CdSe Nanocrystal Assemblies on Anodized TiO2 Nanotubes: Optical

Article pubs.acs.org/JPCC

CdSe Nanocrystal Assemblies on Anodized TiO2 Nanotubes: Optical, Surface, and Photoelectrochemical Properties Bratindranath Mukherjee, York R. Smith,† and Vaidyanathan (Ravi) Subramanian* Chemical and Materials Engineering Department, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557, United States S Supporting Information *

ABSTRACT: The in situ preparation of high-quality organically synthesized cadmium selenide (CdSe) nanocrystals on TiO2 nanotubes (T_NT) prepared by anodization is presented. The deposition of CdSe nanocrystals has been facilitated under a high-pressure and low-viscosity solvothermal process. The formation, growth, and assembly of CdSe nanocrystals in the form of a dense film on T_NT have been studied using thermal (thermogravimetry and calorimetry) and optical (UV−vis and microscopy) techniques. It has been concluded that an organic treatment, followed by annealing under a nitrogen atmosphere at reduced temperatures, helps control CdSe nanocrystal morphology without causing significant particle size growth. Photoelectrochemical measurements indicate that the electrode assembly consisting of T_NT and CdSe can achieve a stable photocurrent density of 6.7 mA/cm2 and a charge-separation efficiency of 35%.

1. INTRODUCTION Chalcogenide nanocrystal-sensitized semiconductor oxide films are promising materials as third-generation photovoltaic solar cells due to their low cost and high efficiency.1−3 The advantages of chalcogenide nanocrystals as photosensitizers over organic chromophore include: (i) tunable bandgap through size, shape,4 and composition control,5,6 (ii) high molar extinction coefficient, and (iii) large intrinsic dipole moments for enhanced charge separation.7,8 A typical solar cell architecture can be described as a layer of nanocrystals sandwiched between two interpenetrating layers of a wide bandgap semiconductor and a redox couple or hole-conducting material (e.g., I 3 − /I − , S x 2 − /S 2 − , Co(phen) 3 2 + /Co(phen)33+(phenanthroline), CuSCN, spiro-OMETAD ((2,2(,7,7(-tetrakis-(N,N-dipmethoxyphenylamine) 9,9(-spirobifluorene)).9,10 With proper band alignment of the participating components, photogenerated charge carriers from the chalcogenide nanocrystals are separated by the wide bandgap semiconductor and the redox couple/hole conductor selectively, giving rise to the photovoltaic effect. Recent developments in interface engineering, new redox couples, and the possibility to employ multiple layers of nanocrystals have together contributed to an increase in the power conversion efficiency of such systems from ∼1 to ∼5%.11 Despite the perceived benefits of chalcogenide nanocrystals and recent developments, the power conversion efficiencies still remain around 4 to 5%, which is comparatively less than dyesensitized solar cells.8,12 Variations in the structural and physicochemical properties of chalcogenides and their © XXXX American Chemical Society

associated electrolytes are the main reason for several disparities. For example, defect sites create low-lying acceptor levels leading to alternative nonradiative pathways for exciton decays.13 Chalcogenide nanocrystal-sensitized solar cells are incompatible with the widely studied I3‑/I− redox couple due to photoelectrochemical instability.10 As a result, a suitable redox couple is required, which in turn requires optimization of the counter electrode to maintain electrocatalytic activity.14,15 Finally, in most cases, the oxide surface is not completely covered by chalcogenide nanocrystals due to the inherent limitations of different deposition processes; the exposed large bandgap oxide surface is vulnerable for a back reaction leading to higher dark current and reduced photovoltage.11 Recent efforts have focused on the introduction of a recombination barrier layer between large bandgap oxides such as TiO2 and chalcogenide nanocrystals.16,17 This layer serves as a protective layer between the nanocrystals and the electrolyte to enhance the photovoltage and the photocurrent by further reducing dark current and corrosion/hole-mediated oxidation of chalcogenide nanocrystals. Grafting a molecular dipole between the nanocrystals and the nanometer-sized protective layer facilitates electron injection without suffering a loss in the open circuit voltage.18 Efforts have also been made to understand the role of chain length when using a bifunctional linker molecule to bind presynthesized chalcogeReceived: September 14, 2011 Revised: May 31, 2012

