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TiO2 Nanobelts/CdSSe Quantum Dots Nanocomposite Shen V. Chong,*,† Narayanaswamy Suresh,‡ James Xia,† Najeh Al-Salim,† and Hicham Idriss§ Industrial Research Limited, P. O. Box 31310, Lower Hutt 5040, New Zealand, The MacDiarmid Institute for AdVanced Materials and Nanotechnology, Victoria UniVersity of Wellington, P. O. Box 600, Wellington 6140, New Zealand, and Department of Chemistry, The UniVersity of Auckland, PriVate Bag 92019, Auckland 1142, New Zealand ReceiVed: April 2, 2007; In Final Form: May 9, 2007
This work presents the successful noncovalent attachment of ∼5 nm diameter cadmium-sulfur-selenium (CdSSe) quantum dots on strips of anatase TiO2 nanobelts. The TiO2 nanobelts were hydrothermally synthesized from a strong alkaline solution and subsequently heat-treated to achieve the anatase phase. The self-assembledmonolayer (SAM) technique was employed to attach the quantum dots onto the nanobelts. Due to the hydrophobic nature of the quantum dots, the surface of the nanobelts was first self-assembled with a layer of hydrophobic organic layer before both mixtures were added together. The resulting nanostructure assembly and composition was confirmed via transmission-electron-microscopy (TEM) imaging, Raman spectroscopy, UV-visible absorption spectroscopy (UV-vis), and X-ray photoelectron spectroscopy (XPS). Both Raman and UV-vis spectroscopies indicate evidence of interactions between the quantum dots and nanobelts. The visible-light sensitizing effect of the quantum dots was demonstrated in photocurrent experiments.
Introduction The photophysics and chemistry of titanium dioxide are some of the most studied topics in literature over the past decades. This is mostly attributed to the ability of TiO2 in absorbing UV light, in the region of ∼380 nm or less wavelength, creating an electron and hole pair which can be channeled for chemical and electronic processes. Various endeavors have been pursued to improve and enhance the functional properties of titania.1,2 A more recent approach has been in the syntheses of quasione-dimensional titanium oxide in the form of nanotubes, nanowires, and nanobelts with the view that dimension confinement in combination with the novel shapes will unveil interesting physical and electrical properties in this metal oxide.3-8 Although the energy gap of these single-crystalline nanostructural titania show little alteration compared with bulk TiO2, they do however display an increase in photoactivity due to better charge transport in the crystallites, a higher surface defect content (higher electron trapping states), and furthermore an important property is their inherent high aspect ratio such as a large surface area.6-8 Moreover, the size and shape of nanocrystalline semiconductors are known to be important factors in influencing the nature of the bonding and packing density of sensitizing molecules or compound, which has a significant impact on the charge-transferring processes from the sensitizer to the semiconductor.9 The exotic surface geometry that these nanostructural titania possess are excellent candidates to explore such factors. The assembling of quantum dots (Q-dots) on the surfaces of TiO2 has been investigated over the past few decades as an alternativetodye-sensitizedvisible-lightharvestingapplications.10-14 Among the Q-dots, the cadmium-based semiconductors have been widely studied on titania electrodes.13-20 Upon visible* Corresponding author. E-mail:
[email protected]. † Industrial Research Ltd. ‡ Victoria University of Wellington. § The University of Auckland.
light irradiation, these Q-dots are capable of injecting electrons into the conduction band of TiO2 (and other wider-gap metal oxides), generating photocurrent and enhancing titania’s photoactivities. Until recently, the assembly of Q-dots on TiO2 has been carried-out only on nanoparticle polycrystalline surfaces. Nanostructural TiO2 such as nanotubes, nanowires, and nanobelts can be advantageous for being single-crystalline in nature compared with nanoparticle titania in addition to having a high surface-to-volume ratio. The assembling of Q-dots on TiO2 nanotubes and nanowires has recently been reported by Shen and co-workers21,22 in which these investigators have found more efficient electron diffusion in these single-crystalline nanomaterials, while Kukovecz et al.23 reported a novel synthetic route to prepare titanate nanotubes decorated with CdS nanoparticles via a complex-assisted process. In this work we report the noncovalent attachment of CdSSe Q-dots on the surfaces of TiO2 nanobelts. A saturated or near-saturation coverage of the Q-dots on the surface of TiO2 nanobelts as a first investigation of the behavior of this nanocomposite will be presented. The advantages of using CdSxSe1-x Q-dots over CdS or CdSe counterparts include the ability to tune the energy gap of cadmium sulfoselenide between the energy level of the same-sized CdSe and CdS nanocrystals, as well as observed higher photoluminescence quantum efficiency of up to 85% and photostability for CdSxSe1-x Q-dots.24 The use of nanobelts is to simplify the geometrical factor in the nanomorphology TiO2 structure as these belts are rectangularly shaped and have preferential exposed surfaces of either (100) or (010)3-6 - an analogy of large pieces of single crystal. Photocurrent measurements in an electrochemical cell have been conducted to probe the photosensitizing effects in this coupled system. Experimental Methods TiO2 nanobelts were synthesized hydrothermally at 180200 °C in a Teflon-lined stainless steel autoclave of 12 mL capacity according to literature methods.3-6 In a typical batch
10.1021/jp072579u CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007
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Figure 1. TEM of anatase TiO2 nanobelts in different magnifications (a and c), and CdSSe Q-dots/TiO2 nanobelts composite (b). Inset in a: Indexed electron diffraction pattern of anatase in [201] zone axis.
