Journal of Materials Chemistry A...ZnO. The photocatalytic activity can be greatly inﬂuenced by the relative concentration ratio of surface defects to bulk defects, crystallization

Journal ofMaterials Chemistry A

PAPER

Defect-related p

Tsinghua University, Department of Chemis

tsinghua.edu.cn

† Electronic supplementary informationsurface area and Barrett–Joyner–Halendaand PL spectra of the as-prepared ZnO sa

Cite this: J. Mater. Chem. A, 2014, 2,15377

Received 28th May 2014Accepted 8th July 2014

DOI: 10.1039/c4ta02678k

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

hotoluminescence andphotocatalytic properties of porous ZnOnanosheets†

Di Liu, Yanhui Lv, Mo Zhang, Yanfang Liu, Yanyan Zhu, Ruilong Zong and Yongfa Zhu*

A porous ZnO nanosheet with a near-rectangular morphology has been successfully prepared through a

simple solvothermal-annealing method using Zn5(OH)6(CO3)2 as a pore-directing agent. Moreover, the

features of ZnO can be easily tuned by changing the annealing temperature. The evolution of defects

along with the increase of annealing temperature has been revealed as follows: the content of surface

oxygen vacancy of the as-prepared samples first increases and then decreases, however, the content of

impurities decreases gradually. A clear relationship between the defects and photoluminescence/

photocatalytic characteristics of ZnO is observed. The defect-related emission mechanism of the visible

photoluminescence (PL) for the as-prepared ZnO samples has been proposed. In addition, the samples

also show good activities for photo-degradation of phenol under UV light irradiation. ZnO-500 �C(annealed at 500 �C) exhibits the best photocatalytic activity, which is superior to that of commercialZnO. The photocatalytic activity can be greatly influenced by the relative concentration ratio of surface

defects to bulk defects, crystallization performance and specific surface area.

Introduction

ZnO has attracted much attention in the bottom-up engineeringof nanostructures due to its plasticity in morphology and itsunique electronic properties with a wide band-gap of 3.37 eVand a large exciton binding energy of 60 meV. Especially, ZnObears many intriguing properties such as high photosensitivity,thermal stability, low cost and biological inertness, which evokegreat interest and desire for exploration in photocatalyticapplications. More recently, two-dimensional (2D) porous ZnOnanosheets have attracted great research interest because oftheir signicantly enhanced properties in photocatalysis,photoluminescence and gas sensing applications. Porous 2Dnanomaterials with nanoscale thickness are promising candi-dates because their special structures can usually provide alarge surface-to-volume ratio that can improve the photoelectricperformance of the semiconductors. Hierarchically porous ZnOspherical nanoparticles which were prepared through a self-assembly pathway had excellent photocatalytic properties.1

Zhou et al.2 reported the gas sensing and photoluminescenceproperties of porous ZnO nanosheets mediated by microwavesand surfactants, respectively. It is also reported that differentmetal oxides could be massively obtained by thermal-

try, Beijing, China. E-mail: zhuyf@mail.

(ESI) available: XRD patterns, BET(BJH) pore size distribution, XPS, DRSmples. See DOI: 10.1039/c4ta02678k

hemistry 2014

decomposition of the corresponding layered metal hydroxidesalt (LMH) compounds.3 Zinc hydroxide salts could be used asprecursors to prepare nanostructured ZnO.4 In general, theseabove-mentioned methods usually involved template/pore-directing reagents and/or a complicated synthesis process witha unique precursor. Unfortunately, they may suffer fromcontamination, which may have a signicant effect on thestructure and photocatalytic performance of the product, due tothe uncompleted removal of the additives either by chemicaletching or thermal treatment. At the same time, the productsusing the above methods usually have irregular morphologiesthough possessing porous and nano-sized structures. Herein, atemplate-, surfactant- and polymer-free low-temperatureaqueous solution route to fabricate highly porous ZnO nano-sheets with a relatively regular morphology has been developed.Investigations of the annealing effects on the crystallinity, size,morphology, specic surface area, defects and surface proper-ties had also been carried out.

So far, various visible emissions in ZnO nanocrystals such asviolet, blue, green, yellow, orange, and red emissions have beenreported. Although the emissions in the visible regions areuniversally considered to be associated with the intrinsic orextrinsic defects in ZnO, extensive controversies on the cleardefect centers have existed for more than two decades.5 Theviolet emission at �421 nm may be attributed to the defect ofthe interstitial zinc in ZnO based on a defect-level calculation.6

Another viewpoint which ascribed the emission at �424 nm tothe transition between shallow donors (oxygen vacancy: VO) tothe valence band (VB) was also proposed.7 The peak at 460 nm

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Journal of Materials Chemistry A Paper

was related to the Zn vacancy.8 However, a vague explanation(oxygen and zinc vacancies or interstitials and their complexes)was given for the �466 nm emission peak.9 The 490 nm emis-sion is controversial; some studies reported that the transitionshould be attributed to the existence of oxygen defects on thesurface of the photocatalyst.10 It is reasonable that there existsinterstitial zinc in ZnO lattices, even as the dominant intrinsicdefect, since the as-prepared nanoparticles are very quicklyformed.11 Previously, Bylander experimentally determined thatthe Zn interstitial (Zni) level is 0.22 eV below the conductiveband-edge.12 Recently, Zeng et al. revealed that the blue emis-sion was strongly related to the interstitial zinc in ZnO, sup-ported by the coincidence of the emission peak energy with thetheoretical prediction, visible emissions with excitationdependence and the correlation between the anneal-induced PLevolution and the change of EPR spectra.13 Visible (green andyellow) emission in ZnO has also been attributed to the pres-ence of Zn(OH)2 on the surface,14 however, annealing at >150 �Ccan result in removal of O–H groups.15 Though the origin of thegreen emission is generally referred to the deep level or trappedstate emission, there is no universally accepted mechanism.There are a few hypotheses to explain the origin of the greenemission. One commonly cited viewpoint is that the greenemission originates due to the radiative recombination of aphoto-generated hole with an electron occupying the oxygenvacancy.16 The other interpretation argues that grain boundary-induced depletion regions lead to the formation of a deeplytrapped, doubly charged oxygen vacancy (VþþO ) state whichundergoes radiative recombination with a CB electron to yieldPL of energy approximately 2.20 eV (564 nm).17 Complex defectsinvolving a transition from the interstitial zinc to a deepacceptor level such as an oxygen vacancy is another perspectivereported on the green emission.18 As for the photoluminescencewith lower energies, it has been suggested that the yellowemission is commonly attributed to interstitial oxygen (Oi).19

