Post on 08-May-2020
Sol–gel synthesis and photocatalytic performanceof ZnO toward oxidation reaction of NO
E. Luevano-Hipolito1• A. Martınez-de la Cruz1
Received: 28 August 2015 / Accepted: 17 October 2015 / Published online: 27 October 2015
� Springer Science+Business Media Dordrecht 2015
Abstract ZnO oxide was prepared by different routes of synthesis such as pre-
cipitation, solvothermal, solvothermal assisted with polyethylene glycol and the
sol–gel method. The physical properties of the oxides were studied by X-ray powder
diffraction, scanning electron microscopy, UV–Vis diffuse reflectance spectroscopy
and adsorption–desorption N2 isotherms. The photocatalytic activity of ZnO sam-
ples was evaluated in the oxidation reaction of nitric oxide (NO). The conversion
degree of NO reached by each sample used as photocatalyst was associated with
their physical properties. By far the best performance was obtained with the sample
prepared by sol–gel, in which a degree of conversion of about 70 % was reached.
Beyond the elimination of NO, the selective formation of innocuous nitrate ions as
the main product of reaction ([80 %) was also confirmed. The effect of the relative
humidity and the charge of photocatalyst in the conversion degree of NO was
analyzed.
Keywords Oxides � Semiconductors � Chemical synthesis � Sol–gel chemistry �Catalytic properties
Introduction
Zinc oxide is a very important material in the ceramic industry due to its excellent
properties, such as chemical stability, broad range of radiation absorption,
photostability, hardness, rigidity, low toxicity, and low cost [1]. For these reasons,
ZnO is frequently incorporated as raw material in cement and glasses for numerous
applications in the field of construction. Currently, the interest in the development
& A. Martınez-de la Cruz
azael70@yahoo.com.mx
1 CIIDIT, Facultad de Ingenierıa Mecanica y Electrica, Universidad Autonoma de Nuevo Leon,
Ciudad Universitaria, 66451 San Nicolas de los Garza, N. L., Mexico
123
Res Chem Intermed (2016) 42:4879–4891
DOI 10.1007/s11164-015-2327-4
of sustainable materials for construction applications is increasing due to the
importance of preserving the environment. Undoubtedly, one of the most interesting
technologies focused in this direction is heterogeneous photocatalysis, in which a
semiconductor oxide is activated by solar radiation to induce the elimination of a
great variety of pollutants.
ZnO is a semiconductor oxide that has proven be an efficient photocatalyst in the
reaction of the degradation of numerous organic pollutants in wastewater [2–5]. In
this sense, there are many reports about the synthesis of ZnO by different methods
in order to modify their chemical and physical properties, especially their
morphology and textural properties. The synthesis of ZnO oxide has been reported
in previous works by the precipitation of inorganic salts, by solvothermal, sol–gel,
thermal decomposition, combustion and microwaves, and by use of ultrasound [6–12].
Furthermore, some template molecules, such as, for example, polyethylene glycol
(PEG), have been used to promote the formation of nanostructures and porosity in the
final product [13].
In the scientific literature, ZnO is recognized as an efficient photocatalyst in
the degradation of organic pollutants in aqueous medium, with high rates of
mineralization of the compounds [2, 3]. However, there are few studies using
ZnO as a photocatalyst in gaseous reactions and most concern the degradation
of organic pollutants such as formaldehyde [14, 15]. In the field of NOx
elimination, a reduced number of works have reported the activity of ZnO
oxide. For example, Huang et al. [16] studied the activity of ZnO with different
morphologies in the removal of NOx gases. In particular, they found that ZnO
with a flower-like morphology showed the highest photocatalytic activity under
UV light irradiation even compared with the commercial TiO2 P-25 Degussa.
On the other hand, Kowsari and Bazri [17] also studied the photocatalytic
activity of ZnO in NOx removal from the air, finding a low degree of
conversion of 23 % for NOx using ZnO particles with the same type of
morphology. Likewise, Wei et al. [18] investigated the photocatalytic activity
of ZnO spheres in deNOx catalytic properties. ZnO spheres used as a
photocatalyst were able to reduce the concentration of NO even with visible
radiation of 510 nm. This situation was associated with the presence of
impurities of residual carbon. As a common feature of these three works, the
synthesis of ZnO was carried out by the solvothermal method which involved
complex organic compounds as additives.
