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Article

Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures

by
Walker Vinícius Ferreira do Carmo Batista
1,
Aniely Pereira de Souza
1,
Tais dos Santos Cruz
1,
Dilton Martins Pimentel
1,
Danila Graziele Silva de Avelar
1,
Sarah Karoline Natalino Oliveira
1,
Wanessa Lima de Oliveira
2,
Danilo Roberto Carvalho Ferreira
2,
Márcio Cesar Pereira
3,
Rondinele Alberto dos Reis Ferreira
4 and
João Paulo de Mesquita
1,*
1
Department of Chemistry, Universidade Federal dos Vales do Jequitinhonha e Mucuri, Diamantina 39100-000, MG, Brazil
2
Nuclear Technology Development Center, Belo Horizonte 31270-901, MG, Brazil
3
Institute of Science, Engineering and Technology, Universidade Federal dos Vales do Jequitinhonha e Mucuri, Teófilo Otoni 39803-371, MG, Brazil
4
Faculty of Chemical Engineering, Universidade Federal de Uberlândia, Uberlândia 38408-100, MG, Brazil
*
Author to whom correspondence should be addressed.
Compounds 2025, 5(2), 21; https://doi.org/10.3390/compounds5020021
Submission received: 28 March 2025 / Revised: 20 May 2025 / Accepted: 4 June 2025 / Published: 10 June 2025

Abstract

:
In this study, we report the synthesis and characterization of hybrid heterostructures composed of red carbon, an organic semiconductor polymer, and the perovskite (NH4)3ZnCl5. Red carbon was synthesized via the polymerization of carbon suboxide (C3O2), exhibiting strong light absorption and distinctive optical properties. The hybrid material was obtained by crystallizing (NH4)3ZnCl5 in the presence of red carbon, leading to significant modifications in the optical characteristics of the perovskite. Comprehensive analyses, including X-ray diffraction, FTIR spectroscopy, UV-vis spectroscopy, and cyclic voltammetry, confirmed the formation of a type I heterostructure with enhanced luminescence and potential for advanced optical applications. The energy band alignment suggests that red carbon can function effectively as both a hole and electron transport medium. This work underscores the potential of (NH4)3ZnCl5@red carbon hybrid heterostructures in the development of next-generation optoelectronic devices, including sensors and LEDs.

