controlled growth of zinc oxide microrods by hydrothermal process

Controlled growth of zinc oxide microrods by hydrothermal processon porous ceramic supports for catalytic application Supamas Danwittayakula,b, Joydeep Duttab,c,⇑ a National Metal and Ma

Controlled growth of zinc oxide microrods by hydrothermal process

on porous ceramic supports for catalytic application

Supamas Danwittayakula,b, Joydeep Duttab,c,⇑

a

National Metal and Materials Technology Center, 114 Thailand Science Park, Paholyothin Rd., Klong Luang, Pathumthani, Thailand

b

Center of Excellence in Nanotechnology, Asian Institute of Technology, P.O Box 4, Klong Luang, Pathumthani 12120, Thailand

c

Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, P.O Box 33, 123 Al Khoud, Oman

a r t i c l e i n f o

Article history:

Received 16 July 2013

Received in revised form 1 October 2013

Accepted 3 October 2013

Available online 14 October 2013

Keywords:

ZnO microrods

Hydrothermal process

Porous ceramic substrates

Catalytic application

a b s t r a c t

The growth of zinc oxide (ZnO) microrods on porous ceramic substrates by mild hydrothermal process was studied One-dimensional ZnO microrods were grown on ZnO nanoparticle seeded substrates by using equimolar concentration of zinc nitrate and hexamethylenetetramine at temperatures lower than

100 °C We found that the growth of ZnO microrods on alumina and diatomite substrates were affected due to hydrolysis of substrate surfaces Stunted ZnO microrod growth onc-alumina and diatomite sub-strates were attributed to arise due to the degradation of hexamine molecules in the growth solution Adjusting the pH prior to the growth of ZnO microrods on both alumina and diatomite lead to the growth

of ZnO microrods similar to what is observed on flat glass substrates Cordierite does not hydrolyze easily and hence ZnO microrods with aspect ratio as high as 24, were obtained without any pH control of the growth solution Copper nanoparticles deposited on ZnO microrods were utilized as a catalyst for meth-anol steam reforming and about 14% hydrogen yield was obtained with almost 90% methmeth-anol conversion

at reforming temperature of 350 °C

Ó 2013 Elsevier B.V All rights reserved

1 Introduction

Porous ceramics are important for many industries where high

surface area, chemical physical and thermally resistant materials

are a requirement[1,2] Applications of porous ceramics extends

from filtration to use in mechanical seals and as catalyst supports

Zinc oxide (ZnO) nanostructures have recently attracted

consid-erable attention due to diverse potential applications[3] ZnO

crys-tals have anisotropic structure with polar and non-polar surfaces

that lead to the possibility of growing nanorod or nanowire

struc-tures[4] One-dimensional structures like nanorods or nanowires

of ZnO have been used in a wide range of applications from gas

sensors, solar cells, optoelectronics to piezoelectric devices It is

generally agreed that varying the morphology suits different

appli-cations of ZnO nanomaterials[4] For gas sensor applications, high

surface area of the sensing parts in devices can improve sensitivity

and allow the possibility to miniaturize devices[5] While highly

dense ZnO microrods on a substrates are preferred for gas sensor

and solar cell applications[6], ZnO based nanogenerators require

gap between nanorods for bending during the energy harvesting

processes [7–9] ZnO as a component in catalysts for hydrogen

production through methanol reforming process is also being pur-sued by several groups including ours[10–14] Copper–zinc oxide– alumina (Cu/ZnO/Al2O3) has been used as a catalyst for methanol synthesis and it was demonstrated that the use of catalyst support enhanced catalytic stability due to its ability to withstand higher temperatures[14,15]

