Journl of Mterils Chemistry A Novel one-dimensionl Bi2O3-Bi2WO6 p-n hierrchicl heterojunction with enhnced photoctlytic ctivity Journl: Journl of Mterils Chemistry A Mnuscript ID: TA-ART-01-2014-000274.R1 Article Type: Dte Sumitted y the Author: 14-Mr-2014 Complete List of Authors: peng, yin; nhui norml university, College of Chemistry nd Mterils Science yn, mei; nhui norml university, College of Chemistry nd Mterils Science chen, qing; nhui norml university, College of Chemistry nd Mterils Science fn, cong; University of Science nd Technology of Chin, Division of Nnomterils nd Chemistry zhou, hi; nhui norml university, College of Chemistry nd Mterils Science Xu, An-Wu; Hefei Ntionl Lortory for Physicl Sciences t Microscle, University of Science nd Technology of Chin, Division of Nnomterils nd Chemistry
Pge 1 of 8 Journl of Mterils Chemistry A Novel one-dimensionl - p-n hierrchicl heterojunction with enhnced photoctlytic ctivity Yin Peng, Mei Yn, Qing-Guo Chen, Cong-Min Fn, Hi-Yn Zhou nd An-Wu Xu* 5 Receipt/Acceptnce Dt [DO NOT ALTER/DELETE THIS TEXT] Puliction dt [DO NOT ALTER/DELETE THIS TEXT] DOI: 10.1039/000000x [DO NOT ALTER/DELETE THIS TEXT] 10 15 20 25 30 35 40 45 A novel one-dimensionl (1D) nnorods- nnosheets p-n junction photoctlyst ws prepred y three steps synthetic route. The otined products were chrcterized y X- ry diffrction (XRD), scnning electron microscopy (SEM), trnsmission electron mic-roscopy (TEM), high-resolution trnsmission electron microscopy (HRTEM), X-ry photoelectron spectroscopy (XPS), N 2 -sorption/desorption nd Brunuer- Emmett-Teller surfce re (BET). rods with the dimeter of out 200 nm were otined y clcining Bi(OHC 2 O 4 ) 2H 2 O precursor. nnosheets verticlly grow onto the rods long the long xil direction. The photoctlytic ctivity to degrde Rhodmine B (RhB) nd phenol under solr/visile light y p-n junction - nnorods ws investigted. The result demonstrtes tht the novel - p-n heterostructures disply higher photoctlytic ctivity thn single nnorods or flowers. The enhncement of the photoctlytic ctivity of the - p-n junction structures cn e scried to the strong visile light sorption nd the effective seprtion of photogenerted electrons nd holes y the internl electrosttic field in the junction region. More importntly, 1D p-n heterostructures mde of ordered nnosheets is eneficil for trnsport of photogenerted crriers, nd incresing the rte of photoctlytic rection. This work would offer new insight into the design nd friction of dvnced mterils with heterojunction structures for photoctlytic pplictions nd optoelectronic devices. Introduction With the environmentl pollution increse, green chemistry hs ecome necessry requirement for the sustinle development of humn society. Among the vrious green chemicl techniques, semiconductor photoctlysis hs een considered n costeffective, sustinle nd the most promising green chemicl technologie ecuse it represents n esy wy to complete degrdtion of orgnic pollutions y utilizing the solr energy. 1 3 However, some trditionl photoctlysts (such s TiO 2, ZnO, SnO 2 ) cn sor only ultrviolet light due to their wide nd gp nd hve moderte photoctlytic ctivity due to the high The Key Lortory of Functionl Moleculr Solids, Ministry of Eduction, College of Chemistry nd Mterils Science, Anhui Norml University, Wuhu 241000, Chin Division of Nnomterils nd Chemistry, Hefei Ntionl Lortory for Physicl Sciences t Microscle, Deprtment of Chemistry, University of Science nd Technology of Chin, Hefei 230026, P. R Chin. Fx: (+86) 551-6360 2346; E-mil: nwuxu@ustc.edu.cn. Electronic Supplementry Informtion (ESI) ville: [detils of ny supplementry informtion ville should e included here]. See http://dx.doi.org/10.1039/000000x/ recomintion rte of photogenerted crriers. 4 6 So, the development of photoctlysts performnce under visile light constitutes the key point. A significnt numer of new photoctlysts do perform dequtely under visile light ut typiclly disply poor performnce with respect to TiO 2 commercil references (e.g., Deguss P25) under sunlight ecuse the enhnced visile light sorption nd fst recomintion of chrge recomintion occurs simultneously. 7 For tht reson, insted of using single semiconductor, comining two or more semiconductors with pproprite nd positions to improve the photoctlytic performnce is n estlished ide ecuse it cn led to n enhnced chrge seprtion nd interfcil chrge-trnsfer efficiency. 8-10 Especilly, it is noted tht the friction of p-n junction photoctlyst is elieved to e the very effective ecuse of the existence of n internl electric field. 