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Novel carbon nit

aMax-Planck Institute of Colloids and Inter

Research Campus Golm, 14424 Potsdam

mpikg.mpg.debUniversity of Applied Science Zittau/Gorlitz

Germany

† Electronic supplementary information (Espectra of CsCl/ZnCl2–C3N4, schemes of hespectra, EA and surface areas of productsheating rates, holding times and ratiosmaterials made in pure ZnCl2; diffuse rand C3N4/ZnO composites, CO2 and Nisotherms and predictions of gas uptakedegradation data. See DOI: 10.1039/c4ra0

Cite this: RSC Adv., 2014, 4, 40803

Received 6th August 2014Accepted 14th August 2014

DOI: 10.1039/c4ra08236b

www.rsc.org/advances

This journal is © The Royal Society of C

ride composites with improvedvisible light absorption synthesized in ZnCl2-basedsalt melts†

Christian Fettkenhauer,a Jens Weber,b Markus Antoniettia and Dariya Dontsova*a

Poly(triazine imide)-based carbon nitride materials with BET surface areas up to 200 m2 g�1 were

synthesized in ZnCl2 containing salt melts without the use of hard templates. We found that the

composition, structural order, optical properties and morphology of the products can be adjusted by

careful selection of synthesis parameters. The nature of the salt eutectic and precursor concentration in

the melt have an especially large influence, with ZnCl2 being a reactive solvent. This novel synthesis

route provides access to easily processable materials with improved optical absorption in the visible

range that can be used as composite photocatalysts, CO2 adsorbents or nanocomposite fillers.

Introduction

Graphitic carbon nitride materials are light element composedpolymeric semiconductors which have recently foundnumerous applications as photo-1 and electrocatalysts,2 oxida-tion catalysts,3 catalyst supports and nanocomposite llers,4

and are reported to be promising even for photovoltaic appli-cations.5 For most applications, carbon nitrides with a specialmorphology and relatively high surface areas are needed.Typically, this is achieved by using various silica templates,which need to be removed aer the synthesis, mostly by reac-tion with an HF source.6 The latter step however requiresadditional safety precautions and put severe restrictions onscalability of carbon nitrides production. Alternative methodsto adjust morphology and surface areas of nal productsinclude, for example, supramolecular preorganization ofmonomers,7 “so-templating”8 and solvothermal synthesis.9

Besides, C3N4-related materials such as crystalline poly(triazineimides) (PTI) that have well-dened morphologies and possessincreased surface areas can be prepared using LiX/KX salt melts(X ¼ Cl, Br) as reaction media.10

faces, Department of Colloid Chemistry,

, Germany. E-mail: dariya.dontsova@

, Department of Chemistry, 02763 Zittau,

SI) available: O 1s, Zn 3p and Cl 2p XPSating procedures; WAXS patterns, FTIRsynthesized at different temperatures,

; WAXS patterns and FTIR spectra ofeectance spectra of NaCl/ZnCl2–C3N4

2 adsorption–desorption experimentalof CO2 and N2 at 303 K, Rhodamine B8236b

hemistry 2014

In general, molten salt(s) can serve as a solvent for high-temperature materials synthesis, as catalyst (for example, ZnCl2in trimerization reactions) but also as “so template” fortailoring micro- and mesoporosity of the resulting products.Previously, ZnCl2 and ZnCl2-containing melts were successfullyused for the synthesis of porous covalent triazine-basedframeworks11 and porous functional carbons.12

In this contribution, we further explore the utility of salt meltsynthesis for the preparation of carbon nitrides, with the mainfocus on increasing the surface areas of products, as well as oncontrolling their structure and morphology by varying thesynthesis parameters. We report on the preparation of newcarbon nitride hybrid materials using zinc chloride containingeutectic mixtures, their characterization by means of X-raydiffraction (XRD), elemental analysis (EA), X-ray photoelectronspectroscopy (XPS), scanning electron microscopy (SEM), FTIR-spectroscopy and gas adsorption, and discuss potentialapplications.

Experimental sectionChemicals and materials

Lithium chloride (99%), sodium chloride (99.5%), potassiumchloride (99%), cesium chloride (99%) and Rhodamine B (95%)were purchased from Sigma Aldrich. Zinc chloride (98%) waspurchased from Alfa Aesar, melamine (99%) from AcrosOrganics and dicyandiamide (98%) from Merck. All the chem-icals were used without further purication.

Synthesis procedure

Salts and melamine were ground together in a glovebox(mBraun Unilab, O2 < 0.1 ppm, H2O < 0.1 ppm) under argonatmosphere according to the eutectic compositions (Table 1).

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Table 1 Composition and melting points of eutectic salt mixtures used in this paper

Salt mixture A/B(/C)A content,wt%

B content,wt%

C content,wt%

Molar contentof ZnCl2

Melting temperature,�C (ref. 17)

ZnCl2 100 — — 1.00 318LiCl/ZnCl2 8.5 91.5 — 0.770 294NaCl/ZnCl2 23.7 76.3 — 0.580 270KCl/ZnCl2 36.3 63.7 — 0.490 230CsCl/ZnCl2 48.3 51.7 — 0.575 263NaCl/KCl/ZnCl2 10.7 13.8 75.5 0.600 203

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Reaction mixtures (�5–10 g) were transferred into porcelaincrucibles and covered with lids. Crucibles were placed in ovenand heated under constant nitrogen ow (15mLmin�1, SchemeS1a†). The crude products were removed from the crucibles andwashed rst with ethanol (50–100 mL), then with deionizedwater (50–100 mL) and nally with 1 M HCl solution (50–100mL). Each step was carried out for 24 hours. Final products wereisolated by ltration, thoroughly washed with deionized water(500 mL) and dried in a vacuum oven at 50 �C for 15 h.

