g-C3N4 Composite Catalysts

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Interface effects in sunlight driven Ag/g-C3N4 composite catalysts: Study of the toluene photo-degradation quantum efficiency Olga Fontelles-Carceller, Mario J. Muñoz-Batista, M Fernandez-Garcia, and Anna Kubacka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10434 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on December 30, 2015

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ACS Applied Materials & Interfaces

Interface effects in sunlight driven Ag/g-C3N4 composite catalysts: Study of the toluene photodegradation quantum efficiency Olga Fontelles-Carceller, Mario J. Muñoz-Batista,* Marcos Fernández-García, Anna Kubacka* Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28049-Madrid, Spain

Abstract Metallic silver (ranging from 1 to 10 wt. %) was deposited onto a graphite like carbon nitride photocatalyst through a microemultion method. Surface, morphological and structural properties of the resulting materials were characterized using BET and porosity measurements, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, UV-vis and photoluminescence spectroscopy. The activity of the composite samples under sunlight-type and visible illumination was measured for toluene photo-degradation and was analyzed by means of the reaction rate and the quantum efficiency parameter. To obtain the latter observable, the lamp emission properties as well as the radiation field interaction with the catalyst inside the reactor were modeled and numerically calculated. The stability of the samples under both illumination conditions was also studied. The results evidence that the composite samples containing 1 to 10 silver wt. % outperform carbon nitride for sunlight-type and visible illumination but the optimal use of the charge generated after light absorption is obtained for the sample with 1 wt. % of silver acording to the quantum efficiency calculation. The study shows that the optimum silver – g-C3N4 contact is able to outperform TiO2 reference systems (nano-TiO2 and P25) under sunlight illumination and points out that this occurs as a direct consequence of the charge handling through the interface between catalyst components. This indicates that composite systems based in g-C3N4 can be competitive in sunlight triggered photodegradation processes to eliminate tough polluctants such as toluene, rendering active and stable systems.

Keywords: Photo-catalysis; Carbon nitride; Silver; Toluene; Sunlight; Visible E-mail: [email protected] (Mario J. Muñoz-Batista); [email protected] (Anna Kubacka)

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1. Introduction Heterogeneous photocatalysis by semiconductors is an exciting technology applied to the abatement of pollutants in both liquid and gas phases. In this context, most of the studies have been focused on inorganic oxide/chalcogenide semiconductors, such as TiO2, ZnO, CdS and its structural and superficial modifications, being the anatase TiO2 the most widely material studied.1–4 However, the limited usage of visible light has restrained the photocatalytic application of the most utilized semiconductors, particularly anatase, in the field of environmental remediation. To purify contaminated air, alternatives to anatase photocatalysts should adsorb light over a wide electromagnetic wavelength range as well as exhibiting strong oxidative power and significant stability under reaction conditions. Recently, graphitic carbon nitride (g-C3N4) has been one of the most studied either as a single phase or as part of a photocatalyst because of its notable visible light absorption (Band Gap ~ 2.7 eV) and environmental stability.5–16 Nevertheless, the photocatalytic efficiency of the pure gC3N4 is limited by the high recombination rate of its photogenerated electron–hole pairs.6,7,9,17,18 To enhance its photocatalytic properties, many methods or procedures have been essayed. In particular combining g-C3N4 with different metals like Ag, Au, Pt to form composites or heterostructures, provides a feasible route toward improving the mentioned good photocatalytic properties of the bare carbon nitride.6,10,19,20 The combination of silver (and other metals) with g-C3N4 provides an ideal situation with potential improvement of the activity due to the fact that the surface of the semiconductor would likely be enriched in hole-related species, opening a way to enhancing the degradation capabilities in pollutant photooxidation processes. In this contribution, metallic silver was contacted with a g-C3N4 material with different weight ratios. The main aim of this work is to optimize the performance of the carbon 2 ACS Paragon Plus Environment

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nitride material to allow it to be competitive with titania based systems for photodegradation processes of pollutants under sunlight excitation. While graphic carbon nitride has been shown outstanding properties in hydrogen generation from water,21,22 its performance in elimination of pollutants seems, as mentioned above, relatively poor12,23 and thus requires optimization. To provide a rigorous and quantitative analysis, here we present results based on reaction rate and, more importantly, quantum efficiency calculations under both, sunlight-type and visible irradiation. The evaluation of the true quantum yield of the reaction requires the measurement and modelling of the radiation reached the surface of the material and the fraction effectively used in photo-catalytic steps. As literature reports consider different levels of calculation(s) concerning light-matter and chemical components of the efficiency parameter, we will report the efficiency observable within most typical formulations, such as the apparent quantum efficiency, as well as the most accurate ones, the so-called true quantum efficiency.1,2 Moreover, as mentioned, we will also carry out a comparison with titania-based systems under sunlight to test the performance of the Ag/g-C3N4 composite system against well known and highly active materials. Based in the reaction rate we provide evidence that the most active composite samples contain high silver load (5-10 wt.). These materials show similar activity-enhancement factors (with respect to the g-C3N4 reference) under sunlight-type and visible light illumination conditions. On the other hand, the quantum efficiency parameter displays an increasing value over the reference material results under both illumination conditions for the 1wt. % Ag over the g-C3N4 support. To interpret such behavior on physical basis here we propose a multitechnique examination of the materials with the help of a group of tools containing X-ray diffraction, transmission electron microscopy, 3

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X-ray photoelectron, ultraviolet and photo-luminescence spectroscopies, as well as N2 Physisorption measurements. Applying these tools we compile evidence indicating that photoactivity shows strong dependence of the illumination wavelength as an effect of the component playing the role of main (light) absorber and the subsequent interface handling of charge carriers. More importantly, the study shows that such interface is able to generate a system competitive with titania under sunlight, indicating the adequacy of carbon nitride materials in obtaining maximum profit of a green and renewable energy source as the sun.