11,12 As one of the simplest Aurivillius oxides with lyered structure, is specil for its good photoctlytic performnce under visile light irrdition. 13 15 Bre presents photosorption properties from UV light to visile light with wvelength of shorter thn c. 450 nm, 16,17 which overlps smll prt of the solr spectrum. Moreover, the rpid recomintion of photoinduced electron-hole pirs seriously limits the energy-conversion efficiency. To roden the rnge of visile-light photoresponse nd promote the seprtion of photogenerted crriers of, we intend to design composite photoctlyst y coupling with nrrow ndgp semiconductor with mtched nd potentils. The well-estlished heterojunction structure could e employed to restrict the recomintion of the chrge crriers nd enhnce the quntum yield. 18 The electrons excited y visile light cn e trnsferred to from the nrrow ndgp semiconductor, which fvors the chrge seprtion nd lso improves the visilelight photoctlytic ctivity of the heterostructure drmticlly. is p-type semiconductor with nd gp of 2.8 ev nd hs proved to e photoctlyst for wter splitting nd pollutnt decomposition under visile-light irrdition. However, the photoctlytic ctivity of pure is lso low ecuse of the high recomintion proility of photogenerted electrons nd holes. 19 21 It is expected tht the formtion of p-n junction structure etween n-type nd p-type will gretly enhnce the photoctlytic efficiency. Recently, - p- n junction structures with microspheres, 22 hollow microspheres, 23,24 nd flowers, 25,26 hve een synthesized nd exhiit etter photoctlytic ctivies thn pure or smples. However, - p-n heterojunction with highly ordered nnosheets grown on one-dimensionl (1D) nnorods hve not een reported. Compre with ove mentioned heterostructures, 1D p-n heterojunction photoctlysts hve roused gret concern ecuse of their high surfce res, This journl Royl Society of Chemistry [JOURNAL], 200X, 00, 0000 1 50 55 60 65 70 75 80 85 90 95
Journl of Mterils Chemistry A Pge 2 of 8 100 105 110 115 120 125 130 135 140 145 150 155 160 remrkle nd directionl trnsport chrcteristics of electrons nd holes, nd thus their enhnced photoctlytic ctivities. 27 Here, we report for the first time the synthesis of novel nnords- ordered nnosheets p-n junction vi simple method. This 1D heterostructure displys superior photoctlytic ctivity for degrdtion of the phenol nd RhB under solr/visile light irrdition. Experimentl Section Photoctlyst preprtion All the regents used in our experiment were nlyticl grde nd used s received without further purifiction. Bi(OHC 2 O 4 ) 2H 2 O nnorods were first synthesized vi hydrotherml method. Bi(NO 3 ) 3 5H 2 O (2.911 g) nd N 2 C 2 O 4 (1.206 g) were dissolved seprtely in 20 ml distilled wter. Then the N 2 C 2 O 4 solution ws dded into the Bi(NO 3 ) 3 suspension solution with vigorous mgnetic stirring. The mixed suspension solution ws poured into stinless steel utoclve with Teflon liner nd heted t 120 ºC for 40 h. The otined solid smple ws wshed with deionized wter nd nhydrous ethnol, nd then dried t 60 ºC for 6 h. Then, -Bi(OHC 2 O 4 ) 2H 2 O smples were synthesized vi solvotherml process. 0.341 g of Bi(OHC 2 O 4 ) 2H 2 O nnorods were dded into the 15 ml of Bi(NO 3 ) 3 5H 2 O ethylene glycol (EG) solution under mgnetic stirring, nd then 15 ml of N 2 WO 4 2H 2 O queous solution ws dded drop-wise nd stirred for nother 30 min. The mixture ws seled into Teflon-lined stinless steel utoclve nd treted t 160 ºC for 20 h. After eing cooled down to room temperture nturlly, the products were collected nd wshed severl times with deionized wter nd solute ethnol, nd dried t 60 ºC for 4 h. flowers were otined in the sence of Bi(OHC 2 O 4 ) 2H 2 O nnorods in ove rective system. Finlly, the - heterojunctions were otined y clcining -Bi(OHC 2 O 4 ) 2H 2 O composites t 400 ºC for 2 h in ir. The s-otined smples with the molr rtio of Bi(OHC 2 O 4 ) 2H 2 O: 2:1.5, 2:3, 2:10, 2:2 nd 2:4, were lelled WBP1, WBP2, WBP3, WBP4 nd WBP5 respectively. And the corresponding clcined products - were lelled,, WB3, WB4 nd WB5, respectively. rods were otined y clcintions of Bi(OHC 2 O 4 ) 2H 2 O precursors t 400 ºC for 2 h in ir. Photoctlytic ctivity mesurements Photoctlytic ctivity of the prepred - smples ws tested y decomposing phenol nd RhB under solr/visile light irrdition. The light source ws 300 W Xe lmp (PLS- SXE300/300UV, Trusttech Co., Ltd. Beijing). In typicl experiment, the photoctlyst (100 mg) ws dded into 100 ml of phenol (or RhB) (10 mg/l) to produce suspension for the degrdtion rection t room temperture under ir. Before