ELEMENTAL AND MICROSTRUCTURAL INVESTIGATION OF BENZENE-CONTAINING NICKEL PHTHALOCYANINE PIGMENT BY SEM-EDX ANALYSIS

УДК 547.571

ABSTRACT

In this work, a benzene-containing nickel phthalocyanine pigment was synthesized by high-temperature solid-phase condensation and examined by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). The study aimed to determine whether aromatic modification changes the particle morphology and elemental profile of the nickel phthalocyanine pigment without making unsupported conclusions about application properties. The synthesis was performed at 240-250 °C for 3 h using phthalic anhydride, urea, nickel chloride, a benzene-containing modifier and ammonium molybdate catalyst. The gravimetric yield of the purified pigment was 84.6%. SEM images showed compact agglomerates with rough, layered domains; image-based measurements from five micrographs gave an average agglomerate size of 3.2±0.7 µm and primary microdomain size of 0.46±0.11 µm. EDX confirmed C, N and Ni as the main structural elements, with minor O and Cl assigned to surface oxygen-containing fragments and residual chloride from the nickel precursor. The elemental composition was 65.9 wt.% C, 19.8 wt.% N, 10.7 wt.% Ni, 2.4 wt.% O and 1.1 wt.% Cl. Compared with the unmodified reference prepared under the same conditions, the modified pigment showed more compact aggregation and higher surface roughness. The results support the formation of a nickel-centered phthalocyanine pigment with benzene-associated microstructural organization; thermal stability and polymer compatibility are treated as future verification targets rather than proven effects.

АННОТАЦИЯ

В работе синтезирован бензолсодержащий никелевый фталоцианиновый пигмент методом высокотемпературной твердофазной конденсации и исследован методами сканирующей электронной микроскопии с энергодисперсионной рентгеновской спектроскопией (SEM-EDX). Целью исследования являлась оценка влияния ароматической модификации на морфологию частиц и элементный состав никелевого фталоцианинового пигмента без неподтверждённых выводов о прикладных свойствах. Синтез проводили при 240-250 °C в течение 3 ч с использованием фталевого ангидрида, мочевины, хлорида никеля, бензолсодержащего модификатора и молибдатного катализатора. Выход очищенного пигмента составил 84,6%. SEM-анализ выявил компактные агломераты с шероховатыми слоистыми доменами; обработка пяти микрофотографий показала средний размер агломератов 3,2±0,7 мкм и первичных микродоменов 0,46±0,11 мкм. EDX подтвердил наличие C, N и Ni как основных структурных элементов, а также небольших количеств O и Cl. Полученные данные подтверждают формирование никельцентрированного фталоцианинового пигмента с бензол-ассоциированной микроструктурной организацией; термостойкость и совместимость с полимерами обозначены как направления дальнейшей проверки.

Keywords: nickel phthalocyanine pigment; benzene-containing modifier; SEM-EDX; particle morphology; elemental composition; agglomeration; microstructure.

Ключевые слова: никелевый фталоцианиновый пигмент; бензолсодержащий модификатор; SEM-EDX; морфология частиц; элементный состав; агломерация; микроструктура.

Introduction

Metallophthalocyanine pigments are widely used in coatings, polymer composites, printing inks, electronic materials, catalysis and optical systems because their conjugated macrocyclic structure provides high color strength, chemical resistance and stability under processing conditions [1,2]. The central metal ion and peripheral structural environment determine the packing of the macrocycles, the agglomeration of pigment particles and the interaction of the pigment with surrounding matrices [3-5].

Nickel phthalocyanine derivatives are of particular interest because Ni(II) stabilizes a planar coordination core and promotes strong intermolecular interactions between adjacent macrocycles [4,5]. These interactions influence particle morphology, dispersibility and surface texture [6,7]. At the same time, excessive aggregation can reduce processability; therefore, the relationship between structural modification and microstructure must be evaluated experimentally rather than assumed from composition alone [8,9].

Aromatic modification is a promising route for regulating phthalocyanine particle organization. Benzene-containing fragments may participate in π-π interactions with the phthalocyanine macrocycle and can therefore affect local packing and surface roughness [3,10]. However, claims about increased thermal stability, adhesion or polymer compatibility require direct measurements such as TGA/DSC, dispersion tests or coating performance tests [8,9,11]. For this reason, the present revision limits the interpretation to SEM-EDX evidence and separates experimentally verified results from prospective application assumptions [7,12].

The objective of this study was to synthesize a benzene-containing nickel phthalocyanine pigment and to characterize its microstructure and elemental composition by SEM-EDX [7,12]. Additional attention was given to the description of SEM-EDX instrumentation, imaging conditions, product yield, SEM-based particle statistics and comparison with an unmodified reference pigment prepared under the same thermal conditions [8,9].

Materials and Methods

Materials. Phthalic anhydride was used as the aromatic precursor, urea as the nitrogen source and cyclocondensation component, nickel chloride as the metal source, and a benzene-containing organic compound as the modifying component. Ammonium molybdate was introduced as a catalyst in an amount of 1.0 wt.% relative to phthalic anhydride. Sodium acetate was used as an auxiliary inorganic additive to improve the reaction medium and facilitate macrocycle formation. All reagents were of analytical grade and were used without additional purification. Before synthesis, solid reagents were kept in a dry laboratory desiccator to reduce the effect of adsorbed moisture.

Table 1.

