An Integrated, Multipart Experiment: Synthesis ... - ACS Publications

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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An Integrated, Multipart Experiment: Synthesis, Characterization, and Application of CdS and CdSe Quantum Dots as Sensitizers in Solar Cells Christina A. Bauer,* Terianne Y. Hamada, Hyesoo Kim, Mathew R. Johnson, Matthew J. Voegtle, and Matthew S. Emrick Chemistry Department, Whittier College, Whittier, California 90608, United States S Supporting Information *

ABSTRACT: Quantum dots (QDs) are useful for demonstrating the particle-in-a-box (PIB) model utilized in quantum chemistry, and can readily be applied to a discussion of both thermodynamics and kinetics in an undergraduate laboratory setting. Modifications of existing synthetic procedures were used to create QDs of different sizes and compositions (CdS passivated with polymer, and CdSe passivated with oleic acid/ trioctylphosphine). These were investigated by spectroscopy, to which standard 3D PIB mathematical models were applied to determine their effective size. The data were compared to those from other methods for students to see the validity of the PIB model. For CdSe QDs, an empirical formula was applied to the spectroscopic data. In the case of CdS, the synthesized QDs were studied with X-ray diffraction, from which one can also estimate the size of the QDs. Finally, the QDs were utilized as the light-harvesting layer in photovoltaic cells by attachment to a layer of surface-modified titania (TiO2) nanoparticles on conductive glass, and the surface chemistry tested via water contact-angle measurements. The photoresponse of these cells was measured using basic electrochemistry equipment for a selection of QDs, and these results were considered in relation to the light source used for excitation (CdS QDs absorb UV light, and a voltage was only measurable upon exposure to UV light). Students are able to synthesize, characterize, and apply their materials to a functional purpose. Ultimately, students drafted reports in the form of an ACS-style communication, allowing for a tie-in of typical lab reports to real-world journal publications. KEYWORDS: Upper-Division Undergraduate, Physical Chemistry, X-ray Crystallography, Undergraduate Research, Surface Science, Spectroscopy, Semiconductors, Quantum Chemistry, Nanotechnology, Electrochemistry

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INTRODUCTION Nanotechnology is a relevant, contemporary topic in chemistry. Incorporation of the topic of nanoparticles into the curriculum is important and becoming more prevalent.1,2 Semiconductor nanoparticles are a useful platform for exposure to such nanomaterials, while reinforcing and learning new topics, such as spectroscopy, diffraction, particle-in-a-box, semiconductor theory, surface chemistry, kinetics, and thermodynamics. Additionally, quantum dots (QDs) are receiving considerable attention because of their potential usage in several areas of advanced applications and in alternative energy storage and conversion devices.3−6 Besides their size-tunable luminescence color, their emission of light is bright and nearly monochromatic with a very narrow emission band, making them especially pleasing to the eye.7,8 Furthermore, their molar extinction coefficient is much larger than traditional organic dyes, making for strong absorbers to harness sunlight for photovoltaics, although the exact role that QDs play is still under investigation.9 QD syntheses and spectroscopy have been the topic of a few Journal of Chemical Education protocols,10−12 © XXXX American Chemical Society and Division of Chemical Education, Inc.

and a unique application and characterization method is described here for the undergraduate laboratory. Semiconductor materials can generate electron−hole pairs (called excitons) when excited by a suitable source (light in this case), and their recombination can result in light emission (or luminescence). Specifically, an absorption event results in the promotion of an electron from the valence band to the conduction band, leaving a “hole” behind in the valence band. The exciton has a composition-specific diameter and can move freely through typical, bulk semiconductor materials as the energy levels are close enough to create a continuum of levels.13 This effect is independent of size at the macroscale. However, when the material is made such that the exciton diameter is comparable to the size of the particle in which they are contained, quantization occurs as the particles are confined to a “box” (or a “dot” in this case, as they are approximately Received: August 9, 2017 Revised: March 5, 2018

