Summary of thesis in materials science: Fabrication and photocatalytic, electrophotocatalytic properties of Cu2O with nano-structured covering layers

Objective of the thesis: Successfully fabricate Cu2O thin film having good crystal structure. Fabricate layers protecting Cu2O electrode from photocorrosion. Study the photocatalytic, electro-photocatalytic water splitting properties of the Cu2O electrode.

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY …………… *****…………… LE VAN HOANG FABRICATING RESEARCH AND PHOTOCATALYTIC, ELECTRICAL-PHOTOCATALYTIC PROPERTIES OF Cu2O WITH NANOSTRUCTURE COVERING LAYERS Major : Materials for optics, optoelectronics and photonics Code : 9.44.01.27 SUMMARY OF THESIS IN MATERIALS SCIENCE HA NOI - 2019 The thesis was completed at: Institute of Materials Science – Vietnam Academy of Science and Technology Supervisors: Prof Dr Nguyen Quang Liem Assoc Prof Dr Ung Thi Dieu Thuy Reviewer 1: Reviewer 2: Reviewer 3: The dissertation will be defended at Graduate University of Science and Technology, 18 Hoang Quoc Viet street, Hanoi Time: ., , 2019 The thesis could be found at: - National Library of Vietnam - Library of Graduate University of Science and Technology - Library of Institute of Science Materials INTRODUCTION With the increasing population and economic boom, the demand for energy escalates everyday However, the major source of energy, fossil fuel, is depleting and its price is projected to rise Therefore, finding clean, renewable and e nvironmentally friendly energy sources is an urgent and practical issue of the entire world, not just any country One of those clean and limitless energy sources is solar energy The question is how can we convert this massive source into other types of energy that can be stored, distributed and utilized on demand Besides solar cell, another method is to store solar energy in the bond of H2 molecules through photoelectrochemical (PEC) cells, also known as artificial leaf This process is similar to the photosynthesis in nature: using sunlight to split water into H2 O2 The photoelectrochemical cell has the cathode made of p-type semiconductor and the anode made of n-type semiconductor Among p-type semiconductor cathodes, Cu2O has been researched extensively Since Cu2O has a small band gap in the range of 1.9 – 2.2 eV, it is efficient in absorbing visible light The maximum theoretical solar-to-hydrogen conversion efficiency of Cu2O is approximately 18% Moreover, Cu2O is neither expensive nor toxic, and can be easily synthesized from abundant natural compounds Nonetheless, one major drawback of Cu2O, which limits its usage in water splitting, is its susceptibility to photo-corrosion The standard redox potentials of the Cu2O/Cu and CuO/Cu2O couples lie within Cu2O's band gap so the preferred thermodynamic process of photogenerated electrons and holes are reducing Cu+ into Cu0 and oxidizing Cu+ into Cu2+, respectively Thus, there are groups concentrating on improving the stability and photocurrent of Cu2O In Vietnam, there are not many researches on Cu2O, most of which focus on synthesizing Cu2O nanoparticles for environmental treatment or fabricating Cu2O thin film by CVD The research on Cu2O thin film synthesized by electrochemical method for the water splitting process in PEC cells is still new Therefore, we choose to conduct the thesis "Fabrication and photocatalytic, electrophotocatalytic properties of Cu2O with nano-structured covering layers" Objective of the thesis Successfully fabricate Cu2O thin film having good crystal structure Fabricate layers protecting Cu2O electrode from photocorrosion Study the photocatalytic, electro-photocatalytic water splitting properties of the