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J. Phys. Chem. C 2010, 114, 2806–2813
Enhanced Conversion of Light at TiO2 Photonic Crystals to the Blue of a Stop Band and at TiO2 Random Films Sensitized with Q-CdS: Order and Disorder Maysaa El Harakeh and Lara Halaoui* Department of Chemistry, American UniVersity of Beirut, Beirut, Lebanon 110236 ReceiVed: October 12, 2009; ReVised Manuscript ReceiVed: December 18, 2009
Significant enhancements in the conversion of light to current were observed at the blue edge of TiO2 inverse opals (i-TiO2-o) and at highly disordered TiO2 films (i-TiO2-d) sensitized with Q-CdS in sulfide electrolyte. i-TiO2-o with stop bands centered at 390 and 450 nm were modified with mercaptopropionic acid-Q-CdS with absorption edges tuned to the red or to the blue of the stop-band edges. A 4.7 average enhancement factor was measured at the blue edge of the stop band when it coincided with low Q-CdS absorption, while a 1.4-1.8 average gain was measured at the red edge. The blue-edge gain can be ascribed to localized or slowed light in the low refractive index medium and was found to extend 30-70 nm to the blue of the stop-band center. Light localization effects were suppressed when the stop-band edges overlapped with appreciable absorption. A highly disordered TiO2 film fabricated by replicating a template from 150, 190, and 243 nm diameter polystyrene spheres exhibited a similar gain per adsorbed Q-CdS in the same spectral window as the blue edge of the photonic crystal when quantum dot absorption was low. This gain is ascribed to slowed light resulting from the interference of multiple internal scattering events in the disordered medium. Introduction Photonic crystals (PC) are ordered structures that exhibit a periodic modulation of the refractive index on the order of the wavelength of light. Similar to the development of an electronic band gap in a semiconductor, a photonic gap or a stop band opens up in photonic crystals.1-3 PCs found several applications as low threshold lasers,4 optical polarizers,5 waveguides,6 and light-emitting diodes7 and, lately, gained attention in solar cells8-16 and photocatalysis.17-21 Interest in PCs for energy conversion is based on the possibility of slowing or confining light at certain frequencies within this medium. The dispersion curve, frequency w versus wavevector k, exhibits significant bending near the band edges of PCs.6,8,22 The slope, which is proportional to the group velocity of lightsthe speed at which the wave modulation propagatessdecreases near the edges where it approaches zero; thus, the group velocity is greatly reduced of what has been termed “heavy” or “slow” photons.22,23 Suppressed group velocities have been observed near the blue and red edges of stop bands using light interferometry measurements.24-26 Reducing the group velocity of light can enhance the absorption probability by matter in the crystal by increasing matter-photon interaction time,8,27 and spectral effects on absorption and emission have been reported.8,9,21,28-30 PCs have been exploited in energy conversion by two ways. The first couples PCs to a semiconductor film to enhance the conversion efficiency over a wide spectral range8-16 via resonant modes created in the film by its intimate contact with the crystal,11,14 as demonstrated when coupling nc-TiO2 to an inverse opal (i-TiO2-o) in dye-sensitized solar cells (DSSCs).8-14 The second directly exploits the lower group velocity of light near the stop band to increase photon transit time.9,17-21,27,31 Reports appeared on enhancing the photocatalytic degradation of dyes in PCs, predominantly at the red edge of a stop band, except * To whom correspondence should be addressed. E-mail: Lara.Halaoui@ aub.edu.lb.
for a recent report of enhanced photoisomerization of azobenzene by 1.35-fold at the blue edge of i-SiO2-o.20 Another possible mechanism of slowing light is by multiple scattering in disordered media or at defects in real PCs. Light localization or slow photons near a photonic gap can be maintained even with a degree of disorder in PCs,18 as an amorphous semiconductor retains electronic gaps or pseudogaps. In the event of appreciable disorder in a medium, the constructive interference of counter-propagating waves could lead to enhanced backscattering, a weak light localization regime.32-35 Enhanced backscattering has been reported in polystyrene opals and in opals of air spheres in titania.35 Eventually, with significant disorder in a medium, Anderson localization could be reached,3,36,37 causing complete halt of light transport. John predicted that disorder in PCs could lead to Anderson localization of light.3 The effects of order and disorder in real PCs and of disorder in random media in causing enhancement of light conversion are important phenomena to understand for fundamental and practical reasons. We aimed to investigate photonic effects in inverse opals (i-TiO2-o) and disordered films (i-TiO2-d) sensitized with quantum dots (QD), for the purpose of understanding effects of order and disorder on enhancing the light conversion efficiency in these media and to couple phenomena of quantum confinement and slowed light to improve the performance of QD-sensitized wide band gap semiconductors. The study of QD solar cells is driven by a prediction that it may be possible to exceed the 31% Schockley-Queisser thermodynamic limit38 for conversion of sunlight at a bulk semiconductor because quantum confinement could potentially render more efficient the utilization of excess kinetic energy before it is dissipated as heat.39 One architecture investigated for QD solar cells consists of sensitizing a wide band gap semiconductor including nc-TiO2 films with QDs.40,41 Diguna et al. published, to our knowledge, the only report of QD-sensitized i-TiO2-o (viz., with Q-CdSe), motivated by the open structure for improved infiltration (by QD and electrolyte) and reported 0.9-2.7% power efficiency.42