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DOI: 10.1021/acs.jchemed.7b00593 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

which impart solubility in nonpolar solvents via the long hydrocarbon chains. See Figure 2.

spherical particles). The key is to make the particles small enough that quantum confinement is operative and the energy levels are dictated by the particle size (Figure 1). This usually

Figure 2. CdS and CdSe nanoparticles must be capped with a passivating agent in order to kinetically inhibit them from forming the bulk material. This is achieved with (a) a branched polymer, PEI, that surrounds the CdS and imparts water solubility or (b) a combination of oleic acid and trioctylphosphine surrounding the CdSe nanoparticles, which imparts solubility in nonpolar solvents.

Figure 1. Semiconductors have a bandgap between the valence (composed of bonding orbitals) and conduction (composed of antibonding orbitals) bands. As the number of atoms in a macroscopic crystal changes, the position of the energy levels stays constant as the bands are the result of multiple overlapping orbitals (left). Not until the particles reach single nanometer sizes do the individual orbital energies become evident (right). Smaller sizes result in larger energy gaps and therefore higher energy light absorption and emission. Once excited (purple “up” arrow), the electrons relax in the conduction band (thin arrows down) and emit light when returning to the valence band (glowing orange or blue down arrows). (Not to scale.)

It is known that longer reaction times result in larger particles in the case of CdSe.11 However, size correlations to synthetic conditions for CdS QDs synthesized with branched PEI in methanol are not reported in detail in the literature. Studies with linear PEI have related increased concentrations of reactants to smaller nanoparticles.18 With the use of both spectroscopy and X-ray diffraction (XRD), it is possible to estimate the particle size and crystalline domain size. The theory of XRD is an important subject that is worthwhile of its own separate discussion, and will only mentioned here as needed to explain size determinations. Briefly, diffraction occurs when light is scattered by a periodic array with long-range order, producing constructive interference at specific angles. By analyzing the diffraction geometry and the corresponding peak intensities, one obtains information concerning the crystal structure responsible for diffraction. Typically, the experimental setup measures the intensity of the X-ray scattering from the sample as a function of angle between the X-ray source and detector (recorded as 2θ), as shown in Figure 3. Because destructive interference of diffracted X-rays is minimized in smaller crystals (due to a smaller number of repeat units), the diffraction peak is broadened. Standard methods such as Scherrer analysis can be used to calculate the crystallite size for QDs.19,20 This is then correlated to the emission color observed, which provides another method to estimate the nanoparticle size. Discussions of crystallinity and domain size can then result. As we are utilizing relatively lower temperatures in the QD syntheses here, we expect that the QD size may be larger than the crystallite size, and hence, XRD is likely to underestimate the true QD size. Solar cells harvest energy in the form of light, converting it to electrical energy. As QDs are strong light absorbers, with extinction coefficients that surpass even the strongest organic absorbers, they are suitable for usage as light harvesters in dyesensitized solar cells. When an appropriate material is placed next to the QDs, the excited QD electrons can transfer to it (titanium dioxide in this case), resulting in a voltage when the circuit is complete. This energy transfer requires that the absorbing layer be in very close proximity to the conducting layer. Surface reaction of the TiO2 with 3-mercaptopropionic

