti(4) is the starting point for the interaction between hydrogen and the (002) surface. From an energetic point of view, the saturation of dangling bonds with H2 lowers the energy of the oxide slabs separated with vacuum, with an energy reduction about 0.063eV to transform H2+slab into H2-Ti(4). The energy changes associated with each absorbing configurations are shown in Fig. 3 (g),



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suggesting that only Ti(4) is the stable site for absorbing the hydrogen molecules. Similarly, it was reported that the Ti site is also the preferred site for the adsorption of hydrogen molecules on the (110) surface of rutile TiO2.36 Setting the total energy of H2-Ti(4) as reference, two possible configurations of atomic H on the surface can be considered. The first is the H1-O(2)-Ti(4)-H2, wherein the H2 atom is bonded to the Ti(4) and the H1 atom is attached to the O(2) site, as shown in Fig. 3(c). The energy for this configuration is 0.17eV higher than that for H2-Ti(4). The second configuration is H1-O(2)O(2)-H2, as displayed in Fig. 3(d), which is formed by keeping the bonded H1-O(2) and changing the adsorption site of H2 from Ti(4) to O(2) (the other O(2) on the right side of Ti(4)). Its configurational energy is 0.3eV higher than that for the reference. For the presence of atomic H in the sublayers, we consider the likely configurations of H-O(2)-diaO(2)-H and H-O(2)-sub2O-H (Fig. 3 (e) and (f)). Both configurations lead to energy decrease, -0.64eV and -0.46eV respectively. We can therefore conclude that energetically speaking, there is a tendency to dissociate the molecular hydrogen on the (002) surface into atomic species, which would be driven into the oxide slab through minimization of system energy. It is worth pointing out that physical absorption of gaseous molecules involves the van der Waals (VDW) effect, which could be corrected by the DFT-D3 method. Table 1 compares the effect of the VDW correction on the total energy. Overall the total energy for each configuration increases for about 0.6 eV, without considerable effect on the relative energetic changes among the different configurations for the hydrogen adsorption. We find that the surface energy of (002) is 0.084 eV/ Å2, which is nearly twice as much as that of (110) surface (0.046 eV/ Å2). Indeed, the (002) plane has the highest surface energy in rutile TiO2, while the (110) plane has the lowest energy, being consistent with previous study. 37

It is therefore necessary to investigate the response of hydrogen absorption over the low

energy surface for comparison. The Ti sites on the (110) surface are bonded with 5 atoms instead of 4, and we therefore start with the H2-Ti(5) configuration as a reference state. The energies for various likely structural configurations for hydrogen adsorption associated with the rutile TiO2 (110) slab is summarized in Fig.4. It is apparent that the configuration for the adsorption of molecular hydrogen on the Ti(5) site is of the lowest energy, and there is thus no energetic driver to dissociate the molecular species. This suggests that the (002) surface


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would have more significant impact to hydrogen adsorption, owing to the energetic advantage to dissociate the adsorbed hydrogen molecules into atomic species, thence probably leading to remarkable modification of the electronic structures through migration of dissociated hydrogen atoms below the outer surface.

3. Dissociation of hydrogen molecules on rutile TiO2 (002) surface On the basis of energetic analysis, there is a tendency to dissociate hydrogen molecules adsorbed onto the (002) surface. Let us look at the dissociation pathways, by taking the H2-Ti(4) configuration as the reactant and the H1-O(2)-Ti(4)-H2 configuration as the product. Through the CI-NEB simulation, four transitional images to represent the most likely configurations are identified along the reaction path, with their corresponding configurational energies being shown in Fig. 5 (a). It can be seen that as the H2 molecule getting close to the free (002) surface, the bond between the two hydrogen atoms in a molecule, H1 and H2, is gradually elongated and the molecular bond is finally broken. This is illustrated in Fig.5 (b) to (e). From Fig.5 (b) to (e), the distance between H1 and H2 is more than doubled from 0.76 to 1.68 Å when there is little interaction between the two hydrogen atoms. The configurational energy of the system increases when the bond between the two hydrogen atoms is stretched from image (b) to (d), beyond which the energy drops steeply. The configuration of Fig. 5(e) corresponds to new bonds with the two hydrogen atoms, H1-O(2) and H2-Ti(4), with bonding length of 0.99 and 1.78 Å respectively. In this way, the chemical state of the adsorbed hydrogen changes from the molecular state (physical adsorption) into an atomic state (chemical adsorption). The potential barrier for such a dissociation process is only 0.68 eV, which corresponds to an activation temperature of 269±53K using the standard Redhead analysis.34,35 This implies that the dissociation of the hydrogen molecules on the (002) surface of rutile TiO2 can be kinetically enabled around the room temperature. The searching direction of this reaction path is shown in Fig. 1(b) by the bent black arrow. The charge distribution maps of the configurations for images (b) to (e) in Fig.5 along the dissociation path are shown in Fig. 6 (a) to (d). While the charge maps in Fig. 6(a, b) exhibit strong bonding between the two hydrogen atoms, it is seen that increasing the atomic interval reduces charge density between H1 and H2, leading to lowered attractive force between them.



