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CdS quantum dots co-sensitized TiO2 nanosheets array film photoelectrodes
Journal of Alloys and Compounds 647 (2015) 402e406
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Photoelectrolchemical performance of PbS/CdS quantum dots cosensitized TiO2 nanosheets array film photoelectrodes Huizhen Yao a, Xue Li a, Li Liu a, Jiasheng Niu a, Dong Ding a, Yannan Mu a, b, Pengyu Su a, Guangxia Wang a, Wuyou Fu a, Haibin Yang a, * a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Physics and Chemistry, Heihe University, Heihe 164300, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 18 June 2015
Herein, PbS/CdS quantum dots (QDs) co-sensitized titanium dioxide nanosheets array (TiO2NSs) films were reported for the first time. The TiO2NSs films exposed {001} facets were vertically grown on transparent conductive fluorine-doped tin oxide (FTO) glass substrates by a facile hydrothermal method. The PbS/CdS QDs were assembled on TiO2NSs photoelectrode by successive ionic layer adsorption and reaction (SILAR). The X-ray diffraction pattern (XRD) and transmission electron microscopy (TEM) verified that QDs with a diameter less than 20 nm were uniformly anchored on the surface of the TiO2NSs films. The QDs co-sensitization can significantly extend the absorption range and increase the absorption property of the photoelectrode by UVevis absorption spectra. The optimal photoelectrolchemical (PEC) performance of PbS/CdS QDs co-sensitization TiO2NSs was with photocurrent density of 6.12 mA cm2 under an illumination of AM 1.5 G, indicating the TiO2NSs films co-sensitized by PbS/CdS QDs have potential applications in solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanosheets array PbS/CdS QDs Photoelectrolchemical
1. Introduction TiO2 is known as a promising semiconductor material due to the features of appropriate electronic band structure, non-toxic, photostability and high chemical inertness [1]. It has been widely used in many fields such as photocatalysis [2], photovoltaic cells [3], and photoelectrochemical (PEC) cells [4,5]. Nevertheless, the wide band gap of TiO2 limits its application under sunlight, grabbing only about 3e5% of the whole solar spectrum. Various tactics have been developed to improve PEC performance of TiO2, such as the doping of nonmetal or metal ions [6e8], the feasible surface modification of dyes [9,10] or semiconductor materials [11,12]. The TiO2 nanostructures sensitized by narrow band gap inorganic semiconductors have been shown an effective photoelectrode. The nanosized semiconductors with large extinction coefficient can be tuned to match the solar spectrum and possess the characteristics of rapid charge separation. Moreover, semiconductor sensitizers can generate multiple charge carriers with a single photon [13e16]. Recently, multifarious narrow band gap semiconductors
* Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.06.132 0925-8388/© 2015 Elsevier B.V. All rights reserved.
quantum dots, including CdS [17,18], CdSe [19,20], PbS [21,22], CuInS2 [23] and CdTe [24,25] etc, have been studied for PEC cell based on TiO2 nanostructures. Among the sensitizers, CdS with direct band gap of 2.43 eV can give rise to wider light absorption range compared to TiO2 [26,27]. In particular, PbS has shown much promise as an impressive sensitizer due to its reasonable band gap of about 0.41 eV in the bulk material, which can allow extension of the absorption band toward the near infrared part of the solar spectrum [28,29]. And PbS with a large exciton Bohr radius of about 20 nm can lead to extensive quantum size effects [30]. The photoelectrodes of TiO2 nanostructures sensitized by QDs are equipped with enhanced the ability of light harvesting and enormously improved PEC properties. Moreover, considering interfacial electronic energy alignment, co-sensitized photoelectrodes incorporating at least two different types QDs is beneficial to promote charge separation and transfer between semiconductors [31,32]. One of the sensitizer can act as a buffer layer to control the band alignments of semiconductor at the interface, modifying the surface states and reducing recombination of photogenerate carriers [33]. QDs co-sensitized one-dimensional TiO2-based film nanostructures photoelectrodes for sensitized solar cells has been investigated, including nanotubes [34], nanorods [35]. However, few works have been carried out on QDs co-sensitized {001} facet-
H. Yao et al. / Journal of Alloys and Compounds 647 (2015) 402e406
dominated single crystalline anatase TiO2NSs array film photoanode. The TiO2NSs exposed high energy facets with intriguing surface and electronic properties have a substantial effect on the surface separation and transfer of photogenerated electronehole pairs [36]. Based on the above understanding, we designed the photoelectrods of PbS/CdS coesensitized TiO2NSs array films with CdS as an under layer and PbS as an outer layer, which should be a promising photoanode for its wide absorption spectrum, high electron injection efficiency and fast electrons transference. In this work, the PbS/CdS QDs co-sensitized TiO2NSs array films photoelectrodes were investigated for the first time. The TiO2NSs grown in situ uprightly on transparent conductive fluorine-doped tin oxide (FTO) glass substrate provided a direct channels for electron transport and the sufficient internal surface area provided the favorable terms for the sufficient QDs loading, thereby increasing the photocurrent efficiency. The TiO2NSs array films sensitized by narrow band gap QDs photoelectrodes significantly enhanced the light harvesting in the visible region and the PEC properties of PbS/CdS QDs co-modified TiO2NSs array films photoelectrodes were significantly improved in contrast with that of bare TiO2NSs array films. The results indicate that the novel photoelectrods are of excellence with respect to PEC properties and can be used as a promising photovoltaic material. 