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1 Elctroic Supplmtary Matrial (ESI) for Joural of Matrials Chmistry A. This joural is Th Royal Socity of Chmistry 2016 Elctroic Supplmtary Iformatio Photolctrochmical Watr Oxidatio usig a Bi 2 MoO 6 / MoO 3 Htrojuctio Photoaod Sythsisd by Hydrothrmal Tratmt of a Aodisd MoO 3 Thi Film Shi N Lou, Jaso Scott, Akihid Iwas, Ros Amal ad Yu Hau Ng * Particls ad Catalysis Rsarch Group, School of Chmical Egirig, Th Uivrsity of Nw South Wals, Sydy NSW 2052, Australia. Dpartmt of Applid Chmistry, Faculty of Scic, Scic Uivrsity of Tokyo, 1-3 Kagurazaka, Shijuku-ku, Tokyo , Japa *Corrspodig author: Yu Hau Ng (yh.g@usw.du.au)

2 Fig. S1: SEM imag of a Mo foil purchasd from Sigma Aldrich bfor aodisatio A scaig lctro microscopy (SEM) imag of th Mo foil is show i Fig. S1. Th Mo foil xhibits a flat ad rough surfac with o distict morphological faturs. Fig. S2: XRD pattr of a as-aodisd MoO 3 thi film i 0.5 wt% NaF aquous lctrolyt at a aodisatio voltag of 2.5 V for 20 mi Th as-aodisd, A-MoO 3 thi film is amorphous, as show by th XRD pattr i Figur S2

3 Fig. S3: Chroo-ampromtry of Bi 2 MoO 6 /MoO 3 thi film udr visibl light illumiatio udr a applid bias of 0.1 V vs Ag/AgCl i 0.1 M Na 2 SO 4 aq. lctrolyt, Pt as coutr lctrod, illumiatio sourc: 300 W X lamp with a cut off filtr, λ 420. Light-o cycl: 50 s. Th chroo-ampromtry profil of th Bi 2 MoO 6 /MoO 3 thi film udr a pottial of 0.1 V vs Ag/AgCl ad visibl light illumiatio (λ 420 m) is show i Fig. S3. Th Bi 2 MoO 6 /MoO 3 thi film xhibitd four rproducibl photocurrt cycls, showig a magitud of 94 µacm -2, which illustrats its stability ad visibl light activity. Dtails of calculatio of xpctd O 2 from matchig photocurrt magitud udr assumptio of 100% Faradic fficicy Th xpctd amout of O 2 gas is calculatd from th masurd photocurrt udr th assumptio of 100% Faradaic covrsio fficicy, as show i Eq 1 4. Th amout of lctric charg, Q, gratd by th photocurrt, I, ca b calculatd usig th itgral of I with rspct to tim, as show i Eq. (1). Th watr oxidatio ractio, ivolvs th trasfr of 4 mols of lctros pr mol of O 2 producd, as show i Eq. (2). By applyig Faraday s costat, as show i Eq (3), th xpctd amout of O 2 gas producd at th masurd photocurrt lvl ca b calculatd usig Eq (4). Q = t I dt 0 Eq. (1) Whr, Q is th lctric charg (C), I is th photocurrt (A) ad t is th tim (s); 2H 2 O O 2 + 4H Faraday = , coulombs/faraday ( C mol 1 ) Eq. (2) Eq. (3)