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Actually, on the basis of a free hydrogen molecule in a vacuum box, we note that the attractive force between H atoms in a hydrogen molecule reaches maximum when the interatomic distance is 1.17 Å (1.56 times of the equilibrium spacing), over which the interaction between the atoms begins to weaken. This would lead to easier migration of the two hydrogen atoms departing from each other.38 In the case of Fig. 6(d), the dominant bonding is changed to O(2)-H1 and Ti(4)-H2 instead, and there is little overlap in charge orbitals between the two H atoms. We therefore summarize the dissociation process of the adsorbed hydrogen molecule in three types of states: free, physically adsorbed, and chemically adsorbed hydrogen. Initially, a free hydrogen molecule is physically adsorbed on the Ti(4) site, Fig. 6(a). Charge redistribution, or charge screening, helps to polarize the H atoms and stabilize the system via some slight reduction of the bonding length. The electric field between O-2 and Ti+4 are behind the charge redistribution and associated charge polarization between the two H atoms, which is exhibited in Fig. 6 (b), wherein the charge distribution around atoms H1 and H2 turns to be asymmetric. Such asymmetry is attributed to higher electronegativity of O than that of H and Ti (in the sequence O > H > Ti), which leads to preferred charge transfer from the H1 atom to O, while the H2 atom tends to accept some charge from Ti. In this way, the H1 next to O(2) will lose some charge with increased positive valence (H1δ+), and the H2 close to Ti(4) will gain some charge instead (H2σ-). We notice the different extents and signs of charge transfer for H1 and H2, i.e. H1δ+ and H2σ-. Charge polarization between the H atoms is further enhanced, as exhibited in Fig.6 (c), where H1 is tilted and pulled further towards O(2) and the bond between Ti(4) and H2 is also slightly strengthened with some reduction of Ti(4)-H2 spacing. One can see that such a charge polarization process helps to separate the two H atoms from each other, thus leading to radical reduction of interaction between them. This in the end leads to rather strong chemical bonding between O(2) and H1 and the formation of the chemically adsorbed product H1+-O(2)-Ti(4)-H2- configuration on the (002) rutile surface, as shown in Fig. 6 (d). Through this process, H1 acts as an electron donor to provide some charge to the O(2) site, and H2 on the other hand plays the role as a weak electron acceptor to gains a little charge from the Ti(4) site. Fig. 7 shows charge difference owing to adsorption, with Figs. 7(a), 7(b) and 7(c)


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corresponding to those in Fig. 6(a), 6(c) and 6(d). It is apparent that physical adsorption of H2 to a Ti(4) atom leads to some charge redistribution between the hydrogen molecule and the Ti(4) atom. It is also noted that the H1 closer to the O(2) loses charge and the H2 close to the Ti(4) gains some charge. The above theoretical justification is supported by ab initio molecular dynamics (AIMD) simulation,24 using a 3×3×1 supercell and the NVT ensemble with a time step of 2 fs with a Nosé–Hoover thermostat at 300K. Fig. 8 shows a few snapshots to reveal the evolution process from a physically adsorbed hydrogen molecule into chemically adsorbed atoms. The current AIMD has offered no evidence for the migration of the dissociated H atoms into sublayers at 300K. It is useful to note that the concentration of hydrogen in the slab model adopted in this work is roughly 38 ppm in weight, a rather low dosage for hydrogen sensing.

