Powder Technology 253 (2014) 360–367

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On the electrical conductivity and photocatalytic activity ofaluminum-doped zinc oxide

P. Zhang a, R.Y. Hong a,b,c,⁎, Q. Chen a, W.G. Feng d

a College of Chemistry, Chemical Engineering and Materials Science & Key Laboratory of Organic Synthesis of Jiangsu Province, Soochow University, SIP, Suzhou 215123, Chinab College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, Chinac Key Laboratory of Environmental Materials and Environmental Engineering of Jiangsu Province, Yangzhou University, Yangzhou 321000, Chinad Suzhou Nanocomp Inc., Suzhou New District, Suzhou 215011, China

⁎ Corresponding author at: College of Chemistry, ChemScience, Key Laboratory of Organic Synthesis of JiangsuSIP, Suzhou 215123, China. Tel./fax: +86 512 6588 2057.

E-mail address: rhong@suda.edu.cn (R.Y. Hong).

0032-5910/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.powtec.2013.12.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 June 2013Received in revised form 22 November 2013Accepted 1 December 2013Available online 8 December 2013

Keywords:Aluminum-doped zinc oxideElectrical conductivityPhotocatalytic activityPost-calciningPhotodegradation

Aluminum-doped zinc oxide (AZO) conductive powders have been successfully prepared by a simple chemicalcoprecipitation method. The obtained powders with different post-calcining atmospheres were characterizedusing X-ray powder diffraction (XRD), scanning electron microscopy (SEM/EDS), and dynamic light scattering(DLS). The resistivity-dependent photocatalytic activity was studied by degradation of methyl orange (MO) inaqueous solution. The result showed that photodegradation of methyl orange dyes obeyed the rule of a pseudofirst-order kinetics reaction and the AZO photocatalytic activity was related to the resistivity under differentpost-calcining atmospheres.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Industrial wastewater pollution control in the dyeing and printingindustries is of importance due to the complex composition, high chro-maticity, large emissions, high toxicity, and low biodegradability ofwastewater. Weathering of organic dyes through oxidation, hydrolysisor other chemical reactions occurring in the wastewater phase can pro-duce toxic metabolites [1–4]. However, chemical and biological degra-dation methods have been shown to be ineffective [5]. Currently,oxide semiconductor photocatalysis has attracted a great deal of atten-tion in wastewater treatment because of its high photocatalytic activity,mild reaction conditions, and low energy consumption [6–8].

Among various oxide semiconductor photocatalysts, ZnO has beenwidely used for its electric and photonic properties and oxidation resis-tance [9]. Specifically, ZnOnanoparticles can be used for their antibacte-rial and shielding ultraviolet radiation properties [10,11]. Moreover,ZnO has the photocatalytic capacity to decompose organic dyes to CO2

and H2O.We have reported that the ZnO nanoparticles possess antibac-terial property and photocatalytic activity [12,13]. However, becauseZnOhas awide band gap of about 3.37 eV and a high electron excitationbinding energy of about 60 meV at room temperature, ZnO resistivity is

ical Engineering and MaterialsProvince, Soochow University,

ghts reserved.

very high and can only be excited under UV irradiation, leading to lim-ited application [14], so further research and development is warranted.

The incorporation of other elements into the ZnO photocatalystcould decrease its resistivity and extend its photoresponse, which isfound to be an effective route to enhance its photocatalytic perfor-mance. Low resistivity and high catalytic efficiency of ZnO can be gainedby incorporating the following elements: Co, Sb, Al, In, N and Cu. For ex-ample, Chang et al. [15] have reported that the photocatalytic perfor-mance of N-doped ZnO was superior to the undoped ZnO undersimulated daylight irradiation. Wang et al. [16] have demonstratedthat Ag-doped ZnO has improved photocatalytic activities owing tothe increased surface defects caused by the enhanced oxygen vacancies.Lim et al. [17] have reported that theAg/ZnO–SnO2 catalyst can improvethe photodegradation rate of methyl orange. Perazolli et al. [18] havestudied the influence of the atmosphere on SnO2 plus ZnO calcining,verifying that Zn promotes densification due to the formation of oxygenvacancies in the structure. However, to the best of our knowledge, therelationship of electrical conductivity and photocatalytic activity of ma-terials has not been reported so far. So an attempt has been made tostudy the relationship of electrical conductivity and photocatalyticactivity. Additionally, the effect of the post-calcining atmosphere onthe conductivity and photocatalytic activity should be investigated.

