CN112760618B - Modification method of bismuth vanadate photoelectrode - Google Patents
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Abstract
The invention belongs to the field of photoelectrodes, and particularly relates to a modification method of a bismuth vanadate photoelectrode. By adopting the scheme, the BiVO 4 Surface deposition of Al on photoelectrode 2 O 3 An oxide layer is further treated by chemical solution to remove the oxide layer, and BiVO with passivated surface is obtained 4 A thin film photoelectrode. After the surface passivation, the photo-generated electron hole pairs no longer tend to be compounded at the surface state, so that the effect of inhibiting the compounding of carriers is achieved, the service life of the carriers is prolonged, the surface catalytic reaction is promoted, and the photoelectrochemical efficiency is improved.
Description
Technical Field
The invention belongs to the field of photoelectrodes, and particularly relates to a modification method of a bismuth vanadate photoelectrode.
Background
As a kind of oxygen with moderate band gapCompound semiconductor, biVO 4 Is a photoelectric catalytic material with great potential, however, the potential of the material is not completely developed at present, and the slow reaction kinetics of the semiconductor electrode/electrolyte interface is an important influencing factor. The improvement of the surface properties of bismuth vanadate photoelectrode is aimed at, wherein factors such as inhibition of bulk phase recombination, charge transmission rate and carrier migration at a phase interface need to be considered heavily. As an oxide semiconductor, with Fe 2 O 3 And the like, biVO 4 Many defects are present in the semiconductor crystal lattice, oxygen vacancies, lattice disorder and the like are the main causes of surface defects, and are currently based on Fe 2 O 3 The research on the surface defects and the optimization and modification thereof have been deeply and widely carried out, but the research is specific to BiVO 4 The modification studies of surface defects are relatively scarce.
Based on BiVO 4 The optimization and modification research of photoelectrode mostly focuses on doping, cocatalyst loading, heterojunction compounding and the like at present, and the work related to surface state passivation is not common. For example, liang et al (Liang YQ, messinger j. Improving BiVO) 4 photoanodes for solar water splitting through surface passivation[J]Physical Chemistry Chemical Physics,2014,16 (24): 12014-12020.) by electrochemical loading and subsequent removal of BiVO 4 NiO of the surface x Deeply research BiVO 4 -H 2 O interfacial process, niO x The partial removal of (a) improved its photoelectrochemical properties, reaching 43% IPCE at 450nm at 1.23v vs. rhe. This article states that NiO is supported on the surface x Has heterogeneity and has three existing forms, namely an oxygen evolution catalytic center, a carrier recombination center and a surface passivation center, and after electrochemical treatment, only part of the surface passivation center is left in BiVO 4 Surface, thus BiVO 4 The surface passivation and the improvement of the photoelectrochemical properties are very limited.
Eisenberg et al (Eisenberg D, ahn HS, bard AJ. Enhanced Photoelectrochemical Water Oxidation on Bismuth Vanadate by Electron Electroposition of Amorphous Titanium Dioxide [ J]Journal of the American Chemical Society,2014,136 (40): 14011-14014.) in W dopingHetero BiVO 4 Electrodepositing a layer of amorphous TiO on the surface 2 The photocurrent efficiency is improved by 5.5 times, and the photocurrent initial potential is advanced by 500mV. However amorphous TiO 2 The self-body has no photoresponse and photoelectrochemical activity, and the authors analyze that the self-body has the effects of inhibiting surface recombination and passivating surface defects, so that the photoelectrochemical efficiency is improved. Although some work is currently carried out on the surface passivation of bismuth vanadate photoelectrode, the related research is still in the primary stage, and the surface passivation mechanism needs to be further and deeply explored.
Disclosure of Invention
The invention aims to provide a modification method of a bismuth vanadate photoelectrode so as to improve the photoelectrochemical efficiency of the bismuth vanadate photoelectrode.
In order to achieve the purpose, the invention adopts the technical scheme that: after depositing aluminum oxide on the surface of the bismuth vanadate photoelectrode by an atomic layer, removing the aluminum oxide to obtain a passivated bismuth vanadate photoelectrode, namely the modified bismuth vanadate photoelectrode.