10.1021/jp909764u 2010 American Chemical Society Published on Web 01/25/2010
Conversion of Light at TiO2 PCs and TiO2 Random Films However, photonic effects were not investigated. In this paper, we report the observation of photonic effects resulting in a significant amplification in the conversion of light to current at Q-CdS dots adsorbed on TiO2 inverse opals (i-TiO2-o) at the blue edge of the stop band and on disordered TiO2 films (iTiO2-d) (not coupled with nc-TiO2), in the first example of amplified photocurrent generation in a different solar cell, uncoupled to a regular semiconductor film, other than the coupled bilayer of the DSSC.8-15 Experimental Methods Materials. Suspensions of monodisperse carboxylate-modified polystyrene spheres (10 wt % in water; ) 150, 190, and 243 nm; σ ) 2%; Seradyn Co., Indianapolis, IN U.S.A.) were stored at 4 °C and used as purchased. Cadmium perchlorate hydrate, (Cd(ClO4)2, Aldrich), sodium sulfide nonahydrate (Na2S · 9H2O, Alfa Aesar), polyoxyethylene(5)nonyl phenyl ether surfactant (Igepal CO-520, Aldrich), boric acid (H3BO3, 99.5%, Aldrich), ammonium hexafluorotitanate ((NH4)2TiF6, 99.99%, Aldrich), titanium isopropoxide (Ti(OCH(CH3)2)4, Aldrich), nanocrystalline TiO2 slurry (nc-TiO2, ) 13 nm, Solaronix, Switzerland), mercaptopropionic acid (99%, Acros), ammonium hydroxide (28-30 wt %, Acros), sulfuric acid (98%, Acros), hydrogen peroxide (35%, Aldrich), nitric acid (Acros), hydrochloric acid (Fisher), ethanol (Recaptur), methanol (Fisher), toluene (Scharlau), and isopropanol (Acros) were used without further purification. Deionized water (resistivity g 18 ΜΩ · cm, Nanopure Diamond) was used in atomic absorption measurements; double-distilled water was used in other experiments. Synthesis of MPA-Modified Q-CdS. Mercaptopropionic acid (MPA)-modified Q-CdS nanoparticles (NPs) were synthesized by adding 20 µL (or 35 or 50 µL) of 1 M Cd(ClO4)2 in ethanol and 20 µL (or 35 or 50 µL, respectively) of 0.25 M Na2S · 9H2O in methanol under stirring to 25 mL of ethanol containing MPA at a concentration of 2 µL of MPA/100 mL of ethanol. Different ratios of MPA/Cd resulted in varying the Q-CdS size and, hence, the absorption edge. Preparation of Photonic Crystal and Disordered Colloidal Templates. TiO2 films were deposited on fluorine-doped SnO2 on glass substrates (FTO, coated one side, Solaronix, R ) 15 Ω/sq). Substrates were cleaned by sonication in water for 10 min and in isopropanol for 30 min in an ultrasonic bath (Nickel Electro Ltd.), rinsed in water, and dried in air. i-TiO2-o and i-TiO2-d films were prepared by replication of polystyrene (PS) colloidal crystals or disordered PS templates on FTO, respectively. Substrates were vertically fully immersed in a PS suspension in water (sonicated for 40 min to break aggregation) with 0.002% Igepal (to form a better meniscus between the FTO surface and solution), and ca. 3/4 of the solvent volume was allowed to evaporate under ambient (Barnstead thermolyne oven, 48 000) at 50 °C. PCs were prepared from a 0.05% or 0.1% suspension of 190 nm spheres or 0.1% of 243 nm PS spheres, whereas disordered templates were assembled from a 0.05% PS suspension containing 35% 150 nm/30% 190 nm/35% 243 nm spheres. The upper part, assembled before significant particle sedimentation, served as template. Opals and disordered colloidal films had thicknesses ranging from ∼2.5 to 5 µm, as measured by an Ambios XP-1 profilometer. TiO2 films of similar thicknesses were compared in the studies. Preparation of Inverse Opal, Disordered, and nc-TiO2 Films. PS templates were replicated with TiO2 following a reported liquid-phase deposition (LPD) procedure.8,9 Colloidal films were immersed vertically in 1.2% (w/v) titanium isopropoxide in (absolute) ethanol solution containing 0.12% (w/v)
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2807 nitric acid for 5 min, then held vertically in air for at least 1 h, intended to form a TiO2 seed layer. The films were then dipped vertically for 30 min in 0.2 M (NH4)2TiF6 and 0.25 M H3BO3(aq) (at pH 2.9 adjusted with 1 M HCl) at 50-52 °C, rinsed thoroughly in water, and air-dried. The infiltrated films were calcined at 400 °C under ambient for 8 h, resulting in either i-TiO2-o or i-TiO2-d films. TEM and XRD of films prepared by this method previously showed growth of anatase TiO2 with a 10 nm crystallite size.9 The TiO2 refractive index is calculated to be 2.0 from the stop-band peak of 450-i-TiO2-o in water using the Bragg equation. nc-TiO2 films (3-5 µm thick) were deposited on FTO from a slurry of 13 nm TiO2 NPs (anatase, Eg) 3.2 eV) via a squeegee