Origin of the Black Color of NiO Used as Photocathode in p-Type Dye

Article pubs.acs.org/JPCC

Origin of the Black Color of NiO Used as Photocathode in p‑Type Dye-Sensitized Solar Cells Adèle Renaud,† Benoit Chavillon,† Laurent Cario,*,† Loïc Le Pleux,‡ Nadine Szuwarski,‡ Yann Pellegrin,‡ Errol Blart,‡ Eric Gautron,† Fabrice Odobel,‡ and Stéphane Jobic*,† †

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP3229, 44322 Nantes cedex 3, France ‡ CEISAM, Université de Nantes, CNRS, UMR 6230, 2 rue de la Houssinière, 44322 Nantes cedex 3, France ABSTRACT: We report herein on the origin of the black color of NiO, the widespread p-type semiconductor used as photocathode in p-type dye-sensitized solar cells (DSSCs). Clearly, the presence of nickel metal (Ni0) was revealed by Xray diffraction analyses and X-ray photoelectron spectroscopy in all NiO samples we have prepared according to chemical routes usually employed for DSSC applications. Thus, the black color of the cathodes is correlated to the existence of Ni0 and not to the increased amount of Ni2+/Ni3+ mixed valence as commonly suggested in the literature. Surprisingly, the presence of elemental nickel does not seem to affect drastically the photovoltaic performances of DSSCs that is rather controlled by the amount of chemisorbed dye. Sintering of NiO samples at higher temperature (i.e., from 450 to 900 °C) triggers the progressive oxidation of Ni0 and ultimately leads to pale green NiO samples, but to larger particles size that results in lower conversion efficiencies.

■

INTRODUCTION Sensitization of p-type semiconductors (p-SC) with a molecular dye has witnessed a growing interest in recent years.1,2 This is mainly related to the perspective of fabricating tandem dyesensitized solar cells2−5 and dye-sensitized photoelectrosynthetic cells6,7 when efficient p-type dye-sensitized solar cells (pDSSCs) will be developed. Practically, p-DSSCs work according to a similar operating principle (i.e., charge photoinjection into a SC) as conventional n-DSSCs commonly called Grätzel cells (in reference to their inventor).8 Only the nature of the photoinjected charges differs. Electrons travel from the dye to the n-SC (e.g., TiO2, ZnO) in conventional nDSSCs,9,10 while the opposite direction is taken in p-DSSCs in which the sensitizer excited state decays by hole injection into the valence band of a p-SC.2 So far, photovoltaic cells based on the sensitization of a p-SC are almost exclusively made of nickel oxide.2−5,11−17 Recently, a very limited number of other inorganic materials (e.g., CuO,18 CuAlO2,19 CuGaO2,20,21 or CuCrO222) have been tested. Unfortunately, conversion efficiencies remained low (a few tenths of percents) until Powar et al. brought about a real breakthrough with tris(1,2diaminoethane)cobalt(II/III) complexes as redox mediator and PMI-6T-TPA as sensitizer leading to the overall photoconversion efficiency (η) of 1.3% with an open circuit photovoltage (Voc) of 709 mV and a short circuit photocurrent (Jsc) of 4.44 mA·cm−2, but the photocathodes were still built upon NiO.23 If nickel oxide can undoubtedly be viewed as a valuable material that served to give a proof of principle of the feasibility of p-DSSCs, we may wonder whether NiO fulfills all of the © 2013 American Chemical Society