The orange-red emission centred at �640–650 nm is alsocommonly attributed to the presence of excess oxygen in thesamples.20 In general, the origins of these defect-related emis-sions of ZnO are still highly controversial, thus an in depthinvestigation on the emission mechanism of the visible PL inZnO is still necessary. At the same time, it can also be of greatimportance to deepen the understanding of defects in ZnO andachieve a good controllability of optical properties of ZnO. Inthis study, with the support of the experimental conrmationon the defect evolution, EPR spectra, PL measurements andtaking samples fabricated under an inert or reducing atmo-sphere as contrasts, a reasonable interpretation to the defect-related emission mechanism of the visible PL for the as-prepared ZnO samples has been carried out. In addition, pho-tocatalytic properties of the as-prepared ZnO samples areinvestigated in detail.

Experimental sectionMaterials preparation

All chemicals used were analytical-grade reagents withoutfurther purication. In a typical reaction, Zn(NO3)2$6H2O

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(1.7847 g) and NH4HCO3 (0.9487 g) in a molar ratio of 1 : 2 weredissolved separately in 90 mL distilled water and stirred atroom temperature for 10 min to obtain clear solutions. Later,the NH4HCO3 solution was quickly injected into theZn(NO3)2$6H2O solution with stirring. The mixture becameturbid immediately aer adding the Zn(NO3)2$6H2O solution.The turbid solution was kept at room temperature for 30 minwith stirring. The resulting white precipitate was treated in anultrasonic water bath for 10 min, collected by centrifugation,and washed with distilled water and ethanol. For the sol-vothermal step, Zn(NO3)2$6H2O (3.6140 g), hexamethylenetet-ramine (1.7033 g) and the above white precipitate were mixedwith a solvent composed of distilled water (54 mL) and meth-anol (108 mL), stirred at room temperature for 1 h. Next, themixture was pretreated in an ultrasonic water bath for 30 minand then was hydrothermally treated at 150 �C for 24 h in aTeon-lined autoclave of 230 mL capacity. When the reactionswere completed, the autoclave was cooled to room temperaturenaturally. The white precipitates were collected by centrifuga-tion, washed with deionized water and ethanol several times,and nally dried in air at 80 �C for 10 h. For the thermaltreatment step, the above collected products aer the sol-vothermal step were used as starting materials and annealed ata given temperature of 300, 400, 500, 600 and 700 �C for 2 h inair with the heating rate of 5 �C min�1, and the products weredenoted as ZnO-300 �C, ZnO-400 �C, ZnO-500 �C, ZnO-600 �Cand ZnO-700 �C, respectively. Additional experiments wereconducted to examine the performance of samples annealedunder different atmospheres in a tube furnace. Samples fabri-cated in the same way with ZnO-500 �C except for the atmo-sphere were labeled as ZnO-500 �C-N2. ZnO-700 �C-R wasobtained by annealing the sample of ZnO-700 �C at 400 �C for 15min in a reducing environment (10% hydrogen–90% argonmixture) with the heating rate of 10 �C min�1. The operation ofannealing under different atmospheres was carried out in atube furnace with the airow velocity set at 60 mL min�1.

Characterization

X-ray diffraction (XRD) patterns of the powders were recorded atroom temperature by using a Bruker D8 Advance X-ray diffrac-tometer. The sizes and morphologies of the samples werecharacterized with the aid of a LEO-1530 eld Scanning ElectronMicroscope (SEM) and a HITACHI HT7700 Transmission Elec-tron Microscope (TEM). High-resolution transmission electronmicroscopy (HRTEM) images were obtained by using a JEOLJEM-2010F eld emission transmission electron microscopewith an accelerating voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area measurements were performed byusing a micromeritics (ASAP 2010V5.02H) surface area analyzer.The nitrogen adsorption and desorption isotherms weremeasured at 77 K aer degassing the samples on a Sorptomatic1900 Carlo Erba Instrument. Zeta-potential measurements wereperformed using a HORIBA SZ-100 series analyzer. The DiffuseReectance Spectra (DRS) of the samples were recorded in therange of 200 to 800 nm using a Hitachi U-3010 spectrometerequipped with an integrated sphere and BaSO4 was used as a

