Photocatalytic Synthesis and Photovoltaic Application of Ag-TiO2

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Photocatalytic Synthesis and Photovoltaic Application of Ag-TiO2 Nanorod Composites Qipeng Lu,†,‡,§ Zhenda Lu,† Yunzhang Lu,‡,§ Longfeng Lv,‡,§ Yu Ning,‡,§ Hongxia Yu,† Yanbing Hou,*,‡,§ and Yadong Yin*,† †

Department of Chemistry, University of California, Riverside, California 92521, United States Key laboratory of Luminescence and Optical Information, Ministry of Education and §Institute of Optoelectronic Technology, Beijing JiaoTong University, Beijing 100044, China

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S Supporting Information *

ABSTRACT: A photocatalytic strategy has been developed to synthesize colloidal Ag-TiO2 nanorod composites in which each TiO2 nanorod contains a single Ag nanoparticle on its surface. In this rational synthesis, photoexcitation of TiO2 nanorods under UV illumination produces electrons that reduce Ag(I) precursor and deposit multiple small Ag nanoparticles on the surface of TiO2 nanorods. Prolonged UV irradiation induces an interesting ripening process, which dissolves the smaller nanoparticles by photogenerated oxidative species and then redeposits Ag onto one larger and more stable particle attached to each TiO2 nanorod through the reduction of photoexcited electrons. The size of the Ag nanoparticles can be precisely controlled by varying the irradiation time and the amount of alcohol additive. The Ag-TiO2 nanorod composites were used as electron transport layers in the fabrication of organic solar cells and showed notable enhancement in power conversion efficiency (6.92%) than pure TiO2 nanorods (5.81%), as well as higher external quantum efficiency due to improved charge separation and transfer by the presence of Ag nanoparticles. KEYWORDS: Photocatalytic synthesis, photovoltaics, nanocomposite, nanorod, titania, solar cell

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enhance the conductivity and achieved a better power conversion efficiency (PCE) than TiO2 nanoparticles. In addition to the benefits of its high crystallinity, the elongated shape of TiO2 nanorods is also believed to provide better connectivity than particles for enhancing charge transport.10 Here we report that further improvement of the PCE in the inverted polymer solar cells can be achieved by employing TiO2 nanorods decorated with silver nanoparticles (AgNPs) in the cell construction. A number of recent examples have demonstrated that formation of such composite nanostructures could improve the optical, electronic, magnetic, and catalytic properties of semiconductor and metal nanoparticles.9,11−15 In the areas of solar energy conversion and photocatalysis, metal nanoparticles are often introduced to TiO2-based semiconductor materials to enhance the separation of the photoinduced charge carriers during photocatalytic processes.16−20 In these cases, Fermi level equilibration is believed to improve the efficiency of the interfacial charge-transfer process in the resulting composite structures.21,22 Although many metals have been proposed for this purpose, silver (Ag) remains as the primary choice due to its relatively high stability

rganic solar cell (OSC) represents a promising photovoltaic technology with many advantages, including transparency, flexibility, lightweight, and compatibility with high throughput and low-cost fabrication processes.1,2 In conventional bulk heterojunction polymer solar cells, a low work-function metal is used as the top cathode for efficient electron extraction, which however is chemically unstable, making this device architecture not viable for large area processing. In addition, the transparent ITO anode may be etched easily by very acidic electron donor layer, such as the typical poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT/PSS), causing device degradation.3,4 To address these challenges, an inverted cell geometry has been proposed as a more practical architecture for existing roll-to-roll processes. By reversing the polarity of charge collection, the inverted structure allows the use of more air stable, higher work-function metals such as Ag and Cu in combination with an appropriate interfacial layer to collect holes, while a transparent electrode modified by metal oxides or metal carbonates is used to collect electrons.5 Among different types of cathode-modifying materials, TiO2 has been regarded as one of the promising candidates due to its high chemical and thermal stability, hole blocking property, and suitable electron selectivity.6−9 As the electrical conductivity of amorphous TiO2 is not sufficient to afford even a reasonable performance, Li et al. proposed the use of crystalline anatase TiO2 nanorods to © 2013 American Chemical Society