Reference g-C3N4 was prepared by heating DCDA with theramp of 2.3 �C min�1 up to 550 �C and subsequent holding atthis temperature for 4 h under constant nitrogen ow (15 mLmin�1) in a covered crucible. The nal product was thoroughlyground.

Dye photodegradation experiments

For photodegradation experiments Rhodamine B (RhB) wasused as amodel dye. 5 mg of a catalyst were dispersed in 5mL ofRhB solution in deionized water (10 mg L�1) in a cuvette. Thesuspension was agitated in dark for 30minutes in order to reachdark-adsorption equilibrium, and then illuminated with a LEDmodule emitting at 420 nm (12 W, OSA Opto Lights) from thexed distance of 5 cm. The RhB degradation was monitored asthe decrease of its concentration over irradiation time using UV/Vis spectrophotometer. For this purpose, aer a certain irradi-ation time t, an aliquot of 300 mL of reaction mixture was taken,diluted with 1.7 mL of deionized water, kept for 30 minutes indark to allow for catalyst precipitation, then UV-Vis absorptionspectrum was measured. For calculation of the rate constants,dye absorbance (at 550 nm) aer dark equilibration (t ¼ 0 min),A0, and at t ¼ 60 min, A, were used. Rate constants k werecalculated as follows:

k ¼ ln (A/A0) � (60 min)�1

Characterization

Powder X-ray diffraction patterns were measured on a BrukerD8 Advance diffractometer with CuKa radiation (l ¼ 0.15148nm) equipped with a scintillation counter detector applying astep size 0.05� 2q and counting time of 3 s per step. FT-IRspectra were recorded on a Varian1000 FT-IR spectrometerequipped with an attenuated total reection unit with diamondapplying a resolution of 4 cm�1. Nitrogen adsorption–

40804 | RSC Adv., 2014, 4, 40803–40811

desorption measurements were performed aer degassing thesamples at 150 �C for 20 hours using a Quantachrome Quad-rasorb SI-MP porosimeter at 77.4 K. CO2 and N2 adsorption–desorption isotherms at 273, 283 and 303 K were conducted ona Quantachrome Autosorb-1MP instrument aer prior degass-ing. The specic surface areas were calculated by applying theBrunauer–Emmett–Teller (BET) model to adsorption isotherms(N2 at 77.4 K) for 0.05 < p/p0 < 0.3 using the QuadraWin 5.05soware package. Elemental analysis was accomplished ascombustion analysis using a Vario Micro device. SEM imageswere obtained on a LEO 1550-Gemini microscope. Opticalabsorbance spectra of powders were measured on a ShimadzuUV 2600 equipped with an integrating sphere. The absorptionspectra of RhB solutions were recorded on a T70 UV/VIS spec-trophotometer (PG instruments Ltd.). The emission spectrawere recorded on LS-50B, Perkin Elmer instrument. The exci-tation wavelength was 300 nm. EDX investigations were con-ducted on a Link ISIS-300 system (Oxford Microanalysis Group)equipped with a Si(Li) detector and an energy resolution of 133eV. X-ray photoelectron spectroscopy (XPS) was performed on aMultilab 2000 (Thermo) spectrometer equipped with Al Kaanode (hn ¼ 1486.6 eV). All spectra were referenced to the C 1speak of adventitious carbon at 285.0 eV. For quanticationpurposes, survey at a pass energy of 50 eV and high-resolutionspectra at pass energy of 20 eV were recorded and analyzed byXPS Peak 4.1 soware (written by Raymund Kwok). The spectrawere decomposed assuming line shapes as sum functions ofGaussian (80%) and Lorentzian (20%) functions. Raw areasdetermined aer subtraction of a Shirley background13 werecorrected according to following sensitivity factors14 (C 1s: 0.25;N 1s: 0.42; O 1s: 0.66; Cl 2p: 0.73; Zn 3p: 0.75). Etching Ar+

bombardment was performed at 2 kV and 18 mA of ioniccurrent.

Results and discussionGeneral considerations

The synthesis of graphitic carbon nitride (g-C3N4) usually startsfrom dicyandiamide (DCDA) and proceeds by a number ofsequential condensation processes that occur between �230and 550 �C (Scheme 1a). We selected zinc chloride as a maincomponent of the salt melt due to the two following reasons.First of all, ZnCl2 is a moderate-strength Lewis acid and wasexpected to solubilize the precursor and at least some of thecondensation intermediates owing to strong Lewis acid–base

This journal is © The Royal Society of Chemistry 2014

Scheme 1 (a) Idealized condensation scheme of dicyandiamide(DCDA) to graphitic carbon nitride; (b) idealized structure of poly(-triazine imide)/Li+Cl�.16

Fig. 1 (a) WAXS patterns and (b) FTIR spectra of bulk-C3N4 andproducts synthesized in salt melts containing ZnCl2. Vertical lineindicates 1350 cm�1 position.

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interactions. Besides, it has a low melting temperature that caneven be lowered to Tm < 300 �C in eutectic mixtures with alkalimetal chlorides (see Table 1). On the other hand, melamine waschosen as a carbon nitride precursor due to the fact that itscondensation temperature towards melam is estimated to beroughly 335 �C (ref. 15) (Scheme 1a). That way, we hope toensure melting of the solvent before the onset of the melaminecondensation reaction.

