and BiOCOOH have similar structures with the general formula [Bi 2 ][X m O 2 ] layers. Recently, to extend the light absorption spectra of (BiO) 2

Transcription

1 Pure Appl. Chem. 2018; 90(2): Conference pper Xin Feng, Xinwei Li, Wen Cui, Fn Dong* nd Tierui Zhng* An ion-exchnge strtegy for I-doped nnopltes with enhnced visile light photoctlytic NOx removl Astrct: A simple ion-exchnge method ws developed for I-doped nnopltes from the replcement of COOH ions with I ions in the interlyers of. The s-prepred ctlysts were chrcterized y XRD, SEM, TEM, XPS, UV-vis DRS PL nd photocurrent genertion. The photoctlytic ctivity of the s-prepred ctlysts ws evluted y removl of NO in ir t pp level under visile light irrdition. As expected, the I-doped (IHB-X, the X represents the molr rtio of KI to ) displyed incresed visile light sorption nd enhnced chrge seprtion due to I-doping. At sturte I-doping content, the ctlyst with optimized electronic structure demonstrted the highest NO removl of 49.7 % nd excellent photochemicl stility. This present work hs demonstrted new strtegy for modifiction of lyered photoctlyst vi ion exchnge. Keywords: ; chrge seprtion; ICGC-6; interlyered I-doping; visile-light photoctlysis; NO removl. Introduction Bismuth-sed photoctlysts hve roused wide reserch interests due to their unique electronic structure nd excellent photoctlytic performnce [1 8]. Among these mterils, BiOX (X = Cl, Br, I), (BiO) 2 CO 3 nd hve similr structures with the generl formul [Bi 2 O 2 ][X m ] (X = hlogen or other groups) tht usully possesses lternting [Bi 2 O 2 ] 2+ sheets nd X sls (m = 1, 2 or 3 rrely). This unique structure enefits the certin ions tht enter into the spce etween the lyers, nd replce the X, COOH, or CO 3 2 ions nd coordinte with [Bi 2 O 2 ] lyers. Recently, to extend the light sorption spectr of (BiO) 2 CO 3 or BiOX into visile light region, diverse pproches hve een developed, such s surfce decortion, doping nd heterojunction construction [9 18]. is recently discovered photoctlyst tht cn only e excited y UV light due to its wide nd gp [19, 20]. Chen et l. [19] prepred with different shpe through fcile nd templte-free solvotherml process, including sphericl-like, sponge-like, tremell-like nd flower-like Article note: A collection of invited ppers sed on presenttions t the 6 th interntionl IUPAC Conference on Green Chemistry (ICGC-6), Venice (Itly), 4 8 Septemer *Corresponding uthors: Fn Dong, Chongqing Key Lortory of Ctlysis nd New Environmentl Mterils, College of Environment nd Resources, Chongqing Technology nd Business University, Chongqing , Chin, e-mil: dfctu@126.com; Tierui Zhng, Key Lortory of Photochemicl Conversion nd Optoelectronic Mterils, Technicl Institute of Physics nd Chemistry, Chinese Acdemy of Sciences, Beijing , Chin; nd University of Chinese Acdemy of Sciences, Beijing 10049, Chin, e-mil: tierui@mil.ipc.c.cn. (T. Zhng) Xin Feng, Xinwei Li nd Wen Cui: Chongqing Key Lortory of Ctlysis nd New Environmentl Mterils, College of Environment nd Resources, Chongqing Technology nd Business University, Chongqing , Chin 2017 IUPAC & De Gruyter. This work is licensed under Cretive Commons Attriution-NonCommercil-NoDerivtives 4.0 Interntionl License. For more informtion, plese visit:

