郭天宇,范祥瑞,白德豪,祁 彧,王紅濤*

異質(zhì)結(jié)CeO2/BiOBr的構(gòu)筑及其光催化降解羅丹明B

郭天宇1,2,范祥瑞1,白德豪3,祁 彧1,王紅濤1*

(1.太原理工大學(xué)環(huán)境科學(xué)與工程學(xué)院,山西 晉中 030600；2.氣體能源高效利用山西省重點(diǎn)實(shí)驗(yàn)室,山西 太原 030024；3.中石油煤層氣有限責(zé)任公司忻州分公司,山西 忻州 036600)

通過(guò)水熱法合成了CeO2、BiOBr和不同物質(zhì)的量比的CeO2/BiOBr復(fù)合光催化劑,采用不同的測(cè)試手段對(duì)光催化劑的形貌、結(jié)構(gòu)和光電化學(xué)性能進(jìn)行了表征,并通過(guò)可見(jiàn)光照射下羅丹明B (RhB)的降解來(lái)評(píng)價(jià)其光催化性能.結(jié)果表明,引入適量CeO2(10%)的CeO2/BiOBr (CB-10)異質(zhì)結(jié)顯著地增強(qiáng)了光催化活性,在30min內(nèi)可降解99%的RhB,且表現(xiàn)出優(yōu)異的可回收性和循環(huán)穩(wěn)定性.另當(dāng)pH值為3,復(fù)合催化劑與RhB濃度分別為0.5g/L、20mg/L時(shí),降解效率在20min內(nèi)可達(dá)最高(96.1%).根據(jù)自由基捕獲試驗(yàn)和催化劑的能帶結(jié)構(gòu),提出了CeO2/BiOBr異質(zhì)結(jié)光催化過(guò)程可能的反應(yīng)機(jī)理.研究結(jié)果可為制備Bi 基復(fù)合催化劑提供一種新的思路與方法.

光催化降解；CeO2/BiOBr；異質(zhì)結(jié)；羅丹明B

高濃度的有機(jī)染料廣泛存在于紡織工業(yè)廢水中,造成了環(huán)境污染與水資源短缺,工業(yè)廢水中的污染物具有很強(qiáng)的穩(wěn)定性和持久性,對(duì)人類(lèi)具有累積毒性與致癌性[1-2].其中,羅丹明B (RhB)作為一種具有熒光效應(yīng)的合成染料,具有色澤深、結(jié)構(gòu)復(fù)雜、化學(xué)穩(wěn)定性強(qiáng)等特點(diǎn)[3-4].因此,亟需開(kāi)發(fā)一種高效、綠色且廉價(jià)的去除廢水中有機(jī)污染物的處理技術(shù).

目前,已有去除RhB等有機(jī)污染物的方法可分為化學(xué)、物理和生物三種方法,即絮凝、膜分離、吸附、高級(jí)氧化、電化學(xué)、光催化、生物降解等工藝[5].然而,物理與化學(xué)方法主要是通過(guò)降低染料濃度,將染料從液相轉(zhuǎn)移到固相,容易造成二次污染[6-7].光催化技術(shù)作為一種新的光化學(xué)技術(shù),具有高效、綠色環(huán)保、成本低、反應(yīng)條件溫和及壽命長(zhǎng)等優(yōu)點(diǎn)[3,8-9],且光催化材料具有無(wú)毒、經(jīng)濟(jì)等特點(diǎn),因此去除有機(jī)染料的效率相對(duì)較高.