In the present work, the possibility of four routes of synthesis of ZnO to enhance
the photocatalytic activity of the oxide in the oxidation reaction of NO will be
explored . The samples of ZnO were prepared by simple precipitation, solvothermal,
solvothermal assisted with PEG and sol–gel methods. Beyond the previous studies
of ZnO as a photocatalyst in this type of reaction, which only followed the depletion
of NO concentration during the photocatalytic reaction, in this work the formation
of nitrates and nitrites ions (NO�3 =NO�
2 Þ to confirm the deep oxidation of NO until
innocuous products was also determined.
4880 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
Experimental
Synthesis of ZnO
ZnO samples were prepared by four different routes of synthesis in order to
correlate their resulting physical and chemical properties with their photocatalytic
activity in the oxidation reaction of nitric oxide (NO). In the synthesis by simple
precipitation (ZnO-P), an inorganic salt was precipitated in basic media. For this
purpose, 0.02 mol of zinc nitrate [Zn(NO3)2�6H2O] (99 %; Fermont) was dissolved
in 100 mL of deionized water under continuous stirring. Afterwards, the pH of the
solution was adjusted to 11 with a solution of 14 M of NH4OH. This process was
accompanied with vigorous stirring for 1 h, and then the suspension was maintained
at room temperature overnight. Once the time was elapsed, the white precipitate was
collected and washed three times with deionized water and ethanol. The precipitate
was dried at 70 �C and was later heated at 300 �C for 24 h in order to obtain
polycrystalline powders. The second method involved a solvothermal process
without the presence of additives (ZnO-S). For this experiment, a solution of
Zn(NO3)2�6H2O was prepared in basic media as described in the previous method,
but now the suspension formed was placed in an autoclave at 150 �C for 4 h. The
powders obtained were washed three times with deionized water and ethanol. In a
third synthesis, the solvothermal procedure previously described for ZnO-S was
modified by addition of 0.0025 mol of PEG (PEG1000). In this case, the presence of
PEG was to increase the porosity of the particles formed (ZnO-SPEG). Finally, ZnO
was also synthesized by the sol–gel method. This route required the preparation of
two solutions (A and B). Solution A was prepared by dissolving 0.01 mol of zinc
acetate [Zn(CH3COO)2�2H2O] (99 %; DEQ) in ethanol. The second solution
(B) was a solution 1 M of NH4OH. Solution B (50 mL) was added dropwise into
solution A in order to carry out the hydrolysis of the zinc acetate. The resulting
solution was maintained under vigorous stirring for 1 h and then aged for 1 day at
room temperature. The obtained gel was dried at 70 �C for 24 h. After the drying
process, the sample was heated at 300 �C for 24 h in order to promote the formation
of ZnO oxide (ZnO-SG).
Characterization
The structural characterization of the ZnO samples was carried out by X-ray powder
diffraction using a Bruker D8 Advance diffractometer with Cu Ka radiation (40 kV,
30 mA). A typical run was made with a step size of 0.05� and a dwell time of 0.5 s.
The morphology of the powders was analyzed by scanning electron microscopy
using a FEI Nova NanoSEM 200 microscope with an accelerating voltage of 30 kV.
The UV–Vis diffuse reflectance absorption spectra of the ZnO samples were
obtained in an Agilent Technologies UV–Vis–NIR spectrophotometer (model Cary
5000 series) equipped with an integrating sphere. The specific surface area
measurements were carried out by the adsorption–desorption N2 isotherms
performed in a Bel-Japan Minisorp II surface area and pore size analyzer. The
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4881
123
isotherms were evaluated at -196 �C after a pretreatment of the samples at 150 �Cfor 24 h.
Photocatalytic experiments
The photocatalytic experiments were performed at room temperature in a
continuous flow reactor designed according to ISO 22197-1. The photocatalytic
reactor was made of stainless steel with a volume of 0.8 L. The device has an
integrated window made of tempered glass in its superior part in order to allow the
passage of radiation. The photocatalyst (50 mg) dispersed previously in ethanol was
deposited over an area of 0.08 m2 in a glass substrate with the help of a small brush.