This journal is © The Royal Society of Chemistry 2014

Paper Journal of Materials Chemistry A

reference. The photocurrent was measured on an electro-chemical system (CHI-660B, China). 254 nm UV light wasobtained from an 11 W germicidal lamp (Institute for ElectricLight Sources, Beijing). A standard three-electrode cell with theITO/ZnO sample as the working electrode, a platinum wire asthe counter electrode, and a standard calomel electrode (SCE)as the reference electrode was used in photoelectric studies. 0.1M Na2SO4 was used as the electrolyte solution. Potentials weregiven with reference to the SCE. The photoelectric responses ofthe photocatalysts as light on and off were measured at 0.0 V.Raman spectra were obtained by using a HORIBA HR800confocal microscope Raman spectrometer employing an Ar-ionlaser (514.5 nm). A �50 Lwd, Olympus objective lens was usedto focus the laser on the samples. All spectra were calibratedwith respect to silicon wafer at 520.7 cm�1. Fourier TransformInfrared Spectra (FT-IR) were recorded on a Bruker VERTEX 700spectrometer in the frequency range of 4000–600 cm�1 with aresolution of 4 cm�1. The EPR measurement of the photo-catalyst powder was carried out using an Endor spectrometer(JEOL ES-ED3X) at room temperature. The g factor was obtainedby taking the signal of manganese. The EPR spectrometer wascoupled to a computer for data acquisition and instrumentcontrol. Magnetic parameters of the radicals detected wereobtained from direct measurements of the magnetic eld andmicrowave frequency. The PL spectra of the samples wereinvestigated using the Perkin-Elmer LS55 spectrophotometerequipped with a xenon (Xe) lamp with different excitationwavelengths. X-ray Photoelectron Spectroscopy (XPS) wasmeasured using a PHI 5300 ESCA system. The beam voltage was3.0 kV, and the energy of the Ar ion beam was 1.0 keV. Thebinding energies were normalized to the signal for adventitiouscarbon at 284.8 eV.

Photocatalytic experiments

The photocatalytic performance of the as-prepared samples wasevaluated by photocatalytic degradation of phenol under UVlight irradiation. The samples (25 mg) were dispersed in the

Fig. 1 FT-IR spectra of the as-prepared ZnO samples in the wavenumbenlarged FT-IR spectra of (a)).


50 mL phenol aqueous solution (20 ppm). Before the lightirradiation, the suspensions were rst ultrasonic dispersed inthe dark for 15 min and then magnetically stirred for 30 min toreach the absorption–desorption equilibrium. Under ambientconditions and stirring, the mixed suspensions were exposed toUV irradiation produced by a 100W high pressure Hg lamp withthe main wave crest at 365 nm. At certain time intervals, 2 mL ofthe mixed suspension were extracted and centrifuged to removethe photocatalyst. The degradation process was monitored byHPLC analysis with a UV detector at 270 nm. The mobile phasewas methanol and water (60% : 40%), and the ow rate was1 mL min�1. For comparison purposes, the ZnO powderpurchased commercially was also used for the photo-decom-position experiments. To investigate the active species gener-ated in the photocatalytic degradation process, the experimentsof active species (hydroxyl radical (cOH), hole (h+) and cO2�)capture were carried out by using tert-butylalcohol (t-BuOH) andethylenediamine tetraacetic acid disodium salt (EDTA-2Na) andpurging N2 under UV light irradiation, respectively.

Results and discussionFormation of porous ZnO nanosheets

In this work, the thermal treatment of the precursor was set inthe range of 300–700 �C. XRD patterns of the as-preparedsamples are shown in Fig. S1.† The pure hydrozincite andhexagonal ZnO are also listed for comparison purposes. TheXRD peaks of the precursor can be indexed to a mixture ofhydrozincite (Zn5(OH)6(CO3)2) (JCPDS no. 72-1100) and hexag-onal ZnO (JCPDS no. 70-2551). However, the products aercalcination can be readily indexed to pure hexagonal ZnO. Noother peaks are observed in samples aer calcination, implyingthe successful transformation of the precursor into ZnO. Withan increase in the calcination temperature, the peak intensitygradually increases and the intensity ratio R of I(100) to I(002)becomes stronger, indicating that the crystallinity can beimproved by increasing the calcination temperature and thegrowth orientation of the ZnO samples along the [100] direction

er ranges of (a) 4000–600 cm�1 and (b) 4000–750 cm�1 (the partial

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Fig. 3 SEM images of (a) ZnO-400 �C, (b) ZnO-500 �C, (c) ZnO-600 �C and (d) ZnO-700 �C.


strengthens. In addition, the peaks gradually narrow down withincrease of the calcination temperature, corresponding to theincrease of the crystallite size of the ZnO samples.

FT-IR spectra of the ZnO samples are shown in Fig. 1. Thestrong peaks at 1500, 1388, 1047, 836, and 708 cm�1 correspondto the vibration and bending modes of CO3

2�.21 The obscuredshoulder peak at 1630 cm�1 arises from the bending modedH2O of the water molecule. The presence of the “Zn–OH” bondgives rise to absorption at around 900–880 cm�1.22 In compar-ison with the precursor, it can be found that the peaksbelonging to the CO3

2� group decrease obviously aer thecalcination treatment, and almost completely disappear for theZnO-700 �C sample, indicating the full transformation of theprecursor into ZnO. The broad band at 3428 cm�1 related tohydroxyl groups and water molecules was also greatly weakenedaer calcination. Based on the FT-IR results, it can beconcluded that with increase of the calcination temperature,the residual impurity in products due to the incomplete pyrol-ysis of the precursor decreases gradually.

