Properties of Semiconductors: Synthesis of Oriented ZnO for

Laboratory Experiment pubs.acs.org/jchemeduc

Properties of Semiconductors: Synthesis of Oriented ZnO for Photoelectrochemistry and Photoremediation Emma Koenig, Ari Jacobs, and George Lisensky* Beloit College, 700 College Street, Beloit, Wisconsin 53511, United States S Supporting Information *

ABSTRACT: Semiconductors are an important class of materials; preparing ZnO nanorods allows semiconducting properties to be easily observed. The week before lab, groups of four students take 15 min to setup two fluorine-doped tin oxide glass (FTO) slides in a zinc nitrate and hexamethylenetetramine solution stored at 90 °C until the next lab. Hexagonal ZnO nanorods oriented along the c-axis are produced, as shown by X-ray diffraction (XRD) and scanning electron microscopy (SEM) images. Semiconductor properties are observed by measuring the band gap by UV−visible absorption spectroscopy and exposing samples to UV light to excite electrons into the conduction band. One pair of students obtains SEM and photoelectrochemical measurements, while the other pair obtains an absorption spectrum, XRD scan, and methylene blue kinetic data. The primary pedagogic goal of the experiment is to reinforce semiconductor concepts where two teams collaborate to summarize their evidence that ZnO is a semiconductor. KEYWORDS: Laboratory Instruction, Second-Year Undergraduate, Inorganic Chemistry, Collaborative/Cooperative Learning, Conductivity, Electrochemistry, Kinetics, Photochemistry, Semiconductors, X-ray Crystallography

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INTRODUCTION A coarse look at the periodic table shows metals on the left side and nonmetals on the right side with semiconductors in between. Many of the technologically important semiconductors share a common motif with C (diamond), Si, and Ge, where each atom makes four bonds to its neighbors in cubic or hexagonal unit cells (Figure S1). This is also possible for isoelectronic binary mixtures like AlP, AlAs, GaN, GaP, GaAs, GaSb, InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, and CsTe and solid solutions like AlxGa 1−xN, AlxGa 1−xP, AlxGa 1−xAs, InxGa1−xN, GaxAs1−xP, AlxGayIn1−x−yP, or CuInxGa1−xSe2 that retain an average electron count of four valence electrons per atom (Figure 1). ZnO, the subject of this experiment, is member of this semiconductor family. Semiconductors are characterized by having a small band gap between a filled and an empty band. Semiconductor electrical conduction occurs when electrons are excited across the band gap and electrons are then free to move to other orbitals of similar energy (Figure 2). Semiconductor materials are important for light-emitting diodes1,2 and solar energy conversion3 because their band gaps can occur in or close to the visible region of the spectrum. ZnO samples are grown on TEC-15 fluorine-doped tin oxide (FTO) coated 1 in. glass squares (∼20 ohm/cm). FTO is surface-conducting, transparent, and a technologically important material with many uses:4 • electrically heated glass windows for commercial refrigeration • heat-reflecting windows for energy conservation • touch screens and flat-panel displays for computers, tablets, cell-phones, and TVs © 2017 American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Matching colors in the semiconductor portion of the periodic table indicate complementary pairs. Selecting one element from each matching column preserves an isoelectronic valence electron count with an average of four valence electrons. Zn has two less valence electrons than Si, and O has two more valence electrons than Si, so solid ZnO has the same average number of valence electrons as solid Si.

• top layers for thin-film photovoltaics and organic lightemitting diodes • static control and EMI/RFI shielding for military, medical displays, and cameras. Growing ZnO on FTO gives a sample with many advantages. A semiconductor grown on a conducting surface does not need to be coated for scanning electron microscopy (SEM) (Figure Received: November 15, 2016 Revised: March 9, 2017 Published: April 12, 2017 738

DOI: 10.1021/acs.jchemed.6b00887 J. Chem. Educ. 2017, 94, 738−742

Journal of Chemical Education

Laboratory Experiment

helps make the connection that XRD peaks correspond to particular orientations. Semiconductor properties are observed by exposing samples to UV light, which excites electrons into the conduction band. UV−visible absorption is used to measure the band gap energy. Photoelectrochemistry uses electron excitation to observe electrical conductivity. Photoremediation of methylene blue uses electron excitation to form a hydroxyl radical that decomposes methylene blue. All three of these illustrate band properties of a semiconductor. We use this experiment in a nanochemistry course that comes after general chemistry.

Figure 2. Semiconductors have small band gaps between the “filled” valence band and the “empty” conduction band. Shading shows electron occupation for a small band gap, where thermal energy excites electrons to provide some electrical conductivity. In this experiment, UV light is used to excite electrons between the bands.