requires that the particles have nanometer dimensions (quantum effects become important below 10 nm diameter for semiconductor nanoparticles). The Bohr exciton radius for CdS and CdSe semiconductors is ∼3 nm14 and 5.6 nm,15 respectively, which is a readily attainable synthetic size using published methods.4 When the size of the box shrinks, the energy levels are pushed further apart, leading to higher energy emission and a shift of color to the blue side of the spectrum.16 The confinement of the electron−hole pair is an excellent realworld example of the particle-in-a-box model in quantum mechanics. Semiconductor nanoparticles can readily be made with varying diameters. Colloidal methods are employed to crystallize the precursors in the presence of a surface passivating agent. Traditional synthetic methods involve high temperatures (>200 °C) to properly crystallize.4 However, safer methods to form both CdS10 and CdSe11 nanoparticles have been reported in the literature that are relevant in an undergraduate laboratory. These general methods were utilized here with some alterations in order to prepare a range of sizes at the quantities necessary for the various characterization methods. Such methods can be used to illustrate ideas of kinetic vs thermodynamic stability. Generally, nanoparticles are thermodynamically unstable, but kinetically stabilized,17 because the surface area of the particles is very high, which is energetically unfavorable. They can be stable for some time, however, if they are “passivated”, meaning that a ligand is placed around the particles as a shell to prevent the particles meeting one another and aggregating. The CdS QDs are passivated with a polymer, branched polyethylenimine (PEI), which coordinates to the particle surface via amine groups and imparts water solubility. CdSe QDs are passivated with oleic acid and trioctylphosphine, B




Figure 3. X-ray diffraction from crystalline domains: (a) incoming X-rays will diffract off of the lattice. If evenly spaced (b), the diffraction will occur at the same angles, leading to X-ray waves that constructively interfere to generate a larger signal that is detected as a peak and ultimately a pattern of peaks in a diffraction pattern. (c) If the lattice is not evenly spaced, destructive interference occurs, and no peak is evident. When nanosized crystalline domains are present, broadened peaks are observed as the amount of constructive vs destructive interference is minimized.

Cd(OAc)2·2H2O in their dry state by transferring them inside a fume hood. Trioctylphosphine (TOP) was transferred to the round-bottom flask using a cannula and nitrogen atmosphere. Oleic acid was also used as a passivating agent, and octadecene, a high-boiling, nonpolar solvent, was used. The synthesis was conducted at 165 °C, and aliquots were removed from the reaction chamber over short time increments, essentially as quickly as possible starting immediately until the reaction color stabilized. A total of 10 samples were collected. Those that had nearly identical spectroscopic characteristics were combined.

acid (MPA) results in modification of the TiO2 such that functional thiols extend from the surface to which the QDs can bind.21 Crude contact-angle measurements can be used to verify attachment of the quantum dots. Once assembled into a rudimentary device, the photovoltage can be measured.

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EXPERIMENTAL METHODS Students work over a five-week period to perform the different portions of the project. This experiment was employed successfully in a physical chemistry laboratory for three years with 15 students. They worked as a class to synthesize the CdSe QDs, and individually to synthesize CdS QDs. The basic wet synthetic approach is to combine the two QD components in a solvent in which the product material has limited solubility. The product is allowed limited aggregation before being sequestered with a suitable passivating coating. Most syntheses require very high temperatures, which are not easily attainable in a typical undergraduate laboratory, and can be unsafe. We made CdS particles at room temperature with differing parameters to yield various sizes of CdS QDs. We utilized a hot injection method to generate CdSe QDs of desirable size and quantity. See Supporting Information Student Handout for a step-by step guide to perform the syntheses outlined below, along with preparation and instructions for spectroscopic measurements and solar cell preparations. Additionally, see Supporting Information Instructor’s Notes for comments and suggestions to help guide students throughout the experiment.

Characterization of Quantum Dots: Week 2 and Week 3 Spectroscopy

UV−vis absorption spectra of CdS and CdSe nanoparticles were measured using an Agilent 8453 UV−vis spectrometer, and emission measurements were taken with a Horiba Scientific Fluoromax-4 spectrofluorometer, with an excitation wavelength of 350 nm. Dilutions of the sample were conducted as necessary such that the lowest energy absorption peak was below an absorbance of 1.0. Emission from CdSe samples was collected from 410 to 700 nm; CdS sample emission was collected from 375 to 650 nm. X-ray Diffraction (XRD). Characterization of solid materials was conducted with a Rigaku MiniFlex II XRD. The concentrated CdS QD solutions were placed on a glass slide and allowed to evaporate, leaving behind the dried sample. Care was taken to fully cover the sample area on the plate with solution to minimize any background from the glass. Scans were conducted from 2θ = 15° to 45°. (Note: Attempts to isolate solid CdSe were unsuccessful, as the solvent, octadecene, is not easily removed and addition of a nonsolvent, such as acetone, resulted in size separation, rather than precipitation of QDs.)

