RSC Advances

PAPER

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article OnlineView Journal | View Issue

Simple hydrothe

Key Laboratory of Food Colloids and Biotech

School of Chemical and Material Engineering

China. E-mail: dongym@jiangnan.edu.cn; F

Cite this: RSC Adv., 2014, 4, 39167

Received 26th March 2014Accepted 11th August 2014

DOI: 10.1039/c4ra02654c

www.rsc.org/advances

This journal is © The Royal Society of C

rmal preparation of a-, b-, and g-MnO2 and phase sensitivity in catalytic ozonation

Yuming Dong,* Kun Li, Pingping Jiang, Guangli Wang, Hongyan Miao, Jingjing Zhangand Chi Zhang

The investigation of the roles of bonds and phases in catalysis is of great significance for designing novel

catalysts and proposing mechanisms. Hindered by the lack of comparable catalyst samples, the influence

of bonds and phases on the instantaneous reaction between ozone and MnO2 is still unknown. In this

paper, a-, b-, and g-MnO2 were prepared through a uniform hydrothermal method, and the phase

sensitivity of the manganese dioxide catalysts in the ozonation process was investigated. Raman

spectra indicated that bands transformed to a larger range in the window of 550–700 cm�1, which is

considered to be the fingerprint region of manganese dioxides. During catalytic ozonation, ozone

reacted with particular Mn–O bonds to generate more powerful oxygen species, leading to better

oxidation efficiency compared to single ozonation. The active bonds, which are favourable to the

activation of ozone, include bonds belonging to the pyrolusite type structure of g-MnO2,

corresponding to the [MnO6] octahedral frameworks of b-MnO2 and perpendicular to the direction of

the [MnO6] octahedral double chains of a-MnO2. Thermogravimetric analysis confirmed that

both active surface oxygen and lattice oxygen were responsible for the catalytic activity of a-MnO2,

while lattice oxygen and the bonded manganese of MnO2 contributed to the catalytic activity of b- and

g-MnO2.

1. Introduction

Manganese dioxides (MnO2) are promising materials for manytechnological applications such as lithium batteries, oxidantagents, and catalysts.1–4 Their high activities are ascribed totheir tunnel and layered structures, which have edge- orcorner-sharing [MnO6] octahedra. Based on the law of varietiesconnectivity schemes, MnO2 shows great structural exibilityand appears in a number of crystallographic polymorphs suchas a-, b-, and g-MnO2.5 In addition, different connectivityschemes result in the anisotropism of Mn–O bonds, even inthe same [MnO6] octahedron.6 Logically speaking, these bondsdisplay differences in bond length, background energy stateand so on. Nevertheless, depending on the nature of reactants,special Mn–O bonds or planes can be suitable for theabsorption and activation of reactants. The investigation onthe roles of bonds and phases in catalysis is of great signi-cance for designing novel catalysts and proposingmechanisms.

Catalytic ozonation is an attractive technology for organicpollutant degradation that does not require the control oftemperature or pressure.7–11 Several articles12–15 have been

nology (Ministry of Education of China),

, Jiangnan University, Wuxi 214122, P. R.

ax: +86-510-85917763

hemistry 2014

published in the catalytic ozonation eld using Mn-containingcatalysts. Taking advantage of the particular properties ofvarious catalysts, they revealed that certain interactions withreactants lead to a satisfactory consequence in pollutantdegradation. The commonality of these catalysts was thatmanganese oxides played a vital role for catalysis. Some effortshave been made to identify the active site and oxygen species todetermine the mechanism of catalytic ozonation. Limited by thevaried preparation methods of the catalysts, a common essen-tial factor in the catalytic activity could not be discerned.16 Todate, the lack of comparable catalyst samples has limited theunderstanding of the inuence of bonds and phases on theinstantaneous reaction between ozone and MnO2. A uniformpreparation strategy for different kinds of manganese dioxidesis needed.

The objective of this paper is to investigate the relationshipbetween the structure of manganese dioxide and catalyticozonation activity. Using a facile hydrothermal method, a-, b-and g-MnO2 were successfully obtained under similarconditions. They revealed different catalytic activities for theozonation degradation of phenol. Control experiments wereconducted to identify the phase sensitivity of MnO2 in theozonation process. Thermogravimetric analysis was carried outto establish the correlation between surface active oxygen,lattice oxygen and catalytic activity.

