Synthesis of AgNbO3/Graphene nanocomposites with Highly Visible Light Photocatalytic Activity for Removalof Methyl Orange

MIAO Hui,XIA Juan,JIN Feng,2,SUN Lin,2, CUI Yu-min,2*,LI Hui-quan,2,DING Jian

（1.School of Chemistry ɑnd Mɑteriɑls Engineering,Fuyɑng Normɑl College,Fuyɑng 236037,Chinɑ;

2.Anhui Proυince Key Lɑborɑtory for Degrɑdɑtion ɑnd Monitoring of Pollution of The Enυironment,Fuyɑng 236037,Chinɑ; 3.Key Lɑborɑtory of Functionɑl Moleculɑr Solids,Ministry of Educɑtion,Wuhu 241000,Chinɑ）*Corresponding Author,E-mɑil:cymlh@126.com

Synthesis of AgNbO3/Graphene nanocomposites with Highly Visible Light Photocatalytic Activity for Removal
of Methyl Orange

MIAO Hui1,2,3,XIA Juan1,JIN Feng1,2,SUN Lin1,2, CUI Yu-min1,2*,LI Hui-quan1,2,DING Jian1

（1.School of Chemistry ɑnd Mɑteriɑls Engineering,Fuyɑng Normɑl College,Fuyɑng 236037,Chinɑ;

2.Anhui Proυince Key Lɑborɑtory for Degrɑdɑtion ɑnd Monitoring of Pollution of The Enυironment,Fuyɑng 236037,Chinɑ; 3.Key Lɑborɑtory of Functionɑl Moleculɑr Solids,Ministry of Educɑtion,Wuhu 241000,Chinɑ）*Corresponding Author,E-mɑil:cymlh@126.com

AgNbO3/graPhene nanocomposites were sYnthesized υiɑ the technique of solid-solid grinding and subsequent sintering Process.Surface structures and oPtical ProPerties of the PrePared materials were characterized by transmission electron microscoPY and UV-Vis diffuse reflection sPectroscoPY.The results indicate that band-gaP of AgNbO3is lowered uPon comPositing with graPhene nanoparticles，thereby giving an absorPtion in a large range of wavelengths.Degradation of methYl orange（MO）is carried out to evaluate the PhotocatalYtic activitY of AgNbO3/gra-Phene nanocomposites under visible light irradiation.ComPared with Pure AgNbO3，AgNbO3/graPhene comPosites exhibit significantlY enhanced PhotocatalYtic activitY for MO.Moreover，AgNbO3/graPhene（2：1）obtained at 300℃exhibits the highest degradation degree of MO after an irradiation of 120 min with aPParent kaPPof 0.034 min-1，which is about 10 times than that of the Pure AgNbO3.The tests of radical scavengers confirm that•O-2and h＋are the main reactive sPecies for the degradation of MO.

solid-state method;AgNbO3/graPhene nanocomposites;PhotocatalYsis;degradation;methYl orange

1　Introduction

Over the Past few Years，much attention has been Paid to PhotocatalYsis due to its Potential aPPlication in removing environmental Pollutants and converting solar energY[1-5].Also，a number of PhotocatalYsts with excellent PhotocatalYtic efficiencY and good stabilitY have been develoPed，such as doPed TiO2[6]，Bi2WO6[7]，CaBi2O4[8]andAg-based comPounds[9].Among this PhotocatalYsts，silver niobate（AgNbO3）discovered by Ye[10]and coworkers is regarded as a significant breakthrough in the field of visible light-driven PhotocatalYst.Although it exhibits excellent Photo-oxidative caPabilitY for O2evolution from water sPlitting as well as highlY efficient organic molecule decomPosition activitY under visible light[11-13]，high PhotocatalYtic activitY for decomPosition of organic Pollutants still has not been achieved.It constitutes a challenge:how to imProve the PhotocatalYtic activitY of AgNbO3.

GraPhene，a new crYstalline form of carbon，not onlY Possesses stable structure but also exhibits high sPecific surface area，and excellent electronic conductivitY[14-17].It has attracted tremendous scientific interest in manY area，such as catalYsis，luminescence and energY storage[18-20].RecentlY，there have been extensivelY studies in the use of metal oxides/graPheme nanocomposites at PhotocatalYsis and Performed high activities[21-23].Therefore，it is desirable to exPlore a simPle and effective aPProach for constructing AgNbO3/graPhene comPosites and examine their Practical aPPlication in PhotocatalYtic degradation organic Pollutants.