method using a 3M Scotch tape (1 layer) to set the thickness. The solvent was evaporated at 80 °C for ∼1 min, and particles were sintered at 400 °C for 1 h. Films were placed face-down on indium tin oxide glass (ITO, Delta Technologies) substrates while sintering. Film Sensitization with Q-CdS. TiO2 films heated to 120 °C for 1 h were immersed in MPA-Q-CdS/ethanol solution for at least 48 h, followed by rinsing in ethanol for 1 min. Heating to 120 °C was to desorb water molecules that could interfere with the binding of the QDs to TiO2. Photoelectrochemical Measurements. Photocurrents were measured using a CHI model 630A electrochemical workstation (CH instruments) in a 3-electrode quartz photoelectrochemical (PEC) cell with a 2 mm diameter Pt wire as the auxiliary electrode and a homemade Ag/AgCl electrode (saturated KCl) as a reference electrode, in aqueous alkaline sulfide electrolyte (0.1 M Na2S and 0.2 M NaOH, pH ) 12.6). Solutions were deaerated by purging with N2(g) for at least 30 min, and a N2(g) blanket was maintained during measurements. Films were irradiated from a 300 W Xe lamp (model 66901 lamp housing and 68911 power supply, Oriel Instruments) operated at 250 W, attached to a 1/4 m grating monochromator (model 77200, Oriel). The lamp spectrum was measured at the approximated electrode position using a thermopile light detector and power meter (model 70260, Oriel) with 10 s average measurements. For collecting photoaction spectra, amperometric plots were acquired at 0 V versus Ag/AgCl under chopped illumination and photocurrents were determined after dark current subtraction (at t ) 45 s). Chopping of light allowed continuous monitoring of photocurrent versus dark current. The monochromatic incident photon-to-current conversion efficiency at a potential V (denoted % IPCE) is calculated according to
% IPCE (at V) )
J@V(A/cm2) 2
I@λ(W/cm )
×
1240(eV.nm) × 100 λ(nm)
where J is the photocurrent density at an applied potential V (0 V vs Ag/AgCl) and I is the light intensity at λ. Photocurrent density is reported with respect to the illuminated area of the film. Scattering and reflection losses were not accounted for. Imaging and Spectroscopy. SEM and TEM images were acquired (by Dr. Yan Xin, NHMFL, Florida State University) on a Carl Zeiss 1540 EsB SEM and a JEOL-2011 HRTEM operated at 200 kV (with a point resolution of 0.23 nm and a lattice resolution of 0.14 nm), respectively. For TEM imaging, a 10 µL drop of Q-CdS in ethanol solution was cast on a C/Cu grid (SPI) and dried in air. Photoluminescence (PL) spectra of films were acquired using a Jobin Yvon Horiba Fluorolog-3 fluorometer, in front-face detection mode with excitation at 360 nm. UV-visible absorption spectra were collected using a
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Figure 1. UV-visible absorption spectra of Q-CdS solutions termed in reference to the absorption edges: Q-CdS-410 (i), Q-CdS-430 (ii), and Q-CdS-445 (iii) (primary axis). Absorption spectra of i-TiO2-o replicated from PC190 with the void filled with water (a) and with toluene (b), termed 390-i-TiO2-o, or from PC243 in water, termed 450-i-TiO2-o (c) (secondary axis). The inset shows the stop band peaks of PC190 and PC243.
JASCO V-570 UV-visible spectrophotometer. The films’ void structure was filled with water or toluene, as indicated. The coverage by the sensitizer was quantitatively determined by measuring adsorbed Cd amounts using atomic absorption spectroscopy (SOLAAR with ASX-510 autosampler), after desorbing Q-CdS in 1 M HNO3(aq). It was ensured that the photocurrent did not change following a PEC experiment (thus eliminating the possibility of QD desorption in the electrolyte). Calibration plots were measured before each determination using standard concentrations of Cd2+ in HNO3. The concentration of Cd2+ is divided by the geometric (cross-sectional) area of films, reported as nmol Cd/cm2. The % IPCE divided by the adsorbed Cd content (CdSads nmol · cm-2) is denoted normalized IPCE. An enhancement factor (EF) is calculated by dividing the normalized IPCE at i-TiO2-o (or i-TiO2-d) with that at ncTiO2. An average EF is computed by dividing the normalized IPCE at each of i-TiO2-o in a set (or at one film, as indicated in the discussion) with each of the normalized IPCE at nc-TiO2 films. Results and Discussion TiO2 inverse opals (i-TiO2-o) replicated from PCs from 190 nm (PC190) or 243 nm (PC243) polystyrene spheres exhibited respective stop bands centered at ca. 390 and 450 nm in water (n ) 1.33) (Figure 1) and are termed 390-i-TiO2-o and 450-iTiO2-o. A more defined peak is observed for 390-i-TiO2-o (at 407 nm) when filling the void with toluene (n ) 1.55) by shifting from electronic absorption. SEM images of a 450-iTiO2-o film (Figure 2) show the layers’ order through a crack that must have existed in the original fcc colloidal crystal and show the ordered internal titania structure with a 179 ( 9 nm cavity size, indicating 26% shrinkage.8,9,17,42 Mecaptopropionic acid (MPA)-modified Q-CdS were prepared in ethanol with absorption edges at 413 nm (3.0 eV), 433 nm (2.9 eV), and 445 nm (2.8 eV), tuned to the red of the 390-
El Harakeh and Halaoui
Figure 2. SEM images of 450-i-TiO2-o film replicated from an opal assembled from 243 nm diameter polystyrene spheres.