Herein, we studied the Al-doped ZnO (AZO) photocatalysis due to itswidespread attention in recent years not only for its optoelectronic per-formance, but also for its properties of being nontoxic, pollution-free,thermally stable and an ideal substitute for ITO (indium tin oxide). Inthis work, we investigated the effect of the different post-calcining

Fig. 1. Apparatus used for the photocatalytic degradation of methyl orange.

Fig. 2. XRD patterns of ZnO powder and AZO samples with post-calcining under differentatmospheres.

361P. Zhang et al. / Powder Technology 253 (2014) 360–367

atmospheres on AZO electrical conductivity, and also examined theeffect of electrical conductivity of AZO on its photocatalytic activity.The photocatalytic activitywas studied bydegradationofmethyl orange(MO) in aqueous solution. The prepared AZO with excellent electricalconductivity showed better photocatalytic activity in degradation ofMO dye. Furthermore, the effects of different parameters such as bub-bling air and MO initial concentration on MO photodegradation werealso explored.

2. Experimental

2.1. Materials

Zinc sulfate (ZnSO4·7H2O), aluminum sulfate (Al2(SO4)3·18H2O),sodium hydroxide (NaOH), anhydrous sodium carbonate (Na2CO3)and polyethylene glycol 400 (PEG-400) were all of analytical gradeand purchased from Wuxi Chemical Co., Ltd. Methyl orange (MO) wasused to measure the AZO photocatalytic activity. Deionized water wasused throughout the experiments.

2.2. Preparation of AZO conductive powders

2.2.1. Precursor preparationAZO precursors were prepared by a simple chemical coprecipitation

route. Generally, ZnSO4·7H2O and Al2(SO4)3·18H2O with the molarratio of n (Al)∕n (ZnO) = 1.5 at.% were firstly dissolved in deionizedwater, then poured into a three-necked flask, PEG-400 was added at amass ratio to ZnSO4·7H2Omass ratio of 1.5 wt.%. Amixed alkali solutionof NaOH and Na2CO3 with the concentration ratio of c(NaOH)∕c(Na2CO3) = 2 was added to the three-necked flask drop by drop at60 °Cwith stirring (400 rpm) until pH reached a value of 7.2 (measuredby a digital pHmeter). After a 150 min precipitation, the precipitatewasfiltered and washed several times with distilled water, and followed bydrying a fewhours in an electric oven at about 100 °C to obtain the driedpowders. The dried powders were pre-calcinated at 400 °C for 2 h inorder to make ZnCO3·2Zn(OH)2 and Al(OH)3 decompose into oxidesand then ground in a mortar for about 2 min.

2.2.2. Post-calcination process for precursorSample A: These obtained powders were post-calcinated at 900 °C

in normal air for 2 h with a constant heating rate of 10 °C/min.

Sample B: Post-calcinated at 900 °C for 2 h in an inert argon (Ar)atmosphere with a constant heating rate of 10 °C/min.

Sample C: Post-calcinated at 900 °C for 2 h in reducing atmosphere(CO) with a constant heating rate of 10 °C/min.

