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

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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

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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.

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