the solr/visile light irrdition, the suspension ws stirred in the drk for 30 min to ensure n dsorption/desorption equilirium of phenol (or RhB) on the surfce of the photoctlyst. Then, the suspension ws illuminted y the Xe lmp comined with UV cutoff filter (λ 400 nm) under stirring. At given time intervls, ove 4 ml suspension ws withdrwn, nd centrifuged to remove the precipitte. The degrdtion rection process ws monitored y mesuring the concentrtion of phenol (or RhB) s function of irrdition time in the solution with UV-Vis sorption spectr. Additionlly, the recycling experiments were performed for six recycles to test the durility. After ech cycle, 165 170 175 180 www.rsc.org/[journal] [JOURNAL NAME HERE] the photoctlyst ws centrifugtion nd used directly for the next test. Photoctlytic wter splitting In typicl photoctlytic experiment, the prepred - smple (50 mg) ws dispersed in n queous solution (100 ml) of Fe 2 (SO 4 ) 3 (2.1 mm L -1 ). The suspension ws seled in qurtz flsk nd purged with rgon for 1 h to drive wy the residul ir. The photoctlytic oxygen production ws initited y irrditing the suspension with 300 W Xenon lmp (MAX- 302, Ashi Spectr, USA) coupled with UV cut-off filter (λ > 420 nm). The gs product ws nlyzed periodiclly through gs chromtogrph (Agilent 7890A) with therml conductivity detector (TCD). Chrcteriztion Field emission scnning electron microscopy (FE-SEM) imges were recorded on Hitchi S-4800 microscope. Trnsmission electron microscopic (TEM) imges, high-resolution trnsmission electron microscopic (HRTEM) imges nd the selected re 185 electron diffrction (SAED) ptterns were performed on JEOL- 2010 microscope with n ccelerting voltge of 200 kv, nd energy-dispersive X-ry spectroscopy (EDS) nlysis ws performed. X-Ry powder diffrction (XRD) ws crried out on Rigku (Jpn) D/mx -γa X-ry diffrctometer with Cu-Kα 190 rdition (λ = 0.154178 nm). UV-vis diffuse-reflectnce spectrum ws recorded with UV-2450 spectrophotometer in the wvelength rnge of 200-800 nm t room temperture. BSO 4 ws used s the reflectnce stndrd mteril. The X-Ry photoelectron spectroscopy (XPS) ws performed on Perkin- 195 Elmer RBD upgrded PHI-5000C ESCA system. Nitrogen dsorption/desorption mesurements were performed t 77 K using Micromeritics Tristr II 3020 M nlyzer fter the smples were degssed t 180 ºC for 6 h. The Brunuer-Emmett- Teller (BET) surfce re ws estimted y using dsorption dt 200 in reltive pressure rnge from 5 to 0.3. 205 210 215 220 225 Results nd discussion In this study, Bi(OHC 2 O 4 ) 2H 2 O rods were first synthesized, then -Bi(OHC 2 O 4 ) 2H 2 O nnosheet-rod heterostructures were otined y hydrotherml tretment using Bi(OHC 2 O 4 ) 2H 2 O rods s support. Finlly, - p-n junction nnostructures wit nnosheets stnding on rods were prepred y clcintions of -Bi(OHC 2 O 4 ) 2H 2 O. The Bi(OHC 2 O 4 ) 2H 2 O- heterostructures with different Bi(OHC 2 O 4 ) 2H 2 O/ molr rtio of 2:1.5, 2:3 nd 2:10, were lelled s WBP1, WBP2 nd WBP3, respectively. And the corresponding clcined products - ws denoted s, nd WB3, respectively (see experimentl section). The X-ry powder diffrction (XRD) ptterns of the otined products re shown in Fig. S1. All of the diffrction peks in the Fig. S1 could e indexed to ismuth oxlte (Bi(OHC 2 O 4 ) 2H 2 O) precursor reported y Monnereu et l. 28 With the loded- content incresing, the new diffrction peks pper nd their intensities increse grdully (Fig. S1 - d) nd these new peks cn e indexed to orthorhomic (Fig. S1e). These results show tht the heterostructure etween Bi(OHC 2 O 4 ) 2H 2 O nd is formed. Fig. 1 shows the XRD ptterns of otined rods, - p-n heterojunctions nd. All of the diffrction peks shown in Fig. 1 nd f cn e well-indexed to the monoclinic structure of (JCPDS No. 76-1730) nd 2 [JOURNAL], 200X, 00, 0000 This journl Royl Society of Chemistry
Pge 3 of 8 Journl of Mterils Chemistry A 280 262 202 002 131 280 nnoprticles cn e indexed s Bi2WO6 single crystl recorded long the [100] zone xis. The energy dispersive spectroscopy (EDS) nlysis further confirms tht nnosheets only contin O, Bi nd W elements, nd the tomic rtio of Bi/W is clculted to e out 1.85:1, close to 2:1 in Bi2WO6 (Fig. S2). e d c 223 Intensity (.u.) f 002 235 002 120 012 230 orthorhomic Bi2WO6 (JCPDS No. 39-0256), respectively. The shrp diffrction peks of oth Bi2O3 nd Bi2WO6 indicte their good crystllinity. No trces of other phses re detected, confirming the high purity of the smples. Fig. 1c e oviously show two sets of XRD peks of monoclinic Bi2O3 nd orthorhomic Bi2WO6, indicting tht the s-synthesized products re