Fabricating research and photocatalytic, electrical - Photocatalytic properties of Cu2O with nanostructure covering layers

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 MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY The thesis was completed at: Institute of Materials Science – Vietnam Academy of Science and Technology Supervisors: 1. Prof. Dr. Nguyen Quang Liem 2. 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 1 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 và 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 2 Cu 0 and oxidizing Cu + into Cu 2+ , 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, electro- photocatalytic 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 photo- corrosion. 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 p- Cu2O) 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, electro- photocatalytic properties of the fabricated electrodes. + Investigate the mechanism of the photocatalysis, electron and hole mobilities within Cu2O photocathode. Research item 3 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 1 presents the introduction to the photocatalytic water splitting process. Chapter 2 presents the experimental methods used in the thesis. Chapter 3 presents the result of the research on fabricating p- Cu2O, pn-Cu2O thin films and Cu2O thin film coated with TiO2, CdS protective layers. Chapter 4 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 pn- Cu2O homojunction thus improving the photoelectrochemical characteristics such as photocurrent onset potential Vonset, charge carriers separations and the electrode stability increases considerably. 4  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 2 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 1. 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 5 introduction to researches on Cu2O and recent advances in utilizing Cu2O as photocathode for PEC cells are also presented in this chapter. CHAPTER 2. 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 Cu2O 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 3 M lactic acid. The solution pH was increased to 12 by a NaOH 20 M solution. The temperature of the electrochemical solution was kept constant at 50 o C. 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 1 C/cm 2 . 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 2.2. Synthesis curves of p- Cu2O (a) and p-Cu2O thin film on FTO (b) Figure 2.6. Synthesis curves of n-Cu2O on p-Cu2O (a) and pn-Cu2O thin film (b) 6 M CH3COOH. The solution pH was raised to 4,9. The solution temperature was kept at 65 o C. 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/cm 2 . 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 75 o C. 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 400 o C 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 7 coating layers on Cu2O was controlled at 5nm, 10nm, 15nm và 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 400 o C 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). Repeating the above process with monolayer graphene yield multilayer graphene coated electrode. We denote the p-Cu2O and pn- Cu2O 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 3. 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 – 1,5 m. The fabricated p-Cu2O film is homogeneous. Figure 2.11. The schematic of the process of transferring graphene (a) and photograph of Cu2O electrode coated with PPMA/Graphene (b) 8 With the passed charge density of 1 C/cm 2 , the thickness of the Cu2O film was determined by SEM cross-section measurement to be in the range of 1,4 – 1,5 m (Figure 3.1b). The X-ray diffractogram of p- Cu2O 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, 42,55 o , 61,60 o , 73,75 o và 77,45 o 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 spectrum of Cu2p, the peak of the binding energy of the electron pair Cu2p3/2 at 934 eV and Cu2p1/2 correspond to the Cu 2+ ion. Moreover, there exist satellite peaks of Cu2p3/2 and Cu2p1/2 at 942.25 eV and 962.25 eV corresponding to Cu 2+ in CuO or Cu(OH)2. Figure 0.1. SEM image of the surface and cross-section of p-Cu2O Figure 0.4. XRD of the p- Cu2O and pn-Cu2O Figure 0.6. XPS spectrum of p-Cu2O 9 3.1.2 Photo and photoelectrochemical properties of p-Cu2O and pn- Cu2O electrodes Figure 3.7a indicates that p- Cu2O and pn- Cu2O electrodes absorb photon with wavelength shorter than 640 nm, the absorbance increases in the range of photon wavelength from 300 nm to 560 nm. The band gaps of p-Cu2O and pn-Cu2O were calculated to be 1.85 – 1.90 eV (Figure 3.7b). Figure 3.9a shows that p-Cu2O has Vonset  +0.55 V (vs. RHE), pn- Cu2O has Vonset  +0,68 V. Thus, making pn homojunction has had positive effect, shifting the Vonset 0.13 V to the anodic side. The maximum photocurrent density jmax at 0 V vs. RHE if p-Cu2O is Figure 0.8. I – V (a) and I – t (b) characteristic curves of p-Cu2O and pn-Cu2O Figure 0.9. I – t curves of p- Cu2O and pn-Cu2O after two chopped - light cycles Figure 0.7. Absorption spectrum (a), band gaps (b) of p-Cu2O and pn-Cu2O 10 approximately 1.6 mA/cm 2 , 1.3 that of pn-Cu2O (1.25 mA/cm 2 ). However, Figure 3.9b shows that the maximum current density of p- Cu2O mostly contributed to the photoelectrochemical corrosion process. After the I – V measurement, at the first cycle of stability test, the