4. Diffusion of atomic H on the (002) slab of rutile TiO2 Within the H1-O(2)-Ti(4)-H2 configuration owing to the dissociation of the hydrogen molecule, the bonding strength for Ti(4)-H2 is relatively weak, as there is limited overlap of electron orbitals between Ti and H2. It is therefore possible that the second hydrogen atom, H2, would be able to diffuse rather easily away from the Ti(4) site to an O site. Here we start with the H1-O(2)-Ti(4)-H2 configuration as the reactant to look for the most likely path for the migration of H2 over the (002) surface, from the Ti(4) site to a O(2) site. We identify two possible diffusional paths, indicated as gray and yellow bent arrows in Fig. 1. The product configurations are H-O(2)O(2)-H and H-O(2) diaO(2)-H respectively, as shown in Fig. 9. It is seen from the energetic changes following the two paths, Fig. 9, that the energetic barriers associated with these two reactions are 1.57 and 1.1 eV respectively. The first diffusional path from H1-O(2)-Ti(4)-H2 to H-O(2)O(2)-H has a considerably higher energetic barrier to overcome than the other one. This is understandable on the basis of charge transfer analysis discussed in the previous section. Starting from the reactant configuration H1δ+-O(2)-Ti(4)-H2σ-, the charge transfer from Ti(4) to H2 leads to the H2 atom being negatively charged, while the H1 atom donates charge to O(2) to be positively charged. A negatively charged hydrogen atom H2σ- is to experience some coulomb repulsion, before further charge polarization to gain charges from the targeted O(2) site and hence to become a



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positively charged hydrogen species in the end. Such a process would thus result in a higher overall energetic barrier. The activation temperature based on the standard Redhead analysis for the 1.57eV barrier is close to 598±121K, being significantly higher than room temperature. In this sense, it is rather unlikely that such a diffusional pathway would be enabled around the room temperature. The other reaction pathway with a longer distance experiences a much lower energetic barrier of 1.1 eV. In this process, the negatively charged hydrogen atom bond to Ti(4), H2σ-, would be able to gain some charge from the environmental O atoms on the way to be neutralized initially, and thence bonded to the targeted diaO(2) site through gaining charge from the O site. The reaction barrier of 1.1 eV (from the green curve in Fig. 9) is equivalent to an activation temperature about 424±85 K, being significantly lower than that for the first path. Such a lower activation of the latter path makes it more feasible to enable the diffusion process much closer to room temperature.

5. Hydrogen effect on electronic and electrical properties In this part, we attempt for the first time to elaborate the mechanism for hydrogen sensing on the basis of hydrogen absorption via the (002) surface of dense films of rutile TiO2, which were synthesized through self-assembling rutile nanorods. 6-8 Fig. 10 is the XRD pattern and cross section SEM image of the TiO2 thin film, with the inset showing a dense top surface. The SEM image of the fractured cross section exhibits clear an overall columnar structural characteristics, which is consistent with the XRD pattern that only shows the (002) peak owing to self-alignment of nanorods along the [002] direction of the rutile phase. Faint peaks correspond to lattice planes from the FTO coated substrate. The strong tendency for the preferred alignment in the [002] direction lies in the much lower energy of the (110) rutile surface than that of the (002) surface, with minimization in overall surface energy helping grow rutile crystals into nanorods in the [002] direction

6,7

. Further details on the synthesis and characterization of the nanorod thin

films were presented in detail in references 6 and 7.

The architecture of the hydrogen sensor is sketched in Fig. 11(a), where the active material layer is the rutile TiO2 film. Pt interdigitated electrodes were deposited over the top surface of the oxide film. Details about materials and device fabrication were presented in


