RGO改性介孔TiO2薄膜光催化同步去除Ni2+和SDBS

李翠霞,孫會(huì)珍,金海澤,張幽幽,楊 軒,李文生

RGO改性介孔TiO2薄膜光催化同步去除Ni2+和SDBS

李翠霞,孫會(huì)珍,金海澤,張幽幽,楊 軒,李文生*

(蘭州理工大學(xué),省部共建有色金屬先進(jìn)加工與再利用國(guó)家重點(diǎn)實(shí)驗(yàn)室,甘肅 蘭州 730050)

以鈦酸四丁酯(TBT)?天然鱗片石墨為原料,聚乙烯吡咯烷酮(PVP)為介孔模板劑,通過(guò)浸漬-提拉法輔助熱處理和紫外輻照還原制備介孔R(shí)GO-TiO2薄膜,利用XRD?SEM?BET?UV-Vis DRS?FT-IR等對(duì)其結(jié)構(gòu)?形貌及性能進(jìn)行表征.以Ni2+和十二烷基苯磺酸鈉(SDBS)為目標(biāo)污染物,考察了介孔R(shí)GO-TiO2薄膜光催化去除Ni2+和SDBS的反應(yīng)性能,探討了GO加入量及pH值對(duì)其催化性能的影響.在最佳條件下,進(jìn)一步研究Ni2+/SDBS共存體系中Ni2+的光催化還原和SDBS的光催化氧化.結(jié)果表明:GO的加入量為1.0wt%時(shí),介孔R(shí)GO-TiO2薄膜對(duì)單一體系Ni2+和SDBS的光催化效率最高,在此條件下,pH=7.5時(shí)Ni2+還原效率最高,pH=6時(shí)SDBS降解效率最高.綜合以上實(shí)驗(yàn)條件,設(shè)定GO加入量為1.0wt%,pH≈6時(shí),Ni2+/SDBS共存體系中Ni2+和SDBS去除效率均優(yōu)于Ni2+和SDBS單一體系,此時(shí),Ni2+還原率為87.9%,SDBS氧化率為95.5%.分析協(xié)同光催化機(jī)制,TiO2-SDBS表面復(fù)合物在紫外光激發(fā)下,SDBS被氧化同時(shí),光生電子及SDBS氧化產(chǎn)物CO2??自由基同步還原Ni2+.

介孔R(shí)GO-TiO2薄膜；浸漬-提拉法；光催化；協(xié)同作用；Ni2+；SDBS

納米TiO2是一種氧化性強(qiáng)、化學(xué)穩(wěn)定性良好、廉價(jià)、無(wú)毒、對(duì)環(huán)境友好的半導(dǎo)體光催化劑,被廣泛應(yīng)用于廢水處理、空氣凈化、光解水制氫、光催化還原CO2等領(lǐng)域[1-4].但納米TiO2禁帶寬度較大,只能吸收占太陽(yáng)光不到5%的紫外光,且電子空穴復(fù)合率高,導(dǎo)致其光催化性能較差,在實(shí)際應(yīng)用中受限.為了提高TiO2光催化性能,眾多學(xué)者采用貴金屬沉積、離子摻雜、表面光敏化、半導(dǎo)體耦合、負(fù)載等手段對(duì)其進(jìn)行改性[5];其中負(fù)載技術(shù)可有效抑制電子-空穴復(fù)合,進(jìn)而改善納米TiO2粉體的分散性,增大光催化劑與有機(jī)污染物的接觸面積,促進(jìn)TiO2光催化效率.Chandra等[6]采用原位摻入法合成的ZIF- 8@TiO2復(fù)合材料,由于金屬有機(jī)骨架的存在對(duì)MB和RhB光催化效率高于純TiO2.