composite mterils. Moreover, it is noted tht the diffrction peks of Bi2O3 re wekened with incresing Bi2WO6/Bi2O3 molr rtio (see red lels in Fig. 1-e), while the pek intensities of Bi2WO6 increse grdully. JCPDS 76-1730 20 30 40 50 60 70 2-Thet (degree) 240 245 250 255 260 265 270 275 Fig. 1 The XRD ptterns of () Stndrd Crd of Bi2O3, () Bi2O3, (c), (d), (e) WB3 nd (f) Bi2WO6. Fig. 2 displyed the SEM imges of these Bi2O3-Bi2WO6 heterojunctions nd their precursors. Bi(OHC2O4) 2H2O is 1D rods with men dimeter of 450 ± 50 nm nd length of 5±1 µm. Ech rod hs smooth surfce (Fig. 2). After clcintions t 400 ºC for 2 h, Bi(OHC2O4) 2H2O trnsformed to Bi2O3 rods with rough surfce nd porosity due to gs removl (Fig. 2). The otined Bi2WO6-Bi(OHC2O4) 2H2O precursors re lso 1D rodlike structures, nd the thin Bi2WO6 nnosheets grow verticlly onto the surfce of Bi(OHC2O4) 2H2O nnorod (Fig. 2c). Menwhile, the more the loded-bi2wo6 content is used, the more Bi2WO6 nnosheets orderly grow onto the Bi(OHC2O4) 2H2O rods long the long xil direction (Fig. 2e, g), nd the gp etween Bi2WO6 nnosheets ecomes nrrower nd nrrower, finlly ech Bi(OHC2O4) 2H2O rod is completely covered y Bi2WO6 nnosheets. During the formtion of Bi(OHC2O4) 2H2O-Bi2WO6 composite, the preferred outwrd diffusion of Bi ions from Bi(OHC2O4) 2H2O precursor to Bi2WO6 leds to net mteril flux cross the composite interfce. Therefore, some hollow structures (see red rrows in Fig. 2e, g) re formed sed on the Kirkendll effect.29 From Fig. 2d, f, h, it cn e clerly seen tht the morphologies of the otined Bi2WO6-Bi2O3 heterostructures were kept unchnged fter clcining Bi2WO6-Bi(OHC2O4) 2H2O precursors t 400 ºC for 2 h. To further otin informtion out the structure of the smple, the p-n heterojunction ws chrcterized y trnsmission electron microscopy (TEM). As shown in Fig. 3, it cn clerly seen the Bi2WO6 nnosheets verticlly grow onto the surfce of the Bi2O3 nnorods, which is consistent with the result of the SEM mesurements. Fig. 3 shows the high-resolution trnsmission electron microscopic (HRTEM) imge tken from the tip of the Bi2WO6 nnosheet (red squre highlighted in Fig. 3). It is found tht two sets of lttice fringes with interplnr spcing of 2 nm nd 0.27 nm, well correspond to (020) nd (002) plnes of orthorhomic Bi2WO6. The selected re electron diffrction (SAED) pttern (inset in Fig. 3) tken from the This journl Royl Society of Chemistry 285 Fig. 2 The FE-SEM imges of the Bi2O3-Bi2WO6nd their precursors () Bi(OHC2O4) 2H2O, () pure Bi2O3, (c) WBP1, (d), (e) WBP2, (f), (g) WBP3 nd (h) WB3. 290 Fig. 3 () The TEM nd () HRTEM imges of the heterojunction. 295 Quntittive X-ry photoelectron spectroscopy (XPS) nlysis ws crried on the otined heterostructures. The typicl [JOURNAL], 200X, 00, 0000 3
Journl of Mterils Chemistry A Pge 4 of 8 300 305 310 315 full survey nd high-resolution spectr for Bi 4f, W 4f, nd O 1s region were showed in Fig. 4. It cn e seen tht only O, Bi nd W elements exist in the smple (Fig. 4, (i)). High resolution XPS spectrum of W 4f region shows inding energy t 35.5 ev for W 4f 7/2 nd t 37.6 ev for W 4f 5/2, suggesting tht W exists in the chemicl stte of W 6+ (Fig. 5, (i)). 22,30 The inding energy for Bi 4f 5/2 nd Bi 4f 7/2 re 164.5 nd 159.2 ev (Fig. 5c, (i)) respectively, which prove ll the Bi species in the smple re in the form of Bi 3+. However, it is worth noting tht these inding energy vlues re not exctly the sme s those otined from pure or, 30,31 which revel tht interfcil structure is formed nd the locl environment nd electron density of the elements is chnged in some degree. The XPS spectrum for O1s cn e deconvoluted to three peks t 529.6, 530.5 nd 531.9 ev, which cn e ssigned to Bi-O nd W-O in (Fig. 4d (i)) nd Bi-O in, respectively. 23,25,31,32 On the other hnd, ccording to the XPS result of smple, the concentrtion of the surfce Bi 3+ is 26 tom%, wheres tht of W 6+ is 5.82 tom%. The tomic rtio of Bi nd W is out 3.55, which is lrger thn the stoichiometric rtio in. This result revels the co-existence of nd species in the smple, in good ccordnce with the XRD, TEM nd SEM results. 