DOI: 10.1002/cssc.201403168
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Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution Xiaofei Yang,[a, b] Hua Tang,[b] Jingsan Xu,[a] Markus Antonietti,[a] and Menny Shalom*[a] Herein, we show the facile synthesis of an efficient silver phosphate/graphitic carbon nitride (Ag3PO4/g-C3N4) photocatalyst for oxygen production and pollutant degradation by using electrostatically driven assembly and ion-exchange processes. The composite materials demonstrate a sheet-like C3N4 structure, decorated with different Ag3PO4 particles sizes. Detailed analysis of the reactions mechanism by electron-spin resonance and radical-capture agents strongly imply the formation of an in situ Z-scheme by the evolution of small silver nanoparticles in the interface of the materials under illumination. The
Ag nanoparticles improve charge separation within the composite material by acting as a storage and recombination center for electrons and holes from Ag3PO4 and C3N4, respectively. In addition, the photostability of Ag3PO4 is enhanced relative to that of the bulk materials, which results in a stabilized heterojunction. We believe that this work provides new insight into the operation mechanism of composite photocatalysts for water splitting and opens the possibility for advanced photocatalysis based on the higher oxidation power of Ag3PO4.
Introduction The photocatalytic splitting of water into hydrogen and oxygen by semiconductors is receiving significant attention,[1] as it presents a promising approach for the environmentally friendly conversion of solar energy into clean and storable chemical energy. Whereas the generation of H2 by a photocatalyst has been heavily explored over the past years and rates have been significantly improved,[2] the complimentary reaction (i.e., water oxidation), which is considered more difficult owing to the multiple reactions steps of four holes, remains the major obstacle for the overall water-splitting reaction. For photocatalytic O2 evolution, the basic requirement is that the valence band of the semiconductor must be more positive than the oxidation potential of H2O to O2 [1.23 V vs. normal hydrogen electrode (NHE), pH 0]. Furthermore, a significant overpotential is needed to overcome the activation energies in the charge-transfer process between the photocatalyst and water molecules.[3] Therefore, although a variety of materials have been developed over the past few decades, there are very few materials[4] that can directly oxidize water into O2 under illumination. Most recently, the use of a silver orthophosphate (Ag3PO4) semiconductor for visible-light-driven O2 evolution was reported.[5] Ag3PO4 demonstrates an excellent photooxida-
tive capability for O2 evolution, although it is not reductive enough to directly reduce H2O to H2. Despite the high activity of Ag3PO4 for O2 generation, it suffers from poor photostability as a result of self-reduction to silver with the leftover electrons (either in the presence or absence of a sacrificial agent). In addition, the regular synthetic path of Ag3PO4 results in big particles with low surface area, which leads to poor charge separation and comparably low photoactivity. To improve its performance, efforts have been made to design and synthesize Ag3PO4-based hybrid materials for photocatalytic applications. The combination of Ag3PO4 with additional materials can result in smaller particles, higher charge mobility, and in the creation of a local heterojunction at the interface with the supporting material. An ideal complementary material should have suitable electronic properties or band structure to quickly take up the electrons, high surface area, and an acceptable electronic conductivity. It is interesting to note that Ag3PO4-based materials are mostly used for the photodegradation of organic pollutants,[6] although their first use as a photocatalyst was for O2 evolution.[5] A suitable material for hybridization with Ag3PO4 is graphitic carbon nitride (g-C3N4), which has been attracting widespread attention in the photo(electro)catalysis field (i.e., water splitting, electrocatalysis, and more).[2j,n, 7] This is due to its appropriate bandgap, which engulfs both the reduction and oxidation potential of water (2.7 eV) and its unusually high thermal and chemical stability. In the last year, a few synthetic paths to form Ag3PO4/g-C3N4 composite photocatalysts were introduced.[8] However, for all of these reports only the photodegradation of organic pollutants was shown. Moreover, the proposed mechanism for the composite materials involved unfavorable electron injection from the conduction band (CB) of gC3N4 CB to the CB of Ag3PO4, whereas the holes transfer from
[a] Dr. X. Yang, Dr. J. Xu, Prof. M. Antonietti, Dr. M. Shalom Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces 14424 Potsdam (Germany) E-mail: [email protected] [b] Dr. X. Yang, Dr. H. Tang School of Materials Science and Engineering Jiangsu University 301 Xuefu Road Zhenjiang 212013 (P.R. China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403168.
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Full Papers obtain Ag + /g-C3N4 that was subsequently converted into Ag3PO4/g-C3N4 nanocomposites by the addition of trisodium phosphate. The Ag3PO4/C3N4 composites with different amounts of g-C3N4 are denoted as ECNX and DCNX (X is the actual mass of g-C3N4 used, in mg) for ethanol and DMSO, respectively. We note that for simplicity we only show the characterizations of the most active composite material derived from ethanol (ECN400) in the main text, whereas characterization of all the other composite materials is given in the Supporting Information. The crystal structure of the obtained samples was investigated by X-ray diffraction (XRD, Figure 2 a). The peak at 2 q = 27.18 is attributed to the (002) reflection of ECN (interplanar stacking peak of conjugated aromatic systems), whereas the (100) peak at 2 q = 13.38 corresponds to in-plane structural packing motifs of the aromatic systems. Compared to bulk C3N4, ECN exhibits lower intensity and broadened width, which suggests that ECN Results and Discussion was well delaminated throughout sonication. For the composite materials, owing to the high crystallinity of Ag3PO4 alongWell-defined g-C3N4 nanostructures were synthesized by therside the small amount of C3N4 in the composite materials, only mal condensation of hydrogen-bonded supramolecular aggrethe diffraction peaks of body-centered cubic-phase Ag3PO4 gates obtained from equimolecular mixtures of melamine and (JCPDS card No.06-0505) were observed. The chemical struccyanuric acid in absolute ethanol (EtOH) and dimethyl sulfoxture of the obtained g-C3N4 samples was further studied by ide (DMSO).[9] We note that in this C3N4 preparation method using attenuated total reflectance Fourier-transform infrared (compared to the traditional one for preparing g-C3N4, i.e., con(ATR-FTIR) spectroscopy. The spectrum of ECN (Figure 2 b) densation of melamine) higher photoactivity is obtained. In adshows characteristic bands in the n˜ = 1200–1700 cm1 region dition, with this synthetic method it is easier to obtain nanothat correspond to the stretching modes of CN heterocycles. In sheets of C3N4, which can be further used to form composite addition, typical breathing vibrations of the triazine units at apmaterials. Afterwards, the resulting g-C3N4 materials were used proximately n˜ = 810 cm1 are observed. For the as-prepared as the matrix for the growth of Ag3PO4 (for more details see Ag3PO4, a band is observed at approximately n˜ = 1385 cm1, atthe Experimental Section). The reaction conditions for the syntributed to the HOH bending band of absorbed water molethesis of Ag3PO4/g-C3N4 nanocomposites are listed in Table S1 cules, alongside PO stretching vibrations of the PO43 ions at (Supporting Information), and a schematic illustration of the n˜ = 943 and 668 cm1. For the composite material (ECN400), synthetic pathway is shown in Figure 1. Briefly, as-prepared gboth the characteristic bands of C3N4 and Ag3PO4 are clearly C3N4 was ultrasonicated to get stable dispersions with fewobserved. For ECN400, also a redshift in the absorption edge layers g-C3N4, and the dispersed ultrathin g-C3N4 nanosheets (Figure 2 c, from l = 460 nm for pure C3N4 to l 520 nm for were negatively charged with a z potential of approximately ECN400) is observed, which indicates hybridization of ECN and 46 mV. Silver nitrate was then deposited into g-C3N4 to Ag3PO4 to a joint electronic system beyond the bare addition of the spectra. It is notable that drastic quenching of the photoluminescence intensity of ECN was observed upon hybridization of Ag3PO4 (Figure 2 d); this implies that the latter creates new nonradiative paths for charge separation of photogenerated electron and/or holes. The fluorescence quenching is presumably due to the formation of an effective heterojunction at which charge transfer occurs between ECN and Ag3PO4, and this prevents the direct recombination of photoinduced electrons and holes within C3N4. The formation of Figure 1. Schematic illustration of the syntheses of nanostructured g-C3N4 and the Ag3PO4/g-C3N4 nanocomposite.
the valence band (VB) of Ag3PO4 to the VB of g-C3N4 (Figure 6 c), that is, the hybrid would only loose activity because of the continuance of the two unfavorable states instead of adding the relative relativities. Especially, it is noteworthy that the mentioned mechanism is unfavorable for the generation of O2 from water owing to the low photocatalytic activity of gC3N4 towards water oxidation. Herein, we show a synthesis of well-defined g-C3N4/Ag3PO4 composite materials by a two-step procedure to resolve this problem by an appropriate nanostructure. The photocatalytic properties were tested by measuring the O2 evolution and the degradation of the organic dye rhodamine B (RhB) under visible-light illumination in the presence of the g-C3N4/Ag3PO4 composite. Moreover, the charge separation under illumination was studied by electron spin resonance (ESR) in the presence of hole and radical scavengers.