石墨烯獨(dú)特的單層sp2碳原子結(jié)構(gòu)使其具有較大的比表面積、良好的導(dǎo)電性和較快的電子遷移速率[7-8],將其作為載體與TiO2復(fù)合可有效提高光催化性能[9].此外,介孔TiO2具有高孔容量和較大比表面積,有利于吸附污染物及提高光催化劑和產(chǎn)物的擴(kuò)散[10],但TiO2固有的較高載流子復(fù)合率仍無(wú)法避免.近年來(lái),采用溶膠凝膠法制備的介孔還原氧化石墨烯/TiO2(RGO/TiO2)復(fù)合材料能夠有效克服上述問(wèn)題,實(shí)現(xiàn)污水中有機(jī)染料的高效降解[11].但粉體光催化劑回收困難,不利于循環(huán)利用導(dǎo)致應(yīng)用成本增加.本課題組采用浸漬-交替提拉法制備的介孔R(shí)GO/TiO2薄膜在120min時(shí)對(duì)甲基橙溶液的降解率達(dá)92.5%;循環(huán)利用5次后降解率仍達(dá)85%,證實(shí)RGO/TiO2薄膜是一種高效且易于回收的光催化劑[12].

通常工業(yè)廢水體系中同時(shí)存在有毒重金屬離子和難降解有機(jī)污染物,如何在低成本下實(shí)現(xiàn)二者的高效同步去除,提高污水凈化效率是目前亟待解決的難題之一.光催化劑在太陽(yáng)光輻照時(shí)產(chǎn)生分別具有還原性、氧化性的光生電子及空穴用以金屬離子的還原和有機(jī)物的氧化降解,為解決此難題提供了有效途徑.因此,研究光催化劑同時(shí)去除金屬離子和有機(jī)污染物具有重要意義.本課題采用浸漬-交替提拉法,經(jīng)熱處理和紫外輻照還原合成介孔R(shí)GO- TiO2薄膜,并以有毒重金屬離子Ni2+和難降解有機(jī)污染物-表面活性劑十二烷基苯磺酸鈉(SDBS)為目標(biāo)污染物,依次考察了GO加入量、初始溶液pH值兩個(gè)因素對(duì)介孔R(shí)GO-TiO2薄膜去除Ni2+和SDBS光催化活性的影響,在最佳實(shí)驗(yàn)條件下進(jìn)一步研究Ni2+/SDBS共存體系中Ni2+還原和SDBS氧化之間的協(xié)同機(jī)理,為半導(dǎo)體光催化技術(shù)在成分復(fù)雜的工業(yè)污水處理領(lǐng)域應(yīng)用提供實(shí)驗(yàn)基礎(chǔ)和理論指導(dǎo).

1 材料與方法

1.1 樣品制備

所用試劑為分析純的鱗片石墨(225目)(上海膠體化工廠)、鈦酸四丁酯(天津市北辰方正試劑廠)、檸檬酸(萊陽(yáng)化工實(shí)驗(yàn)廠)、聚乙烯吡咯烷酮(天津市致遠(yuǎn)化學(xué)試劑有限公司)、鹽酸(白銀良友化學(xué)試劑有限公司)、無(wú)水乙醇(天津富宇精細(xì)化工有限公司),蒸餾水.

采用改進(jìn)的Hummers法[13],將天然鱗片石墨制成GO分散液.將17.5g檸檬酸(CA)攪拌溶解于82mL無(wú)水乙醇,滴加25mL的鈦酸丁酯攪拌反應(yīng)30min,再緩慢滴加5mL蒸餾水繼續(xù)攪拌30min后陳化,得到淺黃色TiO2前驅(qū)體溶膠.向此溶膠中加入占TiO2質(zhì)量10%的模板劑聚乙烯吡咯烷酮(PVP);將不同量的GO(占TiO2質(zhì)量0.1%,0.5%,1.0%,1.5%)加入上述TiO2(PVP)溶膠中并調(diào)節(jié)混合溶膠的粘度為4.5mPa/s得到GO/TiO2(PVP)前驅(qū)體溶膠.將載玻片分別用3%鹽酸溶液和蒸餾水超聲清洗,再用無(wú)水乙醇沖洗3~4次,經(jīng)100℃烘干;設(shè)定拉膜機(jī)電壓為16V,將清洗過(guò)的載玻片裝于拉膜機(jī)上并分別浸入上述不同GO加入量(0.1%,0.5%,1.0%,1.5%)的GO/TiO2(PVP)前驅(qū)體溶膠中靜置3min后勻速提拉上來(lái),放入100℃的真空干燥箱中干燥25min;以上步驟重復(fù)10次即可制得不同GO加入量交替10層的前驅(qū)體薄膜;將前驅(qū)體薄膜放入馬弗爐中400℃熱處理3h,隨爐冷卻后取出置于無(wú)水乙醇中持續(xù)攪拌同時(shí)經(jīng)高壓汞燈照射6h,即得介孔R(shí)GO-TiO2薄膜.按照上述方法只是不加入GO制備介孔TiO2薄膜.