requirements for the targeted application. Formally, three main drawbacks of NiO can be identified and will have to be tackled in the near future to enhance performances of p-DSSCs. First, the low potential of its valence band (0.3 V vs SCE at pH = 7)5 limits the photovoltage produced by the cell.2 Second, its low electrical conductivity hinders a rapid charge flow and favors charge recombination at the p-SC/dye and p-SC/redox mediator interfaces. Third, the transparency of nanocrystalline NiO films is low, restraining the penetration depth of light, and thus the light-harvesting efficiency (LHE) and the capacity of light capture by the dyes. In the present contribution, we focus on transparency only and we unveil the reason why 1 μm thick NiO photocathodes are opaque and dark, while nickel oxide is reported to be pale green.24 The green hue is commonly assigned to weak parity forbidden spin-allowed d−d transitions due to Ni2+ cations in octahedral sites (NiO adopts the NaCl structure type25). Thus, the observed dark color of NiO films is naturally attributed to a Ni2+/Ni3+ mixed valence 26,27 (correlated to a Ni1−xO nonstoichiometry) and homonuclear heterovalent charge transfers by analogy with what happens in Fe3O4. Moreover, this observation goes along with the wellknown propensity of NiO to be oxidized and the ability of the structure to host Ni vacancies whose presence is needed to account for its p-type conductivity. Nevertheless, this darkness could also arise from the presence of an undetected dark impurity. In order to elucidate the origin of this dark color, we Received: June 5, 2013 Revised: September 27, 2013 Published: October 4, 2013 22478

dx.doi.org/10.1021/jp4055457 | J. Phys. Chem. C 2013, 117, 22478−22483

The Journal of Physical Chemistry C

Article

have embarked on a detailed analysis of black NiO samples prepared according to the different chemical routes reported for DSSC application.4,11,12,26

conditions as NiO nanoparticle-based pastes deposited by doctor-blading technique or screen-printing pastes on FTO substrates.4,11 The synthesis procedure is sketched in Scheme 1

EXPERIMENTAL SECTION Synthesis. NiO powder or film samples were prepared according to chemical routes published by Le Pleux et al.,11 Bach et al.,4 Hagfeldt et al.,26 and Suzuki et al.12 to achieve NiO photocathode, i.e., thick NiO layers on FTO-coated glass substrates. X-ray Powder Diffraction. X-ray powder diffraction patterns were recorded at room temperature in the 10°−80° 2θ range on a Bruker D8 Advance diffractometer using Cu KL2,3 radiation. All data treatments and refinement were carried out with the JANA2006 package.28 X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS, Mg K-L3 = 1253.6 eV) was performed in a Leybold-Heraeus ultrahigh vacuum environment with an analyzer operating in the constant pass energy mode (31.5 eV). Spectra were calibrated in energy using C 1s = 284.7 eV as reference, and the resolution was estimated at about 0.9 eV. Transmission Electron Microscopy (TEM). TEM experiments were performed on a Hitachi H9000NAR (300 kV, Scherzer resolution 0.18 nm). Photocathode Fabrication. According to the method of Bach et al.,4 NiO films on FTO-coated glass substrates (Pilkington 10 ohm TEC7 25 × 25 mm) were subsequently realized in a reproducible manner by screen printing starting from 3 g of NiO dispersed in a paste composed of 10 mL of ethylene cellulose/ethanol (1/9) and 20 mL of terpineol. Ethanol was then eliminated by rotary evaporation. After sintering for 30 min at 450 °C to trigger the decomposition of organic ingredients and to promote the adhesion of NiO to themselves and to the substrate, the 1 μm NiO films were coated with Coumarin 343 (C343-Aldrich) by soaking the films for 48 h in a solution of C343 in acetonitrile (0.25 mmol). The exact amount of dye loaded on each films was determined by desorbtion of the dye with a solution of NaOH (0.1 M in a distilled water/methanol 1/1). The exact concentration of adsorbed species was then determined by UV−visible absorption at the characteristic wavelength of C343 (around 420 nm). I−V Measurements. Solar cell performances were recorded on a Keithley model 2420 digital source meter under simulated solar irradiation illumination AM1.5G (100 mW/cm2). The intensity of the solar simulator was calibrated by a standard Si photovoltaic cell. The overall conversion efficiency (η) of the photovoltaic cell was calculated from the short-circuit photocurrent density (Jsc), the open-circuit photovoltage (Voc), the fill factor (ff) of the cells, and the intensity of the incident light.