2. Experiment details 2.1. Preparation of TiO2NSs array films on FTO substrates The TiO2NSs array films were synthesized by a facile hydrothermal method, similar to our previous work [37]. Typically, titanium butoxide was titanium precursor dissolved in mixed solution of deionized water and hydrochloric acid. Ammonium hexafluorotitanate as a morphology controlling agent was added to the solution to promote the growth of nanosheets. The solution was placed in the Teflon-lined stainless steel autoclave clean FTO substrates. The hydrothermal synthesis was carried out at 170 C for 12 h in an electric oven. Subsequently, the FTO substrates with the samples were taken out and rinsed with deionized water thoroughly after the autoclave was cooled down. Finally, all the samples were annealed at 550 C for 2 h in air atmosphere. The prepared samples were preserved in dry and clean ambient for further characterization. 2.2. Preparation of PbS/CdS QDs co-sensitized TiO2NSs array films photoelectrodes The QDs were deposited onto the TiO2NSs film photoanodes by the simple successive ionic layer adsorption and reaction (SILAR). In a typical process, the TiO2NSs films were first immersed into the solution of 0.50 M Cd(NO3)2$4H2O in ethanol for 5 min and then successively into 0.50 M Na2S$9H2O in a mixture of methanol and deionized water (1:1, v/v). Such an immersion cycle was repeated seven times. The samples sensitized by CdS QDs were annealed at 300 C for 1 h to improve the crystalline. For PbS sensitization, the photoanodes were dipped into the 20 mM Pb(COOH)2 in absolute methanol for 2 min, and then sonication-assisted rinsed with absolute methanol to remove the excessive Pb2þ. The photoanodes were dried in a gentle stream of N2. Subsequently the dried photoanodes were dipped into 20 mM Na2S in methanol/water (1:1 v:v) for 2 min and then sonication-assisted rinsed with methanol. The immersion cycle of the PbS QDs was repeated different times. The achieved QDs coesensitized TiO2NSs films were designated as PbS (n c)/CdS/TiO2NSs (n ¼ 0, 1, 3, 5, 7), hereafter. The “n” in bracket denotes the cycles of the PbS QDs.
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2.3. Material characterizations The composition of crystal structure was identified by X-ray diffractometer (XRD, Rigaku D/max-2500) using Cu Ka radiation (l ¼ 1.5418 Å). The morphology and microstructure of the products were examined by field emission scanning electron microscope (FESEM, JEOL JSM-6700F). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. UVevis absorption was recorded in the range from 300 nm to 1000 nm by a UV-3150 double-beam spectrophotometer. The PEC properties were examined by the conventional threeeelectrode system with a platinum mesh as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. A mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution was used as electrolyte. Sunlight was simulated with a 500 W xenon lamp. The light intensity was calibrated at 100 mW/cm2 by a laser power meter. The active area was strictly kept within 0.25 cm2. 3. Results and discussion Fig. 1 shows FESEM images of the as-prepared TiO2NSs and PbS(5 c)/CdS/TiO2NSs films. As shown in Fig. 1A, the obtained TiO2NSs array films with thickness of about 200 nm are uniform and compact. The TiO2 nanosheets grow in order with sufficient internal surface area and the scarifying active {001} facets. The nanosheets are interlocking each other forming the network. The surface of the TiO2NSs is relative smooth as Fig. 1B shown. Fig. 1C shows that after deposition of PbS and CdS QDs, the TiO2NSs array film keeps original morphology but gets rough. The thickness of the nanosheets has a slight augment. Fig. 1D is the large SEM image of the TiO2NSs sensitized by 5 cycles PbS and 7 cycles CdS QDs. A mass of nanoparticles are observed both on the {001} and {101} facets of TiO2NSs film. The QDs distribute uniformly on the TiO2NSs framework. And the QDs are with diameter of less than 20 nm. The large gaps and pores among the nanosheets contribute to the thorough penetration of QDs into the TiO2NSs film photoelectrodes. The prepared TiO2NSs array films were examined by X-ray diffraction to characterize the crystalline of the samples. Fig. 2 displays the XRD patterns of the as prepared bare TiO2NSs and PbS(5 c)/CdS QDs co-sensitized TiO2NSs film. The diffraction peak of FTO substrates is also presented in Fig. 2a in accordance with JPCDS no. 41e1445. As for the bare TiO2NSs array film, all of the diffraction peaks are indexed to the standard anatase structure of TiO2 (JPCDS no. 21e1272; space group: I41/amd (no.141); a ¼ 0.3785 nm, c ¼ 0.9514 nm). No diffraction peaks from over crystalline phase are surveyed, indicating a high purity of the sample. The diffraction peak is very sharp verifying the high crystalline of the TiO2NSs. (Fig. 2b) As shown in Fig. 2c, after deposition of CdS and PbS QDs, no phase change of TiO2NSs film is observed. However, besides the diffraction peaks of anatase TiO2NSs, the new diffraction peaks appear at 30.13 , 43.86 and 68.88 , corresponding to (200) plane of PbS QDs, (110) plane of CdS QDs and (331) plane of PbS QDs, respectively. In addition, the intensified diffraction peaks at 26.47 is also indexed to (002) plane of CdS QDs. This provides powerful evidence for the coating of CdS and PbS QDs on the surface of TiO2NSs. Other diffraction peaks of the QDs are not observed clearly due to highly dispersed of the QDs on TiO2 surface. Fig. 3 shows the TEM and HTEM images of the as-prepared samples (the bare TiO2NSs and QDs co-sensitized TiO2NSs film) scraped off from the substrates. It is observed that the prepared TiO2 nanostructure is with a rectangular outline and extremely smooth surface (Fig. 3A). The relevant HRTEM image (Fig. 3B) directly shows that the lattice spacing is 0.237 nm, corresponding to the (004) planes of anatase TiO2 (JCPDS no. 21e1272). The TEM
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H. Yao et al. / Journal of Alloys and Compounds 647 (2015) 402e406
Fig. 1. (A) The FESEM image and (B) magnified FESEM image of TiO2 nanosheets array grown on FTO; (C) The FESEM image and (D) magnified FESEM image of PbS (5 C)/CdS QDs cosensitized TiO2 nanosheets array grown on FTO.