2 354 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes hierrchicl nnostructures. The sponge-like hierrchicl nnostructures exhiited the highest photoctlytic ctivity mongst those nnostructures [19]. Afterwrds, Zhu et l. [20] synthesized ultrlong nnowires vi simple solvotherml route, which exhiited highly efficient dsorption performnce for Cr(VI) nd MO due to its positively chrged surfce, lrge porosity, nd good dispersion in wter. To improve the photoctlytic ctivities of pure, coupling wide nd gp semiconductor with nrrow nd gp semiconductor to form heterostructures with stggered lignment of nd edges could gretly extend light responsive rnge nd significntly improve the chrge sepertion. Chi nd Wng [21] fricted the BiOI/BiOCOOOH composites y fcile prtil ion exchnge etween nd KI t cidic condition. To dte, little is known out the effect of nonmetl doping on the properties of. In this work, we report simple ion-exchnge method for I-doped nnopltes to extend the light sorption spectr of into visile light region. The photoctlytic ctivity of the smples ws evluted for removl of NO t pp-level under visile light. The results indicted tht the I-doped nnopltes exhiited highly enhnced visile light photoctlytic ctivity in comprison with s I-doping could effectively improve the visile light sorption nd chrge seprtion. The optimized ctlyst () exhiited the highest photoctlytic ctivity nd excellent stility, which is significnt for its prcticl ppliction. Experimntl section Synthesis of I doped All chemicls used in this study were nlyticl grde nd were used without further purifiction. The synthesis of nnopltes ws reported in our previous work [22]. In typicl process, g of ws dded into 60 ml distilled wter, nd then dispersed for 20 min with ultrsound tretment. Then 40 ml of KI queous solution (AR, KeLong, Chengdu, Chin) ws dded to the ove suspension under continuous stirring for 2 h. The otined smple ws filtered, wshed with wter nd ethnol ech for two times nd dried t 60 C for 12 h to get the finl smples. The molr rtio of KI to ws controlled t 0.10, 0.25, 0.50, 1.00, 2.00 nd 4.00, respectively. Accordingly, the resulted products were leled s IHB-0.10,,,, nd, respectively. Chrcteriztion The crystl structure of the s-otined smples ws nlyzed y X-ry diffrction with Cu-Krdition (XRD: model D/mx RA, Rigku Co., Jpn). The morphologicl structures were exmined y trnsmission electron microscopy (TEM: JEM-2010, JEOL, Jpn). X-ry photoelectron spectroscopy with Al Kα X-rys (hν = ev) rdition operted t 150 W (XPS: Thermo ESCALAB 250, USA) ws used to investigte the surfce properties. The shift of the inding energy due to reltive surfce chrging ws corrected using the C1s level t ev s n internl stndrd. The Brunuer-Emmett-Teller (BET: Gemini VII 2390, Micromeritics, Shnghi, Chin) surfce re nd the pore size distriution of the products were identified y Micromeritics ASAP 2020 pprtus. All the smples were degssed t 150 C prior to mesurements. The UV-vis diffuse reflection spectr were otined from the dry-pressed disk smples using Scn UV-vis spectrophotometer (UV-vis DRS: UV-2450, Aucy, Shnghi, Chin) equipped with n integrting sphere ssemly, using 100 % BSO 4 s reflectnce smple. The solid-stte photoluminescence (PL) spectr were mesured with fluorescence spectrophotometer (PL: FS-2500, Hitchi, Jpn) using Xe lmp s n excittion source with opticl filters.

3 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes 355 Evlution of photoctlytic ctivity The photoctlytic ctivity of the resulting smples ws investigted y oxidtion of NO t pp levels in continuous flow rector t mient temperture. The volume of the rectngulr rector, which ws mde of stinless steel nd covered with Sint-Glss, ws 4.5 L (30 cm 15 cm 10 cm). A 150 W commercil tungsten hlogen lmp ws verticlly plced outside the rector ove the rector. For ech photoctlytic ctivity test experiment, two smple dishes (with dimeter of 12 cm) contining the photoctlyst powders were plced in the center of the rector. The photoctlyst smples (The weight of the photoctlysts used for ech dish ws kept t 0.10 g) were prepred y coting n queous suspension of the smples onto the glss dish nd then dried t 60 C. The NO gs ws cquired from compressed gs cylinder t concentrtion of 100 ppm of NO (N 2 lnce). The initil concentrtion of NO ws diluted to 550 pp y the ir strem supplied y purified ir. The desired reltive humidity (RH) level of the NO flow ws controlled t 60 % y pssing the zero ir strems through humidifiction chmer. The gs strems were premixed completely y gs lender, nd the flow rte ws controlled t 3.3 L/min y mss flow controller. After the dsorption-desorption equilirium ws chieved, the lmp ws turned on. The concentrtion of NOx ws continuously mesured y chemiluminescence NOx nlyzer (Thermo Environmentl Instruments Inc., 42i-TL), which monitors NO, NO 2 nd NOx (NOx represents NO + NO 2 ) with smpling rte of 1.0 L/min. The NO removl rtio (η) ws clculted s η(%) = (1 C/C 0 ) 100 %, where C nd C 0 re concentrtions of NO in the outlet stem nd the feeding strem, respectively. Photoelectrochemicl mesurements Photoelectrochemicl mesurements were conducted using three-electrode qurtz cell equipted on CHI- 660B electrochemicl system. The sturted clomel electrode (SCE) ws used s the reference electrode, nd pltinum wire served s counter electrode. The working electrodes were nd films coted on ITO glss. The electrolyte ws 0.1 M of N 2 SO 4 solution. All the photoelectrochemicl mesurements were crried out under visile light irrdition emitted from 500 W Xe lmp with 420 nm cut-off filter. The verge power of visile light ws controlled t 45 mw/cm 2. Results nd discussion Phse structure To chrcterize the phse structure of the s-prepred smples, XRD ws performed. Figure 1 shows the XRD ptterns of, BiOI nd IHB-X smples. All diffrction peks of the smples re strong nd shrp, indicting the smples re well-crystllized. The diffrction peks of the nd IHB-X smples cn e indexed to the (JCPDS-ICDD Crd No ). The pek t round 2θ = 28.6 corresponds to (102) plne of, nd this pek intensity in ech IHB-X smple is lower thn tht in pure. Menwhile, this pek hs slight shift to higher ngle (Fig. 1), indicting decresed lyer distnce. The decresed pek intensity nd the shifting of the pek position cn e scried to the doping of I ions. The I ions could replce the COOH ions in the lyers of nd co-ordinte with oxygen toms of [BiO] + lyer. Note tht no BiOI phse cn e detected, even lrge mount of I ions were dded. This result indictes tht the I ions were doped into the crystl structure of. As the size of I ions is smller thn tht of COOH ions, the prtil replcement of COOH ions with I ions would result in decresed lyer distnce. The lyer distnce would not decrese s the molr rtio of I to COOH incresed from 1.00 to even 4.00 indicting tht the replcement of COOH ions with I ions hs limit, defining s sturte doping.