1.2 樣品表征

利用D8/axs X射線衍射儀分析樣品晶體結(jié)構(gòu),Cu K輻射,步長(zhǎng)0.02°,掃描范圍2為5°~70°. JSM-6701F冷場(chǎng)發(fā)射型掃描電子顯微鏡觀察樣品形貌.ASAP2020全自動(dòng)比表面積及孔隙分析儀測(cè)定樣品的比表面積及孔徑分布.U-3900H紫外-可見分光光度計(jì)分析樣品的光吸收和可見光響應(yīng)能力.F97Pro熒光分光光度計(jì)檢測(cè)樣品的熒光發(fā)射光譜.vector22型FT-IR紅外光譜儀測(cè)定樣品的紅外光譜.

1.3 光催化性能測(cè)試

室溫下以含Ni2+的硫酸鎳溶液和SDBS溶液為目標(biāo)還原物和降解物,采用250W汞燈為光源(距液面15cm)在自行設(shè)計(jì)的光催化裝置上測(cè)試不同污染物體系光催化降解效率.

Ni2+的分析測(cè)試方法:配制(Ni2+)=20.0mg/L的標(biāo)準(zhǔn)液;取10片薄膜放入100mL濃度為20mg/L的硫酸鎳溶液中,暗吸附30min后打開高壓汞燈,每隔20min取一次試樣,離心取上清液于25mL容量瓶中加水至10mL,再依次加入2mL檸檬酸銨、1mL碘溶液、2mL丁二酮肟溶液、2mLEDTA-2Na溶液至標(biāo)線;以蒸餾水為參比液,靜置一段時(shí)間后于530nm波長(zhǎng)處測(cè)量濃度.

SDBS的分析測(cè)試方法:配制(SDBS)= 20.0mg/L的標(biāo)準(zhǔn)液;取10片薄膜加入到100mL濃度為20mg/L的SDBS溶液(pH=7)中,暗吸附30min后向溶液中緩慢加入2mL過(guò)氧化氫溶液;打開高壓汞燈,每隔20min取一次試樣,離心取上清液移入125mL梨型分液漏斗中并加蒸餾水稀釋到50mL,依次加入12.5mL亞甲藍(lán)溶液、10mL氯仿、3mL異丙醇激烈振蕩,再靜置分層;氯仿相經(jīng)脫脂棉洗水后放入10mL比色皿中于625nm波長(zhǎng)下進(jìn)行光度檢測(cè).

1.3.1 Ni2+體系(pH=7.5)和SDBS體系(pH=6)測(cè)試分別取10片薄膜加入100mL濃度為20mg/L的目標(biāo)降解溶液中,暗吸附30min后打開光源,每隔20min取一次樣,通過(guò)分別測(cè)定不同體系水溶液中Ni2+和SDBS殘余質(zhì)量濃度,不同的是,在SDBS體系測(cè)試中暗吸附后需向溶液中緩慢加入2mL過(guò)氧化氫溶液,開光源繼續(xù)取樣測(cè)定.光催化去除效率可根據(jù)公式(1)計(jì)算:

式中:0為初始濃度,mg/L;C為反應(yīng)時(shí)間為時(shí)濃度,mg/L.