Fig. 2. XRD patterns of (a) FTO substrate, (b) as-prepared anatase TiO2 nanosheets array film grown on FTO substrate, (c) PbS (5 C)/CdS QDs co-sensitized TiO2 nanosheets array film grown on FTO.
image (Fig. 3C) of the QDs co-sensitized TiO2NSs demonstrates thickly dotted nanoparticles are attached to all of the facets of the TiO2 nanosheets. The observation is in good agreement with the previous SEM results. The corresponding HRTEM shown in Fig. 3D shows the microscopic property of the QDs co-sensitized TiO2NSs film. Around the TiO2 crystallite, high crystallites with various orientations and lattice spacings are observed. By carefully measuring the lattice spacing, it is confirmed the crystallites are consistent with CdS and PbS nanostructure. The observed lattice fringes with interplanar spacings 0.336 nm and 0.316 nm correspond to the (002) and (101) plane of hexagonal phase CdS [JCPDS no. 65e3414], respectively. While the lattice fringes in outer layer with interplanar spacings 0.343 nm and 0.297 nm is in accordance with (111) and (200) plane of cubic phase PbS [JCPDS no. 65e2935], respectively. The heterojunction are formed between the QDs and the TiO2NSs and the QDs are good immobilization on TiO2NSs. The UVevisible diffuse reflectance absorption spectras of TiO2NSs and QDs sensitized TiO2NSs photoanodes are shown in Fig. 4, from which enhanced visible light absorption of PbS (n c)/ CdS/TiO2NSs composite structures can be confirmed. The pure TiO2NSs array film absorbs mainly UV light with an absorption edge
at about 380 nm. The UVevisible spectra of CdS/TiO2NSs shows significantly redeshift of the absorption edge at about 560 nm, indicating that the deposition of CdS QDs has significantly extended the photoresponse of TiO2NSs photoanode into the visible light region. Remarkably, in the UVevis absorption spectra of PbS (n c)/ CdS/TiO2NSs, broader absorption bands appear to near-infrared region due to the narrow band gap of PbS semiconductor QDs. The absorption intensity in the whole visible light significantly enhanced with increasing SILAR cycles of PbS QDs, due to the growth of the PbS QDs [38]. The enhanced ability to absorb visible light makes this type of QDs/TiO2NSs a promising application in photovoltaic devices. Fig. 5 shows the photocurrent densityevoltage (JeV) curves measured from the samples of TiO2NSs and PbS (n c)/CdS/TiO2NSs. All samples show negligible current under dark condition. With light illumination, the CdS/TiO2NSs array film has a photocurrent density of 3.24 mA cm2 at 0 V versus Ag/AgCl, higher than 0.48 mAcm2 of the bare TiO2NSs array film. It is mainly attributed that CdS QDs make the absorption intensity increase in the visible light. For PbS (n c)/CdS/TiO2NSs photoanodes, with light illumination, the photocurrent is enhanced by increasing PbS SILAR cycles, indicating that a larger incorporated amount of PbS QDs can induce a higher photocurrent density. However, the output is found to somewhat decrease when the deposition cycles are increased to 7 c. This phenomenon may be attributed to the fact that too much PbS deposited would cause conglomeration and growth of the PbS crystal nucleus. The oversized PbS particles will lose the dominance of QDs that semiconductor QDs can efficiently generate multiple electronehole pairs with one single photon absorption and QDs possess large extinction coefficients [39]. Moreover, excess PbS nanoparticles may act as recombination traps for separated electrons and holes. In comparison, the optimum PbS (5 c)/CdS/TiO2NSs photoanode shows a photocurrent density of 6.12 mA cm2 at 0 V versus Ag/AgCl, which is nearly twice that of the CdS/TiO2NSs array film and twelve times than that of bare TiO2NSs. The enhancement of PEC performance may be attributed to three main points as follows. Firstly, the narrow band gap PbS and CdS QDs broaden absorption range in visible light and near-infrared region, which can boost more photoexcited carrier and increase in the light harvesting capability of the photoelectrode. Secondly, the TiO2
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Fig. 3. (A) The TEM bright-field image and (B) HRTEM image of the bare TiO2 nanosheet, (C) The TEM bright-field image and (D) HRTEM image of PbS (5 C)/CdS QDs co-sensitized TiO2 nanosheet, respectively.