2.2.3. CharacterizationThe structure of the AZO conductive powders was investigated by

X-ray diffraction (XRD) on the Bruker D8ADVANCE X-ray diffractome-ter at a voltage of 40 kV with Cu Kα radiation (λ = 1.5406 Å) in the2θ ranging from 27° to 80° and the step width in terms of Delta_2_thetawas set to 0.01°. The microscopy and the composition of the productswere examined by the scanning electron microscopy (SEM, S-4700field emission, Hitachi, Ltd) and an accessory (EDS) of SEM. The particlesize distribution of the AZO conductive powder was measured usingMalvern HPPS5001 laser particle-size analyzer (DLS) at 25 °C. Beforemeasuring DLS, the AZO conductive powders were dispersed in ethanolunder ultrasonic condition (800 W) for 30 min. The obtained suspen-sionwas diluted to a nearly colorless and transparent liquid tomeasure.The resistivity was tested on a RTS-9 four-point probe resistivity

Table 1The tablet resistivities of ZnO and AZO samples.

Sample ZnO A B C

Resistivity(Ω·cm) 1.7 × 108 4.89 × 106 5.26 × 10−2 1.82 × 10−3

362 P. Zhang et al. / Powder Technology 253 (2014) 360–367

measurement system. Due to the resistivities of samples were affectedby the density and contacts between grains, meaningful measurementof the powder form was difficult. Thus, the resistivity of the AZO pow-ders with a fixed weight of 0.3 g was measured with a self-made Φ15 mmmold under uniaxial pressing at 15 MPa pressure. The achievedresistivity data was an average value from six values obtained at differ-ent positions on one substrate.

2.3. Photocatalytic tests

The photocatalytic activities of Al-doped ZnO conductive powdersunder different post-calcining atmospheres were estimated by moni-toring thedegradation ofMOas amodel pollutant in a proprietary appa-ratus with halogen lamp (500 W/D, LW-ZJ-X, Wuxi, china) as radiationsource as shown in Fig. 1. The photocatalytic degradation was per-formed in a 200 mL cylindrical quartz reactor. The reactor was contain-ing a suspension of the photocatalysts (0.6 g/L) inMO aqueous solution(100 mL, 10−5 M) with air bubbling [13,19] continuously at a certainflow rate (80 mL/min) by a feed pump from the bottom of the reactor.In order to maintain the temperature of the suspension at 25 °C, anexternal cooling jacket with recycled water was used. Prior to the lightirradiation, the MO solutions with AZO catalyst were kept in dark for1 h to reach an adsorption/desorption equilibrium. During the photo-catalytic degradation, the MO solution was sampled (10 mL), centri-fuged at a speed of 6000 rpm for 10 min and then filtered, and theabsorbance was determined periodically every 30 min using a UV–visabsorption measurement (Hitachi U-4100 UV–vis spectrometer). Theintensity of themaximumabsorption peak (466 nm) of theMO solutionwas used to measure the concentration of the residual dye. Finally, thephotocatalytic degradation percentage (PDP) of MO was calculated bythe equation:

PDP% ¼ A0−AA0

� 100% ð1Þ

where, A0 is the equilibrium absorbance of MO at the equilibriumadsorption state in the absence of light, and A is the absorbance of MOaqueous solution after light irradiation.

Fig. 3. SEM micrographs of AZO powders prepared at different post-calcinin

3. Results and discussion

3.1. Analysis of AZO conductive powder

3.1.1. Structure analysisThe XRD spectra of the prepared ZnO and AZO samples post-

calcining at 900 °C for 2 h under different post-calcining atmospheresare shown in Fig. 2. All the diffraction peaks are in good agreementwith those of the hexagonal wurtzite ZnO structure (JCPD no. 36-1451). No signals of the metallic Zn are detected and no peaks corre-sponding with the Al or Al2O3 and in absence of the spinel ZnAl2O4

phase, which suggests that the Al3+ has incorporated into the ZnO lat-tice and effectively replaced the site of Zn2+. The pronounced diffrac-tion peaks in the XRD pattern clearly show that the 2θ angles of ZnOat 31.73°, 34.38° and 36.22° correspond to (100), (002) and (101) planes,respectively. In addition, compared to the ZnO peaks, the (100) peak ofsample A shifts toward high angle, due to the smaller radius of Al3+ ionscompared to Zn2+ ions, the incorporation of Al3+ will lead to thedecrease of lattice constant [20]. However, the peaks of the samples Band Cmove to the low angles about 0.02°, in (100) and (101), comparedwith sample A [21,22]. There exists tensile stress which resulted bylattice distortion, largening the lattice constant, leading to the peaksmoved to small-angle [23,24].