Synthesis of Quantum Dots: Week 1 and Week 2

CdS Synthesis. CdS QDs were synthesized on the basis of the methods described by Winkler et al.10 The 1800 MW polyethylenimine (PEI), Cd(NO3)2·4H2O, and Na2S·9H2O solutions were made in methanol at varying concentrations and temperatures. The Cd and S sources were added in 1 mL intervals to prevent formation of large particles. The CdS QDs were cleaned of excess PEI with fresh methanol and resuspended for spectroscopic characterization. A concentrated slurry was used to prep samples for XRD. CdSe Synthesis. The basic protocol of Landry et al. (2014)11 was used. Care was taken while handling Se and

Construction and Measurement of Solar Cells: Week 4 and Week 5

Preparation of Titania Layer. Solar cells were created by making a multilayer “sandwich” with ITO-coated glass as the conductive, transparent substrate material.22 One side was coated with a thin layer of TiO2 paint on the conductive side. C




Table 1. Reaction Conditions and Calculations for CdS QDs in Methanol Sample

[Cd(NO3)2·4H2O], mM

[Na2S·9H2O], mM

[PEI], mM

Temp, °C

Emission Max λ, nm

Size,a nm

B, fwhm deg

Size,b nm

1 2 3 4 5 6 7

2.0 2.0 2.0 2.0 2.0 20.0 20.0

2.0 2.0 2.0 2.0 2.0 20.0 20.0

0.57 0.57 0.46 0.34 0.57 0.57 0.57

5 10 10 10 25 10 25

449 472 471 471 528 495 495

2.7 3.4 3.4 3.4 5.6 5.3 5.4

4.5

1.9 c 2.2 c 2.4 2.0 2.8

3.9 3.6 4.3 3.0

a

Radius, calculated with eqs 1 and 2 from emission data. bDiameter, calculated with eq 6 from XRD data. Lattice strain is ignored; these are underestimates. cXRD data for samples 2 and 4 were not possible as yields were low.

Functionalization of Titania. The titania is soaked in a solution of MPA in ethanol, and then coated with quantum dots by drop-casting from solution. Water contact-angle testing was done to ensure functionalization. Water “beads up” on hydrophobic surfaces and spreads on hydrophilic ones. Pictures were taken, and crude estimates of contact angles were made by drawing a tangent to the curvature of the water droplet. Assembly of Solar Cells. The conductive side of another piece of ITO glass was coated in a thin layer of graphite by simply “coloring” with a pencil. The two halves of the solar cell were then sandwiched together with binder clips, having iodide/glycol solution on the TiO2 surface. Contact leads were fashioned out of aluminum foil. Potential was then measured with a VersaSTAT 3 potentiostat galvanostat during exposure to visible (simply room light or exposure to a window) and UV (via a simple TLC plate-reader light or sunlight) light. A simple voltmeter can also be used to measure the voltages if a potentiostat is not available.

quantum dots, whereas a higher temperature resulted in smaller quantum dots. Utilizing these effects, it was possible to make a range of CdS QD sizes. Characterization of the solutions by UV−vis spectroscopy showed the main peak that represents the initial formation of the exciton occurs at 360−400 nm. The largest particles scattered more light, resulting in a high background, partially masking the main absorption peak. Emission spectroscopy showed the evident size differences more clearly, with emission maxima ranging from 450 to 580 nm (Figure 4). The energy of