Cu2O electrode To achieve the aforementioned goal, the specific research contents have been conducted: + Research on fabricating p-type Cu2O thin film (denoted as pCu2O) and n-type Cu2O (n-Cu2O) to make pn-Cu2O homojunction by electrochemical synthesis + Study the role of protective layers and the influence of synthesis parameters on the stability and water splitting efficiency of Cu2O electrode, on the basis of scientific information obtained from analysis of micromorphology, structure and photo, electrophotocatalytic properties of the fabricated electrodes + Investigate the mechanism of the photocatalysis, electron and hole mobilities within Cu2O photocathode Research item Nano-structured Cu2O thin film and Cu2O thin film coated with protective layers Research method The thesis was conducted by experimental method For each research content, we have chosen the appropriate method Structure and content of the thesis The thesis consists of 132 pages with 14 tables, 109 figures and graphs and is divided into four chapters: Chapter presents the introduction to the photocatalytic water splitting process Chapter presents the experimental methods used in the thesis Chapter presents the result of the research on fabricating pCu2O, pn-Cu2O thin films and Cu2O thin film coated with TiO2, CdS protective layers Chapter presents the obtained results on p-Cu2O and pn-Cu2O electrodes coated with conducting protective layers: Au, Ti, graphene The last part of the thesis lists the related publications and the references New results obtained in the thesis  We have successfully fabricated p-Cu2O and pn-Cu2O thin films on FTO substrate with high quantity and homogeneity by electrochemical synthesis With the n-Cu2O layer making pnCu2O homojunction thus improving the photoelectrochemical characteristics such as photocurrent onset potential Vonset, charge carriers separations and the electrode stability increases considerably  The thesis has investigated the influence of the thickness and annealing temperature of Au and TiO2 protective layers on the stability of the Cu2O electrode In addition, the thesis has proposed optimized thickness and annealing temperatures for these materials on p-Cu2O and pn-Cu2O electrodes  The thesis is the first work to study the effect of the thickness of CdS and Ti protective layers on the photocatalytic water splitting process on Cu2O electrode This research has shown the very good charge carrier separation ability of the CdS/Cu2O junction and the ability to support the charge transport, moving charge carriers from Cu2O to the electrolyte solution of the Ti layer  The thesis has investigated the effect of graphene mono and multilayer on the photocatalytic water splitting of Cu2O CHAPTER THE PHOTOCATALYTIC WATER SPLITTING PROCESS FOR CLEAN FUEL H2 PRODUCTION USING Cu2O PHOTOCATHODE In this chapter, we present the urgency of developing the clean fuel H2 One of the solutions for synthesizing H2 is the process of photocatalytic water splitting using PEC cells We present in detail the structure, operation principle and energy conversion efficiency evaluation of the PEC cell Cu2O is a material being used as the photocathode for the PEC cell This chapter also shows fundamental physicochemical properties of Cu2O, several methods of fabricating Cu2O thin film However, Cu2O is susceptible to photocorrosion due to its redox potential lying within the band gap We present a few measures to protect Cu2O photocathode such as using protective layers made of metal, oxide as well as other compounds The introduction to researches on Cu2O and recent advances in utilizing Cu2O as photocathode for PEC cells are also presented in this chapter CHAPTER EXPERIMENTAL