Visible light photocatalytic activity of novel MWCNT-doped ZnO electrospun nanofibers

ARTIC

LECopyright © 2014 by American Scientific Publishers

All rights reserved.

Printed in the United States of America

Science of Advanced MaterialsVol. 6, pp. 1–7, 2014

(www.aspbs.com/sam)

Visible-Light Photocatalytic Activity of Novel NiTiO3

Nanowires with Rosary-Like ShapePanpan Jing, Wei Lan∗, Qing Su, Minglang Yu, and Erqing Xie

Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, School of Physical Science andTechnology, Lanzhou University, Lanzhou, 730000, People’s Republic of China; Key Laboratory for Magnetism andMagnetic Materials of Ministry of Education, Lanzhou University, Lanzhou, 730000, People’s Republic of China

ABSTRACT

In this paper, we report the synthesis of novel NiTiO3 nanowires with rosary-like shape prepared by the electro-spinning method. The morphology and structure of NiTiO3 nanowires were characterized by means of scanningelectron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy and high-resolution trans-mission electron microscopy (HR-TEM). XRD patterns, micro-Raman spectra and HR-TEM images revealedthat the annealing temperature is crucial for the morphology and crystallinity of the nanowires. Rosary-likeNiTiO3 nanowires are single-nanoparticle-chains. Notably, each nanocrystalline forming the rosary-like NiTiO3

nanowires is a single crystal. A possible growth mechanism of rosary-like NiTiO3 nanowires is proposed indetail. Due to their visible-light photocatalytic activity, the NiTiO3 nanowires with rosary-like shape show excel-lent performance in light-induced-degradation efficiency for Safranine T and Rhodamine B.

KEYWORDS: Visible-Light Photocatalytic Activity, NiTiO3 Nanowires.

1. INTRODUCTIONSemiconductor photocatalysts have attracted much atten-tion owing to their potential applications in environmentremediation.1–4 For instance, they can be utilized to dealwith polluted water through destroying harmful organicpollutants by producing strongly oxidative species undersolar light irradiation.5�6 Many semiconductor materialshave been extensively investigated and used in pollutantdegradation and water-splitting for years, particularly sinceFujishima and Honda7 have reported the light-inducedwater splitting on TiO2 surfaces. Up to now, TiO2 has beenproved to be the most excellent and stable photocatalystfor the elimination of environment pollutants because ofits favorable properties, including long-term chemical sta-bility, high photocatalytic activity, non-toxicity and lowcost.8–10 However, the photocatalytic efficiency of pureTiO2 crystals is severely limited by its relatively wideenergy gap (3.0 and 3.2 eV for rutile and anatase, respec-tively) and rapid recombination rate of the photogener-ated electron–hole pairs, which cause side effects to itspractical applications.11�12 Thus, it is highly worthy anddesirable to make great efforts to develop the ideal alter-native photocatalysts which can better utilize the solarlight.

∗Author to whom correspondence should be addressed.Email: [email protected]: xx xxxx xxxxAccepted: xx xxxx xxxx

Based on recent scientific breakthroughs and advances,many titanate photochemical catalysts, such as SrTiO3,CoTiO3, ZnTiO3 and NiTiO3, have attracted significantattention.13–16 Recently, there have been reports in rela-tion to the highly effective photodegradation of organicsdyes contaminants obtainable by use of NiTiO3 becauseof its remarkable lattice structure and electron configu-ration. The perovskite-type crystal structure of NiTiO3 isdisplayed in Figure 1, which has the characteristic of thelayered structure consisting of octahedral [TiO6]/[NiO6]clusters. Such lattice structure can offer many stable andsuitable photocatalytic sites. In addition, the band gap ofbulk NiTiO3 is ∼2.18 eV,17 and is beneficial to the absorp-tion of photons in the visible range owing to electronictransitions to the Ti:3d conduction band from a filledNi2+:3d8 band situated above the O−:2p6 band.18 Thefabrication methods of NiTiO3 nanomaterials include co-precipitation process, modified Pechini method and sol–gel method.19–21 Electrospinning has been demonstratedto be an effective and feasible technique for preparing1D ultrafine nanofibres ascribable to its features, includ-ing simple device and operation, economc raw materi-als and easy control of nanofibre dimension.22–25 Herein,NiTiO3 nano-photocatalyst is synthesized by using electro-spinning process, which also is very essential for developingnew applications ofNiTiO3 nanomaterials. Therefore, in thiswork, NiTiO3 nano-photocatalyst have been synthesized byusing electrospinning method, which also is very essential

