Internationale Ausgabe: DOI: 10.1002/anie.201706870Photocatalysis Hot PaperDeutsche Ausgabe: DOI: 10.1002/ange.201706870

Optimizing Optical Absorption, Exciton Dissociation, and ChargeTransfer of a Polymeric Carbon Nitride with Ultrahigh Solar HydrogenProduction ActivityGuigang Zhang,* Guosheng Li, Zhi-An Lan, Lihua Lin, Aleksandr Savateev, Tobias Heil,Spiros Zafeiratos, Xinchen Wang,* and Markus Antonietti*

Abstract: Polymeric or organic semiconductors are promisingcandidates for photocatalysis but mostly only show moderateactivity owing to strongly bound excitons and insufficientoptical absorption. Herein, we report a facile bottom-upstrategy to improve the activity of a carbon nitride to a levelin which a majority of photons are really used to drivephotoredox chemistry. Co-condensation of urea and oxamidefollowed by post-calcination in molten salt is shown to result inhighly crystalline species with a maximum p–p layer stackingdistance of heptazine units of 0.292 nm, which improves lateralcharge transport and interlayer exciton dissociation. Theaddition of oxamide decreases the optical band gap from2.74 to 2.56 eV, which enables efficient photochemistry alsowith green light. The apparent quantum yield (AQY) for H2

evolution of optimal samples reaches 57 % and 10 % at 420 nmand 525 nm, respectively, which is significantly higher than inmost previous experiments.

Polymeric photocatalysts, for example, melon-based carbonnitrides (CNs), with their optical and electronic propertiesbeing feasibly tuned by controlling the polymerizationprocess, have recently found fascinating applications in thephotocatalytic production of H2 from water.[1–7] Obtainingsufficient solar-to-energy efficiency calls for the fast gener-ation of photo-excited charge carriers, that is, the hotelectrons and holes, which are involved in the subsequentwater splitting redox reactions. However, like most conju-gated polymers, which are normally restricted by the intrinsicstrong Coulomb interactions of singlet Frenkel excitons, CNsalso bear sluggish exciton dissociation, indicating a highexciton binding energy (Eb). This is why bulk CN in mostcases only presents moderate photoactivities. Plenty of

strategies have been developed to improve the photocatalyticactivity of CN in the past few years,[8–17] always progressing tomore efficient systems.

The layer stacking distance is theoretically predicted todominate the interlayer exciton dissociation and thus chargemobility.[18–20] In comparison with graphite, the p–p stackingdistance in disordered melon is typically 0.326 nm, 5%shorter than in crystalline graphite. Note that shorterdistances in chemistry can usually be attributed to additionalbonding schemes, in this case presumably intense polarizationand/or charge transfer (CT) interactions between two layers,that is, partial positive and negative charges on neighboringlayers enable tighter packing and promote charge and energytransport within the stacks. Indeed, the stacking distance wasalready varied in the range of 0.1 c by optimization of thepreparation process,[21] such as increasing the condensationtemperature. For instance, when increasing the polymeri-zation temperature from 773 K to 823 K, a slightly decreasedstacking distance (from 0.326 to 0.323 nm) could beobserved.[22] Breakthroughs in the stacking distance areexpected to revolve around the rational modification of thesynthetic strategy. Actually, the stacking distance was com-pressed from 0.326 to 0.32 nm by performing the polymeri-zation process in molten salt to accelerate the sluggishdeamination process, resulting in potassium salt formationat the same time.[23] Interestingly, the resulting materials turnout to be very effective for photocatalytic H2 evolution. Thisprovides a sought-for alternative way to improve the poly-condensation process by “cooking” suitable starting materialsin eutectic salt mixtures with a lower melting point to turn atleast parts of a solid-state condensation process into solutionchemistry.

Beside interlayer exciton splitting, intralayer processescan also contribute to better charge handling, mainly bylocating domains with different electron affinity in the plane.In organic polymer photovoltaics, this is known under theterm donor–acceptor (D–A) polymers.[24] Also in 2D-modi-fied melon, strategies such as doping[25–27] or copolymeriza-tion[28–30] could create such 2D D–A structures with signifi-cantly increased charge carrier yield. In addition, such D–A-modification shifts the optical absorption into the red regionand sometimes even allows photochemical reactions in thisspectral region.

