Homogeneous photosensitization of complex TiO₂ nanostructures for efficient solar energy conversion

Abstract

TiO₂ nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO₂ in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO₂ such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO₂ substrates. PEC tests made to CdSe-sensitized TiO₂ inverse opal photoanodes result in a drastically improved photocurrent level, up to ~15.7 mA/cm² at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction.

Figure 1. Sensitization of TiO₂ inverse opals photoelectrochemical anode by ALDIER.

(a) Schematics of the ALDIER process for uniform QD photosensitization of TiO₂ inverse opals (TiIO). Step 1: coating the TiIO with ALD ZnO layer. Step 2: anion exchange reaction converting the initial ZnO layer to ZnSe. Step 3: cation exchange reactions converting the intermediate ZnSe to CdSe. (b) Schematics of the photoelectrochemical cell: The as-prepared nanostructure serves as the anode, saturated Ag/AgCl as the reference electrode and the Pt foil as counter electrode for hydrogen evolution. (c) Electron energy levels of TiO₂ and CdSe. The photogenerated electrons within CdSe will be transferred to TiO₂, while the holes (not drawn) will be scavenged by the Na₂SO₃ + Na₂S electrolyte solution. CB: conduction band. VB: valence band.

Figure 2. Composition transformation.

(a) XRD patterns of the pristine TiIO, and the derived structures after ALD and ion exchange reactions. The green dashed lines indicate the peaks of FTO, and the blue solid lines indicate peaks of anatase TiO₂. (b) UV-vis diffuse reflection spectra of three photoanode samples. TiIO: pure TiO₂ inverse opal. TiIO/ZnSe: ZnSe-coated TiIO after the anion exchange reaction. TiIO/CdSe: CdSe-coated TiIO after the cation exchange reaction. Insets are the photographs of the samples on FTO-coated glass.

Figure 3. SEM characterization.

(a) Top view of the pristine TiO₂ inverse opal. (b-d) Top view images of the ALDIER samples, where the CdSe layers were converted from the ZnO starting layers obtained by using 60, 90, and 120 ALD cycles, respectively. Scale bars: 100 nm. (e) Side view of the entire cross section of the photoanode in (b) showing the uniform sensitization from top to the bottom. Inset: enlarged view of part of the cross section.

Figure 4. TEM characterization.

(a) Low-magnification TEM image of the CdSe nanoparticle-sensitized TiO₂ inverse opal. (b) Atomic-scale TEM image of one CdSe nanoparticle. Inset is the corresponding fast Fourier transformation pattern. (c) X-ray energy dispersion spectrum (EDS) recorded from the CdSe nanoparticle in (b).

Figure 5. Photoelectrochemical properties of the ALDIER TiO₂ inverse opal photoanodes.

(a) Linear sweep voltammagrams (J-V curves) under dark condition and AM1.5 light illumination for samples by ALDIER technique with different ALD cycles, and one by 6 SILAR cycles. (b) Plot of the photocurrent density at zero bias in (a) versus the ALD ZnO cycle. (c) Photocurrent versus time tests (J−t curves) under chopped light illumination (light/dark cycles of 50 s) at a fixed bias of 0 V vs Ag/AgCl. (d) IPCE profile of the ALDIER photoanode with 60 ALD cycles. For comparison, the corresponding data of the optimized SILAR anode (6 cycles CdSe) are also shown.