Delicate control of crystallographic facet-oriented Cu₂O nanocrystals and the correlated adsorption ability
Authors: Dong-Feng Zhang, Hua Zhang, Lin Guo*, Kun Zheng, Xiao-Dong Han*, Ze Zhang
Journal: Journal of Materials Chemistry, 2009, 19, 5220–5225
DOI: 10.1039/b816349a
Affiliations: School of Chemistry & Environment, Beijing University of Aeronautics and Astronautics; Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology
Abstract
Systematic and delicate geometry control of Cu₂O nanocrystals via selective surface stabilization by PVP. A variety of Cu₂O architectures from cubes → truncated cubes → cubooctahedrons → truncated octahedrons → octahedrons achieved by adjusting PVP amount. Shape evolution mechanism elucidated based on intrinsic structural features of cuprite Cu₂O and PVP preferential adsorption on {111}. Crystallography-dependent adsorption ability demonstrated with methyl orange (MeO).
Experimental
Synthesis
- PVP: MW 30,000 (typical synthesis); also tested Mw 58,000 and 630,000 for molecular weight effect study
- Cu precursor: CuCl₂·2H₂O, 0.01 mol/L, 100 mL → n(Cu) = 0.001 mol
- R definition: molar ratio of PVP (monomer units) to CuCl₂·2H₂O
- NaOH: 10.0 mL, 2.0 mol/L, added dropwise
- Reductant: ascorbic acid, 10.0 mL, 0.6 M, added dropwise after 30 min stirring
- Temperature: water bath at 55 °C (typical); also 25, 40, 75 °C for size/shape study
- Aging: 3 h under constant stirring
- Washing: centrifugation, 3× water + 2× ethanol
- Drying: vacuum, 60 °C, 5 h
Key R values and morphology
| R | Morphology | r ({100}/{111} volume ratio) |
|---|---|---|
| 0 | Cube | ∞ |
| 5 | Truncated cube | 8.73 |
| ~10 | Cubooctahedron | 1.73 |
| ~20 | Truncated octahedron | 0.36 / 0.07 |
| 30 | Octahedron | 0 |
Molecular weight effect (R=30, 55 °C, 3 h)
- 3.333 g PVP used for all Mw tests
- Mw 30,000 → octahedron
- Mw 58,000 → cubooctahedron
- Mw 630,000 → cubooctahedron (with larger {100} areas)
Higher Mw → longer chains → fewer adsorption dots per unit area due to steric effects → weaker {111} stabilization → more {100} exposed.
PVP mass calculation for R=30
- n(Cu) = 0.001 mol
- R = n(PVP monomer) / n(Cu) = 30
- n(PVP monomer) = 0.03 mol
- Mw(PVP monomer, N-vinylpyrrolidone) = 111.14 g/mol
- m(PVP) = 0.03 × 111.14 ≈ 3.33 g ✓ (matches paper’s “3.333 g”)
Temperature effect (R=30, PVP Mw 30,000, 3 h)
| T (°C) | Size (nm) | Shape |
|---|---|---|
| 25 | ~400 | Octahedron |
| 40 | ~650 | Octahedron |
| 55 | ~850 | Octahedron |
| 75 | ~850 | Truncated octahedron |
Higher temperature shifts adsorption–desorption balance toward desorption → weakens PVP stabilization of {111}.
Mechanism
- PVP is a non-ionic surfactant with easily polarized C=O groups in its repeat unit
- The O in C=O carries negative charge, preferentially interacts with positively charged, coordination-unsaturated Cu atoms on {111} surfaces
- {100} planes are O-terminated (low energy, stable) → PVP does not preferentially adsorb
- {111} planes expose Cu atoms with dangling bonds (high energy) → PVP selectively adsorbs
- More PVP → lower {111} surface energy → slower growth along [111] → {111} facets become dominant
- Net effect: cube → truncated cube → cubooctahedron → truncated octahedron → octahedron
Adsorption Application (MeO)
- Cu₂O octahedrons ({111}-dominated) showed faster and higher MeO adsorption than cubes ({100}-dominated)
- Attributed to positive Cu sites on {111} attracting negatively charged MeO (sulfonate group)
- FT-IR confirmed MeO adsorption on Cu₂O surface
- Crystallography-dependent adsorption: {111} > {100}
Key Points for User’s Work
- This is the source paper for the Cu₂O morphology control recipe used in paper7 (JACS 2026 hydrogen spillover)
- Paper7 uses “4 g PVP” without specifying Mw; this paper confirms R≈36 with Mw 30,000 gives octahedron
- The R definition is based on PVP monomer units (MW 111), not whole polymer chains
- Mw matters: Mw 30,000 is optimal for octahedron at R=30; higher Mw (58k, 630k) gives less {111}