As shown in Fig. 2, ZnO-300 �C has a sheet-like structuresimilar to that of the precursor, indicating the effectiveness ofthis synthesis route for the preparation of porous ZnO nano-sheets without collapse of the morphology. Thermal treatmentonly induces Zn5(OH)6(CO3)2 among the precursors to trans-form into ZnO and results in the formation of mesoporousstructures in ZnO nanosheets. Thereby, Zn5(OH)6(CO3)2 can beseen as a pore-directing agent which could release gas, generateholes and decrease the thickness of the sheet during thepyrolysis process. The thickness of one nanosheet in ZnO-300 �C is about 10–30 nm (Fig. 2b). Notably, this sheet-likenanostructured product is quite distinct from previouslyreported LMHs, which adopt 2D sheets without any deniteshape, owing to the turbostratic behavior.23 The nanosheet inthis work presents a relatively regular rectangular morphology.It can be seen from Fig. 2c that the length and width of therectangular nanosheet in ZnO-300 �C are several hundrednanometers and several dozen nanometers, respectively. Thesizes of the mesopores are distributed in the range of 2–50 nm.However, it can be seen from Fig. 3a that relatively regularnanosheets of the precursor have evolved into irregular pieceswhen the calcination temperature is increased to 400 �C. Forthe sample fabricated at a further increased calcination

Fig. 2 SEM image (a) of the precursor, SEM image (b) and TEM image (c

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temperature, that is ZnO-500 �C, both long strip particles andsome pieces appear. Almost only particles can be seen insamples by thermal treatment above 600 �C. Moreover, it isobvious that the crystallite size of the ZnO particles graduallyincreased with increase of the calcination temperature, beingconsistent with the XRD results. In consideration of themorphologies of the precursor and ZnO-300 �C, it can beconcluded that the precursor would become broken and re-construct surfaces during the high-temperature annealingprocess, eventually evolving into discrete spherical particles soas to reduce the surface energy and increase the stability.Thereby, during this process, rearrangement of atoms at thesurface of the sample is proposed to produce some defects onthe surface layers of ZnO.

Considering the details of morphology and structure of theas-prepared ZnO samples, HRTEM is employed. As shown inFig. 4a, the corresponding HRTEM image of ZnO-300 �C haswell-resolved lattice fringes, revealing the single-crystallinenature of the porous nanosheets. It was taken with an electronbeam perpendicular to the extension plane of the sheet. Thelattice fringes have a clear separation of 0.514, 0.277 and 0.246nm corresponding to the d-spacing between {0002} planes,

) of ZnO-300 �C.



{10�11} planes and {10�10} planes in the ZnO wurtzite structure,respectively. The image inserted in Fig. 4a is the Fast FourierTransform (FFT) pattern derived from the HRTEM image. Thediffraction spots in this image construct rectangular arrays andthe pattern can be indexed to the [100] zone of a hexagonalstructure. The FFT analysis indicates a single-crystal structurewith a [100] top/bottom surface. The HRTEM and FFT patternobservations suggest that the preferred growth directions of thenanosheets are all parallel to the [0001] crystallographic direc-tion (c-axis). At the same time, the growth along the [01�10]direction perpendicular to the c-axis is also permitted, that iswhy ZnO sheets form. However, from the HRTEM observation ofZnO-500 �C shown in Fig. 4b(1), less clear lattice fringes can beseen and some disordered area is present across the contactareas shared by adjacent particles. It can be speculated that theabove bottleneck area can be derived from the fragmentation ofthe sheet-like structure, thus leading to some defects in ZnO-500 �C. Fig. 4c shows that the clear lattice spacing of ZnO-700 �Cis about 0.281 nm between adjacent lattice planes which corre-sponds to the d-spacing of the {10�11} planes. Almost no disor-dered areas can be seen from particles of ZnO-700 �C, indicatinga decreased content of defects compared to ZnO-500 �C.

To get further insight into the porous structure and pore sizedistribution of the as-obtained samples, the BET measurementwas carried out. As shown in Fig. S2,† ZnO-300 �C has thehighest specic surface area (39.18 m2 g�1) and a relativelynarrow pore size distribution. As shown in the inset of Fig. S2,†it is found that the size of mesopores is not uniform, which tswell with the TEM results, but mainly distributed in the range of5–30 nm. It is found from Fig. S3(a)† that the product obtainedat a higher calcination temperature has a lower BET specicsurface area. The size of pores increases gradually with the

Fig. 4 HRTEM images of (a) ZnO-300 �C, (b) ZnO-500 �C, (c) ZnO-700 �C.


increase of annealing temperature from observations ofFig. S3(b).† The pore structures become very rare when theannealing temperature is above 500 �C, only a small quantity ofpores attributed to the inter-particle space exists. It is worthnoting that ZnO-700 �C-R has a larger BET specic surface areacompared to ZnO-700 �C. Surface degradation is reported tooccur aer treatment via the reducing gas,24 and this mightaccount for the higher value of the BET specic surface area ofZnO-700 �C-R.

Investigations on surface structure and defects

In order to investigate the surface electric properties of thesamples, the zeta potential measurement has been carried out.The results and test conditions are shown in Table S1.† On thebasis of the FT-IR results, such negatively charged groups con-taining the hydroxyl group and CO3

2� would marginally remainon the surface of ZnO-300 �C because of the incomplete pyrolysisof the precursor, leading to amore negative zeta potential. Whenthe sample is annealed at 400 �C, an increased zeta potentialrelative to ZnO-300 �C is observed. This can be ascribed to thedecrease of residues (hydroxyl group and CO3

2�) and the expo-sure of surface terminated-Zn2+. Interestingly, when theannealing temperature is increased further, the zeta potentialdeclines gradually. It seems that the gradually reduced specicsurface area can be responsible for this phenomenon. It is worthnoting that the two samples annealed at 500 �C which aredifferent only in the calcination atmosphere exhibit differentzeta potentials. Compared to ZnO-500 �C,more oxygen vacanciesare supposed to be created on the surface of ZnO-500 �C-N2because of the lack of supplement of oxygen from the environ-ment. Thus, the larger proportion of surface terminated-Zn2+

may result in a more positive zeta potential value (44.6 mV). Asfor ZnO-700 �C-R, it is concluded that when treated with thereducing gas, oxygen atoms on the surface are supposed to bestripped out and react with H2. This speculation could be veri-ed by a report which reveals the existence of hydrogen diffusinginto and reacting with the material.24 In addition, it can also bespeculated that the divalent zinc may be slightly restored aertreatment under the reducing gas. These two factors wouldaccount for the much more negative zeta potential value(15.5 mV) for ZnO-700 �C-R relative to ZnO-700 �C (35.8 mV).

