Materials Chemistry A

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1 Journl of Mterils Chemistry A Accepted Mnuscript This is n Accepted Mnuscript, which hs een through the Royl Society of Chemistry peer review process nd hs een ccepted for puliction. Accepted Mnuscripts re pulished online shortly fter cceptnce, efore technicl editing, formtting nd proof reding. Using this free service, uthors cn mke their results ville to the community, in citle form, efore we pulish the edited rticle. We will replce this Accepted Mnuscript with the edited nd formtted Advnce Article s soon s it is ville. You cn find more informtion out Accepted Mnuscripts in the Informtion for Authors. Plese note tht technicl editing my introduce minor chnges to the text nd/or grphics, which my lter content. The journl s stndrd Terms & Conditions nd the Ethicl guidelines still pply. In no event shll the Royl Society of Chemistry e held responsile for ny errors or omissions in this Accepted Mnuscript or ny consequences rising from the use of ny informtion it contins.

2 Pge 1 of 8 Journl Nme Journl of Mterils Chemistry A Dynmic Article Links Cite this: DOI: /c0xx00000x ARTICLE TYPE Tetrgonl tungsten ronze-type nnorod photoctlysts with tunnel structure: T sustitution for N nd overll wter splitting Ping Wng, Lriss Schwertmnn, Rolnd Mrschll, nd Michel Wrk*,c Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: /000000x Tetrgonl tungsten ronze-type tntlum (T) sustituted Sr 2 KN O 1 nnorod photoctlysts with tunnel structure were prepred y fcile nd low-cost molten slt method using potssium chloride (KCl) t 80 C for only 2 h. Although ll ntive photoctlysts did not possess ny detectle ctivity in pure wter splitting, fter deposition of NiO x (doule-lyered Ni/NiO) s co-ctlysts, smples of Sr 2 KN O 1 nd Sr 2 KT O 1 cn split pure wter into H 2 nd O 2 in stoichiometric mount ( 2:1), which cn e scried to the improved chrge crrier seprtion nd trnsfer in the presence of NiO x. Furthermore, T sustitution effects on the photoctlytic ehviour were systemticlly investigted for hydrogen production y queous methnol reforming. The verge H 2 formtion rtes of Sr 2 KN - xt x O 1 first decrese with tntlum sustitution for x < 2., presumly due to decresed mount of sored photons nd n ovious reduction of their exposed surfce res, wheres the ctivity is significntly improved for smples contining more T (x > 2.) nd especilly the fully sustituted Sr 2 KT O 1. This cn e explined y stronger driving force for photogenerted conduction nd electrons to reduce wter. Introduction Photoctlytic wter splitting nd hydrogen production y semiconductor photoctlysis for converting solr energy into chemicl energy hs drwn ever-growing interest in oth science nd engineering fields. 1, 2 During the pst few yers, severl types of semiconductor photoctlysts such s ulk-type simple oxides (TiO 2, N 2 O, T 2 O, ZnO nd ZrO 2, etc.) s well s perovskitetype oxides (SrTiO 3, K 4 N 6 O 17, NTO 3, nd A 2 L 2 Ti 3 O 10, (A=C, Sr, nd B), etc.) hve een widely studied. 3-6 It is well known tht the superior photoctlytic ility of lyered oxides cn e ttriuted to the unique structure, compred to the ovementioned ulk-type oxides. 