2. Materials and methods 2.1.Catalyst preparation The graphitic carbon nitride was prepared by calcination of melamine (Aldrich), in a semi-closed system to prevent sublimation, at 580 °C for 4 h using a heating ramp of 5 °C min−1.9,23 Materials having the graphic carbon nitride as main component were synthesized by a single-pot microemulsion preparation method using n-heptane (Scharlau) as the organic medium, Triton X-100 (Aldrich) as a surfactant and hexanol (Aldrich) as a cosurfactant. The g-C3N4 was introduced into the organic phase of the microemulsion. After 30 min of stirring a certain quantity of AgNO3 (0.5 M) aqueous solution was added into the organic phase. After 1 h, a NaBH4 aqueous solution (0.1 M) was quickly added into the solution under continuous vigorous stirring. Water/Ag and water/surfactant molar ratios were, respectively, 110 and 18 for all samples.24,25 The resulting mixture was stirred for 24 h and then centrifuged, and the separated solid precursors were rinsed with ethanol, distillated water and acetone, and dried at 60 °C for


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12 h. The sample names were xAg/g-C3N4 for the composite samples, where x is the wt. % (1, 2, 5 and, 10) of Ag with respect to g-C3N4. Silver content of the samples was measured using Inductive Coupled Plasma – Mass Spectrometry (ICP-MS Nextion 300XX Perkin Elmer). Real content of the samples differs by less than 3 % from the nominal ones. 2.2.Characterization The Brunauer–Emmett–Teller (BET) surface areas and average pore volumes and sizes were measured by nitrogen physisorption (Micromeritics ASAP 2010). XRD profiles were obtained using a Seifert D-500 diffractometer using Ni-filtered Cu Kα radiation with a 0.02° step. The particle sizes were estimated using XRD. UV–vis diffusereflectance spectroscopy experiments were performed on a Shimadzu UV2100 apparatus using BaSO4 as a reference. Photoluminescence spectra were recorded at room temperature on a fluorescence spectrophotometer (Perkin Elmer LS50B). Transmission electron microscopy (TEM) and X-ray energy dispersive spectra (XEDS) were recorded on a JEOL 2100F TEM/STEM microscope. XEDS analysis was performed in STEM mode, with a probe size ~1 nm, using the INCA x-sight (Oxford Instruments) detector. Specimens were prepared by depositing particles of the samples to be investigated onto a copper grid supporting a perforated carbon film. Deposition was achieved by dipping the grid directly into the powder of the samples to avoid contact with any solvent. The silver particle size mean diameter was calculated based on a minimum of 150 particles. XPS measurements were carried out on 4x4 mm2 pellets, 0.5 mm thick, prepared by gentle pressing the powered materials which were previously outgassed in the 5



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prechamber of the instrument up to a pressure < 2·10-8 Torr at room temperature to remove chemisorbed water. The SPECS spectrometer main chamber, working at a pressure < 10-9 torr, was equipped with a PHOIBOS 150 multichannel hemispherical electron analyser with a dual X-ray source working with Al Kα (hν=1486.2 eV) at 120 W, 20 mA using C 1s as energy reference (284.6 eV). Surface chemical compositions were estimated from XP-spectra, by calculating the integral of each peak after subtraction of the "S-shaped" Shirley-type background using the appropriate experimental sensitivity factors by means of CASAXPS (version 2.3.15) software. 2.3.Photocatalytic measurements The continuous flow annular photoreactor schematically depicted in Figure 1 was used to run the gas-phase photo-degradation of toluene (≥99%; Aldrich).7,26 Activity and selectivity for gas-phase photooxidation of toluene were measured in the mentioned apparatus using a thin layer (ca. 0,4 mg cm−2 painted from a suspension in ethanol) coated onto the inner pyrex tube displayed in panel A of Figure 1. The reaction mixture (100 ml min−1) was prepared by injecting toluene (ca. 700 ppmv) into a wet (ca. 75 % relative humidity) 20 vol.% O2/N2 flow before entering the photoreactor. The mixture flowed through the catalyst for ca. 6-8 h in the dark (control test). For the experiment under sunlight-type irradiation, the catalyst was subsequently irradiated by four fluorescent daylight lamps (6W, Sylvania F6W/D) symmetrically positioned outside the photoreactor. Figure 1 (panel C) shows the spectral lamp distribution. For the visible light experiments, were used a flexible polyester filter made from a deep-dyed PET material to absorb ultraviolet rays. The material allows less than 10 % transmission below 390 nanometers. The transmittance spectral distribution of this filter is also presented in Figure 1. Reaction rates were evaluated (vide supra) under steady-state conditions, typically achieved after a few hours from the start of irradiation. No change 6 ACS Paragon Plus Environment

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in activity was detected for all samples within the next 24 h. The concentrations of the reactants and products were analyzed using an on-line gas chromatograph (Agilent GC 6890) equipped with HP-PLOT-Q/HP-Innowax columns (0.5/0.32 mm I.D. × 30 m) and TCD (for CO2 measurement)/FID (organic measurement). The carbon balance was above 95 % in all experiments. 2.4.Quantum efficiency calculation The classical formulation of the quantum efficiency requires the calculation of the ratio between the number of molecules reacting by the number of photons interacting with the catalyst.27–33 〈 〉 ( )