maximum of the p-Cu2O electrode is jmax = 0.17 mA/cm 2 (meaning that 89.37% of p-Cu2O was corroded after the I – V measurement). Meanwhile, the jmax value of pn-Cu2O is 0.64 mA/cm 2 , corresponding to 51,2% corrosion. The measured results are indicated in Table 3.1 and Figure 3.9. The corrosion rate of p-Cu2O electron after 2 cycles of turning the light on – off (chopped – light) is determined from the ratio j’/j. Here, j and j’ are respectively steady current density in the 1st and 2 nd chopped – light cycles. Table 3.1 shows j’/j of p-Cu2O and pn- Cu2O are respectively 0.88 and 0.76. Therefore, the corrosion rate of p-Cu2O electrode is higher than that of pn-Cu2O. The p-Cu2O electrode has trap current density jtrap = 0 mA/cm 2 demonstrating that photogenerated carriers, after moving to the electrode's surface, will participate in the corrosion reaction. Conclusion: We have fabricated p-Cu2O electrode with p-Cu2O having cubic structure, film thickness of roughly 1.4 m by the electrochemical deposition method. Also by this method, a layer of Sample Vonset (V) jmax Current density after 2 cycles of chopped – light j180s ρ 180s (%) 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 pn-Cu2O 0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20 Table 0.1. The parameters of the I – V and I – t characteristic curves measurements of p-Cu2O and pn-Cu2O 11 n-Cu2O was deposited successfully on p-Cu2O to make pn homojunction. This method of synthesizing p-Cu2O and pn-Cu2O electrodes has high reproducibility. The p-Cu2O and pn-Cu2O films fabricated are single crystal which preferably orient on the (111) plane. The band gap of p-Cu2O and pn-Cu2O is in the range of 1.85 – 1.90 eV. The pn-Cu2O homojunction helps increase the Vonset of the electrode, the charge separation under illumination and thus, increases the electrode's stability. 3.2. TiO2 semiconductor layer 3.2.1. Micromorphology, structure of the TiO2 covering on p-Cu2O Figure 3.13 indicates the micromorphology of the X nm-TiO2/p-Cu2O films with different values of X. The crystal structure of the p-Cu2O and pn- Cu2O films coated with TiO2 are shown on the X-ray diffractogram (Figure 3.17). To increase the doping concentration and crystallinity of TiO2 and Cu2O, the samples 50 nm-TiO2/p-Cu2O and 50 nm- TiO2/pn-Cu2O were annealed at temperatures from 300 o C đến 450 o C in 30 minutes in the Ar Figure 0.13. SEM images of p-Cu2O coated with TiO2 at different thicknesses Figure 0.17. XRD patterns of Cu2O with a 50 nm TiO2 layer 12 environment. The micromorphology of the 50nm-TiO2/p- Cu2O samples with different annealing temperatures are shown in Figure 3.19. The crystal structures of the samples after being annealed at different temperatures are demonstrated in the X-ray diffractogram (Figure 3.20). 3.2.2. The effect of the thickness and annealing temperature of the TiO2 layer on the photo and photoelectrochemical properties of Cu2O electrode The photoelectrochemical characterization result of 50nm-TiO2/p- Cu2O and 50nm-TiO2/pn-Cu2O electrodes are shown in Figure 3.23 and Table 3.2. All the samples, after being coated with TiO2 and annealed at different temperatures, decrease the rate of photocorrosion on the electrode. The annealing process decrease the potential barrier between the 2 materials and the amount of Ti 3+ ions. Though increasing the annealing temperature helps increasing the maximum current density, the trap current density and the electrode corrosion rate also increase. We decided to anneal the X nm-TiO2/p- Cu2O samples at 350 o C to investigate the effect of the TiO2 layer thickness. Figure 0.19. SEM images of 50nm-TiO2/p- Cu2O annealed at different temperatures 13 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 Vonset (V) jmax Current density after 2 chopped – light cycles j180s ρ 180s (%) jmax jtrap j j’ j’/j p-Cu2O 0.55 1.60 0.27 0.00 0.27 0.10 0.37 0.04 1.25 50-p 0.55 1.05 0.28 0.05 0.23 0.12 0.52 0.02 7.15 50-p-300 o C 0.50 0.56 0.40 0.00 0.40 0.20 0.50 0.12 30.00 50-p-350 o C 0.58 0.84 0.88 0.37 0.51 0.51 1.00 0.28 34.10 50-p-400 o C 0.56 1.10 0.87 0.43 0.44 0.33 0.75 0.15 17.24 50-p-450 o C 0.57 1.30 1.30 0.50 0.80 0.53 0.66 0.27 20.77 pn-Cu2O 0.68 1.25 0.64 0.10 0.54 0.41 0.76 0.14 11.20 50-pn 0.70 1.21 1.12 0.40 0.72 0.42 0.58 0.12 10.72 50-pn-300 o C 0.50 0.80 0.82 0.24 0.58 0.50 0.86 0.15 18.29 50-pn-350 o C 0.53 0.75 1.06 0.29 0.77 0.70 0.91 0.13 12.27 50-pn-400 o C 0.55 0.86 1.30 0.80 0.50 0.50 1.00 1.18 90.80 50-pn-450 o C 0.55 1.16 1.36 0.40 0.96 0.55 0.57 0.23 16.91 The 50 nm- TiO2/pn-Cu2O sample annealed at 400 o C yields a maximum current density of 1.3 mA/cm 2 . After 2 chopped – light cycles, the photocurrent density was steady (j’/j = 1) and Figure 0.2. I – t curve of 50 nm-TiO2/p- Cu2O (a, b) and 50 nm-TiO2/pn-Cu2O (c, d) annealing at different temperature 14 after 3 minutes of the stability test, the current density only show 9.2% reduction. Therefore, we kept the annealing temperature at 400 o C 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 result, with TiO2 coated p-Cu2O, the optimized annealing temperature is 350 o C, the oprimized thickness is 50 nm. The 50 nm- TiO2/p-Cu2O- 350 o C electrode has the current density jmax at approximately 0.9 mA/cm 2 , which retains 34% after 180s of activity measurement. With TiO2 coated pn-Cu2O, the optimized annea