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Room temperature H2 sensing using functionalizedGaN nanotubes with ultra low activation energy

Prasana Sahoo a, S. Dhara a,*, S. Dash a, S. Amirthapandian b, Arun K. Prasad a,A.K. Tyagi a

a Surface and Nanoscience Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, IndiabMaterials Physics Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India

a r t i c l e i n f o

Article history:

Received 3 July 2012

Received in revised form

20 December 2012

Accepted 28 December 2012

Available online 29 January 2013

Keywords:

Gallium nitride

Hydrogen sensor

Platinum functionalization

* Corresponding author. Tel.: þ91 44 2748050E-mail address: dhara@igcar.gov.in (S. Dh

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.12.1

a b s t r a c t

A single-step synthesis route of square shaped wurtzite GaN nanotubes is reported by

a quasi-vaporesolid process with detailed growth kinetics involving surface energies and

Ga mobility along different crystalline facets. A wet chemical route is used for the func-

tionalization of GaN nanotubes with Pt nanoclusters of average diameter w1.6 (0.4) nm in

order to instigate the formation of localized Schottky barrier, responsible for carrier

transport in the sensing process. Catalytically enhanced dissociation of molecular H2 down

to the lowest detection limit of 25 ppm at room temperature, as compared to those of

reported GaN systems has been shown. We report, for the first time, a very low activation

energy value of 29.4 meV which will be useful in practical sensing of H2 at room temper-

ature without any application of bias.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction process of high pure GaN NTs. To date, contamination free

Nanotubes (NTs) with large surface areas and unidirectional

curvature have received considerable interest in wide range of

applications pertaining to nanofluidics, drug delivery, catal-

ysis, and chemical sensing [1e4]. Direct and wide band gap

GaN is one of the most promising semiconductors in future

high-power optoelectronic devices and sensor applications

[4]. Considerable progress has been achieved toward con-

trolled growth of GaN nanowires and improvising desired

functionality by appropriate surface modification. However,

there is only a few published work on the growth of amor-

phous, polycrystalline, and single-crystalline GaN NTs mostly

employing template or using metal catalysts [5e13]. In such

methods, incomplete removal of template material, partial

inclusion of catalyst into the tube annulus and high porosity

in as-grown samples are major blocks in the fabrication

0x22513; fax: þ91 44 2748ara).2013, Hydrogen Energy P31

direct fabrication of a GaN tubular structures have been

a technical challenge. In the present work, an effort has been

made to synthesize catalyst free growth of GaN NTs via

a quasi vaporesolid (VS) process with an attempt toward

elucidation of the underlying growth mechanism.

While looking toward the device performance using

nanowires or NTs, the role played by various functional

groups in interface with zero-dimensional nanocrystallites

assumes enormous importance [4,14e16]. Decoration of

compound semiconductors with noble metal nanoclusters

like Pd or Pt are of specific interest since these metals act as

catalysts for numerous surface chemical reactions. An oxi-

dation step is usually employed specifically to impart photo-

catalytic H2 generation capability [17,18], and to produce

continuous hot electron flow in nanodiodes [19e22]. In addi-

tion, such a step usually enhances electrochemical activity

0081.

ublications, LLC. Published 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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 03514

[23] and H2 sensing response across wide range of tempera-

ture [24,25]. For example, there were several reports from

Somorjai’s group on the generation of steady state chemi-

current using a planar nanodiode device which employed

5e8 nm sputtered thin film of Pt over thick GaN film, and also

metal-oxide bases [20e22] at times. They have achieved hot

electron generation induced chemi-current yield of

(3.5 � 0.8) � 10�3 under catalytic reaction of O2 or CO gas. The

aforementioned value is found to be one order of magnitude

higher than that for Pt/PdeTiO2 [21]. There are several reports

on Pt catalyzed carbon materials, metal oxides and metal-

oxynitrides used in large scale solar energy assisted H2 pro-

duction from water [26]. Pt-loaded metal nitride nanoclusters

depict superior yield than oxide counterpart toward solar

energy assisted H2 production. However, studies on the met-

alesemiconductor interface structure and crystallinity are

important for its applications in device [27].

In the present study, we have reported a methodology for

synthesizing well faceted square shaped high yield GaN NTs

with possible growth mechanism. These NTs were covalently

functionalized with Pt nanoclusters via a wet chemical route.

Room temperature (RT) H2 sensing with the lowest detection

limit, reported so far using GaN system, has been performed

on these Pt-decorated NTs with detailed analysis of activation

energy of the process.

2. Experimental

2.1. Growth of GaN nanotubes

NTs were synthesized using a hot walled CVD reactor [28e30].

