Experimental Approach for Determining Semiconductor/liquid Junction Energetics by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

Summary

We present a method for the determination of the energy relations of semiconductor/liquid junctions, which are the basis for the successful operation of such renewable solar energy converting systems.

Abstract

Operando Ambient Pressure X-ray photoelectron spectroscopy (operando AP-XPS) investigation of semiconductor/liquid junctions provides quantitative understanding of the energy bands in these photoelectrochemical solar cells. Liquid junction photoelectrochemical cells allow a uniform contact between the light-absorbing semiconductor and its contacting electrolyte phase. Standard Ultra High Vacuum (UHV) based X-ray photoelectron spectroscopy (XPS) has been used to analyze the electronic energy band relations in solid-state photovoltaics. We demonstrate how operando AP-XPS may be used to determine these relationships for semiconductor/liquid systems. The use of "tender" X-ray synchrotron radiation produces photoelectrons with enough energy to escape through a thin electrolyte overlayer; these photoelectrons provide information regarding the chemical and electronic nature of the top ~10 nm of the electrode as well as of the electrolyte. The data can be analyzed to determine the energy relationship between the electronic energy bands in the semiconductor electrode and the redox levels in the solution. These relationships are critical to the operation of the photoelectrochemical cell and for understanding such processes as photoelectrode corrosion or passivation. Through the approach described herein, the major conditions for semiconductor-electrolyte contacts including accumulation, depletion, and Fermi-level pinning are observed, and the so-called flat-band energy can be determined.

Introduction

Semiconductor/liquid junctions have long been investigated due to their simplicity of construction and economical possibility of fuel generation ^1-4, with some such systems obtaining efficiencies over 17%.⁵ These systems operate based on the formation of a rectifying junction at the interface between the semiconductor electrode and the electrolyte. The energetics of semiconductor/liquid junctions are similar to those of a semiconductor/metal, Schottky, junction ³ where an electrolyte assumes the role of the metal. The semiconductor Fermi level, E_F, is the electrochemical potential of the electron in the semiconductor and is analogous to the chemical potential of an electron in solution. In a liquid junction cell the difference in the chemical potential of the electron between the two phases results in the transfer of charge from one phase to another at equilibrium. Since the ions in the electrolyte are free to move while the fixed charges in the semiconductor cannot, a space-charge (or depletion) region forms within the semiconductor with an accompanying electric field. This electric field shifts the Fermi level (electrochemical potential) of the semiconductor to be equal to the chemical potential of the electron in the solution ⁶. The resulting electric field in the semiconductor only exists close (~ 1 µm) to the solution interface and the energy of the electron levels in this region are viewed as being "bent" by the field. The "band bending" in the semiconductor space-charge region results in a barrier to current flow in one direction while allowing conduction in the opposite direction, producing a "rectifying junction". Under illumination, this electrical field in the near-surface region of the semiconductor can separate light-generated electrons and holes, such that the device can be operated in a manner analogous to a solid-state photovoltaic device. Figure 1 demonstrates these basic concepts.

Figure 1: Solid/liquid junction. Illustration showing the band diagram and charge carrier density for (a) flat-band, (b) accumulation, (c) depletion and (d) inversion of an n-type semiconductor/liquid junction with n_e the free electron concentration, n_h the free hole concentration and n_i the intrinsic carrier concentration. The width of space-charge region is show as an accumulation layer d_acc, a depletion layer d_dep or an inversion layer d_inv. For further discussion, see ²⁹. Abbreviations are as follows: CBM: Conduction Band Minimum; VBM: Valence Band Maximum; E_F: Fermi Energy; U: the applied potential with respect to flat band; U_FB the flat band potential; μ_​es​- : the chemical potential in the solution as described in reference ²³. Please click here to view a larger version of this figure.

