Applying ultrafast transient absorption spectroscopy on photocharged BiVO 4

Transcription

1 Applying ultrafast transient absorption spectroscopy on photocharged BiVO 4 Patrick Kwee, Vrije Universiteit Amsterdam June 27, Abstract Bismuth vanadate is a promising semiconductor which can be used for photo-electrochemical water splitting where water is directly converted into hydrogen which is a fuel. It is important that the conversion of water into hydrogen is performed with a high efficiency. In this study ultra-fast transient absorption spectroscopy (TAS) is used on a time scale from 100 fs to 100 µs to investigate the properties of four similar bismuth vanadate samples (BiVO 4 ) before and after exposing them to a solar simulator. In earlier research, it is namely found that a so called photocharging treatment where samples are exposed to a solar simulator during a night, would improve the water splitting performance of the samples. In this study we try to find how photocharging affects the photophysical processes which occur on BiVO 4 samples. It was proposed in the earlier research that photocharging changes the surface or the bulk properties of the samples. However we found no differences between photocharged and untreated samples. This indicates that if there is a photocharged effect, it occurs on time scales longer than 100 µs or they occur on the surface. This is because surface contributions cannot be measured using our method. Another possibility is that the photocharging effect fades within 20 minutes, because this is the minimal time necessary to remove a sample from a solar simulator and to start a measurement. 1

2 Contents 1 Abstract 1 2 Introduction 3 3 Theory Photo-electrochemical water splitting The reactions in water splitting set-up Characteristics of BiVO Ultra-fast transient absorption spectroscopy A features for a semiconductor Interpretation of processes on BiVO 4 samples using TAS Photocharging treatment Hypothesis Method Measurement procedure Set-up Results Comparison with data Ravensbergen et al Comparing photocharged and untreated samples Discussion 29 7 Conclusion 30 8 Appendix Comparing the different scans of the same measurement Sample Sample Sample Sample Time trace at 440 nm for all samples Comparing samples 1 and Comparing samples 3 and

3 2 Introduction Photo-electrochemical (PEC) water splitting is a technique which allows to convert sunlight into hydrogen (H 2 ). Hydrogen can be directly used as fuel and has the potential to replace fossil fuels which are responsible for climate change. To perform water splitting, a light absorbing semiconductor is necessary. In this study, the n-type semiconductor bismuth vanadate (BiVO 4 ) is investigated. This is a promising material which can absorb visible light, is non-toxic and can be produced on a large scale as it is composed of earth-abundant materials[1]. Trześniewski et al.[1] claimed that applying a treatment called photocharging on BiVO 4 samples would improve the performance of solar water splitting compared to untreated samples. Photocharging is a treatment technique where sunlight (simulated by a solar simulator) is used to illuminate the BiVO 4 samples during a night with AM 1.5 light. AM 1.5 represents the solar intensity when the sunlight falls on earth in an angle of 48.2 degrees. In this study, ultra-fast transient absorption spectroscopy[5] is used to investigate the photophysical processes of both photocharged and the untreated samples. Ravensbergen et al.[4] used this technique to unravel the dynamics of the charge carriers in bare BiVO 4 samples. They found four main occurring processes with a corresponding time constant. Hole trapping is associated with the time constant of 5 ps, electron relaxation with the time constant of 40 ps, electron trapping with the time constant of 2.5 ns and trap-limited electron-hole recombination occurs on time scales longer than 10 ns. The aim of this study is to find a possible explanation for the improved performance due to photocharging. It is possible that some processes such as hole trapping or recombination are affected by photocharging. 3 Theory 3.1 Photo-electrochemical water splitting Solid materials consist of a valence band and a conduction band. These are energy levels which determine how well electrons can be transported and thus how well a material conducts. In a metal the bands overlap and in an insulator there is a large energy gap between the bands. In a metal, electrons can move freely which makes it a good conductor while an insulator does not conduct well, as electrons need a lot of energy to overcome the energy gap between the valence and conduction band. Between the metal and the insulator stands the semiconductor, which has a smaller band gap than the insulator. If sufficient energy is absorbed, electrons can be excited to the conduction band which makes a material conduct. There is a so called Fermi energy level[2], which is the energy level at which the probability of being occupied by an electron is 1/2. A n-type semiconductor is donor doped, its Fermi level is closer to the conduction band than to the valence band, because its conduction band contains a large presence of electrons. For photo-electrochemical (PEC) water splitting, a n-type semiconductor is used as photoanode. Other components necessary for PEC water splitting are a metal counter electrode and an electrolyte[2]. All components are immersed in water as shown in Figure 2. The electrolyte is an aqueous solution which can be a base, an acid or a salt. The metal counter electrode accepts electrons from the semiconductor to form hydrogen molecules and is called the cathode. 3

