One-Pot Fabrication of Perforated Graphitic Carbon Nitride Nanosheets Decorated with Copper Oxide by Controlled Ammonia and Sulfur Trioxide Release for Enhanced Catalytic Activity.

In this article, we have judiciously interfaced copper oxides with graphitic carbon nitride (g-C3N4) from thermal reaction of melamine and copper sulfate in a one-pot protocol and manipulated the perforated sheet morphology thereafter. The CCN-X (X = 30, 40, 50, 60, and 70, depending on the wt % of CuSO4·5H2O) nanocomposites were prepared by homogenously mixing different percentages of CuSO4·5H2O with melamine from a solid-state thermal reaction in a furnace in air. Drastic lowering of CuSO4 decomposition temperature due to Cu(II)-amine complex formation and subsequent reduction of Cu(II) species by in situ produced ammonia (NH3) resulted in the production of CuO and catalytic amount of Cu2O, homogeneously dispersed within the perforated g-C3N4 nanosheet. How perforated sheet morphology evolved by combined effect of NH3, released from thermal condensation of melamine ensuring two-dimensional (2D) growth, and sulfur trioxide (SO3), expelled from CuSO4·5H2O facilitating the perforation, yielding better catalytic performance, has been elucidated. Excess NH3 from added NH4Cl removed perforation and ensued a marked decrease in efficacy. However, a high proportion of CuSO4·5H2O ruptured the framework of 2D sheets because of excess SO3 evolution. Among the different nanocomposites synthesized, CCN-40 (CuO-Cu2O/g-C3N4) showed the highest catalytic activity for 4-nitrophenol reduction. Thus, enhanced efficiency of the copper oxide catalyst by interfacing it with an otherwise inactive g-C3N4 platform was achieved.

Introduction

In the era of nanoscience and technology, a momentous leap in the field of metal and metal oxide nanoparticles has been witnessed. Solid oxides are most abundant in earth’s crust, and they are very stable and rugged and would be the future materials if they are tuned properly. The wide range of properties of nanoparticles has attracted the attention of scientists over the decade, and different nanocomposites are being designed to accomplish practical applications.[1] Nanoparticles are popular photocatalysts because of their high optical absorption capacity in the UV–visible region, which consists of a major part of the solar spectrum. Metal oxide nanoparticles are able to harvest solar energy efficiently, and owing to their antiferromagnetic properties, low cost, nontoxicity, and high stability, their wide range of applications as a gas sensor, high-temperature superconductor, lithium battery, optical switch, solar cell, electron field emitters, and other electronic and optical devices are possible. In the biochemical field, metal and metal oxide nanoparticles are used as sensors in cancer therapy, light emitting devices, in surface-enhanced Raman spectroscopy studies, and also for antimicrobial activity.[2−5] Nanocomposites have been synthesized in a multitude of ways such as thermal reactions, electrochemical methods, and photochemical and sonochemical methods.[1,2,6,7] Cu2O, the most studied and oldest semiconducting naturally occurring material, has a wide range of applications in solar H2 generation, catalytic activity, batteries, and degradation of dyes, whereas CuO with high electron-accepting property has found its application in photoelectrodes, electronics, optoelectronics, and other fields of nanotechnology.[8,9] CuO and Cu2O both are p-type transition metal semiconductors with narrow 1.2 and 2.2 eV band gap values and 3d9 and 3d10 electronic configurations, respectively.[10]

In contemporary research, graphitic carbon nitride (g-C3N4) has attracted the attention of scientists because of its similarity with graphene. It has a surfeit of applications in energy conversion, gas storage, solar cells, and gas sensors.[11−13] A general synthetic route of g-C3N4 preparation is polycondensation of common monomers.[11,14−16] It has intriguing features like commendable thermal stability and high in-plane nitrogen content.[11,16−20] The introduction of heteroatoms can alter the composition and properties of carbon nitride, thereby engineering the molecular orbital shape and position relevant for enhancing its ability to react and its selectivity.[17] This in turn helps in the synthesis of composites that are bestowed with a high electron density system within the catalyst by either charge transfer complexation or connection with semiconductor materials.[21−23] Owing to the polymeric nature and weak van der Waals force between adjacent layers, large interlayer spacings similar to those in graphite are present, assisting in intercalation with superior physicochemical properties.[24−26] Apart from this, it has a delocalized π-conjugated system and contains lots of coordination sites called “nitrogen pots”, where the six nitrogen lone-pair electrons interact with the metal ions, locking them in the plane of the highly ordered tris-triazine (C6N7) units linked via planar tertiary amino groups.[24−26] The g-C3N4 layers have a band gap of 2.7 eV, and g-C3N4 is persistently used as a photocatalyst. It has useful applications in degradation of toxic dyes, water splitting, gas storage, antibacterial activity, Friedel craft reactions, NO decomposition, and CO2 reduction.[27−32]

Environmental remediation is of utmost concern for the sustenance of healthy life on earth. Nitrophenols and their derivatives are obstinate toxic compounds incorporated in wastewater from various sources such as herbicide, pesticide, insecticide, and synthetic dye industries. The reduction strategy of 4-nitrophenol was first proposed by our group,[32] along with the plausible mechanism and reaction kinetics,[33] which was later detailed by Ballauff et al.[34−36] 4-Nitrophenol reduction majorly follows the Langmuir–Hinshelwood model, which reveals an induction time caused by dynamic restructuring of the nanoparticle surface.[37] Although 4-nitrophenol is a lethal material, its reduced form is a potent intermediate for the synthesis of various analgesic and antipyretic drugs. Apart from this, it is widely used in photographic developers, anticorrosion lubricants, and hair dyeing agents.[38,39]