1.3.2 Ni2+/SDBS體系(pH≈6)測(cè)試 測(cè)試步驟同上,混合溶液為100mL濃度均為20mg/L的Ni2+與SDBS溶液組成.暗吸附結(jié)束后向混合溶液中緩慢加入2mL過(guò)氧化氫溶液,再打開光源每隔20min取樣后經(jīng)高速離心取上清夜并將所取試樣分為2份,分別按照Ni2+和SDBS體系的測(cè)試方法分析Ni2+和SDBS兩者濃度變化,光催化效率通過(guò)公式(1)計(jì)算.

2 結(jié)果與討論

2.1 RGO-TiO2薄膜結(jié)構(gòu)與性能分析

2.1.1 XRD分析 圖1是GO、TiO2和不同GO加入量RGO-TiO2薄膜的XRD圖譜.TiO2及RGO- TiO2薄膜均在2=25.3°、37.8°、48.2°、54.0°和62.8°處出現(xiàn)了衍射峰,分別對(duì)應(yīng)于銳鈦礦相TiO2(JCPDS No.89-4921)的(101)、(004)、(200)、(105)、(204)晶面,在2=27.5°處衍射峰則對(duì)應(yīng)金紅石相TiO2的(110)晶面,說(shuō)明RGO-TiO2薄膜中TiO2晶型為銳鈦礦相與金紅石相的混合晶相.此外,不同GO加入量RGO-TiO2薄膜樣品經(jīng)紫外光輻照還原后在2= 12.8°處未檢測(cè)到明顯的GO特征峰,表明薄膜樣品中大部分GO被還原,歸因于RGO-TiO2薄膜在乙醇溶液中經(jīng)紫外輻照產(chǎn)生光生電子-空穴對(duì),空穴被乙醇消耗產(chǎn)生乙氧基自由基和氫離子,GO上一些含氧官能團(tuán)與電子相互作用進(jìn)而被還原[14].

圖1 不同樣品的XRD譜

圖2 不同樣品SEM圖及1.0wt%RGO-TiO2薄膜EDS譜

2.1.2 SEM-EDS分析 GO的SEM圖如圖2(a)所示.GO呈現(xiàn)透明薄紗狀,并由于引入含氧官能團(tuán)破壞C=C雙鍵使其表面有很多褶皺,而這種褶皺也保證了GO擁有良好的柔韌性,從而使其在熱力學(xué)條件下更加穩(wěn)定[15].圖2(b、c)分別為純TiO2、1.0wt%RGO-TiO2薄膜樣品的SEM圖,二者均表面平整,但1.0wt%RGO-TiO2薄膜附著均勻的白色微粒,這源于實(shí)驗(yàn)所用檸檬酸和鈦酸四丁酯發(fā)生單齒配位反應(yīng)后與溶膠中GO上的含氧官能團(tuán)反應(yīng)均勻負(fù)載于GO表面,形成具有位阻效應(yīng)的復(fù)雜絡(luò)合物[16],再經(jīng)熱處理及紫外輻照后在薄膜上生成均勻負(fù)載納米TiO2粒子的RGO片.圖(d)為1.0wt%RGO- TiO2薄膜樣品的EDS能譜圖.由圖可知,RGO-TiO2薄膜中的主要元素為Ti、C、O 3種,經(jīng)分析可得,Ti元素來(lái)源于鈦酸四丁酯,O元素由TiO2薄膜和未被完全還原的GO提供,C元素則是RGO和殘留的有機(jī)物提供.此外,還有一些微量元素Si、Ca、Na、部分O等來(lái)源于載玻片,Au元素則是樣品噴金所致.

2.1.3 BET分析 如圖3所示,N2吸附-脫附曲線為典型介孔材料的IV型曲線,具有H2型遲滯回線,表明薄膜樣品中存在狹縫孔結(jié)構(gòu)[17];這主要?dú)w因于模板劑PVP中羰基(C=O)與CA和鈦酸四丁酯(TBT)形成鈦絡(luò)合物發(fā)生交叉配位環(huán)狀分子鏈,該分子鏈與GO上含氧官能團(tuán)發(fā)生反應(yīng),經(jīng)400℃煅燒PVP消失,留下銳鈦礦型TiO2的連續(xù)固體骨架均勻附著在GO片上,形成介孔結(jié)構(gòu),其形成過(guò)程如圖4所示.BET法計(jì)算得到薄膜樣品比表面積為34.57m2/g.由孔徑分布曲線得孔徑分范圍主要集中于2.5~ 8.0nm之間,說(shuō)明1.0wt%RGO-TiO2薄膜具有較窄的孔分布.有望獲得孔徑分布均勻、高比表面積的薄膜樣品,為其吸附污染物提供更多的附著位點(diǎn),進(jìn)而提高其光催化性能.

