Germany 2) Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D Berlin,

Transcription

1 Crystl struture of polymeri rbon nitride nd determintion of its proess-temperture-indued modifitions T. Tyborski, 1, 2 C. Mershjnn, 1, 3 S. Orthmnn, 1 F. Yng, 1 M.-Ch. Lux-Steiner, 1 nd Th. Shedel-Niedrig 1 1) Helmholtz-Zentrum Berlin für Mterilien und Energie GmbH, D Berlin, Germny 2) Mx-Born-Institut für Nihtlinere Optik und Kurzzeitspektroskopie, D Berlin, Germny ) 3) Universität Rostok, D Rostok, Germny Bsed on the rrngement of two-dimensionl melon, we onstrut unit ell for polymeri rbon nitride (PCN) synthesized vi therml polyondenstion whose theoretil diffrtion powder pttern inludes ll mjor fetures mesured in X-ry diffrtion. With the help of tht unit ell, we desribe the proesstemperture-indued rystllogrphi hnges in PCN tht our within temperture intervl between 510 C nd 610 C. We lso disuss further potentil modifitions of the unit ell for PCN. It is found tht both trizine nd heptzine-bsed g-c 3 N 4 n only ount for minor phses within the investigted synthesis produts. Keywords: polymeri rbon nitride, g-c 3 N 4, therml polyondenstion, rystl struture Polymeri rbon nitride (PCN) stnds for very remrkble lss of mterils. It revels phototlyti tivity, both s pristine powder nd s nnorystlline film in heterojuntion photothode. 1 3 Furthermore, PCN only onsists of bundnt elements, n be synthesized vi strightforwrd, slble proess nd offers high hemil nd therml stbility. 4 Light-indued hydrogen relese on the bsis of PCN in n idi queous solution under the presene of n eletron donor hs been demonstrted. 1 At the sme time, synthesis-prmeter nd thus rystl struture dependene of the phototlyti tivity s well s of the photophysis of PCN is doumented. 1,4 6 These fetures ombined in PCN estblish fr-rnging interest on tht lss of mterils, espeilly with respet to sustinble hydrogen prodution. 7 The synthesis of PCN vi bulk therml polyondenstion is hrterized by both the retion-temperturedependent hydrogen ontent nd the degree of polymeriztion within temperture rnge of bout 510 C up to 610 C. 4,8 10 The therml polyondenstion itself is ontinuous proess tht revels severl dominnt phses t ertin tempertures, suh s melmine t 250 C or melem t 450 C. 4,9 While some of these dominnt phses n be exmined from rystllogrphi point of view, the rystllogrphi struture of PCN hs only been desribed insuffiiently. 11 X-ry diffrtion (XRD) dt of PCN revel preferentilly two mjor, slightly retiontemperture-dependent reflexes t 2Θ 27.3 (3.26 Å) nd 2Θ 13.2 (6.70 Å). These reflexes re understood s diffrtion t grphite-like sheets for the former nd s diffrtion t periodi struturl feture within the sheets for the ltter diffrtion ngle. 4 The grphite-like sheets exhibit pores nd onsist of polymerized heptzine units. 4,12,13 However, so fr distint unit ell with onrete spe group for PCN tht ombines the listed struturl spets nd is in good greement with the mesured XRD powder ptterns hs not been identified. The usul wy to determine the rystl struture of ertin mteril on the bsis of its powder pttern is exeuted s follows (1) The unit ell is determined by indexing the peks of the powder pttern. (2) The powder pttern is deomposed into integrted intensities I h,k,l. (3) The spe group is ssigned bsed on the systemti bsene of severl I h,k,l. (4) The phse problem is solved, e.g. with the help of the Ptterson Method. (5) The struture is refined, typilly with Rietveld refinement. This proper struture nlysis ssumes high purity s well s high rystllinity of the investigted mteril whih in the se of PCN synthesized vi therml polyondenstion n both not be fforded. 2,4 This is the point where subjetive judgements nd fesible rguments ome to the fore in order to overome the steps (1)-(3) while proper refinement does not stnd hne. In this work, we ontrst melon-bsed unit ells with trizine - nd heptzine-bsed unit ells nd ompre their theoretil X-ry diffrtion ptterns with mesured ptterns from PCN. Our findings of tht nlysis suggest tht the mjor phse of PCN is bsed on melon sheets s well s tht g-c 3 N 4 n only ount for minor phse within the investigted synthesis produts. We lso disuss geometril modifitions of the melon-bsed PCN unit ell nd demonstrte their influene on X-ry diffrtion ptterns. Furthermore, we use the melon-bsed PCN unit ell in order to desribe rystllogrphi hnges within PCN tht ome long with higher proess tempertures. Detils onerning the rystllogrphi strutures n be found in the supporting informtion. ) Eletroni mil: tybo@physik.fu-berlin.de