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Full Papers Further investigation of the chemical composition of the samples was provided by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of ECN400 (Figure 3 a) reveals the presence of C, N, O, P, and Ag and a C/N atomic ratio of 3:4, which is in good agreement with the nominal composition of as-prepared ECN (Figure S5 a). Two characteristic peaks at binding energies of 368.2 and 374.2 eV that correspond to the Ag 3d5/2 and Ag 3d3/2 orbitals, respectively, were found (Figure 3 b), and this suggests the dominance of AgI in the composite surface. The high-resolution C 1s peak (Figure 3 c) was deconvoluted into two peaks at binding energies of 284.8 and Figure 2. a) XRD patterns, b) ATR-FTIR spectra, c) UV/Vis diffuse reflectance spectra, and d) PL spectra of the as288.2 eV corresponding to prepared samples. carbon atoms in a purely carbonaceous environment and typiheterojunctions increases the lifetime of the electrons and cal aromatic CNC coordination in a graphitic carbon nitride holes and, therefore, their possibility to participate in the phoframework, respectively. The corresponding N 1s peak (Figtocatalytic reaction. More information about the XRD pattern ure 3 d) could be deconvoluted into three peaks at binding enand the ATR-FTIR, UV/Vis diffuse reflectance, and photolumiergies of 398.6, 400.3, and 401.3 eV. The peak at 398.6 eV is asnescence (PL) spectra of the different samples are shown in signed to the aromatic N atom bonded to two C atoms in the Figures S1–S4. triazine or heptazine rings (C=NC), whereas the peak at 400.3 eV is ascribed to the sp2-hybridized N atom bonded to three atoms [CN(C)C or CN(H)C]. The peak at 401.3 eV is attributed to the sp3-hybridized terminal N atom (NH2 or NO) of the triazine rings. A field-emission scanning electron microscopy (FE-SEM) study shows that the bare ECN consists of curved nanosheets with lamellar morphology (Figure S6 a1–a3). For ECN400 (Figure 4 a, b), the surfaces of g-C3N4 are decorated with irregular Ag3PO4 particles (with diameters of 40–300 nm) that are either on the surface or in the pores of g-C3N4. Additional FE-SEM images of g-C3N4 and Ag3PO4/gC3N4 composites are shown in Figures S6 and S7, and energydispersive X-ray spectroscopy (EDX) results further confirm the presence of all elements, that is, Figure 3. a) Full XPS spectrum and b) high-resolution Ag 3d peak, c) C 1s peak, and d) N 1s peak of the Ag3PO4/gC, N, O, P, and Ag, in the comC3N4 composite ECN400. ChemSusChem 0000, 00, 0 – 0
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Full Papers higher photoactivity for the degradation of RhB (95 % in 10 min) than pure ECN. Despite the high photoactivity of Ag3PO4, the incorporation of Ag3PO4 with moderate amounts of ECN (100, 200, and 400 mg) resulted in another clear enhancement in the photocatalytic activity, whereas an excessive amount of ECN (800 mg) resulted in a significant decrease in the degradation rate. Furthermore, it is important to note that the direct photolysis of RhB without ECN400 or the decomposition with ECN400 in the dark are negligible (Figure 5 b). To investigate the photocatalytic mechanism, three different scavengers, p-benzoquinone (BZQ), disodium ethylenediaminetetraacetic acid (Na2-EDTA), and tert-butyl alcohol, were employed for the radical- and hole-trapping experiments. As shown in Figure 5 c, upon using Na2-EDTA as the direct hole scavenger, a remarkable inhibitory effect on the degradation efficiency of RhB over both ECN400 and Ag3PO4 was observed. In these experiments, only less than 4 % RhB was degraded. The addition of BZQ (an O2C radical scavenger) into the photocatalytic system also caused a clear decrease in the degradation efficiency to 58 % for ECN400 and 40 % for Ag3PO4, whereas the presence of tert-butyl alcohol (a COH radical scavenger) had a negligible effect on the photocatalytic activity of RhB. The hole- and radical-trapping experiments imply that direct transfer of photoinduced holes and the coupled electron transfer to generate O2C radicals is the dominant mechanism responsible for the highly efficient visible-light-driven photocatalytic degradation performance (Figure S13). Besides the photocatalytic activity, the stability of a photocatalyst is also important for its practical application. The photodegradation of RhB over ECN400 was retained at over 90 % after three successive photocatalytic cycles, whereas the photodegradation activity of RhB over pure Ag3PO4 decreased markedly to 43 % (Fig-
Figure 4. Low- and high-magnification a, b) FE-SEM images and c, d) TEM images of ECN400 (scale bars in correspond to 100 nm).
posite (Figure S8). In addition, the thickness of ultrasonicated ECN nanosheets was estimated by atomic force microscope (AFM) to be down to 2 nm (Figure S9). The transmission electron microscopy (TEM) images of ECN400 (Figure 4 c, d) confirm the presence of Ag3PO4/ECN heterostructures, in which ECN serves as a support for the Ag3PO4 nanoparticles. Additional TEM images of g-C3N4 and the Ag3PO4/g-C3N4 composites are shown in Figures S10 and S11. Notably, almost no free particles are found outside the g-C3N4 sheets in the TEM images, which implies the successful deposition of Ag3PO4 particles on the gC3N4 surface. Furthermore, the surface area of the materials was studied by nitrogen sorption experiments. Table S2 summarizes the values of the surface areas of the as-prepared samples and the nitrogen adsorption–desorption isotherms, and the corresponding poresize distribution curves are shown in Figure S12. Higher surfaces areas were obtained for pure ECN (50.2 m2 g1) and DCN (62.4 m2 g1). Subsequent introduction of Ag3PO4 leads to a decrease in the specific surface area of the Ag3PO4/g-C3N4 composite as a result of the heavier silver. To evaluate the photocatalytic activity of the obtained samples, the photodegradation of RhB dye as a model system was studied in the presence of pure ECN and Ag3PO4 and the Ag3PO4/g-C3N4 composite photocatalyst under visible-light irFigure 5. a) Photodegradation performance of RhB dye in the presence of the composite materials compared to radiation (Figure 5 a). Ag3PO4 the pure ones, b) control experiments of ECN 400, c) degradation efficiency by using different radical scavengers, alone already exhibited much and d) comparison of the photostability of ECN400 and Ag3PO4.
