Letter pubs.acs.org/JPCL
Improving Hematite’s Solar Water Splitting Efficiency by Incorporating Rare-Earth Upconversion Nanomaterials Ming Zhang,† Yongjing Lin,‡ Thomas J. Mullen,†,§ Wei-feng Lin,† Ling-Dong Sun,∥ Chun-Hua Yan,∥ Timothy E. Patten,† Dunwei Wang,*,‡ and Gang-yu Liu*,† †
Department of Chemistry, University of California, Davis, California 95616, United States Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States § Department of Chemistry, University of North Florida, Jacksonville, Florida 32224, United States ∥ Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing, 100871, P.R. China ‡
ABSTRACT: Confounded by global energy needs, much research has been devoted to convert solar energy to various usable forms, such as chemical energy in the form of hydrogen via water splitting. Most photoelectrodes, such as hematite, utilize UV and visible radiation, whereas ∼40% infrared (IR) energy remains unconverted. This work represents our initial attempt to utilize IR radiation, that is, adding rare-earth materials to existing photoelectrodes. A simple substrate composed of hematite film and rare-earth nanocrystals (RENs) was prepared and characterized. Spectroscopy evidence indicates that the RENs in the composite absorb IR radiation (980 nm) and emit at 550 and 670 nm. The emitted photons are absorbed by surrounding hematite films, leading to improvement of water splitting efficiency as measured by photocurrent enhancement. This initial work demonstrates the feasibility and concept of using RENs for utilizing more solar radiation, thus improving the efficiency of existing solar materials and devices. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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deposition (ALD), known to provide high-quality and uniform films.17,20,22 Subsequently, the RENs were deposited onto the hematite film by casting a solution containing designated concentration of RENs. (Typically, 20 μL of 2 mg/mL REN solution in hexane was dropped on 1 × 1 cm2 substrate and dried in air.) Atomic force microscopy (AFM) was used as a simple tool to characterize the products at each step. As shown in Figure 1B, the FTO surface is relatively flat with an RMS roughness of 12.0 ± 0.7 nm. As we can see from the cursor profile in Figure 1C, mounds are tens to hundreds of nanometers laterally, separated by valleys of similar dimension. Upon the ALD deposition of hematite, the AFM topography in Figure 1D reveals a smoother morphology with a RMS roughness of 9.3 ± 0.5 nm. The corresponding cursor profile shown in Figure 1E indicates the smearing effect, reduction of valleys, and enlargement of mounds. These observations are consistent with the experiments from previous ALD reports,19 that is, precursors filling of valleys and following the existing positive surface features, to a large degree. Upon REN deposition, hexagonal disk-shaped RENs lie flat on surface, where individual RENs are clearly visible, as revealed in Figure 1F. From the AFM topograph, the hexagonal disks are 350 ± 10 nm tall with side length measuring 360 ± 18 nm. The side
here is current and imminent need to construct clean and renewable energy systems to face future energy crises.1−6 Storing solar energy via a chemical fuel of hydrogen from water splitting represents a clean alternative for petroleum fuel.7−11 Most electrodes used for this conversion are semiconductor materials whose band gaps match the UV and visible radiation of solar energy.12−19 Among these materials, α-iron oxide (αFe2O3, or hematite) has gained recent attention owing to its abundance, stability, nontoxicity, suitable band gap, and favorable valence band edge position.18,20−28 Photoabsorption spectra of hematite reveal the materials’ effective absorption in the UV and visible range, up to 610 nm, above which the extinction coefficient diminishes.17,29−31 Further improvement of hematite’s performance should be possible by improving photoabsorption efficiency. This work explores one mechanism toward this goal, that is, providing more visible photons by converting solar radiation at longer than the visible region, which represents ∼40% energy of solar spectrum.32,33 Adding upconversion rare-earth nanocrystals (RENs) of NaYF4:Yb,Er34−40 could, in principle, absorb near IR radiation and emit at the visible region, thus providing additional photons for absorption by hematite films for water splitting. The upconversion RENs were synthesized via a thermodecomposition method previously reported.37,38 The composite hematite electrode was produced following a simple protocol as illustrated in Figure 1A. The hematite film was deposited on a fluorine-doped tin oxide (FTO) glass by atomic layer © 2012 American Chemical Society
Received: September 17, 2012 Accepted: October 15, 2012 Published: October 15, 2012 3188
dx.doi.org/10.1021/jz301444a | J. Phys. Chem. Lett. 2012, 3, 3188−3192
The Journal of Physical Chemistry Letters
Letter
Figure 1. (A) Schematic diagram of the preparation of REN/hematite photoelectrodes. (B) 2 × 2 μm2 AFM topograph of the FTO glass. (C) Corresponding cursor profile as indicated in panel B. (D) 2 × 2 μm2 AFM image of the hematite-coated FTO glass after ALD. (E) Corresponding cursor profile, as indicated in panel D. (F) 2 × 2 μm2 AFM scan of RENs on hematite films. (G) Corresponding cursor profile as indicated in panel F. All scale bars are 500 nm. AFM characterization was conducted using a commercial AFM scanner (MFP-3D, Asylum Research, Santa Barbara, CA). All AFM images were acquired via tapping mode with 60% damping. Silicon probes (AC240-TS, Olympus America, Center Valley, PA) with a force constant of 2 N/m and resonant frequency of 70 kHz were used for the characterization. The AFM images were acquired and analyzed using Asylum MFP3D software developed on the Igor Pro 6.12 platform.
