Decolorization kinetics of the direct red 23 diazo dye
from zinc/cobalt mixed oxide semiconductor using
oxalate as a precursor

Vanildo Souza Leão Neto1 • Thiago Orcelli1 •

Marcelo Rodrigues da Silva2 • Fauze Jacó Anaissi3 •

Keiko Takashima1

Received: 12 August 2015 / Accepted: 1 November 2015 / Published online: 13 November 2015

� Akadémiai Kiadó, Budapest, Hungary 2015

Abstract In this paper, we report the oxidation kinetics of the Direct Red 23 diazo

dye through the UV irradiation of an aqueous suspension of zinc/cobalt mixed oxide

semiconductor at 30 �C. The zinc and cobalt mixed oxide was prepared by the

thermal decomposition of their oxalates, containing 5, 10, and 20 % (w/v) cobalt

and calcined at 400 �C for 12 h. The characterization of these oxides was performed

by X-ray diffraction, thermal and thermo-gravimetric analysis, X-ray dispersive

energy spectrometry, scanning electron microscopy, diffuse reflectance spec-

troscopy, and textural analysis to understand the physical and chemical behavior of

these materials. The suspension, formed by synthesized oxide and the Direct Red 23

diazo dye, was kept for 60 min in the dark and, subsequently irradiated during

210 min. The decolorization rate constant, kobs, was determined under pseudo-first

order conditions, maintaining the semiconductor concentration much larger than the

diazo dye concentration, using the maximum Direct Red 23 absorbance at 503 nm,

and plotting the natural logarithm as a function of irradiation time at 30 �C. All the

synthesized mixed oxides displayed higher photocatalytic activity in comparison to

commercial zinc oxide. From these, the oxide containing 5 % cobalt showed the

highest photocatalytic activity, resulting in a rate constant for the Direct Red 23

diazo dye equivalent to 14.0 9 10-3 min-1 or 93 % decolorization at 30 �C.

Keywords Diazo dye oxidation � Heterogeneous photocatalysis � Photocatalytic

activity � Oxide characterization

& Keiko Takashima

keiko.takashima@gmail.com

1 Laboratório de Processos de Oxidação Avançados, Department of Chemistry, Universidade

Estadual de Londrina, Caixa Postal 10011, Londrina, Paraná 86057-970, Brazil

2 Faculdade de Engenharia, Universidade Estadual Paulista, São Paulo, Bauru, Brazil

3 Department of Chemistry, Universidade Estadual do Centro-Oeste, Guarapuava, Paraná, Brazil

123

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DOI 10.1007/s11144-015-0949-6

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Introduction

Several sectors of industrialized countries face environmental problems related to

water contamination, which deserve special attention, because of the high loading of

organic and inorganic pollutants. Within this segment, textile dyes have caused

significant pollution because of the discarding of the colored effluent into the

environment is a source of aesthetic pollution, besides the adverse effects on aquatic

life [1]. Considering that environmental pollution is a major problem faced by

contemporary society, many studies have been focused on the preservation of

aquatic and atmospheric environments using advanced oxidation processes as a tool.

Most of the dyes used in textile industries are of the azo type, characterized by one

or more azo groups (N=N), bonded to the sp2 carbon atoms, as benzene or

naphthalene rings. Although these dyes are present in approximately 20 % of textile

industry effluents, research about these compounds is very important for several

reasons [2]. Among these, there are removal difficulties by the traditional processes

and due to mutagenic and carcinogenic effects. So, the Direct Red 23 diazo dye,

DR23 (C35H27N7S2O10Na2, C.I. 29160) (Fig. 1) is a molecule model to study the

removal dye of the wastewater.

Advanced oxidation processes (AOPs) have attracted great interest for the

oxidation of organic matter because under appropriate conditions, species are

removed and converted completely into CO2, H2O, and harmless minerals. AOPs

are based on the hydroxyl radical (HO�) generation. This radical is characterized by

non-specificity, high oxidizing power (Eo = 2.8 V), and short lifetime [3, 4].