7 The pst two decdes hve lso witnessed gret efforts on the synthesis of tunnel-structured oxides, such s BTi 4 O 9 nd A 2 Ti 6 O 13 (A = N, K, R), etc. 8, 9 The deficient perovskite tetrgonl tungsten ronze-type (TTB) niotes (A1) 4 (A2) 2 (A3) 4 N 10 O, consisting of corner-shring NO 6 octhedr rrys nd three different tunnels (A1, A2 nd A3) for ction filling, hve een widely investigted s ferroelectric nd piezoelectric mterils in view of their lrge spontneous polriztion nd high dielectric constnts They lso exhiit dominnt electron-hole seprtion nd trnsport vi sttic electric fields etween structure-induced dipole moments in distorted metl-oxygen polyhedr. Therefore, slight structurl modifiction y ltering the ction-site occupncy hs drmtic effect on the chrge moility. However, to our knowledge except the pper y Wu, et l, 1 there hve een no reports on photoctlytic properties of the modified tunnel-structure ronze type niotes. In principle on ll the three ction positions A1, A2 nd A3 ction exchnge cn e done, ut since recent studies in niote/tntlte systems hve shown tht the sustitution of the N site with tntlum (T) could considerly improve the photoctlytic ctivity 16-18, we concentrted on this sustitution. However, most reports of sustitution effects re focused on the photodegrdtion of orgnic compounds. It still remins gret chllenge to directly split wter into H 2 nd O 2 nd to determine the vrition of the T sustitution induced-structure properties nd photoctlytic ehviour. In the present work, y employment of simple nd cost-effective molten slt technique, the Sr 2 KN -x T x O 1 nnorod photoctlysts with different N/T sustitution rtios were synthesized. The T sustitution effect on the photoctlytic ehviour for photoctlytic hydrogen production from methnol reforming ws systemticlly investigted. Experimentl Photoctlysts preprtion Powders of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd were prepred y molten slt method. 19 SrCO 3 ( 99.99%, Aldrich), K 2 CO 3 (>99.9%, Alf Aesr), N 2 O (99.99%, Alf Aesr), nd T 2 O (99.8%, Alf Aesr) were used s strting mterils. The desired stoichiometric molr rtios of the strting mterils were ground together with potssium chloride (KCl: 99.%, Honeywell Riedel-de Hën) t weight rtio of 1:2. The wellmixed powders were heted t different clcintion tempertures This journl is The Royl Society of Chemistry [yer] [journl], [yer], [vol],

3 Journl of Mterils Chemistry A Pge 2 of for 2 h in ir using corundum crucile. After cooling down to room temperture, the mixture ws intensively wshed with distilled wter in order to remove ny residul slts nd then dried in ir t 80 C. The 1.0 wt% NiO x loded smples were prepred y impregntion method: 20, 21 the respective powders were dispersed in suitle mount of Ni(NO 3 ) 2 (99.998%, STREM Chemicls) solution nd heted under constnt stirring until the wter ws completely evported. The resulting powder ws dried t 80 C in ir, followed y clcintion t 200 C for 2 h, nd further treted y reduction under H 2 flow t 0 C for 2 h nd re-oxidtion under O 2 flow t 200 C for 1 h. Chrcteriztion The X-ry diffrction (XRD) ptterns of ll smples were recorded with PANlyticl MPD diffrctometer using Cu-Kα rdition (λ= nm), nd the dt were collected from 10 to 60 (2θ). Sttic N 2 physisorption mesurements were crried out t -196 C using n Autosor-1MP Quntchrome system nd smples were degssed t 200 C for 6 h efore the mesurements. The UV-Vis diffuse reflectnce spectr were mesured using MgO s reference on UV/Vis Vrin Cry 00 spectrophotometer. Bnd gp energies were clculted y nlysis of the Tuc-plots resulting from Kuelk-Munk trnsformtion of diffuse reflectnce spectr. Powder smples were sputter-coted with thin lyer of Au prticles nd then exmined in scnning electron microscopes (SEM) using LEO (Zeiss) 1 Gemini field-emission. The trnsmission electron microscope (TEM) imges were recorded y Philips/FEI Tecni F20 S-TWIN TEM instrument operting t 200 kv. Photoctlytic ctivity for H 2 production Photoctlytic rections were performed in typicl doulewlled inner irrdition-type qurtz rector connected to closed gs evolution system. 