i-TiO2-o or to the blue of the 450-i-TiO2-o stop bands (Figure 1). Pseudopotential calculations43 estimate the sizes of Q-CdS at 2.4, 2.9, and 3.2 nm for 3.0, 2.9, and 2.8 eV, respectively, consistent with the TEM size, revealing a ca. 3.0 nm diameter for 2.9 eV Q-CdS particles (that were not agglomerated) (Figure S1, Supporting Information). For easy reference relative to stop bands, the QDs are denoted Q-CdS-410, Q-CdS-430, and Q-CdS-445. The bifunctional MPA modifies the NP surface using the thiol group and binds to TiO2 via the carboxylate moiety.41 Existing scratches in the i-TiO2-o upper layer (inset of Figure 2) and cuts created with a blade to define the area open the structure to infiltration by Q-CdS/ethanol solution. Q-CdS adsorption on i-TiO2-o was evidenced in photoluminescence spectra (Figure S2, Supporting Information). Incident photon-to-current conversion efficiency (IPCE %) spectra were collected by measuring anodic photocurrents at 0 V versus Ag/ AgCl in the presence of S2- (0.1 M N2S, pH ) 12.6) as a regenerative hole scavenger (cf. Figure S3, Supporting Information). S2- efficiently scavenges photogenerated holes from Q-CdS,44 and the photogenerated electron is injected to the TiO2 conduction band.45 Q-CdS coverage varied for films of comparable thickness, possibly due to surface differences affecting wetting and QD binding. The coverage could not be optimized by the dipping time; this may need a different preparation or post-treatment. Variations in RuL2(SCN)2 adsorbed amounts on similarly fabricated i-TiO2-o were reported by Lee et al., and films with same dye coverage were compared.10 Here, % IPCE values were normalized by amounts of CdS adsorbed (CdSads in nmol/cm2) measured by atomic absorption, and, where applicable, % IPCE values were compared for films with comparable coverage. A proportional increase in % IPCE with Q-CdS coverage (e.g., Figure S4, Supporting Information) and adsorbed amounts larger than needed for an assumed fully packed Q-CdS layer on the outer surface ensured that QDs are uniformly distributed on the internal surface, and all Q-CdS dots at the levels and thicknesses investigated are contributing to the IPCE. For instance, we estimate that 2.6 nmol/cm2 of CdS is needed to cover the outer surface area of 450-i-TiO2-o (∼180 nm sphere size) with an assumedsif unlikelysclosely packed 2.4 nm Q-CdS with a 1
Conversion of Light at TiO2 PCs and TiO2 Random Films
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2809 SCHEME 1: Sketch of a TiO2 Inverse Opal Sensitized with Q-CdS (Yellow Dots) and Processes of Bragg Reflection, Diffuse Scattering, and Multiple Internal Scattering in the Photonic Crystal. An Absorbance Event in the Inverse Opal Is Denoted by a Red Dot (Not to Scale). Right Shows a Sketch of Multiple Internal Scattering at Random Scattering Centers, Leading to Light Localization
Figure 3. (A) Photoaction spectra at a 390-i-TiO2-o film (red line, a) and at three nc-TiO2 films (dotted black line, b) sensitized with Q-CdS410 at 137 nmol/cm2 of CdSads and 129 ( 16 nmol/cm2 of CdSads, respectively. Normalized % IPCE spectra (primary axis) and enhancement factor EF (secondary axis) are shown in the inset. (B) Average normalized % IPCE at Q-CdS-410 sensitized 390-i-TiO2-o (N ) 3) and nc-TiO2 (N ) 7) films. The inset shows average EF vs wavelength. The blue arrows indicate the stop-band position in water.
nm cap, assumed rigid, and similarly for 390-i-TiO2-o. A coverage of 60 nmol/cm2 of CdS on 450-i-TiO2-o (vide infra) is 23 times this hypothetical outer-layer coverage, whereas 137 nmol/cm2 on 390-i-TiO2-o (vide infra) is 53 times this coverage, indicating that the QDs are distributed on the internal, not merely the external, surface. A gain in the conversion efficiency was measured at the red edge of the stop band when it coincided with low Q-CdS absorption. Photoaction spectra (Figure 3) at a 5 µm 390-iTiO2-o film sensitized with Q-CdS-410 at 137 nmol/cm2 of CdSads and at three similarly sensitized (4.7 ( 0.02 µm) ncTiO2 films with comparable coverage (129 ( 16 nmol/cm2) reveal photocurrent onsets consistent with Q-CdS absorption and a % IPCE reaching 37% at 400 nm at Q-CdS/i-TiO2-o. The enhancement factor (EF, ratio of normalized IPCE at i-TiO2-o to nc-TiO2) equaled 1.2 ( 0.3 at 400 nm and 1.6 ( 0.8 at 420 nm (inset of Figure 3A). The minimal gain at 400 nm (EF of 1.24 ( 0.50) and the gain at 420 nm (EF of 1.82 ( 1.00) at the red edge, albeit the wide variation, were confirmed at Q-CdS410/390-i-TiO2-o films (N ) 3) compared with nc-TiO2 films
(N ) 7) with varied coverage (Table S-1, Supporting Information and Figure 3B). The gain at 420 nm, 30 nm to the red of the stop-band center, is attributed to photonic effects slowing light at the red edge.9,17-21 The gain decreased with increased absorbance of Q-CdS-430 and Q-CdS-445, consistent with absorbance suppressing photonic effects.46 EFs of 1.4 ( 0.4 at 400 nm and 1.4 ( 0.3 at 420 nm were measured at Q-CdS430/390-i-TiO2-o films (N ) 3) and decreased to 0.9 ( 0.3 at 440 nm (Figure S5 and Table S-2, Supporting Information). The absence of a gain at low absorbance at 440 nm supports the attribution of a red-edge effect, while the persistence of a gain close to stop-band reflection could result from defects introducing photon states. Although Bragg reflection attenuates the light intensity, internal Bragg reflection can also increase the path length of light in this window (Scheme 1). The EF further decreased to 1.2 ( 0.4 at 420 nm for Q-CdS-445 sensitized 390-i-TiO2-o (Figure S6 and Table S-3, Supporting Information). The same normalized IPCE at 390-i-TiO2-o and nc-TiO2 films with Q-CdS-445 indicates that the yield of electron injection to TiO2 and its collection must be similar at the two structures and that facile infiltration of the electrolyte is an unlikely determining factor in this case. On the other hand, when the stop band was shifted to the red of Q-CdS absorbance, a significantly greater gain was measured to the blue of the stop band. Photoaction spectra (Figure 4A) at a 450-i-TiO2-o film sensitized with Q-CdS-410 at 60 nmol/cm2 of CdSads reveal a 37% IPCE at 400 nm, compared with 15 ( 4.4% at nc-TiO2 (62 ( 4 nmol/cm2, N ) 4). The EF was highest at 420 nm, 30 nm to the blue of the stop-band center, equaling 3.8 ( 1.5 (inset of Figure 4A), and decreased at 400 nm (3.0 ( 1.0). The significant blue-edge gain was confirmed at Q-CdS410/450-i-TiO2-o films with varying coverage (N ) 3, Table 1 and Figure 4B), with a highest EF equal to 4.7 ( 2.6 at 420 nm, significantly larger than the red-edge gain at the same wavelength (measured at 390-i-TiO2-o). Each of the films featured a blue-edge gain, experimental variations notwithstanding, and the enhancement was reproduced when comparing to nc-TiO2 films with 129 ( 16 nmol/cm2 of CdSads (N ) 3; cf. data in Table S-1, Supporting Information) and is thus present irrespective of QD coverage here. The EF also diminished when the blue edge overlapped with greater QD absorption, to 2.0 ( 0.7 at 420 nm for Q-CdS-445 on 450-i-TiO-o films (N ) 3)
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Figure 4. (A) Photoaction spectra at a 450-i-TiO2-o film (red line, a) and at three nc-TiO2 films (dotted black line, b) sensitized with Q-CdS410 at 60 nmol/cm2 of CdSads and 62 ( 4 nmol/cm2 of CdSads, respectively. The inset shows normalized % IPCE spectra (primary axis) and EF (secondary axis). (B) Average normalized % IPCE spectra at Q-CdS-410 sensitized 450-i-TiO2-o (N ) 3) and at nc-TiO2 (N ) 4) (cf. Table 1). The inset shows average EF vs wavelength. Red arrows show the stop-band positions in water.
