Facile synthesis of ZnO particles: Via benzene-assisted co-solvothermal method with different alcohols and its application

Article abstract of DOI:10.1039/c6ra13260j

In this study, ZnO particles with different morphologies were synthesized by a novel co-solvothermal method using benzene. The prepared samples were characterized by Brunauer-Emmett-Teller (BET) measurements, X-ray diffractometry (XRD), scanning electron microscopy coupled with an energy dispersive X-ray detector (SEM-EDX), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrometry (XPS), and H₂-temperature programmed reduction (H₂-TPR). The results showed that the molecular sizes and carbon numbers of the alcohols used in the reaction and the addition of benzene had a great effect on the morphologies, textural properties, and crystalline structures of the material products in our reaction system. Different ZnO morphologies, such as spherical coral-like, carnation-like, rose-like, and plate-like structures, were obtained using methanol, ethanol, propanol, and butanol, respectively. Moreover, Cu particles loaded on ZnO with different morphologies were also investigated for the hydrogenation of CO₂ to CH₃OH. High catalytic activity and selectivity (82.8%) for CH₃OH formation were obtained using ZnO prepared from methanol with Cu doping (Cu/ZnO-Me).

Full text of DOI:10.1039/c6ra13260j

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A facile synthesis of ZnO particles via benzene-assisted co-solvothermVaielw Article Online
DOI: 10.1039/C6RA13260J
method with different alcohols and its application
Panya Maneechakr*, Surachai Karnjanakom and Jittima Samerjit*
Department of Chemistry, Faculty of Science, Rangsit University, Pathumthani 12000,
Thailand
*Corresponding author
E-mail: panya.m@rsc.ac.th; samerjit.j@gmail.com
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DOI: 10.1039/C6RA13260J
ABSTRACT
In this study, ZnO particles with different morphologies are synthesized by a novel co-
solvothermal method with benzene assistance. The prepared samples were characterized by
Brunauer–Emmett–Teller (BET) measurement, X-ray diffractometer (XRD), scanning
electron microscope coupled with an energy dispersive X-ray detector (SEM-EDX), High-
resolution transmission electron microscope (HRTEM), X-ray photoelectron spectrometer
(XPS) and H2-Temperature programmed reduction (H2-TPR). The results found that the
molecular sizes with carbon numbers of alcohols and addition of benzene had great effect on
the morphologies, textural properties and crystalline structures of material products in our
reaction system. The different ZnO morphologies such as spherical coral-like structure,
carnation-like structure, rose-like structure and plate-like structure were obtained using
methanol, ethanol, propanol and butanol, respectively. Moreover, Cu particles loaded on ZnO
with different morphologies were also investigated for hydrogenation of CO2 to CH3OH. The
high catalytic activity with selectivity (82.8%) for CH3OH formation was obtained using ZnO
prepared from methanol with Cu doping (Cu/ZnO-Me).
Key words: ZnO; Solvothermal method; Alcohol; Morphology; Hydrogenation
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DOI: 10.1039/C6RA13260J
1. Introduction
Nowadays, semiconducting materials with controllable morphologies are more
attracted for applications in the industries as well as daily life such as dye-sensitized
solar cells, optoelectronic devices, adsorbents, photocatalysis and biosensors.1-7
A
large number of semiconducting materials such as TiO2, ZnO and CdS have been
identified.8-10 Among them, ZnO is considered as one of most promising material due
to its non-toxicity, low cost and high catalytic activity, leading to widely practical
applications.11,12 However, it is still necessary to find the new structures of this
material for more appropriate applications. Hence, the development of ZnO material is
highly desired and remains a big challenge. Many methods synthesis for synthesis of
ZnO have been widely investigated such as hydrothermal, thermal evaporation, sol–
gel methods.13-15 Nevertheless, synthesis route has some disadvantages, for instance,
impurities of ZnO phases are usually found when template was utilized to modify the
ZnO structure. To date, no report or any guidance on synthesis, formation mechanism
and special morphology of ZnO structure as well as its evolution in a facile benzene-
assisted co-solvothermal method using different alcohols can be observed. From stated
before, it is an interesting for investigating of this novel method.