X-ray photoelectron spectroscopy (XPS) is a widely-used technique for determining both chemical (i.e., oxidation) states and electronic effects such as energy band relations in solid materials. Because of the very small inelastic mean free path (IMFP) of photoelectrons in air, including IMFPs on the mm scale even at millibar pressures⁷, and in order to avoid changes of the probed surfaces during measurements, XPS generally has to be performed under ultra-high vacuum (UHV) conditions. Numerous reviews of the XPS technique have been written ^8-10. In XPS, typically, electrons from core levels of the constituent elements of the sample are ejected into the vacuum by the absorption of X-rays. Upon irradiation with X-rays of an energy hν, electrons are ejected from the sample having a kinetic energy E_Kvac with respect to the vacuum level E_VAC. Figure 2 shows (a) the general geometry of an XPS instrument, (b) a simulated XPS spectra of TiO₂ with core levels (CL), Auger lines and a measurement of the work function, and (c) the relation of photon energy to kinetic and binding energies. The conservation of energy requires

_hν = E_B + E_Kvac + φ (1)

where E_B is the binding energy of the photoelectron from the core level, and φ is the work function of the sample. E_B is referenced to the Fermi level of the sample, E_F. The position of E_F can be determined by measurement of the valence band maximum of a noble metal (i.e. Gold or Silver) and fitting the Fermi function when the photon energy is well known (i.e. Al Kα). Otherwise this procedure is used to calibrate the photon energy, i.e. at electron synchrotrons that produce X-rays of variable energy.

Figure 2: XPS Schematic. Illustration of the XPS method: (a) standard XPS geometry; (b) Simulated XPS spectra of TiO₂ with core levels (CL), Auger lines and work function measurement; (c) Energy band relations for TiO₂ and definitions of kinetic energies E_Kvac, binding energies E_B and work function φ. Please click here to view a larger version of this figure.

Recently, ambient-pressure XPS, AP-XPS, experiments have been made possible due to the construction of differentially pumped electrostatic lens equipped ambient-pressure XPS analyzer systems. One approach to doing XPS at a solid/liquid interface is to separate the vacuum and the solution with a thin membrane through which XPS is carried out{Kolmakov, 2011 #176}^11-13. This technique requires the use of extremely thin membranes of materials such as silicon or graphene, as opposed to allowing measurements on thicker semiconductor materials. While standard XPS is carried out under UHV (10^-9- 10^-11 Torr), in AP-XPS the sample is at tens of Torr pressure while the analyzer remains under HV/UHV conditions. The resulting large pressure difference is realized by multiple stages of differential pumping ^7,14. As a result, measurement conditions much closer to a normal working environment can be realized. Studies on gold oxidation ¹⁵, lithium-oxygen redox reactions ¹⁶, and catalytic reactions ¹⁷ have been carried out in such systems. Further development and refinement of the technique ¹⁸ has allowed use of an electrochemical cell as the sample with the ability to apply a potential difference between the working electrode and the solution in a three-electrode electrochemical cell, which we term operando AP-XPS. The surface of the working electrode under a thin meniscus of electrolyte is analyzed by the operando AP-XPS technique. Figure 3 shows (a) a general schematic of the endstation as well as (b-d) pictures of the various parts of the endstation and (e) the materials under investigation. As a result, the solid working electrode as well as the thin (~13 nm) electrolyte layer can be investigated simultaneously, provided that the photoelectrons have a sufficient kinetic energy to penetrate through the electrolyte overlayer and escape unscattered, i.e. without energy loss, to the analyzer/detector. The use of ~ 4 keV X-rays produces photoelectrons with sufficient kinetic energy (~3.5 keV for Ti 2p and O 1s core levels) to make this possible ¹⁸.

Figure 3: Operando AP-XPS setup. (a) Scheme of the operando XPS setup. The working electrode and the hemispherical electron energy analyzer (HEA) were grounded together. The potential of the working electrode was changed with respect to the reference electrode. The PEC-beaker containing the electrolyte could be lowered whereas the three-electrode mount could be moved in all three directions. (b) View into the high-pressure analysis chamber. The X-ray beam enters through the window on the left, the three-electrode setup is on the top, the electrolyte beaker on the bottom, and the electron analyzer cone is in the center. (c) Three-electrode setup pulled up and in measurement position (compare to (a)). (d) Photo of the actual "tender" X-Ray operando AP-XPS analyzer and the analysis chamber that is directly connected to the analyzer. (e) The energy band relations of the p⁺-Si/TiO₂/H₂O(l.)/H₂O(g.) system under applied potential U. The working electrode (Si) and analyzer are grounded. In the three-electrode configuration the Fermi energy is shifted by U with respect to the reference electrode. The definitions of kinetic energies E_Kvac, binding energies E_B, work function φ and the ionization energy of H₂O (g.) E_IE are given. For p⁺-Si/TiO₂/Ni/H₂O(l.)/H₂O(g.) electrodes, a thin film of Ni/NiO_x would also be present at the solid/liquid interface, and would influence band bending as discussed in the text. For a further analysis of the importance of the Ni/NiO_x film, please see ²⁷. Please click here to view a larger version of this figure.

