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Article Photocatalytic Decomposition of N2O by Using Nanostructured Graphitic Nitride/Zinc Photocatalysts Immobilized on Foam

Kamila Koˇcí 1, Martin Reli 1 , Ivana Troppová 1, Marcel Šihor 1,* , Tereza Bajcarová 2, Michal Ritz 2, Jiˇrí Pavlovský 2 and Petr Praus 1,2

1 Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17. listopadu 15, 708 00 Ostrava-Poruba, Czech Republic 2 Faculty of Materials Science and Technology, VŠB-Technical University of Ostrava, 17. listopadu 15, 708 00 Ostrava-Poruba, Czech Republic * Correspondence: [email protected]; Tel.: +42-0597-327-304

 Received: 12 August 2019; Accepted: 27 August 2019; Published: 30 August 2019 

Abstract: The aim of this work was to deposit cost-effective g-C3N4/ZnO nanocomposite photocatalysts (weight ratios of g-C3N4:ZnO from 0.05:1 to 3:1) as well as pure ZnO and g-C3N4 on Al2O3 foam and to study their photocatalytic efficiency for the photocatalytic decomposition of N2O, which was studied in a home-made batch photoreactor under A irradiation (λ = 365 nm). Based on the photocatalysis measurements, it was found that photocatalytic decomposition of N2O in the presence of all the prepared samples was significantly higher in comparison with photolysis. The photoactivity of the investigated nanocomposite photocatalysts increased in the following order: g-C N /ZnO (3:1) g-C N /ZnO (0.45:1) g-C N /ZnO (2:1) ZnO < g-C N < g-C N /ZnO (0.05:1). 3 4 ≈ 3 4 ≤ 3 4 3 4 3 4 The g-C3N4/ZnO (0.05:1) nanocomposite showed the best photocatalytic behavior and the most effective separation of photoinduced –hole pairs from all nanocomposites. The key roles played in photocatalytic activity were the electron–hole separation and the position and potential of the valence and conduction band. On the other hand, the specific surface area and band gap energy were not the significant factors in N2O photocatalytic decomposition. Immobilization of the photocatalyst on the foam permits facile manipulation after photocatalytic reaction and their repeated application.

Keywords: carbon nitride; ; alumina foam; photocatalysis;

1. Introduction Nitrous oxide is one of the most important greenhouse gases and represents 6.4% of the total global radiative forcing. Its contribution to global warming is important, because it is nearly 300 times higher than that of [1]. N2O is also a significant contributor to the destruction of the layer in the stratosphere. The increase of N2O emissions in the atmosphere (approximately 0.3% per year) is caused particularly by anthropogenic activities [1]. Synthetic in agriculture, combustion (fossil fuel, biomass), and the production of adipic and are among the main contributors to N2O emissions. The decomposition of nitrous oxide into N2 and O2 offers a simple method for its transformation to natural air elements. Decomposition of N2O under UV irradiation on materials (Equation (1)) is one possible solution for its removal:

hν, catalyst 1 N O N + O . (1) 2 → 2 2 2

Catalysts 2019, 9, 735; doi:10.3390/catal9090735 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 735 2 of 13

