Effect of g-C3N4 loading on TiO2-based photocatalysts: UV and visible degradation of toluene

CatalysisScience &Technology

PAPER

2006 | Catal. Sci. Technol., 2014, 4, 2006–2015 This journal is © The R

Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie Curie 2, 28045-Madrid,

Spain. E-mail: [email protected], [email protected]

† Electronic supplementary information (ESI) available: Details for ea,s calculationare included. See DOI: 10.1039/c4cy00226a

Cite this: Catal. Sci. Technol., 2014,

4, 2006

Received 19th February 2014,Accepted 25th March 2014

DOI: 10.1039/c4cy00226a

www.rsc.org/catalysis

Effect of g-C3N4 loading on TiO2-basedphotocatalysts: UV and visibledegradation of toluene†

M. J. Muñoz-Batista, A. Kubacka* and M. Fernández-García*

g-C3N4 was introduced into a highly active anatase TiO2-based photocatalyst using a simple yet effective

impregnation method. The corresponding composite materials were obtained with a carbon nitride

weight content ranging from 0.25 to 4 wt.%, and their surface, morphological and structural properties

were characterized using BET and porosity measurements, X-ray diffraction, infrared, UV-vis and photo-

luminescence spectroscopy, and transmission electron microscopy. They were subsequently tested for

toluene photoelimination under UV and sunlight-type illumination conditions. The photocatalytic perfor-

mance of the materials was analysed quantitatively through calculation of the quantum yield of the reac-

tion under all illumination conditions essayed. We observed that the addition of carbon nitride enhances

the photoactivity and selectivity to CO2 of samples containing 0.5 to 1 wt.% g-C3N4. Physicochemical

characterization presented evidence that the enhanced behaviour is related to the intimate contact

between the two components of the photocatalyst system, which provides two new (i.e. not present in

the parent system) charge carrier handling routes affecting photocatalytic properties upon UV and visible

light illumination.

1. Introduction

Heterogeneous photocatalysis is an advanced oxidation processthat uses nanocrystalline semiconductors broadly applied inthe degradation/transformation of organic pollutants as wellas biological microorganisms. This is essentially based on theexcellent performance and stability of titania, the most promi-nent photocatalytic material, for the mineralization of typicalpollutants or the inactivation of dangerous microorganismstypically carried out under mild conditions, e.g. at room tem-perature and atmospheric pressure, and using oxygen (air) asan oxidant.1–4 However, the main limitation of anatase, thestable polymorph of titania at the nanometric level,4 comesfrom its relatively wide band gap energy, ca. 3.0–3.4 eV, whichrequires UV radiation for triggering photoredox reactions.Because UV radiation only accounts for ca. 3–5% of the solarspectrum, the photocatalytic efficiency of pure titania-basedphotocatalysts under sunlight is limited. The quest for fruitfuluse of the visible part of the solar spectrum, ca. 45%, hasforced implementation of several strategies aiming at profitingfrom the complete solar spectrum, particularly the visibleregion. Among the strategies implemented, the use of visible

light-sensitive phases in intimate contact with TiO2 has beenshown particularly successful.1–4

In the photocatalytic field, a relatively new sensitizermaterial type corresponds to certain C-containing materialssuch as carbon nanotubes, graphene, or carbon nitrides.4

Contrary to classical carbon materials, they show by themselvesinteresting photoredox properties. Also they have capturedattention from the point of view of boosting the activity oftitania. As such, they provide not only significant (classical)adsorption properties but also charge carrier storage after lightexcitation, as well as specific visible light capabilities.5–7

Among the new C-containing materials, nanostructuredcarbon nitrides, particularly g-C3N4, the most stable allotropicphase, have displayed interesting properties in several photo-chemical reactions such as water splitting or dye degrada-tion.8–10 Similarly, the use of g-C3N4 in combination withtitania has been shown to provide outstanding photochemicalactivity in hydrogen production11,12 as well as a series of deg-radation reactions concerning several pollutants such asformaldehyde,13 phenol14 and organic dyes,15–18 including theremoval of Cr(VI).15 In all cases tested, the improvement ofperformance against titania is constant throughout the studies,but the visible light capability is not always better than theone of carbon nitride. This, however, needs critical assessmentas the comparison was presented using bare reaction ratesand not efficiency parameters. Such an issue can limit thegeneralization of the results presented and thus complicate

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Catalysis Science & Technology Paper

the interpretation of the effectiveness of the g-C3N4–TiO2 com-posite materials. The calculation of efficiency parameters,particularly the quantum yield of the reaction, involves themeasurement and modelling of the radiation that reachedthe surface of the material and the fraction effectively used inthe photocatalytic steps. Such a task typically involves, in thefirst place, measurement and modelling of the light sourceemission properties and, subsequently, the light absorptioncapability of the photocatalysts. The latter requires calculationof the radiative transfer by calculating in our case the so-calledlocal superficial rate of photon absorption, ea,s.19–23