在已有報(bào)道的光催化劑中,作為一種典型的p型半導(dǎo)體,BiOBr由于其具有合適的帶隙(E=2.64～ 2.9eV)[10]以及良好的化學(xué)穩(wěn)定性,且具有四方馬氏體和特殊的層狀結(jié)構(gòu),提升了光電化學(xué)性能并促進(jìn)了載流子的分離,有效提高了光催化活性,因而在光催化領(lǐng)域受到了廣泛的關(guān)注,但其應(yīng)用受限于可見(jiàn)光吸收能力差和高載流子復(fù)合速率[10-12].有研究報(bào)道,構(gòu)筑BiOBr異質(zhì)結(jié)結(jié)構(gòu)可以有效地促進(jìn)光生電子空穴對(duì)的分離和提高可見(jiàn)光的利用率[13-14].近年來(lái),氧化鈰(CeO2)因其廉價(jià)、環(huán)境友好及良好的儲(chǔ)放氧能力等突出特點(diǎn)而在光催化領(lǐng)域備受關(guān)注[15-16].在構(gòu)建異質(zhì)結(jié)時(shí),CeO2的引入可以抑制光生電子空穴對(duì)的復(fù)合,增加化學(xué)吸附氧的濃度,從而有利于光催化性能的提高.例如,Wang等[17]構(gòu)筑了CeO2/TiO2異質(zhì)結(jié)結(jié)構(gòu),抑制了光致載流子的重組,擴(kuò)大了光響應(yīng)范圍,從而提高了苯酚的降解率.Liu等[18]合成了具有Z型異質(zhì)結(jié)和氧空位的CeO2/Bi7O9I3花狀納米球,促進(jìn)了電子的傳遞,提高了對(duì)重金屬污染物Hg0的去除率.基于上述分析,本文通過(guò)水熱法制備了CeO2/BiOBr光催化材料,并應(yīng)用于可見(jiàn)光下降解RhB,以探究材料的光催化降解性能,采用多種表征技術(shù)對(duì)CeO2/BiOBr光催化劑微觀結(jié)構(gòu)及光電性能進(jìn)行分析,且根據(jù)自由基捕獲實(shí)驗(yàn)和能帶結(jié)構(gòu),探討了該光催化材料的作用機(jī)制.

1 材料與方法

1.1 試劑

本實(shí)驗(yàn)所有化學(xué)藥品均為分析純.五水合硝酸鉍(Bi(NO3)3·5H2O)、溴化鉀(KBr)、六水合硝酸鈰(Ce (NO3)3·6H2O)、異丙醇(IPA)、對(duì)苯醌(BQ)、乙二胺四乙酸二鈉(EDTA-2Na)、無(wú)水甲醇(CH3OH)均購(gòu)自國(guó)藥集團(tuán)化學(xué)試劑有限公司,乙二醇((CH2OH)2)購(gòu)自天津科密歐化學(xué)試劑有限公司,氫氧化鈉(NaOH)購(gòu)自天津致遠(yuǎn)化學(xué)試劑有限公司.

1.2 材料的制備

1.2.1 CeO2的制備 將2.60g的Ce(NO3)3·6H2O溶解于10mL的去離子水中,磁力攪拌至完全溶解后,得到溶液A;另15mL的去離子水中加入8g的NaOH,同樣待完全溶解后記為溶液B.將溶液B緩慢倒入溶液A中,磁力攪拌30min后,將混合溶液轉(zhuǎn)移至100mL的聚四氟乙烯內(nèi)襯的反應(yīng)釜中,在180oC條件下反應(yīng)12h.為了去除雜質(zhì),將反應(yīng)后的樣品先后用去離子水和無(wú)水乙醇各洗滌三次,最后在80oC干燥12h.

1.2.2 CeO2/BiOBr的制備 取3mmol的Bi(NO3)3·5H2O在磁力攪拌下加入到24mL乙二醇與12mL無(wú)水甲醇的混合溶液中,攪拌至完全溶解,得到溶液A;取3mmol的溴化鉀和物質(zhì)的量比分別為5%、10%以及15%的CeO2分別分散在10mL的去離子水中并超聲10min,得到懸浮液B.將懸浮液B緩慢倒入到溶液A中,并在室溫下攪拌30min后,將上述混合溶液轉(zhuǎn)移到100mL的反應(yīng)釜內(nèi),在180oC加熱12h;待反應(yīng)釜冷卻后,取出樣品,經(jīng)去離子水和無(wú)水乙醇分別洗滌3次,然后離心,得到的樣品在80oC干燥12h后,即得到CeO2/BiOBr復(fù)合光催化劑.根據(jù)所加CeO2量分別記為CB-5, CB-10, CB-15.為了進(jìn)行對(duì)比實(shí)驗(yàn),在不加CeO2時(shí),用相同的方法合成了純相BiOBr.

