Synthesis

larger corundum boat containing varying amounts (typically ~2 g) of polyvinylidene difluoride (PVDF, Aldrich). The nested boats were then placed inside a corundum tube with one closed end and placed in a horizontal tube furnace with a low nitrogen gas flow of 5 l/h. The temperature was set to 380 °C for 24 h and after a final heating step in air at 450 °C for 30 min an orange powder was obtained. Note: For comparability the pristine BiVO4 passed the same calcination procedure solely leaving out the PVDF.

6.2.1.3. Electrode fabrication out of BiVO4 and F:BiVO4 powders

An electrophoretic deposition method (EPD) based on a procedure Abe et al. initially reported for TaON was used to fabricate electrodes for photoelectrochemical investigations.^[96] In a first step, the pristine as well as the F-containing powders were treated for 15 min in a mixer mill (Retsch GmbH, MM301) to provide a small particle sizes (verified by SEM-investigations). 80 mg of the respective BiVO4 powder were dispersed in iodine-containing acetone (total volume 50 ml, I2 content 2 mg/ml) by 20 min of ultrasonication (Emmi-20 HC, 27 kHz) at room temperature. Two fluorine-doped tin oxide coated glass slides (FTO) were immersed parallel in the dispersion with 1.0 cm of distance.

Particles were deposited by application of a potential of 10 V for 5 min. This procedure was applied twice in total. After electrophoretic deposition, the as-prepared electrodes were calcined at 450 °C or 550 °C for 30 min under air to interconnect the deposited particles by sintering.

6.2.2. Synthesis of BiVO4 and Mo-doped BiVO4 thin films

6.2.2.1. Synthesis of BiVO4 thin films

A solution of VO(OEt)3 (123 µl, 0.7 mmol, Aldrich) in 1.5 ml CHCl3 (>99 %, dry, Aldrich) was prepared. After stirring for 10 min, the solution was added to Bi(2-ethyl-hexanoate)3 (445 mg, 0.7 mmol, AlfaAesar) and stirred for 4 h. The obtained red solution was used for dip coating various substrates (Si or FTO-coated glass slides) under controlled conditions (<30 % relative humidity, 30 °C, 300 mm/min withdrawal speed, 2 s dwell time). After solvent evaporation, all deposited films were aged at 100 °C for 12 h followed by calcination at 450 °C for 2 h in air (heating ramp: 0.5 K/min).

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6.2.2.2. Synthesis of Mo:BiVO4 thin films

A solution of VO(OEt)3 (123 µl, 0.7 mmol, Aldrich) in 1.5 ml CHCl3 (>99 %, dry, Aldrich) was prepared. Amounts of 11.3 mg (0.035 mmol), 22.6 mg (0.070 mmol), 33.9 mg (0.105 mmol) or 45.2 mg (0.140 mmol) of MoO2(acac)2 which correspond to a Mo-content with respect to the Bi content of 5, 10, 15 and 20 at% in the resulting Mo:BiVO4-solution, respectively, were added (see Table 6-1 for an overview of different synthesis solution compositions). After stirring for 10 min, the solution was added to Bi(2-ethyl-hexanoate)3 (445 mg, 0.7 mmol, AlfaAesar) and stirred for 4 h. The obtained red solution was used for dip coating Silicon wafers or FTO-coated glass slides under controlled conditions (< 30 % relative humidity, 30 °C, 300 mm/min withdrawal speed, 2 s dwell time). After solvent evaporation all deposited films were aged at 100 °C for 12 h followed by calcination at 450 °C for 2 h in air (heating ramp: 0.5 K/min).