Here we will provide quantitative calculation of the toluenephotoelimination quantum yield in order to obtain a reliablecomparison among the g-C3N4–TiO2 catalysts. The reaction willbe carried out under UV as well as sunlight-type illuminationconditions. Toluene is a typical pollutant present in urbanatmospheres which demands highly active photocatalysts forits elimination.24,25 In addition, and with the aim of inter-preting the quantum yield or efficiency results on a physico-chemical basis, we carried out a multitechnique examinationof the materials using X-ray diffraction, infrared, photo-luminescence and ultraviolet spectroscopy, and transmissionelectron microscopy. Applying these tools, we provide evidencethat the underlying activity-enhancement factors with respectto the titania reference are not the same under UV and visiblelight illumination conditions. In spite of this, the compositeprovides in all cases a route to improve the performance, e.g.the quantum yield and selectivity to total oxidation, of the par-ent materials. We thus observed that the combination of thesetwo materials with a loading of g-C3N4 in the 0.5–1.0 wt.%range yielded photocatalysts overperforming the parent refer-ence materials under UV, visible and sunlight conditions.

2. Materials and methods2.1 Catalyst preparation

Materials were prepared using a single-pot microemulsionpreparation method using n-heptane (Scharlau) as the organicmedium, Triton X-100 (Aldrich) as a surfactant and hexanol(Aldrich) as a cosurfactant. A TiO2 reference sample wasobtained as a first step using a microemulsion into the aque-ous phase and titanium tetraisopropoxide as the precursor.Titanium tetraisopropoxide was introduced into the previouslyresulting microemulsion drop by drop from a mixture withisopropanol (2 : 3). Water/Ti and water/surfactant molar ratioswere, respectively, 110 and 18 for all samples.26,27 The resultingmixture was stirred for 24 h and then centrifuged, and theseparated solid precursors were rinsed with methanol anddried at 110 °C for 12 h. After drying, the solid precursorswere subjected to a heating ramp of 2 °C min−1 up to 500 °C.

The graphitic carbon nitride was obtained by calcinationof melamine (Aldrich) at 580 °C with a heating ramp of 5 °Cmin−1 for 4 h. The incipient wetness impregnation methodwas used to obtain the carbon nitride–TiO2 composites. Forthis, an appropriate amount of g-C3N4 was suspended intomethanol and sonicated for 1 h, deposited on TiO2 and dried

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at 110 °C for 24 h. The sample names were Ti for the TiO2

reference, and xg-C3N4/Ti for the composite ones, where x isthe wt.% (0.25, 0.5, 1, 2 and 4) of g-C3N4 with respect to TiO2.This was confirmed with an error below 5% using ICP-AASspectrometry.

2.2 Characterization details

The Brunauer–Emmett–Teller (BET) surface areas and averagepore volumes and sizes were measured by nitrogen physisorption(Micromeritics ASAP 2010). XRD profiles were obtained using aSeifert D-500 diffractometer using Ni-filtered Cu Kα radiationwith a 0.02° step and fitted using the Von Dreele approach tothe Le Bail method;28 the particle sizes and microstrain weremeasured by XRD using the Williamson–Hall formalism.29

TEM analysis of the materials was carried out using a JEOL2100F TEM/STEM microscope. Energy-dispersive X-ray spec-troscopy (XEDS) was performed in STEM mode with a probesize of ~1 nm using the INCA x-sight (Oxford Instruments)detector. UV–vis transmission or diffuse-reflectance spectros-copy experiments were performed on a Shimadzu UV2100apparatus using, for diffuse-reflectance experiments, BaSO4

as a reference. Photoluminescence spectra were measured atroom temperature on a fluorescence spectrophotometer(Perkin Elmer LS50B). The Fourier transform infrared spectrawere taken in a Bruker Vertex 80 FTIR spectrometer using aMCT detector.

2.3 Photocatalytic experimental details

Gas-phase photooxidation of toluene (Panreac, spectroscopicgrade) was carried out in a continuous flow annular photoreactorusing a set-up described elsewhere30,31 (details in the ESI†).The activity and selectivity for gas-phase photooxidation wereobtained in a continuous flow annular photoreactor containingca. 40 mg of the photocatalyst as a thin layer coating on a pyrextube. The corresponding amount of catalyst was suspended in1 mL of ethanol, painted on a pyrex tube (cut-off at ca. 290 nm),and dried at RT. The reaction mixture (100 ml min−1) was pre-pared by injecting toluene (≥99%; Aldrich) into a wet (ca. 75%relative humidity) 20 vol.% O2/N2 flow before entering thephotoreactor, yielding an organic inlet concentration of ca.700 ppmv. Under such conditions, the reaction rate shows azero-order kinetics with respect to the total flow and organicpollutant/oxygen concentrations. After flowing the mixture for6 h (control test) in the dark, the catalyst was irradiated byfour fluorescent daylight lamps (6W, Sylvania F6W/D) with aradiation spectrum simulating sunlight (UV content of 3%;main emission lines at 410, 440, 540, and 580 nm), symmetri-cally positioned outside the photoreactor. Similar tests werecarried out using UV lamps (Sylvania F6WBLT-65; 6W, maxi-mum at ca. 350 nm). Reaction rates were evaluated (vide supra)under steady-state conditions, typically achieved after ca. 6–10 hfrom the start of irradiation. No change in activity wasdetected for all samples within the next 24 h. The concentra-tions of the reactants and products were analyzed using anon-line gas chromatograph (Agilent GC 6890) equipped with