Oxidation and reduction processes occur on the surface or in the vicinity of the

photo-excited semiconductor particle. Among the processes that produce hydroxyl

radical, heterogeneous photocatalysis has been shown as one of the promising

method in the destruction of a variety of organic compounds over the past decades.

It is based on the irradiation of an n-type semiconductor as TiO2, ZnO, etc., whose

photon energy (hm) must be greater or equal to the band gap energy (Ebg),

mineralizing organic compounds and simultaneously reducing dissolved metals or

other species [5–7]. Consequently, the semiconductor oxides have been widely

investigated in the fighting of environmental pollution by heterogeneous photo-

catalysis. Transition metal oxides draw attention for use as photocatalysts because

they combine the d electrons, nanometric size, and large surface area, in addition to

the lower cost alternative [8].

N=N

NaO3S

OH OH

N=N

SO3Na

CH3C

O

N
N C N H

O

HH

Fig. 1 Structural formula of Direct Red 23 (DR23) acid 3-[[4-(acetylamino)phenyl]azo]-4-hydroxy-7-
[[[[5-hydroxy-6-(phenylazo)-7-sulfo-2-naphtalenyl]-amino]carbonil]amino]-2-naftalenosulfonate of
sodium (IUPAC nomenclature)

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Zinc oxide has been widely studied due to the above mentioned properties, which

make it a large technological potential material. ZnO presents the structure of

wurtzite type, band gap of 3.37 eV, excitons binding energy of 60 meV, high

electron mobility and good transparency [9]. Among other technological applica-

tions used are gas sensors [10], solar cells [11], biosensors [12], etc. ZnO is regarded

as a non-stoichiometric compound, due to the Zn2? excess in the interstitial lattice

positions. Thus, in order to maintain electroneutrality, additional electrons are found

in the oxide lattice, acquiring the n-type semiconductor behavior [13–15]. Since it is

very important to find alternatives to pollutant decontamination, several studies

have been conducted in order to increase the photocatalytic efficiency of the

semiconductor oxides. Among these, doping oxides have been investigated to

generate a p-n junction in the mixed oxides. Hence, the p-type semiconductor as

Co3O4 is also a non-stoichiometric compound with O2- excess in the lattice with

balanced positive charges in the interstices. This means that when a p-type

semiconductor is placed in contact with an n-type, a double layer of charges is

formed in the junction. The p-n junction is based on non-recombination of the

electron–hole pair, which prevents the electron passage to the p-region and the holes

for n-region. This generates a different electrostatic potential on both sides and

produces a lack of carriers, called depletion region in the interface [16, 17]. For

instance, Wang et al. synthesized p-CuO/n-BiVO4 mixed oxide and applied to the

Rhodamine B degradation [18]. Furthermore, da Silva et al. synthesized a mixed

oxide containing n-BiVO4 and p-NiO in order to degrade the methylene blue [19].

In both cases, there was an enhancement of the photocatalytic activity, attributed to

the formation of the p-n junction in relation to the pure semiconductor. Moreover,

the p-n junction formation using n-ZnO attracted attention to improve the

photocatalytic activity of this important oxide [20, 21].

Among the p-type semiconductors, cobalt oxide has been widely used in

reactions for the production of H2 [22], in alkaline batteries [23], organic pollutants

degradation through the catalytic and electrochemical reactions [24]. Cobalt oxide

has five species [CoO2, Co2O3, CoO(OH), Co3O4, and CoO] with a wide range of

applications in various industrial sectors. The Co3O4 spinel has a band gap energy

ranging from 1.4 to 1.8 eV and is stable up to 800 �C [25]. Owing to these

properties, it is expected that cobalt oxide addition in zinc oxide could promote the

p-n junction formation and also, to enhance the photocatalytic activity.