Photocatalytic decomposition of nitrous oxide was studied on Cu embedded on various , e.g., (SiO Al O , Al O , SiO )[2], ZnS [3], TiO [4], TiO doped by Ag [5–7], Ce [8,9]. 2· 2 3 2 3 2 2 2 Graphitic carbon nitride (g-C3N4) is an n-type semiconductor with a unique structure and useful optical, electric, and physicochemical properties. Although g-C3N4 has attracted increasing interest worldwide, its photocatalytic performance is still limited due to fast recombination of charge carriers [10–13]. Enhancing the photoactivity of g-C3N4-based photocatalysts can be achieved by the construction of ’ heterojunctions [13,14]. The combination of different semiconductor photocatalysts may create new materials with optimal performances. The formation of unique Catalysts 2019, 9, x FOR PEER REVIEW 2 of 12 semiconductor–semiconductor junctions can enhance the electron–hole separation for improved photocatalyticGraphitic efficiency carbon and nitride extend (g-C3N the4) is an spectral n-type semiconductor range for with light a unique absorption. structure and Looking useful into the mechanisms,optical, one electric, can identify and physicoche that fourmical dissimilar properties. modelsAlthough of g-C semiconductor–semiconductor3N4 has attracted increasing interest junctions worldwide, its photocatalytic performance is still limited due to fast recombination of charge carriers have been[10–13]. utilized Enhancing to enhance the photoactivity the charge of kinetics, g-C3N4-based including photocatalysts semiconductor can be achieved sensitization, by the Type II, junction,construction and Z-scheme of semiconductors’ [15,16]. heterojunctions In our previous [13,14]. work, The heterojunctioncombination of different photocatalysts semiconductor (g-C 3N4/TiO2 photocatalysts may create new materials with optimal performances. The formation of unique and g-C3N4/WO3 photocatalysts with various weight ratios) were prepared and studied for N2O semiconductor–semiconductor junctions can enhance the electron–hole separation for improved photocatalytic decomposition [17–19]. The formation of heterojunctions contributed to the increase of photocatalytic efficiency and extend the spectral range for light absorption. Looking into the the N2O conversion.mechanisms, one can identify that four dissimilar models of semiconductor–semiconductor junctions Recenthave investigations been utilized to enhance on semiconductors’ the charge kinetics, photocatalysisincluding semiconductor are sensitization, oriented to Type photocatalyst II, immobilizationphase junction, [16]. Theand immobilizationZ-scheme [15,16]. In of our photocatalysts previous work, removesheterojunction any photocatalysts imperfections (g- with the C3N4/TiO2 and g-C3N4/WO3 photocatalysts with various weight ratios) were prepared and studied powder form of the photocatalysts: (i) The difficulties of separating photocatalysts, (ii) the inclination of for N2O photocatalytic decomposition [17–19]. The formation of heterojunctions contributed to the the nanoparticlesincrease of of the photocatalysts N2O conversion. to aggregate, and (iii) the difficulty of applying them to continuous flow reactors [Recent20]. investigations on semiconductors’ photocatalysis are oriented to photocatalyst immobilization [16]. The immobilization of photocatalysts removes any imperfections with the For those reasons, we decided to prepare g-C3N4/ZnO nanocomposites with different weight ratios powder form of the photocatalysts: (i) The difficulties of separating photocatalysts, (ii) the inclination to immobilize them on Al O foam, and to investigate them for N O photocatalytic decomposition of the nanoparticles2 of3 photocatalysts to aggregate, and (iii) the difficulty2 of applying them to under UVAcontinuous (365 nm) flow irradiation. reactors [20]. The g-C3N4/ZnO photocatalysts have not yet been studied for N2O photocatalyticFor decomposition. those reasons, we decided to prepare g-C3N4/ZnO nanocomposites with different weight ratios to immobilize them on Al2O3 foam, and to investigate them for N2O photocatalytic 2. Resultsdecomposition and Discussion under UVA (365 nm) irradiation. The g-C3N4/ZnO photocatalysts have not yet been studied for N2O photocatalytic decomposition. 2.1. Photocatalysts Characterization 2. Results and Discussion The real content of ZnO in prepared samples was determined by atomic absorption measurements 2.1. Photocatalysts Characterization (Table1). The nitrogen physisorption characterization technique was used for the specific surface area determinationThe (Figure real 1content). Each photocatalystof ZnO in prepared displayed samples a mesoporous-macroporous was determined by atomic character.absorption From the measurements (Table 1). The nitrogen physisorption characterization technique was used for the comparisonspecific of textural surface propertiesarea determination of the (Figure prepared 1). Each samples, photocatalyst it is evident displayed that a themesoporous- g-C3N4 addition positivelymacroporous affected the character. properties From of the the comparison photocatalyst’s of textural porous properties structure, of the prepared significantly samples, enhancingit is its specific surfaceevident area that asthe wellg-C3N as4 addition the net positively pore volume, affected e.g., the pureproperties ZnO of showed the photocatalyst’s a specific porous surface area of 2 structure, significantly enhancing its3 specific surface area as well as the net pore volume, e.g., pure 7 m /g and a net pore volume of 61 mm liq/g, while g-C3N4 /ZnO (3:1) showed a specific surface area ZnO showed a specific surface area of 7 m2/g and a net pore volume of 61 mm3liq/g, while g-C3N4 of 68 m2/g and a net pore volume of 339 mm3 /g (Table1). /ZnO (3:1) showed a specific surface area of liq68 m2/g and a net pore volume of 339 mm3liq/g (Table 1).

Figure 1. Nitrogen adsorption/desorption isotherms of ZnO, g-C3N4, and g-C3N4/ZnO. Figure 1. Nitrogen adsorption/desorption isotherms of ZnO, g-C3N4, and g-C3N4/ZnO.

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Catalysts 2019, 9, x FOR PEER REVIEW 3 of 12 Table 1. Structural and microstructural properties and content of ZnO ZnO, g-C3N4, and g-C3N4/ZnO. Table 1. Structural and microstructural properties and content of ZnO ZnO, g-C3N4, and g-C3N4/ZnO. 2 3 2+ Photocatalysts SBET (m /g) V net (mm lig/g) Content of Zn (wt.%) Photocatalysts SBET (m2/g) V net (mm3lig/g) Content of Zn2+ (wt.%) ZnO 7 61 80 ZnO 7 61 80 g-C3N4/ZnO (0.05:1) 8 62 77 g-C3N4/ZnO (0.05:1) 8 62 77 g-C3N4/ZnO (0.45:1) 37 212 55 g-C3N4/ZnO (0.45:1) 37 212 55 g-C N /ZnO (2:1) 58 298 26 3 4 g-C3N4/ZnO (2:1) 58 298 26 g-C N /ZnO (3:1) 68 339 21 3 4 g-C3N4/ZnO (3:1) 68 339 21 g-C N 85 413 0 3 4 g-C3N4 85 413 0