Initial investigations

Initial investigations were conducted at a xed precursor to saltmixture ratio (1 : 5 by weight), while the composition of the saltmixture was varied by choosing different alkali metal chlorides.

The synthesis and work-up procedure yielded colored prod-ucts, which were analyzed by EA, EDX and XPS with regard totheir composition (Table 2). The C/N weight ratios in theproducts range from 0.59 to 0.61 and are slightly higher thanthe value for reference g-C3N4 (0.58) but slightly lower than thestoichiometric value for C3N4 (0.64). This might be explained byincomplete amide condensation and the corresponding pres-ence of terminal NH/NH2-groups. The sum of C, N, and H as

Table 2 Composition of materials synthesized in different ZnCl2 contain

Melt C, wt% N, wt% H, wt% C/N

Ref. bulk C3N4 35.0 60.1 2.06 0.58LiCl/ZnCl2 30.6 50.8 2.70 0.60NaCl/ZnCl2 29.6 49.1 3.04 0.60KCl/ZnCl2 30.1 49.2 3.06 0.61CsCl/ZnCl2 30.2 49.8 3.13 0.59NaCl/KCl/ZnCl2 29.7 50.3 3.13 0.59Pure ZnCl2 31.5 52.5 2.86 0.60

This journal is © The Royal Society of Chemistry 2014

obtained from EA is typically lower than 100%, usually around82–87%. The remaining 13–18 wt% are Zn (3–5%), O (10–15%)and Cl (1–3%) ions as analyzed by EDX spectroscopy and XPS forselected samples. Zinc could be removed completely by washingthe composites with 10 M HCl. This step is however accompa-nied by additional protonation of the product surface18 and washence not employed in the default recipe. The presence ofoxygen in products is partially the result of the aqueous work-upof the reaction mixtures but also due to the strong adsorption ofwater and CO2 under ambient conditions.

XRD and FTIR spectroscopy investigations showed that thesynthesized materials have features typically observed for

ing salt melts (EA data), and calculated BET surface areas of products

, wt% 100 � (C + N + H) Color SBET, m2 g�1

2.8 Yellow 1015.9 Pale yellow 2018.3 Brown 19316.0 Yellow 4216.5 Yellow 6816.9 Brown 7113.1 Pale yellow 58

RSC Adv., 2014, 4, 40803–40811 | 40805

Fig. 2 (a) C 1s and (b) N 1s XPS spectra of carbon nitride synthesized inCsCl/ZnCl2.

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previously described carbon nitride polymers, but differ fromeach other with respect to the degree of structural order. Fig. 1aillustrates that products prepared in NaCl/ZnCl2, CsCl/ZnCl2and NaCl/KCl/ZnCl2 eutectics are mostly X-ray amorphous asthey show only broad halos at 2q � 15� and 27�, LiCl/ZnCl2-derived material shows quite dened reections correspondingto higher crystallinity, while the product obtained from pureZnCl2 is characterized by outstanding structural order. Themain 2q reection around 27� is present in the diffractogramsof all materials and corresponds to interplanar stacking ofcarbon nitride layers. Another reection observed for most ofthe materials (except for NaCl/KCl/ZnCl2 and NaCl/ZnCl2eutectics) at 2q � 12–15� is related to in-plane structuralpacking motifs. The appearance of XRD patterns of productsprepared in ZnCl2 and LiCl/ZnCl2 suggests that these materialsare poly(triazine imide)-like as corroborated by recent investi-gations of acid washed poly(triazine imides) obtained fromLiCl/KCl.19 KCl/ZnCl2-derived material seems to be composed oftwo phases characterized by different crystallinity.

In principle, crystallinity of carbon nitrides prepared indifferent ZnCl2-containing melts increases in the row NaCl/KCl/ZnCl2 � NaCl/ZnCl2 < CsCl/ZnCl2 < KCl/ZnCl2 < LiCl/ZnCl2 <bulk g-C3N4 < ZnCl2, and therefore can be tuned by varying thenature of the salt eutectic used for the synthesis.

Fig. 1b shows that the main IR absorption bands displayedby the synthesized materials are similar to those of reference g-C3N4. Namely, the large absorption band at 1200–1650 cm�1

corresponds to stretching vibrations of CN heterocycles, astrong vibration at �800 cm�1 is attributed to deformationvibrations of triazine or tri-s-triazine ring, and the broad bandbetween 2400 and 3400 cm�1 indicates stretching vibrations ofsurface hydroxyl groups (OH–) and terminal and residualamino-groups (NH2–, NH–). The presence of the distinctabsorption band at �1350 cm�1 in ZnCl2-, LiCl/ZnCl2- and KCl/ZnCl2-derived materials suggests that they are built of triazinerather than of tri-s-triazine rings20 that is in good agreementwith the XRD data. Besides, the amorphous nature of NaCl/ZnCl2, CsCl/ZnCl2 and NaCl/KCl/ZnCl2-derived products causesbroadening and subsequent overlap of the absorption bands,while improved structural order (products from pure ZnCl2,LiCl/ZnCl2 and KCl/ZnCl2) results in more dened vibrationpeaks.