nanosheets film with gaps is conducive to the absorption of QDs and the large nanoshees with light scattering abilities extend the distance that light travels within the photoelectrode film. More photons can be absorbed by the sensitizer molecules, resulting in the improved PEC performance. Last but not the least, CdS and PbS have higher conduction band edges and lower valence band edges than those of TiO2. (Scheme 1) The typeeII structure between the QDs and TiO2 nanostructures is formed, impelling photogenerated electrons injection rapidly from the QDs into TiO2 and holes to opposite direction before recombination occurs.
Fig. 4. Diffuse reflectance absorption spectra of (a) the bare TiO2NSs array films, (b)e(f) the composite films with seven cycles CdS QDs and different SILAR cycles (0 C, 1 C, 3 C, 5 C, 7 C) of PbS QDs, respectively.
4. Conclusions In summary, PbS/CdS QDs co-sensitized TiO2 nanosheets array thin films photoelectrodes were prepared by simple hydrothermal method and sequential successive ionic layer adsorption subsequently. UVevisible absorption spectrum shows that the aseprepared photoanode exhibited strong absorption in almost the
Fig. 5. Photocurrent voltage (JeV) curves of photoelectrodes (a) dark current, (b) bare TiO2NSs, (c) PbS(0 C)/CdS/TiO2NSs, (d) PbS(1 C)/CdS/TiO2NSs, (e) PbS(3 C)/TiO2NSs, (f) PbS(5 C)/CdS/TiO2NSs; (g) PbS(7 C)/CdS/TiO2NSs in the polysulfide electrolyte consists of 0.25 M Na2S and 0.35 M Na2SO3 solution in Milli-Q ultrapure water under illumination of 100 mW cm2.
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Scheme 1. : Architecture of the PbS/CdS QDs co-sentisized TiO2NSs array photoelectrode, energy band of PbS, CdS and TiO2 coupled semiconductor, and photoinduced charge separation and transport in the composite structures.
entire visible spectrum. The photoelectrochemical performance of the PbS/CdS/TiO2NSs was investigated in detailed. After sensitized by seven cycles CdS and five cycles PbS QDs, the optimal photocurrent intensity of the novel photoelectrode was 6.12 mA cm2 at a potential of 0 V versus Ag/AgCl under AM 1.5 G illumination, much higher than that of a bare TiO2 nanosheets array films. According to the excellent photoelectrochemical properties achievement, PbS/ CdS QDs co-sensitized TiO2 nanosheets array films can be a promising photovoltaic device. Acknowledgment This work was financially supported by Science and Technology Development Program of Jilin Province (20110417) and project suppoeted by Graduate Innovation Found of Jilin University (2015024). References [1] H. Yang, C. Sun, S. Qiao, J. Zou, G. Liu, S.C. Smith, H. Cheng, G. Lu, Nature 453 (2008) 638e641. [2] A. Fujishima, K. Honda, Nature 238 (5358) (1972) 37e38. [3] Y.G. Kim, J. Walker, L.A. Samuelson, J. Kumar, Nano Lett. 3 (4) (2003) 523e525. [4] J. Hensel, G. Wang, Y. Li, J.Z. Zhang, Nano Lett. 10 (2) (2010) 478e483. [5] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205e213. [6] S. Hoang, S. Guo, N.T. Hahn, A.J. Bard, C.B. Mullins, Nano Lett. 12 (2012) 26e32. [7] T. Ohno, T. Mitsui, M. Matsumura, Chem. Lett. 32 (2003) 364e365. [8] S. Hoang, S. Guo, C.B. Mullins, J. Phys. Chem. C 116 (2012) 23283e23290. [9] W.Q. Wu, Y.F. Xu, C.Y. Su, D.B. Kuang, Energy Environ. Sci. 7 (2) (2014) 644e649. [10] X.J. Feng, K. Zhu, A.J. Frank, C.A. Grimes, T.E. Mallouk, Angew. Chem. 124 (2012) 2781e2784. [11] A.J. Nozik, M.C. Beard, J.M. Luther, M. Law, R.J. Ellingson, J.C. Johnson, Chem. Rev. 110 (2010) 6873e6890. [12] S. Emin, S.P. Singh, L. Han, N. Satoh, A. Islam, Sol. Energy 85 (2011) 1264e1282. [13] W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854e2860.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Photoelectrolchemical performance of PbS/CdS quantum dots cosensitized TiO2 nanosheets array film photoelectrodes Huizhen Yao a, Xue Li a, Li Liu a, Jiasheng Niu a, Dong Ding a, Yannan Mu a, b, Pengyu Su a, Guangxia Wang a, Wuyou Fu a, Haibin Yang a, * a b
National Key Lab of Superhard Materials, Jilin University, Changchun 130012, China Department of Physics and Chemistry, Heihe University, Heihe 164300, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 18 June 2015
Herein, PbS/CdS quantum dots (QDs) co-sensitized titanium dioxide nanosheets array (TiO2NSs) films were reported for the first time. The TiO2NSs films exposed {001} facets were vertically grown on transparent conductive fluorine-doped tin oxide (FTO) glass substrates by a facile hydrothermal method. The PbS/CdS QDs were assembled on TiO2NSs photoelectrode by successive ionic layer adsorption and reaction (SILAR). The X-ray diffraction pattern (XRD) and transmission electron microscopy (TEM) verified that QDs with a diameter less than 20 nm were uniformly anchored on the surface of the TiO2NSs films. The QDs co-sensitization can significantly extend the absorption range and increase the absorption property of the photoelectrode by UVevis absorption spectra. The optimal photoelectrolchemical (PEC) performance of PbS/CdS QDs co-sensitization TiO2NSs was with photocurrent density of 6.12 mA cm2 under an illumination of AM 1.5 G, indicating the TiO2NSs films co-sensitized by PbS/CdS QDs have potential applications in solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: TiO2 nanosheets array PbS/CdS QDs Photoelectrolchemical