By adopting the scheme, the BiVO 4 Al deposition on the surface of photoelectrode 2 O 3 An oxide layer is further treated by chemical solution to remove the oxide layer, and BiVO with passivated surface is obtained 4 A thin film photoelectrode. After the surface passivation, the photo-generated electron hole pairs no longer tend to be compounded at the surface state, so that the effect of inhibiting the compounding of carriers is achieved, the service life of the carriers is prolonged, the surface catalytic reaction is promoted, and the photoelectrochemical efficiency is improved.
The method for depositing the aluminum oxide on the surface of the bismuth vanadate photoelectrode by the atomic layer comprises the following steps: the bismuth vanadate photoelectrode is arranged in a reaction cavity of ALD equipment, trimethylaluminum is taken as a precursor source, plasma oxygen is taken as an oxygen source, the reaction is carried out under the conditions that the temperature is 140-160 ℃ and the plasma power is 180-220W, the reaction cycle number is 1-100 until Al is generated on the surface of the bismuth vanadate photoelectrode 2 O 3 Stopping the reaction after oxidizing the layer; the reaction temperature is preferably 150 ℃ and the plasma power is preferably 200W.
The ALD apparatus parameters are set as follows:
precursor source sample introduction time is 0.02s, reaction time is 1s, and purging time is 120s;
plasma treatment time 30s, purge time 100s.
The method for removing the aluminum oxide comprises the following steps: and (3) soaking the bismuth vanadate photoelectrode with the aluminum oxide film formed on the surface in alkali liquor, preferably soaking the bismuth vanadate photoelectrode in a sodium hydroxide aqueous solution with the concentration of 1M until the aluminum oxide film is completely dissolved, taking out the bismuth vanadate photoelectrode, washing the bismuth vanadate photoelectrode with deionized water, and airing the bismuth vanadate photoelectrode.
The preparation method of the bismuth vanadate photoelectrode comprises the following steps:
s1, so as to contain at least Bi (NO) 3 ) 3 The mixed solution of KI and p-benzoquinone is an electrodeposition solution, and electrodeposition is carried out under a three-electrode system with FTO as a working electrode, ag/AgCl as a reference electrode and platinum as a counter electrode to obtain a red sheet BiOI film;
s2, mixing 0.2M VO (acac) 2 Uniformly dripping dimethyl sulfoxide solution on the BiOI film obtained in the step S1, placing the BiOI film in a muffle furnace, performing heat treatment for 1-3 hours at the temperature of 420-480 ℃, taking out and cooling;
s3, cleaning the film obtained in the step S2 with a sodium hydroxide solution, cleaning with deionized water, and blow-drying to obtain the pure nano-porous BiVO 4 A film.
Specifically, in step S1, the method for preparing the electrodeposition solution includes: 0.2 to 0.6M of Bi (NO) 3 ) 3 And 2-6M KI are dissolved in deionized water, and then nitric acid solution with the concentration of 1M is used for adjusting the pH value to 1.5-2, so as to obtain solution A; dissolving p-benzoquinone in absolute ethyl alcohol under vigorous stirring to obtain a p-benzoquinone-absolute ethyl alcohol solution with the concentration of 0.20-0.30M, and marking as a solution B; and uniformly mixing the solution A and the solution B to obtain the electrodeposition solution.
In the step S2, VO (acac) 2 And (3) dripping an ethanol solution on the BiOI film, placing the BiOI film in a muffle furnace for heat treatment at 450 ℃ for 2 hours, taking out after the reaction is finished, and naturally cooling to room temperature. In the process, the BiOI film is converted into a film with V on the surface after reaction 2 O 5 BiVO of oxide layer 4 A film.
In the step S3, the film after heat treatment is immersed in 1M sodium hydroxide solution, stirred and cleaned, then washed by deionized water and dried by nitrogen, and the pure nano-porous BiVO is obtained 4 A film. The purpose of washing with sodium hydroxide is to wash out BiVO 4 V of film surface 2 O 5 To obtain pure nano-porous BiVO 4 A film.