Figure 2. (A) High-resolution SEM image of a REN/hematite composite film with a zoom-out image shown as the inset. All scale bars are 500 nm. (B) Typical EDS spectrum of the composite films, in which all peaks are identified with the elements indicated.
length is in good agreement with that measured from transmission electron microscopy (TEM). The chemical composition of the films was determined by scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS), as shown in Figure 2. The SEM image (inset) surveyed a larger area than AFM, from which REN locations were determined and the surface coverage was measured as 60%. Focusing the electron beam at high REN coverage regions, EDS spectra were acquired. A typical spectrum is shown in Figure 2B, from which the following characteristic peaks are revealed: F (0.677 keV), Na (1.041 keV), Y (1.685 keV), and Yb (1.948 keV). The stoichiometry determined from peak intensity in EDS spectrum reveals F/ Na/Y/Yb = 4:1:0.8:0.2. Because of the low doping percentage of Er, the characteristic X-ray peak of Er was typically not observed in the EDS spectrum of our materials.37,38 Our previous study of REN particles confirmed the existence of Er by inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement.38 Elemental peaks of Fe and O from hematite films are also present at 6.4 and 0.53 keV, respectively. Collectively, these EDS studies confirm the chemical integrity of NaYF4:Yb,Er as well as the arrangement of submonolayer RENs on hematite films, which is consistent with the AFM observation. The spectroscopic properties of the composite films were investigated using a 150 mm imaging dual grating monochromator (Acton SP2150, Princeton Instruments) and a 50
Figure 3. Upon excitation at 980 nm, the emission spectra of RENs before (green) and after (red) deposition onto hematite films are acquired. Using 670 nm as internal references (superposition of their intensity), the absorption of 570 nm photons is clearly seen by the much lower intensity upon deposition. Inset shows the absorption spectra of hematite films (black) and the emission spectra of RENs (green).
mW 980 nm diode laser (UH5-50G-980, World Star Tech) as the excitation source. As shown in Figure 3, the REN spectra have been acquired before (green) and after (red) deposition onto hematite films. Because hematite films are known to exhibit little absorption longer than 610 nm, as shown in inset of Figure 3, the emission of RENs at 670 nm can be used as an internal reference for comparing the spectra in Figure 3. The emission peaks observed here are consistent with the known transitions reported for RENs in solutions, the emission at 520 and 550 nm corresponds to the 2H11/2 → 4I15/2 and 4S3/2 → 4 I15/2, whereas 670 nm corresponds to the 4F9/2 → 4I15/2 3189
dx.doi.org/10.1021/jz301444a | J. Phys. Chem. Lett. 2012, 3, 3188−3192
The Journal of Physical Chemistry Letters
Letter
Figure 4. (A) Schematic diagram of the photoelectrochemical measurements. (B) Current density (red) of the REN/hematite electrodes was measured in the duration of the experiments, in which the 980 nm laser was turned on and off. The same electrode without RENs was also measured (green) as a negative control for comparison. This current versus time curve remains unvaried with lasers on or off. (C) I−V measurements were taken for REN/hematite composite films with (red) and without (green) 980 nm illuminations.
that incorporation of RENs into hematite films leads to higher efficiency and performance in solar water splitting because the RENs enable harvesting more photons at IR range than conventional hematite, which absorbs only UV and visible radiation. This concept of using RENs could be applied to general semiconductor photoelectrodes. Because the incident photon to current conversion efficiency (IPCE) of this composite electrode was calculated to be 1.24 × 10−4% at 980 nm, to improve further the efficiency and performance of solar water splitting devices, work is in progress to integrate broad IR radiation materials and improve upconversion efficiency. One possibility is to utilize a variety of RENs to cover a broad range of wavelengths.41−44 Alternatively, the efficiency of the existing composites could be improved by (a) increasing upconversion efficiency by varying and optimizing the host lattice and doping percentage45−50 and (b) increasing absorption of 550 nm REN emission by hematite by optimizing the materials design and production. We hope this concept could be further expanded to other solar devices, in which the addition of new REN-based materials would facilitate more solar absorption and thus higher energy conversion efficiency.
transitions.36−38 The ratio of REN emission, 550/670 nm, changed from 3.4 to 1.7 upon deposition onto hematite films. This trend has been repeatedly seen among all composite materials made, from 3.3 ± 0.1 to 1.7 ± 0.1 for REN spectra before and after deposition onto hematite electrodes, respectively. The decrease of 550 nm emission indicates the absorption by hematite films. The water splitting performance of these composite films was tested using a simple electrochemical cell as shown in Figure 4A.11,17,22 The electrical potential was applied between the working electrode, which was composed of RENs and hematite films, and a reference electrode of Hg/HgO in 1 M NaOH electrolyte solution, with current flowing between working electrode and a Pt mesh counter electrode. The RENs/ hematite electrode was subject to 980 nm radiation with power density at 2000 mW/cm2 (Aixiz Company). The photocurrent was measured as a function of radiation time and potential, respectively. Figure 4B shows a typical chronoamperometry measurement for the REN/hematite composite films upon turning the IR laser on and off with applied potential at 1.43 V versus RHE (i.e., reversible hydrogen electrode). With an applied potential of 1.43 V, the current density increased from 25 to 130 nA/cm2 upon illumination and sharply dropped to baseline level with removal of excitation. In contrast, current density remained low (