1. Introduction TiO2 is known as a promising semiconductor material due to the features of appropriate electronic band structure, non-toxic, photostability and high chemical inertness [1]. It has been widely used in many fields such as photocatalysis [2], photovoltaic cells [3], and photoelectrochemical (PEC) cells [4,5]. Nevertheless, the wide band gap of TiO2 limits its application under sunlight, grabbing only about 3e5% of the whole solar spectrum. Various tactics have been developed to improve PEC performance of TiO2, such as the doping of nonmetal or metal ions [6e8], the feasible surface modification of dyes [9,10] or semiconductor materials [11,12]. The TiO2 nanostructures sensitized by narrow band gap inorganic semiconductors have been shown an effective photoelectrode. The nanosized semiconductors with large extinction coefficient can be tuned to match the solar spectrum and possess the characteristics of rapid charge separation. Moreover, semiconductor sensitizers can generate multiple charge carriers with a single photon [13e16]. Recently, multifarious narrow band gap semiconductors
* Corresponding author. E-mail address: [email protected] (H. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.06.132 0925-8388/© 2015 Elsevier B.V. All rights reserved.
quantum dots, including CdS [17,18], CdSe [19,20], PbS [21,22], CuInS2 [23] and CdTe [24,25] etc, have been studied for PEC cell based on TiO2 nanostructures. Among the sensitizers, CdS with direct band gap of 2.43 eV can give rise to wider light absorption range compared to TiO2 [26,27]. In particular, PbS has shown much promise as an impressive sensitizer due to its reasonable band gap of about 0.41 eV in the bulk material, which can allow extension of the absorption band toward the near infrared part of the solar spectrum [28,29]. And PbS with a large exciton Bohr radius of about 20 nm can lead to extensive quantum size effects [30]. The photoelectrodes of TiO2 nanostructures sensitized by QDs are equipped with enhanced the ability of light harvesting and enormously improved PEC properties. Moreover, considering interfacial electronic energy alignment, co-sensitized photoelectrodes incorporating at least two different types QDs is beneficial to promote charge separation and transfer between semiconductors [31,32]. One of the sensitizer can act as a buffer layer to control the band alignments of semiconductor at the interface, modifying the surface states and reducing recombination of photogenerate carriers [33]. QDs co-sensitized one-dimensional TiO2-based film nanostructures photoelectrodes for sensitized solar cells has been investigated, including nanotubes [34], nanorods [35]. However, few works have been carried out on QDs co-sensitized {001} facet-
H. Yao et al. / Journal of Alloys and Compounds 647 (2015) 402e406
dominated single crystalline anatase TiO2NSs array film photoanode. The TiO2NSs exposed high energy facets with intriguing surface and electronic properties have a substantial effect on the surface separation and transfer of photogenerated electronehole pairs [36]. Based on the above understanding, we designed the photoelectrods of PbS/CdS coesensitized TiO2NSs array films with CdS as an under layer and PbS as an outer layer, which should be a promising photoanode for its wide absorption spectrum, high electron injection efficiency and fast electrons transference. In this work, the PbS/CdS QDs co-sensitized TiO2NSs array films photoelectrodes were investigated for the first time. The TiO2NSs grown in situ uprightly on transparent conductive fluorine-doped tin oxide (FTO) glass substrate provided a direct channels for electron transport and the sufficient internal surface area provided the favorable terms for the sufficient QDs loading, thereby increasing the photocurrent efficiency. The TiO2NSs array films sensitized by narrow band gap QDs photoelectrodes significantly enhanced the light harvesting in the visible region and the PEC properties of PbS/CdS QDs co-modified TiO2NSs array films photoelectrodes were significantly improved in contrast with that of bare TiO2NSs array films. The results indicate that the novel photoelectrods are of excellence with respect to PEC properties and can be used as a promising photovoltaic material. 2. Experiment details 2.1. Preparation of TiO2NSs array films on FTO substrates The TiO2NSs array films were synthesized by a facile hydrothermal method, similar to our previous work [37]. Typically, titanium butoxide was titanium precursor dissolved in mixed solution of deionized water and hydrochloric acid. Ammonium hexafluorotitanate as a morphology controlling agent was added to the solution to promote the growth of nanosheets. The solution was placed in the Teflon-lined stainless steel autoclave clean FTO substrates. The hydrothermal synthesis was carried out at 170 C for 12 h in an electric oven. Subsequently, the FTO substrates with the samples were taken out and rinsed with deionized water thoroughly after the autoclave was cooled down. Finally, all the samples were annealed at 550 C for 2 h in air atmosphere. The prepared samples were preserved in dry and clean ambient for further characterization. 