4 356 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes IHB-0.10 BiOI IHB-0.10 Intensity (.u.) Intensity (. u.) θ ( ) 2θ ( ) Fig. 1: XRD ptterns of s-prepred, BiOI nd IHB with different I contents () nd the corresponding enlrged view (). Fig. 2: SEM () nd HRTEM () imges of. Morphologicl structure The morphology nd microstructure of the otined smples were chrcterized y SEM, TEM, HRTEM nd EDS mpping. In Fig. 2, it cn e een tht the pure consists of numerous two-dimensionl (2D) nnosheets in different sizes. The HRTEM imge shows the lttice distnce of the crystl plne, corresponding to the (012) plne. Compred with the pure nnosheets, the morphology of is irregulr (Fig. 3). The oserved lttice spcing of nm cn e well ssigned to the (012) plne of. The EDS mpping (Fig. 3c f) indictes the elementl distriution of C, O, Bi nd I, therey demonstrting tht the doped I is uniformly distriuted cross the smple. Chemicl composition XPS mesurement ws used to determine the chemicl composition of, IBH-0.25, IBH-1.00, nd smples. The peks (Fig. 4) with inding energies t nd ev cn e scried to Bi4f 7/2 nd Bi4f 5/2 [19 22], respectively. Note tht there pper new peks round nd ev. As the hs lyered crystl structure, the I ions could enter into the spce etween the lyers, nd then replce the COOH ions nd co-ordinte with Bi nd O toms. Thus the new peks of nd ev cn e scried to the formtion of I Bi O ond. Figure 4 shows I3d spectr with two peks t nd ev, ttriuted to the presence of I [23 25]. The inding energy hs witnessed positive chemicl shift with the incresing of I ions contents. From XPS result, the content of doped I is determined to e 6.62, 11.53, nd %, for

5 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes 357 Fig. 3: SEM () nd HRTEM () imges, nd EDS mpping (c f) of. I-Bi-O Bi-O Intensity (.u.) Intensity (.u.) Binding energy (ev) Binding energy (ev) Fig. 4: XPS spectr of Bi4f () nd I3d () for,,, nd.,, nd, respectively. This result confirms tht the I ions content reches the upper limit or sturtion point of doping for. Bet surfce res nd pore structure Figure 5 shows the nitrogen dsorption-desorption isotherms nd the corresponding pore size distriution curves of the smples. According to the Brunuer-Deming-Deming-Teller (BDDT) clssifiction, the mjority of physisorption isotherms cn e clssified into six types. Typiclly, the nd IHB-X hve n isotherm of type IV. The displys stronger sorption t reltively high pressure compring with others, suggesting the presence of more mesopores in this smple. As no mesopores re formed in the nnosheets, the oserved mesopores should e the void spce etween the stcked nnosheets. This result is lso demonstrted y the pore-size distriution curves. Figure 5 shows tht the four smples hve the pek pore sizes t 1.2 nd 2.6 nm. The S BET, pore volume nd pek pore dimeter re summrized in Tle 1. The smple with high surfce re nd pore volume would fcilitte the exposure of ctive sites nd promote the photoctlysis efficiency.

