nanomaterials

Article Mesoporous Graphitic Carbon Decorated with Cu : Efficient Photocatalysts for Degradation of Tartrazine Yellow Dye

Tao Zhang 1, Isis P. A. F. Souza 2, Jiahe Xu 1, Vitor C. Almeida 2,* ID and Tewodros Asefa 1,3,* 1 Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854, USA; [email protected] (T.Z.); [email protected] (J.X.) 2 Laboratory of Environmental and Agrochemistry, Department of Chemistry, State University of Maringá, 5790 Colombo Avenue, Maringá, Paraná 87020-900, Brazil; [email protected] 3 Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, USA * Correspondences: [email protected] (V.C.A.); [email protected] (T.A.); Tel.: +55-44-3011-4500 (V.C.A.); +1-848-445-2970 (T.A.)  Received: 17 July 2018; Accepted: 16 August 2018; Published: 21 August 2018 

Abstract: A series of mesoporous graphitic carbon (mpg-C3N4) materials are synthesized by directly pyrolyzing containing many embedded silica nanoparticles templates, and then etching the silica templates from the carbonized products. The mass ratio of melamine-to-silica templates and the size of the silica nanoparticles are found to dictate whether or not mpg-C3N4 with large surface area and high porosity form. The surfaces of the mpg-C3N4 materials are then decorated with copper (Cu) nanoparticles, resulting in Cu-decorated mpg-C3N4 composite materials that show excellent photocatalytic activity for degradation of tartrazine yellow dye. The materials’ excellent photocatalytic performance is attributed to their high surface area and the synergistic effects created in them by mpg-C3N4 and Cu nanoparticles, including the Cu nanoparticles’ greater ability to separate photogenerated charge carriers from mpg-C3N4.

Keywords: mesoporous graphitic carbon nitride; copper nanoparticles; photodegradation; tartrazine yellow;

1. Introduction The fast-growing global economy has resulted in a vast number of industries, many of which directly or indirectly generate various environmental pollutants, which can cause serious threats to society’s well-being and the development of a sustainable future [1]. Therefore, there is no question that these pollutants must be tackled to overcome their many undesirable consequences. Among many remediation strategies to address environmental pollutants, semiconductor-based solar photocatalysis, which utilizes the abundant solar energy irradiated by the sun to decompose environmentally polluting organic and inorganic species via various light-induced redox reactions over semiconductor materials, is quite promising [2]. While many semiconducting materials, such as TiO2, ZnO, and SnO2, have been extensively studied for this purpose, their large band gap energy hinders the absorption of visible light and results in inefficient utilization of solar energy to photocatalyze reactions over these materials. Graphitic carbon nitride (g-C3N4), a low-cost metal-free polymeric semiconductor with great thermal and chemical stability and good electronic structure and photoactivity in the visible region of electromagnetic radiation, has recently emerged as a promising visible-light photocatalyst for degradation of various pollutant using sunlight, and thus stimulated intensive research works [2,3].

Nanomaterials 2018, 8, 636; doi:10.3390/nano8090636 www.mdpi.com/journal/nanomaterials Nanomaterials 2018, 8, 636 2 of 14

g-C3N4 materials are often synthesized by treating certain nitrogen-rich organic precursors at high-temperature. However, g-C3N4 materials synthesized via high-temperature thermal treatment of precursors such as , dicyandiamide, melamine, thiourea and urea generally possess non-porous structure and relatively low surface area because these precursors tend to undergo a high degree of polycondensation at high temperature [4,5]. Thus, finding synthetic methods and/or precursors that can lead to advanced g-C3N4 materials with high surface area and large porosity is of paramount importance, since these structural features provide the materials with more accessible catalytically active sites and better pathways for mass transport of reactants and products and thus improved photocatalytic performances for decontamination of pollutants, water splitting, reduction, etc. [2,6,7]. Meanwhile, although g-C3N4 has a good band gap energy (2.7 eV) for visible-light photocatalysis, with conduction band (CB) and valence band (VB) potentials located at ca. –1.1 eV and ca. +1.6 eV vs normal hydrogen electrode (NHE), respectively [5], its photo-induced electron-hole pairs still suffer from rapid recombination even when high surface area and large porosity are introduced into the material. As a result, g-C3N4‘s charge carriers are often unable to participate in many photocatalytic reactions and the material shows poor photocatalytic activity [1]. In order to improve charge separation as well as light absorption over g-C3N4, many strategies have been developed. For example, some improvement in the optical properties and electronic structures of g-C3N4 have been realized by depositing metal particles on it [8], doping or co-doping heteroatoms (metals or nonmetals) on it [6,9,10], combining it with carbon materials [11] and constructing heterojunctions over its structures [12]. In the case of metal deposition, the immobilized metal particles on the surfaces of g-C3N4 increase charge separation, usually by capturing the photogenerated electrons and preventing them from combining with the holes. For this, noble metals such as Au [13], Pt [14], Pd [15] and Ag [16] have particularly been used, and their hybrid products with g-C3N4 have shown superior photocatalytic performances. Compared with noble metals, non-noble metals such as Cu are more appealing to use for these purposes because they are more Earth-abundant and can be combined with g-C3N4 to yield low-cost hybrid materials for a broad range of applications [17,18]. In terms of the potential applications of such g-C3N4-based semiconductor composite materials, pollutant degradation is of particular interest. In fact, g-C3N4-based materials have been demonstrated to degrade a variety of organic contaminants, including methyl orange, methylene blue and rhodamine B [5]. Over the past few years, melamine has been successfully used as precursor to synthesize g-C3N4 materials by thermal condensation [19,20]. Melamine has some advantages, especially compared with cyanamide precursors that are more commonly used to make g-C3N4. It is nontoxic and inexpensive. Furthermore, it is less likely to produce toxic and flammable byproducts if additional solution-phase nanocasting synthetic steps, e.g., rigid template materials (such as mesoporous or colloidal SiO2) to create nanopores in g-C3N4, are included [4]. Herein, we show that melamine and colloidal silica nanoparticles with different sizes can be used, as precursor and hard templates, respectively, to produce mesoporous g-C3N4 (mpg-C3N4) materials with three-dimensionally interconnected frameworks, large surface areas and high porosities. The mass ratio of melamine-to-silica nanoparticles is found to be crucial in the formation of high surface area and desirable pore sizes in the final g-C3N4 materials. The pore size in the materials was easily tailored (to be around 12, 22, 47 or 79 nm) simply by changing the sizes of the silica nanoparticles used as templates during the synthesis. By functionalizing the surfaces of the mpg-C3N4 materials with Cu nanoparticles, mpg-C3N4 materials decorated with Cu nanoparticles are produced. The Cu-decorated mpg-C3N4 composite material are then demonstrated, for the first time, to serve as efficient photocatalysts for degradation of tartrazine yellow, a colorful azo-dye that is widely used as a colorant in various food and pharmaceutical products in many countries, but is also implicated in causing allergies, hyperactivity and even cancer if consumed in excess, and is banned in some countries as a result [21]. Nanomaterials 2018, 8, 636 3 of 14

2. Experimental Section

2.1. Materials, Chemicals and Reagents Colloidal silica dispersions (12 nm, Ludox TM-40, 40 wt.% suspension in water and 22 nm, Ludox HS-40, 40 wt.% suspension in water), copper(II) nitrate hemi(pentahydrate) and sodium borohydride were obtained from Sigma-Aldrich (St. Louis, MO, USA). Colloidal silica dispersions (47 nm, SNOWTEX-30LH, 30 wt.% suspension in water and 79 nm, SNOWTEX-ZL, 40 wt.% suspension in water) were purchased from Nissan Chemical America Corporation (Houston, TX, USA). Melamine (99%) and ammonium hydrogen difluoride (95%) were acquired from Alfa Aesar (Haverhill, MA, USA). Tartrazine yellow was obtained from Duas Rodas Company (Campinas, São Paulo, Brazil). All chemicals and reagents were of analytical grade and used as received without further purification. Distilled water was used throughout the experiments.