This study aimed at determining the photocatalytic activity of the prepared mixed

oxide and compared it to that of the commercial ZnO by the means of the

decolorization rate constant of the Direct Red 23 diazo dye at 30 �C. The mixed

oxide was prepared by the thermal decomposition of the zinc and cobalt mixed

oxalate containing 5, 10, and 20 % cobalt and calcined at 400 �C during 12 h. These

materials were characterized by several structural and morphological techniques

such as scanning electron microscopy, X-ray diffraction (XRD), thermogravimetry

and differential thermal analysis, textural analysis, UV/Vis spectroscopy coupled to

diffuse reflectance sphere, among others.

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Experimental

Preparation of the zinc and cobalt mixed oxide

The mixed oxide of zinc and cobalt was prepared according to Muruganandham

et al. [26]. The mixed oxide containing 5 % cobalt was prepared using equal

volumes of 0.40 mol L-1 zinc nitrate [Zn(NO3)2�6H2O, Synth 99.5 %], 2.0 9 10-2

mol L-1 cobalt nitrate [Co(NO3)2�6H2O, Vetec 98.8 %], and 0.60 mol L-1

anhydrous oxalic acid (H2C2O4, Belga 99 %) in deionized water (Milli-Q Plus),

which were boiled separately. At this temperature, solutions of zinc nitrate and

cobalt nitrate were immediately added to the oxalic acid and the heating stopped.

The resulting mixture was kept under agitation (600 rpm—301 Microquı́mica

AMA) at room temperature. The formed precipitate was filtered under vacuum

(Schleicher Schuell 0.47 ± 0.5 mm diameter, pore 0.2 lm) and washed several

times with distilled water, maintained overnight at room temperature, and dried at

100 �C (BioPar S150SD) for 3 h. About 2.0 g of the compound was introduced into

the muffle (Marconi MA385) and heated from 5 to 10 �C min-1 at a calcination

temperature of 400 �C for 12 h. The procedure was repeated for 10 and 20 % (w/v)

cobalt in the mixed oxide.

Characterization

The thermal decomposition of zinc and cobalt oxalate was evaluated by

thermogravimetry (TG) and differential thermal analysis (DTA) at a heating rate

of 10 �C min-1 from 30 to 1200 �C in air atmosphere using alumina as reference

material (Seiko 6300). XRD data was collected in a Rigaku D/Max–2100PC

instrument at room temperature, using Cu Ka radiation (1.5418 Å
´

) radiation,

coupled to a nickel filter, in order to reduce the unfavorable Cu Kb radiation. The

applied tension was 40 kV and the current was 30 mA. The scanning range was

from 10 to 80� with regular step of 1�s-1. Current and voltage were 20 mA and

40 kV, respectively. The band gap energy of the oxides was determined using the

diffuse reflectance spectra (Shimadzu UV-3101PC) by Kubelka–Munk and Tauc

methods. The textural analysis was performed, preheating the sample at 300 �C
under vacuum for 3 h to measure the surface area, pore volume and pore radius

(Quantachrome NovaWin version 10:01). The morphology of the oxides was

studied by scanning electron microscopy (Philips FEI Quanta 200). The distribution

and the surface composition were determined by X-ray dispersive energy

spectroscopy (EDX) using the same microscope. The elemental composition was

determined by X-ray fluorescence (Shimadzu EDX-720).