Different multistep synthetic procedures have been accounted for among the few reports of the copper oxide/g-C3N4 nanocatalyst. Liu et al. synthesized Cu2O/g-C3N4 by a solvothermal method following the reduction of CuCl2·2H2O by vitamin C and then reacting with g-C3N4.[40] Mitra et al. also prepared a similar composite from g-C3N4 and reduction of Cu(OAc)2·H2O using NaBH4.[41] Li et al. prepared CuO/g-C3N4 by the wet impregnation–calcination technique by individually preparing g-C3N4 and Cu2O. In this method, Cu2O was prepared from CuSO4·5H2O, glucose, and NaOH, whereas g-C3N4 was prepared from a reported method.[42] Zhang et al. prepared Cu-Cu2O/g-C3N4 using the precursors g-C3N4, Cu(NO3)2, and NaBH4.[43] We endeavored to embed both CuO and Cu2O onto g-C3N4 to design an ace catalyst for superior application by following a one-pot facile green synthetic route using a single copper precursor and melamine under heat treatment in a furnace in air, for the first time. Here, we have capitalized thermal condensation–polymerization of melamine for two-dimensional (2D) nanosheet formation with obvious interaction of the Cu(II) moiety with ammonia[44] (NH3) at a substantially low temperature and perforation of the 2D nanosheet morphology by the in situ produced sulfur trioxide (SO3) from CuSO4·5H2O decomposition. Reduction of Cu(II) to Cu(I) by NH3[45] and drastic lowering of the decomposition temperature of CuSO4·5H2O in the presence of melamine, leading to the final CuO–Cu2O/g-C3N4 (CCN-X) nanocomposite, are also notable deviations at a temperature of 550 °C. Inspired by the abovementioned properties of the individual copper oxides and g-C3N4, investigation of the catalytic efficiency of as-prepared CCN-X nanocomposites for the 4-nitrophenol reduction reaction was studied in detail.

Results and Discussion

Analytical Instruments

Details have been provided in the Supporting Information.

Considering the best catalytic performance, characterizations were carried out in detail with CCN-40 as the representative catalyst.

X-ray Diffraction (XRD) Analysis

The XRD pattern (Figure ) of the as-obtained composite depicts peaks at 2θ = 32.6, 35.8, 38.9, 49.2, 53.8, 58.6, 61.7, 66.3, 68.2, 72.6, and 75.4°, which can be indexed to the (110), (002)/(1̅11), (200)/(111), (202̅), (020), (202), (311̅), (113̅), (220), (311), and (222̅) lattice planes of CuO, respectively, as confirmed from JCPDS no. 48-1548. No sharp peak was observed, instead a broad hump was observable below 30 because of g-C3N4 layers. The intensity in this region was low, presumably, because of the excess of copper oxide incorporated in g-C3N4 and also the high intensity of CuO masking the less intense signals of g-C3N4. Pure g-C3N4 however shows peaks at 13.0 and 27.4°, which refer to the (100) and (002) planes of hexagonal g-C3N4, as confirmed by JCPDS no. 44-0706. Time-dependent XRD spectra (Figure S1a) reveal the gradual synthesis of CuO–Cu2O and g-C3N4. However, the final product has greater percentage of CuO than that of Cu2O; as a result, the XRD pattern of Cu2O is faint but evident in the final product. The XRD patterns of bulk CuO heated in the presence of melamine (Figure S1b) and CCN-40 in the presence of ammonium chloride (NH4Cl) (Figure S1c) were obtained. The XRD peak of the used catalyst (Figure ) clearly shows the peak of Cu, which explains the gradual loss of catalyst activity.

Figure 1

XRD patterns of the as-synthesized nanocomposite CCN-40.

Figure 12

XRD pattern of the CCN-40 catalyst after a few cycles of 4-nitrophenol reduction in the presence of NaBH4.

X-ray Photoelectron Spectroscopy (XPS) Analysis

The surface composition and oxidation state of the metal oxides were ascertained by XPS analysis. The XPS spectra of the CuO−Cu2O/g-C3N4 composite show the presence of carbon, nitrogen, copper, and oxygen (Figure S2a). From the graph (Figure a), it is evident that the peaks of C 1s located at 284.4, 286.2, and 288.4 eV were for sp2 C=C (graphitic or amorphous carbon formed due to decomposition of carbon nitride due to X-ray irradiation or carbon-containing contaminants), C–NH2, and N=C–N2. The N 1s peak, which was comparatively more complicated, had three asymmetrical peaks at 398.8, 399.6, and 401.1 eV, which can be assigned to C=N–C, N–(C)3 groups linking structural motif (C6N7), or (C)2–NH connected with structural defects or incomplete condensation, C–NH or C–NH2 or N–(C)3 inside the aromatic ring, respectively, in triazine units as observed (Figure b).[12,13,29] The N–(C)3 group consolidates polymerization and the N–(C)3/C–NH peak ratio is a manifestation of the degree of condensation reaction.[13] The peaks at 952.2 and 932.3 eV can be assigned to the binding energy of Cu1+ for Cu 2p1/2 and Cu 2p3/2. The peaks at 933.8 and 953.4 eV and the shakeup satellite peaks at higher binding energies of 942.3 and 961.8 eV can be assigned to Cu2+ present in the CuO–Cu2O/gC3N4 nanocomposite (Figure d).[10] XPS spectra (Figure c) show the main peak of O 1s which on deconvolution gave a peak at 532.8 eV due to lattice oxygen atoms in CuO–Cu2O and the other peak at 535.8 eV for the adsorbed water molecules on the surface. The Cu 2p spectra (Figure S2b) of CCN-40 at 4 h clearly reveal the presence of Cu(II) and Cu(I) during the formation of CCN-40.