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recent work.6-8 The sensitivity for hydrogen sensing is demonstrated by the change in resistance owing to the presence of hydrogen gas in dried ambient air at the room temperature. In Fig. 11(a), the red arrows indicate the moment hydrogen was introduced into the system, and the blue arrows represent the time when hydrogen was flushed out. It can be seen that the device exhibits excellent sensing properties, with detection limit being as low as 1 ppm of hydrogen gas, the response time being as short as 9 s, and a quick recovery time of 20 s. Comparing to previous work on TiO2 based H2 sensor,6 the detecting temperature of the sensor in this work was decreased from above 100°C to room temperature, with the sensing concentration being as low as 1ppm, the response time being as short as 9 s, and a quick recovery time of 20 s.8 The significant advancement with respect to previous result is mainly attributed to the use of an insulating seeding layer of pure TiO2 over the FTO coated substrate, which played a key role not only in preventing short circuiting of the device through the conductive FTO coating, but also in helping produce much denser (002) surface with less oxygen defects 8. The response-recovery characteristics of the device at different hydrogen concentrations are very similar, and the detail for one sensing cycle corresponding to 30ppm H2 is enlarged and displayed in Fig. 11(b), which shows multi-stage characteristics in the resistance response to the presence of hydrogens. The whole sensing cycle can be divided into five stages as following: (A) a drastic resistance decrease in a short response time as H2 is introduced into the sensing chamber; (B) an obvious increase of resistance; (C) the resistance slightly decreases again; (D) approaching steady state; (E) after hydrogen is released from the chamber, the resistance begins to recover. In a short time, the resistance almost returns to the initial value (Inflection points are marked as point 1 to 4, which divide the whole sensing cycle into five stages as mentioned above). Based on transition state searching, we have identified several important states of hydrogen such as H2+slab, H2-Ti(4), H-O(2)-Ti(4)-H and H-O(2)diaO(2)-H, in the process for the adsorption of hydrogen and sequential dissociation over the (002) surface of rutile TiO2. Band structures of these states are shown in Fig. 12 (a) to (d). Before the hydrogen molecule is adsorbed on the Ti(4) site, the overall band structure of the slab containing a hydrogen molecule exhibit a direct band gap, Fig. 12(a), which is consistent with that for the bulk rutile



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phase. The dashed line refers to the overall Fermi level of the whole system. The band structure is intrinsic in nature, which corresponds well with the point 1 in Fig. 11(b) when the device is of highest resistance. When a hydrogen atom is attached on the Ti(4) site exposed on the (002) surface, Fig. 12(b), the Fermi level is lowered towards the maximum of the valence band (VBM), with the VBM being only 0.078 eV below the Fermi energy of the system. Such a band structure suggests that the H2-Ti(4) configuration induces shallow acceptor states in the slab and charge transfers from the Ti(4) site to H2 molecule to form the Ti-H2 configuration, thus resulting in enhanced conductivity, or lowered resistance, such as shown at point 2 in Fig. 11(b). Referring back to Fig. 6(a), charge transfer between the slab and the molecule is dictated by overlapped charge orbitals between Ti(4) and the hydrogen molecule, with Ti passing some charge to the largely gaseous molecule. In other words, the attachment of the hydrogen molecule onto the exposed Ti(4) atom helps to accept more charge from the metal site, thus leading to hole conductivity of the whole system. This is in line with recent work on the doping principles of pristine TiO2 phases.39-41 The sharp decrease of resistance is demonstrative of quick response, as the physical adsorption of a hydrogen molecule to the Ti(4) site is a natural process owing to lowered system energy. In stage B of Fig. 11(b), from point (2) to point (3), an obvious resistance recovery can be observed, when the physical adsorption state H2-Ti(4) is being changed into the chemical adsorption configuration of H1-O(2)-Ti(4)-H2, with a modest activation barrier of 0.68eV. Such a moderate barrier could help stabilize the physically adsorbed H2, without the molecule being quickly dissociated by the active surface into the chemically adsorbed H-O(2)-Ti(4)-H state. This enabled the H2-Ti(4) being detected in the current experiment. We can see from Fig. 12 (c) that the valence band for the slab containing H1-O(2)-Ti(4)-H largely drops back with respect to the Fermi level, returning thence to intrinsic conduction characteristics together with some blue shift in the band gap. In stage C of Fig. 11(b) from point (3) towards a steady state after point (4), H-O(2)-Ti(4)-H, the full dissociation of the two hydrogen atoms in a molecule corresponding to the H-O(2)diaO(2)-H configuration, with both H1 and H2 from a hydrogen molecule being bonded to different O atoms. In this case, the H atoms contribute to donate charge to O atoms, and hence the band structure exhibit donor bands next to the Fermi level, Fig. 12(d). This in