Several techniques have been utilized to synthesize ZnO nano-rod arrays such as chemical vapor deposition, sol–gel and

expensive technique while sol–gel and hydrothermal processes are relatively straight-forward as they do not require any compli-cated systems during synthesis Hydrothermal process is a simple and low temperature technique for the growth of zinc oxide single crystals[16] Vayssieres et al first proposed the low temperature epitaxial growth of ZnO microrods on various substrates from equimolar concentrations of zinc nitrate ((Zn(NO3)2) and hexa-methylenetetramine (hexamine, C6H12N4) precursors [17] Since this early work, ZnO microrods were successfully grown on flat substrates such as glass [4], transparent conducting oxide film

[18], stainless steel [14], paper and flexible polymeric fibers

[19,20], amongst others Though many groups have worked on ZnO microrod growth on solid substrates, there still is a gap in the understanding of hydrothermal growth of ZnO microrods on porous ceramic substrates

In this study, we compare the growth of ZnO microrods on

0925-8388/$ - see front matter Ó 2013 Elsevier B.V All rights reserved.

⇑ Corresponding author at: Chair in Nanotechnology, Water Research Center,

Sultan Qaboos University, P.O Box 33, 123 Al Khoud, Oman Tel.: +968 24143266;

fax: +968 24413532.

E-mail address: dutta@squ.edu.om (J Dutta).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j a l c o m

diatomite and cordierite with an aim to understand and control the

growth of ZnO microrods by hydrothermal process on porous

sub-strates The ZnO microrods on ceramic supports were utilized as a

catalyst support for methanol steam reformation (MSR) Different

catalysts have been used for methanol steam reforming amongst

which copper (Cu) on ZnO supports show very high catalytic

activ-ity and hydrogen selectivactiv-ity[21] In this work, copper

nanoparti-cles were deposited on ZnO microrods (Cu–ZnO MR) by

conventional impregnation technique Prepared Cu–ZnO MR

cata-lysts were then utilized for examining the catalytic activities on

MSR using a packed tubular reactor operating at temperatures up

to 350 °C

2 Experimental

ZnO microrods were grown using a modified method suggested by Guo et al [4]

that compose of ZnO seeding on a substrate followed by a hydrothermal growth

process for the microrod growth [4] Seeding the substrates lead to the formation

of ZnO microrods in a preferential direction [22] Prior to seeding and growth

pro-cesses, all ceramic and reference glass substrates were prepared and cleaned to

re-move any surface contaminants.

The glass slides were immersed in detergent solution, sonicated for 20 min and

then thoroughly washed with deionized water The glass slides were then dried in

an atmospheric oven at 95 °C overnight Three types of ceramic substrates namely

alumina, calcined diatomite and cordierite which were selected to study the growth

of ZnO microrods on porous substrates, and were prepared from commercially

available materials ( Table 1 ) that were uniaxially pressed using a hydraulic press

to form 1–2 mm thick pellets of 20 mm diameter All pellets were then calcined

at 1000 °C for 2 h in the ambient for strengthening, prior to further use All pellets

were then cleaned by sonication in deionized water for 5 min and dried in a furnace

at 200 °C for 1 h in air Substrate densities were examined by following ASTM C20

(2010) protocal [23] Specific surface areas (S.S.A.) of substrates were determined

using gas adsorption technique (Autosorb-1C; Quantachrome Ins.) where 0.5 g of

each sample was outgassed at 300 °C for 5 h prior to the 5-point BET

measure-ments X-ray diffraction analysis (XRD; PANalytical, X’Pert PRO) was carried out

to study the crystal structure of the modified ceramic substrates.

2.1 Zinc oxide seeding process

All chemicals used for ZnO nanoparticle synthesis were of analytical grade Zinc

acetate dihydrate (Zn(CH 3 COO) 2 2H 2 O) and sodium hydroxide obtained from Merck

were used as a zinc and hydroxyl precursors to synthesize ZnO nanoparticles 4 mM

zinc acetate solution was gradually mixed with 4 mM sodium hydroxide in ethanol

(Merck) and then the Zn 2+

sol was allowed to hydrolyze under controlled aging in air at 60 °C for 3 h Hydrolyzed Zn 2+ sol initially formed Zn(OH) 2 gel that turns into

ZnO colloids upon aging Seeding the substrates were then conducted by dipping

ceramic substrates into ZnO colloids for 15 min and then dried in an oven for

15 min; this process was repeated thrice The seeded substrates were then annealed

at 350 °C for 5 h in air and stored in a dehumidified chamber for further use.