The current proof-of-concept demonstration can be seenschematically in Scheme 1 and the Supporting Information,Figure S1. We found out that by co-condensation of simpleurea with oxamide (OA) in molten salt mixtures (KCl/LiCl),

[*] Dr. G. Zhang, Dr. A. Savateev, Dr. T. Heil, Prof. M. AntoniettiDepartment of Colloid ChemistryMax Planck Institute of Colloids and Interfaces14476 Potsdam (Germany)E-mail: guigang.zhang@mpikg.mpg.de

markus.antonietti@mpikg.mpg.de

G. Li, Z. Lan, L. Lin, Prof. X. WangState Key Laboratory of Photocatalysis on Energy and Environment,College of Chemistry, Fuzhou UniversityFuzhou, 350002 (China)E-mail: xcwang@fzu.edu.cn

Dr. S. ZafeiratosICPEES, Institut de Chimie et des Proc8d8s pour l’Energie, l’Envi-ronnement et la Sant8, UMR 7515 CNRS/Universit8 de Strasbourg25 rue Becquerel, 67087 Strasbourg cedex (France)

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this issue could be easily attacked. In this system, we founddiverse packing motifs, with which the interlayer stackingdistance could be reduced from 0.326 to 0.292 nm. Moreover,the visible light absorption threshold of the as-preparedcopolymers could be extended to wavelengths as long as650 nm, which is shown to also make those photons accessiblefor photochemistry.

Typically, CN was prepared from urea, whereas CN-OAwas obtained from urea and oxamide. CN-m and CN-OA-mare the corresponding samples prepared in molten salts. Toexamine the layered-stacking mode of the as-synthesizedpolymers, powder XRD characterizations were first con-ducted. As shown in Figure 1a, all the samples showed twodistinct peaks, albeit with different peak location andintensity. Two identical peaks located at 13.088 and 27.388corresponding to (100) interplanar packing of heptazine unitsand (002) p–p interlayer stacking motif were recorded for CNand CN-OA.[31,32] The (100) peak was shifted from 13.088 to8.088, revealing an extended interplanar packing distance ofabout 0.81 nm for CN-OA-m compared with 0.618 nm forpristine CN. In contrast, the (002) peak was shifted up from27.488 to 28.288, reflecting the fact that the interlayer stackingdistance was compacted because of a strong van der Waalsattraction between the neighboring heptazine layers. HR-TEM further demonstrates the evident change in theinterlayer stacking distance. As shown in Figure 1b, we findfor some remarkably crystalline species a heptazine p–p layerstacking (002) of CN-OA-m with apparent interlayer dis-tances of 0.292 nm. As polyheptazine units are slightlycorrugated, this distance is to be taken with a grain of salt,but a corresponding image of pristine CN shows a stackingdistance of 0.326 nm. In this species, the in-plane arrangementdistance of nitrogen-linked heptazine units (100) was 0.96 nm,again much larger than that of pristine CN (0.618 nm). Theevident change in the stacking mode is most probablyascribed to well-condensed polymers with few structuraldefects and also to simultaneous K-salt formation, as evi-denced with the FTIR, XPS and elemental analyses discussedbelow.

FTIR spectroscopy shows that two new absorption bandsat 987 and 1605 cm@1 related to the symmetric and asymmet-ric vibration of NC2 bonds in metal–NC2 groups are revealedfor CN-m and CN-OA-m (Figure S1),[33] indicating thepresence of K-NC2 groups. C, N, K, and O were examined

by XPS analysis of the as-prepared CN-OA-m (SupportingInformation, Figure S2). Residual K is derived from moltensalt and no chlorine is observed, consistent with previousstudies.[23, 34] Compared with pristine CN, CN-OA-m exhibitsquite similar C1s and N1s spectra, implying the sp2 hybridizedheptazine units were hardly changed after the modification inthe stacking geometry. The additional weak peak of N1s at396.7 eV is attributed to the negatively charged C@N@@Cgroup, which could neutralize the positive K+ (Figure S2 c).Note that the metal location position may be differentdepending on the synthesis strategies.[34b,c] EELS spectra(Supporting Information, Figure S3) reveal that both CN andCN-OA-m are composed of sp2-hybridized carbon and nitro-gen atoms, as seen by the presence of a well-developed 1s!p* transition for both carbon and nitrogen.[35] The structuralrefinement in the molten salt also modifies the texture of thepolymer crystals (Supporting Information, Figures S4 andS5). SEM elemental mapping (Supporting Information, Fig-ure S6) confirms the homogeneous distribution of C, N, and Kinside the material (Supporting Information, Table S1). TheC/N molar ratio of CN-OA-m (0.72) is slightly higher thanthat of pristine CN (0.71). The specific surface area of CN-OA-m (85 m2 g@1) only weakly increased compared to that ofpristine CN (63 m2 g@1).

