Sample Preparation Methods

Composite photocatalysts used throughout the experiments in this dissertation were simple physical mixtures of metal and semiconductor nanoparticles. The spe-cific methods used to prepare these photocatalysts are described in the following sections. In general, all of the nanoparticles were suspended in ethanol, then mixed

to create composite suspensions. Using ethanol as the solvent was important. Water could not be used because its surface tension is too high and it did not properly wet the substrates, which caused uneven drying and sometimes led to solids flaking off when submerged in water. Acetone could not be used, as PVP-capped metal nanoparticles are not stable in it—they instantly “crash out” of solution and form a black precipitate.

3.3.1 Organic Decomposition Photocatalysts (Chapter 4)

Semiconductor (TiO₂ and N-TiO₂) and metal (Ag and Au) nanoparticles were independently suspended in pure ethanol and sonicated. Semiconductor suspensions were typically 1 g semiconductor particles to 99 g pure 200-proof ethanol. Metal nanoparticle samples were ∼25 mg per mL of ethanol. Single-component samples (for example, TiO₂-only or Ag-only) were prepared by drop-coating these suspensions onto 1-cm² SiO₂ substrates and drying in a stagnant ambient atmosphere. For the purposes of this document, drop-coating means applying a solution to a substrate using a micropipette. Aliquots of no more than 100 L per square centimeter seemed to produce the best results (i.e. the most consistent homogeneous films). If thicker films were required, which was generally the case, multiple drop-coating cycles were performed, with sufficient time in between to ensure that all ethanol had evaporated.

The total weight of films was determined by knowing the density of solids in each solution and simply tracking the total volume of solution applied to a substrate.

For semiconductors, the amount of solids in a solution was determined by drying a known volume of the solution (several mL) and weighing the resulting solids. For metal particles, it was not expedient to waste this much of a nanoparticle solution, so a known volume of the solution (∼10 L) was applied to a fragment of Si wafer.

Ten SEM images were then taken from different spots on the wafer, particles were

measured and counted, and the total mass of the Ag particles was estimated.

Composite suspensions were prepared by combining the pure nanoparticle sus-pensions and thoroughly mixing using agitation and sonication. Weight percent of the two constituents was determined by first determining the solids density of each individual solution and then simply tracking the mixing ratios. The amount of metal nanoparticles in the various experiments was varied and has been indicated in the respective chapters. Composite photocatalyst samples (for example, Ag/TiO2) were prepared by the same drop-coating method using the mixed nanoparticle suspen-sions, resulting in a physical mixture of the two types of particles on the substrate.

All photocatalysts samples used in the experiments contained constant weight (as well volume and surface area) of semiconductor particles. Figure 3.3a shows UV-visible extinction spectra measured in diffuse reflectance mode (see the Spectroscopy section below).

3.3.2 Water Splitting Photoelectrodes (Chapter 5)

The N-TiO₂ powder was suspended in 200-proof ethanol (EtOH) at a ratio of 99 g EtOH per gram of N-TiO₂; the solution was sonicated for one hour. Com-posite solutions were prepared by mixing the Ag nanocube colloidal solution or Au nanosphere solution with the N-TiO₂ in ethanol mixture and sonicating again for one hour. Composites were 5% metal by weight in both cases for all of the water splitting experiments. The samples were prepared by drop-coating the nanoparticle solution onto 1-in² conductive substrates (glass coated with indium tin oxide) with a micropipette; the samples were allowed to dry in a stagnant atmosphere. It was observed that the stagnant atmosphere was necessary to produce consistent, reason-able homogeneous films. It seems that if uncovered the substrates dried too quickly, which caused large-scale agglomeration of solids that sometimes resulted in some of

300 400 500 600 700 800 Wavelength [nm]

Extinction [a.u.]

N-TiO₂ TiO₂

Ag/N-TiO₂ Composite Au/N-TiO₂ Composite

300 400 500 600 700 800

Wavelength [nm]

Extinction [a.u.]

Au/N-TiO₂ Ag/N-TiO₂

N-TiO₂

(a) (b)

Figure 3.3: UV-visible extinction spectra for composite samples used in Chapters 4 and 5. (a) UV-visible extinction spectra for N-TiO2-only, Ag cube/N-TiO2 composite and Au sphere/N-TiO2

composite photocatalysts, used in the methylene blue decomposition experiments in Chapter 4.

(b) UV-visible extinction spectra for TiO2-only, N-TiO2-only, Ag cube/N-TiO2 composite and Au sphere/N-TiO2 composite photo-electrodes, used in the water splitting experiments in Chapter 5.

the material flaking off of the substrate when submersed in water. The resulting photo-electrode films were ∼0.75 µm thick (measured by ellipsometry). All sam-ples contained 2 mg N-TiO2 (constant semiconductor weight, volume, surface area).

Figure 3.3b shows UV-visible extinction spectra (diffuse reflectance mode) of the samples.

3.3.3 Photo-electrodes for Distance Dependence Studies (Chapter 6)

For the distance dependence studies presented in Chapter 6, the preparation meth-ods discussed above were insufficient. Essentially, drop coating multiple layers of different nanoparticle suspensions did not produce samples that were consistent or uniform in thickness. Instead, samples were prepared by spin coating successive layers of the constituent materials onto an inert transparent conductive support (fluorine-doped tin oxide (FTO)). Spin coating is a process by which the substrate is affixed to a plate that spins at a constant and controllable rate (a constant rate of 4,000 rpm was used in all of the steps discussed below). The suspension is then dropped onto the spinning substrate with a micropipette, as before. This produces much more uniform and consistent samples compared to the drop coating method discussed. The main drawback of spin coating is that, even at low spinning speeds, most of the applied solution spins off of the substrate and thus is wasted.

For the photocatalyst electrodes used in Chapter 6, the first step was spin coating a suspension of semiconductor particles (TiO₂ or N-TiO₂ in ethanol) on the FTO support. The semiconductor particles were suspended in pure (200 proof) ethanol at a ratio of 99 g ethanol to each gram of semiconductor (the semiconductor suspensions were sonicated for at least 15 minutes prior to each use). The weight of semiconduc-tor particles was constant for each sample. A dilute solution of polyethylene glycol (PEG) in ethanol (2.5% PEG by weight) was then applied on top of the

semicon-ductor via spin coating. The thickness of the PEG layer was varied by changing the volume of PEG solution applied. Finally, a constant volume of Ag nanocube suspension in ethanol was applied on top via spin coating. A UV-visible extinction spectrum of one of the finished composite samples is shown in Figure 3.4. The figure also shows a schematic of the composite sample geometry.

The thickness of the spacer layer was estimated by applying the same volumes of PEG solution to a clean Si wafer with the same area as the FTO support, and measuring the resulting PEG thickness using ellipsometry, as shown in Figure 3.4a.

Ellipsometry is an optical technique that can be used to analyze thin films by ob-serving the change in the polarization of light that is reflected off of the sample.

Measurements were performed using a J. A. Woollam BASE-160 ellipsometer with EC-270 control module. Fitting the measured spectra with a model comprising a 1 micron thick layer of Si and a variable-thickness PEG layer (refractive index of

∼1.4) allowed the estimation of the thickness of the PEG layer. The Si substrate was necessary because the ellipsometry technique relies on transmission through a very thin sample and then reflection off of the underlying substrate.