2 2 A. EXPERIMENTAL SECTION Synthesis of PCN Diyndimide (Aldrih, 99 %, bout 2.5 g) ws heted to tempertures between 490 C nd 610 C (in 20 -steps) t rte of 25 min 1 in ermi ruible within muffle furne under ontinuous N 2 gs flow. In first proessing step, the therml tretment lsts for five minutes fter whih the resulting produts were grounded. In seond proessing step, the grounded mterils were heted to the respetive proessing tempertures for nother three hours under N 2 gs flow, fter wht they were llowed to ool down pssively in the furne. Struturl hrteriztion Struturl phse nlysis of the PCN powders ws performed by X-ry diffrtometry (Bruker AXS D8, Cu Kα - line) in Θ 2Θ onfigurtion. Approximtely 100 mg of eh PCN powder ws nlyzed for one hour between 2Θ = 10 nd 2Θ = 90. The resulting powder ptterns hve been bkround-orreted with n exponentil funtion in order to minimize the influene of diffuse sttering. Sine they do not exhibit nrrow reflexes, the powder ptterns hve been smoothed for the purpose of emphsizing their fetures with respet to the noise. Struture nlysis Eh disussed theoretil unit ell ws nlyzed in numeril X-ry diffrtion experiment. These nlysis were performed with the progrm Dimond 3.2i nd reviewed with the progrm PowderCell The following prmeters (djustble in the softwre) were used: X-ry lbortory soure with wvelength λ = Å, 2Θ min = 10 nd 2Θ mx = 90, tivted Lorentz ftor, tivted Polriztion ftor nd disbled displement ftor. Depending on the prtiulr nlyzed unit ell, the softwre lultes the orresponding struture ftors nd thus the orresponding diffrtion pttern. The physil bkground of tht lultion is the independent-tom model (IAM), in whih the eletroni hrge density is exlusively loted t the toms. All disussed numeril vlues for reflex positions or seprtion distnes suh s d s or d hve been mesured with the progrm Dimond 3.2i on the bsis of the orresponding tomi positions nd rystl prmeters. The men seprtion distne d is defined s d = i=1 d i in whih d i re the seprtion distnes of the (five) prtiulr pirs of nitrogen toms tht re loted fe to fe with eh other long the heptzine hins (figure 5). B. Grphiti rbon nitride g-c 3N 4 A trizine-bsed struture for g-c 3 N 4 ws theoretilly suggested by Teter nd Hemley in Next to the unit ell (upper inset), figure 1 shows the theoretil diffrtion pttern of tht mteril in ombintion with mesured XRD pttern of PCN. This omprison revels fundmentl struturl differenes between PCN nd trizine-bsed g-c 3 N 4. Espeilly the intense reflexes of PCN t diffrtion ngles smller thn 2Θ = 20 nnot be ttributed to g-c 3 N 4. Similrities n be found for the g-c 3 N 4 -reflexes (011) t 2Θ = 25.40, (002) t 2Θ = nd (020) t 2Θ = FIG. 1. Top: Unit ell for trizine-bsed g-c 3N 4 with spe group P 6m2 (187), = Å, = Å nd 14 toms (8N(blue) +6C(grey)) together with the diffrtion pttern of PCN (blk urve) nd the most intense diffrtion reflexes of n idel trizine-bsed g-c 3N 4-rystl (blue lines). 12 Bottom: Struture of heptzine-bsed g-c 3N 4 synthezised by Bojdys et l. with spe group P 6 3m (185), = Å, = Å nd its most intense diffrtion reflexes (grey lines). 21 Bojdys et l. hve demonstrted n ionotherml synthesis route for substne they lso ll g-c 3 N Their mteril (lower inset figure 1) exhibits the lrgest rystllites known so fr for ny grphite-like rbon nitride, showing dimeters of bout 200 nm. The orresponding struture nlysis revels the hexgonl spe group P6 3 m (185). However, XRD nlysis lso indites tht this is diverse mteril ompred to both the trizinebsed g-c 3 N 4 nd the PCN synthesized vi therml polyondenstion whih neessittes the onsidertion of different unit ell for PCN. C. Unit ell for PCN We onstrut unit ell for PCN on the bsis of poly(minoimino)heptzine [C 6 N 7 (NH 2 )(NH)] n, lso known s melon, whose struture hs been nlyzed by