Catal. Sci. Technol., 2014, 4, 2006–2015 | 2007

Table 1 Morphological properties of the samplesa

SampleBET surfacearea (m2 g−1) Pore volume (cm3 g−1) Pore size (nm)

Ti 91.8 0.123 5.20.25g-C3N4/Ti 91.5 0.119 5.20.5g-C3N4/Ti 89.4 0.119 5.151g-C3N4/Ti 92.6 0.118 5.12g-C3N4/Ti 90.9 0.119 5.254g-C3N4/Ti 88.2 0.104 4.7g-C3N4 26.8 0.103 15.1

a Standard error: BET, 1.5 m2 g−1; porosity, 8%.

Fig. 1 XRD diffractograms of the Ti and g-C3N4 references andxg-C3N4/Ti samples.

Table 2 XRD-derived parameters for the anatase phase of the

samplesa

SampleSize (nm)TiO2

Microstrain <ξ2>1/2

(× 10−3) TiO2

TiO2 anatasecell parameters (Å)

a b

Ti 12.1 2.085 3.789 9.4770.25g-C3N4/Ti 12.2 2.10 3.785 9.4710.5g-C3N4/Ti 12.05 2.10 3.790 9.4751g-C3N4/Ti 12.0 2.11 3.787 9.4712g-C3N4/Ti 12.0 2.09 3.787 9.4764g-C3N4/Ti 12.1 2.10 3.787 9.474

a Standard errors: 12% (size), 10% (strain), ±0.004 Å (cell parameters).

Catalysis Science & TechnologyPaper

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 yield or efficiency

The classical formulation of the quantum efficiency requiresthe calculation of the ratio between the number of mole-cules reacting and the number of photons interacting withthe catalyst:32,33

Q.E.Rate mol m s

Photon rate Einstein m s%

2 1

2 1100 (1)

To evaluate the denominator of eqn (1), we first evaluatethe local superficial rate of photon absorption, ea,s, of the sam-ples. The mathematical model of the present sample–reactorsystem allowing such calculations is presented in the ESI.† Thisprovides an “exact” calculation of the number of photons persurface area and unit time. Such calculation considers the reac-tor–photocatalyst geometry as well as the optical properties ofthe photocatalyst. Once ea,s is obtained in the desired units(eqn (1)) we need to consider the selectivity of the reaction astwo different molecules were obtained in the toluene photo-oxidation reaction. A general equation for the quantum efficiencywould be obtained using a correction factor (here called γ) ofthe ea,s presented as follows:

(2)

where i runs over all products of the reaction, Si is the frac-tional selectivity to product i, and ni is the inverse of the num-ber of charge carrier species required to obtain the specificproduct i. This correction factor accounts for the number ofcharge carrier species (and in turn photons) necessary to pro-duce one molecule of a specific product and requires mea-surement of the reaction selectivity. This is possible in thecase of toluene photooxidation due to both its relatively wellestablished mechanism as well as the limited number ofproducts detected in a gas-phase experiment.24,25,34

3. Results and discussion

The physicochemical characterization results for the xg-C3N4/Tisamples are summarized in Table 1.

Although the g-C3N4 reference presents a relatively lowsurface area of 26.8 m2 g−1, the composite materials display asurface area very close to that of the titania reference withinthe experimental error (ca. 89.5 ± 1.5 m2 g−1). This is anexpected result due to the relatively mild treatment (in com-parison with the anatase calcination treatment) carried out tocontact the carbon nitride and anatase components. Asshown in Table 1, this conclusion is extensible to all morpho-logical properties including average pore volume and size ofthe materials.

TOL i

i in S

2008 | Catal. Sci. Technol., 2014, 4, 2006–2015

The X-ray diffraction patterns of the g-C3N4–TiO2 materialsare shown in Fig. 1. The technique only detects the presenceof the anatase (I41/amd space group) phase. Table 2 lists thedata for the primary particle size, strain and cell parametersof the anatase phase in all samples.

The nanostructured anatase phase displays a particle sizealways close to 12 nm. Furthermore, results concerning strainand cell parameters strongly suggest that the contact with thecarbon nitride phase does not alter the main structuralparameters of anatase. In particular, the absence of dopingcan be foreseen. The constancy of the anatase structural and



morphological characteristics is important in order to inter-pret the photocatalytic ability of the composite samples. Suchconstancy is, as previously mentioned, a relatively expectedresult due to the mentioned mild treatment applied to titania(with respect to its calcination) to facilitate the contact withthe carbon nitride component. In Fig. 1 we can also observethat the g-C3N4 nanomaterial displays a XRD profile typical ofits graphitic structure, dominated by the interlayer-stacking(002) peak.35,36 As said, no evidence of its presence in thecomposite materials can be achieved using XRD.