2.2 Hydrothermal growth of zinc oxide microrods

Zinc nitrate hexahydrate (Zn(NO 3 ) 2 6H 2 O) purchased from Merck was used as a

zinc precursor during the ZnO microrod growth The seeded substrates were

in-serted in an equimolar solution of zinc nitrate and hexamine heated to 95 °C for

up to 10 h In a sealed chemical bath, equimolar (5–10 mM) solution of zinc nitrate

and hexamine was replenished every 5 h to ascertain the ready availability of zinc

ions in the growth solution [24] The substrates after ZnO growth were finally

an-nealed at 350 °C for 1 h in the ambient prior to further use Each specimen was

investigated using field emission scanning electron microscope (FESEM,

JEOL-6301) working at 20 kV to record the morphology of microrods Shape and sizes of ZnO microrods were determined from the micrographs by using standard image analysis software (ImageJ software).

2.3 A catalyst support application of ZnO microrods on cordierite subtrates 2.3.1 Preparation copper/zinc oxide microrod catalysts (Cu–ZnO MR) Copper nanoparticles used in this work have been prepared as a colloidal dis-persion by heterogeneous precipitation The synthesis was carried out in an aque-ous solution under constant stirring using 0.46 mM copper nitrate as a copper precursor and hydrazine as a reducing agent [25,26] Polyvinylpyrolidone (PVP)

5 wt% in deionized water was added for stabilization of the colloids Copper nano-particles were deposited on ZnO microrods grown on the substrates by impregna-tion technique The ZnO microrod supports were immersed in the copper colloidal suspensions at 95 °C for 2 h Excess copper nanoparticles which did not attach to the microrods were then removed by rinsing the samples with deionized water The immersed samples were finally calcined at 300 °C for 1 h in air Each specimen was investigated using field emission scanning electron microscope (FESEM, JEOL-6301) working at 20 kV to record the morphology of copper nanoparticles on ZnO microrods Copper and zinc contents were determined using inductively coupled plasma-optical emission spectrometer (ICP-OES: Horiba, Activa) First, the catalyst samples were weighed and heated at 95 °C and then soaked in strong sulfuric acid for 3 h to allow metals and metal oxides to be released from the substrates Zinc oxide, copper and copper (I) oxide are readily dissolved in sulfuric acid while copper (II) oxide forms copper sulfate before dissolving in water Adjusted final volume of released metal ions in the solution were used to determine the contents of copper and zinc using ICP-OES.

2.3.2 Steam reforming of methanol The steam reforming of methanol were carried out at atmospheric pressure in a packed electrically heated tubular reactor of 20 mm diameter in a 20 cm long heated zone schematically represented in Fig 1 Methanol steam reforming was performed at varying temperatures ranging from 200 °C to 350 °C in the presence

of 0.5 g of as-prepared catalysts that were ground and packed in the reactor Prior

to methanol steam reforming process, catalyst were activated by flowing 60 mL/ min of 5% H 2 in argon at 300 °C for 1 h Water to methanol ratio of 0.8 mol was used for all the reforming experiments An ultrasonic transducer was used to generate aerosols of the reactant which was then carried into the reaction zone by flowing

20 mL/min of argon gas through the aerosol generator chamber Gas products were collected and analyzed by a gas chromatograph (GC, Buck Scientific) connected to a thermal conductivity detector (TCD) Packed columns of Hyesep D (polyvinylben-zene, PVB) and molecular sieve 13x were used to separate the gas mixtures.