2.2. Preparation of PbS/CdS QDs co-sensitized TiO2NSs array films photoelectrodes The QDs were deposited onto the TiO2NSs film photoanodes by the simple successive ionic layer adsorption and reaction (SILAR). In a typical process, the TiO2NSs films were first immersed into the solution of 0.50 M Cd(NO3)2$4H2O in ethanol for 5 min and then successively into 0.50 M Na2S$9H2O in a mixture of methanol and deionized water (1:1, v/v). Such an immersion cycle was repeated seven times. The samples sensitized by CdS QDs were annealed at 300 C for 1 h to improve the crystalline. For PbS sensitization, the photoanodes were dipped into the 20 mM Pb(COOH)2 in absolute methanol for 2 min, and then sonication-assisted rinsed with absolute methanol to remove the excessive Pb2þ. The photoanodes were dried in a gentle stream of N2. Subsequently the dried photoanodes were dipped into 20 mM Na2S in methanol/water (1:1 v:v) for 2 min and then sonication-assisted rinsed with methanol. The immersion cycle of the PbS QDs was repeated different times. The achieved QDs coesensitized TiO2NSs films were designated as PbS (n c)/CdS/TiO2NSs (n ¼ 0, 1, 3, 5, 7), hereafter. The “n” in bracket denotes the cycles of the PbS QDs.
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2.3. Material characterizations The composition of crystal structure was identified by X-ray diffractometer (XRD, Rigaku D/max-2500) using Cu Ka radiation (l ¼ 1.5418 Å). The morphology and microstructure of the products were examined by field emission scanning electron microscope (FESEM, JEOL JSM-6700F). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope. UVevis absorption was recorded in the range from 300 nm to 1000 nm by a UV-3150 double-beam spectrophotometer. The PEC properties were examined by the conventional threeeelectrode system with a platinum mesh as the counter electrode and a saturated Ag/AgCl electrode as the reference electrode. A mixture of 0.25 M Na2S and 0.35 M Na2SO3 aqueous solution was used as electrolyte. Sunlight was simulated with a 500 W xenon lamp. The light intensity was calibrated at 100 mW/cm2 by a laser power meter. The active area was strictly kept within 0.25 cm2. 3. Results and discussion Fig. 1 shows FESEM images of the as-prepared TiO2NSs and PbS(5 c)/CdS/TiO2NSs films. As shown in Fig. 1A, the obtained TiO2NSs array films with thickness of about 200 nm are uniform and compact. The TiO2 nanosheets grow in order with sufficient internal surface area and the scarifying active {001} facets. The nanosheets are interlocking each other forming the network. The surface of the TiO2NSs is relative smooth as Fig. 1B shown. Fig. 1C shows that after deposition of PbS and CdS QDs, the TiO2NSs array film keeps original morphology but gets rough. The thickness of the nanosheets has a slight augment. Fig. 1D is the large SEM image of the TiO2NSs sensitized by 5 cycles PbS and 7 cycles CdS QDs. A mass of nanoparticles are observed both on the {001} and {101} facets of TiO2NSs film. The QDs distribute uniformly on the TiO2NSs framework. And the QDs are with diameter of less than 20 nm. The large gaps and pores among the nanosheets contribute to the thorough penetration of QDs into the TiO2NSs film photoelectrodes. The prepared TiO2NSs array films were examined by X-ray diffraction to characterize the crystalline of the samples. Fig. 2 displays the XRD patterns of the as prepared bare TiO2NSs and PbS(5 c)/CdS QDs co-sensitized TiO2NSs film. The diffraction peak of FTO substrates is also presented in Fig. 2a in accordance with JPCDS no. 41e1445. As for the bare TiO2NSs array film, all of the diffraction peaks are indexed to the standard anatase structure of TiO2 (JPCDS no. 21e1272; space group: I41/amd (no.141); a ¼ 0.3785 nm, c ¼ 0.9514 nm). No diffraction peaks from over crystalline phase are surveyed, indicating a high purity of the sample. The diffraction peak is very sharp verifying the high crystalline of the TiO2NSs. (Fig. 2b) As shown in Fig. 2c, after deposition of CdS and PbS QDs, no phase change of TiO2NSs film is observed. However, besides the diffraction peaks of anatase TiO2NSs, the new diffraction peaks appear at 30.13 , 43.86 and 68.88 , corresponding to (200) plane of PbS QDs, (110) plane of CdS QDs and (331) plane of PbS QDs, respectively. In addition, the intensified diffraction peaks at 26.47 is also indexed to (002) plane of CdS QDs. This provides powerful evidence for the coating of CdS and PbS QDs on the surface of TiO2NSs. Other diffraction peaks of the QDs are not observed clearly due to highly dispersed of the QDs on TiO2 surface. Fig. 3 shows the TEM and HTEM images of the as-prepared samples (the bare TiO2NSs and QDs co-sensitized TiO2NSs film) scraped off from the substrates. It is observed that the prepared TiO2 nanostructure is with a rectangular outline and extremely smooth surface (Fig. 3A). The relevant HRTEM image (Fig. 3B) directly shows that the lattice spacing is 0.237 nm, corresponding to the (004) planes of anatase TiO2 (JCPDS no. 21e1272). The TEM
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Fig. 1. (A) The FESEM image and (B) magnified FESEM image of TiO2 nanosheets array grown on FTO; (C) The FESEM image and (D) magnified FESEM image of PbS (5 C)/CdS QDs cosensitized TiO2 nanosheets array grown on FTO.