3.1.2. Electrical conductivityThe electrical resistivity of the prepared samples is shown in Table 1.

Compared to the AZO with post-calcining in air, the samples B and Cshow significant differences. Furthermore, the resistivity of the threesamples is lower than the prepared ZnO powders in air, which wasconfirmed by other studies [25]. The Al-doped ZnO mechanism couldbe described as [26]:

ZnCO3 � 2Zn OHð Þ2� � � xH2O ¼ ZnCO3 � 2Zn OHð Þ2 þ xH2O↑ ð2Þ

ZnCO3 � 2Zn OHð Þ2 ¼ 3ZnOþ CO2↑þ 2H2O↑ ð3Þ

2Al OHð Þ3 ¼ Al2O3 þ 3H2O↑ ð4Þ

ZnOþ Al2O3→ZnZn þ 2Al�Zn þ 3O�O þ 1=2 O2↑þ 2e−: ð5Þ

In AZO conductive materials, two kinds of donors could be consid-ered: (I) a native donors, consisting of interstitial Zn or O vacancy and(II) Al substitution atom. Some researchers have also confirmed thatthe low resistivity is mainly due to the Al3+ on the substitutional sitesof Zn2+, the oxygen vacancies, and Zn interstitial atoms [27–29].According to the Eq. (5), each Zn2+ is replaced by Al3+, a free electronis released to the conduction band, which leads to the decrease in theresistivity, according to Table 1, we know that the post-calcining atmo-sphere considerably affects electrical properties. Sample A exhibited a

g atmospheres: a) in air; b) in argon; c) in reducing atmosphere (CO).

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high resistivity, possibly due to the difficulty of replacing Zn2+ withAl3+ in an oxygen-rich atmosphere. In such a system, the high partialpressure of oxygen would cause the ZnO to hardly lose any oxygenatoms according to Eq. (5), leading to a carrier concentration decrease.Samples B and C can be interpreted as oxygen atoms gradually diffusing

Fig. 4. EDS analysis of AZO powders prepared at different post-calcining atmospheres:a) in air; b) in argon; c) in reducing atmosphere (CO).

out from the AZO under the atmosphere without oxygen, allowing Al3+

to diffuse into the interior of the ZnO and more readily replacing Zn2+.Therefore more Al3+ can replace Zn2+ in the ZnO lattice, causing anincrease in the carrier concentration to rapidly decrease the resistivityof samples B and C, the results are in accordance with the reports ofBerardan et al. [30] and Myoung et al. [31]. From Table 1, we alsoknow that the resistivity of sample C decreases one order of magnitudecompared to that of sample B. The reducing atmosphere (CO) is able toconsume the oxygen atoms diffusing out from the ZnO, leading to thereaction of Eq. (5) proceeding continually and facilitating the effectivereplacement of Zn2+ with Al3+.

3.1.3. Morphological analysisSEM is one of the promising techniques for themorphology study of

the samples and it gives important information regarding the shape andsize of the particles. The surface morphology of the Al-doped ZnO parti-cles with different post-calcining atmospheres is shown in Fig. 3. Themorphologies of samples are approximately hexagonal in accordancewith the above XRD analysis. And it can be seen that the average grainsize and surface roughness of AZO particles decrease as reducing theoxygen molecule of AZO surrounding atmosphere in post-calcining.Sample C shows that the synthesized particle sizes are uniformlydistributed on the surface. This result is attributed to the AZO post-calcining under CO reducing atmosphere. This kind of atmosphere cancause more defects such as oxygen vacancies, Zn interstitial and Al3+

substitutes Zn2+. The existence of more defects greatly decreasesthe grain size and the surface roughness due to the reaction that Al3+

substitutes Zn2+ completely and uniformly.