(Figure 5), consistent with absorbance suppressing localization effects.46 It is notable that the variability in the enhancement increased with the increase in EF, leaving one to question if this may be inherent to the measurement at low absorbance and in the presence of scattering events rather than to only differences between samples. The blue-edge gain can be ascribed to slow group velocity of light at the blue edge of the stop band. At the band edges of a photonic gap, light can be described as a standing wave; to the red edge, the wave has a maximum amplitude in the high refractive index medium and nodes in the low refractive index medium, while at the blue edge, light intensity will be localized in the low refractive index medium.8,26,47 This mechanism can be maintained in the presence of disorder in the photonic crystal. Q-CdS particles (2-3 nm) adsorbed on i-TiO2-o via an ∼1 nm MPA linker are partly solvated in the aqueous medium in the cavity and can absorb light with high intensity in that medium (Scheme 1). The blue-edge gain is greater than measured at the
El Harakeh and Halaoui red edge for this system and larger than reported red-edge gains at TiO2 PCs, whereas the 1.4 to 1.8-fold red-edge gain is consistent with reported numbers at i-TiO2-o.9,17-19 A blue-edge EF of 1.35 was reported for the photoisomerization of azobenzene on silica PC in air, also greater than the observed rededge gain.20 Although better infiltration by the electrolyte could be thought to have contributed to a greater gain at the 450-iTiO2-o, the difference in pore size compared to 390-i-TiO2-o in this respect is not great to account for the significant difference. The decrease in EF with increasing absorbance and with shifting away from the blue edge supports the attribution of the gain to photonic effects at the blue edge, improving absorbance by the QDs. A greater blue-edge gain relative to the red-edge gain is consistent with measurements of slower group velocity of light at the blue edge of a PC opal26 compared with the red edge, attributed to diffuse scattering at defects in the material band, affecting (increasing), to a greater extent, the group velocity at the red edge.26,27 The gain extended 70 nm to the blue of the stop-band center, which can be caused by variation in the number of layers, as the particles sediment during template assembly, causing broadening of the blue-edge effect.26 A long-range linear increase in thickness and short-range thickness fluctuations have been observed along the growth direction in PCs prepared via the evaporation self-assembly.48,49 A second mechanism that affects light propagation in real PCs (and in random media) is internal scattering at disordered regions. Real PCs are not perfectly ordered; disorder is inherent to the PC preparation method and could consist of point and line defects, grain boundaries, drying cracks, stacking faults, and a cavity size distribution and depends on the size distribution and the experimental conditions.50,51 Although the presence of a stop band and the SEM images are evidence of the presence of a degree of order in the structure, the scattering background in the UV-visible spectra (cf. Figure 1) along with the broad peaks and the absence of total reflection at stop-band frequenciesscausing an EF larger than 1 within these frequenciess indicate the existence of inherent disorder in the photonic crystals. Single scattering and multiple scattering can take place at these defects. Mie scattering occurs at particles with a size comparable to the wavelength of light;52-54 such scatterers could be ∼150 to 200 nm TiO2 particles formed from sphere vacancies in the template.50 On the other hand, Rayleigh scattering is effective for very small scattering points relative to the wavelength of light (2πr/λ , 1).52 These defects (diameter ∼30 nm or less) can result from filling areas from sphere random displacements or the size distribution and from surface roughness.50 Diffuse scattering can decrease the light intensity reaching the interior of the film, but scattering can also lead to a shorter mean free path in the film’s volume (Scheme 1). Enhanced backscattering, known as the weak light localization regime and a precursor to Anderson localization, has been observed by Koenderink et al. in photonic crystals of polystyrene spheres and air spheres in TiO235 and was modeled by diffusion theory in random media after taking into account the effect of the stop band. It has also been hypothesized by John that strong photon localization can be reached in photonic crystals with disorder in a “delicate interplay between order and disorder”.3 Disorder will play a role in affecting light propagation in real photonic crystals, and internal scattering at defects could have contributed to the blue-edge and red-edge gains and to extending the gains over a wide spectral range. However, there is no evidence that the 450-i-TiO2-o is more disordered than 390-iTiO2-o (cf. widths at half-height of stop-band peaks in Figure 1 for the inverse opals and PCs) to ascribe the significantly
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TABLE 1: % IPCE and Normalized % IPCE/CdSads at 450-i-TiO2-o (N ) 3), n-TiO2 (N ) 4), and i-TiO2-d (N ) 5) Sensitized with Q-CdS-410a CdSads (nmol/cm2)
thickness (µm)
% IPCE (normalized % IPCE) at 380 nm
% IPCE (normalized % IPCE) at 400 nm
% IPCE (normalized % IPCE) at 420 nm
63.3 (1.07) 19.9 (0.9) 28.6 (1.9) 1.29 ( 0.52 27.7 ( 9.3 0.45 ( 0.16 46.1 (0.71) 34.0 ( 10.3 (1.30 ( 0.44) 1.18 ( 0.47
38.9 (0.66) 9.7 (0.44) 19 (1.26) 0.79 ( 0.42 15 ( 4.4 0.24 ( 0.08 31.1 (0.48) 21 ( 7.7 (0.81 ( 0.34) 0.74 ( 0.33
14.4 (0.25) 4.3 (0.2) 7 (0.46) 0.30 ( 0.14 4.5 ( 1.6 0.073 ( 0.03 14.2 (0.22) 8.1 ( 2.7 (0.31 ( 0.11) 0.29 ( 0.11
450-i-TiO2-o/Q-CdS-410 (N ) 3)
60 22 15
4.5 5.3 3.6
nc-TiO2/Q-CdS-410 (N ) 4)
62 ( 4
4.3 ( 1
i-TiO2-d/Q-CdS-410 (N ) 5)
65 26 ( 2 (N ) 4)
5.3 4.0 ( 0.5
a
N is the number of samples. Normalized % IPCE values are in italics. Normalized % IPCE averages for each films’ set are in bold.