We have recently demonstrated that the combination of atomic-layer deposition (ALD)-grown TiO₂ with a Ni catalyst can effectively stabilize a variety of semiconductors in alkaline media, including Si, GaP, GaAs ¹⁹, CdTe ²⁰, and BiVO₄²¹ against photocorrosion. This advancement enables the use of technologically advanced semiconductors for energy converting devices such as solar fuel generators. Further investigation of the working principles of TiO₂ in these systems was undertaken to evaluate the nature of the semiconductor/liquid junction in the presence or absence of Ni ^22-24. Direct observation of these junctions using the operando AP-XPS approach produces data which demonstrate the working principles (accumulation, Fermi level pinning, depletion, inversion) behind these systems. Furthermore, this approach provides a tool by which a semiconductor/liquid junction or photocatalyst^25,26 may be interrogated such that the fundamental operating characteristics may be understood and optimized. We describe herein the manner in which such investigations may be undertaken, the conditions that are required for these experiments to work, and the means by which the data collected may be understood. We describe, in sections 1-2, the preparation of the electrodes which were used in our experiments, before presenting more general directions (sections 3-5) regarding the collection of data using this approach.

Protocol

1. Preparation of Semiconductor for Analysis

1. Clean a p⁺, (100)-oriented boron doped Czochralski-grown Si wafer with a resistivity of ρ < 0.005 Ωcm. First soak for 2 min in a 3:1 (by volume) "piranha" solution of concentrated H₂SO₄ (98%) to 30% H₂O_2(aq).
2. Etch for 10 s in a 10% (by volume) solution of HF_(aq).
3. Immediately after step 1.2, etch in a 5:1:1 (by volume) solution of H₂O, 36% hydrochloric acid, and 30% hydrogen peroxide for 10 min at 75 °C before moving the sample into the ALD chamber.
4. Deposit the TiO₂ from a tetrakis(dimethylamido)titanium (IV) (TDMAT) precursor in an ALD reactor. Set the sample temperature to 150 °C.
5. Carry out the deposition beginning with a pulse of TDMAT for 0.1 s, followed by a purge for 15 s with N₂ at 20 sccm (with constant N₂ flow/purge over the complete deposition period).
6. Proceed with a 0.015 s pulse of H₂O before another 15 s purge with N₂. This completes one full ALD cycle. Repeat this process (1.4-1.5) for 1500 cycles to provide films ~ 70 nm in thickness.
7. Where desired, deposit Ni at a rate of ~ 2 nm per min by use of a RF sputtering power of 150 W for 20 s - 300 s in a sputtering system. Use Ar as the sputtering gas at a pressure of approximately 8.5 milliTorr.

Figure 4: ALD. (a) Illustration of one full ALD cycle for the growth of TiO₂ on Si/SiO₂. (b) Pressure variation in the ALD reactor during one cycle with times of precursor, oxygen, and purging pulses. Please click here to view a larger version of this figure.

2. Construction of Electrodes for the Endstation

1. Cut strips of the semiconductor sample into rectangles approximately 1 cm x 3.5 cm in size.
2. Using a scribe, scratch In/Ga eutectic into the back of the Si wafer to create an ohmic contact to the Si.
3. Cut pieces of 1 mm thick glass to approximately 0.8 cm x 4 cm in size as the support. Add 1-sided copper tape to the glass to cover it, with the sticky side of the copper tape on the glass as the back contact.
4. Add silver paint to the copper tape on the glass support and the back of the Si wafer. Push the back of the Si wafer to the copper tape on the glass support and let dry.
5. Use epoxy (stable in 1 M KOH) to encapsulate the edges and back of the sample such that only the front of the TiO₂/Si sample can be contacted by solution.
6. For the counter electrode, use a Pt or Ni foil.