Amorphous CsCl/ZnCl2–C3N4 sample was investigated byXPS in order to unravel the structure of the carbon nitridepolymer and intercalated Zn2+ species. The C 1s spectrum(Fig. 2a) shows a main carbon species with a binding energy of288.2 eV corresponding to CN3 bonds.21 In addition to the C 1speak of adventitious carbon at 285.0 eV, there is a weak peak at283.3 eV which results from carbide impurities in the sample.The deconvolution of the N 1s signal reveals three contributions(Fig. 2b): at 401.2 eV (NHx groups), 399.7 eV (C–N]C) and 397.6eV (deprotonated N-sites). The number of contributions as wellas their chemical shis matches the calculated values for N 1selectrons in PTI prepared in LiCl/KCl.22 The C/N ratio is thencalculated as a ratio of the area of the main carbon peak to thetotal area of nitrogen peaks. The value of 0.59 ts the one forpoly(triazine-imide) reported by Wirnhier et al.10a and differs

40806 | RSC Adv., 2014, 4, 40803–40811

from fully condensed heptazine-based C3N4 (0.64). Further-more, the ratio of amino-nitrogen atoms (NHx groups) to ringnitrogen atoms (C–N]C) calculated as a ratio of the corre-sponding peak areas, is equal to 0.49 and is close to the theo-retical value of 0.5 for the idealized structure of PTI((C3N3)2(NH)3). For heptazine-based structure this value wouldbe much lower because only uncondensed terminal amino-groups would contribute to the XPS spectrum and only weakpeaks of –NHx are reported for this case.21 All these ndingsprove that the carbon nitride component of the composite isbased on poly(triazine imide).

XPS investigations further conrmed the presence of O, Znand Cl in the sample. O 1s signal (Fig. S1a†) consists of threecontributions: at 529.3 eV (assigned to O–Zn bonds23), 531.5 eV(–OH groups)24 and 533.0 eV (adsorbed water).25 The ratiobetween these three contributions is 1 : 16 : 8. The presence ofZn–O bonds is further conrmed by Zn 3p signal (Fig. S1b†)which shows two main and almost equal contributions: at 88.4eV for Zn–O bonds26 and at 89.6 eV for Zn–Cl and Zn–OH bonds.The peak of Cl 2p (Fig. S1c†) at 198.1 eV, by-turn, indicates thepresence of Zn–Cl bonds. Basing on these ndings, we concludethat the intercalated Zn species are mainly zinc oxide and zincoxychloride. The latter one is the product of hydrolysis of ZnCl2during the work-up procedure. The qualitative differencesbetween surface and bulk of the product were analyzed bycomparing the XPS spectra before and aer Ar-ion

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bombardment of the sample which allows removing the surfacelayer of the material. The Cl 2p signal aer ion bombardmentreveals two peaks that correspond to different Zn–Cl bonds(Fig. S2a†). The contribution at 198.2 eV is due to zinc oxy-chloride while the one at 199.5 eV indicates the presence of zincchloride27 which didn't hydrolyze being entrapped in thestructure. The presence of ZnCl2 in the bulk of the product isalso supported by Zn 3p signal (Fig. S2b†) which shows now anadditional contribution at 91.4 eV due to Cl–Zn–Cl bonds. Theamount of zinc oxychloride in the bulk of the product is muchhigher than the amount of zinc oxide, though at the surfacetheir quantities were almost even. This nding together withthe fact that the amount of Zn in the bulk of the material is atleast 3 times higher than at the surface illustrates the difficultyto remove the intercalated Zn species ascribed to the diffusionlimitations. In contrast, the oxygen content in the bulk of theproduct is at least two times lower than at the surface that isexplained by the removal of surface hydroxyl groups andsurface-adsorbed water. The weight content of the elementscalculated from XPS data is in good agreement with the valuesobtained from EA, ICP and EDX measurements (Table S1†).

The morphology of the prepared materials was investigatedby SEM. It was found to be fairly homogeneous within each ofthe samples and variant depending on the nature of alkali metalchloride constituting the salt melt (Fig. 3). Carbon nitrides areobtained as crystalline densely-packed nanosheets of 100–200nm in diameter from pure ZnCl2 and LiCl/ZnCl2 eutectic melts.NaCl/ZnCl2, CsCl/ZnCl2 and NaCl/KCl/ZnCl2 melts give amor-phous nanoparticles with the diameters of 50 � 10 nm, 60 � 10nm and 40 � 10 nm, respectively. KCl/ZnCl2 eutectic gives riseto the mixed product morphology containing larger sheets (1–2

Fig. 3 Representative SEM images of carbon nitrides synthesized in (a)ZnCl2, (b) LiCl/ZnCl2, (c) KCl/ZnCl2, (d) NaCl/ZnCl2, (e) CsCl/ZnCl2 and(f) NaCl/KCl/ZnCl2.

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mm in diameter), and spherical nanoparticles aggregates (�300nm in diameter).

ZnCl2 salt melt derivedmaterials have higher specic surfaceareas as determined by Brunauer–Emmett–Teller (BET) analysisof the gas adsorption data compared to the reference g-C3N4

(see Table 1). However, the values are much lower than thosereported for other types of materials prepared in ZnCl2 melts(CTFs11 and carbons12). The appearance of nitrogen adsorption–desorption isotherms suggests the absence of any kind of innerporosity in nal MCl/ZnCl2–C3N4 products, which is typical forcarbon nitrides prepared in salt melts.10a Hence the estimatedsurface area values are reecting only the external surface areaof material particles, in good agreement with the results of SEMinvestigations.