The surface elemental compositions of the as-preparedsamples of ZnO-300 �C, ZnO-500 �C and ZnO-700 �C werestudied by XPS analysis, as shown in Fig. S4.† The carbon peakis attributed to the residual carbon from the sample andadventitious hydrocarbon from XPS instrument itself. TheZn2p3/2 peak at 1022 eV and the O1s peak at 531 eV can beassigned to Zn and O elements in ZnO, respectively. The high-resolution XPS spectra of O1s are illustrated in Fig. S4(b)–(d).†The specic oxygen-related chemical bond species versus BE arelisted in Table S2.† The O1s spectra in all cases could be ttedwith two peaks, labeled as O1 (lower energy) and O2 (higherenergy). The O1 peak, located at 529–530.7 eV, can be attributedto lattice oxygen (OL) in a wurtzite ZnO structure. The O2 peak,at 531–532 eV, is typically assigned to loosely bound oxygen (OH)on the surface, such as OH groups or O2� ions in oxygen-

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decient regions.25 In addition, the peak at 532.9 eV can beattributed to the chemisorbed oxygen (Oa).26 The OL/Zn ratiocan reect the crystallinity. For a perfect ZnO crystal, the OL/Znratio should be equal to 1. As shown in Table S3,† the calculatedOL/Zn ratios are 0.60, 0.78 and 0.98 for ZnO-300 �C, ZnO-500 �Cand ZnO-700 �C respectively, indicating the increase in crys-tallinity with increasing the annealing temperature. However,OH/O1s rst increases and then decreases with increase of theannealing temperature, which may reect the variation in thecontent of oxygen deciency of these three samples.

Fig. 5 shows the Raman spectrum of each sample. Accordingto group theory analysis, the A1 + E1 + 2E2 modes are Ramanactive.27 The two higher peaks at 99 and 437 cm�1 can beassigned to the ZnO nonpolar optical phonons E2-(high) vibra-tion mode, characteristic of the wurtzite lattice. The muchweaker peak at 380 cm�1 is attributed to the transverse opticalmodes of A1T. The other two weaker and broader peaks at 203and 331 cm�1 can be assigned to the secondary Raman scat-tering arising from zero-boundary phonons 2-TA(M) and2-E2(M), respectively.28 The high intensity of the E2 mode and theweak A1T mode may indicate a good crystalline quality.29 Inaddition, the low band centered at 580 cm�1 is a superpositionof the A1(LO) mode at 574 cm

�1 and the E1(LO) mode at583 cm�1 (ref. 30) and it was reported to be related to the oxygendeciency.29,31 It can be seen from Fig. 5, the intensity of peaks at99 and 437 cm�1 and the value of I99 & 437cm�1/I380cm�1 increaseswith the increase of the annealing temperature, indicating theincreasing crystalline quality. Although the intensity of the peakat 580 cm�1 is too low to be observed for samples annealedunder 600 �C, I99 & 437cm�1/I580cm�1 for ZnO-600 �C and ZnO-700 �C can be clearly identied. The value of I99 & 437cm�1/I580cm�1of ZnO-700 �C is superior to that of ZnO-600 �C, thus implying adecreased content of oxygen deciency for ZnO-700 �C.

The DRS are used to determine the optical properties of theas-prepared samples. As expected, ZnO shows the characteristicspectrum with its fundamental absorption sharp edge rising at�390 nm. It can be seen from Fig. S5† that the sample annealedat a low temperature (300 �C or 400 �C) shows a slight redshi ofthe band edge and a slight absorption in the visible region. Thiscan be ascribed to the existence of impurity originated from theincomplete pyrolysis of the precursor under the low annealingtemperature, which is also conrmed by XRD and FT-IR results.

Fig. 5 Raman spectra of the as-prepared ZnO samples.

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Typically, an EPR spectrum reveals different defects in metaloxide, such as vacancies and interstitials. As reported previ-ously, the surface modication can reduce the number ofsurface vacancies and defects and thus reduce the para-magnetic resonance centers, conrming that the paramagneticresonance centers are on the surfaces of the ZnO nano-materials.32 As shown in Fig. 6a, there exist more than threetypes of signals corresponding to different paramagneticcenters. However, the origin of these EPR features is still highlycontroversial. Based on the previous reports,33 the signals atg1 ¼ 2.0512, g2 ¼ 2.0173, and g3 ¼ 2.0046 can be ascribed tochemisorbed O2c

� on the surface. The O2� ion vacancies, as anintrinsic donor in ZnO,34 are stable and can trap electrons underthe interaction of the external magnetic eld to form para-magnetic centers.32 Then, g-factor close to the free-electronvalue (g¼ 2.0021) is generally attributed to an unpaired electrontrapped in an oxygen vacancy site (g ¼ 1.9948,35 2.0190,32 and2.010636). It is also noteworthy that, because of the presence ofoxygen ion vacancies on the crystallite surfaces, the Zn+ ionwhich has a 4s1 orbital will also appear on the ZnO surface andthus produce a paramagnetic signal.33b In this case, the g-factordue to the Zn+ ion should be equal to the free electron g-factor(g ¼ 2.0021 (ref. 37)). According to the previous reports, thesignal with g ¼ 1.96 appears oen owing to shallow donorimpurities in ZnO,38 though some holding the view that itoriginates from the oxygen vacancies.39 Recently, through anexperiment of illumination, Vladislav et al. concluded that theEPR signal at g¼ 1.96might be due to one electron being weaklybound to ionized impurity atoms.40 Here, we are inclined toattributing the EPR signal with g z 1.99 to VþO based on thefollowing reasons: it is expected that the signal that was due toVþO can be observed upon light excitation, thus the assignmentto a light-sensitive center with gz 1.99 is more likely.41 Anotherargument in favor of the assignment of the g z 1.99 EPR signalto VþO is the tetrahedral symmetry of the paramagnetic center.