As well-known that CO2 is a main reason on global warming problem as
becoming more and more serious in recent years because of the increase of the
concentration of CO2 in air atmosphere. To solve this problem, the use of
hydrogenation for conversion of CO2 to CH3OH over Cu/ZnO catalysts is an
interesting method in the present. Recently, it reported that different ZnO
morphologies had a significant effect on its interaction with Cu in the hydrogenation
of CO2 to CH3OH.16 They also found that a strong interaction between Cu with ZnO
has good selectivity of methanol from CO2 hydrogenation.
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In this work, synthesis of ZnO particles by a facile template-free co-solvotheVriemw aArlticle Online
DOI: 10.1039/C6RA13260J
method using different alcohols, i.e., methanol, ethanol, propanol and butanol, were studied
with assistance of benzene in order to control the morphological structures by just tuning the
concentration of benzene. The properties of as-prepared ZnO such as morphology, textural
properties and crystalline structure were characterized. The possible synergistic mechanisms
of organic solvents on ZnO formation were revealed. The Cu/ZnO catalyst was also applied
for hydrogenation of CO2 to CH3OH.
2. Experimental
2.1 Preparation of ZnO materials
All reagents were of analytical grade, purchased from Sigma-Aldrich, and used as
received without further purification. In a typical synthesis, 5 g of Zn(NO3)2.6H2O was
dissolved in 25 ml of methanol (0.60 mol) under vigorous stirring at room temperature
until obtained clear solution. Then, 25 ml of benzene as a co-solvent or directing agent
was added into the solution with continuously stirred for 10 min, and followed by
adding 2.4 g of urea for adjusting pH = 10 and precipitation during reaction process.
After complete dissolution, the mixture solution was transferred into a Teflon-lined
stainless autoclave and heated at 120 °C for 24 h under the solvothermal process. After
cooling down to room temperature, the white solid product was collected by filtration
and washed with deionized water for several times, and finally dried in the oven at 80
°C for 12 h. Finally, the ZnO product was stabilized by calcination at 200 °C for 4 h and
kept in a desiccator for further characterization and hydrogenation application.
Comparing the use of different alcohols, ZnO products were synthesized by using
ethanol (0.42 mol), propanol (0.32 mol) and butanol (0.27 mol) as solvents instead of
methanol, respectively. The ZnO products were labels as Me, Et, Pr and Bu
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corresponding to methanol, ethanol, propanol and butanol, respectively. For Cu/ZnO
DOI: 10.1039/C6RA13260J
sample, 1 wt.% of Cu was doped on the prepared ZnO sample by impregnation method.
Firstly, a certain amount of prepared ZnO sample was added into the Cu(NO3)2.3H2O
solution and stirred at 35 °C for 6 h. Then, the resulting suspension solution was gently
dried by heat at 90 °C. The obtained solid product was calcined at 500 °C for 6 h.
2.2 Characterization of ZnO materials
The X-ray powder diffraction (XRD) patterns and the crystallite sizes of samples
were determined by using X-ray diffraction (Bruker D8 Advance AXS X-ray
Diffractometer) with CuKα radiation =1.5406 Å in the 2θ scan range of 20-70°. The
morphologies of samples were investigated by the scanning electron microscope (SEM
S-4800; Hitachi) equipped with an X-ray spectrometer for energy dispersive
spectroscopy (EDS) analysis. High-resolution transmission electron microscopy
(HRTEM) images were obtained from a Philips CM200, model JEOL JEM-2100
transmission electron microscope working at an accelerating voltage of 200 kV. Before
HRTEM observation, the sample was dispersed at first in ethanol under ultra-sonication
and then dropped on a carbon-coated copper grid, which was dried at room temperature.
The specific surface areas of samples were obtained from N2 adsorption and desorption
isotherms using a Quantachrome Autosorb 1 gas adsorption analyzer at -196 °C. All the
samples were degassed at 150 °C in vacuum for 2 h prior to measurements. The surface
areas were calculated from Brunauer–Emmett–Teller (BET) method, and the pore size
and pore volume were determined by the Barrett–Joyner–Halenda (BJH) method from
desorption branch of the isotherms. X-ray photoelectron spectroscopy (XPS)
measurement was analyzed on a PerkinElmer PHI 5000C system with a Mg Kα X-ray
source (hv = 1253.6 eV). The reduction behaviour of Cu/ZnO sample was performed
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using H2-Temperature-programmed reduction (TPR) with a gas mixture of 5 vol% H2 in
DOI: 10.1039/C6RA13260J
N2 at a flow rate of 50 ml/min.