3.1.4. Elementary composition analysisFig. 4 shows the results of EDS analysis of the samples, the results

indicate the presence of Zn, O, and Al peaks with intensity proportionalto their respective concentrations. This ruled out any unintentionaldoping of impurities other than Al. All the EDS data are summarized inTable 2, themolar ratios of Al/Znwere in good agreementwith thenom-inal compositions, within the experimental error. However, the contentof Zn, Al and O in every sample under different post-calcining atmo-spheres was entirely different. Samples B and C owned lower O contentand higher Zn content than sample A, with a little higher content of Al atthe same time. So resulted in the defects of oxygen vacancies, zinc inter-val and aluminum dopant, in accordance with the above resultsconcerning electrical properties.

3.1.5. DLS analysisThe particle size distribution was analyzed using a DLS analyzer, as

shown in Fig. 5. The average size of the particles was 458.6 nm in air,220.2 nm in argon atmosphere, and 105.7 nm in the reducing atmo-sphere. The results showed that the low oxygen environment coulddecrease the particle size. It may be due to the reason that the AZO sur-face activity is larger in oxygen-rich atmospheres, causing the particlesto still be growing, so that sample B and C particlesweremeasured to besmaller than those of sample A. The particle size of sample C is smallerthan that of sample B due to the fact that the reducing atmosphere(CO) could consume the oxygen atoms diffusing out from the ZnO,promoting the reaction of Eq. (5) quickly and allowing more Al3+ toeffectively substitute Zn2+.

Table 2The quantitative analysis of compositional elements present for samples.

Samples Mole percentage of the elements (%)

O Al Zn

A 38.81 1.09 60.00B 28.42 1.17 70.41C 15.78 1.29 82.93

Fig. 7. The corresponding kinetic analysis of sample A photocatalysis associated with afirst-order reaction for MO under light irradiation.

Fig. 5. Size distribution of AZO powders prepared at different post-calcining atmospheres:a) in air; b) in argon; c) in reducing atmosphere (CO).

364 P. Zhang et al. / Powder Technology 253 (2014) 360–367

3.2. Photocatalytic analysis

3.2.1. Photocatalytic kineticsAs a metal-doped ZnO photocatalyst, AZO has been used for photo-

catalytic degradation of organic pollutants in aqueous solution. Inthe present study, methyl orange (MO) dye is selected as model com-pound to evaluate the photocatalytic activity of AZO conductive pow-der. Fig. 6 (a) and (b) represents the variation of MO absorbance indark and irradiated conditions with the photocatalyst as AZO post-

Fig. 6. The time-dependent absorption spectra of MO solution in the presence of samp

calcining under air. Fig. 6 (a) shows that the adsorption–desorptionequilibrium was attained within 60 min in dark, therefore, prior to thephotocatalysis, theMO suspension should be kept 1 h in dark to reduceerror. Fig. 6 (b) represents theMO absorbance variation under the sam-ple A photocatalyst in the irradiation.MO gives rise to a large absorptionpeak at 466 nm. As the irradiation time increases, the peak intensitydecreases. According to the Lambert–Beer law:

A ¼ Kbc ð6Þ

where b is the absorption layer thickness; c is the concentration oflight-absorbing substance; and K is the extinction coefficient. In dilutesolution, the absorbance is proportional to the concentration, so theconcentration of MO dye in aqueous solution is reducing. We alsoproved that the AZO degrades MO dye that is a pseudo-first order reac-tion under photocatalysis for low dye concentrations. According to themodel of Langmuir–Hinshelwood [32]:

r ¼ −dc=dt ¼ k·K· c= 1þ K·cð Þ ð7Þ

where r is the reaction rate, c is the MO dye concentration; and k andK are the constants for reaction rate and apparent adsorption,respectively.

le A at different conditions: a) without light irradiation; b) under light irradiation.

Fig. 8. Photocatalytic activities of AZO samples for photodegradation of MO under light irradiation (a) and the fitting curves of ln (A0/At) vs. irradiation time (b).