TheThe X-ray X-ray powder powder diffraction diffraction (XRD) (XRD) patterns patterns of of all all investigated investigated photocatalysts photocatalysts are are shown shown in in FigureFigure 2.2. Two Two different di fferent phases phases based based on on zinc, zinc, zinc zinc oxide, oxide, and and zinc zinc cyanamide cyanamide were were observed observed in in all all mixturesmixtures with with higher contentscontents ofof g-Cg-C33NN44.. TheThe zinczinc oxide oxide phase phase was was markedly markedly more more significant significant than than the thezinc zinc cyanide phase. phase. Graphitic Graphitic carbon carbon nitride nitride was confirmedwas confirmed by the by presence the presence of the reflectionof the reflection at position at position14.9◦ 2Theta 14.9° (d 2Theta= 0.695 (d nm) = 0.695 indexed nm) asindexed the (100) as planethe (100) and attributedplane and toattributed the in-plane to the tri-s-triazine in-plane tri-s- units triazineforming units one-dimension forming one-dimension melon strands. melon The strands. most typical The most reflection typical was reflection found around was found 32.2 ◦around2Theta 32.2°(d = 0.3222Theta nm) (d and= 0.322 could nm) be and interpreted could be asinterpreted long range as interplanar long range stackinginterplanar of the stacking aromatic of the systems aromatic with systemsthe hkl planewith (002).the hkl Unfortunately, plane (002). thisUnfortunately, reflection was this overlapped reflection with was the overlapped zinc cyanamide with reflectionthe zinc cyanamide(211) in all reflection measured (211) samples. in all measured samples. 3 4 ForFor the the obtained obtained g-C g-C3NN/ZnO4/ZnO photocatalysts, photocatalysts, nearly nearly all all of of the the characteristic characteristic diffraction diffraction peaks peaks of of zinczinc oxide oxide were were observed observed in in the the XRD XRD pattern pattern and and confirmed confirmed the the presence presence of of the the ZnO ZnO phase. phase. In In the the 3 4 3 4 casecase of of the the g-C 3NN4phase,phase, the the (002) (002) peak peak of of g-C g-C3NN 4waswas almost almost not not observed observed.. The The relatively relatively poor poor 3 4 crystallinitycrystallinity ofof thethe g-C g-C3NN4 phase phase indicates indicates that that simultaneous simultaneous crystallization crystallization of the ZnOof the phase ZnO interferes phase 3 4 interfereswith the condensationwith the condensation process and process interlayer and stackinginterlayer patterns stacking of thepatterns g-C3N of4 phase,the g-C asN shown phase, by as its 3 4 shownweak dibyff ractionits weak intensity diffraction [21 intensity]. The XRD [21]. patterns The XRD in patterns g-C3N4/ ZnOin g-C photocatalystsN /ZnO photocatalysts indicate that indicate these thattwo these phases two occur phases separately occur separately in the composite in the andcomposite they do and not havethey do strong not crystalhave strong lattice crystal incorporation. lattice incorporation.This result was This also result confirmed was also by transmissionconfirmed by electron transmission microscopy electron (TEM). microscopy (TEM). AllAll phases phases in in the the XRD XRD patterns patterns are are marked marked by by different different symbols symbols (Figure (Figure 22).). All phases were identifiedidentified using using the the PDF-2 PDF-2 database database issued issued by by the the international international center center diffraction diffraction data. data. Zinc Zinc oxide oxide correspondscorresponds to PDF-2PDF-2 cardcard no. no. 01-080-0075 01-080-0075 and and zinc zinc cyanamide cyanamide to PDF-2to PDF-2 card card no. 01-070-4898.no. 01-070-4898. All XRDAll XRDpatterns patterns were were recorded recorded using using co-radiation co-radiation with wavelengthwith wavelength lambda lambda= 1.78 = Å. 1.78 For Å. this For reason, this reason, all peaks all peakswere shiftedwere shifted to the higherto the higher 2Theta 2Theta angles (basedangles on(based the Braggson the equation)Braggs equation) than Cu than radiation, Cu radiation, which is whichmore common.is more common.

(a) (b)

FigureFigure 2. 2. XRDXRD patterns patterns of ZnO, of ZnO, g-C g-C3N4,3 andN4, g-C and3N g-C4/ZnO3N4 /(ZnOa) and (a XRD) and fragment XRD fragment of g-C3N4/ZnO of g-C3N4 (0.45:1;/ZnO 2:1,(0.45:1; and 2:1,3:1) andphotocatalysts 3:1) photocatalysts (b). (b).

The UV–vis diffuse reflectance spectra of g-C3N4, ZnO, and g-C3N4/ZnO photocatalysts are compared in Figure 3a. The band gaps’ energy values of the prepared nanocomposites were

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CatalystsThe 2019 UV–vis, 9, x FOR di PEERffuse REVIEW reflectance spectra of g-C3N4, ZnO, and g-C3N4/ZnO photocatalysts4 of are 12 compared in Figure3a. The band gaps’ energy values of the prepared nanocomposites were estimated estimatedby plotting by the plotting Kubelka–Munk the Kubelka–Munk function, function, as shown as in shown Figure in3b. Figure The largest 3b. The band largest gap band was gap 3.09 was eV 3.09and eV it belongs and it belongs to pure to ZnO. pure OnZnO. the On other the other hand, hand, pure g-Cpure3N g-C4 had3N4 had a band a band gap gap energy energy of 2.64of 2.64 eV. eV.The The nanocomposites nanocomposites band band gap gap energies energies were were 3.00, 3.00, 2.75, 2.75, 2.75, 2.75, and and 2.75 2.75 eV eV forfor g-Cg-C33N4/ZnO/ZnO (0.05:1), (0.05:1), g-C3NN44/ZnO/ZnO (0.45:1), (0.45:1), g-C 3NN44/ZnO/ZnO (2:1), (2:1), and and g-C g-C33NN44/ZnO/ZnO (3:1), (3:1), respectively. respectively. The presence ofof g-Cg-C33N4 inin thethe nanocomposites nanocomposites extended the light light absorption absorption of the the composites composites into into the the visible visible region region compared compared toto pure ZnO photocatalyst.

(a) (b)

Figure 3. UV-vis didiffuseffuse reflectancereflectance spectraspectra ((aa)) andand TaucTauc plotsplots ((bb)) ofof ZnO,ZnO, g-Cg-C33NN44, and g-C3N44/ZnO./ZnO.