Scheme 1. Synthesis Route of NiO from Ni(OCOCH3)2· 4H2O Annealed at 450 °C for 30 min in Air

■

with photographs of NiO-P and NiO-F that clearly reveal their black hue.11 X-ray patterns of NiO-F and NiO-P are displayed in Figure 1, a and b, respectively. All diffraction peaks of the

■

RESULTS AND DISCUSSION In order to elucidate the origin of the black color of NiO films used in DSSC applications, we have first considered NiO synthesized according to Le Pleux et al. under hydrothermal conditions11 from nickel acetate with hexamethylenetetramine in aqueous solution. This led to the green Ni(OCOCH3)2· 4H2O material recovered as powdered samples after filtration or directly as films deposited on a FTO substrate. This green intermediate was then converted into NiO samples (NiO-P and NiO-F for powder and film samples, respectively) upon heating in air at 450 °C for 30 min, i.e., in the same “sintering”

Figure 1. X-ray diffraction patterns recorded for (a) the film NiO-F, (b) the powder NiO-P obtained by annealing the Ni(OCOCH3)2· 4H2O precursor in air at 450 °C for 30 min, and (c) the NiO(Inframat) used in DSSCp according to the U. Bach method.21 For comparison, the pattern of a powder obtained by annealing the Ni(OCOCH3)2·4H2O precursor in air at 900 °C for 1 h is displayed in (d). The diffraction peaks indexed with the (hkl) notation correspond to the main NiO phase. The main diffraction peaks of the Ni metal are indicated by a black triangle (▼) while the asterisks (∗) point out the main diffraction peaks of the SnO2 substrate. 22479



Article

film can be unambiguously attributed to the NiO phase (file 778-429 JCPDS) and the SnO2 substrate. In contrast, XRD investigation on the bulk material reveals, in addition to NiO, a side phase (extra peaks at 2θ = 44.4° and 51.8°) that can be assigned to the presence of Ni metal (JCPDF database file 652865) wherein the atomic ratio is estimated to be about 5% via a Rietveld analysis. The low intensity of these peaks compared to those of NiO straightforwardly explains why they are hardly detectable by X-ray diffraction only on films a few micrometers thick in standard conditions. Interestingly, these peaks from elemental Ni disappear once the sample is annealed at 900 °C for 1 h in air. In order to probe the different oxidation states of Ni in NiOP, an XPS study was initiated at the Ni 2p edge with a green NiO powder purchased from Aldrich (Aldrich Chemical Co., 99.99%) used for comparison (hereafter labeled NiO-ref) (see Figure 2a). Both samples exhibit a 2p3/2 peak at 853.7 eV with

than from a high concentration of the Ni2+/Ni3+ mixed valence system, even if the mixed valence cannot be called into question to explain p-typeness of NiO. To test this explanation, we attempted to eliminate Ni0 metal from the NiO samples by heating the Ni(OCOCH3)2·4H2O precursor at higher temperatures. Pictures displayed in Scheme 2 highlight the gradual Scheme 2. Photographs of (a) NiO Powders Prepared from Ni(OCOCH3)2·4H2O by Annealing for 1 h at 450, 600, 700, 800, and 900°C and (b) NiO Inframat Powder Unannealed or Annealed for 1 h at 450, 500, 600, 700, 800, and 900°C, and Photocathodes Fabricated with These Powders

color change from black to green when the annealing temperature (TA) was raised from 450 to 900 °C. This goes along with a progressive disappearance of Ni0 with TA. Figure 2b displays the Ni 2p3/2 XPS spectra of NiO powders obtained at different TA. A clear decrease of the Ni0 2p3/2 peak (red vertical line for emphasis) is detected when the annealing temperature is raised and the Ni0 2p3/2 peak is not detectable for the green powder. X-ray diffraction pattern shown in Figure 1d confirms that Ni0 is absent from this green sample annealed at 900 °C. This study gives therefore a clear indication that the black coloration of NiO-P comes from the presence of nickel metal. At this stage, attempts to localize nickel metal in NiO samples were initiated with transmission electron microscopy (TEM) analysis. Figure 3 shows a TEM picture of NiO-P where 10 nm diameter crystalline nanoparticles can be imaged. These particles have a selected area electron diffraction pattern

Figure 2. (a) XPS study at the Ni 2p3/2 edge of several NiO samples used in DSSC and given in the literature. (1) Le Pleux et al.,11 (2) Bach et al.,4 (3) Hagfeldt et al.,21 (4) Suzuki et al.,12 and (5) NiO-ref. (b) Evolution of the Ni 2p3/2 XPS spectra of NiO samples prepared from Ni(OCOCH3)2·4H2O by annealing for 1 h at 450, 600, 700, 800, and 900 °C. The NiO-ref refers to the green commercial NiO (Aldrich), and ∗ mark the expected location of Ni3+ peaks.

