Nanophotonics Class 4 - Density of states

R)
R
E = E0
+
1 2
u(R)
·
∂2E ∂u(R)∂u(R
)
·
u(R
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R,R
+···
Energy derivatives & perturbation theory
H = H0 + λivi
i
E[λ] = E0 −
fiλi
+
1 2
hij λiλj + · · ·
i
ij
Energy derivatives & perturbation theory
H = H0 + λivi
i
E[λ] = E0 −
fiλi
+
1 2
hij λiλj + · · ·
i
ij
fi
=−
∂E ∂λi
λ=0
=
− Ψ0|vi|Ψ0
Energy derivatives & perturbation theory
H = H0 + λivi
i
E[λ] = E0 −
fiλi
+
1 2
Density-functional perturbation theory
forces, response functions, phonons, and all that
Stefano Baroni
Scuola Internazionale Superiore di Studi Avanzati & DEMOCRITOS National Simulation Center Trieste - Italy

精心整理1.a c c e p t a n c e t e s t i n g(W A T:w a f e r a c c e p t a n c e t e s t i n g)?3.A C C E S S：一个E D A（E n g i n e e r i n g D a t a A n a l y s i s）系统?6.A l i g n m a r k(k e y)：对位标记?9.A m m o n i a?氨24.A s p e c t r a t i o n：形貌比（E T C H中的深度、宽度比）?25.A u t o d o p i n g：自搀杂（外延时S U B的浓度高，导致有杂质蒸发到环境中后，又回掺到外延层）?28.B e n c h m a r k：基准?30.B o a t：扩散用（石英）舟?32.C h a r a c t e r w i n d o w：特征窗口。