A mixture 3 ppm of NO stabilized in N2 was used as inlet gas. The concentration of
gas was adjusted to 1 ppm in NO by using air (20.5 vol% O2 and 79.5 vol% N2)
and the flow rate of gas was adjusted to 1 L min-1. The source of light irradiation
were two fluorescent black lamps (TecnoLite) of 20 W each, emitting between 365
and 440 nm. The main contribution of the radiation source came from the emission
line at 365 nm, and in minor proportion from two emission lines located in the
visible region at 405 and 437 nm. Figure 1 shows the emission spectra of the UVA
lamp used. The spectrum was obtained using a Jaz spectrometer and the data were
collected using the software OceanViewTM. The software collected data of the UVA
lamp irradiance registered in the instrument at each wavelength. The concentration
of NO was continuously measured with a chemiluminescent NOx analyzer
(EcoPhysics CLD88p) with a sampling rate of 0.3 L min-1. The products of
reaction, nitrate and nitrite ions were identified and quantified by the analysis of
50 mL of deionized water used in the washing of the photocatalyst after the
Fig. 1 Lamp spectrum and absorbance of the ZnO samples prepared by different methods
4882 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
photocatalytic reaction. For this purpose, the photocatalyst was dispersed in
deionized water and sonicated for 30 min in order to desorb the nitrate and nitrite
ions from the powder. Then, the dispersion was centrifuged to obtain a crystalline
solution which was used in the analysis. The concentration of nitrate and nitrite ions
was measured in a Hach colorimeter through the reduction of nitrate to nitrite using
cadmium as the catalyst and by the diazotization method, respectively. To avoid
interference in the results due to the presence of nitrates or nitrites adsorbed in the
photocatalysts during the synthesis step, before the photocatalytic reaction, the
samples were washed several times with deionized water until the accumulative
mass of ions (NO�2 =NO�
3 Þ was constant after three successive washings.
Results and discussion
Characterization
The formation of ZnO samples was followed by X-ray powder diffraction. The
X-ray diffraction patterns of ZnO-P, ZnO-S, ZnO-SPEG, and ZnO-SG samples are
shown in Fig. 2. In general, all diffraction lines were correctly indexed with the
hexagonal wurtzite structure of ZnO according to the JCPDS card no. 36-1451. All
samples were highly crystalline but with some differences between them. According
to the results, the samples obtained by the solvothermal treatments (ZnO-S and
ZnO-SPEG) were characterized by a remarkably high crystallinity in comparison
with the samples obtained by the precipitation and sol–gel routes, in which an
Fig. 2 X-ray powder diffraction patterns of ZnO samples prepared by different methods
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4883
123
additional thermal treatment of 300 �C was applied. The temperature used in the
solvothermal methods (150 �C) promotes a better solubility of the chemical species,
and consequently the possibility of the formation of crystalline materials without a
posterior thermal treatment. The crystallinity of the samples increased in the
following order: ZnO-SG\ZnO-P\ZnO-S\ZnO-SPEG.
The morphology and particle sizes of the ZnO samples were analyzed by SEM.
Figure 3 shows some representative SEM images of samples prepared by different
routes of synthesis. Agglomerates with a mean size of 300–450 nm were observed
in the sample ZnO-P (Fig. 3a, b). On other hand, the samples ZnO-S, ZnO-SPEG,
and ZnO-SG developed a morphology of bars with different sizes. Figure 3c–f
shows the SEM images of ZnO samples prepared under the solvothermal treatment.
It seems that, when PEG was introduced into the reaction medium, the particle size
increased and some small primary particles were deposited on the surface of the bars
(Fig. 3f). The morphology of the sample ZnO-SG was characterized by the
formation of similar bars but with considerably smaller length and width. The
differences in the size of the bars is possibly due to the crystallization process
(nucleation and growth) associated with each route of synthesis. In this sense, in the
sol–gel synthesis after the hydrolysis and condensation reactions, polycondensed
and cross-linked species are formed which act as nucleation centers for the
subsequently growth of ZnO particles [19]. Furthermore, the acetate ions act as a
stabilizer of the colloidal sol which prevents the growth of ZnO particles by the sol–
gel method. In contrast, the precipitation method involves a fast and spontaneous
reaction of the inorganic salt without the use of additional agents to limit the growth
of ZnO particles. In the solvothermal method, the growth of ZnO particles is favored
by increasing the solubility of the inorganic salts. Table 1 shows the mean particle
size for all the ZnO samples prepared in this work. These values correspond with the
average obtained from the analysis of 100 particles of each sample by scanning
electron microscopy.