two satellites at 855.4 and 860.7 eV, characteristic of Ni2+ cations in NiO thin films.29,30 However, we note that NiO-P shows an additional peak at 853.1 eV. This peak can be assigned to the 2p3/2 transition of Ni0 according to the Handbook of X-ray Photoelectron spectroscopy (853.3 eV)31 and literature (852.7 ± 0.4 eV).29,31 Hence, as expected from X-ray analyses, XPS confirms the presence of Ni0 metal in our sample. In contrast, the characteristic 2p3/2 peaks of nickel(III), predicted at 856.5 and 857.8 eV according to the literature,29,30,32 are not observed either in NiO-P or in NiOref. All these results suggest therefore that the black color of the NiO samples originates from the presence of Ni0 metal rather

Figure 3. TEM micrograph and SAED pattern of NiO-P. 22480



Article

(SAED) compatible with that expected for NiO (fcc structure) and do not appear coated by any species. Moreover, if no isolated Ni0 blocks are identified, diffraction analyses on large selected areas reveal rings of nickel oxide as well as three diffraction spots of nickel metal. This suggests that a very small amount of isolated nickel metal nanoparticles may be present mixed with a large amount of NiO nanoparticles. Thus, our results on NiO prepared according to Le Pleux’s route demonstrate that the annealing temperature chosen to produce NiO films is not high enough to completely oxidize Ni(OCOCH3)2·4H2O in air. At this stage, it is worth noting that most preparation routes of NiO films for DSSCs applications reported in the literature are based on similar thermal treatments in air.4,12,26 In that respect, we may infer that all these preparation routes systematically lead to the presence of Ni0 metal and explain the black color found in thick photocathodes. This assumption was checked by synthesizing some NiO powder samples using the same procedures as reported by Bach et al.,4 Hagfeldt et al.,26 and Suzuki et al.12 to prepare NiO films. All these powders are black and their Ni 2p3/2 XPS spectra (Figure 2a) highlight the presence of nickel metal. Note also that the XRD pattern of NiO (Inframat) prepared according to the U. Bach method4 reveals the presence of Ni0 in the material (Figure 1c). Consequently, even if we cannot fully rebut the role of the Ni2+/Ni3+ mixed valence as the origin of the black color, we can clearly assert the presence of Ni metal in numerous black NiO used as photocathode in ptype DSSCs and definitely suggest that this color is associated with Ni0 rather than Ni3+ (NB: Ni3+ is not detected at all by XPS). An important issue is therefore to clarify the role on the DSSC performances of this small amount of Ni0 metal nanoparticles within the NiO films. Namely, we may naturally wonder whether the presence of Ni0 particles may minimize charge recombination via the formation of an ohmic contact between the particles or favor the charge flow via the presence of metallic species, which would enhance the conductivity. To address this issue, a series of NiO-based DSSCs were mounted with variable amounts of Ni0 metal. Practically, different NiO batches were prepared from nanostructurated black NiO powder commercially available (Inframat, 99.9%) annealed for 1 h between 450 (remained black) and 900 °C (turned green) to trigger the oxidation of Ni0 metal. Room temperature X-ray and magnetization curves measurements confirmed the presence of 0.7 atom % of Ni0 in the black powder annealed at 450 °C while no traces of Ni metal were found in the green NiO powder annealed at 900 °C. NiO films on FTO substrates were subsequently prepared according to the method of Bach et al.4 (see Experimental Section for details). Note in Scheme 2 that using the NiO Inframat instead of NiO prepared from Ni(OCOCH3)2·4H2O a similar gradual color change from black to green is observed for powders and photocathodes when the annealing temperature is increased. Figure 4a displays the photocurrent−photovoltage characteristics of the photocathodes prepared with NiO-Inframat sample annealed at different temperatures (shown in scheme 2). Table 1 summarizes all the photovoltaic performances of these cells. Clearly, we observed that the Voc does not depend heavily on the annealing temperature (Figure 4b). Conversely, the Jsc is highly reduced when TA increases (Figure 4c). This loss of photocurrent could stem from either a reduction of the amount of conducting Ni0 metal or the increase of particles size and the