ƞ = 〈 〉 ( ) × 100

(1)

The numerator of Equation 1 was obtained from experimental catalytic experiments (described in the previous section) and using Equation 2.31 reaction rate =

& (〈'()* 〉+, - 〈'()* 〉)./ ) 0 1

(2)

Where Q is the volumetric flow rate of the stream fed, 〈2345 〉67 and 〈2345 〉489 are the averaged toluene concentrations of the inlet and outlet streams of the reactor, respectively, : is the surface area of the catalysts illuminated (calculated as described in ref. 32) and ; the mass of catalyst as a thin-layer coating on the Pyrex surface. To evaluate the denominator of Equation 1, we need to calculate the average superficial rate of photon absorption (< =,? ) for the catalyst with consideration of the number of charge carrier species (selectivity factor) involved in generating chemistry, defined by Equation 3.8,32 〈Photon Rate〉 = < =,? C

(3) 7



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Were C is a dimensionless constant associated with the selectivity defined by the Equation 4. C = ∑6 E6 C6

(4)

In equation 4, F runs over all products of the reaction, C6 is the fractional selectivity to product F, and E6 is the inverse of the number of charge carrier species required to obtain the specific F product. The reaction mechanism of toluene is relatively complex and has been actively investigated.17,26,32,34–39 Although the details of the mechanism are unknown, the degradation is well established to be produced by hole-related hydroxyl radicals as active oxidative species in photocatalysts.26,36,40–42 Using this information we provided a general framework for analyzing the number of hydroxyl radicals consumed in the formation of benzaldehyde and carbon dioxide, the only products detected at the gas phase in our experiments.32 The balance of oxygen, hydrogen and charge species provides a system of linear equations which can be solved, rendering a region of existence having physical meaning (i.e. with non-negative values for all coefficients) for each parameter. Based on this and considering that oxygen does not play a relevant role in the initial steps of the reaction, we obtained n values of 4 and 36 for the production of benzaldehyde and CO2, respectively. Other valid n values maintain the ratio between the charges utilized by both products and thus gives the same trends when comparing different catalysts.32 The local superficial rate of photon absorption is defined by Equation 5. < =,? (x) = H?8I (x) J0 ?

(5)


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Where J0 ? is the fraction of light absorbed by the sample and H?8I the radiation flux at each position (x,y,z) of the catalytic film (See details in Supporting information). This last observable can be obtained using Equation 6. O

H?8I = K LH M + HM

(6)

Where L

H = RSLHP↑ + RTLHP↑ + RSLHM↓ + RTLHM↓

O

W

W

W

(7)

W

H = RSHPV + RT HPV + RS HM- + RT HM-

L ↑ L ↑ L ↓ L ↓ W V W V W - W RS HP , RT HP , RS HM , RT HM , RS HP , RT HP , RS HM , RT HM

(8) = X( H , J3Y , J3Z , J[Y , J[Z )

(9)

Details of the model procedure followed to obtain H?8I are presented in the Supporting Information file. The model considers all optical events (light transmission, absorption and reflectance) occurring on the reaction system and provides a formulation to describe H?8I at each point of the catalytic surface as a function of the local net radiation flux H7 as well as the fraction(s) of light transmitted (FT) or reflected (FR) at each component (sample, s, or glass, g) of the reactor.

3. Results and discussion Figure 2 displays the X-ray diffraction patterns of the g-C3N4 reference and xAg/g-C3N4 materials. In this figure it can be observed the typical g-C3N4 XRD profile, characteristic of its graphitic structure and dominated by the interlayer-stacking (002) peak.8,43 The technique also detects the presence of a metallic Ag (Fm3m (no. 225) space group; JCPDS file No. 04-0783) phase through the sample series. In fact, the 9



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silver contribution is barely observed for a 1 wt. % content but it is easily detected in the remaining samples. Table 1 summarizes the physico-chemical characterization results for the xAg/gC3N4 samples. In this table, it is shown that the g-C3N4 reference presents a relatively low surface area of 13.7 m2 g-1 and the composite materials display surface area with a moderate increasing trend with silver content. Note that variations from one sample to other having nearest Ag content within the series are small (in cases within experimental error) indicating that this observable is in all cases dominated by the carbon nitride component. This conclusion can be extended to the interpretation of other morphological properties, as demonstrated by the pore volumes and sizes reported in Table 1. Table 1 also contents data for the primary particle size of such Ag phase for all samples as obtained from the analysis of the Scherrer analysis of the (101) reflection of the corresponding XRD pattern. The Ag component, when detected by XRD, displays a particle size in the nanometric range, between ca 16 and 26 nm. It can be noted a clear increase of the particle size related with the silver loading. The structural analysis of the materials was completed with the help of transmission electron microscopy. Figure 3 provides some representative micrographs for selected samples. In the micrographs, silver nanoparticles are clearly visible as darker zones (chemical composition checked using XEDS) on top of the characteristic polymeric morphology of the carbon nitride component. The mean particle diameters of Ag calculated from TEM measurements of the different samples are displayed in Table 2. The mean volume-surface averaged particle diameters (d4/3) were also estimated and included in Table 3. The later observable, sensitive to the volume of particles, gives 10 ACS Paragon Plus Environment