Fig. 9. The degradation rate of MO in the presence of different catalysts after the lightirradiation 150 min.

365P. Zhang et al. / Powder Technology 253 (2014) 360–367

r ¼ −dc=dt ¼ ·K· c K·c ≦ 1� � ð8Þ

∫−dc=c ¼ ∫k·K·dt

lnc0=c ¼ k·K·t ð9Þ

lnc0=c ¼ k′·t ð10Þ

c0=c ¼ A0=A ð11Þ

lnA0=A ¼ k′·t ð12Þ

where c0 and c are the concentration of dye in the aqueous solution attime t = 0 and t respectively and k′ = k·K is the pseudo-first orderrate constant, A0 and A are the absorbance of MO in the dilute aqueoussolution at time t = 0 and t respectively. The rate constant of sampleA photocatalysis obtained from the linear fitting curve of equation ln(A0/At) vs irradiation time t is plotted in Fig. 7. The ln (A0/At) is a functionabout irradiation time t, and the linear fitting degree R2 = 0.9879 ishigh, suggesting that the photocatalytic reaction is close to thepseudo-first order reaction.

3.2.2. Photocatalytic activity versus electrical conductivityFig. 8 shows the variation of MO absorbance as a function in light ir-

radiation time under the different photocatalysts we have successfullyprepared with different electrical conductivities under different post-calcining atmospheres. Firstly, we can see that the Al-doped ZnO ex-hibits more activity than pure ZnO, which may be ascribed to the Al3+

entering into the ZnO lattice to substitute Zn2+ site to make the particlesize become small and carrier concentration increase. Secondly, thephotocatalytic activities of AZO with different post-calcining atmo-spheres are different. Compared to samples A and B, sample C, post-calcining under reducing atmosphere (CO), has higher catalytic activity.Because under reducing atmosphere, the generated oxygen of Eq. (5) isconsumed, prompted the larger generation of oxygen vacancy, zinc in-terval, andmade the Al3+ effectively substitute Zn2+ sites, so the carrierconcentration increases tomake the resistivity decreases [33,34]. As thecarrier concentration increases, the intermediate energy gap betweenvalence band and conduction band becomes more active. Therefore,during light irradiation, the free electrons could be easily decomposedfrom the AZO, resulting in a positively-charged hole appearing andallowing the better the electrical conductivity, the more free electronsand holes the better. The hole (h+) can promote H2O to generate H+

and •OH, electron can activate O2 to be superoxide anion radicals •O2−

in air, and also react with H2O to form H2O2, which could further yieldreactive •OH. The generated radicals can cause MO dye to decompose.

The major initial steps in the mechanism under light irradiation timeare summarized by the following equations [35,36]:

AZOþ hv→AZO hþ þ e−� �

ð13Þ

hþ þ H2O→·OHþHþ ð14Þ

e− þ O2→·O2− ð15Þ

·O2− þH2O→H2O2→2·OH ð16Þ

MO dyeþ ·O2−or ·OH→the decomposed products: ð17Þ

The degradation rate of MO after the light irradiation of 150 min isshown in Fig. 9. Degradation using AZO post-calcining under a reducingatmosphere (CO) reached 95.9%. So it is concluded that the enhance-ment in photocatalytic activity is attributed to the excellent electricalconductivity of AZO.

3.2.3. The influence of some parameters

3.2.3.1. MO initial concentration. In general, the higher the initial concen-trations ofMOdyewith treatment, the longer the timeof removing con-taminants, and MO degradation rate is lower. From Fig. 10, we couldnotice that the MO dye degradation rate was enhanced initially with

Fig. 10. The effect of MO initial concentration on photocatalytic degradation in thepresence of zinc oxide.