METHODS IN THE THESIS In this chapter, we present in detail the experimental processes used in this thesis 2.1 Fabrication of Cu2O thin film and protective layers 2.1.1 Synthesis of p-type and pn-type Cu O films a Fabrication of p-type Cu2O (p-Cu2O) photoelectrode The FTO substrate was used as the working electrode The electrolyte solution contains 0.4 M CuSO4 and M lactic acid The solution pH Figure 2.2 Synthesis curves of pCu2O (a) and p-Cu2O thin film on FTO NaOH 20 M solution (b) The temperature of the electrochemical solution was kept constant at was increased to 12 by a 50oC To create the Cu2O film, a potential of + 0,2 V vs RHE was applied on the FTO electrode The thickness of the Cu2O film was controlled by fixing the charge density at C/cm2 b Fabrication of n-type Cu2O on p-type Cu2O electrode – forming pn-Cu2O homojunction The solution used to fabricate n-type Cu2O comprised of 0.02 M Cu(CH3COO)2 and 0.08 Figure 2.6 Synthesis curves of n-Cu2O on p-Cu2O (a) and pn-Cu2O thin film (b) M CH3COOH The solution pH was raised to 4,9 The solution temperature was kept at 65oC The n-type Cu2O (n-Cu2O) film was synthesized by applying a potential of +0,52 V vs RHE The charge density passed through FTO and p-Cu2O working electrodes was fixed at 0.45 C/cm2 2.1.2 Electron beam evaporation to deposit TiO2 layer We coated TiO2 layers with different thicknesses on p-Cu2O and pn-Cu2O electrodes by the electron beam evaporation method The source material Ti3O5 used for evaporation was of 99,9% purity The thickness of TiO2 layers on Cu2O was controlled at 10 nm, 20 nm, 50 nm and 100 nm 2.1.3 Chemical bath deposition of CdS layer We synthesized the CdS layer by the chemical bath deposition method from the precursor solution of 0,036 M Cd(CH3COO)2 and 0,035 M (NH2)2CS The thickness of the CdS layer was controlled by varying the deposition time (from 30 to 300s) on Cu2O electrode at 75oC We continued to deposit a 10 nm layer of Ti on the CdS/Cu2O film by thermal evaporation The electrodes were then annealed in Ar environment at 400oC in 30 minutes 2.1.4 Sputtering Au film We used the radio frequency magnetron sputtering method to coat a Au layer on p-Cu2O and pn-Cu2O electrodes We varied the sputtering duration (60s, 100s, 200s and 300s) to fabricate Au layers with different thicknesses on Cu2O electrode 2.1.5 Thermal evaporation to deposit Ti layer We use the thermal evaporation method to deposit Ti layers with different thicknesses on p-Cu2O and pn-Cu2O electrodes The Ti source for evaporation was of 99,9% purity The thickness of Ti coating layers on Cu2O was controlled at 5nm, 10nm, 15nm 20 nm After depositing Ti on Cu2O, the sample was annealed in Ar environment to increase the interaction between the Ti protective layer and the light absorber layer The annealing temperature was 400oC and the time was 30 minutes 2.1.5 Monolayer graphene coating The Cu2O electrode was coated with graphene by transferring monolayer graphene on Cu substrate on Cu2O electrode (Figure 2.11a) Figure 2.11 The schematic of the process of transferring graphene (a) and photograph of Cu2O electrode coated with PPMA/Graphene (b) Repeating the above process with monolayer graphene yield multilayer graphene coated electrode We denote the p-Cu2O and pnCu2O electrodes with graphene coating as X Gr/p-Cu2O and X Gr/pn-Cu2O, with X being the number of coated graphene layers, respectively CHAPTER RESULT OF THE FABRICATION OF p-Cu2O WITH n-Cu2O, n-TiO2 AND n-CdS PROTECTIVE LAYERS 3.1 Characteristics of p-Cu2O and pn-Cu2O electrodes 3.1.1 Morphology, structure of p-Cu2O and pn-Cu2O electrodes Figure 3.1a shows that p-Cu2O has a cubic structure, the size of the edges is approximately – 