Figure 2

XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s and (d) Cu 2p, of the CCN-40 nanocomposite.

Fourier Transform Infrared (FTIR) Analysis

From graph (Figure ), the characteristic stretching vibrational modes of s-triazine derivatives like C–N heterocycles are observable from 900 to 1700 cm–1, thereby consolidating that nitrogen is chemically bonded to carbon in the g-C3N4 moiety. On comparing the peaks between pure g-C3N4 and as-synthesized CCN-40, we observe a slight decrease in wavenumber for the latter, thus advocating presumably the presence of a nitrogen–copper interface in the composite (Figure S3). The sharp peak at 804 cm–1 is the contribution of the breathing mode of the triazine unit. We observe broad low-intensity peaks at a higher wavenumber at around 3200–3500 cm–1, which can be assigned to the stretching vibration of N–H (primary −NH2 and secondary =N–H), suggesting the hydrogenation of some nitrogen atoms in g-C3N4.[4−6] These peaks also suggest a bridging amine. Moreover, an absorption peak observable at 2175 cm–1 can be attributed to the cyano terminal group (C≡N) and cumulated double bond (−N=C=N– and such similar bonds).[20] The peaks at 601, 512, and 432 cm–1 are assigned to the Au mode and 2Bu modes, respectively. The Cu(II)–O stretching along the [1̅01] direction is likely to induce a peak at 601 cm–1, whereas stretching along the [101] direction shows the peak at 512 cm–1.[46,47] Faint peaks in the range from 600 to 660 cm–1 in the IR active mode prove the existence of a small amount of another phase, i.e., Cu2O, for the Cu(I)–O vibrational band.[48]

Figure 3

FTIR spectrum of the as-synthesized CCN-40 nanocomposite.

Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), and High-Resolution Transmission Electron Microscopy (HRTEM) Analyses

Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) assisted in the determination of the morphology of the nanocomposite. The FESEM and TEM images shows bare sheet-like morphology of amorphous g-C3N4 (Figure a). FESEM and TEM images of CCN-X divulges structural feature (Figure b and Figure S4a,b,c) of perforated sheet-like structure decorated with particle deposited on it. From HRTEM analysis the fringe spacing of copper oxides was obtained in the nanocomposite which was further confirmed with SAED pattern. Curled nanosheets were observed from representative TEM images of gC3N4. The composite however has CuO–Cu2O nanoparticles of 10–20 nm size trapped on the g-C3N4 sheets, as displayed in the TEM images. The figures obtained were from samples subjected to ultrasonication, used to disperse the material prior to characterization, which clearly establish the strong binding of the composite and stability of the 2D nanosheets. The fringe patterns and selected area electron diffraction (SAED) pattern for CCN-30, CCN-40, CCN-50, and CCN-60 give d-spacings of 2.3 nm related to the (200/111) plane of CuO or the (111) plane of Cu2O and 1.8 nm for (202̅) of CuO. The used catalyst CCN-40 was also analyzed (Figure c) to obtain heavily deposited Cu, confirmed by the fringe pattern for both copper oxide and Cu. We also observe (Figure ) the gradual synthesis of perforated CCN-40 at intervals of 1 h. We realize that initially a 2D sheetlike morphology is dominant, which at a time of 4 h gradually shows perforations probably due to the expulsion of SO3 during decomposition of the CuSO4·5H2O molecule but at a much lower temperature (∼550°C) than usual. With time, the gaps become more prominent, resulting into perforated 2D nanosheets in the final product at the end of 6 h. It is worth mentioning that the FESEM images of CCN-40 using 250, 500, and 1000 mg of NH4Cl during synthesis were recorded, showing a gradual decrease in the perforation with an increase in NH4Cl content at the start of the synthesis. This observation guaranteed the role of NH3 not only for the extended graphitic sheet formation out of condensation and subsequent polymerization reaction but also for the prevention of the formation of perforated morphology. To study the effect of excess CuSO4·5H2O on the morphology of the CCN-X, nanocomposite CCN-90 was also prepared and the FESEM image was recorded (Figure S5). We see a rupture of the nanosheet structure and formation of aggregated nanoplates. This presumably happens because the excess SO3 causes a large number of perforations in the nanosheets which eventually results in the ruptured framework of the 2D sheet. Thus, the optimum amount of the precursor compounds balances the evolution of NH3 and SO3 from the proposed one-pot reaction. Precisely, NH3 is evolved first and then SO3, which assists in nanosheet formation[49] and its subsequent perforation in a later step. Drastic lowering of CuSO4·5H2O decomposition temperature with melamine may be explained by Cu(II)–amine complex formation[50] and its reduction by the in situ produced NH3 under heating. How excess NH3 blocks the creation of perforated nanosheet morphology is verified experimentally as mentioned above (Figure S6).

Figure 4

SEM, TEM, and HRTEM images of (a) g-C3N4, (b) CCN-40, and (c) used CCN-40: (i) FESEM image, (ii) TEM image, and (iii) HRTEM image revealing the fringe pattern (inset: respective SAED patterns).