Sci. Adv. Mater. 2014, Vol. 6, No. 3 1947-2935/2014/6/001/007 doi:10.1166/sam.2014.1735 1

Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape Jing et al.

ARTIC

LE

Fig. 1. Crystal structure of the perovskite NiTiO3 with two cells in thea-axis, and viewed from the (0,−2,−1) plane.

for developing new applications of perovskite-type titanatenanomaterials.

2. EXPERIMENTAL DETAILS2.1. Preparation of NiTiO3 NanowiresAll chemical reagents were of analytical grade andused without further purification. NiTiO3 nanowires wereprepared by the following procedure. First, 1.021 gof Ti(OC4H9�4 (98.0% purity, Mw = 340�36, TianjinGuangfu Fine Chemical Research Institute, China) and0.894 g of Ni(NO3�2 ·6H2O (98.0% purity, Mw = 290�79,Tianjin Guangfu Technology Development Co. ltd. China)(nTi:nNi = 1:1) were dissolved together in a mixtureof ethanol and acetic acid. The mixture was stirredfor an hour until it became a clear green solution.A 10% poly(vinyl pyrrolidone) (PVP, 99.0% purity,Mw = 1,300,000, Alfa Aesar) solution was obtained bydissolving 0.9 g PVP in a mixture of 4.5 g N ,N -Dimethylformamide (DMF, 99.5% purity, Tianjin BaishiChemical Co. ltd. China) and 4.5 g acetic acid. After theabove two solutions were mixed and stirred vigorouslyfor 2 h, a homogeneous viscous precursor of PVP/NiTiO3

composite was acquired. Second, the precursor was trans-ferred to a glass syringe equipped with a stainless nee-dle with an inner diameter of about 0.4 mm. In a typicalelectrospinning process, a distance of 16 cm and an elec-tric potential difference of 17.00 kV offered by a high-voltage power supply (HVPS) were maintained betweenthe syringe tip (cathode) and aluminum fence (anode).The electrospun PVP/NiTiO3 hybrid nanofibers were col-lected on the aluminum collector. Third, the as-electrospunPVP/NiTiO3 nanofibers were heat treated in air at differenttemperatures (700 �C, 800 �C, 900 �C, and 1000 �C) for2 h at a constant heating rate of 2 �C/min. The differentannealing times (1 h, 2 h, 3 h) were carried out, 2 h wasfound to be the optimal.

2.2. CharacterizationThe morphology of samples was observed by means offield emission scanning electron microscopy (FESEM,

Hitachi S-4800) and transmission electron microscopy(TEM, FEI Tecnai F30). X-ray diffraction (XRD, D/MAX-2400, 1.5406 Å), micro-Raman spectroscopy (HoribaJobin Yvon LabRAM-HR800, 532 nm laser) and high-resolution transmission electron microscopy (HRTEM, FEITecnai F30) were employed to determine the structuralproperties of the samples. UV-visible absorption spectrumwas recorded using a spectrophotometer (TU-1901) toinvestigate the photocatalytic performance.