1,5 m The fabricated p-Cu2O film is homogeneous With charge the passed density of C/cm , the thickness of the Cu2O determined film was by SEM Figure 0.1 SEM image of the surface and cross-section of p-Cu2O cross-section measurement to be in the range of 1,4 – 1,5 m (Figure 3.1b) The X-ray diffractogram of pCu2O and pn-Cu2O shows the fabricated Cu2O is a single crystal without impurities such as Cu or CuO (Figure 3.4) The diffraction peaks at 2 values: 29,70o, 36,70o, o o 42,55 , 61,60 , 73,75 o 77,45 o Figure 0.4 XRD of the pCu2O and pn-Cu2O match with the crystal planes (110), (111), (200), (220), (311) and (222) Figure 3.6 is the XPS spectra of p-Cu2O film On the XPS Figure 0.6 XPS spectrum of p-Cu2O spectrum of Cu2p, the peak of the binding energy of the electron pair Cu2p3/2 at 934 eV and Cu2p1/2 correspond to the Cu2+ ion Moreover, there exist satellite peaks of Cu2p3/2 and Cu2p1/2 at 942.25 eV and 962.25 eV corresponding to Cu2+ in CuO or Cu(OH)2 Table 0.1 The parameters of the I – V characterization and the stability test of the 50 nm-TiO2/p-Cu2O and 50 nm-TiO2/pn-Cu2O electrodes annealed at different temperatures Sample p-Cu2O 50-p 50-p-300oC 50-p-350oC 50-p-400oC 50-p-450oC pn-Cu2O 50-pn 50-pn-300oC 50-pn-350oC 50-pn-400oC 50-pn-450oC The 50 Vonset jmax (V) 0.55 0.55 0.50 0.58 0.56 0.57 0.68 0.70 0.50 0.53 0.55 0.55 1.60 1.05 0.56 0.84 1.10 1.30 1.25 1.21 0.80 0.75 0.86 1.16 Current density after chopped – light cycles jmax jtrap j j’ j’/j 0.27 0.00 0.27 0.10 0.37 0.28 0.05 0.23 0.12 0.52 0.40 0.00 0.40 0.20 0.50 0.88 0.37 0.51 0.51 1.00 0.87 0.43 0.44 0.33 0.75 1.30 0.50 0.80 0.53 0.66 0.64 0.10 0.54 0.41 0.76 1.12 0.40 0.72 0.42 0.58 0.82 0.24 0.58 0.50 0.86 1.06 0.29 0.77 0.70 0.91 1.30 0.80 0.50 0.50 1.00 1.36 0.40 0.96 0.55 0.57 j180s ρ 180s (%) 0.04 0.02 0.12 0.28 0.15 0.27 0.14 0.12 0.15 0.13 1.18 0.23 1.25 7.15 30.00 34.10 17.24 20.77 11.20 10.72 18.29 12.27 90.80 16.91 nm- TiO2/pn-Cu2O sample annealed o at 400 C yields a maximum current density of 1.3 mA/cm After chopped – light cycles, the photocurrent density was steady (j’/j = 1) and Figure 0.2 I – t curve of 50 nm-TiO2/pCu2O (a, b) and 50 nm-TiO2/pn-Cu2O (c, d) annealing at different temperature 13 after minutes of the stability test, the current density only show 9.2% reduction Therefore, we kept the annealing temperature at 400oC and investigate the influence of TiO2 film thickness on the photocatalytic activity and stability of pn-Cu2O The result of I – V characterization and electrode stability are indicated in Figure 3.24c, d and Table 3.3 We have investigated the photoelectrochemical characteristics of the p-Cu2O and pn-Cu2O electrodes coated with TiO2 thin film of different thickness and annealed at different temperatures As indicated by the with result, TiO2 coated p-Cu2O, the optimized annealing temperature o 350 C, is the oprimized thickness is 50 nm The 50 nm- Figure 0.3 I – t and I – t curves of p-Cu2O (a, b) and pn-Cu2O (c, d) coverd different thickness of TiO2/p-Cu2OTiO2 350 oC electrode has the current density jmax at approximately 0.9 mA/cm2, which retains 34% after 180s of activity measurement With TiO2 coated pn-Cu2O, the optimized annealing temperature is 400 oC, the TiO2 thickness is in the range of 50 nm – 100 nm The 50 nm-TiO2/pnCu2O-400oC electrode has the current density jmax of roughly 1.3 mA/cm2, which retains 91% after 180s of activity measurement 14 Table 0.2 The parameters of the I – V characterization and the stability test of the X nm-TiO2/p-Cu2O-350oC, X nm-TiO2/pn-Cu2O400oC samples Current density after j180s ρ 180s chopped – light cycles (%) jmax jtrap j j’ j’/j 1.02 0.71 0.20 0.51 0.20 0.39 0.04 5.63 1.30 0.66 0.09 0.57 0.36 0.63 0.12 8.18 