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more weight to the larger particles and is more appropriate than the mean particle size (particularly for particle size distributions significant skewed) for comparison with data obtained from physical techniques such as X-ray line broadening or magnetic methods.44 The semi-quantitative agreement with the averaged size values obtained from X-ray (Table 1) can be noticed. XPS measurements were also carried out to study the chemical properties of the samples. Representative examples (for the 1Ag/g-C3N4 and 10Ag/g-C3N4 samples) of the Ag 3d XPS peak of the composite samples are present in panels A and B of Figure 4. Both the energy of the Ag 3d peaks as well as their characteristic energy difference are indicative of the metallic state of the noble metal in all samples.45,46 Figure 4 also shows the C1s (C) and N1s (D) XPS peaks corresponding to the 1Ag/g-C3N4 specimen as an illustrative result. The summary of the C- and N-containing species binding energies of the different chemical entities contributing to the C1s and N1s peaks of all composite samples and reference systems is provided in Table S1 at the Supporting Information file. As can be observed in Fig. 4, the C1s plot displays at lower energy the C-C contribution mostly coming from surface residues/contamination while the remaining contributions are exclusively ascribable to carbon nitride structural moieties, e.g. bridging carbons between aromatic moieties (C3-N; 286.0-286.2 eV) or at the aromatic rings (N-C-N; 287.6-287.8 eV). These two chemical moieties are also contributing to the N1S peak in addition to the N-H moiety and the broad pi excitation.7,9,12,47 The results reported in Table S1 give evidence of the minimal disturbance of C- and N-containing chemical entities in the composite samples with respect to the g-C3N4 reference. So, silver deposition makes little chemical modification of the carbon nitride support. 11



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Figure 5 displays the UV-visible spectra for the composite materials. The spectra show features dominated by the semiconducting nature of the g-C3N4 component, displaying the characteristic intensity decay expected for a band gap at a energy of ca. 2.7 eV (Table 3; see refs. 9,12 for details). Compared with the bare g-C3N4 reference, the UV– vis spectra of xAg/g-C3N4 samples showed an additional absorption peak about 515 nm, characteristic of the surface plasmon resonance band of nanostructured metallic silver.48 In spite of it, the band gap values calculated for all samples are rather constant, indicating that silver has a limited influence in the absorption properties of the major carbon-containing phase. The photochemical properties as measured by the reaction rate under sunlight-type and visible illumination and calculated using Equation 3 are reported in Figure 6. Note that results are presented after 24 h under reaction conditions and thus likely representative of a pseudo-steady state. A stability test (running for a total of 60 hours) for the 1Ag/gC3N4 sample under sunlight-type and visible illumination conditions is presented in Fig. S5 at the Supporting Information file. The test shows the excellent stability displayed by the materials here analyzed. Focusing on the binary xAg/g-C3N4 samples, we can see that the presence of Ag increases the activity upon both, sunlight-type and visible excitations. With sunlight-type illumination we observed a maximum enhancement of ca. 4.3 (with respect to the g-C3N4 pure) for the 5Ag/g-C3N4 sample while the enhancement factor under visible, for the best sample (10Ag/g-C3N4), is ca. 28. In part this is due to the limited efficiency of the carbon nitride under excitation with a wavelength well above 440-450 nm (see corresponding bar at Figure 6). Photooxidation of toluene typically produces CO2 as total oxidation product, and benzaldehyde as partial oxidation products. In this case, the presence of silver seems not


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to affect significantly the selectivity toward benzaldehyde and CO2 as can be deduced from the selectivity data shown in Figure 7. More interesting is the analysis of the photocatalytic properties in terms of calculating the true quantum efficiency, which considers: the effect of photon absorption, the reaction rate and the chemical selectivity results as discussed in Section 2.4. First to note is that the optical properties of the materials and concretely the H?8I observable only varies moderately within the series, displaying a decreasing trend (of ca. 35. % for both illumination conditions; see Table 4) from the reference to the 10Ag/g-C3N4 sample. However, the local superficial rate of photon absorption reported in Table 4 displays an opposite trend, increasing with the silver content of the material. Such increase is roughly independent of the illumination source as it has a magnitude of ca. 3.0. The variation in the local superficial rate of photon absorption is partially counteracting the increasing trend (with silver content) observed in the reaction rate in Figure 6 and strongly influences of quantum efficiency observable. The results of the quantum efficiency calculation upon all the illumination conditions employed here are displayed in Figure 8. The quantum efficiency maximum in our calculation is obtained for the 1Ag/g-C3N4 sample under both sunlight-type and visible excitation. The trend among samples displayed in Figure 8 and not only the existence of a maximum through the series is also similar under the two illumination conditions tested in this work. To interpret the activity enhancement, we first carried out photoluminescence experiments under UV (385 nm) and visible light (420 nm) excitation. The optical characterization results are presented in the A and B panels of Figure 9. UV excitation at 385 nm allows scanning the potential de-excitation channels of the g-C3N4 component related to the ca. of 3 % of UV light existing in the sunlight-type lamp. On 13



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the other hand, under 420 nm light excitation we attempted to (further) analyze the interaction effect among the catalyst component on the charge carriers photo-handling but this time using a wavelength of the visible region. We would like to note that, according to Figure 5, these two wavelengths mostly excite the carbon nitride component of the catalytic solids. As documented previously, under a 385 nm light excitation, the g-C3N4 reference shows a broad, featureless peak at ca. 490 nm.8,49,50 Silver presence decreases the photoluminescence intensity, providing evidence of a direct effect in charge recombination due to the well known ability of silver to actuate as an electron sink.2 This will increase hole-type number and availability at the carbon nitride phase. The photoluminescence intensity decrease parallels the silver content of the sample. Under visible light (420 nm) the spectra of the composite samples are also dominated by a broad, featureless peak characteristic of the carbon nitride component. Other peaks observed at higher wavelengths (ca. 650 nm) come from an incomplete elimination of the source features (due to their rather small intensity). This is demonstrated using silica or other large band gap “internal” references (result not shown). Interestingly, it is important to note that the photoluminescence study shows similar UV (385 nm) and visible (420 nm) light excitation trends along the sample series and are fully in line with the reaction rate values presented in Figure 6. This fact indicates that the dominant factor in the different photochemical behavior observed in the presence of Ag is related to charge recombination phenomena. As it is well know that the photoluminescence is inversely related to the recombination between photogenerated electron hole pair.51 The results displayed in panels A/B of Figure 9 can be thus rationalized considering that silver could inhibit the recombination process of charge carrier generated in the carbon nitride component by detracting electron species from the semiconductor, subsequently 14 ACS Paragon Plus Environment