Received: September 13, 2013 Revised: October 24, 2013 Published: October 28, 2013 5698

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Figure 1. (A) TEM image of the TiO2 nanorods. (B−E) TEM images of Ag-TiO2 nanorod composites formed by UV irradiation for 30 min (B), 60 min (C), 180 min (D), and 600 min (E). (F) Dependence of the intensity of the plasmon band at 440 nm and the size of AgNPs on the irradiation time.

against oxidation in comparison to more active transition metals and its significantly lower cost than other noble metals. Several chemical- or photoreduction processes have been developed for the synthesis of Ag-TiO2 composite structures,16,23−26 albeit with very limited control in the size and spatial distribution of the AgNPs. In some cases, the Ag and TiO2 nanoparticles are in fact produced separately, thus limiting charge transfer. Therefore, it is technically interesting to develop a simple yet effective method for the synthesis of Ag-TiO2 nanocomposites with precise dimensional control, which may have important consequences for solar energy harvesting.27,28 In this work, we present a robust photodeposition approach for the synthesis of Ag-TiO2 nanorod composites with wellcontrolled dimensions and demonstrate their use as a cathodemodifying layer in OSCs with increased short-circuit current density (Jsc) due to the enhanced electrical conductivity than pure TiO2 nanorods. AgNPs are also believed to act as effective antennas for incident light and lead to improved photogeneration of excitons in polymer solar cells.29,30 Systematic analyses including absorption spectroscopy and external quantum efficiency measurement confirm that AgNPs could enhance the charge separation and transfer rate at the TiO2 interface, leading to a strong improvement in device performance (19.1% enhancement in PCE). To ensure controllable deposition of AgNPs to TiO2 surface, our synthesis takes advantage of the photoexcitation of TiO2 nanorods under UV illumination to produce electrons for reduction of Ag precursors directly on TiO2 surface. The photocatalytic reaction was carried out in a nonpolar solvent in order to limit the diffusion of reductive species and further confine the reduction of Ag(I) and growth of AgNPs to the TiO2 surface. In the first step, oleic acid capped anatase TiO2 nanorods with a narrow size distribution (3.0 nm × 40 nm) were synthesized using a high-temperature pyrolysis reaction reported previously (Figure 1A).31 Silver precursor was then introduced to the reaction system by dissolving AgNO3 in oleylamine and then mixing with the dispersion of TiO2

nanorods in toluene. In a typical photocatalytic reaction, a clear solution containing TiO2 nanorods, silver source and toluene was irradiated under a UV lamp with wavelength at 365 nm for up to 600 min. Aliquots were collected after different periods of irradiation, and products were precipitated with the addition of ethanol and then redispersed in toluene. TEM images in Figure 1B−E show the morphology evolution of AgTiO2 nanorod composites after irradiation for 30, 60, 180, and 600 min, respectively. Formation of AgNPs on TiO2 nanorods can be clearly observed thanks to the higher contrast of Ag than TiO2. Interestingly, although each TiO2 nanorod contains multiple AgNPs, there is always one particular Ag nanoparticle that appears to be dominantly larger than others deposited on the same nanorod. This became more unequivocal when the system was irradiated for a longer period of time. The measured average diameter of these large AgNPs after irradiation for different periods of time was plotted in Figure 1F, showing a size increase near linearly from ∼2.6 nm after 30 min of irradiation to 7.5 nm after 600 min of irradiation. Consistently, the intensity of the characteristic plasmonic resonance peak of AgNPs at ∼440 nm also increases, mainly due to the enhanced scattering of the larger AgNPs (Supporting Information Figure S1). The growth of AgNPs on TiO2 nanorods involves a ripening process, as illustrated in Scheme 1. At the very early stage of the reaction, the electrons resulting from the photoexcitation of TiO2 nanorods reduce Ag(I) ions into metallic silver that is deposited on the surface of TiO2 nanorods in the form of small Scheme 1. The Proposed Formation Mechanism of Ag-TiO2 Nanorod Composites