4 Before electrons can be transferred to the metal counter electrode, a different reaction which is independent of light has to occur. This is a charge transfer reaction where the electrolyte accepts electrons from the semiconductor until the Fermi level of the semiconductor equals the energy level of the electrolyte and this is illustrated in Figure 1. Figure 1: The light independent charge transfer reaction is shown here[2]. In (a), the semiconductor and the electrolyte are isolated. In (b) they are contacted and the Fermi level of the semiconductor becomes equal to the redox level of the electrolyte which bends the valence and conduction bands upward. The valence band and the conduction band at the interface of the electrolyte and the n-type semiconductor are bent upward during the process until an electrochemical equilibrium is reached. A result of this equilibrium is that a space charge layer is formed in the n-type semiconductor which separates the electrons and holes. The holes move towards the interface of the electrolyte and the semiconductor, while the electrons move in opposite direction where they can be transferred to the metal counter electrode by an external circuit. The reason why the n-type semiconductor is used, is thus because the space charge layer separates the holes and the electrons[6] The reactions in water splitting set-up The energy which is required to excite electrons in the n-type semiconductor, is provided by photons. Depending on the band gap, energy is absorbed in the visible region, which is between 400 and 800 nm, or in the UV region. Light in the IR region has not enough energy to split water, because for water splitting at least 1.23 ev is necessary if losses are not taken into account. The excited electrons are transferred to a metal counter electrode, and at the interface of this counter electrode and the electrolyte, the reduction reaction takes place. At the interface of the electrolyte and the semiconductor oxidation reaction occur because here are holes (h + ) which are positively charged. Four of these holes react with four hydroxide ions (OH ), if the electrolyte is an alkaline electrolyte, and the end products of this reaction are two water molecules (H 2 O) and one oxygen molecule (O 2 ) as shown in the following equation: 4OH + 4h + 2H 2 O + O 2. (1) The actual production of hydrogen (H 2 ) takes place at the counter electrode where 4

5 four electrons and four water molecules are converted into 2H 2 molecules as shown in the following equation: 4H 2 O + 4e 2H 2 + 4OH. (2) Water splitting is in competition with another process called recombination. This is the recombination of an electron and a hole and if this happens, the electron can no longer be transferred. The holes have to be mobile as well because they move to the surface to drive catalysis. To increase the efficiency of water splitting, the recombination time should be as long as possible or at least longer than the time necessary to split water. Figure 2: The set-up for actual water splitting[3]. Light falls on a photoanode (BiVO 4 ), here an electron is excited and is transferred to the counter electrode. At the interface of BiVO 4 and electrolyte oxidation reaction takes place and at the interface of the counter electrode and the electrolyte reduction reaction takes place. The aim of this study is to investigate the photoanode thus the power source and the counter electrode are excluded in this experiment Characteristics of BiVO 4 Bismute vanadate (BiVO 4 ) is a metal oxide photoanode which is composed of VO 4 and BiO 8 structural units[4]. It is non-toxic and can be produced on a large scale and consists of earth-abundant materials. Besides it is stable in an aqueous environment with a ph value close to 7, which is necessary to it use in water. BiVO 4 has an energy band gap of 2.4 ev which allows to absorb visible light. 2.4 ev corresponds to 520 nm. BiVO 4 is thus a promising semiconductor. 3.2 Ultra-fast transient absorption spectroscopy Dynamic processes on a bismuth vanadate sample can take place on a pico-second time scale[4], thus a fast technique of measuring is required to investigate these dynamic pro- 5

6 cesses. Ultrafast transient absorption spectroscopy (TAS) allows to measure from 100 fs to 100 µs and is therefore applied in this study. During a single measurement, a pump pulse and a probe pulse are sent through the BiVO 4 sample (see Figure 3) and the probe pulse can be delayed by a value τ[5]. The pump excites the sample and with the probe the absorption spectrum of the excited state can then be determined. The spectrum of the ground state is determined when the sample is unpumped (this is when the pump beam is blocked by a chopper before it reaches the sample) and the spectrum of the pumped sample minus the spectrum of the unpumped sample is defined as A. This can be viewed as a function of time and as a function of the wavelength. Figure 3: In this figure, it is illustrated how the pump and the probe passes through a sample[5]. The pump which excites the sample is blocked after passing the sample while the probe is not blocked. 3.3 A features for a semiconductor When measuring A for a semiconductor, three features can be distinguished and these are shown in Figure 4. The features are ground state bleach (1), electron absorption (2) and hole absorption (3). Ground state bleach gives a negative A peak because the difference between pumped and unpumped is then negative. This is the case because a fraction of the electrons is excited while the rest is in the ground state. After excitation, less electrons are in the ground state and thus more ground state absorption is found in the unpumped sample compared to the pumped sample. Electron absorption is measured when the absorption spectrum of the excited electrons (which are in the conduction band) is regarded and this gives a positive contribution. This is because in a pumped sample, more electrons are in the excited state than in the unpumped state. Hole absorption also gives a positive contribution. This is because there are more holes if the sample is pumped compared to when the sample is unpumped. Holes are in the valence band. 6

7 Figure 4: The valance band is indicated in green and the conduction band is indicated in pink. The green dots (a) are excited electrons and the pink dots (b) are the holes in the valence band Interpretation of processes on BiVO 4 samples using TAS The processes which occur after excitation are explained by Ravensbergen et al.[4] who applied TAS to BiVO 4 samples. According to their results, four main processes occur (and some compete with each other) and these processes are hole trapping, electron relaxation, electron trapping and recombination. The difference absorption spectrum as shown in Figure 5 is characterized by a negative peak around 440 nm which corresponds to ground state bleach. In Figure 6 the time trace at 440 nm is shown where this negative peak can be observed. Hole trapping is associated with the time constant of 5 ps and is characterized by a positive peak around 475 nm. This contribution is due to excited state absorption. The broad absorption tail beyond 700 nm corresponds to the absorption of free holes and is also associated with the time constant of 5 ps. The time constants 40 ps and 2.5 ns both correspond to electron-hole recombination. This process is in competition with other processes. For the time constant of 40 ps the competing process is the relaxation of an excited electron in the conduction band, while for the time constant of 2.5 ns the competing process is trapping of electrons. Trap-limited recombination is associated with a time constant between 10 ns and 10 µs. Here the recombination of an electron or hole is temporary limited because it is trapped. It should be noted that the time constants 40 ps, 2.5 ns and 10 ns do not correspond to a specific wavelength. 7