The scheme aims at the BiVO 4 The photoelectrode has surface defects, and the main component of the surface defects is oxygen vacancies, so that the surface state passivation effect can be realized by covering the surface of the photoelectrode with a metal oxide thin layer to reduce the concentration of the surface oxygen vacancies. Amorphous Al by plasma enhanced atomic layer deposition 2 O 3 Nano-porous BiVO (BiVO) 4 The surface is chemically treated by alkaline solution to remove Al on the surface 2 O 3 BiVO in the process 4 The photoelectrode realizes surface passivation, so that the photoelectrochemical property of the photoelectrode is obviously improved.
Drawings
FIG. 1 shows Al 2 O 3 BiVO before and after atomic layer deposition 4 A film XRD spectrogram;
FIG. 2 is Al 2 O 3 BiVO before and after atomic layer deposition 4 Film SEM pictures;
FIG. 3 shows different cycle periods of Al 2 O 3 BiVO before and after atomic layer deposition 4 Thin film photocurrent-voltage curves;
FIG. 4 shows Al 2 O 3 BiVO before and after atomic layer deposition and chemical treatment 4 A thin film XPS spectrum;
FIGS. 5-7 BiVO before and after surface passivation 4 The photocurrent-voltage curves of the photoelectrode are respectively 1,10,100 in ALD cycle;
FIG. 8 BiVO before and after surface passivation 4 Electrochemical impedance spectroscopy of the photoelectrode;
FIG. 9 is surface passivated BiVO 4 An equivalent circuit diagram of the photoelectrode;
FIG. 10 shows surface-passivated BiVO 4 (ii) steady state fluorescence spectra of;
fig. 11 is a schematic diagram of the mechanism of surface passivation.
Detailed Description
The technical solution of the present invention is further described below with reference to examples.
1. Examples of the embodiments
1. Reagent and instrument device
Reagent: bismuth nitrate pentahydrate (Bi (NO) 3 ) 3 ·5H 2 O), vanadyl acetylacetonate (VO (acac) 2 ) Potassium iodide (KI), concentrated nitric acid (HNO) 3 ) Absolute ethanol (C) 2 H 5 OH), p-benzoquinone (C) 6 H 4 O 2 ) Dimethyl sulfoxide (C) 2 H 6 OS), sodium hydroxide (NaOH), trimethylaluminum (Al (CH) 3 ) 3 ). The reagents are all products sold in the market, and no special treatment is carried out before use.
The instrument equipment comprises: electrochemical workstation model CHI-760D, plasma enhanced atomic layer deposition device (calix cormin electronics technologies, ltd).
2. Preparation of pure nanoporous BiVO 4 Photoelectrode
Electrodeposition solution: taking 0.002mol of Bi (NO) 3 ) 3 Mixing the solution with 0.02mol of KI, dissolving the mixture in 50mL of deionized water, and dropwise adding 1M dilute nitric acid to adjust the pH to 1.7 to obtain a solution A; dissolving 0.5g of p-benzoquinone in 20mL of absolute ethanol under vigorous stirring to prepare a 0.23M solution to obtain a solution B; and uniformly mixing the solution A and the solution B to obtain the electrodeposition solution.
Electrodeposition was carried out in a three-electrode system with cleaned FTO as the working electrode, saturated Ag/AgCl as the reference electrode, and a platinum sheet as the counter electrode. And (3) performing electrodeposition under the bias voltage of-0.2V for 5min to obtain a red flaky BiOI film, taking out the red flaky BiOI film after deposition is finished, washing the red flaky BiOI film by deionized water to remove residual solution on the surface, and storing the red flaky BiOI film after nitrogen blow-drying.
VO (acac) 2 Dissolving in dimethyl sulfoxide to obtain 0.2M solution, dripping 0.2-0.5mL of the solution onto the surface of BiOI film to uniformly distribute, placing in a muffle furnace, heat treating at 450 deg.C for 2 hr, heating for 2 hr, and naturally cooling to room temperatureAnd (4) room temperature.
Taking out the FTO film, putting the FTO film into 1M NaOH solution, and washing off V on the surface under stirring 2 O 5 And after being washed by deionized water, the mixture is blown dry by nitrogen, and pure nano-porous BiVO can be obtained 4 A film.
3、BiVO 4 Photoelectrode modification
The prepared electrode is placed in a reaction cavity of ALD equipment, and a cleaning step is carried out before reaction, so that the purity of the cavity and a sample before reaction is ensured. Trimethyl aluminum is adopted as a precursor source, plasma oxygen is adopted as an oxygen source, the reaction temperature is 150 ℃, and the plasma power is 200W.