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producing an increase in the activity toward pollutant degradation. This occurs, as mentioned, for both UV and visible photons (below ca. 450 nm) and would thus indicate that the silver effect in charge recombination is similar (e.g. similar trend among samples) for both types of illumination and would produce similar results, with rough independence of the light source characteristics. This behavior is, as previously mentioned, mimicked by the reaction rates presented in Figure 6. At visible light wavenumbers above 500 nm, the contribution of silver to light absorption is significant, as expected from current knowledge and confirmed by UVvisible results displayed in Figure 5.48,46 When silver is dominating the optical absorption properties of the solids, the photoluminescence experiments are qualitatively different from the results presented above. Panel C of Figure 9 indicates the presence of a single contribution at ca. 575 nm, observed as a shoulder over the strong decay corresponding to the tailing region of the excitation peak centered at 515 nm. Such weak shoulder for the carbon nitride reference clearly indicates the presence of localized, mid gap states, likely associated with defects of the structure. 12 As previously mentioned for panel B of Figure 9, additional, small peaks above 600 nm come from the source as they are also observed using wide band gap oxides like MgO or ZrO2 (results not shown). The most important point is that the intensity of the peak at ca. 575 nm is relatively affected by the presence of silver, practically disappearing already for the sample with the lowest silver content. Absence of dependence with the silver content is thus noticed for the radiative de-excitation channels studied upon 515 nm excitation. The photoluminescence experiment would allow to interpreting that the optimum use of incoming photons is not achieved with the “most active” sample (10Ag/g-C3N4) but with the 1Ag/g-C3N4 sample, as can be seen in Figure 8 for both sunlight-type and 15



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visible illumination conditions. For the two illumination conditions, the similar trends as well as the presence of a maximum for the same silver content of the quantum efficiency parameter displayed in Figure 8 can be interpreted as a function of charge handling properties between components of the system as a function of the main absorbed of light. As suggested by the photoluminescence results (Figure 9), the recombination suppression by the synergistic interaction between components when the carbon nitride is the main absorber phase has a similar behavior though the sample series under sunlight and visible excitation but this positive effect seems not enough to compensate the modification of the composite material in presence of silver for samples having a noble metal content above 1 wt. %. The situation is different when the main absorber is the silver component, above ca. 500 nm. In such case, although the composite system may handle charge efficiently with respect to the single components alone, the charge species generated now at the silver electronic structure may not have enough energy to attack efficiently the toluene molecule, a rather complicate molecule to be degraded.

8,9,32,36,37

Absence of chemically-relevant silver content effects in the

electronic interplay between catalyst components after excitation with light with wavenumber above ca. 500 nm seems thus grounded in the above mentioned fact. The above discussion indicates that photons absorbed by the silver component may not produce radical species able to attack the toluene molecule. This will be thus only possible to be carried out by hot electrons originated in the carbon nitride component. The quantum yield optimum as a function of the silver content is thus a trade off effect between the sinergistic interaction occurring between the two components of the system and two possible effects related to a shadowing of the active sites of the reaction by silver as well as the relatively low efficiency of the charge species originated at the silver phase to generate chemistry by excitation with visible light above ca. 500 nm. 16 ACS Paragon Plus Environment

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Note however that shadowing effects triggered by the noble metal at the carbon nitride surface are proportional to the total area expected to be inhibited if the active sites of the reaction are considered to be present (as should be) at the carbon nitride component. The “shadow” surface area can simply be calculated considering the number of particles and the inverse of the square of the primary particle radius (1/R2 –i.e. assuming spherical particles). Such effect would imply a decrease of activity from 1 to 10 Ag wt. % of roughly 3.7 times (higher than the one experimentally detected) and, importantly, would not display the behavior through the sample series presented in Figure 8. This would indicate the relatively minor importance of such contribution to explain the behavior of the quantum efficiency parameter throughout the sample series. The profiting of the solar-type photons by the optimum 1Ag/g-C3N4 sample was compared with a pure anatase-TiO2 reference. This reference sample provides significantly higher activity than the P25 reference in toluene photo-degradation under sunlight-type illumination.34 Figure 10 displays the toluene photo-degradation efficiency values for these two (1Ag/g-C3N4 and TiO2) systems. Panel A of this figure provides evidence of the significantly higher efficiency achieved for the Ag/g-C3N4 system with respect to titania independently of the way (or approximation level) the efficiency parameter is calculated. The most accurate calculation (using the local superficial rate of photon absorption coefficient values –ea,s- and taking into account the selectivity of the reaction) renders an enhancement factor of 2.5 with respect to the titania reference. This factor is competitive with those reached with titania systems (≈ 1.5-4.5) modified by doping with cation or anions as well as with surface additives like cerium oxides or others specifically designed for optimizing visible light absorption and handling.17,34,35,42,52 17