To provide information of the carbon nitride componentin g-C3N4–TiO2 composite samples, we carried out an infra-red study. Fig. 2 shows the infrared spectra of the materials.In the case of the g-C3N4 reference, from high to lowerwavenumbers, we detected the presence of N–H stretchingvibration contribution(s) at the 3500–2500 cm−1 region. Thisis related to residual NHx groups, although the presence ofadsorbed water molecules may also need to be considered.11

After it, in the 1600–1200 cm−1 region, we can observe severalcontributions mostly associated with the N–C stretching modesof heterocyclic compounds. Finally, around 850–800 cm−1 wenote additional contributions coming from the breathingmodes of tris-s-triazine (C6N7-based) building blocks.37 Thesame contributions can be observed in composite catalystshaving a carbon nitride content equal or superior to 1 wt.%. Ofcourse, in some regions, typically water or hydroxyl relatedones, they are overlapped with intrinsic TiO2 contributions.The region below ca. 1600 cm−1 displays, however, clear carbonnitride like fingerprints in the composite catalysts. Specifically,in the 1600–800 cm−1 region, although some of the bandspresent differences between the composites and the g-C3N4

reference, the constancy of the tris-s-triazine breathingmode frequency indicates that the carbon nitride componentmaintains its main structural characteristics in the g-C3N4–TiO2

materials. Differences in other (N–C stretching) bands areattributable to the effects of the interaction between the two


Fig. 2 FTIR spectra of the Ti and g-C3N4 references and xg-C3N4/Tisamples.

semiconductor components and/or (less likely) to minormorphological differences concerning the carbon nitride.

To analyze the interaction between the components, we car-ried out a TEM analysis of the samples. The g-C3N4 referenceshows its characteristic platelet-like, layered morphology (Fig. 3).The presence of carbon nitride is, however, difficult to track inthe composite materials due to both its low weight content inthe samples and its low atomic number constituents. In spiteof this, visual inspection of Fig. 3 evidences the presence ofg-C3N4 platelets surrounded by crystalline oxide (see upper-leftcorner of the micrograph as a representative example), indicat-ing the close relationship established between the componentsin the samples. The presence of the carbon nitride phase at theupper-left corner of the graph was corroborated by XEDS. Inaddition, this technique measured an average carbon nitridecontribution corresponding to 3.05 wt.% (reasonably close tothe formal composition) in the micrograph presented in Fig. 3.

The optical properties of the catalysts were examinedusing UV-visible spectroscopy (Fig. 4). The composite mate-rials show spectra dominated by the semiconducting natureof titania, displaying the intensity decay expected for a bandgap energy of 3.2 eV (Table 3).


Fig. 3 TEM images of the g-C3N4 (upper panel) and 4g-C3N4/TiO2

(lower panel) samples.

Fig. 4 UV-vis spectra of the Ti and g-C3N4 references and xg-C3N4/Tisamples.

Table 3 Band gap energy of the samplesa

Sample Band gap (eV)

Ti 3.20.25g-C3N4/Ti 3.00.5g-C3N4/Ti 3.051g-C3N4/Ti 3.052g-C3N4/Ti 3.04g-C3N4/Ti 3.0g-C3N4 2.7

a Standard error: 0.05 eV.

Fig. 5 Surface-area-normalized reaction rate for toluene photooxidation.A. Sunlight irradiation. B. UV irradiation. Lines are only guides.


The band gap was calculated considering titania as anindirect band gap semiconductor.38 The composite materialspresent a slightly inferior but essentially constant band gapenergy through the series. g-C3N4, as an indirect band gapsemiconductor, shows a band gap energy of ca. 2.7 eV, inaccordance with literature reports.7 The UV-vis spectra of thecomposite catalysts present intensities above ca. 400 nm; afact not observed in titania (see the inset in Fig. 4). Compari-son with the g-C3N4 reference makes it obvious that this isa contribution from the carbon nitride component whichexpands the absorption capability of the TiO2-based compos-ite materials into the visible region of the electromagneticspectrum.

The photochemical properties of the materials undersunlight-type and UV illumination are presented in, respec-tively, the upper and bottom panels of Fig. 5. The figure pre-sents pseudo steady-state reaction rates after 24–30 h, withconstant rate values (within the experimental error) for morethan 20–24 h. This indicates that the catalysts have reason-able stability. The reaction rates, as a function of the sample'sg-C3N4 content, present an inverse U shape in both cases. Amaximum is always obtained in the intermediate region for0.5 or 1.0 wt.% samples.