320 Intensity(.u.) Intensity(.u.) (i) (ii) O1s C1s Bi4f W4f 1000 800 600 400 200 0 c (ii) (i) Bi4f 170 168 166 164 162 160 158 Intensity(.u.) Intensity (.u.) (ii) (i) W4f 46 44 42 40 38 36 34 32 d (ii) (i) O1s 528 530 532 534 Fig. 4 () Survey XPS spectrum of the smple, High-resolution XPS spectr of () W 4f, (c) Bi 4f nd (d) O 1s for smple. (i) 325 efore use; (ii) fter eing used to degrde RhB dye for six times. Adsorption nd desorption experiments using N 2 were crried out t 77 K. Fig. 5 displys the nitrogen sorption isotherms of the - heterostructures, pure nd 330 smples. The shpe of the isotherm is type IV isotherm with type H3 hysteresis loop t high reltive pressures ccording to the IUPAC clssifiction, which indictes tht these smples re mesoporous structures in the pore dimeter rnge of 2 50 nm. 33,34 This result cn e further confirmed y the corresponding pore 335 size distriution, s shown in Fig. 5. Considering the oserved morphology of the smples, the smller pores with shrp pek t out 2.6 nm my e generted during the crystl growth process, wheres the lrger pores (20 30 nm) could e generted during therml tretment process. Tle 1 gives the BET surfce 340 re nd porous volume of different smples. It cn e found tht the BET specific surfce res of the - p-n junction structures re ll higher thn tht of pure nnorods (1.9 m 2 g -1 ), ut lower thn tht of flowers (15.4 m 2 g -1 ). nd hve much lrger pore volume thn tht of 345 other smples, nd nnorods hve the smllest pore volume. 350 355 360 365 370 375 380 www.rsc.org/[journal] [JOURNAL NAME HERE] Previous studies show tht suitle conformtion of pores llows light wves to penetrte deep inside the photoctlysts nd leds to high moility of chrge. 35-38 It is speculted tht the pores in the heterostructures nd flowers llow the penetrtion of light wves nd phenol or RhB molecules in solution deep into the photoctlysts, which my gretly promote the photoctlytic ctivity. Volume Asored (cm 3 g -1 ) Fig. 5 () Nitrogen dsorption-desorption isotherm nd () the corresponding pore size distriution of the different smples. Tle 1 The surfce re surfce nd pore volume of different smples. The opticl property of - heterojunctions ws exmined using UV-vis diffuse-reflectnce spectrum (DRS). As shown in Fig. 6, The sorption edge of the pure nd is ~437 nd 428 nm, which shows their visile light sorption. After comining the two semiconductors, - heterojunctions show more intensive sorption within the visile light rnge in comprison with pure or, nd the visile light sorption ility of the composite is grdully enhnced with loded- increse. However, too much loded- (such s smple WB3) will decrese the visile light sorption. For semiconductors, their opticl nd gp cn e clculted from the sorption spectr using the eqution αhν = A(hν E g ) 2/n, in which α, h, ν, A, nd E g re the sorption coefficient, Plnck constnt, light frequency, constnt nd nd gp, respectively. 39 In the eqution, n decides the chrcteristics of the trnsition in semiconductor, here n = 2 for Bi 2 WO 40 6 nd n = 4 for Bi 2 O 41 3. The energy of the nd gp is clculted y extrpolting the stright line to the sciss xis. The nd gp of pure nd ws estimted to e 2.89 nd 2.71 ev, respectively. Asornce (.u.) WB3 Pore Volume ( cm 3. g -1. nm -1 ) 0.2 0.4 1.0 0 40 80 120 160 Reltive Pressure ( p/p0) Pore Dimeter (nm) WB3 (αhν) 2/n (ev) 2/n WB3 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 200 300 400 500 600 700 800 Wvelength (nm) hν (ev) 2.0 2.2 2.4 2.6 2.8 3.0 Pore Dimeter (nm) 4 [JOURNAL], 200X, 00, 0000 This journl Royl Society of Chemistry
Pge 5 of 8 Journl of Mterils Chemistry A 385 390 395 400 405 410 415 420 Fig. 6 () UV-Vis diffuse reflectnce spectr of Bi 2O 3, Bi 2WO 6 nd Bi 2O 3-Bi 2WO 6 smples; () the plots of (αhν) 2/n vs. hν (n = 4 for Bi 2O 3 nd n = 2 for Bi 2WO 6). The photoctlytic ctivity of the smples ws evluted y decomposing toxic orgnic compounds in queous solution, such s phenol. Fig. 7 depicts the correltion curves etween the concentrtion chnges of phenol molecules nd the irrdition durtions in the presence of photoctlysts. It is oserved tht compring with pure nd, the photoctlytic ctivities of - p-n junction photoctlysts grdully increse with the loded- content incresing nd rech the mximum vlue for smple. After tht, the ctivity of - p-n junction photoctlyst (WB3) sequentilly decreses. Oviously, the photoctlytic ctivity of the smples is significntly ffected y the content in - junctions. According to the SEM imges (Fig. 2) of - p-n heterojunctions, for low content in smple, sprse nnosheets grown on rods re oserved, so only smll numer of p-n junctions re generted, leding to low photoctlytic ctivity. When the content is incresed in the smple, lrge numer of p-n junctions re formed, thus resulting in the highest photoctlytic ctivity for the optiml smple. With the content further incresing dense nnosheets lmost cover the rods, s oserved in WB3 smple (Fig. 2h), which decreses the light irrdition on rods nd the p-n junction interfces. This shielding effect of dense nnosheets