Graphical Abstract

1. Introduction

In the last decade, ternary compounds based on the perovskite structure have attracted great attention from the scientific community, especially because of the power conversion efficiency (PCE) [1]. However, perovskite halides have been considered for several other applications becoming a prominent field of research in recent years because of their distinct photo-physical features that include a tunable band-gap, long diffusion length, and high carrier mobility [2,3]. Despite their fantastic photoelectric properties, several strategies have been used to improve their performance and expand their field of application. The development of heterostructures with a wide range of materials has been considered to have positive impacts on the properties and performance of devices based on metal halide perovskites [3,4,5,6]. In this context, hybrid heterostructures of metal halide perovskites using carbon-based materials (graphene, carbon dots) [7,8] and organic semiconductors have received increasing attention because of their unique optical and electrical characteristics [9,10,11].
Metal halide perovskite-based optoelectronics has experienced an unprecedented development in the last decade, while further improvements in the efficiency, stability, and economic gains of such devices require novel engineering concepts. The use of carbon nanoparticles as versatile auxiliary components of perovskite-based optoelectronic devices is one strategy that offers several advantages in this respect.
On the one hand, the development of hybrid heterostructures based on halide perovskites is an interesting and viable strategy to improve the properties of optoelectronic energy devices [12], sensors [8], and photocatalysis [3]; on the other hand, it can contribute to a better understanding of the physical–chemical processes involved at the interface between the compounds and strategies for the protection of the structure and surface of perovskite-based material, motivating the synthesis and future development of new and more efficient halide perovskite-based heterostructures [6,7,13,14].
Here, we report, for the first time, a study on forming a hybrid heterostructure among a metal halide perovskite, (NH4)3ZnCl5, and an organic semiconductor called “red carbon”. The (NH4)3ZnCl5 crystallizes in the orthorhombic Pnma space group. In general, A3BX5-based perovskite structures have been much less explored compared to the widely studied ABX3 counterparts, although A3BX5-based materials will demonstrate superior properties for photonic applications [15].
Red carbon is a conjugated organic polymer/oligomer with semiconductor properties derived from carbon suboxide (C3O2) [16]. Carbon suboxide and other “oxycarbons” were first reported in 1873 by Brodie, during research on the effect of an electric current on carbon monoxide [17]. Currently, this structure is obtained mainly from the dehydration of malonic acid [16,18,19]. A solid state investigation using single-crystal X-ray diffraction revealed that the crystal belongs to the Pnma space group, being constituted by two non-crystallographically equivalent molecules in the asymmetric unit: a = 986.9(2), b = 1206.0(2), c = 516.0(1) pm [18]. It presents a conjugated molecular structure formed of pyrone units with a band gap of ~1.9 eV, high molar absorptivity, and good ability to form films [16,20]. The small band gap and the positions of their valence and conduction bands are suitable for the formation of both type I and type II heterostructures with both metal halide perovskites that are activated by visible light and those with a band gap in the ultraviolet region of the electromagnetic spectrum [21]. In addition to the different optical and electronic properties, the “carbon networks” present a significant and homogeneous concentration of oxygenated functional groups that can contribute to the formation of heterostructures such as perovskites. Furthermore, the synthesis of carbon-based materials with a homogeneous and controllable molecular chemical structure is essential for the elucidation of reaction mechanisms in different applications [22,23].
Despite its interesting properties, it feels manageable and easy, but the work in the literature on red carbon is still very rarely reported [24,25]. Odziomek et al. investigated the photocatalytic properties of carbon red in the oxidation of thioanisole and benzyl alcohol. On the one hand, if the oxidation of sulfurous compost was not very selective, the oxidation of alcohol resulted in a 74% yield of benzaldehyde [16]. Ba et al. used red carbon as a precursor for the preparation of an oxocarbon framework containing dispersed Cu2O clusters. The new material was tested for the electroreduction of nitrate under acidic conditions and presented a high yield rate of ammonium (NH4+) at 3.31 mmol h−1 mgcat−1 with a Faradaic efficiency of 92.5% at −0.4 V (vs RHE) [24]. Giusto et al. reported the use of the material for the preparation of thin films and its use as a platform for amine sensing [20].
Since one of the main challenges associated with the preparation of heterostructures based on perovskite-type materials is the establishment of heterojunctions, here, we propose, for the first time, the use of red carbon in the preparation of hybrid heterostructures with (NH4)3ZnCl5 structures. We explore the impact of red carbon on a perovskite that does not absorb in the visible region of the electromagnetic spectrum, thus being less suitable for photovoltaic applications [26].

2. Materials and Methods

2.1. Chemical Reagents

Hydrochloric acid 38%, ammonium chloride 99.5%, and zinc chloride 96% were provided by Êxodo Científica (Sumaré, Brazil). Malonic acid 98% was obtained from ACS Cientifica (Sumaré, Brazil). Acetic anhydride 97% was obtained from Dinâmica (Indaiatuba, Brazil), ethyl alcohol 99.5% was obtained from Vetec, and isopropyl alcohol 99.8% were obtained from Exodo (Sumaré, Brazil). All chemicals were used directly without further purification.

2.2. Preparation of the Materials

2.2.1. Preparation of Red Carbon

Red carbon was prepared according to the procedure published elsewhere [16]. Briefly, 10 g of malonic acid was mixed with 19.6 g of acetic anhydride in a glass round-bottom flask. The mixture was heated to 90 °C to dissolve the malonic acid, and then, the temperature was raised to 140 °C. The reaction was carried out under these conditions for 5 h. During this time, changes in color and viscosity of the system could be visually observed, changing from a clear solution to a yellow solution and finally a very dark suspension. The resulting material was filtered and washed with ethyl ether and then dried in an oven at 90 °C.