2.3 Catalytic evaluation
Catalytic activity of as-prepared sample was evaluated using a fix-bed reactor. In
short, 0.1 g of catalyst diluted with quartz sand was loaded into the quartz tube. Before
catalytic test, catalysts were reduced with H2 flow (flow rate = 50 ml/min) at 350 °C
for 1 h under atmospheric pressure. Then, the reactor was cooled to 180 °C and the
reactant gas flow was introduced. The reaction conditions fixed for all experiments
were 240 °C, 3 MPa, 0.54 mol/(g-cat h) and molar feed composition of H2/CO2 = 3/1.
The products were analyzed online by a gas chromatograph equipped with a thermal
conductivity detector.
3. Results and discussion
Fig. 1a shows the XRD pattern of as-synthesized products. As revealed, the typical
diffraction peaks at 31.8, 34.6, 36.3, 47.7, 56.7, 62.9, 66.6, 68.1 and 69.2° are attributed to
the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes,
respectively.17 No other crystalline impurities of ZnO structure are detected in the XRD
pattern, indicating that high phase purity of ZnO structure is obtained in our method. In order
to investigate the crystallite size of each product, the Scherrer’s equation (ep.1), is applied for
the calculation as follows:
Where K is the Scherer constant (K=0.89), λ is X-ray radiation wavelength (λ = 1.5406 Å), θ
the Bragg’s angle at which the peak is observed measured in radians, and β is the full width
of diffraction line at half of the maximum intensity (FWHM).18 The average crystallite size of
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the samples are calculated and listed in Table 1. One can see that ZnO cryDstOaI:l1l0it.1e03Vi9se/wiCzA6erRticAl1e3O2n6li0nJe
increases when the carbon number or molecular sizes in the alcohols increases, suggesting
that the alcohol with longer alkyl chain based on the relative of carbon number is favorable
for the formation of ZnO with higher crystallinity. It should be noted that different kinds of
alcohols are utilized as a solvent in an equal volume but moles of alcohol molecules are
different in the system, i.e., methanol > ethanol > propanol > butanol. In this case, during the
reactions, smaller alcohol molecule size can be intercalated the Zn(OH)2 layer structures
more easily as large amounts of alcohol molecules show the better growth and arrangement
of ZnO unit cells.19 As stated above, even the molecular size has a great effect but polarity of
alcohol is also a main important factor. The stronger polarity is favorable for making the
hydrogen bonds between alcohol molecules with hydroxides surface, for instance, the
methanol has strongest polarity which improves the CH3-O groups combination or adsorption
with the surface hydroxyl groups of ZnO crystallites.19 As expected, the ZnO-Me exhibits a
relatively lower crystallinity than others. In addition, it reveals that nitrate groups derived
from zinc compound could affect to adsorb on product surface, resulting in the different
crystallite formation as well as morphologies with pore structures.20 However, an increase of
carbon number results larger molecular size of alcohol, which could lead to that nitrate
groups adsorbing on product surface becomes more and more difficult due to effects of
polarity and steric. Further evidence of the formation of ZnO with high purity is the EDX
analysis. The Zn and O peaks can be clearly observed in Fig 1b. It should be mentioned here
that C peak was also detected which obtained from the supporting carbon tape (Fig. 1b).