19, 22 The rection temperture ws mintined t 10 C to prevent ny therml ctlytic effect with doule-wlled qurtz jcket filled with flow of cooling wter from thermostt (LAUDA). A 00 W Hg mid-pressure immersion lmp (Peschl UV-Consulting) ws used s light source. High-purity Argon (6.0) ws used s crrier gs for the rection products, of which continuous gs flow ws set to 0 NmL/min controlled y Bronkhorst mss flow controller. The evolved gses were nlyzed online using multi-chnnel nlyzer (Emerson) equipped with detector for the determintion of the concentrtion of hydrogen (therml conductivity detector), oxygen (prmgnetism) nd cron dioxide (IR). In typicl run, 0.1 g of photoctlyst powder ws suspended in 00 ml distilled wter. In cse of photoctlytic methnol reforming, n dditionl 0 ml of methnol ws dded. Prior to irrdition, the whole system including the photoctlysts ws purged with rgon to remove ir completely. Results nd Discussion Synthesis of mterils Opticl nd physicl properties In order to determine the optimum clcintion temperture, Sr 2 KT O 1 ws prepred under different clcintion tempertures from 80 to 100 C (higher thn the melting point of KCl, 774 C) nd the photoctlytic ctivity of these smples ws proed y photoctlytic methnol reforming for H 2 production. As shown in Figure 1, the diffrction peks of the Sr 2 KT O 1 (JCPDS -034) ecome shrper nd stronger with incresing temperture, indicting the increse of the verge crystllite size Intensity /.u. 100 o C 90 o C 80 o C thet / degree Figure 1. XRD ptterns of typicl Sr 2 KT O 1 smples synthesized with different clcintion tempertures for 2 h. Tle 1. Effect of clcintion temperture on BET surfce re nd photoctlytic ctivity for H 2 production. Temperture ( C) BET re (m 2 /g) [] Rte of H 2 (mmol/h) [] [] mesured y the Brunuer-Emmett-Teller (BET) method; [] rection conditions: 0.1 g of photoctlysts; 0 ml of methnol dissolved in 00 ml of wter; 00W Hg lmp. nd the improvement of the crystllinity of Sr 2 KT O 1 crystls. As shown in Tle 1, the smple clcined t 80 C shows BET surfce re of 8. m 2 /g, nd n increse in clcintion temperture resulted in the significntly decresed surfce res of.3 nd 2.7 m 2 /g for smples clcined t 90 C nd 100 C, respectively. Presumly enhnced prticle growth results in loss of ctive surfce sites with incresing temperture, leding to n incresingly detrimentl effect on the photoctivity for H 2 production. Therefore, we chose for the preprtion of the smples with different N/T rtios Sr 2 KN -x T x O 1 (x= 0, 1, 2., 4 nd ) temperture of 80 C. Structurl properties X-ry powder diffrction ptterns of Sr 2 KN -x T x O 1 (x= 0, 1, 2., 4 nd ) with different molr N/T rtios prepred t 80 C for 2 h re shown in Figure 2. All the powders re wellcrystllized nd no significnt shifts re oserved in the diffrction ptterns. This indictes tht T + hs diffused into the tetrgonl tungsten ronze (TTB) structure to form solid solution in which T + occupies the N lttice sites, ecuse the effective ionic rdii of N + nd T + ions in the Sr 2 KN -x T x O 1 re similr (64 pm), nd on the contrry, the effective ionic rdius of the T + ion is remrkly smller thn those of the Sr 2+ ion nd the K + ion. 23 As shown in Figure 2, the pttern of our Sr 2 KN O 1 cn e indexed to TTB structure on sis of the 2 Journl Nme, [yer], [vol], This journl is The Royl Society of Chemistry [yer]