0.84 0.88 0.37 0.51 0.51 1.00 0.28 3.10 0.93 0.86 0.30 0.56 0.23 0.41 0.10 11.63 0.47 0.70 0.30 0.40 0.57 1.40 0.60 85.72 0.73 0.93 0.25 0.68 0.68 1.00 0.45 48.39 0.86 1.30 0.80 0.50 0.50 1.00 1.18 90.80 0.44 1.09 0.81 0.27 0.27 1.00 1.29 118.34 Sample Vonset jmax (V) 10-p 20-p 50-p 100-p 10-pn 20-pn 50-pn 100-pn +0.58 +0.56 +0.58 +0.58 +0.46 +0.47 +0.55 +0.47 3.3 The CdS layer 3.3.1 Morphology and structure of the CdS covered Cu2O electrode Figure 0.4 SEM images of p-Cu2O samples coated with CdS at different times The micromorphology of the p-Cu2O eletrodes after n-CdS deposition at different times is shown in Figure 3.28 15 The chemical composition and crystal structure of the sample are characterized by X-ray diffraction (Figure 3.32), X-ray photoelectron spectroscopy (Figure 3.33a) and Raman spectroscopy (Figure Figure 0.32 XRD pattern of p-Cu2O after coating CdS 3.33b) Figure 0.33 EDX spectrum (a) and Raman spectrum of the 300s-CdS/p-Cu2O electrode (b) 3.3.2 photoelectrochemical properties of CdS protected Cu2O The photoelectrochemical measurement results of CdS coated pCu2O electrodes are shown in Figure 3.34 and Table 3.4 Figure 0.34 I – V (a) and I – t (b) curves of CdS coated p-Cu2O 16 The Cu2O electrodes coated with CdS shows noticeable charge carrier separation due to the pn heterojunction Because the CdS layer is thick, the generated electrons are trapped at the interface between n-CdS and p-Cu2O (very high jtrap) However, this very thick n-CdS layer coats uniformly on the surface of Cu2O, preventing H+, thus slowing down the Cu2O from interacting with photoelectrochemical corrosion process After 180s of I – t measurement, 20% of the 300s-CdS/Cu2O sample was corroded Table 0.3 The parameters of the photoelectrochemical characterization of CdS coated p-Cu2O Sample Vonset (V) jmax p-Cu2O 30 s-p 60 s-p 120 s-p 180 s-p 300 s-p 1.60 1.03 1.19 0.70 0.48 0.68 +0.55 +0.51 +0.49 +0.38 +0.49 +0.49 Current density after j 180s chopped – light cycles jmax jtrap j j’ j’/j 0.17 0.00 0.17 0.15 0.88 0.02 1.63 0.82 0.81 0.70 0.86 0.54 1.65 0.85 0.80 0.65 0.81 0.50 0.57 0.25 0.33 0.27 0.81 0.19 1.30 0.86 0.44 0.40 0.91 0.92 2.44 1.87 0.57 0.55 0.97 1.95 ρ 180s (%) 98.75 66.87 69.70 66.67 29.23 20.08 We have studied the effect of CdS deposition time on the photoelectrochemical characteristic and stability of the Cu2O electrode The 300s deposition time, corresponding to a CdS thickness of 600nm, shows the highest current density  2.4 mA/ cm2 This electrode also possess the highest stability Only 20% of the activity is lost after 180s of photocatalytic stability measurement CHAPTER THE INFLUENCE OF CONDUCTIVE LAYERS ON THE PHOTOELECTROCHEMICAL CHARACTERISTIC OF THE Cu2O ELECTRODE 17 4.1 H+ reduction catalytic activity of Au NPs and Au coated Cu2O electrode Figure 0.3 Au protective mechanism on p-Cu2O (a) and pn-Cu2O (b) 4.1.1 H+ reduction catalytic activity of Au NPs 4.1.2 Morphology and structure of Au coated Cu2O electrodes The Au layer was chosen for purposes: conducting protective layer and catalyst for the hydrogen evolution reaction (Figure 4.3) The electrodes with different Au layer thicknesses are denoted as Xnm-Au/p-Cu2O and Xnm-Au/pn-Cu2O, with Xnm being the thickness of the Au layer The Au coated electrodes annealed 30 minutes in the Ar environment at different temperatures are denoted as Xnm-Au/p-Cu2O-YoC, with YoC being the annealing temperature Figure 4.2 is the SEM images of the pn-Cu2O electrode coated with Au for different sputtering durations On the X-ray Figure 0.6 SEM iamges of Au coated pn-Cu2O electrode with different sputtering times Figure 0.9 XRD pattern of Au coated Cu2O electrode before and after PEC measurement 18 