3 3 Wng et l. nd Lotsh et l.1,10 The reltive positions of the toms within the melon sheet were extrted digitlly from figure 12 in Ref. 10 with the id of the imge dt proessing progrm ImgeJ 1.43u.10,22,23 In this proedure, the entrl points of the illustrted grey nd blk dots served s the tomi oordintes for nitrogen nd rbon. We dded oordintes for the missing hydrogen toms. With these tomi oordintes, we onstrut theoretil unit ell for PCN. The reltive lignment of the toms in the unit ell n be modified mthemtilly suh tht, for exmple, bukling or different stking motifs n be introdued. Sine the grphil melon representtion in Ref. 10 nd the extrtion of the tomi oordintes re inherently orrelted with minor unertinties, we initilly ssume the lowest possible symmetry (spe group P 1) in our unit ell. The ell prmeters hve been hosen to be in line with the mesured XRD dt of our PCN smples s well s with the findings from Lotsh et l.10 In this wy, we suggest unit ell for PCN synthesized vi therml polyondenstion of diyndimide with the following prmeters: spe group P 1, = 16.2 A, b = 12.1 A, = A nd α = β = γ = 90, in whih nd γ exhibit proess-temperture dependene (figure 4 nd figure 5). Figure 2 shows unit ell for A-A-stked PCN with flt melon sheets. The orresponding theoretil diffrtion pttern is opposed to mesured powder pttern of PCN in the upper prt of figure 2. This omprison revels tht espeilly the strong reflexes (2 10), (210) t 2Θ = nd (001) t 2Θ = mth the mesured powder pttern with respet to their ngulr positions nd intensities persusively. The evident pek between the mjor reflexes n be understood s diffrtion t the hkl-plnes (3 10) nd (310) t 2Θ = The pirs (2 10), (210) nd (3 10), (310) re eh overlpping reflexes. Three modifitions need to be disussed in the further proedure ompred to flt A-A stked PCN: bukling within singulr melon sheets, different stking motifs nd the influene of the proess temperture.4 1. Bukling Compred to infinite C6 N8 sheets, in whih the energeti dvntge of sinusoidl bukling with n mplitude of 0.7 A prllel to the plne norml is lulted to be 11 kj mol 1 per neighboring N-N pir, the melon sheets in PCN very likely feture relted bukling.4,24 hkl-plne (101) hkl-plne (001) ds b hkl-plne (101) b hkl-plne (210) hkl-plne (210) FIG. 2. Trilini unit ell for PCN with flt melon sheets, n A-A-stking motif nd 72 toms (36N + 24C + 12H). Bottom: PCN ltties long -diretion nd with view into the stked melon sheets, eh with highlighted hkl-plnes. Top: Mesured XRD pttern (blk urve) nd the most intense diffrtion reflexes of the orresponding idel PCN (blue lines). FIG. 3. Trilini unit ell for PCN with sinusoidlly bukled melon sheets (prllel to --plne, 0.7 A mplitude) nd n A-A-stking motif. Bottom: PCN lttie with view into the stked melon sheets, highlighted hkl plnes nd trilini unit ell. Top: Mesured XRD pttern (blk urve) together with the most intense theoretil diffrtion reflexes of the orresponding PCN lttie (blue lines). In the speil se of sinusoidl bukling prllel to the --plne, s demonstrted in figure 3, ssoited fetures in the diffrtion pttern re rising reflexes (1 01)

4 4 nd (101), both t 2Θ = Emerging reflexes (0 11) nd (011), t 2Θ = 28.21, re the result of sinusoidl bukling prllel to the b--plne (not shown). A sinusoidl bukling prllel to the -diretion nd long the -b-digonl uses wek emerging ( 101),(101) - nd (0 11),(011) - s well s (111),( 111),(1 11),( 1 11) - reflexes (not shown). The orresponding bukling-indued inrese of N-N-distnes between nerest neighbors in one heptzine unit ounts for roughly Å, the distne d (figure 5) between the in-plne heptzine hins inreses by bout Å while the intrplnr N- N-distnes of seond nerest neighbors extend by bout Å. Comprble, minor hnges in the theoretil diffrtion ptterns n be observed for slightly vried bukling mplitudes. 