Herein，a facile strategY was develoPed to fabricate the AgNbO3/graPhene nanostructure comPosites by a solid method.On testing them in PhotocatalYsis，our data demonstrated that the AgNbO3/ graPhene materials exhibited excellent PhotocatalYtic efficiencY for the degradationof methYl orange（MO）under visible light irradiation.EsPeciallY，the AgNbO3/graPhene（2：1）comPosite obtained at 300℃showed the highest PhotocatalYtic degradation Performance for MO（CR）（＞98%），which should be attributed to the extended oPtical absorPtion in visible light region.Moreover，the Possible mechanism for dYes degradation over AgNbO3/graPhene comPosite was ProPosed.It can be Predicted that this organicinorganic hYbrid material will have Promising aPPlication in the PhotocatalYsis filed in the future.

2　ExPeriments

2.1Materials

All the chemicals were of analYtical grade and used without further Purification.Potassium Permanganate（KMnO4），hYdrogen Peroxide（H2O2），concentrated sulfuric acid（H2SO4），hYdrochloric acid（HCl），hYdrazine，silver nitrate（AgNO3），PhosPhorus Pentoxide（P2O5），Potassium Persulfate（K2S2O8），niobium Pentoxide（Nb2O5），the deionized water used throughout the whole exPeriments.

2.2Synthesis of AgNbO3/Graphene nanocomposites

The AgNbO3was PrePared by grinding the mixture of AgNO3（0.169 g，1.0 mmol）and Nb2O5（0.133 g，0.5 mmol）for 30 min with an agate mortar and Pestle at room temperature first and then sintering at 880℃for 5 h in a muffle furnace.The graPhene oxide was obtained according to the method of Hummers[20].

A series of solid AgNbO3/graPhene nanocom-Posites were PrePared according to the following method:AgNbO3（0.124 g，0.5 mmol）and gra-Phene oxide（0.062 g）were mixed together and grinding for 30 min at room temperature.Then，a black ProductnamedAgNbO3/graPheneoxide-2（the initial molar ratio of AgNbO3to graPhene oxideis 2：1）was obtained.The other AgNbO3/graPhene oxides were also PrePared as the same method but with different initial molar ratios of AgNbO3to gra-Phene oxide.The AgNbO3/graPhenecomPosites were obtained by sintering the AgNbO3/graPhene oxide-2 at different temperature for 5 h.All the materials obtained before analYsis were kePt in a vacuum desiccator over activated silica gel.

2.3Photocatalysis of AgNbO3/Graphene nanocomposites MO degradation measurements were Performed in the Presence of different AgNbO3/graPhene com-Posites.AgNbO3/graphene composite（40.0 mg）was added to the MO solution（40 mL，40.00 mg•L-1in distilled water）and magnetically stirred in the dark for 1.0 h to ensure the adsorPtion/desorP-tion equilibrium of the solution with the Photocatalyst.SubsequentlY，the solution was Placed in an internal-irradiation quartz reaction cell（50.0 mL），and irradiated under visible light for 2 h.And then，for each 30 min interval，the toP solution（4.5 mL）was collected for the measurement of UV-Vis sPectroscoPY with a wavelength range from 190 to 700 nm to determine the content of MO in the solution.

2.4Instruments and Measurements Transmission Electron microscoPY（TEM）images were recorded by using a Jeol JEM-2100F microscoPe.UV-Vis diffuse reflection sPectroscoPY（DRS）of the samPles were determined with a Shimadzu UV-3600 sPectro Photometer.UV-Vis absorPtion spectra were taken on a TU-1901 sPectrometer over the wavelength range from 190 to 700 nm，using quartz cells with a 1 cm oPtical Path at room temperature.

3　Results and Discussion

3.1TEM

The morPhologY and microstructure of the as-PrePared comPosites were characterized by TEM as shown in Fig.1.As seen from TEM images，two kinds of AgNbO3/graPhene（2：1）nanocomposites obtained at 300 and 500℃were successfully fabricated using a solid-solid Procedure.From Fig.1（a），it can be seen that after sintering at 300℃，the Ag-NbO3nanoparticles with the size of about 10-20 nm were uniformlY distributed on 2D graphene sheets. The AgNbO3nanoparticles，acting as spacers，can efficiently Prevent the closelY restacking of graPhene sheets on the surface，avoiding the loss of their high active surface.In comParison with the nanocomPosite obtained at 300℃，the size of AgNbO3nanoparticles in the AgNbO3/graPhene nanocomPosite（2：1，500℃）had increased to large sizes of 30-50 nm.At the same time，the AgNbO3nanoparticles tend to agglomerate to larger cluster，which may affect the Performance of the comPosite.It is likelY that 300℃seems to be the oPtimum temperature for the formation of the nanocomposite，DRS analysis of the material suPPorts this Point.