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Panel B of Figure 10 allows the study of the efficiency as a function of the main variables used to compute the observable and, particularly, provides some guidelines for comparison with efficiency values reported in the literature for carbon nitride based systems. To this end, we can first analyze the values obtained for the true (using the local superficial rate of photon absorption coefficient values –ea,s- of Table 4) or apparent (using the averaged local net radiation flux -q, Table 4) quantum efficiency calculated according to the IUPAC rules.53 Note that the plot shows results concerning the apparent quantum efficiency as this is the parameter usually reported in the literature. Maximum sunlight efficiencies in the 0.1-1 % range are calculated using Ptpromoted carbon nitride materials for hydrogen production.21,22 Such literature values are calculated using the source intensity and not the IUPAC recommended averaged local net radiation flux at catalyst surface and are usually not obtained under kinetic relevant conditions. Figure 10B also points another factor of importance when comparing with efficiency values reported for hydrogen production. As the photooxidation of organic pollutant renders several final products consuming the charge carrier species involved in the rate determining steps of the reaction,32,34 its calculation requires careful consideration of the selectivity of the reaction. Taking into account the selectivity makes also significant differences while comparing catalysts (as illustrated in the panel A of Figure 10 between the 1Ag/g-C3N4 sample and the titania reference). In any case, although all these issues make complex the comparison of efficiency values of hydrogen production and organic pollutant photo-degradation processes, values presented in the literature for the first are likely significantly higher than the ones here reported for the photo-elimination of toluene. Summarizing the study suggests that the main effect driving the behavior of the photodegradation of toluene in a Ag/g-C3N4 composite material is the electronic interaction 18 ACS Paragon Plus Environment

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between components. In Figure 11 we present the schematic representation of the main synergistic effects playing a positive catalytic role in the system. As mentioned, only when the electron flow is from the carbon nitride to the silver component, such effect significantly boosts the photo-degradation of toluene as measured by the quantum efficiency parameter. The photoluminescence data would indicate that this is of importance when the solid is excited below ca. 450 nm. Such an effect leads to a significant activity enhancement of the carbon nitride photocatalytic performance upon both UV and visible excitation, indicating the significant profiting of sunlight in the mentioned wavelength range. Comparison with a highly active titania reference further indicates the potential of the Ag/g-C3N4 composite system for the elimination of pollutants under sunlight-type excitation. The radical species responsible for the attack of the pollutant seems thus activated by photons with relatively high energy. However, the nature of such species is unknown. The literature reports show that the g-C3N4 material is not usually able to form OH-type radicals due to the fact that the top of its valence band is at a energy lower than the chemical potential of the OH-/OH• pair.54,55 However, in a previous study we showed that the samples can produce both hole- and electron-related species under UV and visible light illumination.56 The OH• radicals are thus likely formed via indirect routes such as O2•- + H2O H2O2•- OH- + OH•. So, in the present reaction it seems that several radical species, particularly bare holes as well as O2•-, H2 O2•-, and/or OH• can be involved in the degradation of toluene. Also important, the post-reaction characterization of the materials shows that the Ag/gC3N4 system displays high quantum efficiency but also a reasonable stability after prolonged (24 h) toluene photo-degradation tests upon all illumination conditions tested. 19



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To examine the solid evolution we first used XRD. Main changes observed are related to silver component stability. As shown in Figure 2B, silver maintains a dominant metallic state and displays a moderate growth of primary particle size after reaction under all illumination conditions tested in this work (Table 1). This was corroborated using TEM (Table 2). The silver particle size (and shape) modification and maybe the potential effect of surface species present after reaction has also consequences in the UV-visible spectra detecting a shift of the plasmon resonance region from ca. 515 nm to 560 nm.48 We however note that Table 1 shows moderate changes in size under reaction conditions and their effects in the optical properties of the samples are similar under all illumination condition tested. So absence of differential aging effects either as a function of the excitation wavelength or as a function of the silver content of the materials can be noticed. On the other hand, Table 3 displays the carbon nitride band gap values before and after reaction. In accordance of the minimum effects detected with XRD for the semiconductor, the band gap measured for each sample is not varying within experimental error (ca. 0.03 eV). Overall, the characterization results indicate the reasonable stability of the xAg/g-C3N4 composite samples and, importantly, the limited effect that sample evolution would have to interpret the main trends of the photo-activity of the samples in the photodegradation of toluene. Conclusions In this contribution, Ag/g-C3N4 composite materials were synthesized with a content of metallic silver in the 1 to 10 wt. % range. The microemulsion procedure used in the preparation step allows a reasonable good dispersion of the metallic phase onto the polymeric carbon nitride phase. We investigated the photo-degradation of toluene using 20 ACS Paragon Plus Environment

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these composite materials under sunlight-type and visible illumination conditions. Quantitative comparison of photocatalytic properties was carried out through the calculation of the quantum efficiency parameter. Using this approach, we provide evidence that the presence of silver in the 1 to 10 wt.% range enhances the activity of the semiconductor in toluene photo-degradation for sunlight and visible irradiation but the best quantum efficiency is obtained for 1 wt. % of silver, irrespective of the illumination conditions. The photoluminescence study indicates that the interaction of the carbon nitride and the silver improves charge separation and that this enhancement can be proportional to the silver loading, at least for excitation below ca. 450 nm. However, a quantum efficiency maximum at 1 wt. % of silver strongly indicates that the presence of a positive effect on charge recombination, generated at the interface between components, is counteracted by a negative effect dominating for higher loadings and concerning the limited photochemical role (in the case of toluene degradation) of charge species generated by light absorption occurring at the noble metal. The performance of the composite systems for the optimum silver – carbon nitride contact is able to provide a rather efficient and reasonably stable system for degradation of pollutants under sunlight, outperforming titania references. This indicates the potencial of the composite material for using a green and removable energy source as the sun for photo-degradation processes.