Fig. 3 SEM images of ZnO samples prepared by different methods: a, b ZnO-P; c, d ZnO-S; e, f ZnO-PS; g, h ZnO-SG
4884 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
The UV–Vis diffuse reflectance spectra of ZnO samples are presented in Fig. 4a.
The diffuse reflectance data was converted with the Kubelka–Munk function
according to F(R) = (1 - R)2/2R to obtain the energy band gap value [20]. As
shown in Fig. 4b, the energy band gap values can be obtained from the UV–Vis
spectra by plotting [F(R?)hm]2 versus photon energy (hm). These values are reported
in Table 1. All energy band gap values are higher than 3 eV which involve the
oxides only being absorbed in the UV region. The samples were absorbed in a
region of the spectrum which corresponds to the location of the maximum emission
of the lamp used as the radiation source (Fig. 1). The difference of the absorption of
radiation between the samples is of note. This is a very important factor because UV
light absorption (hm C Eg) starts the charge generation which promotes the
oxidation of molecules on the surface of the photocatalyst. This fact is more
evident for the samples ZnO-P and ZnO-SG, which have the largest and smallest
particle sizes, respectively.
Table 1 shows the specific surface area values of the ZnO samples prepared by
the four methods. The highest value of specific surface area was obtained for the
sample prepared by sol–gel, 16 m2 g-1, which was almost five times higher than
observed in the sample prepared at the same temperature but by the precipitation
method. These results are in agreement with the particle size and morphology
described in the SEM images. In samples obtained by the solvothermal method, the
values of the specific surface area reached were 2.48 and 3.46 m2 g-1 for ZnO-S
and ZnO-SPEG, respectively. In this case, the introduction of PEG into the reaction
medium does not have a significant effect in increasing the value of the specific
surface area. Figure 5 shows the adsorption–desorption N2 isotherms for ZnO
samples prepared by the different methods. In general, the isotherms can be
categorized as type II, which indicates a non-porous material with a high energy of
adsorption [21].
Photocatalytic activity
ZnO samples prepared by different methods were tested as photocatalysts in the
oxidation reaction of NO in order to investigate its potential capacity to purify air.
Figure 6 shows the evolution of the degree of conversion (%) of NO, under 1 h of
Table 1 Physical properties of ZnO prepared by different synthesis routes
Sample Mean particle
size (nm)
Band gap (eV) BET surface
area (m2 g-1)
ZnO-P 325 3.20 3.41
ZnO-S 1550a, 450b 3.23 2.48
ZnO-SPEG 2050a, 570b 3.22 3.46
ZnO-SG 130a, 40b 3.27 16.02
a Lengthb Width
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4885
123
lamp irradiation, when used as a photocatalyst for the four synthesized ZnO oxides.
As can be seen in Fig. 6, the highest degree of conversion (70 %) was reached for
the sample prepared by sol–gel (ZnO-SG). The high activity of the ZnO-SG sample
can be associated with its physical properties such as low particle size, high surface
area and high UV light absorption. The degree of conversion of NO reached with
the other ZnO samples was only 2, 7 and 15 % for ZnO-P, ZnO-S, and ZnO-SPEG,
respectively.
In order to obtain a higher degree of conversion with ZnO-SG, the mass of the
photocatalyst was increased from 50 to 100 mg (Fig. 7). In this new experiment, the
degree of conversion of NO reached was 95 % after 2 h of UV irradiation,
Fig. 4 Kubelka–Munk-transformed reflectance spectra of the ZnO samples prepared by differentmethods
4886 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
increasing to 35 % the elimination of the pollutant with respect to the first
experiment in which a charge of 50 mg of photocatalyst was used. One additional
experiment was performed in the presence of relative humidity (70 ± 3 % RH) to
study its effect in NO conversion when ZnO-SG was used as a photocatalyst under
UV light. The inset of Fig. 7 shows the degree of conversion of NO with two
Fig. 5 Adsorption–desorption N2 isotherms of samples Filled symbols adsorption and open symbolsdesorption
Fig. 6 Evolution of NO conversion using ZnO photocatalyst prepared by different methods under UVlight
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4887
123
different concentrations of molecules of water vapor (\0.4 ppm and 70 %). When
the relative humidity increased from ppm levels to 70 %, the conversion degree
increased slightly, but after 90 min of reaction the photocatalyst seems to be
passivated, decreasing its conversion degree of NO to 60 % after 120 min. This
phenomenon is a consequence of the competition for adsorptive sites between the
NO and water molecules at this level of relative humidity.