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HAZARDS Standard laboratory safety precautions should be used during the syntheses, including wearing nitrile gloves, protective clothing, and safety goggles. Conduct all the portions of this synthesis in a well-ventilated fume hood, including preparation of reactant solutions. A majority of the hazards are toxic if inhaled, swallowed, or placed in contact with skin and eyes. Cadmium is toxic and must be disposed of separately. Wear gloves when handling any of the reagents here, and weigh the Cd(NO3)2·4H2O in a fume hood. Any cadmium-containing solutions should be collected in a separate waste container for proper disposal. Octadecene, oleic acid, and trioctylphosphine vapors should be avoided and only removed from the hood for disposal once cooled. Metallic selenium is toxic if inhaled and may be harmful if swallowed or absorbed through skin. Proper use of a cannula needs to be demonstrated by the instructor, along with air-sensitive transfer techniques. Additionally, X-ray diffraction must be used in accordance with Radiation Safety.

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Figure 4. Size differences in CdS QDs evident by spectroscopy (left). The absorption spectra are shown as solid lines, and the emission spectra as dashed lines. Size differences are also visually evident in powdered samples after exposure to a blacklight. These are shown on glass slides, as prepared for XRD studies (right).

the emitted photons represents the sum of the bandgap of the bulk material and the additional change in energy due to quantum confinement (ΔEqc) in small crystals. Using emission data, the size can be calculated via eqs 1 and 2:10 Ephoton(λmax ) =

hc = E bandgap + ΔEqc λmax

(1)

Here, Ephoton represents the maximum emitted energy, h is Planck’s constant, c is the speed of light, λmax is the maximum wavelength of the emitted photon, and Ebandgap is the bulk bandgap energy for macroscopic CdS (∼2.42 eV). The energy is determined via of the emission for the CdS QDs. Once this quantum confinement energy is determined, the particle size can be extracted from eq 2:

RESULTS AND DISCUSSION

CdS Quantum Dots

The CdS QD synthesis was conducted similarly to that previously reported, but alterations to the procedure resulted in an array of particle sizes via changing the concentration, reactant ratios, and temperature. Although we initially utilized different sizes of the polymer (600, 1800, 10,000 MW), we had the most success with 1800 MW. Table 1 summarizes the conditions for each reaction. We found that polymer/CdS ratio alterations did not affect the size as much as the concentration or temperature did. Higher concentrations resulted in larger

ΔEqc =

h2 ⎛ 1 1 ⎞ ⎟ ⎜ + 2 mh* ⎠ 8R ⎝ me*

(2)

Here, R is the particle radius, me* is the effective mass of the electron, mh* is the effective mass of the hole, and ΔEqc is the quantum confinement energy. Note that, for CdS, me* = 0.19me and mh* = 0.80me, where me is the mass of the electron. Note the similarity to the one-dimensional particle-in-the-box (PIB) energy: D


Journal of Chemical Education E=


n p2h2 8mL2

fwhm of the diffraction peak. Broadening is primarily due to a limited number of repeat units and is important in nanostructured materials, although lattice strain due to the amount of surface versus bulk atoms is also a factor. The crystalline domain size can be calculated using the Scherrer equation

(3)

Here, np = principal quantum number, m is the mass of the particle, and L is the length of the box. There is an inverse relationship between length and energy, just as larger QDs emit light at a lower energy (higher λmax). A complementary method, XRD, was also used to determine crystalline particle size (prepped samples shown in Figure 4). The first {111} diffraction peak in the zinc blende cubic structure of CdS occurs at 26.5°, as seen in Figure 5. The

average domain size =

0.9λ Β cos θ

(6)

where θ is the maximum peak angle (in degrees) and B is the fwhm in radians (multiply θ by π/180). After instrumental correcting for broadening, the diffraction sizes were determined. The data for both emission- and XRD-determined sizes are shown in Table 1. Note that the XRD diameters are consistently smaller than that calculated from emission data (reported as radii). This is expected, as the QD nanocrystals are unlikely to consist of a single crystal under these lowtemperature synthetic conditions. More likely, they will be composed of smaller crystalline domains, as shown in Figure 5. We note that the domain is no more than one-third of the size of the actual QD, according to the PIB model. In addition to the likely smaller domains, the crystals are likely to have strain, which also would broaden the XRD peaks. This is not accounted for here, making these calculated sizes underestimates. CdSe Quantum Dots