The spectra spectra are are seen seen in in Fi Figuregure 44.. The The spectra spectra of of samples samples g-C g-C3N34N, g-C4, g-C3N34/ZnON4/ZnO (3:1), (3:1), g- Cg-C3N34N/ZnO4/ZnO (2:1), (2:1), and and g-C g-C3N34N/ZnO4/ZnO (0.45:1) (0.45:1) primarily primarily contain contain bands bands that that are are characteristic characteristic for for graphitic graphitic carbon nitride [22–24]. [22–24]. The The regions regions are are labelled as N1, N2, and N3. The The region region N1 contains bands at 1 3280, 3255, 3160, and 3090 cm −1. .These These bands bands can can be be assigned assigned to to the the stretching stretching vibration vibration of of N-H N-H bonds. bonds. The region N2 contains intensive bands at 1680 (s (shoulder),houlder), 1635, 1575, 1545, 1545, 1460, 1460, 1430 1430 (shoulder), (shoulder), 1 1410, 1335 (shoulder), 1320, 1280, (shoulder),(shoulder), 1240,1240, andand 12101210 cmcm−−1.. Characteristic Characteristic bands bands of of the the skeletal skeletal “breathing” vibration of aromatic rings of nitrogen heterocyclic compounds and stretching vibration bonds between carbon and nitrogen were found in the N2 region. The The N3 N3 region contains only a 1 medium intensive band at 815 cm−1, ,which which is is due due to to the the breathing breathing mode mode of of the the triazine triazine unit. unit. Moreover, Moreover, thethe spectra ofof samplessamples g-Cg-C33N4/ZnO/ZnO (3:1), g-C3N44/ZnO/ZnO (2:1), and g-C3NN44/ZnO/ZnO (0.45:1) (0.45:1) also contain 1 bands of of complex complex cyanide cyanide compounds compounds at at2085 2085 and and 2045 2045 cm cm−1 [25].− [ 25The]. bands The bands have havea medium a medium or weak or intensityweak intensity (see region (see region CN). The CN). relative The relative red shifts red shiftsof theof complex the complex cyanide cyanide compounds compounds compared compared with 1 thewith normal the normal wavenumber wavenumber of 2250 of cm 2250−1 are cm due− areto the due conjugation to the conjugation effect of the effect extended of the extendedC-N aromatic C-N systemaromatic [21]. system The [spectra21]. The of spectra samples of g-C samples3N4/ZnO g-C 3(0.05:1)N4/ZnO and (0.05:1) ZnO andonly ZnO contain only medium contain mediumor weak bandsor weak of bandscarbonates of (see region (see C1 region and C1C2) and and C2) a broad and a broadband of band zinc of ox zincide oxide(see region (see region Z). The Z). 1 carbonateThe bands bands can canbe assigned be assigned to toasymmetric asymmetric stretching stretching vibration vibration of of carbonate carbonate (1450 cm−−1)) and and 1 symmetric deformation deformation vibration vibration of of carbonate carbonate (885 (885 cm cm−1−), ),respectively respectively [25]. [25 The]. The band band of ofvibration vibration of 1 zincof zinc oxide oxide [26] [26 is] isseen seen at at 480 480 cm cm−1−. This. This band band is is seen seen also also in in the the spectrum spectrum of samplesample g-Cg-C33N4/ZnO/ZnO (0.45:1).

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FigureFigure 4. 4. InfraredInfraredInfrared spectra spectraspectra of ofof ZnO, ZnO,ZnO, g-C g-Cg-C333NN444,, and and g-Cg-C33NN44/ZnO./ZnO./ZnO.

The RamanRaman spectraspectra areare seenseen inin FigureFigure5 .5. TheThe spectraspectra areare inin thethe “full-scale”“full-scale” mode,mode, thusthus thethe presentedpresented spectraspectra havehave didifferentfferent scales scales of of intensity intensityintensity axes. axes.axes.The TheThespectra spectraspectra of ofof samples samplessamples g-C g-Cg-C3N33N444,,, g-Cg-Cg-C333NN444/ZnO//ZnOZnO (3:1),(3:1), g-Cg-Cg-C333N44/ZnO/ZnO/ZnO (2:1),(2:1), and and g-C33NN444/ZnO/ZnO/ZnO (0.45:1)(0.45:1) (0.45:1) onlyonly containcontain thethe ba bandsbands characteristic for graphiticgraphitic carboncarbon nitridenitride (i.e.,(i.e., [[22,24,27])22,24,27]) inin thethe regionsregions labelledlabelledlled N1,N1,N1, N2,N2,N2, N3,N3,N3, N4,N4,N4, andandand N5.N5. The The N1 N1 region region contains contains 1 1 strongstrong bandsbands atat 12301230 andand 12151215 cmcm−−11 andandand weakweak weak bandsbands bands atat at 11501150 1150 andand and 11151115 1115 cmcm cm−−1−1.. TheThe. The N2N2 N2 regionregion region containscontains contains aa 1 1 amedium medium band band at at770 770 cm cm−−11 andand− and aa strongstrong a strong bandband band atat 705705 at cm 705cm−−11 cm.. TheThe− . N3N3 The regionregion N3 region containscontains contains twotwo mediummedium two medium bandsbands 1 1 1 bandsat 485 and at 485 470 and cm−− 47011 andand cm oneone− andweakweak one bandband weak atat 455455 band cmcm− at−11.. MediumMedium 455 cm− bandsbands. Medium atat 355355 bands andand 210210 at 355cmcm−−11 and areare 210inin thethe cm N4N4− areand in N5 the regions, N4 and respectively. N5 regions, respectively.The assignment The of assignment thethe above-mentionedabove-mentioned of the above-mentioned RaRaman bands Raman of graphitic bands ofcarbon graphitic nitride carbon has not nitride been haspublis nothed been yet. published We can suppose yet. We the can bands suppose are related the bands to vibration are related of the to vibrationtri-s-triazinetri-s-triazine of the ringring tri-s-triazine (i.e.,(i.e., [27]).[27]). TheThe ring spectraspectra (i.e., [ 27ofof ]). samplessamples The spectra g-Cg-C33N4 of4/ZnO/ZnO samples (0.05:1)(0.05:1) g-C andand3N 4 ZnOZnO/ZnO containcontain (0.05:1) twotwo and bandsbands ZnO containof carbonate two bands(see region of carbonate C1 and (seeC2) regionand three C1 andbands C2) of and zinc three oxide bands (see ofregion zinc Z1, oxide Z2, (see and region Z3). The Z1, 1 Z2,carbonate and Z3). bands The carbonateare presented bands at are 1080 presented cm−−11 at(symmetric(symmetric 1080 cm− stretchingstretching(symmetric vibration)vibration) stretching andand vibration) atat 700700 and cmcm at−−11 1 700(symmetric(symmetric cm− (symmetric deformationdeformation deformation vibration),vibration), vibration), respectivelyrespectively respectively [25].[25]. TheThe [25 ].bandsbands The bandscharacteristiccharacteristic characteristic forfor zinczinc for zincoxideoxide oxide areare 1 arepresented presented at 435, at 435, 330, 330, and and 185 185cm−− cm11.. − .