2.2. Synthesis of Melamine-Derived Mesoporous g-C3N4 (mpg-C3N4) First, melamine (1 g, 7.92 mmol) and different amounts of 12 nm colloidal silica (0.25, 1, 2 or 4 g) were mixed with hot water (80 ◦C, 80 mL), and the solution was stirred for 1 h at 80 ◦C in a closed beaker. The beaker was then left open, and the solution was kept at 100 ◦C to let the solvent in it evaporate completely. The resulting composite materials comprising melamine-silica nanoparticles were pyrolyzed in a semi-closed or a partially covered combustion boat in a tube furnace under argon atmosphere at 550 ◦C for 3 h at a ramp of 20 ◦C min−1. In order to create nanoporous structures in the materials, the silica templates were removed from the resulting yellow powders by treating them in 4 M ammonium hydrogen difluoride solution for 24 h at room temperature. The solid products were recovered via filtration, washed with copious amount of water and then ethanol, and finally kept at ◦ 60 C to dry. The resulting materials were denoted as 12-mpg-C3N4−x, where x represents the initial mass ratio of melamine-to-colloidal silica templates, which is 0.25, 0.5, 1 or 4. As a control material for the studies, bulk g-C3N4 was prepared without including 12 nm silica nanoparticles templates in the precursor. In addition, by using colloidal silica templates with sizes of 22, 47 or 79 nm, but keeping the melamine-to-silica nanoparticles ratio as 1:1 and the synthetic procedures and conditions the same, three other mesoporous g-C3N4 materials, named y-mpg-C3N4-1, where y represents the size of colloidal silica templates (22, 47 or 79 nm), were synthesized.

2.3. Synthesis of Cu Nanoparticles-Decorated 22-mpg-C3N4-1 Materials

Two Cu-decorated 22-mpg-C3N4-1 materials containing different amounts of Cu nanoparticles were synthesized by mixing two different concentrations of aqueous Cu2+ solutions (5 or 10 wt.% 2+ 2+ Cu ) with 22-mpg-C3N4-1, followed by the reduction of the Cu ions with NaBH4 solution. First, 2+ 22-mpg-C3N4-1 (300 mg) was dispersed in water (40 mL) containing Cu ions (15 mg, 0.23 mmol), and then a desired amount of NaBH4 (50 mg, 1.32 mmol) was slowly added into the mixture. The mixture was stirred for 4 h at room temperature and then filtered. The solid product was washed with water and then ethanol and dried in an oven at 60 ◦C overnight. The product had 5 wt.% Cu and was named as 5Cu-22-mpg-C3N4-1. The corresponding material containing 10 wt.% Cu was obtained in a similar manner but using a higher amount of Cu2+ ions (30 mg, 0.47 mmol). The material was named 10Cu-22-mpg-C3N4-1.

2.4. Characterization The Brunauer–Emmett–Teller (BET) surface areas and pore properties of the materials were ◦ analyzed by N2 porosimetry (at −196 C) using a Tristar-3000 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA). Before each measurement, the sample was degassed at 80 ◦C for 12 h under a flow of N2 gas to remove any possible gas impurities adsorbed on the sample’s surfaces. Based on the adsorption–desorption data and isotherm, the surface areas and the pore size distributions were calculated with the Brunauer–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda Nanomaterials 2018, 8, 636 4 of 14

(BJH) method, respectively. Transmission electron microscope (TEM) images of the materials were taken with a JEOL 1200EX transmission electron microscope (JEOL Inc., Akishima, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns of the materials were obtained with a Philips X’Pert XRD diffractometer (Philips, Almelo, the Netherlands) operating with Cu Kα radiation as the X-ray source. The XRD patterns were recorded in a 2θ range between 10◦ and 80◦ with a step size (2θ) of 0.02◦ and a scan rate of 0.6◦ min−1. Thermogravimetric analyses of the materials were conducted with a thermogravimetric analysis (TGA7) instrument (PerkinElmer, Waltham, MA, USA) at a heating rate of 5 ◦C min−1 and under a flow of air passing over the samples at a rate of 20 mL min−1. The UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the materials in a spectral range of 250–800 nm were obtained with a Lambda 950 spectrophotometer (PerkinElmer, Waltham, MA, USA). Small angle X-ray scattering (SAXS) patterns of the materials were acquired using a Brüker AXS HI-STAR Area Detector Diffraction system (Brüker, Billerica, MA, USA) equipped with an Enraf-Nonius FR-571 rotating anode X-ray generator operating with a graphite monochromator (Cu Kα; λ = 1.5418 Å). The surface chemical compositions of the materials were analyzed with a Thermo Scientific K-Alpha X-ray photoemission spectrometer (XPS) (Thermo Fisher Scientific, Waltham, MA, USA) operating with Al Kα X-ray source (hv = 1486.6 eV). For high-resolution spectra of individual peaks, the scans were performed with an energy resolution of 0.1 eV. Energy dispersive X-ray fluorescence (EDXRF) was performed using DX-95 EDXRF spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with Rh tube.

2.5. Photocatalytic Degradation of Tartrazine Yellow Dye

The photocatalytic performances of the as-prepared mpg-C3N4 materials containing different loadings of Cu nanoparticles as well as the control material bulk C3N4 for degradation of tartrazine dye in solutions were evaluated by subjecting the samples to UV light emitted by a Germicide lamp, 18 W (Osram, Münich, Germany) in a jacketed beaker at room temperature, as in our previous report [21]. Blank experiments were also performed without putting the photocatalysts in the solution. Typically, prior to the test, 100 mg of the catalyst (with a concentration of 0.5 g L−1) was mixed with 200 mL tartrazine yellow dye solution (with a concentration of 10 mg L−1). The mixture was stirred for 30 min in the dark to let the solution and the catalyst reach adsorption–desorption equilibrium. The light was then turned on. During photocatalytic reaction, 4.0 mL of a suspension was sampled every 30 min using 0.45 µm syringe filter (MilliporeSigma, Burlington, MA, USA), and the concentration of tartrazine yellow in the supernatant was measured and quantified using Cary 50 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).

3. Results and Discussion

The synthetic procedures that were employed to make the set of mesoporous C3N4 and Cu-decorated mesoporous C3N4 materials (denoted as y-mpg-C3N4-x and Cu-22-mpg-C3N4 respectively) are illustrated in Scheme1 and also described in the Experimental section above. First, an optimal mass ratio of melamine-to-silica nanoparticles that can render the material with high surface area and desired pore sizes, replicating the sizes of the silica nanoparticles templates, was determined. Briefly, melamine was mixed with different amounts of 12 nm colloidal silica templates and the solvent was evaporated resulting in melamine/silica nanoparticles composite materials. After subjecting the composite materials to thermal condensation and removing the silica templates from the carbonized composite products, a set of 12-mpg-C3N4−x materials was obtained, in which x represents the mass ratio of melamine-to-silica nanoparticles used (0.25, 0.5, 1, or 4). Another set of materials, y-mpg-C3N4-1, where y represents the size of colloidal silica templates, was synthesized using 22, 47 or 79 nm silica nanoparticles as templates with an optimized x value (x = 1). Two different amounts of Cu2+ (5 or 10 wt.%) were then deposited onto the surfaces of one of the mpg-C3N4 materials, 22-mpg-C3N4-1. 2+ After reducing the Cu ions with NaBH4, photocatalytically active Cu-modified mpg-C3N4 materials, denoted 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg-C3N4-1, respectively, were obtained. Nanomaterials 2018, 8, 636 5 of 14 Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 14

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SchemeScheme 1. 1. SchematicSchematic illustration illustration of the procedure of the procedure used to synthesize used to y- synthesizempg-C3N4-x yand-mpg-C Cu-223N-mpg4-x - and

