The concept of photo-electrochemical (PEC) solar hydrogen generation has been demonstrated based on metal-oxide nanoparticle and nanowire films.1,2 Compared to bulk materials, these nanostructures can provide large surface areas and short diffusion lengths for photogenerated charge carriers, representing a new class of photo-electrode. Titania (TiO2) has attracted much attention as photo-electrode because of its favorable electronic-band structure, superior photochemical stability in aqueous solution, and low material cost.3 However, the large band-gap energy of this semiconductor limits light absorption only to the UV region, which comprises fewer than 5% of sunlight photons. Enhancing TiO2's visible-light absorption will improve the solar-to-hydrogen conversion efficiency.
Several approaches have been developed to improve TiO2's PEC performance. To date, elemental doping4 and quantum-dot (QD) sensitization5 of metal-oxide nanostructures have been explored separately for PEC hydrogen generation, yet little work has been done on combining the two approaches. Recently, we reported the first example of cadmium selenide (CdSe) QD-sensitized and nitrogen (N)-doped TiO2 nanomaterials for PEC hydrogen generation.6 We observed a significant synergistic effect between QD sensitization and N doping in both anatase TiO2nanoparticle films and vertically aligned rutile TiO2 nanowire arrays. (Anatase and rutile TiO2 have the same chemistry but different structures.)
We synthesized nanocrystalline TiO2 nanoparticles and vertically aligned nanowire arrays on transparent, conducting substrates.6 These nanomaterials were N doped by annealing them in ammonia and sensitized with CdSe QDs using chemical-bath deposition. We performed PEC measurements on four kinds of TiO2nanocomposite films, pristine TiO2, N-doped TiO2(TiO2:N), QD-sensitized TiO2 (CdSe-TiO2), and QD-sensitized, N-doped TiO2 (CdSe-TiO2:N), using a three-electrode cell with a platinum-coil counter electrode and silver/silver chloride as reference. The linear sweep voltammograms recorded from four samples show a general trend in photocurrent density, CdSe-TiO2:N > CdSe-TiO2 > TiO2~ TiO2:N (see Figure 1). Pristine TiO2 and TiO2:N samples exhibited similar photocurrent densities, which indicates that sole N doping has no obvious effect on the photocurrent. Both TiO2 and TiO2:N samples showed a great enhancement in photocurrent after CdSe sensitization, because visible-light absorption was improved (as we expected). Significantly, the CdSe-TiO2:N sample exhibited the greatest photocurrent density, almost twice that of CdSe-TiO2. These results clearly demonstrate a strong synergistic effect in combining CdSe QD sensitization and N doping.
Figure 1. Linear sweep voltammograms collected from the four titania (TiO2) nanocomposite films at a scan rate of 10mV/s under light illumination of 100mW/cm2. TiO2:N: Nitrogen (N)-doped TiO2. CdSe-TiO2: Cadmium selenide quantum-dot-sensitized TiO2. CdSe-TiO2:N: N-doped CdSe-TiO2. I: Intensity. Ag/AgCl: Silver/silver chloride reference for potential (V) measurements.
We developed a preliminary model to explain this synergy (see Figure 2). Compared to the pristine TiO2 sample, TiO2:N has a higher density of partially occupied oxygen-vacancy states (Vo) located at ~0.4eV above the CdSe valence-band edge that can facilitate hole transfer from CdSe to TiO2 following photo-excitation of the CdSe QDs. This interfacial hole transfer is believed to improve the PEC photocurrent of CdSe-TiO2nanocomposite films in two ways. First, it can reduce electron-hole recombination in the CdSe QD. Second, the holes transferred to the Vo levels in TiO2 can either oxidize the electrolyte on site or be further transported through the TiO2 network to other oxidation sites, with the latter being especially important for thick nanocrystalline films. Recombination between holes transferred to the Vo levels and electrons in the conduction band of TiO2 or CdSe is expected to be insignificant, since the coupling between the localized Vo states and the delocalized conduction band should be weak.
Figure 2. Proposed model for electron transfer at the CdSe/TiO2interface in a CdSe-TiO2:N sample. Red arrows highlight hole (h+) and electron (e- ) transfer from CdSe to TiO2. Black dashed arrows highlight possible electronic transitions between the different energy levels in TiO2. VB, CB: Valence, conduction bands. E versus NHE: Electrode versus normal hydrogen electrode. Vo: Oxygen vacancy state. No: N doping at O sites. hν: Light energy.
These studies have shown that the photoactivity of TiO2 can be substantially enhanced by uniquely integrating the QD-sensitization and element-doping approaches. This work provides useful insights for developing new nanocomposite structures tailored for PEC hydrogen generation and other applications through band engineering. Further investigations will include direct and detailed kinetic studies of interfacial carrier transfer between CdSe QDs and TiO2, which could lead to a better understanding of the origin of the synergistic effect.
Yat Li acknowledges support of this work by the University of California at Santa Cruz's new faculty startup fund and the National Science Foundation's Faculty Early Career Development (CAREER) program DMR-0847786. Jin Zhong Zhang thanks the Basic Energy Sciences program of the US Department of Energy (05ER4623A00) and the National Natural Science Foundation of China (grant 20628303) for support.
Jin Zhong Zhang, Yat Li
Department of Chemistry and Biochemistry
University of California at Santa Cruz
Santa Cruz, CA
Jin Zhong Zhang is professor of chemistry and biochemistry. His primary research interests include synthesis, characterization, and application of metal, metal oxide, and semiconductor nanomaterials. His group specializes in optical spectroscopy and ultrafast-laser techniques.
Yat Li is assistant professor of chemistry and biochemistry. His research focuses on fabrication and characterization of 1D nanostructures for energy conversion and photonics applications. His group specializes in chemical-vapor deposition and metalorganic chemical-vapor-deposition techniques.
4. X. Yang, A. Wolcott, G. Wang, A. Sobo, R. C. Fitzmorris, F. Qian, J. Z. Zhang, Y. Li, Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting, Nano Lett. 9, pp. 2331-2336, 2009.