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Full Papers material, most probably as a result of the higher relative reaction rate, that is, O2 is not only generated, but at higher concentrations it is also consumed by the secondary reduction process. Supporting this picture, in the presence of a sacrificial agent no clear decrease in the O2 concentration was observed because of the kineticdominant process of AgNO3 reduction (instead of O2). The photostability of ECN400 was evaluated by measuring the recycled sample three times (Figure 6 c). After three photooxidation cycles, the best sample still preserved more than 40 % of the initial oxygen generation. We note that in spite of the photostability enhancement for Figure 6. O2 evolution of a) ECN-derived samples in the presence of AgNO3 ; b) ECN400 and Ag3PO4 in the presthe composite material, we still ence or absence of AgNO3 ; c) over-recycled ECN400; and d) by using mixed ECN and Ag3PO4, ECN400, and no observed some decomposition photocatalyst. We note that without a photocatalyst the O2 concentration did not change. of active Ag3PO4 into metallic Ag after the recycling experiments (Figure S14), which we attribute to a missing balance ure 5 d). The decrease in the photocatalytic activity can be atbetween the (in this system too fast) water oxidation and a sectributed to the decomposition of Ag3PO4 to Ag by the remainondary reduction process. To confirm the advantage of the ing electrons (self-reduction) under light irradiation in the abin situ growth of Ag3PO4 on carbon nitride, we measured the sence of a sacrificial agent for the electrons. The self-reduction of Ag3PO4 in the composite materials is clearly strongly inhibitphotoactivity of physically mixed C3N4 and Ag3PO4 in the presed, which is seen as direct evidence for efficient electron transence of AgNO3 (Figure 6 d). The physically mixed material exfer from Ag3PO4 to C3N4, as it will be discussed below. hibited activity that was five times lower than that of the optimized material, which indicates that the hybridization of ECN Figure 6 a shows the results of the water splitting/O2 evoluand Ag3PO4 plays an important role in the material photoactivition experiments as a function of time in the presence of ECN, Ag3PO4, and Ag3PO4/ECN composites under white-light irradiaty over O2 evolution. Furthermore, a similar tendency in terms tion in the present of silver nitrate (AgNO3) as a sacrificial elecof both efficiency and photostability was observed for the tron acceptor. A negligible amount of O2 was evolved from the Ag3PO4/DCN composites (Figure S15) irradiated ECN, whereas for pure Ag3PO4 the O2 concentration To further investigate the mechanisms beyond the high acincreased to 10 mmol L1 after 30 min. For the Ag3PO4/ECN tivity of ECN400, we studied the generation of active radicals composites, the O2 generation was already enhanced by 30 % involved in the photocatalytic system by in situ ESR analysis of (up to 13 mmol L1) composited with only a minor ECN content the obtained samples with 5,5-dimethyl-1-pyrroline-N-oxide (ECN100). By increasing the carbon nitride content further (DMPO), a spin trapping reagent, in MeOH and H2O. As shown (ECN200 and ECN400), the O2 concentration increased (20 and in Figure 7 a, incubating Ag3PO4 with DMPO in H2O under light 25 mmol L1, respectively). At higher C3N4 concentration shows a rapid production of an ESR spectrum with seven char(ECN800, 35 % ECN), the evolved O2 concentration strongly acteristic bands. The ESR spectrum can be assigned to a derivative of DMPO, 5,5-dimethyl-2-ketopyrrolidino-1-oxyl (DMPOX), decreased to 5 mmol L1. In the absence of AgNO3 (Figure 6 b), which is generally produced by the interaction between active the concentration of evolved O2 over Ag3PO4 increased to hydroxyl radicals and DMPO–COH adducts.[10] DMPOX produc8 mmol L1 in 20 min, whereas the concentration of evolved O2 1 tion is maximized after approximately 4 min from the irradiatover ECN400 reached a capacity of 32 mmol L (fourfold ined Ag3PO4 system. A similar phenomenon was observed also crease). However, when both two samples achieved their highest O2 evolution efficiency, subsequent light irradiation (more with the incubation of ECN400 and DMPO under light. Moreover, in accordance with the photocatalytic measurements, than 3 h) caused a gradual decrease in evolved O2. We attriDMPOX production appears faster with a higher intensity (Figbute this to the in situ consumption of the as-generated O2 by ure 7 b). For comparison, DMPO spin-trapping experiments of further oxygen reduction promoted by the as-generated elecAg3PO4 and ECN400 in the presence of the sacrificial agent trons at the carbon nitride subphase. Moreover, after oxygen saturation the O2 reduction rate was faster in the composite AgNO3 were performed (Figure 7 c). It was found that the typiChemSusChem 0000, 00, 0 – 0
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Full Papers ductor heterojunction, the expected charge separation and transfer are usually governed by the thermodynamics driving force (Figure 8 c) of the system (positions of the conduction and valence bands). Taking into account the positions of the related energy levels in the ECN400 composite in the conventional heterojunction structure under illumination, the electrons and holes would accumulate in the CB of Ag3PO4 and the VB of ECN, respectively. However, in this scenario, the oxidation of water either to O2 or to active COH becomes practically impossible because of the weak energetic driving force along with the weaker photocatalytic properties of ECN for oxiFigure 7. ESR spectra of radical adducts trapped by DMPO in a) aqueous dispersion of Ag3PO4 ; b) aqueous disperdation reactions. However, all sion of ECN400; c) Ag3PO4 and ECN400 in the presence or absence of AgNO3 ; and d) ECN, Ag3PO4, and ECN400 our experiments (photocatalytic methanol dispersions. degradation and radical-capture experiments, O2 evolution, and ESR results) suggest that the charge-transfer mechanism of the cal bands from DMPOX disappear with the addition of AgNO3 Ag3PO4/ECN composite should be different from that of the under 2 min of irradiation, which suggests that fewer COH radicals are generated at the earlier stage. For all the samples, no previously described, conventional heterostructure systems, ESR signal in the dark was observed, and this indicates that and the energies of the two subsystems rather add to each light irradiation is crucial to the generation of active radicals. other, presumably to a kinetically preferred coupling of the CB Additionally, six characteristic bands corresponding to DMPO– of Ag3PO4 and the VB of ECN, which essentially balances those O2C adducts were detected in MeOH under light for all three two electronic states. As a result, a Z-scheme photocatalytic mechanism for the samples (Figure 7 d). The intensity of the DMPO–O2C adduct degradation of RhB and O2 evolution is proposed (Figure 8 d). from ECN400 is higher than that of the ECN one and similar to that of the Ag3PO4 sample. In addition, no ESR signal from In the beginning, the system behaves similar to an ordinary DMPO–COH adducts was observed from only a ECN or DCN heterojunction, and electrons can pass from the CB of ECN to aqueous dispersion under visible-light irradiation (Figure S16). Ag3PO4 (Figure 8 c, path 1), or they are generated in Ag3PO4 as On the basis of the above experimental evidence, we prosuch. These electrons have enough reduction power to convert pose a photocatalytic mechanism of water splitting for the Ag3PO4 into Ag. Consequently, under illumination, the twophase (Ag3PO4/ECN) system transforms into a three-phase Ag3PO4/ECN composite with an emphasis on O2 evolution to occur through a Z-scheme. Figure 8 shows a schematic reprecomposite (Ag3PO4/Ag/ECN), even in the absence of AgNO3 sentation of different photocatalytic processes. From the ther(see also Figure S14 b–d). Interestingly, this Ag3PO4 reduction modynamic limitations it is clear that under illumination only seems to occur mainly at the interface of the materials, more holes from the Ag3PO4 subsystem can generate OH radicals specifically exactly the point at which the ECN subphase and Ag3PO4 electronically communicate. The formation of Ag nano(Figure 8 a, b) because of their strong oxidation potential. On the contrary, in ECN COH radicals can only be generated by the particles at the interface of the material opens a faster chargereduction of dissolved oxygen in solution. In addition, the transfer channel connecting the CB of Ag3PO4 to the VB of holes of both systems have a suitable driving force for the ECN, which then seems to be kinetically more energetically fadirect oxidation of RhB. This hypothesis is in accordance with vorable. The new charge-transfer process consists of Ag nanothe ESR spectra and the reactive species trapping experiments, particles as a recombination center for CB-Ag3PO4 electrons which clearly demonstrate the dominance of holes and O2C (Figure 8 d, path 2) and VB-C3N4 holes (Figure 8 d, path 3), which is also in good agreement with the positions of the radicals in the photocatalytic process. Moreover, the driving energy levels.[8c] As a consequence, a larger amount of elecforce for O2 evolution is dramatically higher for Ag3PO4 than for ECN as a result of the position of the valence band and trons accumulate in the CB of ECN, which is coupled to higher also as a result of the known fact that silver derivatives are oxiactivity in the reduction of oxygen into active superoxide radidation catalysts. In the construction of a conventional semiconcals (Figure 8 d, path 4) (which further react to hydroxyl radi-
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Figure 8. Schematic for the energy-band structure of a) Ag3PO4 and b) ECN; c) conventional mechanism; and d) proposed Z-scheme mechanism for charge transfer. The band-gap values of Ag3PO4 and ECN were estimated to be 2.45 and 2.70 eV, respectively (see also the Supporting Information for more details). The EVB of Ag3PO4 and ECN were calculated to be 2.90 and 1.53 eV vs. NHE, respectively, whereas their corresponding ECB were 0.45 and 1.17 eV vs. NHE, respectively.
cals), whereas the holes in the VB of Ag3PO4 present superior activity in the oxidation of water into O2 and in the direct oxidation of RhB (Figure 8 d, path 5). We believe that the current kinetic limiting factor of the system is electron accumulation within ECN, which leads to a higher recombination of electrons and holes within carbon nitride (Figure 8 d, path 6), along with a shorter electron–hole lifetime. A smaller concentration of holes within ECN also hinders their recombination with Ag3PO4 electrons, which leads to further self-reduction to Ag and the coupled (slow) self-destruction of the photocatalytic tandem system. This in fact could be tested by more efficient sacrificial electron acceptors or by the employment of reduction catalysts, which were left out for simplicity in the present system.
sion into Ag3PO4 nanoparticles. The composite materials possess a sheet-like C3N4 structure with variable Ag3PO4 particles sizes, depending on the solvent and the molar ratio of the starting material. The composite materials exhibited high photocatalytic performance in both oxygen evolution and RhB degradation along with an improvement in photostability. The reaction mechanism was carefully studied by radical-capture agents and electron spin resonance. Our results show that under illumination, an in situ Z-scheme is created, which involves the spontaneous formation of Ag nanoparticles in the interface of the materials, which then enables the recombination of electrons from Ag3PO4 and holes from g-C3N4. This nanoscopic metallic Ag mediator thereby promotes the “addition path” of the band-gap energies and thereby the possibility for efficient O2 evolution under illumination while keeping the high reduction potential of carbon nitride. We believe that this work opens the possibility to design more efficient photocatalysts for the production of solar fuels and other highenergy, uphill reactions.
Conclusions In summary, we reported the synthesis of Ag3PO4/g-C3N4 photocatalysts by the assembly of Ag + on the surface of delaminated graphitic C3N4 nanostructures, followed by their converChemSusChem 0000, 00, 0 – 0
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ferent amounts (100, 200, 400, and 800 mg) of ECN and DCN are denoted as ECN100, ECN200, ECN400, ECN800 and DCN100, DCN200, DCN400, DCN800, respectively.