Table 6-1. Composition of the precursor solution used for the synthesis of X% Mo:BiVO4 thin film photoanodes.

sample Bi(-OHex-2-Et)3 VO(OEt)3 MoO2(acac)2

0% Mo:BiVO4 445 mg /

0.70 mmol

140 mg / 123 µl /

0.70 mmol -

5% Mo:BiVO4

445 mg / 0.70 mmol

140 mg / 123 µl / 0.70 mmol

11.3 mg / 0.035 mmol 10% Mo:BiVO4 445 mg /

0.70 mmol

140 mg / 123 µl / 0.70 mmol

22.6 mg / 0.070 mmol 15% Mo:BiVO4

445 mg / 0.70 mmol

140 mg / 123 µl / 0.70 mmol

33.9 mg / 0.105 mmol 20% Mo:BiVO4 445 mg /

0.70 mmol

140 mg / 123 µl / 0.70 mmol

45.2 mg / 0.140 mmol

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6.2.3. Variation of Synthesis parameters

6.2.3.1. Synthesis of BiVO4 and Mo-doped BiVO4 thin films with worm-like morphology A solution of 123 µl (0.7 mmol) of VO(OEt)3 in 1.5 ml CHCl3 containing 112.5 mg of BASF Pluronic© F127 (ethylene oxide- block -propylene oxide- block -ethylene oxide, (PEO)101-(PPO) 56-(PEO)101) corresponding to 50 w% of BiVO4 in solution was prepared.

For Mo-doped BiVO4 thin films amounts of 11.3 mg (0.036 mmol), 22.6 mg (0.07 mmol), 33.9 mg (0.10 mmol) or 45.2 mg (0.14 mmol) of MoO2(acac)2 which correspond to a Mo-content of 5, 10, 15 and 20 at% with respect to the Bi Mo-content in the resulting Mo:BiVO4 -solution, respectively, were added. After stirring for 10 min, the solution was added to Bi(2-ethyl-hexanoate)3 (445 mg, 0.7 mmol) and stirred for 4 h. The obtained red solution was used for dip coating Silicon wafers or FTO-coated glass slides under controlled conditions (<30 % relative humidity, 30 °C, 300 mm/min withdrawal speed, 2 s dwell time). After solvent evaporation all deposited films were aged at 100 °C for 12 h followed by calcination at 450 °C for 2 h in air (heating ramp: 0.5 K/min).

6.2.3.2. Variation of the calcination procedure

For the variation of the calcination procedure 10% Mo:BiVO4 thin films were prepared. A solution of VO(OEt)3 (123 µl, 0.7 mmol) in 1.5 ml CHCl3 (>99 %, dry) was prepared.

MoO2(acac)2 (22.6 mg, 0.07 mmol) corresponding to 10% Mo with respect to the Bi content within the synthesis solution was added to the solution. After stirring for 10 min, the solution was added to Bi(2-ethyl-hexanoate)3 (445 mg, 0.7 mmol) and stirred for 4 h.

The obtained red solution was used for dip coating FTO-coated glass slides under controlled conditions (< 30 % relative humidity, 30 °C, 300 mm/min withdrawal speed, 2 s dwell time). After solvent evaporation all deposited films were aged at 100 °C for 12 h in air. Final calcination temperatures were varied from 350 °C to 550 °C in increments of 50 K. Note: The heating ramp was kept constant at 0.5 K/min for all final calcination times.

6.2.3.3. Variation of CoPi deposition methods

For all CoPi depositions 10% Mo:BiVO4 thin films prepared according to the method described in chapter 6.2.2.2 were used.

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6.2.3.3.1. Electrodeposition of CoPi

An electrochemical cell equipped with a RHE reference electrode, a Pt counter electrode and a 10% Mo:BiVO4 thin film as working electrode described in chapter 6.4 (vide infra) was used for CoPi electrodeposition. The cell was filled with potassium phosphate solution (20 ml, 0.1 mol/l, Carl Roth) containing Co(NO3)2. (0.5 mmol/l, Aldrich). The deposition was carried out at an applied potential of 1.9 V vs RHE in the dark without stirring and without IR-correction and was stopped after a certain amount of charge was transferred.

6.2.3.3.2. Photodeposition of CoPi

The electrochemical cell equipped with RHE reference electrode, Pt counter electrode and the 10% Mo:BiVO4 thin film as working electrode described in chapter 6.4 (vide infra) was used for CoPi electrodeposition. The cell was filled with potassium phosphate solution (20 ml, 0.1 mol/l, Carl Roth) containing Co(NO3)2. (0.5 mmol/l, Aldrich). Without application of any bias potential the working electrode was irradiated by white light (400-700 nm) of an intensity of 100 mW/cm² in frontside mode (frontside PD) or in backside mode (backside PD). The deposition was carried out without stirring. By variation of the irradiation time between 20 min and 40 min the amounts of deposited CoPi was varied.