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EXPERIMENTAL OVERVIEW Oriented ZnO hexagonal rods are grown by aqueous thermal decomposition of hexamethylenetetramine serving as a kinetic pH buffer8 and subsequent formation of ZnO on FTO

S2). The sample can be used as an electrode for photoelectrochemical (PEC) measurements (Figure S3). The transparent surface means the sample can be used directly for ultraviolet (UV) absorbance measurements (Figure S4). The surface-coated sample is a convenient size to use directly for Xray diffraction (XRD) measurements of crystal orientation (Figure S5). One of the reasons that so many instruments can be used in a short period of time is that the same sample works in all of them. At the end of the experiment, the ZnO can be dissolved with HCl and the FTO glass squares reused. This laboratory experiment was adapted from published ZnO syntheses,5−7 modified for timing and additional applications of ZnO to demonstrate semiconductor properties. Reaction times of 1−10 h for the hydrothermal synthesis have been reported. Under the 90 °C conditions used here, we find a minimum of 3 h reaction time is needed after which we prefer to pour off the hot reaction solution and rinse to minimize precipitation. These do not fit well with our laboratory scheduling. We have tried starting the experiment during class time in the morning but recommend leaving the solution in the oven from one lab period to the next. Characterization of the ZnO is done by XRD and SEM to examine crystal orientation. The XRD of powdered ZnO shows random orientation of the characteristic ZnO peaks (Figure 3). In this experiment, ZnO is grown from seeds, and this maximizes the [002] peak. The same orientation along the c-axis can be seen as hexagonal faces in the SEM. This

Zn 2 + + 2OH− → Zn(OH)2 → ZnO + H 2O

The tin oxide surface can act as seed crystals for the ZnO growth5 (Figure S6), but adding ZnO nanoparticles9 as smaller seeds gives more reproducible results. The initial 15 min of this experiment involves a group of four students setting up duplicate samples 1 week before analysis. After 4 h to 1 week with vertical samples in the oven at 90 °C in sealed autoclavable centrifuge tubes, the samples are rinsed vigorously (Figure S7). The rest of the experiment is finished in one 3 h lab period with 20 students. One pair of students does the SEM and PEC experiments on one sample, and the other pair does the absorption spectroscopy, XRD, and degradation of methylene blue experiments on the matched sample. Working in groups reduces instrument time and increases collaboration because both pairs are required to share results to answer the report questions. The experimental plan is outlined in the Supporting Information (Figure S8).

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HAZARDS Zinc oxide nanoparticles can be toxic to aquatic life, so any unused seed solution should be collected for proper disposal. Leftover synthesis solutions should be handled according to local regulations. Avoid exposure to UV light irradiation. Normal chemical precautions are required while using KOH, hexamethylenetetramine, and zinc nitrate.

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RESULTS AND DISCUSSION

Characterization of ZnO

ZnO can be hydrothermally grown from tin oxide or ZnO seeds. In both cases, SEM shows a hexagonal face with preferential growth along the c-axis (Figures 4a and 5a). Orientation of the ZnO crystals along the c-axis can be confirmed by XRD in comparison with that of randomly oriented ZnO powder (Figure 3) since ZnO grown from seeds has [002] as the tallest peak (Figures 5b and 6b). Another useful comparison is with the non-FTO side of the glass, where the absence of seeds does not give oriented rods (Figure S9). Using 5 nm ZnO seeds gives smaller nanorods with higher coverage and higher XRD counts; coverage is also more consistent with less streaking (Figure S10). Using 5 nm ZnO seeds on regular glass gives similar results to Figure 5 but

Figure 3. Portion of the XRD scan for ZnO powder and the planes of atoms that contribute to each peak. Peaks are labeled with the Miller indices, the inverse intercepts along the crystal lattice axes. For example, [100] is parallel to the bc plane, and [002] is parallel to the ab plane and perpendicular to the c-axis. If the sample was crystalline, the relative heights of the peaks would vary with crystal orientation. 739




Figure 4. (a) SEM of hydrothermally grown (3 h, 90 °C) ZnO on FTO shows hexagonal morphology. Surface tin oxide is not smooth at the micron scale (Figure S6) and can act as seeds for nucleation of the ZnO crystals. (b) XRD for the same sample indicates crystals primarily oriented along the c-axis, consistent with the hexagonal faces seen in the SEM. The additional peaks are due to FTO.