RSC Adv., 2014, 4, 39167–39173 | 39167

RSC Advances Paper

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

2. Experimental2.1 Materials

Potassium permanganate (KMnO4) and tetrahydrate manga-nese chloride (MnCl2$4H2O) were analytical grade and usedwithout further purication. Deionized water was usedthroughout all experiments.

2.2 Preparation of catalysts

The catalysts were synthesized by a simple hydrothermalmethod. In a typical procedure, a mixed solution (100 mL)containing MnCl2 and KMnO4 (total amount of 20 mmol) wasstirred for 2 h and transferred into a Teon-lined autoclave. Themolar ratios of MnCl2 and KMnO4 are listed in Table 1. Theautoclaves were then maintained at 160 �C for 6 h. The obtainedbrown precipitates were washed with water, dried at 80 �C andnamed Mn1, Mn2 and Mn3 (Table 1).

2.3 Characterization of catalysts

The compositions and crystalline phases of the products wereidentied by powder X-ray diffraction (XRD) on a D8 X-raydiffractometer (Bruker AXS, German) using Cu Ka radiation(l ¼ 1.5406 A). Scanning electron microscopy (SEM; S4800,Hitachi Co. Ltd., Japan) was employed to observe themorphologies and sizes. Thermogravimetric analysis (TGA) anddifferential thermal gravimetry (DTG) were performed with aMettler TG/DTA b1/1100 SF analyser at a ramp rate of10 �C min�1. Micro-Raman spectroscopy was conducted on anInvia-Reex spectrometer (Renishaw). In a typical procedure,gases ozone was bubbled into water containing catalyst (1 g L�1,25 �C). At specic time, a uniform thickness of sample wascollected on amicroscope slide. The specic surface areas of thecatalyst samples were measured according to the Brunauer–Emmet–Teller (BET) method using nitrogen adsorption at77.4 K on an ASAP 2020 instrument (Micromeritics, USA) aerdegassing under 120 �C for 2 h.

2.4 Ozonation procedure and analytical methods

Catalytic activities were tested in the ozonation degradation ofphenol. In each experiment, 200 mL of phenol aqueous solution(c0,phenol ¼ 300 mg L�1) with a catalyst concentration of 1 g L�1

was used. Aer stirring for 10 min, 0.75 mg min�1 of ozone owfrom a 3S-A5 laboratory ozoniser (Tonglin Technology, China)was bubbled into the above solution at 25 �C. Samples were

Table 1 Preparation conditions and characterisation resultsa

SampleRaw materialsratio n(Mn2+)/n(MnO4

�) Phase

Mn1 6 : 4 Pyrolusite: g-MnO2

Mn2 7 : 3 Pyrolusite: b-MnO2

Mn3 5 : 5 Cryptomelane: a-MnO2

a SEM images of catalysts: (a) Mn1, (b) Mn2 and (c) Mn3.

39168 | RSC Adv., 2014, 4, 39167–39173

collected every 15 min. Single ozonation (without any catalyst)was also carried out under the same conditions.

The concentration of phenol was determined by high-performance liquid chromatography (HPLC, WATERS, USA)with a UV absorbance detector (UV 2487). The absorbancedetection system for phenol was set at 210 nm. Elution wascarried out by pumping methanol (HPLC/Spectro)–water(3 : 1 v/v) at a ow rate of 1.0 mL min�1. The ozone absorbancein aqueous solution was measured by a UV-vis spectrometer at258 nm with a TU-1900 spectrophotometer (Beijing).

3. Results and discussion3.1 Catalyst characterization

The XRD patterns of the as-synthesized samples are shown inFig. 1. Depending on the ratio of raw materials, these manga-nese dioxide samples revealed obvious differences in crystallinephase, despite having the same synthetic procedures andconditions. With a rawmaterial molar ratio [n(Mn2+)/n(MnO4

�)]of 6 : 4, the product is g-MnO2 (JCPDS 14-0644); several sharpdiffraction peaks corresponding to the (120), (131) and (300)planes are observed. When the ratio is changed to 7 : 3, thecrystalline phase is b-MnO2 (JCPDS 24-0735), with sharp peaksmainly corresponding to the (110), (101), (111), (211) and (312)planes. At a ratio of 5 : 5, the a-MnO2 crystalline phase (JCPDS44-0141) with (110), (200), (310), (211), (301), (411) and (521)planes was obtained. Taking the intensities and half-widths ofthe diffraction peaks into account, the degree of the crystallinityfollows the order: Mn2 > Mn3 > Mn1.