4 Pge 3 of 8 Journl of Mterils Chemistry A reported dt of ulk Sr 2 KN O 1 crystls (JCPDS , spce group P4m elonging to the point group 4mm). However, the most intense reflection is (410), insted of (311, 420), nd the x= 0 (211) (410) x= 1 x= 2. x= 4 x= (311) Sr KN O JCPDS (420) 2 1 Sr 2 KT O 1 JCPDS thet / degree Figure 2. ) XRD ptterns of Sr 2 KN -x T x O 1 with x=0, 1, 2., 4 nd prepred t 80 C for 2 h, ) Enlrged XRD ptterns of the Sr 2 KN -x T x O 1 rnging from 2θ = c x= 0 x= 1 x= 2. x= 4 x= 10 Figure 3. Representtive SEM imges of Sr 2 KN O 1 nnorods () Sr 2 KN 2. T 2. O 1 () nd Sr 2 KT O 1 nnorods (c), respectively. intensity of (211) reflection decreses y more thn hlf. This grees very well with reported results, nd cn e ttriuted 1 to the nisotropic growth of the Sr 2 KN O 1 prticles leding to strong (001) orienttion. It is importnt to note tht some unknown impurity (< 3%) ws oserved for Sr 2 KN O 1, s indicted y the rrows in Figure 2. Interestingly, no impurity peks were detected with incresing x nd the phenomenon of the 20 preferred (001) orienttion growth grdully egn to e wekened for these solid solutions, indicting tht the nisotropic grin growth is inhiited y T + sustitution. After complete sustitution of T + for N +, the Sr 2 KT O 1 phse ws derived ccording to the JCPDS The diffrction pek t out splits into two peks (Figure 2). This suggests tht when sustituting N + with T +, the crystl structure trnsition to symmetric center ppered from tetrgonl point group 4mm to tetrgonl point group 4/mmm), in good greement with the 27, 28 literture. Representtive SEM imges of Sr 2 KN O 1 (), Sr 2 KN 2. T 2. O 1 () nd Sr 2 KT O 1 (c) smples re shown in Error! Reference source not found.. The smples disply nnorod-shped morphology with the dimeters in the rnge of nm nd the lengths up to few microns, suggesting tht 3 the eutectic KCl slt fvors the formtion of nnorod morphology. With incresing T/N rtio most of the nnorods ecome more uniform in size, ut lso more gglomerted, nd grow shorter in length, implying tht the nisotropic grin growth cn e to some extent e suppressed y the T + sustitution, which is in ccordnce with the XRD nlysis. A similr suppression cused y T + sustitution ws lso found in other cermic systems like (K 0.44 N 0.2 Li 0.04 ) 29, (N 0.96-x T x S 0.04 )O 3, nd Sr 0.3 B 0.47 N 2-x T x O 6. The microstructure, the lttice prmeters nd growth properties 4 of the Sr 2 KN O 1 nd Sr 2 KT O 1 nnorod smples were further studied y TEM, high-resolution TEM (HRTEM), nd selected re electron diffrction (SAED). Figure 4 shows TEM imge of typicl Sr 2 KN O 1 nnorod t low mgnifiction. The HRTEM imge of the Sr 2 KN O 1 smple tken from the top prt 0 of the Sr 2 KN O 1 nnorod (s mrked y white rectngle in Figure 4) nd the corresponding SAED pttern re presented in Figure 4 nd Figure 4c, respectively. The lttice fringes of the Sr 2 KN O 1 [001] nd [0] plnes re found to e out 0.39(0) nm nd 0.31(2) nm, respectively nd the SAED pttern cn e ssigned to tetrgonl Sr 2 KN O 1 single crystls with the electron em closely prllel to [010] direction. These oservtions unmiguously indicte tht the Sr 2 KN O 1 nnorod is high qulity single crystl with tetrgonl structure, grown long the [001] direction (mrked y white rrow in 60 Figure 4). This journl is The Royl Society of Chemistry [yer] Journl Nme, [yer], [vol],

5 Journl of Mterils Chemistry A Pge 4 of 8 10 c 001 Figure 4. TEM imges of the Sr 2 KN O 1 smple: () Lowmgnifiction TEM imge of typicl Sr 2 KN O 1 nnorod, () HRTEM tken from the top prt of the Sr 2 KN O 1 nnorod s mrked y white rectngle in (), the white rrow indictes growth direction of the Sr 2 KN O 1 nnorod long the [001] fcet plnes, (c) corresponding SAED pttern. A typicl Sr 2 KT O 1 nnorod cn e oserved in the lowmgnifiction TEM imge of Figure. The HRTEM imge in Figure, which corresponds to the re mrked y white rectngle in Figure, shows the sme crystl plnes nd the sme growth direction (s indicted y white rrow) s illustrted in Figure 4. Therefore, cler evidence for preferentil growth direction of Sr 2 KN O 1 nd Sr 2 KT O 1 nnorod smples could e further found nd oth of them hve the sme [001] growth direction. Therefore, it is suggested tht the use of molten slt s rection medium plys dominnt role for the formtion of nnorod morphology nd their preferentil growth, while presumly the growth inhiition cused y T sustitution in these T sustituted smples, s oserved in ove XRD nd SEM results, is limited. Figure c shows representtive TEM imge of typicl 1.0 wt.