8 Figure 5: The spectrum of a sample in air at a time delay of 1 ps. The characteristics are the negative peak at 440 nm, the positive peak at 475 nm and the broad absorption tail at longer wavelengths. In Figure 5 the spectrum of a sample for a time delay of 1 ps is plotted. This is the spectrum which is expected if the sample is measured in air. Because a pixel in the detector was off during the measurements, the showed example contains a sharp decrease, following a sharp increase immediately after the peak at 475 nm. In Figure 7, the time trace at 475 nm is shown which contains a coherent oscillation. This corresponds to an early trapped fraction of holes at the surface as proposed by Ravensbergen et al.[4]. Coherent oscillation is not present in the time trace at 700 nm, because only transitions at λ <700 nm are associated with surface-trapped holes[8]. The time trace at 700 nm is shown in Figure 8 where a decay can be observed, after a peak is reached. This represents a decrease of absorbed of free holes and also contains information about the 8

9 recombination. The corresponding time constant is 5 ps. In our study, we compare our data to the data of Ravensbergen et al. for the same delays and time traces and we also view the difference absorption spectrum for a time delay of 1 ps, 100 ps and 10 ns for our samples. The time traces at 440 nm, 475 nm and 700 nm are regarded. Figure 6: The time trace of a sample in air at 440 nm on a short time scale. Figure 7: The time trace of a sample in air at 475 nm. From 0 to 3 ps a coherent oscillation is visible. Figure 8: The time trace of a sample in air at 700 nm. From 0 to 3 ps a exponential decay with a time constant of 5 ps is visible. 3.4 Photocharging treatment Trześniewski et al.[1] introduced a treatment technique which they called photocharging and with this they measured a larger photocurrent and higher photovoltage. Photocharging means that samples BiVO 4 are illuminated by a solar simulator in an open circuit during a night with AM 1.5 light. AM 1.5 represents the solar intensity when the sunlight falls on earth in an angle of 48.2 degrees. Trześniewski et al. found that the photocharging effect was best visible if a PBA PH10 buffer was used, while in a PEC cell, ph7 is usually used. They plotted the photocurrent density as a function of the photovoltage and the catalytic efficiency as a function of the photovoltage. Although an increased catalytic efficiency was found after photocharging, it could be not explain why. The surface morphology and the crystal structure were investigated but no differences between before and after photocharging were found in both cases. Trześniewski et al. hypothesized that the enhancement after photocharging was due to passivated electronic surface states which would cause an increase of the photovoltage. This is because the presence of the surface states has a negative effect on the performance of semiconductors and passivation of these states would improve the surface properties. Another suggestion is that the enhancement is due to a bulk process and this would explain the reduction of V 5+ to V 4+ which they observed. 3.5 Hypothesis It is expected that the samples which are photocharged give a similar outcome as the samples which are untreated, but that the peaks at 475 nm are shifted for example. A shift in the peak of 475 nm in the A spectrum would indicate that there are trapped states at wavelengths other than 475 nm. It is also possible that the signal in the time traces 9

10 at 475 nm and 700 nm decays slower because the recombination time would be longer. Four samples are compared and these samples have a number from 1 to 4. Although the samples are similar, the treatment of samples 1 and 2 is different from that of samples 3 and 4. Sample 1 and 2 are measured in air and sample 3 and 4 in buffer. It is expected that the results of samples 1 and 2 are similar because these are both measured in air. The same goes for samples 3 and 4 which are both measured in the buffer. It is also expected that the data of sample 1 (reference) and sample 2 (reference) are similar to the data of Ravensbergen et al. because in both cases, similar samples are measured in air. The data of sample 3 (reference) and sample 4 (reference) are expected to be similar to the reference of sample 1 and 2, because it is expected that the buffer itself does not affect the absorption spectrum of the samples. 4 Method 4.1 Measurement procedure One BiVO 4 sample is made by applying spray pyrolysis[4] and later divided into four pieces. The samples are FTO coated glass with 200 nm BiVO 4. Two sets of measurements are done, each on a different day. For the first set of measurements, ultrafast transient absorption spectroscopy is applied on the four untreated BiVO 4 samples. Sample 1 and 2 are measured in air, while sample 3 and 4 are measured in a PBA PH10 buffer. As a preparation for the second set of measurements on the next day, all samples (including 1 and 2) were put in the PBA PH10 buffer and then illuminated by a solar simulator during a night (from 6pm until 9am), as it was demonstrated that this would improve the measurements[1]. Before the second set of measurements started, sample 1 and 2 were removed from the buffer in order to measure it in air. Samples 3 and 4 were measured in buffer in such a way that the pump and probe first passed the buffer before it reached the samples. For the analysis of the data, Matlab is used. To investigate the consistency of each set of measurements, multiple scans are performed. Especially for the samples which are illuminated by the solar simulator, it is important to know what happens to the photocharged effect as a function of time and thus to understand the durance of this effect. This is investigated by studying what happens between the first and last scan from which it can be determined if there is a trend. Besides the different scans of the same measurements, the averages of samples 1 and 2 only and the averages of sample 3 and 4 only are compared. Sample 1 and 2 are namely measured under the same circumstances (in air) and so are sample 3 and 4 (in buffer). From this it can be determined if the results for the samples in air and in buffer are consistent or not. At last the samples 1 and 2 are compared to 3 and 4 from which it can be concluded whether or not the measurements in buffer give a similar outcome as the measurements in air. The difference absorption spectrum is regarded as a function of the wavelength and is regarded for a 1 ps, 100 ps and 10 ns delay. Besides the difference absorption spectrum is regarded as a function of time. This is the case when the time trace at 475 nm and 700 nm are viewed. Each graph consists of data corresponding to untreated samples and photocharged samples. 10