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dx.doi.org/10.1021/jp208879f | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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nides to TiO2. Conversely, a direct attachment route through chemical bath deposition (CBD) or successive ion layer absorption and reaction (SILAR) have shown the most promising results.19−21 The SILAR method deposits multiple layers of different chalcogenide nanocrystals with the proper band alignment (covering the full visible light spectrum) and delivers the best efficiency.12,22 Direct deposition methods such as CBD and SILAR have limited application on coating substrates such as nanotubular morphologies. Applying these methods to a nanotubular morphology, for example, results in nanocrystals mainly forming around the tube mouth and not along the length of the tube due to surface roughness. This results in considerable exposure of the large bandgap semiconductor oxide giving rise to increased dark current. TiO2 has been used extensively as an underlying substrate for depositing chalcogenide nanocrystals.12,23−26 Mesoporous TiO2 films are cost-competitive, transparent, eco-friendly, and amenable to film formation on planar as well as nonplanar surfaces. However, TiO2 nanoparticulate films are prone to extensive recombination losses at grain boundaries. Recently, the nanotubular architecture of TiO2 prepared by a simple onestep anodization process has attracted significant interest.27−30 The nanotubes offer a significantly lower recombination of photogenerated charges compared with their nanoparticulate counterparts.31,32 They also provide a higher surface area and can be prepared in planar33 as well as nonplanar34 architectures. However, a significant challenge remains to ensure the utilization of the high surface area, which predominantly exists along the inside and outside wall surfaces of the nanotubes. This study presents three key results. First, we report details of a solvothermal approach to uniformly distribute CdSe nanocrystals on anodized TiO2 nanotube arrays (T_NT) resulting in a dense coating along T_NT walls, rim of the mouth, and the intertubular spaces. High-quality CdSe nanocrystals nucleate, grow, and deposit on T_NT under high-pressure/temperature achieved using a solvothermal-based coating technique. Second, the effects of a chemical treatment (organic ligand), followed by thermal treatment (annealing under controlled conditions) on the properties (crystal size) of the CdSe nanocrystals and its effects on the photoelectrochemical responses of the T_NT-CdSe assembly, have been correlated. Finally, on the basis of the results of complementary optical, thermal, and microscopy techniques, a mechanism for the formation, growth, and deposition of CdSe nanocrystals on T_NT has been proposed. The method developed here may be useful in the following synthesis approaches: (i) controlled deposition of other small bandgap chalcogenides such as CdX (X = S and Te) and (ii) sensitizing T_NT or other oxide architectures grown on nonplanar substrates such as wires with dense coatings.

Scheme 1. Details of the Step-Wise Process for the Synthesis of CdSe Nanocrystal Assembly on T_NT Including the Treatment Conditions (Organic Ligand and Thermal Treatment) during the Synthesis Process

and were annealed for 2 h under a N2 atmosphere to achieve crystallization. Further details of the anodization can be found elsewhere.35 2.1.2. Deposition of CdSe Nanocrystals on T_NT. A onestep solvothermal coating technique was utilized to prepare nanocrystal deposits on T_NT (Scheme 1, step 2). A typical procedure involved using a solution consisting of 1 mM Cdmyristate (prepared from cadmium acetate and myristic acid and recrystallized from toluene) mixed with 1 mM Se (99.5% from ACROS Organics) in 800 μL of tri-n-octylphosphine (TOP - STREM chemicals) and 50 mL of toluene as solvent. The mixture was loaded into a 125 mL Teflon-lined stainlesssteel autoclave (PARR reactor) along with the substrates (T_NT/Ti) and held stable vertically using a Teflon support. The objective was to use a one-pot solvothermal approach to prepare and deposit CdSe nanocrystals on T_NT. The PARR reactor, with its contents, was maintained at 190 °C for various time periods between 9 and 18 h. The substrates with a brownish red coating were collected after the PARR reactor was cooled under natural convection to room temperature. The treated T_NT substrate was cleaned with hot toluene (80 °C) to remove loosely attached nanocrystals from the substrates. These cleaned samples, before any further treatment, are referred to as T_NT/CdSe. For analysis purposes, the nanocrystals present in the solution were precipitated from the toluene using centrifugation. A cosolvent, ethanol, was added to expedite the separation (ethanol, isopropanol, and toluene were of AR grade and supplied by Fisher). 2.1.3. T_NT/CdSe Surface Treatment. The T_NT/CdSe was kept in a thiolacetic acid (TAA) (ACROS organics) solution (200 μL in 20 mL isopropanol) for 12 h. The objective of this step was to perform CdSe surface treatment to facilitate low-temperature annealing. The TAA-treated sample was then washed with isopropanol (Scheme 1, step 3). The TAA-treated samples are referred to as T_NT/CdSe_TAA. Annealing of the samples before and after using TAA was performed (Scheme 1,

2. EXPERIMENTAL SECTION 2.1. Synthesis. 2.1.1. Preparation of the Nanotube Substrate by Anodization. Titanium (Ti) foils (99% purity, 0.2 mm thick) supplied from ESPI International were degreased by ultrasonication in acetone for 15 min prior to anodization. The cleaned Ti foil was anodized in a twoelectrode cell at 40 V (DC) for 2 h with platinum foil as the cathode (Scheme 1, step 1). A fluorinated solution (0.5% w/w) of ethylene glycol and water (10% w/w) mixture was used as electrolyte. The anodized samples were then ultrasonicated for 5 sec in deionized water to remove postanodization surface deposits. The anodized T_NT samples were mostly amorphous B