Lattice parameter (nm)

SBET(m2 g�1)a b c

0.63279 1.04294 0.40319 2.530.43953 0.43953 0.28542 2.930.97401 0.98102 0.28427 28.78

Fig. 1 XRD patterns of (a) Mn1, (b) Mn2 and (c) Mn3.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 Evolution of phenol concentration of phenol in single ozona-tion and catalytic ozonation.

Paper RSC Advances

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

The lattice parameters calculated from the XRD patterns arelisted in Table 1. From these lattice parameters, obviousdifferences among these samples can be observed. The struc-tures and morphologies of samples were also substantiallyaffected by the ratio of raw materials. As shown in Fig. 2, bothMn2 and Mn3 have one-dimensional rod morphologies withdiameters and lengths in the range of 0.5–3.2 mm. Mn1 exhibitsrods with dimensions of 3–5 mm, along with discrepant particlesthat wrap around the surface of the rods.

As listed in Table 1, the BET surface areas of the samples are2.53 (Mn1), 2.93 (Mn2), and 28.78 (Mn3) m2 g�1. In spite of theirsimilar rod morphologies, the samples exhibit signicantdifferences in surface area, which may be caused by theirmicrostructures. The larger surface areas of Mn3 mostly resul-ted from the one-dimensional (2� 2) tunnels in a-MnO2, whichhas a size of 0.46 nm � 0.46 nm (this will be discussed in detailin the Raman spectra analysis).17 By adjusting the [n(Mn2+)/n(MnO4

�)] ratio in the simple hydrothermal method, a-, b- andg-MnO2 were prepared under the same procedures and condi-tions. The a-, b-, and g-MnO2 phases had 2 � 2 tunnels, 1 � 1tunnels, and layer structures, respectively.18 The K+ ions canserve as templates in the formation of these tunnel or layerstructures,19 although the tunnels may need more cations to bestable. Hence, a- and b-MnO2 phases were produced at high K+

ion concentrations, while the g-MnO2 phase was obtained at acomparatively lower K+ ion concentration. Compared with a-MnO2, b-MnO2 has a large tunnel size that is suitable for theinsertion/extraction of more K+ ions. These results are consis-tent with the literature.18,20–23 The preparation method usedherein is easy to realize in a large-scale reactor, and it mayprovide a way to synthesize different kinds of MnO2 and furtherinvestigate the inuence of phase.

3.2 Catalytic ozonation activity

As a probe action for catalytic ozonation activity, the degrada-tion efficiency of phenol in presence of catalyst and ozone wasinvestigated. The removal of phenol along with ozonation timeis presented in Fig. 3. Generally, all the catalysts reveal highactivity towards the oxidation of phenol. About 22% of phenolwas removed in a single ozonation of 1 h, and 69% of phenolwas removed in the presence of Mn3. Comparing the perfor-mance of these MnO2 catalysts, the trend of Mn3 > Mn1zMn2in terms of efficiency was observed. This observation indicatesthat the crystallography of MnO2 inuenced the catalyticozonation performance.

Ozone decomposition experiments were then introduced toillustrate the interaction between catalyst and ozone. First, thecatalyst was added into the O3-saturated solution. As shown in

Fig. 2 SEM images of (a) Mn1, (b) Mn2 and (c) Mn3.

This journal is © The Royal Society of Chemistry 2014

Fig. 4a, all the catalysts showed excellent activity towards theremoval of ozone. Taking promotion percentage into account,the activities of these catalysts follow the order: Mn3 > Mn1 >Mn2. This sequence is consistent with the catalytic ozonationactivity. Based on this result, it can be concluded that a stronginteraction existed between ozone and catalyst.