Fig. 2 shows the SEM images of as-synthesized products. As observed, different
morphologies of ZnO structures are obviously obtained when the different types of alcohols
are used. Fig. 2a shows SEM image of ZnO-Me with spherical coral-like structure. When
ethanol is used to replace of methanol, carnation-like structure is formed (Fig. 2b) When
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propanol and butanol are applied, rose-like structure and plate-like structure are obtained,
DOI: 10.1039/C6RA13260J
respectively (Fig. 2c and d). From these results, the different alcohols have a great effect on
formation of ZnO structures. In the initial stage of reaction, Zn(NO3)2.6H2O is dissolved into
different alcohols and then formed metastable zinc alkoxide (Zn(OH)m(OR)n). As the reaction
proceeded, the condensation between (Zn(OH)m(OR)n) molecules took place and Zn(OH)x
crystallites are formed.21 Due to large amounts of OH groups on the surface of these
crystallites and alcohol molecules existing in the system, the –OR groups may readily bond to
the surface of Zn(OH)x crystallites through H bonds and electrostatic force, while alkyl
chains head away from the surface. In case of ZnO-Me, due to methanol molecules possess
small steric bulkiness, the corresponding crystallites could self-assemble into tight spherical
particles. For ZnO-Et, ZnO-Pr and Zn-Bu, their crystallites bonding formed into loose plate
shapes. HRTEM images of samples was also observed as can be seen in Fig. 2a-d. The
results exhibit lattice fringes of the ZnO crystal, which corresponds to the (002) plane of
ZnO. It should be concluded here that the certain morphologies in each sample can be
obtained by assistance of benzene. In this work, volume ratio of alcohol to benzene (25 ml to
25 ml) is an optimum condition to obtain the certain ZnO structure in our preliminary
experiments. When volume ratio volume ratios of propanol to benzene are changed, irregular
and non-uniform of ZnO structures are clearly observed (Fig. 2). In this study, ZnO-Pr was
selected as an example for investigation on the change of ZnO morphology when benzene
amount is adjusted. Fig. 2e-h show the SEM images of product obtained from using different
volume ratio of propanol to benzene. When amount of benzene is added an increase or a
decrease in the system from optimum value (25 ml), irregular shape of ZnO-Pr can be
observed. It demonstrates that benzene has a great influence on the morphology of products.
Here, it is possible that benzene bears quite weak polarity and dissolubility in contrast with
alcohol, but both of them affected the hydrolyzation of Zn-alcoholic complex and the
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formation of products. When the benzene is added in optimum amount, the benVzieewnAerticle Online
DOI: 10.1039/C6RA13260J
molecules could be adsorbed on the surface of amorphous Zn(NO3)2 particles by
physicochemical reaction process, leading to the decrease in the surface free energy of
them.22 These behaviors affect to the growth of crystallites along a certain direction to some
extent and promote the formation of certain structures. Moreover, in Fig. 3, when ZnO-Pr is
calcined at high temperature of 500 °C for 4 h, its morphology can further changed to worm-
like structure due to hydration reaction and agglomeration.
Fig. 4 shows the N2 adsorption-desorption isotherms of ZnO-alcohol products. All
products exhibit an International Union of Pure and Applied Chemistry (IUPAC) type IV
isotherm with a significant hysteretic loop above relative pressures P/P0 of 0.45, indicating
the existence of ink-bottle mesoporous in these materials. The textural properties of ZnO-
alcohol products are listed in Table 1. It can be seen that when the carbon number of alcohol
increased, the surface area decreased while pore size increased. It is possible that weak
crystalline structure has a lot of defects on agglomerated particles (secondary particles),
resulting that the larger amount of pores between single-particle without agglomeration
(primary particles) and these pores are very small. It is why ZnO-Me has higher surface area
even the morphology is tight spherical. In contrast, when the primary crystallites are better
crystalline, the corresponding secondary particles have few defects and most pores arise from
the packing spaces between secondary particles. Thus, lowest surface area and largest pore
size should be ZnO-Bu. Fig. 5 shows XPS patterns of ZnO synthesized by different alcohols.
It is found that the binding energy of O 1s for the ZnO-Me (532 eV) is lower than others and
become lower and lower when increasing the C number of alcohol for ZnO synthesis,
indicating that electrons are easier to be excited from the ZnO-Me and the ZnO-Me has more
oxygen vacancies and larger polar planes when comparing with others.23
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In this study, hydrogenation of CO2 to CH3OH was also investigated oDvOeIr: 101.10w39t/.C%6RA13260J
Cu/ZnO samples. The fix-bed reactor was used for performance evaluation of catalyst. The
reaction conditions fixed for all experiments were 240 °C, 3 MPa, 0.54 mol/(g-cat h) and
molar feed composition of H2/CO2 = 3/1. The catalytic activity results of as-prepared
catalysts for CO2 hydrogenation are shown in Fig. 6. One can see that the best catalytic
activity for CO2 hydrogenation to CH3OH with a 18% conversion of CO2 and 82.8%
selectivity of CH3OH is obtained using Cu/ZnO-Me. It is probably due to stronger interaction
between Cu and ZnO and more amounts of surface oxygen vacancies in Cu/ZnO-Me.23 In
addition, in case of largest surface area of ZnO-Me, Cu should be better well-dispersed than
others, resulting in the increasing of active sites. To support the explained assumptions, H2-
TPR and XPS analyses for each catalyst are also investigated and the results are shown in Fig