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Supporting Information shows the TGA and DTA plot of CdSe prepared by the solvothermal process. The CdSe sample showed ∼16% weight loss up to 450 °C. The DTA plot showed endothermic peaks ∼340 and 407 °C, which indicated the possible removal of organics. Although annealing up to 450 °C in air assisted with the removal of carbonaceous material in the absence of a ligand treatment, such a high temperature could induce negative effects (such as sintering and agglomeration of CdSe nanocrystals). Therefore, a lower temperature is desired to remove carbonaceous material without triggering any negative effects. The TAA treatment was applied with the intent to examine if the heat treatment to remove the organics could be carried out at a lower temperature (below 450 °C). TAA can remove longer chain organics. (Myristic acid might be formed by decomposition of cadmium myristate.) Supplementary Figure 3b of the Supporting Information shows the TGA and DTA plot for CdSe after TAA treatment. The TGA plot in Supplementary Figure 3b shows that the removal of organic is only ∼3.5%. The organic removal step was initiated below 200 °C and decayed gradually until a temperature of 300 °C was reached. Interestingly enough, beyond 300 °C, the decay was much slower (circled region in Supplementary Figure 3b insert) to reach a stable weight. The insert of Supplementary Figure 3b has an ordinate axis similar to Supplementary Figure 3a to highlight this point. The DTA did not show any distinct peaks similar to those noted in Supplementary Figure 3a. Therefore, TGA-DTA analysis suggested that one possible approach to reduce annealing temperature could involve treatment with an alternate short chain ligand such as TAA. Furthermore, to confirm organic removal, a FTIR study was undertaken. The objective of this study was to analyze the CdSe samples before and after heat treatment. FTIR results shown in Supplementary Figure 3c of the Supporting Information indicated the presence of characteristic peaks at ∼2800 cm−1 before heat treatment, which corresponds to −CH stretching (indicating the presence of organics). After heat treatment of the TAA-based sample in N2 at 350 °C, this peak was no longer present. On the basis of this observation, it was concluded that the organics were removed by heat treatment from the CdSe. The preliminary TGA-DTA analysis and FTIR study indicated the removal of the long chain organic molecule (myristic acid through a possible ligand exchange with TAA) was the basis for this reduction in temperature. Further studies are underway to examine the details of this process. 3.1.3. Physical Characterization of Particle Morphology. Supplementary Figure 4 of the Supporting Information shows the top- and side-view SEM images of the bare T_NT. The T_NTs have an average outer diameter of 110 ± 10 nm and a wall thickness of ∼15 nm. Distinct intertubular spaces between the T_NTs are also evident from the top- and side-view images, as marked in the Figure. The average length of the nanotube is determined to be ∼5 μm. For a more detailed discussion regarding T_NT morphologies, readers are referred to other published literature.28,32,37,38 Figure 1a shows the top view of the samples after the solvothermal deposition before TAA or heat treatment (T_NT/CdSe). Distinctly fine nanocrystal deposits forming a dense coating along the rim of the T_NT arrays can be observed. The particle sizes of the deposits are estimated as ∼5 nm from a close-up of Figure 2a. The deposits, if any, on the inside of the T_NT are not clearly evident from the top-view images. Interestingly, the intertubular spaces are clearly sealed shut with no space evident, suggesting that the

step 4). The T_NT/CdSe_TAA and T_NT/CdSe samples were annealed in nitrogen (350 °C) or air (350 or 450 °C) for 3 h. These samples are referred to as T_NT/CdSe_TAA_N2 and T_NT/CdSe_Air, respectively. For the TGA-DTA and FTIR analysis, CdSe samples only were used. These CdSe samples without T_NT are referred to as follows: TAA and no annealing (CdSe/TAA), TAA and annealing in nitrogen (CdSe/TAA_N2), TAA and annealing in air (CdSe/TAA_Air) at 350 °C, and TAA and annealing in air at 450 °C (CdSe/ TAA_Air). 2.2. Characterization. 2.2.1. Surface and Optical Characterization. X-ray diffraction measurements were recorded using a Philips12045 B/3 diffractometer at a scan rate of 1°/min with the aid of Cu Kα radiation. Morphological and compositional characterization was carried out using a Hitachi S-4700 scanning electron microscope equipped with an OXFORD X-ray energy-dispersed spectrometer (EDS). Highresolution SEM images were recorded in UHR mode using a through-lens SE detector located at a working distance of 4 mm and operated under an accelerating voltage of 5 kV. Diffuse reflectance (DRUV−vis) studies were performed in a Shimadzu UV-2501PC spectrophotometer equipped with diffuse reflectance accessories. Thermal analysis of the sample was performed in N 2 (50 mL/min) with a Perkin-Elmer simultaneous thermogravimetric analysis differential thermal analyzer (TGA-DTA) instrument (TA 6000). Fourier transform infrared (FTIR) analysis was performed using a PerkinElmer instrument. 2.2.2. Photoelectrochemical Characterization. Photoelectrochemical studies were carried out in a three-electrode cell with T_NT-based substrate as the working electrode, Pt wire as the counter electrode, Ag/AgCl (in 3 M KCl) as the reference electrode, and 0.5 M Na2S solution as the electrolyte. Photoelectrochemical data were obtained using an Autolab PGSTAT 30 electrochemical analyzer. The working electrode was irradiated with a 500 W Newport Xenon lamp equipped with 0.5 M CuSO4 solution as a far UV cutoff filter.36 Incident photon-to-current conversion efficiency (IPCE) was measured in a two-electrode system with an Oriel Instruments optical power meter and a Newport 77250 monochromator.