Cu-22-mpg-CC3N4-1 materials,3N4 where-1 materials, y and x whererepresenty andthe sizex represent of the silica the nanoparticles size of the silicaused as nanoparticles templates and used asthe templatesScheme mass ratio 1. Schematicand of melamine the mass illustration-to ratio-silica of ofnanoparticles melamine-to-silicathe procedure used used for tothe nanoparticles synthesize synthesis ofy- mpgusedthe materials-C for3N4 the-x and, synthesisrespectively. Cu-22-mpg of the- materials,C3N4-1 materials, respectively. where y and x represent the size of the silica nanoparticles used as templates and Thethe structures mass ratio of melamine12-mpg-C-to3N-silica4−x were nanoparticles first investigated used for the by synthesis N2 porosimetry. of the materials The results, respectively. showed that the surface area and porosity of the materials depended on the initial mass ratio of melamine-to- The structures of 12-mpg-C3N4−x were first investigated by N2 porosimetry. The results The structures of 12-mpg-C3N4−x were first investigated by N2 porosimetry. The results showed showedcolloidal that silica the (12 surface nm) templates area and used porosity to synthesize of the the materials materials depended (Figure 1). on Bulk the g- initialC3N4, which mass was ratio of synthesizedthat the surface without area using and porositycolloidal of silica the materials templates, depended exhibited on low the surfaceinitial mass area ratio (3 m of2/g) melamine and non--to- melamine-to-colloidal silica (12 nm) templates used to synthesize the materials (Figure1). Bulk g-C 3N4, colloidal silica (12 nm) templates3 used to synthesize the materials (Figure 1). Bulk g-C3N4, which was porous structure (0.04 cm /g). When colloidal silica nanoparticles templates were used, the surface2 which was synthesized without using colloidal silica templates, exhibited low surface area2 (3 m /g) and synthesized without using colloidal silica 2templates, exhibited low surface area (3 m /g) and non- area of the materials first increased,3 from 7 m /g for 12-mpg-C3N4-0.25 (which was synthesized using non-porous structure (0.04 cm3 /g). When colloidal silica nanoparticles templates were used, the surface porous structure (0.04 cm /g). When colloidal silica nanoparticles2 templates were used, the surface melamine-to-silica nanoparticles mass ratio 2of 0.25) to 136 m /g for 12-mpg-C3N4-1 (which was 2 areaarea of theof the materials materials first first increased, increased, from from 7 7 m m/g/g forfor 12-mpg-C12-mpg-C3NN44-0.25-0.25 (which (which was was synthesized synthesized2 using using synthesized using melamine-to-silica nanoparticles mass ratio of 1),2 but then decreased to 9 m /g for melamine-to-silica nanoparticles mass ratio of 0.25) to 136 m 2/g for 12-mpg-C33N4 4-1 (which was 12melamine-mpg-C3N-4to-4- silica(which nanoparticles was synthesized mass using ratio the of highest 0.25) to ratio 136 of m melamine/g for 12-to-mpg-silica-C nanoparticlesN -1 (which ofwas 2 synthesized4).synthesized The pore using volume using melamine-to-silica melamineof the mater-to-ialssilica exhibited nanoparticles nanoparticles a similar mass mass trend, ratioratio ofnamely,of 1),1), but it then first decreased decreasedincreased tofrom to 9 9 m m 20.06/g/g for for 12-mpg-C N3 -44 (which was synthesized using the highest ratio of melamine-to-silica nanoparticles cm123/g-mpg for 312-C-4mpgN -4 -(whichC3N4-0.25 was to synthesized 0.27 cm3/g for using 12- mpgthe highest-C3N4-1, ratio but ofthen melamine decreased-to -tosilica 0.05 nanoparticles cm3/g for 12- of ofmpg 4). -TheC The3N 4pore- pore4). It volume is volume also worth of ofthe the noting mater materials ialsthat exhibitedonly exhibited 12-mpg a similar-C a3 similarN4- 1trend, (the trend, pore namely, s namely,izes it of first which it increased first are increased centered from at0.06 from 3 3 3 3 3 3 0.06cacm. 11.7 cm/g nm /gfor) for 12showed- 12-mpg-Cmpg- Cpore3N4s3-0.25 Nclose4-0.25 to to 0.27 the to 0.27 cmsize/g cmof for colloidal/g 12 for-mpg12-mpg-C silica-C3N templates4-1, 3butN4 -1,then used but decreased thento synthesize decreased to 0.05 the cm to material. 0.05/g for cm 12 -/g forLowmpg 12-mpg-C or- Chigh3N4- melamine4).3N It4-4). is also It-to isworth-silica also noting worthmass ratio that noting only(i.e., that x12 =- mpg0.25 only -orC 12-mpg-C 34)N 4did-1 ( thenot 3pore resultN4-1 sizes (thein materials of pore which sizes theare ofporecentered which size at are centeredof cawhich. 11.7 atdirectly nm ca.) showed 11.7 correspond nm) pore showeds close to each pores to thesilica closesize of to colloidal the size; silicain of both colloidal templates cases, silica the used materials templates to synthesize were used largely the to material. synthesize non- theporousLow material. orand high had Low melamine a small or high amount-to melamine-to-silica-silica of mass large ratio pores. (i.e., mass x = ratio0.25 or (i.e., 4) didx = 0.25not result or 4) in did materials not result the in pore materials size theof pore which size directly of which correspond directly correspondto each silica to nanoparticle each silica; nanoparticle; in both cases, inthe both materials cases, were the materials largely non were- largelyporous non-porous and had a andsmall had amount a small of amountlarge pores. of large pores.

Figure 1. (a) N2 adsorption–desorption isotherms of 12-mpg-C3N4−x materials synthesized using

different mass ratios of melamine-to-silica nanoparticles and (b) their corresponding pore size Figuredistributions.Figure 1. 1. ((a a)N) N2 adsorptionadsorption–desorption–desorption isotherms isotherms of 12 of-mpg 12-mpg-C-C3N4−x 3materialsN4−x materials synthesized synthesized using usingdifferent different mass mass ratios ratios of melamine of melamine-to-silica-to-silica nanoparticles nanoparticles and (b and) their (b) corres theirponding corresponding pore size pore sizedistributions. distributions.

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BasedBased on on N2 N porosimetry2 porosimetry results results presented presented above, above, the processes the processes leading leading to the toformation the formation of mpg- of Cmpg-C3N4 and3N Cu4 and-loaded Cu-loaded mpg-C mpg-C3N4 were3N proposed4 were proposed as illustrated as illustrated in Scheme in Scheme 2. For 2the. For lowest the lowest mass ratio mass ofratio melamine of melamine-to-silica-to-silica nanoparticles nanoparticles (x = 0.25 (x, where= 0.25, the where mass the of masssilica ofnanoparticles silica nanoparticles is much ishigher much thanhigher that than of melamine that of melamine),), the carbonized the carbonized material d materialoes not maintain does not its maintain porous itsstructure porous after structure removal after ofremoval silica due of to silica the dueinsufficie to thent insufficient amount of amount melamine of melamineand the small and amount the small of amount carbon product of carbon forming product fromforming it, which from can it, which easily can collapse easily. collapse.The small The proportion small proportion of large pores of large in poresit seem in itto seemreplicate to replicate some aggregatedsome aggregated silica nanoparticles silica nanoparticles templates templates.. For the For thehighest highest mass mass ratio ratio of of melamine melamine-to-silica-to-silica nanoparticlesnanoparticles (x ( x= =4), 4), the the lack lack of of many many silica silica nanoparticles nanoparticles templates templates can can inhibit inhibit the the formation formation of of carboncarbon structures structures around around the the particles particles and and thereby thereby the the formation formation of of porous porous carbon carbon structure structure in in the the materialmaterial.. In Inaddition, addition, the the formation formation of compact of compact g-C g-C3N4 3shellsN4 shells around around the silica the silicaspheres spheres templates templates,, due todue the torelatively the relatively higher amount higher amount of melamine of melamine used for the used synthesis, for the synthesis,can prevent can ammonium prevent ammonium hydrogen difluoridehydrogen from difluoride reaching from the reaching silica templates the silica and templates etching andthem etching all. As them a result, all. Asthe asilica result, templates the silica cantemplatesnot all be cannot removed all be from removed the carbonized from the carbonized product and product porous and structure porouss structures cannot be cannot formed be in formed this material.in this material. This was This actually was actuallycorroborated corroborated by thermogravimetric by thermogravimetric analysis analysis(TGA) (Figure (TGA) S1), (Figure which S1), clearlywhich revealed clearly revealedthat 12-mpg that-C 12-mpg-C3N4-4 had3 Nhigher4-4 had amount higher of amount residue of associated residue associated with silica with (ca. 6.79 silica wt.(ca.%) 6.79 compared wt.%) compared with 12-mpg with-C 12-mpg-C3N4-1 (ca. 31.12N4-1 wt. (ca.%). 1.12 However, wt.%). However,the low surface the low area surface in the area former in the wasformer likely was to likelybe caused to be also caused by the also lack by theof porous lack of structure porous structure in the material in the material,, since most since of most the silica of the wassilica removed was removed,, despite despitea large amount a large amountof it being of itleft being behind left, and behind, yet the and material yet the materialshowed low showed surface low areasurface. Based area. on Based the above on the results, above results,the optimal the optimal mass ratio mass of ratio melamine of melamine-to-silica-to-silica nanoparticles nanoparticles for the for synthesisthe synthesis of high of high surface surface area area mpg mpg-C-C3N4 3materialsN4 materials with with desired desired pore pore sizes sizes,, i.e., i.e., pores pores corresponding corresponding toto the the size size of of the the silica silica nanoparticles, nanoparticles, was was determined determined to to be be around around 1. 1.