2.2.2. Preparation of (NH4)3ZnCl5 Perovskites and Heterostructure

The perovskite (NH4)3ZnCl5 was synthesized in a hydrochloric acid solution. Briefly, 1.36 g of zinc (II) chloride was dissolved in 1 mL of 1 M hydrochloric acid, and 0.80 g of ammonium chloride was dissolved in 3 mL of hydrochloric acid (1 M) together with 0.5 mL of isopropyl alcohol. The solutions were combined and heated at 80 °C until the solvent volume was reduced to 2 mL, during which a color change from clear to yellowish was observed. The solution was then placed in an ice bath until crystals formed. The crystals were filtered using a sintered glass funnel, washed with isopropyl alcohol and ether, and then transferred to a Petri dish to be dried in desiccators for 24 h. The red carbon@(NH4)3ZnCl5 heterostructures were prepared following the synthesis methodology for (NH4)3ZnCl5 with the addition of 2% red carbon.

2.3. Characterizations

FTIR spectra were obtained with an IRSPIRIT spectrometer (Shimadzu, Kyoto, Japan) using an ATR module (QATR-S). X-ray diffraction (XRD) data were obtained with a Shimadzu diffractometer (XRD6000) to evaluate changes in the crystal structure after alkaline exfoliation. Rietveld refinement was performed to determine the crystal structure from X-ray diffraction data, from which it was possible to obtain detailed structural parameters and quantification of the crystalline phases present in the sample. All adjustments were conducted using FullProf software, version 2023. The morphology of the samples was observed using a Tescan (Brno, Czechia) scanning electron microscope (model VEGA3 LMH). Energy-dispersive X-ray Spectroscopy (EDS), coupled to a microscope, was used for elemental composition and mapping. Reflectance diffuse data were obtained using a spectrometer UV-Vis Varian Cary 50 using a Diffuse-Reflection Accessory DiffusIR (PIKE). Stationary fluorescence analyses were performed on a PerkinElmer (Waltham, MA, USA) EnSpire spectrofluorometer with a slit width of 10 nm, using a Brandplates-pureGrade white flat-bottom plate with 96 wells.
The cyclic voltammograms were obtained with a Metrohm Autolab PGSTAT 128N potentiostat/galvanotate (Herisau, Switzerland) at room temperature using a conventional three electrode arrangement with a Pt wire counter electrode and a saturated Ag/AgCl reference electrode. The glass carbon (Ø 0.9 mm) was used as a working electrode. The electrochemical measurements were obtained for a 0.01 mgmL−1 red carbon solution within a potential window of −1.4 and 2.0 V with a scan speed of 50 mVs−1. A 0.1 M MgClO4 (Aldrich, St. Louis, MO, USA) solution prepared in acetonitrile was used to support the electrolyte, previously purged with nitrogen for 15 min. For the electrochemical characterization of the perovskite-based material, a paste was prepared combining the solid and magnesium perchlorate solution. The mixture was stirred until a homogeneous consistency was achieved and then applied to the surface of a glassy carbon electrode, which had been previously cleaned with alumina and dried with nitrogen gas. Cyclic voltammetry was performed at a scan rate of 50 mV/s, with a potential range from −2 V to 2 V. Based on work published elsewhere [7,27], the energy level relative to the vacuum level for the valence and conduction bands was determined using Equations (1) and (2)
E V B = e ( E O x + 4.64 )
E C B = E V B + E B G
where EVB is the valence band energy, ECB is the conduction band energy, and EOx is the onset of oxidation potential. EBG is the electrochemical or optical band-gap. The optical band gap was estimated using a Tauc plot.

3. Results

Figure 1 shows the synthetic route used in the preparation of the materials studied in this work.