2. Stking motifs If one ssumes the stking of the melon sheets in PCN to be of the form A-B, the (001)-reflex is repled by (002)-reflex in the orresponding diffrtion pttern nd the mount of toms per unit ell is doubled, generlly. A displement of every seond melon sheet 0.5 long the -xis leds to diffrtion pttern tht fetures forbidden reflexes ( 310) nd (310) only (not shown). In exhnge reloted nd enhned qud reflex (311),( 311),(3 11),( 3 11) ppers t 2Θ = (not shown). In the se of displement 0.5 b long the b-xis, the reflexes ( 210) nd (210) vnish wheres strong qud reflex (211),( 211),(2 11),( 2 11) emerges t 2Θ = (not shown). A ombintion of these two displements ( b) lso leds to vnishing (2 10),(210) - nd rising (211),( 211),(2 11),( 2 11) reflexes (not shown). 3. Proess-temperture-indued influene on the rystl struture of PCN FIG. 4. Mesured XRD ptterns of PCN synthesized t 510 C (top) nd t 610 C (bottom), together with the most intense theoretil reflexes of flt, A-A-stked PCN with two different unit ell ngles γ. Figure insets show n enlrgement of the peks t 2Θ = 13.2 nd 2Θ = 27.2, respetively, for three different proess tempertures. Inresing the proess temperture of PCN leds to signifint hnges in the mesured diffrtion ptterns. 4,21 As n be seen in figure 4, the higher the proess temperture, the weker nd broder is the pek t 2Θ = At the sme time the pek t 2Θ = 27.2 is shifted towrds lrger diffrtion ngles. While the wekening nd brodening of the 2Θ = pek is lerly observble, one n only observe slight shift towrds higher diffrtion ngles for the 2Θ = pek. Next to the speified hnges, deresing FWHM of the pek from roughly 1.6 to 1.3 is observble. These proess-temperture-indued hnges n be understood on the bsis of the trilini PCN suggestion. During the therml polyondenstion, the heptzine units begin to hemilly ondenste t tempertures of round 510 C, thus forming PCN. Further heting to tempertures of round 610 C redues the hydrogen ontent vi mmoni relese, pointing to n enhned rystllinity. 4 An enhned rystllinity long the -xis is lerly observble in the shift of the pek nd its redued FWHM, inditing n inresed number of stked melon sheets per rystllite s well s redued stking distne d s (figure 2). Neessrily orrelted to the deresing distne between the sheets is the deresing distne of prtiulr nitrogen toms. Sine ll nitrogen toms in PCN exhibit n unbound pir of eletrons, lrge N-N-distnes re energetilly benefiil. So one

5 5 single melon sheet distorts its two-dimensionl ordering suh, tht the inner nitrogen distnes inrese. For tht reson, the ngle γ of PCN s trilini unit ell is slightly shifted to vlues < 90, s shown in figure 5. FIG. 5. Left: Sheme of the temperture-indued distortion of PCN s rystl struture together with setion of distorted unit ell (γ = 87 ) nd the men seprtion distne between two heptzine hins d. Note tht the sheme is illustrted exessively for n enhned lrity. Right: Reltion between γ, d, ds nd the proess temperture (in flt, A-A-stked PCN). While the stking distne ds stys unhnged with respet to deresing ngle γ, the heptzine units re slightly distorted nd the men seprtion distne d between the heptzine hins is inresed from 3.11 A to 3.19 A with slope of A / C. This distortion influenes the theoretil diffrtion pttern of PCN suh, tht the overlpping reflexes (2 10), (210) nd (3 10), (310) re displed ginst eh other (figure 4). A very similr behvior n be observed for n enlrged ngle γ. But enlrging γ is orrelted to deresed distnes d, whih mkes this kind of distortion unplusible. Slightly vrying the ngles α nd β ends up in only minorly hnged theoretil diffrtion ptterns. The disussed results n be found for the flt s well s for bukled PCN. D. DISCUSSION As mentioned before, the pplied independent-tom model restrits the eletron density t the toms. Thus, the eletron density is desribed by the spherilly verged density of the isolted toms.25 The model works very onviningly for solely hevy toms in the unit ell. This is beuse in tht se, the vlene