Figure 5

FESEM images of the growth of CCN-40 at intervals of 1 h.

EDX Analysis

The evenly distributed elements in CCN-X were recorded in energy-dispersive X-ray (EDX) spectroscopy analysis. The elemental area mapping (Figure ) of the elements revealed the presence of C, N, Cu, and O well distributed throughout the material.

Figure 6

Energy-dispersive X-ray (EDX) analysis of CCN-40 and its elemental area mapping showing the distribution of elements.

Brunauer–Emmett–Teller (BET) Analysis

The FESEM and TEM images clearly display the perforations in the as-prepared nanocatalyst, thereby promising a high surface area for reaction. From the nitrogen adsorption–desorption isotherm, the Brunauer–Emmett–Teller (BET) surface area of the CCN-40 nanocatalyst was established to be 37.0485 m2 g–1 (Figure S7a). The Barrett–Joyner–Halenda measurement was implemented to obtain pore diameter of 138.392 Å (Figure S7b).

TGA Analysis

The thermogram depicts the change in sample mass as a function of temperature and delivers stoichiometric, heat stability, and compositional information for a complete reaction considering the precursors as well as the intermediates. We have taken samples of CuSO4·5H2O, mixture of the precursors (optimum proportions), and CCN-40 product for thermal study. The samples were heated at changing temperature in an oxygen atmosphere, and the mass readings were recorded by a thermobalance at intervals. From the thermogram (Figure S8a) of CuSO4·5H2O, we perceived that five molecules of water of hydration leave in three distinct steps at three temperatures within 250 °C. CuSO4 thus formed remains stable till 550 °C, and after which, CuSO4 releases SO3 and forms CuO within ∼700 °C. The reported thermogram of melamine[51] shows a large decrease in mass from 250 to 360 °C and a small decrease from 370 to 500 °C, which accounts for the volatilization of NH3. Another step is observed at 500 °C, which can be assigned to condensation of melem to g-C3N4, and above 540 °C, g-C3N4 undergoes mass loss due to decomposition till 610 °C (Figure S8b). In the case of the mixture with optimum ratio of precursors (Figure ), we could interpret the steps of formation of the CCN-40 nanocomposite. The five molecules of water of crystallization were completely removed from CuSO4·5H2O within the temperature of 250 °C with a weight loss of 22% (theoretical 12.5% i.e., 2.5 for each water molecule). The excess weight loss was probably due to expulsion of NH3 during condensation of melamine, which also begins around this temperature. We observe the release of NH3 between 250 and 450 °C, with the formation of portions of melam, melem, and g-C3N4 and also the sublimation and expulsion of portions of melamine derivatives within this temperature (as it is an open system analysis).[19] It is very interesting to note that temperature-dependent XRD results unequivocally confirm the decomposition of CuSO4·5H2O within 550 °C and the presence of CuO. Although we know that the decomposition temperature decreases with the incorporation of impurity, for a large decrease of about 100 °C, impurity alone cannot be held responsible. Surely, the chemical affinity of Cu2+ to NH3 must be the root cause of such lowering of decomposition temperature. This drastic lowering of CuSO4·5H2O decomposition temperature with melamine can be elucidated by the Cu(II)–amine complex.[50] Formation of SO3 is revealed with a weight loss of 7% (theoretical weight loss of SO3, 10.3%), with a signature of formation of CuO within 550–650 °C, which forms at a lower temperature due to the presence of melamine.[52−54] However, TGA analysis is done in an open system, which has a different environment with respect to that in the synthetic protocol followed by us, which is carried out in a closed system. From the XRD and XPS analyses, we can envisage the onset of the final reaction step at around 550 °C, the synthesis temperature for the nanocomposite. Here, we see a lowering of decomposition temperature of g-C3N4 due to in situ incorporation of copper oxide particles and quintessential coordination of Cu atoms with nitrogen atoms of the framework.[14,15,18]

Figure 7

Thermogravimetric analysis (TGA)-differential scanning calorimetry (DSC) analysis of a uniform mixture of CuSO4·5H2O and melamine.

Reaction steps:

