Sample characterization

Spectroscopic ellipsometry

Spectroscopic ellipsometry measurements were performed on a J.A. Woollam Co., M-2000X, 210 – 1000nm ellipsometer, both in-situ (i.e. mounted to the ALD reactor) and ex-situ. Ex-situ measurements were performed under three angles θ, 70°, 65° and 60°. In-situ measurements were performed under a fixed angle of 68°. Optical models rely on Tauc-Lorentz oscillators for Bi2O3, MnOx and Mn2V2O7 and Cauchy models for SiO2, Al2O3, the Bi-thd surface layer and VOx. The CompleteEASE software of Woollam was used to define and fit all optical models.

39 X-ray photoelectron spectroscopy

XPS measurements were executed using a SPECS PHOIBOS 100 hemispherical analyzer and a monochromatic X-ray source (SPECS FOCUS 500 monochromator, Al Kα radiation, 1486.74 eV). XPS survey and fine spectra were collected at a normal angle from the surface.

The pass energy was set to 30 and 10 eV for survey and fine spectra, with step sizes of 0.5 and 0.05 eV, respectively. The atomic sensitivity factor are 24.47, 15.23, 10.23, 3.1, and 1.0 for Bi 4f, Mn 2p, V 2p, O 1s, and C 1s, respectively.

All samples were exposed to air prior to analysis. The peak originating from adventitious carbon at the known binding energy of 284.8 eV was used to calibrate the energy scale of the spectra, if not explicitly stated otherwise. Shirley background subtraction was used to fit the photoemission lines in the fine spectra. The base pressure of the system was ~ 10^-9 mbar and the load lock was pumped until at least pressures below 3x10^-7 mbar were achieved.

UVVis spectroscopy

UVVis measurements were conducted using a PerkinElmer Lambda 950 spectro-photometer with an integrating sphere. The samples were placed inside the integrating sphere with an offset of ~7.5° from the incident light, and the transflectance (TR, i.e., transmittance T + reflectance R) was measured.

X-ray diffraction

XRD measurements were conducted under grazing incident conditions (0.5°) on a Panalytical X’Pert Pro MPD using Cu Kα radiation (λ = 1.5406 Å) with an acceleration voltage of 40 kV and an X-ray current of 40 mA. Scans were performed from 10 – 80° (Bi2O3 and CuBi2O4) or 10 – 70° (Mn2V2O7) with a step width of 0.04°. Measurement times varied from 5 – 60 h, depending on the sample thickness.

Scanning electron microscopy and Energy-dispersive X-ray spectroscopy Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) measurements were performed on a LEO GEMINI 1530 scanning electron microscope with an acceleration voltage of 10 kV. Samples deposited on FTO substrates with electronic contact to the instrument were used to minimize charging effects. One sample was investigated by RBS (see below) as an internal reference for the EDX measurements to determine stoichiometries.

Time-resolved microwave conductivity

TRMC measurements were performed using a wavelength-tunable laser (NT230-50-SH/SF, EKSPLA) at 410 nm as excitation source with a pulse length of 7 ns. The X-band (8400 – 8700 MHz) microwave probe was generated by a voltage-controlled oscillator (Sivers IMA VO3262X).

40 Photoelectrochemical measurements

Measurements were performed in three-electrode setup using a Ag/AgCl reference electrode (E⁰Ag/AgCl = 0.199 V vs normal hydrogen electrode, XR300 in KClaq,sat, Radiometer Analytical), a platinum wire as counter electrode and an EG&G PAR 273A potentiostat. The illuminated area was defined by a rubber sealing ring to 0.2376 cm².

AM 1.5 illumination was generated by a solar simulator (WACOM, type WXS-50S-5H, class AAA). If not stated otherwise chopped light was used in frontside or backside illumination as mentioned in the respective chapters.

IPCE measurements used a Xe lamp (LSH302, LOT) with a monochromator (Acton Research Spectra Pro 2155). The incident light intensity was measured by a calibrated photodiode (PD300R-UV, Ophir) behind a PEC cell with a bare FTO coated glass substrate without electrolyte or quartz window to normalize for backside illumination through the substrate.

Measurements of Mn2V2O7 used the following electrolytes: 1) 0.1M K2B2O7 solution adjusted to pH 9.2 with KOH and 0.5M Na2SO3 as hole scavenger; 2) 0.1M K2B2O7 solution adjusted to pH 9.2 with KOH and 5x10^-2M [Fe(CN)6]^3-/4- as hole scavenger (from potassium salts); 3) 0.1M KOH (pH 13) and 5x10^-2M [Fe(CN)6]^3-/4- as hole scavenger (from potassium salts).

