Periodic Properties of CH₃NH₃Pb(I_1‑xBr_x)₃ Solid Solution Perovskite Semiconductors

Procedure modified by Fabian Dauzvardis, Megan Young and George Lisensky, Beloit College, from "Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells", J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, and S. I. Seok, Nano Lett. 2013, 13, 1764-1769.

For solid solutions with the same arrangement of atoms, the size of a unit cell depends on the size of the atoms in the unit cell. Consider samples of the CH₃NH₃Pb(I_1‑xBr_x)₃ semiconductor. Since I^- and Br^- are different sizes, changing the value of x between 0 and 1 should change the unit cell size. Since the distance between nuclei helps determine how strongly electrons are held, changing the value of x should also change the band gap energy. In this experiment you will study CH₃NH₃Pb(I_1‑xBr_x)₃ for an assigned value of x, measure the unit cell size by powder x-ray diffraction, and estimate the band gap energy by visible absorption spectroscopy. Combining class values will let us see the overall relationship between composition, atomic size, and electron energy levels.

Caution: these samples contain lead. Care should be taken not to accidentally ingest them. Clean up all spills. Dispose of the material in an appropriate waste container.

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/nanolab/perovskite/index.html) shows a movie of that step.

Procedure

Wear eye protection

Chemical gloves required

Fumehood required

Synthesis and Powder X-ray Diffraction (XRD)

Align the wide end of an aluminum XRD holder with the end of a glass microscope slide. Use a marker to draw the XRD sample position on the microscope slide. Write the value of x in the CH₃NH₃Pb(I_1‑xBr_x)₃ formula on the slide. Show your labeled slide to TA or instructor before going on.

Heat a hotplate to 120°C in a hood. (You can use a cooking thermometer to measure the temperature of the top of the hotplate.) Place the microscope slide with the drawn side down onto the middle of the hotplate. Repeatedly dip a pasteur pipet (no bulb) into your assigned solution and tap the pipet onto the hot microscope slide to transfer solution and cover the sample area. When you have filled the sample area and the liquid has evaporated, take the slide to the XRD instrument. Keep the sample on a hotplate near the instrument until it is your turn.

If a sample phase-separates (a color change occurs on cooling) reheat the sample.

Record the XRD spectrum of the sample on the glass slide. Use a second blank slide to give a thicker piece of glass in the instrument holder. Scan from 27.5° to 30.5° (step width 0.02° and a 1 sec count gives a 3 minute scan).

Measure the peak position, 2θ, and use Bragg's Law, λ = 2d sinθ, to determine the spacing between planes, d. The X-ray wavelength depends on the target in the instrument X-ray tube. Common targets are Cr (2.28970Å), Fe (1.93604Å), Co (1.78897Å), Cu (1.54056Å), or Mo (0.70930Å) metals. To do the calculation in Jade, click on Find PeaksDouble the d-spacing to get the unit cell size (this is the 200 peak.)

(The crystal structure of CH₃NH₃PbI₃ has lower symmetry than the crystal structure of CH₃NH₃PbBr₃ but can be analyzed as a pseudocubic unit cell.)

If a shoulder is apparent in the XRD of the solid solutions, reheat the sample and obtain the XRD pattern again. The figure shows the XRD change upon additional heating (thick line) for a phase-separated sample.

Synthesis and Visible Spectroscopy

Use a straightedge and a diamond pencil to score the center of a 22x40mm cover slip. Draw the line only once. Use about the same amount of pressure that you would use to write on paper with a regular pencil. Fold away along the line to produce two 11x40mm pieces of glass that will fit in the visible spectrometer.

Prepare a sample for visible spectroscopy measurements by using a micropipet to add 10µL of mixture to half a cover slip on a 120° hotplate. Heat for 5-10 minutes.

Extrapolate the two linear portions of the visible spectra to find their intersection. This wavelength corresponds to the minimum energy needed to excite an electron from the conductance band to the valence band which is called the band gap energy, E=hc/λ. If two steps are observed, reheat the sample.

Materials

• glass microscope slides, standard 25 x 75 mm
• aluminum XRD sample holders and markers
• 6-inch glass Pasteur pipets
• hotplates (Heat settings for samples in the center of the hotplate: Corning 190, Cimarec 160. Higher settings will be needed if the sample is placed on the edge of the hotplate.)
• microscope cover slips, 22x40mm, Corning 2940-224
• diamond tipped pencil
• visible spectrometer
• x-ray powder diffractometer (XRD)

Stock Solutions

• Dissolve 0.61g PbBr₂ and 0.18g CH₃NH₃Br in 3 mL dimethylformamide (DMF) with stirring to give a clear 0.55 M stock solution of CH₃NH₃PbBr₃.
• Dissolve 0.76g PbI₂ and 0.26g CH₃NH₃I in 3 mL dimethylformamide (DMF) with stirring to give a yellow 0.55 M stock solution of CH₃NH₃PbI₃.
• Prepare the series of CH₃NH₃Pb(I_1‑xBr_x)₃ solutions from the 0.55 M stock solutions:

  x	(CH3NH3)PbI3	(CH3NH3)PbBr3
  1.00	0 µL	250 µL
  0.90	25 µL	225 µL
  0.80	50 µL	200 µL
  0.70	75 µL	175 µL
  0.60	100 µL	150 µL
  0.50	125 µL	125 µL
  0.40	150 µL	100 µL
  0.30	175 µL	75 µL
  0.20	200 µL	50 µL
  0.10	225 µL	25 µL
  0.00	250 µL	0 µL

  Two adjustable micropipettes (one for each stock solution) minimizes tip usage. Store the solutions in capped labelled vials. Each solution is enough to prepare 5 samples.
  

Preparation of solids used to make the stock solutions

• 7.45 g Pb(NO₃)₂ were dissolved in 50 mL water to make a 0.45 M solution.
• 3.74 g KI were dissolved in 50 mL water to make a 0.45 M solution.
• 2.68 g KBr were dissolved in 50 mL water to make a 0.45 M solution.
• 50 mL of 0.45 M KI were mixed with 25 mL of 0.45 M Pb(NO₃)₂. The yellow precipitate was isolated by filtration and washed with cold water. Yield: 3.89 g PbI₂.
• 50 mL of 0.45 M KBr were mixed with 25 mL of 0.45 M Pb(NO₃)₂. The white precipitate was isolated by filtration and washed with cold water. Yield: 3.06 g PbBr₂.
• 44 mL HBr (48% in water) was placed in a 250-mL round-bottom flask in an ice bath and stirred with a magnetic stirrer. 28 mL CH₃NH₂ (40% in water) was added gradually. The flask was then rotory evaporated at 70ºC for 2 hours to remove water and leave behind yellowish crystals. After filtration and washing with 100% cold ethanol, 26.88g of colorless crystalline solid CH₃NH₃Br was obtained, 75% yield.
• 50 mL HI (55% in water) was placed in a 250-mL round-bottom flask in an ice bath and stirred with a magnetic stirrer. 27.4 mL CH₃NH₂ (40% in water) was added gradually. The flask was then rotory evaporated at 70ºC for 2 hours to remove water and leave behind dark crystals. After filtration and washing with 100% cold ethanol, 29.92g of colorless crystalline solid CH₃NH₃Br was obtained, 64% yield.

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