diffractogram, there appears diffraction peaks of Au at 2 values of 38,25o, 44,50o and 64,75o, corresponding to the crystal planes (111), (200) and (220) of Au (Figure 4.7) 4.1.3 The photo and PEC properties of Au coated Cu2O electrodes Figure 0.5 I-V characteristic curve and stability of p-Cu2O (a, b) and pn-Cu2O (c, d) electrodes coated with Au at different thicknesses Among the Au coated p-Cu2O electrodes, the one with 100 nm Au coating has the highest stability The electrodes with thinner Au coating show higher current density However, the thin Au layer is not enough to protect the Cu2O electrode from photocorrosion The high photocurrent density is mostly contributed by the electrode corrosion process After minutes of stability test, the remaining photocurrent density is 30% of the initial photocurrent density With Au coated pn-Cu2O electrodes, the 200nm-Au/pn-Cu2O has the highest current density and stability of approximately 0.76 mA/ cm2 After minutes of stability test, the remaining current density is 50% of the initial current density 19 Figure 4.17 illustrate the current density versus time curve after chopped – light cycles at V vs RHE at Sun illumination The electron accumulation is better seen at the Au/electrolyte interface when coating the Au layer on the p-Cu2O and pn-Cu2O electrodes (Figure 4.17, blue and purple curve) In this case, we have observed a positive Figure 0.17 I – t curves of Cu2O and Au coated Cu2O in the 1st on – off cycle current when the light was turned off This has proven that the photogenerated electrons have been trapped inside the Au coating Therefore, the Au layer has an important contribution as a catalyst and protective layer for Cu2O photoelectrode 4.2 Ti protective layer 4.2.1 Morphology, structure of the Ti coated Cu2O electrode Figure 4.19 is SEM images of 20nm-Ti/pCu2O and 20nm- Ti/pn-Cu2O electrodes before and after thermal annealing The composition and structure of Ti coated Cu2O electrode was analyzed by Xray diffraction (Figure Figure 0.6 SEM images of Ti coated Cu2O phủ Ti before and after annealing 4.21), X-ray photoelectron spectroscopy and Raman spectroscopy 20 In the XPS spectrum (Figure 4.24), the characteristic region of Ti 2p in the 20 nm-Ti/p-Cu2O electrode shows the peaks 2p3/2 at 458 eV and 2p1/2 at 463,76 eV, corresponding to TiO2 Figure 0.7 XPS spectrum of Ti 2p3/2 of 20 nm-Ti/p-Cu2O Figure 0.21 XRD pattern of Ti coated Cu2O 4.2.2 The photoelectrochemical properties of the Ti coated Cu2O electrode Table 0.4 The parameters of the photoelectrochemical measurement of the Ti coated Cu2O samples Sample Vonset jmax (V) Current density after j180s ρ 180s chopped – light cycles jmax jtrap j j’ j’/j p-Cu2O +0.55 1.60 0.17 0.00 0.17 0.15 0.88 0.02 1.25 5nm-Ti/p 10nm-Ti/p 15nm-Ti/p 20nm-Ti/p pn-Cu2O 5nm-Ti/pn 10nm-Ti/pn 15nm-Ti/pn 20nm-Ti/pn +0.56 1.75 0.70 +0.54 1.63 0.56 +0.53 1.40 0.73 +0.57 1.30 1.20 +0.68 1.25 0.64 +0.54 1.60 1.65 +0.53 0.82 1.10 +0.52 1.00 1.14 +0.55 1.36 0.50 0.23 0.15 0.31 0.59 0.10 0.49 0.20 0.29 0.05 21 0.47 0.42 0.90 0.41 0.40 0.97 0.42 0.32 0.76 0.61 0.48 0.79 0.54 0.41 0.76 1.16 0.91 0.78 0.90 0.69 0.77 0.85 0.76 0.89 0.45 0.45 1.00 0.27 0.22 0.28 0.22 0.14 0.45 0.42 0.38 0.40 38.57 39.29 38.36 18.33 11.20 27.27 38.18 33.33 29.42 The parameters of the photoelectrochemical and I – V, I – t measurements of the Ti coated Cu2O electrodes are indicated in Table 4.4 The 5nm-Ti/p-Cu2O sample has 0.15 mA higher maximum photocurrent density and times the jmax value compared to p-Cu2O, proving that the 5nm Ti coating has reduced the electrode corrosion The maximum photocurrent density decreases when increasing the Ti coating thickness from – 20 nm In addition, jmax and jtrap tend to