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Extraordinary room-temperature hydrogensensing capabilities of porous bulk PteTiO2

nanocomposite ceramics

Wan Ping Chen a,*, Yao Xiong a, Ye Sheng Li b, Ping Cui c, Shi Shang Guo a,Wei Chen c, Zi Long Tang b, Zijie Yan d, Zhenyu Zhang c

a School of Physics and Technology, Wuhan University, Wuhan, Hubei 430072, Chinab School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua

University, Beijing 100084, Chinac International Center for Quantum Design of Functional Materials (ICQD), Hefei Laboratory for Physical Sciences at

Microscale (HFNL), and Synergetic Innovation Center of National Quantum Information and Quantum Physics,

University of Science and Technology of China, Hefei, Anhui 230026, Chinad Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, NY 13699, United States

a r t i c l e i n f o

Article history:

Received 31 October 2015

Received in revised form

22 December 2015

Accepted 23 December 2015

Available online 14 January 2016

Keywords:

Hydrogen

Sensing

Metal oxide

Semiconductors

Catalysis

* Corresponding author.E-mail address: wpchen@whu.edu.cn (W

http://dx.doi.org/10.1016/j.ijhydene.2015.12.10360-3199/Copyright © 2015, Hydrogen Ener

a b s t r a c t

Metal oxide semiconductors (MOSs) are among the most promising materials for sensing

hydrogen in distributed use, yet prevalent hydrogen sensors based on porous thick-film

MOSs have the standing and serious drawback of demanding high working tempera-

tures. Here we report on the surprising experimental observation of porous bulk PteTiO2

nanocomposite ceramics that exhibit ultrahigh hydrogen sensitivities of the order of 104

for 1000 ppm H2 in N2 at room temperature, with fast response and short recovery times of

10 s and 20 s, respectively. We also demonstrate that hydrogen diffuses in the bulk ce-

ramics with an unusually high diffusion coefficient, and a novel hydrogen sensing

mechanism rooted in hydrogen chemisorption on TiO2 is established. The central findings

help to enrich our understanding of hydrogen interaction with MOSs, and are also expected

to be instrumental in developing related technologies with hydrogen as the energy carrier.

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Hydrogen is expected to have immense potentials in future

fuel cells and hydrogen-powered automobiles, which will

unavoidably face mounting safety concerns [1]. Among the

several different classes of hydrogen sensors already available

on the market, the one based on metal oxide semiconductors

.P. Chen).51gy Publications, LLC. Publ

(MOSs) is especially attractive for sensing hydrogen in

distributed use, as it is relatively compact, durable, and simple

in operation [2]. Unfortunately, prevalent commercial MOS

hydrogen sensors such as those based on porous thick-film

SnO2 all have to work at temperatures (300e700 �C) much

higher than room temperature [3], which naturally requires

high power consumption, and also results in undesirable high

cross-sensitivity and decreased lifetime. Various

ished by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 1 ( 2 0 1 6 ) 3 3 0 7e3 3 1 23308

nanostructured MOSs with large surface-to-volume ratios

have been successfully synthesized in the past decades,

which have been found especially promising for photo-

catalysis and low-temperature gas sensing [4e7]. While low-

dimensional nanostructured MOSs can be directly applied

for photocatalysis [8,9], for gas sensing nanostructured MOSs

have to be connected to electric circuits. For this reason one-

dimensional (1D) MOS nanomaterials are relatively suitable

and have been fabricated as MOS hydrogen sensors with

impressive low-temperature hydrogen sensing capabilities

[10,11]. Nevertheless, such materials still encounter the

standing shortcomings of difficulties in forming low-

resistance contacts, low mechanical and thermal robustness

[12], and strong adsorption of other unwanted gases in sur-

rounding atmosphere [13,14], hindering their potential prac-

tical gas-sensing applications. So for gas-sensing, MOSs in

bulk forms, such as in porous thick films, are highly preferred

in numerous circumstances. It is strange that bulk MOSs have

been rarely investigated for room-temperature hydrogen

sensing applications up to date, and few systematic in-

vestigations have been conducted on room-temperature re-

actions between hydrogen and MOSs, either. A clear

mechanistic understanding for room-temperature hydrogen

sensing of MOSs is thus still lacking, in many cases with

mutually contradictory assumptions [2,6,15].