42

As discussed above, ZnO-500 �C-N2 is supposed to have a highercontent of oxygen vacancy compared to ZnO-500 �C because ofthe lack of oxygen supplement. As shown in Fig. 6b, the g¼ 1.99signal of ZnO-500 �C-N2 is obviously superior to that of ZnO-500 �C, further verifying that it should originate from VþO.Actually, the g ¼ 1.9568 EPR signal can consist of two lines g ¼1.955 (Hi (ref. 42) or Zni (ref. 43)) and 1.958 whichmay be causedby different defects. With respect to ZnO-700 �C, it can be seenfrom the EPR spectrum of ZnO-700 �C-R (Fig. 6c) that the signalcorresponding to VþO appeared and the g factor at 1.95 exhibitedan obvious increase. As reported before,39c when ZnO nano-crystals are annealed in a reducing atmosphere, more oxygenvacancies and zinc interstitials are created. Therefore, weattributed the g factor at 1.955 to Zni. ZnO materials preparedusing the wet chemical routes generally have intermediatecomplexes or unreacted precursors. In this work, because of theincomplete pyrolysis of the precursor, some impurities ratherthan oxygen vacancies would dominate on the surface of thesample annealed at a low calcination temperature. Then, it canbe seen from ZnO-500 �C which has a certain amount ofimpurities conrmed by the observation of XRD and FT-IR thatthe g factor signal at a high magnetic eld is closer to 1.958,


Fig. 6 EPR spectra measured at room temperature from ZnO samples prepared under different conditions.


suggesting a correlation between g ¼ 1.958 and the impurities.As shown in Fig. 6a, the g factor signal of 1.99 and 1.96 both rstincrease and then decrease with an increase in the annealingtemperature, indicating that the same trend exists in thecontent of surface oxygen vacancies and shallow donor centers.As discussed above, re-construction of the surface during themorphology-evolution process would lead to the existence ofdefects on it. However, when further increasing the tempera-ture, on one hand the crystallization performance of the samplecan be further optimized, on the other hand the defects on thesurface can be compensated to some extent. Hereby, combinedwith the characterizations of SEM (HRTEM), XPS and Ramanspectra, the EPR results further shed light on the evolution ofthe content of surface oxygen vacancies for the as-preparedsamples. It is reasonable to believe that the content of surfaceoxygen vacancy rst increases and then decreases with increaseof the annealing temperature. At the same time, the XRD, FT-IRand DRS results have shown that the content of impuritydecreases gradually.

Defect-related photoluminescence

In this work, the luminescence properties of the ZnO sampleshave been investigated in detail. The use of different excitationwavelengths is useful in examining the luminescence spectra of


materials containing multiple defect levels. A better research onthe nature of defect centers which are involved in the visibleemission can be obtained by treating samples in an inert orreducing atmosphere, as the gas ow would have a signicantinuence on the type and content of defects in ZnO. For thesample annealed in N2, a higher concentration of oxygenvacancies is supposed to be created owing to the deciency ofoxygen supplement as discussed above. When ZnO crystals areannealed in a hydrogen atmosphere, more oxygen vacancies andzinc interstitials are created.39c Moreover, a higher surface area tovolume ratio for the sample annealed under a reducing atmo-sphere might favor a high-level surface and sub-surface oxygenvacancy.44 In this work, as discussed above, with an increase inthe annealing temperature, the content of surface oxygenvacancies rst increases and then decreases, the content ofimpurities decreases gradually and the content of Zni remainsunclear. PL spectra of ZnO-500 �C by using different excitationwavelengths are shown in Fig. 7. It can be seen that the emissionat 490 nm always appeared even when the excitation wavelengthwas increased to 420 nm, implying that the initial level of thisemission exists at least 0.22 eV below the conduction band edge.It is also noteworthy that, with the increase of the excitationwavelength, the position of the peak of green emission from thissame sample showed signicant red-shis. Moreover, the

J. Mater. Chem. A, 2014, 2, 15377–15388 | 15383

Fig. 7 PL spectra of ZnO-500 �C by using different excitationwavelengths.