3. Preparation of Electrolyte(s) and Materials for Beamline Experiments

1. Prepare (or purchase) all necessary electrolytes. For 1.0 M KOH, add 56 g of KOH (semiconductor grade, 99.99%) to 1.0 L of water (18.2 M cm at 25 °C resistivity) and let cool.
2. Use a pH meter to measure the pH of the resulting solution, and add KOH pellets or H₂O until true pH 14 solution is prepared.
3. Add excess electrolyte (100 mL) to a clean beaker and place under vacuum to degas the solution. Also prepare a beaker of pure water (18.2 M cm at 25 °C resistivity), degassed in a similar manner. A properly degassed electrolyte will remain at a constant vapor pressure under static vacuum.
4. Clean the beaker that will be used to hold the electrolyte by immersing in aqua regia. Prepare aqua regia by adding 1 part (15 mL) nitric acid (concentrated) to 3 parts (45 mL) 36% hydrochloric acid. Clean the beaker by washing with copious amounts of water (18.2 M cm at 25 °C resistivity).

4. Photoelectron Spectroscopy Energy Calibration

1. Mount a gold foil onto the sample holder arm in the working electrode slot, and test contact to the arm with a multimeter.
2. Insert the sample holder arm into the endstation with a copper gasket between the endstation and sampler holder arm flange. Attach the sample holder arm to the endstation through a copper gasket.
3. Open valve to vacuum for the sample chamber to pull vacuum on this chamber. Once pressure has decreased below 20 Torr, open the analyzer cone by removing the wobble stick.
4. Lower the sample using z-axis controls into sampling height. Turn on the detector; focus sample in x- and y- axes by measuring count rate for Au 4f photoelectrons; a maximum (ideally > 200,000 counts per second) indicates a focused spot.
5. Collect XPS data (by selecting the appropriate core level and starting the scan from within the software) for the Au 4f core level, and record the peak positions. Calibrated binding energy for the metallic Au 4f^7/2 core level is 84.0 eV. For a more detailed description please see section 5.6.
6. Collect XPS data for the Au Fermi edge (near zero binding energy). Use as a secondary calibration if necessary.
7. Turn off the detector; back the sample away from the detector cone and raise the sample before placing wobble stick on the detector cone to isolate the detector. Flood the chamber with N₂ to remove the vacuum. Remove the sample holder arm.

5. Photoemission Measurement and Data collection

1. Mount working electrode (semiconductor sample), a Pt foil counter electrode, and a leakless Ag/AgCl reference electrode, to the sample holder arm. Mount the working electrode to face the collection cone. Rinse with water to ensure removal of any KCl or dust. Ensure that the liquid nitrogen in the vacuum traps (which condenses evaporated electrolyte) is full, and once full, refill within every 2 hours.
2. Attach sample holder arm to the endstation. Place the electrolyte beaker on the beaker holder platform and fill with degassed electrolyte; place degassed water beaker inside chamber as well (as a sacrificial electrolyte). Ground the working electrode metallic outside contact on the sample holder arm to the instrument; attach the working, counter, and reference electrode leads to their respective leads from the potentiostat.
3. Slowly open the valve to apply vacuum until a stable vacuum near 15 Torr is reached.
4. Remove wobble stick and lower the electrodes into the beaker (z-axis) (Figure 5a), without approaching the detector cone (x-axis). Record the height of the detector cone opening, as well as the height of the top of the electrolyte, in terms of the z-axis position.
   1. If necessary, take preliminary cyclic voltammogram (CV) measurements or undertake oxidation reactions to ensure a hydrophilic surface. Ensure that the potentials under which the experiment will be run do not cause substantial bubbles to approach the detector cone.²⁷
5. Based on the CV data, choose an initial potential and set this potential. Turn on the detector. Retract by approximately the difference in height between the cone opening and the electrolyte surface level and focus in the x and y positions (Figure 5b) by analyzing the count rate of the sample material, not the O 1s count rate. Collect sample and water XPS data and adjust x, y, and z values until a spot is found which contains both liquid water, near 536 eV binding energy, and the sample core levels (here, Ti 2p and Ni 2p) in the XPS data.
   ​NOTE: Preferably, choose a potential range in which the hydrophilic nature of the electrode surface can be maintained based on the CV data that will produce X-ray data which will provide information about the electrochemical nature of the system.
6. Collect all necessary XPS data at this potential (Figure 5c), including sample core levels such as Ti 2p and Ni 2p, as well as O 1s data, by selecting these core level areas within the detection software and performing data collection.
   1. Generally, open the scan selection window within the software by selecting "setup" from the "run" pull-down menu. On the setup page, select a scan and click "edit" to to modify the energy parameters of the scan. Alternateively, enter a new scan profile, by selecting "new,". Select a "check mark" next to a scan to select the scan to be carried out, and click "start" to start the acquisition.
   2. Scan until a sufficient signal-to-noise ratio is observed in the data; this may take between 15 min and multiple hours.
7. Retract the sample, in the x-axis, from the detector cone. Lower the sample by the aforementioned electrolyte-cone vertical distance and set a new potential; then, retract by the same distance, focus, and repeat the XPS data collection procedure as described in 5.5-5.6.
8. When data collection is complete, turn off the detector, retract the sample in the x and z directions and place the wobble stick onto the detector cone. Flood the chamber with N₂ to bring the chamber back to atmospheric pressure and remove the sample holder arm; remove the electrodes from the arm and the liquids from the sampling chamber.