用文字或数字描述的包含工艺所有特性的一个方形区域。

?36.C I M：c o m p u t e r-i n t e g r a t e d m a n u f a c t u r i n g的缩写。

用计算机控制47.C p k：工艺能力指数，详见p r o c e s s c a p a b i l i t y i n d e x。

?48.C y c l e t i m e：圆片做完某段工艺或设定工艺段所需要的时间。

49.D a m a g e：损伤。

50.D e f e c t d e n s i t y：缺陷密度。

单位面积内的缺陷数。

?51.D e p l e t i o n i m p l a n t：耗尽注入。

一种在沟道中注入离子形成耗尽晶体管的注入工艺。

（耗尽晶体管指在栅压为零的情况下有电流流过的晶体管。

）?52.D e p l e t i o n l a y e r：耗尽层。

可动载流子密度远低于施主和受主的固的部分划片槽区域。

DOI: 10.1126/science.1246913, 990 (2014);343 Science Tae Woo Kim and Kyoung-Shin Choi Catalysts for Solar Water Splitting Photoanodes with Dual-Layer Oxygen Evolution 4Nanoporous BiVOThis copy is for your personal, non-commercial use only.clicking here.colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to othershere.following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles): April 7, 2014 (this information is current as of The following resources related to this article are available online at/content/343/6174/990.full.html version of this article at:including high-resolution figures, can be found in the online Updated information and services, /content/suppl/2014/02/12/science.1246913.DC1.html can be found at:Supporting Online Material /content/343/6174/990.full.html#ref-list-1, 1 of which can be accessed free:cites 37 articles This article/cgi/collection/chemistry Chemistrysubject collections:This article appears in the following registered trademark of AAAS.is a Science 2014 by the American Association for the Advancement of Science; all rights reserved. The title Copyright American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the Science o n A p r i l 7, 2014w w w .s c i e n c e m a g .o r g D o w n l o a d e d f r o manisotropic structures,local GCR acceleration that may result in a more defined excess near the heliotail (21),and asymmetric influences on the heliosphere by the interstellar magnetic field (49–53).The modeled and observed anisotropies show some differences (e.g.,the region from right ascension =–30to 90°shows weaker modeled than observed anisotropies).That a similar ordering of the global cosmic-ray anisotropy maps is generated simply by taking into account IBEX observations of low-energy interstellar neutral atoms to deduce the interstellar velocity,and the IBEX ribbon to de-duce the local interstellar magnetic field direction reveals consistency with independent observations across 10orders of magnitude in particle energy (keV energies in the IBEX ribbon compared to 10-TeV cosmic ray energies).Thus,local inter-stellar conditions play a key role in ordering very-high-energy (TeV)cosmic rays in the immediate interstellar environment of the Sun.References and Notes1.N.A.Schwadron,H.E.Spence,R.Came,Eos 92,297(2011).2.D.J.McComas et al .,Science 336,1291–1293(2012).3.D.J.McComas et al .,Science 326,959–962(2009).4.N.A.Schwadron et al .,Science 326,966–968(2009).5.H.O.Funsten et al .,Science 326,964–966(2009).6.S.A.Fuselier et al .,Science 326,962–964(2009).7.J.Heerikhuisen et al .,Astrophys.J.708,L126–L130(2010).8.J.Heerikhuisen,N.V.Pogorelov,Astrophys.J.738,29(2011).9.N.A.Schwadron,D.J.McComas,Astrophys.J.764,92(2013).10.P.C.Frisch et al .,Astrophys.J.760,106(2012).11.P.C.Frisch,N.A.Schwadron,/abs/1310.2922(2013).12.D.J.McComas,M.A.Dayeh,H.O.Funsten,G.Livadiotis,N.A.Schwadron,Astrophys.J.771,77(2013).13.K.Nagashima,K.Fujimoto,R.M.Jacklyn,J.Geophys.Res.103,17429(1998).14.D.L.Hall et al .,J.Geophys.Res.104,6737(1999).15.G.Guillian et al .,Phys.Rev.D Part.Fields Gravit.Cosmol.75,062003(2007).16.M.Amenomori et al .,in Proceedings of the 9th AnnualInternational Astrophysics Conference ,AIP Conference Proceedings ,J.Le Roux,G.P.Zank,A.J.Coates,V.Florinski,Eds.(American Institute of Physics,College Park,MD,2010),vol.1302,p.285.17.M.Amenomori;The Tibet AS g Collaboration,Astrophys.SpaceSci.Trans.6,49–52(2010).18.A.A.Abdo et al .,Phys.Rev.Lett.101,221101(2008).19.A.A.Abdo et al .,Astrophys.J.698,2121–2130(2009).20.R.Abbasi et al .,Astrophys.J.740,16(2011).21.P.Desiati,zarian,Astrophys.J.762,44(2013).22.G.Giacinti,G.Sigl,Phys.Rev.Lett.109,071101(2012)llement et al .,Science 307,1447–1449(2005).24.M.Opher,E.C.Stone,T.I.Gombosi,Science 316,875–878(2007).25.M.Witte,Astron.Astrophys.426,835–844(2004).26.E.Möbius et al .,Science 326,969–971(2009).27.E.Möbius et al .,Astrophys.J.Suppl.Ser.198,11(2012).28.M.Bzowski et al .,Astrophys.J.Suppl.Ser.198,12(2012).29.L.J.Gleeson,W.I.Axford,Astrophys.J.154,1011(1968).30.M.A.Forman,L.J.Gleeson,Astrophys.Space Sci.32,77–94(1975).31.A.Shalchi,I.Büsching,zarian,R.Schlickeiser,Astrophys.J.725,2117–2127(2010).32.I.Büsching,M.S.Potgieter,Adv.Space Res.42,504–509(2008).33.V.S.Ptuskin,I.V.Moskalenko,F.C.Jones,A.W.Strong,V.N.Zirakashvili,Astrophys.J.642,902–916(2006).34.J.R.Jokipii,Science 307,1424–1425(2005).35.J.R.Jokipii,Science 316,839–840(2007).36.L.C.Lee,J.R.Jokipii,Astrophys.J.206,735(1976).37.A.H.Minter,S.R.Spangler,Astrophys.J.458,194(1996).38.S.Chandrasekhar,G.Munch,Astrophys.J.115,103(1952).39.J.R.Jokipii,E.N.Parker,Astrophys.J.155,799(1969).40.J.W.Armstrong,B.J.Rickett,S.R.Spangler,Astrophys.J.443,209(1995).41.J.R.Jokipii,in Proceedings of the 2nd Guillermo HaroConference ,J.Franco,A.Carraminana,Eds.