3.1. Red Carbon Characterization

Red carbon is formed from the polymerization of carbon suboxide (C3O2) in a relatively simple procedure, recently developed by Odziomek et al. [16]. In this alternative procedure to the use P2O5, the dehydration of malonic acid is carried out using acetic anhydride (Figure 1A). During the reaction, acetic anhydride reacts with a molecule of malonic acid, releasing water molecules (dehydration) to form an intermediate structure containing four carbonyls. The suboxide carbon monomer is then formed from a structural rearrangement of this intermediate by releasing two molecules of acetic acid for each monomer unit (C3O2), which polymerizes spontaneously into highly conjugated light-absorbing structures [16]. Figure S1 shows digital images of red carbon suspensions (0.06 mgmL−1) in different solvents under ambient and UV light.
Figure 2A shows the X-ray diffractogram obtained for the product of the reaction between malonic acid and acetic anhydride. The diffraction pattern obtained for the precursor malonic acid is shown in Figure S2. The observed profile presents three regions of intense diffractions, centered respectively at 19.5, 25.1 and 33.2 2 θ . The diffraction peak observed at 44.1 2 θ originates from the diffraction of the aluminum sample holder. The peak at 25.1 2 θ , corresponding to an interplanar distance of 3.5 Å, is attributed to the spacing of the polypyrrone chains, with the shoulder observed at 22.9 2 θ , possibly due to the presence of structures with different degrees of crystallinity. The other diffraction peaks are associated with the interplanar distances of the molecular structures that form the polymer chains of the semiconductor.
The FTIR spectrum shown in Figure 2B presents the characteristic absorption bands of the functional groups present in the polymeric structure of red carbon. The presence of cyclic esters is confirmed by the band centered at 1736 cm−1, characteristic of the C=O stretching, in addition to the absorptions observed at 1170 and 1130 cm−1, characteristic of C-O stretching. The presence of C=C bonds is confirmed by the absorption at 1611 cm−1 and 1520 cm−1. Although the intense absorption of the band centered at 796 cm−1 is still little discussed, with some authors attributing it to the presence of ether-like functional groups at the edges of graphene structures [3,8], we believe that the absorption is associated with the vibrations of the pyrone-like units, which form the polymeric structure of the material, similarly to the triazine rings observed in graphitic carbon nitrides.
Figure 2C shows the image of red carbon in a solid state and its dispersion in acetonitrile. Under ambient light, the red carbon solution in acetonitrile appears reddish-yellow in color, while under UV light ( λ m a x = 360 nm), the solution exhibits strong emission in the green region of the electromagnetic spectrum (inset in Figure 2C). On the other hand, in water and DMF, the red carbon dispersion emits in the blue and yellow regions, respectively (Figure S1).
The UV-Vis spectrum of the red carbon obtained by diffuse reflectance of the solid sample presents an optical maximum absorption around the 400 nm region that extends to ~700 nm. On the other hand, the spectrum of the sample in acetonitrile shows an absorption starting at 600 nm. The bathochromic shift observed in the spectrum of the solid sample has been attributed to the presence of intermolecular interactions of the material’s oligomeric structures [16].
According to the molecular structure, the polymer presents two main chromophores, C=C (π-π transitions) and C=O (π-π and n-π transitions), in a broad conjugated system, which justifies its high absorption in the visible region of the spectrum (Figure 2D). Due to the characteristics of the π-π transitions that are allowed (unlike the n-π, which are forbidden by symmetry) [28], therefore being more intense and that undergo a more accentuated bathochromic shift, the spectrum does not present these band distinctions. From the TAUC plot, the band gap was estimated at 1.95 eV (Figure S3).
The electronic band structure of the material was investigated using cyclic voltammetry. Figure 3 shows a typical cyclic voltammogram obtained for the red carbon solution with a scan rate of 50 mVs−1. The voltammogram shows the presence of well-defined redox peaks at 1.54 V and −0.70 V corresponding to the oxidation and reduction processes of the material structure, respectively. The onsets of oxidation (EOx) and reduction (ERed) potentials were estimated at 1.34 and −0.29 V, which corresponds to an electrochemical band gap of ~1.63 eV. From these results, the energy relative to the vacuum level of the top of the valence band and bottom of the conduction band was estimated at −5.98 and −4.35 eV, respectively.