7. Fig. 7a shows H2-TPR results for investigating on reduction behavior of Cu/ZnO catalysts.
H2 was feed to CuO on sample with an increasing of temperature. During this process, CuO
can be reduced to Cu with an appropriate temperature range based on interaction between
metal oxides with supports. It should be mentioned here that all catalysts exhibit reduction
profiles as a broad band of H2 consumption below 350 °C. This indicates that appeared peak
has only CuO reduction while ZnO reduction is not found due to its reduction properties
generally (>500 °C). The H2-TPR patterns of all Cu/ZnO samples can be attributed to the two
step-reduction of Cu2+ to Cu+ with Cu+ to Cu0.24 Interestingly, the reduction temperature of
CuO species in Cu/ZnO-Me is higher than others, indicating that Cu prepared on ZnO-Me
has strongest interaction and best dispersion. In addition, it can be clearly seen that alcohol
has also effect to improve the interaction of ZnO with Cu. Fig 7b shows the XPS patterns of
Cu/ZnO catalysts. As expected, binding energy value of Cu 2p3/2 of Cu/ZnO-Me is lower
than another catalyst, resulting from better electrons transfer and more amount of oxygen
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vacancy. This is that why Cu/ZnO-Me has better catalytic activity on hydrogenation ofVCiewOAr2ticle Online
DOI: 10.1039/C6RA13260J
to CH3OH.
4. Conclusions
In summary, based on a facile benzene-assisted solvothermal method with different
alcohol from C1 to C4, the different ZnO morphologies such as spherical coral-like structure,
carnation-like structure, rose-like structure and plate-like structure were obtained. The carbon
number, molecular sizes and polarities of alcohols had a great effect on the formation
mechanism of ZnO structures. The benzene had a significant synergistic influence with
alcohols on the morphology and formation process of ZnO product. Cu/ZnO-Me showed the
best catalytic activity for hydrogenation of CO2 to CH3OH with high selectivity due to
characteristic of its physicochemical properties.
Acknowledgments
The authors wish to acknowledge Department of Chemistry, Faculty of Science,
Rangsit University for supporting all instruments and chemicals.
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Table caption
Table 1 Textural properties of ZnO products synthesized by different alcohols.
Figure captions
Fig. 1 (a) XRD patterns of ZnO products synthesized by different alcohols and (b) EDX
spectrum of ZnO product.
Fig. 2 SEM images of (a) ZnO-Me, (b) ZnO-Et, (c) ZnO-Pr, (d) ZnO-Bu and ZnO-Pr
products synthesized by different volume ratios of propanol to benzene: (e) 5 to 45, (f) 15to
35, (c) 25 to 25, (g) 35 to 15 and (h) 45 to 5. HRTEM images were inserted in (a), (b), (c) and
(d)
Fig. 3 SEM image of ZnO-Pr (propanol to benzene = 25 to 25) which calcined at 500 °C for
4 h.
Fig. 4 N2 adsorption-desorption isotherms of ZnO products synthesized by different alcohols.
Fig. 5 XPS patterns of ZnO products synthesized by different alcohols.
Fig. 6 Performance testing of as-prepared catalysts for the hydrogenation of CO2.
Fig. 7 (a) H2-TPR profiles and (b) XPS patterns of 1 wt.% Cu loaded ZnO products
synthesized by different alcohols.
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Table 1 Textural properties of ZnO products synthesized by different alcohols.
Surface area
(m2/g)
63.2
41.6
37.7
Pore volume
(cm3/g)
0.22
0.51
0.87
Pore diameter
(nm)
2.3
3.4
5.1
6.4
Crystallite size
Sample
(nm)
35.8
36.5
37.3
39.9
ZnO-Me
ZnO-Et
ZnO-Pr
ZnO-Bu
20.9
0.92
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Fig. 3
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Fig. 5
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