Herein, we present some striking findings on room-

temperature hydrogen sensing capabilities of bulk MOSs. We

show that a porous bulk PteTiO2 nanocomposite ceramic

material, readily fabricated from TiO2 and Pt nanoparticles

through pressing and sintering, exhibits extraordinary room-

temperature hydrogen sensitivities of the order of 104 for

1000 ppm H2 in N2, with fast response and short recovery

times of 10 s and 20 s, respectively. We further reveal a fast

diffusion of hydrogen at room temperature in the porous bulk

ceramics, and propose a hydrogen sensing mechanism rooted

in chemisorption of hydrogen on TiO2. These central findings

help to enrich our understanding of hydrogen interactionwith

MOSs, and are also expected to be instrumental in developing

related technologies with hydrogen as the energy carrier.

Experimental methods

Material preparation

Commercial TiO2 nanoparticles, P25 (ca. 80% anatase and 20%

rutile), purchased from the Degussa Co., were used as the

startingmaterial. The TiO2 powderwas dispersed in deionized

water and magnetically stirred. Pt powder (particle sizes of

~10 nm) was added to the suspended system according to a

weight ratio of 5:95 for Pt:TiO2. After stirring for 2 h, the sys-

tem was dried at 110 �C in oven. The dried powder was then

pressed into cylindrical pellets (~1.7 mm in thickness and

~12 mm in diameter) by applying about 10 MPa pressure

through a hydraulic press. Pellets of pure P25 were also made

for comparing purpose.

These pellets were then heat treated in air for 2 h at a series

of temperatures, from 250 to 950 �C with a step of 100 �C. Due

to the large specific surface area of the nanoparticles, a

sintering-induced decrease in diameter was detected when

the pellets were treated at temperatures as low as 450 �C.Through DC magnetron sputtering, a pair of rectangular Pt

electrodes were coated on the major surfaces of those pellets

intended for hydrogen-sensing measurement.

Hydrogen sensing measurement

During the sensing measurements, a pellet was placed in a

sealed chamber (of volume ~350 ml) with a gas inlet and

outlet. In the response process, N2 was used as the carrier gas,

and H2 was mixed with N2 in appropriate ratios to realize

desirable hydrogen concentrations. In the recovery process,

air gas was pumped into the chamber. The total gas flow rate

was kept at about 300 ml/min. A 15 V DC voltage was applied

between the two electrodes and the electric current was

measured to calculate the real-time resistance response of the

sensor to hydrogen. All the data acquisition was carried out

automatically through a computer.

Results and discussion

The transition from anatase to rutile was observed in the

pellets heat-treated at and above 650 �C, while such transition

was not observed in those pellets heat-treated at and below

550 �C [16]. We have thus chosen 550 �C as the sintering

temperature for those pellets used in further hydrogen-

sensing investigation, at which there was a 3.5% decrease in

diameter after sintering. Fig. 1a shows a schematic of a sensor

and a scanning electron microscopy (SEM) image of the

PteTiO2 ceramics sintered at 550 �C. TiO2 grains are ~30 nm in

diameter and numerous nanosized pores are present. Ac-

cording to energy dispersive spectroscopy (EDS) analysis, the

Pt clusters mostly exist as aggregates of a few micrometers in

size and ~10 mm apart from one another, with a typical

aggregate partially revealed in the bottom-right corner of

Fig. 1a. Obviously, porous bulk PteTiO2 nanocomposite ce-

ramics have thus been prepared through convenient pressing

and sintering.

The sintered pellets of pure P25 show no detectable

response to hydrogen at room temperature. In contrast,

PteTiO2 nanocomposite ceramics exhibit surprising and

extraordinarily strong room-temperature hydrogen sensing

behaviors as shown in Fig. 1b. Upon exposure to 1000 ppm H2

in N2, the resistance of the ceramics quickly decreases by a

factor of over 6000 at room temperature, indicating a sensi-

tivity of over 6000 in terms of Ra/Rg (where Ra and Rg are the

electrical resistances of the sensor in clean air and in the

measuring gas, respectively). Previously, such high hydrogen

sensitivities at room temperature could only be achieved

using much less robust MOS nanostructures, such as net-

works of Pd-decorated SnO2 nanowires [10]. Considering that

prevalent commercial SnO2 thick-film hydrogen sensors have

to work at elevated temperatures, the ultrahigh room-

temperature sensitivities of the PteTiO2 ceramics demon-

strated here represent a breakthrough discovery in bulk MOS-

based hydrogen sensing.