coting on the rods mkes the photoctlytic ctivity of WB3 smple decrese. When,, nd P25 re used s photoctlysts, the degrdtion efficiencies of phenol re 100%, 44%, 13% nd 2% in 60 min under solr light irrdition, respectively. Moreover, when ws used s photoctlyst, the sorption peks t 270 nm nd 208 nm disppered completely in 120 min (Fig. 7). The totl orgnic cron (TOC) ws mesured, nd results show tht the phenol molecules cn e thoroughly minerlized. Aove fcts clerly show tht the sotined p-n junction structure owns outstnding photoctlytic cpcity nd cn e used in environmentl tretment nd protection. The UV-vis sornce spectr of phenol using different smples s photoctlysts re shown in Fig. S3. C/C 0 1.0 0.4 0.2 P25 WB3 0 20 40 60 80 100 120 425 Time (min) Wvelength(nm) Fig. 7 () Photoctlytic degrdtion curves of phenol (10 mg/l) using different photoctlysts, () UV-Vis sorption spectr of phenol using s phtoctlyst under solr light irrdition. 430 In order to further evlute the photodegrdtion cpcity of - p-n junction structures, the degrdtion of RhB queous solution under solr light irrdition ws crried out. As shown in Fig. 8, - p-n junction photoctlysts disply etter degrdtion efficiency for RhB thn pure nd 435, nd smple displys the est photoctlyticl ctivity which cn degrde 100% RhB in 60 min. Moreover, the sorption peks of the RhB t 554 nm nd 200 nm dispper completely fter 60 min rection, nd no new peks pper (Fig. 8) using s photoctlyst. This lies in two fcts: i) the 440 removl of the ethyl groups nd clevge of the whole Asornce (.u.) 0.9 120 min 90 min 0.3 60 min 30 min dsorption 0 min 200 250 300 350 400 445 450 455 460 chromophore structure (cycloreversion) occur simultneously for RhB molecules during the photoctlytic process; ii) ll RhB molecules re completely degrded. The comprison of the degrdtion of RhB using - p-n heterojunctions ( WB5) s photoctlysts is given in Fig. S4. It cn e found tht with the content increse ( < WB4 < ), more nd more p-n junctions re formed, which results in the highest photoctlytic ctivity for the optiml smple. However, with further incresing the content ( < WB5 < WB3), dense nnosheets shield the light irrdition on rods nd the p-n junction interfces nd then result in the lowest photoctlytic ctivity for WB3 smple. hs lso good photoctlytic performnce for the degrdtion of RhB queous solution under visile light irrdition (λ > 400 nm) (Fig. S5), nd displys higher photoctlytic ctivity thn / heterostructures reported in ref 24 nd 25. However, the degrdtion mechnism of RhB molecules is different from tht under solr light irrdition. The UV-Vis sornce spectr of RhB molecules (Fig. S5) fter photoctlytic rection indicte tht the removl of the ethyl groups is the first stge nd then clevge of the whole chromophore structure (cycloreversion) occurs in RhB 42-44, 25 molecules y the visile light irrdition. 465 470 475 480 485 C/C 0 1.0 0.4 WB3 0.2 P25 0 20 40 60 80 Time (min) 2.4 0min 200 300 400 500 600 700 800 Wvelength (nm) Fig. 8 () The degrdtion curves of RhB using different photoctlysts, () UV-Vis sorption spectr of RhB queous solutions using s phtoctlyst under solr light irrdition. Inset in is photogrphs of RhB solutions y different rection time. To investigte the stility of photoctlytic performnce in solr light region, the smple ws used to degrde RhB dye in six repeted cycles, nd the results re shown in Fig. 9. It is noteworthy tht photoctlyst exhiits good photostility under solr light irrdition (Fig. 9), nd its photoctlytic efficiency only reduces 2% fter six repeted cycles. From the SEM imge, we lso find tht still retined the originl structure (Fig. 9) fter six repeted cycles. The XPS spectrum of the fter eing reused for six times to degrde RhB dye is lso crried out to prove the photostlility of smple under solr light irrdition. As shown in Fig. 4 (ii), Bi species re still in the form of Bi 3+, nd W exists in the form of W 6+, demonstrting its high stility in the process of photoctlysis. C/C 0 1.0 0.4 0.2 one two three four five six Asornce (.u.) 2.0 1.6 1.2 0.4 0 80 160 240 320 400 480 Time (min) 80min 80 min 60 min 40 min 20 min dsorption 0 min This journl Royl Society of Chemistry [JOURNAL], 200X, 00, 0000 5