The second relevant change lies in the modification of theoptical properties. As shown in Figure 2a, the salt treatedsamples, for example, CN-OA-m, show a notable enhancedoptical absorption in both UV and visible light regions. Theintrinsic absorption band originates from the p!p* electron

Scheme 1. Proposed interlayer charge carrier transfer of (left) pristineCN (d= 0.326 nm) and (right) optimized CN-OA-m with shortenedlayer stacking distance (d = 0.316 nm). Figure 1. a) Powder XRD patterns of CN, CN-OA, CN-m, and CN-OA-

m. b) HR-TEM image of CN-OA-m.

Figure 2. a) UV/Vis absorption spectra of CN, CN-M, CN-OA, and CN-OA-m. b) Mott–Schottky plots of CN and CN-OA-m electrodes.

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transition of the sp2 hybridization of C and N in the CNframework is clearly red-shifted to the visible light region byabout 0.2 eV, and the band gap is decreased from 2.74 to2.56 eV. We attribute the reinforced p!p* electron transitionin the conjugated aromatic ring system to the tighter andbetter packing of the joint heptazine system.[34] Moreover, anobvious new absorption band forms in the region from 462 nmto 700 nm. This new band is usually ascribed to the n!p*electron transition involving the lone pairs of the edgenitrogen atoms in the heptazine units.[31] It is noteworthythat this electron transition is forbidden for perfectly sym-metric and planar units, but the present structure is chargedand polarized, and the shortened stacking distance betweentwo layers allows access to this electron transition of CN-OA-m with an only slightly diminished resonator strengthcompared to the p!p* transition. The creation of newabsorption bands can potentially be utilized for photochem-istry with visible light with wavelengths longer than 500 nm,however, then leaving a differently localized electron hole inthe chemical structure. Mott–Schottky curves (Figure 2b andthe Supporting Information, Figure S7) revealed that therelative conduction band minimum (CBM) of CN-OA-m was@1.6 V, very similar to that of the pristine CN (@1.4 V),consistent with the UPS results (Supporting Information,Figure S8). These positions thermodynamically enable therapid proton reduction for H2 evolution.

The excitonic processes of the samples were monitored bysteady-state photoluminescence (PL) measurements. In Fig-ure 3a, CN-OA-m exhibits an obviously decreased emissionintensity with respect to pristine CN. Time-resolved fluores-cence spectra monitored at the corresponding emission peaksgive the mean radiative lifetimes (At) of the recombiningcharge carriers (Figure 3a, inset). The At of CN, CN-m, CN-OA, and CN-OA-m were 15.37, 8.07, 11.31, and 5.65 ns,respectively (for lifetime components see the SupportingInformation, Table S2). As this decreased singlet excitonlifetime of the optimized samples obviously implies enhancedexciton dissociation, only the very fast charge pairs canrecombine. It is most likely that shortened layer distance inthe crystalline polymers accelerates the charge transfer over

the layers, promoting the singlet excitons to dissociate(tunneling of electrons and holes is usually very differentbecause of the different orbital symmetries), then alsopromotes the fast charge carrier migration from bulk tointerface without recombination. The improved charge trans-port can be very classically confirmed by the decreasedhemicycle radius measured by using electrochemical impe-dance spectroscopy (EIS) and the increased photocurrent,reflecting a lower electric resistance of the material as shownin Figure 3b.

To check the influence of this optimization on photo-catalysis in general, photocatalytic H2 evolution was con-ducted for all as-prepared samples. Optimizations of thepolymerization temperature are included in the SupportingInformation, Figure S9. In the first place, we examined thephotocatalytic activities by using a white LED as the lightsource. Figure 4a shows that both CN-m and CN-OA-mpossess dramatically enhanced H2 evolution activities incomparison with pristine CN. In particular, CN-m exhibitsan ultra-high H2 evolution activity (150 mmolh@1), which iscirca 30 times higher than that of CN (5 mmolh@1). CN-OA-mshows a slightly lower activity (84 mmolh@1) than CN-m butstill much higher (ca. 17 times) than that of CN.