Syntheses of CuO–Cu2O/g-C3N4 and the Mechanistic Insight

This is the first report, to the best of our knowledge, of the synthesis of CuO–Cu2O/g-C3N4 by the pyrolysis of melamine in a semiclosed system, which also contained a copper precursor salt, by a one-pot method by heating in a furnace. More specifically, weighed amounts of melamine and CuSO4·5H2O were taken in the mortar and evenly ground following the grinding–mixing protocol (GMP)[55] so as to obtain a homogenous physical mixture, which was then taken in a covered porcelain crucible. Thereafter, it was kept under heat treatment at 550 °C with a heating rate of 5 °C/min and maintained at this temperature for 4 h. A schematic representation of the process is given in Scheme . Thus one-pot method yielded the CCN-X nanocomposite. The time-dependent XRD pattern (Figure S1a) shows the syntheses of CuO–Cu2O along with g-C3N4 gradually. The pentahydrate compound loses five water molecules in the temperature range of 150–250 °C to form a very pale blue compound and further strong heat decomposes it to CuO.[56,57] The dehydration of the molecule occurs with decomposition of the tetraaquacopper(2+) molecule. In the second step, two more water molecules are lost, producing a diaquacopper(2+) moiety. Dehydration is completed by the removal of the last hydrogen-bonded water molecule. At about 650 °C, CuSO4 decomposes to CuO and SO3. Cu(II) has a rich coordination chemistry because of which in the present precursor the Cu is directly coordinated to four water molecules in equatorial positions. The axial coordination is fulfilled by the O atoms from the sulfate group in the lattice. The fifth water molecule is hydrogen-bonded between a sulfate group in the axial and water molecule in the equatorial position. Although the four water molecules are removed in the temperature range of 150–200 °C, the fifth one is different in nature and is removed at ∼250 °C, preceding the decomposition of sulfate anion and formation of CuO within 550 °C (Figure ). A unique observation noted was that the decomposition temperature of CuSO4 decreases by about ∼100 °C because of the presence of melamine. The onset of sublimation of melamine is around 296 °C, which peaks at 345 °C.[58] However, under slow heat treatment, reflections of melamine disappear with volatilization of NH3 initiating at 250 °C, giving rise to a new phase of melam and melem. These derivatives eventually disappear to give g-C3N4 at the synthesis temperature of 550 °C. Presumably, with an increase in temperature, melamine sublimes and, owing to thermal condensation, forms polymeric g-C3N4 intact nanosheets, whereas CuSO4·5H2O loses all its water molecules and gets entrapped in the nanosheet moiety. Because of the semiclosed system, melamine does not sublime, instead condenses to form the g-C3N4 moiety. At a higher temperature, ∼550 °C, CuSO4 expels SO3 and decomposes to CuO. We obtain direct evidence of lowering of decomposition temperature of CuSO4·5H2O and g-C3N4 from thermal analysis performed. Melamine has nitrogen groups, and also its condensation releases NH3, which reduces few Cu(II) to Cu(I), as evident in the XPS analysis.[45,59,60] In the present case, the well-established reducing property of NH3 favored the progress of the reaction. However, partial reduction of CuO occurs and proceeds till the formation of Cu2O and not further to Cu(0). To confirm our findings, we had also heated commercial CuO with melamine to establish this partial reduction step and observed the XRD peaks of both CuO and Cu2O in the final product (Figure S1b). Thus, CuO−Cu2O entrapment on the g-C3N4 framework was made possible, thereby creating the CuO−Cu2O/g-C3N4 nanocomposite. It is worth mentioning that no other copper precursors, viz., CuCl2, Cu(OAc)2, and Cu(NO3)2, were able to yield any nanocomposite and remain as two distinctly separate compounds as black copper oxide powder and yellow g-C3N4 powder. This observation confirms the selectivity of the thermal reaction for catalyst formation with CuSO4·5H2O.

Scheme 1

Schematic Representation of Thermochemical Reactions of Melamine and CuSO4·5H2O

A well-defined 2D sheet of carbon nitride is very commonly reported. Indirect synthesis of the copper oxide/g-C3N4 catalyst has been reported previously. Boron nitride sheets are also frequently observed, whereas aluminum nitride and P3N5 sheets are fewer in number because of their preparation protocol. In all of the cases, high electronegativity of N (3.45) provokes the formation of N3– with any element having a lower electronegativity value. g-C3N4 when heated to about 600 °C gradually decomposes, and the product yield is low. The CN compound has an unbroken 2D nanosheet structure at 550 °C. During the condensation process, the expelled NH3 probably acts like a binder to the triazine units to accomplish a 2D structure, which is most likely due to the hydrogen-bonding property of the amine groups.[44] However, the CCN-40 nanocomposite has a perforated 2D nanosheet morphology, which is probably the result of SO3 released due to the decomposition of entrapped CuSO4 within the g-C3N4 nanosheet. The presence of melamine and released NH3 also reduces a small amount of Cu(II) to Cu2O.[45,60,62] When we add NH4Cl in small proportion in the reaction mixture, we observe a gradual decrease in the perforation of the CCN-40 nanocomposite, as observed from the FESEM images (Figure S6), which is emblematic of the NH3-assisted growth of the 2D nanosheet. It is confirmed that NH3 released from both melamine condensation and NH4Cl assists in the formation of a 2D morphology of the nanocomposite. It is observed that the catalyst obtained with the added proportion of NH4Cl from 250 to 1000 mg does not exhibit any change in the chemical composition, as revealed from XRD peaks (Figure S1c). However, the catalytic activity of NH4Cl assisted the CCN-40 nanocatalyst drastically and decreases the reaction rate of 4-nitrophenol reduction due to decreased perforation.

Nitrophenol Reduction

Because of the negative effects of 4-nitrophenol on the environment and human life, it is necessary to design a cheap earth-abundant catalyst, which can be efficiently utilized to reduce it into more useful 4-aminophenol. The reduction of 4-nitrophenol with NaBH4 is a well-studied reaction as it has kinetics that is convenient to monitor with a spectrophotometer. The reaction takes place in aqueous medium, making it a benchmark reaction to test metal and metal oxide nanoparticles.[37] The protocol has been used worldwide to investigate the behavior of catalysts using a UV–visible spectrophotometer. Hence, we designed a catalyst by tuning the concentration of CuSO4·5H2O in the nanocomposite, to reduce the nitrophenols effectively to more useful amine derivatives, leading to sustenance of environment. In this protocol, we study the reaction by UV–visible spectrophotometry and make important inferences about the catalyst and its surface chemistry. In the absorption spectra obtained (Figure S9a), there occurs a prominent very pale yellow peak of aqueous solution of 4-nitrophenol at ∼315 nm. When 33.32 times more concentration of NaBH4 was added, the color of the solution deepened to bright yellow, giving absorption at a red-shifted value of ∼400 nm for the nitrophenolate anion, which behaves as an oxidant in the presence of reductant BH4–. The bright yellow color is the result of the extended conjugation, which occurs in basic medium within the nitrophenolate anion. It is noteworthy to remark that at the mentioned concentration of NaBH4 in the absence of any catalyst, the reaction is unable to proceed, as observable in UV–visible spectra (Figure S9b). However, with the addition of the as-prepared catalyst, there occurs a rapid reduction. The reduction is also unable to proceed in the complete absence of NaBH4. Hence, we employed our nanocatalyst for the reduction process, which was carried out efficiently within a span of few minutes. Initially, the nitrophenolate anion absorption maximum was tracked at ∼400 nm, which gradually decreased at the expense of a new concomitant peak at ∼301 nm for 4-aminophenol species. The kinetic study of the reduction reaction was carried out exploring the gradually decreasing peak at ∼400 nm monitored with a UV–visible spectrophotometer. The two isosbestic points at ∼311 and ∼280 nm confirm the yield of a single product.[8]