Scheme 2. Proposed processes leading to different 12-mpg-C3N4-x materials from melamine/silica Scheme 2. Proposed processes leading to different 12-mpg-C3N4-x materials from melamine/silica nanoparticles composite precursors containing different mass ratios of melamine-to-silica nanoparticles composite precursors containing different mass ratios of melamine-to-silica nanoparticles. nanoparticles.

The synthetic procedure was easily extended to produce a series of y-mpg-C N -1 materials with The synthetic procedure was easily extended to produce a series of y-mpg-C3N3 4-41 materials with differentdifferent pore pore sizes sizes (with (with yy = =12, 12, 22, 22, 47 47 or or 79 79 nm, nm, representing representing the the different different sizes sizes of of silica silica nanoparticles nanoparticles usedused as as templates templates to to create create the the pores) pores) (Figure (Figure 2).2). When When the the size sizess of of the the silica silica nanoparticles nanoparticles templates templates used to synthesize the materials were increased, the surface areas of the y-mpg-C N -1 materials used to synthesize the materials were increased, the surface areas of the y-mpg-C33N4-41 materials 2 2 slightlyslightly decreased decreased (from (from 136 136 m2 m/g to/g 73 to m 732/g m), but/g), their but corres theirponding corresponding pore volume pore volumess increased increased (from (from 0.27 cm3/g to 1.05 cm3/g). Quantitative results obtained by N porosimetry analyses of the 0.27 cm3/g to 1.05 cm3/g). Quantitative results obtained by N2 porosimetry2 analyses of the materials arematerials compiled are in compiled Tables 1 inand Tables 2. 1 and2.

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Figure 2. (a) N2 adsorption–desorption isotherms of different y-mpg-C3N4-1 materials synthesized Figure 2. (a)N2 adsorption–desorption isotherms of different y-mpg-C3N4-1 materials synthesized fromfrom melamine/silicamelamine/silica nanoparticles nanoparticles composite composite precursors precursors containing containing different different mass ratios mass of melamine-ratios of to-silicamelamine nanoparticles-to-silica nanoparticles and (b) their and corresponding (b) their corresponding pore size distributions. pore size distributions.

TableTable 1.1.Brunauer–Emmett–Teller Brunauer–Emmett–Teller (BET) (BET surface) surface area, area, average average Barrett–Joyner–Halenda Barrett–Joyner–Halenda (BJH) (BJH pore) pore size, size, and pore volume of different 12-mpg-C3N4-x materials. and pore volume of different 12-mpg-C3N4-x materials.

Size of SiO2 Mass Ratio of Melamine-to- BET Surface Average Pore Pore Volume Size of SiO2 Mass Ratio of Melamine-to-SiO2 BET Surface Average Pore Pore Volume Nanoparticles (y = 12) SiO2 Nanoparticles (x) Area (m2/g) Size (nm) a (cm3/g) Nanoparticles (y = 12) Nanoparticles (x) Area (m2/g) Size (nm) a (cm3/g) 12 - b 3 - c 0.04 12 - b 3 - c 0.04 12 0.25 7 - c 0.06 12 0.25 7 - c 0.06 12 120.5 0.5 110 110 6.26.2 0.220.22 12 121 1136 136 11.711.7 0.270.27 12 124 49 9 - c- c 0.050.05 aa Pore sizesize is is measured measured based based on the on adsorption the adsorption data. b Nodata. colloidal b No melamine silica nanoparticles was used were in used this incase. this case.The c Melamine was carbonized and the carbon materials obtained from it was characterized for comparison. No definitec valuecolloidal was obtainedsilica nanoparticles here, perhaps duewere to dried the largely and non-poroustheir pores structure were characterized in the material. for comparison. No definite value was obtained here, perhaps due to the largely non-porous structure in the material.

Table 2. BET surface area, average BJH pore size, and pore volume of different y-mpg-C3N4-1 materials. Table 2. BET surface area, average BJH pore size, and pore volume of different y-mpg-C3N4-1 materials. Size of SiO2 Mass ratio of Melamine-to-SiO2 BET Surface Average Pore Pore Volume Nanoparticles (y) Nanoparticles (x = 1) Area (m2/g) Size (nm) a (cm3/g) Mass ratio of Melamine-to- Size of SiO2 BET Surface Average Pore Pore Volume 12SiO2 Nanoparticles 1 136 11.7 0.27 Nanoparticles22 (y) 1Area 118(m2/g) Size 22.5 (nm) a 0.56(cm3/g) (x = 1) 47 1 92 43.6 0.88 1279 1 1 136 73 ca.11.7 80 1.050.27 22 1 118 22.5 0.56 a Pore size is measured based on the adsorption data. 47 1 92 43.6 0.88 79 1 73 ca. 80 1.05 The structures of y-mpg-C N -1 materials were further examined by transmission electron a Pore size3 4 is measured based on the adsorption data. microscope (TEM) (Figure3a–d). When x = 1, the material showed pores with sizes of ca. 12 nm

(FigureThe3a), structures corroborating of y the-mpg pore-C3N size4-1 obtainedmaterials by were N 2 porosimetry further examined for this materialby transmission (12-mpg-C electron3N4-1) earliermicroscope (Table (TEM)1). The (Figure TEM images3a–d). When of the x y=-mpg-g-C 1, the material3N4-1 materials,showed pores which with were sizes synthesized of ca. 12 nm by keeping(Figure 3a), the corroborating same x value the but pore bychanging size obtained the sizesby N2 of porosimetry the silica nanoparticles for this material templates, (12-mpg clearly-C3N4- showed1) earlier that (Table all the 1). materials The TEM possessed images of nanoporous the y-mpg- structures.g-C3N4-1 materials, However, which their pore were sizes synthesized were highly by dependentkeeping the on same the sizes x value of the but silica by templateschanging usedthe sizes to synthesize of the silica them, nanoparticles and generally templates, increased clearly as the sizesshowed of colloidal that all silicathe materials templates possessed were increased nanoporous from 12 structures. nm to 79 nm. However, In addition, their TEM pore images sizes were alsohighly acquired dependent for 5Cu-22-mpg-Con the sizes of the3N4 silica-1 and templates 10Cu-22-mpg-C used to synthesize3N4-1 (Figure them,3e,f). and The generally images increased of these materialsas the sizes showed of colloidal many silica dark templates spots or areaswere associatedincreased from with 12 Cu nm species. to 79 nm. Overall, In addition, the N2 TEMporosimetry images andwere TEM also resultsacquired above for clearly5Cu-22- indicatedmpg-C3N that4-1 and mesoporous 10Cu-22-mpg g-C3-NC34Nmaterials4-1 (Figure with 3e, highf). The surface images area of andthese mesopore materials sizes showed could many be produced dark spots by a facileor areas synthetic associated method with using Cu melamine species. asOverall, precursor the and N2 aporosimetry proper amount and of TEM silica results nanoparticles above clear as templates.ly indicated Furthermore, that mesoporous the pore g sizes-C3N in4 materials the materials with could high besurface tuned area by varyingand meso thepore sizes sizes of the could silica be nanoparticles produced by useda facile as templatessynthetic method to synthesize using themelamine materials. as precursor and a proper amount of silica nanoparticles as templates. Furthermore, the pore sizes in

Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 14

Nanomaterialsthe materials2018 could, 8, 636 be tuned by varying the sizes of the silica nanoparticles used as templates8 of to 14 synthesize the materials.

