LETTER pubs.acs.org/NanoLett
Branched TiO2 Nanorods for Photoelectrochemical Hydrogen Production In Sun Cho,† Zhebo Chen,‡ Arnold J. Forman,‡ Dong Rip Kim,† Pratap M. Rao,† Thomas F. Jaramillo,*,‡ and Xiaolin Zheng*,† †
Department of Mechanical Engineering and ‡Department of Chemical Engineering, Stanford University, California 94305, United States
bS Supporting Information ABSTRACT: We report a hierarchically branched TiO2 nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm2 total power density), the branched TiO2 nanorod array produces a photocurrent density of 0.83 mA/cm2 at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO2/electrolyte interface. KEYWORDS: TiO2, nanorods, branched nanorods, photoanode, photoelectrochemical hydrogen production, charge transport/ transfer
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ince the seminal demonstration of photoelectrochemical (PEC) water splitting using n-type TiO2 by Honda and Fujishima in 1972,1 considerable efforts have been devoted to develop suitable photoanode materials and to design their nanostructures in order to further improve the solar-to-hydrogen conversion (STH) efficiency.27 Among various photoanode materials, metal oxides, predominantly TiO2, WO3 and α-Fe2O3, have been extensively studied for PEC water splitting because of their high resistance to photocorrosion, chemical stability, nontoxicity, and low cost.1,5,813 Early efforts included the study of metal oxide nanoparticle (NP) films1418 because of their large surface area to volume ratios. However, the NP films suffer from high charge recombination loss since the electron mobility in a NP film is about 2 orders of magnitude lower than that of a bulk single crystal due to the electron trapping/scattering at grain boundaries.3,5,19,20 For instance, the electron mobilities of TiO2 in a NP film and a bulk single crystal are about 0.01 and 1 cm2/ V-sec, respectively.21,22 Additionally, photon absorption in a NP film is often limited by light reflection at the surface.35,15 Recent work has focused on one-dimensional (1D) nanostructured metal oxide photoelectrodes, such as nanotubes (NTs)6,2326 and nanorods (NRs)19,20,2731 because of enhancements in charge separation, charge transport, and light absorption. However, photoanode NRs and NTs, compared to NPs, have smaller surface areas, which can negatively impact r 2011 American Chemical Society
charge transfer process, especially with the case of water oxidation as the kinetics are sluggish. In addition, some of the 1D nanostructures are polycrystalline, which, similar to a NP film, can limit charge carrier mobility due to trap states at grain boundaries.32,33 Therefore, it is highly desirable to synthesize single crystalline 1D nanostructures with enhanced surface-to-volume ratios. Herein, we report the synthesis and characterization of single crystalline branched TiO2 nanorods (B-NRs) as a model photoanode nanostructure for efficient PEC water splitting devices. B-NRs have better charge transport and light absorption properties than NP films and larger surface areas for more efficient carrier collection than bare NRs. Though it is unlikely that a pure TiO2 photoelectrode alone could ultimately serve as an efficient solar water splitting device due to its wide bandgap, the approach and device geometry demonstrated with TiO2 in this work can be leveraged to other, more promising semiconducting materials to greatly improve their efficiencies. Material Synthesis and Characterization. To demonstrate the advantages of B-NRs in PEC applications, we compared TiO2 films of three different nanostructures: NPs, NRs, and B-NRs, as Received: August 24, 2011 Revised: October 12, 2011 Published: October 14, 2011 4978
dx.doi.org/10.1021/nl2029392 | Nano Lett. 2011, 11, 4978–4984
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Figure 1. (a) Schematic description and corresponding SEM images of (b,e) TiO 2 nanoparticles, (c,f) nanorods, and (d,g) branched nanorods. (bd) Cross-sectional view and (eg) top view. The insets of c and d show high-magnification SEM images of NRs and B-NRs, respectively.
