Enhanced Photocatalytic Degradation of Methyl Orange Using CdTe Quantum Dot/BiOI‑Coated TiO₂ Hollow Microspheres under Simulated Sunlight

Abstract

Hollow, heterostructured nanocomposites are a proven strategy for boosting photocatalytic activity. We engineered a ternary TiO₂/CdTe/BiOI system in which TiO₂ hollow microspheres serve as the scaffold, CdTe quantum dots (QDs) act as electron donors, and BiOI flakes provide visible‑light harvesting and charge‑separation enhancement. The resulting TiO₂(H)/CdTe/BiOI composite achieved 99.7 % degradation of methyl orange (MO) within 90 min under simulated sunlight, surpassing all binary and solid‑sphere counterparts. The performance gains are attributed to synergistic light absorption, increased specific surface area, and efficient interfacial charge transfer, as confirmed by XRD, SEM/TEM, BET, XPS, UV‑Vis, Mott–Schottky, PL, and photocurrent measurements. This work demonstrates a scalable route to high‑performance, environmentally benign photocatalysts for water purification.

Background

Semiconductor photocatalysis offers a promising, green route for environmental remediation, including self‑cleaning surfaces, water treatment, and air purification. TiO₂ is the benchmark material owing to its low cost, stability, and nontoxicity, yet its wide bandgap limits activity to the ultraviolet (UV) region and promotes rapid charge recombination. Strategies to extend the light response into the visible spectrum—ion doping, noble‑metal loading, heterojunctions, and sensitization—have been explored. Among them, heterojunctions with narrow‑bandgap partners are especially effective in enhancing visible‑light absorption while suppressing recombination.

Bismuth oxyhalides, particularly BiOI (bandgap 1.72–1.9 eV), have emerged as attractive partners due to their layered structure, stability, and intrinsic internal electric field that facilitates charge separation. When coupled with TiO₂, a type‑II heterojunction is expected to transfer electrons from BiOI to TiO₂, thereby expanding the light‑harvesting window. However, binary systems still suffer from incomplete charge separation and limited active surface area.

Cadmium telluride (CdTe) is a p‑type II‑VI semiconductor with a direct bandgap of 1.44 eV and high absorption coefficient, making it an excellent sensitizer. Prior work has shown CdTe/TiO₂ composites to outperform bare TiO₂ in degrading organic pollutants. Integrating CdTe QDs with BiOI on TiO₂ creates a ternary heterostructure that could combine the strengths of each component, yet systematic studies remain scarce.

In this study, we synthesized CdTe QDs/BiOI‑modified TiO₂ microspheres using a two‑step approach and compared solid and hollow TiO₂ scaffolds. We examined how the scaffold morphology and the energy band alignment influence visible‑light photocatalytic degradation of MO.

Methods

Materials

All reagents were analytical grade and used as received: titanium isopropoxide (TTIP, 97 %), bismuth nitrate pentahydrate (99 %), cadmium chloride dihydrate (99 %), sodium tellurite (98 %), N‑acetyl‑L‑cysteine (98 %), potassium borohydride (97 %), NaOH, KNO₃, KBr, HCl, H₂O₂, ethylene glycol, and ethanol.

Synthesis of TiO₂ Solid and Hollow Microspheres

TiO₂ solid microspheres (TiO₂(S)) were prepared by adding TTIP to a KNO₃/ethanol mixture, aging for 12 h, and calcining at 450 °C for 2 h. Hollow microspheres (TiO₂(H)) were obtained via a hydrothermal route: TiO₂·nH₂O was treated with H₂O₂ and NaOH, then heated at 180 °C for 4 h, followed by acid etching, washing, drying, and calcination identical to TiO₂(S).

Decoration with CdTe QDs

TiO₂ powders (2 g) were dispersed in water, and N‑acetyl‑L‑cysteine, CdCl₂·2.5H₂O, and KBH₄ were added sequentially. Na₂TeO₃ was introduced slowly, followed by heating to 100 °C and reflux for 6 h. After washing and drying, TiO₂/CdTe was obtained.

Fabrication of TiO₂/CdTe/BiOI Ternary Composites

TiO₂/CdTe (258 mg) was suspended in ethylene glycol (EG) and mixed with a Bi(NO₃)₃ solution (627.6 mg in 28 mL EG). KI (214.8 mg) was added, and the mixture was stirred for 1 h before hydrothermal treatment at 80 °C for 3 h. The precipitates were washed, dried, and labeled TiO₂/CdTe/BiOI. Binary TiO₂/BiOI was prepared analogously without CdTe.

Characterization

XRD (Rigaku D‑MAX2500), SEM (JSM‑6700F), TEM (JEM‑2100), BET (NOVA 1000e), XPS (XSAM800), UV‑Vis diffuse reflectance (CARY500UV), photoluminescence (Shimadzu RF‑5301), Mott‑Schottky (CHI760E), and transient photocurrent measurements were performed to probe structure, composition, surface area, electronic states, and charge‑carrier dynamics.