3. RESULTS 3.1. Surface Characterization of the Films. 3.1.1. Confirmation of Deposits on T_NT. The outcome of the solvothermal deposition approach was examined initially by color change of the samples. Supplementary Figure 1 shows a photograph of the T_NT before and after solvothermal treatment of the Supporting Information. The image indicates a change in the coloration from metallic gray to brown-red. This suggests the formation of chalcogenide deposits on the T_NT. Subsequent EDS analysis over a broad region was performed to identify the deposited elements. The EDS data (shown in Supplementary Figure 2 of the Supporting Information) indicated the presence of Ti, O, Cd, and Se. 3.1.2. Thermal Characterization Using TGA- DTA. Because organics were used in the solvothermal synthesis of the nanocrystals deposits, identifying the thermal treatment conditions for removal of stabilizing organics with minimal effect on the CdSe particle size was required. TGA-DTA analysis was performed to determine the minimum annealing temperature required for: (i) removal of the organic material and (ii) retention of the CdSe nanocrystal size (prevents sintering/aggregation). Supplementary Figure 3a of the C



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Figure 2a−c shows a higher magnification lateral view images of the T_NT arrays after coating, followed by heat treatment in nitrogen for different intervals of time. Figure 2a,b shows the cross-sectional view of the T_NT/CdSe_N2 sample after 12 and 15 h. The deposition of CdSe nanocrystals in the form of small island clusters can be noted along the T_NT walls at 12 h. Individual particle size is noted to be between 8 ± 2 nm. Figure 2c reveals a dense coating of CdSe nanocrystals on the T_NT walls after 18 h. Coverage inside the tube is not clear from these images. However, it can be argued that if the outside walls (i.e., narrower intratubular space), which provide strong mass transfer resistance, are densely coated, then the inside walls are most likely coated as well: albeit not as densely as the intertubular spaces. 3.1.4. Surface Characterization of Particle Morphology Using XRD. Figure 3a shows the XRD pattern of the annealed

Figure 1. High-magnification SEM images of the top view of (a) T_NT after CdSe coating (T_NT/CdSe) and a close up of a section indicating nanocrystal location, (b) T_NT/CdSe after heat treatment in air at 450 °C, and (c) T_NT/CdSe treated with TAA, followed by thermal annealing under nitrogen at 350 °C (T_NT/CdSe_TAA_N2). A closeup of panel c shows the particle morphology as well.

solvothermal process leads to the formation of densely packed nanocrystals. Figure 1b shows the as-deposited CdSe nanocrystals after heat treatment in air at 450 °C before TAA treatment (T_NT/ CdSe_Air). The fine deposits as noted in Figure 1a, are no longer evident. Instead, a continuum of particles with indistinct interparticle interface is evident along the T_NT mouth and especially at the intertubular spaces. We attribute this to sintering-induced nanocrystal growth, resulting in an increase in the size and fusion of the nanocrystals. Finally, Figure 1c shows the SEM images of the CdSe on T_NT after TAA treatment, followed by annealing in nitrogen at 350 °C (T_NT/ CdSe_N2). The average particle size appears to be ∼8 ± 2 nm, as observed from the close up image shown in the Figure. Therefore, a slight increase from ∼5 to ∼8 nm after TAA, followed by heat treatment under N2, is noted. The nitrogen annealing at 350 °C after the TAA treatment seems to help to decrease significantly the aggregation of the nanocrystals. This observation highlights the benefit of the applied approach to treat the T_NT/CdSe samples.

Figure 3. XRD of the (a) T_NT after heat treatment at 450 °C, indicating the formation of crystalline TiO2 and (b) T_NT/CdSe_N2 following solvothermal treatment indicating the formation of CdSe on T_NT (note “A” − anatase, “T” − titanium).

T_NT nanotube arrays. Annealing of T_NT arrays at 450 °C in nitrogen leads to the formation of an anatase phase and can be indexed to a standard JCPDS card (PDF no. 21-1272, labeled “A”). Signals from the underlying Ti foil are also observed and can be indexed with a JCPDS card (PDF no. 441294, labeled “T”). Figure 3b shows the XRD pattern of the T_NT/CdSe_N2. Solvothermally synthesized deposits typically manifest in the cubic zinc blend structure.39 Because the strongest peak, the (111) plane for CdSe, is located at almost the same position as that of (101) peak of anatase and considering the fact that CdSe loading is just 5%, it is difficult to identify the (111) peak of CdSe nanoparticles. Therefore, to

Figure 2. Cross-section view of several T_NT/CdSe_TAA_N2 samples with solvothermal treatment times varied from (a) 12, (b) 15, to (c) 18 h, indicating the extent of CdSe nanocrystal formation. (It is to be noted that the samples were subjected to TAA treatment, followed by heat treatment in nitrogen at 350 °C, after various solvothermal times prior to imaging using SEM.) D