Figure 3. 3. TransmissionTransmission electron electron microscope microscope (TEM (TEM)) images images of the of series the series of y-mpg of y--mpg-CC3N4-1 and3N4 -1Cu and-22- Cu-22-mpg-Cmpg-C3N4-1 materials3N4-1 materials synthesized synthesized using using different different sizes sizes of silica of silica nanoparticles nanoparticles as as templates templates and modifiedmodified with different loadings loadings of of Cu: Cu: (a (a) )12 12-mpg-C-mpg-C3N3N4-41-1;; (b ()b 22) 22-mpg-C-mpg-C3N34N-14; -1;(c) (47c)- 47-mpg-Cmpg-C3N43-N1; 4(-1;d) (79d)-mpg 79-mpg-C-C3N43-1N; 4(e-1;) 5Cu (e) 5Cu-22-mpg-C-22-mpg-C3N4-31N; and4-1; ( andf) 10Cu (f) 10Cu-22-mpg-C-22-mpg-C3N4-1.3N 4-1.

Since 22-mpg-C3N4-1 and Cu-22-mpg-C3N4-1 were used as representative materials to evaluate Since 22-mpg-C3N4-1 and Cu-22-mpg-C3N4-1 were used as representative materials to evaluate the photocatalytic activity of Cu-doped mpg-C3N4 materials for degradation of tartrazine yellow dye, the photocatalytic activity of Cu-doped mpg-C3N4 materials for degradation of tartrazine yellow dye, the followingfollowing characterizations were carried out only forfor thesethese twotwo materials.materials. UponUpon increasing the relative amount of Cu2+2+ ions used to synthesize the different Cu-22-mpg-C3N4-1 materials, the colors relative amount of Cu ions used to synthesize the different Cu-22-mpg-C3N4-1 materials, the colors of the samples slowly changed from yellow to light greengreen (Figure(Figure4 4a).a). UV-VisUV-Vis diffusediffuse reflectancereflectance spectroscopy (DRS) was used to determine the optical properties of these materials between 250 nm to 800 nm (Figure 4b). The intrinsic absorption edge of 22-mpg-C3N4-1 was found to be ca. 459 nm, to 800 nm (Figure4b). The intrinsic absorption edge of 22-mpg-C 3N4-1 was found to be ca. 459 nm, which is in line with the value reported for g-C3N4 materials in the literature [22]. After decorating its which is in line with the value reported for g-C3N4 materials in the literature [22]. After decorating its surfaces with 5 wt.% and 10 wt.% of Cu (producing 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg-C3N4-1 surfaces with 5 wt.% and 10 wt.% of Cu (producing 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg-C3N4-1 materials), thethe absorption absorption edge edge shifted shifted to higher to higher wavelengths, wavelengths, to ca. 498 to nmca. and498 ca.nm 514 and nm, ca respectively,. 514 nm, respectively, and the intensity of the absorption band increased; this suggests that the modification and the intensity of the absorption band increased; this suggests that the modification of mpg-C3N4 withof mpg Cu,-C at3N least,4 with up Cu, to 10at least, wt.% enhancesup to 10 wt.% the material’s enhances ability the material to absorb’s ability visible to light absorb [23]. visible light [23]. The band gap energy (Eg) of the materials were calculated based on the spectra obtainedThe band with gap UV-Vis energy DRS (Eg) and of the using materials the transformed were calculated Kubelka–Munk based on the equationspectra obtained (Figure 4withb): [ ( ) ]2 [UVF(R-Vis)·hv DRS]2= αand= usingA(hv −theE transformed), where F( KubelkaR) = (1–−MunkR)2/ 2equationR, where (Figurehv is the 4b): incident F R ·hv photon = α = energy, A(hv– 2 g Eg) , where F(R)= (1–R) /2R , where hv is the incident photon energy, R is the percentage of R is the percentage of reflected light, Eg is the band gap energy and A is constant, the value of which reflected light, E is the band gap energy and A is constant, the value of which depends on the depends on the transitiong probability [24,25]. The value of Eg for 22-mpg-C3N4-1 was determined to be transition probability [24,25]. The value of for 22-mpg-C3N4-1 was determined to be 2.71 eV, 2.71 eV, whereas the values of Eg for 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg-C3N4-1 were determined towhereas be 2.49 the and values 2.41 eV, of respectively. for 5Cu-22 This-mpg result-C3N indicates4-1 and 10 thatCu the-22- valuempg-C of3NE4-1decreases were determined as the amount to be 𝐸𝐸𝑔𝑔 g of2.49 Cu and in mpg-C2.41 eV, Nrespectively.-1 is increased. This result These indicates results are that consistent the value with of those decreases reported as the for amount g-C N inof 3 4 𝐸𝐸𝑔𝑔 3 4 theCu literature,in mpg-C3N where4-1 is aincreased. steady decrease These results in band are gap consistent energy is with observed those𝑔𝑔 asreported more metal for g is-C loaded3N4 in the on 𝐸𝐸 g-Cliterature,3N4 [26 where–28]. Overall,a steady decrease the DRS in results band suggestedgap energy that is observed Cu/mpg-C as more3N4 composite metal is loa materialsded on g- haveC3N4 significantly[26–28]. Overall, improved the DRS light results absorption; suggested they that can Cu/mpg thus be-C potentially3N4 composite better materials photocatalysts have significantly compared with pure g-C3N4.

Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 14 Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 14 improved light absorption; they can thus be potentially better photocatalysts compared with pure g- improvNanomaterialsed light2018, absorption8, 636 ; they can thus be potentially better photocatalysts compared with pure9 of g 14- C3N4. C3N4.

Figure 4.4. ((aa)) PhysicalPhysical appearanceappearance of of 22-mpg-C 22-mpg-CN3N4-1,-1, 5Cu-22-mpg-C5Cu-22-mpg-C3NN4-1,-1, andand 10Cu-22-mpg-C10Cu-22-mpg-C3NN4-1-1 Figure 4. (a) Physical appearance of 22-mpg-C33N44-1, 5Cu-22-mpg-C33N44-1, and 10Cu-22-mpg-C33N4-1 materials (from left to right). (b) UV-Vis diffuse reflectance spectra (DRS) of 22-mpg-C3N4-1, 5Cu-22- materials (from left to right); (b) UV-Vis diffuse reflectance spectra (DRS) of 22-mpg-C3N4-1, materials (from left to right). (b) UV-Vis diffuse reflectance spectra (DRS) of 22-mpg-C3N4-1, 5Cu-22- mpg-C3N4-1, and 10Cu-22-mpg-C3N4-1 and (c) the corresponding plots of transformed Kubelka– 5Cu-22-mpg-C3N4-1, and 10Cu-22-mpg-C3N4-1 and (c) the corresponding plots of transformed mpg-C3N4-1, and 10Cu-22-mpg-C3N4-1 and (c) the corresponding plots of transformed Kubelka– Kubelka–MunkMunk function versus function the versus energy the of energy incident of light. incident light. Munk function versus the energy of incident light.