preparation, 0.2 g of anatase TiO2 powder was placed in the autoclave and filled to 80% capacity with 10 mol/L NaOH(aq). After thorough mixing, the reaction vessel was sealed and placed in an oven for at least 24 h. The obtained product, after being naturally cooled to room temperature, was filtered and washed repeatedly with 0.1 mol/L HCl(aq) and deionized water until a neutral pH was observed from the filtrate. The final step involved a wash with absolute ethanol before being dried first in air and then in a 70 °C oven overnight. The anatase phase TiO2 nanostructured product was obtained by heating the dried sample under a flow of dry air at 727 °C (or 1000 K) for at least 3 h. Green-emitting CdSSe quantum dots were prepared by the single injection of a S and Se source mixture into a hot cadmium precursor reaction medium.24 The cadmium mixture was prepared by dissolving cadmium oleate (0.2 mmol) in trioctylphosphine oxide, TOPO (0.01 mol), and heated to 300 °C under argon atmosphere. Another solution prepared under argon containing selenium (0.024 mmol) and sulfur powder (0.6 mmol) dissolved in trioctylphosphine (4.5 mmol) was quickly injected into the cadmium solution. After 5 min the resulting orange mixture was removed from heating and cooled quickly by quenching it with the addition of absolute ethanol. The yellow precipitate was separated by centrifugation and washed several times with ethanol. After drying in vacuum the solid was dissolved in a minimum amount of toluene. The hydrophobic nature of these Q-dots dictates that one should first hydrophobically functionalize the surface of the
titania to enable the self-assembling of these Q-dots via hydrophobic interactions between the two components. The reaction of carboxylic acids with TiO2 is among the best understood reaction in surface science. Because of their strong adsorption energy via their carboxylate function to the Ti centers, they are easily self-assembled on the surface of TiO225,26 - a longchain alkyl carboxylic acid with methyl group termination is an excellent candidate to serve this purpose. TiO2 nanobelts (0.2 g, 2.5 mmol) were suspended in absolute ethanol in an ultrasonic bath and treated with octanoic acid (3 mmol) and left to selfassemble for 2 h with occasional shaking. The mixture was centrifuged, and the solid was washed several times with ethanol to remove excess octanoic acid. The solid was dried under vacuum and resuspended in dry toluene before the solution was divided into two equal portions. To one portion of the nanobelt suspensions, 0.1 mL of the green-emitting TOPO-capped CdSSe quantum dot solution was added and left to self-assemble for 1 h with occasional shaking followed by another two lots of 0.1 mL of the Q-dots solution, which were added with a time space of 30 min. The mixture was centrifuged, and the solid was washed several times with toluene to remove excess Q-dots. Quenching of the fluorescence of activated CdSSe by TiO2 was observed during the self-assembling process, which is an indication of the attachment of these Q-dots onto titania. The resulting TiO2/Q-dots composite was kept in suspension in toluene. Transmission electron microscopy (TEM) and SAED (selected area electron diffraction) were obtained using a JEOL
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Figure 2. Raman spectra of TiO2 anatase powder (lower curve, black), TiO2 nanobelts (upper curve, grey), and Q-dots/TiO2 nanobelts composite (inset) obtained from two different excitation sources.