Photocatalysis

The photocatalytic activity was determined inside a wooden box (50 9 50 9

50 cm3) with internal walls covered with aluminum foil and closed frontally with

black curtain in a double wall borosilicate glass reactor (200 mL) with water

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circulation at 30.0 ± 0.1 �C (Microquı́mica MBQTC99-20). The suspension

formed by 0.7 g L-1 oxide in 150 mL of 7.5 9 10-5 mol L-1 DR23 diazo dye

(Chimical—30 % dye content) was stirred at 600 rpm (Fisatom 752) in the dark for

60 min and irradiated by a mercury vapor lamp without bulb (Phillips HPL-N 125W

with wavelengths from *350 to 450 nm) positioned horizontally at 22 cm of the

suspension surface during 210 min. Aliquots of 1.5 mL were collected at

predetermined times and UV–Vis spectra (200–700 nm) recorded (Hitachi

U3000). The decolorization rate constant, kobs, was determined under pseudo-first

order conditions , maintaining the semiconductor concentration much larger than

the diazo dye, using the maximum of Direct Red 23 absorbance at 503 nm, with a

molar absorptivity of 2.17 9 104 L cm-1 mol-1, and plotting the natural logarithm

as a function of irradiation time at 30 �C. The decolorization percentage of DR23

was determined spectrophotometrically by measuring dye absorbance at kmax

503 nm. The decolorization efficiency (%) was calculated as given in Eq. 1, where

A0 is the initial dye absorbance, and A is the absorbance after irradiation.

Efficiency %ð Þ ¼ A0 � A

A0

� 100 ð1Þ

Results and discussion

Fig. 2 shows the TG and DTA decomposition of 5 % cobalt in zinc and cobalt

mixed oxalate. The mass losses occurred at the same temperatures for 10 and 20 %

cobalt. The initial loss at 175 �C is an endothermic process that corresponds to the

dehydration as shown in Eq. 2 [27].

Fig. 2 TG/DTA of zinc and cobalt mixed oxalate containing 5 % Co with 10 �C min-1 in air atmosphere

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ZnC2O4�CoC2O4� 3:7ð ÞH2O ! ZnC2O4�CoC2O4 þ 3:7ð ÞH2O ð2Þ

The second loss at 400 �C took place exothermically and corresponds to the

oxidation of the mixed oxalate (ZnC2O4�CoC2O4) in mixed oxide (ZnO�Co3O4),

CO2, and CO, as shown in Eq. 3.

3ZnC2O4�3CoC2O4 ! 3ZnO�3Co3O4 þ 5CO2 þ 7CO ð3Þ

The Co3O4 spinel is stable up to 800 �C, whose crystal structure releases oxygen

above 900 �C, forming ZnO�CoO that corresponds to the third loss at 904 �C shown

in Eq. 4 [28].

3ZnO�3Co3O4 ! 3ZnO�3CoO þ O2 ð4Þ

The thermal decomposition of zinc and cobalt oxalate in other proportions

occurred also in three stages in air atmosphere as shown in TG/DTA curves in

Fig. 2. Even though the oxide is stable from 400 to 800 �C, the samples were

calcined at 400 �C, due to the existing defects in the structure, which are dependent

on the calcination temperature, that is, the lower sample crystallization, the lower

e-/h? pair recombination, and consequently, the higher catalytic activity.

Fig. 3 displays the XRD patterns of commercial and prepared zinc oxide, mixed

oxide of zinc and cobalt containing 5, 10, and 20 % cobalt, respectively, and cobalt

oxide synthesized using the same methodology [26]. The 2h angles for both

commercial and prepared ZnO (31.7�, 34.4�, 36.2�, 47.5�, 56.6�, 62.9�, 66.2�, 67.9�,
and 69.0�) are related to the wurtzite hexagonal structure, according to the data base

used (Crystmet and PCPDFWin version 2003) and none related to the starting

oxalate, confirming the oxide formation at 400 �C [29]. According to the thermal

Fig. 3 X-ray diffractogram of (a) commercial zinc oxide; (b) synthesized zinc oxide; (c) synthesized
cobalt oxide and, zinc and cobalt mixed oxides containing (d) 5, (e) 10, and (f) 20 % Co

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Fig. 4 SEM of a commercial
ZnO, b prepared zinc oxide, and
c zinc and cobalt mixed oxide
containing 5 % Co at 912,000
magnitude