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Figure 2. TEM images of Ag-TiO2 nanorod composites prepared by (A) UV irradiation of the mixed solution for 600 min; (B) UV irradiation for 60 min and aging in dark for 540 min; and (C) UV irradiation for 60 min, washing with ethanol twice, followed by UV irradiation for another 540 min.

nanoparticles. Prolonged UV irradiation reduces more ions, leading to gradual growth of the AgNPs. On the other hand, the holes resulting from photoexcitation also produce oxidative species such as hydroxide radicals that can dissolve the AgNPs through oxidation reaction. Such an etching effect is more profound on small particles as they are more energetically unstable, leaving behind only larger particles on TiO2 surface. The large nanoparticles continue to grow as they receive more Ag atoms from further reduction of the Ag(I) ions. From the photocatalysis point of view, the preferential growth of one large Ag nanoparticle on each nanorod can also be attributed to the fact that metal particles can serve as electron sinks that accumulate photoelectrons generated on the TiO2 rod and further promote the deposition of silver on the already large AgNPs. The creation of both reductive and oxidative species through the photoexcitation of TiO2 is believed to drive such a ripening process, as observed in our earlier work involving reconstruction of silver nanoplates by UV irradiation.24 In the current case, the photocatalytic process assisted by the TiO2 nanorods greatly enhances the ripening rate and ensures only one Ag nanoparticle is ultimately deposited on each TiO2 rod to form nanocomposites. We have carried out control experiments to confirm the proposed photocatalytic ripening mechanism. TiO2 nanorods and Ag(I) precursor were mixed and irradiated under UV for 60 min in the same manner as a normal procedure. Then the mixture was separated into three parts: one was kept under UV for 540 min, the second was stored in the dark, and the third was washed twice, redispersed in toluene, and then irradiated for the same amount of time. As we described above, composites containing one Ag particle per TiO2 rod were formed under the first set of reaction conditions (Figure 2A). In the second case, as shown in Figure 2B, multiple small AgNPs deposited on TiO2 rods did not show obvious morphology change after storage in dark for 540 min. This result suggests that the photocatalytic process does promote the ripening of AgNPs. In the last case, after removal of the possible remaining Ag(I) source followed by 540 min irradiation, one large Ag nanoparticle per TiO2 nanorod could still be obtained, indicating that its growth is driven primary not by the reduction of remaining Ag(I) precursor but by consuming the small nanoparticles on the rods. Similar to many other photocatalytic processes, the current synthesis can be promoted by introducing hole scavengers to selectively remove the holes produced during the photoexcitation of TiO2 under UV irradiation.32,33 In this case, when 1-hexadecanol was added to the reaction, it sped up the growth of the AgNPs. As shown in Figure 3, when the reactions were

Figure 3. (A−C) TEM images of the Ag-TiO2 nanorod composites prepared under UV irradiation for 60 min: (A) without the presence of 16-OH, (B) with a low concentration (0.1 mM) of 16-OH, and (C) with a high concentration (0.5 mM) of 16-OH. (D) Plots summarizing the evolution of Ag particle size with irradiation time for the three samples.