The specific reaction steps and parameters are set as follows: precursor source sample introduction time is 0.02s, reaction time is 1s, and purging time is 120s; plasma time 30s, purge time 100s.
The above is the basic parameter of a reaction period, the number of reaction cycles (1,10,100) is set, and the film growth thickness is about 0.1nm per cycle according to the ellipsometer test.
Completely immersing the film photoelectrode into NaOH aqueous solution containing 1M, stirring the solution for about half an hour to ensure that the aluminum oxide on the surface is fully contacted with the solution and completely reacted, then taking out the electrode film, washing with deionized water and airing at room temperature, and naming the electrode film as p-BiVO 4 。
2. Atomic layer deposition of Al 2 O 3 For BiVO 4 Research on photoelectrochemical property action
1、Al 2 O 3 Deposition of BiVO 4 Thin film structural characterization
BiVO according to X-ray diffraction analysis, as shown in FIG. 1 4 The film has good crystallinity, and accords with the standard spectrogram JCPDS No.14-0688 of monoclinic scheelite bismuth vanadate. Al loading by atomic layer deposition 2 O 3 The deposition to BiVO can be accurately controlled by controlling the number of deposition cycles 4 Al of film 2 O 3 To achieve optimum performance. As can be seen in FIG. 1, 100 cycles of ALD deposited Al 2 O 3 After that, no significant change was observed in the XRD signal of the film, either wayThe surface is that the thickness of the atomic layer deposition is very low, the thickness which can grow even if 100 deposition cycle periods does not exceed 10nm, and on the other hand, the atomic layer deposition film is not subjected to high-temperature heat treatment, the reaction temperature is 150 ℃, and the film is generally in an amorphous state. Thus, atomic layer deposited Al 2 O 3 In BiVO 4 The surface of the film exists in an amorphous form, has extremely low thickness, does not have an X-ray diffraction signal, and is resistant to BiVO 4 The crystal structure of the film also has no influence.
The morphology of the films was characterized by Scanning Electron Microscopy (SEM) and the results are shown in fig. 2, where a (left) is the pre-deposition picture and b (right) is the post-deposition picture. It can be seen that BiVO 4 The film has good particle appearance and a three-dimensional nano-network structure, and the particle size of a single particle is about 100nm. The porous structure enables the film to have a large specific surface area, can be in full contact with a solution and can undergo a hydrolytic oxidation reaction, and the small particle size is also beneficial to transmission of photo-generated electrons and holes. However, atomic layer deposition of Al 2 O 3 Then, in BiVO 4 The presence of the film surface is not directly observable because the amount deposited is very small, the thickness is very low and it is present in an amorphous form.
2、Al 2 O 3 Deposition of BiVO 4 Photocurrent property of
The above results indicate that the surface is covered with Al 2 O 3 The thin layer is of low thickness and does not have crystallinity, which makes it difficult to directly test its presence using conventional instrumental means. However, al 2 O 3 Deposited on BiVO 4 After the surface of the film, the photoelectrochemistry property of the film is remarkably influenced, and the result is shown in fig. 3. From FIG. 3, it can be seen that the unloaded nanoporous BiVO 4 The film has good photocurrent activity, the initial potential is about 0.12V vs. RHE, and the photocurrent reaches 0.51 mA-cm under the bias voltage of 1.05V vs. RHE -2 . While passing through Al of different thickness 2 O 3 The photocurrent density exhibited a different drop after deposition. Wherein 100 cycles of Al 2 O 3 The photocurrent of the deposited sample dropped sharply to 10 muA-cm -2 Rank, reduced by twoGrade, which shows that the presence of alumina severely inhibits the photoelectrochemical water splitting reaction from proceeding, probably because of Al 2 O 3 Is an oxide material with larger resistivity, and can seriously obstruct the charge transmission and migration on the surface of the electrode when the thickness of the oxide material is larger, and photogenerated holes reach BiVO 4 After the surface, it is difficult to pass Al 2 O 3 The layer reacts with water, resulting in a dramatic decrease in photoelectrochemical properties. In addition, 10 cycles and 1 cycle of Al 2 O 3 The photocurrent of the deposited samples is also greatly reduced, but the reduction range is slowed down along with the reduction of the deposition thickness, and the 10 periods of Al are carried out 2 O 3 Rhe at lower bias (below 1.35v vs. rhoe) for the deposited samples, the photocurrent was very low, and with continued increase in applied bias, the photocurrent showed a near linear, sharp increase in photocurrent, probably because, although the transport of photogenerated carriers was hindered, the bias was large enough to provide enough driving force to allow photogenerated holes to pass through Al 2 O 3 The layer reacts with water. And 1 cycle of Al 2 O 3 The photocurrent trend of the deposited sample is closer to that of the unmodified pure sample, and the amplitude of the drop is also minimal. From the trend of the change of the deposition thickness and the corresponding change of the photocurrent, the amorphous Al can be seen 2 O 3 Deposited BiVO 4 The photocurrent is not increased but reduced to different degrees because of the amorphous Al 2 O 3 Has extremely poor conductivity and plays a role in blocking charge transmission and migration.