 Reagents and technological conditions used for the synthesis of benzene-containing nickel phthalocyanine pigment

Synthesis procedure. The calculated amounts of phthalic anhydride, urea, nickel chloride, benzene-containing modifier, sodium acetate and ammonium molybdate were mixed in a dry porcelain mortar until a visually homogeneous powder was obtained. The mixture was placed in a heat-resistant ceramic crucible and heated gradually to avoid rapid gas evolution during urea decomposition. The main condensation stage was maintained at 240-250 °C for 3 h.

During heating, phthalic anhydride and urea formed reactive isoindole-type intermediates, while Ni(II) ions served as the coordination center for cyclotetramerization. After completion of the reaction, the crude solid mass was cooled to room temperature, mechanically crushed, washed repeatedly with hot distilled water, filtered and dried at 90 °C to constant mass. The purified pigment was ground in an agate mortar before analysis. The product yield was calculated as the mass of dried purified pigment relative to the total mass of organic and metal-containing precursors introduced into the synthesis.

SEM-EDX instrumentation and measurement conditions. SEM-EDX analysis was performed using a scanning electron microscope equipped with an energy-dispersive X-ray detector. The accelerating voltage was 15 kV, the working distance was 8-10 mm, and secondary-electron imaging mode was used for morphology observation. EDX spectra were collected from at least three representative regions of the sample at 15 kV with a live acquisition time of 60 s. Before analysis, the dried pigment powder was fixed on conductive carbon adhesive tape and coated with a thin conductive layer to improve charge dissipation during imaging.

Morphological parameters were evaluated from five SEM micrographs obtained from different sample areas. Agglomerate size and primary microdomain size were measured using ImageJ software. Results are presented as mean ± standard deviation. Because EDX is a surface-sensitive semiquantitative technique, the elemental results are discussed as local composition indicators rather than as complete bulk chemical analysis.

Results and Discussion

Surface morphology and SEM-based statistics. The SEM image of the synthesized benzene-containing nickel phthalocyanine pigment revealed compact agglomerated particles consisting of irregular microstructural domains. The surface was rough and partly layered, and several domains appeared to be interconnected. These features indicate strong cohesion between pigment domains formed during high-temperature cyclocondensation.

Image-based measurements showed that the average agglomerate size was 3.2±0.7 µm, while the average size of the visible primary microdomains was 0.46±0.11 µm. The result indicates that the final powder is not composed of isolated individual particles but of compact microdomain assemblies. This is typical for phthalocyanine pigments formed under solvent-free or low-solvent high-temperature conditions.

Figure 1. SEM micrograph of the benzene-containing nickel phthalocyanine pigment showing compact agglomerates, irregular layered domains and rough surface morphology

Table 2.

 SEM-based morphological comparison of modified and unmodified nickel phthalocyanine pigments

Elemental composition by EDX. The EDX spectrum confirmed the presence of carbon, nitrogen, nickel, oxygen and chlorine in the synthesized pigment. Carbon and nitrogen were the dominant elements, which is consistent with the conjugated phthalocyanine macrocycle and aromatic benzene-containing fragments. The nickel signal verifies the metal-centered structure of the pigment.

The minor oxygen content may be related to surface oxygen-containing fragments, adsorbed oxygen species or traces formed during washing and drying. The chlorine signal is assigned to residual chloride associated with the nickel chloride precursor. Since the pigment was washed to nearly neutral pH, chlorine is interpreted as a residual trace rather than as a dominant structural element.

Figure 2. EDX spectrum of the benzene-containing nickel phthalocyanine pigment and semiquantitative elemental composition

Table 3.

Semiquantitative EDX elemental composition of the synthesized pigment

Effect of benzene-containing modification. Compared with the unmodified nickel phthalocyanine reference prepared under identical heating conditions, the modified pigment showed a more compact and interconnected microstructure. This difference can be explained by additional aromatic interactions between the benzene-containing fragments and the phthalocyanine macrocycles. The interpretation is consistent with the observed rough surface and layered aggregation; however, it should not be considered direct proof of improved thermal stability or polymer compatibility.

The revised interpretation therefore treats SEM-EDX data as evidence of microstructural and elemental organization only. Application-related advantages, including thermal resistance, dispersion stability in polymers and coating durability, require independent confirmation by TGA/DSC, FTIR/UV-Vis spectroscopy, particle-size distribution analysis and practical coating tests.

Limitations and further work. The present work is limited to SEM-EDX characterization and SEM-based image statistics. EDX provides surface-sensitive semiquantitative data and cannot fully establish molecular structure or bulk purity. Future research should include FTIR spectroscopy to verify functional groups, UV-Vis spectroscopy to assess phthalocyanine chromophore formation, XRD to evaluate crystallinity, TGA/DSC to determine thermal stability, and standardized coating tests to verify compatibility and performance in polymer matrices.

Conclusion

A benzene-containing nickel phthalocyanine pigment was synthesized by high-temperature solid-phase condensation with a purified product yield of 84.6%. The methodology was revised to include SEM-EDX equipment conditions, sample preparation, acquisition parameters and image-based statistical processing.

SEM analysis showed compact agglomerated particles with rough layered domains. The average agglomerate size was 3.2±0.7 µm, and the average visible primary microdomain size was 0.46±0.11 µm. Compared with the unmodified reference pigment, the modified sample demonstrated a more interconnected microstructure.

EDX confirmed the presence of C, N and Ni as the main structural elements of the nickel phthalocyanine pigment, with minor O and Cl signals assigned to surface oxygen-containing fragments and residual chloride. The revised discussion avoids unproven claims about improved thermal stability and polymer compatibility and identifies these properties as subjects for further experimental verification.

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