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analysis (Fig. 2) the total organic matter decomposition occurred at 400 �C,

confirming its formation from the diffractograms. Beyond that, the commercial ZnO

displayed lower intensity and greater enlargement in the peaks in comparison to the

prepared one, indicating a lower crystallinity in the commercial sample. Meanwhile

for the zinc and cobalt mixed oxide, no peak relating to cobalt oxide structure was

observed, simply the decrease of the relative intensities with increase in the cobalt

proportion. Thus, it is suggested that the substitution of zinc by cobalt atom had

taken place, due to the approximately same ionic radii, 74 and 82 pm for zinc and

cobalt, respectively. This means that there was no formation of a new phase in the

mixed oxide, as well as phase segregation. The relative intensity of 2h peak at 36.2�
displayed 2087.09 counts in the oxide containing 5 % cobalt, while with 10 % Co,

1997.80 counts and, 1897.24 counts for 20 %, attributed to the zinc by cobalt

replacement in the structure, decreasing the zinc diffraction intensity at the

reference angle.

The images obtained from SEM of the commercial zinc oxide (Fig. 4a), prepared

zinc oxide (Fig. 4b), and mixed oxide containing 5 % cobalt (Fig. 4c) with

magnification of 12,000 times are shown in Fig. 4. It is observed that the prepared

ZnO (Fig. 4b) presents larger and more compact particles in comparison to the

commercial ZnO (Fig. 4a). This means that there was change in the oxides

morphology, resulting in different behaviors during the azo dye adsorption on the

surface. The mixed oxide images containing 5 % cobalt (Fig. 4c) show a similar

format, but smaller than the prepared zinc oxide with cobalt oxide particles

aggregated to the surface. The mixed oxides containing 10 and 20 % cobalt showed

comparable morphology with respect to 5 % cobalt with increase of the particle size

with increase of the dopant percentage.

The elements EDX of the investigated oxides were performed to study the

composition and distribution of elements. As expected, commercial zinc oxide and

the preparation showed the zinc and oxygen, while the mixed oxide containing 5 %

cobalt showed zinc, oxygen, and cobalt. The distribution of these elements was

uniform in these oxides. The mixed oxide containing 10 and 20 % cobalt exhibited

the same characteristics in relation to 5 %, suggesting that there was not segregation

phase, when the cobalt proportion was enhanced in the oxide.

The transition metal percentages in the mixed oxides were obtained quantita-

tively using X-ray fluorescence. It was found 4.00 % cobalt with standard deviation

Table 1 Band gap energy of commercial ZnO and zinc and cobalt mixed oxide containing different

cobalt percentage and surface area calculated from BET model

% Zn % Co Eg (eV) A (m2 g-1) Vpore (cm3 g-1) rpore (nm)

100* 0 3.37 6.440 – –

95 5 3.12 37.144 0.1119 6.026

90 10 3.22 29.785 0.1029 6.910

80 20 2.77 26.400 0.1221 6.709

* Commercial ZnO—nuclear

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of 0.013 to 5 % cobalt, while the oxide containing 10 % cobalt presented 9.71 %

with standard deviation of 0.019 %. Conversely, the oxide containing 20 % cobalt

showed 13.98 % with standard deviation of 0.023 %. These differences could be

caused by possible experimental errors, from precursors or concentration of

contaminating reagents. In addition, the synthesis of mixed oxide containing 20 %

cobalt could not be repeated, for lack of the same material used in all experiments

for characterization and photocatalysis. Furthermore, the cobalt oxide amount on the

mixed oxide surface is important in the discussion of dye adsorption capacity [30,

31]. As a function of this, it is expected that the increase of the cobalt percentage

decreases the available area as well as the photocatalytic activity, taking into place

that the adsorption and photocatalytic activity are directly linked [32, 33].