11 4.2 Set-up The set-up as shown in Figure 9, consists of an optical parametric amplifier (OPA) and two lasers which are seeded by a single 80 MHz oscillator which serves as the master clock. This allows the lasers to be electronically synchronized. Both laser systems are amplified Ti:sapphire laser systems. One of the lasers is called the Libra (output power is 4.5 W) and is used to generate the probe beam. The other laser is called the Legend (output power is 3.0 W) and this laser is used to generate the pump beam. The output wavelength of both lasers is 800 nm and the repetition rate of is 1 khz which means that they give a pulse each 1 ms. The OPA has no specific purpose in our set-up, the output wavelength is equal to the input wavelength which is 800 nm. The output wavelength of the Legend is doubled using a beta barium borate (BBO) crystal and this gives a pump beam with a wavelength of 400 nm. The probe beam is a broadband white beam generated by focussing the output of the Libra on a CaF 2 plate. It is a collimated beam that is focussed on the sample. The intensity of the pump can be varied but for this experiment it is set at 400 nj/pulse, because in is used earlier research[4] and is more comparable to sunlight than higher intensities. The polarization of the pump beam is set at 54.7 degrees with respect to the probe beam. This is called the magic angle and avoids polarization and photoselection effects[5]. In the path of the pump there is an optical delay line which can delay the pump by increasing the distance between the mirrors at (1) and (2). This delay line is connected to a computer which controls the positions. We chose to place the delay line in the path of the pump, because the probe is more sensitive to changes in the alignment of the mirrors than the pump. The delays which can be achieved using this delay line are in the range of fs to ns scale and the maximum delay that can be achieved is 3.8 ns[7]. In addition to the delay of 3.8 ns, delay steps of 12.5 ns can be used possible and these delay steps are controlled by a signal delay generator (SDG). The set-up consists of a chopper which blocks the pump at specific moments. At these moments the absorption spectrum of the ground state or unpumped sample is measured such that the difference absorption spectrum between the excited state (or pumped sample) and the ground state can be determined. The buffer solution or electrolyte used for our experiments is PBA PH10 buffer. This buffer is a 0.1 M solution of H 3 PO 4, H 3 BO 3 and CH 3 COOH (0.1 M each) titrated to ph10 with KOH. 11

12 Figure 9: The set-up, adapted from Ravensbergen[7], consists of two synchronized laser systems, the Legend from which the pump beam is generated and the Libra from which the probe beam is generated. The optical parametric amplifier (OPA) in our set-up has no specific purpose, The input and output wavelength of the OPA is 800 nm. The pump can be delayed by varying the distance between mirrors at (1) and (2). The chopper blocks the pump at some moments in order to measure the ground state absorption. At the moments that the chopper does not block the pump, the excited state absorption is measured. The difference between the excited state absorption spectrum and the ground state absorption spectrum is defined as A. The pump is blocked by a beam stopper which prevents the pump to reach the detector. 5 Results As explained in the measurement procedure, our data will be compared to earlier research done by Ravensbergen et al.. Besides the averages of sample 1 and 2, the averages of sample 3 and 4 and the samples in air and in buffer will be compared. For each set of measurements the A can be plotted as a function of time in picoseconds or as a function of the wavelength in nanometer. In the first case the spectrum is plotted as a function of wavelength for different delays. We compared our samples for delays of 1 ps, 100 ps and 10 ns. In the second case, the time traces for two specific absorption wavelengths are regarded and these are 475 nm and 700 nm as was done in earlier research[4]. All samples are either photocharged or untreated and the samples which are untreated are referred to as reference. In the appendix (chapter 8), the different scans for each measurement are compared. From this it can be concluded that the scans are consistent and similar. In the appendix the data of sample 1 compared to sample 2 is also shown and from this it can be concluded that the measurements in air are consistent. The same conclusion can be drawn for sample 3 compared to sample 4. It should be noted that when comparing two samples in air or in buffer, the peaks at 475 nm are different which is due to laser fluctuations. Because we are interested in the differences of the shape between the samples, we regard the normalized data in the following sections. 12

13 5.1 Comparison with data Ravensbergen et al. Here we compare the normalized raw data of the different samples with the normalized raw data of Ravensbergen et al. for delays of 0.5 ps, 20 ps, 1 ns and 0.1 µs. This is done to discuss the similarities and differences between our data, because our methods and used set-up are similar. A difference between in method is however that our measurements are done with an excitation energy of 400 nj/pulse while the measurements of Ravensbergen et al. are done with a 50 nj/pulse excitation energy. In Figure 10 the time delay of 0.5 ps is viewed and here the data of Ravensbergen et al. from 2014 is indicated with the black lines. In this figure it can be observed that the recent data contains less noise than the data from 2014 and that the negative peak at 440 nm of the data in 2014 is more negative than for our measurements. Besides the data of 2014 shows an increasing trend in the region >550 nm while this is not the case in our data. Figure 10: The data from Ravensbergen et al.[4] (from 2014) compared to our data for a delay of 0.5 ps. While in Figure 10 the negative peak of the data of Ravensbergen et al. is lower than in our data, the negative peak at a delay larger than 20 ps (thus at 1 ns and 0.1 µs) seems to completely disappear. This is shown in Figures 11, 12 and 13. Apart from the negative peak at 440 nm, the lines are similar. 13