To determine whether the removal of ozone was driven byabsorption or decomposition, we continued bubbling gaseousozone into water and added the catalysts aer 30 min. As shownin Fig. 4b, absorbance of the O3-saturated aqueous solutionwithout any catalyst remained at about 0.80 aer 30 min. Theaddition of catalyst led to a sharp decline in ozone absorbance,and the absorbance value remained at a relatively low level inthe following 20 min in presence of the three catalysts. The

Fig. 4 (a) Decomposition of ozone in O3-saturated aqueous disper-sions of various catalysts; (b) ozone concentration in aqueousdispersions of various catalysts with the continued bubbling of ozone.

RSC Adv., 2014, 4, 39167–39173 | 39169

RSC Advances Paper

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

results demonstrate that the ozone removal resulted fromcatalytic decomposition.

Fig. 6 (a) Evolution of phenol concentration in single ozonation andcatalytic ozonation with catalysts calcined at 300 �C; (b) evolution ofphenol concentration in ozonation and catalytic ozonation withcatalysts calcined at 600 �C.

3.3 Active surface atoms

The existence of surface active oxygen in MnO2 has beenconrmed by XPS (X-ray photoelectron spectroscopy), TGA andother characterization methods.24,25 Several articles have alsomentioned the importance of surface active oxygen, which playsa vital role in many reactions.26–28 Here, we employ TGA to detectthe relationship between the catalytic activity and the surfaceand bulk oxygen atoms of these catalyst samples. Fig. 5 showsthe TGA and DTG plots of each catalyst in the temperature rangeof 40–700 �C.

For Mn1 and Mn2, the weight loss can be categorized intotwo segments. The initial weight loss before 300 �C was due tothe removal of physically- and chemically-absorbed water. Thesubsequent weight loss from 400–560 �C was caused by thephase transformation from MnO2 to Mn2O3 (accompanied bythe change in lattice oxygen and bonded manganese). Theweight loss of 7.2% from 400–560 �C is smaller than the theo-retical value of 9.2%, indicating the incomplete transformationfrom MnO2 to Mn2O3. In the case of Mn3, the weight lossincludes three steps: the rst step under 240 �C was caused bythe removal of physically- and chemically-absorbed water; thesecond step from 240–300 �C possibly corresponded to theevolution of surface active oxygen; and the successive over-lapping weight loss (total of 5.67%) from 400–640 �C originatedfrom the transformation of MnO2 to Mn2O3 (accompanied bythe change in lattice oxygen and bonded manganese).

In order to clarify which type of active atoms are responsiblefor ozonation catalysis, several control experiments were carriedout. First, three catalyst samples were calcined at 600 �C for 3 h;these catalysts did not show any catalytic activity compared tothe single ozonation (Fig. 6b). This indicated that all of thecatalysts completely lost their activities aer calcination.Second, the samples were calcined at 300 �C for 3 h. For Mn1and Mn2, the intrinsic activities aer calcination exhibitednearly no difference compared to the original catalysts, while asmall decrease in activity was observed for Mn3 (Fig. 6a).

Considering the catalytic activity, weight loss analysis, andactivity change aer calcination for Mn1 and Mn2, it can beconcluded that the lattice oxygen and the bonded manganese of

Fig. 5 TGA and DTG plots of the catalysts under N2 atmosphere.

39170 | RSC Adv., 2014, 4, 39167–39173

MnO2 contributed to the catalytic activity these materials. In thecase of Mn3, the notable phenomena are the partial loss ofcatalytic activity aer calcination at 300 �C and the completeactivity loss aer calcination at 600 �C. Combined with the TGAresults, the activity of Mn3 can be considered to have twoorigins: the lattice oxygen and bonded manganese of MnO2 andthe active surface oxygen. With a surface active oxygenpercentage of 1.86%, Mn3 exhibited a signicantly differentcatalytic ozonation activity. The quantity of surface activeoxygen had an obvious inuence on the catalytic activity ofMn3.29 Consequently, active surface oxygen should play animportant role in the activity of Mn3.