2.3. PhotocatalysisThe photocatalytic activity of the samples in solutionwas verified as follows: 30 mg of powder (NiTiO3-800/900/1000 �C) was added to the 40 mL aqueous solu-tion of safranine T (ST, 15 mg/L) or Rhodamine B (RhB,15 mg/L) stored in a reactor. Before the visible light irra-diation, the suspension was treated by ultrasonic for 5 minand magnetically stirred in the dark for 30 min to ensurethat the powder sample was well dispersed and reachedadsorption-desorption equilibrium. A 175 W high pressuremercury lamp was used with a UV filter 420 nm cut off,and 4–5 mL suspension was taken and centrifuged duringthe irradiation at given time intervals. Finally, the filtrateswere analyzed by recording the changes in the absorptionbands of ST and RhB using a TU-1901 spectrophotometer.

3. RESULTS AND DISCUSSIONFigures 2(a)–(d) illustrate FESEM images of NiTiO3-X(X represents the annealing temperature) nanowiresobtained after as-electrospun nanofibers were annealed at(a) 700 �C, (b) 800 �C, (c) 900 �C and (d) 1000 �C,respectively. In Figure 2(a), the surface of NiTiO3-700

�Cnanowires is clean and smooth, and the mean diameterof the nanowires is about 140 nm. The sample shownin Figure 2(b), however, is composed of huge amountsof nanowires with rough surface, because the nanowiresare constructed of numerous nanoparticles assembled ran-domly. Moreover, when the annealing temperature isincreased up to 900 �C, the NiTiO3 nanowires acquire anovel rosary-like shape. In addition, the average diame-ter of the nanowires is reduced to approximately 100 nm.Nevertheless, if the heating temperature reaches 1000 �C,the rosary-like structures are disrupted due to the amalga-mation and growth of nanoparticles, which is revealed infigure (d). Throughout these FESEM photographs, it canbe seen that the diameter of NiTiO3 nanowires decreasesfirstly with the increase of annealing temperature up to900 �C and then increases by further increase of the heat-ing temperature.Figure 3 shows the structural properties of the NiTiO3-

X nanowires. As observed in Figure 3(a), with the increaseof the annealing temperature from 700 �C to 900 �C, allthe characteristic XRD peaks of the NiTiO3 nanowires dis-play remarkably intense, narrow and sharp. All of them,which match well with the rhombohedral structure [JCPDS

2 Sci. Adv. Mater., 6, 1–7, 2014

Jing et al. Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape

ARTIC

LE

Fig. 2. SEM images of NiTiO3 nanowires annealed at (a) 700 �C, (b) 800 �C, (c) 900 �C and (d) 1000 �C.

No. 33-0960], are demonstrated clearly by Figure 3(b).The calculated lattice parameters are a = 5�030 Å andc = 13�790 Å. However, when the electrospun nonofibersare annealed at the higher temperature of 1000 �C, theXRD signals attenuate abruptly. Figure 3(c) shows themicro-Raman scattering spectra (532 nm excitation) in150–900 cm−1 range of the NiTiO3-X nanowires post-annealed for 2 h in air. The Raman spectra show obvi-ously active vibration modes of NiTiO3 nanowires withthe perovskite phase. The Raman peaks locate at 184, 225,241, 284, 340, 391, 457, 498, 610 and 704 cm−1 respec-tively, which are consistent with previous reports.26 Thefull width half maximum and the intensity of the Ramanpeaks vary with an increase in the temperature at which theNiTiO3 nanowires were heated. This point reveals that thenovel NiTiO3-900

�C nanowires with the rosary-like shapeexhibit the best crystallinity, which is consistent with thecoclusionsof the XRD analysis. Figure 3(e) clearly showsthat the NiTiO3-900

�C nanowires possess a rosary-likeshape of single-nanoparticle chains. The HRTEM imageof a single-nanoparticle chain is illustrated in Figure 3(f).It can be determined that each nanoparticle is a monocrys-talline and lattice fringes have an interplanar spacing of0.223 nm, corresponding to the perovskite phase NiTiO3