3. Study of surface passivation
1. Surface passivation pair BiVO 4 Structural influence
Although amorphous Al 2 O 3 Deposition loading results in a reduction in photocurrent, but this is mainly due to its poor conductivity, and the effect on surface properties after loading is not properly reflected. Thus to amorphous Al 2 O 3 Deposition-supported nanoporous BiVO 4 The film is further chemically treated by immersing the film in NaOH solution to remove excessive Al on the surface 2 O 3 . Can be measured as thin as possible from X-ray photoelectron spectroscopy (XPS)The elemental composition of the surface of the film and its valence information, the results are shown in fig. 4. The samples in the figure are deposited with Al for 1 cycle 2 O 3 First, it is evident from the XPS Al 2p signal that surface Al 2 O 3 Significant changes occurred. Amorphous Al 2 O 3 Loaded BiVO 4 The Al on the surface has a distinct 2p peak at a bonding energy of 74.3eV, indicating that Al 2 O 3 BiVO is successfully deposited by an atomic layer deposition method 4 The surface, and additionally the signal at its lower binding energy of about 69.5eV is the V3 s orbital, which is not generally used for analysis of V elements, and is commonly used for analysis of V2 p signals. And the surface passivation sample p-BiVO after the chemical treatment of the alkaline solution 4 Little XPS signal of Al is observed because of surface Al 2 O 3 The reaction with alkali solution was successfully removed, indicating that the excess Al was removed by chemical solution 2 O 3 Is effective. The existence form of oxygen on the surface of the photoelectrode is a key factor influencing surface passivation, and unmodified pure BiVO can be seen from an XPS O1 s spectrogram 4 There are two main peaks, 529.6eV and 531.0eV respectively, corresponding to the lattice oxygen and the surface oxygen in the film. When amorphous Al 2 O 3 After loading, the lattice oxygen signal is basically not changed, because the loading amount is low, the influence is only generated on the outermost layer of the material, and the crystal structure of the material is not influenced. The surface oxygen signal changed significantly, the position of the peak was shifted to 531.5eV towards higher binding energy, and the relative intensity was also increased significantly. This is because the surface oxygen composition becomes amorphous Al 2 O 3 The surface property of the film is obviously changed due to the oxygen in the water. Further removing amorphous Al 2 O 3 After the surface, the peak intensity at the position is obviously reduced, and the signal is proved to be from the surface Al from the side surface 2 O 3 . The peak position did not change significantly, shifted slightly to the lower binding energy by about 0.1-0.2eV, and still higher than the unmodified surface oxygen signal, indicating that after loading and removal processing, biVO 4 The existence form of oxygen on the surface of the film is obviously changed although Al 2 O 3 Al in the alloy is completely removedBut the tightly bound surface oxygen it forms is still present, which may be crucial for surface passivation modification. In addition, it can be seen from XPS Bi and V signals that Bi 4f of the three samples before and after modification 5/2 And 4f 7/2 The signal peak positions are respectively located at 164.2eV and 158.9eV, V2p 1/2 And V2 p 3/2 The signal peak positions are respectively located at 524.0eV and 516.3eV, and no obvious change is generated, which shows that Al 2 O 3 BiVO after thin layer loading and treatment 4 The metal element in (2) hardly influences, does not change the crystal structure of the metal element, and only influences the existence form of surface oxygen.
