Parami Amashakya

The term absorption is used when gases dissolve in a liquid. In your case, the term adsorption should be used. Adsorption is a spontaneous process and occurs with a decrease in the Gibbs energy of the surface. Your dye should be adsorbed on the surface of zinc oxide. Only in comparison with others will the adsorption be better or worse. It is not clear from your question on what basis you concluded that the dye is not adsorbed.

Okay, let's troubleshoot why your natural dye isn't being absorbed by your electrodeposited ZnO layer. Here's a breakdown of the common reasons, along with explanations and potential solutions:

1. Dye and ZnO Surface Compatibility:

• Insufficient Chemical Affinity: A strong bond between the dye and the ZnO surface is crucial for absorption. If the dye lacks functional groups that interact well with ZnO (e.g., carboxylic acids, hydroxyls, amines), absorption will be poor. Natural dyes vary widely in their chemical composition and affinity for oxide surfaces.

• Surface Charge of ZnO: ZnO's surface can have a positive or negative charge depending on the solution's pH. If the dye has the same charge as the ZnO surface, electrostatic repulsion will prevent absorption. You should consider the charge of the dye in your solution (possible deprotonation or protonation) and compare it with the point of zero charge (PZC) of ZnO (typically around 9).

• Dye Molecule Size: The pores in your electrodeposited ZnO layer might be too small for the dye molecules to penetrate. Natural dyes can have quite large and complex molecules.

2. ZnO Layer Characteristics:

• Density of the ZnO Layer: A dense ZnO layer with low porosity will prevent the dye from entering. Electrodeposition can produce dense or porous layers depending on the parameters (current, voltage, temperature, electrolyte composition).

• Presence of a Protective Layer: An oxide or hydroxide layer might have formed on the ZnO surface, preventing direct interaction with the dye.

• Contaminated Surface: The ZnO surface could be contaminated with oils, grease, or other impurities that block dye absorption.

• ZnO Crystallinity: The crystallinity of your ZnO layer can influence absorption. A more organized crystalline structure might offer fewer anchoring sites for the dye.

3. Dye Application Procedure:

• Dye Concentration: The concentration of the dye in the solution might be too low.

• Insufficient Soaking Time: The immersion time of the ZnO layer in the dye solution might be too short to allow significant absorption.

• Temperature: The temperature of the dye solution could be too low. A higher temperature can increase the mobility of dye molecules and improve absorption.

• pH of the Dye Solution: The pH of the dye solution can influence the charge and solubility of the dye, thereby affecting its absorption.

• Agitation: Lack of agitation can limit the contact of the dye with the ZnO surface.

4. Dye Solvent:

• Solvent Incompatibility: The solvent used to dissolve the dye might not be appropriate. It should allow good dye solubility and facilitate its interaction with the ZnO surface. For example, water is a common solvent, but some dyes are better solubilized in ethanol or other organic solvents.

What Can You Do to Improve Absorption?

1. Dye Selection: Choose a natural dye with functional groups capable of binding to ZnO (COOH, OH, NH2).

2. ZnO Surface Cleaning: Clean the ZnO surface with an appropriate solvent (e.g., ethanol, acetone) to remove contaminants.

3. ZnO Surface Treatment:

• Acid Treatment: A mild acid treatment can slightly etch the ZnO surface, creating more anchoring sites for the dye. Be very careful and use a weak acid at a low concentration for a short period.

• Plasma Treatment: Plasma treatment can modify the ZnO surface and improve its adhesion to the dye.

4. Dye Solution Optimization:

• Increase Concentration: Increase the dye concentration in the solution.

• Adjust pH: Adjust the pH of the dye solution to promote adsorption on the ZnO surface.

• Solvent Choice: Try different solvents (water, ethanol, methanol, etc.) to see which gives the best absorption.

5. Application Procedure Optimization:

• Increase Soaking Time: Increase the immersion time of the ZnO layer in the dye solution.

• Increase Temperature: Increase the temperature of the dye solution (without degrading the dye).

• Agitation: Gently agitate the dye solution during soaking.

6. Characterization of the ZnO Layer: Analyze the morphology and structure of your ZnO layer (e.g., by scanning electron microscopy (SEM) or X-ray diffraction (XRD)) to understand its porosity and crystallinity.

By combining these suggestions, you should be able to significantly improve the absorption of the natural dye onto your electrodeposited ZnO layer. It's important to test different combinations of parameters to find the optimal conditions for your specific system.

ZnO is amphoteric an oxide but generally insoluble in water which could limit its interaction with water based dies second reason is say as discussed by Freddy Harcel