(113) plane, which agrees with the JCPDS card No. 33-0960. The selected-area electron diffraction (SAED) pat-tern presented in the inset further suggests that the wellcrystallized monocrystalline nanoparticles construct the

rosary-like NiTiO3 nanowires. Figure 3(d) illustrates opti-cal absorption spectra of the NiTiO3 samples. It is clearlyindicated that all the prepared samples have good responsenot only in the UV range but also in the visible region.A broad absorption edge situated at ∼410 nm indicatesthe optical band gap attributed to the O2− → Ti4+ charge-transfer interaction. Two absorption bands around 448 and511 nm are observed, due to the crystal field splitting ofNiTiO3, such that the 3d8 band associated with Ni2+ ionssplits up into two sub-bands at about 490 and 560 nm.They are called Ni2+ → Ti4+ charge-transfer bands.26 Fora crystalline semiconductor, the optical absorption near theband edge follows the equation:27

�h� = A�h�−Eg�n/2 (1)

where �, �, Eg and A are the absorption coefficient, thelight frequency, the band gap and a constant, respectively.Among them, n decides the characteristics of the transitionin a semiconductor. The optical band gap (Eg) value ofNiTiO3-900

�C is calculated to be about 2.17 eV accord-ing to the equation from the onset of the absorption edge,which is consistent with previous studies (2.18 eV).28

Thus, NiTiO3 nanowires can be expected to act as goodvisible-light photocatalyst.To explain the formation of the NiTiO3 nanowires

with rosary-like shape, a feasible mechanism showed inScheme 1 is proposed as follows. First, when the initial as-electrospun nanofibers (Scheme 1(a)) are heated at lower

Sci. Adv. Mater., 6, 1–7, 2014 3

Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape Jing et al.

ARTIC

LE

Fig. 3. (a) Typical XRD patterns and (b) an enlarged view of the 2� region from 32� to 37� of all samples. The reference JCPDS card (No. 33-0960)corresponded to perovskite-type NiTiO3. (c) Raman spectra and (d) UV-vis absorption spectra of all samples. (e) Typical TEM and (f) HRTEM imagesof the NiTiO3-900

�C nanowires; the inset in (f) is SAED pattern.

Scheme 1. Schematic diagrams of the formation mechanism of NiTiO3

nanowires with rosary-like shape.

temperature, such as 700 �C, which can offer an envi-ronment providing a thermodynamic condition to disorga-nize and deompose the micelles composed of PVP andother ions, and induce a controlled interaction of Ni2+ andTiO2−

3 ions, nanocrystals are formed and growth followedby Ostwald ripening.29 These nanocrystals form NiTiO3

nanowires with smooth surface (Scheme 1(b)). Second,if the temperature is increased gradually to around 800 �C,the crystal grains in the NiTiO3 nanowires begin to growup non-uniformly, hence these nanowires are piled uprandomly by numerous nanograins (Scheme 1(c)). Third,as the annealing temperature reaches 900 �C, the crys-tal grains further grow, and novel nanowires with rosary-like structure are formed by single-nanoparticle-chains(Scheme 1(d)). Based on the detachment mechanism,30

when the annealing temperature rises further to 1000 �C

4 Sci. Adv. Mater., 6, 1–7, 2014

Jing et al. Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape

ARTIC

LEor more, the kinetic process of mass transportation leadsto the severe stress (F1 �= F2 �= Fi) between nanoparti-cles and amalgamation and growth of part of them dueto the presence of the Rayleigh instability. The NiTiO3

nanowires with rosary-like structure break into severalindependent sections (Scheme 1(e)). Therefore, the anneal-ing temperature 900 �C is an important factor for the for-mation of NiTiO3 nanowires with rosary-like shape. Shortand thick NiTiO3 nanorods can be obtained under highertemperature conditions.On the basis of the structural characterization described

above, the NiTiO3 nanowires with rosary-like structure pos-sess an outstanding crystallinity and surface morphology,which are very important to guarantee photodegradationefficiency.31 Therefore, NiTiO3-900

�C could be appliedto the photocatalytic degradation of ST and RhB, whichare always chosen as model pollutants to evaluate thelight-induced-catalytic activity of photocatalysts.32�33 Thecharacteristic absorptions of ST at 553 nm and RhB at554 nm are selected to monitor the level of photocatalytic

Fig. 4. UV-vis absorption spectra of ST photocatalytic decomposition with (a) NiTiO3-900�C, (b) NiTiO3-800

�C and (c) NiTiO3-1000�C nanowires.