1.3 光催化材料的表征

采用X射線粉末衍射儀(XRD, Rigaku MiniFlex X),以Cu Kα為輻射源測(cè)定樣品的晶型和物相組成, 掃描范圍為2=10°～80°.通過(guò)掃描電子顯微鏡(SEM, SU8180)觀察樣品的形貌和結(jié)構(gòu).利用X射線光電子能譜(XPS, Thermo ESCALAB 250XI)表征樣品的元素組成和價(jià)態(tài).采用物理吸附儀(Micromeritics, Tristar Ⅱ 3020)分析N2吸附-脫附曲線和孔徑分布.通過(guò)紫外可見(jiàn)漫反射光譜(UV- Vis DRS, UV-1800)測(cè)定樣品的光學(xué)吸收性能.使用光致發(fā)光光譜(PL, Hitachi F-4700)對(duì)樣品的電子-空穴分離效率進(jìn)行分析.采用電化學(xué)工作站(VersaSTAT-3, Ametek Princeton, USA)測(cè)試樣品的電化學(xué)阻抗和瞬態(tài)光電流.在三電極體系中,使用Pt網(wǎng)和Ag/AgCl電極分別作為對(duì)電極和參比電極,涂覆光催化材料的ITO電極作為工作電極. 0.5mol/L Na2SO4作為電解液溶液.

1.4 光催化降解實(shí)驗(yàn)

利用光反應(yīng)設(shè)備(PLS-SEX300, perfect, China)在可見(jiàn)光照射下降解RhB染料,來(lái)測(cè)定合成材料的光催化活性.采用300W裝有420nm濾光片的氙燈作為可見(jiàn)光光源.首先,將光催化劑加入到100mL的RhB溶液中,并轉(zhuǎn)移到光反應(yīng)器中,在黑暗環(huán)境下將混合溶液進(jìn)行磁力攪拌30min,以達(dá)到吸附-脫附平衡.然后,開(kāi)啟光源,在可見(jiàn)光照射下進(jìn)行光催化降解,實(shí)驗(yàn)期間每隔5min取出3mL反應(yīng)液,進(jìn)行離心分離后,使用0.22μm聚醚砜濾膜進(jìn)行過(guò)濾,所得濾液用紫外可見(jiàn)分光光度計(jì)(=554nm)測(cè)量RhB的濃度.此外,研究了催化劑用量(0.1, 0.3, 0.5, 0.7和0.9g/L), RhB濃度(10, 20, 30和40mg/L),以及溶液pH值(3, 5, 7, 9, 11)對(duì)RhB去除效率的影響.

1.5 自由基捕獲實(shí)驗(yàn)

為了探究光催化材料的活性成分,對(duì)RhB降解反應(yīng)進(jìn)行自由基捕獲實(shí)驗(yàn).使用1mmol的乙二胺四乙酸二鈉(EDTA-2Na)、對(duì)苯醌(BQ)和異丙醇(IPA)分別用來(lái)捕獲空穴(h+)、超氧自由基(·O2-)和羥基自由基(·OH).

1.6 數(shù)據(jù)處理與表示方法

利用Excel 2016軟件對(duì)RhB降解過(guò)程中的濃度進(jìn)行分類(lèi)整理,并用降解公式進(jìn)行RhB的降解效率計(jì)算:E= (1-C/0) × 100%,其中,E為RhB的降解率,0和C分別為反應(yīng)前和反應(yīng)時(shí)刻的RhB濃度;通過(guò)Origin 2018軟件繪制RhB濃度隨降解時(shí)間的變化曲線并對(duì)RhB降解的動(dòng)力學(xué)進(jìn)行擬合.

2 結(jié)果與討論

2.1 材料的表征

2.1.1 XRD分析 采用X射線衍射儀對(duì)所制備樣品的晶相結(jié)構(gòu)進(jìn)行分析,如圖1所示.由曲線可以看出,在2為10.93°, 21.97°, 25.20°, 31.70°, 32.22°, 39.30°, 46.22°, 50.67°, 57.15°和76.67°處的特征峰與四方晶相BiOBr(JCPDs No. 09-0393)的(001), (002), (101), (102), (110), (112), (200), (104), (212)和(310)晶面衍射峰相吻合[19-20].CeO2樣品在2為28.55°, 33.08°, 47.48°, 56.33°處出現(xiàn)的衍射峰分別對(duì)應(yīng)于立方螢石相CeO2的(111), (200), (220), (311)晶面(JCPDS No. 34-0394)[21-22].此外,在復(fù)合材料中,CeO2在2為28.55°處可觀察到一個(gè)衍射峰,但這個(gè)衍射峰較弱,說(shuō)明其結(jié)晶度較低[23]或CeO2的含量較低[24],同時(shí)說(shuō)明了CeO2成功地負(fù)載到了BiOBr材料中.隨著CeO2含量的增加,BiOBr材料的特征峰并沒(méi)有發(fā)生明顯的變化,說(shuō)明CeO2的加入并沒(méi)有改變BiOBr樣品的結(jié)晶度和晶體類(lèi)型.