It has been postulated in several works that nitrate and nitrite ions are the final
products of the complete oxidation of NO [22–24]. Nevertheless, few works show
evidence of this fact. For this purpose, the samples used as photocatalysts were
washed with deionized water several times before the photocatalytic reaction, in
order to eliminate nitrate and nitrite ions from its surface. The analysis of successive
washings of the photocatalyst revealed the presence of an important concentration
of nitrate ions, whose presence was attributed to impurities of the reagents used in
the synthesis of ZnO. Although the concentration of NO�3 ions detected was low, at
the same time it was considerably higher than the maximum theoretical value
expected from the conversion of NO to NO�3 : Figure 8 shows the accumulated mass
of nitrates and nitrites after each washing during eight treatments with deionized
water. As can be seen, it was necessary for up to eight washes to maintain the
accumulated mass of nitrates and nitrites at a constant level, which means that ions
removable from the surface of the oxide were eliminated to almost 100 %.
Nevertheless, even in the eighth washed the solution analyzed revealed the presence
of nitrates and nitrites at trace level, showing the difficulty to remove completely the
ions from the surface of photocatalyst. For the photocatalytic experiment, 100 mg of
washed ZnO (ZnO-SG) were used as the photocatalyst in the oxidation reaction of
NO. Figure 8 shows a remarkable increase in the accumulated mass of nitrates and
Fig. 7 NO conversion using two different mass of photocatalyst and (inset) under different values ofrelative humidity using ZnO-SG as photocatalyst
4888 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
nitrites ions when the sample was washed after the photocatalytic experiment (wash
# 9). Due to the tendency observed in the previous eight washings, the amount of
nitrates and nitrites extracted from the sample during the ninth washings was
associated with the deep oxidation of NO. Under this assumption, the level of
concentration of nitrites and nitrates revealed that the elimination of NO takes place
by oxidation until NO�3 = 62 %. When the mass of the photocatalyst was reduced in
the photocatalytic experiment to 50 mg, the conversion degree from NO to NO�3
was slightly increased to 65 %. The mass of nitrites generated in the photocatalytic
reaction was significant lower compared with the mass of nitrates. This can be
explained by the fact that, once the nitrite ions are formed on the photocatalyst
surface, they are oxidized to NO�3 by holes [22]. For this reason, the conversion
degrees of NO to NO�2 only reached values of 5.3 and 2.9 % for experiments with a
charge of photocatalyst of 100 and 50 mg, respectively. These results are in
agreement with previous works in which the concentration of nitrite ions was quite
low in comparison with that of the nitrate ions [23–25]. On the other hand, when the
photocatalytic experiment was performed in the presence of 70 % RH and 50 mg of
ZnO-SG, the conversion degree from NO to NO�3 was the highest (82 %) in
comparison with the previous experiments described. In the same way, the
conversion degree of NO to NO�2 only reached a value of 1.4 %.
Taking into account these results, it is possible to conclude that the mass of the
photocatalyst has an important effect on the conversion degree of NO, but its
contribution to the selective oxidation of NO to NO�3 is insignificant. In contrast, the
selective oxidation of NO to the formation of nitrates was considerably improved by
the presence of water molecules (70 % HR), which can be associated with the
contribution of hydroxyl radicals to the mechanism of oxidation of NO. In any case,
Fig. 8 Accumulative mass of nitrates (triangles) and nitrites (circles) generated in the experimentsperformed using ZnO-SG as photocatalyst under UV light
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4889
123
the oxidation of NO to NO�3 was the predominant process over the partial oxidations
of NO to the formation of NO2 or NO�2 : This is a relevant point because it confirms
that the elimination of NO by the action of a ZnO-SG photocatalyst takes place by a
deep oxidation to innocuous products such as NO�3 in a major proportion.
Conclusions
Zinc oxide was prepared successfully by four different routes of synthesis. Each
method provided an oxide with different physical properties such as surface area,
particle size and light absorption. According to SEM images, the oxides synthesized
by the solvothermal and sol–gel methods have particles with the morphology of bars
but with differences in their size. The oxide with the smallest particle size, largest
surface area and highest light absorption exhibited the highest conversion of NO
under steady state, reaching a removal of 70 %. The efficiency of the process was
promoted when the mass of the photocatalyst was increased from 50 to 100 mg, due
to a higher dispersion of the photocatalyst on the glass substrate. The presence of
nitrate and nitrite ions was successfully confirmed. The selective oxidation of NO to
the formation of nitrates was considerably improved by the presence of water
molecules (70 % HR), which can be associated with the contribution of hydroxyl
radicals to the mechanism of oxidation of NO. The results obtained shown that NO�3
is the main product of the photocatalytic oxidation reaction of NO when ZnO is
synthesized by the sol–gel method used as photocatalyst.