Figure 4. (a) Photoresponses obtained under AM 1.5 illumination on the best solar cells made of commercial black NiO (Inframat). Curve (1), raw NiO (Inframat); curves (2−7), annealed NiO for 1 h in air at 450, 500, 600, 700, 800, and 900 °C, respectively. (b) Average Voc and (c) average Jsc evolutions according to TA of black NiO (mean values obtained on about 10 cells). The error bars correspond to 2 times the standard deviations.

Table 1. Photovoltaic Performances of DSSCs Based on NiO (Inframat) Unannealed or Annealed at Different Temperatures unannealed 450 500 600 700 800 900

Voc (mV)

Jsc (mA·cm‑2)

ff (%)

η (%)

95 85 105 95 105 115 115

0.662 0.489 0.212 0.198 0.106 0.047 0.038

35 33 33 33 34 31 28

0.022 0.014 0.007 0.006 0.004 0.002 0.001

associated loss of specific surface area (see Figure 5a). Indeed, this would limit the amount of chemisorbed dye on the NiO film and therefore the LHE and consequently the Jsc. To disentangle the role of Ni0 metal and particle size effect, we have determined the sensitizers load by UV−vis spectroscopy (Figure 5b). As expected, the amount of coumarin steadily decreases as the particle size increases. However, Figure 5c shows a linear dependence between the amount of coumarin chemisorbed on the surface of the films (prepared either with NiO unannealed or annealed at different temperatures) and the photocurrent delivered by the DSSC. This is the expected behavior because the photocurrent is generally related to the LHE and therefore to the amount of chemisorbed dye on the SC. We can thus conclude that the decrease of the photocurrent observed when the NiO precursor is annealed at high temperature is mainly driven by the increase of the particle size (decrease of surface area) and not by the removal 22481



Article

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Dr. Adèle Renaud and Dr. Benoit Chavillon contributed equally to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project was part of the PERLE and PERLE2 programs financed by Région Pays de la Loire. C. Payen is thanked for his help in performing susceptibility measurements.