FigureFigure 5. 5. RamanRaman spectraspectra ofof ZnO,ZnO, g-Cg-C333NN444,, and and g-C g-C33NN44/ZnO./ZnO./ZnO.

Figure 6a–d6a–d demonstrate demonstrate thethe TEM TEM images images of of pure pure ZnO, ZnO, g-C g-C33N3N444,,, andandand g-Cg-Cg-C333N4/ZnO/ZnO/ZnO (3:1) (3:1) photocatalysts,photocatalysts, respectively. The The ZnO ZnO nanopartic nanoparticleslesles (Figure(Figure (Figure 66a)6a)a) havehavehave hexagonalhexagonalhexagonal shapesshapesshapes andandand the thethe particle size is about 50 nm while pure g-C33N44 (Figure(Figure 6b)6b) showsshows aa 2D2D lamellarlamellar structure.structure. TheThe TEMTEM micrographs of g-C33N44/ZnO/ZnO (3:1)(3:1) (Figure(Figure 6c)6c) testifytestify thatthat thethe ZnOZnO nanoparticlesnanoparticles werewere notnot supportedsupported

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particle size is about 50 nm while pure g-C3N4 (Figure6b) shows a 2D lamellar structure. The TEM micrographsCatalysts 2019 of, 9 g-C, x FOR3N PEER4/ZnO REVIEW (3:1) (Figure6c) testify that the ZnO were not supported6 of 12 well on the surface of g-C3N4. It is possible to infer that the salt (from preparation) can be observed in well on the surface of g-C3N4. It is possible to infer that the salt (from preparation) can be observed the g-C3N4/ZnO samples (Figure6d). It is also evident from the results of the EDS analysis (Figure6e), whichinconfirmed the g-C3N4/ZnO not only samples the presence(Figure 6d). of It Zn, is also C, O, evident and N from elements the results but alsoof the Na EDS element. analysis The (Figure presence 6e), which confirmed not only the presence of Zn, C, O, and N elements but also Na element. The of the salt was also confirmed by Raman and infrared spectroscopy. presence of the salt was also confirmed by Raman and infrared spectroscopy.

(a) (b)

(c) (d)

(e)

Figure 6. TEM images of ZnO (a), g-C3N4 (b), and g-C3N4/ZnO (3:1) (c,d) and EDS analysis of g- Figure 6. TEM images of ZnO (a), g-C3N4 (b), and g-C3N4/ZnO (3:1) (c,d) and EDS analysis of C3N4/ZnO (3:1) (e). g-C3N4/ZnO (3:1) (e).

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Photoelectrochemical measurementsmeasurements of of all all photocatalysts photocatalysts are are shown shown in Figurein Figure7. It 7. is It evident is evident that thatthe magnitudethe magnitude of the of generated the generated current current decreased decreased with an increasingwith an increasing amount of amount g-C3N4 ofpresented g-C3N4 presentedat the photocatalysts at the photocatalysts until the g-Cuntil3N the4 was g-C3 inN4 the was majority. in the majority. Higher Higher photocurrents photocurrents measured measured in the inpresence the presence of the of ZnO the sampleZnO sample show show that there that there are significantly are significantly more more charge charge carriers carriers produced produced in ZnO in ZnOcompared compared to g-C to3N g-C4. The3N4. photocurrent The photocurrent decreased decreased with an with increasing an increasing amount amount of g-C3 Nof4 ,g-C which3N4, formed which Catalysts 2019, 9, x FOR PEER REVIEW 7 of 12 formeda significantly a significantly smaller smaller photocurrent. photocurrent. Photoelectrochemical measurements of all photocatalysts are shown in Figure 7. It is evident that the magnitude of the generated current decreased with an increasing amount of g-C3N4 presented at the photocatalysts until the g-C3N4 was in the majority. Higher photocurrents measured in the presence of the ZnO sample show that there are significantly more charge carriers produced in ZnO compared to g-C3N4. The photocurrent decreased with an increasing amount of g-C3N4, which formed a significantly smaller photocurrent.

Figure 7. PhotocurrentPhotocurrent generation at electrodes made of ZnO,ZnO, g-Cg-C33N4,, and and g-C g-C3NN44/ZnO/ZnO recorded recorded at at 1 1