(Cambridge Univ.Press,Cambridge,1999),p.70.42.J.Giacalone,J.R.Jokipii,Astrophys.J.520,204–214(1999).43.P.Blasi,E.Amato,J.Cosmol.Astropart.Phys.2012,011(2012).44.A.D.Erlykin,A.W.Wolfendale,Astropart.Phys.25,183–194(2006).45.I.Büsching,M.Pohl,R.Schlickeiser,Astron.Astrophys.377,1056(2001).46.M.Pohl,D.Eichler,Astrophys.J.766,4(2013).47.I.Busching et al .,Astrophys.J.619,314–326(2005).48.C.Evoli,D.Gaggero,D.Grasso,L.Maccione,Phys.Rev.Lett.108,211102(2012).49.G.P.Zank,Space Sci.Rev.89,413–688(1999).50.R.Ratkiewicz et al .,Astron.Astrophys.335,363(1998).51.N.V.Pogorelov,G.P.Zank,Astrophys.J.636,L161–L164(2006).52.M.Opher,E.C.Stone,P.C.Liewer,Astrophys.J.640,L71–L74(2006).53.N.V.Pogorelov,J.Heerikhuisen,G.P.Zank,S.N.Borovikov,Space Sci.Rev.143,31–42(2009).54.R.Schönrich,J.Binney,W.Dehnen,Mon.Not.R.Astron.Soc.403,1829–1833(2010).55.H.O.Funsten et al .,Astrophys.J.776,30(2013).Acknowledgments:We thank all those who made IBEXpossible.IBEX is primarily funded by NASA ’s Explorers Program (Contract no.NNG05EC85C).IBEX data are available at /researchers/publicdata.shtml.IceCube cosmic ray data are available from /science/data.Supplementary Materials/content/343/6174/988/suppl/DC1Supplementary Text References (56–67)21August 2013;accepted 29January 2014Published online 13February 2014;10.1126/science.1245026Nanoporous BiVO 4Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water SplittingTae Woo Kim and Kyoung-Shin Choi *Bismuth vanadate (BiVO 4)has a band structure that is well-suited for potential use as aphotoanode in solar water splitting,but it suffers from poor electron-hole separation.Here,we demonstrate that a nanoporous morphology (specific surface area of 31.8square meters per gram)effectively suppresses bulk carrier recombination without additional doping,manifesting an electron-hole separation yield of 0.90at 1.23volts (V)versus the reversible hydrogen electrode (RHE).We enhanced the propensity for surface-reaching holes to instigate water-splitting chemistry by serially applying two different oxygen evolution catalyst (OEC)layers,FeOOH and NiOOH,which reduces interface recombination at the BiVO 4/OEC junction while creating a more favorableHelmholtz layer potential drop at the OEC/electrolyte junction.The resulting BiVO 4/FeOOH/NiOOH photoanode achieves a photocurrent density of 2.73milliamps per square centimenter at a potential as low as 0.6V versus RHE.N-type bismuth vanadate (BiVO 4)has re-cently emerged as a promising photoanode for use in water-splitting photoelectro-chemical cells because it absorbs a substantial portion of the visible spectrum (bandgap energy,~2.4eV)and has a favorable conduction band (CB)edge position very near the thermodynamic H 2evolution potential (1,2).However,the solar-to-hydrogen (STH)conversion efficiency achieved with BiVO 4to date has been far below what isexpected because the material suffers from poor electron-hole separation yield (f sep )(2–6).Previ-ous efforts to improve the f sep of BiVO 4mainly focused on doping studies,which were intended to improve its poor electron transport properties (2,6–12).Here,we demonstrate that a high-surface-area,nanoporous BiVO 4electrode composed of par-ticles smaller than its hole diffusion length can effectively increase f sep without additional dop-ing.Furthermore,we investigated the effect of an oxygen evolution catalyst (OEC)layer on the in-terfacial recombination at the BiVO 4/OEC junc-tion,water oxidation kinetics,and the Helmholtz layer potential drop at the OEC/electrolyte junction using two different OECs,FeOOH and NiOOH.Our understanding of the BiVO 4/OEC/electrolyte junction resulted in the development of a new strategy to serially apply dual layers of OECs that can optimize both the BiVO 4/OEC and the OEC/electrolyte junctions simultaneously,enabling effi-cient utilization of surface-reaching holes for solar water oxidation.Department of Chemistry,University of Wisconsin –Madison,Madison,WI 53706,USA.