Fig. 2. XRD patterns of (a) FTO substrate, (b) as-prepared anatase TiO2 nanosheets array film grown on FTO substrate, (c) PbS (5 C)/CdS QDs co-sensitized TiO2 nanosheets array film grown on FTO.
image (Fig. 3C) of the QDs co-sensitized TiO2NSs demonstrates thickly dotted nanoparticles are attached to all of the facets of the TiO2 nanosheets. The observation is in good agreement with the previous SEM results. The corresponding HRTEM shown in Fig. 3D shows the microscopic property of the QDs co-sensitized TiO2NSs film. Around the TiO2 crystallite, high crystallites with various orientations and lattice spacings are observed. By carefully measuring the lattice spacing, it is confirmed the crystallites are consistent with CdS and PbS nanostructure. The observed lattice fringes with interplanar spacings 0.336 nm and 0.316 nm correspond to the (002) and (101) plane of hexagonal phase CdS [JCPDS no. 65e3414], respectively. While the lattice fringes in outer layer with interplanar spacings 0.343 nm and 0.297 nm is in accordance with (111) and (200) plane of cubic phase PbS [JCPDS no. 65e2935], respectively. The heterojunction are formed between the QDs and the TiO2NSs and the QDs are good immobilization on TiO2NSs. The UVevisible diffuse reflectance absorption spectras of TiO2NSs and QDs sensitized TiO2NSs photoanodes are shown in Fig. 4, from which enhanced visible light absorption of PbS (n c)/ CdS/TiO2NSs composite structures can be confirmed. The pure TiO2NSs array film absorbs mainly UV light with an absorption edge
at about 380 nm. The UVevisible spectra of CdS/TiO2NSs shows significantly redeshift of the absorption edge at about 560 nm, indicating that the deposition of CdS QDs has significantly extended the photoresponse of TiO2NSs photoanode into the visible light region. Remarkably, in the UVevis absorption spectra of PbS (n c)/ CdS/TiO2NSs, broader absorption bands appear to near-infrared region due to the narrow band gap of PbS semiconductor QDs. The absorption intensity in the whole visible light significantly enhanced with increasing SILAR cycles of PbS QDs, due to the growth of the PbS QDs [38]. The enhanced ability to absorb visible light makes this type of QDs/TiO2NSs a promising application in photovoltaic devices. Fig. 5 shows the photocurrent densityevoltage (JeV) curves measured from the samples of TiO2NSs and PbS (n c)/CdS/TiO2NSs. All samples show negligible current under dark condition. With light illumination, the CdS/TiO2NSs array film has a photocurrent density of 3.24 mA cm2 at 0 V versus Ag/AgCl, higher than 0.48 mAcm2 of the bare TiO2NSs array film. It is mainly attributed that CdS QDs make the absorption intensity increase in the visible light. For PbS (n c)/CdS/TiO2NSs photoanodes, with light illumination, the photocurrent is enhanced by increasing PbS SILAR cycles, indicating that a larger incorporated amount of PbS QDs can induce a higher photocurrent density. However, the output is found to somewhat decrease when the deposition cycles are increased to 7 c. This phenomenon may be attributed to the fact that too much PbS deposited would cause conglomeration and growth of the PbS crystal nucleus. The oversized PbS particles will lose the dominance of QDs that semiconductor QDs can efficiently generate multiple electronehole pairs with one single photon absorption and QDs possess large extinction coefficients [39]. Moreover, excess PbS nanoparticles may act as recombination traps for separated electrons and holes. In comparison, the optimum PbS (5 c)/CdS/TiO2NSs photoanode shows a photocurrent density of 6.12 mA cm2 at 0 V versus Ag/AgCl, which is nearly twice that of the CdS/TiO2NSs array film and twelve times than that of bare TiO2NSs. The enhancement of PEC performance may be attributed to three main points as follows. Firstly, the narrow band gap PbS and CdS QDs broaden absorption range in visible light and near-infrared region, which can boost more photoexcited carrier and increase in the light harvesting capability of the photoelectrode. Secondly, the TiO2
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Fig. 3. (A) The TEM bright-field image and (B) HRTEM image of the bare TiO2 nanosheet, (C) The TEM bright-field image and (D) HRTEM image of PbS (5 C)/CdS QDs co-sensitized TiO2 nanosheet, respectively.