The inset in the figure (a) shows the colour changes of the ST solutions. (d) RhB is photodegradation is catalyzed by NiTiO3-900�C nanowires. (e) The

photodegradation level of dyes as a function of irradiation time for novel NiTiO3-900�C nanowires.

decomposition. As illustrated in Figure 4(a) and the inset,the NiTiO3-900

�C nanowires induce a significant pho-todegradation reaction to ST for 90 min under visible lightsupplied by a 175 W high pressure mercury lamp with a420 nm cut off UV filter. Moreover, the absorbance peak at553 nm well-nigh disappears after 150 min, which indicatesthat the ST is wholly disintegrated. It should be noticedthat the NiTiO3-900

�C nanowires can also promote thedecomposition of RhB to a certain degree, as shown inFigure 4(d). Figures 4(b) and (c) present analysis resultsof the photodegradation efficiency of the NiTiO3-800

�Cand the NiTiO3-1000

�C nanowires. To further demonstratethe photocatalytic decomposition of the dyes related to theNiTiO3 nanowires, the self-photodegradation reaction ofabove two dyes without nanophotocatalysts have also beenimplemented under the same conditions. According to thedyes concentration (%) after various intervals of reactiontime could be computed using the following equation:34

%�dyes�= Ct/C0×100% (2)

Sci. Adv. Mater., 6, 1–7, 2014 5

Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape Jing et al.

ARTIC

LE

Scheme 2. The photocatalytic mechanism schematic diagrams ofNiTiO3-900

�C nanowires under visible light irradiation.

where C0 is the initial dyes absorbance and Ct is theabsorbance after various intervals of time (T ). As thecurves of Ct/C0 and the time T showed in Figure 4(e), wecan understand clearly that the NiTiO3-900

�C nanowireseffectively promote the photodecomposition of ST andRhB.A simplified illustration of the photocatalytic mech-

anism for NiTiO3 nanowires with rosary-like structureunder visible light irradiation is shown in Scheme 2(a).(1) NiTiO3, whose band gap is ∼2.17 eV, can be excitedby lower-energy photons (h�) in visible light region.Abundant electrons jump into the conductive band andholes are left in the valence band.(2) The photo-generated electrons and holes migrateswiftly to the surface of NiTiO3 nanowires to react withother molecules and ions. For example, electrons canreact with O2/O2 +H+ dissolved in solution and gener-ate strong oxidizing O−

2 /HO2, respectively. In this case,the electronic structure of NiTiO3 is composed of octahe-dral [TiO6]/[NiO6] clusters, the [NiO6] clusters can reactwith the OH− to generate OH, while the species gen-erated [TiO6] clusters react with oxygen molecules inaqueous solution.35 Furthermore, holes can partially reactwith OH− and create the OH• radicals possessing strongoxidability. Holes can also be involved in the oxidation-reduction reaction directly.(3) The oxidizing and non-oxidizing species react withthe dye molecules absorbed on the surface of NiTiO3

nanowires.