2. BiVO with passivated surface 4 Thin film photoelectrochemical testing
According to the above analysis, the subsequent chemical treatment removed the amorphous Al from the surface 2 O 3 And the surface of the thin film is obviously changed, and the positive influence on the photoelectrochemical performance of the thin film photoelectrode can be expected due to the removal of an oxide layer of which the surface hinders charge transmission and the passivation effect of the surface. The photocurrent results after processing are shown in fig. 5-7. It can be seen that the photoelectrochemical properties of the processed films with different thicknesses are improved, and the initial expected effect is achieved. First, al is cycled over 1 atomic layer deposition 2 O 3 After the load sample is passivated by NaOH solution, the photocurrent value of the load sample is obviously improved and reaches 1.06 mA-cm at 1.05V vs -2 Compared with pure BiVO 4 The improvement is about 1 times, and more than 6 times compared to the sample with the alumina coating on the surface. Combined with results of previous testing, 1 layer of Al 2 O 3 Is very low in thickness and loading but is still at BiVO 4 The surface produces a severe charge transport inhibition, and when Al is completely removed, the resistive layer no longer acts as a barrier, and if no other effects occur, the photocurrent value should be close to that before the deposition. The photoelectrochemical efficiency is improved instead, indicating that such deposition and removal processes result in activation of the electrode surface, probably because the adverse effects of surface states on photoelectrochemistry are suppressed and electron-hole pairs are suppressedAnd (4) compounding. The influence of NaOH alkaline solution on the film can be basically eliminated because BiVO 4 When the film is prepared, excessive V is generated on the surface after heat treatment 2 O 5 The latter is removed by soaking in NaOH solution, so that the alkaline solution treatment is a step in the preparation process and does not have obvious influence on the photoelectrochemical properties of the product. For more cycle times Al 2 O 3 Deposited BiVO 4 After the electrode is chemically treated, the photocurrent is obviously improved due to the removal of the surface resistance layer. Photocurrent after passivation of 10 ALD cycle samples and 100 ALD cycle samples reached 0.77 and 0.74mA · cm at 1.05v vs. rhe -2 . This indicates that Al is not particularly limited 2 O 3 BiVO can be treated by any deposition thickness 4 The surface has passivation effect, and the alkaline solution treatment can easily remove the excessive Al on the surface 2 O 3 And (4) removing. Different thickness of Al 2 O 3 After removal, the 1-cycle sample had a higher photocurrent at a lower applied bias, probably due to lower thickness and thus excess Al 2 O 3 The removal is more thorough, and the surface passivation effect is better.
The electrochemical impedance spectrum analysis can further analyze the function of surface passivation in photoelectrochemistry, the electrochemical impedance spectrum considers a system as an equivalent circuit, each component and interface of a thin film electrode are each element (resistance R, capacitance C and the like), the structure of the equivalent circuit and the numerical value of each element can be measured through EIS test and further selecting a proper equivalent circuit and fitting through ZsimWin software, the electrode dynamics process of the photoelectrochemical system is analyzed through the electrochemical meaning of the elements, the result is shown in figure 8, 1 ALD cycle sample is tested, the test is carried out under the illumination condition, the external bias voltage is 1.05V vs. RHE, the test range is 100mHz to 100kHz, and the AC amplitude is 10mV. In the figure, the scatter diagram is the data obtained by the test, and the solid line is the fitting result according to the equivalent circuit, it can be seen that the fitting value and the test value have good agreement, which indicates that the selection of the equivalent circuit is appropriate, and the selected equivalent circuit is as shown in fig. 9 in consideration of the existence of the surface state. In the photoelectrochemistry of a semiconductor,the low frequency response corresponds to the semiconductor/electrolyte interface charge transfer process, while the high frequency response corresponds to the semiconductor bulk charge transfer process. As can be seen from the impedance spectrum, al 2 O 3 After deposition, the low frequency response impedance increased significantly, indicating that it acted to disfavor charge transport, which is consistent with electrochemical test results. To remove Al 2 O 3 And then, the diameter of the semicircle is obviously reduced, and the charge transmission is effectively enhanced, so that the improvement of the photocurrent efficiency is promoted. According to the fitting numerical values in table 1, the influence before and after modification can be compared more intuitively. Although the fitting values of the electrochemical impedance spectrum do not necessarily represent the actual situation because the process of the photoelectrochemical reaction is quite complicated, the relative sizes and rules of the values can help us to generally understand the reaction process in photoelectrochemistry. The interface charge transfer resistance is obviously reduced after the surface is passivated, which greatly promotes the transmission of charges, thereby being more beneficial to the surface reaction. In addition Q SS The values also decreased significantly, which is consistent with relevant studies in the literature, indicating that Al 2 O 3 The removal process after deposition serves as surface passivation.