Initially, a Si substrate coated with 3 nm of Au was placed

horizontally on the top of a high pure alumina boat (99.95%) at

a distance of w20 mm from the Ga (99.999%, Aldrich) source,

which was kept at the upstream side of the same boat. The

growth was carried out within the hot zone of the tubular

furnace. The furnace tube was initially evacuated to 1 mbar

base pressure and subsequently the temperature of the

chamber was slowly raised to 500 �C with a ramping rate of

w25 �C per minutes. The system was continuously evacuated

for 10 min at 500 �C. The temperature of the system was fur-

ther increased to 900 �C following a fast ramping rate of

w40 �C perminute whilemaintaining the same base pressure.

After the chamber temperature was raised to 900 �C, the base

pressure was slowly increased to 1 atm in 10 min by bleeding

high purity NH3 gas at a flow rate of 10 sccm. The reaction was

terminated after duration of 2 h. The system was allowed to

cool down slowly to RT in the same NH3 ambient. A large

quantity of light yellowish material was found to be coated

uniformly throughout the Si substrate. To identify the growth

mechanism of NTs, the role of the most decisive growth pa-

rameters, namely, growth rate, temperature and NH3 partial

pressure were taken into consideration.

2.2. Chemical functionalization and metallization

The NTs were further functionalized with Pt nanoclusters

using lysine [HO2CCH(NH2)(CH2)4NH2] as a capping agent fol-

lowing a standard procedure [31]. Initially, GaN NTs were

treated with a diluted piranha solution (50% aqueous solution

containing H2SO4:H2O2: 3:1) for 5 min, and then washed with

deionized water (w18 MU) followed by a drying process using

N2 gas. The hydroxylated samples were treated with solution

containing 1 mM of lysine followed by 1 mM of HPtCl4 in

continuous stirring condition for 30 min. A freshly prepared

solution of 0.1 M NaBH4 was added slowly into the mixture in

order to achieve the desired decoration of Pt nanoclusters

surrounding the NTs. The product was washed and dried

following the above mentioned process. The Pt coated sam-

ples were heated at 300 �C for 30 min to remove lysine [31].

Contact probes were made by sputtering a 5 nm thick Cr fol-

lowed by 80 nm thick Au films on inter-digitated electrodes

with 1 mm spacing for electrical measurements.

2.3. Characterization methodologies

Morphological features of the NTs were examined by field

emission scanning electron microscopy (FESEM; Model Zeiss

SUPRA 40). High resolution transmission electron microscopy

(HRTEM; Libra 200 Zeiss) and selected area electron diffraction

(SAED) were employed for structural characterization. Micro-

Raman scattering (inVia Renishaw spectrometer) with

514.5 nm line of Arþ laser excitation (<1 mm laser spot at the

sample surface) using 3000 gr mm�1 grating and micro-PL

with 325 nm line of HeeCd excitation using 2400 gr mm�1

grating assisted with thermoelectric cooled CCD detector in

the back scattering configuration were performed for optical

characterization of the as-grown sample. A liquid N2 stage

(Linkam) was used for temperature dependent (80e800 K)

measurements. In-situ H2 sensing was carried out by using

a custom built in-house gas exposure facility.

3. Results and discussion

3.1. Morphological studies

Fig. 1aec shows typical FESEM images of the pristine NTs on

Si substrate. Highly dense GaN layer of thickness w20 mm is

seen over the Si substrate, as revealed from the tilted cross-

sectional image (Fig. 1a). A close view of the magnified

image at middle region (Fig. 1b) reveals that NTs are homo-

epitaxially grown over a layer of GaN nanoclusters with

overall thickness w2 mm (Fig. 1a). The underlying GaN

nanocluster layer may have acted like a buffer layer, which

has initiated the NT formation. On mechanical scratching

it is found that the as-grown GaN NT layer can be

smoothly removed from the nanocluster interface region

(Supplementary Information Fig. S1). It is worthwhile men-

tioning that NTs can be isolated without being con-

taminated with nanocluster impurity. The surface layer, as

observed from the side (Fig. 1c) and normal views (Fig. 1d),

reveals that the GaN NTs are homogeneously distributed

throughout the surface. The typical images of the Pt deco-

rated GaN NTs are shown in Fig. 1deg. Most of the NTs

exhibit nearly square shaped cross-section (Fig. 1f) with wall

thickness of about w10e15 nm and average side dimensions

of nearly w50 nm (Fig. 1g). Typical length extends to several

micrometers.