shown in Figure 1a. The NP film was prepared by screen printing TiO2 NPs (Degussa P25) onto fluorine-doped tin oxide (FTO) substrates, followed by calcination at 450 °C for 1 h in air. The resulting film is about 4.4 μm in thickness and is highly porous (Figures 1b,e). The NR array was synthesized on TiO2-seeded FTO substrates based on a previously reported hydrothermal method34 but with improved synthesis parameters (see Experimental Methods, Supporting Information Figure S1 to S4). These NRs have an average diameter of 65 nm, length of 2.0 μm, and surface coverage density around 6.3 109 NRs/cm 2 (Figures 1c,f and 2a). The TiO2 NRs reported here, compared to NRs from previous reports,20,30,3537 have smaller diameters and reduced aggregation, especially at the base of the NRs (Supporting Information Figure S5). Both improvements lead to larger surface to volume ratios, a desired feature for PEC applications. The B-NRs consist of a NR trunk and many branches. The NR trunks were synthesized with the same conditions as those used for the NRs and the branches were synthesized by a solution phase method in a subsequent growth step (see Experimental Methods). The morphology of the branches, such as diameter and length, can be tuned by varying the synthesis conditions (Supporting Information Figure S5 and S6). Figures 1d,g show typical morphologies of the B-NRs, where short needle-shaped branches were grown uniformly on the entire surface of the NR trunks. The morphology and crystal structure of the TiO2 NRs and B-NRs were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The NRs are singlecrystalline tetragonal-rutile phase TiO2 with a [001] growth
Figure 2. TEM and HRTEM images of TiO2 NRs and B-NRs. (a) TEM image of a single nanorod. The inset shows a HRTEM image of the rectangular region. (b,c) TEM images of a single branched nanorod. The branches have a cone shape with an average length of 90 nm and a base diameter about 15 nm. (d) HRTEM image of a single branch, indicating that the braches grow along the [001] direction, the same as the NRs. 4979
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Figure 3. Optical characterization of TiO2 films of NPs, NRs, and B-NRs. (a) Transmittance (it should be noted that T 6¼ 100 A, i.e., the loss of transmission across the visible is primarily due to scattering and reflection), (b) reflectance (diffuse and specular) and (c) absorption plus scattering (A + S = 100 RT) of films of TiO2 NPs, NRs, and B-NRs grown on an FTO substrate over a wavelength of 300700 nm. Plot (d) shows the absorption plus scattering of three samples grown on quartz, illustrating the substrate dependence of the scattered light. The integrated absorption plus scattering values (shown in parentheses) were calculated by integrating over the wavelength range from 300 to 420 nm, i.e., bandgap absorption. A vertical dotted line in the plots indicates the 420 nm wavelength corresponding to the bandgap of rutile TiO2.
direction (Figure 2a), which is consistent with TiO2 NRs reported previously.34 The TEM image in Figure 2b shows that the branches densely and uniformly cover the surface of the NRs. Close inspection of the branches (Figure 2c) reveals that they possess a cone shape with an average length of 90 nm, a base diameter around 15 nm and a surface coverage density around 1.6 1011 branches/cm2 (per unit area of NR) (Figure 2b). A HRTEM image of a single branch (Figure 2d) shows well resolved lattice fringes even at the outer surface, indicating good crystallinity of the branches. The lattice constants are 0.29 and 0.23 nm respectively in the parallel and perpendicular directions to the length of the branches, suggesting that the branches are also single-crystalline tetragonal-rutile phase TiO2 with the same [001] growth direction as that of the trunks. Similar nanodendrite TiO2 morphologies were recently synthesized and used for dye-sensitized solar cells (DSSCs),3537 but our TiO2 B-NRs have smaller diameters, higher surface coverage, and a lower degree of aggregation, which are all desired for both DSSCs and PEC cells. On the basis of the surface coverage densities and dimensions of the NRs and the B-NRs, the roughness factor of the NRs and B-NRs, defined as the ratio of the total surface area of TiO2 to its projected FTO substrate area is estimated to be 32 and 130, respectively. A comparable 2 μm thick NP film (consisting of NPs 30 nm in diameter) has a roughness factor of about 210, though not all of this area is accessible to the electrolyte due to the presence of sealed pore spaces and sintering. As the B-NR structure quadruples the NR surface area, it
helps to close the surface area gap between highly conductive, lower surface area single crystal NR electrodes and less conductive, higher surface area polycrystalline NP electrodes. Light Absorption. The light absorption properties of the three types of TiO2 films (NPs, NRs and B-NRs) were measured and compared over a wavelength range from 300 to 700 nm (see Experimental Methods). Figure 3ac shows the transmittance (T), reflectance (R), and absorption plus scattering (A + S = 100 RT) spectra of the three films on FTO substrates. For photons with energies above the bandgap of rutile TiO2 (λ < 420 nm), the integrated light absorption plus scattering values over a wavelength range of 300420 nm are 76, 89, and 91% for NPs, NRs, and B-NRs, respectively on the FTO substrate. A significant, nonzero baseline (λ > 420 nm) was observed for all three samples (Figures 3c,d). A small portion of this comes from light absorbed in the FTO substrate38 and surface defect states of TiO23941 However, the majority of this baseline arises from scattering events in the TiO2 film which efficiently couple a significant portion of the photons into the glass (or quartz) substrate where the photons may be reflected internally and exit without being measured in this experimental setup,42 (i.e., light loss through the sides of the glass or quartz substrate). Evidence for this scattering phenomenon appears in the form of a substrate dependent baseline across the spectrum (Figure 3, panel c versus panel d). The remaining signal in the UV which rises rapidly is due to absorption within the TiO2. The spectra indicate that the TiO2 NRs and B-NRs have very similar light absorption properties and exhibit less transmission 4980
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Figure 4. Photoelectrochemical properties of TiO2 B-NRs, NRs, and NPs. (a) Chopped JV curves under Xenon lamp (UV portion of spectrum matched to AM 1.5G, 88 mW/cm2) illumination using three electrode setup (TiO2 working, Pt counter, Ag/AgCl reference electrode, scan rate of 20 mV/s). (b) JV curves under illumination from a class AAA solar simulator (100 mW/cm2) and two electrode setup (TiO2 working and Pt disk electrode, scan rate of 10 mV/s). The dark current densities (all