366 P. Zhang et al. / Powder Technology 253 (2014) 360–367

the decrease of initial concentration of MO within irradiation time of130 min, but after 130 min, the degradation was quickly improvedwhen initial concentration was about 10−5 mol/L. Higher MO initialconcentration is disadvantageous to the degradation, it might be thata large amount of intermediates and MO dye was able to absorb thelight, so the light intensity weakened and inhibited the photocatalyticreaction. Furthermore, more MO was adsorbed on the surface ofphotocatalyst, which led to the decrease of the generated active radical[37]. However, when the initial concentration ofMOdye reached below10−6 mol/L, MO was quickly decomposed, but after 60 min, the degra-dation rate tended to a constant. The lowMO initial concentration led tothe photocatalytic reaction becoming slow, this might be because thereaction rate (r) was directly related to initial concentration of MO,when MO is consumed, the photocatalytic reaction became slow. Theresults showed that the initial concentration of MO also had a powerfulinfluence on the degradation rate of MO.

3.2.3.2. Bubbling air. According to Eq. (15), introducing oxygen insuspension is considered advantageous to the degradation of MO sinceit increases the formation rate of superoxide anion radicals (•O2

−) byacting as an electron donor and allows the gain of the active grouphydroxyl radicals (•OH) from the superoxide anion radicals. Eq. (17)shows that the active groups promoted the MO dye decomposition.Therefore, the effect of oxygen was further investigated. In order to

Fig. 11. The effect of bubbling air to the reaction vessel on photocatalytic degradation inthe presence of zinc oxide.

reduce the cost, bubbling air, instead of bubbling oxygen, was used inour work [38], as shown in Fig. 11. The pseudo-first order rate constantk′ almost remains equal within 30 min, because at first the oxygen onthe solution interface and the dissolved oxygen in the solution are suffi-cient to promote the generation of the superoxide anion radicals. How-ever, it is interesting to find thatwhen theMOsolutionwas irradiated inthe presence of bubbling air, the reaction rate constant k′ increased rap-idly, and the degradation rate of MO reached 25.3%, a 12.9% improve-ment over that without bubbling air. Thus, it can be demonstratedthat bubbling of air is able to considerably promote the efficiency ofMO photocatalytic degradation.

4. Conclusions

The Al-doped ZnO conductive powders were synthesized bycoprecipitation under different post-calcining atmospheres. The obtainedpowders act as photocatalysts to degrade MO. The following conclusionscan be drawn from our experiments and analyses.

(1) Compared to ZnO, the Al-doped ZnO improves the electricalconductivity and conforms to the hexagonal wurtzite structureof ZnO. In the present investigation Al-doped ZnO with post-calcining under reducing atmosphere (CO) is found to showbetter electrical conductivity.

(2) Photocatalytic studies reveal that AZO photocatalysts showbetter photocatalytic activity than ZnO, and the prepared AZOwith excellent electrical conductivity shows better photocatalyticactivity in degradation of MO dye and we also find AZO degradesMO dye is a pseudo-first order reaction under photocatalysis.

(3) Furthermore, the photocatalysis results also show that the appro-priate initial concentration of MO and the presence of bubblingair can significantly improve MO degradation efficiency. Thepresence of bubbling air in photocatalytic reactions can promotephotocatalytic degradation efficiencies and the higher or lowerMO initial concentration is disadvantaged to the degradation.These results demonstrate the importance of choosing the opti-mum photodegradation parameters to obtain high degradationefficiency, which is essential for practical application of theprocess.

Acknowledgments

The project was supported by the National Natural Science Founda-tion of China (NSFC, Nos. 21246002 and 20876100), the National BasicResearch Programof China (973 Program,No. 2009CB219904), NationalPost-doctoral Science Foundation (No. 20090451176), the Key Lab. ofEnvironmental Materials and Environmental Engineering of YangzhouUniversity (No. K11025), the Technology Innovation Foundation ofMOST (No. 11C26223204581), the Natural Science Foundation of Jiang-su Prov. (No. BK2011328), the 333 Talent project (2013) of Jiangsu Prov.,the Project Academic ProgramDevelopment of JiangsuHigher EducationInstitutions (PAPD) and the Minjiang Scholarship of Fujian Prov.

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