shells re only minor prt of the totl eletron density t whih the X-ry photons re sttered. It ffords unuries for light toms. For exmple, the bond length of ovlent C-H bond is typilly underestimted within the IAM piture sine the hydrogens eletron density is signifintly distributed over the bond.25,26 Nevertheless, the IAM hs been the bsis of X-ry struture nlysis sine its ineption.25 Espeilly for PCN, whose X-ry diffrtion ptterns do not llow n dvned struture nlysis, the IAM offers n estblished tool in order to determine unit ell for PCN. We hve lso nlyzed the influene of tomi disple- ment ftors on the diffrtion ptterns of PCN. For tht purpose, we hve used unit ell for flt, A-A-stked PCN nd implemented n isotropi displement ftor of 0.04 A 2 for eh tom, ording to the melem rystl investigted by Ju rgens et l. nd A. Sttler et l.9,11 The orresponding findings re slightly vried totl reflex intensities nd intensity rtios. For exmple, the (001)reflex intensity is deresed by bout 15 % nd the sum intensity of (210) nd (2 10) by bout 4 %. This results in modified intensity rtio of (I[(210)] + I[(2 10)]) /I[(001)] from pproximtely 0.41 for disbled displement ftors (figure 2) to 0.46 for enbled displement ftors (not shown). The proposed unit ell for PCN nd its modifitions do not llow inversion symmetry. However, Lotsh et l. desribe melons in-plne symmetry with the plne group p2gg, whih inludes 2-fold rottions, glide-refletions nd trnsltions.10 In order to onserve the 2-fold rottionl symmetry within the melon sheets nd thus inversion symmetry within the PCN unit ells, the following proedure my be pplied: One ould onstrut PCN unit ell with the disussed ell prmeters, but only inlude hlf of the toms per unit ell, suh tht n inversion mps the missing toms to the pproprite tomi positions. The orresponding spe group, whih llows this symmetry opertion, is the spe group P 1. The theoretil diffrtion ptterns for inversionsymmetri PCN (spe group P 1 ) re very similr to those of PCN without inversion symmetry (spe group P 1). For exmple, the sum intensity of the (210) nd (2 10)-reflex is deresed by bout 4 % for n inversion symmetry in flt, A-A-stked PCN, ompred to the non-inversion-symmetri se, while the diffrtion ngle stys unhnged. We propose trilini rystl lttie for PCN, sine this is the only rystl system for whih 6= b 6= nd γ 6= 90 whih is neessry for the desription of the disussed proess-temperture-indued modifitions in PCN. Vrying α nd β leds to shifted (001) - nd (210),(2 10)-reflexes in the diffrtion ptterns towrds higher diffrtion ngles nd modified intensities. For α = 85 or α = 95 nd untouhed residul lttie prmeters, the (210) - nd (2 10)-reflexes re displed by bout 2Θ = 0.02, wheres the (001)-reflex is displed by bout 2Θ = For β = 85 or β = 95 the (210) - nd (2 10)-reflexes re displed by bout 2Θ = 0.03, wheres the (001)-reflex is displed by bout 2Θ = Seyfrth et l. hve disussed some possible rrngements of stked melon sheets bsed on theoretil lultions. Their findings led to signifint reltive displements.27 Implementing these displements into the unit ell for flt PCN results in theoretil diffrtion ptterns tht re ll not suffiient to reonstrut the mesured ones (see supporting informtion). The only treble hint for grphiti rbon nitride gc3 N4 within the synthesized PCN is the wek but omnipresent pek round 2Θ = 44 in the mesured diffr-

6 6 tion ptterns (figure 1). This one n be relted to the (200)-reflex of trizine-bsed g-c 3 N 4 or to the (300)- reflex of heptzine-bsed g-c 3 N 4 (ppendix). Thus, inor se, g-c 3 N 4 is only minor outome of the therml polyondenstion of diyndimide. This is lso onsistent with the reported hydrogen ontent of 1 2 wt% in the synthesized PCN, sine the theoretil vlue for its hydrogen ontent is expeted to be 1.27 wt%. 