Fig. 1 e FESEM image of GaN NTs grown in Si substrates; a) cross-sectional view and b) magnified view of the transition

region from nanoclusters to NTs, c) top view; Large scale view of Pt decorated NTs with d) top view, e) side view and f) high

resolution image; g) square shaped facets of the NTs with uniform Pt decoration on the walls.

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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 0 3515

3.2. Crystallographic structural studies

The structure of individual GaN NTs was investigated by

HRTEM analysis. The typical tube like contrast is observed

(Fig. 2a). The areas close to the edge are appearing darker than

the central zone. The NTs exhibit uniform cross section and

extend to several micrometers in length (inset Fig. 2a). The

external surface is relatively rough as seen from the HRTEM

image (Fig. 2b). The typical segment of the open end of a NT is

shown in Fig. 2c. Lattice pattern shows family of m-planes

with growth direction along [1120]. A smooth transition takes

place between the side faces of the NT as indicated from the

HRTEM image (Fig. 2d). Further studies suggest that the tube

surfaces are composed of (0001) planes and the tube axis is

subtended along [1120] direction which is normal to a-plane.

The high resolution lattice image of the transition region, with

interface region at the corner overw2 nm, is shown in Fig. 2e.

The m-planes, orthogonal to c-plane are indicated in the high

resolution image. A typical SAED pattern along one of the

facets shows zone axes along [0001] of wurtzite (wz)-GaN

phase (Fig. 2f). Double spots in the selected area electron dif-

fraction (SAED) pattern indicate twin like features, which are

essentially from the opposite facet of the NT having same

crystalline orientation [0001] with a small tilt. The tilt is likely,

as the facets are not exactly parallel to each other. The

observation, however establishes the fact that all the facets

are oriented along [0001] direction. The Pt decorated NTs

(Fig. 3a) exhibit relatively rougher external surface, as com-

pared to that for the pristine NTs (Fig. 3b). The typical high

resolution image reveals that the Pt nanoclusters are uni-

formly spread over the GaN NT with different crystalline

orientations (Fig. 3c). The d-spacings are indexed to cubic Pt

along with the presence of wz-GaN as shown by SAED pattern

in the outset of Fig. 3c. Fig. 3d shows the bright field image of Pt

decorated NTs and the corresponding dark field image is

depicted in Fig. 3e. These images are recorded by using <111>

diffraction spot of the Pt nanoclusters. Average diameter of Pt

nanocluster is found to be w1.6 (0.4) nm. Energy dispersive

X-ray (EDX) analysis shows (Supplementary Information

Fig. S2) that the present method of Pt functionalization pro-

vides an excellent spatial distribution of w1.5% Pt across

a depth extending up to 10 mm from the surface layer.

3.3. Vibrational and optical properties

Fig. 4a shows the typical micro-Raman spectra of pristine NTs

at different incident laser power using 514.5 nm laser line. At

low laser power (2 mW), the Raman spectra reveals three

fundamental modes with frequencies around 541, 571 and

732 cm�1, which correspond to symmetry allowed E1(TO),

E2(high) and A1(LO) phonon modes of wz-GaN phase, respec-

tively [28,29]. Additional peaks at 252 and 421 cm�1 are pri-

marily attributed to zone boundary (ZB) phonons arising from

the finite size effect of GaN NTs [28]. Surface optical (SO)

modes at 624 and 692 cm�1 appear as a result of surface dis-

order induced perturbation of the surface potential [28]. Low

temperature micro-PL spectra of NTs show strong emission in

the region of photon energy 3.0e3.6 eV (Fig. 4b). The PL

emission lines at RT (300 K) consist of broad and asymmetric

line shape with weak band-edge-emission. Pt functionaliza-

tion leads to a decrease in the intensity of emission line by two

orders of magnitude as compared to that for the emission

Fig. 2 e a) TEM images of a NT with visible side facets, b) tube with open end, c) lattice images of the NT shows presence of

family of m-planes and growth direction along [1120], d) smooth transition between side facets and e) corresponding lattice

image; f) SAED pattern of NTs shows wz-GaN phase with inset showing the region from which the pattern is generated.

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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 03516

spectra of control NTs as a result of quenching effect due to

carrier transfer taking place between GaN and Pt nano-

clusters. As temperature decreases to 80 K, the emission

process is dominated by a peak at 3.47 eV with a series of

Fig. 3 e TEM images NTs a) Pt functionalized NT and b) control

walls with different crystalline planes. Outset shows SAED patt

bright and e) dark filed image of Pt-decorated GaN NTs are reco

emission peaks centered at 3.24 and 3.30 eV. The peak at

3.47 eV corresponds to the recombination of excitons bound to

neutral donors (I2) [32,33]. The I2 emission shows broad line

shape of w65 meV, which is blue shifted by 18 meV. This is

NT, c) HRTEM images of Pt nanoclusters decorated on NT

ern for the wz-GaN NT and cubic Pt NC; Corresponding d)

rded using <111> diffraction spot of Pt NCs.