V vs. Ag/AgCl Ag/AgCl in deoxygenated 0.1 M KNO 3.. Figure 7. Photocurrent generation at electrodes made of ZnO, g-C3N4, and g-C3N4/ZnO recorded at 1 2.2. PhotocatalyticV vs. Decomposition Ag/AgCl in deoxygenated of N2O 0.1 M KNO3. 2.2. Photocatalytic Decomposition of N2O The influence of prepared nanocomposites on the photocatalytic decomposition of nitrous oxide The influence2.2. Photocatalytic of prepared Decomposition nanocomposites of N2O on the photocatalytic decomposition of nitrous oxide was studied from 0 to 22 h (Figure8). The photolysis was also conducted in order to confirm the was studied fromThe influence0 to 22 ofh prepared (Figure nanocomposites 8). The photol on ysisthe photocatalytic was also conducted decomposition in of order nitrous to oxide confirm the photocatalytic activity of the prepared samples. The time dependences of the nitrous oxide conversion photocatalyticwas studiedactivity from of 0 tothe 22 hprepared (Figure 8). Thesample photols. ysisThe was time also conducteddependences in order of to confirmthe nitrous the oxide of all g-C3Nphotocatalytic4/ZnO nanocomposites, activity of the includingprepared sample pures. ZnOThe time and g-Cdependences3N4 photocatalysts, of the nitrous areoxide depicted in conversion of all g-C3N4/ZnO nanocomposites, including pure ZnO and g-C3N4 photocatalysts, are Figure8. Itconversion is evident of that all g-C each3N4 prepared/ZnO nanocomposites, photocatalyst including proved pure to ZnO be muchand g-C more3N4 photocatalysts, e ffective in are comparison depicted indepicted Figure in 8. Figure It is evident 8. It is evident that thateach each prep preparedared photocatalystphotocatalyst proved proved to be tomuch be moremuch effective more effective with photolysis. The g-C3N4/ZnO (0.05:1) and pure g-C3N4 and ZnO showed a significantly higher in comparisonin comparison with photolysis. with photolysis. The Theg-C g-C3N34N/ZnO4/ZnO (0.05:1)(0.05:1) and and pure pure g-C 3Ng-C4 and3N 4 ZnOand showed ZnO ashowed a efficiency than other photocatalysts. significantlysignificantly higher efficiency higher efficiency than thanother other photocatalysts. photocatalysts.

(a) (b)

Figure 8. The time dependences of N2O conversion (a) and N2O conversion after 22 h of irradiation (b) over ZnO, g-C3N4 and g-C3N4/ZnO and photolysis under UVA (λ = 365 nm) irradiation.

(a) (b)

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The highest N2O conversion under UVA irradiation was reached over the g-C3N4/ZnO (0.05:1) photocatalyst. The N2O conversion in the presence of this photocatalyst was 11% after 22 h of UVA irradiation. For comparison, the N2O conversion without photocatalyst was only 6% after 22 h of UVA irradiation. The same foam with photocatalyst was used for repeated tests three times. This confirms the stability and ability to reuse these materials without losing photocatalytic activity. It is hard to compare the observed photocatalytic results with the literature, as it is usually measured under different reaction conditions (geometry of photoreactor, irradiation source, concentration of N2O, amount of photocatalyst, whether it is immobilized, and also the irradiation time). Each of these conditions influences the photocatalytic decomposition of N2O in a different way and therefore accurate comparison is impossible. Also, not many research groups have investigated the photocatalytic decomposition of N2O. Therefore, the obtained results were compared with our previous works. N2O conversion for the g-C3N4/BiVO4 and g-C3N4/WO3 composites after 20 h of irradiation was 12.5% and 14.5%, respectively. However, both previous sets of photocatalysts were measured in a different photoreactor (different geometry) and therefore, from that point of view, the observed N2O conversion of approximately 10% after 20 h of irradiation seems to be comparable to the previous results.

2.3. Proposed Mechanism for Photocatalytic Activity Enhancement

+ When g-C3N4 is combined with ZnO, the generated (e−) and holes (h ) can migrate by two possible mechanisms: Type II heterojunction and the Z-scheme system. In the case of the type II - heterojunction of g-C3N4/ZnO, the generated e in the conduction band (CB) of g-C3N4 can transfer to the CB of ZnO and simultaneously, the h+ in the valence band (VB) of ZnO can migrate to the VB of g-C3N4. In the case of the Z-scheme system of g-C3N4/ZnO, e− in the CB of ZnO are directly + recombined with the h in the VB of g-C3N4 at the heterojunction interface, and so the recombination + - of e− and h in ZnO and g-C3N4 is inhibited. In this case, the e are accumulated in the CB of g-C3N4 and h+ in the VB of ZnO. Due to these two mechanisms, a prolonged lifetime of electrons and holes is gained [14]. The band edge positions of materials were calculated via Equations (2) and (3) [28]:

EVB = χ E + 0.5Eg, (2) − 0

E = EVB Eg, (3) CB − where χ is the of the material, EVB is the VB potential, ECB is the CB potential, Eg is the band gap energy of a material, and E0 is the energy of free electrons (4.5 V vs. NHE). The χ values for g-C3N4 and ZnO are 4.72 and 5.95 eV, respectively [28]. The EVB of g-C3N4 and ZnO were 1.75 and 3.05 eV, respectively. The E of g-C N and ZnO were 1.31 and 0.15 V, respectively. CB 3 4 − − The formation of the Z-scheme system of g-C3N4/ZnO can be confirmed by the hydroxyl radical ( OH) trapping test using modified photoluminescence (PL) measurements [29]. After the • photoexcitation, the h+ with sufficient oxidation potential can be either directly involved in the photocatalytic degradation reactions or generate active species. The reaction between terephthalic acid (TA) and hydroxyl radicals ( OH) can generate hydroxyterephthalic acid (HTA), which is detectable • by the PL spectrometer (signal with maximal intensity centered at 425 nm) [30]. The PL peak intensity of the HTA is in proportion to the amount of produced OH. • Since the photoluminescence intensity increased in the presence of each photocatalyst over time, with the exception of pure g-C3N4, the presence of hydroxyl radicals was confirmed (Figure9a). + According to the band structures of ZnO and g-C3N4, the photoinduced h on g-C3N4 cannot oxidize the adsorbed H O molecules to OH because the VB potential of g-C N (+1.75 V vs. NHE) is less 2 • 3 4 positive than the standard potential of OH/OH (2.4 V vs. NHE). In the case of Z-scheme • − formation, the photogenerated h+ remain on the ZnO. The h+ accumulation on ZnO is in accordance with the migration mechanism of the direct Z-scheme. The creation of the Z-scheme system was observed by Wang et al. [14], Liu et al. [31], and Yu et. al. [21]. In our case, it was not confirmed that Catalysts 2019, 9, x FOR PEER REVIEW 9 of 12

Catalysts 2019, 9, 735 9 of 13 with the migration mechanism of the direct Z-scheme. The creation of the Z-scheme system was observed by Wang et al. [14], Liu et al. [31], and Yu et. al. [21]. In our case, it was not confirmed that g-C3NN44/ZnO/ZnO nanocomposite nanocomposite had had contact contact with with the the Z-sche Z-schememe system system because because the the nanocomposites nanocomposites were were not provenproven toto have have a highera higher PL intensityPL intensity than purethan ZnO.pure TheZnO. PL The intensity PL intensity decreased decreased with an increasing with an increasingcontent of ZnO,content which of ZnO, can producewhich can hydroxyl produce radical. hydroxyl radical.