The energy band-gap of commercial zinc oxide and mixed oxides shown in

Table 1 were obtained from the diffuse reflectance spectra using the Kubelka–Munk

and Tauc methods [34, 35] displayed in Fig. 5. All the synthesized oxides showed

band gap energy (Eg) lower in relation to ZnO (3.26 eV), that is, 3.12 eV to 5 % Co

(Fig. 5, inset), 3.22 eV to 10 % and, 2.77 eV to 20 %. Therefore, the dopant

increase in the zinc oxide structure decreases the band gap energy thereof, due to the

lower band gap energy of cobalt oxide (1.4–1.8 eV) in the region of visible light

[36, 37].

These results show that the synthesis was satisfactory, given that one of the

objectives was to reduce the band gap energy. Conversely, this parameter is not the

only one, which can justify the increase or decrease in the photocatalytic activity

[31]. As previously mentioned, the photocatalytic activity of the semiconductor is

related to the area available for adsorption. Among the surface areas of the oxides,

calculated from the BET model, the largest surface area took place to the oxide

containing 5 % cobalt (35.617 m2 g-1). According to Table 1, the pore radius of the

Fig. 5 UV/Vis transmittance spectra of the zinc and cobalt mixed oxides containing (a) 5, (b) 10 and,
(c) 20 % cobalt. Inset Band gap energy of the mixed oxide containing 5 % cobalt evaluated by Kubelka–
Munk and Tauc

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oxides was around 6 nm. This means that these materials are classified as

mesoporous for presenting a radius between 2 and 50 nm [38]. The DR23

adsorption on the mixed oxide surface decreased with the increase of the cobalt

percentage in the dark during 60 min. The adsorption was measured by the

difference between the initial and after 60 min DR23 absorptions. The commercial

ZnO presented 13 % of DR23 (7.5 9 10-5 mol L-1) adsorption, meanwhile the

mixed oxide containing 20 % of Co was about 45 %, 10 % Co was 56 %, while for

5 % Co about 64 %. Therefore, it was attributed that the oxide with larger surface

area has more available active sites that facilitate the species adsorption.

In order to verify the Direct Red 23 diazo dye (7.5 9 10-5 mol L-1)

decolorization, the photolysis was performed, as a control experiment, using UV

irradiation during 210 min, whose decolorization efficiency was of 2 %. The

photocatalytic activity of the mixed oxides and the commercial ZnO was

determined under the pseudo-first order condition with a very large amount of

semiconductor (0.7 g L-1) containing respectively 5, 10, and 20 % cobalt in a

DR23 solution (7.5 9 10-5 mol L-1) at natural pH around 7.00 at 30.0 �C. The

decolorization rate constants, kobs, of DR23 mediated by mixed oxides as well as by

commercial ZnO are presented in Table 2.

In this table, the highest kobs of 14.00 9 10-3 min-1 occurred, following a first

order behavior (r = 0.9970), when it was used the mixed oxide containing 5 %

cobalt, calcined at 400 �C during 12 h. It was observed a 93 % decolorization after

150 min UV irradiation, with an intensity of 1274 mW cm-2, in accordance to the

initial absorbance decay at 30 �C, as shown in Fig. 6.

The mixed oxide containing 10 and 20 % cobalt displayed respectively, kobs of

8.00 and 6.10 9 10-3 min-1 during 180 min under UV irradiation, suggesting that

the proportion of increase of cobalt in the zinc oxide structure reduces the

photocatalytic activity of the oxide, as observed in Table 2. Therefore, the more

cobalt present on the surface, the larger the difficulty of diazo dye adsorption and

decolorization. Furthermore, the rate constants are in agreement with the surface

areas sizes obtained from the BET method as shown in Table 1. The mixed oxide

containing 5 % cobalt displayed the highest surface area, indicating the largest

available area for the dye adsorption. In addition, the pH changes during irradiation

are shown in Table 2. Considering that the pKa of ZnO is approximately 11.1 [39],