*Corresponding author.E-mail:kschoi@28FEBRUARY 2014VOL 343SCIENCE990REPORTSNanoporous BiVO4electrodes were prepared by first electrochemically depositing BiOI elec-trodes and then applying a dimethyl sulfoxide (DMSO)solution of vanadyl acetylacetonate [VO(acac)2]onto their surface and heating in air at450°C for2hours(experimental deatails are available in the supplementary materials,ma-terials and methods).A schematic overview of the synthesis procedure is shown in fig.S1.The specific advantage of using BiOI is that its two-dimensional(2D)crystal structure enables elec-trodeposition of extremely thin plates(~20nm) with sufficient voids between them(Fig.1A). These voids inhibit grain growth of BiVO4dur-ing the conversion process,resulting in nano-porous BiVO4electrodes.In a previous attempt to prepare nanoporous BiVO4(13),we used an NH4OH solution of V2O5as the vanadium source,which could not easily wet the BiOI surface because air in the voids between the BiOI plates renders the surface highly hydrophobic(fig.S2).Thus,the distribu-tion of V2O5was uneven,few BiVO4nucleation processes were induced within a single2D BiOI sheet,and the resulting electrodes manifested limited porosity(Fig.1,B and C).The use of comparatively hydrophobic VO(acac)2/DMSO solution overcame this problem and resulted in a remarkable increase in surface area.The top-view and side-view scanning electron microsco-py(SEM)images show the formation of much smaller BiVO4nanoparticles(mean particle size= 76T5nm)(fig.S3)creating a3D nanoporous network(Fig.1,D to F).N2adsorption-desorption-isotherm measure-ments show that the nanoporous BiVO4electrode contains micropores within BiVO4particles aswell as mesopores and macropores between BiVO4nanoparticles(fig.S4and table S1)(14).Thespecific surface area of the nanoporous BiVO4electrode was estimated to be31.8T2.3m2/gbased on a fitting analysis using the Brunauer-Emmett-Teller equation(14).BiVO4electrodesprepared by using other synthesis methods(suchas metal organic decomposition,spray deposition,or direct electrodeposition of BiVO4)possess lim-ited surface areas,and no attempts to measure sur-face areas of these samples were reported(2).The purity and crystal structure of the nano-porous BiVO4electrode(monoclinic scheelitestructure)were confirmed with x-ray diffraction(fig.S5).The bandgap of the nanoporous BiVO4electrode was estimated to be~2.50to2.55eV,using ultraviolet-visible absorption spectra(fig.S6),which is slightly larger than the bandgapof BiVO4samples prepared by other methodsthat result in larger grain composition(~2.4eV)(13,15).The photoelectrochemical properties werefirst examined in the presence of1M sodiumsulfite(Na2SO3),which served as the hole scav-enger.The oxidation of sulfite is thermodynami-cally and kinetically more facile than oxidation ofwater(11,13,15–18),and therefore,measuringphotocurrent for sulfite oxidation enables inves-tigation of the photoelectrochemical properties ofBiVO4independently of its poor water oxidationkinetics.A typical photocurrent-potential(J-V)curve of the sulfite oxidation with nanoporousBiVO4is shown in Fig.2A.A very early pho-tocurrent onset potential,0.1V versus reversiblehydrogen electrode(RHE),and a rapid increase inphotocurrent in the0.2V<E versus RHE<0.6Vregion,representing an excellent fill factor,resultedin a photocurrent density of3.3T0.3mA/cm2at a potential as low as0.6V versus RHE.Theincident photon-to-current conversion efficiency(IPCE)and the absorbed photon-to-current con-version efficiency(APCE)of the nanoporous BiVO4at0.6V versus RHE are60and72%,respec-tively,at420nm(Fig.2B).Photocurrent density obtained for sulfite oxi-dation was used to calculate f sep by using Eq.1,where J PEC is the measured photocurrent densityand J abs is the photon absorption rate expressedas current density,which is calculated assuming100%APCE(calculation details are available inthe supplementary materials,materials and meth-ods)(4,6,19–21).J abs of the nanoporous BiVO4electrode was calculated to be4.45mA/cm2.f sepis the yield of the photogenerated holes that reachthe surface,and f ox is the yield of the surfacereaching holes that are injected into the solutionspecies(21).J PEC=J abs×f sep×f ox(1)For sulfite oxidation with extremely fast oxidationkinetics,surface recombination is negligible,andf ox is~1.Therefore,f sep is obtained by dividingJ PEC by J abs(Fig.2A,inset).The result shows thatthe nanoporous BiVO4electrode achieves f sep=0.70T0.03and0.90T0.03at0.6V and1.23Vversus RHE,respectively,which is remarkablebecause a typical f sep value for a BiVO4photo-anode is below0.3at1.23V versus RHE(4,6).The highest f sep achieved recently by Abdi andcoworkers using gradient doping was~0.6at1.23V versus RHE(12).The hole diffusion lengthof BiVO4was recently reported to be~100nmwhen using single-crystal