Al2O3:SiOC Nanocomposites from Phenyltrimethoxysilane‑Modified Alumina—Synthesis, Structure, and Photoluminescence

Background

Recent work has shown that silica nanoparticles with carbonized surfaces—generated by pyrolysis of phenylmethoxy groups—exhibit strong visible PL under UV light [1]. Such materials, which emit broad‑band visible light at room temperature without rare‑earth activators, are promising substitutes for expensive rare‑earth‑doped ceramic phosphors in white‑light sources based on compact gas‑discharge lamps or LEDs. Although carbon‑containing SiO₂ (SiO₂:C) has been reported to show broadband visible PL [2–5], the exact nature of the emission centers remains unclear. A prevailing hypothesis attributes these centers to carbon nanoclusters that form on the high‑surface‑area silica matrix [1, 2, 5]. Validating this hypothesis requires detailed PL studies of carbonized surfaces in nanostructured systems. Fumed alumina, with its high specific surface area and robust mechanical and chemical properties, is an attractive template for such investigations [6–11]. In this study we investigate UV‑ and X‑ray‑excited PL in superfine Al₂O₃ powder whose surface has been intentionally carbonized via PhTMS grafting and subsequent thermal treatment.

Methods

The starting material was pyrogenic Al₂O₃ powder (specific surface area 89 m²/g; particle size 30–50 nm). The powder was dispersed in a toluene solution of PhTMS (1.73 mL PhTMS per 10 mL toluene) and refluxed at 70 °C for 4 h in the presence of triethylamine as a catalyst, facilitating the grafting of phenylmethoxy groups onto the alumina surface. The resulting “phenyl‑alumina” was dried and then annealed at 400, 500, or 600 °C for 30 min under a flow of pure nitrogen at atmospheric pressure.

Structural analysis was performed by Fourier‑transform infrared (FTIR) spectroscopy (Bruker Vertex 70 V, transmission mode, KBr pellet, 400–5000 cm⁻¹). PL measurements were carried out with a 290‑nm semiconductor laser (5 mW) for UV excitation and with 13–14 keV X‑rays for high‑energy excitation. UV‑excited PL spectra were recorded on an Edinburgh Instruments LIFESPEC II spectrometer; X‑ray‑excited PL was collected using an MDR‑2 monochromator and a FEP‑106 photomultiplier.

Results and Discussion

IR Spectroscopy

Figure 1 compares the FTIR transmission spectra of pristine alumina and phenyl‑alumina. In the pristine sample, broad absorption bands at 540 and 800 cm⁻¹ arise from Al–O stretching vibrations in tetrahedral and octahedral coordination, respectively [13, 14]. After grafting, additional bands appear: 2800–3000 cm⁻¹ (C(sp³)–H), 3000–3100 cm⁻¹ (C(sp²)–H), and sharp peaks at 1136, 1430, and 1590 cm⁻¹ (C=C in phenyl rings) along with a broad 980–1200 cm⁻¹ band centered at 1033 cm⁻¹ indicating siloxane network formation [16, 17].

Upon annealing, the phenyl‑related bands diminish progressively (Fig. 2). At 400 °C, the C=C vibrations are largely suppressed, and the siloxane band shifts from 1033 to 1070 cm⁻¹, signalling a transition from polymeric to ceramic silica structure. A shoulder near 450–460 cm⁻¹ (Si–O–Si rocking) appears, confirming silica network formation. The residual phenyl signatures persist even at 600 °C, yet no amorphous pyrolytic carbon band (~1600 cm⁻¹) is observed, suggesting that carbon diffuses into the Al₂O₃ lattice during annealing [18].

Photoluminescence

The as‑prepared alumina shows a weak broad PL band (300–600 nm) under 290 nm excitation, comprising peaks near 335, 390–400, and 470 nm. These features are attributed to F⁺ (oxygen‑vacancy electron), P⁻ (anion‑cation vacancy pair), and F₂ centers, respectively [9, 19–20]. After PhTMS grafting, a prominent UV band at 340 nm emerges, likely due to excimer states of densely grafted phenyl groups [21–23]. Annealing removes this band as phenyl groups decompose. However, a new visible PL component develops, with integrated intensity peaking at 500 °C. The 410 nm peak and 500 nm shoulder remain stable across 400–600 °C, suggesting emission from silica surface or carbonized silica structures, consistent with the FTIR evidence of SiO₂ formation.

X‑ray‑excited PL at 90 K reveals a narrow green band (~550 nm) only in the 400 °C annealed sample, attributable to self‑trapped excitons in the newly formed silica network [24]. This band is absent in pristine alumina and the ungrafted sample, reinforcing the role of silica precipitation in the visible PL enhancement.

Conclusions

We have synthesized Al₂O₃:SiOC nanocomposites by grafting phenyltrimethoxysilane onto fumed alumina and annealing at 400–600 °C. Hydroxyl groups on the alumina surface are replaced by phenylsiloxane linkages, which upon pyrolysis form silica precipitates on the particle surface. No carbon precipitation is detected after the organosilicon groups decompose. The evolution of the visible photoluminescence spectrum is directly linked to the formation of carbon‑rich silica on the alumina surface, offering a pathway to tunable PL in Al₂O₃‑based nanocomposites.

Abbreviations

Al₂O₃:C:

Carbon‑doped aluminum oxide

Al₂O₃:SiOC:

Alumina/organosilicon nanocomposite

F⁺-center:

Oxygen vacancy with trapped electron

F₂-center:

Two adjacent F‑centers

F-center:

Oxygen vacancy with two trapped electrons

F_S ⁺-center:

Surface analog of F⁺-center

FTIR:

Fourier transform infrared

IR:

Infrared

P⁻-center:

Anion‑cation vacancy pairs

PhTMS:

Phenyltrimethoxysilane

SiO₂:C:

Carbonized nanocomposite materials based on silica