Another vitally important characteristic of a sensor is its

response and recovery times. As shown in Fig. 1b, the ce-

ramics exhibit a 90% response time of ~10 s and a

Fig. 1 e (a) A SEM image taken from a fractured surface of a PteTiO2 ceramic pellet. A schematic of a sensor is shown in the

top-left inset, where two Pt electrodes are coated on the same surface of the pellet to measure its resistance change upon

hydrogen exposure. (b) Real-time electrical resistance response to 1000 ppm H2 in N2 for five successive cycles at room

temperature. (c) Electrical resistance response to hydrogen at different hydrogen concentrations at room temperature.

(d) Sensitivity dependence on hydrogen concentration.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 1 ( 2 0 1 6 ) 3 3 0 7e3 3 1 2 3309

comparatively short 90% recovery time of ~20 s. In contrast,

the 1D/quasi-1D nanostructure-based sensors that exhibit

fast response will often encounter the common drawback of

too slow room-temperature recovery [17,18]. Furthermore, the

overall response and recovery behaviors shown in Fig. 1b have

little variations in five successive sensing cycles, indicating

high repeatability in hydrogen sensing.

The ceramics also show excellent sensing capability over a

large range of hydrogen concentrations. As shown in Fig. 1c

and d, the sensitivity decreases with decreasing hydrogen

concentration, as expected; yet even at 30 ppm H2 in N2, the

sensitivity is still ~200. This is still an extraordinarily high

sensitivity at such a low hydrogen concentration.

Aside from ultrahigh sensitivities, the PteTiO2 composite

ceramics also possess high hydrogen sensing selectivity,

shown by the lack of any visible responses upon in-gassing of

CO and NO2 at room temperature. This high selectivity is tied

to the drastically different catalytic reactions of the Pt aggre-

gates with H2, CO, and NO2 [11].

So far we have demonstrated that those PteTiO2 composite

ceramic materials are ideal for hydrogen sensing in many

important aspects, including the readily accessible and stable

bulk form, ultrahigh room-temperature sensitivities, fast

response and short recovery times, high repeatability, and

high gas selectivity. Next we turn our attention to the likely

underlying room-temperature hydrogen sensing mecha-

nisms, which for bulk ceramic materials have so far been

largely overlooked. We first designed a special experiment to

study the diffusion of hydrogen in the bulk PteTiO2 ceramics,

in which a pair of Pt electrodes were coated on the two

opposite major surfaces of a sufficiently thick (~1.7 mm) sin-

tered pellet, with its side surface sealed by epoxy as illustrated

in Fig. 2a. This configuration was adopted to avoid possible

formation of a short circuit on the surface layer between the

electrodes, to ensure that the electrical resistance change

must reflect the evolution of the bulk property during

hydrogen sensing. A representative hydrogen absorption and

sensing curve is shown in Fig. 2b. Compared with the results

shown in Fig. 1b, this configuration again yields ultrahigh

sensitivity, but its response to hydrogen clearly indicates

multiple electronic/ionic processes, reflected by the two

stages with distinctly different slopes. As the decrease in

resistance is primarily related to some hydrogen-induced re-

actions, the ultrahigh sensitivity demonstrates that a low-

resistance path has been established along the thickness di-

rection of the pellet when exposed to hydrogen, indicating

that hydrogen is accessible to the very interior of the pellet at

room temperature.