Acknowledgment We wish to thank to the CONACYT for its invaluable support through the Project
167018.
References
1. A.K. Radzimska, T, Jesionowski. Materials 7, 2833–2881 (2014)
2. S. Ameen, M.S. Akhtar, H. Seo, H. Shin, Mater. Lett. 113, 20–24 (2013)
3. J. Gao, A.V. Teplyakov, Catal. Today 238, 111–117 (2014)
4. R. Nagaraja, N. Kottam, C.R. Girija, B.M. Nagabhushana, Powder Technol. 215–216, 91–97 (2012)
5. Y. Peng, J. Ji, X. Zhao, H. Wan, D. Chen, Powder Technol. 233, 325–330 (2013)
6. D. Raoufi, Renew. Energy 50, 932–937 (2013)
7. N. Kiomarsipour, R.S. Razavi, Ceram. Int. 40, 11261–11268 (2014)
8. S. Rani, P. Sury, P.K. Shishodia, R.M. Mehra, Solar Energy Mater. Sol. Cells 92, 1639–1645 (2008)
9. R.C. Wang, C. Tsai, Appl. Phys. A 94, 241–245 (2009)
10. K. Hembram, D. Sivaprahasam, T.N. Rao, J. Eur. Ceram. Soc. 31, 1905–1913 (2011)
11. F. Kharchouche, E. Savary, A. Thuault, S. Marinel, S. d’Astorg, M. Rguiti, S. Belkhiat, C. Courtois,
A. Leriche, Ceram. Int. 40, 13697–137071 (2014)
12. Y. Azizian-Kalandaragh, A. Khodayari, M. Behboudnia, Mater. Sci. Semicond. Process. 12, 142–145
(2009)
13. F. Wang, X. Qin, D. Zhu, Y. Meng, L. Yang, Y. Ming, Mater. Lett. 117, 131–133 (2014)
14. Y. Liao, C. Xie, Y. Liu, H. Chen, H. Li, J. Wu, Ceram. Int. 38, 4437–4444 (2012)
15. A. Rezaee, H. Rangkooy, A. Khavanin, A.J. Jafari, Environ. Chem. Lett. 12, 353–357 (2014)
16. Y. Huang, C. Guo, L. Huang, Q. Dong, S. Yin, T. Sato, Int. J. Nanotechnol. 10, 30–37 (2013)
17. E. Kowsari, B. Bazri, Appl. Catal. A 475, 325–334 (2014)
18. Y. Wei, Y. Huang, J. Wu, M. Wang, C. Guo, Q. Dong, S. Yin, T. Sato, J. Hazard. Mater. 248–249,
202–210 (2013)
4890 E. Luevano-Hipolito, A. Martınez-de la Cruz
123
19. S. Sakka, Handbook of Sol–Gel Science and Technology. 1. Sol–Gel Processing (Kluwer Academic
Publishers, Osaka, 1968), pp. 351–352
20. A.E. Morales, E.S. Mora, U. Pal, Rev. Mex. Fıs. 5(53), 18–22 (2007)
21. J.B. Condon, Surface Area and Porosity Determinations by Physisorption (Elsevier, Amsterdam,
2006), pp. 8–9
22. C.H. Pollema, E.B. Milosavljevic, J.L. Hendrix, L. Solujic, J.H. Nelson, Mon. Chem. 123, 333–339
(1992)
23. S. Devahasdin, C. Fan Jr, K. Li, D.H. Chen, J. Photochem. Photobiol. A 156, 161–170 (2003)
24. S. Chin, E. Park, M. Kim, J. Jeong, G. Bae, J. Jurng, Powder Technol. 206, 306–311 (2011)
25. T. Sano, N. Negishi, K. Koike, K. Takeuchi, S. Matsuzawa, J. Mater. Chem. 14, 380–384 (2004)
Sol–gel synthesis and photocatalytic performance of ZnO toward… 4891
123