% NiO x loded Sr 2 KT O 1 smple. It cn e clerly seen tht distinguishle nno-nio x co-ctlyst prticles re nchored on n individul Sr 2 KT O 1 nnorod. The nno-nio x co-ctlyst prticles consist of solid Ni metl core nd righter NiO outer lyer. Furthermore, the inset on the lower right shows lttice resolved HRTEM imge of nno-nio x prticle, in which the lttices of the out lyer-cuic NiO cn e determined to e the [111] nd [200] crystl plnes with d-spcings of 0.24 nm nd 0.21 nm, respectively. The results demonstrte tht the doulelyered structure of metllic Ni nd metl oxide NiO is creted on the surfce of the Sr 2 KT O 1 nnorods photoctlyst y reduction-reoxidtion processes. The formtion of Ni metl t the surfce of photoctlyst results in the formtion of Schottky contct t the interfce, nd thus provides n opportunity to fcilitte electron trnsport to the NiO shell to enhnce the photoctlytic ctivity of the photoctlysts for overll wter splitting s will e discussed elow. 4 Journl Nme, [yer], [vol], This journl is The Royl Society of Chemistry [yer] Opticl nd physicl properties The UV-Vis diffuse reflectnce spectr of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd re shown in Figure 6. The sorption edges of Sr 2 KN -x T x O 1 oviously shift to shorter wvelengths 4 with incresing T content. On sis of Kuelk Munk trnsformtion of the diffuse reflectnce spectr, the nd-gp energies from Tuc plots were lrger thn the pprent nd-gp energies if direct trnsition is ssumed for light sorption y the smples. As indicted in Figure 6, the ssumption of indirect 0 trnsition is in good greement with pprent nd gp energies, thus suggesting tht the s-prepred oxides re indirect nd gp semiconductors. Figure 6c shows the nd gps of Sr 2 KN O 1, Sr 2 KN 4 T 1 O 1, Sr 2 KN 2. T 2. O 1, Sr 2 KN 1 T 4 O 1 nd Sr 2 KT O 1 eing estimted to out 3.19, 3.24, 3.32, 3.1 nd 3.89 ev, respectively. The nd gp of Sr 2 KN O 1 is lmost consistent with the reported vlue (3.24 ev). 1 The dt indicte tht the nd gps do not linerly increse with the T sustitution; for high T contents the nd gp widening is more prominent. 60 On the other hnd, the BET surfce res of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd decrese lmost linerly with the sustitution of T, s shown in Figure 6c. The surfce re of sprepred Sr 2 KN O 1 (19.1 m 2 /g) ws improved y more thn 60 times compred with tht of the sintered smple (0.31 m 2 /g) y 6 the conventionl solid stte rection method, 1 which my provide more ctive sites, reduce the recomintion of photogenerted electron-hole pirs, nd thus gretly enhnce photoctlytic efficiency. This cn e scried to the lower rection temperture nd the shorter rection times (t 80 C for 2 h) possile due to 19, 31, the intimte contcts in the molten slt moderted rection.

6 Pge of 8 Journl of Mterils Chemistry A c 001 NiO x Figure. TEM imges of the Sr 2 KT O 1 smple: () Lowmgnifiction TEM imge of typicl Sr 2 KT O 1 nnorod, () HRTEM tken from the edge of the Sr 2 KT O 1 nnorod s mrked y white rectngle in (). The white rrow indictes growth direction of the Sr 2 KT O 1 nnorod long the [001] fcet plnes, (c) The TEM imge of the 1.0 wt.% NiO x loded Sr 2 KT O 1 nnorod with inset of NiO x prticle. 