Fig. 4 e a) Raman spectra of Pt-decorated GaN NTs at room temperature showing all the symmetry allowed peaks

corresponding to wz-GaN phase. Surface optical (SO) modes are also shown along with zone boundary (ZB) modes. b) Low

temperature micro-PL spectra of GaN NTs using 325 nm laser excitation. The spectra possess strong I2 line at low

temperatures.

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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 0 3517

attributed to the size effect in the NTs. As revealed by HRTEM

analysis, the NTs have wall thickness ofw10 nmwhich is less

than that of the exciton Bohr radius (w11 nm) of GaN [34]. The

prominent peaks observed at 3.3 and 3.24 eV have been

assigned to zero phonon line and their phonon replica of the

free-to-bound (FB) emission line, respectively [32].

3.4. Nanotube growth mechanism

The growth mechanism of GaN NTs proceeds via a quasi VS

process. As depicted schematically in Supplementary Fig. S3,

it is postulated that the nucleation and further growth of the

NTs may occur in following stages: 1) generation of ultra-

small, nanosized GaN clusters on the substrate surface, 2)

self-agglomeration and coalescence induced formation of

facets, and 3) further attachment of incoming vapor phase

clusters oriented to specific crystallographic facets [29,35e37].

In the present study, experiments were conducted with fast

ramping rates, initially, to provide enough population density

of Ga followed by the atomic nitrogen flux to start the nucle-

ation process aided by Au catalyst. Under this condition, un-

like vaporeliquidesolid (VLS) process the reaction occurs

faster than the effective incubation time stimulating the for-

mation of agglomerated nanoclusters. An increased incuba-

tion time is anticipated in the VLS process wherein AueGa

interfacial energy drives a directional growth [35]. Upon

increasing the incoming Ga and N flux rate, the nanoclusters

undergo further self-agglomeration process. The nucleation of

facets leading to formation of NT possibly stems from

agglomeration of GaN clusters, whichmay form a ring shaped

nucleation front. Further, reactant species from the vapor

phase gets exposed continually over these nucleated facets

giving rise to the formation of GaN NTs. The observed square

or rectangular shape of NTs possibly results due to the dif-

ferential adatom migration behavior among different crys-

tallographic faces [37]. Faceting of nucleated grains stems

from the fact that certain crystal faces are energetically dis-

favored to co-ordinate with adatoms, thereby restricting the

growth directions. Similar structures of GaN NTs are also

reported in the literature. The calculated surface energy for

polar (0001) and non-polar (1120) surfaces of GaN are 100 and

123 meV/A, respectively [38]. Although, the difference of sur-

face energy between the non-polar and polar surface is rela-

tively small, nevertheless, they do behave differently to

incoming adatom species. As a matter of fact, polar surfaces

prefer systematic adsorptions, whereas non-polar surfaces

maintain their stoichiometry by systematic atomic diffusions.

The interaction between adsorbate and surface strongly de-

pends on themost decisive parameter such as incoming Ga or

N flux ratio. In Ga rich condition, a non-stoichiometric surface

having GaeGa dimers is stable. N-rich condition increases the

nucleation rate of GaN due to the higher strength of GaeN

bond and shorter diffusion length of Ga than that of GaeGa

bond [36,39,40]. It is thus apparent that, a N-rich condition

promotes stoichiometric growth of a non-polar surface. The

higher nucleation rate in the (1120) surface over (0001) surface

is expected to stimulate the axial growth of the NTs along

[1120]. On the other hand, relatively lower surface energy

along (0001) plane than that of the (1120) plane promotes

formation of (0001) facets in the square symmetry, while the

GaN system is in the hexagonal symmetry (Fig. 2).