Maximum enhancement factors of ca. 1.4/1.3 were observedfor, respectively, UV and sunlight-type illumination. After themaximum, the activity decay is sufficiently strong as to obtain


samples with activity below that of the Ti reference. In bothcases, UV and sunlight-type excitation, the composite with4 wt.% g-C3N4 displayed lower activity than the pure g-C3N4

reference.The presence of the carbon nitride polymer also affects

selectivity although moderately (Fig. 6). All samples generatebenzaldehyde and CO2 as reaction products. Total oxidationis favored in some composite samples both under UV andsunlight-type illumination conditions. Composite sampleswith lower quantities of the carbon nitride component dis-play a reasonable similarity with respect to the parent titaniareference, although with a modest increase in CO2 production.The 4g-C3N4/Ti sample shows however a more similar behav-iour to the carbon nitride reference material. We can thus see acontinuous shifting in selectivity with the increase in the car-bon nitride component; total oxidation seems continuouslyfavoured up to the 1g-C3N4/Ti sample. For this sample, thepartial oxidation pathway becomes increasingly important.

As mentioned, here we aim to analyze quantitatively thedifferences among our samples. The first step in calculatingquantitative quantum efficiencies corresponds to the numeri-cal calculation of the local superficial rate of photon absorp-tion (ea,s) at the sample surface. In our set-up (ESI†) thisrequires calculation on a cylindrical geometry and thus twoindependent geometrical variables are needed. This is presentedin Fig. 7 (UV) and 8 (sunlight-type) in Cartesian coordinates.The two dimensional behaviour of all samples is identicaland thus differences can be expressed by a single number.


Fig. 6 Yields toward benzaldehyde and CO2 production undersunlight (A) and UV (B) irradiation. Standard error: ±10%.

Fig. 7 Local rate of photon absorption (Einstein cm−2 s−1) under UVirradiation. From top to bottom: Ti, 0.25g-C3N4/Ti, 0.5g-C3N4/Ti,1g-C3N4/Ti, 2g-C3N4/Ti, 4g-C3N4/Ti and g-C3N4.


Under UV the ea,s coefficient increased with the carbon nitridecontent of the photocatalysts by ca. 5% while under sunlightthe behaviour is more acute and goes up to 50%. The secondfactor to be considered is the one related to eqn (2). Thisrequires the selectivity data from Fig. 6 as well as the constantvalues n for benzaldehyde and CO2. For the former we cancount the hole-related species for the initial steps leading tobenzaldehyde (Scheme 1).

In this case, it is well known that only one charge carrierspecies is required to provide the radical species presentedat the first stage depicted in Scheme 1. In the presence of(molecular) oxygen, such an intermediate evolves in the desiredproduct, i.e. benzaldehyde.25,39–41 For the production of CO2

the situation is less clear but, on average, four hole-relatedspecies (and corresponding O2˙

− species) are required.39–41 Theratio between the number of charge carrier species used forCO2 and benzaldehyde generation is in any case the mostimportant parameter to obtain an accurate measurement ofthe quantum yield throughout the series of samples.

Calculation of the quantum yield or efficiency (Q.Y./Q.E.)rendered the values presented in Fig. 9. First to note isthat the Q.Y. values are rather small, always below 10−3.Q.Y. values below 1 seem typical for gas-phase photocatalyticreactions.42,43 Concerning this issue, we must point out that


a simpler calculation of the so-called photonic efficiency(which does not account for the effect of the optical prop-erties of the samples in light absorption, with their realsurface exposed to reactants and other variables)33,42,43 pre-sents in the present case up to two or three orders of mag-nitude variation between the true (quantum) and apparentphotonic efficiencies.

The maximum in our calculation is, in any case, obtainedfor the 0.5g-C3N4/Ti sample both for UV and sunlight-typeexcitation. A more pronounced maximum is observed in thefirst case while in the second we observed a flat maximumregion covering two samples: 0.5g-C3N4/Ti and 1g-C3N4/Ti.The plot indicates that optimum profiting of light in the“whole” UV-visible electromagnetic region is achieved in thecomposition region limited by these two photocatalysts. Wethus observed a consistent increase in the efficiency of thereaction when carbon nitride is contacted with titania in ade-quate proportions.

To interpret the physical origin of the enhancement factor,we carried out a photoluminescence experiment under UV



Fig. 8 Local rate of photon absorption (Einstein cm−2 s−1) undersunlight irradiation. From top to bottom: Ti, 0.25g-C3N4/Ti, 0.5g-C3N4/Ti,1g-C3N4/Ti, 2g-C3N4/Ti, 4g-C3N4/Ti and g-C3N4.

Scheme 1 Simplified reaction mechanism of toluene photooxidation.