14 Figure 11: The normalized data from Ravensbergen et al.[4] (from 2014) compared to our data for a delay of 20 ps. 14

15 Figure 12: The normalized data from Ravensbergen et al.[4] (from 2014) compared to our data for a delay of 1 ns. 15

16 Figure 13: The normalized data from Ravensbergen et al.[4] (from 2014) compared to our data for a delay of 0.1 µs. In Figure 14, the time trace at 440 nm of our data and the data of earlier research (indicated in black) are plotted on a short scale. The difference absorption spectrum is normalized. In Figure 15, the time trace at 475 nm of our data and the data of earlier research are shown. In Figure 16, the time trace at 700 nm of our data and the data of earlier research are plotted on a short scale. In Figure 14 the black line stays below A=0 while the recent data goes above A=0 as a function of time. In Figure 15 the normalization factor is based on the highest peak which is at a different point for the data of 2014 compared to our data. In Figure 16 it seems that the peak in our data decays faster as a function of time than the peak in earlier data. The differences in all time traces indicate that our samples are different than those used in

17 Figure 14: The time trace at 440 nm where the data from Ravensbergen et al. data from sample 1 and 2 are plotted. 17

18 Figure 15: The time trace at 475 nm where the data from Ravensbergen et al. data from sample 1 and 2 are plotted 18

19 Figure 16: The time trace at 700 nm where the data from Ravensbergen et al. data from sample 1 and 2 are plotted The general difference between our results and the data of Ravensbergen et al. is that at a time delay of at least 20 ps, no negative peak at 440 nm can be observed in our data while this is clearly the case in the data of Ravensbergen et al.. The absence of the significant negative peak in our measurements can be due a decrease in amplitude of the negative peak at 440 nm or due to an increase in amplitude of the overlapping positive peak at 475 nm. To investigate why there is no negative peak, the time trace at 440 nm of our data is examined and fitted using an exponential. It is expected the time constant τ 1 is 0.5 ps, but this gives no converging fit. If a double exponential is used instead where the first time constant is fixed at 0.5 ps, a converging fit is found. The other time constant τ 2 is on average 3.4 ps. In Figure 17 the time trace at 440 nm of sample 1 (reference) is fitted on a region from 0 to 10 ps with a double exponential. The fits of the other samples are shown in section 8 Appendix and are also fitted using a double exponential. 19

20 Figure 17: The time trace at 440 nm of sample 1 (reference) on an interval from 0 to 10 ps is fitted using τ 1 =0.5 ps and τ 2 =3.48 ps. 5.2 Comparing photocharged and untreated samples In this section our four samples are compared for a time delay of 1 ps, 100 ps and 10 ns. In Figure 18 the normalized spectrum is shown for the four samples. The brown lines correspond to sample 1, the green lines correspond to sample 2, the blue lines correspond to sample 3 and the red lines correspond to sample 4. The dashed lines represent the photocharged samples while the reference is indicated with the solid lines. In Figure 18 the lines corresponding to samples 1 and 2, which are measured in air, almost overlap in the region <500 nm. This is however not the case if samples 3 and 4 (measured in buffer) are compared. In general it seems that the data of sample 3 is odd and this is can be explained by the calibration of the wavelength which is slightly different for sample 3. If the photocharged samples are compared to the untreated samples no significant differences can be observed. 20

21 Figure 18: The normalized spectrum for all samples for a time delay of 1 ps. 21

22 Figure 19: The normalized spectrum for all samples for a time delay of 100 ps. 22

23 Figure 20: The normalized spectrum for all samples for a time delay of 10 ns. The time trace at 475 nm on a short time scale is shown in Figure 23 and here a difference between the normalized data of sample 3 and the other samples is also visible. No differences between the photocharged and untreated samples can be observed. In the normalized time trace at 475 nm on a long scale or long semi-logarithmic time scale (see Figures 22 and 21), the lines of sample 3 and the other samples seem to overlap thus on a long time scale there is no significant difference between the samples. Besides because the normalized data of sample 3 (reference) almost overlaps with the normalized data of sample 3 (photocharged) it can be assumed that sample 3 is not odd because of photocharging effects. The same assumption can be made when studying the time trace at 700 nm as shown in Figures 24, 25 and 26 on different time scales. In the normalized time trace at 475 nm on a long semi-logarithmic time scale, at 10 4 ps there are two odd points which are likely due to an issue in the software which was used for 23

24 the measurements, thus we assume that it is not due to photocharging effects. Figure 21: The time trace at 475 nm for all samples on a semi-logarithmic scale. 24