Photocatalytic Performance

Photodegradation of 5 mg L⁻¹ MO was conducted in a 50 mL quartz beaker with 140 mg catalyst under a 500 W Xe‑arc lamp (simulated sunlight). Samples were stirred in the dark for 30 min to reach adsorption–desorption equilibrium, then irradiated for 180 min. Aliquots were taken every 45 min, centrifuged, and analyzed at 465 nm. Reusability was tested over three cycles with fresh MO solution each time. Radical trapping experiments employed EDTA‑2Na (h⁺), KBrO₃ (e⁻), BQ (•O₂⁻), and IPA (•OH).

Results and Discussions

XRD Analysis – All samples exhibit anatase TiO₂ peaks (JCPDS #84‑1285). Additional peaks at 29.7°, 31.7°, 45.5°, and 51.3° confirm BiOI formation (JCPDS #73‑2062). CdTe peaks are absent due to their nanometric size and low loading.

Morphology (SEM/TEM) – TiO₂(S) and TiO₂(H) form 200–400 nm spheres. BiOI flakes decorate TiO₂ surfaces, with larger, flake‑like particles on solid spheres and smaller, densely packed flakes on hollow spheres. TEM shows lattice fringes of TiO₂ (0.238 nm), CdTe (0.33 nm), and BiOI (0.282 nm), confirming heterojunction formation.

Surface Area (BET) – TiO₂(S)/CdTe/BiOI and TiO₂(H)/CdTe/BiOI exhibit 77.7 m² g⁻¹ and 91.6 m² g⁻¹, respectively. The hollow architecture increases pore volume and active sites.

Structural Evolution – Starting from amorphous TiO₂, calcination yields solid microspheres, while hydrothermal treatment yields hollow spheres. Subsequent BiOI crystallization occurs on TiO₂ surfaces, with morphology dictated by scaffold surface area.

XPS – Ti 2p peaks shift 0.8 eV to lower binding energy in TiO₂/CdTe and TiO₂/CdTe/BiOI, indicating strong CdTe–TiO₂ interaction. O 1s shifts suggest partial oxidation of CdTe. Te 3d spectra confirm CdTe presence.

Optical Properties – TiO₂ (S) shows UV absorption only. TiO₂(S)/CdTe/BiOI and TiO₂(H)/CdTe/BiOI display strong visible‑light absorption up to ~650 nm. Bandgaps decrease to 2.57 eV (solid) and 2.45 eV (hollow).

Mott–Schottky – All composites are n‑type. Flat‑band potentials: −0.76 V (S), −0.80 V (S/BiOI), −0.85 V (H/BiOI) vs SCE, translating to CBM of −0.52, −0.56, and −0.60 V vs NHE. VBM positions are 2.5, 2.01, and 1.85 V vs NHE, respectively, enabling reduction of O₂ to •O₂⁻ and oxidation of h⁺.

Photocatalytic Degradation – TiO₂(S)/CdTe/BiOI achieves 88.4 % MO removal in 180 min, while TiO₂(H)/CdTe/BiOI reaches 99.7 % in 90 min. Solid‑sphere TiO₂(S)/BiOI and TiO₂(S)/CdTe show 46.3 % and 57.5 % after 180 min, respectively. Reuse tests indicate only minor decline after three cycles.

Active Species – EDTA‑2Na (h⁺ scavenger) suppresses activity entirely, confirming holes as the primary oxidant. KBrO₃ (e⁻) accelerates degradation, indicating electron participation. BQ (•O₂⁻) shows modest influence; IPA (•OH) has negligible effect, suggesting minimal hydroxyl radical involvement.

Photoluminescence & Photocurrent – PL intensity is lowest for TiO₂(H)/CdTe/BiOI, reflecting efficient charge separation. Transient photocurrent is highest for TiO₂(S)/CdTe, with TiO₂(H)/CdTe/BiOI slightly lower but still superior to bare TiO₂. These trends corroborate the enhanced charge dynamics in the ternary system.

Mechanism – Under visible light, BiOI and CdTe QDs absorb photons and generate electrons. In the p–n junction, electrons transfer from BiOI CB to TiO₂ CB, while CdTe electrons also move to TiO₂ CB via type‑II alignment. Holes remain in BiOI and CdTe VBs, driving oxidation of MO. The hollow TiO₂ scaffold further facilitates charge migration and provides abundant active sites, leading to superior photocatalysis.

Conclusions

We have developed a facile two‑step synthesis to produce TiO₂/CdTe/BiOI ternary heterostructures on both solid and hollow TiO₂ microspheres. The hollow design, coupled with CdTe QDs and BiOI flakes, synergistically enhances visible‑light absorption, charge separation, and surface area, achieving >99 % methyl orange degradation in 90 min under simulated sunlight. This strategy illustrates how structural regulation and multi‑component integration can be leveraged to design high‑performance, eco‑friendly photocatalysts for wastewater treatment.