In our investigation, we studied in detail the UV–vis spectra of the 4-nitrophenolate anion upon addition of the as-prepared catalyst in the presence of NaBH4 in aqueous medium. We acquired absorption spectra in each case for our as-prepared CCN-X nanocomposites (Figure ), CuO, Cu2O, g-C3N4, and various combinations (Figure S10). From the linear plot of ln(At/A0) versus time (min) (Figure a), pseudo-first-order kinetics was established as expected from all of the catalysts employed. The presence of excess BH4– ions makes the reaction independent of this species. The reaction was optimized (Figure S11) for conducting all of the comparative study. The rate constant was obtained from the above graph, which reveals the reactivity of the species under the reaction conditions, using the following equationFor a comparison of the catalytic activity, all of the CCN-X catalysts were separately employed to study the 4-NP reduction under the same experimental protocol. The ln(At/A0) versus time (min) graph gave a straight line for all of the catalysts with a negative slope explaining similar kinetics, thereby leading to an impactful conclusion of their rates. The rates of the reaction obtained from the ln(At/A0) versus time (min) graph for bulk CuO, Cu2O, and g-C3N4 were 2.29 × 10–2 (R2 = 0.99), 5.01 × 10–2 (R2 = 0.97), and 5.66 × 10–3 (R2 = 0.95), respectively. From the plot (Figure b) a comparative account of the percentage of conversion of 4-nitrophenol to 4-aminophenol was revealed. We observed that a bare minimum of 16.27% of 4-nitrophenol was converted by pure Cu2O, 13.42% by CuO, and 2.4% by pure g-C3N4 at the end of about 4 min, with 1 mg of each of the catalyst used. However, the ln(At/A0) versus time plot for CCN-30 gave rate constant 7.24 × 10–2 (R2 = 0.95), which converts only 19.71% nitrophenol. Interestingly, CCN-40, CCN-50, and CCN-60 gave remarkable catalytic rate constants: 1.19 (R2 = 0.96), 1.16 (R2 = 0.98), and 0.82 (R2 = 0.96) min–1 with a % conversion of 95.28, 94.22, and 75.82, respectively. The straight line proves its similarity in kinetics with bulk CuO, bulk Cu2O, and g-C3N4. These newly reported nanocomposites, especially CCN-40, are capable of reducing ∼100% 4-nitrophenol within a small time span of 5–6 min. Again, as we move toward CCN-70, we observe a gradual decrease in the rate constant to 4.57 × 10–2 min–1 (R2 = 0.97), which converts 8.39% 4-nitrophenol in 4 min. The results achieved led to the order of the rate constants as follows: kCCN-40 > kCCN-50 > kCCN-60 > kCCN-30 > kCu > kCCN-70 > kCuO > kg-C. The root cause of the change in the catalytic activity originates from the copper center. The active copper center is low in the case of CCN-30, and overloading of copper is understandable from CCN-60 onward. In both the cases, exposure of active catalytic sites is insufficient in comparison to that in CCN-40, which gives the optimum exposure of active catalytic sites in the nanocatalyst. However, all of the prepared catalysts consolidated the efficiency of the nanocatalyst in comparison to that of individual g-C3N4, CuO, and Cu2O.

Figure 8

Absorption spectra of 4-nitrophenol reduction in the presence of NaBH4 with as-prepared catalysts (a) CCN-30, (b) CCN-40, (c) CCN-50, (d) CCN-60, and (e) CCN-70.

Figure 9

(a) Comparative study of the ln(At/A0) vs time (min) plot for different catalysts used. (b) Plot showing comparative percent formation of product in different catalysts.

It was confirmed from the analysis that both CuO and Cu2O remain present in the CCN-X catalyst and the catalyst is obtained exclusively from CuSO4·5H2O. Thereafter, we also tried to synthesize the catalyst using CuO and Cu2O separately as precursor compounds together with proportionate amount of melamine and obtained a black powder in both cases. The catalysts are indicated as CCN-B and CCN-C having rate constants 3.20 × 10−3 and 5.25 × 10−3, and % conversion of 4-nitrophenol 7.75 and 11.15, respectively. However, they were not as good as the catalysts, CCN-X, and the rate of the reaction was much lower. Furthermore, we employed different mixtures containing variable proportions of CuO and Cu2O (1:4, 1:1, and 4:1, indicated as CCN-D, CCN-E, and CCN-F, respectively) together with a constant quantity of g-C3N4 to study the catalysis, and the obtained rate constants were 1.50 × 10–2, 1.94 × 10–3, and 1.95 × 10–3 min–1, respectively. The results also consolidated the impactful conclusion of the superior catalytic activity of Cu2O over CuO.