6 358 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes Adsored volume (cm 3 /g) Reltive pressure (P/P 0 ) Pore dimeter (nm) dv/dd (cm 3 /g) Fig. 5: N 2 dsorption-desorption isotherms () nd pore size-distriution curves () of nd IHB-X. 100 Tle 1: The S BET, pore volume, pek pore dimeter, nd NO removl rtio of the smples. Smple S BET (m 2 /g) Pore volume (cm 3 /g) Pek pore dimeter (nm) NO η (%) / / / / / Light sorption, chrge seprtion The pure presents white color, while the smples fter I-doping chnge from ple to drk yellow with the increse of I-doping. Seen from the UV-vis DRS spectr (Fig. 6), the pure displys n sorption edge round 380 nm. With the increse of I-doping from 0.25 to 1.00, the sorption edge shows red-shift from 380 to 600 nm nd the light sorption spectr re rodened to 650 nm in visile light region. The enhnced visile light sorption should e scried to I-doping resutling in the nd gp nrrowing. However, even with more mount of I ions dded, the sorption edge nd light sorption spectr of IHB nd do not chnge oviously. As shown in Fig. 6, the nd gp energies of,,, nd smples re estimted to e 2.78, 1.99, 1.61, 1.61 nd 1.61 ev, respectively, which is lower thn tht of the pure with vlue of 3.2 ev. It lso ws found tht the nd gp enengies do not chnge nymore when the molr rtio of KI to reches t The XPS nd UV-vis Asornce (.u.) BiOI Wvelength (nm) Bnd gp (ev) (αhv) 1/ Eg = 3.20 ev Eg = 2.78 ev Eg = 1.99 ev Eg = 1.61 ev Eg = 1.61 ev Eg = 1.61 ev 5 6 Fig. 6: UV-vis DRS spectr () nd plots of (hν) 1/2 vs. photo energy () of, BiOI nd IHB with different I contents.

7 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes 359 Intensity (.u.) IHB Current (A*cm 2 ) IHB Wvelength (nm) Time (s) Fig. 7: PL spectr with UV light irrdition () nd photocurrent genertion () for nd under visile light irrdition (UV-vis light, [N 2 SO 4 ] = 0.5 M). DRS results indicte tht the I-doping could chieve sturte point when the interlyer spces mong the lyers hve een fully interted with I ions. To mesure the recomintion rte of the photogenerted electron-hole pirs, PL spectroscopy is pplied. A low PL intensity indictes high chrge seprtion efficiency [26]. As cn e seen in Fig. 7, pure gives high pek intensity. With the increse of I-doping, the PL pek intensity significntly decreses, indicting lrgely suppressed recomintion of photoinduced electron-hole pirs. The result implies tht the I-doping cn improve seprtion of photogenerted electron-hole pirs nd thus enhnce the photoctlytic performnce. Photocurrent genertion ws crried out for nd I-doped () electrodes to evlute the chrge seprtion efficiency. Figure 7 shows tht the stedy nd prompt photocurrent genertion is otined during on nd off cycles of visile light illumintion. The pure shows certin photocurrent responses. The chrge trnsport in this s-prepred smple proceeds quickly, mking the chnges of on nd off currents nerly verticl. The photocurrent of the I-doped electrode is much higher thn tht of the pure electrode shown in Fig. 7. The photocurrent enhncement of I-doped cn e scried to the enhnced photo-generted electrons/holes seprtion. Photoctlytic ctivity nd stility NO with concentrtion t pp level is one of the dominnt ir pollutnts, which is very stle nd cnnot e photolyzed under light irrdition without photoctlysts. Figure 8 shows the vrition of NO concentr- C/C 0 (%) BiOI C/C 0 (%) Time (min) Time (min) Fig. 8: Visile light photoctlytic ctivity of pure, BiOI nd IHB with different I contents () nd the repeted visile light photoctlytic ctivities of () for NO removl.