Table 1

Properties of the ceramic substrates used in this work.

area (m 2

/g)

Substrate density (g/cm 3

) Alumina

(c-Al 2 O 3 )

Calcined

diatomite

Jiangxi, China

3 Results and discussion

3.1 ZnO microrod microstructures

Hydrothermal growth of ZnO micro and nanostructures depend

on several synthesis parameters like pH, temperature of hydrolysis,

precursor concentration, etc.[24] Essentially the concentration of

zinc ions and the availability of hydroxyl groups in the growth

solution balance the consumption or removal of these ions until

it finds an energetically favorable position leading to the epitaxial

growth of the ZnO microrods[17,27]

Growth of ZnO microrods on the three ceramic substrates were

found to be different with exactly similar precursor solution and

growth conditions possibly arising from the ionization reactions

that take place on surfaces of porous ceramic substrates leading

to a change in pH of the growth solution Strength of ionization

reactions depend on the pH and temperature of reaction[28–30]

It has already been reported that during initial stages of ZnO

growth on glass substrates, growth solution changes from being

slightly acidic to mildly basic, within about one hour of the onset

of the hydrothermal process[24,31] The growth solution turns

ba-sic during growth process because of hydrolysis reaction as shown

in Eq.(1) Baruah et al reported that the growth rate of ZnO

micro-rod was maximum in basic conditions during hydrothermal

syn-thesis [23] as hydroxyl ions dominate the reaction process and

the hydrolysis reaction shifts backward following the Le Châtelier’s principle, resulting in a higher growth rate

In order to attain ZnO microrod growth by hydrothermal pro-cess on ceramic substrates similar to what is achieved on glass sub-strates, an experiment was designed to monitor the surface state of these substrates upon continuous exposure to water [4,18] In

Fig 2, we have plotted the changes in pH of deionized water upon continued soaking of the porous substrates in 25 mL deionized water We found that water in the presence of alumina and diato-mite substrates showed rapidly increasing pH during first immer-sion period (30 min) while cordierite and glass substrates did not influence any substantial changes in the pH over 150 min of soaking

The specific surface area and densities of the three ceramic sub-strates used in this work are summarized inTable 1.c-Al2O3and diatomite showed almost comparable specific surface areas (10 m2/g) but diatomite was twice as dense as thec-Al2O3 sub-strates (Table 1) Alumina also has larger terminated surfaces than calcined diatomite and cordierite[32–34] Cordierite has a higher specific surface areas (14 m2/g) and also higher monolith density (1.7 g/cm3) It is well known that alumina hydrolyzes readily in contact with water forming hydroxides and forms complexes such

as Al(OH)2+, AlðOHÞþ

2, Al(OH)3and AlðOHÞ

4that can raise the pH to-wards the basic region[29,30,35]

X-ray diffraction (XRD) shows that the cordierite samples matched with JCPDS No 09-0472 showing (Mg,Fe)2Al4Si5O18while quartz (JCPDS No 82-1403), muscovite (JCPDF No 21-0993) and anorthite (89-1460) were found in calcined diatomite Corundum (calcined alumina, Al2O3) structure was confirmed for the alumina substrates (JCPDS No 76-0144) These results are shown inFig 3

As mentioned before, ZnO microrods grow faster in the basic re-gion, and hence the growth rate of ZnO microrods on alumina and diatomite substrates should be faster than on cordierite and glass substrates[24] Scanning electron micrograph (SEM) of ZnO micro-rods shows the homogeneous growth on glass substrate with an average rod thickness of 170 nm (Fig 4) The uniformity of the ZnO microrods on glass substrate is evident with the formation

of microrods 3lm in length and 170 nm in diameter Aspect ra-tio of ZnO microrods grown on cordierite substrates estimated from the ratio of length over width was found to be 24 Morphol-ogy of ZnO microrods grown on cordierite substrate were similar to Fig 2 Change in pH of deionized water upon continued soaking of the porous

substrates in 25 mL deionized water.

