Rubidium Sulfate (Rb₂SO₄) Doped Titanium-Based TiO₂ Thin-Film Photocatalyst: In-Situ Hydrothermal Growth + Calcination

Textile and dyeing wastewater often contains complex organic dyes that are difficult to remove by conventional methods. Titanium dioxide (TiO₂) is widely used as an n-type semiconductor photocatalyst because it is stable, recyclable, low-toxicity, and cost-effective. However, standard TiO₂ mainly responds to ultraviolet light due to its wide bandgap, which limits performance under visible light.

This page summarizes a practical preparation route for a rubidium sulfate (Rb₂SO₄) modified titanium-based TiO₂ thin film. The approach grows an oxide film directly on a titanium sheet via in-situ hydrothermal synthesis, then densifies and activates the film through controlled calcination. By introducing Rb⁺ (metal dopant) and S-containing species (non-metal dopant) from rubidium sulfate, the film morphology and charge behavior are improved, enabling faster pollutant mass transfer to active sites and higher photocatalytic degradation efficiency for organic dyes.

Materials & reagents (typical)

• Titanium sheets (cut to a consistent size; a common lab size is 3 cm × 1.5 cm).
• Nitric acid solution: HNO₃ at 0.2 mol/L (prepared using H₂O₂ as described in the source process).
• Rubidium sulfate: Rb₂SO₄ at 0.02–0.04 mol/L (key raw material for Rb + S introduction).
• Oxalic acid dihydrate: C₂H₂O₄·2H₂O at 0.4–0.7 mol/L.
• Deionized water for ultrasonic cleaning.

Safety note: HF (hydrofluoric acid) and pressurized hydrothermal equipment require strict safety controls, trained personnel, proper PPE, ventilation, and compliant waste handling.

Step 1 — Titanium sheet polishing (surface activation)

1. Mechanical polishing: sand the titanium sheet sequentially using 600, 800, 1000, and 1200 grit sandpaper to remove oxide and scratches.
2. Chemical polishing: immerse the polished sheet in a polishing solution with volume ratio V(HF):V(HNO₃) = 1:10 for 60 seconds.
3. Ultrasonic cleaning: rinse and ultrasonically clean the sheet in deionized water multiple times until the surface is free of residue.

Purpose: a clean, activated titanium surface improves nucleation and adhesion of the in-situ grown TiO₂ film.

Step 2 — Hydrothermal in-situ growth (Rb₂SO₄ introduced here)

1. Prepare the mixed precursor solution in a Teflon-lined autoclave insert:
   • HNO₃: 0.2 mol/L
   • Rb₂SO₄: 0.02–0.04 mol/L
   • C₂H₂O₄·2H₂O: 0.4–0.7 mol/L
2. Load the cleaned titanium sheet into the insert, ensuring it is fully contacted by the solution.
3. Hydrothermal reaction: seal the reactor and run at 80–200°C for the chosen duration. A commonly used preferred condition is 80°C for 24 hours.

Why rubidium sulfate matters in this step: Rb₂SO₄ is the controlled source of Rb⁺ (metal dopant) and S-containing species (non-metal dopant), directly influencing film formation and micro-structure during in-situ growth.

Step 3 — Natural cooling & air drying (film stabilization before calcination)

1. After the hydrothermal run, allow the reactor to cool naturally to room temperature.
2. Remove the titanium sheet and air-dry at room temperature to obtain a titanium sheet with a grown oxide film (a yellow-toned film can appear prior to calcination in typical runs).

Step 4 — Calcination (density, phase optimization, stronger photocatalysis)

1. Place the dried sheet into a muffle furnace.
2. Calcine at 400–600°C for 1.0–2.5 hours.
3. A widely used preferred setting is 450°C for 1 hour.

Calcination improves film density and stability, enhances charge separation, and helps form a more active TiO₂ phase (commonly anatase-dominant), which supports higher photocatalytic efficiency and better reusability.

Example process window (for morphology tuning)

Typical test setup referenced in the source process: methyl orange at 20 mg/L, 80 W high-pressure mercury lamp (UV + partial visible), with performance assessed by absorbance change after irradiation (e.g., 5 h), and optional synergy testing with small H₂O₂ addition.

Process summary (what you obtain)

• A titanium-supported TiO₂ thin film grown by in-situ hydrothermal synthesis.
• Rb and S co-doping introduced via rubidium sulfate (Rb₂SO₄), enabling structural and electronic modification.
• A film surface that can include microsphere-based architecture with additional nano-features, improving effective surface area and mass transfer.
• After calcination, a denser, more stable film that is less prone to detachment and supports repeated use.

A. Rubidium sulfate is a high-leverage dopant raw material (Rb + S in one input)

• Dual-function source: Rb₂SO₄ provides Rb⁺ (metal dopant) and S-containing species (non-metal dopant) simultaneously, supporting co-doping without complicated precursor systems.
• Micro-dose, macro impact: even low Rb⁺ levels can strongly influence oxide film formation and the development of microsphere-like structures, improving active site accessibility.
• Improved light utilization: S incorporation can introduce visible-light response (reported red-shift behavior), helping TiO₂ move beyond UV-only activity.
• Better charge behavior: Rb-related modification supports electron–hole separation and longer carrier lifetime, which directly translates into higher photocatalytic efficiency.

B. Thin-film architecture beats powder handling in real wastewater systems

• No powder agglomeration: the active layer is a film on titanium, reducing clustering issues typical of particulate photocatalysts.
• Easy recovery & reuse: the catalyst is a solid sheet—simple to retrieve from reactors and suitable for repeated cycles.
• Low catalyst loss: calcination densifies the film, improving adhesion and reducing shedding into water (lower secondary contamination risk).
• Higher effective area: microsphere/needle-like surface features can increase surface area and accelerate pollutant mass transfer to reactive sites.

C. Scalable process controls (ideal for consistent production)

• Clear concentration windows: Rb₂SO₄ dosing can be controlled by molarity (0.02–0.04 mol/L) for repeatability.
• Simple unit operations: polishing → hydrothermal growth → drying → calcination (no complex coating equipment required).
• Optimization-ready: morphology and performance can be tuned by hydrothermal time/temperature and calcination profile.
• Synergy with advanced oxidation: in UV + H₂O₂ systems, the film catalyst can further accelerate dye degradation via complementary oxidative pathways.