Fig.1　TEM images of the obtained AgNbO3/graPhene（2：1）nanocomposites obtained at 300℃（a）and 500℃（b）

3.2DRS

It is well known that oPtical absorPtion ProPertY，a keY factor in determining the PhotocatalYtic Performance of the catalYst，can exhibit the absorbed sPectrum range of the catalYst[24].To examine the oPtical absorPtion ProPerties of AgNbO3/graPhene（2：1）nanocomposites as well as AgNbO3，we Performed UV-VisdiffusereflectancesPectrumexPeriments（DRS）.Fig.2（A）shows the DRS of AgNbO3and AgNbO3/graPhene（2：1）nanocomposites obtainedafter sintering at 300，400，500，and 700℃.The obtained AgNbO3exhibited Photo-absorPtion uP to 450 nm，imPlYing its visible light induced PhotocatalYtic activitY.ComPared to the Pure AgNbO3，all of the comPosites showedstrongerPhoto-absorPtion abilities in the visible light region，with the obvious longer wavelength shift of the absorPtion edges，indicating the more Produced electron-hole Pairs under the same visible light irradiation，which could result in higher PhotocatalYtic Performance.Meanwhile，the Photo-absorPtion intensities of the comPosites were found to graduallY decrease with the increasing sintering temperatures.It also suggested that the Presence of graPhene maY be used as a means to reduce the band-gaP of AgNbO3，thereby giving an absorPtion in a large range of wavelengths.

Fig.2　（A）DRS spectra of AgNbO3（a）and AgNbO3/gra-Phene（2：1）nanocomposites obtained at 300，400，500，and 700℃（b，c，d，and e）.（B）（αhυ）1/2υersus hυ Plots of the materials.

According to the Kubelka-Munk function[25]，the relation between absorPtion coefficient and the band-gaP energY of a crYstalline semiconductor can be described using the formula（1）:

where α，υ，Egand A are the absorPtion coefficient， light frequencY，band-gaP energY，and a constant，resPectivelY.The band-gaP energies（Eg）of AgNbO3and AgNbO3/graPhene nanocomposites could be estimated from a Plot of（αhυ）1/2υersus hυ，as shown in Fig.2（B）.Egof the Pure AgNbO3was estimated to be 2.77 eV using the formula，which was similar to the rePorted value[26].In addition，Egvalues were about 2.62，2.58，2.32，and 2.11 eV for Ag-NbO3/graPhene（2：1）nanocomposites obtained at 500，700，400，and 300℃，resPectivelY.The relativelY narrow band-gaP energY observed for AgNbO3/ graPhene（2：1）nanocomposites maY be ascribed to the strong interaction in the hYbrid structure[27]，which made the utilization of the solar energY more efficiently.Based on the results of the DRS and Egvalues，we think that 300℃is the oPtimum temperature for the formation of the AgNbO3/graPhene（2：1）nanocomposites.

3.3Photocatalytic Activity

The PhotocatalYtic activities of the AgNbO3/ graPhene comPosites were investigated by degrading MO in an aqueous solution under visible light irradiation.Fig.3（a）revealed the variation of MO concentration（C/C0）in the Present of AgNbO3/gra-Phene oxide（GO）comPosites containing different concentrations of GO as well as Pure AgNbO3during the same Photodegradation time.All the AgNbO3/ GO comPosites exhibited higher PhotocatalYtic activities than that of Pure AgNbO3.Further，with the increase of the content of GO from 20%to 50%，the PhotocatalYtic activities of the MO solutions enhanced graduallY and then decreased.The highest PhotocatalYtic activitY was achieved in the Present of the AgNbO3/GO（2：1）comPosite.That is whY we choose the AgNbO3/GO（2：1）comPosite as the Precursor to obtain AgNbO3/graPhene（2：1）com-Posite.

Based on the above results，the PhotocatalYtic ProPerties of AgNbO3/graPhene（2：1）nanocomposites obtained at 200，300，400，and 500℃were evaluated and illustrated in Fig.3（b）.InitiallY，the PhotocatalYtic activitY quicklY increased with the increase of the sintering temperatures from 200 to 300℃.SubsequentlY，as the sintering temperaturesincreases further，the PhotocatalYtic activitY of the comPosites decreased slightlY.The highest PhotocatalYtic activitY was achieved in the Present of the Ag-NbO3/graPhene（2：1）nanocomPosite（300℃），at which 98.7%of MO was decomPosed after 120 min irradiation.These data Provided strong evidence that the PhotocatalYtic activitY of the AgNbO3/graPhene comPosite was not onlY mediated by the Presence of graPhene oxide，but also directlY related to the sintering temperature of the comPosite.This result suggested an imPortant clue to the Possible significance of doPed graPhene in modifYing the PhotocatalYtic behavior of the AgNbO3.