Though the underlying mechanisms of photodegradationof NiTiO3 nanowires for ST and RhB are different, themain reasons driving their demolition should be ascribedto the redox potential, the molecule masses, the structuralstability and the absorption of dyes molecules. It is shouldbe noted that NiTiO3 nanowires with rosary-like structureboth induce degradation reaction for ST and RhB with dif-ferent degree under visible light irradiation. In addition,

the photocatalysis efficiency of NiTiO3-900�C is much

higher than that of NiTiO3-800�C nanowires and NiTiO3-

1000 �C nanorods, which should be explained by twofollowing arguments. First, besides having a strong pho-toresponse in the visible region of all samples, NiTiO3-900 �C nanowires possesses finer crystallinity and fewerdefects, which are indicated in the XRD and Raman data.Second, as illustrated in Scheme 2(b), the favorable sur-face of the NiTiO3-900

�C with rosary-like structure allowsmore efficient multiple reflections of the visible light,resulting in effective utilization of the light source andenhanced catalysis efficiency.

4. CONCLUSIONSNiTiO3 nanowires with rosary-like shape are synthe-sized using the electrospinning technique. Based on thecharacterization of SEM, XRD, micro-Raman and TEM,a possible formation mechanism and the ideal tectonic con-dition of rosary-like NiTiO3 nanowires are proposed andexplored. One special point is that each nanograin formingNiTiO3-900

�C nanowires with rosary-like shape is a sin-gle crystal. The photocatalytic activity measurements indi-cate that the NiTiO3 samples catalyze photodecompositionfor ST and RhB with different degrees of efficiency undervisible light irradiation. The light-degradation efficiency ofthe NiTiO3-900

�C nanowires is the best among all theNiTiO3 samples synthesiuzed. The present investigation onthe NiTiO3 nanowires with rosary-like shape may open abright perspective and direction to research NiTiO3 andother complex oxides for exclusive applications.

Acknowledgments: The authors would like to acknowl-edge the financial support from the National Natural Sci-ence Foundation of China (50802037), the Natural ScienceFoundation of Gansu Province (No. 1208RJZA199),Beijing research program (No. D121100001812002) andthe Project-sponsored by SRF for ROCS, SEM.

References and Notes1. J. Zhang, J. G. Yu, Y. M. Zhan, Q. Li, and J. R. Gong, Nano Lett.

11, 4774 (2011).2. O. K. Varghese, M. Paulose, T. J. L. Tempa, and C. A. Grimes, Nano

Lett. 9, 731 (2009).3. M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemannt,

Chem. Rev. 95, 69 (1995).4. F. Xu, Y. T. Shen, L. T. Sun, H. B. Zeng, and Y. N. Lu, Nanoscale

3, 5020 (2011).5. B. Yue, Q. Y. Li, H. Iwai, T. Kako, and J. H. Ye, Sci. Technol. Adv.

Mater. 12, 034401 (2011).6. Y. P. Bi, S. X. Ouyang, N. Umezawa, J. Y. Cao, and J. H. Ye, J. Am.

Chem. Soc. 133, 6490 (2011).7. A. Fujishima and K. Honda, Nature 238, 37 (1972).8. X. B. Chen, L. Liu, P. Y. Yu, and S. S. Mao, Science 331, 746

(2011).9. J. Thomas, K. P. Kumar, and S.Mathew, Sci. Technol. Adv. Mater.

3, 59 (2011).10. X. J. Xu, X. S. Fang, T. Y. Zhai, H. B. Zeng, B. D. Liu, X. Y. Hu,

Y. Bando, and D. Golberg, Small 7, 445 (2011).

6 Sci. Adv. Mater., 6, 1–7, 2014

Jing et al. Visible-Light Photocatalytic Activity of Novel NiTiO3 Nanowires with Rosary-Like Shape

ARTIC

LE11. S. S. Soni, M. J. Henderson, J. Bardeau, and A. Gibaud, Adv. Mater.