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°C (as shown in TGA-DTA data of Supplementary Figure 3 of the Supporting Information). Therefore, heat treatment for T_NT/CdSe with myristate is performed in air, instead. Heat treatment at 350 °C in air (Figure 4d) proves that CdSe nanocrystal growth does not occur. This is attributed to the incomplete removal of the myristate ligand. However, further heating to 450 °C in air (Figure 4e) not only results in complete removal of myristate ligand but also a rapid increase in size to ∼20−25 nm (as noted from the narrower (111) peak). The distinct shoulder observed in the spectrum 4d between 30 and 35° following the treatment in air suggests surface oxidation. This step also results in a rapid crystal growth, followed by a complete phase transformation to a hexagonal wurtzite structure (JCPDS card no. 77-2307) and also additional peaks at 33.2 and 38.4° arising from oxidation of CdSe to CdO (JCPDS card no. 75-0594). Note that these additional peaks are not observed in samples annealed under nitrogen. From these four observations, it can be concluded that: (i) treating T_NT/CdSe with TAA followed by heat treatment under nitrogen results in the removal of organics at a lower temperature (350 °C as opposed to 450 °C); (ii) annealing in nitrogen after TAA treatment results in the maintenance of the nanocrystal size close to the dimensions prior to heat treatment in air and/or in nitrogen; and (iii) nitrogen annealing is preferable if surface oxidation is to be avoided. 3.1.5. Optical Characterization of the CdSe Deposits Using Spectroscopy. Figure 5 shows: the UV−vis diffuse reflectance

confirm the presence of CdSe, the signals noted at the other 2θ values were also examined. Distinct peaks at 2θ values of 42.4 and 49.7° are identified as (200) and (311) planes of CdSe, as indicated by the JCPDS card (PDF no. 19-0191). The broadening of the (200) CdSe peak in Figure 3b suggests the possibility of the small crystallite size of ∼7 nm (as estimated using the Scherrer equation: Dp = (0.94λ/(β1/2 cos θ)), where β = full width at half-maxima, 2θ is the peak position, and λ is the wavelength of the X-ray radiation). Note that this estimated value is in close agreement with the nanoparticle size estimated from the SEM analysis. Therefore, the XRD analysis confirms the presence of CdSe nanocrystals on T_NT and complements the SEM results. Figure 4 shows a series of diffractograms of CdSe nanocrystal deposits on glass slides (without T_NT) studied to examine

Figure 4. XRD of the (a) plain CdSe nanoparticles, after treatment with (b) TAA and no annealing (CdSe/TAA), (c) TAA and annealing in nitrogen (CdSe/TAA-N2), (d) TAA and annealing in air (CdSe/ TAA_Air) at 350 °C, and (e) TAA and annealing in air (CdSe/ TAA_Air) at 450 °C. (Note: All samples were prepared without T_NT.)

the effects of TAA treatment and annealing in the presence of air and nitrogen . Several key observations can be noted by performing a comparative analysis of the XRD spectra of CdSe following different treatment steps. First, XRD of the as-deposited and unannealed CdSe films before the organic ligand treatment shows broad and distinct CdSe peaks [(200) and (311) peaks] corresponding to cubic CdSe, as shown in Figure 4a. Second, as shown in Figure 4b, the spectrum of the CdSe samples after exposure to TAA shows a distinct peak (circled) close to (111). Third, annealing of the sample shown in Figure 4b under nitrogen at 350 °C results in the complete elimination of this distinct peak (Figure 4c). A possible explanation of this observation is that TAA reacts with surface Cd atoms to form a crystalline complex, which upon heat treatment in nitrogen, decomposes to either a thin amorphous CdS layer or contributes to a S-rich alloyed CdSe on the surface.40 Fourth, on comparing the (111) peaks of CdSe deposits on a glass plate in Figure 4a,b, it is noted that TAA treatment maintains the nanocrystal size at ∼4 nm (estimated using Scherrer equation) prior to heat treatment. Furthermore, comparing the (111) peaks of CdSe, in Figure 4b,c, it is observed that the nanocrystal sizes increases from 4 to 7 nm after TAA treatment (followed by heat treatment in nitrogen). It is to be noted that myristate containing CdSe is not annealed under nitrogen because it does not result in complete elimination of myristate ligand even after heating up to 450

Figure 5. Diffuse reflectance UV−visible spectra of (a) TiO2 nanotubes (T_NT), (b) CdSe-coated TiO2 nanotubes after TAA treatment (T-NT/CdSe_TAA), (c) CdSe-coated TiO2 nanotubes (T_NT/CdSe), (d) CdSe-coated T_NT nanotubes after heat treatment in air at 350 °C (T_NT/CdSe_Air), and (e) CdSe-coated T_NT nanotubes after heat treatment in nitrogen at 350 °C (T_NT/ CdSe_TAA_N2).