■

(1) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. DyeSensitized Nanostructured p-Type Nickel Oxide Film as a Photocathode for a Solar Cell. J. Phys. Chem. B 1999, 103, 8940−8943. (2) Odobel, F.; Le Pleux, L.; Pellegrin, Y.; Blart, E. New Photovoltaic Devices Based on the Sensitization of p-type Semiconductors: Challenges and Opportunities. Acc. Chem. Res. 2010, 43, 1063−1071. (3) Nattestad, A.; Fergusson, M.; Kerr, R.; Cheng, Y.-B.; Bach, U. Dye-sensitized Nickel(II) Oxide Photocathodes for Tandem Solar Cell Applications. Nanotechnology 2008, 19, 295−304. (4) Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y.-B.; Mishra, A.; Bäuerle, P.; Bach, U. Highly Efficient Photocathodes for Dye-Sensitized Tandem Solar Cells. Nat. Mater. 2010, 9, 31−25. (5) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. DyeSensitized Nanostructured Tandem Cell-First Demonstrated Cell with a Dye-Sensitized Photocathode. Sol. Energy Mater. Sol. Cells 2000, 62, 265−273. (6) Li, L.; Duan, L.; Wen, F.; Li, C.; Wang, M.; Hagfeldt, A.; Sun, L. Visible Light Driven Hydrogen Production From a Photo-Active Cathode Based on a Molecular Catalyst and Organic Dye-Sensitized ptype Nanostructured NiO. Chem. Commun. 2012, 48, 988−990. (7) Tong, L.; Iwase, A.; Nattestad, A.; Bach, U.; Weidelener, M.; Gotz, G.; Mishra, A.; Bäuerle, P.; Amal, R.; Wallace, G. G.; et al. Sustained Solar Hydrogen Generation Using a Dye-Sensitised NiO Photocathode/BiVO4 Tandem Photo-Electrochemical Device. Energy Environ. Sci. 2012, 5, 9472−9475. (8) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (9) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (10) Jose, R.; Thavasi, V.; Ramakrishna, S. Metal Oxides for DyeSensitized Solar Cells. J. Am. Ceram. Soc. 2009, 92, 289−301. (11) Le Pleux, L.; Chavillon, B.; Pellegrin, Y.; Blart, E.; Cario, L.; Jobic, S.; Odobel, F. Simple and Reproducible Procedure to Prepare Self-Nanostructured NiO Films for the Fabrication of p-Type DyeSensitized Solar Cells. Inorg. Chem. 2009, 48, 8245−8250. (12) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. Syntheses of NiO Nanoporous Films Using Nonionic Triblock CoPolymer Templates and Their Application to Photo-cathodes of pType Dye-Sensitized Solar Cells. J. Photochem. Photobiol. A 2008, 199, 1−7. (13) Huang, Z.; Natu, G.; Ji, Z.; Hasin, P.; Wu, Y. p-Type DyeSensitized NiO Solar Cells: A Study by Electrochemical Impedance Spectroscopy. J. Phys. Chem. C 2011, 115, 25109−25114. (14) Mori, S.; Fukuda, S.; Sumikura, S.; Takeda, Y.; Tamaki, Y.; Suzuki, E.; Abe, T. Charge-Transfer Processes in Dye-Sensitized NiO Solar Cells. J. Phys. Chem. C 2008, 112, 16134−16139. (15) Zhu, H.; Hagfeldt, A.; Boschloo, G. Photoelectrochemistry of Mesoporous NiO Electrodes in Iodide/Triiodide Electrolytes. J. Phys. Chem. C 2007, 111, 17455−17458. (16) Zhang, X. L.; Huang, F.; Nattestad, A.; Wang, K.; Fu, D.; Mishra, A.; Bäuerle, P.; Bach, U.; Cheng, Y.-B. Enhanced Open-Circuit

Figure 5. (a) Evolution of the specific surface area and the particles size with annealing temperature. The SEM pictures evidence the progressive increase of the particle size from about 10 nm for the unannealed NiO Inframat to about 300 nm for NiO Inframat annealed at 900 °C. (b) Absorption spectrum of desorbed C343 dye issued from photocathodes prepared according to the U. Bach method4 with unannealed NiO Inframat and annealed NiO Inframat at 450, 500, 600, 700, 800, and 900 °C. (c) Evolutions of Jsc of C343 sensitized NiO solar cells with the amount of adsorbed C343. Error bars correspond to two standard deviations.

of Ni0 metal. In that respect, the presence of a small amount of Ni0 metal in the films does not seem to affect drastically the photovoltaic performances of the DSSCs.

■

CONCLUSION We have demonstrated that the black color of NiO films used generally as photocathode in DSSCs comes from the presence of a small amount of Ni0 metal within the NiO films and not from the mixed valence Ni2+/Ni3+ species. Most NiO films prepared for p-type DSSCs are produced by annealing the nickel hydroxide precursor (that eventually decompose) at relatively low temperature (450−500 °C) to prevent the melting of the FTO-coated glass and crystal growth. This annealing temperature is not sufficiently high to fully oxidize Ni0 metal in the films. When a higher annealing temperature is used, a nickel metal free NiO sample with green color is indeed obtained. This study, made with black NiO and green NiO films, shows that they exhibit similar photovoltaic performances if we take into account the diminished surface area of NiO film prepared at higher temperature. Another important finding of this study is the origin of black color of NiO film, which is due to the presence of Ni0 rather than the Ni2+/Ni3+ mixed valence.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Fax: (33)2.40.37.39.95. Phone: (33)2.40.37.39.16. E-mail: [email protected]. *Fax: (33)2.40.37.39.95. Phone: (33)2.40.37.39.16. E-mail: [email protected]. 22482