BiVO4(22).The meanparticle size of BiVO4composing the nanoporousBiVO4electrode shown in Fig.1D is76T5nm(fig.S3),and the particle size obtained from theXRD peaks(fig.S5)when using the Scherrerequation is27T2nm.Therefore,the nanoporosityincorporated into BiVO4electrodes in this studyappears to be ideal for effectively suppressingbulk carrier recombination,resulting in a recordhigh f sep.Photocurrent from the nanoporous BiVO4forwater oxidation shown in Fig.3A(black line)isconsiderably lower than the photocurrent for sul-fite oxidation(Fig.2A),indicating that the ma-jority of the surface-reaching holes were lost tosurface recombination because of the poor catalyticnature of the BiVO4surface for water oxidation(2).To improve water oxidation kinetics,we photo-deposited a thin FeOOH or NiOOH layer on thenanoporous BiVO4surface as an OEC layer inorder to assemble BiVO4/FeOOH and BiVO4/NiOOH electrodes.Their thicknesses were opti-mized so as to maximize photocurrent generation(fig.S7).It has been previously demonstratedthat FeOOH interfaces well with BiVO4(13,15),whereas NiOOH is known to be a more activeOEC than FeOOH(less overpotential required toFig.1.Morphologies of nanoporous BiVO4 electrodes.(A)SEM im-age of BiOI.(B and C) Top-view and side-view SEM images of BiVO4 electrode prepared using NH4OH/V2O5.(D to F)Top-view and side-view SEM images of nanoporous BiVO4prepared using DMSO/VO(acac)2. SCIENCE VOL34328FEBRUARY2014991REPORTSachieve the same current density)as a dark electrocatalyst on a conducting substrate(fig.S8) (23–25).The photocurrents for water oxidation from the resulting BiVO4/FeOOH and BiVO4/NiOOH photoanodes were markedly higher than those from the bare BiVO4electrode(Fig.3A and table S2),but their photocurrents were still lower than that generated for sulfite oxidation at the bare BiVO4electrode.This comparison suggested that neither BiVO4/FeOOH nor BiVO4/NiOOH engages all surface-reaching holes in the oxygen evolution reaction,instead losing a portion to surface recombination at the BiVO4/OEC junc-tion.The interface states formed at the BiVO4/ OEC junction can serve as recombination centers and cause surface recombination.The fact that BiVO4/FeOOH generated higher photocurrent than did BiVO4/NiOOH,although NiOOH shows faster water oxidation kinetics as an elec-trocatalyst,suggests that the interface recombi-nation at the BiVO4/NiOOH junction is more substantial than that at the BiVO4/FeOOH junc-tion.This can be easily confirmed by comparing photocurrents for sulfite oxidation by BiVO4, BiVO4/FeOOH,and BiVO4/NiOOH(Fig.3,B and C,and table S3).Because the interfacial hole transfer rates for sulfite oxidation on the BiVO4, FeOOH,and NiOOH surfaces should be equally fast,any difference observed in photocurrents for sulfite oxidation by BiVO4,BiVO4/FeOOH,and BiVO4/NiOOH should be mainly due to the re-combination at the BiVO4/OEC junction.The com-parison shows that photocurrent for BiVO4/ FeOOH is very close to that for BiVO4,whereas the photocurrent for BiVO4/NiOOH is consider-ably lower,which indicates that the interface recom-bination at the BiVO4/NiOOH junction is indeed more substantial.In addition to the interface recombination at the BiVO4/OEC junction,slow water oxidation kinetics at the OEC/solution junction can cause additional surface recombination during water oxidation(26,27).This additional surface re-combination can be shown as the difference inphotocurrent for sulfite oxidation and water oxi-dation(Fig.3,B and C).When the rate of inter-facial hole transfer for water oxidation is slowerthan the rate of holes entering the OEC layer,holes are accumulated in the OEC layer and atthe BiVO4/OEC junction,which in turn increasesthe electron current from the CB of BiVO4to theOEC layer for surface recombination(27).Be-cause FeOOH has slower water oxidation kineticsthan that of NiOOH,the difference in photo-current for water oxidation and sulfite oxida-tion is more pronounced for BiVO4/FeOOH thanBiVO4/NiOOH(Fig.3,B and C).However,when the effects of interface recombination atthe BiVO4/OEC junction and water oxidationkinetics are combined,BiVO4/NiOOH loses moresurface-reaching holes to surface recombinationand generates lower photocurrent than doesBiVO4/FeOOH for water oxidation(Fig.3A).Regardless of more substantial surface re-combination,BiVO4/NiOOH shows an earlierphotocurrent onset and generates higher photo-current than does BiVO4/FeOOH in the low biasregion(E<0.44V versus RHE)for water oxi-dation(Fig.3A and table S2).This means thatBiVO4/NiOOH has a more negative flatband po-tential(E FB)than that of BiVO4/FeOOH.Thephotocurrent onset potential for sulfite