the ones observed on glass substrates (Fig 5) The morphology

could however be controlled by varying Zn2+concentration in the

starting precursor solution In contrast, morphology of ZnO rods

grown on alumina and diatomite substrates exhibited very large

diameter of the microrods as shown inFigs 6 and 7which are

visu-ally noticeable in the case of microrods growth with 10 mM Zn2+

For lower concentration of Zn2+(5 mM) in the starting solution,

no clear evidence of ZnO microrod growth could be observed on

c-alumina (Fig 6a) At slightly higher precursor concentrations

(7–10 mM), microrods with lower aspect ratios are formed as

shown inFig 6b and c.Table 2summarizes length, diameter and

aspect ratios of ZnO microrods grown on different substrates

(Figs 4–7) As can be observed in Fig 6b and c and Fig 7a–c,

ZnO microrod growth was stunted on alumina or diatomite

sub-strates and no growth perpendicular to the substrate were

observed as it is seen for ZnO microrod growth on glass substrates

(Fig 4) Baruah and Dutta[24]reported that the growth rate of ZnO

microrods were highest in both lateral ½1 0 1 0 and longitudinal

[0 0 0 1] directions in basic conditions and that ZnO nanorods are

known to get eroded in acidic conditions with the final growth

depending upon the competition between growth and dissolution

processes This explains why thicker ZnO microrods grow on

alu-mina substrates Similar justification holds true for the ZnO

micro-rod growth on diatomite substrates since it consists of comparable

hygroscopic materials as alumina, leading to a similar change in pH

to basic region, albeit not as strong as in the case of growth on

alu-mina substrates

In order to grow ZnO microrods with high aspect ratio (ratio of

length to width of the microrods) similar to what we obtained on

glass and cordierite substrates, several experiments of ZnO

micro-rod growth were performed under controlled pH conditions ZnO

seeded onc-alumina and diatomite substrates were immersed in

equimolar concentrations of zinc nitrate and hexamine (10 mM)

and pH of the growth solutions were then adjusted to about 6.4

by gradually adding 1% HNO3and then the other growth procedure were followed as reported Dense ZnO microrods with high aspect ratio (20) were successfully grown on both alumina and diato-mite substrates (Fig 8) confirming that surface ionization strongly affects the growth of ZnO microrods on porous substrates The observed larger diameter of ZnO microrods grown on alu-mina and diatomite substrates as compared to the microrods grown on glass substrates can be attributed to arise due to the con-tribution of hexamine for the growth of anisotropic ZnO microrods The initial growth solution at the substrate interface is slightly acidic about pH 5.8–6.2 which probably leads to decomposition

is well known to degrade in not only acidic conditions but also at elevated temperatures[36] Tada[37]reported that in acidic aque-ous solutions, protonated hexamine ((CH2)6N4H+) decomposes to protonated ammonia raising pH of the solutions which can thus subsequently lead to accelerated growth of ZnO microrods Baruah and Dutta[4]reported that thermal degradation releases the

ZnO Hexamine is a chelating agent that attaches to ZnO microrods securing ZnO non polar facets (1 0 1 0) from the deposition of Zn2+, therefore degradation of hexamine lead to a lack of complete cov-erage on these facets hindering the Zn2+attachment This leads to a growth in both lateral and longitudinal directions in the ZnO microrods on alumina and diatomite substrates Moreover, as ob-served in earlier section, alumina and diatomite hydrolysis raises

pH of growth solution to basic regions that can promote ZnO growth[24]

3.2 Copper nanoparticle deposition on zinc oxide microrods (Cu/ZnO MR)

Uniform copper nanoparticles of 20 nm size were obtained after stirring the precursor for 30 min at room temperature (Fig 9a) Cu colloidal suspension was yellowish-brown in color with the optical absorption peak at about 570 nm Spherical copper nanoparticles

of about 50 nm in sizes are firmly attached on ZnO microrods as shown inFig 9b There are two major causes of an increase in cop-per nanoparticle size from 20 nm to 50 nm Firstly, 20 nm sized particles formed the copper colloids but during the deposition pro-cess, copper colloid suspension was heated at 95 °C for 2 h in which ZnO nanorods samples were immersed, as reported in the literature [38,39] Upon prolonged heating (for 2 h) at 95 °C, agglomeration of copper nanoparticles occurred leading to a parti-cle size of 50 nm Secondly, the synthesized samples (Cu/ZnO microrods) were annealed at 300 °C for 1 h which increases the size of the nanoparticles The contents of copper and zinc on the