Article

Voltage of p-type DSC with Highly Crystalline NiO Nanoparticles. Chem. Commun. 2011, 47, 4808−4810. (17) Zhang, X. L.; Zhang, Z.; Chen, D.; Bäuerle, P.; Bach, U.; Cheng, Y.-B. Sensitization of Nickel Oxide: Improved Carrier Lifetime and Charge Collection by Tuning Nanoscale Crystallinity. Chem. Commun. 2012, 48, 9885−9887. (18) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. Photoelectrochemical Characteristics of Cells with Dyed and Undyed Nanoporous p-Type Semiconductor CuO Electrodes. J. Photochem. Photobiol., A 2008, 194, 143−147. (19) Nattestad, A.; Zhang, X.; Bach, U.; Cheng, Y.-B. Dye-Sensitized CuAlO2 Photocathodes for Tandem Solar Cell Applications. J. Photon Energy 2011, 1, 011103. (20) Renaud, A.; Chavillon, B.; Le Pleux, L.; Pellegrin, Y.; Blart, E.; Boujtita, M.; Pauporté, T.; Cario, L.; Jobic, S.; Odobel, F. CuGaO2: a Promising Alternative for NiO in p-Type Dye Solar Cells. J. Mater. Chem. 2012, 22, 14353−14356. (21) Srinivasan, R.; Chavillon, B.; Doussier-Brochard, C.; Cario, L.; Paris, M.; Gautron, E.; Deniard, P.; Odobel, F.; Jobic, S. Tuning the Size and Color of the p-Type Wide Band Gap Delafossite Semiconductor CuGaO2 with Ethylene Glycol Assisted Hydrothermal Synthesis. J. Mater. Chem. 2008, 18, 5647−5653. (22) Xiong, D.; Xu, Z.; Zhang, W.; Chen, W.; Xu, X.; Wang, M.; Cheng, Y.-B. Hydrothermal Synthesis of Ultrasmall CuCrO2 Nanocrystal Alternatives to NiO Nanoparticles in Efficient p-Type DyeSensitized Solar Cells. J. Mater. Chem. 2012, 22, 24760−24768. (23) Powar, S.; Daeneke, T.; Ma, M. T.; Fu, D.; Duffy, N. W.; Weidelener, M.; Mishra, A.; Bäuerle, P.; Spiccia, L.; Bach, U. Highly Efficient p-type Dye-Sensitized Solar Cells Based on Tris(1,2diaminoethane)Cobalt(II)/(III) Electrolytes. Angew. Chem. 2013, 125, 630−633. (24) Anthony, J. W.; Bideaux, R. A.; Bladh, K. W.; Nichols, M. C. Handbook of Mineralogy; Mineralogical Society of America: Chantilly, VA, 2005; http://www.handbookofmineralogy.org/. (25) Adler, D.; Feinleib, J. Electrical and Optical Properties of Narrow-Bands Materials. Phys. Rev. B 1970, 2, 3112−3134. (26) Boschloo, G.; Hagfeldt, A. Spectroelectrochemistry of Nanostructured NiO. J. Phys. Chem. B 2001, 105, 3039−3040. (27) Korosec, R. C.; Bukovec, P. Sol-gel Prepared NiO Thin Films for Electrochromic Applications. Acta Chim. Slov. 2006, 53, 136−147. (28) Petricek, V.; Dusek, M.; Palatinus, L. The Crystallographic Computing System; Academy of Science of Czech Republic: Prague, Czech Republic, 2006. (29) Biesinger, M. C.; Payne, P. P.; Lau, L. W. M.; Gerson, A.; Smart, R. S. C. X-ray Photoelectron Spectroscopic Chemical State Quantification of Mixed Nickel Metal, Oxide and Hydroxide Systems. Surf. Interface Anal. 2009, 41, 324−332. (30) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Inc.: Eden Prairie, MN, 1995. (31) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Powell, C. J.; Rumble Jr, J. R. NIST Standard Reference Database 20, version 3.4 (web version), 2003; http:/srdata.nist.gov/xps/. (32) Bakar, W. A. W. A.; Othman, M. Y.; Ali, R.; Yong, C. K. Nickel Oxide Based Supported Catalysts For the In-Situ Reactions of Mathanation and Desulfurization in the Removal of Sour Gases From Simulated Natural Gas. Catal. Lett. 2009, 128, 127−136.

22483


Origin of the Black Color of NiO Used as Photocathode in p-Type Dye

Recommend Documents