Synthesis of C3N4 Carbon nitrides from ethanol (ECN) and DMSO (DCN) were synthesized by using equimolar amounts of melamine and cyanuric acid according to the modified method reported in previous papers.[9] Whereas melamine and cyanuric acid are insoluble in EtOH, they are both soluble in DMSO. Therefore, for g-C3N4 derived from DMSO, the two starting materials were first separately dissolved in DMSO. The slow addition of one clear solution into the other resulted in the generation of a white precipitate. For the g-C3N4 sample derived from EtOH, the two compounds were mixed in EtOH and stirred gently for several hours. The cyanuric acid–melamine (CM) complexes were collected by centrifugation, dried under vacuum, and calcined at 550 8C to generate nanostructured ECN. In a typical synthesis, for ECN, melamine (1.26 g,10 mmol) and cyanuric acid (1.29 g, 10 mmol) were ground and mixed in ethanol for 8 h in a beaker. For DCN, equimolar amounts of melamine and cyanuric acid were separately dissolved in DMSO with ultrasonication. The two clear solutions were mixed slowly and stirred gently for 30 min to give white precipitates. The white powders precipitated from both ethanol and DMSO were collected by centrifugation and washed with the original solvent (3 ). The resulting white powders were dried at 60 8C under vacuum to give the corresponding hydrogen-bonded supramolecular cyanuric acid–melamine complexes E-CM and D-CM. The final ECN and DCN products were subsequently obtained by calcination of E-CM and D-CM at 550 8C under an atmosphere of N2 for 4 h at a heating rate of 2.3 8C min1. Bulk C3N4 was synthesized as a reference sample by directly placing melamine (2 g) in a crucible followed by calcination at 550 8C under an atmosphere of N2 for 4 h (the same thermal treatment conditions as those used for the synthesis of ECN and DCN).
Characterization The morphologies of the as-prepared samples were examined by field-emission scanning electron microscopy (FE-SEM, JEOL, JSM7001F) and transmission electron microscopy (TEM, JEOL, JEM2100). The AFM images and thickness of the ECN nanosheets were obtained by using atomic force microscopy (AFM, MFP-3DSA). Powder XRD measurements were performed with a Bruker D8 Advance diffractometer by using CuKa1 radiation (l = 1.5406 ). FTIR spectra were collected by using a Thermo Scientific FTIR spectrometer (Nicolet is5, iD5 ATR-Diamond mode). The light-harvesting ability was evaluated by using UV/Vis diffuse reflectance spectroscopy (Shimadzu UV-2600 spectrophotometer). The photoluminescence spectra were collected with a PerkinElmer LS-50B, and the surface electronic states were analyzed by using X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C). Nitrogen sorption measurements were accomplished with N2 at 77 K after degassing the samples at 150 8C under vacuum for 20 h by using a Quantachrome Quadrasorb SI porosimeter. The apparent surface area was calculated by applying the Brunauer–Emmett–Teller (BET) model to the isotherm data points of the adsorption branch. ESR measurements were conducted with a Bruker A300 Spectrometer Model.
Photocatalytic degradation experiments and O2 evolution Rhodamine B (RhB) was chosen as a model dye for the photocatalytic degradation experiments of the as-prepared samples. A lightemitting diode (LED) white-light module served as the light source for photocatalytic degradation of RhB. Typically, RhB solutions (50 mL, 20 mg L1) containing the samples (40 mg) were put in a glass beaker, first ultrasonicated and then stirred in the dark to ensure absorption–desorption equilibrium. After visible-light illumination, aliquots (3 mL) of the samples were taken out at regular time intervals and separated through centrifugation. The degradation efficiencies of RhB were monitored by using UV/Vis absorption spectroscopy to measure the concentration of RhB remaining in solution. The changes in the concentration were determined by the ratio C/C0, in which C0 is the initial concentration of RhB and C is the remaining concentration of RhB. The concentration of RhB was calculated by recording variations in the absorption band maximum (553 nm) in the UV/Vis spectra. For comparison, control experiments were performed in which, one, a blank sample of only RhB was employed under light irradiation and, two, mixtures of RhB and the catalyst were kept in the dark while other reaction conditions were kept constant. For radical-trapping experiments, 5 mm p-benzoquinone, disodium ethylenediamintetraacetate, and tert-butyl alcohol were employed as the scavengers for superoxide radical, hydroxyl radicals and hole, respectively.
Formation of the supramolecular complexes was confirmed (Figure S1 a) by XRD measurements of the hydrogen-bonded melamine–cyanuric acid complexes (E-CM, D-CM). Additionally, the ATRFTIR spectra of melamine, cyanuric acid, and the hydrogen-bonded supramolecular melamine–cyanuric acid complex were also recorded (Figure S2 a). It was clearly shown that the C = O stretching bands of cyanuric acid were shifted from n˜ = 1694 and 1755 cm1 to n˜ = 1734 and 1781 cm1, whereas the triazine ring vibration of melamine was observed to shift from n˜ = 807 to 765 cm1, which confirmed the generation of a hydrogen-bonded supramolecular melamine–cyanuric acid complex.
Synthesis of Ag3PO4/C3N4 composites The Ag3PO4/C3N4 composites were prepared by the combination of electrostatically driven assembly and the ion-exchange method. In a typical synthesis, C3N4 (400 mg) was ground gently and then ultrasonicated in deionized water to give C3N4 aqueous dispersions with well-defined nanostructures. AgNO3 (1.53 g, 9 mmol) was then dissolved in deionized water (30 mL), and this solution was added dropwise to the corresponding C3N4 dispersion. The obtained mixture was further stirred overnight to promote the adsorption of Ag + on the surface of C3N4. The addition of an aqueous solution of Na3PO4 (0.1 m, 30 mL) slowly into the suspended solution resulted in the immediate formation of a green precipitate, and the mixture was stirred for another 5 h at room temperature. The precipitate was centrifuged, washed with deionized water (3 ) and with ethanol (2 ), and dried at 60 8C under vacuum to give different kinds of Ag3PO4/C3N4 composites. The Ag3PO4/C3N4 composites with dif-
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Photocatalytic O2 evolution was monitored by an oxygen probe (PreSens Precision Sensing GmbH, Fibox 3 fiber optic oxygen transmitter) in a sealed double-layered flask connected to a water-cooling system. Before illumination, the oxygen probe was calibrated with temperature compensation by standard calibration solution 0 (oxygen-free water) and calibration solution 100 (air-saturated water). Under light irradiation, the temperature of the reaction system was maintained almost constant by connecting cooling water. Furthermore, the oxygen meter was temperature compensated and was equipped with a precision temperature sensor,
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Full Papers which indicated that oxygen measurements in environments with changing temperature could be taken and optimum results could be obtained. For each O2 evolution reaction, the photocatalyst powder (0.3 g) was first dispersed by ultrasonication for 10 min in deionized water (100 mL) containing an aqueous solution of AgNO3 (10 g L1). The mixture was then degassed for 30 min by N2 to remove oxygen before light irradiation with a LED white-light module. The photocatalytic O2 evolution efficiencies over different photocatalysts were determined by measuring the evolved O2 produced from water under regular irradiation time intervals. Additionally, a variety of control experiments including a blank sample, dark conditions, and photocatalyst in the absence of the scavenger AgNO3 were performed for comparison. The band structure of the Ag3PO4/ECN composite could be calculated according to the following empirical equations [Eqs. (1) and (2)]: E VB ¼ cE e þ 0:5 E g
ð1Þ
E CB ¼ E VB E g
ð2Þ
[3] [4]
in which EVB is the valence band edge potential and ECB is the conduction band edge potential; c is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; and Ee is the energy of free electrons on the hydrogen scale ( 4.5 eV vs. NHE).
[5]
[6]
Acknowledgements M.S. acknowledges “Minerva Fellowship”, and X.Y. is thankful for a fellowship from the Max Planck Society. This work was also financially supported by the National Natural Science Foundation of China (51102116), Natural Science Foundation of Jiangsu Province (BK2011480), and Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-Aged Teachers, Excellent Young Teachers Program of Jiangsu University, China.
[7]
[8]
Keywords: nanostructures · nitrides · oxygen evolution · photocatalysis · silver phosphate [9]
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Received: October 23, 2014 Revised: December 16, 2014 Published online on && &&, 0000
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FULL PAPERS X. Yang, H. Tang, J. Xu, M. Antonietti, M. Shalom*
Composite drawing: Well-defined gC3N4/Ag3PO4 composite materials are fabricated by using electrostatically driven assembly and ion-exchange processes. Clear evidence for the formation of a Z-scheme between the g-C3N4/ Ag3PO4 photocatalyst with outstanding efficiency in oxygen evolution and pollutant degradation is presented. The composite material is superior in both efficiency and stability than pristine Ag3PO4 and C3N4 materials.
&& – && Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution
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Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution Xiaofei Yang,[a, b] Hua Tang,[b] Jingsan Xu,[a] Markus Antonietti,[a] and Menny Shalom*[a] Herein, we show the facile synthesis of an efficient silver phosphate/graphitic carbon nitride (Ag3PO4/g-C3N4) photocatalyst for oxygen production and pollutant degradation by using electrostatically driven assembly and ion-exchange processes. The composite materials demonstrate a sheet-like C3N4 structure, decorated with different Ag3PO4 particles sizes. Detailed analysis of the reactions mechanism by electron-spin resonance and radical-capture agents strongly imply the formation of an in situ Z-scheme by the evolution of small silver nanoparticles in the interface of the materials under illumination. The
Ag nanoparticles improve charge separation within the composite material by acting as a storage and recombination center for electrons and holes from Ag3PO4 and C3N4, respectively. In addition, the photostability of Ag3PO4 is enhanced relative to that of the bulk materials, which results in a stabilized heterojunction. We believe that this work provides new insight into the operation mechanism of composite photocatalysts for water splitting and opens the possibility for advanced photocatalysis based on the higher oxidation power of Ag3PO4.
Introduction The photocatalytic splitting of water into hydrogen and oxygen by semiconductors is receiving significant attention,[1] as it presents a promising approach for the environmentally friendly conversion of solar energy into clean and storable chemical energy. Whereas the generation of H2 by a photocatalyst has been heavily explored over the past years and rates have been significantly improved,[2] the complimentary reaction (i.e., water oxidation), which is considered more difficult owing to the multiple reactions steps of four holes, remains the major obstacle for the overall water-splitting reaction. For photocatalytic O2 evolution, the basic requirement is that the valence band of the semiconductor must be more positive than the oxidation potential of H2O to O2 [1.23 V vs. normal hydrogen electrode (NHE), pH 0]. Furthermore, a significant overpotential is needed to overcome the activation energies in the charge-transfer process between the photocatalyst and water molecules.[3] Therefore, although a variety of materials have been developed over the past few decades, there are very few materials[4] that can directly oxidize water into O2 under illumination. Most recently, the use of a silver orthophosphate (Ag3PO4) semiconductor for visible-light-driven O2 evolution was reported.[5] Ag3PO4 demonstrates an excellent photooxida-
tive capability for O2 evolution, although it is not reductive enough to directly reduce H2O to H2. Despite the high activity of Ag3PO4 for O2 generation, it suffers from poor photostability as a result of self-reduction to silver with the leftover electrons (either in the presence or absence of a sacrificial agent). In addition, the regular synthetic path of Ag3PO4 results in big particles with low surface area, which leads to poor charge separation and comparably low photoactivity. To improve its performance, efforts have been made to design and synthesize Ag3PO4-based hybrid materials for photocatalytic applications. The combination of Ag3PO4 with additional materials can result in smaller particles, higher charge mobility, and in the creation of a local heterojunction at the interface with the supporting material. An ideal complementary material should have suitable electronic properties or band structure to quickly take up the electrons, high surface area, and an acceptable electronic conductivity. It is interesting to note that Ag3PO4-based materials are mostly used for the photodegradation of organic pollutants,[6] although their first use as a photocatalyst was for O2 evolution.[5] A suitable material for hybridization with Ag3PO4 is graphitic carbon nitride (g-C3N4), which has been attracting widespread attention in the photo(electro)catalysis field (i.e., water splitting, electrocatalysis, and more).[2j,n, 7] This is due to its appropriate bandgap, which engulfs both the reduction and oxidation potential of water (2.7 eV) and its unusually high thermal and chemical stability. In the last year, a few synthetic paths to form Ag3PO4/g-C3N4 composite photocatalysts were introduced.[8] However, for all of these reports only the photodegradation of organic pollutants was shown. Moreover, the proposed mechanism for the composite materials involved unfavorable electron injection from the conduction band (CB) of gC3N4 CB to the CB of Ag3PO4, whereas the holes transfer from
[a] Dr. X. Yang, Dr. J. Xu, Prof. M. Antonietti, Dr. M. Shalom Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces 14424 Potsdam (Germany) E-mail: [email protected] [b] Dr. X. Yang, Dr. H. Tang School of Materials Science and Engineering Jiangsu University 301 Xuefu Road Zhenjiang 212013 (P.R. China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403168.
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Full Papers obtain Ag + /g-C3N4 that was subsequently converted into Ag3PO4/g-C3N4 nanocomposites by the addition of trisodium phosphate. The Ag3PO4/C3N4 composites with different amounts of g-C3N4 are denoted as ECNX and DCNX (X is the actual mass of g-C3N4 used, in mg) for ethanol and DMSO, respectively. We note that for simplicity we only show the characterizations of the most active composite material derived from ethanol (ECN400) in the main text, whereas characterization of all the other composite materials is given in the Supporting Information. The crystal structure of the obtained samples was investigated by X-ray diffraction (XRD, Figure 2 a). The peak at 2 q = 27.18 is attributed to the (002) reflection of ECN (interplanar stacking peak of conjugated aromatic systems), whereas the (100) peak at 2 q = 13.38 corresponds to in-plane structural packing motifs of the aromatic systems. Compared to bulk C3N4, ECN exhibits lower intensity and broadened width, which suggests that ECN Results and Discussion was well delaminated throughout sonication. For the composite materials, owing to the high crystallinity of Ag3PO4 alongWell-defined g-C3N4 nanostructures were synthesized by therside the small amount of C3N4 in the composite materials, only mal condensation of hydrogen-bonded supramolecular aggrethe diffraction peaks of body-centered cubic-phase Ag3PO4 gates obtained from equimolecular mixtures of melamine and (JCPDS card No.06-0505) were observed. The chemical struccyanuric acid in absolute ethanol (EtOH) and dimethyl sulfoxture of the obtained g-C3N4 samples was further studied by ide (DMSO).[9] We note that in this C3N4 preparation method using attenuated total reflectance Fourier-transform infrared (compared to the traditional one for preparing g-C3N4, i.e., con(ATR-FTIR) spectroscopy. The spectrum of ECN (Figure 2 b) densation of melamine) higher photoactivity is obtained. In adshows characteristic bands in the n˜ = 1200–1700 cm1 region dition, with this synthetic method it is easier to obtain nanothat correspond to the stretching modes of CN heterocycles. In sheets of C3N4, which can be further used to form composite addition, typical breathing vibrations of the triazine units at apmaterials. Afterwards, the resulting g-C3N4 materials were used proximately n˜ = 810 cm1 are observed. For the as-prepared as the matrix for the growth of Ag3PO4 (for more details see Ag3PO4, a band is observed at approximately n˜ = 1385 cm1, atthe Experimental Section). The reaction conditions for the syntributed to the HOH bending band of absorbed water molethesis of Ag3PO4/g-C3N4 nanocomposites are listed in Table S1 cules, alongside PO stretching vibrations of the PO43 ions at (Supporting Information), and a schematic illustration of the n˜ = 943 and 668 cm1. For the composite material (ECN400), synthetic pathway is shown in Figure 1. Briefly, as-prepared gboth the characteristic bands of C3N4 and Ag3PO4 are clearly C3N4 was ultrasonicated to get stable dispersions with fewobserved. For ECN400, also a redshift in the absorption edge layers g-C3N4, and the dispersed ultrathin g-C3N4 nanosheets (Figure 2 c, from l = 460 nm for pure C3N4 to l 520 nm for were negatively charged with a z potential of approximately ECN400) is observed, which indicates hybridization of ECN and 46 mV. Silver nitrate was then deposited into g-C3N4 to Ag3PO4 to a joint electronic system beyond the bare addition of the spectra. It is notable that drastic quenching of the photoluminescence intensity of ECN was observed upon hybridization of Ag3PO4 (Figure 2 d); this implies that the latter creates new nonradiative paths for charge separation of photogenerated electron and/or holes. The fluorescence quenching is presumably due to the formation of an effective heterojunction at which charge transfer occurs between ECN and Ag3PO4, and this prevents the direct recombination of photoinduced electrons and holes within C3N4. The formation of Figure 1. Schematic illustration of the syntheses of nanostructured g-C3N4 and the Ag3PO4/g-C3N4 nanocomposite.
the valence band (VB) of Ag3PO4 to the VB of g-C3N4 (Figure 6 c), that is, the hybrid would only loose activity because of the continuance of the two unfavorable states instead of adding the relative relativities. Especially, it is noteworthy that the mentioned mechanism is unfavorable for the generation of O2 from water owing to the low photocatalytic activity of gC3N4 towards water oxidation. Herein, we show a synthesis of well-defined g-C3N4/Ag3PO4 composite materials by a two-step procedure to resolve this problem by an appropriate nanostructure. The photocatalytic properties were tested by measuring the O2 evolution and the degradation of the organic dye rhodamine B (RhB) under visible-light illumination in the presence of the g-C3N4/Ag3PO4 composite. Moreover, the charge separation under illumination was studied by electron spin resonance (ESR) in the presence of hole and radical scavengers.