The CdSe QD synthesis resulted in a variety of colors and sizes. As the reaction proceeds, the particles grow rapidly. Withdrawals of samples were conducted as quickly as possible after the reactants were combined and stopped once the reaction color reached red, whereby nonvisible, infrared emission will occur. The progression in sizes was evident both visually and via spectroscopy, as shown in Figure 6. The sizes were

Figure 5. Primary diffraction peak from two different CdS QDs (top). Differences in the width of the band are the result of crystalline size. Bottom left shows a TEM image of a nanoparticle with two crystalline domains evident and a representation in blue. A single crystalline nanoparticle is shown on the right, where the crystalline domain is the same size as the whole particle, also represented in gray. Note that these representative TEM images are of silver nanoparticles, used for instructional purposes here.

presence of a smaller shoulder peak at 43°, which occurs for the {220} diffraction plane, indicates that these are likely dominated by the cubic structure, but a full discussion of the crystal structure is beyond the scope of this study. Note that there is some precedence for faster reactions favoring the cubic phase, but cubic versus hexagonal wurtzite structure is not always distinguishable at this size.18,23 The lattice spacing can be determined from the Bragg equation: nλ = 2d sin θ

(4)

where n is the order, λ is the wavelength of the incoming X-rays (1.54 Å for Cu Kα), d is the spacing between atoms, and θ is the diffraction angle. For a cubic cell, one can simply equate the lattice parameter, a, to the distance d via: 1 h2 + k 2 + l 2 = 2 d a2

Figure 6. Size differences in CdSe QDs evident by spectroscopy (bottom). The absorption spectra are shown as solid lines, with the emission spectra as dashed lines. The progression of size is also visually evident in solutions after exposure to a blacklight (top).

(5)

where h, k, l represent Miller indices in three dimensions. For the first observed diffraction, h = k = l = 1. Knowing the wavelength of the X-rays and the lattice parameter (a = 5.83 Å), one can determine the expected 2θ for comparison to experimental data. Students will calculate 2θ = 26.4° for comparison to their measured peak. Regardless of the phase, the width of the diffraction peak can be used to determine crystalline size via measurement of the

calculated by two methods. First, eqs 1 and 2 were again used, as they apply to any 3D particle-in-the-box, where Ebandgap, me*, and mh* are unique for each nanocrystal composition. Here, the bandgap for CdSe at room temperature is ∼1.74 eV, me*(CdSe) = 0.13me, and mh*(CdSe) = 0.45me.11 Additionally, an empirical equation was utilized to determine particle size. It has been reported that the diameter of the particles (D) can be determined with the following equation:24 E




D = 0.0566e 0.0071λ

hydrophobic CdSe films (which are terminated with an oleic group or octyl group). Simple imaging with a cellphone camera allows for contact-angle measurements. The CdS surfaces wet thoroughly; a contact-angle measurement was not feasible on this superhydrophilic surface, and the angle approaches 0. Conversely, the CdSe surfaces had reproducible results of approximately 135−140° for all nanoparticle sizes tested. The solar cells were then fully assembled into the multilayer sandwich arrangement that will allow electrons, which are generated after absorption of light via QDs, to complete a circuit. The voltage is recorded with the light both on and off. The full assembly is demonstrated in Figure 8, with a schematic

(7)

where λ is the emission wavelength. This was derived by correlating electron microscope image analysis of the size to the emission spectra. These sizes were consistently smaller than the basic PIB model, as summarized in Table 2. However, the Table 2. CdSe QD Experimental Conditions and Size Analysis Sample