The amount of Cu in Cu-22-mpg-C3N4-1 was estimated by comparing the weight of the residue The amount of Cu in Cu-22-mpg-CCu-22-mpg-C3N4--11 was was estimated estimated by by comparing comparing the the weight weight of the residue that remained in Cu-mpg-C3N4 at the end of thermogravimetric analysis (TGA) in air with respect to that remained in in Cu Cu-mpg-C-mpg-C33NN4 4atat the the end end of of thermogravimetric thermogravimetric analysis analysis (TGA) (TGA) in inair air wi withth respect respect to that of 22-mpg-C3N4-1 (Figure 5). While 22-mpg-C3N4-1 gave ca. 1.12 wt.% residue, which is thtoat that of of22 22-mpg-C-mpg-C3N34N-14 -1(Figure (Figure 5).5). While While 22 22-mpg-C-mpg-C3N34N-14 -1gave gave ca ca.. 1.12 1.12 wt. wt.%% residue residue,, which which is associated with residual silica, since the C3N4 in it should be lost in the form of CO2 and NO2 during associated with residual silica,silica, sincesince thethe CC33N4 in it should be lost in thethe formform ofof COCO22 and NO2 during calcination, 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg-C3N4-1 gave ca. 6.07 wt.% and 12.24 wt.% residue, calcination, 5Cu-22-mpg-C5Cu-22-mpg-C33N4--11 and and 10Cu 10Cu-22-mpg-C-22-mpg-C3NN44--11 gave gave ca ca.. 6.07 6.07 wt. wt.%% and and 12.24 12.24 wt. wt.%% residue, residue, which should be largely due to CuO. From these values, the amounts of Cu in 5Cu-22-mpg-C3N4-1 which shouldshould bebe largelylargely due due to to CuO. CuO. From From these these values, value thes, the amounts amounts of Cu of inCu 5Cu-22-mpg-C in 5Cu-22-mpg3N-4C-13N and4-1 and 10Cu-22-mpg-C3N4-1 were determined to be ca. 3.73 wt.% and 8.68 wt.%, respectively. In 10Cu-22-mpg-Cand 10Cu-22-mpg3N4-C-13N were4-1 determinedwere determined to be ca. to 3.73 be wt.%ca. 3.73 and wt.% 8.68 wt.%, and respectively.8.68 wt.%, respectively. In addition, itIn is addition, it is worth noting that the presence of Cu NPs in mpg-C3N4 facilitated the materials’ thermal worthaddition, noting it is thatworth the noting presence that of the Cu presence NPs in mpg-C of Cu3 NPsN4 facilitated in mpg-C the3N4materials’ facilitated thermal the materials’ decomposition. thermal decomposition. While 22-mpg-C3N4-1 was stable up◦ to ca. 480 °C in air and decomposed only in the Whiledecomposition. 22-mpg-C While3N4-1 22 was-mpg stable-C3N4 up-1 was to ca.stable 480 upC to in ca. air 480 and °C decomposedin air and decomposed only in the only range in the of range of ◦480–625 °C, the Cu-functionalized counterparts, 5Cu-22-mpg-C3N4-1 and 10Cu-22-mpg- range480–625 of 480C,– the625 Cu-functionalized°C, the Cu-functionalized counterparts, counterparts, 5Cu-22-mpg-C 5Cu-223-Nmpg4-1- andC3N4 10Cu-22-mpg-C-1 and 10Cu-22-3mpgN4-1,- C3N4-1, decomposed in lower temperature ranges of◦ 440–575 °C and◦ 390–560 °C, respectively. Cdecomposed3N4-1, decomposed in lower in temperature lower temperature ranges of 440–575ranges ofC 440 and–575 390–560 °C andC, respectively.390–560 °C, Furthermore,respectively. Furthermore, the decreased thermal stability of Cu-22-mpg-C3N4-1 materials compared with mpg- Furthermorethe decreased, thermalthe decreased stability thermal of Cu-22-mpg-C stability of3N Cu4-1-22 materials-mpg-C compared3N4-1 materials with mpg-Ccompared3N4-1 with indirectly mpg- C3N4-1 indirectly indicated the successful decoration of the surfaces of mpg-C3N4 with Cu Cindicated3N4-1 indirectly the successful indicated decoration the successful of the surfaces decoration of mpg-C of theN withsurfaces Cu nanoparticles.of mpg-C3N4 Thiswith is notCu nanoparticles. This is not unprecedented as the presence of Cu3 (metallic)4 particles has been reported nanoparticlesunprecedented. T ashis the is not presence unprecedented of Cu (metallic) as the presence particles of has Cu been (metallic) reported particles to facilitate has been heat reported transfer to facilitate heat transfer during calcination and promote the combustion of g-C3N4 [29]. toduring facilitate calcination heat transfer and promote during thecalcination combustion and promote of g-C3N the4 [29 combustion]. of g-C3N4 [29].

Figure 5. Thermogravimetric analysis (TGA) curves obtained in air for 22-mpg-C3N4-1, 5Cu-22-mpg- Figure 5. Thermogravimetric analysis (TGA) curves obtained in air for 22-mpg-C3N4-1, 5Cu-22-mpg- FigureC3N4-1 5.andThermogravimetric 10Cu-22-mpg-C3N analysis4-1 materials. (TGA) curves obtained in air for 22-mpg-C3N4-1, 5Cu-22-mpg- C3N4-1 and 10Cu-22-mpg-C3N4-1 materials. C3N4-1 and 10Cu-22-mpg-C3N4-1 materials. Powder X-ray diffraction (XRD) patterns of 22-mpg-C3N4-1 and Cu-22-mpg-C3N4-1 materials Powder X-ray diffraction (XRD) patterns of 22-mpg-C3N4-1 and Cu-22-mpg-C3N4-1 materials werePowder obtained X-ray to further diffraction characterize (XRD) patternsthe materials of 22-mpg-C (Figure3 6).N4 -1The and XRD Cu-22-mpg-C patterns of3 22N4--1mpg materials-C3N4-1 were obtained to further characterize the materials (Figure 6). The XRD patterns of 22-mpg-C3N4-1 wereshowed obtained two pronounced to further characterizediffraction peaks the materials centered (Figureat ca. 27.5°6). The and XRD 13.0°, patterns corresponding of 22-mpg-C to the N(002)-1 showed two pronounced diffraction peaks centered at ca. 27.5° and 13.0°, corresponding to the3 (002)4 showedand (100) two reflections pronounced that diffractionare due to the peaks interlayer centered stacking at ca. 27.5 of ◦theand graphite 13.0◦, corresponding-like, conjugated to aromatic the (002) and (100) reflections that are due to the interlayer stacking of the graphite-like, conjugated aromatic andsystems (100) and reflections the in-plane that arestructure due to repeating the interlayer motif stacking of trigonal of the N-linkages graphite-like, of tri- conjugateds-triazene in aromatic g-C3N4, systems and the in-plane structure repeating motif of trigonal N-linkages of tri-s-triazene in g-C3N4, systemsrespectivel andy [ the5,30] in-plane. This result structure indirectly repeating confirm motifed ofthe trigonal formation N-linkages of g-C3N of4. tri-s-triazeneWhile the XRD in patterns g-C3N4, respectively [5,30]. This result indirectly confirmed the formation of g-C3N4. While the XRD patterns respectively [5,30]. This result indirectly confirmed the formation of g-C3N4. While the XRD patterns