圖3 1.0wt%RGO-TiO2薄膜的氮?dú)馕?脫附等溫線

2.1.4 UV-Vis DRS及FT-IR分析 由圖5(a)可知, TiO2與1.0wt%RGO-TiO2薄膜的光吸收帶邊分別為410和470nm,表明引入GO使TiO2光吸收范圍拓展至可見光區(qū)域.二者的帶隙能(E)可通過(guò)以下公式:

式中:是普朗克常數(shù),6.626×10-26JS;是光速,3× 108m/s.

圖4 TiO2-PVP-GO作用機(jī)理

Fig.4 TiO2- PVP-GO action mechanism diagram

通過(guò)計(jì)算可得TiO2和1.0wt%RGO-TiO2薄膜的E分別為3.0和2.6eV,可見TiO2中引入RGO可使其帶隙變小,其主要原因是RGO與TiO2復(fù)合后TiO2的O(2p)軌道與RGO的C(2p)軌道發(fā)生了軌道雜化,致使TiO2價(jià)帶邊緣上升從而減小其帶隙[18-19].

圖5 TiO2、1.0wt%RGO-TiO2薄膜UV-Vis DRS和不同樣品的FT-IR圖

GO、TiO2、1.0wt%GO-TiO2和1.0wt%RGO- TiO2薄膜樣品的FT-IR圖如圖5(b)所示.在3386cm-1處較寬的吸收峰對(duì)應(yīng)GO中羥基(-OH)的伸縮振動(dòng)[20],在2954,1727,1600,1289,1056cm-1處出現(xiàn)CH2的反對(duì)稱伸縮振動(dòng)[21]、GO羧基中C=O的伸縮振動(dòng)、水分子變形振動(dòng)、GO表面C-O-C及醇的C-OH伸縮振動(dòng).表明GO主要含有-OH、-COOH、-C=O 3種含氧官能團(tuán).對(duì)比GO和GO-TiO2的FT-IR圖, RGO-TiO2在3386和1727cm-1處吸收峰消失或減弱,表明GO部分被還原.同時(shí),RGO-TiO2薄膜樣品在1626cm-1處的吸收峰表明石墨烯片骨架的振動(dòng)[22],位于1278cm-1處的吸收峰應(yīng)源于PVP中C-N的伸縮振動(dòng)[23],可能是由于400℃熱處理后RGO-TiO2薄膜中殘留少量的PVP所致.RGO-TiO2薄膜樣品在低于1000cm-1處的吸收帶屬于TiO2中Ti-O-Ti鍵的振動(dòng),這可能是Ti-O-Ti和Ti-O-C鍵共同作用的結(jié)果[24].而Ti-O-C鍵的存在表明,GO上羧酸官基團(tuán)和TiO2納米顆粒表面的羥基發(fā)生強(qiáng)有力相互作用最終形成通過(guò)化學(xué)鍵結(jié)合的RGO-TiO2薄膜[25].

2.1.5 PL分析 光生電子和空穴的分離效率是影響光催化效率的重要因素.一般來(lái)說(shuō),較低的熒光強(qiáng)度表示光生電子-空穴對(duì)復(fù)合率較低,即光生載流子的壽命較長(zhǎng)[26].如圖6,相比于a(TiO2),各RGO-TiO2薄膜的熒光強(qiáng)度均有所降低,說(shuō)明引入GO可有效抑制光生電子-空穴對(duì)的復(fù)合,延長(zhǎng)載流子的壽命.其中GO添加量為1.5%的樣品e電子-空穴分離效率相對(duì)最佳,說(shuō)明其更有利于提高光催化劑的活性.