3.2. Characterization of (NH4)3ZnCl5 Perovskite@Red Carbon Heterostructures

Figure 4A,B show the crystallization products of solutions containing NH4Cl and ZnCl2 in the absence and presence of red carbon. In the absence of organic structures, the formation of crystals with a well-defined needle-like morphology of millimeters in length is observed. On the other hand, the crystallization of the perovskite halide in the presence of red carbon does not present a well-defined pattern indicating the interaction of the organic semiconductor with the ternary phase during the crystallization process, which is confirmed by the yellowish coloration of the crystals to the detriment of the translucent needles observed for the pure crystals.
The yellowish coloration can be better verified in Figure S4, which shows images of the suspensions of both materials in acetonitrile after two minutes in an ultrasound bath.
The X-ray diffractogram is consistent with the crystalline phase (NH4)3ZnCl5 (Card number 30–69). The Rietveld refinement results revealed significant information about the composition of the sample prepared with NH4Cl and ZnCl2. The diffraction patterns obtained for these precursor reagents are shown in Figure S2. The qualitative analysis performed with the Search-Match 2.0.2.0 and X’Pert HighScore Plus 3.0 software indicated the presence of the (NH4)2ZnCl4 and (NH4)3ZnCl5 phases. These results highlight that (NH4)3ZnCl5 is the predominant phase, while (NH4)2ZnCl4 is only a residual byproduct. The predominance of (NH4)3ZnCl5 suggests that the experimental conditions, such as temperature and concentration of the reagents, favored the formation of this main phase over the other observed phase. The X-ray diffraction pattern obtained for the (NH4)3ZnCl5 sample attests to the excellent crystallinity of this material. The Rietveld method reveals a structure indexed in an ortorrombic lattice (Pnma space group) with lattice parameters a = 7.21 Å, b = 12.74(4) Å, c = 9.28 Å, and α = β = γ = 90° [29]. (NH4)3ZnCl5 crystallizes in an orthorhombic system (space group Pnma) and is isostructural with various A3BX5-type halides, such as (NH4)3ZnBr5 and CsZnI5. Its structure features isolated [ZnCl4]2− tetrahedra and additional chloride ions, with NH4+ ions occupying interstitial sites and stabilizing the framework via hydrogen bonding. The Zn2+ center is tetrahedrally coordinated by Cl ions. This structural arrangement is distinct from tetragonal variants like Cs3CoCl5. The compound forms preferentially in ammonium-chloride-rich conditions, and X-ray diffraction is used to distinguish it from similar phases. Intermediate stoichiometries may yield mixed phases containing both (NH4)2ZnCl4 and (NH4)3ZnCl5 [30,31].
To examine the ratios of the elements, energy dispersive spectroscopy (EDS) analyses were carried out on a single crystal, and the results obtained (see Supplementary Information, Figure S5, Table S1) are consistent with the stoichiometric ratio of the compound (NH4)3ZnCl5, which presents 18, 22, and 60 wt% for NH4+, Zn, and Cl, respectively.
EDS element-mapping was performed to prove the uniformity of the bulk crystalline of (NH4)3ZnCl5 and confirm the presence of the organic structures in the material composition of (NH4)3ZnCl5@red carbon (Figure 5).
The FTIR analysis obtained for (NH4)3ZnCl5 crystals is typical of this crystalline phase with the presence of stretching and deformation bands of the ammonium ion at 3190, 2975, 2788, and 1380 cm−1 [32]. The spectrum of the sample crystallized in the presence of red carbon is predominantly of the (NH4)3ZnCl5 structure, but the bands at 1160 and principally at 1734 cm−1 confirm the presence of the polymeric semiconductor associated with the perovskite structure.