10 The nd structure of photoctlysts is one of the importnt fctors ffecting the photonic efficiency. The vlence nd (VB) potentils of ll Sr 2 KN -x T x O 1 solid solutions should e quite similr ecuse of the sme crystl structure nd the sme oritls (O 2p oritls) comprising the VB. Thus, the difference of nd 1 gp energies is due to the oritls forming the conduction nd (CB). According to the report y Scife, 33 for oxides contining no prtly filled d-levels, the following eqution cn e used to pproximtely determine the flt nd potentil: V f ( NHE ) = 2.94 E g 20 where V f nd E g represent flt nd potentil nd nd gp, respectively. The nd structures of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd re schemticlly illustrted in Figure 7. The CB of Sr 2 KN O 1 consists of N 4d, while tht of Sr 2 KT O 1 is from T d. The CB formed y the empty T d oritls lies t 2 considerly more negtive potentil thn the N 4d oritls. Thus, with incresing T sustitution, the lrger nd gp will led to the decresed mount of sored photons. On the contrry, it lso mens tht the CB potentil ecomes more negtive thn the wter reduction potentil (<0 V vs. NHE t ph 0), suggesting tht the driving force of Sr 2 KN -x T x O 1 for wter reduction y electrons in the CB ecomes lrger, thus promoting the trnsfer of photoelectrons. It cn e concluded tht the T sustitution molr rtio hs gret effect on the nd structures nd thus plys crucil role in determining the efficiency of H 2 3 production. 34 Furthermore, the ottom level of the CB ws much more negtive thn the reduction potentil of H + /H 2 (0 ev), while the top level of the VB ws more positive thn the oxidtion potentil of O 2 /H 2 O (+1.23 ev). It is therefore resonle to infer tht these photoctlysts might e le to split wter into H 2 nd O 2 y the pproprite excited energy. 4 R % x= x=4 x=2. x=1 x=0 ( F(R) * hv ) 1/2 x= Wvelength (cm -1 ) Photon Energy (ev) c BET surfce re ( m 2 /g ) X Figure 6. () UV-Vis diffuse reflection spectr of Sr 2 KN - xt x O 1 with x= 0, 1, 2., 4 nd, () Tuc plots (for indirect nd gp trnsition) clculted from the Kuelk-Munk trnsformtion, nd (c) Bnd gp energies nd BET surfce res of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd, respectively. x=1 x=0 x=4 Bnd gp ( ev ) x= (1) This journl is The Royl Society of Chemistry [yer] Journl Nme, [yer], [vol], 00 00

7 Journl of Mterils Chemistry A Pge 6 of V vs. NHE t ph= ev N 4d 3.24 ev T d X CB H + /H 2 O 2 /H 2 O 3.32 ev 3.1 ev 3.89 ev O 2p VB Figure 7. A schemtic illustrtion of nd structures of Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd. Effect of T sustitution on photoctlytic H 2 production y reforming of queous methnol solution The photoctlytic H 2 production rtes over co-ctlyst-free Sr 2 KN -x T x O 1 with x=0, 1, 2., 4 nd from photoctlytic reforming of methnol re shown in Figure 8. Note tht the ctivities of photoctlysts with different T sustitution molr rtio were compred y the verge rte of H 2 production in the first 1 h nd the sme photoctlytic trend cn e otined y t lest three repeted mesurements. For comprison, commercil TiO 2 nnoprticles (Evonik P2) were lso tested for photoctlytic H 2 production under the sme conditions. The Sr 2 KN -x T x O 1 solid solutions photoctlysts exhiited superior photoctlytic ctivity thn P2. And it is clerly seen tht the photoctlytic ctivity of Sr 2 KT O 1 is much higher thn tht of Sr 2 KN O 1. With the incresed content of T sustitution, the verge formtion rtes of H 2 decresed first, the lowest ctivity ws oserved for the Sr 2 KN 2. T 2. O 1, nd then egn to increse in the cse of x > 2.. It might e possile tht the crystl sizes nd crystllinity of the solid solutions negligily influence the photoctlytic ctivity ecuse their crystl sizes nd high crystllinity were not significntly chnged y T sustitution from the XRD results. Therefore, the ovious decrese of H 2 formtion rte (x < 2.) cn e relted to the