Journl of Mterils Chemistry A Pge 6 of 8 490 495 Fig. 9 () Cycling times of the photoctlytic degrdtion of RhB in the presence of under solr light irrdition, () the SME imge of the fter six repeted cycles. To determine the reltive positions of conduction nd (CB) nd VB edges, the totl densities of sttes of VB for, nd were mesured, s shown in Fig. 10. In contrst to, the VB mximum of is down-lowered from 1.30 to 1.71 ev. Compred to, the VB mximum of is up-shifted from 2.24 to 1.71 ev. The shift of VB mximum position for cn e ttriuted to the formtion of heterojunction, s confirmed y XRD nd TEM results. 540 545 www.rsc.org/[journal] [JOURNAL NAME HERE] splitting into O 2 were performed (Fig. S6) using - s photoctlyst. It cn e found O 2 production is out 70 ml g -1 in 4 h under 300 W Xe light irrdition. As well known, mny fctors influence the photoctlytic ctivity of photoctyst, such s BET surfce re, size of prticles, etc. Prepred flower owns lrger BET surfce re thn tht of, ut its photoctlytic ctivity is much poorer thn tht of (Fig. 7, 8). This is minly ttriuted to the formtion of p-n junction in the smple. 500 505 510 515 520 525 Intensity (.u.) 1.30 ev 1.70 ev 2.24 ev -1 0 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 10 VB-XPS spectr of Bi 2O 3 nd Bi 2WO 6 nd smples. According to the VB edges of nd, nd comined with nd gp derived from DRS, the CB edge potentils of the two semiconductors cn thus e otined y using the eqution of E CB = E VB E g. So, the energy nd structures of nd is esy to e generted (Fig. 11). For p-type, its Fermi energy level is close to the vlence nd, while for n-type, its Fermi energy level is close to the conduction nd. When the two semiconductors re in contct to form p-n junction (Fig. 11), there is diffusion of electrons from to due to their different Fermi energy level, resulting in ccumultion of negtive chrges in close to the junction. At the sme time, the holes trnsfer from to, leving positive section in ner the junction. Menwhile, the energy nds of shift upwrd long the Fermi level nd those of the shift downwrd long its Fermi level (Fig. 11). With equilirtion of nd Fermi levels, the diffusion of electrons from to stops. Therefore, n equilirium stte is formed nd n inner electric field will lso e generted t the interfce. Under the solr/visile light irrdition, with nrrow nd gp is excited nd photoelectrons nd holes re generted. The excited electrons on the conduction nd of p-type trnsfer to tht of n-type, while the holes remin in the vlence nd of p-type. Furthermore, the migrtion rte of the photogenerted electrons nd holes could e promoted y the internl electric field in the - p-n heterojunctions nd the photoctlytic ctivity is lrgely enhnced. According to Fig. 10 nd Fig. 11, it cn e seen tht the electronic structure of - p-n junction structure mtches well with the redox potentil of wter into hydrogen nd oxygen molecules, nmely, the ottom level of the conduct nd hs to e more negtive thn the reduction potentil of H + /H 2 (0 530 535 ev vs norml hydrogen electrode (NHE)); the top level of the vlence nd hs to e more positive thn the oxidtion potentil of O 2 /H 2 O (1.23 ev). The experiments of photoctlytic wter 550 555 560 565 570 Fig. 11 Schemtic digrm for () energy nd of Bi 2WO 6 nd Bi 2O 3 nd () the formtion of p-n junction nd the possile chrge seprtion. Conclusions In summry, the novel p-n junction photoctlysts of rods- nnosheets hve een prepred for the first time. nnosheets verticlly grow on the nnorods long the xil direction. The otined - p-n heterojunctions exhiit higher photoctlytic ctivity thn pure nd for the degrdtion of phenol nd RhB under solr/visile light irrdition. Phenol nd RhB cn e completely degrded in 60 min under solr light irrdition using - p-n heterojunction s photoctlyst. This good photoctlytic ctivity is scried to the synergistic effects: () extended sorption in the visile light region; () formtion of p- n junction enhncing the seprtion of photogenerted crriers; (c) 1D ordered nnostructure is fvourle for high efficient nd directionl trnsport nd seprtion of electrons nd holes. This study provides generl nd effective method to fricte unique 1D - p-n junction photoctlyst on lrge scle. Moreover, this route would offer new insight into the design nd friction of other dvnced mterils with heterojunction structures for photoctlytic pplictions. 575 580 Acknowledgements This work is supported y the specil funding support from the the Ntionl Bsic Reserch Progrm of Chin (2011CB933700, 2010CB934700,) nd the Ntionl Nturl Science Foundtion of Chin (21271165, 21101006). Notes nd references Additionl figures nd tle re given in supporting informtion. 1 H. Tong, S. Ouyng, Y. Bi, N. Umezw, M. Oshikiri nd J. Ye, Adv. Mter., 2012, 24, 229 251. 2 R. Ashi, T. Morikw, T. Ohwki, K. Aoki nd Y. Tg, Science, 2001, 585 293, 269 272. 6 [JOURNAL], 200X, 00, 0000 This journl Royl Society of Chemistry