Cu nanoparticle modified ZnO microrods determined using

and zinc contents are about 1.3% and 3.7% by weight while Cu/Zn weight ratio is about 2.8 The copper nanoparticles are mixed

Fig 4 ZnO microrods grown on glass substrate by hydrothermal process with

20 mM Zn 2+

after 20 h of growth The precursor solution (equimolar concentration

of Zn(NO 3 ) 2 and hexamine) was changed every 5 h during the growth process.

Fig 5 Morphology of ZnO microrods grown on cordierite substrates by hydrothermal process after 10 h of growth The precursor solution (equimolar concentration of

phases of copper in the as-prepared catalysts (copper metal form,

copper (I) oxide and copper (II) oxide) However, prior to the

refor-mation process, the catalyst were reduced by flowing 5% hydrogen

in argon gas at 300 °C for 1 h to convert all phases into copper

metal phase During the reformation process, hydrogen gas pro-duced from the reforming reaction also adds as a self-reducing agent for the catalyst system

3.3 Catalytic activities on MSR

Fig 10shows the methanol reforming results using Cu nanopar-ticles on ZnO microrods (Cu–ZnO MR) grown on cordierite sub-strate It was found that the prepared Cu–ZnO MR catalyst exhibited high methanol conversion (about 90%) even at a low operating temperature of 200 °C, but low hydrogen yield was ob-served under these conditions (4% at 200 °C) At elevated temper-atures, higher hydrogen yields could be obtained (14% hydrogen yield at 350 °C working temperature) As the hydrogen yields were lower than the estimated levels, this can be attributed to the for-mation of intermediate compounds like methoxy (CH3O), formal-dehyde (CH2O), methylformate (CH3COOH) and formate (HCOOH) species which could not be further dehydrogenated to produce hydrogen gas[40–42] The active site O(ads)in Eqs.(2)–(6)can be

a surface Cu–O–Cu group, with Cu in the oxidation state +1 (Cu(II)

is inactive for MSR)[43–46] Methanol can then react at this site leading to adsorbed methoxy and OH species (Eq (2)) followed

Fig 6 Morphology of ZnO rods grown onc-Al 2 O 3 substrates by hydrothermal process after 10 h of growth The precursor solution (equimolar concentration of Zn(NO 3 ) 2 and hexamine) was changed every 5 h during the growth process; (a) 5 mM of Zn 2+

, (b) 7 mM of Zn 2+

and (c) 10 mM of Zn 2+

; insets of (a) and (c) is the high magnification of top view ZnO microrod.

Fig 7 Morphology of ZnO rods grown on calcined diatomite substrates by hydrothermal process after 10 h of growth The precursor solution (equimolar concentration of Zn(NO 3 ) 2 and hexamine) was changed every 5 h during the growth process; (a) 5 mM of Zn 2+

, (b) 7 mM of Zn 2+

and (c) 10 mM of Zn 2+

.

Table 2

Summary of width, length and aspect ratio of zinc oxide rods grown on glass,

cordierite, alumina and calcined diatomite.

Zn 2+ conc.