2.1.2 形貌和結(jié)構(gòu)分析 通過(guò)SEM來(lái)觀察BiOBr和CeO2/BiOBr樣品的微觀形貌和結(jié)構(gòu),結(jié)果見(jiàn)圖2.圖2(a), (b)表明所制備的BiOBr由緊密堆積的納米片和尺寸較小的納米片所組成.適量的CeO2引入(CB-5和CB-10樣品),使納米片的尺寸變小,且呈現(xiàn)由納米片自組裝成的類(lèi)花狀結(jié)構(gòu)(圖2(c)-(f)),此結(jié)構(gòu)有利于增加光反射并增強(qiáng)有效光反應(yīng)表面,從而提高光催化性能[25].進(jìn)一步,過(guò)量的CeO2(CB-15樣品)破壞了BiOBr類(lèi)花狀結(jié)構(gòu),呈現(xiàn)出尺寸較小的類(lèi)正方形納米片形貌(圖2(g), (h)).可以看出, CeO2的引入及其含量的變化會(huì)影響B(tài)iOBr納米片的形貌、結(jié)構(gòu)和尺寸大小.然而,并未觀察到明顯的CeO2形貌,這可能是由于SEM只能檢測(cè)到樣品的表面形貌,且XRD已經(jīng)證實(shí)了CeO2的存在,可推測(cè)出在復(fù)合催化劑中CeO2的含量較少,被BiOBr覆蓋在了材料的內(nèi)部.EDS譜圖進(jìn)一步證實(shí)了CB-10樣品由Bi, O, Br和Ce 4種元素所組成(圖2(i)-(l)),同時(shí)表明了CeO2/BiOBr復(fù)合材料的成功合成.

圖1 BiOBr、CeO2和CeO2/BiOBr樣品的XRD譜圖

圖2 不同樣品的SEM圖和CB-10樣品的EDS圖(i～l)

a,b. BiOBr;c,d. CB-5; e,f. CB-10; g,h. CB-15

2.1.3 XPS分析 CB-10樣品的化學(xué)組成與元素價(jià)態(tài)用XPS進(jìn)行分析.從XPS全譜可知(圖3(a)), CB-10復(fù)合材料存在Bi、O、Br、Ce和C元素,說(shuō)明CeO2成功地復(fù)合到BiOBr中,C 1s峰的出現(xiàn)來(lái)源于設(shè)備中的校準(zhǔn)峰.圖3(b)為Bi 4f的高分辨XPS能譜,在158.98eV以及164.30eV處的特征峰與BiOBr中的Bi3+4f7/2和4f5/2相對(duì)應(yīng)[26].O 1s的XPS能譜圖中(圖3(c))位于結(jié)合能529.91和531.31eV處的兩個(gè)峰分別歸功于吸附氧和CB-10中的晶格氧(Bi-O鍵和Ce-O鍵)[27-28].從Br 3d的XPS能譜圖中可知(圖3(d)),結(jié)合能位于67.99與69.03eV處的兩個(gè)特征峰分別對(duì)應(yīng)于Br 3d5/2和Br 3d3/2[29].在Ce 3d精細(xì)能譜中(圖3(e)),可以擬合為8個(gè)特征峰,其中,916.71, 906.52, 900.72, 898.05, 889.21和882.20eV對(duì)應(yīng)Ce4+, 902.11和884.12eV對(duì)應(yīng)Ce3+, 表明Ce3+和Ce4+在CeO2中共存[21,30].

