Optical and Photocatalytic Properties of Spinel ZnCr2O4 NanoparticlesSynthesized by a Hydrothermal Route

Cheng Peng and Lian Gao*,w

State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics,Chinese Academy of Sciences, Shanghai 200050, China

In this paper, a simple hydrothermal route has been developed tosynthesize ZnCr2O4 nanoparticles. Experimental results show thatthe as-prepared ZnCr2O4 nanoparticles have an average particlesize ofo5 nm. The ZnCr2O4 nanoparticles have a direct band gapabout 3.46 eV and exhibit blue emission in the range of 300–430nm, centered at 358 nm when excited at 220 nm. Furthermore, thenanoparticles show apparent photocatalytic activities for the de-gradation of methylene blue under UV light irradiation.

I. Introduction

IN the last 20 years or so, nanoscale materials, the basis of nano-science and nanotechnology, have attracted a great deal of at-

tention. Because of the small size and large surface areas that canbe exposed to the second phase, nanoscale materials have enhancedproperties than ordinary materials. For example, integrating ahigh-quality film of silicon nanoparticles 1 nm in size directlyonto silicon solar cells improves power performance by 60% in theultraviolet range of the spectrum.1 Nanosized TiO2 powders showextraordinarily high photocatalytic activities compared with com-mercial microsized powders.2

Metal oxides have been an extremely active field and have beenwidely studied in recent years. Spinel oxides are a family of com-plex metal oxides. Because spinel oxides are of special importancefor luminescence materials,3 magnetic materials,4,5 sensors,6,7 cat-alysts,8–10 and lithium batteries,11 the impetus to explore the novelnanostructured spinel oxides is given to us. With a normal spinelstructure (AB2O4), ZnCr2O4 has the crystal group Fd3m with lat-tice constants of a58.280 A. Zn21 and Cr31 ions occupy tetra-hedral and octahedral positions, respectively.12 In the literature,several methods have been used to prepare ZnCr2O4materials suchas direct reaction from ZnO and Cr2O3 mixtures,12–14 mechanicalball milling,15 glycothermal reaction,16 and the microemulsionmethod.17 However, some of the above-mentioned methodsgenerate products of poor quality, and some need high temper-ature, high energy consumption, or organic solvents in the syn-thesis process, which make the production expensive. Gassensing,17 humidity sensing,18–20 and magnetic properties13 ofthe products prepared by these methods were also reported. Butto date, to our knowledge, there were few reports on the opticaland photocatalytic properties of ZnCr2O4 nanomaterials.

In this paper, we synthesized ZnCr2O4 nanoparticles via asimple hydrothermal route and investigated the optical andphotocatalytic properties for the first time.

II. Experimental Procedure

(1) Preparation

0.595 g Zn(NO3)2 � 6H2O and 1.6 g Cr(NO3)3 � 9H2O weredissolved in 20 mL distilled water to form a clear aqueoussolution. NaOH solution (50 mL; 4M) was slowly dropped intothe mixed solution vigorously stirred to adjust the pH. Theobtained suspension was transferred into Teflon-lined 100 mLcapacity autoclaves. Hydrothermal reaction was conducted at2201C for 48 h in an oven. After the reaction was completed, theproduct was collected and washed with distilled water threetimes, and then washed with alcohol two times. Finally, greenpowders were obtained after being dried at 601C for 4 h.

(2) Characterization

The powder-phase composition was identified by X-ray diffrac-tion (XRD; Model D/MAX-RB, Rigaku Co., Tokyo, Japan),using CuKa (l5 1.5418 A), over the scan range of 101–801. Themorphology and size of the powders were observed using atransmission electron microscope (TEM; Model JEM-2100F,JEOL, Tokyo, Japan) equipped with an energy-dispersive X-rayspectrometer (EDS). The optical absorption spectrum was re-corded on a Perkin-Elmer, Lambda 950 UV–vis spectrometer,(Perkin-Elmer, Waltham, MA) using BaSO4 as reference sam-ple, in the range of 200–800 nm. The photoluminescence spec-trum was recorded on a Perkin-Elmer LS-55 photoluminescenceinstrument, using a Xe lamp excited at 220 nm with a slit widthof 5 nm at room temperature.

(3) Photocatalytic Experiments

Phenol (C6H6O) and methylene blue (C16H18N3SCl � 3H2O) wereused as supplied. Photocatalytic experiments were carried out byadding the required quantity of ZnCr2O4 powders into a 450 mLPyrex photoreactor (Shanghai Xinhu Glass Ltd., Shanghai, China)containing 400 mL of 1.06 mM phenol or 0.0535 mM methyleneblue solution, respectively. The cooling water in a quartz cylindricaljacket round the lamp was used to keep the reaction temperatureconstant (201C). Before irradiation, the suspensions were magnet-ically stirred in the dark for 30 min to ensure equilibrium of thesolution. The stirred suspensions were then illuminated by meansof a 300 W medium-pressure Hg lamp with a 340 nm cutoff filterfor UV–vis light irradiation. The concentration of the catalyst was0.4 g/L and the pH of the solution was not adjusted. Oxygen wasbubbled into the aqueous suspension at a flow rate of 100 mL/min.At given time intervals, 4 mL aliquots were sampled, diluted with16 mL water, and centrifugated to remove the particles. The con-centrations of resultant solution were analyzed by the Perkin–Elmer Lambda 950 UV–vis spectrometer at 270 and 665 nm forphenol and methylene blue, respectively.