Hence, the nanocomposite formed from 40% CuSO4·5H2O and 60% melamine by weight, i.e., CCN-40, emerged as the most competent catalyst with a rate constant of 1.19 min–1. The rate for our as-synthesized CCN-40 is ∼52 times higher than that of bulk CuO, ∼24 times than that of bulk Cu2O, and ∼210 times higher than that of bare g-C3N4. The catalyst activity parameter (ratio of rate constant and catalyst dose; i.e., ka = rate constant in s–1/weight of the catalyst used in mg) of as-prepared CCN-40 was found to be 2.0 × 10–2 mg–1 s–1. The percentage of conversion of the substrate was plotted, and the validation of our ace catalyst was revealed from the order of completion of the product. Hence, we can assimilate from the above results that Cu2O is presumably a better catalyst than CuO for the reaction, and their cumulative effect is manifested in CCN-40.

We have also judiciously exploited our catalyst for the reduction of various nitroarenes. The nitroarenes we investigated were 2-nitrophenol, 2-nitroaniline, 4-nitroaniline, 3-nitroaniline, and nitrobenzene. From the structures of the said compounds, extended conjugation is expected in three former cases but absent in the two latter compounds. Hence, we can infer that these two compounds, 3-nitroaniline and nitrobenzene, will be the least stable anionic species, thereby resulting in the most reactive compound for reduction. Their absorption spectra depicted (Figure a,b) a rapid reduction within a span of few minutes. However, in the case of 2-nitrophenol, 2-nitroaniline, and 4-nitroaniline, extended conjugation is evident from their slower rate of reduction because of stable anionic species, which are comparatively less reactive. However, among these three compounds, 2-nitroaniline and 2-nitrophenol are more reactive because of steric interaction of the bulky −NH2 and −OH groups in the meta position, respectively, as witnessed (Figure c,d). 4-Nitroaniline has the most stable anion in the alkaline medium, thereby making it less reactive and increasing the time for completion of the reduction process as evident (Figure e).

Figure 10

Absorption spectra of (a) 3-nitroaniline, (b) nitrobenzene, (c) 2-nitroaniline, (d) 2-nitrophenol, and (e) 4-nitroaniline in the presence of NaBH4 as the reducing agent.

Again, we also conducted a study to draw a linear relationship of the rate of catalysis with the extent of perforation of the CCN-40 material. NH3-mediated 2D morphology evolution and subsequent perforation of the nanocomposite gave us a clue to study the effect of NH4Cl. Interestingly, we observed that with the increase in the proportion of NH4Cl the FESEM images show a distinct decrease in the extent of perforation. This proves that introduction of NH4Cl induces facile formation of 2D morphology without the scope for perforation. However, the decrease in perforation of the 2D sheets results in poor catalysis, as observed from the absorption plot (Figure a–c), because of the decreased surface area for reaction. The rate constants of CCN-40 with 250, 500, and 1000 mg of NH4Cl are 1.23 × 10−1, 8.85 × 10−2, and 2.17 × 10−3 min−1, giving the catalyst activity parameters 2.05 × 10−3, 1.48 × 10−3, and 3.62 × 10−2 mg−1 s−1, respectively, and their corresponding conversion percentage of 4-nitrophenol being 8.74%, 4.66% and 0.33%. The observed results (Figure d) clearly show that increase in perforation of the 2D sheets ensures improved catalytic activity because of the increase in the effective surface area for the reaction to occur, as conveyed in Scheme .

Figure 11

Absorption spectra of CCN-40 with (a) 250 mg, (b) 500 mg, and (c) 1 g of NH4Cl, showing a decrease in perforation within 2D nanosheets.

Scheme 2

Pictorial Representation of the Synthetic Procedure

In general, the catalytic activity of noble metals has been the main focus for such kind of reduction, but in recent years, a plethora of research has been conducted with metal oxide nanocomposites due to their low cost, facile syntheses, and stability. Our findings of enhanced catalysis with our as-synthesized nanocomposite are in line with these reports and can be marked as remarkable as observed. The reduction is believed to be in sync with the most commonly accepted Langmuir–Hinshelwood model.[37] In this model, the catalyst surface acts as a scaffold, where the nitrophenolate anion as well as BH4– gets adsorbed and prepares the environment for the facile reduction reaction. Being in the vicinity, the substrate extracts the hydrogen molecule from the reductant and gets reduced to aminophenol. The two steps by which catalysis occurs are as follows: (i) adsorption of the substrate and (ii) desorption of products. Hence, the ease of these two processes enhances the efficiency of a catalyst. The perforated skeleton of well dispersed CuO-Cu2O on g-C3N4[62,63] provides a nanocatalyst with high active site for the substrate and reductant to bind readily, thereby attracting the anionic species from the solution into the catalyst framework making the reduction mechanism facile as depicted in Scheme . We postulated that the high reactivity of the evolved CCN-40 nanocomposite can only be due to the presence of in situ generated Cu(0) as copper oxides are unable to transfer electron effectively. CuO–Cu2O having Cu(II) and Cu(I) species undergoes in situ reduction to give Cu(0), which oxidizes back to give copper oxide again in the alkaline solution.[45] Hence, the in situ generated Cu(0) catalyzed the reduction process through adsorption of 4-nitrophenol and BH4–, followed by hydride transfer, where the nitro group is reduced to the amine group through nitroso and hydroxylamino intermediates, completed by desorption of 4-aminophenol. With the final step of desorption, the CCN-40 nanocomposite could catalyze the reduction again.[9] Hence, the active Cu(0) species help in facile catalysis. Nitrogen though being an electronegative species has lone pair of electrons available. If we observe the framework of g-C3N4, we can assume that the CuO–Cu2O particles are interfaced with the nitrogen atoms of the g-C3N4 structure with their available lone pair. Hence, such electron-rich vicinity presumably also helps in the facile conversion of Cu(II) and Cu(I) to transient Cu(0) species, leading to consequential facile hydrogenation of nitro compounds. As NaBH4 is a strong reducing agent, the copper oxide gets reduced to Cu(0) over time, as evident from XRD patterns (Figure ), and there is a drop in catalytic activity after about seven cycles.[63]