Pge 7 of 8 Journl of Mterils Chemistry A 590 595 600 605 610 615 620 625 3 Y. Peng, S. C. Qin, W. S. Wng nd A. W. Xu, Cryst, Eng, Comm., 2013, 15, 6518 6525. 4 D. E. Skinner, D. P. Colomo, J. J. Cvleri nd R. M. Bowmn, J. Phys. Chem., 1995, 99, 7853 7856. 5 C. Ye, Y. Bndo, G. Shen nd D. Golerg, J. Phys. Chem. B, 2006, 110, 15146 15151. 6 D. Chu, J. Mo, Q. Peng, Y. Zhng, Y. Wei, Z. Zhung nd Y. Li, ChemCtChem, 2011, 3, 371 377. 7 X. Zhng, K. Udw, Z. Liu, S. Nishimoto, C. Xu, Y. Lu, H. Ski, M. Ave, T. Mrkoi nd A. Kujishim, J. Photoch. Photoio. A, 2009, 202, 39 47. 8 X. Wng, Q. Xu, M. R. Li, S. Shen, X. L. Wng, Y. C. Wng, Z. C. Feng, J. Y. Shi, H. X. Hn nd C. Li, Angew. Chem. Int. Ed., 2012, 51, 13089 13092. 9 T. Kwhr, Y. Konishi, H. Td, N. Tohge, J. Nishii nd S. Ito, Angew. Chem. Int. Ed., 2002, 41, 2811 2813. 10 X. Wng, S. Shen, S. Q. Jin, J. X. Yng, M. R. Li, X. L. Wng, H. X. Hn nd C. Li, Phys. Chem. Chem. Phys., 2013, 15, 19380 19386. 11 H. G. Kim, P. H. Borse, W. Choi nd J. S. Lee, Angew. Chem. Int. Ed., 2005, 44, 4585 4589. 12 B. Nikookht, J. Bonevich nd A. Herzing, J. Phys. Chem. C, 2011, 115, 9961-9969. 13 C. Zhng nd Y. F. Zhu, Chem. Mter., 2005, 17, 3537 3545. 14 F. Amno, A. Ymkt, K. Nogmi, M. Osw nd B. Ohtni, J. Am. Chem. Soc., 2008, 130, 17650 17651. 15 G. S. Li, D. Q. Zhng, J. C. Yu nd M. K. H. Leung, Environ. Sci. Technol., 2010, 44, 4276 4281. 16 L. Wu, J. H. Bi, Z. H. Li, X. X. Wng nd X. Z. Fu, Ctl. Tody, 2008, 131, 15 20. 17 Z. J. Zhng, W. Z. Wng, M. Shng nd W. Z. Yin, J. Hzrd Mter., 2010, 177, 1013 1018. 18 M. K. Lee nd T. H. Shih, J. Electrochem. Soc., 2007, 154, 49 51. 19 Z. L. Xu, I. Tt, K. Hirogki, K Hisd, T. Wng, S. Wng nd T. Hori, RSC Adv., 2012, 2, 103 106. 20 Y. Xu nd M. A. A. Schoonen, Am. Minerl., 2000, 85, 543 556. 21 L. Zhou, W. Z. Wng, H. L. Xu, S. M. Sun nd M. Shng, Chem. Eur. J., 2009, 15, 1776 1782. 22 M. Ge, Y. F. Li, L. Liu, Z. Zhou nd W. Chen, J. Phys. Chem. C, 2011, 115, 5220-5225. 23 X. N. Li, R. K. Hung, Y. H. Hu, Y. J. Chen, W. J. Liu, R. S. Yun nd Z. H. Li, Inorg. Chem., 2012, 51, 6245 6250. 24 M. S. Gui nd W. D. Zhng, J. Phys. Chem. Solids, 2012, 73, 1342 1349. 25 M. S. Gui, W. D. Zhng, Q. X. Su nd C. H. Chen, J. Solid Stte 630 Chem., 2011, 184, 1977 1982. 26 Z. Q. Li, X. T. Chen nd Z. L. Xue, J. Colloid Interfce Sci., 2013, 394, 69 77. 27 Z. Dong, S. J. Kennedy nd Y. Wu, J. Power Sources, 2011, 196, 4886 4904. 635 28 O. Monnereu, L. Torter, P. Llewellyn, F. Rouquerol nd G. Vcquier, Solid Stte Ionics., 2003, 157, 163-169. 29 W. S. Wng, M. Dhl nd Y. D. Yin, Chem. Mter. 2013, 25, 1179 1189. 30 J. Wu, F. Dun, Y. Zheng nd Y. Xie, J. Phys. Chem. C, 2007, 111, 640 12866 12871. 31 Y. Schuhl, H. Bussrt, R. Deloel, M. LeBrs, J. Leroy, L. G. Gengemre nd J. Rimlot, J. Chem. Soc., Frdy Trns., 1983, 79, 2055-2069. 32 Y. Wng, Y. Wng, Y. Meng, H. Ding, Y. Shn, X. Zho nd X. Tng, J. 645 Phys. Chem. C, 2008, 112, 6620 6626. 650 655 660 665 670 33 H. G. Yu, R. Liu, X. F. Wng, P. Wng nd J. G. Yu, Appl. Ctl. B, 2012, 111 112, 326 333. 34 G. I. N. Wterhouse, G. A. Bowmker nd J. B. Metson, Phys. Chem. Chem. Phys., 2001, 3, 3838-3845. 35 X. Wng, J. C. Yu, C. Ho, Y. Hou nd X. Fu, Lngmuir, 2005, 21, 2552 2559. 36 L. Z. Zhng nd J. C. Yu, Chem. Commun., 2003, 16, 2078 2079. 37 C. S. Guo, M. Ge, L. Liu, G. D. Go, Y. C. Feng nd Y. Q. Wng, Environ. Sci. Technol., 2010, 44, 419 425. 38 Y. W. Mi, S. Y. Zeng, L. Li, Q. F. Zhng, S. N. Wng, C. H. Liu nd D. Z. Sun, Mter. Res. Bull., 2012, 47, 2623 2630. 39 M. A. Butler, J. Appl. Phys., 1977, 48, 1914 1920. 40 Y. Y Guo, G. K. Zhng, H. H. Gn nd Y. L. Zhng, Dlton Trns., 2012, 41, 12697 12703. 41 H. Y. Jing, J. J. Liu, K. Cheng, W. B. Sun nd J. Lin, J. Phys. Chem. C, 2013, 117, 20029 20036. 42 T. Wtne, T. Tkizw nd K. Hond, J. Phys. Chem., 1977, 81, 1845 1851. 43 J. D. Zhung, W. X. Di nd P. Liu, Lngmuir, 2010, 26, 9686-9694. 44 K. Yu, S. G. Yng, H. He, C. Sun, C. G. Gu nd Y. M. Ju, J. Phys. Chem. A, 2009, 113, 10024 10032. This journl Royl Society of Chemistry [JOURNAL], 200X, 00, 0000 7
Journl of Mterils Chemistry A Pge 8 of 8 Grphic Astrct: 1.0 5 C/C0 0.4 0.2 Phenol RhB 0 10 20 30 40 50 60 Time (min) A novel one-dimensionl nnorods- nnosheets p-n junction photoctlyst ws prepred. This - heterostructure exhiits high photoctlytic ctivity for the degrdtion of phenol nd Rhodmine B (RhB) under solr/light light irrdition, which is scried to the extended sorption in the visile light region nd the effective seprtion of photogenerted electrons nd holes y the internl electrosttic field in the junction region. This journl Royl Society of Chemistry [JOURNAL], 200X, 00, 0000 1