The optical properties of the (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon were investigated via UV–vis spectroscopy. Figure 6 shows the electronic spectra obtained for these materials. For the (NH4)3ZnCl5 sample, absorption begins at 400 nm with a direct band gap, estimated from the Tauc plot, of 3.6 eV. On the other hand, in the composite material, absorption begins at 700 nm, with a strong intensity from 500 nm with a maximum at approximately 386 nm. This result is similar to that observed for red carbon, but with small shifts in wavelengths due to the interaction with the (NH4)3ZnCl5 structure. The band gap was estimated at 2.7 eV. The intermediate value between 1.95 to 3.6 eV, observed in the composite band structure coupling, enabling electron excitation from the valence band of one material to the conduction band of the other.
From Equations (1) and (2), the band structure of (NH4)3ZnCl5 was determined. The energy of the conduction band top was estimated to be −2.57, while the valence band bottom was calculated to be −6.17 eV (Figure S6). According to the energy band alignments of the semiconductors involved, the inorganic semiconductor (NH4)3ZnCl5, and the organic semiconductor red carbon, the results indicate the formation of a type I heterostructure, in which the conduction band bottom (CB) and valence band bottom (VB) of the inorganic semiconductor are larger and smaller, respectively, than those observed for the organic semiconductor, which presents a narrow band gap (Figure 6B). Type I heterojunctions, both holes and electrons generated in semiconductor A (NH4)3ZnCl5 will move to semiconductor B (red carbon). Then, it can recombine within semiconductor B or at the interface, effectively improving the luminescence properties and performance [33].
The photoluminescence spectra obtained for the samples studied are shown in Figure 7. The photoluminescence spectrum of the (NH4)3ZnCl5 sample shows the presence of two peaks, one more intense at 435 nm (2.85 eV) and another with lower intensity at 495 nm (2.50 eV). The presence of two peaks for perovskite-based structures has been associated with different factors including the presence of two components [34], coexistence of radiative direct and indirect transitions, strain-induced defects in the bulk, surface defect states, and optical effects caused from an extensive self-absorption effect [35] and is normally observed in quasi-2D structures [36]. For red carbon, as observed for other carbon nanostructures, for example, carbon dots, the different emission peaks may be associated with differences in sizes, crystallinity, and compositions of the structures that make up the sample or even surface defects. In the spectrum of the composite material (red carbon@(NH4)3ZnCl5), a significant increase in the intensity of the emissions is observed. The increase in intensity corroborates the interpretation regarding the formation of a type I heterostructure. It is also possible to observe a significant broadening of the emission bands, especially in the band located in the visible region of the electromagnetic spectrum, with a significant increase in emission at wavelengths greater than 500 nm. In the nominalized spectrum (Figure 7B) of the heterostructure, an inversion of the relative intensities between peaks centered at 439 and 496 nm is also observed due to the contribution of the emission originating in the red carbon structure.
Finally, in addition to the structural and optical characterization presented, the red carbon@(NH4)3ZnCl5 hybrid heterostructure demonstrates properties that make it a promising candidate for practical applications, such as environmental monitoring, food quality control, and biomedical diagnostics. The formation of a type I heterojunction and the observed enhancement in photoluminescence suggest strong potential for the development of optoelectronic devices—particularly sensors that rely on changes in the electrochemical or optical response of the material upon interaction with target analytes.