3.4 Raman spectra study

Almost all MnO2 structures can be described as a close-packednetwork of oxygen atoms in which Mn4+ cations are differentlydistributed to form [MnO6] octahedra (as shown in Fig. 7a). Bysharing opposite octahedral edges or corners, the endlesschains form and link to neighbouring octahedral chains. In thispattern, tunnel openings can be formed by basal layers.30

The Raman spectra of these catalysts treated with ozone fordifferent times are shown in Fig. 8. Compared with the twosharp bands situated at 550–700 cm�1 (this situation is suitablefor these catalysts), other bands corresponding to the stretchingmode of the Mn–O bond or skeletal vibrations of the [MnO6]chains are weak in intensity. In addition, the variations inRaman shi or peak intensity for these bands and peaks werenegligible, even with a long period of ozone treatment. Thus, we

This journal is © The Royal Society of Chemistry 2014

Fig. 7 Polyhedral representations of the crystal structures of varietiesof MnO2: (a) [MnO6] octahedron, (b) cryptomelane-type a-MnO2, (c)pyrolusite-type b-MnO2 and (d) pyrolusite-type g-MnO2.

Fig. 8 Raman spectra of (a) Mn1, (b) Mn2 and (c) Mn3 after ozonetreatment for different times.

Paper RSC Advances

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

only focus on the stronger peaks with signicant changes andapplied the same principle in the following analysis.

First, we analyse the case of a-MnO2 (Mn3). Without anyozone treatment, two sharp diagnostic high-frequency Ramanbands originating from the Ag spectroscopic species are situatedat 635 and 576 cm�1 (Fig. 8c). On the basis of the principle forspectrum choice and analysis, 6Ag+6Bg+3Eg

30–32 spectroscopicspecies are Raman active, and een peaks or bands can beobserved. However, some modes are difficult to observe due tothe low polarisabilities and the overlap of incompletely resolvedmodes. According to previous studies, the above-mentioned twosharp peaks belong to the Mn–O stretching mode in [MnO6]octahedra. For cryptomelane-type a-MnO2, [MnO6] octahedrashare edges with opposite octahedra to form an endless chain,which is in turn linked to neighbouring octahedral [MnO6]chains to form (2 � 2) or (1 � 1) tunnels (Fig.7b). The peaksituated at 635 cm�1 might be related to Mn–O vibrationsperpendicular to the direction of the [MnO6] octahedral doublechains, and the Raman band at 579 cm�1 may correspond to thedisplacement of oxygen atoms relative to manganese atomsalong the octahedral double chains.31

Aer treatment with ozone in water for 30 and 60 min, thepeak at 635 cm�1 shied to 643 and 649 cm�1, respectively.According to the quantum theory of Raman spectra, the inter-action of a molecule with laser radiation can be analysed interms of an energy-transfer mechanism. In other words, achange of state is accompanied by the gain or loss of one ormore quanta of energy. Thus, Raman spectral features havebeen described in terms of frequency shis, or, simply, the wavenumber relative to that of the excitation.33,34 For this reason, theRaman shi canmeasure the energy required for a bond to shifrom the background state to the excited state. A larger Ramanshi indicates a greater change in energy for the activation ofbonds. In the presence of ozone, a little ozone energy maytransfer to the Mn–O bond, resulting in the Raman shi of the

This journal is © The Royal Society of Chemistry 2014

Mn–O bond. Interestingly, the Raman shi returned to itsoriginal value when it was exposed to air for several hours. Thisindicates that the changes in Raman shi arising from ozonetreatment are unstable, and the metastable state can recover tothe original stable state by itself. This phenomenon is alsoobserved in the other catalyst samples in this work. In conclu-sion, ozone reacted with the Mn–O bonds within [MnO6] octa-hedra perpendicular to the double chains to achieve a moreunstable state by energy transfer from ozone to the Mn–O bond.As a result, ozone became a more powerful reactive oxidationspecies. In addition, ozone treatment changes the band shapeand intensity of Mn3. The slope between the peaks at 635 cm�1

and 576 cm�1 became sharp aer treatment with ozone, con-rming the existence of active oxygen, which decreased the

RSC Adv., 2014, 4, 39167–39173 | 39171

RSC Advances Paper

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

polarisability of the Mn–O bond. Aer ozone treatment, theaverage number of Mn–O bonds that can be irradiated byRaman laser increased, and the intensity of the Mn–O bondsthen increased as well.35