rise This phenomenon happens because when the thickness of the Ti layer increases, the quantity of photogenerated electrons trapped at the interface between Cu2O and Ti increases, accelerating the self reduction process from Cu2O to Cu0 at the interface between Cu2O and Ti and thus, the corrosion rate Therefore, for p-Cu2O, the optimized Ti coating thickness is approximately – 10 nm The same conclusion can be drawn for the pn-Cu2O electrode Therefore, a – 10 nm thick Ti coating on the pn-Cu2O yields optimized charge separation and transport from the light absorber to the interface with the electrolyte 4.3 Graphene protective layers 4.3.1 Morphology, structure of graphene coated electrode Figure 0.8 SEM images of graphene coated electrodes before and after catalytic activity measurement On the SEM images (Figures 4.28a, b), thin layers on p-Cu2O and pn-Cu2O can be observed 22 By analysis of Raman spectrum of the electrode, it can be proven that graphene layers exist on top of the Cu2O layer (Figure 4.29) On the Raman spectrum, we have observed peaks at 1580 cm-1 (G-band) and Figure 0.9 Raman spectrum of 3-Gr/p-Cu2O 2616 cm-1 (2D-band) 4.3.2 The PEC properties of graphene coated Cu2O electrodes Figure 0.10 I – V characteristic and stability of the p-Cu2O (a, b) and pn-Cu2O (c, d) electrodes coated with graphene The I–V characteristics and the parameters of the photoelectrochemical measurement of graphene coated Cu2O samples are indicated in Figure 4.32 and Table 4.5 The light LSV of 23 the p-Cu2O sample shows reduction peaks of Cu2O at +0.27 V and +0.07 V vs RHE These reduction events are related to the photocorrosion of Cu2O to create Cu With the sample with monolayer graphene, the peak at +0.27 V is still observable However, for samples with and layers of graphene, these peaks cannot be observed This result show that the samples coated with and layers of graphene has higher p-Cu2O electrode protection Because the photogenerated electrons move to the surface of Cu2O then move to the graphene layer, slowing down the reduction of Cu2O to Cu0 on the electrode surface This shows that the graphene layer coated on p-Cu2O slows the corrosion of Cu2O, thus increases the electrode stability When coating layers of graphene, the resistance of the coating increases, accelerating the corrosion process However, when coating layers of graphene, the current density jmax rises after the I – V measurement This fact clearly indicates that the electrons are trapped at the interface between pCu2O and graphene The value jmax is times that of p-Cu2O The value jtrap at the interface between p-Cu2O and graphene increases roughly times compared to when monolayer and 2-layer graphene were coated The result can be explained by the fact that when stacking graphene layers, the area of graphene islands on Cu2O increases Therefore, the area of the interface between Cu2O and the electrolyte solution decreases The stability of Cu2O is improved However, when the graphene layers stack, the resistance of the coating and the defects increases, increasing the number of electrons trapped at the interface Cu2O and graphene 24 Table 0.5 The parameters of the photoelectrochemical measurement of graphene coated Cu2O samples Sample Vonset jmax (V) p-Cu2O +0.55 1.60 1-Gr/p-Cu2O +0.56 1.14 2-Gr/p-Cu2O +0.51 1.72 3-Gr/p-Cu2O +0.51 1.13 pn-Cu2O +0.68 1.25 1-Gr/pn-Cu2O +0.52 1.03 2-Gr/pn-Cu2O +0.52 1.02 3-Gr/pn-Cu2O +0.52 1.25 Current density after j180s 180s chopped – light cycles (%) jmax jtrap j j’ j’/j 0.17 0.00 0.17 0.15 0.88 0.02 1.25 0.65 0.17 0.48 0.43 0.90 0.27 41.54 0.50 0.12 0.38 0.32 0.85 0.19 38.00 1.35 0.46 0.89 0.68 0.77 0.27 20.00 0.64 0.10 0.54 0.41 0.76 0.14 11.20 0.55 0.00 