nanosheets film with gaps is conducive to the absorption of QDs and the large nanoshees with light scattering abilities extend the distance that light travels within the photoelectrode film. More photons can be absorbed by the sensitizer molecules, resulting in the improved PEC performance. Last but not the least, CdS and PbS have higher conduction band edges and lower valence band edges than those of TiO2. (Scheme 1) The typeeII structure between the QDs and TiO2 nanostructures is formed, impelling photogenerated electrons injection rapidly from the QDs into TiO2 and holes to opposite direction before recombination occurs.
Fig. 4. Diffuse reflectance absorption spectra of (a) the bare TiO2NSs array films, (b)e(f) the composite films with seven cycles CdS QDs and different SILAR cycles (0 C, 1 C, 3 C, 5 C, 7 C) of PbS QDs, respectively.
4. Conclusions In summary, PbS/CdS QDs co-sensitized TiO2 nanosheets array thin films photoelectrodes were prepared by simple hydrothermal method and sequential successive ionic layer adsorption subsequently. UVevisible absorption spectrum shows that the aseprepared photoanode exhibited strong absorption in almost the
Fig. 5. Photocurrent voltage (JeV) curves of photoelectrodes (a) dark current, (b) bare TiO2NSs, (c) PbS(0 C)/CdS/TiO2NSs, (d) PbS(1 C)/CdS/TiO2NSs, (e) PbS(3 C)/TiO2NSs, (f) PbS(5 C)/CdS/TiO2NSs; (g) PbS(7 C)/CdS/TiO2NSs in the polysulfide electrolyte consists of 0.25 M Na2S and 0.35 M Na2SO3 solution in Milli-Q ultrapure water under illumination of 100 mW cm2.
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Scheme 1. : Architecture of the PbS/CdS QDs co-sentisized TiO2NSs array photoelectrode, energy band of PbS, CdS and TiO2 coupled semiconductor, and photoinduced charge separation and transport in the composite structures.
entire visible spectrum. The photoelectrochemical performance of the PbS/CdS/TiO2NSs was investigated in detailed. After sensitized by seven cycles CdS and five cycles PbS QDs, the optimal photocurrent intensity of the novel photoelectrode was 6.12 mA cm2 at a potential of 0 V versus Ag/AgCl under AM 1.5 G illumination, much higher than that of a bare TiO2 nanosheets array films. According to the excellent photoelectrochemical properties achievement, PbS/ CdS QDs co-sensitized TiO2 nanosheets array films can be a promising photovoltaic device. Acknowledgment This work was financially supported by Science and Technology Development Program of Jilin Province (20110417) and project suppoeted by Graduate Innovation Found of Jilin University (2015024). References [1] H. Yang, C. Sun, S. Qiao, J. Zou, G. Liu, S.C. Smith, H. Cheng, G. Lu, Nature 453 (2008) 638e641. [2] A. Fujishima, K. Honda, Nature 238 (5358) (1972) 37e38. [3] Y.G. Kim, J. Walker, L.A. Samuelson, J. Kumar, Nano Lett. 3 (4) (2003) 523e525. [4] J. Hensel, G. Wang, Y. Li, J.Z. Zhang, Nano Lett. 10 (2) (2010) 478e483. [5] H.A. Atwater, A. Polman, Nat. Mater. 9 (2010) 205e213. [6] S. Hoang, S. Guo, N.T. Hahn, A.J. Bard, C.B. Mullins, Nano Lett. 12 (2012) 26e32. [7] T. Ohno, T. Mitsui, M. Matsumura, Chem. Lett. 32 (2003) 364e365. [8] S. Hoang, S. Guo, C.B. Mullins, J. Phys. Chem. C 116 (2012) 23283e23290. [9] W.Q. Wu, Y.F. Xu, C.Y. Su, D.B. Kuang, Energy Environ. Sci. 7 (2) (2014) 644e649. [10] X.J. Feng, K. Zhu, A.J. Frank, C.A. Grimes, T.E. Mallouk, Angew. Chem. 124 (2012) 2781e2784. [11] A.J. Nozik, M.C. Beard, J.M. Luther, M. Law, R.J. Ellingson, J.C. Johnson, Chem. Rev. 110 (2010) 6873e6890. [12] S. Emin, S.P. Singh, L. Han, N. Satoh, A. Islam, Sol. Energy 85 (2011) 1264e1282. [13] W. Yu, L.H. Qu, W.Z. Guo, X.G. Peng, Chem. Mater. 15 (2003) 2854e2860.
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