20, 1493 (2008).12. P. Y. Dong, Y. H. Wang, L. N. Guo, B. Liu, S. Y. Xin, J. Zhang,

Y. R. Shi, W. Zeng, and S. Yin, Nanoscale 4, 4641 (2012).13. Z. K. Zheng, B. B. Huang, X. Y. Qin, X. Y. Zhang, and Y. Dai,

J. Colloid Interface Sci. 358, 68 (2011).14. P. N. Kapoor, S. Uma, S. Rodriguez, and K. J. Klabunde, J. Mol.

Catal. A� Chem. 229, 145 (2005).15. Z. X. Chen, A. Derking, W. Koot, and M. P. Dijk, J. Catal. 161, 730

(1996).16. Y. H. Ni, X. H. Wang, and J. M. Hong, Mater. Res. Bull. 44, 1797

(2009).17. Y. Shimodaira, H. Kato, H. Kobayashi, and A. Kudo, J. Phys. Chem.

B 110, 17790 (2006).18. P. Salvador, C. Gutierrez, and J. B. Goodenough, Appl. Phys. Lett.

40, 188 (1982).19. A. V. Murugan, V. Samuel, S. C. Navale, and V. Ravi, Mater. Lett.

60, 1791 (2006).20. Y. J. Lin, Y. H. Chang, W. D. Yang, and B. S. Tsai, J. Non-Cryst.

Solids 352, 789 (2006).21. K. P. Lopes, L. S. Cavalcante, A. Z. Simoes, R. F. Goncalves, M. T.

Escote, J. A. Varela, E. Longo, and E. R. Leite, J. Sol–Gel Sci.Technol. 45, 151 (2008).

22. A. Greiner and J. H. Wendorff, Angew. Chem. Int. Ed. 46, 5670(2007).

23. F. A. Sheikh, T. Cantu, J. Macossay, and H. Kim, Sci. Adv. Mater.3, 216 (2011).

24. C. Q. Zhu, Y. R. Li, Q. Su, B. A. Lu, J. Q. Pan, J. W. Zhang, E. Q.Xie, and W. Lan, J. Alloys Compd. 575, 333 (2013).

25. C. Q. Zhu, B. A. Lu, Q. Su, E. Q. Xie, and W. Lan, Nanoscale4, 3060 (2012).

26. K. P. Lopes, L. S. Cavalcante, A. Z. Simoes, J. A. Varela, E. Longo,and E. R. Leite, J. Alloys Compd. 468, 327 (2009).

27. W. Lan, M. Zhang, G. B. Dong, P. M. Dong, Y. Y. Wang, and H. Yan,Mater. Sci. Eng. B 139, 155 (2007).

28. J. Zhang, F. J. Shi, D. F. Chen, J. M. Gao, Z. X. Huang, X. X. Ding,and C. C. Tang, Chem. Mater. 20, 2937 (2008).

29. L. S. Cavalcante, J. C. Sczancoski, M. S. Li, E. Longo, andJ. A. Varela, Colloids and Surfaces A� Physicochem. Eng. Aspects396, 346 (2012).

30. C. J. Dalmaschio and E. R. Leite, Cryst. Growth Des. 12, 3668(2012).

31. H. Yu, S. X. Ouyang, S. C. Yan, Z. S. Li, T. Yu, and Z. G. Zou,J. Mater. Chem. 21, 11347 (2011).

32. Y. L. Lai, M. Meng, and Y. F. Yu, Appl. Catal. B 100, 491(2010).

33. J. J. Li, S. Q. Liu, Y. Y. He, and J. Q. Wang, MicroporousMesoporous Mater. 115, 416 (2008).

34. Y. Wang, Y. R. Su, L. Qiao, L. X. Liu, Q. Su, C. Q. Zhu, and X. Q.Liu, Nanotechnology 22, 225702 (2011).

35. L. S. Cavalcante, F. M. C. Batista, M. A. P. Almeid, A. C. Rabelo,I. C. Nogueir, N. C. Batista, J. A. Varela, M. R. M. C. Santos,E. Longo, and M. S. Li, RSC Adv. 2, 6438 (2012).

Sci. Adv. Mater., 6, 1–7, 2014 7