25 Figure 22: The time trace at 475 nm for all samples on a long time scale. 25

26 Figure 23: The time trace at 475 nm for all samples on a short time scale. 26

27 Figure 24: The time trace at 700 nm for all samples on a semi-logarithmic scale. 27

28 Figure 25: The time trace at 700 nm for all samples on a long time scale. 28

29 Figure 26: The time trace at 700 nm for all samples on a short time scale. 6 Discussion When comparing our data to the data of Ravensbergen et al. we found in our data that the expected negative peak at 440 nm was not present or at least lower than in the earlier research. This can be explained by a decrease of ground state bleach or an increase of the overlapping absorption of trapped holes. Ravensbergen et al. showed that the hole absorption peak grows on a 5 ps constant. That the 440 nm kinetics in our data can be largely fitted with a timeconstant close to 5 ps could indicate that in our samples more holes are trapped than in the samples used by Ravensbergen et al. and this could be further investigated. 29

30 In general we observed no significant differences between the different samples or between the untreated and the photocharged samples if we either studied the (normalized) spectrum for different delays or the time trace at 475 and 700 nm. The odd data points are likely due to an issue in the software used for the measurements and not due to a photocharging effect. The reason why the peaks at 475 nm for sample 1 (both reference and photocharged) at 1 ps delay are higher than the peaks of sample 2, is likely because sample 1 was measured at a thicker spot than sample 2. We found that by moving the sample a few mm and thus by changing the measured spot, the height of the peaks changed significantly. Trześniewski et al. hypothesized that the photocharging effect could be due to changing bulk properties or due to changing surface properties. The changing bulk properties would explain the reduction of V 5+ to V 4+ while the changing surface properties would cause the electronic surface states to be passivated which would explain the increase in photovoltage. It is possible that photocharging effects are on the surface, because these contributions cannot be measured using our methods. To investigate if there are changes on the surface due to photocharging, we suggest to investigate BiVO 4 samples using Kelvin Probe Force Microscopy[9] (KPFM) before and after photocharging. This is a non-contact method which makes use of an atomic force microscope (AFM) and this could give information about the surface potential for the untreated and photocharged samples. If the photocharged effect is a bulk process, we did not observe it using TAS. This could indicate that the differences between the processes of photocharged and untreated samples are possibly only visible on time scales longer than 100 µs. From other research done by Ma et al.[10] it was namely concluded that water splitting occurs on the ms-s time scale. It is also possible that photocharging sustains for a shorter period than 20 minutes. The time between the moment that a sample was removed from the solar simulator until the start of the measurement was namely at least 20 minutes. However it was claimed by Trześniewski et al. that the effect can be sustained for at least several hours, thus the hypothesis that the photocharging effect only sustains for a shorter period than 20 minutes would not be consistent with their results. A suggestion for further research (besides the use of KPFM) would be to investigate photocharged samples on time scales longer than 100 µs. Besides photocharging effects on different n-type semiconductors such as TiO 2, Fe 2 O 3 or WO 3 could be investigated. 7 Conclusion We viewed the (normalized) difference absorption spectra for different delays and for corresponding time traces at 440 nm, 475 nm and 700 nm. The behaviour of the untreated samples is similar to the behaviour of the photocharged samples. Our data is similar to data from earlier research except for a negative peak at 440 nm in the difference absorption spectrum. From our measurements we conclude that if there is a photocharging effect we have not found it using TAS. Further research is required to investigate how photocharging enhances the performance of BiVO 4 samples. Our suggestions are to apply TAS on time scales longer than 100 µs and to apply KPFM to investigate the surface potential before and after photocharging the BiVO 4 sample. Another suggestion is to investigate photocharging effects on the performance of a different n-type semiconductor such as 30

31 TiO 2, Fe 2 O 3 or WO 3. References [1] Photocharged BiVO 4 photoanodes for improved solar water splitting, B. J. Trześniewsk and W. A. Smith, J. Mater. Chem. A, 2016, 4, , DOI: /C5TA04716A. [2] Towards highly efficient bias-free solar water splitting, F.F. Abdi, PhD thesis (2013). Delft University of Technology. ISBN [3] Electrochemistry: Photocatalysts in close-up J. Hofkens and M.B.J. Roeffaers Nature 530, (2016) doi: /530036a [4] Unraveling the Carrier Dynamics of BiVO 4 : A Femtosecond to Microsecond Transient Absorption Study, J. Ravensbergen, F.F. Abdi, J.H. van Santen, R.N. Frese, B. Dam, R. van de Krol and J.T.M. Kennis, J. Phys. Chem. C, 2014, 118 (48), pp doi: /jp509930s. [5] Ultrafast transient absorption spectroscopy: principles and applications to photosynthetic systems, R. Berera, R. van Grondelle and J.T.M. Kennis, Photosynth Res (2009) 101: , doi: /s y. [6] Photoelectrochemical Hydrogen Production, R. van de Krol and M. Grätzel, Springer: New York, 2012, doi: / [7] Photophysics of solar fuel materials, J. Ravensbergen, PhD thesis (2015). Vrije Universiteit Amsterdam. ISBN [8] Electron-Phonon Coupling Dynamics at Oxygen Evolution Sites of Visible-Light- Driven Photocatalyst: Bismuth Vanadate. N. Aiga, Q. Jia, K. Watanabe, A. Kudo, T. Sugimoto, and Y. Matsumoto. J. Phys. Chem. C, 2013, 117 (19), pp DOI: /jp [9] Deconvolution of Kelvin probe force microscopy measurements - methodology and application, Machleidt T., Sparrer E., Kapusi D., and Franke K.-H., Meas. Sci. Technol. 20, (2009) / /20/8/ [10] Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation, Y. Ma, S.R. Pendlebury, A. Reynal, F. Le Formal and J.R. Durrant, Chemical Science, 2014, pp , DOI: /c4sc00469h. 31