Figure 5. (a) SEM of hydrothermally grown (4 h at 90 °C) ZnO on FTO using previously prepared ∼5 nm ZnO seeds shows hexagonal morphology but smaller diameter nanorods. (b) XRD for the same sample indicates crystals almost exclusively oriented along the c-axis, consistent with the hexagonal faces seen in the SEM. Increased surface coverage compared to Figure 4 means the unlabeled FTO peaks are smaller.

misses the advantages of having a conducting surface (see above). The UV−vis absorption spectrum can be used to determine the band gap of ZnO. Photons with less energy than the band gap are not absorbed and photons with greater energy are absorbed (Figure 2), resulting in a cutoff filter spectrum with an absorption onset that indicates the band gap energy (Figure 6). Extrapolating the vertical portion and the longer wavelength horizontal portion to find their intercept gives the band gap of these samples as 384 nm (3.2 eV). This is in the near-ultraviolet portion of the spectrum, and ZnO is a wide-band-gap semiconductor. Photoelectrochemistry

These ZnO samples are not very conductive. Shining UV light on ZnO excites electrons from the filled valence band to the empty conduction band, where they can travel to the conductive FTO glass. Solution PEC measurements of ZnO nanorods on FTO, in 0.01 M KOH with a carbon counter electrode and a constant applied voltage of 0 V vs Ag/AgCl, show that ZnO conducts many times better in the presence of UV light (Figure 7). The effect is reversible when the light is blocked and repeatable many times (Figure S11 and supplemental movie). PEC measurements using different light sources are a possibility. Larger responses are seen for light sources with more intensity greater than the band gap energy (Figures S12 and S13). Investigation using controlled wavelengths for

Figure 6. UV−vis spectrum (curved lines) and band gap determination (linear extrapolations for values between the indicated points) for three different samples of ZnO nanorods on FTO. The three samples have different opacities but similar band gap onsets. The intercept for the lowest curve is at 384 nm.

excitation also shows conductivity that corresponds with energy greater than that of the band gap (Figure S14). 740




Figure 7. Photoelectrochemical measurement of ZnO nanorods on FTO in 0.01 M KOH with a constant applied voltage of 0 V vs Ag/ AgCl alternately illuminated (high current) with a Zelco UV pocket fluorescent lantern (4 W) or room light (low current). Excitation of electrons across the band gap makes the sample more conductive and indicates that ZnO is a semiconductor.

Figure 9. Photoexcitation creates electron−hole pairs in a semiconductor. As the band energies in the solid (EFermi) match solution energetics (Eredox), band bending causes holes to energetically float and move toward the solution while electrons energetically sink and move into the solid. The holes create hydroxyl radicals that oxidize solution species such as methylene blue.

In addition to measurements made at a constant voltage, cyclic voltammetry was used to measure current response as a function of voltage (Figure 8). Under illumination, the sample acted as a conductor and followed Ohm’s law,10 with the slope of the line indicating the inverse of the resistance (V = iR so i = V/R).

loss by glass absorbance, to give a half-life of 90 min (Figure S15). The degradation of methylene blue follows a first order rate law and can be used to calculate half-life of the methylene blue with and without ZnO: Differential Rate Law: d[A]/dt = −k[A] Integrated Rate Law: [A] = [A]0 exp( −kt ) Data Analysis: ln[A] = ln[A]0 − kt

Thus, a plot of ln[A] versus t is a straight line with slope −k and a half-life of ln(2)/k (Figure 10). The raw data for this plot are available in Figure S16 and Table S1.

Figure 8. Cyclic voltammetry of ZnO nanorods on FTO showing iV characteristics as the voltage is scanned in the dark (lower curve, blue) and illuminated with 365 nm light from a UVP UVLS-24 EL series 4 W UV lamp (upper curve, red). The arrow indicates the starting point and direction for the cyclic voltage scan. Figure 10. Photodegradation of 5 mL of 0.010 M methylene blue in the presence of 6 cm2 FTO without (circles) and with (squares) ZnO nanorods. The light source is a UVP single midrange UV transilluminator model TM-40 gel viewer (4 × 8 W). The half-life of methylene blue with ZnO under these conditions is 90 min. The raw data are shown in Figure S16 and Table S1.

Degradation of Methylene Blue

Photoexcitation creates electron−hole pairs in a semiconductor. Band bending occurs near the interface as the band energies in the solid match solution energetics. As electrons drop to lower energy levels, they will move away from the interface into the solid and holes will move toward the interface to create hydroxyl radicals11 that oxidize solution species such as methylene blue (Figure 9). In this photoremediation experiment 0.010 M methylene blue, a nontoxic dye that is not degraded by ultraviolet light, is used to simulate an organic pollutant in water. Absorbance spectroscopy is used to measure the disappearance rate of methylene blue.12 To increase the rate, a strong UV source is used, and the solutions are illuminated from above to prevent

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STUDENT LEARNING The student instructions include laboratory conclusion questions whose answers are provided in the instructor materials. The majority of these questions (2−5) involve calculations using the experimental data, and almost all (85%) of our students did these correctly. Questions about preferred orientation of crystal growth (1) and the properties of 741



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semiconductors (6) are more central as an indication of conceptual understanding. Most students were able to pair up the XRD of an oriented sample, which primarily shows just one peak, with the SEM images showing oriented hexagonal faces and recognize that the two techniques provide confirming evidence. Our students understood prior to this experiment the basic concept of a semiconductor band gap and could easily connect the increase in conductivity upon illumination with the band gap concept. Some had a more difficult time understanding the visible spectrum of a semiconductor, which does not show a peak but a cutoff band edge. Many did not think to include the separation of holes and electrons at the surface of an illuminated semiconductor and the degradation of methylene blue as illustrating the chemical properties of a semiconductor.