spectra of the T_NT (5a), as synthesized T_NT/CdSe (5c), T_NT/CdSe composites after TAA treatment (5b), and heat treatment (5d,5e). T_NT samples show a main band edge of ∼380 nm, corresponding to 3.26 eV, which is in accordance with the bandgap of anatase. A shoulder of ∼460 nm can be attributed to oxygen deficiency in the sample, as typically observed in N2 annealed T_NT substrates.41 T_NT/CdSe samples show a broad absorption in the visible region. A blueshifted band edge at 645 nm (obtained from first derivative) equivalent to 1.92 eV compared with 1.74 eV of the bulk CdSe, demonstrates size quantization (5c). However, the calculated E



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bandgap is still less when compared with that observed for a particle size of ∼4 to 5 nm42,43 obtained using the Scherrer formula from XRD. This shift can be attributed to interparticle interaction once the CdSe nanocrystals deposit and assemble closely (as seen in SEM images − 2b and 2c) on the T_NT surface. A secondary shoulder within 450−500 nm (possibly from a second excitonic peak) is observed. Similar absorption features are retained after TAA treatment (5b). A slight blue shift in the band edge and attenuation of the absorption can be attributed to the reaction of TAA on CdSe nanoparticles, followed by a reduction of effective particle size. However, after annealing at 350 °C for 3 h in air (5d) as well as nitrogen (5e), both visible light absorbance shoulders are noted to merge and smoothen into a single broad absorption. This may be attributed to annealing assisted removal of the organics leading to direct contact between the CdSe and T_NT. The absorbance studies show that CdSe deposition does occur after the solvothermal process and complements the results from XRD and SEM analysis. 3.1.6. Photoelectrochemical Properties of the T_NT/CdSe Films. Photoelectrochemical measurements provide information on the responses of a photoactive material under illumination by correlating voltage (E), current (J), and time (t).44,45 Figure 6a shows the current−voltage (J/E) characteristics of the T_NT and the T_NT/CdSe films (before and after heat treatment) under a constant illumination of ∼90 mW/cm2 intensity in a three-electrode configuration. Ag/AgCl is used as the reference electrode. The measurement process follows a similar approach to that discussed elsewhere.46,47 T_NT shows a photocurrent of 1 mA/cm2 at 0 V bias, and the zero-current potential (ZCP) is noted at 0.95 V (vs Ag/AgCl). T_NT with the CdSe deposits shows a lower ZCP of 0.8 V and short circuit current of 0.35 ma/cm2 before heat treatment. After TAA treatment (T_NT/CdSe_TAA), the voltage and current values revert back to nearly the same as T_NT. Heat treatment by either annealing in air or nitrogen results in a significant increase in photocurrent. The best results are obtained with nitrogen annealing T_NT/CdSe_TAA_N2 at 350 °C for 3 h, demonstrating a photocurrent up to 6.7 mA/cm2. Because the surface treatment was noted to have an impact on the photoelectrochemical responses, its role is discussed here. The as-deposited CdSe is hydrophobic in nature, whereas the TAA treatment makes the CdSe relatively hydrophilic. This is the basis for the difference in the photocurrent noted before the heat treatment. However, in either case, photogenerated charges face resistance to flow due to the presence of the organics. Heat treatment in air at 450 °C removes the organics but increases the current only slightly. The reason for this is that heating at 450 °C in air causes the CdSe to sinter and increase in size, reducing surface area (Figures 2−4). Alternately, surface treatment with TAA, followed by heat treatment in nitrogen at a lower temp of 350 °C, results in the removal of organics. The particle sintering at higher temperature can therefore be avoided, and surface area can be maintained. Therefore, the role of surface treatment using TAA includes: (i) lowering of the organic removal temperature, (ii) minimizing sintering, and (iii) improving the photoelectrochemical responses through better electronic coupling between the T_NT and the CdSe. Figure 6 shows a plot of current at zero potential for T_NT/ CdSe_TAA_N2 versus the solvothermal treatment time. The CdSe loading is varied by changing the time the T_NT is exposed to the solvothermal solution. The Jsc increases from

Figure 6. (a) J−E characteristics under illumination of (a) T_NT/ CdSe, (b) T_NT, (c) T_NT/CdSe_TAA, and (d) T_NT/ CdSe_TAA_N2 electrodes. The J−E characteristics were obtained in a three-arm electrochemical cell with Ag/AgCl as the reference and a Pt wire as a counter electrode under a UV−vis illumination of 90mW/ cm 2 . (b) Photocurrent obtained using T_NT/CdSe_TAA_N 2 prepared by varying the solvothermal duration between 12 and 16 h. The photocurrent values reported here are noted at 0 V versus Ag/ AgCl and were obtained in a three-arm electrochemical cell under a UV−vis illumination of 90 mW/cm2.