oxidationwith fast oxidation kinetics should be very closeto E FB.The results show that BiVO4has the mostnegative onset potential(0.11T0.02V versusRHE),followed by BiVO4/NiOOH(0.12T0.02Vversus RHE),and then BiVO4/FeOOH(0.18T0.02V versus RHE)(table S3and fig.S9A).TheE FB s obtained by Mott-Schottky plots of BiVO4,BiVO4/FeOOH,and BiVO4/NiOOH show thesame trend;BiVO4has the most negative E FB(0.10T0.03V versus RHE),followed by BiVO4/NiOOH(0.11T0.03V versus RHE),and thenBiVO4/FeOOH(0.15T0.02V versus RHE)(table S4and fig.S10).It is unlikely that the shift in E FB is due to thechange in charge carrier density of BiVO4be-cause the addition of an extremely thin OEC layershould not affect the doping level or carrier densitywithin the BiVO4electrode.This assumption isalso supported by the comparable slopes in theMott-Schottky plots for these three electrodes ateach frequency(table S4).Then,the difference inE FB should be caused by the change in theHelmholtz layer potential drop(V H),which is theonly other factor that can affect the E FB,as shownin Eq.2,where f SC is the work function of thesemiconductor versus vacuum,and4.5is the scalefactor relating the H+/H2redox level to vacuum(28).At the BiVO4/electrolyte junction,the domi-nant charges that affect the solid side of theHelmholtz double layer come from the adsorptionof H+and OH–ions on the BiVO4surface,whichdepends on the solution pH and the point of zeroz potential(pH PZZP)of BiVO4(Eq.3)(28,29).E FB(NHE)=f SC+V H–4.5(2)V H=0.059(pH PZZP–pH)(3)The pH PZZP of BiVO4is reported to be between2.5and3.5,and therefore,V H should be negativein a pH7solution(30,31).However,when a thinlayer of FeOOH or NiOOH is deposited onBiVO4,because the Helmholtz double layer isnow formed at the OEC/solution junction the V Hat the solid/solution interface is no longer deter-mined by the pH PZZP of BiVO4but by the pH PZZPof the OEC layer.This means that the pH PZZP ofOEC is an important factor to consider in opti-mizing the photoanode/OEC junction because itcan affect the E FB of the photoanode.The pH PZZP of FeOOH is reported to bebetween7and9(32,33).Thus,the resultingmore positive V H at the FeOOH/solution junctionwill shift the E FB of BiVO4/FeOOH positively,which is in agreement with the observed shiftdirection of the E FB.The pH PZZP of NiOOHcannot be straightforwardly determined becausechemical composition of NiOOH varies withpH.However,it is known that NiOOH hasa Fig. 2.Photoelectrochemical properties of nanoporous BiVO4electrode for sulfite oxidation.(A)J-V curve of nanoporous BiVO4electrode measured in a0.5M phosphate buffer(pH7)containing1MNa2SO3as hole scavenger under AM1.5G,100mW/cm2illumination(scanrate,10mV/s).Dark current is shown as a dashed line.(Inset)f sep cal-culated from the J-V curve after dark current is subtracted.(B)IPCE(redcircles)and APCE(blue triangles)measured in the same solution at0.6Vversus RHE.28FEBRUARY2014VOL343SCIENCE 992REPORTSnegative z potential(~–20mV)in a pH7solu-tion(34),meaning that pH PZZP of NiOOH is lower than7,and the E FB of BiVO4/NiOOH is ex-pected to be more negative than that of BiVO4/FeOOH,which again agrees with the observedE FB shift.The pH PZZP of a material depends on itsspecific surface termination and solution compo-sition.Therefore,the most reliable estimations ofthe V H s of electrodes discussed in this study canbe obtained when the z potential measurement isperformed by using the electrodes and the solu-tion that were used in this study.The z potentialsmeasured for our nanoporous BiVO4,BiVO4/FeOOH,and BiVO4/NiOOH in a0.5M phosphatebuffer(pH7)were–36T3,–8T3,and–36T4mV,respectively.These results indicate thatthe V H s at the BiVO4/solution and the NiOOH/solution junctions should indeed be more nega-tive than that at the FeOOH/solution junction.On the basis of our new understanding of theBiVO4/OEC and the OEC/electrolyte interfaces,we deposited consecutive layers of FeOOH andNiOOH,simultaneously optimizing the BiVO4/OECand the OEC/electrolyte junctions.The FeOOH at theBiVO4/OEC junction will reduce the interfacerecombination,whereas the NiOOH at the OEC/electrolyte junction will decrease V H to achieve amore negative E FB for BiVO4while realizingfaster water oxidation kinetics than if FeOOHwas used as the outermost layer.The photocurrent onset for sulfite oxidation(fig.S9A and table S3)as well as the Mott-Schottkyplot(fig.S10and table S4)of the resulting BiVO4/FeOOH/NiOOH photoanode shows that its E FB iscomparable with that of BiVO4/NiOOH,indicatingthat the E FB of the BiVO4photoanode is indeedaffected by the pH PZZP of the outermost OEClayer.In addition,BiVO4/FeOOH/NiOOH andBiVO4/FeOOH show comparable J-V curves forsulfite oxidation,confirming that the BiVO4/FeOOHjunction effectively reduces the interface recom-bination at the BiVO4/OEC junction(Fig.3,Band D).As a result,BiVO4/FeOOH/NiOOH showsFig.3.Effect of OECs on photocurrents for water oxidationandsulfiteoxida-tion.