0.55 0.37 0.67 0.25 45.46 1.12 0.29 0.83 0.63 0.76 0.28 25.00 1.13 0.15 0.98 0.67 0.68 0.34 30.09 CONCLUSION With the aim of fabricating Cu2O thin film for electro – photocatalytic water splitting application, the thesis has concentrated on synthesizing Cu2O thin film by electrochemical method The fabricated film has high homogeneity, stability and can be made at large scale From these Cu2O thin films, we investigate the influence of protective layers coated on the electrode's photoelectrochemical characteristics From the obtained result, some conclusions can be drawn: We have successfully fabricated p-Cu2O and pn-Cu2O thin films on FTO substrate with high quantity and homogeneity by electrochemical synthesis technique With the n-Cu2O layer making pn-Cu2O homojunction help improve photoelectrochemical characteristics such as photocurrent onset potential Vonset, charge carrier separation and the electrode's stability increase considerably 25 An n type semiconductor such as n-TiO2 and n-CdS coating helps improve the charge separate However, the photo-electrons which have been trapped at the interface between protective layer and Cu2O increase when increse the thickness of protective layers For p-Cu2O, the optimized TiO2 thickness is 50 nm and annealed at 350oC For pn-Cu2O, the optimized TiO2 thickness is 50 nm and annealed at 400oC The best time deposition of CdS is 180 – 300s The annealing process helps to increase the linkage and reduce the potential barrier between Cu2O and the Au, Ti and graphene conductive material The thickner protective layer was deposited, the more photo-electrons were trapped at the interface The optimal thickness and annealing temperature of the Au layer are 100-200nm and 400oC A thin Ti layer 5-10nm has good support for the electronic separation and the movement of electrons from Cu2O to the surface between Ti and electrolyte solution The graphene layers coated on Cu2O electrodes increase the optical current density and the stability of electrodes 26 LIST OF PUBLICATIONS Hoang V Le, Ly T Le, Phong D Tran, Jong-San Chang, Ung Thi Dieu Thuy and Nguyen Quang Liem, “Hybrid amorphous MoSx-graphene protected Cu2O photocathode for better performance in H2 evolution”, International Journal of Hydrogen Energy, available online May 2019 (IF: 4.229) Hoang V Le, Phong D Tran, Huy V Mai, Thuy T.D Ung, Liem Q Nguyen, “Gold protective layer decoration and pn homojunction creation as novel strategies to improve photocatalytic activity and stability of the H2-evolving copper (I) oxide photocathode”, International Journal of Hydrogen Energy 43 (2018) 21209-21218 (IF: 4.229) Hoang V Le, Thi Ly Le, Ung Thi Dieu Thuy, Phong D Tran, “Current perspectives in engineering of viable hybrid photocathodes for solar hydrogen generation”, Advances in Natural Sciences: Nanoscience and Nanotechnology (2018) 023001 (13p) Tien D Tran, Mai T.T Nguyen, Hoang V Le, Duc N Nguyen, Quang D Truong, Phong D Tran, “Gold nanoparticles as an outstanding catatyst for the hydrogen evolution reaction”, Chem Commun 54 (2018) 3363-3366 (IF: 6.29) 27 ... p -Cu2O and pnCu2O electrodes with graphene coating as X Gr/p -Cu2O and X Gr/pn -Cu2O, with X being the number of coated graphene layers, respectively CHAPTER RESULT OF THE FABRICATION OF p -Cu2O WITH. .. electrophotocatalytic properties of Cu2O with nano-structured covering layers" Objective of the thesis Successfully fabricate Cu2O thin film having good crystal structure Fabricate layers protecting Cu2O electrode... fabricating p-type Cu2O thin film (denoted as pCu2O) and n-type Cu2O (n -Cu2O) to make pn -Cu2O homojunction by electrochemical synthesis + Study the role of protective layers and the influence of synthesis