Table 1 surface passivation of BiVO 4 Equivalent circuit fitting parameter values of
The steady state fluorescence spectrum is an effective means for researching carrier separation and recombination in the semiconductor photoelectric catalytic material, the fluorescence spectrum of a sample is shown in figure 10, and the selected excitation wavelength is 325nm. Pure BiVO 4 A relatively intense fluorescent signal is shown around 510nm, consistent with the band gap of the sample, indicating that there is significant recombination of the photo-generated electron-hole pairs in the sample. And after the surface passivation, the fluorescence peak of the sample can not be seen almost, because the surface passivation inhibits the recombination of photon-generated carriers, the photon-generated electron-hole pair has better separation efficiency, and the photoelectrochemistry water decomposition efficiency is improved.
Combining the above analysis to provideThe mechanism of the surface passivation is shown in fig. 11. BiVO 4 The surface state of the semiconductor surface caused by oxygen vacancy, lattice defect and the like is sometimes beneficial to photoelectrochemical reaction, can improve the carrier concentration, but can be used as a recombination center of a photogenerated electron-hole pair in many times, so that the electron and the hole are recombined on the semiconductor surface instead of reacting with water, and the photoelectrochemical efficiency is reduced. By depositing ultra-thin Al 2 O 3 An oxide layer is further treated by chemical solution to obtain BiVO with passivated surface 4 A film. After the surface passivation, the photo-generated electron hole pairs no longer tend to be compounded at the surface state, so that the effect of inhibiting the compounding of carriers is achieved, the service life of the carriers is prolonged, the surface catalytic reaction is promoted, and the photoelectrochemical efficiency is improved.
The invention optimizes and modifies BiVO 4 Starting with the surface property of the electrode, the nano-porous BiVO is prepared by an electrochemical synthesis method 4 Film and by atomic layer deposition of Al of different thickness on its surface 2 O 3 An oxide layer is further treated by chemical solution to obtain BiVO with passivated surface 4 The photoelectrochemical property of the film is obviously improved, and the photocurrent reaches 1.06 mA-cm at 1.05V vs. RHE -2 Compared with pure BiVO 4 The improvement is 108 percent.
Research results show that BiVO can be effectively performed by the atomic layer deposition method 4 A layer of Al is loaded on the surface of the film 2 O 3 The thickness of a deposition layer can be controlled according to different deposition cycle times, and due to the fact that the deposition temperature is not high, an amorphous film is deposited on the deposition layer, and the thickness is low, signals cannot be detected through X-ray diffraction, and the morphology of the film cannot be directly observed through a scanning electron microscope. Due to amorphous Al 2 O 3 The conductivity of the conductive layer is poor, the effect of blocking charge transmission and migration is achieved, and the photocurrent is reduced on the contrary. However, after further chemical treatment with alkaline solution, the photocurrent activity is obviously improved, and analysis such as XPS shows that BiVO 4 The existence form of the middle surface oxygen is changed, thereby playing the role of surface passivation. Hair brushDeposition of ultra-thin oxide layer by atomic layer deposition and subsequent removal of excess Al 2 O 3 Realizes the BiVO 4 The passivation effect of the surface state inhibits the recombination of photo-generated electron-hole pairs generated by the surface state, realizes the improvement of the hydrogen production efficiency by photoelectrochemical water decomposition, and is used for BiVO 4 The research of surface passivation has certain guiding significance.