SUMMARY The primary pedagogic goal of the experiment is to reinforce semiconductor concepts where two teams collaborate to summarize their evidence that ZnO is a semiconductor. From photoelectrochemistry, students should observe the change in semiconductor current in the presence of UV light and the large difference in resistance of the sample with and without illumination. From the degradation of methylene blue, students should observe that both light and ZnO are needed, which implicates band gap absorption in the mechanism. Students also gain experience interpreting XRD data and methylene blue kinetic data. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00887.

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REFERENCES

(1) Ellis, A. B.; Nordell, K. J.; Kuech, T. F.; Stockman, S. A.; Lisensky, G. C.; Condren, S. M. LEDs: New Lamps for Old and a Paradigm for Ongoing Curriculum Modernization. J. Chem. Educ. 2001, 78 (8), 1033−1040. (2) Wagner, E. P. Investigating Bandgap Energies, Materials, and Design of Light-Emitting Diodes. J. Chem. Educ. 2016, 93 (7), 1289− 1298. (3) Grätzel, M. Photoelectrochemical cells. Nature 2001, 414 (6861), 338−344. (4) Pilkington Glass. NSG TEC http://www.pilkington.com/ products/bp/bybenefit/specialapplications/tecglass/ (accessed Mar 9, 2017). (5) Vayssieres, L.; Keis, K.; Lindquist, S.-E.; Hagfeldt, A. PurposeBuilt Anisotropic Metal Oxide Material: 3D Highly Oriented Microrod Array of ZnO. J. Phys. Chem. B 2001, 105 (17), 3350−3352. (6) Guo, M.; Diao, P.; Cai, S. Hydrothermal growth of well-aligned ZnO nanorod arrays: Dependence of morphology and alignment ordering upon preparing conditions. J. Solid State Chem. 2005, 178 (6), 1864−1873. (7) Guo, M.; Diao, P.; Cai, S. Hydrothermal growth of perpendicularly oriented ZnO nanorod array film and its photoelectrochemical properties. Appl. Surf. Sci. 2005, 249 (1−4), 71−75. (8) Ashfold, M. N. R.; Doherty, R. P.; Ndifor-Angwafor, N. G.; Riley, D. J.; Sun, Y. The kinetics of the hydrothermal growth of ZnO nanostructures. Thin Solid Films 2007, 515 (24), 8679−8683. (9) Hale, P. S.; Maddox, L. M.; Shapter, J. G.; Voelcker, N. H.; Ford, M. J.; Waclawik, E. R. Growth Kinetics and Modeling of ZnO Nanoparticles. J. Chem. Educ. 2005, 82 (5), 775−778. (10) Zheng, Z. Q.; Yao, J. D.; Wang, B.; Yang, G. W. Lightcontrolling, flexible and transparent ethanol gas sensor based on ZnO nanoparticles for wearable devices. Sci. Rep. 2015, 5, 11070. (11) Yu, J. C.; Chan, L. Y. L. Photocatalytic Degradation of a Gaseous Organic Pollutant. J. Chem. Educ. 1998, 75 (6), 750−751. (12) Tayade, R. J.; Natarajan, T. S.; Bajaj, H. C. Photocatalytic Degradation of Methylene Blue Dye Using Ultraviolet Light Emitting Diodes. Ind. Eng. Chem. Res. 2009, 48 (23), 10262−10267.

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Experimental protocol including student directions and laboratory questions (PDF, DOCX) Instructor notes and answer key (PDF, DOCX) Movie of the PEC experiment (MPG) Additional supporting figures (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

George Lisensky: 0000-0002-1000-406X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge partial support of this research by NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288); Beloit College Sanger Research and Honors Term programs for additional support; Jiaqi Luo for helping to collect kinetic data; and the 2012, 2013, 2014, and 2016 Beloit College nanochemistry classes and the 2013, 2014, 2015, and 2016 Chemistry Collaborations, Workshops and Communities of Scholars (cCWCS) workshops (DUE-1022895) for helping refine this experiment. 742


Properties of Semiconductors: Synthesis of Oriented ZnO for

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