3.25 m to ∼7 mA/cm2 when the solvothermal process is extended from 12 to 18 h. No significant coating is obtained below 12 h, whereas multiple layers of nanocrystals are observed for a longer coating time (coating mechanism, vide infra Section 4). Multiple layers not in direct contact with T_NT do not contribute with direct electron injection into T_NT; therefore, deposition beyond 18 h is not pursued in this work. Nominal loadings vary from 1 to 5 wt % (as estimated using EDS). The responses of the T_NT/CdSe_TAA_N2 films under discontinuous illumination are shown in Figure 7 and compared with bare T_NT substrate. These measurements are indicative of the reproducibility of photoresponses.36,48,49 Both the samples show an instantaneous change in current upon illumination. The current retracts to its original values almost instantaneously as well, once the illumination is switched off. This trend is repeated with every on−off cycle and indicates that the anodes are stable under photoillumination. The reproducible and stable photoresponses are attributed to the existence of a good CdSe-nanotube interface that allows efficient electron injection from CdSe to T_NT. An effective approach for determining the performance of a photoanode is to measure the incident photon to current F



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works.4,20,21,53 A schematic of the energy band alignment of the CdSe nanocrystals and the T_NT is shown in Supplementary Figure 5 of the Supporting Information.

4. DISCUSSION 4.1. Mechanism of the Formation of the Nanocrystals. The solvothermal technique demonstrated in this work addresses significant improvement in two distinct areas: (i) one-step formation, growth, and assembly of high quality organically synthesized CdSe nanocrystals on T_NT arrays and (ii) the option to perform surface modification of T_NT/CdSe through an organic ligand treatment leading to the formation of thermally stable CdSe nanocrystals on T_NT. Stable photoelectrochemical responses using the method outlined in this work have been demonstrated in the previous section. On the basis of the findings from thermal, physical, surface, and optical characterization, a mechanism for the formation and assembly of CdSe nanocrystals on T_NT is proposed. 4.1.1. One-Step Formation Followed by Growth and Direct Assembly of CdSe Nanocrystals along T_NT Arrays. High-quality nanocrystals are prepared using a low boiling noncoordinating solvent, toluene, at ∼200 °C. The interaction between the Cd and Se precursors results in the formation of CdSe nanocrystals. It is possible (currently further test is ongoing) that the release of the myristic acid from the cadmium precursor serves as a capping agent for the nanocrystals. Subsequently, under the solvothermal conditions of high temperature and pressure, CdSe nanocrystal dispersion is expected to penetrate deep inside the nanotubes due to a concentration gradient between the mouth and the tube depth. Both viscosity and surface tension is substantially reduced under solvothermal conditions, which in turn allow the particle dispersion to wet the T_NT wall effectively and introduce instability in the dispersion, which leads to irreversible deposition on the T_NT arrays. The basis of this process lies in a recent experimental study on the microrheology of the sticking transition of hard and soft spheres.54 The study shows that when the distance between the particles and substrate becomes comparable to the diameter of the particle, the vertical component of the bulk diffusion coefficient reduces to zero and the particles become trapped at the surface. With a further time-dependent reduction of the particle-substrate distance, the horizontal component of the bulk diffusion coefficient also goes to zero; as a result the particles irreversibly stick to the substrate. To validate this hypothesis, a set of the absorbance measurements from the postsolvothermal T_NT/CdSe films (Figure 9) as well as the CdSe solution (Figure 10) during the solvothermal synthesis was performed. These Figures show a series of difference absorbance spectra of the T_NT after various intervals of solvothermal treatment. It is noted that no CdSe deposits are observed up to ∼6 h (Figure 9, spectra a−d) into the solvothermal process. Between 6 and 9 h, a slow growth of CdSe deposits is detected. A parallel examination of the absorbance measurements from the remnant CdSe nanocrystal solutions shows that CdSe particles form up to ∼6 h (a−c). This is evidenced from the characteristic peaks of the CdSe, as noted in the solution. Furthermore, a red shift is observed between 6 and 18 h (d−f), indicating nanocrystal growth. The complete absorbance profiles of the remnant solution in various stages of the solvothermal process are shown in Figure 10.

Figure 7. Effects of several on−off cycles on the photocurrent responses of (a) T_NT and (b) T_NT/CdSe_TAA_N2 in 0.5 M Na2S electrolyte. The photocurrent values were obtained in a threearm electrochemical cell under a UV−vis illumination of 90 mW/cm2.

conversion efficiency (IPCE).50,51 IPCE is a measure of the number of electrons generated for every 100 photons absorbed by the photoactive material and is estimated using the following equation.45,52 IPCE(%) =

1240 × isc(A/cm 2) Iinc(W/cm 2) × λ(nm)

× 100 (1)

where isc is the short circuit current, Iinc is the incident light power, and λ is the wavelength of light. Figure 8 shows the

Figure 8. Incident photon to current conversion efficiency (IPCE %) of (a) T_NT and (b) T_NT/CdSe_TAA_N2 was estimated using a photoelectrochemical cell under UV−vis illumination of 90 mW/cm2 and plotted with respect to the wavelength (λ, nm) of illumination.