Fig. 9 Quantum yield of the Ti and g-C3N4 references and xg-C3N4/Tisamples under UV and sunlight-type illumination. Lines are only guides.


and visible light excitation. The corresponding experimentalresults are presented in the two panels of Fig. 10. UV excita-tion at 280 nm allows scanning of all potential de-excitationchannels of the titania component. Above 2.0 eV the photo-luminescence titania samples display the presence of two orthree peaks.44–46 In addition, low energy peaks are less impor-tant in generating photochemistry and always related to thepresence of defective Ti (interstitial or lattice) states.47 Thepeaks of about 2.0 eV can be grouped into two types of transi-tions corresponding to high-energy (sometimes called green)and low-energy (red) de-excitation channels. Although we lacka definitive interpretation of the photoluminescence spectraof anatase materials, the first transition mostly probes thede-excitation of excited electrons in localized (typically oxygen-related) states by trap-related holes, while the second probesthe de-excitation of electron localized states by holes at thevalence band.44–46

In the upper panel of Fig. 10 we can observe an anatase-type photoluminescence pattern in the composite samplesup to the 1g-C3N4/Ti sample but with enhanced intensity.Due to the fact that this showed enhanced activity, thisstrongly suggests that the composite system provides a newroute for de-excitation involving the charge carrier species ofthe two components. This fact is further suggested by the


Fig. 10 PL spectra of the Ti and g-C3N4 references and xg-C3N4/Tisamples. A. λext = 280 nm. B. λext = 420 nm. Note the scale factor of Ti.


photoluminescence graph presented in the ESI.† Comparisonof the composite photoluminescence spectra correspondingto the appropriate g-C3N4/TiO2 physical mixture reinforcesthe conclusion that composite materials display additionalde-excitation channels not present in the parent materials.According to the band diagram (see Scheme 2) the compo-nent interaction would generate a de-excitation channel with-drawing electrons from the anatase phase and permittingholes, the active species detected at anatase surfaces in tolu-ene photodegradation,23,25,39,40 to be involved in the chemicalsteps. Such interpretation is consistent with the higher inten-sity increase observed in the ca. 2.75–2.25 eV region ofFig. 10. The new de-excitation channel evidences the creationof a Z-“enhancement scheme” of the reaction (Scheme 2). Thishas been claimed previously to exist in other carbon nitridecontaining composite samples with oxide semiconductors.48

Above the 1g-C3N4/Ti sample (i.e. samples with highercarbon nitride content), we detected a photoluminescencepattern resembling that of the carbon nitride parent compoundbut with decreasing intensity. This may be rationalized in termsof the previously mentioned reduction of holes at the carbonnitride component (Scheme 2) although a unique interpretationof the behaviour is not possible.

Upon visible light excitation (420 nm), a different de-excitationbehaviour is reflected in the luminescence spectra (lower panelof Fig. 10). Here the spectra of the composite samples aredominated by a broad, featureless peak characteristic of thecarbon nitride component.7 This is an expected result consid-ering the band gap energies of the two component materialsof the photocatalysts (Table 3).

Upon visible light excitation, a decrease in the photo-luminescence intensity is observed, of which the minimumwas reached with samples having carbon nitride contentsbelow or equal to 0.5 wt.%. Above such minimum the intensityincreases. The behaviour can be interpreted as if an additional


Scheme 2 Schematic view of the charge carrier(s) behavior inxg-C3N4/Ti samples under UV, visible and sunlight-type illumination.Solid lines correspond to main de-excitation channels of the singlecomponents observed by photoluminescence (color code indicative oftheir characteristic energy). Dashed lines are de-excitation channelscharacteristic of the composite samples.

de-excitation channel is present in the composite materials.The de-excitation channel observed under visible light excita-tion is also depicted in Scheme 2. This channel may withdrawelectrons from the carbon nitride phase and may involve thelocalized states of one or both components. The different effi-ciencies of the de-excitation channel in the different samplescan be related to the specific energy situation of the localizedstates with respect to the corresponding Fermi edge of thecomposite.49 The correct energy position may allow maximumoverlapping with the corresponding final states of the excitedelectrons, increasing the likehood of charge carrier transferbetween components. This hypothesis needs however furtherassessment. In any case, the interpretation may suggest thatthe g-C3N4 surface is active under visible light excitation andwould contribute to the overall activity of the materials. In suchcase, the active species for toluene photoattack is not obviousas it is well known that carbon nitride does not generate holeswith enough chemical potential to produce OH˙ radicalspecies.50 This fact may differentiate the carbon nitride fromits anatase counterpart. The situation is relatively complex toanalyse; on the one hand, the presence of a carbon nitride“active” surface is in agreement with the majority of previouslyreported contributions concerning g-C3N4-containing mate-rials in contact with photoactive semiconductors.11–18 On thecontrary, in our case, the UV and sunlight-type chemicalbehaviour(s) through the sample series is (are) relativelysimilar (Fig. 9), indicating that the hypothetical contributionof the second de-excitation mechanism may not determinethe series behaviour upon visible light excitation.