III. Results and Discussion

(1) Phase Formation and Morphology

Figure 1 shows the XRD pattern and EDS of the sample. All thereflection peaks can be readily indexed to cubic phase of spinel

M. Junfeng—contributing editor

*Member, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: liangaoc@online.sh.cn

Manuscript No. 24111. Received December 17, 2007; approved January 17, 2008.

Journal

J. Am. Ceram. Soc., 91 [7] 2388–2390 (2008)

DOI: 10.1111/j.1551-2916.2008.02417.x

r 2008 The American Ceramic Society

2388

ZnCr2O4 (JCPDS No. 73-1962). No other phase was detected.EDS analysis confirms the pure nature of the ZnCr2O4 product:only Zn, Cr, and O elements were detected from the product; theatom ratio of these elements Zn:Cr:O is 13.55:27.05:59.40, closeto the theory ratio 1:2:4. EDS data further prove the formationof pure cubic phase of ZnCr2O4 product.

The morphology and microstructure of the sample are theninvestigated with TEM. Figure 2(a) reveals that the productconsists of a large quantity of nanoparticles. The ZnCr2O4

nanoparticles are irregular in shape and have a nearly uniformparticle size o5 nm. The morphology is also confirmed by theHRTEM image (Fig. 2(b)). Because the particles are very small,the boundaries observed on the HRTEM image are blurred.However, the HRTEM image shows clear lattice fringes,indicating the crystal nature of the particles.

(2) Optical Properties

Figure 3 shows the optical absorption spectrum of the ZnCr2O4

nanoparticles. There are three absorption peaks in the wave-length range. The peaks at about 440 and 600 nm of the spec-trum can be indexed as two typical bands of octahedral Cr31

ions. They have been assigned to d–d transitions described as4A2g-

4T2g and 4A2g-4T1g transitions, respectively.21,22 The

strong peak in the UV region could be attributed to band gapabsorption of the ZnCr2O4 nanoparticles. The optical band gap(Eg) of semiconductors can be calculated on the basis of theoptical absorption spectrum by the equation

ðAhnÞn ¼ Bðhn� EgÞwhere hn is the photo-energy, A is absorbance, and B is a constantrelative to the material when n depends on whether the transitionis direct (n52) or indirect (n51/2).23,24 The optical band gap forthe absorption peak can thus be deducted by extrapolating thelinear portion of the (Ahn)n–hn curve to zero. From the functioncurve of (Ahn)1/2–hn, no linear relation was found, indicating thatthe as-prepared ZnCr2O4 sample is a direct band gap semicon-ductor. The inset of Fig. 3 shows the (Ahn)2–hn curve for thesample. The band gap of the ZnCr2O4 particles is about 3.46 eV.As is known to us, theoretically, as long as the absorbed light en-ergy is larger than the band gap, the semiconductors can be excitedto produce electrons and holes, which may be used as semicon-ductor photocatalysts. Therefore, photocatalytic activities of theZnCr2O4 particles under UV light irradiation could be expected.

Figure 4 depicts the room-temperature photoluminescenceemission spectrum of ZnCr2O4 nanoparticles, which was takenunder excitation with a 220 nm line. The ZnCr2O4 nanoparticlesexhibit a broad emission in the range of 300–430 nm, centered at358 nm (about 3.46 eV in photon energy). In general, emissionspectra can be divided into two broad categories: the near-band-

Fig. 1. X-ray diffraction pattern of as-prepared ZnCr2O4 powders; theinset is energy-dispersive X-ray spectrometer analysis.

Fig. 2. (a) TEM image and (b) HRTEM image of as-prepared ZnCr2O4 powders.

Fig. 3. Optical absorption spectrum of ZnCr2O4 nanoparticles; theinset is the (Ahn)2–hn curve.

July 2008 Communications of the American Ceramic Society 2389

edge emissions and deep-level emissions.25,26 Obviously, thepeak at 358 nm originates from the free exciton emission around3.46 eV from the wide-band-gap ZnCr2O4 nanoparticles. More-over, the photoluminescence in the visible region might beattributed to the deep-level emissions. It is probable that thesuperposition of the emission peaks leads to such a broad peakin the range of 300–430 nm. Thus, a Gaussian peak fit is tried todeconvolute the spectrum into two peaks, which are centered at356 and 465 nm, respectively.

(3) Photocatalytic Activities

The photocatalytic activities of the obtained particles are evaluatedby the decomposition of phenol and methylene blue, as shown inFig. 5. Under UV light irradiation, the ZnCr2O4 particles showapparent photocatalytic activities to the two organic pollutants,especially for methylene blue. The decomposition rate of methyleneblue reaches 87% in 2 h, which is comparable to the efficiency toTiO2 nanoparticles and commercial P-25 under the same experi-mental conditions.27 It takes 3 h to decompose methylene bluemostly under UV light irradiation. This reveals that ZnCr2O4

has the potienial to be used as a new kind of semiconductorphotocatalyst.

IV. Conclusions

In summary, pure phase ZnCr2O4 nanoparticles were synthe-sized by a simple hydrothermal route. The as-prepared sampleshowed optical and photocatalytic properties. It was found to be

a direct band gap semiconductor and the optical band gap wasabout 3.46 eV. The ZnCr2O4 nanoparticles exhibited broademission in the range of 300–430 nm, centered at 358 nmwhen excited at 220 nm. Furthermore, the particles also showedapparent photocatalytic activities under UV light irradiations.

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Fig. 4. Room temperature photoluminescence emission spectrum ofZnCr2O4 nanoparticles excited at 220 nm.

Fig. 5. Photocatalytic activities of ZnCr2O4 nanoparticles.

2390 Communications of the American Ceramic Society Vol. 91, No. 7