4. Conclusions

The (NH4)3ZnCl5 structure was successfully crystallized in the presence of red carbon, a semiconductor organic polymer derived from the suboxide molecule (C3O2). The red carbon was characterized using different techniques, and its band structure was determined from cyclic voltammetry measurements in an organic electrolyte support. On the other hand, the obtained composite presented significant changes in optical properties with a red shift in absorption and a significant increase in emissions in the visible region of the electromagnetic spectrum due to the formation of a type I heterostructure, which has potential applications in the development of different optical devices. Considering the energies of the conduction band bottom (CB) and valence band bottom (VB), red carbon could also be considered an alternative for transporting holes for traditional hybrid metal halide structures in the development of optoelectronic devices. Thus, the incorporation of red carbon represents a promising strategy for the improvement and development of next-generation optoelectronic devices for a wide range of applications, including sensors and light-emitting devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/compounds5020021/s1, Figure S1: (A) Red carbon powder. (B) Digital images of red carbon suspensions (0.06 mgmL−1) in different solvents under ambient (B) and UV light (C), Figure S2: X-ray diffractograms obtained for the precursor reagents used in the synthesis of red carbon (malonic acid) and (NH4)3ZnCl5 (zinc chloride, PDF card 39-887, and ammonium chloride, PDF card 7-7), Figure S3: The Tauc plots obtained for red carbon for the direct transition, Figure S4: Images of (NH4)3ZnCl5 (white) (NH4)3ZnCl5@red carbon (yellow) suspensions, Figure S5: Energy dispersive X-ray spectroscopy (EDS) spectra of (NH4)3ZnCl5. The measured atomic percentages of the main elements (Zn, N, Cl) in the perovskite are summarized in Table S1, Figure S6: Cyclic voltammograms obtained for (NH4)3ZnCl5 anchored on a surface glass carbon electrode in a supporting electrolyte of 0.1 M magnesium perchlorate/acetonitrile; Table S1: EDS elemental analysis of the (NH4)3ZnCl5 sample.

Author Contributions

Methodology, W.V.F.d.C.B., A.P.d.S., T.d.S.C., D.M.P., D.G.S.d.A., W.L.d.O., D.R.C.F., M.C.P. and R.A.d.R.F.; Formal analysis, W.V.F.d.C.B., T.d.S.C., W.L.d.O. and D.R.C.F.; Investigation, W.V.F.d.C.B., A.P.d.S., T.d.S.C., D.M.P., S.K.N.O., W.L.d.O., D.R.C.F., M.C.P. and R.A.d.R.F.; Writing—original draft, D.M.P., D.G.S.d.A., S.K.N.O., W.L.d.O., M.C.P. and J.P.d.M.; Supervision, J.P.d.M.; Funding acquisition, J.P.d.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES 001) and by FAPEMIG through grants APQ-02754-24 and APQ-02629-17.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

