II-VI semiconductor heterojunctions have emerged as promising candidates for photocatalytic water splitting due to their tunable bandgaps, efficient charge separation, and favorable band alignment. Among these, ZnO/CdS and CdSe/ZnTe heterostructures exhibit distinct advantages in solar energy conversion, particularly in visible-light absorption and charge carrier dynamics. This article examines the fundamental properties of these heterojunctions, focusing on band alignment, charge separation mechanisms, and stability under irradiation, while excluding catalysis-specific processes or reactor engineering aspects.

Band alignment is a critical factor determining the efficiency of II-VI heterojunctions for water splitting. The ZnO/CdS system demonstrates a type-II staggered alignment, where the conduction band minimum (CBM) of CdS lies above that of ZnO, while the valence band maximum (VBM) of ZnO is below that of CdS. This alignment creates a built-in electric field that drives electron transfer from CdS to ZnO and hole transfer in the opposite direction. The conduction band offset of approximately 0.5 eV facilitates electron injection, while the valence band offset of 1.2 eV promotes hole migration. Similarly, CdSe/ZnTe heterojunctions exhibit a type-II alignment with a conduction band offset of 0.3 eV and a valence band offset of 0.8 eV. These energy level differences ensure spatial separation of photogenerated electrons and holes, reducing recombination losses.

Charge separation efficiency in these heterostructures is governed by interfacial quality and carrier mobility. In ZnO/CdS, the electron mobility in ZnO (200 cm²/V·s) significantly exceeds that of CdS (50 cm²/V·s), enabling rapid electron transport away from the interface. Transient absorption spectroscopy measurements reveal electron transfer timescales of 1-10 picoseconds in optimized ZnO/CdS nanostructures. For CdSe/ZnTe, the hole mobility in ZnTe (100 cm²/V·s) surpasses that of CdSe (10 cm²/V·s), favoring efficient hole extraction. The lattice mismatch between these materials, however, presents challenges. ZnO and CdS have a 7% lattice mismatch, while CdSe and ZnTe exhibit 12% mismatch, necessitating interface engineering to minimize defect-assisted recombination.

Stability under irradiation remains a key concern for II-VI heterojunctions in photocatalytic applications. ZnO/CdS systems show susceptibility to photocorrosion, with CdS undergoing oxidation under prolonged illumination. Studies indicate that 30% of CdS degrades after 20 hours of continuous operation under AM1.5 illumination. Strategies to mitigate this include ZnS shell passivation, which reduces degradation to 10% under identical conditions. CdSe/ZnTe demonstrates better stability, with only 15% performance loss after 50 hours, attributed to ZnTe's higher chemical inertness. Both systems exhibit stability improvements when operated at neutral pH, with degradation rates doubling under acidic or alkaline conditions.

The optical absorption characteristics of these heterojunctions directly influence their water-splitting performance. ZnO/CdS combines ZnO's UV absorption (bandgap 3.3 eV) with CdS's visible-light response (2.4 eV), extending the usable spectrum to 520 nm. Quantum efficiency measurements show 45% photon-to-electron conversion at 400 nm, dropping to 15% at 500 nm. CdSe/ZnTe covers an even broader range, with CdSe absorbing up to 710 nm (1.75 eV) and ZnTe contributing to the 550 nm (2.25 eV) region. The tandem absorption enables 60% quantum efficiency at 450 nm and maintains 25% at 650 nm.

Interfacial defect states play a significant role in the performance of II-VI heterojunctions. Deep-level transient spectroscopy reveals trap densities of 10¹² cm⁻³ at untreated ZnO/CdS interfaces, which can be reduced to 10¹⁰ cm⁻³ through annealing at 400°C. CdSe/ZnTe interfaces typically show lower trap densities around 10¹¹ cm⁻³ due to the more compatible ionic radii of the constituent elements. These defects act as recombination centers, with time-resolved photoluminescence measurements showing carrier lifetimes decreasing from 10 ns to 1 ns as defect density increases from 10¹⁰ to 10¹² cm⁻³.

The morphology of these heterostructures significantly impacts their water-splitting efficiency. ZnO nanowire/CdS core-shell structures demonstrate enhanced performance compared to planar junctions, with surface area increases leading to 3-fold higher hydrogen evolution rates. Optimal nanowire dimensions of 100 nm diameter and 1 μm length provide both high surface area and efficient carrier collection. For CdSe/ZnTe, quantum dot heterostructures show particular promise, with 5 nm CdSe dots on ZnTe films achieving 2 mA/cm² photocurrent at 0 V vs RHE, compared to 0.5 mA/cm² for bulk heterojunctions.

Doping strategies further optimize II-VI heterojunction performance. In ZnO/CdS, aluminum doping of ZnO (10¹⁸ cm⁻³) increases conductivity by an order of magnitude while maintaining optical transparency. For CdSe/ZnTe, copper doping in ZnTe (10¹⁷ cm⁻³) enhances hole conductivity without compromising the band alignment. These doping levels represent optimal concentrations, as higher doping introduces additional recombination centers.

Temperature effects on heterojunction performance reveal interesting trade-offs. While increasing temperature from 25°C to 60°C improves reaction kinetics, it also accelerates degradation rates. ZnO/CdS shows a 40% increase in hydrogen production at 60°C but suffers from doubled degradation rates. CdSe/ZnTe exhibits better high-temperature stability, with 50% higher activity at 80°C and only 30% faster degradation.

Long-term stability studies under operational conditions provide crucial insights. Continuous illumination testing shows that ZnO/CdS maintains 70% of initial activity after 100 hours, while CdSe/ZnTe retains 85% under identical conditions. The primary degradation mechanism in both systems involves interfacial oxidation, as confirmed by X-ray photoelectron spectroscopy analysis showing increasing oxygen content at the interfaces over time.

Recent advances in interface engineering have led to improved performance metrics. Atomic layer deposition of ultrathin Al₂O₃ interlayers (0.5 nm) in ZnO/CdS increases stability by 300% while maintaining 90% of the original activity. For CdSe/ZnTe, sulfur passivation of the interface reduces trap density by an order of magnitude, leading to 50% higher quantum efficiency.

The choice of sacrificial reagents in testing conditions affects measured performance. Using Na₂S/Na₂SO₃ as hole scavengers, ZnO/CdS achieves 8% solar-to-hydrogen efficiency, while CdSe/ZnTe reaches 6%. These values represent upper limits, as practical water-splitting systems without scavengers typically show 2-3% efficiency due to the additional overpotential required for oxygen evolution.

Scalability considerations for these materials highlight challenges in large-area fabrication. Chemical bath deposition of CdS on ZnO enables relatively uniform coatings over 100 cm² areas, with less than 10% performance variation across the substrate. CdSe/ZnTe fabrication remains more challenging, with molecular beam epitaxy required for high-quality interfaces, limiting practical applications to smaller areas.

Future development directions for II-VI heterojunctions in water splitting include advanced interface engineering, novel architecture design, and hybrid systems combining multiple heterojunctions. The integration of these materials with co-catalysts and protective layers continues to show promise for achieving both high efficiency and long-term stability required for practical solar hydrogen production.