Claims (10)
1. A modification method of a bismuth vanadate photoelectrode comprises the following steps: the method is characterized in that: and after depositing aluminum oxide on the surface atomic layer of the bismuth vanadate photoelectrode, removing the aluminum oxide to obtain a passivated bismuth vanadate photoelectrode, namely the modified bismuth vanadate photoelectrode.
2. The method for modifying a bismuth vanadate photoelectrode according to claim 1, wherein the method comprises the following steps: the method for depositing the aluminum oxide on the surface of the bismuth vanadate photoelectrode by the atomic layer comprises the following steps: putting a bismuth vanadate photoelectrode into a reaction cavity of an ALD device, reacting at 140-160 ℃ and 180-220W plasma power by taking trimethylaluminum as a precursor source and plasma oxygen as an oxygen source, wherein the reaction cycle is 1-100 until Al is generated on the surface of the bismuth vanadate photoelectrode 2 O 3 After oxidation, the reaction was stopped.
3. The method for modifying a bismuth vanadate photoelectrode according to claim 2, wherein the method comprises the following steps: the ALD apparatus parameters are set as follows:
precursor source sample introduction time is 0.02s, reaction time is 1s, and purging time is 120s;
plasma treatment time 30s, purge time 100s.
4. The method for modifying a bismuth vanadate photoelectrode according to claim 2, wherein the method comprises the following steps: the reaction temperature was 150 ℃ and the plasma power was 200W.
5. The method for modifying the bismuth vanadate photoelectrode according to claim 1, wherein the method comprises the following steps: the method for removing the aluminum oxide comprises the following steps: and (3) soaking the bismuth vanadate photoelectrode with the aluminum oxide film formed on the surface in alkali liquor until the aluminum oxide film is completely dissolved, taking out the bismuth vanadate photoelectrode, cleaning with deionized water, and airing.
6. The method for modifying a bismuth vanadate photoelectrode according to claim 5, wherein the method comprises the following steps: the alkali liquor is sodium hydroxide aqueous solution with the concentration of 1M.
7. The method for modifying a bismuth vanadate photoelectrode according to claim 1, wherein the method comprises the following steps: the preparation method of the bismuth vanadate photoelectrode comprises the following steps:
s1, so as to contain at least Bi (NO) 3 ) 3 The mixed solution of KI and p-benzoquinone is an electrodeposition solution, and electrodeposition is carried out under a three-electrode system with FTO as a working electrode, ag/AgCl as a reference electrode and platinum as a counter electrode to obtain a red sheet BiOI film;
s2, mixing 0.2M VO (acac) 2 Uniformly dripping dimethyl sulfoxide solution on the BiOI film obtained in the step S1, placing the BiOI film in a muffle furnace, performing heat treatment for 1-3 hours at the temperature of 420-480 ℃, taking out and cooling;
s3, cleaning the film obtained in the step S2 with a sodium hydroxide solution, cleaning with deionized water, and blow-drying to obtain the pure nano-porous BiVO 4 A film.
8. The method for modifying a bismuth vanadate photoelectrode according to claim 7, wherein the method comprises the following steps: in the step S1, the preparation method of the electrodeposition solution comprises: 0.2 to 0.6M of Bi (NO) 3 ) 3 And 2-6M KI are dissolved in deionized water, and then nitric acid solution with the concentration of 1M is used for adjusting the pH value to 1.5-2, so as to obtain solution A; dissolving p-benzoquinone in absolute ethyl alcohol under vigorous stirring to obtain a p-benzoquinone-absolute ethyl alcohol solution with the concentration of 0.20-0.30M, and marking as a solution B; and uniformly mixing the solution A and the solution B to obtain the electrodeposition solution.
9. The method for modifying a bismuth vanadate photoelectrode according to claim 7, wherein the method comprises the following steps: and in the step S2, the BiOI film is placed in a muffle furnace to be subjected to heat treatment for 2 hours at the temperature of 450 ℃, and is taken out after the reaction is finished, and is naturally cooled to the room temperature.
10. The method for modifying a bismuth vanadate photoelectrode according to claim 7, wherein the method comprises the following steps: in the step S3, the thermally treated BiOI film is immersed in 1M sodium hydroxide solution, stirred and cleaned, washed by deionized water and dried by nitrogen, and the pure nano-porous BiVO is obtained 4 A film.
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