2.1.4 比表面積和孔結(jié)構(gòu)分析 圖4(a)為BiOBr和CB-10樣品的N2吸附-脫附等溫線圖,這兩種材料均表現(xiàn)出IV型等溫線和H3滯后環(huán),表明樣品中存在介孔結(jié)構(gòu),且屬于片狀粒子堆積形成的狹縫孔[31-32]. BiOBr和CB-10樣品的比表面積分別為11.79和13.01m2/g.圖4(b)顯示了樣品的孔徑分布,通過(guò)BJH法計(jì)算樣品的孔徑大小,得出BiOBr和CB-10的平均孔徑分別為14.80和12.77nm.

圖3 CB-10樣品的XPS光譜圖

圖4 BiOBr和CB-10樣品的氮?dú)馕?脫吸等溫線和孔徑分布曲線

2.2 光學(xué)與光電性能

利用紫外-可見(jiàn)漫反射光譜儀(UV-Vis DRS)來(lái)研究材料的能帶結(jié)構(gòu)和對(duì)可見(jiàn)光的吸收能力.如圖5(a)所示,BiOBr的吸收帶邊在440nm附近,由于CeO2的引入,導(dǎo)致CeO2/BiOBr復(fù)合光催化劑的吸收邊相較于純BiOBr均發(fā)生了一定程度的紅移.此結(jié)果表明,BiOBr與CeO2復(fù)合后,拓寬了可見(jiàn)光的利用范圍.半導(dǎo)體的帶隙可以根據(jù)Tauc方法獲得[33],其公式為=(-E)/2,其中,,,E和分別是吸收系數(shù)、普朗克常數(shù)、光頻率、帶隙能量以及常數(shù),BiOBr與CeO2均為間接半導(dǎo)體且值為4[34],得出BiOBr, CeO2和CB-10樣品的帶隙值分別為2.54, 2.68和2.48eV (圖5(b)),即CB-10的帶隙值較小,這有利于電子的激發(fā)和轉(zhuǎn)移,從而提高了光催化活性[35].

為了分析光生-電子空穴對(duì)的分離效率,對(duì)BiOBr和CB-10樣品進(jìn)行了光致發(fā)光光譜(PL)表征,結(jié)果如圖 5(c)所示.可以看出,相比于純BiOBr和CeO2, CB-10材料的PL峰強(qiáng)度更低,較低的峰強(qiáng)度表明光致載流子的復(fù)合速率較低[36],說(shuō)明CeO2的引入有效抑制了光生電子-空穴對(duì)的重組,產(chǎn)生了更多活躍的光生電子-空穴對(duì),這有利于光催化活性的提高.

為進(jìn)一步探究電荷分離和轉(zhuǎn)移效率,分別對(duì)光電流響應(yīng)和電化學(xué)阻抗進(jìn)行了測(cè)試.如圖6(a)所示,顯然CB-10具有更強(qiáng)的光電流響應(yīng),其電流密度約為2.0μA/cm2,比純BiOBr高出1.42倍,說(shuō)明BiOBr與CeO2構(gòu)建的異質(zhì)結(jié)結(jié)構(gòu)提升了光生載流子的遷移效率,有效地阻礙了電子-空穴對(duì)的復(fù)合[37].從圖6(b)可以看出,CB-10的弧半徑明顯小于BiOBr的弧半徑,通常情況下,弧半徑越小說(shuō)明電荷界面?zhèn)鬏旊娮柙叫38],表明復(fù)合光催化材料有效地降低了電荷的傳輸電阻,從而有利于光催化活性的提升.這一結(jié)果與其瞬態(tài)光電流結(jié)果相一致.