(a) (b)

Figure 9. PLPL spectra spectra in in presence presence of of TA TA in in NaOH NaOH solution solution (excitation (excitation wave: wave: 365 365 nm), nm), irradiated irradiated for for40

min40 min in the in thepresence presence of ZnO, of ZnO, g-C3N g-C4, and3N4 ,g-C and3N g-C4/ZnO3N4 (/aZnO) and ( aa) comparison and a comparison of the PL of intensity the PL intensity against irradiationagainst irradiation time for timeof g-C for3N of4/ZnO g-C3 Nnanocomposites4/ZnO nanocomposites (b). (b).

According to to the the literature, literature, the the standard standard reduction reduction potential potential of ofN2O/N N2O2 /Nis 2+1.355is +1.355 V [32]. V [The32]. conductionThe conduction bands’ bands’ potentials potentials of both of pure both and pure prepared and prepared photocatalysts photocatalysts (g-C3N4 and (g-C ZnO)3N4 and are more ZnO) negativeare more ( negative−1.19 and ( −0.171.19 V, and respectively)0.17 V, respectively) than the standard than the reduction standard potential reduction of potential N2O/N2. ofTherefore, N2O/N2. − − bothTherefore, pure photocatalysts both pure photocatalysts showed similar showed photocatalytic similar photocatalytic activity. activity. The photocatalytic activity of g-C33N4/ZnO,/ZnO, with the exception of g-C 3NN44/ZnO/ZnO (0.05:1), (0.05:1), was was lower lower with regard toto purepure photocatalysts.photocatalysts. TheseThese threethree nanocompositesnanocomposites (g-C(g-C33NN44//ZnOZnO (0.45:1),(0.45:1), g-Cg-C33N4/ZnO/ZnO (2:1), and and g-C g-C3N3N4/ZnO4/ZnO (3:1)) (3:1)) had had not notonly only similar similar photocatal photocatalyticytic activity activity but also but band also gap band energy. gap Theenergy. photocatalytic The photocatalytic activity activity of nanocomposites of nanocomposites (g-C3 (g-CN4/ZnO3N4 /ZnO(0.45:1), (0.45:1), g-C3 g-CN4/ZnO3N4/ ZnO(2:1), (2:1), and and g- Cg-C3N34N/ZnO4/ZnO (3:1)) (3:1)) decreased decreased both both by by the the presence presence of of cyanide cyanide compounds compounds formed formed during during preparation preparation (Figure4 4)) andand byby thethe imperfectimperfect contact contact of of g-C g-C33N4 andand ZnO, ZnO, which which is visible visible in Figure 66c.c. The detection of cyanide compounds in g-C 3NN44/ZnO/ZnO indicates indicates that that simultaneous simultaneous ZnO ZnO crystallization crystallization could could interfere interfere with the thermal polymerization of melamine toto generategenerate aa well-definedwell-defined g-Cg-C33N44 phase. Based Based on the previous discussiondiscussion about about XRD XRD patterns patterns and infrared and infrared spectra, suchspectra, interference such interference in ZnO crystallization in ZnO crystallizationcan result in relatively can result poor in relatively crystallinity poor and crystallinity give rise to and defect give states rise withinto defect the states g-C3 Nwithin4 phase the [21 g-]. CThese3N4 phase defect states[21]. These can catch defect generated states chargecan catch carriers; gene however,rated charge they arecarriers; not accessible however, for they the adsorbed are not accessiblemolecule offor nitrous the adsorbed oxide, which molecule consequently of nitrous oxide, to awhich decrease consequently in photoactivity. leads to a decrease in photoactivity.A slightly different situation is the case of g-C3N4/ZnO (0.05:1), which proved to have the highest photocatalyticA slightly different activity. situation This nanocomposite is the case of containedg-C3N4/ZnO only (0.05:1), 4% of which g-C3 Nproved4, which to have means the mosthighest of photocatalyticthe holes in VB activity. of g-C 3ThisN4 probablynanocomposite recombined contained with only some 4% of of the g-C electrons3N4, which from means the most CB of of ZnO. the holesIn the in meantime, VB of g-C the3N4 photogeneratedprobably recombined electrons with in some the CB of ofthe g-C electrons3N4 and from a big the part CB of of the ZnO. electrons In the meantime,in the CB ofthe ZnO, photogenerated which was electrons not used in for the recombination CB of g-C3N4 withand a holes big part of g-Cof the3N 4electrons, reduced in Nthe2O CB to ofN ZnO,and whichO . The was photoexcited not used for recombination holes in the VB with of ZnO holes oxidized of g-C3N4,O reducedto O N[192O]. to This N2 and result O shows−. The 2 • − • − 2 photoexcitedthat only a small holes loading in the ofVB g-C of3 NZnO4 in g-Coxidized3N4/ZnO O− canto O increase2 [19]. This the eresultffectiveness showsof that photogenerated only a small loadingelectron–hole of g-C separation.3N4 in g-C3N4/ZnO can increase the effectiveness of photogenerated electron–hole separation.In heterogeneous photocatalysis, on the contrary to , the specific surface (SBET) is not the mostIn heterogeneous crucial parameter. photocatalysis, It is obvious on inthe this cont work,rary to where catalysis, g-C3 Nthe4/ ZnOspecific (0.05:1) surface and (S g-CBET)3 isN 4nothad the a mostsimilarly crucial high parameter. photoactivity It is though obvious the in SBET thisof work, g-C3N 4wherewas more g-C3 thanN4/ZnO 10 times (0.05:1) higher and than g-C in3N the4 had case a