Table 2 Decolorization rate constant, kobs, for DR23 (7.5 9 10-5 mol L-1) in the presence of com-

mercial ZnO and synthesized mixed oxide (0.7 g L-1) and pH change as a function of irradiation time at

30 �C

% Zn % Co kobs (10-3 min-1) Decolorization (%) DpH r

100* 0 3.00 45.94 6.58–7.11 = 0.53 0.9744

95 5 14.00 93.00 6.80–6.93 = 0.13 0.9970

90 10 8.00 87.00 6.90–7.21 = 0.31 0.9673

80 20 6.10 78.00 7.17–7.65 = 0.48 0.9946

* Commercial ZnO—nuclear

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it means that the semiconductor surface in this pH range is positively charged as

ZnOH2
?. Thus, the approximation of a negatively charged species as sulfonate

group of the diazo dye is facilitated. With respect to the commercial ZnO, the diazo

dye DR23 decolorization followed the first order kinetic behavior (r = 0.9774),

giving a kobs of 3.00 9 10-3 min-1 and approximately 46 % decolorization after

210 min irradiation at 30 �C. Therefore, the decolorization rate constants of DR23

mediated by synthesized mixed oxides with respect to the commercial ZnO, were

almost five-fold larger for the oxide containing 5 % cobalt, nearly tripled for 10 %,

and doubled for 20 %. This may be justified by the depletion region formation in the

p-n junction, which slows down the electron (e-)/hole (h?) pair recombination,

0.0

0.2

0.4

0.6

0.8

A
bs

or
ba

nc
e

Wavelength / nm

0 min

150 min

300 400 500 600 700

0 40 80 120 160
-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

ln
 a

bs

Time / min

a

b

Fig. 6 a Decay of DR23 (7.5 9 10-5 mol L-1) absorbance mediated by mixed oxide containing 5 %
cobalt (0.7 g L-1) as a function of the UV light irradiation time at 30 �C during 150 min. b Natural
logarithm of (a) as a function of irradiation time at 30 �C

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enhancing the hydroxyl radical generation and subsequent dye decolorization. Dong

et al. observed an increase in the photocatalytic activity, attributed to the p-n

junction formation, when Co3O4/ZnO was decolorized the methylene blue [21]. On

the other hand, the ZnO, synthesized and calcined at 400 �C, decolorized 15 %

during the first 8 min. The irradiation was continued for 45 min more and there was

no more DR23 decolorization, indicating that the synthesized ZnO does not present

the first order behavior (r = 0.7745). Because of this, this result was not considered

for comparison with other oxides.

Conclusion

The mixed oxides were formed at 400 �C with complete oxidation of all organic

matter precursors. Furthermore, the cobalt substituted the zinc in the mixed oxides

and no formation of a new phase in the material was verified by XRD. All the

prepared mixed oxides presented larger photocatalytic activities with respect to

ZnO, due to the depletion region forming p-n junction and presented kobs for DR23

decolorization larger than the commercial ZnO (3.0 9 10-3 min-1) at natural pH at

30 �C. The mixed oxide containing 20 % Co was about two times faster

(6.1 9 10-3 min-1); for 10 % Co about three-fold (8.0 9 10-3 min-1); while for

5 % about fivefold (14.0 9 10-3 min-1) higher, decolorizing about 93 % diazo dye

after 150 min irradiation at 30 �C.

Acknowledgments The authors thank the Fundação Araucária (22850/2011) for the financial support.

Vanildo Souza Leão Neto thanks to CNPq for the IC scholarship and Thiago Orcelli thanks to CAPES by

scholarship.

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	Decolorization kinetics of the direct red 23 diazo dye from zinc/cobalt mixed oxide semiconductor using oxalate as a precursor
	Abstract
	Introduction
	Experimental
	Preparation of the zinc and cobalt mixed oxide
	Characterization
	Photocatalysis

	Results and discussion
	Conclusion
	Acknowledgments
	References