圖6 BiOBr和CB-10的瞬態(tài)光電流響應(yīng)曲線和電化學(xué)阻抗譜

2.3 光催化活性

為研究所制備樣品的光催化性能,本文設(shè)計(jì)在可見(jiàn)光照射下,選擇100mL濃度為20mg/L的RhB為光催化降解的目標(biāo)污染物(催化劑用量為50mg),如圖7(a)所示.在黑暗條件下,將光催化劑與染料持續(xù)攪拌30min后,所有樣品在暗反應(yīng)中均表現(xiàn)出一定的吸附性能.在光照與未添加催化劑的條件下, RhB濃度幾乎未發(fā)生變化,表明其具有較強(qiáng)的穩(wěn)定結(jié)構(gòu),可以忽略光解對(duì)反應(yīng)的影響.而在加入催化劑后,CeO2對(duì)RhB染料幾乎沒(méi)有降解作用, BiOBr, CB-5, CB-10和CB-15在20min光照下, RhB的降解率分別達(dá)到了83.3%, 85.8%, 94.7%和83.9%,即CeO2的引入后,使得復(fù)合材料的光催化性能獲得了提升,當(dāng)CeO2的負(fù)載量為10%時(shí),光催化性能最高.這是因?yàn)镃eO2與BiOBr之間形成了異質(zhì)結(jié),提高了可見(jiàn)光的利用率和光生電子-空穴對(duì)的轉(zhuǎn)移及分離.當(dāng)CeO2負(fù)載量進(jìn)一步增多時(shí),光催化活性反而降低,這可能是因?yàn)檫^(guò)量的CeO2抑制了光生載流子的轉(zhuǎn)移所致[39].通過(guò)偽一級(jí)反應(yīng)動(dòng)力學(xué)對(duì)數(shù)據(jù)進(jìn)行擬合,如圖7(b)所示, CB-10的降解速率常數(shù)為0.200min–1,且分別為BiOBr (0.108min–1)、CB-5 (0.116min–1)及CB-15 (0.112min–1)的1.85、1.72和1.79倍,進(jìn)一步證明了CB-10材料顯示了優(yōu)異的光降解性能.

圖7 可見(jiàn)光下RhB在BiOBr及其復(fù)合材料上的光降解曲線及其偽一階動(dòng)力學(xué)

通過(guò)改變催化劑投加量(0.1～0.9g/L)、RhB初始濃度(10～40mg/L)和初始溶液pH值(3～11),評(píng)估了不同條件下復(fù)合材料的RhB降解效率.如圖8(a)所示,當(dāng)投加量從0.1g/L增加到0.5g/L時(shí),降解效率得到了顯著提升,這是由于隨著催化劑用量的增加,為反應(yīng)提供了更多的活性位點(diǎn)以及吸附了更多的RhB分子,從而提高了光降解效率[40-41].當(dāng)催化劑用量提升至0.7g/L時(shí),光催化活性?xún)H獲得小幅提升,且在25min后仍能達(dá)到99%左右.在用量繼續(xù)提升至0.9g/L時(shí),光解效率反而出現(xiàn)下降趨勢(shì),這主要是因?yàn)楣獯呋瘎┖吭龃蠛髮?dǎo)致溶液濁度變大,從而引起光散射增強(qiáng)導(dǎo)致可見(jiàn)光的穿透率降低[1].因此,以光解效率最佳情況下即催化劑用量為50mg (0.5g/L)作進(jìn)一步研究(圖8(b)).可以看出,當(dāng)RhB溶液濃度為10, 20, 30和40mg/L時(shí),反應(yīng)30min后,其降解效率分別為100%, 99.8%, 86.7%和76.4%,也就是說(shuō),隨著RhB染料濃度的提高,光降解效率出現(xiàn)下降的現(xiàn)象,這是因?yàn)檩^高的RhB染料濃度會(huì)使光子傳輸路徑和透光率降低,并進(jìn)一步減少了可利用的自由基活性基團(tuán)[42].因此,選取擁有相似降解效率的較高初始污染物濃度(20mg/L)作為后續(xù)的研究.

反應(yīng)pH值是影響實(shí)際廢水中光催化降解的一個(gè)重要因素,pH值對(duì)改變催化劑和有機(jī)染料的表面電荷起著重要的作用,從而影響光催化降解速率[43].本文在催化劑用量為0.5g/L與RhB溶液濃度為20mg/L的條件下,通過(guò)改變pH值,來(lái)分析其對(duì)降解效率的影響.由圖8(c)可以看出,在酸性和中性條件下(pH=3, 5, 7), RhB的去除效率獲得有效的提升,當(dāng)pH值為3時(shí),降解效率最大,反應(yīng)20min后,可達(dá)到96.1%.然而,在堿性溶液中(pH=9和11),去除效率呈現(xiàn)明顯的下降.即在酸性條件下很容易將RhB染料進(jìn)行降解,這主要?dú)w功于酸性條件有利于RhB分子的吸附,可以產(chǎn)生更多的質(zhì)子和光生氧化劑[44].