Catalysts 2019, 9, 735 10 of 13

of g-C3N4/ZnO (0.05:1). Many authors have quoted the band gap energy as one of the most important parameters. The photoelectrochemical measurements validated the capability of all nanocomposites to absorb UVA. Moreover, both pure photocatalysts, which had similar photoactivity, have different band gap energy. Consequently, the band gap energy is not the most decisive parameter. The most significant factor, which is valid in photocatalytic reactions generally, is the position and potential of the valence and conduction band.

3. Materials and Methods

3.1. Preparation of ZnO, g-C3N4, and g-C3N4/ZnO Nanocomposites

ZnO was prepared by precipitation [33]. G-C3N4 was formed by heating of melamine [18,34]. In a synthesis procedure of g-C3N4/ZnO, the determined amounts of ZnO and g-C3N4 were dispersed in deionized water and evenly mixed (4 h). Subsequently, the mixture was dried (at 60 ◦C in air) and calcined [18]. The weight ratios of prepared g-C3N4/ZnO photocatalysts were set at 0.05:1, 0.45:1, 2:1, and 3:1, respectively. The prepared nanocomposites (0.1 g) were anchored on a commercial support VUKOPOR (Lanik Ltd., Boskovice, Czech Republic) [35,36]. For detailed preparation information, see the Supplementary Materials.

3.2. Characterization of g-C3N4/ZnO Nanocomposite and Photocatalytic Decomposition of N2O

The products were characterized in detail by N2 physisorption (Micromeritics), X-ray powder diffraction (XRD, Rigaku, Tokyo, Japan), UV–Vis diffuse reflectance spectra (DRS, IRS-2600Plus), transmission electron microscopy (TEM, JEOL 2100 equipped with an EDS detector, Tokyo, Japan), Fourier transform infrared spectroscopy (FT-IR, Nexus 470 ThermoScientific, Waltham, MA, USA), Raman spectroscopy (DXR SmartRaman (ThermoScientific), photoluminescence (PL) measurements (FLSP920 Series, Edinburgh Instruments, Livingston, UK), and photoelectrochemical measurements (Instytut Fotonowy, Krakow, Poland), and their photocatalytic activity was studied for the photocatalytic decomposition of N2O according to the reported method [18,29]. The photocatalytic inactivity of the bare foam was confirmed via blank tests and compared to the blank test without foam.

4. Conclusions

g-C3N4/ZnO nanocomposites with different ratios of components as well as pure ZnO and g-C3N4 were prepared with success. Each photocatalyst was coated on commercial Al2O3 foam. The prepared nanocomposites were investigated for N2O photocatalytic decomposition for the first time. g-C3N4 addition positively affects the properties of the photocatalysts’ porous structure, significantly enhancing its specific surface area as well as the net pore volume. In all mixtures with higher contents of g-C3N4, two phases based on zinc were observed, -zinc oxide (markedly more significant) and zinc cyanamide. ZnO nanoparticles have hexagonal shapes and pure g-C3N4 shows a 2D lamellar structure. Based on the experimental and characterization results, N2O conversion in the presence of photocatalysts was higher in comparison with photolysis; in the case of the best photocatalysts, g-C3N4/ZnO (0.05:1) was almost twice higher. FT-IR characterization showed that samples of the g-C3N4/ZnO (3:1), g-C3N4/ZnO (2:1), and g-C3N4/ZnO (0.45:1) photocatalysts also contained cyanide compounds, which together with an imperfect contact of g-C3N4 and ZnO caused a decrease of photocatalytic activity. The key role played in photocatalytic activity was the electron–hole separation and the position and potential of the valence and conduction band. On the other hand, the specific surface area and band gap energy were not significant factors in N2O photocatalytic decomposition. The immobilization on commercial Al2O3 foam proved to be promising. A short-term test confirmed the photocatalysts’ stability and reproducible activity. Immobilization might be an efficient way to bring photocatalysis closer to an application, since the powder form is not usable for several reasons, such as filtration from the liquid phase or blowing off by the gas stream. No such drawbacks exist with the immobilized photocatalyst on the foam. Catalysts 2019, 9, 735 11 of 13

Supplementary Materials: The experiment parts are available online at http://www.mdpi.com/2073-4344/9/9/735/ s1. Author Contributions: Validation, K.K., M.R. and M.Š.; analysis, J.P., M.Š., T.B. and M.R.; preparation of photocatalyst, I.T.; writing—original draft preparation, K.K.; writing—review and editing, M.R.; visualization, M.R.; supervision, P.P. Funding: This research was funded by ERDF “Institute of Environmental Technology–Excellent Research” (No. CZ.02.1.01/0.0/0.0/16_019/0000853), the Grant Agency of the Czech Republic (No. 16-10527S) and by using Large Research Infrastructure ENREGAT supported by the Ministry of Education, Youth and Sports of the Czech Republic under project No. LM2018098. Acknowledgments: The work was supported from ERDF “Institute of Environmental Technology—Excellent Research” (No. CZ.02.1.01/0.0/0.0/16_019/0000853), the Grant Agency of the Czech Republic (No. 16-10527S) and by using Large Research Infrastructure ENREGAT supported by the Ministry of Education, Youth and Sports of the Czech Republic under project No. LM2018098“. Authors thank Alexandr Martaus (IET, VŠB-TU Ostrava) for the XRD measurements. Conflicts of Interest: The authors declare no conflict of interest.

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