decresed numer of sored photons nd lower specific surfce res. With higher T sustitution (x > 2.), the increse of H 2 formtion rte cn e minly ttriuted to the stronger driving force nd more effective photoelectron trnsfer from the CB s descried ove Rte of H 2 production ( mmol h -1 ) X 4 P2 Figure 8. Formtion rtes of hydrogen over Sr 2 KN -x T x O 1 with x= 0, 1, 2., 4 nd nnorod photoctlysts (Conditions: 0.1 g of photoctlysts; 0 ml of methnol dissolved in 00 ml of wter, 00 W Hg lmp). Photoctlytic Overll Wter Splitting All ntive photoctlysts did not possess photoctlytic ctivity in pure wter, proly due to the low chrge seprtion nd trnsfer efficiency. While 1.0 wt% NiO x ws loded s coctlyst, the photoctlytic overll wter splitting into stoichiometric mounts of H 2 nd O 2 cn e effectively promoted over prototypicl exmples of Sr 2 KN O 1 nd Sr 2 KT O 1 photoctlysts. Figure 9 shows the time course of H 2 nd O 2 evolution from pure wter over 1.0 wt% NiO x /Sr 2 KN O 1 nd 1.0 wt% NiO x /Sr 2 KT O 1 photoctlysts under UV light irrdition for h. After some short induction period of out 1 minutes of irrdition, the formtion of H 2 nd O 2 in stoichiometric rtio (H 2 /O 2 2/1) ws oserved nd there ws no pprecile decrese y on-line monitoring of gs evolution, indicting tht the smples re stle under UV light irrdition. Therefore, it is demonstrted tht the presence of the doulelyered nno-nio x prticles s co-ctlysts on the surfce of photoctlysts cn improve the photoctlytic ctivity for overll wter splitting. Bsed on the ove TEM results, the formtion of Ni metl core t the surfce of photoctlyst results in the formtion of Schottky contct t the interfce, nd thus fcilitte electron trnsport to the NiO shell nd suppress the ckwrd rection of wter formtion, leding to n enhncement of photoctlytic ctivity for overll wter splitting Furthermore, the verge rtes of H 2 /O 2 evolution over Sr 2 KN O 1 nd Sr 2 KT O 1 photoctlysts were estimted to e out 36.2/1.2 nd 84./46.3 µmol/h, respectively, thus the ltter ws more thn 2 times higher thn the former, in greement with the ove-presented comprison vlue from photoctlytic H 2 production y methnol reforming. These experimentl results not only proved the photoctlyticl ilities of the tunnel structured photoctlysts for overll wter splitting, ut lso consistently confirmed the ove resonle prediction resulting from the theoreticl clcultion of the nd structures. 6 Journl Nme, [yer], [vol], This journl is The Royl Society of Chemistry [yer]

8 Pge 7 of 8 Journl of Mterils Chemistry A Yield of gs evolution ( µmol) Sr 2 KN O 1 ( H 2 O 2 ) Sr 2 KT O 1 ( H 2 O 2 ) Time (h) Figure 9. H 2 nd O 2 evolution from pure wter over 1.0 wt% NiO x loded Sr 2 KN O 1 nd Sr 2 KT O 1 for h (Conditions: 0.1 g of photoctlysts; 00 ml of H 2 O, Hg lmp). Conclusions The novel Sr 2 KN T -x O 1 nnorod photoctlysts with tunnel structure were prepred y molten slt technique for 2 h t 80 C. The effective improvement of the photoctlytic H 2 production ctivity y T sustitution hs een demonstrted in methnol 10 reforming. Compred to ulk TiO 2 smple, the Sr 2 KN T -x O 1 nnorod photoctlysts showed much higher ctivity. Moreover, y loding of NiO x co-ctlyst on prototypicl Sr 2 KN O 1 nd Sr 2 KT O 1 photoctlysts, the decomposition of wter into H 2 nd O 2 t stoichiometric mount ( 2:1) under UV 1 irrdition ws oserved, which cn e ttriuted to the improved chrge seprtion nd trnsfer in the presence of doule-lyer structure NiO x co-ctlyst. Therefore, it cn e concluded tht the tunnel structure photoctlysts re new type of photoctlyst mterils for overll wter splitting from the view point of the 20 tunle components ecuse it consists of three different tunnels nd octhedrl rrys. Our ongoing work will continue to study on nd gp engineering y controlling ction filling of three different tunnels nd octhedrl rrys, for extending the photosensitivity of photoctlytic oxides into the visile-light 2 region. 