The slope of the first response stage is much lower than

that of the second stage, and the much faster second stage

resembles well the monotonous behavior shown in the first

configuration (see Fig. 1). So the first stage should correspond

to the hydrogen diffusion process over the relatively long

distance along the thickness direction of the sample in the

Fig. 2 e (a) Schematic of a PteTiO2 nanocomposite ceramic

pellet with two electrodes coated on its two opposite major

surfaces, and the side surface of the pellet was sealed by

epoxy. (b) Real-time electrical resistance response to

1000 ppm H2 in N2 for three successive cycles (bottom) at

room temperature, with a typical response upon hydrogen

exposure magnified (top).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 1 ( 2 0 1 6 ) 3 3 0 7e3 3 1 23310

second configuration. The timewhen themeasured resistance

begins to decrease upon hydrogen exposure is taken as t ¼ 0.

During hydrogen diffusion along the thickness (~1.7 mm) di-

rection of the pellet, hydrogenated layers grow from each of

the two major surfaces inwards. Since the presence of

hydrogen leads to a dramatic decrease in the resistivity of

TiO2, the measured resistance in the first stage of response in

Fig. 2 is dominated by that of the central un-reacted layer,

which is proportional to the thickness of (1.7 � 2x). At any

given time t, when the measured resistance is R, the corre-

sponding x value is calculated according to

R=R0 ¼ ð1:7� 2xÞ=1:7, where R0 is the resistance measured in

air. The diffusion coefficient D is then calculated using the

formula of diffusion length x ¼ 2ffiffiffiffiffiffi

Dtp

, as shown in Fig. 3. The

transition from the first to the second stage corresponds to the

Fig. 3 e Plot of x (thickness of the hydrogenated layer from

one side of the pellet) vs t1/2.

effective merging of the two reaction fronts around the center

of the pellet. Based on this picture, we estimate the effective

room-temperature diffusion coefficient of hydrogen in the

ceramics to be 4.2� 10�5 cm2/s, which is rather high for typical

bulk diffusion processes in oxide materials and should be

responsible for the observed fast response and short recovery

times of the bulk materials. As a matter of fact, for pure P25

pellets sintered at 550 �C, the presence of numerous cylin-

drical mesopores has been revealed, and the Bru-

nauereEmmetteTeller (BET) specific surface area of the

ceramics is calculated to be around 23 m2/g [16]. Such a BET

specific surface area is close to one half of that of as-received

P25. So the bulk ceramics prepared from TiO2 nanoparticles

through convenient pressing and sintering is highly porous

with cylindrical mesopores, which are ideal for gas-sensing

applications via molecular diffusion and explain the

observed high bulk diffusion coefficient of hydrogen as

detailed above.

Given that the sensitivities of those commercial hydrogen

sensors based-on thick-film MOSs have remained relatively

low up to date, it seems a little surprising that such ultrahigh

hydrogen sensitivities can be readily obtained for PteTiO2

nanocomposite ceramics in this study. Commercial MOS

thick-film hydrogen sensors are usually fabricated through

drop-coating, tape-casting or screen-printing, in which thick-

films are formed frompastes of activematerial, namelyMOSs,

a low softening temperature glass frit, and organic carrier [2].

In a subsequent firing, organic carrier is burnt out while glass

frit forms a suitablematrix for the activematerial. As the glass

is inert to target gas, it will seriously decrease the overall

sensitivities of thick-films to the target gas due to a series

connection between glass frit and MOSs. In this study, glass is

not present and the distance between the two electrodes can

be connected continuously with TiO2 nanograins whose

resistance is sensitive to hydrogen. Compared with commer-

cial thick-film MOSs hydrogen sensors, the nanoceramics in

this study not only have relatively high specific surface area as

discussed above, the negative influence of glass on the sen-

sitivities have also been avoided. So ultrahigh hydrogen sen-

sitivities at room temperature have been observed for them.