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turn reduces resistivity of the material through electron conduction. However, as is discussed in the previous section with respect to Fig. 9, the reaction path from H-O(2)-Ti(4)-H to H-O(2)diaO(2)-H has rather limited possibility to happen at room temperature, due to the activation barrier of 1.1eV, so that the full potential of hydrogen dissociation cannot be realized in reduction of resistivity of the material. Overall, it is reasonable to conclude that when the (002) surface of the rutile slab is subject to an ambient containing hydrogen molecules at the room temperature, the process is dictated largely by the physical adsorption and partial transition into the chemical H1-O(2)-Ti(4)-H2 configurations. The former enables lowered conductivity via formation of holes, while the latter helps to reduce the hole densities. Enhancement of conductivity via donor states from the full dissociation of the two hydrogen atoms from each molecule can only become important at higher temperatures, when recent experimental work showed that the sensitivity can be further enhanced.6 Being consistent with the current theoretical elaboration, the stage A associated with the physical adsorption of H2 on Ti(4) was not detected at higher temperatures 6, when the chemical dissociation of H2 was promoted by thermal excitation to overcome the 0.68 eV energy barrier. The great advantage to have a sensor working close to the room temperature enables simpler sensor architecture without a heating element, which increases energy and device cost. On the other hand, one would not endeavor to exert a working temperature lower than the room temperature, for the same reason of sensing cost. Once hydrogen is emptied from the sensing chamber, the resistance bounces back to the initial value quickly. This is consistent with the lower activation barrier in the opposite direction for releasing any dissociated hydrogen atoms, albeit being limited in percentage, on the surface, being only 0.45eV as shown in Fig 5. Reorganization of partially dissociated hydrogen atoms on the surface, H1-O(2)-Ti(4)-H2, into molecules is after all energetically preferred. It is worth emphasizing that unlike at elevated temperatures,6 sensing of hydrogen at room temperature could not be realized using gold electrodes.8

Such experimental observation

that a material of significantly higher work function (close packed plane: 5.9 eV for Pt v.s. 5.3 eV for Au)42 is needed to enable sensing hydrogen at the room temperature, is consistent with the current theoretical discovery that the physical adsorption is dominant when the



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temperature is low, since it is a natural process through energy minimization. At higher temperatures, the dissociation of adsorbed hydrogen molecules could experience extensive chemical dissociation, thus providing the basis for significant doping through chemical bonding to O atoms. Since the electron affinity of TiO2 is only about 4.0 eV,43 a large work function of the Pt electrodes makes it easier to form Schottky junctions with the TiO2 that tends to be n-type due to the natural tendency to contain oxygen vacancies. The physical adsorption, dictated by the H2-Ti(4) configuration, is p-type in nature, so that molecular attachment at the exposed Ti sites on the (002) surface helps to turn the surface next to the electrodes into p-type. This leads to local elimination of the Schottky barrier, as a p-type semiconductor requests larger metal work function for the formation of an ohmic contact. On the other hand, chemical adsorption of hydrogen at elevated temperatures will improve the conductivity of the material significantly via the introduction of high density of donor states, when a high Schottky barrier is not essential for hydrogen sensing. 6 A detailed discussion was made in our recent work that compared the hydrogen sensitivity of dense films of self-aligned rutile nanorods with other TiO2 sensors such as hollow anatase nanowires made of poly-crystals, concluding that such [002] oriented nanorod films is essential for highly enhanced sensitivity.6 This is in line with the current theoretical work that offers fundamental support to experimental studies,6,8 in particular that the high surface energy of the exposed (002) surface enables higher capacity in turning physically adsorbed molecule into chemically adsorbed atomic species. Owing to the high stability in the TiO2 phase and the reliable electrical contacts on a dense and planar surface of the active oxide film, the device was extremely stable in service, such that little change in hydrogen response has been observed in sensors used in the recent work after 26 months. 8 Unlike previous work to test hydrogen response through mixing hydrogen with inert gases such as nitrogen, 6 the current sensor was tested in dry air, which is close to an ambient relevant to practical sensing of the hydrogen gas with a rather simple device. An important additional factor to affect the sensitivity is the presence of water vapor in the environment, as any condensation of water molecules could be affecting the surficial conductivity. This would request engineering measures such as anti-foggy treatment or moderate heating to prevent water condensation over the active sensing materials. A wider


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study concerning selectivity with the presence of various gaseous species is outside the scope of the current work, which is largely focused on the effect of hydrogen on the electronic structures.

Ⅳ. Conclusions First principle calculations have been carried out to study the impact of hydrogen adsorption on the surfaces of rutile TiO2 on the electronic characteristics and associated electric conductivity. The results show that the high energy surface (002) is more effective in adsorbing hydrogen than the close packed (110) surface. This is in accord with the experimental finding that the (002) surface of the rutile TiO2 exhibited significantly better performance for hydrogen sensing. The complex process for hydrogen sensing through change in electric resistance is consistent with theoretically identified reaction pathways. At room temperature, the dictating surface adsorption configuration is physiochemical adsorption of the hydrogen molecules on the Ti sites exposed on the high energy (002) surface. Further dissociation of the adsorbed molecules can be enabled at moderately elevated temperatures to overcome an insignificant activation barrier, leading to chemical bonding of dissociated hydrogen atoms to the oxygen atoms on the surface.