32 8 Appendix 8.1 Comparing the different scans of the same measurement In our measurements it can be observed that the time between the first and the last scan of the same sample does not affect the outcome systematically for sample 1, 2, 3 and 4. This is because for each graph where a delay between the pump and the probe for a specific sample is investigated, there is a peak in A around 475 nm in the spectrum which does not depend on the moment of the scan (except for sample 2 reference and sample 3 reference) Sample 1 In Figure 27, the spectrum of sample 1 (reference) for a time delay of 1 ps is plotted. For all samples and the shape of the spectrum is similar. It shows a negative peak around 440 nm which corresponds to ground state bleach[4], a positive peak around 475 nm and a broad absorption tail which corresponds to absorption of charge carriers. Around 480 nm a sharp peak is shown which is because this pixel in the detector was off. From Figure 27 it can be observed that all scans give similar results. The peak at 475 nm is slightly higher for the first and second scan (scan0 and scan1) compared scan2 and scan3 but this difference is not significant. Because the spectra almost overlap, it is possible to take the average of the scans. In Figure 28, the spectrum of sample 1 (photocharged) for a delay of 1 ps is plotted for different scans. The height of the peak at 475 nm for the scans is independent of the moment at which the scan is done. The heights do vary more significantly than for sample 1 (reference), especially scan2 is above the average of the other scans. It is still possible to take the average of these scans. 32

33 Figure 27: The spectrum of sample 1 (reference) at a time delay of 1 ps. 33

34 Figure 28: The spectrum of sample 1 (photocharged) at a time delay of 1 ps. The time traces corresponding to the spectra at 475 nm and 700 nm for scans of both sample 1 (reference) as sample 1 (photocharged) are investigated. Figure 29 shows the different scans for the time trace of 475 nm for sample 1 (reference). It appears that t 0 shifts to a negative value, as scan0 is at t 0 = 0 (where t 0 is expected) while scan1, scan2 and scan 3 have a t 0 with a more negative value than the previous scan (t 0,scan0 > t 0,scan1 > t 0,scan2 > t 0,scan3 ). On average the value of t 0 is shifted by -0.1 ps. The shift also occurs for the time trace at 700 nm as shown in Figure 30, but here the shift of t 0 on average is ps. The time traces of the different scans for 475 nm overlap for sample 1 (photocharged) as shown in Figures 31 and the t 0 value is shifted ps. The time traces of the different scans for 700 nm also overlap for sample 1 (photocharged) as shown in Figure 32 and the t 0 value is moved 0.43 ps to the right on average in this case. 34

35 Figure 29: The time trace of sample 1 (reference) at 475 nm. 35

36 Figure 30: The time trace of sample 1 (reference) at 700 nm. 36

37 Figure 31: The time trace of sample 1 (photocharged) at 475 nm. 37

38 Figure 32: The time trace of sample 1 (photocharged) at 700 nm. In all cases considering sample 1, the average of the different scans can be taken and by doing so for both reference and photocharged, the behaviour of the spectra as a function of time (using delays) can be investigated. This is shown in Figure 33 where the red lines correspond to sample 1 (reference) and the green lines correspond to sample 1 (photocharged). The characteristics of the raw data from Figure 33 are partly consistent with Ravensbergen et al.[4] who found that the peak of the spectrum at a delay of 20 ps was greater than that at a delay of 0.5 ps and that the peak for a longer delay then decreases. In our data the peak of sample 1 (photocharged) is larger than the peak of sample 1 (reference). Not all characteristics are consistent with Ravensbergen et al. because in our data the negative peak at 440 nm has disappeared for a delay of 100 ps which was not the case in earlier research. 38

39 Figure 33: The spectra of sample 1 for different time delays: 1 ps (solid line), 100 ps (dotted line) and 10 ns (dashed line). The red lines represent the averages of sample 1 (reference) and the green lines represent sample 1 (photocharged) Sample 2 The comparison of different scans for sample 2 is done in the same way as for sample 1. As shown in Figure 34, the scans corresponding to sample 2 (reference) at a delay of 1 ps have a similar shape but do not overlap. It seems that the peak at 475 nm decreases after each scan which can be due to a decreasing the intensity of the pump during the measurement. From Figure 35, it can be observed that the peaks corresponding to sample 2 (photocharged) at a time delay of 1 ps are consistent, because the data almost overlap. It is thus possible to take the average of the scans. 39

40 Figure 34: The spectrum of sample 2 (reference) at a time delay of 1 ps. 40

41 Figure 35: The spectrum of sample 2 (photocharged) at a time delay of 1 ps. When regarding the time trace of sample 2 (reference) at 475 nm which is shown in Figure 36, it can be observed that the value of t 0 is consistent but not at t 0 = 0. On average t 0 is moved 0.15 ps to the right. A similar observation can be made for the time trace of sample 2 (reference) at 700 nm which is shown in Figure 37, but here the time axis is shifted 0.55 ps to the right. Figures 38 and 39 show the time traces for sample 2 (photocharged) at 475 nm and 700 nm and these figures are similar to Figures 36 and 37 which correspond to sample 2 (reference). In Figure 36, t 0 is moved 0.15 ps to the right and in Figure 37, t 0 is shifted 0.57 ps to the right. 41