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Full Papers Further investigation of the chemical composition of the samples was provided by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of ECN400 (Figure 3 a) reveals the presence of C, N, O, P, and Ag and a C/N atomic ratio of 3:4, which is in good agreement with the nominal composition of as-prepared ECN (Figure S5 a). Two characteristic peaks at binding energies of 368.2 and 374.2 eV that correspond to the Ag 3d5/2 and Ag 3d3/2 orbitals, respectively, were found (Figure 3 b), and this suggests the dominance of AgI in the composite surface. The high-resolution C 1s peak (Figure 3 c) was deconvoluted into two peaks at binding energies of 284.8 and Figure 2. a) XRD patterns, b) ATR-FTIR spectra, c) UV/Vis diffuse reflectance spectra, and d) PL spectra of the as288.2 eV corresponding to prepared samples. carbon atoms in a purely carbonaceous environment and typiheterojunctions increases the lifetime of the electrons and cal aromatic CNC coordination in a graphitic carbon nitride holes and, therefore, their possibility to participate in the phoframework, respectively. The corresponding N 1s peak (Figtocatalytic reaction. More information about the XRD pattern ure 3 d) could be deconvoluted into three peaks at binding enand the ATR-FTIR, UV/Vis diffuse reflectance, and photolumiergies of 398.6, 400.3, and 401.3 eV. The peak at 398.6 eV is asnescence (PL) spectra of the different samples are shown in signed to the aromatic N atom bonded to two C atoms in the Figures S1–S4. triazine or heptazine rings (C=NC), whereas the peak at 400.3 eV is ascribed to the sp2-hybridized N atom bonded to three atoms [CN(C)C or CN(H)C]. The peak at 401.3 eV is attributed to the sp3-hybridized terminal N atom (NH2 or NO) of the triazine rings. A field-emission scanning electron microscopy (FE-SEM) study shows that the bare ECN consists of curved nanosheets with lamellar morphology (Figure S6 a1–a3). For ECN400 (Figure 4 a, b), the surfaces of g-C3N4 are decorated with irregular Ag3PO4 particles (with diameters of 40–300 nm) that are either on the surface or in the pores of g-C3N4. Additional FE-SEM images of g-C3N4 and Ag3PO4/gC3N4 composites are shown in Figures S6 and S7, and energydispersive X-ray spectroscopy (EDX) results further confirm the presence of all elements, that is, Figure 3. a) Full XPS spectrum and b) high-resolution Ag 3d peak, c) C 1s peak, and d) N 1s peak of the Ag3PO4/gC, N, O, P, and Ag, in the comC3N4 composite ECN400. ChemSusChem 0000, 00, 0 – 0
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Full Papers higher photoactivity for the degradation of RhB (95 % in 10 min) than pure ECN. Despite the high photoactivity of Ag3PO4, the incorporation of Ag3PO4 with moderate amounts of ECN (100, 200, and 400 mg) resulted in another clear enhancement in the photocatalytic activity, whereas an excessive amount of ECN (800 mg) resulted in a significant decrease in the degradation rate. Furthermore, it is important to note that the direct photolysis of RhB without ECN400 or the decomposition with ECN400 in the dark are negligible (Figure 5 b). To investigate the photocatalytic mechanism, three different scavengers, p-benzoquinone (BZQ), disodium ethylenediaminetetraacetic acid (Na2-EDTA), and tert-butyl alcohol, were employed for the radical- and hole-trapping experiments. As shown in Figure 5 c, upon using Na2-EDTA as the direct hole scavenger, a remarkable inhibitory effect on the degradation efficiency of RhB over both ECN400 and Ag3PO4 was observed. In these experiments, only less than 4 % RhB was degraded. The addition of BZQ (an O2C radical scavenger) into the photocatalytic system also caused a clear decrease in the degradation efficiency to 58 % for ECN400 and 40 % for Ag3PO4, whereas the presence of tert-butyl alcohol (a COH radical scavenger) had a negligible effect on the photocatalytic activity of RhB. The hole- and radical-trapping experiments imply that direct transfer of photoinduced holes and the coupled electron transfer to generate O2C radicals is the dominant mechanism responsible for the highly efficient visible-light-driven photocatalytic degradation performance (Figure S13). Besides the photocatalytic activity, the stability of a photocatalyst is also important for its practical application. The photodegradation of RhB over ECN400 was retained at over 90 % after three successive photocatalytic cycles, whereas the photodegradation activity of RhB over pure Ag3PO4 decreased markedly to 43 % (Fig-
Figure 4. Low- and high-magnification a, b) FE-SEM images and c, d) TEM images of ECN400 (scale bars in correspond to 100 nm).
posite (Figure S8). In addition, the thickness of ultrasonicated ECN nanosheets was estimated by atomic force microscope (AFM) to be down to 2 nm (Figure S9). The transmission electron microscopy (TEM) images of ECN400 (Figure 4 c, d) confirm the presence of Ag3PO4/ECN heterostructures, in which ECN serves as a support for the Ag3PO4 nanoparticles. Additional TEM images of g-C3N4 and the Ag3PO4/g-C3N4 composites are shown in Figures S10 and S11. Notably, almost no free particles are found outside the g-C3N4 sheets in the TEM images, which implies the successful deposition of Ag3PO4 particles on the gC3N4 surface. Furthermore, the surface area of the materials was studied by nitrogen sorption experiments. Table S2 summarizes the values of the surface areas of the as-prepared samples and the nitrogen adsorption–desorption isotherms, and the corresponding poresize distribution curves are shown in Figure S12. Higher surfaces areas were obtained for pure ECN (50.2 m2 g1) and DCN (62.4 m2 g1). Subsequent introduction of Ag3PO4 leads to a decrease in the specific surface area of the Ag3PO4/g-C3N4 composite as a result of the heavier silver. To evaluate the photocatalytic activity of the obtained samples, the photodegradation of RhB dye as a model system was studied in the presence of pure ECN and Ag3PO4 and the Ag3PO4/g-C3N4 composite photocatalyst under visible-light irFigure 5. a) Photodegradation performance of RhB dye in the presence of the composite materials compared to radiation (Figure 5 a). Ag3PO4 the pure ones, b) control experiments of ECN 400, c) degradation efficiency by using different radical scavengers, alone already exhibited much and d) comparison of the photostability of ECN400 and Ag3PO4.