4. H2 sensing and mechanism for sensing

The dissociation of H2 on the Pt-decorated GaN NTs is

investigated as a function of time over a wide range of H2

concentration (25e200 ppm) in N2 background, and temper-

ature (RT to 473 K) as shown in Fig. 5a and b. The change in

the reference value of the resistance with the variation in H2

concentrations, as obviously observed in Fig. 5b, is due to the

fact that the resistance value has not recovered completely

during the sequential measurement period. Initially, the

sensor active area is exposed to N2 and no noticeable change

in the resistance of the sensor probe has been observed

(Supplementary Information Fig. S4a). However, the material

shows good sensitivity to H2 with response time of 120 s and

recovery time of about 5 min at RT. At RT, Pt-decorated GaN

Fig. 5 e Time dependent change in resistance of Pt decorated GaN NTs at a) RT (300 K) and b) 373 K with different H2

concentration; c) relative sensitivity to H2 at different temperatures for Pt coated and control NTs; d) Semi-log Arrhenius plot

of jSj versus 1/T shows a linear response. The slope is proportional to the activation energy that is w29.4 meV.

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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 03518

NTs show relative sensitivities of 0.4 at 25 ppm, and 0.95 at

100 ppm level for H2, which are factor of 10 as compared to

that shown by control NTs. However, the relative sensitiv-

ities of H2 increase to 3.8 at 25 ppm and 10.8 at 100 ppm at

373 K (Fig. 5c). The reported value of 25 ppm of H2 at RT using

the NTs, in the present study, is the lowest ever reported in

the GaN system. Wright et al. [24] reported the previous

lowest value with a relative response of 1.7 at 200 ppm of H2

for 7 nm Pt sputter-coated GaN nanowire with longer expo-

sure time w10 min at RT and 0.1 V bias condition. By com-

parison, Pt functionalized GaN NTs are found to be more

sensitive than Pt sputter-coated GaN nanowires. The uni-

form distribution of Pt nanoclusters over NTs, thus is ad-

vantageous over sputter-coated film, and found to be more

effective in catalytic dissociation of molecular H2 at RT. A

semi-log Arrhenius plot of the jSj versus 1/T, (where sensi-

tivity S ¼ DR/R0, and DR change in resistance with respect to

the H2 concentration and R0 resistance of the PteGaN system

prior to the introduction of H2), has been used to calculate

the adsorption activation energy of 29.4 meV (Fig. 5d). The

value obtained is found to be the lowest ever reported, as

compared to those for other GaN systems. The calculated

activation energy is well below the reported value of

w300 meV for Pt and Pd sputtered GaN nanowires [24,25],

which confirms the superiority in sensitivity and respon-

sively of our GaN NT based H2 sensor [41,42]. Initially, the

response of the Pt-functionalized GaN NT senor to N2 gas has

been evaluated. For selectivity purpose, however we have

also shown that the present sensor is not sensitive to O2

(Supplementary Information Fig. S4b).

There is growing evidence which indicates that Schottky

barrier formed between the metal and semiconductor surface

gets altered as a result of chemical reaction at the interface

[43]. Upon interaction with H2 the excess electrons, generated

as a result of catalytic dissociation of H2 on Pt active sites, can

easily exceed the Schottky barrier and travel ballistically

through the metal conduction band. These electrons possibly

reach the semiconductor surface where it contributes to the

conduction process resulting in the decrease of resistance at

Pt-decorated GaN NTs interface. The low activation energy of

Pt-decorated GaN NTs interface and small Pt nanocluster size

distribution enhances the catalytic dissociation of H2 through

a non-adiabatic energy transfer route leading to a high value

of sensitivity. Such a H2 sensor has also an important

advantage over others in terms of the fact that the sensing

elements can be effectively integrated with GaN based elec-

tronics leading to further miniaturization.

5. Conclusion

In conclusion, the growth of well crystallinewz-GaN NTs with

regular square facets using CVD technique has been reported.

The NTs nucleate via a quasi VS process followed by forma-

tion of self-agglomeration induced facets. A growth kinetic is

proposed for the formation of square facets in the hexagonal

symmetry of the wurtzite phase. The differential growth rate

along different crystallographic facets is the driving force for

the axial growth. Functionalizing NTs with well dispersed Pt

nanoclusters exhibits noble catalytic properties. H2 induced

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 3 8 ( 2 0 1 3 ) 3 5 1 3e3 5 2 0 3519

decrease in the resistance has been measured to the lowest

value of 25 ppm level at RT with a very low activation energy

value of 29.4 meV for the Pt-decorated GaN NT surfaces, as

compared to other GaN systems. This will be extremely useful

for low power application in emergency cases as detection of

hydrogen is very important in the nuclear reactor assembly.

Acknowledgments

We thank M. Kamruddin for helping in FESEM analysis. We

also thank N. Satish Kumar for helping in the sensor

measurements. We acknowledge J. Basu of Physical Met-

allurgy Division, IGCAR for useful discussion. One of us (PS)

acknowledges Department of Atomic Energy for the finan-

cial aid.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2012.12.131.

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