Next, we discuss the case of b-MnO2 (Mn2). The Ramanspectrum of Mn2 (Fig. 8b) displays two sharp bands: the band at760 cm�1 is attributed to the B2g mode and involves antisym-metric Mn–O vibrations, while the more intense band at665 cm�1 results from the A1g mode.36 This mode correspondsto the skeletal vibration of the Mn–O bond in the [MnO6]octahedral frame. The two bands shied to 766 and 668 cm�1,respectively, when the ozone treatment time was extended to60 min. This phenomenon implied that ozone acted preferen-tially on the Mn–O bonds in the [MnO6] octahedra.

Finally, we discuss the case of g-MnO2 (Mn1). The [MnO6]octahedron chains formed (2� 1) tunnels, as shown in Fig.7d.37

Mn1 is characterized by two sharp peaks at 652 and 576 cm�1

due to the stretching mode of the Mn–O bond in [MnO6] octa-hedra. It should be emphasized that the more intense band at576 cm�1 belongs to ramsdellite g-MnO2. It has been reportedthat a sharp band appears at 654 cm�1 when Pr (the rate ofpyrolusite intergrowth) increased to 61%.37 Hence, the band at652 cm�1 should correspond to pyrolusite intergrowth. WhenMn1 was treated with ozone, some alternations also occurred.Aer treatment for 30 min, the band at 576 cm�1 did notchange, while the peak at 652 cm�1 shied to 663 cm�1. Whenthe treatment time was extended to 60min, two bands appearedat 660 and 579 cm�1. Therefore, pyrolusite g-MnO2 reacts moreeasily with ozone than ramsdellite g-MnO2. In other words,pyrolusite-type g-MnO2 determined the catalytic activity of Mn1,while ramsdellite-type g-MnO2 played a relatively small role.

4. Conclusion

a-, b- and g-MnO2 have been prepared by a hydrothermalmethod. The preparation method is easy to realize in a large-scale reactor and may provide a way to synthesize differentkinds of MnO2 and further investigate the inuence of phase.Based on Raman shis during ozonation, the sensitivity of thebonds to catalytic activity was determined. The observed activebonds are the Mn–O bonds perpendicular to the direction of the[MnO6] octahedral double chains of a-MnO2, the Mn–O bondsin the [MnO6] octahedral frameworks of b-MnO2 and the Mn–Obonds belonging to the pyrolusite-type structure of g-MnO2.TGA results conrmed that active surface oxygen was positivelycorrelated with the catalytic activity of a-MnO2, and that thelattice oxygen and bonded manganese of MnO2 contributed allthe catalytic activity of b- and g-MnO2.

Acknowledgements

The authors gratefully acknowledge the support from theNational Natural Science Foundation of China (no. 20903048,21005031, 21275065), the National Key Technology Researchand Development Program (2012BAD32B03-4), the Funda-mental Research Funds for the Central Universities

39172 | RSC Adv., 2014, 4, 39167–39173

(JUSRP21113, JUSRP51314B), and MOE & SAFEA for the 111Project (B13025).

Notes and references

1 X. Liu, Q. Wang, H. Zhao, L. Zhang, Y. Su and Y. Lv, Analyst,2012, 137, 4552–4558.

2 Y. Hasegawa, K. Fukumoto, T. Ishima, H. Yamamoto,M. Sano and T. Miyake, Appl. Catal., B, 2009, 89, 420–424.

3 H. C. Genuino, S. Dharmarathna, E. C. Njagi, M. C. Mei andS. L. Sui, J. Phys. Chem. C, 2012, 116, 12066–12078.

4 E. E. McCabe and C. Greaves, Chem. Mater., 2006, 18, 5774–5781.

5 M. M. Thackeray, Prog. Solid State Chem., 1997, 25, 1–4.6 C. M. Julien, M. Massot and C. Poinsignon, Spectrochim. Acta,Part A, 2004, 60, 689–700.