圖6 TiO2及不同GO加入量RGO-TiO2薄膜的熒光光譜圖

2.2 光催化性能

2.2.1 GO加入量對(duì)Ni2+還原率及SDBS氧化率的影響 從圖7可以看出單純引入H2O2的SDBS體系中,經(jīng)過(guò)80min光催化反應(yīng)后其降解率僅有1%,說(shuō)明在缺少光催化劑的情況下H2O2對(duì)SDBS的去除效果很差.在上述體系以及Ni2+溶液中引入光催化劑后,在暗吸附階段對(duì)SDBS的去除率均有大幅度提高,其中RGO-TiO2薄膜對(duì)SDBS和Ni2+的吸附率均高于TiO2薄膜.暗吸附30min時(shí)顯示不同GO加入量RGO-TiO2薄膜對(duì)Ni2+和SDBS的吸附率均較TiO2高,由于RGO-TiO2薄膜中RGO表面大量的π電子及其獨(dú)特的二維單原子層網(wǎng)絡(luò)結(jié)構(gòu)與Ni2+和SDBS相互作用,提高其吸附性能[27].光照80min后加入不同量GO的RGO-TiO2薄膜較純TiO2有更好的光催化性能,并隨著GO加入量的增加對(duì)Ni2+的光催化還原效率和SDBS的光催化氧化效率呈先上升后降低的趨勢(shì),在光照時(shí)間內(nèi),整體上GO加入量為1.0wt%時(shí)光催化效率最好.由圖7(a)可知, 80min時(shí)1.0wt%與1.5wt%RGO-TiO2薄膜對(duì)Ni2+還原率接近且均達(dá)到67%左右;由圖7(b)可知,80min時(shí)1.0wt%與1.5wt%的RGO-TiO2薄膜對(duì)SDBS的降解率接近且達(dá)到87.9%左右;歸因于薄膜中RGO快速轉(zhuǎn)移電子,降低光生電子、空穴復(fù)合率,同時(shí)薄膜中介孔結(jié)構(gòu)為光催化反應(yīng)提供更多附著位點(diǎn),從而提高光催化性能.

2.2.2 初始溶液pH值對(duì)Ni2+還原率和SDBS氧化率的影響 基于以上研究,以1.0wt%RGO-TiO2薄膜為光催化劑,研究pH值對(duì)1.0wt%RGO-TiO2薄膜光催化還原Ni2+和光催化氧化SDBS效率影響.

由圖8(a)可知,1.0wt%RGO-TiO2薄膜隨著初始溶液pH值增大Ni2+光催化還原效率整體呈上升趨勢(shì),光照80min時(shí)初始溶液pH=7.5和9的還原率接近且為73%左右.總體來(lái)看,在一定催化時(shí)間內(nèi)初始溶液pH=7.5的反應(yīng)條件最佳,這源于此條件下薄膜對(duì)Ni2+的有效吸附.此外,TiO2等電點(diǎn)為6.8,當(dāng)初始溶液pH>6.8時(shí)TiO2表面帶負(fù)電易吸附Ni2+進(jìn)而利于還原;反之,當(dāng)pH<6.8時(shí)TiO2表面帶正電,與Ni2+相斥,不利于Ni2+還原.當(dāng)pH=9時(shí),其還原效率增加不明顯,可能是由于溶液中多余的OH-與Ni2+反應(yīng)生成沉淀致使Ni2+還原效率受影響.

由圖8(b)可知,隨著pH值增大,1.0wt%RGO- TiO2薄膜對(duì)SDBS的氧化率呈先增大后降低的趨勢(shì),pH=6時(shí)SDBS氧化率最高,80min光催化效率達(dá)到83%.在強(qiáng)酸性條件下由于過(guò)量的H+消耗了OH使降解效率減小;而弱堿性條件使H2O2快速分解導(dǎo)致降解率減小.