carried out in the presence of 0.1 and 0.5 mM 1-hexadecanol for 60 min, the average diameter of the AgNPs increased from 3.59 nm in the standard case to 5.02 and 5.83 nm, respectively (Supporting Information Figures S2−S4). We also monitored the size change of the AgNPs with time and plotted the results in Figure 3D, which clearly shows that a higher concentration of 1-hexadecanol leads to a larger size of AgNPs for any given period of reaction. The results also indicated that the hole scavenger takes effect at the early stage of the reaction so that the initial nanoparticles are larger than those prepared at the standard condition. Ripening of these particles at the later stage leads to an even larger surviving nanoparticle on each nanorod. We have explored the potential application of the Ag-TiO2 nanorod composites in photovoltaics by incorporating them in the construction of OSCs. The devices were fabricated in an inverted structure through a low-temperature solution process in which the critical heterojunction was formed between the spin-cast layers of Ag-TiO2 nanocomposites and poly(4,8-bisalkyloxybenzo (l,2-b:4,5-b′) dithiophene-2,6-diyl-alt-(alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diyl) (PBDTTT5700

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Figure 4. (A) Diagram showing the energy levels of device components referenced to the vacuum level. (B) J−V curves of PBDTTT-C/PCBM devices using TiO2 nanorods or Ag-TiO2 nanorod composites as the electron transport layers. (C) External quantum efficiency (EQE) of the devices and EQE enhancement.

be attributed to the plasmonic effect,28,34,35 which enhances charge extraction in the electron transport layer. The combined effect of enhanced electrical conductivity and the plasmonic effect results in improved efficiency of the exciton separation and charge carrier transport among the interlayers and cathode and consequently, significantly improved JSC and FF. On the other hand, a considerably large interface area between active layer and Ag nanoparticles may also increase the probability of hole−electron recombination.36 Thus the efficiency of charge extraction is expected to decrease, leading to a lower shortcircuit current density for samples containing Ag NPs with sizes beyond 5.5 nm. In summary, we have reported a photocatalytic process for the synthesis of Ag-TiO2 nanorod composites, each of which contains a single Ag nanoparticle attached to one TiO2 nanorod. The formation of such unique hybrid structures can be attributed to an interesting photoinduced ripening process, in which the photogenerated oxidative species dissolve small AgNPs and then the photoexcited electrons reduce the ions and redeposit them back onto a larger and more stable Ag particle. The size of the AgNPs deposited on the individual TiO2 nanorods can be precisely controlled either by varying the irradiation time or the amount of alcohol additive. When used as an electron transport layer in OSCs, the Ag-TiO2 nanorod composites show notable increased power conversion efficiency (6.92%) than pure TiO2 nanorods (5.81%), as well as enhanced external quantum efficiency due to the improved charge separation and transfer by the presence of AgNPs. It is expected that the Ag-TiO2 nanorod composites may also find applications as electron transport layers for enhancing the efficiency of other organic optoelectronic devices such as organic light-emitting diodes.

C), (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) polymers. Due to its lower work function, the Ag-TiO2 nanorod composite acts as the electron transport layer. The theoretical energy level diagram for the inverted configuration is illustrated in Figure 4A. It can be perceived that the AgNPs attached to the TiO2 nanorods may serve as charge-extracting interlayers and therefore enhancing the cell performance. In order to confirm the effect of AgNPs, we have compared the device performance for OSCs fabricated by incorporating pure TiO2 nanorods and Ag-TiO2 nanorod composites prepared by UV irradiation for various periods of time. Under Air Mass 1.5 Global (AM1.5G) simulated solar illumination (100 mW/cm2), as summarized in Table 1 and Supporting Information Figure S5, the inverted Table 1. Photovoltaic Performance of OSCs Fabricated Using Different Materials As Electron Transport Layers electron transport layer TiO2 TiO2−Ag TiO2−Ag TiO2−Ag TiO2−Ag TiO2−Ag

(30 min) (3 h) (5 h) (7 h) (10 h)

JSC (mA/cm2)

Voc (V)

FF (%)

PCE (%)