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Full Papers material, most probably as a result of the higher relative reaction rate, that is, O2 is not only generated, but at higher concentrations it is also consumed by the secondary reduction process. Supporting this picture, in the presence of a sacrificial agent no clear decrease in the O2 concentration was observed because of the kineticdominant process of AgNO3 reduction (instead of O2). The photostability of ECN400 was evaluated by measuring the recycled sample three times (Figure 6 c). After three photooxidation cycles, the best sample still preserved more than 40 % of the initial oxygen generation. We note that in spite of the photostability enhancement for Figure 6. O2 evolution of a) ECN-derived samples in the presence of AgNO3 ; b) ECN400 and Ag3PO4 in the presthe composite material, we still ence or absence of AgNO3 ; c) over-recycled ECN400; and d) by using mixed ECN and Ag3PO4, ECN400, and no observed some decomposition photocatalyst. We note that without a photocatalyst the O2 concentration did not change. of active Ag3PO4 into metallic Ag after the recycling experiments (Figure S14), which we attribute to a missing balance ure 5 d). The decrease in the photocatalytic activity can be atbetween the (in this system too fast) water oxidation and a sectributed to the decomposition of Ag3PO4 to Ag by the remainondary reduction process. To confirm the advantage of the ing electrons (self-reduction) under light irradiation in the abin situ growth of Ag3PO4 on carbon nitride, we measured the sence of a sacrificial agent for the electrons. The self-reduction of Ag3PO4 in the composite materials is clearly strongly inhibitphotoactivity of physically mixed C3N4 and Ag3PO4 in the presed, which is seen as direct evidence for efficient electron transence of AgNO3 (Figure 6 d). The physically mixed material exfer from Ag3PO4 to C3N4, as it will be discussed below. hibited activity that was five times lower than that of the optimized material, which indicates that the hybridization of ECN Figure 6 a shows the results of the water splitting/O2 evoluand Ag3PO4 plays an important role in the material photoactivition experiments as a function of time in the presence of ECN, Ag3PO4, and Ag3PO4/ECN composites under white-light irradiaty over O2 evolution. Furthermore, a similar tendency in terms tion in the present of silver nitrate (AgNO3) as a sacrificial elecof both efficiency and photostability was observed for the tron acceptor. A negligible amount of O2 was evolved from the Ag3PO4/DCN composites (Figure S15) irradiated ECN, whereas for pure Ag3PO4 the O2 concentration To further investigate the mechanisms beyond the high acincreased to 10 mmol L1 after 30 min. For the Ag3PO4/ECN tivity of ECN400, we studied the generation of active radicals composites, the O2 generation was already enhanced by 30 % involved in the photocatalytic system by in situ ESR analysis of (up to 13 mmol L1) composited with only a minor ECN content the obtained samples with 5,5-dimethyl-1-pyrroline-N-oxide (ECN100). By increasing the carbon nitride content further (DMPO), a spin trapping reagent, in MeOH and H2O. As shown (ECN200 and ECN400), the O2 concentration increased (20 and in Figure 7 a, incubating Ag3PO4 with DMPO in H2O under light 25 mmol L1, respectively). At higher C3N4 concentration shows a rapid production of an ESR spectrum with seven char(ECN800, 35 % ECN), the evolved O2 concentration strongly acteristic bands. The ESR spectrum can be assigned to a derivative of DMPO, 5,5-dimethyl-2-ketopyrrolidino-1-oxyl (DMPOX), decreased to 5 mmol L1. In the absence of AgNO3 (Figure 6 b), which is generally produced by the interaction between active the concentration of evolved O2 over Ag3PO4 increased to hydroxyl radicals and DMPO–COH adducts.[10] DMPOX produc8 mmol L1 in 20 min, whereas the concentration of evolved O2 1 tion is maximized after approximately 4 min from the irradiatover ECN400 reached a capacity of 32 mmol L (fourfold ined Ag3PO4 system. A similar phenomenon was observed also crease). However, when both two samples achieved their highest O2 evolution efficiency, subsequent light irradiation (more with the incubation of ECN400 and DMPO under light. Moreover, in accordance with the photocatalytic measurements, than 3 h) caused a gradual decrease in evolved O2. We attriDMPOX production appears faster with a higher intensity (Figbute this to the in situ consumption of the as-generated O2 by ure 7 b). For comparison, DMPO spin-trapping experiments of further oxygen reduction promoted by the as-generated elecAg3PO4 and ECN400 in the presence of the sacrificial agent trons at the carbon nitride subphase. Moreover, after oxygen saturation the O2 reduction rate was faster in the composite AgNO3 were performed (Figure 7 c). It was found that the typiChemSusChem 0000, 00, 0 – 0
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Full Papers ductor heterojunction, the expected charge separation and transfer are usually governed by the thermodynamics driving force (Figure 8 c) of the system (positions of the conduction and valence bands). Taking into account the positions of the related energy levels in the ECN400 composite in the conventional heterojunction structure under illumination, the electrons and holes would accumulate in the CB of Ag3PO4 and the VB of ECN, respectively. However, in this scenario, the oxidation of water either to O2 or to active COH becomes practically impossible because of the weak energetic driving force along with the weaker photocatalytic properties of ECN for oxiFigure 7. ESR spectra of radical adducts trapped by DMPO in a) aqueous dispersion of Ag3PO4 ; b) aqueous disperdation reactions. However, all sion of ECN400; c) Ag3PO4 and ECN400 in the presence or absence of AgNO3 ; and d) ECN, Ag3PO4, and ECN400 our experiments (photocatalytic methanol dispersions. degradation and radical-capture experiments, O2 evolution, and ESR results) suggest that the charge-transfer mechanism of the cal bands from DMPOX disappear with the addition of AgNO3 Ag3PO4/ECN composite should be different from that of the under 2 min of irradiation, which suggests that fewer COH radicals are generated at the earlier stage. For all the samples, no previously described, conventional heterostructure systems, ESR signal in the dark was observed, and this indicates that and the energies of the two subsystems rather add to each light irradiation is crucial to the generation of active radicals. other, presumably to a kinetically preferred coupling of the CB Additionally, six characteristic bands corresponding to DMPO– of Ag3PO4 and the VB of ECN, which essentially balances those O2C adducts were detected in MeOH under light for all three two electronic states. As a result, a Z-scheme photocatalytic mechanism for the samples (Figure 7 d). The intensity of the DMPO–O2C adduct degradation of RhB and O2 evolution is proposed (Figure 8 d). from ECN400 is higher than that of the ECN one and similar to that of the Ag3PO4 sample. In addition, no ESR signal from In the beginning, the system behaves similar to an ordinary DMPO–COH adducts was observed from only a ECN or DCN heterojunction, and electrons can pass from the CB of ECN to aqueous dispersion under visible-light irradiation (Figure S16). Ag3PO4 (Figure 8 c, path 1), or they are generated in Ag3PO4 as On the basis of the above experimental evidence, we prosuch. These electrons have enough reduction power to convert pose a photocatalytic mechanism of water splitting for the Ag3PO4 into Ag. Consequently, under illumination, the twophase (Ag3PO4/ECN) system transforms into a three-phase Ag3PO4/ECN composite with an emphasis on O2 evolution to occur through a Z-scheme. Figure 8 shows a schematic reprecomposite (Ag3PO4/Ag/ECN), even in the absence of AgNO3 sentation of different photocatalytic processes. From the ther(see also Figure S14 b–d). Interestingly, this Ag3PO4 reduction modynamic limitations it is clear that under illumination only seems to occur mainly at the interface of the materials, more holes from the Ag3PO4 subsystem can generate OH radicals specifically exactly the point at which the ECN subphase and Ag3PO4 electronically communicate. The formation of Ag nano(Figure 8 a, b) because of their strong oxidation potential. On the contrary, in ECN COH radicals can only be generated by the particles at the interface of the material opens a faster chargereduction of dissolved oxygen in solution. In addition, the transfer channel connecting the CB of Ag3PO4 to the VB of holes of both systems have a suitable driving force for the ECN, which then seems to be kinetically more energetically fadirect oxidation of RhB. This hypothesis is in accordance with vorable. The new charge-transfer process consists of Ag nanothe ESR spectra and the reactive species trapping experiments, particles as a recombination center for CB-Ag3PO4 electrons which clearly demonstrate the dominance of holes and O2C (Figure 8 d, path 2) and VB-C3N4 holes (Figure 8 d, path 3), which is also in good agreement with the positions of the radicals in the photocatalytic process. Moreover, the driving energy levels.[8c] As a consequence, a larger amount of elecforce for O2 evolution is dramatically higher for Ag3PO4 than for ECN as a result of the position of the valence band and trons accumulate in the CB of ECN, which is coupled to higher also as a result of the known fact that silver derivatives are oxiactivity in the reduction of oxygen into active superoxide radidation catalysts. In the construction of a conventional semiconcals (Figure 8 d, path 4) (which further react to hydroxyl radi-
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Figure 8. Schematic for the energy-band structure of a) Ag3PO4 and b) ECN; c) conventional mechanism; and d) proposed Z-scheme mechanism for charge transfer. The band-gap values of Ag3PO4 and ECN were estimated to be 2.45 and 2.70 eV, respectively (see also the Supporting Information for more details). The EVB of Ag3PO4 and ECN were calculated to be 2.90 and 1.53 eV vs. NHE, respectively, whereas their corresponding ECB were 0.45 and 1.17 eV vs. NHE, respectively.
cals), whereas the holes in the VB of Ag3PO4 present superior activity in the oxidation of water into O2 and in the direct oxidation of RhB (Figure 8 d, path 5). We believe that the current kinetic limiting factor of the system is electron accumulation within ECN, which leads to a higher recombination of electrons and holes within carbon nitride (Figure 8 d, path 6), along with a shorter electron–hole lifetime. A smaller concentration of holes within ECN also hinders their recombination with Ag3PO4 electrons, which leads to further self-reduction to Ag and the coupled (slow) self-destruction of the photocatalytic tandem system. This in fact could be tested by more efficient sacrificial electron acceptors or by the employment of reduction catalysts, which were left out for simplicity in the present system.
sion into Ag3PO4 nanoparticles. The composite materials possess a sheet-like C3N4 structure with variable Ag3PO4 particles sizes, depending on the solvent and the molar ratio of the starting material. The composite materials exhibited high photocatalytic performance in both oxygen evolution and RhB degradation along with an improvement in photostability. The reaction mechanism was carefully studied by radical-capture agents and electron spin resonance. Our results show that under illumination, an in situ Z-scheme is created, which involves the spontaneous formation of Ag nanoparticles in the interface of the materials, which then enables the recombination of electrons from Ag3PO4 and holes from g-C3N4. This nanoscopic metallic Ag mediator thereby promotes the “addition path” of the band-gap energies and thereby the possibility for efficient O2 evolution under illumination while keeping the high reduction potential of carbon nitride. We believe that this work opens the possibility to design more efficient photocatalysts for the production of solar fuels and other highenergy, uphill reactions.
Conclusions In summary, we reported the synthesis of Ag3PO4/g-C3N4 photocatalysts by the assembly of Ag + on the surface of delaminated graphitic C3N4 nanostructures, followed by their converChemSusChem 0000, 00, 0 – 0
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ferent amounts (100, 200, 400, and 800 mg) of ECN and DCN are denoted as ECN100, ECN200, ECN400, ECN800 and DCN100, DCN200, DCN400, DCN800, respectively.