It is exciting to observe that CN-m (509 mmolh@1) and CN-OA-m (210 mmolh@1) show ultrahigh activity when we usecolloid chemistry to improve local electric fields at the surfaceby ion adsorption in the Helmholtz layer and by performingthe same reactions in modelled sea water (3 wt.% NaCl).Interestingly, this improvement only works for the highlycrystalline samples, while no obvious improvement was foundfor CN and CN-OA. When the same amounts of LiCl or KClwere used instead of NaCl, a very similar H2 evolution rate(Supporting Information, Figure S10) was obtained for CN-munder the same conditions. Other simple salts, such asK2HPO4 (described as an electron mediator),[9] KBr, andNa2SO4, also promote the H2 evolution, however, theactivities are lower than that when using chlorides. Regardingthe standard redox potential of chlorine (ca. 1.36 V), oxida-tion of Cl@ is clearly within the reactivity of the hole of thephotocatalysts. However, no free chlorine could be detected,that is, the buffering species (for example, oxychloride orhypochloride) stays surface bound.

Figure 3. a) Room temperature (298 K) steady-state PL spectra. Inset:Time-resolved fluorescence kinetics monitored at the correspondingemission peaks of CN, CN-OA, CN-m, and CN-OA-m. b) Electrochem-ical impedance spectroscopy (EIS) Nyquist plots under visible lightirradiation. Inset: Transient photocurrent conducted with a bias poten-tial of @0.2 V of CN and CN-OA-m.

Figure 4. Photocatalytic H2 evolution activities of the as-preparedsamples under a) white and b) green LED light irradiation.

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The advantage of the optimized CN-OA-m was thesignificantly enhanced light absorption above 500 nm, andthat is why we further examine the photocatalytic activitieswith longer wavelengths (Supporting Information, Fig-ure S11). When performing photocatalytic H2 evolution witha green LED (l = 525 nm; Figure 4b), CN and CN-m arebarely active in this wavelength region. In contrast, CN-OAand CN-OA-m express very unusual H2 evolution activitieswith green LED illumination. Having such an extra in-planeoptical absorption does not necessarily translate into photo-chemical activity, as the generated hole has in our case a loweroxidation power (note that, as discussed above, that thereduction power of the electron is similar) and a differentlocation on the material to react from. However, CN-OA-mpresents a comparably high activity (17 mmolh@1), nicely seenin comparison with other counterparts, demonstrating thatthis material is indeed active in the green region. Only few ofconjugated polymers have been reported to be active in thiswavelength.[35] The apparent quantum yield (AQY) of theoptimized samples in the presence of NaCl is observed toreach 57% and 10 % at 420 and 525 nm, respectively, which isindeed much higher than most of the current polymericphotocatalysts. This is probably because the chlorine that istightly bound in the Helmholtz layer around the particles actsas electron donor, thereby accelerating the reaction to therate of the reduction side, which is however very fast. TEOAunder those conditions is still needed to close the catalyticcycle, but the reaction takes place in an accelerated fashionand thus significantly promotes the photocatalytic activi-ties.[36]

Moreover, this optimized polymeric photocatalyst showsa robust recyclability and an absence of light and solutioncorrosion. No evident decay in the activity and no structuraldifference could be detected (Supporting Information, Figur-es S12 and 13) after four runs of continuous reaction, againillustrating the extraordinary stability for sustainable appli-cations. The modification in the charge carrier transfer couldalso benefit the photocatalytic water oxidation process(Supporting Information, Figure S14).

In summary, a facile strategy has been presented to tailorthe texture and electronic band structure of polymeric carbonnitrides. Owing to an obviously reduced interlayer stackingdistance and extended spectral absorption, more photons canbe turned into dissociated, reactive surface charges, asreflected by a dramatically improved visible light H2 evolu-tion activity, especially in the spectral region above 500 nm.This study highlights the synergistic optimization of crystalstructure and D–A-copolymerization to significantly advancethe exciton dissociation and hot charge carrier yield. It opensa new avenue to optimize the physicochemical properties ofpolymeric light collectors, which offers many opportunitiesfor advanced photochemistry in general and their sustainableutilizations.

Acknowledgements

G.G.Z. thanks the Alexander von Humboldt Foundation fora postdoctoral fellowship. This work was financially sup-

ported by Max Planck Society and the National BasicResearch Program of China (2013CB632405), the NationalNatural Science Foundation of China (21425309 and21761132002), and the 111 Project.

Conflict of interest

The authors declare no conflict of interest.

Keywords: carbon nitride · exciton dissociation · H2 production ·van der Waals stacking · water splitting

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Manuscript received: July 6, 2017Revised manuscript received: August 17, 2017Accepted manuscript online: August 28, 2017Version of record online: September 18, 2017

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