Synergistic Cooperation of Rutile TiO2 {002}, {101} and {110} Facets

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Surfaces, Interfaces, and Applications

Synergistic Cooperation of Rutile TiO2 {002}, {101} and {110} Facets for Hydrogen Sensing Xiaoyan Zhou, Zhuo Wang, Xiaohong Xia, Guosheng Shao, Kevin Homewood, and Yun Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07816 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Applied Materials & Interfaces

Synergistic Cooperation of Rutile TiO2 {002}, {101} and {110} Facets for Hydrogen Sensing Xiaoyan Zhou, † Zhuo Wang, ‡ , ф Xiaohong Xia, † ,* Guosheng Shao, ‡ , ф Kevin Homewood,† Yun Gao†, * †

Ministry-of-Education Key Laboratory for the Green Preparation and

Application of Functional Materials, Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, School of Materials Science & Engineering, Hubei University, Wuhan 430062, China ‡

State Center for International Cooperation on Designer Low-carbon &

Environmental Materials, Zhengzhou University, 100 Kexue Avenue, Zhengzhou 450001, China ф

Zhengzhou Materials Genome Institute, Zhongyuanzhigu, Xingyang 450100,

China AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected] Xiaoyan Zhou and Zhuo Wang contributed equally to this paper

Abstract: An oriented TiO2 thin film based hydrogen sensor has been demonstrated to have excellent sensing properties at room temperature. The exposed high energy surface offers a low energy barrier for H2 adsorption and dissociation. In this work, rutile TiO2 with {101} and {002} facets exposed was 1

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controllably synthesized by adjusting the ethanol content of the hydrothermal solvent. The crystalline structure, morphologies and H2 sensing performance of the samples varied with the relative ratios of {002} and {101} facets. By increasing the ethanol content, the (002) orientation growth was enhanced and the (101) orientation growth was restrained, the size of the nanorods composing the thin film was reduced and the density of the film was increased. All of the prepared TiO2 nanorod array film based hydrogen sensors performed very well at room temperature. TiO2 hydrogen sensor with both {110} and {002} facets exposed gave a faster response, as well as better repeatability and stability than those with only {002} facets. DFT simulations have been adopted to reveal the surface interaction of H2 and the TiO2 surface. The results suggested that H2 tended to be adsorbed and dissociated on the (002) and (101) surface. There is very small active barrier for atomic H to recombine into H2 molecules on the (110) surface. Thin films with lower density, where more (110) surface is exposed, offered more space for H2 regeneration, leading to shorter response and recovery times as well as higher sensitivity. The (002), (101) and (110) surfaces of rutile TiO2 synergistically cooperated to complete the whole H2 sensing process. KEYWORDS: Hydrogen sensor; TiO2 thin film; Synergistic; Surface interaction; Simulation

1. Introduction Due to the advantages of low cost, stable physical and chemical properties and 2


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abundant material availability, wide-band-gap metal oxide semiconductors (SnO2, NiO, TiO2, CuO, ZnO etc.) have been extensively applied in various fields, such as solar cells, lithium ion batteries, electro- or photo-catalysis and gas sensors.1-6 The functionality of these semiconductor oxides is strongly dependent on the crystal structure especially the exposure of high energy surfaces, on which chemical reactions such as water splitting and gas dissociation can be easily realized.7-10 The exposed facets introduce more atom steps, edges, kinks, and dangling bonds, which usually have high chemical activity.11,12 Hierarchical SnO2 nano-flowers with high-index {113} and {102} facets exposed have shown high NO2 gas sensing sensitivity and selectivity.13 Fe3O4 with exposed {110} facets has shown a significant potential in lithium storage with good cycle performance and specific capability.14 Zheng et.al reported that hierarchical NiO mesocrystals with exposed high energy {100} facets exhibiting an exceptional electrochemical performance when fabricated as an electrode material for supercapacitors.15 TiO2 with exposed reactive facets have shown excellent performance in photo-catalysis.16-17 Recently, TiO2 with two high energy facets exposed has attracted broad interest where the synergistic or heterojunction effect was considered to have a positive impact on the functionality of TiO2. Liu et al. found that anatase TiO2 with different {001} and {101} facet ratios had a synergistic eﬀect on photocatalytic hydrogen evolution reaction activity.18 Roy et al. demonstrated that anatase TiO2 with an optimal {001} and {101} facets ratio improves the photocatalytic activity in water splitting by reducing carrier recombination.18 Yu et.al showed that the co-exposed 3



