Do BaTiO3/ZnO nanocomposite photocatalysts degrade mixed antibiotics and dyes effectively?

The gain depends on which pollutant is degraded

The paper asks whether combining barium titanate (BaTiO3, BTO) with zinc oxide (ZnO) produces a photocatalyst better than either oxide alone for persistent wastewater pollutants. The answer is not a single yes: the composite's advantage appears mainly against antibiotics, and against dyes it mostly recovers the activity that pure ZnO already provides.

In single-pollutant tests, the ZnO-rich composite BTO/ZnO-3 raised the mass-normalized rate constant for ciprofloxacin from 0.24015 min⁻¹ g⁻¹ on pristine BTO to 0.48113, roughly two-fold, and about 1.7 times the 0.28792 measured for ZnO. For tetracycline the same composite reached 0.6440 min⁻¹ g⁻¹ versus 0.18596 for BTO, about 3.5-fold, and about 1.4 times ZnO's 0.45450.

For the dyes the ordering changes. Methylene blue rose from 0.0236 min⁻¹ g⁻¹ on BTO to 0.3001 on BTO/ZnO-3, about 13-fold, but pure ZnO reached 0.3437, so the composite kept roughly 87% of ZnO's activity. Methyl orange went from 0.0312 to 0.2125, about 6.8-fold over BTO, yet stayed near 75% of ZnO's 0.2820. These rate constants are normalized by catalyst mass, so they compare activity per gram under the same 0.5 g/L loading, not per surface area.

Two separate oxide phases held in contact

BTO and ZnO nanoparticles were each made hydrothermally, then combined by stirring and ultrasonication in ethanol at BTO:ZnO mass ratios of 80:20, 50:50 and 20:80, followed by calcination at 600 °C for 2 h. The resulting powders are labelled BTO/ZnO-1, -2 and -3.

X-ray diffraction and Rietveld refinement show the composites are superpositions of cubic BaTiO3, hexagonal wurtzite ZnO and a minor BaCO3 impurity, with unchanged peak positions. Measured ZnO fractions rose from 11.96 wt% in BTO/ZnO-1 to 36.32 wt% in BTO/ZnO-2 and 77.13 wt% in BTO/ZnO-3, close to the nominal ratios. The authors read this as distinct crystalline domains in intimate contact rather than a solid solution, which is the structural basis for calling the interface a heterojunction.

Electron microscopy supports that picture: fine ZnO particles decorate BTO spheres and fill spaces between them, and ZnO surface coverage increases from BTO/ZnO-1 to BTO/ZnO-3. That graded coverage matters because it changes how much ZnO surface is exposed to the pollutant solution.

Light absorption and surface area do not explain the gain

A composite can look better simply because it absorbs more light or offers more surface. The paper rules out both as the main cause. Optical band gaps of the composites were about 3.48–3.51 eV, essentially identical to pristine BTO, so the enhanced activity cannot be attributed to a changed absorption edge.

Nitrogen physisorption gave low and closely similar BET surface areas across the series, 6.55–11.50 m²/g, and these values do not follow the activity trend. X-ray photoelectron spectroscopy confirmed Ba²⁺, Ti⁴⁺ and Zn²⁺ oxidation states and showed both oxide phases exposed at the composite surface.

The authors therefore attribute the improvement to charge behaviour at the BTO/ZnO interface rather than to texture or light harvesting. Photoluminescence and band-edge analysis are described as consistent with, but not proof of, charge separation at that interface, so the heterojunction explanation remains an interpretation supported by several lines of evidence rather than a directly measured charge-transfer rate.

Adsorption and direct photolysis are minor removal routes

Before irradiation, each catalyst suspension was stirred in the dark for 60 min to reach adsorption–desorption equilibrium, and the concentration after that step served as C0. Only a modest decrease in C/C0 occurred in the dark for all catalysts, and dark-adsorption runs monitored over the full 240 min confirmed that adsorption alone contributes little.

Control experiments compared dark adsorption, direct photolysis under UV without catalyst, and photocatalysis under UV with catalyst for pristine BTO, BTO/ZnO-3 and pure ZnO. Removal was dominated by the catalyzed, illuminated condition. This matters for reading the rate constants: they describe light-driven degradation of the parent pollutants, measured at their characteristic absorption maxima, not complete mineralization to CO2 and water.

Photogenerated holes dominate, especially for dyes

Scavenger tests identify which reactive species do the work. Adding EDTA to block photogenerated holes cut methylene blue degradation from 93.8% to 41.3% and methyl orange from 62.8% to 2.7%, while tert-butanol, which traps hydroxyl radicals, had a much weaker effect. For the dyes, holes are therefore the dominant oxidant and hydroxyl radicals are secondary.

For the antibiotics the picture is more mixed. EDTA lowered ciprofloxacin degradation from 88.9% to 81.3% and tetracycline from 95.0% to 85.8%, and tert-butanol produced only minimal change. The authors interpret this as direct hole oxidation at the catalyst surface combined with additional parallel oxidation routes.

Superoxide radicals appear minor: benzoquinone inhibition was limited for methylene blue, and reliable benzoquinone data could only be obtained for that dye because its strong UV absorption overlapped the analytical bands of the other pollutants. The paper proposes an S-scheme charge-transfer pathway that preserves strongly oxidizing ZnO holes, but the band-edge potentials behind it were estimated from optical band gaps and Mulliken electronegativities, so the authors themselves treat those positions as qualitative indicators rather than measurements.

The four-pollutant mixture works, with semi-quantitative readings

The practical test is the quaternary mixture of ciprofloxacin, tetracycline, methylene blue and methyl orange, prepared so that 100 mL contained all four contaminants. The selected composite degraded the mixture following pseudo-first-order kinetics, and four repeat runs gave error bars reported as one standard deviation, which the authors present as good repeatability.

The measurement has a stated constraint. Because the pollutants' absorption bands partially overlap in the mixed system, concentrations were read from each pollutant's characteristic absorption maximum and should be regarded as semi-quantitative estimates of removal. The mixed-system results therefore establish that simultaneous degradation occurs and is reproducible, not precise per-pollutant removal percentages.

Binary antibiotic and binary dye mixtures were also tested, extending the evidence beyond single-pollutant conditions that the authors note have dominated earlier BTO heterostructure work.

A composition trade-off rather than a universal winner

Across the three compositions, BTO/ZnO-3, the ZnO-rich sample, gave the highest mass-normalized rates for both antibiotics and both dyes, making it the best single compromise in this series. The paper's own comparison table places it against other BaTiO3-based photocatalysts, but the authors caution that reported loadings, pollutant concentrations, reactor setups and light sources vary widely, so that table is qualitative and indicative rather than a performance ranking.

The conditions also define the boundary. Activity was measured under UVC lamps emitting maximally at 273 nm, with 0.5 g/L catalyst, 10 ppm initial ciprofloxacin and tetracycline, and 5 ppm methylene blue and methyl orange in single-pollutant tests. The composites' band gaps near 3.48–3.51 eV mean the demonstrated gains belong to UV-driven operation, and the paper does not report visible-light performance or real effluent testing for this system.

The most useful reading is a priority rule: if the target is antibiotic removal, BTO/ZnO-3 outperforms both pristine BTO and pure ZnO on a per-mass basis; if the target is dye removal, pure ZnO remains the more active reference and the composite mainly offers a single material that handles both classes at once. That trade-off, not a blanket claim of superiority, is what the evidence supports.

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