interval value of the energy of the radiative transition undergradually increased excitation wavelength is close to that ofexcitation energy. Hereby, it can be concluded that the emissionmust be derived frommultiple initial states to the same acceptorlevel which may be related to the oxygen vacancy, rather than thesame donor level to the VB, since the energy of emission shouldbe either the same or absent under the latter condition. As theintrinsic defects in ZnO, these three kinds of defects includingoxygen vacancies, interstitial zinc and impurities would formmultiple defect-related levels in the band gap. Regarding thenative donor, the electronic structure indicates that only the zincinterstitials or the zinc antisite can explain the n-type conduc-tivity of undoped ZnO, whereas the oxygen vacancy with thelowest formation energy should be dominant under n-typeconditions.45 According to the previous experiments and theo-retical investigations, the oxygen vacancy has a deep level in theband gap.46 Lv et al.47 reported that the shallow oxygen-vacancystates would appear above and partly overlapping with the VB ofZnO. Lin et al.6 calculated the energy levels of various defectcenters, such as the vacancy of oxygen and zinc, interstitialoxygen and zinc in ZnO. The energy gap between the conductionband and the level of oxygen vacancies is considered to be 2.28 eVand that from the interstitial zinc level to the VB is 2.9 eV. Theyare very consistent with the energy of the blue emissions in ourstudy. Sesha Vempati et al. also claim that peaks centred at 424and 451 nm can be due to transitions from (extended) interstitialzinc defect states to zinc vacancy states or the VB through abroadly based analysis of the PL output.48 Zeng et al.13b,49 alsoreported that the initial states of violet and blue emissionsshould be correlated with Zni. In this work, it is found that thereexists a good positive correlation between the evolution ofintensity of the visible PL and the signal of z1.96 g factor, asshown in Fig. S6† and Fig. 6a respectively, further conrmingthat the initial levels of the radiative transition should be asso-ciated with the shallow donors (impurity and Zni). Therefore,based on the reports mentioned above and investigation on thelevels of the radiative recombination, together with the strongintensity of emissions at 408–490 nm which indicates a biggertransition probability compared to emissions at 500–600 nm, weattribute the blue emissions to the radiative recombination ofelectrons from the local defect level of shallow donor centers with

15384 | J. Mater. Chem. A, 2014, 2, 15377–15388

holes at the VB, whereas the green emissions are attributed to theradiative transition from the shallow donor levels to the deeplytrapped holes at VþþO . Actually, the number of the trapped holesat VþþO is far less than that at the VB, thus leading to a biggerradiative transition probability of electrons from the shallowdonor centers to the VB, that is to say a stronger intensity of theblue emissions. Moreover, the shallow donor centers, as theinitial states of the visible radiative transition, are supposed tocontain multiple sub-states resulting in multiple emissions andbroad emission peaks. It is noteworthy that the defect-relatedgreen luminescence is generally reported to be connected withthe specic surface area and surface states of the sample,50whichimplies that the green luminescence is associated with defectsnear the surface. A signicant decrease in the visible PL aersurfactant coating is indicative of the fact that the major part ofthe visible emission originates from centers at the nanostructuresurface.51 Our attribution of green emission to the shallowdonors to the deeply trapped holes at VþþO (surface oxygenvacancies) does not work against the view of green luminescenceoriginating fromdefects near the surface. In this work, alongwiththe increase of crystallite size of the ZnO samples, the surface tovolume ratio decreased gradually. It is reasonable to believe thatthe decreased PL intensity for the sample annealed at a highcalcination temperature could also be associated with thedecreased specic surface area and the content of the shallowdonor centers. Fig. 8 shows the PL spectra of the as-preparedsamples and their proposed mechanisms. The intensity of theemission peak centered at�408 nm of the samples annealed at ahigher temperature was particularly higher compared to otheremission peaks. Since the content of impurity decreased a lot forZnO-700 �C due to its signicantly improved crystallizationperformance at a higher annealing temperature, thus theproportion of Zni in the shallow donors increases, indicating thatthe emission at �408 nm might be attributed to the radiativetransition of electrons from the shallow donor level related to Znito the VB. Thus, the higher proportion of Zni in ZnO-600 �C &700 �C may lead to the higher emission intensity at �408 nm.Furthermore, as shown in Fig. 9b, aer annealing in 10%hydrogen (in argon) at 400 �C for 15 min, more oxygen vacanciesand zinc interstitials are created in ZnO-700 �C-R, the intensity ofthe blue emission peak at �408 nm increased as expected, con-rming our speculation above. Fig. 9a also shows that theintensity of PL of ZnO-500 �C is superior to that of ZnO-500 �C-N2.On one hand, this can be partly attributed to the larger BETspecic surface area of ZnO-500 �C. On the other hand, comparedto ZnO-500 �C-N2, ZnO-500 �C exhibits a higher intensity of the gsignal at 1.96 corresponding to the shallow donor centersalthough lower at g¼ 1.99 which is related to the oxygen vacancy.As a result, it is reasonable to conclude that the initial levelsformed by shallow donor centers would have a much highereffect on the PL emission compared to the accepter levels.

Photocatalytic performance

It is widely accepted that the e–h separation efficiency plays adecisive role in the photocatalytic reaction:52 the higher thephotocurrent is, the higher the e–h separation efficiency is.


Fig. 8 PL spectra of the as-prepared ZnO samples with excitation at325 nm and proposed mechanisms for blue and green emissions.


Fig. 10a shows the transient photocurrent response of ITO/ZnOelectrodes in several on–off cycles of UV (l¼ 254 nm) irradia-tion. The photocurrent trend follows the order ZnO-500 �C >ZnO-600 �C > ZnO-700 �C > ZnO-400 �C > ZnO-300 �C. That is tosay, the transient photocurrent of the as-prepared samplesgradually enhanced with an increase in the calcinationtemperature: when the temperature reaches 500 �C, the sampledisplays the highest photocurrent, which is ca. 6 times that ofZnO-300 �C, indicating the most efficient separation of photo-generated electrons and holes; however, further increasing thecalcination temperature, the photocurrent decreases. This