For hydrogen interaction and sensing of MOSs at elevated

temperatures, the most widely accepted mechanism involves

the chemisorption of oxygen on MOSs [2], which leads to the

formation of electron depleted layers beneath the surface of

MOSs. When hydrogen, and other reducing gases as well, re-

acts with such chemisorbed oxygen, the thickness of the

depleted layers will decrease, and the resistance of the MOSs

will decrease accordingly. To verify whether this prevailing

mechanism is valid for the room-temperature hydrogen ab-

sorption and sensing behaviors observed in this study, we

have conducted some special measurements as shown in

Fig. 4. The ceramics display relatively high and steady con-

ductivity in a mixed atmosphere of 1000 ppm H2 in N2. When

the hydrogen flow was stopped, the surrounding atmosphere

was changed to N2 and the resistance was found to increase

slowly in N2. And when the atmosphere of 1000 ppm H2 in N2

was restored, the initial high conductivity corresponding to

the mixed atmosphere was promptly restored. As O2 was ab-

sent in this process, the drastic increase/decrease in the

resistance could hardly be explained in terms of oxygen

Fig. 4 e Real-time electrical resistance response to different

kinds of atmospheres at room temperature for a PteTiO2

nanocomposite ceramic pellet. The Pt electrodes were

coated on the same major surface of the pellet.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 1 ( 2 0 1 6 ) 3 3 0 7e3 3 1 2 3311

chemisorption. As a matter of fact, many studies have shown

that for oxygen chemisorption on MOSs, the leading oxygen

species is O� which starts to be dominant at temperatures

above 150e220 �C [19,20]. So for the room-temperature

hydrogen absorption and sensing behaviors observed in this

study, a mechanism rooted in hydrogen chemisorption on

TiO2 is highly favored: First, molecular hydrogen can easily

penetrate into the bulk of the porous ceramic materials.

Subsequently, the Pt aggregates in the ceramics are effective

in decomposing the incoming molecular hydrogen [21]. In the

third step, such decomposed hydrogen atoms can efficiently

migrate into the surfaces or PteTiO2 interfaces of the TiO2

nanoparticles via a spill-over process [22,23]. Lastly and most

crucially, the hydrogen atoms become chemisorbed on TiO2

Fig. 5 e X-ray diffraction pattern taken from the surface of a

P25 ceramic pellet, which had been electrochemically

hydrogenated for 20 h and then immersed in a CuSO4

solution for 10 min.

nanoparticles with their electrons denoted [24]. Expressing a

molecular hydrogen asH2ðgÞ, and a chemisorbed hydrogen ion

on TiO2 as H$i , the overall reaction in the response stage can be

described as:

H2ðgÞ�������!Pt;TiO2 2H$i þ 2e0; (1)

where e0 represents a released electron contributing to the

conductivity.

To further understand hydrogen chemisorption on the

TiO2 porous ceramics, we have also tried to produce such

hydrogen chemisorption in an electrochemical way [25], in

which a sintered P25 pellet was used as the cathode to elec-

trolyze water in a NaOH solution. To our surprise, the chem-

isorbed hydrogen was found to be so reactive that such

hydrogen atoms effectively reduce Cu2þ in aqueous solutions

to Cu2O deposited on TiO2 at room temperature, as shown in

Fig. 5. With such a high reactivity, the increase/decrease in

resistance in Fig. 4 now can be satisfactorily understood: In N2,

chemisorbed hydrogen atoms will combine with one another

and leave from TiO2 in the form of molecules, resulting in the

relatively slow resistance increase; when the surrounding

atmosphere is in air, chemisorbed hydrogen will react with

oxygen more intensively [26], leading to the fast recovery.

Conclusion

We have achieved some novel and surprising findings on the

room-temperature hydrogen absorption and sensing behav-

iors of properly functionalized bulk porous PteTiO2 nano-

composite ceramics. Intriguing aspects included fast effective

hydrogen diffusivity, ultrahigh hydrogen sensitivity, fast

response and short recovery times, and high gas selectivity.

Collectively, these results point to a novel hydrogen sensing

mechanism rooted in hydrogen chemisorption on TiO2. These

findings help to enrich our microscopic understanding of

room-temperature hydrogen interactions with different

MOSs, and should also have important application potentials

in various technological developments with hydrogen as the

energy carrier in view of the stability, readiness for mass

production, and suitability in everyday-life circumstances

inherent with bulk MOSs.

Acknowledgments

We thank Mr. Peng Lu for his help on hydrogen sensing

measurement. This work was supported by the National

Natural Science Foundation of China (Nos. 61434002, J1210061,

and 11204286), the National High Technology Research and

Development Program of China (No. 2013AA031903), and the

National Key Basic Research Program of China (No.

2014CB921103).

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