14.99 15.36 15.94 16.46 16.39 15.63

0.71 0.71 0.71 0.71 0.71 0.71

54.57 59.08 58.86 59.19 56.23 57.18

5.81 6.44 6.66 6.92 6.54 6.34

PBDTTT-C/PCBM OSC containing Ag-TiO2 nanorod composites all show notable increased cell performance in both the short-circuit current density (JSC) and the fill factor (FF), but the optimal performance of the inverted OSCs is achieved for the composite sample irradiated for 5 h, corresponding to AgNPs with an average diameter of ∼5.5 nm. Figure 4B compares the density−voltage (J−V) characteristics of the devices fabricated by using pure TiO2 nanorods and the optimized composite sample, showing significant enhancement in the short-circuit current density (from 14.99 to 16.46 mA/cm2) and the fill factor (from 54.57 to 59.19%) when TiO2 nanorods are decorated with AgNPs. Accordingly, this yields an improvement in PCE from 5.81 to 6.92%. The external quantum efficiency (EQE) spectra of the TiO2 rods and Ag-TiO2 nanocomposite devices are presented in Figure 4C and Supporting Information Figure S6, along with the EQE enhancement ratio. The peak EQEs of the device based on pure TiO2 rods are around 65% at around 450 nm. When AgNPs are introduced onto the TiO2 nanorods, the result is a constant improvement in the wavelength range of 350 to 500 nm, especially a 10% increase around 400 nm, which corresponds to the position of the Ag plasmon peak. These results evidence that the increased Jsc and FF may also

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental sections, additional absorption spectra, HRTEM, and TEM images for Ag-TiO2 nanorod composites, SEM images of the spin-coated films, additional J−V curves, and external quantum efficiency plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (Y.H.) [email protected]. *E-mail: (Y.Y.) [email protected]. Notes

The authors declare no competing financial interest. 5701

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(28) Zhang, D.; Choy, W. C. H.; Xie, F.; Sha, W. E. I.; Li, X.; Ding, B.; Zhang, K.; Huang, F.; Cao, Y. Adv. Funct. Mater. 2013, 23, 4255. (29) Choi, H.; Ko, S.-J.; Choi, Y.; Joo, P.; Kim, T.; Lee, B. R.; Jung, J.W.; Choi, H. J.; Cha, M.; Jeong, J.-R. Nat. Photonics 2013, 7, 732. (30) Xu, M.-F.; Zhu, X.-Z.; Shi, X.-B.; Liang, J.; Jin, Y.; Wang, Z.-K.; Liao, L.-S. ACS Appl. Mater. Interfaces 2013, 5, 2935. (31) Joo, J.; Kwon, S. G.; Yu, T.; Cho, M.; Lee, J.; Yoon, J.; Hyeon, T. J. Phys. Chem. B 2005, 109, 15297. (32) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (33) Shkrob, I. A.; Sauer, M. C. J. Phys. Chem. B 2004, 108, 12497. (34) You, J.; Li, X.; Xie, F.-X.; Sha, W. E. I.; Kwong, J. H. W.; Li, G.; Choy, W. C. H.; Yang, Y. Adv. Energy Mater. 2012, 2, 1203. (35) Xiao, Y.; Yang, J. P.; Cheng, P. P.; Zhu, J. J.; Xu, Z. Q.; Deng, Y. H.; Lee, S. T.; Li, Y. Q.; Tang, J. X. Appl. Phys. Lett. 2012, 100, 013308. (36) Wang, C. C.; Choy, W. C.; Duan, C.; Fung, D. D.; Wei, E.; Xie, F.-X.; Huang, F.; Cao, Y. J. Mater. Chem. 2012, 22, 1206.

ACKNOWLEDGMENTS Financial support for this project was provided by the U.S. Department of Energy (DE-FG02-09ER16096). Y.Y. also thanks the Research Corporation for Science Advancement for the Cottrell Scholar Award and DuPont for the Young Professor Grant. Y.H. acknowledges additional funding from the National Natural Science & Foundation of China (NSFC) (61275175, 61036007, and 61125505) and the 111 Project (B08002). Q.L. and H.Y. acknowledge the fellowship support by the China Scholarship Council (CSC).

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