8 360 X. Feng et l.: An ion-exchnge strtegy for I-doped nnopltes tion (C/C 0 %) with irrdition time over the s-prepred smples under visile light irrdition. As previous reported, nnosheets exhiit no visile light ctivity due to the lrge nd gp. All the smples fter I-doping exhiit decent visile light photoctlytic ctivity for NO removl. When the molr rtio of I to is incresed to 1.00, the NO removl rtio is incresed to 49.7 %. Further incresing the molr rtio to 2.00 nd 4.00, the NO removl rtion is decresed to 45.3 % for IHB-2 nd 44.5 % for IHB-4, which cn e scried to decresed surfce res (Tle 1). Note tht the NO removl is grdully decresed due to the occuption of the surfce with the finl products. An idel photoctlyst should mintin photochemicl stility nd durility under repeted irrdition so tht it cn e reused [27 29]. Figure 8 shows the repeted photoctlytic ctivity of under visile light irrdition, which does not hve ovious dectivtion even fter five recycles. As demonstrted in Figs. 6 nd 7, I-doping could nrrow the nd gp nd enhnce the chrge seprion of, which in turn directly contriute to the highly promoted visile light photoctlytic ctivity. Conclusions A simple ion-exchnge method ws developed for synthesis of I-doped nnosheets vi the replcement of COOH ions with I ions. The concept of sturte-doping in lyered photoctlyst ws proposed. After I-doping, the light sorption spectr of cn e extended into the visile light region. The I-doped exhiited highly enhnced visile-light photoctlytic removl of NO in ir, which cn e scried to the incresed visile light sorption nd the promoted chrge seprtion. The concept of promoting visile-light photoctlysis through nion-exchnge could lso e extended to other lyered semiconductor photoctlysts. Acknowledgement: This reserch is finncilly supported y the Ntionl Nturl Science Foundtion of Chin ( , , nd ), the Innovtive Reserch Tem of Chongqing (CXTDG ), the Nturl Science Foundtion of Chongqing (cstc2016jcyja0481, cstc2015jcyja0061, cstc2017jcyjbx0052), nd the Chongqing Eduction Commission (KJ , Kj ). References [1] S. Tokung, H. Kto, A. Kudo. Chem. Mter. 13, 4626 (2001). [2] M. Ou, Q. Zhong, S. L. Zhng, H. Y. Nie, Z. J. Lv, W. Ci. Appl. Ctl. B: Environ. 193, 160 (2016). [3] C. Zhng, Y. F. Zhu. Chem. Mter 13, 3537 (2005). [4] F. Qin, G. F. Li, R. M. Wng, J. L. Wu, H. Z. Sun, R. Chen. Chem.- Eur. J. 18, (2012). [5] Z. Ni, W. Zhng, G. Jing, X. Wng, Z. Lu, Y. Sun, X. Li, Y. Zhng, F. Dong. Chin. J. Ctl. 38, 1174 (2017). [6] L. Q. Ye, J. Y. Liu, C. Q. Gong, L. H. Tin, T. Y. Peng, L. Zn. ACS Ctl. 8, 1677 (2012). [7] F. Dong, W. K. Ho, S. C. Lee, Z. B. Wu, M. Fu, S. C. Zhou, Y. Hung. J. Mter. Chem. 21, (2011). [8] F. Dong, Q. Y. Li, Y. J. Sun, W. K. Ho. ACS Ctl. 4, 4341 (2014). [9] S. J. Peng, L. L. Li, H. T. Tn, Y. Z. Wu, R. Ci, H. Yu, X. Hung, P. N. Zhu. J. Mter. Chem. A 1, 7630 (2013). [10] T. Xiong, H. W. Hung, Y. J. Sun, F. Dong. J. Mter. Chem. A 3, 6118 (2015). [11] F. Dong, S. C. Lee, Z. B. Wu, Y. Hung, M. Fu, W. K. Ho, S. C. Zou, B. Wng. J. Hzrd. Mter. 195, 346 (2011). [12] F. Dong, T. Xiong, Z. W. Zho, Y. J. Sun, M. Fu. CrystEngComm. 15, (2013). [13] L. Q. Ye, J. Y. Liu, Z. Jing, T. Y. Peng, L. Zn. Appl. Ctl. B: Environ. 1, 142 (2013). [14] J. Co, B. Y. Xu, H. L. Lin, B. D. Luo, S. F. Chen. Dlton Trns. 41, (2012). [15] Y. S. Xu, W. D. Zhng. Appl. Ctl. B: Environ. 140, 306 (2013). [16] N. Ling, J. T. Zi, M. Xu, Q. Zhu, X. Wei, X. F. Qin. J. Mter. Chem. A 2, 4208 (2014). [17] F. Dong, T. Xiong, S. Yn, H. Wng, Y. Sun, Y. Zhng, H. Hung, Z. Wu. J. Ctl. 344, 401 (2016). [18] F. Dong, X. Feng, Y. X. Zhng, C. F. Go, Z. B. Wu. RSC Adv. 5, (2015). [19] J. Y. Xiong, G. Cheng, Z. Lu, J. L. Tng, X. L. Yu, R. Chen. CrystEngComm 13, 2381 (2011). [20] L. L. Yng, Q. F. Hn, X. Wng, J. W. Zhu. Chem. Eng. J. 262, 169 (2015).

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