催化劑的可循環(huán)性和光化學(xué)穩(wěn)定性是作為工業(yè)實(shí)際應(yīng)用重要的評(píng)價(jià)指標(biāo).為了評(píng)價(jià)BC-10的穩(wěn)定性,進(jìn)行了3次RhB光降解循環(huán)實(shí)驗(yàn).操作步驟為: CB-10樣品在經(jīng)過(guò)光催化降解實(shí)驗(yàn)后,通過(guò)真空抽濾分離反應(yīng)后的光催化劑,用去離子水和無(wú)水乙醇洗滌并離心,在80 ℃烘箱中干燥12h后用于下一次循環(huán)實(shí)驗(yàn).圖8(d)顯示,催化劑1st～3rd的降解效率分別為99%、92%和85%.此結(jié)果表明,雖然降解效率在三次循環(huán)實(shí)驗(yàn)中呈現(xiàn)下降趨勢(shì),但降解效率仍能達(dá)到85%以上,表明CB-10具有良好的穩(wěn)定性和可循環(huán)性.通過(guò)XRD圖可知(圖9(a)),反應(yīng)前后CB-10樣品的衍射峰沒(méi)有明顯的變化,證實(shí)了催化劑具有較好的結(jié)構(gòu)穩(wěn)定性.此外,催化劑的SEM圖像與反應(yīng)前催化劑具有相似的納米片結(jié)構(gòu)(圖9(b)),同樣表明了催化劑的結(jié)構(gòu)穩(wěn)定性.

圖9 CB-10光催化循環(huán)前后的XRD圖譜和循環(huán)后的SEM圖

2.4 光催化反應(yīng)機(jī)理

為了探究光催化過(guò)程中的主要活性基團(tuán),進(jìn)行了自由基捕獲試驗(yàn).試驗(yàn)過(guò)程與光催化降解RhB染料過(guò)程大致相同,僅在降解之前加入EDTA-2Na、BQ和IPA分別作為空穴(h+)、超氧自由基(?O2–)和羥基自由基(?OH)的捕集劑[45].結(jié)果如圖10所示,在加入IPA后,光催化活性幾乎沒(méi)有受到影響,僅降低了2.8%,而EDTA-2Na和BQ的引入則顯著降低了光催化性能,即在光照射30min后,RhB降解率分別從99.8%降至53.8%和58.14%,表明光催化過(guò)程受到了強(qiáng)烈抑制,證實(shí)了h+和?O2–是主要的活性物種,而?OH在光催化降解過(guò)程中幾乎不起作用.

圖10 不同捕獲劑30min光照下的RhB降解曲線

通過(guò)CB-10樣品的能帶結(jié)構(gòu)進(jìn)一步探索了其光催化機(jī)理.如圖11(a), (b)所示, BiOBr和CeO2的XPS-VB光譜顯示其價(jià)帶值(VB)分別為1.50和1.86eV,通過(guò)方程VB=g+CB[46]計(jì)算BiOBr和CeO2的導(dǎo)帶(CB)位置分別為-1.04和-0.82eV.基于以上分析, CB-10復(fù)合材料降解RhB的機(jī)理如圖11(c)所示.在可見(jiàn)光照射下, BiOBr和CeO2吸收光能后,在價(jià)帶上的電子躍遷至導(dǎo)帶,從而產(chǎn)生光生電子和空穴. BiOBr導(dǎo)帶電子的電勢(shì)(-1.04eV)比CeO2半導(dǎo)體導(dǎo)帶電子的電勢(shì)(-0.82eV)更負(fù), CeO2價(jià)帶空穴的電勢(shì)(1.86eV)比BiOBr價(jià)帶空穴的電勢(shì)(1.50eV)要正.這時(shí)BiOBr導(dǎo)帶上的電子會(huì)轉(zhuǎn)移到CeO2的導(dǎo)帶上,同時(shí)激發(fā)空穴從CeO2價(jià)帶上移動(dòng)到BiOBr的價(jià)帶上,這歸因于傳統(tǒng)的Ⅱ型交錯(cuò)帶排列和電位差[30].因此,電子和空穴分別聚集在CeO2的導(dǎo)帶和BiOBr的價(jià)帶上.通過(guò)兩者間緊密接觸界面上的內(nèi)電場(chǎng),這種電子-空穴對(duì)的遷移方式顯著加快了電荷載體的分離效率.此外,由于CeO2的導(dǎo)帶電位比O2/?O2–的氧化還原電位(-0.33eV)更負(fù),BiOBr價(jià)帶的氧化還原電位小于OH–/?OH的氧化電位(2.38eV),因此電子將吸附在催化劑表面的氧氣還原成超氧自由基(?O2–),而不會(huì)產(chǎn)生?OH自由基,可推出h+和?O2–在降解過(guò)程中起主要作用,這與捕獲實(shí)驗(yàn)的結(jié)果相符.