2011 high-resolution instrument with a LaB6 filament operated at 200 kV. Raman spectra were collected using a Jobin-Yvon LabRam HR spectrometer with 325 and 514 nm excitation sources. X-ray photoelectron spectroscopy (XPS) was undertaken in a Perkin-Elmer (PHI) surface-analysis chamber (1 × 10-10 Torr) with cylindrical mirror analyzer using the KR line of monochromatized Al (1486.6 eV, 250 W, 13.5 kV) as the radiation source. A constant pass energy of 25 eV was used for all scans with a step size of 0.1 eV unless otherwise stated. All spectra were referenced to the carbon 1s peak at 284.6 eV binding energy. Results and Discussion Electron micrographs of the synthesized TiO2 nanostructured material showed they are mostly in nanobelt form, that is, a rectangularly shaped cross-section instead of a cylindrical structure (nanowires), with length up to several micrometers long and typical thickness and width of 20-50 nm and 50150 nm, respectively. Figure 1a presents a typical morphology of two overlapping nanobelts annealed at 727 °C, while the electron diffraction micrograph (inset) confirming their singlecrystalline nature. Powder X-ray diffraction (not shown) and Raman spectroscopy (Figure 2) confirm that these titania are in the anatase phase, with the latter exhibiting the characteristic anatase-phase Raman bands27 of A1g + 2B1g + 3Eg at 133.5, 187.2, 386.9, 506.6, and 630.1 cm-1 observed under the irradiation of 514 nm argon ion laser. The first overtone of B1g at 796 cm-1 is not visible in both the TiO2 anatase powder and nanobelts spectra. Upon the self-assembling of octanoic acid, and the subsequent noncovalent attachment of Q-dots, a strong fluorescent background due to the Q-dots at this excitation wavelength blanketed the TiO2 Raman bands. However the Raman lines are visible under the irradiation of 325 nm (Liconix He-Cd laser) excitation source, with a notable upshift in peak centers for the Eg band and the Ti-O stretching mode above 850 cm-1, which is normally observed in low-dimensional titanium oxide nanomaterials and is assigned to a stretching
mode of terminal or apical titanium-oxygen bonds protruding into the interfacial/interlayer space28-30 (An analogy of this latter bond is the apical WdO bond, of which vibrational stretching is observed on both layered WO3‚H2O and oxygen-deficient monoclinic WO3 at ∼950 cm-1).31,32 The former band (Eg) shifts from 631.2 cm-1 for bare TiO2 nanobelts to 638.2 cm-1 for the nanobelts with octanoic acid and Q-dots attached. The shift is even larger for the band above 850 cm-1, from 855.1 to 867.4 cm-1. The direct attachment of CdS Q-dots on TiO2 nanoparticles usually results in a red shift of the titania Raman bands due to a charge transfer from the Q-dots to the TiO2 conduction band, resulting in the softening of the phonon modes.33 However, in our case, the extra layer of octanoic acid selfassembled monolayer (SAM) is causing a different effect on the TiO2 surface. Xu and co-workers have observed a blue shift in their surface-decorated TiO2 nanoparticles with dodecylbenzenesulfonic and stearic acids,34 in which the authors attributed the Raman blue-shift to being caused by a compressive stress exerted by the attached SAMs on the surface bonds. In our case, the larger blue shift observed for the terminal Ti-O Raman stretching band at 855.1 cm-1 further ascertains a compressive force is acting on the titania surface, first by the octanoic SAMs and then furthermore with the addition of CdSSe Q-dots. This force might be dominating over the charge-transfer effect from the nanosized semiconductor as in our case the Q-dots are farther away from the TiO2 surface due to the extra SAMs which act as a barrier for the transferring of electrons from the Q-dots to titania under the present conditions. The tethering of CdSSe Q-dots on the surface of TiO2 nanobelts is further confirmed by TEM. Figure 1b shows that the spherical Q-dots, with average diameter size of ca. 5 nm, are randomly distributed across the surface of the nanobelts. Energy-dispersive X-ray analysis (EDX) indicates an effective molecular formula of CdS0.7Se0.3. XPS shows the coverage of these Q-dots is around 21% with respect to titanium (Cd/Ti atomic ratio ) 0.21; Figure 3) on the surface. In addition, a phosphorus 2p core level emission at around 131.2 eV binding
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Figure 3. XPS of the core-level emissions from Cd 3d (inset) and Ti 2p of the CdSSe Q-dots/TiO2 nanobelts composite.