7 B. K. -Hordern, M. Ziołek and J. Nawrocki, Appl. Catal., B,2003, 46, 639–669.

8 J. Nawrocki and B. K. -Hordern, Appl. Catal., B, 2010, 99, 27–42.

9 L. Zhao, Z. Sun, J. Ma and H. Liu, Environ. Sci. Technol., 2009,43, 2047–2053.

10 H. Yang, S. Yang, L. Wu and W. Liu, Catal. Commun., 2011,15, 99–102.

11 J. Bing, L. Li, B. Lan, G. Liao, J. Zeng, Q. Zhang and X. Li,Appl. Catal., B, 2012, 115–116, 16–24.

12 S. Tong, W. Liu, W. Leng and Q. Zhang, Chemosphere, 2003,50, 1359–1364.

13 Y. M. Dong, H. X. Yang, K. He, S. Q. Song and A. M. Zhang,Appl. Catal., B, 2009, 85, 155–161.

14 A. Lv, C. Hu, Y. Nie and J. Qu, Appl. Catal., B, 2010, 100, 62–67.

15 M. Sui, J. Liu and L. Sheng, Appl. Catal., B, 2011, 106, 195–203.

16 M. Polverejan, J. C. Villegas and S. L. Suib, J. Am. Chem. Soc.,2004, 126, 7774–7775.

17 W. Hong, S. Iwamoto, S. Hosokawa, K. Wada, H. Kanai andM. Inoue, J. Catal., 2011, 277, 208–216.

18 X. Wang and Y. D. Li, Chem.–Eur. J., 2003, 9, 300–306.19 Y.-F. Shen, S. L. Suib and C.-L. O'Young, J. Am. Chem. Soc.,

1994, 116, 11020–11029.20 S. Devaraj and N. Munichandraiah, J. Phys. Chem. C, 2008,

112, 4406–4417.21 Y. Khan, S. K. Durrani, M. Mehmood and M. R. Khan, J.

Mater. Res., 2011, 26, 2268–2275.22 W.-M. Chen, L. Qie, Q.-G. Shao, L.-X. Yuan, W.-X. Zhang and

Y.-H. Huang, ACS Appl. Mater. Interfaces, 2012, 4, 3047–3053.23 D. Su, H.-J. Ahn and G. Wang, J. Mater. Chem. A, 2013, 1,

4845–4850.24 V. P. Santos, M. F. R. Pereira, J. J. M. Orfao and

J. L. Figueiredo, Appl. Catal., B, 2010, 99, 353–363.25 T. Rao, M. Q. Shen, L. W. Jia, J. J. Hao and J. Wang, Catal.

Commun., 2007, 8, 1743–1747.26 X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang andW. Shen,

Appl. Catal., B, 2006, 62, 265–273.27 H. Chen, A. Sayari, A. Adnot and F. Larachi, Appl. Catal., B,

2001, 32, 195–204.

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

13

Aug

ust 2

014.

Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 29

/10/

2014

07:

34:1

5.

View Article Online

28 M. A. Peluso, L. A. Gambaro, E. Pronsato, D. Gazzoli,H. J. Thomas and J. E. Sambeth, Catal. Today, 2008, 487,133–135.

29 T. Gao, H. Fjellvag and P. Norby, Anal. Chim. Acta, 2009, 648,235–239.

30 T. Gao, M. Glerup, F. Krumeich, R. Nesper, H. Fjellvag andP. Norby, J. Phys. Chem. C, 2008, 112, 13134–13140.

31 C. M. Julien, M. Massot and C. Poinsignon, Spectrochim.Acta, Part A, 2004, 60, 689–700.

32 G. Thrrell and J. Corset, Raman Microscopy: Developments andApplications, 1996, pp. 2–5.

This journal is © The Royal Society of Chemistry 2014

33 G. Gouadec and P. Colomban, Prog. Cryst. Growth Charact.,2007, 53, 1–56.

34 Q. Tu and C. Chang, Nanomedicine, 2012, 8, 545–558.35 T. Gao, H. Fjellvag and P. Norby, Nanotechnology, 2009, 20,

055610.36 Y. Xiong, Y. Xie, Z. Li and C. Wu, Chem.–Eur. J., 2003, 9,

1645–1651.37 C. Julien, M. Massot, S. Rangan, M. Lemal and D. Guyomard,

J. Raman Spectrosc., 2002, 33, 223–228.

RSC Adv., 2014, 4, 39167–39173 | 39173