(mM)

Diatomite

Fig 8 Morphology of ZnO rods grown onc-alumina and calcined diatomite substrates by hydrothermal process after 10 h of growth Prior to the growth process, pH of growth solution after immersing the substrate was adjusted to about 6.4 The precursor solution (10 mM of equimolar concentration of Zn(NO 3 ) 2 and hexamine) was changed

by the dehydrogenation of the methoxy species, giving rise to

ad-sorbed formaldehyde with simultaneous reduction at the Cu site

(Eq (3)) Desorption of formaldehyde from the copper surface

can occur upon the oxidation of formaldehyde into formate species

possibly through an intermediate dioxymethylene group (Eqs.(4)

and (5)) [43,47] During ideal SRM conditions, formate would

decompose to CO2(Eq.(6)) instead of forming CO [48–50] Cu(I)

sites react with H2O (Eqs.(8) and (9), where H(ads) is a H atom

bound to a Cu metal site) and is reactivated Thus Cu undergoes

an oxidation–reduction cycle, catalyzing the abstraction of

hydro-gen atoms from the adsorbed species with subsequent

recombina-tion to form gaseous H2 (Eq.(10)) The determining step in this

mechanism is the dehydrogenation of adsorbed methoxy groups

[51,52] For this catalyst carbon dioxide evolution is more

prefer-ential than carbon monoxide and hence it is suitable for fuel cell

applications

CH3OHðgÞþ OðadsÞ! CH3OðadsÞþ OHðadsÞ ð2Þ

CH3OðadsÞþ OHðadsÞ! CH2OðadsÞþ H2OðadsÞ ð3Þ

CH2OðadsÞþ OðadsÞ! CH2OðadsÞ ð4Þ

CH2O2ðadsÞþ OðadsÞ! HCOOðadsÞþ OHðadsÞ ð5Þ HCOOðadsÞþ OðadsÞ! CO2ðadsÞþ OHðadsÞ ð6Þ

4 Conclusions

We have successfully grown ZnO microrods on porous ceramic substrates showing morphology similar to what is achieved on flat glass substrates by a hydrothermal process ZnO microrods grow faster on alumina and diatomite substrates compared to cordierite and glass substrates which was attributed to arise from the surface basicity of the alumina substrates due to its hydrolysis in the pres-ence of water Large ZnO microrods were found on alumina and diatomite substrates upon hydrothermal growth which were ar-gued to occur due to the degradation of hexamine into ammonia and formaldehyde effectively exposing the non-polar facets of the ZnO crystals and thus adding to the homogeneous growth in all directions Surface hydrolysis of alumina and diatomite sub-strates could be circumvented controlling the pH of the growth solution during hydrolysis Cordierite which is not hygroscopic was found to be an appropriate porous ceramic substrate for the hydrothermal growth of ZnO microrods with high aspect ratio, similar to what is achieved on flat glass substrates Proper under-standing of ZnO microrod growth on porous substrates allows its applications as catalytic support for methanol steam reformation

as the deposition of Cu–ZnO microrod on cordierite substrates could be utilized to reform methanol resulting in 90% conversion with 14% hydrogen yield at 350 °C

Acknowledgements The authors would like to acknowledge partial financial support from the Center of Excellence in Nanotechnology at the Asian Insti-tute of Technology, National Nanotechnology Center (NANOTEC) and National Metal and Materials Technology Center (MTEC), belonging to the National Science & Technology Development Agency (NSTDA), Thailand The authors would also like to acknowl-edge the support of the Research Council of Oman for partial support

Fig 9 Scanning electron micrographs; (a) copper nanoparticles prepared by using hydrazine as a reducing agent, and (b) copper nanoparticles decorated ZnO nanorods prepared by a typical impregnation process at 95 °C for 2 h.

Table 3

Copper and zinc contents in the catalyst samples determined by ICP-OES.

Fig 10 Catalytic activities on methanol steam reforming using Cu (nps)/ZnO

nanorods grown on cordierite ceramic The catalysts were activated by flowing

5%H 2 gas (60 mL/min) at 300 °C for 1 h before determination; (a) % methanol

conversion, (b) % hydrogen selectivity, (c) % carbon dioxide selectivity and (d) %

Appendix A Supplementary material

Supplementary data associated with this article can be found, in

the online version, athttp://dx.doi.org/10.1016/j.jallcom.2013.10

019

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