{001} and {101} facets of anatase TiO2 promoted photocatalytic CO2-reduction activity by forming a ‘surface heterojunction’ within a single TiO2 particle.19 Zheng et.al proposed that co-exposed {101} and {001} facets improved photo-generated carrier separation thus enhanced photocatalytic efficiency.20 In this work, TiO2 nanorod arrays thin film with (002) orientation were used as a hydrogen sensor material, and the synergistic cooperation of the (002), (101) and (110) surfaces in the process of hydrogen sensing is tested and the fundamental mechanism of the synergistic effect is studied. In our previous work, (002) oriented rutile TiO2 thin film was demonstrated to have excellent hydrogen sensing properties at room temperature.21-23 We also found that the orientation of the thin film and the exposed high-energy surface could be adjusted by controlling the hydrothermal solvent for the growth of TiO2 thin films.24 Following this strategy, in this work, we prepared TiO2 nanorod array thin films with highly reactive {101} and {002} facets by adjusting the ratio of ethanol content in the hydrothermal reaction with the aim of improving the performance of the already existing, room temperature working, TiO2 based hydrogen sensor. Previous reports on TiO2 based hydrogen sensors are summarized in Table 1. It is seen that the working temperature of TiO2 based hydrogen sensors has been brought down to room temperature by modifying the TiO2 nanotube with Pd particles37, 51 or Ni particles.31 The detection limit has been decreased to less than 20 ppm by surface modification with metals,26, 45 or with metal oxide combinations.30 A high sensitivity of more than 90% has been achieved in Pd modified TiO2 nanotubes.26, 48, 49 The 4


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fastest response and recovery time was reported to be less than 10 s in Pd modified TiO2 nanowires39 and nanofilms.49 However, such low detection limits, high sensitivity and fast response have not been achieved at the same time in one device. Krsko et al. reported that a flexible H2 sensor based on TiO2/polyimide was able to detect H2 concentration as low as 1 ppm in dry air at room temperature. However, the corresponding response and recovery time were about 100 min and 240 min, respectively.50 Wei et al. studied the H2 sensing performance of Pd nanoclusters decorated tubular TiO2 films and found that, although the response/recovery time was as fast as 20 s, the best detection limit of H2 was only 500 ppm.51 Table 1. Summary of TiO2 based H2 sensors. * indicates the literature data has been standardized by defining sensitivity as S=(Rair-RH2)/Rair, where R0 is the resistance before the introduction of H2, RH2 is the resistance in the presence of H2, and defining the response/recovery time as the time taken for the resistance drop/recover to 90%*

△R. T - working temperature, DL – Detection Limit, Cc – H2 concentration, S – sensitivity, Res – response time, Rec – recovery time, Ref – reference. T / oC

Materials

DL

Cc / ppm

S%

Res/s

Rec/s

Ref

/ppm Pd/TiO2 nanotube

~25

--

1000

~99*

~50

~50

25

TiO2 nanotube

20~150

200

200

60*

23

13

28

TiO2 nanofilm

340

300

10000

99*

100*

600*

29

TiO2/polyaniline

~25

--

Pure H2

3*

200*

150*

30

400

>20

20

80

12 -14

270-30

31

3+

Ce /TiO2 nanopowder

nanoparticles SnO2/TiO2 nanoparticles Ni/TiO2 nanotube

0 ~25

>50

50

7.7

5


~100

~20*

32


TiO2

nanorod

thin

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100

100

100

10*

30

70

22

Pt/TiO2/MWCNTs

150

>5000

5000

~8*

150–200

--

33

Ni/TiO2 nanotube

200

>1000

1000

13.7

80

100*

34

100-200

--

1000

20-30

--

--

35

25

--

800

~40*

83

130

36

25

>1000

10000

8*

150*

50*

37

film

Al-/V-

doped

TiO2

nanotube Polyaniline/TiO2 nanoparticles Pd/TiO2 nanotube

0 Ni/TiO2 nanotube

100-200

50

20000

25

80

--

38

Pd/TiO2 nanowire

200

500

5000

10*

Synergistic Cooperation of Rutile TiO2 {002}, {101} and {110} Facets

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