We finally would like to further stress that the behav-iour of the photoluminescence spectra depicted in Fig. 10for the two excitation wavelengths tested indicates thecontact between the two components of the composite sys-tem. This contact seems critical to maximise/optimise thephotocatalytic properties of samples having a carbon nitridecontent between 0.5 and 1 wt.%. So, independent of theexact electronic pathway used to establish the electroniccontact between components, its effectiveness seems in allcases (i.e. under UV and visible excitation) strongly relatedto the extension of the interface contact. The complete inter-pretation of the photocatalytic properties is neverthelesscomplex and they may not be exclusively driven by thetwo de-excitation mechanisms discussed in the compositematerials but can be additionally affected by factors relatedto the shadowing of the anatase surface (in the presenceof the carbon nitride component): small, unnoticed but sig-nificant modification of the interaction between componentsas a function of the carbon nitride content, morphologyeffects related to the modest variations presented in Table 1,small enhancement effects of the pollutant adsorptioncapability in the presence of carbon nitride in the compositesamples (although the increment in surface area in Table 1would also indicate the modest potential of this point),as well as other unknown contributions which may need tobe considered for full interpretation of the photocatalyticproperties.



Conclusions

In this work we synthesized g-C3N4–TiO2 composite materialswith the content of carbon nitride in the 0.25 to 4 wt.% range.A simple impregnation method provides high surface areamaterials resembling the anatase component. Physicochemicalcharacterization indicates that the anatase phase is unalteredin the presence of g-C3N4 while the minority componentmaintains its main structural and electronic properties.

The performance of the g-C3N4–TiO2 composite materialswas tested for gas-phase toluene photoelimination underUV and sunlight-type illumination conditions. Full calcula-tion of the quantum yield of the reaction allows comparisonof the activity of the samples on a quantitative basis. Fromphotocatalytic results we provide evidence that the presenceof carbon nitride in the 0.5 to 1 wt.% range enhances boththe activity and selectivity (to total oxidation) of the reactionirrespective of the illumination conditions. Analysis ofde-excitation after light excitation indicates that the perfor-mance of the photocatalysts is intimately related to the pres-ence of de-excitation channels characteristic of the compositematerial, i.e. not present in the single parent components.Under UV excitation the component contact provides a Z-schemeroute for charge carrier recombination which increases theprobability of the anatase active hole species to reach thesurface and be involved in the chemical steps of photo-degradation. Under visible excitation we observe the predomi-nance of an alternative channel which would eliminate theelectrons of the carbon nitride component and may boost theperformance of its surface activity during degradation of tolu-ene. In both cases (UV and visible light), the interface contactbetween the two components appears to critically influencethe recombination and thus the overall photoactivity perfor-mance of the g-C3N4–TiO2 materials.

Acknowledgements

A. Kubacka and M. J. Muñoz-Batista thank the MINECO forsupport through, respectively, the postdoctoral “Ramón yCajal” and predoctoral FPI programs. Financial support fromMINECO is also acknowledged (project CTQ2010-14872/BQU).

Notes and references

1 M. R. Hoffman, S. T. Martin, W. Choi and D. W. Bahneman,
Chem. Rev., 1995, 95, 69.
2 O. Carp, C. L. Huisan and A. Reller, Prog. Solid State Chem.,
2004, 32, 33.
3 H. Thu, M. Karkmaz, E. Puzenat, C. Guillard and J. M. Herrmann,
Res. Chem. Intermed., 2005, 31, 449.
4 A. Kubacka, G. Colón and M. Fernández-García, Chem. Rev.,
2012, 112, 1555.
5 A. Kohgkand and P. V. Kamat, ACS Nano, 2007, 1, 13.
6 W. D. Wang, P. Serp, P. Kalack and J. L. Faria, J. Mol. Catal.
A: Chem., 2005, 235, 199.7 Y. Qu and X. Duan, Chem. Soc. Rev., 2013, 42, 2568.


8 X. Wang, K. Maeda, X. Chen, K. Tanabe, K. Domen, Y. Hou,
X. Fu and M. Antonietti, J. Am. Chem. Soc., 2010, 131, 1680.
9 L. Gei and C. Han, Appl. Catal., B, 2011, 100, 108.
10 S. Chu, Y. Wang, X. Fan and Z. Zou, ACS Catal., 2013, 3, 912. 11 H. Yan and H. Yang, J. Alloys Compd., 2011, 509, L26. 12 S. Obregón and G. Colón, Appl. Catal., B, 2014, 144, 775. 13 J. Yu, S. Wang, J. Low and W. Xiao, Phys. Chem. Chem. Phys.,
2013, 15, 16883.14 C. Miranda, H. Mansilla, J. Yáñez, S. Obergón and G. Colón,

J. Photochem. Photobiol., A, 2013, 253, 16.15 K. Shidharan, E. Jang and T. J. Park, Appl. Catal., B,

2013, 142–143, 718.16 X.-J. Wang, W.-Y. Yang, F.-T. Li, Y.-B. Xue, R.-H. Liu and

Y.-J. Llao, Ind. Eng. Chem. Res., 2013, 52, 17140.17 N. Boohprokov, N. Wetchakon, D. Waxler and