4 O2 = Q F Eq. (4) Whr, O2 is th xpctd amout of oxyg volvd (µmol), Q is th lctric charg (C), is th umbr of mols of lctros cosumd for vry mol of O 2 volvd, (hc, = 4) ad F is th Faraday s costat as show i Eq. (3). Origi of dark currt i MoO 3 ad Bi 2 MoO 6 /MoO 3 films Th obsrvd dark currt was ot attributd to parasitic oxidatio of th Mo foil. If th dark currt was du to th mtallic Mo foil udr +v applid bias, th phomo would b aalogous to aodisatio of th mtal foil, i.. lctrochmical oxidatio of mtal to grat currt i th dark. Durig aodic oxidatio of a mtal foil, th currt profil will stadily dcay ovr tim (i th rag from 10 mis to hours), as show i th figur blow. Th stady dcay of currt is du to th formatio of a oxid barrir layr o th mtal foil, thus ladig to th highr rsistac of th lctrochmical systm [1]. W ar o of th activ rsarch groups i th aodisatio fild ad such currt profils ar typically obsrvd i various kids of mtal foil (.g. Mo, W, Ti) [1]. Howvr, th currt profil of our activity masurmt (Fig. 6) is stabl ovr ight hours udr 1 V vs Ag/AgCl. A dcay of currt du to th formatio of a oxid barrir layr was ot obsrvd. Thrfor, oxidativ corrosio of th mtal foil is ulikly. Furthrmor, i a ustabl photoaod xpricig parasitic oxidatio of th udrlyig mtal, Faradaic fficicy for oxyg volutio will ot rach 100% as obsrvd i our work (bcaus th hols gratd ar cosumd to oxidis th mtal istad of th watr). W attribut th dark currt to th itrisic charactristics of MoO 3. MoO 3 has b usd as th cathod matrial i Li-battry systms [2] as MoO 3 has a layrd structur which facilitats th itrcalatio/ditrcalatio of Li +. I this work, th MoO 3 ad Bi 2 MoO 6 /MoO 3 films xhibitd a +v dark currt from 0.7 to +0.5 V vs Ag/AgCl. Wh th sam currt profil is prstd usig Li + /Li as th rfrc lctrod, th rsultig voltag rag ( V vs Li + /Li) typically yilds a dark currt which drivs from th procss of catio ditrcalatio [2] (Not that catio itrcalatio ito MoO 3 ca b iducd by th photoxcitd lctros withi MoO 3 or by lctrochmical itrcalatio durig th cathodic CV sca. Ths phoma hav b rportd wh usig WO 3 ad MoO 3 [1d, 3]. Plas s th rfrcs blow for furthr dtails). Hc, th dark currt obsrvd i this work is attributd to th itrisic proprty of th udrlyig MoO 3 but ot from th Mo foil. As xprimtal vidc, w also coatd a layr of MoO 3 particls o a coductig carbo fibr lctrod (Toray papr) to avoid th prsc of Mo foil. As ca b s i Figur S4, th MoO 3 -coatd carbo fibr film provids a dark

5 currt across th widow of th applid voltag. O th othr had, th currt gratd udr illumiatio (Figur S4 ad Figur 6 i th mauscript) is sigificatly distict from th dark currt ad thus th photoactivity ca b valuatd. Figur S4. Dark ad light currt profils of MoO 3 particls coatd o coductig carbo fibr papr. Rfrcs [1] (a) C. Ng, C. Y, Y. H. Ng, R. Amal, Crys. Growth Ds. 2010, 10, 3794; (b) J. H. Yu, Y. H. Ng, C. Y, A. J. Mozr, G. G. Wallac, R. Amal, ACS Appl. Matr. Itrfacs 2011, 3, 1585; (c) P. Wag, Y. H. Ng, R. Amal, Naoscal 2013, 5, 2952; (d) S. N. Lou, Y. H. Ng, C. Ng, J. Scott, R. Amal, ChmSusChm 2014, 7, [2] (a) L. Zhou, L. Yag, P. Yua, J. Zou, Y. Wu, C. Yu, J. Phys. Chm. C 2010, 114, 21868; (b) X. Wag, R. Nspr, C. Villviill, P. Novak, Adv. Ergy Matr. 2013, 3, 606; (c) L. Norochim, J. Wag, D. Wxlr, Z. Chao, H. K. Liu, J. Powr Sourcs 2013, 228, 198. [3] (a) C. Ng, Y. H. Ng, A. Iwas, R. Amal, PCCP 2011, 13, 13421; (b) C. Ng, A. Iwas, Y. H. Ng, R. Amal, ChmSusChm 2013, 6, 291; (c) C. Ng, Y. H. Ng, A. Iwas, R. Amal, ACS Appl. Matr. Itrfacs 2013, 5, 5269; (d) S. N. Lou, N. Yap, J. Scott, R. Amal, Y. H. Ng, Sci. Rp. 2014, 4, 7428.