Time, s

1 2 3

5 7 9

4 5

12 15

6

20

a

Visible Color

Emission Max λ, nm

Sizea (Emission)

Sizeb (Emission)

Pale yellow Yellow Yellow/ orange Orange Orange/ red Red

527 529 532

1.96 1.97 1.97

2.32 2.34 2.34

535 539

2.03 2.06

2.49 2.56

541

2.10

2.66

b

Radius calculated with eqs 1and 2. Diameter calculated with eq 7.

comparison of PIB theory to empirical data is instructive and reinforces this simple model again. Reasons for differences involve polydispersity of the QD size, trap-states due to dangling surface bonds,13 and electron−electron repulsion considerations in the QD. We note that samples were kept and observed for stability. After approximately 4 weeks, aggregation and diminished emission was observed, demonstrating the kinetic stability of these nanoparticles. Solar Cells

Figure 8. Solar cell setup. (a) Order of the layers: contacts are made to the aluminum foil. The “sandwiches” are made by facing the conductive ITO layer on the glass toward the inside. Binder clips are used to hold the device tightly together. The function of each layer is shown in part b, where incoming light (that matches the absorption of the QDs) is absorbed to promote an electron from the valence band (VB) to the conduction band (CB), and then transferred to the CB of a titania nanoparticulate film, followed by transfer to the ITO layer. Meanwhile, holes are transferred to the iodine, which behaves as a redox shuttle to allow replenishing of electrons to the QDs.

Once students have prepared the TiO2 nanoparticle films and linked QDs to them, small drops of water were placed on each film to confirm binding of the QDs, as shown in Figure 7. Water completely spread on the hydrophilic CdS films (which are coated with a water-soluble polymer) and “beaded” on the

of the path taken by electrons in the device.25 Voltage measurements were carried out for a selection of CdS and CdSe QD solar cells. A large and small particle representative sample of each QD type was tested, as summarized in Table 3. Interestingly, students can immediately see a correlation between voltage and the absorption spectra collected earlier. The wavelength of absorption occurs in the UV portion of the spectrum for CdS (Figure 4), with maxima between 355 and Table 3. Voltage Data Collected from CdS and CdSe Solar Cells

Figure 7. Contact-angle measurements after QDs were bound to the titania layer confirm binding of QDs. Water droplets were imaged with a camera, and the angle was roughly measured with a protractor. Hydrophobic CdSe QDs are shown in parts a and c. The measured angle is shown in d. Hydrophilic CdS QDs are shown in b, where the droplet spread out and absorbed into the titania layer.

Sample/Light Source

Baseline (V)

Maximum Potential under Illumination (V)

CdS batch 2/blacklight 365 nma CdS batch 5/blacklight 365 nm CdSe batch 1/room light CdSe batch 5/room light

0.040

0.22

0.070

0.49

0.070 0.000

0.70 0.94

a

Data shown Figure 9. Note that the blacklight was held at a constant distance from each CdS solar cell for consistent measurements.

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these methods, will be achieved, as students must explain their combined data in the discussion portion of the final paper. Fifth, application of nanomaterials is demonstrated with a contemporary purpose, whereby the connection between the absorption profile and the voltage output of the rudimentary devices is made by the students to connect quantum theories to device function. Success in the experiments is necessary to continue on the following week, and evidence of understanding the series of experiments culminates in the ability to articulate this in a unique paper, for which they must find their own relevant references and present real data in a coherent way. Students are given a chance to first write a paper draft to get initial feedback along the way (see Supporting Information Example of Student Report). Furthermore, students must share data, fostering communication and discussion of the goals listed here. Success in all five goals was noted within the final reports.