Figure 4. UV-visible diffused reflectance spectra of TiO2 nanobelts and the Q-dots/TiO2 nanobelts composite, and the transmittance spectrum of the CdS0.7Se0.3 solution. The computed midpoints of the absorption edge are at 356, 364, and 514 nm for TiO2 nanobelts, the Q-dots/TiO2 nanobelts composite, and CdS0.7Se0.3 Q-dots, respectively.
energy was also detected (not shown), indicating the Q-dots are still capped with TOPO after their self-assembling. The oxidation state of titanium in the nanobelts are mainly in the +4 form centered at 459.3 eV binding energy (Ti 2p3/2) with a full width at half-maximum of ca. 1.6 eV, similar to that of a fully oxidized TiO2 single-crystal surface.35 The XPS Cd 3d5/2, centered at 405.2 eV, indicates cadmium in the +2 oxidation state and as a chalcogenide.36 UV-vis transmittance spectrum (Figure 4) of the Q-dots solution displays an absorption edge onset above 490 nm. When these Q-dots are coupled with TiO2 nanobelts, the spectrum clearly displays the characteristic absorption of CdS0.7Se0.3 and the optical absorption of this coupled system is extended into the visible region. To explore the practical response of this nanocomposite in the visible region of the solar spectrum, photoelectrochemical experiment was conducted in a simple three-electrode cell using a focused 150 W quartz-halogen lamp (350-5000 nm) as the excitation light source. It is well-known that both the conduction band edge potentials of CdS and CdSe,14,37 > -0.5 and ∼ -1.0 V (vs NHE), respectively, are more negative than that of TiO2 (-0.45 V vs NHE).38 Similarly the conduction band edge potential of CdSxSe1-x Q-dots in principle should also be more negative than that of TiO2, between -0.5 and -1.0 V (vs NHE). This together with the photoabsorbance results infers the irradiation of visible light (420-700 nm) is energetically sufficient to excite an electron from the valence band of CdS0.7Se0.3 across the band gap (∼2.4 eV) to its conduction band. Since the conduction band of TiO2 is lower in energy compare with that of this Q-dot, the excited electron can be transferred from the conduction band of the latter to the former, while the
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Figure 5. Photocurrent response of TiO2 nanobelts (lower curve) and the Q-dots/TiO2 nanobelts composite (upper curve) on ITO electrodes. The coating on ITO was prepared by “drop-casting” the suspension on the conducting glass, dried first in air and then for 2 h at 70 °C followed by another 2 h at 100 °C. The potential was controlled by a PAR 363 potentiostat/galvanostat power supply.
hole remains in the valence band of the Q-dot. This process increases the charge separation of the electron-hole pairs, which can be channeled for various photochemical processes, such as photocurrent or photocatalysis. Figure 5 shows the photocurrent response of TiO2 nanobelts/ Q-dots composite coating and bare TiO2 nanobelts coated on ITO electrodes. The photocurrent density was registered at a constant potential of +500 mV versus Ag/AgCl (KCl, saturated) in an electrolyte consisted of 0.4 mol/L Na2S(aq) and 0.1 mol/L Na2SO3(aq), with Pt as the counter electrode. Upon the irradiation of light, the composite film clearly shows an enhancement in photocurrent and with good reproducibility. This indicates the TiO2 nanobelts are able to accept electrons from the excited Q-dots and transport them to the ITO surface to generate anodic photocurrent. The holes left in the valence band of CdS0.7Se0.3 are scavenged by sulfide and sulfite redox couple to complete the redox process.15 There is still a very small photocurrent response on the bare TiO2 nanobelts due to the presence of an ultraviolet component of the quartz-halogen lamp. Summary A nanocomposite consisting of ∼5 nm sized CdS0.7Se0.3 quantum dots hydrophobically attached to anatase TiO2 nanobelts has been successfully prepared, and its photoactivities are studied in this work. TEM, Raman, and XPS all showed the presence of Q-dots on the surface of the nanobelts. Raman spectroscopy under UV-laser excitation indicated a compressive force is acting on the surface of TiO2 upon the assembling of octanoic acid SAMs and Q-dots. This force was postulated to dominate over the charge-transferring effects, which explained the observed blue-shift in certain Ti-O Raman bands. The visible-light sensitizing effect of the Q-dots was demonstrated in a photocurrent experiment, which shows a 3-4-fold increase in photocurrent in the coupled system. Acknowledgment. The authors thank Dr. Sebastiampillai Raymond and Dr. Tony Bittar for their assistance in the UVvis diffused reflectance measurements. Supporting Information Available: Figures of an XRD pattern of TiO2 nanobelts in the anatase phase, TEM showing the average size of the CdS0.7Se0.3 quantum dots on TiO2 nanobelts, and XPS phosphorus 2p of the trioctylphosphine oxide used to cap the Q-dots. This material is available free of charge via the Internet at http://pubs.acs.org.
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