(A)J-V curves of BiVO4 (black solid line),BiVO4/FeOOH (blue),BiVO4/NiOOH(green), BiVO4/FeOOH/NiOOH(red), and BiVO4/NiOOH/FeOOH (pink)for water oxidation measured in a0.5M phos-phate buffer(pH7)under AM 1.5G illumination.Dark cur-rent is shown as a dashed line.(B to E)J-V curves of(B)BiVO4/ FeOOH,(C)BiVO4/NiOOH,(D) BiVO4/FeOOH/NiOOH,and(E) BiVO4/NiOOH/FeOOH com-paring photocurrent for sul-fite oxidation(dashed)and water oxidation(solid)mea-sured with and without the presence of1.0M Na2SO3 as hole scavenger.Photocur-rent for sulfite oxidation by BiVO4is shown as the black dashed line for comparison. The mean values and SDs of photocurrent onset poten-tials and photocurrent densi-ties are summarized in tables S2andS3.Fig.4.Photoelectrolysis of water by BiVO4/FeOOH/NiOOH photo-anode.(A)ABPE obtained using a three-electrode system.(B)J-t curvemeasured at0.6V versus counter electrode in a phosphate buffer(pH7)under AM1.5G illumination.(C)Detection of H2and O2at0.6V versuscounter electrode. SCIENCE VOL34328FEBRUARY2014993REPORTSimpressive overall performance for water oxi-dation,reaching a photocurrent density of 2.8T 0.2mA/cm 2at 0.6V versus RHE (Fig.3A and table S2),which is markedly better than those of BiVO 4/FeOOH and BiVO 4/NiOOH and is al-most comparable with the performance of bare BiVO 4for sulfite oxidation.When NiOOH was first deposited on the BiVO 4surface and FeOOH was added as the outermost layer to form BiVO 4/NiOOH/FeOOH (reversed OEC junction),the resulting E FB s de-termined by sulfite photocurrent onset (fig.S9A and table S3)and Mott-Schottky plot (fig.S10and table S4)are comparable with those of BiVO 4/FeOOH,again confirming that the E FB of the BiVO 4photoanode is affected by the pH PZZP of the outermost OEC.Also,the J -V curve for sulfite oxidation by BiVO 4/NiOOH/FeOOH was comparable with that by BiVO 4/NiOOH,con-firming that a BiVO 4/NiOOH junction is not fa-vorable for interface recombination (Fig.3,C and E).As a result,BiVO 4/NiOOH/FeOOH shows the lowest photocurrent for water oxidation.These results prove that the photocurrent enhancement achieved by the BiVO 4/FeOOH/NiOOH photo-anode for photoelectrolysis of water is truly due to the simultaneous optimization of the BiVO 4/OEC and OEC/electrolyte junctions,using an op-timum dual OEC structure.The applied bias photon-to-current efficiency (ABPE)of the BiVO 4/FeOOH/NiOOH electrode calculated by using its J -V curve,assuming 100%Faradaic efficiency,is plotted in Fig.4A (35).The maximum ABPE of 1.75%achieved by the system is impressive because it is obtained by using unmodified BiVO 4as a single photon ab-sorber.Moreover,this efficiency is achieved at a potential as low as 0.6V versus RHE,which is a highly favorable feature for assembling a tandem cell or a photoelectrochemical diode (12,36,37).The ABPE obtained by using a two-electrode system (working electrode and a Pt counter electrode),which achieves the maximum ABPE of 1.72%,is also shown in fig.S11(35).The long-term stability of BiVO 4/FeOOH/NiOOH was tested by obtaining a J -t curve.A photocurrent density of 2.73mA/cm 2,obtained by applying 0.6V between the working and counter electrodes,was maintained for 48hours without showing any sign of decay,proving its long-term stabil-ity (Fig.4B).The O 2measurement made by using a fluorescence O 2sensor confirmed that the photocurrent generated at 0.6V versus counter-electrode was mainly associated with O 2produc-tion (>90%photocurrent-to-O 2conversion effi-ciency)(Fig.4C).The same results were obtained when the measurement was performed at 0.6V versus RHE.H 2production at the Pt counter electrode was also detected with gas chromatography (GC)(Fig.4C).The molar ratio of the produced H 2:O 2was 1.85:1.The slight deviation from the stoi-chiometric ratio of 2:1is due to our imperfect manual sampling method of H 2for GC analysis.Because this outstanding performance was achieved by using simple,unmodified BiVO 4(no extrinsic doping and no composition tuning)as the only photon absorber,further improve-ment of the cell efficiency is expected when var-ious strategies of tuning compositions or forming heterojunctions and tandem cells are used to enhance photon absorption and electron-hole separation.References and Notes1.A.Kudo,K.Omori,H.Kato,J.Am.Chem.Soc.121,11459–11467(1999).2.Y.Park,K.J.McDonald,K.