Supporting Information. The section presents details concerning the mathematical procedure to obtain the efficiency parameter, data related to the C1s and N1s XPS analysis, as well as a long term activity run measurement to test catalytic stability. 21



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Acknowledgements Financial support by Fundación General CSIC (COMFUTURO programme) is acknowledged. M. J. Muñoz-Batista thanks “Ministerio de Economía y Competividad” MINECO for support thought the predoctoral FPI program.

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Table. 1. Morphological properties for the g-C3N4 reference and xAg/g-C3N4 samples.a Sample

g-C3N4 1Ag/g-C3N4 2Ag/g-C3N4 5Ag/g-C3N4 10Ag/g-C3N4

Ag average particle size (nm) Area BET (m²/g) Fresh Used for sunlight- Used for visible type photoreaction photoreaction 13.7 n.d n.d. n.d. 12.4 15.7 22.8 21.8 15.3 21.8 24.4 25.0 18.7 25.9 26.7 27.5 17.1 a Standard error: size 0.5 nm, BET 1.5 m g−1; pore size 8%.

Pore volume (cm³/g)

Pore size (nm)

0.087 0.085 0.091 0.111 0.098

27.0 35.9 25.8 25.0 23.7

n.d., not detected.

Table. 2. Mean particle size (d) and volume-surface averaged mean particle size (d4/3) observables obtained from TEM particle size distribution measurements of the 1Ag/gC3N4 and 10Ag/g-C3N4 catalysts before and after reaction under sunlight-type illumination. Sample 1Ag/g-C3N4 10Ag/g-C3N4

d (nm) Fresh 4.6 8.0

d4/3 (nm) Used 5.0 8.9

Fresh 13.8 22.3

Used 14.9 24.5


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Table 3. Band gap values for the g-C3N4 reference and xAg/g-C3N4 samples.a Sample


Band Gap (eV) Fresh Used for Used for visible sunlight-type photoreaction photoreaction 2.71 2.70 2.71 2.81 2.72 2.73 2.75 2.72 2.71 2.81 2.82 2.82 2.85 2.84 2.86 a Standard error: 0.03 eV

Table 4. Values for the average local net radiation flux and local superficial rate of photon absorption at the catalytic films.a Sample


Sunlight-type Visible b,^ ]^_` ]^_` a ab,^ -2 -1 -2 -1 -2 -1 (mol m s ) (mol m-2 s-1) (mol m s ) (mol m s ) -8 -8 -8 7.85 × 10 1.28 × 10 6.06 × 10 8.85 × 10-9 7.29 × 10-8 1.49 × 10-8 5.62 × 10-8 1.05 × 10-8 -8 -8 -8 6.15 × 10 2.45 × 10 4.72 × 10 1.81 × 10-8 6.14 × 10-8 2.84 × 10-8 4.71 × 10-8 2.12 × 10-8 -8 -8 -8 5.36 × 10 3.74 × 10 4.09 × 10 2.83 × 10-8 a Standard error: 5.3 % and 6.4 %, respectively.

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A

B

1.0

Sunlight-type UV filter

0.8

UV filter transmittance Relative lamp emition

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0.6

0.4

0.2

0.0

300

400

500

600

700

Wavelength / nm

C

Figure 1. (A) Photocatalytic annular reactor. (B) Side section view. (1) gas inlet, (2) gas outlet, (3) lamps, (4) catalyst sample. (C) Intensity distribution of the sunlight-type lamp and Transmittance of the UV filter.


Page 31 of 40

g-C3N4

Ag (111)

A

1Ag/g-C3N4 2Ag/g-C3N4

Intensity / a.u.

g-C3N4

5Ag/g-C3N4

(002)

g-C3N4

10Ag/g-C3N4

Ag (200)

Ag (220)

(100)

20

40

Ag (311)

60

Ag (222)

80

2θ / degrees

B

10Ag/g-C3N4 VIS 10Ag/g-C3N4 ST 10Ag/g-C3N4 5Ag/g-C3N4 VIS

Intensity / u.a.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5Ag/g-C3N4 ST 5Ag/g-C3N4 2Ag/g-C3N4 VIS 2Ag/g-C3N4 ST 2Ag/g-C3N4 1Ag/g-C3N4 VIS 1Ag/g-C3N4 ST 1Ag/g-C3N4 g-C3N4 VIS g-C3N4 ST g-C3N4

20

40

60

80

2θ / degrees

Figure 2. XRD patterns for the xAg/g-C3N4 samples and reference systems. (A) Fresh samples; (B) comparative of selected samples before and after reaction under sunlighttype (ST) and visible (VIS) illumination. XRD peaks are labeled with the chemical symbol/formula of the corresponding components.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

Figure 3. TEM images of the 1Ag/g-C3N4 (A,B: fresh; C,D: after reaction under sunlight-type illumination) and 10Ag/g-C3N4 (E,F: fresh; G,H: after reaction under sunlight-type illumination) catalysts. Arrows indicate the location of a few silver nanoparticles. 32 ACS Paragon Plus Environment

Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


368.3

Experimental spectrum Model

A

374.3

376

374

372 370 368 Binding Energy (eV) 368.3


366

B

374.3

376

374

372

370

368

366

Binding Energy (eV)

C

287.7


286.1

290

288

286

284.6

284

Binding Energy / eV


282


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 34 of 40

397.7

D

399.1

400.2

403.6

406

404

402

400

398

396

Binding Energy / eV Figure 4. Ag 3d XPS spectra of the 1Ag/g-C3N4 (A) and 10Ag/g-C3N4 (B) samples; C1s (C) and N1s (D) XPS spectra of the 1Ag/g-C3N4 sample.