Acknowledgement The work was financially supported in part by the Innovate UK, the National Natural Science Foundation of China (Nos. 51001091, 111174256, 91233101, 51602094, 11274100) and the Fundamental Research Program from the Ministry of Science and Technology of China (no. 2014CB931704).

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Table 1 The total energies for the configurations of H2+slab, H2-Ti(4) and H-O(2)-Ti(4)-H with or without van der Waals correction.

Configurations H2 + slab H2 - T(4) H1-O(2)-Ti(4)-H2

Energy without VDW (eV) -241.18 -241.31 -241.08


Energy with VDW (eV) -246.73 -246.94 -246.75

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Figure 1 (a) Side view of the (110), and (b) top view of the (002) surfaces of the rutile TiO2 phase. Atoms of specific interest are labeled. Red, blue and gray spheres represent O, Ti and H atoms correspondingly. Bent arrows stand for searching directions for reaction paths.



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Figure 2 (a) Side view of the (002) slab of rutile TiO2, with red and blue spheres representing O and Ti atoms respectively. (b) Change of surface energy of the slab against number of layers.


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Figure 3 Energies for adsorption of hydrogen species on the (002) rutile surface with varied structural configurations: (a) H2+slab for hydrogen molecule far away from TiO2 surface, with charge density map illustrating that H2 is free of interactions with TiO2 surface or neighboring hydrogen. The configurations from (b) to (f) are named as H2-Ti(4), H-O(2)-Ti(4)-H, H-O(2)O(2)-H, H-O(2)diaO(2)-H and H-O(2)sub2O-H. Red, blue and gray spheres represent O, Ti and H atoms correspondingly. The reference energy as shown in (g) is that for the H2-Ti(4) configuration. Integers in the brackets are number of bonds for the atoms.



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Figure 4 Energies for adsorption of hydrogen species on the (110) rutile surface with varied structural configurations, with respect to that of H2-Ti(5) configuration. The configurations from left to right are H2 molecule far away from the (110) surface, H adsorbing on the (110) surface, and H adsorbing on the subsurface in sequence. Red, blue and gray spheres represent O, Ti and H atoms correspondingly. Integers in the brackets are number of bonds for the atoms.


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Figure 5 (a) Reaction path for H2 dissociation on the rutile TiO2 (002) surface. (b – e) are structural configurations formed along the transitional path.



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Figure 6 (a – d) Charge maps for transitional configurations in Figure 5. (e) Summarizes the process for the dissociation of an adsorbed hydrogen molecule in three stages.


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Fig. 7 Charge difference owing to adsorption, with (a), (b) and (c) corresponding to those in Fig. 6(a), 6(c) and 6(d).



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Fig. 8 Snapshots to show the dissociation trajectory evolution of adsorbed hydrogen on the (002) rutile surface. The black spheres are the dissociating molecule, showing increasing bond length (a,b) over time until bond breakage (c).


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Figure 9 Possible reaction paths for H diffusion, setting H1-O(2)-Ti(4)-H2 as the reactant. Gray and green solid line correspond to the paths defined in Figure 2, taking the H-O(2)O(2)-H and H-O(2)diaO(2)-H configuration as the products.



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Figure 10 (a) XRD pattern and (b) cross section SEM pictures of the TiO2 thin film with the top surface SEM image inserted


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Figure 11 (a) Change in the electric resistance at room temperature owing to the presence of H2 with concentration from 1ppm to 100ppm mixed in dry air. The inset is a schematic diagram for the sensor, using TiO2 thin film made of nanorods self-assembled in the [002] direction as the active layer and interdigitated Pt electrodes. (b) An enlarged view for a typical sensing cycle at 30ppm of hydrogen.



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Figure 12 Band structure of (a) H2+slab, (b) H2-Ti(4), (c) H-O(2)-Ti(4)-H, (d) H-O(2) diaO(2)-H, where the black dotted lines represent the Fermi level.


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Table of Contents Graphic 146x67mm (220 x 220 DPI)


Fundamental Pathways for the Adsorption and Transport of Hydrogen

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