The structural order and morphology of the products seemto correlate with the amount (c) of ZnCl2 in the salt melt at axed melamine to salt weight ratio (1 : 5) (see Table 1). Thus, atc(ZnCl2)�0.6, products are obtained as amorphous sphericalnanoparticles. Being a strong Lewis acid, ZnCl2 interacts withcyano-groups of precursors and intermediates. This interactionfacilitates their solubilization in the salt melt, but is also thesource of the discussed structural complexity. The secondcomponent of the salt mixture (MCl, M ¼ Li+, Na+, K+, Cs+)allows tuning the solubility of the reaction intermediates in thesalt melt and thus inuences the onset of “polymer–salt melt”phase separation. Li+ and K+ cations favor the solubilization ofthe reaction intermediates that results in slower phase separa-tion and allow the growth of crystalline poly(triazine imides).Na+ and Cs+ seem to cause rather quick supersaturation of themelt with oligomeric intermediates that leads to the formationof numerous amorphous nanoparticles, which cannot crystal-lize into more extended networks.

Inuence of synthesis parameters

In order to control morphology and crystallinity of salt meltderived carbon nitrides, the inuence of various reactionparameters on product properties was studied using NaCl/ZnCl2as an example eutectic. Among these are synthesis temperatures(400, 450, 500, 550, 600 �C) (Fig. S3 and Tables S2, S3†), heatingrates (2.5, 5, 10, 20, 40 �C min�1) (Fig. S4 and Table S4†),holding times (2, 4, 6, 8, 10 hours) (Fig. S5 and Table S5†),precursor to salt ratios (1 : 1, 1 : 2, 1 : 5, 1 : 10, 1 : 20) (Fig. S6and Table S6†), synthesis atmospheres (N2, air, ampoule) andtype of C3N4 precursors (DCDA, melamine). The results of thesestudies are briey discussed below, but for more detailedinformation the reader is referred to the ESI.†

We found that the microstructure of products is mainlydened already at 400 �C. This means that the nal morphologyis determined by precipitation and crystallization of interme-diary condensates at the point of vanishing solubility in the saltmelt. No signicant changes in FTIR spectra and WAXS dif-fractograms can be observed when the synthesis temperature isfurther increased from 400 to 600 �C (Fig. S3†). However, thedegree of condensation of the polymeric networks increaseswith raising the reaction temperature, resulting in slightlyimproved C/N ratios (0.59 to 0.61) and narrowing the band gap

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of the resulting semiconductors, as already visually observed asan intensication of products colors. In the case of applying aone-step heating procedure (Scheme S1b†), materials obtainedat 550 �C are characterized by slightly higher values of the BETsurface areas if compared to those synthesized at 600 �C (144and 115m2 g�1, respectively) while providing the same C/N ratio(0.61, Table S3†), thus 550 �C was selected as the temperature ofchoice for running the condensation. This is in good agreementwith previous observations however on bulk condensation ofC3N4 precursors.

Variations of the heating rate and holding time (Fig. S4 andS5†) have only minor inuence on products structures, thoughthe best products compositions characterized by a C/N ratio of0.61 were obtained at 10� min�1 ramp and 6 hours holdingtime. Other studied parameters gave lower C/N ratios in theproducts (Tables S4 and S5†).

As the morphology is given by a product precipitation atrather early stages of the condensation, the concentration of theprecursor in the melt must have crucial impact on themorphology and crystallinity of ZnCl2-derived carbon nitrides.Here, three different regimes were established: both low (1 : 1precursor to salts weight ratio) and high (1 : 10, 1 : 20 wt ratios)melamine concentrations give highly crystalline products, whileconcentrations in between (1 : 2, 1 : 5 wt ratios) lead rather toamorphous polymeric structures (Fig. S6†). The crystallinespecies have sheet-like morphology characterized by differentdiameters: 0.3–1 mm for 1 : 1 ratio, 1.5–2 mm for 1 : 10, and 50–80 nm for 1 : 20. The intermediary concentrations yield nano-particles 50–100 nm in size (Fig. 4). The BET surface areas ofproducts aer acidic work-up vary between 60 and 190 m2 g�1

(Table S6†).

Fig. 4 SEM images of materials synthesized in NaCl/ZnCl2 salt meltsusing different precursor to salt ratios: (a) 1 : 1, (b) 1 : 2, (c) 1 : 5, (d)1 : 10, (e), (f) 1 : 20.

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We can only speculate about the origin of this morphologychanges. We suggest that at high precursor concentrations, amolecular complex between melamine and ZnCl2 is initiallyformed, which consumes all ZnCl2. Further transformations donot occur in a solution anymore but in solid state, and ratherthe structure of complexes between ZnCl2 and intermediatesdetermines the high order in the nal products. Upondecreasing melamine concentration, the initially formedmolecular complex is redissolved at higher temperatures, butwhile polymerizing quickly reaches supersaturation andprecipitates as amorphous nanoparticles. In some cases (NaCl/ZnCl2, CsCl/ZnCl2, NaCl/KCl/ZnCl2) even spinodal decomposi-tion of the polymer–salt melt phase is assumed to take place,yielding highly microporous materials (see below). At lowprecursor concentration, demixing of phases occurs slowly, andthe polymer (PTI) has enough time to crystallize. It ejects thesolvent because the cohesion energy of carbon nitride is higherthan the secondary valence to the metal center.