Measurements of CuBi2O4 were executed in 0.1M Na2HPO4/NaH2PO4 buffer (pH ~7 phosphate buffer) and 6% H2O2 as electron scavenger (30%, stabilized).

Rutherford Backscattering Spectrometry

Rutherford backscattering spectrometry (RBS) was performed at the Ion Beam Facility (IBF) at DIFFER. The measurements used a 2.0 MeV ⁴He ion beam at a scatter angle of 163°. Data was analyzed using the simnra 7.01 and multisimnra software.

Mass spectrometry

Mass spectrometry (MS) analysis was performed using a Netzsch thermobalance STA409C coupled with a mass spectrometer to detect the decomposition of various components of the sample during annealing. The Mn2V2O7 powder was placed in an Al2O3 crucible and heated with a heating rate of 10 K/min. The sample was kept under a constant argon flow of 80 mL/min to carry the vaporized sample to the mass spectrometer. Electron ionization with a tungsten filament was used to generate positive ions. Each mass/charge ratio was measured for 2 s.

Transmission electron microscopy

Cross-sectional transmission electron microscopy (TEM) images were recorded with a Philips CM12/STEM equipped with a LaB6 cathode, Super TWIN lens and a 2k x 2k CCD camera from Gatan (Orius SC 830) operated at 120 kV. Bi2O3 films deposited on a [100] n-type silicon substrate are cut into small pieces and glued face to face using epoxy resin. This

41

compound is sliced and polished till a sample thickness of 4 to 6 µm is reached. The sample is further thinned by argon bombardment (5kV, 2mA) till it becomes transparent for electrons.

Atomic force microscopy

Topographical measurements were conducted on an NTMDT NTEGRA Atomic force microscopy (AFM) in tapping mode at room temperature and ambient pressure.

Raman spectroscopy

Raman measurements were conducted on a Horiba HR800 spectrometer with a HeNe laser as a monochromatic light source. The HeNe laser is a 500:1 polarized 20 mW laser with a wavelength of 632.8 nm. The scattered light was detected with a cooled Si CCD camera with 1800 l/mm grid.

42

3 ALD of Bi

O

Parts of this chapter are adapted from:

M. Müller, K. Komander, C. Höhn, R. van de Krol, A. C. Bronneberg, ”Growth of Bi2O3 Films by Thermal- and Plasma-Enhanced Atomic Layer Deposition Monitored with Real-Time Spectroscopic Ellipsometry for Photocatalytic Water Splitting”, ACS Appl. Nano Mater.

2019, 2, 10, 6277 ¹¹⁵

Bismuth containing ternary oxides such as BiVO4, CuBi2O4, BiMn2O5, Bi2WO6 or BiFeO3

attracted much attention as photoabsorbers for PEC applications.^55,116–118 Among these BiVO4

and CuBi2O4 are the most thorough studied ones. However, ALD of such ternary oxides has been shown to be challenging. ALD of photoactive BiVO4 is a successful example, as reported by Stefik et al.^88,89 Even the previously described optimization from planar to nanostructured geometries has recently been reported by Lamm et al.¹¹⁹ The main challenge in these reports is the limited reactivity of the bismuth precursor [BiPh3]. Only one monolayer of bismuth oxide can be grown on a heterosurface, i.e. on V2O5, in a thermal process.⁸⁸ The binary ALD process of Bi2O3 stops after a closed Bi2O3 film is deposited. Therefore the stoichiometry of a ternary film can only be tuned to some extend by adjustment of the cycle ratio. As a consequence only vanadium rich BiVO4 films can be grown.⁸⁸ A phase pure BiVO4 film can only be achieved if a selective subsequent V2O5 etching or an inhibition of the V2O5 ALD growth by alcohols is employed.^88,89

Therefore, these reports only provide useful information on one specific ALD process of BiVO4. This process cannot be easily extended towards new ALD processes of other bismuth-based materials. A universal binary ALD process of Bi2O3 is therefore still a bottleneck limiting the deposition of bismuth based multinary oxides. This bottleneck will be addressed in this chapter.

A few studies on ALD of Bi2O3 exist, even a publication on various precursors is available.^64,120–124 Unfortunately only two of these precursors are commercially available, which is an essential prerequisite for a universally usable ALD process. These are the above mentioned [BiPh3] and [Bi(tmhd)3]. As binary ALD of Bi2O3 from [BiPh3] was not yet shown and the precursor is only available in the USA, [Bi(tmhd)3] was chosen in this study and its structure is shown in Figure 17.

Figure 17: Chemical structure of [Bi(tmhd)3] or tris(2,2,6,6-tetramethyl-3,5-heptanedionato)bismuth(III).

43