Figure 5: Operando AP-XPS data acquisition. (a) Sample is dipped into the electrolyte, CVs are recorded and the potential U is set. (b) The sample is pulled up and placed in measurement position (while maintaining electrical contact of all three electrodes with the electrolyte). (c) Beam shutter is opened and the measurement spot is illuminated by X-rays. Sample position is corrected, if necessary, and core level spectra are recorded. Please click here to view a larger version of this figure.

Results

Representative results are shown in Figures 6, 7, and 8. Figure 6 shows the collected O 1s and Ti 2p core level spectra for a TiO₂ electrolyte in 1.0 M KOH solution, stacked with respect to the applied potential. Figure 7 shows the plotted core level water O 1s and Ti 2p peak positions, as collected from Figure 6 as well as from data in which a TiO₂/Ni/electrolyte sample was investigated in the same electrolyte. Figure 8 shows a brief summary of our conclusions from ...

Discussion

The most critical steps in the technique for data collection are the application of voltage and the collection of the XPS data. The semiconductor preparation is necessarily crucial but can be generalized to any system where the semiconductor/liquid junction is stable enough to be investigated. However, for the choice of electrolyte, a number of experimental parameters must be considered. First, there must be sufficient interaction (hydrophilic or hydrophobic) between the solid electrode and the electrolyte in order to fo...

Materials

Name	Company	Catalog Number	Comments
p+-Si(100)	Addison	3P-111	Resistivity < 0.005 Ω - cm
H2SO4	Sigma Aldrich	339741	99.999%
H2O2	Sigma Aldrich	216763	30%
HF	Sigma Aldrich	339261	99.99%
millipore H2O	EMDMillipore	Milli-Q® Advantage A10	18.2 MΩ
HCl	Sigma Aldrich	320331	ACS Reagent, 37%
Tetrakis(dimethylamido)titanium(IV) (TDMAT)	Sigma Aldrich	469858	99.999%
N2	Praxair	NI 6.0 RS	>99.9999%
Ni target	AJA International	7440-02-0	>99.99%
In/Ga	Sigma Aldrich	495425	>99.99%
Hysol 9460	Ellsworth Adhesives	83128	Dual cartridge
KOH	Sigma Aldrich	306568	Semiconductor grade, 99.99%
Liquid Nitrogen	Praxair	NI 5.0
Gold foil	Sigma Aldrich	326496	99.99%
HNO3	Sigma Aldrich	438073	ACS Reagent, 70%
1-sided copper tape	adafruit	1128	For electrode construction
glass microscope slides	VWR	48300-025	For electrode construction
Ag/AgCl reference electrode	eDaq	ET072-1
Platinum foil	Sigma Aldrich	349348	99.99%
SP-300 Biologic Potentiostat	Biologic	SP-300
Scienta r4000 HiPP-2 Detector APPES	Scienta	HiPP-2