Products prepared in pure ZnCl2

In order to elaborate on the reactive nature of pure ZnCl2 andthe role of the second component of the eutectic salt mixture tomoderate this reactivity, we also investigated the condensationof melamine in pure ZnCl2 at different precursor concentra-tions. Materials derived from pure ZnCl2 melts have typicallydense structures with heterogeneous morphologies, whichcomplicate the removal of the intercalated Zn ions, especially atlow precursor to salt ratios (1 : 1, 1 : 2). Some illustrative SEMimages of products obtained from ZnCl2 melts at differentprecursor concentrations are shown in Fig. 5, while Fig. S7a†presents typical WAXS patterns of composites. Similar to NaCl/ZnCl2 eutectic, three regimes determining the productmorphology and crystallinity are observed. At 1 : 1 ratio, bothnanoparticle and nanosheet morphologies can be recognized,at 1 : 2 ratio the resulting material is obtained as amorphousspherical nanoparticles, while at 1 : 5 and 1 : 10 ratios theproducts are mostly crystalline, organized into sheet-likestructures covering broad range of shapes and dimensions

Fig. 5 Selected SEM images of materials synthesized in ZnCl2 usingdifferent precursor to salt ratios: (a) 1 : 1, (b) 1 : 2, (c) 1 : 5 and (d) 1 : 10.

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Fig. 7 (a) UV-visible light diffuse reflectance spectra and (b) steady-state photoluminescence spectra of g-C3N4 and MCl/ZnCl2–C3N4 (M¼ Li, Na, K, Cs) excited at 300 nm.

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(from the nano- to micrometer range). As a consequence, theBET surface areas of those products are typically low.

Due to the very strong donor–acceptor interactions of ZnCl2with the precursor, zinc cyanamide (also called zinc carbodii-mide, Zn(CN2)) is formed as a by-product when performing thereaction in pure ZnCl2. At 1 : 10 precursor to salt ratio, the yieldof ZnCN2 is �50%. When the ratio is further lowered down to1 : 20, solely ZnCN2 is formed (Fig. S7a and b†). These resultsprove the precoordination between the salt and the monomervia N–Zn donor–acceptor bonds formation and point out thealternative reaction pathway in the case of ZnCl2 if compared toLiX/KX (X ¼ Cl, Br) melts. LiX/KX melts only weakly solubilizepoly(triazine imides), therefore phase demixing occurs early,and big crystalline structures can grow from that melts. On thecontrary, strong binding of Zn to condensation intermediatesensures their solubilization, and at later condensation stagesthe reaction progresses towards the thermodynamically morestable Zn(CN2) rather than to C3N4. Thus, ZnCl2 should bealways considered as a reactive solvent in the case of carbonnitride synthesis.

Zn(CN2) can be easily removed by washing the productmixture with diluted acid, or converted to zinc oxide by re-heating the composite in air (2 hours at 400 �C; note thatcontrary to carbon nitride Zn(CN2) is not oxidation stable). Inthis way, PTI/ZnO composites can be obtained. Fig. 6 shows aTEM image and SAED pattern of such a composite containing�40 wt% C3N4 and �60 wt% ZnO (crystallite size: 13 � 2 nm)and having a BET surface area of 270 m2 g�1 (Fig. S8†). Thephotocatalytic activity of this material is discussed below.

Optical absorption and emission properties of ZnCl2-derivedcarbon nitrides are compared with those of g-C3N4 in Fig. 7aand b, respectively. As it can be seen, NaCl/ZnCl2- and CsCl/ZnCl2-derived materials absorb visible light over a wide wave-length range. If compared to g-C3N4, the absorption band edgeof these composites is obviously smeared to longer wavelengths.The appearance of the absorption spectra of NaCl/ZnCl2–C3N4,CsCl/ZnCl2–C3N4 and PTI/ZnO composite (Fig. S9†) is typical fordyade materials, such as carbon@TiO2.28 In a dyade, twocomponents form a joint electronic system, and the altered

Fig. 6 TEM image and SAED pattern of PTI/ZnO composite.

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absorption spectrum likely points out at charge transferbetween carbon nitride and Zn containing species (such asZnO). At the same time, the steady state photoluminescence ofthe products fromNaCl/ZnCl2 and CsCl/ZnCl2 excited at 300 nmis negligible, that implies that the relaxation of excitons occursvia non-radiative pathways: charge transfer at heterojunctionand/or non-radiative recombination at the impurity states in thecrystal lattice.

The product obtained from LiCl/ZnCl2 melt on the otherhand absorbs less visible light and is characterized by slightlystronger emission than g-C3N4. Both absorption and emissionspectra indicate that this semiconductor has a wider band-gapcompared to the reference carbon nitride. This is in goodagreement with the assignment that this structure is based ontriazine imide moieties, as previously reported salt melt prod-ucts.19,22 The optical properties of the material synthesized inKCl/ZnCl2 lie in between those of LiCl/ZnCl2 and NaCl/ZnCl2- orCsCl/ZnCl2-derived composites, that is in agreement with XRDand SEM data discussed above.

Composites obtained aer only aqueous work-up of thereaction mixtures

Materials, which were only washed with water, but not with 1 MHCl showed a C/N weight ratio of 0.70–0.85. This suggests the

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formation of some carbonaceous species other than C3N4 (e.g.ZnCO3). This goes along with an overall low content of C, N, H(sum: 40–50 wt%, Table S7†) and the presence of otherelements. Small amounts of zinc cyanamide, zinc cyanide andzinc oxychloride might also be present, according to XRDstudies (Fig. S10†). Acid washing step efficiently removes allthese by-products, so that the nal C/N ratio is �0.6 again, asmentioned above.

The water-washed MCl/ZnCl2-derived composites typicallypossess higher specic surface areas than the acid-treated ones.The most striking example is NaCl/ZnCl2–C3N4, whose BETsurface area dropped from 700 m2 g�1 (value aer waterwashing) to 193 m2 g�1 aer the acidic treatment (Fig. S11†).Such behavior may be explained by the collapse of the internalstructure during acidic work-up or the removal of high surfacearea impurities, such as nanoparticles onto the hybrid. In therst case, the composite material aer water washing may beenvisaged as a poor, amorphous analogue of metal organicframeworks, in which triazine imide (oligomeric) moieties playthe role of organic ligands, which are interconnected by Zn2+

ions, ZnCl2, ZnO clusters, or other species. Such microporouscomposite materials show very promising adsorption proper-ties, which will be discussed later.