IPCE spectra of the T_NT/CdSe_TAA_N2 and bare T_NT samples. The T_NT photoanode did not show any reasonable photocurrent in the visible region of the spectra where as a maximum IPCE of 35% for T_NT/CdSe_TAA_N2 is observed at ∼420 nm. It is noteworthy to mention that the IPCE profile is similar to the absorbance spectra (Figure 5). These observations from photoelectrochemical measurements (J/E, J-t, and IPCE) indicate that a major contribution to photogenerated electrons occurs from CdSe through visible light irradiation and subsequent transport of the photogenerated electrons from the CdSe conduction band to the TiO2 nanotube conduction band. The general mechanism for photogenerated charge transport in chalcogenide−TiO 2 assemblies has been discussed in detail in other G



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T_NT arrays (Scheme 2) is proposed. The first step in the mechanism is the formation of the CdSe nanocrystals under the solvothermal conditions and its growth (Scheme 2, step a). Subsequently, driven under solvothermal conditions, an effective penetration via diffusion and wetting occurs ∼6 h into the process (Scheme 2, step b). This step brings the nanocrystals and T_NT surface close enough for deposition along the walls (Scheme 2, step c). This deposition is obviously favored by surface roughness, and thereby denser deposition is observed around the rim of the nanotubes between 6 and 12 h (Scheme 2, step d). 4.1.2. Unique Aspects of This Synthesis Approach. Various methods to synthesize CdSe, including another solvothermal approach, have been outlined in the text (refs 1 and 19−21). It is also important to mention that Wang et al.55 and Wu et al.56 have reported on studies with unique 1D TiO2 nanobelts and their applicability to immobilize PbS nanoparticles to build a QD/TiO2 structure. The work presented here differs from these published works in that: (i) a particulate film of CdSe nanocrystal is prepared using a pressure-based process not only on the T_NT mouth but also on the wall as a dense continuous coating and (ii) unlike deposition of preformed QDs, nanocrystals of CdSe form on the T_NT from the respective ions in situ under solvothermal conditions. The solvothermal method developed here assumes significance because it may be useful in: (i) the deposition of other chalcogenides CdX (X = S,Te) or PbX (X = S,Se) on planar substrates; (ii) the deposition of chalcogenides along the walls of 1D tubes grown on intricately shaped nonplanar substrates such as tubes, wires, or coils; and (iii) further customization that can potentially be useful in sensing and catalysis.

Figure 9. Difference absorbance spectra of T_NT/CdSe films obtained after (a) 1, (b) 2, (c) 4, (d) 6, (e) 9, (f) 12, (g) 15, and (h) 18 h of exposure to solvothermal process. The difference absorbance measurements were obtained by subtracting the diffuse reflectance data of T_NT from all T_NT/CdSe_TAA_N2 samples.

5. CONCLUSIONS A solvothermal-based synthesis approach for deposition of high-quality CdSe nanocrystals on TiO2 nanotubes has been demonstrated. The effects of an organic ligand, TAA, and postdeposition thermal treatment under controlled conditions on the physical and optical properties of T_NT/CdSe have been systematically examined using SEM, XRD, and absorbance (and supplemented by TGA-DTA and FTIR). The key observations are: (i) the combination of a ligand treatment followed by annealing under nitrogen is reported to assist in significantly

Figure 10. Absorbance spectra of the remnant CdSe solution obtained after (a) 1, (b) 2, (c) 4, (d) 6, (e) 9, and (f) 12 of exposure in the high-pressure cell to the T_NT during the solvothermal process.

On the basis of the absorbance analysis of the solvothermal deposition process, a mechanism of CdSe nanocrystal formation, growth, and assembly inside and around the

Scheme 2. Step-by-Step Process Explaining the (a) Interaction between Cd and Se Ions, (b) Formation of CdSe Nanocrystals, (c) Growth and Coaxial Alignment of the CdSe Nanocrystals, and (d) Assembly of the CdSe Nanocrystals on the T_NT Walls, Rim of the Mouth, and the Intertubular Cavity during the Solvothermal Synthesis Process

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reducing the aggregation of the CdSe and (ii) this specific treatment is highly beneficial to improve the photoelectrochemical responses of the T_NT/CdSe. Using absorbance as a primary probe of the T_NT/CdSe and supported by other techniques, a mechanism of CdSe formation, growth, and assembly on T_NT arrays has been discussed. A maximum photocurrent of 6.7 mA/cm2, Voc of ∼1 V, and chargeseparation efficiency of 35% is obtained using the T_NT/ CdSe/TAA-N2 assembly in a photoelectrochemical cell.

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ASSOCIATED CONTENT

S Supporting Information *

Photographs of the T_NT without and with CdSe, EDS, results of thermal analysis (TGA-DSC), FTIR, SEM images of T_NT, and relative band energy positions of the T_NT and CdSe are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1 775-784-4686. Fax: +1 775-327-5059. E-mail: ravisv@ unr.edu. Present Address †

University of Utah, Metallurgical Engineering Department, William Browning Building 135 S 1460 East, Salt Lake City, Utah 84112. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.S.V. would like to thank the Department of Energy for the funds that supported this project (DE-EE0000272). This work was performed at UNR and was partially supported by funds from the UNR Office of Vice President for Research through junior faculty startup grants to support Y.R.S. Assistance with editorial corrections by Janel Fontana is acknowledged.

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