2.2.3 Ni2+與SDBS協(xié)同光催化性能 近年來(lái)重金屬離子和有機(jī)污染物協(xié)同去除方面已經(jīng)取得了許多成果(表1).實(shí)現(xiàn)了Cr(VI)、Pb(II)、Ni(II)等重金屬離子與有機(jī)污染物RhB、SDBS、MB、苯酚、MO的同時(shí)去除.本文制備的介孔R(shí)GO- TiO2薄膜比表面積大且由于RGO優(yōu)良的導(dǎo)電性可有效提高電子空穴的分離效率,從而提高光催化效率.

基于以上分析,綜合考慮Ni2+和SDBS酸堿環(huán)境,取10片1.0wt%RGO-TiO2薄膜作為光催化劑,加入初始濃度為40mg/L([Ni2+]=[SDBS]=20mg/L)Ni2+/ SDBS共存體系的混合溶液(pH≈6)中,研究二者的協(xié)同效應(yīng)對(duì)光催化性能的影響.

如圖9所示,由于較低pH值更有利于催化劑通過(guò)靜電作用吸附金屬陽(yáng)離子[28].在Ni2+/SDBS共存體系中,SDBS陰離子的存在有助于減緩光催化劑表面吸附Ni2+而導(dǎo)致對(duì)其余Ni2+的排斥.SDBS陰離子可以作為一個(gè)橋接離子,通過(guò)靜電作用將Ni2+拖近光催化劑表面最終形成了Ni2+-SDBS-Ni2+夾心絡(luò)合物,進(jìn)一步促使二者的去除率升高.由圖可知在Ni2+/SDBS共存體系下,H2O2對(duì)SDBS的去除率同樣很低;引入光催化劑后Ni2+的還原率和SDBS的氧化率較其單一體系均有所提高.光照80min時(shí),Ni2+單一體系及Ni2+/SDBS共存體系中Ni2+還原率分別為67%和87.9%;SDBS在其單一體系及Ni2+/SDBS共存體系中其氧化率分別為86%和95.5%,表明Ni2+與SDBS在Ni2+/SDBS共存體系中光催化還原率和氧化率均高于單一體系,說(shuō)明兩者之間具有較強(qiáng)的協(xié)同作用.

表1 1.0wt%RGO-TiO2薄膜及其他光催化劑協(xié)同處理重金屬離子與有機(jī)污染物光催化效率的對(duì)比

圖9 SDBS氧化對(duì)Ni2+光催化還原效率的影響

插圖為Ni2+還原對(duì)SDBS光催化氧化效率的影響

Ni2+與SDBS協(xié)同效應(yīng)機(jī)理如圖10所示.通常情況下在單一金屬離子光催化還原體系中,離子還原主要依賴于對(duì)光催化劑所產(chǎn)生的光生電子的吸收.而在體系中引入能夠自敏化的有機(jī)污染物后,經(jīng)光源輻照有機(jī)污染物產(chǎn)生電子并將其注入半導(dǎo)體光催化劑的導(dǎo)帶,還原吸附在光催化劑表面的金屬離子,這個(gè)現(xiàn)象被稱之為電荷轉(zhuǎn)移絡(luò)合[34].在Ni2+/ SDBS共存體系中,當(dāng)RGO-TiO2薄膜受到紫外光照射時(shí),RGO-TiO2-SDBS絡(luò)合物通過(guò)光敏化以及激發(fā)光催化劑產(chǎn)生的電子進(jìn)入TiO2導(dǎo)帶后被表面吸附的Ni2+捕獲以促進(jìn)其還原.此外在本實(shí)驗(yàn)條件下, RGO-TiO2在光輻照作用下產(chǎn)生光生電子-空穴對(duì)后,空穴的消耗主要存在2個(gè)途徑(氧化SDBS和光生電子-空穴對(duì)復(fù)合).而SDBS在光催化氧化過(guò)程中直接或間接消耗空穴可抑制光生電子-空穴對(duì)復(fù)合,提高光催化效率的重要因素[35].SDBS經(jīng)自敏化提供一個(gè)電子后會(huì)轉(zhuǎn)變?yōu)殛?yáng)離子自由基(SDBS?+),并經(jīng)過(guò)一系列的氧化反應(yīng)而被轉(zhuǎn)化分解[36].在SDBS的氧化過(guò)程中產(chǎn)生的CO2??陰離子自由基具有較強(qiáng)的還原性可以被進(jìn)一步應(yīng)用于促進(jìn)Ni2+的還原,從而實(shí)現(xiàn)了Ni2+與SDBS的高效協(xié)同催化降解.