Fig. 9 PL spectra of (a) ZnO-500 �C and ZnO-500 �C-N2, (b) ZnO-700


result is in accordance with the photocatalytic activity of the as-prepared samples which is shown in Fig. 10b and c. It is alsoworth noting that the samples fabricated under differentatmospheres differ in the photocurrent response. It exhibits theorder ZnO-500 �C > ZnO-500 �C-N2, which is also consistent withthe photocatalytic activity. However, when ZnO-700 �C wastreated with the reducing gas, the photocurrent decreasesobviously. Photocatalytic tests indicated that the as-preparedZnO samples showed good activity for photo-degradation ofphenol under UV (365 nm) light irradiation. ZnO-500 �Cexhibits the best photocatalytic activity, which is superior tothat of commercially purchased ZnO. The data are tted usingthe pseudo rst-order kinetic equation ln(C/C0) ¼ �kt. Theapparent rate constant for phenol photodegradation is about0.0166 min�1 for ZnO-500 �C. It is well known that the photo-catalytic activity of a catalyst is related to its microstructure,such as the crystal plane, crystallinity, surface properties, BETspecic surface area and so on. Based on the XRD results,specic crystal planes for the as-prepared samples do not exist.As a result, the difference in the photocatalytic activity of thesesamples is not due to specic crystal planes. Ming Kong and co-workers reported that both surface and bulk defects in TiO2nanocrystals play very important roles in photocatalysis, whiledecreasing the relative concentration ratio of bulk defects tosurface defects signicantly improves the e–h separation effi-ciency and thus enhances the photocatalytic activity.53 Thesurface defects may serve as charge carrier traps as well asadsorption sites where the charge transfers to the adsorbedspecies and prevents the e–h recombination, whereas bulkdefects only act as charge carrier traps where e–h recombine inthe photocatalytic process. As for the surface photochemicalreaction, a photo-catalyst with a large specic surface area canprovidemore active sites for the reaction and then facilitates thediffusion and mass transportation of the pollutants andhydroxyl radicals during the photochemical reaction. Fu et al.found that it is the synergy of the BET surface areas and crys-tallinity that results in a great photocatalytic activity.54 With anincrease in the annealing temperature, the content of surface

�C and ZnO-700 �C-R.

J. Mater. Chem. A, 2014, 2, 15377–15388 | 15385

Fig. 10 (a) Photocurrents of the as-prepared ZnO sample electrodes, under UV-light irradiation (l¼ 254 nm), (b) UV-photocatalytic degradationratio on phenol by the as-prepared ZnO samples, (c) Degradation rate constant k calculated from (b). (d) The plots of the photoinduced carrierstrapping in the system of photodegradation of phenol by ZnO-500 �C.


oxygen vacancy of samples rst increases and then decreases,while the crystalline performance enhances (that is to say thebulk defects decrease) and the specic surface area of samplesgradually decreases. Thereby, it can be proposed that the higherrelative concentration ratio of surface defects to bulk defectstogether with a moderate BET specic surface area can besupposed to be responsible for the enhanced photocatalyticactivity of ZnO-500 �C. Although samples calcined at hightemperatures have much lower BET specic surface areas, theystill show decent activities, indicating that among these factorsinuencing photocatalytic activity, the relative concentrationratio of surface defects to bulk defects rather than BET specicsurface area would govern the photocatalytic activity of the as-prepared samples in this work. It is important to detect mainoxidative species in the photocatalytic process for elucidatingthe photocatalytic mechanism. The main oxidative species inthe photocatalytic process could be detected through trappingexperiments of hydroxyl radicals, holes and cO2� by usingt-BuOH,55 EDTA-2Na,56 and purging N2 (ref. 57) under UV lightirradiation, respectively. Aer UV light irradiation, as shown inFig. 10d, the photocatalytic activity of ZnO-500 �C decreasesslightly by the addition of hydroxyl radical scavengers (t-BuOH)or purging N2 gas (cO

2� scavenger) and decreases largely withthe addition of hole scavengers (EDTA-2Na), indicating that theholes are the main oxidative species for ZnO-500 �C. In order to

15386 | J. Mater. Chem. A, 2014, 2, 15377–15388

nd out the stability of ZnO-500 �C, cyclic degradation experi-ments are conducted over it. The photodegradation rate of4 rounds of continuous recycle photodegradation of phenol(15 ppm) is 98.1%, 83.7%, 72.2%, 64.3%, respectively (as shownin Fig. S7†), indicating that the stability of the as-preparedsample still remains a challenge.

Conclusions

Porous ZnO nanosheets with a regular rectangular morphologyhave been synthesized by using Zn5(OH)6(CO3)2 in the precursoras a pore-directing reagent. Annealing effects on the crystal-linity, size, morphology, specic surface area, surface propertiesand defects had been investigated in detail. The experimentalresults show that with an increase in the annealing tempera-ture, the content of surface oxygen vacancy of the as-preparedsamples rst increases and then decreases, however, thecontent of impurity and the BET surface specic area decreasegradually. Moreover, in depth investigation and interpretationof the defect-related emission mechanism of the visible PL ofthe as-prepared samples had been accomplished, among whichthe blue emissions can be attributed to the radiative recombi-nation of electrons from the local defect level of shallow donorcenters with holes at the VB, whereas the green emissions areattributed to the radiative transition from the shallow donor



levels to the deeply trapped holes at VþþO . In addition, it is foundthat the higher relative concentration ratio of surface defects tobulk defects together with a moderate BET specic surface areaare responsible for the enhanced photocatalytic activity of ZnO-500 �C. This work can be favorable for further understandingthe structure–activity relationship of the photocatalysts.

Acknowledgements

This work was partly supported by the National Basic ResearchProgram of China (973 Program) (2013CB632403), the NationalHigh Technology Research and Development Program of China(2012AA062701), and the National Natural Science Foundationof China (21373121).

Notes and references

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Journal of Materials Chemistry A...ZnO. The photocatalytic activity can be greatly inﬂuenced by the relative concentration ratio of surface defects to bulk defects, crystallization