3 Acknowledgements We gretly pprecite Prof. Dr. M. Muhler (RUB, Germny) for his support, Dr. T. Reinecke (RUB, Germny) for XRD mesurements. P.W. thnks for the scholrship support from the Chin Scholrship Council (CSC No ). R.M. grtefully cknowledges funding in the Emmy-Noether progrm (MA 392/3-1) of the Germn Science Foundtion DFG. Notes nd references Lortory of Industril Chemistry, Fculty of Chemistry nd Biochemistry, Ruhr-University Bochum, Universitetsstrsse 10, Bochum, Germny Institute of Physicl Chemistry, Justus-Lieig-University Giessen, Heinrich-Buff-Ring 8,3392 Giessen, Germny c Institute for Chemistry, Technicl Chemistry, Crl von Ossietzky University Oldenurg, Crl-von-Ossietzky-Str. 9-11, Oldenurg, Germny. Tel: ; Fx: ; E-mil: michel.wrk@uni-oldenurg.de 1. P. Yng nd J. M. Trscon, Nt. Mter., 2012, 11, A. J. Brd nd M. A. Fox, Acc. Chem. Res., 199, 28, S. T. Mrtin, A. T. Lee nd M. R. Hoffmnn, Environ. Sci. Technol., 199, 29, F. E. Osterloh, Chem. Mter., 2008, 20, 3.. A. Kudo nd Y. Miseki, Chem. Soc. Rev., 2009, 38, X. B. Chen, S. H. Shen, L. J. Guo nd S. S. Mo, Chem. Rev., 2010, 110, Y. Miseki, H. Kto nd A. Kudo, Energy Environ. Sci., 2009, 2, M. Kohno, T. Kneko, S. Ogur, K. Sto nd Y. Inoue, J. Chem. Soc., Frdy Trns., 1998, 94, S. Ogur, M. Kohno, K. Sto nd Y. Inoue, Appl. Surf. Sci., 1997, 121, A. Mgneli nd B. Blomerg, Act Chem. Scnd., 191,, A. Simon nd J. Rvez, CR. Chim., 2006, 9, S. Lnfredi, C. X. Crdoso nd M. A. L. Nore, Mter. Sci. Eng. B, 2004, 112, E. Grci-Gonzlez, A. Torres-Prdo, R. Jimenez nd J. M. Gonzlez-Clet, Chem. Mter., 2007, 19, M. Prdes, N. Mso, H. Beltrn, E. Cordoncillo nd A. R. West, Inorg. Chem., 2013, 2, J. J. Wu, F. Q. Hung, Z. C. Shn nd Y. M. Wng, Dlton Trns., 2011,, L. W. Zhng, H. B. Fu, C. Zhng nd Y. F. Zhu, J. Phys. Chem. C, 2008, 112, S. M. Zhng, G. K. Zhng, S. J. Yu, X. G. Chen nd X. Y. Zhng, J. Phys. Chem. C, 2009, 113, O. Plsyuk nd P. A. Mggrd, J. Solid Stte Chem., 2012, 191, P. Wng, P. Chen, A. Kostk, R. Mrschll nd M. Wrk, Chem. Mter., 2013, 2, K. Domen, A. Kudo, T. Onishi, N. Kosugi nd H. Kurod, J. Phy. Chem., 1986, 90, 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. Edit., 2012, 1, L. Schwertmnn, M. Wrk nd R. Mrschll, Rsc Adv., 2013, 3, R. D. Shnnon, Act Crystllogr. A, 1976, 32, C. Durn, G. L. Messing nd S. Trolier-Mckinstry, Mter. Res. Bull., 2004, 39, Z. P. Yng, L. L. Wei, Y. F. Chng nd B. Liu, J. Mter. Sci., 2007, 42, Z. P. Yng, L. L. Wei nd Y. F. Chng, J. Eur. Cerm. Soc., 2007, 27, L. L. Wei, Z. P. Yng, Y. F. Chng nd R. I. Gu, J. Am. Cerm. Soc., 2008, 91, L. Wei, Z. Yng, H. Ren nd X. Chen, J. Am. Cerm. Soc., 2010, 93, Z. P. Yng, R. Gu, L. L. Wei nd H. M. Ren, J. Alloy. Compd., 2010, 04, Z. P. Yng, Y. F. Chng nd L. L. Wei, Appl. Phys. Lett., 2007, D. Arney, C. Hrdy, B. Greve nd P. A. Mggrd, J. Photochem. Photoiol. A, 2010, 214, N. McLm, P. P. Shoo, L. Fuoco nd P. A. Mggrd, Cryst. Growth Des., 2013, 13, D. E. Scife, Sol. Energy, 1980, 2, B. Siritnrtkul, K. Med, T. Histomi nd K. Domen, ChemSusChem, 2011, 4, M. Tin, W. Shnggun, J. Yun, S. Wng nd Z. Ouyng, Sci. Technol. Adv. Mter., 2007, 8, Y.-C. Chiou, U. Kumr nd J. C. S. Wu, Appl. Ctl. A: Gen., 2009, 37, Z. Zou, J. Ye, K. Sym nd H. Arkw, Nture, 2001, 414, T. K. Townsend, N. D. Browning nd F. E. Osterloh, ACS nno, 2012, 6, This journl is The Royl Society of Chemistry [yer] Journl Nme, [yer], [vol],

9 Journl of Mterils Chemistry A Pge 8 of 8 Grphicl strct 8 Journl Nme, [yer], [vol], This journl is The Royl Society of Chemistry [yer]