Influence of Peptizing Acid on TiO₂ Phase Composition and Photocatalytic Efficiency: A Comparative Study of Sulfuric, Nitric, and Acetic Acids

Abstract

We synthesized TiO₂ nanoparticles from titanium isopropoxide by a low‑temperature peptization route using sulfuric, nitric, and acetic acids as peptizing agents. Comprehensive XRD, TEM, BET, Raman, UV‑vis, FTIR, and XPS analyses revealed that acetic acid promotes pure anatase formation, nitric acid induces a 3:1 anatase‑rutile mix, and sulfuric acid yields a 95:5 anatase‑rutile ratio. Photocatalytic testing on crystal violet, methylene blue, and p‑nitrophenol demonstrated that the nitric‑acid‑peptized sample delivered the highest degradation rates—exceeding commercial P25 by up to 20 %—thanks to its enhanced electron‑hole separation, larger pore diameter (≈15 nm), and higher photocurrent density (0.545 mA cm⁻²). These findings highlight the critical role of acid type in tailoring TiO₂ nanostructures for advanced environmental remediation.

Background

Titanium dioxide (TiO₂) remains the benchmark semiconductor for photocatalysis, owing to its chemical stability, low cost, and suitable band structure. Anatase, rutile, and brookite are the three polymorphs, with anatase generally considered the most active phase for organic pollutant degradation. Phase composition, crystallite size, and porosity strongly influence photocatalytic performance, yet the impact of peptizing acid chemistry on these parameters has been underexplored. Previous studies have shown that strong acids can suppress or promote the anatase‑to‑rutile transition during hydrothermal or sol‑gel processing. Here, we systematically investigate how sulfuric, nitric, and acetic acids affect TiO₂ phase evolution and subsequent photocatalytic activity.

Materials and Methods

Peptization Procedure

Ti(OR)₄ (isopropoxide) was aged (15 days, 25 °C, 50 % RH) to form an amorphous gel. The xerogel was dispersed in 1 N peptizing acid (100 mL) and sonicated at 40 °C for 10 min. The resulting colloid was centrifuged, washed, and calcined at 500 °C for 3 h. Samples were designated TiO₂‑ace (acetic), TiO₂‑nit (nitric), and TiO₂‑sul (sulfuric).

Characterization

XRD (Cu Kα), TEM, BET (N₂ physisorption), Raman, UV‑vis diffuse reflectance, FTIR, and XPS were employed to determine phase composition, particle size, surface area, optical band gap, and surface chemistry. Phase fractions were calculated via the Spurr–Myers equation, and crystallite sizes by Scherrer’s formula.

Photocatalytic Tests

100 mg of catalyst was suspended in 100 mL of 10 ppm dye solution (CV, MB, or p‑NP). After 45 min adsorption equilibrium, the mixture was irradiated by 13 W m⁻² UV light (8 × F20 lamps). Degradation was monitored by UV‑vis spectroscopy, and removal efficiencies were computed as ξ = (1 – C/C₀) × 100 %. Reusability was assessed over five cycles with simple washing and drying.

Results and Discussion

Phase Analysis

TiO₂‑ace exhibited 100 % anatase (particle size ≈ 48 nm). TiO₂‑sul contained 95 % anatase (≈ 23 nm) with minor rutile (≈ 5 %). TiO₂‑nit showed a 67 % rutile / 33 % anatase mixture (rutile ≈ 50 nm, anatase ≈ 41 nm). The stronger acids (HNO₃, H₂SO₄) favor rutile formation, while the weak acetic acid preserves anatase.

Morphology and Surface Area

TEM revealed that TiO₂‑sul forms dense 7 nm agglomerates, TiO₂‑nit shows 10–20 nm spherical particles plus larger sheets, and TiO₂‑ace consists of 15–20 nm spheres. BET measurements gave 115 m² g⁻¹ (ace), 98 m² g⁻¹ (sul), and 82 m² g⁻¹ (nit). Nitric‑acid peptization produced a slightly lower surface area but larger pore diameter (~15 nm), enhancing reactant diffusion.

Optical Properties

UV‑vis absorption edges shifted from 372 nm (sul) to 383 nm (ace) to 402 nm (nit). Band gaps were 3.12 eV (sul), 2.99 eV (ace), and 2.97 eV (nit). The reduced band gap in nit correlates with the higher rutile content.

Photocatalytic Performance

In 120 min, TiO₂‑nit achieved 99 % degradation for CV and MB, and 95 % for p‑NP, surpassing commercial P25 (≈ 85 % for p‑NP). TiO₂‑ace and TiO₂‑sul reached only 70–80 % for p‑NP. The superior activity of TiO₂‑nit is attributed to efficient inter‑phase charge transfer (rutile CB → anatase CB, anatase VB → rutile VB) and its high photocurrent density (0.545 mA cm⁻²). Reuse experiments showed >90 % activity after three cycles, decreasing to 75 % by the fifth cycle due to minor catalyst loss.

Conclusions

Peptizing acid chemistry decisively controls TiO₂ phase composition, morphology, and photocatalytic behavior. Nitric‑acid peptization yields a mixed anatase–rutile nanostructure with optimal electron‑hole separation, delivering the best dye‑degradation performance among the acids studied. These results provide a straightforward, scalable route to tailor TiO₂ nanocatalysts for environmental applications.