3 結(jié)論

3.1 采用溶劑熱法成功地制備了具有異質(zhì)結(jié)結(jié)構(gòu)的CeO2/BiOBr光催化材料,表征結(jié)構(gòu)表明,BiOBr呈現(xiàn)緊密堆積的納米片形貌,且CeO2成功地引入到了BiOBr的結(jié)構(gòu)中,顯著提高了RhB降解的光催化性能.

3.2 CeO2引入后,改善了CeO2/BiOBr的可見(jiàn)光吸收能力、禁帶寬度和電子-空穴對(duì)復(fù)合速率等性能,提高了BiOBr光催化劑對(duì)RhB的去除效率.當(dāng)CeO2摻雜量為10%時(shí),CeO2/BiOBr復(fù)合光催化劑降解效率最高,在30min內(nèi),可降解99%的RhB染料.

3.3 經(jīng)過(guò)3次循環(huán)實(shí)驗(yàn)后,CB-10光催化材料具有良好的循環(huán)穩(wěn)定性,降解效率在30min內(nèi)仍能達(dá)到85%以上.

3.4 自由基捕獲實(shí)驗(yàn)證明,h+和?O2–是主要的活性物種.

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Construction of CeO2/BiOBr heterojuction for photocatalytic degradation of Rhodamine B.

GUO Tian-yu1,2, FAN Xiang-rui1, BAI De-hao3, QI Yu1, WANG Hong-Tao1*

(1.College of Environment Science and Engineering, Taiyuan University of Technology, Jinzhong 030600, China；2.Shanxi Key Laboratory of Gas Energy Efficient and Clean Utilization, Taiyuan 030024, China；3.Xinzhou Branch of Petrochina Coalbed Methane Co. LTD, Xinzhou 036600, China)., 2023,43(11)：5845～5854

In this study, CeO2, BiOBr and CeO2/BiOBr photocatalysts were synthesized by a hydrothermal method, and the morphology, structure and photoelectrochemical properties of the photocatalysts were characterized by different techniques. The photocatalytic performance was evaluated by degrading Rhodamine B (RhB) under visible light irradiation. The experimental results showed that the CeO2/BiOBr (CB-10) catalyst with the appropriate amount of CeO2(10%) exhibited enhanced photocatalytic activity, recyclability and cycling stability and the degradation rate of RhB reached 99% within 30min. Furthermore, the catalyst attained the highest degradation efficiency (96.1%) within 20min when the catalyst dosage was 0.5g/L, the RhB concentration was 20mg/L and pH value was 3. The possible reaction mechanism of CeO2/BiOBr heterojunctional material in photocatalytic process was proposed based on the radical capture test and the energy band structure of material. This study can provide a new idea and method for the preparation of Bi-based composite catalysts.

photocatalytic degradation；CeO2/BiOBr；heterojunction；Rhodamine B

X703.5

A

1000-6923(2023)11-5845-10

郭天宇(1989-),女,山西太原人,講師,博士,主要從事環(huán)境催化領(lǐng)域的研究.發(fā)表論文10余篇.tyguo1117x@163.com.

郭天宇1,2,范祥瑞1,白德豪,等.異質(zhì)結(jié)CeO2/BiOBr的構(gòu)筑及其光催化降解羅丹明B [J]. 中國(guó)環(huán)境科學(xué), 2023,43(11):5845-5854.

Guo T Y, Fan X R, Bai D H, et al. Construction of CeO2/BiOBr heterojuction for photocatalytic degradation of Rhodamine B [J]. China Environmental Science, 2023,43(11):5845-5854.

2023-04-23

國(guó)家自然科學(xué)基金資助項(xiàng)目(51572185);山西省應(yīng)用基礎(chǔ)研究計(jì)劃項(xiàng)目(202203021211158,20210302123176)

* 責(zé)任作者, 副教授, wanghongtao@tyut.edu.cn