J.P.d.M. acknowledge grants from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, 102190/2022-4). The authors gratefully acknowledge UFVJM for institutional support and the technical assistance provided by MULTIFART and LMMA through the FAPEMIG projects APQ-00370-22 and APQ-003088-21.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Simplified reaction mechanism based on activation of malonic acid by acetic anhydride for obtained red carbon. (B) Schematic illustration of the steps to obtain (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon.
Figure 1. (A) Simplified reaction mechanism based on activation of malonic acid by acetic anhydride for obtained red carbon. (B) Schematic illustration of the steps to obtain (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon.
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Figure 2. (A) XRD pattern and (B) FTIR spectra obtained for red carbon. (C) Image of the material in powder form and in acetonitrile solution under ambient light (top) and exposed to a UV lamp (bottom). (D) UV-Vis spectrum obtained for the solid material and in an acetonitrile solution (0.06 mgmL−1). Inset: magnification of the region located between 340 and 460 nm of the spectrum obtained for the solution.
Figure 2. (A) XRD pattern and (B) FTIR spectra obtained for red carbon. (C) Image of the material in powder form and in acetonitrile solution under ambient light (top) and exposed to a UV lamp (bottom). (D) UV-Vis spectrum obtained for the solid material and in an acetonitrile solution (0.06 mgmL−1). Inset: magnification of the region located between 340 and 460 nm of the spectrum obtained for the solution.
Compounds 05 00021 g002
Figure 3. Cyclic voltammogram obtained for a solution of 0.01 mgmL−1 of “red carbon” dispersed in a supporting electrolyte of 0.1 M magnesium perchlorate/acetonitrile. The scan started at 0 V in the cathodic direction with a scan speed of 50 mVs−1.
Figure 3. Cyclic voltammogram obtained for a solution of 0.01 mgmL−1 of “red carbon” dispersed in a supporting electrolyte of 0.1 M magnesium perchlorate/acetonitrile. The scan started at 0 V in the cathodic direction with a scan speed of 50 mVs−1.
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Figure 4. Digital images of (NH4)3ZnCl5 crystals in the absence (A) and presence (B) of red carbon. (C) XRD pattern with Rietveld refinements and (D) FTIR spectra obtained for potassium (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon.
Figure 4. Digital images of (NH4)3ZnCl5 crystals in the absence (A) and presence (B) of red carbon. (C) XRD pattern with Rietveld refinements and (D) FTIR spectra obtained for potassium (NH4)3ZnCl5 and (NH4)3ZnCl5@red carbon.
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Figure 5. EDS element-mapping images of (NH4)3ZnCl5@red carbon. (A) SEM image, (BF) elemental distributions, zinc (red), oxygen (violet), nitrogen (green), chlorine (yellow), and carbon (cyan).
Figure 5. EDS element-mapping images of (NH4)3ZnCl5@red carbon. (A) SEM image, (BF) elemental distributions, zinc (red), oxygen (violet), nitrogen (green), chlorine (yellow), and carbon (cyan).
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Figure 6. (A) UV–Vis spectra obtained for (NH4)3ZnCl5 and red carbon@(NH4)3ZnCl5. The absorbance values were obtained using the Kubelka–Munk function from the normalized reflectance data. (B) Valence band top and conduction band bottom energies estimated for (NH4)3ZnCl5 and red carbon from electrochemical UV-Vis spectroscopy measurements.
Figure 6. (A) UV–Vis spectra obtained for (NH4)3ZnCl5 and red carbon@(NH4)3ZnCl5. The absorbance values were obtained using the Kubelka–Munk function from the normalized reflectance data. (B) Valence band top and conduction band bottom energies estimated for (NH4)3ZnCl5 and red carbon from electrochemical UV-Vis spectroscopy measurements.
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Figure 7. (A) Photoluminescence spectra obtained for (NH4)3ZnCl5, red carbon@(NH4)3ZnCl5, and red carbon with a 250 nm excitation. (B) Normalized results.
Figure 7. (A) Photoluminescence spectra obtained for (NH4)3ZnCl5, red carbon@(NH4)3ZnCl5, and red carbon with a 250 nm excitation. (B) Normalized results.
Compounds 05 00021 g007
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Batista, W.V.F.d.C.; de Souza, A.P.; Cruz, T.d.S.; Pimentel, D.M.; de Avelar, D.G.S.; Oliveira, S.K.N.; de Oliveira, W.L.; Ferreira, D.R.C.; Pereira, M.C.; Ferreira, R.A.d.R.; et al. Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures. Compounds 2025, 5, 21. https://doi.org/10.3390/compounds5020021

AMA Style

Batista WVFdC, de Souza AP, Cruz TdS, Pimentel DM, de Avelar DGS, Oliveira SKN, de Oliveira WL, Ferreira DRC, Pereira MC, Ferreira RAdR, et al. Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures. Compounds. 2025; 5(2):21. https://doi.org/10.3390/compounds5020021

Chicago/Turabian Style

Batista, Walker Vinícius Ferreira do Carmo, Aniely Pereira de Souza, Tais dos Santos Cruz, Dilton Martins Pimentel, Danila Graziele Silva de Avelar, Sarah Karoline Natalino Oliveira, Wanessa Lima de Oliveira, Danilo Roberto Carvalho Ferreira, Márcio Cesar Pereira, Rondinele Alberto dos Reis Ferreira, and et al. 2025. "Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures" Compounds 5, no. 2: 21. https://doi.org/10.3390/compounds5020021

APA Style

Batista, W. V. F. d. C., de Souza, A. P., Cruz, T. d. S., Pimentel, D. M., de Avelar, D. G. S., Oliveira, S. K. N., de Oliveira, W. L., Ferreira, D. R. C., Pereira, M. C., Ferreira, R. A. d. R., & de Mesquita, J. P. (2025). Synthesis and Optical Properties of Red Carbon@(NH4)3ZnCl5 Hybrid Heterostructures. Compounds, 5(2), 21. https://doi.org/10.3390/compounds5020021

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