Figure 5. Photoaction spectra at a 450-i-TiO2-o film (N ) 3; 4.0 ( 0.2 µm thick; red line, a) and nc-TiO2 films (N ) 4; 3.6 ( 0.5 µm thick; dotted black line, b) sensitized with Q-CdS-445 at 41 ( 6 nmol/ cm2 of CdSads and 65 ( 5 nmol/cm2 of CdSads, respectively. The inset shows the normalized % IPCE (primary axis) and EF (secondary axis) spectra. The red arrow marks the position of the stop band.
greater gain at the former to scattering at disordered regions without taking into consideration the effect of the stop band. Understanding the effect of inherent disorder in affecting light propagation in these PCs and decoupling the roles of order and disorder requires further systematic studies on varying the extent of disorder and the sphere dimension in these materials. Lopez and co-workers showed the presence of Mie resonances in random materials assembled from one size spheres, termed photonic glass.55 The existing resonances were shown to lead to a spectral dependence of the scattering mean free path and light diffusion constant that depends on the scattering center size. Such a mechanism cannot explain the difference in the EF at 420 nm measured at the blue edge versus the red edge of the inverse opals stop bands since the 180 nm cavity size in this system (for 450-i-TiO2-o or ∼140 nm for 390-i-TiO2-o) is too small to sustain such Mie resonances at this wavelength (nd/λ would need to be ∼2 or larger55). Interestingly, a similar gain was achieved by invoking significant disorder in a medium, ascribed to a different localization mechanism. The effect was studied at i-TiO2-d replicated from 150, 190, and 243 nm diameter PS spheres template (35%:30%:35%). The significant randomness is observed by SEM (Figure 6) and causes an absence of a stop band (Figure S7, Supporting Information). Figure 7 compares pho-
Figure 6. SEM images of i-TiO2-d replicated from a template from three polystyrene sizes of 150, 190, and 243 nm (scale bars are 1 µm).
toaction spectra at i-TiO2-d and nc-TiO2 films (N ) 4 each) sensitized with Q-CdS-410 at 26 ( 2 and 62 ( 4 nmol/cm2 of CdSads, respectively. The EF equaled 4.8 ( 2.4 at 420 nm and 5.1 ( 2.5 at 440 nm (Figure 7, inset). The same trend was observed at an i-TiO2-d film of similar coverage as that of the nc-TiO2 films (Figure S8, Supporting Information, and Table 1). The gain in this case is attributed to multiple internal scattering significantly slowing light. The EF also decreased significantly when the 400-420 nm range coincided with greater QD absorption, equaling 1.6 ( 0.7 at 400 nm and 1.3 ( 0.7 at 420 nm at Q-CdS-445/i-TiO2-d films (N ) 3; Table S-4, Supporting Information), consistent with a model of light localization being suppressed by absorbance. The magnitude and spectral dependence of the EF are similar at i-TiO2-d and at 450-i-TiO2-o to the blue of the inverse opal stop band (Figure 7, inset), except for the dip within stop-band frequencies at the later. Ozin and co-workers studied the effect of disorder on the gain of dye photodegradation at i-TiO2-o and concluded, in that case, that the gain was mainly caused by structural order rather than disorder.18 On the other hand, a 2.6-fold enhancement was measured per adsorbed Ru-dye at 680 nm at i-TiO2-d films from a 1:2 ratio of 150 nm/243 nm PS sphere template, greater than the 1.5 EF at the red edge of an i-TiO2-o stop band.9 Depending on the degree of disorder in a medium, multiple elastic scattering can lead to light localization by interference of counterpropagating waves, leading to significant enhancements in light
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El Harakeh and Halaoui Acknowledgment. We thank the University Research Board at AUB for financial support of this work. We thank Dr. Yan Xin at the NHMFL, Florida State University, Tallahassee, Florida, for acquiring SEM and TEM images, and we thank the NSF (DMR-9625692) and NHMFL under cooperative agreement DMR-0084173. Supporting Information Available: TEM image of Q-CdS430; PL spectra; I-t curve under chopped illumination at Q-CdS-410 on 390-i-TiO2-o; photoaction spectra at 390-i-TiO2-o sensitized with Q-CdS-445 at different nanoparticle loadings; photoaction spectra, EF spectra, and data tables at 390-i-TiO2-o and nc-TiO2 films sensitized with Q-CdS-430 and with Q-CdS445; UV-visible of disordered films; photoaction spectra, EF spectra, and data tables at i-TiO2-d films sensitized with Q-CdS410 and Q-CdS-445 at various QD coverages. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 7. Photoaction spectra of i-TiO2-d films (N ) 4; red line, a) and nc-TiO2 films (N ) 4; dotted black line, b) sensitized with Q-CdS410 at 26 ( 2 nmol/cm2 of CdSads and 62 ( 4 nmol/cm2 of CdSads, respectively. The inset shows the normalized % IPCE (primary axis) and the EF (secondary axis) spectra at i-TiO2-d films. The average EF at 450-i-TiO2-o films sensitized with Q-CdS-410 is presented (open symbols) for comparison.