4,21 One finds synthesis produts for proess tempertures below 510 C or bove 610 C whose mesured diffrtion ptters devite signifintly from the ones of PCN. 4,5,21 This is minly used by onsiderble mount of nonpolymerized melem within the synthesis produts for proess tempertures below 510 C nd degrdtion of PCN for proess tempertures bove 610 C. 4,9,11 The proess-temperture dependene of the photophysis of PCN n be extended to tempertures from bout 400 C to 610 C. The reson for tht is the building unit C 6 N 10 H x, melem in the non-polymerized nd heptzine in the polymerized stte, whih is omnipresent in tht temperture rnge nd in whih the photophysis is found to tke ple. 6,9 These units re more densely pked for higher proess tempertures, thus enhning the eletroni intertion between them whih influenes PCN s optil properties. 6 Furthermore, we suggest the spe group P 6(174) for heptzine-bsed g-c 3 N 4. From rystllogrphi point of view, this is the only hexgonl spe group tht inludes the symmetry opertions for whih the sum of Wykoff multipliities is identil to the generted toms per heptzine-bsed g-c 3 N 4 unit ell. A-A-stked s well s A-B-stked heptzine-bsed g-c 3 N 4 is thinkble (see supporting informtion). To onlude our nlysis bout the rystl struture of PCN nd the ssoited disussion, we sum up our findings s follows. We expet PCN to minly onsist of bukled, A-A-stked PCN nd/or flt, A-A-stked PCN with proess-temperture-dependent unit ell prmeters nd γ. For these rystl strutures, the strongest ordne between the mesured diffrtion ptterns of PCN nd the orresponding theoretil diffrtion ptterns n be found. Nevertheless, due to the speifi hrter of the mesured diffrtion ptterns with its brod peks, we n only drw qulittive piture. In tht piture, we n not expliitely exlude ny of the disussed potentil forms of PCN. We rther ssume the PCN to be onglomerte of mny rystllites with slightly different rystl strutures nd/or defets, in whih bukled nd flt, A-A-stked PCN is the dominnt phse. The dimeters of the rystllites re pproximtely 15 nm, estimted with the Sherrer eqution on the bsis of the reflexes (210) nd (001) of flt PCN. 28,29 We lso expet n morphous ontent in the PCN minly resulting in non-speifi diffrtion intensity between the two mjor peks. Furthermore, PCN seems to be the result of severl other synthesis routes. 30,31 We hve shown tht trilini rystl struture (P 1 or P 1) with = 16.2 Å, b = 12.1 Å, 3.26 Å Å, α = β = 90 nd 87 γ 90 bsed on bukled nd flt melon sheets n explin ll mjor fetures of the diffrtion ptterns of PCN. The strong mesured pek t 13.2 is identified to be superposition of the reflexes ( 210) nd (210), the mesured pek t 17.9 is ssigned to superposition of the reflexes ( 310) nd (310), while the mesured pek t 27.2 n be relted to the reflexes (001) or (002). Different bukling nd stking motifs re probble rystllogrphi devitions tht re inluded within the frmework of trilini PCN. An inresing proess temperture leds to more dense pking long the -xis nd lso to distortion of the melon sheets. The unit ell ngle γ is redued to vlue of bout 87, leding to non-overlpping ( 210),(210) nd ( 310),(310) reflexes for high proess tempertures, nd thus to brodening of the respetive peks in the mesured XRD ptterns. We hve ssumed hexgonl unit ell P 6(174) for heptzine-bsed g-c 3 N 4 nd illustrted tht the heptzine-bsed s well s the trizine-bsed g- C 3 N 4 n only be minor synthesis outome of the therml polyondenstion of diyndimide or relted retnts. ACKNOWLEDGMENTS The uthors would like to thnk F. Zmponi, M. Wörner nd T. Elsässer from the Mx-Born-Institute für Nihtlinere Optik und Kurzzeitspektroskopie for professionl dvies nd the oppurtunity to finish this work. Finnil support by the Germn Bundesministerium für Bildung und Forshung (exellene luster projet Light2Hydrogen, grnt No. 03IS2071F) is grtefully knowledged. 1 X. C. Wng, K. Med, A. Thoms, K. Tknbe, G. Xin, J. M. Crlsson, K. Domen, nd M. Antonietti, Nture Mterils 8, 76 (2009). 2 F. Yng, M. Lublow, S. Orthmnn, C. Mershjnn, T. Tyborski, M. Rusu, S. Kubl, A. Thoms, R. Arrigo, M. Häveker, nd T. Shedel-Niedrig, ChemSusChem 5, 1227 (2012). 3 F. Yng, V. Kuznietsov, M. Lublow, C. Mershjnn, A. Steigert, J. Kler, A. Thoms, nd T. Shedel-Niedrig, J. 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