Applications

Among potential applications for at least some of ZnCl2-derivedcarbon nitride composites, we would like to underline thefollowing two: CO2 adsorbents and photocatalysts.

The highly microporous NaCl/ZnCl2-derived C3N4 composite(water-washed only) was investigated with respect to its CO2

adsorption properties (Fig. 8 and S12, S13†). It shows quite highCO2 uptake of 3.6 mmol g�1 at 0 �C and 2.5 mmol g�1 at 30 �C.The steep rise of the amount of adsorbed CO2 at low pressuresindicates the presence of rather small micropores, which couldindeed be interesting for separation applications. The N2

uptake at 0 �C and 30 �C was also studied and initial calculationwith respect to the CO2/N2 selectivity a were undertaken. Idealadsorbed solution theory (IAST)29 gives a ¼ 225 (0 �C) and a ¼100 (30 �C) at 1 bar and a gas composition of CO2/N2 of 0.15/0.85

Fig. 8 CO2 and N2 adsorption–desorption isotherms of microporousNaCl/ZnCl2-derived C3N4 composite (after water washing step).

40810 | RSC Adv., 2014, 4, 40803–40811

that reects an approximate composition of a ue gas. Suchselectivities are indeed competitive to some reported zeolites30

and MOFs31 given also the simplicity of synthesis and puri-cation procedures as well as the relatively high product yield of50% (based on total C content in the precursor and theproduct).

The photocatalytic properties of ZnCl2-derived carbonnitrides were evaluated in a model reaction assay, the photo-degradation of Rhodamine B (RhB) under blue light (l ¼ 420nm) irradiation. Here, two selected examples of remarkableperformance are the microporous NaCl/ZnCl2-derived C3N4

hybrid (water-washed), again, and the C3N4/ZnO (40 wt%/60wt%) nanocomposite. The estimated rate constants of RhBdegradation accomplished by these two photocatalysts are k0 ¼21 � 10�3 min�1 and k0 ¼ 12 � 10�3 min�1, respectively; theseare �10 and �5 times higher than the one observed for thereference g-C3N4 (2.2 � 10�3 min�1, Fig. S14†). We mainlyattribute these enhanced activities to the increased surfaceareas of the composites and improved absorption of visiblelight due to the formation of the dyadic structures. Additionally,the C3N4/ZnO hybrid is characterized by an improved crystal-linity of ZnO NPs that results in better transport of the photo-generated and transferred electrons to the surface-adsorbedRhB and O2 molecules.

Conclusions

In summary, condensation of melamine in ZnCl2-containingeutectic salt melts gives rise to a broad range of novel carbonnitride-based composite materials with the improved absorp-tion in visible light range due to the formation of dyadic systembetween C3N4 and ZnO clusters or other Zn2+ containingspecies. Unlike LiX/KX (X ¼ Cl, Br), ZnCl2 plays a role of thereactive solvent during the synthesis of carbon nitrides, andbinds strongly to the condensation intermediates. Adjustmentof precursor concentration and a proper selection of the alkalimetal chloride constituent of MCl/ZnCl2 melt give a possibilityto change the on-set of phase demixing, tune the interactionsstrength between the condensation intermediates and thesolvent and vary the solubility of the intermediates in the melt.Overall, one can direct the reaction from zinc cyanamide to bothcrystalline poly(triazine imides) or MOF-like hybrid materials.The latters have surface areas up to 700 m2 g�1 and turned outto be interesting as highly performing CO2 adsorbents andphotocatalysts.

Acknowledgements

The authors want to gratefully acknowledge the Max PlanckSociety, Dr J. Hartmann for EDXmeasurements and Dr G. Clavelfor TEM measurements.

Notes and references

1 (a) X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin,J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater.,2009, 8(1), 76–80; (b) K. Maeda, X. Wang, Y. Nishihara,

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D. Lu, M. Antonietti and K. Domen, J. Phys. Chem. C, 2009,113(12), 4940–4947.

2 M. Shalom, S. Gimenez, F. Schipper, I. Herraiz-Cardona,J. Bisquert and M. Antonietti, Angew. Chem., 2014, 126(14),3728–3732.

3 (a) Y. Wang, J. Zhang, X. Wang, M. Antonietti and H. Li,Angew. Chem., 2010, 122(19), 3428–3431; (b) Y. Wang, Y. Di,M. Antonietti, H. Li, X. Chen and X. Wang, Chem. Mater.,2010, 22(18), 5119–5121; (c) Y. Wang, H. Li, J. Yao, X. Wangand M. Antonietti, Chem. Sci., 2011, 2(3), 446–450; (d)F. Su, S. C. Mathew, G. Lipner, X. Fu, M. Antonietti,S. Blechert and X. Wang, J. Am. Chem. Soc., 2010, 132(46),16299–16301.

4 M. Naffakh, V. Lopez, F. Zamora and M. A. Gomez, SoMater., 2010, 8(4), 407–425.

5 (a) Y. Zhang, T. Mori, L. Niu and J. Ye, Energy Environ. Sci.,2011, 4(11), 4517–4521; (b) Y. Zhang, T. Mori, J. Ye andM. Antonietti, J. Am. Chem. Soc., 2010, 132(18), 6294–6295.