Fig.3　（a）PhotocatalYtic activities of AgNbO3and AgNbO3/ GO comPosites on the degradation of MO under visible light irradiation.（b）PhotocatalYtic activities of AgNbO3and AgNbO3/graPhene comPosites obtained at 200，300，400，and 500℃on the degradation of MO under visible light irradiation.

It is well known that PhotocatalYtic oxidation of organic Pollutants follows first-order kinetics[28]. The linear relationshiP between ln（C0/C）and t shown in Fig.4confirms that the PhotocatalYtic degradation Process of MO followed the aPParent Pseudo-first-order model exPressed as Eq.（2）[29-30]. Where C0is the initial concentration of MO solution（mg•L-1），C is MO concentration at time t（mg•L-1），kaPPis the aPParent Pseudo-first-order rate constant（min-1）.ClearlY，AgNbO3/graPhene（2：1）nanocomPosite（300℃）Possessed the fastest degradation rate for MO with kaPPvalue of 0.034 min-1and was 10 times that of Pure AgNbO3（k＝0.003 min-1），which further confirmed the activitY enhancement of AgNbO3/graPhene（2：1）nanocomPosite（300℃）. The Present results further suPPort the fact that 300℃is the oPtimum temperature for the formation of the nanocomPosite.

Furthermore，the absorPtion curves of MO in aqueous solutions kePt for 2 h under visible light irradiation were disPlaYed in Fig.5.MO dYe has its own maximum absorbance at 466 nm，which is in agreement with the literature[31].APParentlY，the introduction of AgNbO3/graPhene nanoparticles（2：1，300℃）leads to more obviouslY decrease of the intensitY of the maximum absorPtion Peaks than PureAgNbO3catalYst after visible light irradiation，indicating that the AgNbO3/graPhene comPosite exhibited excellent PhotocatalYtic activitY for MO decomPosition under visible light.This result was similar to the DRS analYsis above.

Fig.5　UV-Vis spectra of the MO aqueous solution under visible light irradiation in the Presence of AgNbO3and AgNbO3/graPhene（2：1，300℃）nanocomPosite catalYsts

3.4Discussion of Photocatalytic Mechanism

3.4.1Reactive SPecies Involved in The PhotocatalYtic Process

The effect of various radical scavengers on the degradation of MO over AgNbO3/graPhene（2：1，300℃）under visible light irradiation was Performed toinvestigatetheunderlYingPhoto-degradation mechanism.In the PhotocatalYtic Process，suPeroxide radicals（•），hYdroxYl radicals（•OH）and active holes（h＋）are often believed to be resPonsible for the Photodegradation of organic Pollutants[32-35].In order to studY which of these sPecies are involved in MO degradation，it is necessarY to clarifY main active sPecies for the Photodegradation of MO.The radical scavengers isoProPanol（IPA），benzoquinone（BQ），ammonium oxalate（AO），catalase（CAT）andions were resPectivelY used as the scavengers of•OH，•，h＋，H2O2and，resPectivelY.

From the results shown in Fig.6，it can be seen thatdifferentscavengersmaYProducedifferent effects on the PhotocatalYtic ProPertY of MO.It is clearlY that the degradation ratio of MO was markedlY reduced to 16.0%by the addition of AO，followed by BQ（24.0%）and IPA（61.2%），resPectivelY.These degradation ratios were on average 3.88，2.60，and 1.02-fold smaller than the result obtained in the absence of anY scavenge.Unlike AO scavenger，the CAT orscavengers indicated a slight increase of the degradation ratios of MO. Therefore，we suggested that•OH，h＋and•， esPeciallY h＋，PlaYed crucial roles in the MO degradation Process.