Scheme 3

Mechanism of 4-Nitrophenol Reduction in a Perforated Nanosheet Morphology

Conclusions

In brief, we can conclude by emphasizing the novel synthetic strategy to evolve the nanocomposite of CuO–Cu2O/g-C3N4 from low-cost precursors by heat treatment. It is noteworthy that only CuSO4·5H2O suffice to form a nanocomposite from the one-pot reaction with melamine, whereas other Cu-salts could not form any composite and remained separated as dark copper oxide and yellow g-C3N4 powder after the reaction. Various indirect multistep reaction procedures have been reported previously, but the one-pot synthesis protocol is reported for the first time for the synthesis of perforated nanosheets of copper oxide-decorated g-C3N4 nanocomposite. A definite proportion of the precursor materials is mandatory for effective catalyst preparation. This article also entails, from the evidence acquired, that a catalytic amount of Cu2O plays a vital role in catalysis. The effects of NH4Cl and excess CuSO4·5H2O uniquely control the 2D morphology of the nanosheet. In situ produced NH3 (250–500 °C) for 2D morphology and SO3 evolution (∼550 °C) for perforation of the nanosheets have been documented. The unequivocal activity of the as-prepared catalyst has also been reported for the first time for nitroarene reduction, which can open gateways to vital advancement in the field of industrial as well as environmental chemistry.

Experimental Section

Chemicals

All of the used chemicals were of AR grade. Details are provided in the Supporting Information.

Syntheses of CuO–Cu2O/g-C3N4 (CCN-X)

Different CuO–Cu2O-loaded g-C3N4 nanocomposites were prepared from the one-pot method by heating the well-ground precursors in a furnace. In a typical synthesis, 500 mg of melamine was mixed separately with 30, 40, 50, 60, and 70% by weight of CuSO4·5H2O in a mortar to get a uniform homogenous mixture in all of the cases. The solid-state grinding–mixing protocol was ensured for a reproducible thermochemical reaction of the chosen precursors. The mixture was then separately taken in a covered porcelain boat and heated in a tube furnace in air. The well-ground mixture was heated to 550 °C at 5 °C/min from room temperature and kept at that temperature for 4 h. Thus, we obtained CuO–Cu2O-captured g-C3N4 nanocomposites, CCN-X. During this heat treatment in the presence of CuSO4·5H2O, melamine condenses to form g-C3N4 with CuO–Cu2O-immobilized nanocomposites, whereas in its absence, melamine produced neat g-C3N4 only in the habitual way. During this process, melamine condenses to form g-C3N4 with the CuO–Cu2O nanoparticle trapped in between. The nanocomposites formed from variable weights of CuSO4·5H2O and constant quantity (500 mg) of melamine were termed as CCN-30, CCN-40, CCN-50, CCN-60, and CCN-70 according to the weight percent of CuSO4·5H2O used. Also, we took the mixture of the CCN-40 precursor and added different amounts (250, 500, and 1000 mg) of NH4Cl as a source of NH3 during syntheses. The as-obtained compounds were then characterized.

Catalytic Reduction of 4-Nitrophenol

The reaction mixture was taken in a quartz cuvette, under ambient conditions, containing 3 mL of 5 × 10–5 M 4-nitrophenol and 1.6 × 10–3 M NaBH4. Then, respective amount of the as-prepared CCN-X catalyst (1 mg quantity) was added in the reaction medium, and the decrease in yellow coloration of the solution was monitored using a UV–visible spectrophotometer. This experiment was individually conducted with all of the catalyst systems, viz., CCN-30, CCN-40, CCN-50, CCN-60, and CCN-70. To have a neat comparative account of catalytic activity, commercial CuO and Cu2O and g-C3N4 were also employed individually for 4-nitrophenol reduction. The CCN-40 nanocomposites prepared from 250, 500, and 1000 mg of NH4Cl were also tested for their comparative catalytic activity. We carried out rest of the detailed study on the reduction reaction parameters and other nitroarene reductions with a minimal amount of 0.20 mg of the as-obtained CCN-40 composite for an insightful investigation of the reaction. Each experiment was conducted five times.

Recycling of Catalysts for 4-Nitrophenol Reduction

After the completion of the first cycle of reduction of 4-nitrophenol, the catalyst was collected by centrifugation and washed five times with water. A fresh batch of 3 mL of solution of 4-nitrophenol (5 × 10–5 M) and NaBH4 (1.6 × 10–3 M) was taken in a cuvette, and the collected catalyst was added to it. Immediately, the cycle of catalysis was monitored by a UV–visible spectrophotometer. All of the reaction cycles were performed in a similar way.