363 nm. Therefore, the wavelength needed to initiate the electron excitation and subsequent transfer within the solar cell should also require UV light. As expected, CdS QD solar cells were only responsive to UV light, showing little voltage generation with only visible room light. CdSe QDs were sensitive to room light and window-filtered sunlight, however, as the absorption occurs in the visible portion of the spectra (Figure 6, between 465 and 520 nm). Background measurements were made with the light off by shielding the sample with aluminum foil. In most cases, the baseline consistently moved up over time, but the overall voltage difference remained nearly consistent. There was a strong response that increased with QD size for CdSe QDs. This is likely due to larger extinction coefficients in larger QDs. As the contact-angle measurements were similar between samples, the coverage with CdSe QDs is likely to be similar and therefore relation to QD size is reasonable to consider. Furthermore, the room light covers much of the visible spectrum, and can excite each QD sample similarly. Meanwhile, the CdS QDs yielded a relatively lower signal. A size correlation with voltage is not as clear-cut in this case, as the narrow excitation will not equally excite each sample. Furthermore, large particles scatter more, and PEI can absorb/scatter some of the light. An example of CdS sample 2 QD solar cell voltage measurements is shown in Figure 9.

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CONCLUSIONS This project allows for students to learn about multiple topics spanning the classical and quantum chemistry curriculum, including concepts in materials chemistry, while also learning about nanotechnology. First, conclusions can be made regarding control of kinetics with temperature, overall concentration, and reactant ratios. In the case of CdS, increasing the amount of capping polymer does not make a notable difference in the size of CdS QDs, but the temperature of the synthesis and concentration of reactants were observed to have an effect. CdSe QDs continued to grow with time at high temperatures, while other parameters were kept constant. It was noted that the CdSe quantum dots were not thermodynamically stable; by the end of the semester, aggregation and precipitation were evident, with loss of visible emission. Second, students applied 3D PIB-type equations to these novel materials, and saw reasonable agreement with empirical data. Size-dependent properties for QDs were quite evident, as is the case for typical microscopic systems such as cyanine dyes. Initial ideas of crystallinity and correlation with particle size and crystalline domains were also possible to introduce as part of this project. Surface functionalization and characterization by a simple water contact-angle measurement allowed for clearly evident differences in hydrophobic versus hydrophilic QDs and was used as a method to verify they had been attached through the MPA linker. Finally, successful application to functional solar cells resulted in a purposeful project that introduced ideas about electrochemistry and molecularly tailorable electronics using semiconductors. This laboratory has been carried out on three separate occasions, with multiple variations and allowances for specific student projects of interest, whereby group efforts can be made to choose the particular research direction. Final laboratory reports were presented in the format of an ACS-style communication article, and students could see their work in the format of a “real” journal article.

Figure 9. Voltage measurements from CdS sample 2. The square waves represent light on (maxima) and light off (minima).

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LEARNING GOALS AND OUTCOMES Five of the most important learning goals are described in this section, which essentially results in one major goal targeted per week. First and foremost, when this project is complete, students will have gained the ability to successfully synthesize and observe differences in properties that are unique to nanosized particles. All of the characterization measurements that follow are intended to reinforce this. The relationship of visible color to the spectroscopic measurements is evident, and the students keep this in mind throughout the experiments. Second, students will learn the concept of kinetic and thermodynamic stability differences, which will also be evident as the size of CdSe QDs increases with time, and the quantum dots eventually precipitate. Third, comparison to both empirical data and theoretical particle-in-a-box models demonstrates the utility of a real-world quantum model. In calculations, modeling the movement of an electron and making a connection to the absorption/emission spectra makes this clear to students. Fourth, the use of different methods to characterize the size, along with appreciation of the benefits and limitations behind

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00593. Laboratory student handout (PDF, DOCX) Instructor notes (PDF, DOCX) Example of student report (PDF, DOCX) G



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christina A. Bauer: 0000-0002-7687-1676 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.A.B. acknowledges the Whittier College Chemistry Department for use of facilities. M.S.E. acknowledges the Howard Hughes Medical Institute (Grant 52007571) through the Precollege and Undergraduate Science Education Program for funding.

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REFERENCES

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