harvesting. This effect would exist where it is most needed, where absorption is low. The significant gain at disordered films sensitized with Q-CdS-410, comparable to the blue-edge EF at inverse opals, can be likely caused by such a mechanism (Scheme 1). Appreciable absorbance appears herein to suppress more the gain at disordered films than at i-TiO2-o (cf. Figure S9, Supporting Information, Q-CdS-445 case)sand understanding this would require further studying. Conclusion Amplification of light conversion has, therefore, been demonstrated for the first time at QD-sensitized titania inverse opal solar cellssnot coupled to nc-TiO2sat the blue edge of the stop band and in highly randomized media, and the gain was larger than red-edge gains measured at this system or reported in the literature.9,17-21 The blue-edge gain could be caused by slower light at the blue edge of the stop band localized in the low refractive index medium. Reported differences in the group velocity of light at the red and blue edges are consistent with the observation of greater blue-edge gain. Real photonic crystals are not perfectly ordered, and disorder will influence light propagation in these media. It is conceivable that disorderseither by variable layer thickness or by scattering at defect sitesscauses the gain to extend to a wide range to the blue of the stop band. It is also conceivable that disorder has contributed to the observed gains at the inverse opals. When very significant in a random medium, disorder can cause light localization by interference of waves, and a similar gain was observed at a disordered film with three sizes of pores. Exploiting slow light to the blue of a stop band or in disordered films offers a greater amplification of photoreactions or light conversion, while avoiding stop-band reflectivity losses in absorbance problematic when exploiting a red-edge effect. A future systematic study by a controlled variation of disorder while varying the cavity sizes and size ratios, in the presence and absence of a stop band, will be conducted to decouple various factors contributing to slowing light in real PCs and in highly random media.
(1) (a) Carlson, R. J.; Asher, S. A. Appl. Spectrosc. 1984, 38, 297. (b) Flaugh, P. L.; O’Donnel, S. E.; Aher, S. A. Appl. Spectrosc. 1984, 38, 847. (2) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (3) John, S. Phys. ReV. Lett. 1987, 58, 2486. (4) Yamomoto, Y.; Slusher, R. E. Phys. Today 1993, 46, 66. (5) (a) Tyan, R.; et al. J. Opt. Soc. Am. A 1997, 14, 1627. (b) Ohtera, Y.; Sato, T.; Kawashima, T.; Tamamura, T.; Kawamaki, S. Electron. Lett. 1999, 35, 1271. (6) Mekis, A.; et al. Phys. ReV. Lett. 1996, 77, 3787. (7) Xu, Z.; Cao, L.; Tan, Q.; He, Q.; Jin, G. Opt. Commum. 2007, 278, 211. (8) Nishimura, S.; Abrams, N.; Lewis, B. A.; Halaoui, L. I.; Mallouk, T. E.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. J. Am. Chem. Soc. 2003, 125, 6306. (9) Halaoui, L. I.; Abrams, N.; Mallouk, T. E. J. Phys. Chem. B 2005, 109, 6334. (10) Lee, S. A.; Abrams, N. M.; Hoertz, P. G.; Barber, G. D.; Halaoui, L. I.; Mallouk, T. E. J. Phys. Chem. B 2008, 112, 14415. (11) Mihi, A.; Miguez, H. J. Phys. Chem. B 2005, 109, 15968. (12) Mihi, A.; Lopez-Alcaraz, F. J.; Miguez, H. Appl. Phys. Lett. 2006, 88, 193110. (13) Mihi, A.; Calvo, M. E.; Anta, J. A.; Miguez, H. J. Phys. Chem. C 2008, 112, 13–17. (14) Colodrero, S.; Mihi, A.; Anta, J. A.; Ocana, M.; Miguez, H. J. Phys. Chem. C 2009, 113, 1150–1154. (15) Colodrero, S.; Mihi, A.; Ha¨ggman, L.; Ocan˜a, M.; Boschloo, G.; Hagfelt, A.; Mı´guez, H. AdV. Mater. 2009, 21, 764–770. (16) (a) O’Brien, P. G.; Kherani, N. P.; Zukotynski, S.; Ozin, G. A.; Vekris, E.; Tetreault, A.; Chutinan, A.; John, S.; Mihi, A.; Miguez, H. AdV. Mater. 2007, 19, 4177. (b) Suezaki, T.; O’Brien, P. G.; Chen, J. I. L.; Loso, E.; Kherani, N. P.; Ozin, G. A. AdV. Mater. 2009, 21, 559. (17) Chen, J. I. L.; von Freymann, G.; Choi, S. Y.; Kitaev, V.; Ozin, G. A. AdV. Mater. 2006, 18, 1915. (18) Chen, J. I. L.; von Freymann, G.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2007, 129, 1196. (19) Chen, J. I. L.; Loso, E.; Ebrahim, N.; Ozin, G. A. J. Am. Chem. Soc. 2008, 130, 5420. (20) Chen, J. I. L.; Ozin, G. A. AdV. Mater. 2008, 20, 4784–4788. (21) Li, Y.; Kunitake, T.; Fujikawa, S. J. Phys. Chem. B 2006, 110, 13000. (22) (a) Sakoda, K. J. Opt. Soc. Am. B 1999, 16, 361. (b) Scalora, M.; Dowling, J. P.; Bowden, C. W.; Bloemer, M. J. Phys. ReV. Lett. 1994, 73, 1368. (c) Dowling, J. P.; Scalora, M.; Bloemer, M. J.; Bowden, C. W. J. Appl. Phys. 1994, 75, 1896. (d) Tocci, M. D.; Scalora, M.; Bloemer, M. J.; Dowling, J. P.; Bowden, C. W. Phys. ReV. A 1996, 53, 2799. (23) Vlasov, Y. A.; Petit, S.; Klein, G.; Ho¨nerlage, B.; Hirlimann, Ch. Phys. ReV. E 1999, 60, 1030. (24) Imhof, A.; Vos, W. L.; Sprik, R.; Lagendijk, A. Phys. ReV. Lett. 1999, 83, 2942. (25) von Freymann, G.; John, S.; Wong, S.; Kitaev, V.; Ozin, G. A. Appl. Phys. Lett. 2005, 86, 053108. (26) Galisteo-Lo´pez, J. F.; Galli, M.; Patrini, M.; Balestreri, A.; Andreani, L. C.; Lo´pez, C. Phys. ReV. B 2006, 73, 125103. (27) Chen, J. I. L.; von Freymann, G.; Choi, S. Y.; Kitaev, V.; Ozin, G. A. J. Mater. Chem. 2008, 18, 369. (28) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528. (29) Yoshino, K.; Lee, S. B.; Tatsuhara, S.; Kawagishi, Y.; Ozaki, M.; Zakhidox, A. Appl. Phys. Lett. 1998, 73, 3506.