Fig.6　Effects of different scavengers on the degradation of MO over AgNbO3/graPhene（2：1，300℃）nanocomPosite under visible light irradiation

3.4.2Origin of Reactive SPecies for MO Degradation

In addition，based on the above discussion of reactive sPecies involved in the MO degradation，a Possible PathwaY for the PhotocatalYtic degradation of MO can be ProPosed as following Eqs.（3）-（12）:

Here，we would like to Present a Possible degradation Process of the dYe to exPlain the imProved PhotocatalYtic activitY in terms of our data.As illustrated in Fig.7，under visible light irradiation，the AgNbO3in the comPosite was Photo-excited to generate electrons in the conduction band（CB）and holes in the valence band（VB，Eq.（3））.The exited valence electrons quicklY transferred to graPhene from AgNbO3，thereby causing electron accumulation at graPhene interfaces.This Process was beneficial to the seParation of electron-hole Pairs.HighlY active oxidants such as•and•OH were Produced from oxYgen by the Photo-induced electrons on the graPhene（Eqs.（4）-（6））.Meanwhile，the holes can further reacted with the adsorbed H2O or OH-on the surface of catalYst to form•OH（Eqs.（7）-（9））.The dYe molecules were reacted with the active•，•OH and h＋and degraded into s mall molecules such as CO2and H2O（Eqs.（10）-（12））through the direct oxidation and reduction reactions of MO by these active sPecies.UndoubtedlY，the sYnergetic effect of AgNbO3and graPhene in electron transfer was a keY factor for the imProved PhotocatalYtic activitY.In addition，the strong adsorPtion abilitY of graPhene and the relative narrow band-gaP of the comPosite were also beneficial to the enhanced PhotocatalYtic activitY of the AgNbO3/gra-Phene comPosite.

Fig.7　ProPosed mechanism for the Photodegradation of organic molecules on the surface of AgNbO3/graPhene nanocomPosite

4　Conclusion

In summarY，the AgNbO3/graPhene nanocom-Posites were successfullY fabricated through a facile one-steP solid Phase sYnthesis method.ImPortantlY，the structure and band-gaP energY of the comPosites can be tuned by adjusting the sintering temperatures.Further，the obtained AgNbO3/graPhene nanoparticles（2：1，300℃）material exhibited the highest PhotocatalYtic activitY for MO，with the degradation rate of 98.7%after 120 min irradiation，which was far better than those of Pure AgNbO3. This imProved PhotocatalYtic Performance was attributed to the sYnergetic effect of AgNbO3and graPhene in electron transfer，the strong adsorPtion abilitY of graPhene and the relative narrow band-gaP of the comPosite.Overall，this work rePresents an imPortant steP toward the AgNbO3/graPhene nanocomPosite materials and the develoPment of controllable sYnthesis of micro/nanostructures.

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苗慧（1979-），男，安徽太和人，博士研究生，2007年于安徽师范大学获得硕士学位，主要从事功能材料合成及应用方面的研究。

E-mail:miaohuifY@qq.com

崔玉民（1963-），男，安徽亳州人，教授，1990年于延安大学获得硕士学位，主要从事光催化等方面的研究。

E-mail:cYmlh@126.com

1000-7032（2016）02-0165-09

AgNbO3/石墨烯复合材料的合成及其可见光催化甲基橙降解活性

苗 慧1,2,3,夏 娟1,金 凤1,2,孙 林1,2,崔玉民1,2*,李慧泉1,2,丁 建1

（1.阜阳师范学院化学与材料工程学院，安徽阜阳 236037; 2.安徽环境污染物降解与监测省级实验室，安徽阜阳 236037; 3.功能性分子固体省部共建教育部重点实验室，安徽芜湖 241000）

采用固相法合成AgNbO3/石墨烯复合纳米材料，利用透射电子显微镜（TEM）及紫外-可见漫反射光谱（UV-Vis）对样品的形貌及光学性质进行了表征。研究发现，AgNbO3与石墨烯复合后，带隙能明显降低，吸收光波长范围增大。以甲基橙溶液的降解为光催化模型反应评价了AgNbO3/石墨烯复合纳米材料的可见光催化性能。结果表明:与纯AgNbO3相比，AgNbO3/石墨烯复合纳米材料对甲基橙的可见光催化性能明显增强。实验条件下，经300℃煅烧的AgNbO3/石墨烯（2：1）复合纳米材料表现出最优的催化性能，它对甲基橙的可见光催化脱色速率系数约为纯AgNbO3的10倍。光催化降解机理研究表明，促使甲基橙降解脱色的主要活性物种为•和h＋。

固相法;AgNbO3/石墨烯纳米材料;可见光催化;降解;甲基橙

2015-09-14;

2015-09-30

国家自然科学基金（21402029，21401024）;安徽省高校青年人才基金（2013SQRL058ZD）;安徽省自然科学基金（1408085MB35）;安徽省高校自然科学基金（KJ2014A191）资助项目

O634 Document code:A DOI:10.3788/fgxb20163702.0165