42 Figure 36: The time trace of sample 2 (reference) at 475 nm. 42

43 Figure 37: The time trace of sample 2 (reference) at 700 nm. 43

44 Figure 38: The time trace of sample 2 (photocharged) at 475 nm. 44

45 Figure 39: The time trace of sample 2 (photocharged) at 700 nm. In Figure 40, the average spectrum for the three time delays (1 ps, 100 ps and 1 ns) are plotted for sample 2 (reference) and sample 2 (photocharged). This figure is partly consistent with Ravensbergen et al. because the peak of sample 2 (photocharged) for a delay of 100 ps is higher than the peak for the delay of 1 ps. This is however not the case for sample 2 (reference). It is possible that sample 2 (reference) was measured at a thicker part than for sample 2 (photocharged). This is because if Figures 34 and 35 are studied again, it appears that the peak of all scans for sample 2 (reference) are higher than the peaks for sample 2 (photocharged), except for scan2. Another explanation might be that the light intensity during the measurements of sample 2 (reference) was higher than during the measurements of sample 2 (photocharged). 45

46 Figure 40: The spectra of sample 2 for different time delays: 1 ps (solid line), 100 ps (dotted line) and 10 ns (dashed line). The red lines represent the averages of sample 2 (reference), and the green lines represent sample 2 (photocharged) Sample 3 The different scans for sample 3 (reference) are shown in Figure 41 and these are similar but do not exactly overlap. It seems that the peak at 475 nm decreases after each scan. This is not the case for the scans of sample 3 (photocharged) as shown in Figure 42. The graphs in this figure show similarities with the scans of sample 3 (reference). 46

47 Figure 41: The spectrum of sample 3 (reference) at a time delay of 1 ps. 47

48 Figure 42: The spectrum of sample 3 (photocharged) at a time delay of 1 ps. The time trace of sample 3 (reference) at 475 nm is shown in Figure 43 and in this figure, it can be observed that the time axis of all scans is moved to the right. On average this shift is 0.24 ps. Figure 44 shows the time trace corresponding to sample 3 (reference) at 700 nm and here the time axis of all scans also moved to the right. On average t 0 is shifted 0.7 ps. Figures 45 and 46 are the figures corresponding to sample 3 (photocharged) at 475 nm and at 700 nm. They are similar to Figures 43 and 44 but the t 0 values are shifted by respectively 0.15 ps and 0.63 ps. 48

49 Figure 43: The time trace of sample 3 (reference) at 475 nm. 49

50 Figure 44: The time trace of sample 3 (reference) at 700 nm. 50

51 Figure 45: The time trace of sample 3 (photocharged) at 475 nm. 51

52 Figure 46: The time trace of sample 3 (photocharged) at 700 nm. Figure 47 shows the average spectrum for the three time delays (1 ps, 100 ps and 1 ns), plotted for sample 3 (reference) and sample 3 (photocharged). This figure is consistent with Ravensbergen et al. because the peak for a delay of 100 ps is higher than the peak for the delay of 1 ps. It can be observed that the peaks of sample 3 (photocharged) are higher for a delay of 1 ps and 100 ps compared to the peaks of sample 3 (reference) at the same delays, but not for a delay of 10 ns. 52

53 Figure 47: The spectra of sample 3 for different time delays: 1 ps (solid line), 100 ps (dotted line) and 10 ns (dashed line). The red lines represent the averages of sample 3 (reference), and the green lines represent sample 3 (photocharged) Sample 4 The spectrum for the different scans of sample 4 (reference) for a delay of 1 ps is similar, but the height of the peak at 475 nm varies for each scan. The spectra are shown in Figure 48. The scans shown in Figure 49 represent sample 4 (photocharged) at a time delay of 1 ps and these scans are similar to the scans of sample 4 (reference). 53

54 Figure 48: The spectrum of sample 4 (reference) at a time delay of 1 ps. 54

55 Figure 49: The spectrum of sample 4 (photocharged) at a time delay of 1 ps. The different scans shown in Figure 50 correspond to the time trace of sample 4 (reference) at 475 nm and these scans are consistent. An overall positive shift of t 0 can be observed and the average shift is 0.12 ps. The different scans corresponding to the time trace of sample 4 (reference) at 700 nm are also consistent and the average shift is 0.62 ps. These scans are shown in Figure 51. Figures 52 and 53 show the time trace of sample 4 (photocharged) at respectively 475 nm and 700 nm and the figures are similar but have different shifts of t 0. These shifts are on average respectively 0.15 ps and 0.67 ps. 55

56 Figure 50: The time trace of sample 4 (reference) at 475 nm. 56

57 Figure 51: The time trace of sample 4 (reference) at 700 nm. 57

58 Figure 52: The time trace of sample 4 (photocharged) at 475 nm. 58

59 Figure 53: The time trace of sample 4 (photocharged) at 700 nm. The spectra of sample 4 can be compared by taking the averages of the different scans. This is done for the three time delays (1 ps, 100 ps and 1 ns) and this is shown in Figure 54. The red lines correspond to sample 4 (reference) and the green lines correspond to sample 4 (photocharged). The red lines are consistent with Ravensbergen et al. as the peak for the 100 ps delay is higher than the peak for the 1 ps delay. This is not the case for sample 4 (photocharged), where the peaks decrease if the delay is increased. The peaks shift to lower wavelengths if the delay is increased. It is interesting to note that the peaks of sample 4 (reference) and sample 4 (photocharged) around 475 nm for the 1 ps delay overlap but the other peaks do not. The green dashed line which represents sample 4 (photocharged) for a delay of 10 ns is below the red dashed line which represents sample 4 (reference) for a delay of 10 ns. 59