Synthesis of C3N4 Carbon nitrides from ethanol (ECN) and DMSO (DCN) were synthesized by using equimolar amounts of melamine and cyanuric acid according to the modified method reported in previous papers.[9] Whereas melamine and cyanuric acid are insoluble in EtOH, they are both soluble in DMSO. Therefore, for g-C3N4 derived from DMSO, the two starting materials were first separately dissolved in DMSO. The slow addition of one clear solution into the other resulted in the generation of a white precipitate. For the g-C3N4 sample derived from EtOH, the two compounds were mixed in EtOH and stirred gently for several hours. The cyanuric acid–melamine (CM) complexes were collected by centrifugation, dried under vacuum, and calcined at 550 8C to generate nanostructured ECN. In a typical synthesis, for ECN, melamine (1.26 g,10 mmol) and cyanuric acid (1.29 g, 10 mmol) were ground and mixed in ethanol for 8 h in a beaker. For DCN, equimolar amounts of melamine and cyanuric acid were separately dissolved in DMSO with ultrasonication. The two clear solutions were mixed slowly and stirred gently for 30 min to give white precipitates. The white powders precipitated from both ethanol and DMSO were collected by centrifugation and washed with the original solvent (3 ). The resulting white powders were dried at 60 8C under vacuum to give the corresponding hydrogen-bonded supramolecular cyanuric acid–melamine complexes E-CM and D-CM. The final ECN and DCN products were subsequently obtained by calcination of E-CM and D-CM at 550 8C under an atmosphere of N2 for 4 h at a heating rate of 2.3 8C min1. Bulk C3N4 was synthesized as a reference sample by directly placing melamine (2 g) in a crucible followed by calcination at 550 8C under an atmosphere of N2 for 4 h (the same thermal treatment conditions as those used for the synthesis of ECN and DCN).
Characterization The morphologies of the as-prepared samples were examined by field-emission scanning electron microscopy (FE-SEM, JEOL, JSM7001F) and transmission electron microscopy (TEM, JEOL, JEM2100). The AFM images and thickness of the ECN nanosheets were obtained by using atomic force microscopy (AFM, MFP-3DSA). Powder XRD measurements were performed with a Bruker D8 Advance diffractometer by using CuKa1 radiation (l = 1.5406 ). FTIR spectra were collected by using a Thermo Scientific FTIR spectrometer (Nicolet is5, iD5 ATR-Diamond mode). The light-harvesting ability was evaluated by using UV/Vis diffuse reflectance spectroscopy (Shimadzu UV-2600 spectrophotometer). The photoluminescence spectra were collected with a PerkinElmer LS-50B, and the surface electronic states were analyzed by using X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C). Nitrogen sorption measurements were accomplished with N2 at 77 K after degassing the samples at 150 8C under vacuum for 20 h by using a Quantachrome Quadrasorb SI porosimeter. The apparent surface area was calculated by applying the Brunauer–Emmett–Teller (BET) model to the isotherm data points of the adsorption branch. ESR measurements were conducted with a Bruker A300 Spectrometer Model.
Photocatalytic degradation experiments and O2 evolution Rhodamine B (RhB) was chosen as a model dye for the photocatalytic degradation experiments of the as-prepared samples. A lightemitting diode (LED) white-light module served as the light source for photocatalytic degradation of RhB. Typically, RhB solutions (50 mL, 20 mg L1) containing the samples (40 mg) were put in a glass beaker, first ultrasonicated and then stirred in the dark to ensure absorption–desorption equilibrium. After visible-light illumination, aliquots (3 mL) of the samples were taken out at regular time intervals and separated through centrifugation. The degradation efficiencies of RhB were monitored by using UV/Vis absorption spectroscopy to measure the concentration of RhB remaining in solution. The changes in the concentration were determined by the ratio C/C0, in which C0 is the initial concentration of RhB and C is the remaining concentration of RhB. The concentration of RhB was calculated by recording variations in the absorption band maximum (553 nm) in the UV/Vis spectra. For comparison, control experiments were performed in which, one, a blank sample of only RhB was employed under light irradiation and, two, mixtures of RhB and the catalyst were kept in the dark while other reaction conditions were kept constant. For radical-trapping experiments, 5 mm p-benzoquinone, disodium ethylenediamintetraacetate, and tert-butyl alcohol were employed as the scavengers for superoxide radical, hydroxyl radicals and hole, respectively.
Formation of the supramolecular complexes was confirmed (Figure S1 a) by XRD measurements of the hydrogen-bonded melamine–cyanuric acid complexes (E-CM, D-CM). Additionally, the ATRFTIR spectra of melamine, cyanuric acid, and the hydrogen-bonded supramolecular melamine–cyanuric acid complex were also recorded (Figure S2 a). It was clearly shown that the C = O stretching bands of cyanuric acid were shifted from n˜ = 1694 and 1755 cm1 to n˜ = 1734 and 1781 cm1, whereas the triazine ring vibration of melamine was observed to shift from n˜ = 807 to 765 cm1, which confirmed the generation of a hydrogen-bonded supramolecular melamine–cyanuric acid complex.
Synthesis of Ag3PO4/C3N4 composites The Ag3PO4/C3N4 composites were prepared by the combination of electrostatically driven assembly and the ion-exchange method. In a typical synthesis, C3N4 (400 mg) was ground gently and then ultrasonicated in deionized water to give C3N4 aqueous dispersions with well-defined nanostructures. AgNO3 (1.53 g, 9 mmol) was then dissolved in deionized water (30 mL), and this solution was added dropwise to the corresponding C3N4 dispersion. The obtained mixture was further stirred overnight to promote the adsorption of Ag + on the surface of C3N4. The addition of an aqueous solution of Na3PO4 (0.1 m, 30 mL) slowly into the suspended solution resulted in the immediate formation of a green precipitate, and the mixture was stirred for another 5 h at room temperature. The precipitate was centrifuged, washed with deionized water (3 ) and with ethanol (2 ), and dried at 60 8C under vacuum to give different kinds of Ag3PO4/C3N4 composites. The Ag3PO4/C3N4 composites with dif-
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Photocatalytic O2 evolution was monitored by an oxygen probe (PreSens Precision Sensing GmbH, Fibox 3 fiber optic oxygen transmitter) in a sealed double-layered flask connected to a water-cooling system. Before illumination, the oxygen probe was calibrated with temperature compensation by standard calibration solution 0 (oxygen-free water) and calibration solution 100 (air-saturated water). Under light irradiation, the temperature of the reaction system was maintained almost constant by connecting cooling water. Furthermore, the oxygen meter was temperature compensated and was equipped with a precision temperature sensor,
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Full Papers which indicated that oxygen measurements in environments with changing temperature could be taken and optimum results could be obtained. For each O2 evolution reaction, the photocatalyst powder (0.3 g) was first dispersed by ultrasonication for 10 min in deionized water (100 mL) containing an aqueous solution of AgNO3 (10 g L1). The mixture was then degassed for 30 min by N2 to remove oxygen before light irradiation with a LED white-light module. The photocatalytic O2 evolution efficiencies over different photocatalysts were determined by measuring the evolved O2 produced from water under regular irradiation time intervals. Additionally, a variety of control experiments including a blank sample, dark conditions, and photocatalyst in the absence of the scavenger AgNO3 were performed for comparison. The band structure of the Ag3PO4/ECN composite could be calculated according to the following empirical equations [Eqs. (1) and (2)]: E VB ¼ cE e þ 0:5 E g
ð1Þ
E CB ¼ E VB E g
ð2Þ
[3] [4]
in which EVB is the valence band edge potential and ECB is the conduction band edge potential; c is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; and Ee is the energy of free electrons on the hydrogen scale ( 4.5 eV vs. NHE).
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Acknowledgements M.S. acknowledges “Minerva Fellowship”, and X.Y. is thankful for a fellowship from the Max Planck Society. This work was also financially supported by the National Natural Science Foundation of China (51102116), Natural Science Foundation of Jiangsu Province (BK2011480), and Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-Aged Teachers, Excellent Young Teachers Program of Jiangsu University, China.
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[8]
Keywords: nanostructures · nitrides · oxygen evolution · photocatalysis · silver phosphate [9]
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Received: October 23, 2014 Revised: December 16, 2014 Published online on && &&, 0000
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FULL PAPERS X. Yang, H. Tang, J. Xu, M. Antonietti, M. Shalom*
Composite drawing: Well-defined gC3N4/Ag3PO4 composite materials are fabricated by using electrostatically driven assembly and ion-exchange processes. Clear evidence for the formation of a Z-scheme between the g-C3N4/ Ag3PO4 photocatalyst with outstanding efficiency in oxygen evolution and pollutant degradation is presented. The composite material is superior in both efficiency and stability than pristine Ag3PO4 and C3N4 materials.
&& – && Silver Phosphate/Graphitic Carbon Nitride as an Efficient Photocatalytic Tandem System for Oxygen Evolution
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ChemSusChem 0000, 00, 0 – 0
www.chemsuschem.org
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ÝÝ These are not the final page numbers!