Conversion of Light at TiO2 PCs and TiO2 Random Films (30) Vlasov, Y. A.; Luterova, K.; Pelant, I.; Ho¨nerlage, B.; Astratov, V. N. Appl. Phys. Lett. 1997, 71, 1616. (31) Yip, C.; Chiang, Y.; Wong, C. J. Phys. Chem. C 2008, 112, 8735. (32) var der Mark, M. B.; van Albada, M. P.; Laggendijk, A. Phys. ReV. B 1988, 37, 3575. (33) Wolf, P.-E.; Maret, G. Phys. ReV. Lett. 1985, 55, 2696. (34) Albada, M. P.; Lagendijk, A. Phys. ReV. Lett. 1985, 55, 2692. (35) Koenderink, A. F.; Megens, M.; van Soest, G.; Vos, W. L.; Legendijk, A. Phys. Lett. A 2000, 268, 104. (36) Anderson, P. W. Phys. ReV. 1958, 109, 1492. (37) (a) John, S. Phys. ReV. Lett. 1984, 53, 2169. (b) Soukoulis, C. M.; Economou, E. N.; Grest, G. S.; Cohen, M. H. Phys. ReV. Lett. 1989, 62, 575. (c) Garcia, N.; Genack, A. Z. Phys. ReV. Lett. 1991, 66, 1850. (d) Stoytchev, M.; Genack, A. Z. Phys. ReV. B 1997, 55, R861. (e) Wiersma, D. S.; Bartolini, P.; Lagendijk, A.; Righini, R. Nature 1997, 390, 671. (f) Schuurmans, F. J. P.; Megens, M.; Vanmaekelbergh, D.; Lagendjik, A. Phys. ReV. 1999, 83, 2183. (38) Shockley, W.; Queisser, H. J. J. Appl. Phys. 1961, 32, 510–519. (39) Nozik, A. J. Physica E 2002, 14, 115–120. (40) (a) Vogel, R.; Pohl, K.; Weller, H. Chem. Phys. Lett. 1990, 174, 241. (b) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183– 3188. (c) Zaban, A.; Mic´ic´, O. I.; Gregg, B. A.; Nozik, A. J. Langmuir 1998, 14, 3153. (41) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (42) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, V. Appl. Phys. Lett. 2007, 91, 023116. (43) Rama Krishna, M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95, 8309.
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2813 (44) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S. J. Am. Chem. Soc. 1977, 99, 2839. (45) Gerischer, H.; Luebke, M. J. Electroanal. Chem. 1986, 204, 225. (46) von Freymann, G.; John, S.; Schulz-Dobrick, M.; Vekris, E.; Te´treault, N.; Wong, S.; Kitaev, V.; Ozin, G. A. Appl. Phys. Lett. 2004, 84, 224. (47) Meade, R. D.; Rappe, A. M.; Brommer, K. D.; Joannopoulos, J. D. J. Opt. Soc. Am. B 1993, 10, 328. (48) Lozano, G.; Mı´guez, H. Langmuir 2007, 23, 9933. (49) Schimmin, R. G.; DiMauro, A. J.; Braun, P. V. Langmuir 2006, 22, 6507. (50) Rengarajan, R.; Mittleman, D.; Rich, C.; Colvin, V. Phys. ReV. E 2005, 71, 016615. (51) Palacios-Lido´n, E.; Jua´rez, B. H.; Castillo-Martı´nez, E.; Lo´pez, C. J. Appl. Phys. 2005, 97, 063502. (52) (a) He, G. S.; Qin, H.; Zheng, Q. J. Appl. Phys. 2009, 105, 023110. (b) Mishenchenko, M. I.; Travis, L. D.; Lacis, A. A. Scattering, Absorption, and Emission of Light by Small Particles; Cambridge University Press: Cambridge, U.K., 2002. (c) Bohren, C. F.; Huffman, D. R. Absorption and scattering of light by small particles; John Wiley & Sons: New York, 1983. (53) Ferber, J.; Luther, J. Sol. Energy Mater. Sol. Cells 1998, 54, 265. (54) Hore, S.; Nitz, P.; Vetter, C.; Prahl, C.; Niggemann, M.; Kern, R. Chem. Commun. 2005, 2011. (55) Garcı´a, P. D.; Sapienza, R.; Bertolotti, J.; Martı´n, M. D.; Blanco, ´ .; Altube, A.; Vin˜a, L.; Wiersma, D. S.; Lo´pez, C. Phys. ReV. A 2008, 78, A 023823.
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