6 (a) M. Groenewolt and M. Antonietti, Adv. Mater., 2005,17(14), 1789–1792; (b) K. Kailasam, J. D. Epping,A. Thomas, S. Losse and H. Junge, Energy Environ. Sci.,2011, 4(11), 4668–4674; (c) F. Goettmann, A. Fischer,M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2006,45(27), 4467–4471.

7 (a) M. Shalom, S. Inal, C. Fettkenhauer, D. Neher andM. Antonietti, J. Am. Chem. Soc., 2013, 135(19), 7118–7121;(b) Y.-S. Jun, E. Z. Lee, X. Wang, W. H. Hong, G. D. Stuckyand A. Thomas, Adv. Funct. Mater., 2013, 23(29), 3661–3667.

8 (a) Y. Wang, X. Wang, M. Antonietti and Y. Zhang,ChemSusChem, 2010, 3(4), 435–439; (b) Y. Zhang,Z. Schnepp, J. Cao, S. Ouyang, Y. Li, J. Ye and S. Liu, Sci.Rep., 2013, 3, 2163.

9 (a) Q. Guo, Y. Xie, X. Wang, S. Lv, T. Hou and X. Liu, Chem.Phys. Lett., 2003, 380(1–2), 84–87; (b) Q. Guo, Y. Xie,X. Wang, S. Zhang, T. Hou and S. Lv, Chem. Commun.,2004, 1, 26–27.

10 (a) E. Wirnhier, M. Doblinger, D. Gunzelmann, J. Senker,B. V. Lotsch and W. Schnick, Chem.–Eur. J., 2011, 17(11),3213–3221; (b) S. Y. Chong, J. T. A. Jones, Y. Z. Khimyak,A. I. Cooper, A. Thomas, M. Antonietti and M. J. Bojdys, J.Mater. Chem. A, 2013, 1(4), 1102–1107.

11 (a) P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int.Ed., 2008, 47(18), 3450–3453; (b) P. Kuhn, A. Forget,J. Hartmann, A. Thomas and M. Antonietti, Adv. Mater.,2009, 21(8), 897–901; (c) P. Kuhn, A. Thomas andM. Antonietti, Macromolecules, 2008, 42(1), 319–326.

12 N. Fechler, T.-P. Fellinger and M. Antonietti, Adv. Mater,2013, 25(1), 75–79.

This journal is © The Royal Society of Chemistry 2014

13 D. A. Shirley, Phys. Rev. B: Solid State, 1972, 5(12), 4709–4714.14 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor,

R. H. Raymond and L. H. Gale, Surf. Interface Anal., 1981,3(5), 211–225.

15 (a) M. J. Bojdys, Uber neue Allotrope und Nanostrukturen vonKarbonitriden, Universitat Potsdam, Potsdam, 2009; (b)B. V. Lotsch and W. Schnick, Chem.–Eur. J., 2007, 13(17),4956–4968.

16 K. Schwinghammer, B. Tuffy, M. B. Mesch, E. Wirnhier,C. Martineau, F. Taulelle, W. Schnick, J. Senker andB. V. Lotsch, Angew. Chem., Int. Ed., 2013, 52(9), 2435–2439.

17 G. J. Janz, C. B. Allen, J. R. Downey Jr and R. P. T. Tomkins,Physical Properties Data Compilations Relevant to EnergyStorage, National Standard Reference Data System, NewYork, 1978.

18 Y. Zhang, A. Thomas, M. Antonietti and X. Wang, J. Am.Chem. Soc., 2008, 131(1), 50–51.

19 Y. Ham, K. Maeda, D. Cha, K. Takanabe and K. Domen,Chem.–Asian J., 2013, 8(1), 218–224.

20 V. G. Manecke and D. Wohrle, Macromol. Chem. Phys., 1968,120(1), 176–191.

21 A. Thomas, A. Fischer, F. Goettmann, M. Antonietti,J.-O. Muller, R. Schlogl and J. M. Carlsson, J. Mater. Chem.,2008, 18(41), 4893–4908.

22 E. J. McDermott, E. Wirnhier, W. Schnick, K. S. Virdi,C. Scheu, Y. Kauffmann, W. D. Kaplan, E. Z. Kurmaev andA. Moewes, J. Phys. Chem. C, 2013, 117(17), 8806–8812.

23 M. C. Biesinger, L. W. M. Lau, A. R. Gerson andR. S. C. Smart, Appl. Surf. Sci., 2010, 257(3), 887–898.

24 N. Boshkov, K. Petrov, S. Vitkova, S. Nemska andG. Raichevsky, Surf. Coat. Technol., 2002, 157(2–3), 171–178.

25 B. J. Coppa and R. F. Davis, Appl. Phys. Lett., 2003, 82(3), 400–402.

26 B. R. Strohmeier and D. M. Hercules, J. Catal., 1984, 86(2),266–279.

27 J. C. Klein and D. M. Hercules, J. Catal., 1983, 82, 424–441.28 L. Zhao, X. Chen, X. Wang, Y. Zhang, W. Wei, Y. Sun,

M. Antonietti and M.-M. Titirici, Adv. Mater., 2010, 22(30),3317–3321.

29 A. L. Myers and J. M. Prausnitz, AIChE J., 1965, 11(1), 121–127.

30 F. Akhtar, Q. Liu, N. Hedin and L. Bergstrom, Energy Environ.Sci., 2012, 5(6), 7664–7673.

31 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald,E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem.Rev., 2011, 112(2), 724–781.

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