B. Inceesungrom, J. Colloid Interface Sci., 2014, 417, 402.18 L. Zhang, D. Jing, X. She, H. Liu, D. Yang, Y. Lu, J. Li,

Z. Zheng and L. Gou, J. Mater. Chem. A, 2014, 2, 2071.19 G. E. Imoberdorf, A. E. Cassano, O. M. Alfano and

H. A. Irazoqui, AIChE J., 2006, 52, 1814.20 L. Zhang and W. A. Anderson, Chem. Eng. J., 2010, 65, 1513.
21 Q. L. Yu, M. M. Ballari and H. J. H. Brouwers, Appl. Catal., B,
2010, 99, 58.22 A. L. Zazueta, H. Destallaits and G. L. Puma, Chem. Eng. J.,

2013, 217, 475.23 M. J. Muñoz-Batista, A. Kubacka, M. N. Gómez-Cerezo,

D. Tudela and M. Fernández-García, Appl. Catal., B,2013, 140–141, 626–635.

24 J. Mo, Y. Zhang, Q. Xu, Y. Zhu, J. J. Lamson and R. Zhao,
Appl. Catal., B, 2009, 89, 570–576.
25 M. J. Muñoz-Batista, M. N. Gómez-Cerezo, A. Kubacka,
D. Tudela and M. Fernández-García, ACS Catal., 2014, 4, 63.
26 P. G. De Gennes and C. Taupin, J. Phys. Chem., 1982, 86,
2294.
27 M. Fernández-García, X. Wang, C. Belver, J. C. Hanson and
J. A. Rodriguez, J. Phys. Chem. C, 2007, 111, 674.
28 A. Le Bail, H. Duroy and J. L. Forquet, Mater. Res. Bull.,
1988, 23, 447.
29 G. K. Williamson and W. H. Hall, Acta Metall., 1953, 1, 22.
30 A. Fuerte, M. D. Hernández-Alonso, A. J. Maira, A. Martínez-Arias,
M. Fernández-García, J. C. Conesa, J. Soria and G. Munuera,Chem. Commun., 2001, 2718.

31 A. Fuerte, M. D. Hernández-Alonso, A. J. Maira, A. Martínez-Arias,
M. Fernández-García, J. C. Conesa, J. Soria and G. Munuera,J. Catal., 2002, 212, 1.
32 B. Ohtani, Chem. Lett., 2008, 37, 167.
33 C. Baumanis and D. W. Bahnemann, J. Phys. Chem. C, 2008,
112, 19097.34 M. Sleiman, P. Conchon, C. Ferronato and J.-M. Chovelon,

Appl. Catal., B, 2009, 86, 159.35 M. Groenewolt and M. Antonietti, Adv. Mater., 2005,

17, 1789.36 S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397.
37 J. S. Zhang, J. H. Sun, K. Maeda, K. Domen, P. Liu,
M. Antonietti, X. Z. Fu and X. C. Wang, Energy Environ. Sci.,2011, 4, 675–678.



38 M. Fernández-García, A. Martínez-Arias, J. C. Hanson and
J. A. Rodríguez, Chem. Rev., 2004, 104, 4063.
39 J. Mo, Y. Zhang, Q. Xu, Y. Zhu, J. J. Lamson and R. Zhao,
Appl. Catal., B, 2009, 89, 570.
40 M. Sleiman, P. Conchon, C. Ferronato and J.-M. Chovelon,
Appl. Catal., B, 2009, 86, 159.
41 Y. Feng, L. Li, M. Ge, C. Gou, J. Wang and L. Liu, ACS Appl.
Mater. Interfaces, 2010, 2, 3134.
42 M. Peláez, N. T. Nolan, S. C. Pillai, M. K. Severy, P. Falaras,
A. G. Kontos, P. S. M. Dunlop, J. A. Bryne and D. D. Dionysiou,Appl. Catal., B, 2012, 125, 331.
43 A. Kubacka, M. J. Muñoz-Batista, R. Rachwalik, B. Bachiller-Baeza
and M. Fernández-García, J. Catal., 2014, 309, 428.

44 T. Tachikawa and T. Majima, J. Am. Chem. Soc., 2009, 131, 8485.
45 X. Wang, Z. Feng, J. Shi, G. Jia, S. Shen, J. Zhou and C. Li,
Phys. Chem. Chem. Phys., 2010, 12, 7083.46 C. C. Mercado, Z. Seeley, A. Bandyopadhyay, S. Bose and

J. L. McHale, ACS Appl. Mater. Interfaces, 2011, 3, 228.47 E. Finnazi, C. D. Valentin and G. Pacchioni, J. Phys. Chem. C,

2009, 113, 3382.48 Z. Jin, N. Murakami, T. Tsubota and T. Ohno, Appl. Catal.,

B, 2014, 150–151, 479.49 V. Singh, J. Castellanos Beltrán, J. Casamada Ribot and

P. Nagpal, Nano Lett., 2014, 14, 597.50 K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti and

K. Domen, J. Phys. Chem. C, 2009, 113, 4940.


Effect of g-C3N4 loading on TiO2-based photocatalysts: UV and visible degradation of toluene

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