-S.Choi,Chem.Soc.Rev.42,2321–2337(2013).3.Y.Liang,T.Tsubota,L.P.A.Mooij,R.van de Krol,J.Phys.Chem.C 115,17594–17598(2011).4.F.F.Abdi,R.van de Krol,J.Phys.Chem.C 116,9398–9404(2012).5.F.F.Abdi,T.J.Savenije,M.M.May,B.Dam,R.van de Krol,J.Phys.Chem.Lett 4,2752–2757(2013).6.D.K.Zhong,S.Choi,D.R.Gamelin,J.Am.Chem.Soc.133,18370–18377(2011).7.H.S.Park et al .,J.Phys.Chem.C 115,17870–17879(2011).8.W.J.Jo et al .,Angew.Chem.Int.Ed.Engl.51,3147–3151(2012).9.S.P.Berglund,A.J.E.Rettie,S.Hoang,C.B.Mullins,Phys.Chem.Chem.Phys.14,7065–7075(2012).10.K.Zhang,X.J.Shi,J.K.Kim,J.H.Park,Phys.Chem.Chem.Phys.14,11119–11124(2012).11.Y.Park,D.Kang,K.-S.Choi,Phys.Chem.Chem.Phys.16,1238–1246(2014).12.F.F.Abdi et al .,mun 4,2195(2013).13.K.J.McDonald,K.-S.Choi,Energy Environ.Sci 5,8553–8557(2012).14.T.Allen,Particle Size Measurement (Chapman and Hall,London,1997).15.J.A.Seabold,K.-S.Choi,J.Am.Chem.Soc.134,2186–2192(2012).16.R.Sarala,M.A.Islam,S.B.Rabin,D.M.Stanbury,Inorg.Chem.29,1133–1142(1990).17.D.M.Stanbury,Adv.Inorg.Chem.33,69–138(1989).18.J.-F.Reber,K.Meier,J.Phys.Chem.88,5903–5913(1984).19.H.Dotan,K.Sivula,M.Grätzel,A.Rothschild,S.C.Warren,Energy Environ.Sci 4,958–964(2011).20.J.M.Foley,M.J.Price,J.I.Feldblyum,S.Maldonado,Energy Environ.Sci 5,5203–5220(2012).21.M.Zhou et al .,ChemSusChem 5,1420–1425(2012).22.A.J.E.Rettie et al .,J.Am.Chem.Soc.135,11389–11396(2013).23.L.Trotochaud,J.K.Ranney,K.N.Williams,S.W.Boettcher,J.Am.Chem.Soc.134,17253–17261(2012).24.R.D.L.Smith et al .,Science 340,60–63(2013).25.M.W.Louie,A.T.Bell,J.Am.Chem.Soc.135,12329–12337(2013).26.C.Y.Cummings,F.Marken,L.M.Peter,A.A.Tahir,K.G.Wijayantha,mun.(Camb.)48,2027–2029(2012).27.L.M.Peter,K.G.Wijayantha,A.A.Tahir,FaradayDiscuss.155,309–322,discussion 349–356(2012).28.J.Nozik,Annu.Rev.Phys.Chem.29,189–222(1978).29.S.R.Morrison,in Electrochemistry at Semiconductor andOxidized Metal Electrodes (Plenum Press,New York,1980),pp.49–78.30.N.C.Castillo,A.Heel,T.Graule,C.Pulgarin,Appl.Catal.B 95,335–347(2010).31.S.Obregón,G.Colón,J.Mol.Catal.Chem.376,40–47(2013).32.R.M.Cornell,U.Schwertmann,The Iron Oxides:Structure,Properties,Reactions,Occurrences and Uses (Wiley VCH,New York,2003).33.D.M.Sherman,Geochim.Cosmochim.Acta 69,3249–3255(2005).34.N.Hernández,R.Moreno,A.J.Sánchez-Herencia,J.L.Fierro,J.Phys.Chem.B 109,4470–4474(2005).35.Z.Chen et al .,J.Mater.Res.25,3–16(2010).36.M.G.Walter et al .,Chem.Rev.110,6446–6473(2010).37.L.Tong et al .,Energy Environ.Sci 5,9472–9475(2012).Acknowledgments:We acknowledge support from the Center for Chemical Innovation of the National Science Foundation (POWERING THE PLANET:grant CHE-1305124).We thank M.A.Woo for electrodepositing FeOOH and NiOOH on FTO substrates and testing their electrochemical water oxidation performances.Supplementary Materials/content/343/6174/990/suppl/DC1Materials and Methods Figs.S1to S11Tables S1to S4References (38–41)7October 2013;accepted 31January 2014Published online 13February 2014;10.1126/science.1246913A Direct Quantitative Measure of Surface Mobility in a Glassy PolymerY.Chai,1T.Salez,3J.D.McGraw,4*M.Benzaquen,3K.Dalnoki-Veress,3,4E.Raphaël,3J.A.Forrest 1,2†Thin polymer films have striking dynamical properties that differ from their bulk counterparts.With the simple geometry of a stepped polymer film on a substrate,we probe mobility above and below the glass transition temperature T g .Above T g the entire film flows,whereas below T g only the near-surface region responds to the excess interfacial energy.An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-T g data.The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk T g .The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible.This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.The last decades have seen a considerable interest in the dynamical and rheological properties of glassy materials (1,2).Re-cent efforts (1,3–5)have focused on elucidating the nature of glassy dynamics both in the bulk and in systems,such as thin films or colloids,28FEBRUARY 2014VOL 343SCIENCE994REPORTS。

(a)
N-type
N d N d
P-type
N a N a
q φbi
(b)
Ec Ef Ev
V φbi
(c)
xN
0
xP
x
Can the built-in potential be measured with a voltmeter?
Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 4-3
4.5.1 Peak Electric Field
Arbitrarily choose the voltage at x = xP as V = 0.
xn N
xp P
x Ec
φbi , built-in potential Ef Ev
On the N-side, qN d V ( x) = D − ( x − xN )2 2ε s qN d = φbi − ( x − xN )2 2ε s
• Is this an accurate way to determine Nl ? Nh ?
Modern Semiconductor Devices for Integrated Circuits (C. Hu) Slide 4-14
4.5 Junction Breakdown
I Forward Current V B, breakdown voltage Small leakage Current R A V
N-region
(a)
P-region
Ef Ec
Ef is constant at equilibrium Ec and Ev are known relative to Ef