Nanomaterials 2018, 8, 636 10 of 14 Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 14 ofof both both Cu Cu-22-mpg-C-22-mpg-C3N3N4-41-1 materials materials showed showed the the prominent prominent peak peak as associatedsociated with with C C3N3N4,4 they, they did did not not showshow any any diffraction diffraction peak peak associated associated with with Cu Cu species, species, due due possibly possibly to to either either the the small small size size or or the the high high dispersiondispersion of of the the Cu Cu nanoparticles nanoparticles in them [[28]28].. There isis somesome evidence,evidence, atat least, least, for for the the latter, latter, based based on onTEM TEM images images (Figure (Figure2e,f). 2e Energy,f). Energy dispersive dispersive X-ray X fluorescence-ray fluorescence (EDXRF) (EDXRF) was carried was carried out to furtherout to furtherdetermine determine the amount the amount of Cu of in Cu 10Cu-22-mpg-C in 10Cu-22-mpg3N4-C-13N material4-1 material (Figure (Figure S2). S The2). The result result showed showed that thatnot not only only Cu Cu was was present present in the in materialthe material but but also also its amount its amount was was ca. 14.1 ca. 14.1 wt.%, wt which.%, which is close is close to what to whatwas usedwas used for its for synthesis. its synthesis Small. angelSmall X-rayangel scatteringX-ray scattering (SAXS) (SAXS) pattern ofpattern 22-mpg-C of 223-Nmpg4-1- materialC3N4-1 material(see Figure (see S3)Figure showed S3) showed a Bragg a reflection Bragg reflection in 2θ-region in 2 - ofregion 0.15◦ –0.75of 0.15°◦, indicating–0.75°, indicating the presence the presence of some ofordered some ordered pores with pores d-spacing with d-spacing of ca. 26.1 of nm,ca. 26.1 which nm, is which also in is line also with in line the resultswith the obtained results fromobtained TEM 𝜃𝜃 fromimages TEM and images by N 2andporosimetry. by N2 porosimetry.

FigureFigure 6. 6. XX-ray-ray diffraction diffraction (XRD (XRD)) patterns patterns of of 22 22-mpg-C-mpg-C3N3N4-41,-1, 5Cu 5Cu-22-mpg-C-22-mpg-C3N3N4-14-1 and and 10Cu 10Cu-22-mpg--22-mpg- CC3N3N4-14-1 materials. materials.

XX-ray-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) (XPS) analysis was performedperformed toto determinedetermine the the composition composition of of 10 Cu-22-mpg-C3N4-1 material as well as the chemical states of the elements existing in it. The 10Cu-22-mpg-C3N4-1 material as well as the chemical states of the elements existing in it. The survey surveyspectrum spectrum showed showed that only that C, only O, N C, and O, N Cu and were Cu present were present in the materialin the mate (Figurerial (Figure7a). The 7a). C 1s The peak C 1s in peakthe spectrumin the spectrum showed showed two peaks two centered peaks centered at ca. 284.3 at eVca. and284.3 288.0 eV eV,and corresponding 288.0 eV, corresponding to sp2−bonded to spcarbon2−bond atomsed carbon of C–C atoms and of N–C=N C–C and groups, N–C=N respectively groups, respectively (Figure7b) [ (Figure5]. The N7b) 1s [5 peak]. The in N the 1s spectrumpeak in thewas spec deconvolutedtrum was deconvoluted into four peaks into withfour peaks binding with energy binding values energy of 398.5,values 399.9, of 398.5, 401.1 399.9, and 401.1 404.1 eV eV, and 404.1 eV, which can be ascribed2 to sp2−bonded N atoms of C–N=C species, N atoms in tertiary which can be ascribed to sp −bonded N atoms of C–N=C species, N atoms in tertiary N–(C)3 groups, NN–( atomsC)3 groups, in N–H N moietiesatoms in and N-πHexcitation, moieties and respectively π excitation, (Figure respectively7c) [ 5,31]. The(Figure oxidation 7c) [5 state,31]. ofThe Cu oxidation state of Cu was determined on the basis of the position of the Cu 2p3/2 peak (Figure 7d). was determined on the basis of the position of the Cu 2p3/2 peak (Figure7d). Two peaks at binding Two peaks at binding energies of ca. 932.4 and 952.2 eV were observed, corresponding to Cu 2p3/2 0 energies of ca. 932.4 and 952.2 eV were observed, corresponding to Cu 2p3/2 and Cu 2p1/2 of Cu 0 1 andand/or Cu 2p Cu1/21+ of. TheCu additionaland/or Cu small⁺. The peaks additional observed small at peaks ca. 933.9 observed and 954.0 at ca. eV 933 could.9 and be assigned954.0 eV tocould Cu2+ 2 0 bespecies, assigned which to Cu were⁺ species likely, formedwhich were from likely the oxidation formed offrom Cu 0theon oxidation the surfaces of Cu of the on Cu the nanoparticles surfaces of theduring Cu nanoparticles exposure to airduring and humidityexposure [to32 air]. and humidity [32].

Nanomaterials 2018, 8, 636 11 of 14 Nanomaterials 2018, 8, x FOR PEER REVIEW 11 of 14

Figure 7 7.. ((aa)) XPS XPS s surveyurvey spectra spectra of of 10Cu 10Cu-22-mpg-C-22-mpg-C3NN44--1;1. High High-resolution-resolution XPS XPS spectra spectra of of ( (bb)) C C 1s 1s peak peak;; (c) N 1s peak and ( d) Cu 2p peak of 10Cu-22-mpg-C10Cu-22-mpg-C3N44--1.1.

The photocatalyticphotocatalytic activities activities of theof the materials materials for degradationfor degradation of tartrazine of tartrazine yellow dyeyellow in solutions, dye in solutions,a colorful a additive colorful thataddi istive widely that is used widely in variousused in various consumer consumer products products worldwide worldwide but is but also is often also oftenregarded regarded as a cause as a ofcause allergies, of allergies, hyperactivity hyperactivity and even and cancer, even were cancer, evaluated were evaluated (Figure8). (Figure The necessary 8). The necessarycomparative comparative and control and experiments control experiments were also were performed. also performed. In one control In one experiment, control experiment, the extent the of extentphotolysis of photolysis of tartrazine of tartrazine yellow without yellow thewithout presence the presence of catalyst of overcatalyst an irradiationover an irradiation time of240 time min of 240was min found was to found be only to ca. be 5.3%. only Thisca. 5.3% revealed. This thatrevealed the photodegradation that the photodegradation of this dye of in this the absencedye in the of absencea catalyst of was a catalyst very slow. was The very photodegradation slow. The photodegradation of the dye in the of presencethe dye in of the a sample presence of 22-mpg-C of a sample3N 4of-1 22was-mpg not-C good3N4-1 either, was not as only good ca. either 18.8%, as tartrazine only ca. yellow18.8% tartrazine was photodegraded yellow was in photodegraded the same time period.in the 3 4 sameThis suggestedtime period. that This as-synthesized suggested that mpg-C as-synthesized3N4 was mpg not a-C goodN was photocatalyst not a good photocatalyst for the degradation for the degradationof tartrazine of yellow. tartrazine However, yellow the. However, photocatalytic the photocatalytic performance ofperformance the material of for the degradation material for of degradationtartrazine yellow of tartrazine dye in solution yellow dye was in significantly solution was improved significantly after improved its surfaces after were its decorated surfaces were with decoratedCu nanoparticles, with Cu nanoparticles, especially with especially about 10 with wt.% about of Cu 10 nanoparticles.wt.% of Cu nanoparticles. Specifically, Specifically the extents, the of 3 4 extentphotodegradations of photodegradation of tartrazine of tartrazine yellow dye yellow in 240 dye min in 240 in themin presence in the presence of 5Cu-22-mpg-C of 5Cu-22-mpg3N4--1C andN - 3 4 110Cu-22-mpg-C and 10Cu-22-3mpgN4-1-C wereN -1 found were tofound be ca. to 32.8% be ca. and 32.8% ca. 81.9%, and ca. respectively, 81.9%, respectively, which were which significantly were 3 4 significantlyhigher than thehigher one obtainedthan the inone the obtained presence in of the 22-mpg-C presence3N of4-1. 22 For-mpg comparison,-C N -1. For the comparison, performance the of 3 4 performance10Cu-47-mpg-C of 310CuN4-1-47 (a- materialmpg-C N that-1 (a is synthesizedmaterial that using is synthesized 47 nm silica using nanoparticles 47 nm silica as nanoparticles templates but ascontaining templates the but same containing amount, the 10 same wt.%, amount, Cu nanoparticles) 10 wt.%, Cu was nanoparticles also evaluated.) was This also material evaluated resulted. This materialin ca. 42.5% resulted photodegradation in ca. 42.5% photodegradation of tartrazine yellow of tartrazine in the same yellow period in the of same time. period This materialof time. Th hadis material had a surface2 area is 87 m2/g, which is lower than that of 10Cu-22-mpg-C3N2 4-1 (i.e., 97 m2/g). a surface area is 87 m /g, which is lower than that of 10Cu-22-mpg-C3N4-1 (i.e., 97 m /g). These results Thesesuggest results that, besides suggest the that surface-deposited, besides the surface Cu nanoparticles,-deposited Cu higher nanoparticles surface area, higher enhances surface the ability area enhances the ability of Cu-doped mpg-C3N4 materials to photocatalytically degrade tartrazine yellow of Cu-doped mpg-C3N4 materials to photocatalytically degrade tartrazine yellow dye in solutions. dye inDuring solutions photocatalytic. degradation, efficient charge separation is important, because it allows the chargesDuring notphotocatalytic to be compromised degradation, by undesirable efficient charge charge separation recombination is important, processes andbecause to participate it allows thein various charges redox not to reactions. be compromised The available by undesirable photogenerated charge recombination electrons on the processes conduction and band to participate can then in various redox reactions. The available photogenerated•− electrons on the conduction band can then react with O2 and form superoxide radicals (O2 ). The resulting superoxide radicals as well as react with O2 and form superoxide radicals (O2•−). The resulting superoxide radicals as well as the photogenerated holes will help the photo-oxidative degradation of various compounds [2,33,34].