Page 35 of 40

g-C3N4

A

B

1Ag/g-C3N4 2Ag/g-C3N4 5Ag/g-C3N4

200 300 400 500 600 700 800 900

515 560

Kubelka-Munk

10Ag/g-C3N4

Kubelka-Munk

400

500

600

700

Wavelength / nm

Wavelength / nm

Figure 5. UV-vis of xAg/g-C3N4 samples and reference systems. (A) Fresh samples; (B) comparative of selected samples before and after reaction under sunlight-type (dashed line) and visible (dotted line) irradiation. A single color is used for each sample.

-10

6x10

-1

-10

-2

Reaction rate / mol m s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5x10

Sunlight-type Visible

-10

4x10

-10

3x10

-10

2x10

-10

1x10

0 g-C 3

N4

10A 1A g 2Ag 5A g g /g - C /g - C /g - C 3N 4 3N4 3N4 /g-C3N 4

Figure 6. Surface-area-normalized reaction rate for toluene photo-oxidation.



Sunlight-type

Visible 10Ag/g-C3N4

10Ag/g-C3N4

5Ag/g-C3N4

5Ag/g-C3N4

2Ag/g-C3N4

2Ag/g-C3N4

1Ag/g-C3N4

1Ag/g-C3N4

g-C3N4

g-C3N4

-80

-60

-40

-20

0

20

40

60

80

Selectivity (%) CO2 Bz

Figure 7. Selectivity to CO2 and Benzaldehyde (Bz) for reference and xAg/g-C3N4 samples.

-3

2.0x10

QE VIS 3.0x10-3

QE ST

-3

1.6x10

2.0x10

-3

1.0x10

-3

-3

1.2x10

-4

8.0x10

g-C 3

N4

1Ag

Visible Quantum efficiency / %

Sunlight-type Quantum efficiency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

0.0 2Ag 5Ag 10A /g-C /g-C /g-C g 3N 4 3 N4 3N4 /g-C3N 4

Figure 8. Quantum efficiencies for reference and xAg/g-C3N4 samples under sunlighttype (ST) and visible (VIS) illumination.


Page 37 of 40

Energy / eV 2,8 2.8

2,6 2.6

A

2,4 2.4

2,2 2.2

λext = 385 nm

2,0 2.0

g-C3N4

PL intensity / a.u.

1Ag/g-C3N4 2Ag/g-C3N4 5Ag/g-C3N4 10Ag/g-C3N4

450

500

550

600

650

Wavelenght // nm nm Wavelength Energy / eV 2,6 2.6

2,4 2.4

B

2,2 2.2

2,0 2.0

1,8 1.8

g-C3N4

λext = 420 nm

1Ag/g-C3N4

PL intensity / a.u.


500

600

700

Wavelenght // nm nm Wavelength Energy / eV 2,2 2.2

2,0 2.0

C

1,6 1.6

1,8 1.8

λext = 515 nm

1,4 1.4

g-C3N4 1Ag/g-C3N4

PL intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



600

650

700

750

800

850

900

Wavelenght// nm nm Wavelength

Figure 9. Photoluminescence spectra for the reference and xAg/g-C3N4 samples. (A) λext = 385 nm, (B) λext = 420 nm, (C) λext =515 nm.



A

Sunlight-type -3

1.6x10

-3

-3

1.2x10

-3

1.0x10

-4

8.0x10

-4

6.0x10

-4

4.0x10

Q.E. or App.Q.E. / %

1.4x10

-4

N q 1Ag/g-C 3 4 q TiO 2 a,s -C N e 1Ag/g s 3 4 a, e TiO 2

2.0x10 0.0

No Se lec tivi

Se lec

tivi ty

ty

Sunlight-type

B

-3

1.6x10

-3

1.4x10

-3

1.2x10

-3

1.0x10

-4

8.0x10

-4

6.0x10

-4

4.0x10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

-4

N q 1Ag/g-C 3 4 q TiO 2 a,s -C N e 1Ag/g s 3 4 a, e TiO 2

2.0x10 0.0

No Se lec tivi ty

Se lec tivi ty

Fig. 10. Analysis of the photocatalytic performance of the 1Ag/g-C3N4 catalysis. (A) True or Apparent Quantum efficiency values as a function of the calculation level; the 1Ag/g-C3N4 catalysis performance is compared with a nano-TiO2 reference; (B) Analysis of main physico-chemical variables affecting efficiency; projection of quantum efficiency values of 1Ag/g-C3N4 catalyst values in basal planes of previous plot. Red dashed lines are only visual guides.


Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. Schematic representation of the main electronic events occurring after light absorption and having impact in the photo-degradation of toluene.



TOC

Sunlight-type -3

1.6x10

-3

1.4x10

-3

1.2x10

-3

1.0x10

-4

8.0x10

-4

6.0x10

-4

4.0x10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 40

-4

C 3N 4 q TiO 2 a,s g-C 3N 4 e 1Ag/ a,s e TiO 2

2.0x10

q 1Ag/g-

0.0

Se lec tiv ity

No Se lec tiv ity


g-C3N4 Composite Catalysts

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