6.2.3.3.3. Photo-assisted electrodeposition

A slightly modified PED method based on methods reported by Gamelin and Grätzel^[101] as well as Ma and Durrant^[102] was performed to functionalize 10% Mo:BiVO4 photoanodes by CoPi. The electrochemical cell equipped with a RHE reference electrode, a Pt counter electrode and a 10% Mo:BiVO4 thin film as working electrode described in chapter 6.4 (vide infra) was used for CoPi electrodeposition. The cell was filled with potassium phosphate solution (20 ml, 0.1 mol/l, Carl Roth) containing Co(NO3)2. (0.5 mmol/l, Aldrich). The working electrode was irradiated by white light (400-700 nm) of an intensity of 100 mW/cm² in frontside illumination mode. At the same time a bias potential of 1.2 V vs RHE was applied. The deposition was carried out without stirring and without IR-correction and was stopped after a certain amount of charge was transferred.

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6.2.4. Fluorination of BiVO4 and 10% Mo:BiVO4 thin films

BiVO4 and 10% Mo:BiVO4 thin films on FTO-coated glass slides were prepared by the methods elucidated in chapter 6.2.2.1 and 6.2.2.2. The thin films were fluorinated by the solid-vapor reaction using the indirect gas flow method described in chapter 6.2.1.2 (Figure 6-1). A maximum of samples were fluorinated at once. The amounts of PVDF were varied from 1-3 g in increments of 1 g, the temperature was varied from 350 °C to 400 °C in increments of 10 K. A final heat treatment at 450 °C for 30 min in air was applied as finishing step.

Figure 6-1. Scheme of the tube furnace used for fluorination of BiVO4 and Mo:BiVO4 thin films.

6.2.5. Synthesis of WO3/BiVO4 heterojunctions 6.2.5.1. Synthesis of WO3 thin films

To fabricate WO3 thin films on FTO substrates an adjusted sol-gel-approach based on the method reported by Brezesinski et al.^[103] was used. WCl6 (385 mg, 1 mmol, AlfaAesar) was dissolved in ethanol (1.5 ml, 98%, Carl Roth). After stirring for 4 h, the deep blue sol was used for dip coating of FTO substrates under controlled conditions (200 mm/min, 55% relative humidity, room temperature, 2 s dwell time). After evaporation of the solvent, as-coated thin films were aged at 100 °C for 12 h and subsequently calcined at 550 °C for 30 min in air (heating ramp: 0.5 K/min) resulting in colorless, transparent thin films.

6.2.5.2. Synthesis of WO3/BiVO4 and WO3/Mo:BiVO4 thin film heterojunctions

The synthesis of BiVO4 and Mo:BiVO4 thin films were performed according to chapter 6.2.2.1 and 6.2.2.2. Instead of FTO-coated glass slides WO3 thin films deposited on FTO-coated glass slides were used as substrates.

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6.2.5.3. Optimized synthesis of WO3 thin films

A solution of 4.5 mg BASF Pluronic^© F127 in 1.5 ml ethanol (1.5 ml, 98%) was prepared.

Afer 10 min of ultrasonication (27 kHz, room temperature), WCl6 (385 mg, 1 mmol) was added to the clear, colorless solution. After stirring for 4 h, the blue sol was used for dip coating FTO substrates under controlled conditions (100 mm/min, relative humidity 25 %, 30 °C, 8 s dwell time). After evaporation of the solvent the as-coated thin films were aged at 100 °C for 12 h and subsequently calcined at 550 °C for 30 min in air (heating ramp:

0.5 K/min) resulting in colorless, transparent thin films.

6.2.5.4. Optimized Synthesis of WO3/BiVO4 and WO3/Mo:BiVO4 thin film heterojunctions The optimized synthesis of WO3/BiVO4 and WO3/Mo:BiVO4 thin film heterojunctions was performed according to the method described in chapter 6.2.3.1. Instead of 112.5 mg F127 only 3.0 mg were used. Optimized WO3 thin films on FTO substrates were used as substrates.