Nanomaterials 2018, 8, 636 12 of 14

Nanomaterials 2018, 8, x FOR PEER REVIEW 12 of 14 the photogenerated holes will help the photo-oxidative degradation of various compounds [2,33,34]. UnfortunatelyUnfortunately,, plain g-Cg-C3NN44 oftenoften suffers suffers from from rapid electron electron-hole-hole recombination, and thus shows negligible photocatalytic photocatalytic activity. activity. However, However, in in the the presence presence of ofmetals metals such such as Cu, as Cu, which which can cantrap trap the photogeneratedthe photogenerated electrons, electrons, the recombination the recombination rate of rate the ofphotogenerated the photogenerated electron electron-hole-hole pairs in pairs g-C3N in4 isg-C significantly3N4 is significantly minimized. minimized. Hence, Hence, not surprisingly, not surprisingly, the thedecoration decoration of ofmpg mpg-C-C3N3N4 4withwith Cu nanoparticles facilitated the the photodegradation of of tartrazine tartrazine yellow yellow over over the the materials materials.. In In particular, particular, 10Cu-22-mpg-C10Cu-22-mpg-C33NN44 showshoweded superior superior photocatalytic photocatalytic activity activity compared with plain 22-mpg-C22-mpg-C3N4--1.1. Although 10Cu-47-mpg-C10Cu-47-mpg-C33N4--11 also also showed showed better better photocatalytic activity than plain 22-mpg-C22-mpg-C3N4--1,1, 22 its activity was inferior toto thatthat ofof 10Cu-22-mpg-C10Cu-22-mpg-C33NN44-1-1 (the(the surfacesurface areaarea ofof whichwhich isis 9797 mm /g);/g); this this was most likely due to its lower surfacesurface areaarea (87(87 mm22/g) and and thus thus less less exposed exposed catalytic catalytic activ activee sites sites [35]. [35]. ThereforeTherefore,, it can be said that, in addition addition to to decorating decorating its its surfaces surfaces with with Cu Cu nanoparticles, nanoparticles, introducing introducing high sur surfaceface areaarea in mpg-Cmpg-C33N4 cancan improve the photocatalytic activity of g-Cg-C33N4 forfor degrada degradationtion of tartrazine yellow dye.

Figure 8 8.. DegradationDegradation profiles profiles of oftartrazine tartrazine yellow yellow over over the theas-prepared as-prepared mpg mpg-C-C3N4 and3N4 Cuand-mpg Cu-mpg--C3N4 photocatalysts.C3N4 photocatalysts.

4. Conclusions Conclusions A facile synthetic route that produces highly photocatalytically active Cu nanoparticles-loadednanoparticles-loaded mesoporous g-Cg-C33N4 (mpg(mpg-C-C33NN44) )from from melamine melamine using using colloidal colloidal silica silica nanoparticles nanoparticles as as hard hard templates templates has been developed. The synthetic conditions that produce meso mesoporosityporosity in the materials have been investigatedinvestigated,, and and the the mechanisms mechanisms that that lead lead to to mesoporosity mesoporosity in in the the materials materials have have been been proposed. proposed. The surface area, area, pore pore size size and and pore pore volume volume of of mpg mpg-C-C3N34N could4 could be betuned tuned by bychanging changing the thesize size of the of silicathe silica nanoparticles nanoparticles used used as templates as templates to synthesize to synthesize the thematerials. materials. Modification Modification of the of thesurface surfacess of mpgof mpg-C-C3N43 Nby4 Cuby Cunanoparticles nanoparticles has hasresulted resulted in improved in improved lightlight absorption, absorption, charge charge separation, separation, and photocatalyticand photocatalytic activity activity over overthe materials the materials.. The resulting The resulting Cu nanoparticles Cu nanoparticles-modified-modified mpg mpg-C-C3N4 (Cu3N4- mpg(Cu-mpg-C-C3N4) 3materialsN4) materials were were found found to photocatalytically to photocatalytically degrade degrade tartrazine tartrazine yellow yellow dye, dye, with with activity activity much better than that of purepure mpg-Cmpg-C33N4.. In In particular, particular, the Cu-mpg-CCu-mpg-C33N4 mmaterialaterial synthesized synthesized from melaminemelamine-to-silica-to-silica nanoparticlesnanoparticles mass mass ratio ratio of 1:1of and1:1 and then then modified modified with 10with wt.% 10 Cu wt.% showed Cu superiorshowed superiorphotocatalytic photocatalytic activity toward activity the toward photodegradation the photodegradation of tartrazine of yellow tartrazine dye. Theyellow photocatalytic dye. The photocatalyticperformances ofperformance the materialss of also the dependedmaterials also on their depended specific on surface their specific area, besides surface the area, amount besides of the Cu amountloaded on of them.Cu load Theed presenton them. work The providespresent work a new provide synthetics a new strategy synthetic to prepare strategy three-dimensionally to prepare three- dimensionallyinterconnected interconnected porous g-C3N4 materialsporous g- withC3N4 largematerials surface with area, large high surface porosity area and, high great porosity potential and for greatremediation potential of for common remediation environmental of common pollutants. environmental pollutants.

AuthorSupplementary Contributions Materials:: T.Z.The conceived, following aredesigned available and online performed at http://www.mdpi.com/2079-4991/8/9/636/s1 the experiments; I.P.A.F.S. performed. pAuthorhotodegradation Contributions: experiments;T.Z. conceived, J.X. processed designed the data; and T.Z. performed and T.A. drafted the experiments; the manuscript; I.P.A.F.S. V.C.A. performed and T.A. participatedphotodegradation in the development experiments; of J.X. the processed basic ideas the of data; the wo T.Z.rk andand T.A.reviewed drafted and the edited manuscript; the manuscript. V.C.A. and T.A. participated in the development of the basic ideas of the work and reviewed and edited the manuscript. Funding: This research was funded by the US National Science Foundation (Grant No.: NSF DMR-1508611).

Nanomaterials 2018, 8, 636 13 of 14

Funding: This research was funded by the US National Science Foundation (Grant No.: NSF DMR-1508611). Acknowledgments: T.A. and T.Z. thank the Rutgers Energy Institute (REI) for the partial financial support to T.Z., enabling him to participate in this research work. The authors thank Hongbin Yang for his help with XPS analysis. Conflicts of Interest: The authors declare no conflicts of interest.

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