圖10 Ni2+與SDBS在RGO-TiO2薄膜上的協(xié)同作用機(jī)理

3 結(jié)論

3.1 采用浸漬-交替提拉法成功合成介孔R(shí)GO- TiO2薄膜;薄膜SEM顯示TiO2納米粒子均勻分散在薄膜表面;薄膜BET測(cè)試表明RGO-TiO2復(fù)合材料具有介孔結(jié)構(gòu),且比表面積較大;UV-Vis DRS圖譜顯示介孔R(shí)GO-TiO2薄膜光吸收范圍較TiO2發(fā)生了紅移.

3.2 GO加入量為1.0wt%時(shí)RGO-TiO2薄膜在紫外光下對(duì)Ni2+和SDBS的降解效率最佳,且在此條件下,pH=7.5時(shí)有利于Ni2+還原,pH=6時(shí)有利于SDBS降解.

3.3 GO加入量為1.0wt%,pH≈6時(shí),Ni2+/SDBS共存體系中Ni2+還原率和SDBS降解率均較其單一體系高,此時(shí)Ni2+還原率和SDBS降解率分別為87.9%和95.5%;歸因于SDBS的氧化中間產(chǎn)物CO2??可對(duì)Ni2+進(jìn)一步還原,從而促進(jìn)了Ni2+和SDBS同時(shí)的去除.

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Simultaneous removal of Ni2+and SDBS by RGO modified mesoporous TiO2thin films photocatalytic.

LI Cui-xia, SUN Hui-zhen, JIN Hai-ze, ZHANG You-you, YANG Xuan, LI Wen-sheng*

(Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China)., 2021,41(4)：1663~1671

The mesoporous RGO-TiO2thin films were synthesized by dipping-coating assisted heat treatment and ultraviolet lamp irradiation reduction with tetrabutyl titanate (TBT), natural flake graphite as raw materials and polyvinylpyrrolidone (PVP) as mesoporous template. The structure, morphology and properties of samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area (BET), UV-Vis diffuse reflectance spectra (UV-Vis DRS) and Fourier transform infrared (FTIR). The reaction property of the removal of Ni2+and SDBS by photocatalysis of mesoporous RGO-TiO2thin films were evaluated with Ni2+and SDBS as target pollutants. The GO amount and the effects of pH on its catalytic performance were discussed. The photocatalytic reduction of Ni2+and photocatalytic oxidation of SDBS in Ni2+/SDBS co-existed system were further studied under the optimum condition. The results showed that the mesoporous RGO-TiO2thin film had the highest photocatalytic efficiency for single system Ni2+and SDBS with the content of 1.0wt% GO; The reduction efficiency of Ni2+and the degradation efficiency of SDBS were the highest when the pH values were 7.5 and 6, respectively. In summary, the removal efficiency of Ni2+and SDBS in the co-existed system was better than that of the single system under the additions that the amount of GO was 1.0wt% and pH≈6. The reduction rate of Ni2+was 87.9% and the oxidation rate of SDBS was 95.5%. In this present contribution, the mechanism of synergistic photocatalysis was further explored. It can be concluded that the Ni2+synchronous reduction occured by the photogenerated electrons and the oxidation producted CO2??free radical when the TiO2-SDBS surface complex was oxidized under the excitation of ultraviolet light.

mesoporous RGO-TiO2thin films；dipping-coating；photocatalysis；synergy；Ni2+；SDBS

X703.5

A

1000-6923(2021)04-1663-09

李翠霞(1972-),女,河北盧龍人,副教授,博士,主要從事無(wú)機(jī)功能材料的制備及其在環(huán)境領(lǐng)域中的應(yīng)用研究.發(fā)表論文30余篇.

2020-08-31

國(guó)家自然科學(xué)基金資助項(xiàng)目(51764039)

* 責(zé)任作者, 教授, liws@lut.edu.cn