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Protocol to modify the surface of nano-Cu2O using facet controlling and MOF shell coating

Summary The morphology of nano-Cu2O profoundly determines its catalytic performance. Here, we provide two universal and reliable techniques to modify the surface of nano-Cu2O. First, we detail steps for the systematic tuning of the exposed facets of nano-Cu2O ranging from low index to high index using reductant-controlled technique in the presence of sodium dodecyl sulfate. Second, we describe steps for facet-directed precipitation in which the morphology-dependent ZIF-8 (a type of zeolitic imidazolate frameworks) shells on different nano-Cu2O are well introduced. For complete details on the use and execution of this protocol, please refer to Luo et al. (2022).


SUMMARY
The morphology of nano-Cu 2 O profoundly determines its catalytic performance. Here, we provide two universal and reliable techniques to modify the surface of nano-Cu 2 O. First, we detail steps for the systematic tuning of the exposed facets of nano-Cu 2 O ranging from low index to high index using reductant-controlled technique in the presence of sodium dodecyl sulfate. Second, we describe steps for facet-directed precipitation in which the morphology-dependent ZIF-8 (a type of zeolitic imidazolate frameworks) shells on different nano-Cu 2 O are well introduced. For complete details on the use and execution of this protocol, please refer to Luo et al. (2022).

BEFORE YOU BEGIN
Materials with specific nanoscopic or microscopic morphologies have extensive perspective of application, especially in heterogeneous catalysis, optical devices, and batteries (Lai et al., 2019;Liu et al., 2022;Diehl et al., 2022;Joshi et al., 2021). As an outstanding semiconductor material, Cu 2 O has been widely investigated in photo or electrocatalysis, due to its relatively low cost, abundance of Cu on earth and promising selectivity of specific product, whose surface morphology will profoundly influence its catalytic performance (Ma et al., 2021;Wu et al., 2019;Liu et al., 2021;Gao et al., 2020). In the past research, Cu 2 O nanomaterials with low index facets such as {100}, {110} and {111}, have been well-studied, both in synthesis and applications (Huang et al., 2012;Aran-Ais et al., 2020;Hua et al., 2014). Further, systematical synthesis of nano-Cu 2 O from low index facets to high index facets, and universal surface coating approach with stable metal organic framework material were proposed in our previous work (Luo et al., 2022).
Herein, the protocol describes those two strategies to modify the surface of nano-Cu 2 O (Scheme 1). For process i, Cu 2 O nanoparticles with different morphologies are successfully synthesized through sonicating by tuning the usage of reductant in the presence sodium dodecyl sulfate (SDS), which is beneficial for the formation of concave (Wang et al., 2013). For process ii, facet-directed precipitation method is carried out to in-situ coat ZIF-8 (2-Methylimidazole zinc salt, a type of zeolitic imidazolate frameworks) shell on Cu 2 O nanoparticles under low oxygen condition. Before synthesis, the following preparations should be made.

Preparation of the reagents and equipment
A complete list of reagents and equipment can be found in the ''key resources table''.

MATERIALS AND EQUIPMENT
PLA models During the sonicated process, we build a 3D printed PLA model to prevent the beaker from moving in the ultrasonic cleaner (Details see Figure 1A).
Alternatives: Any tool that can prevent the beaker from moving casually during sonicating will do.

Ultrasonic cleaner
During the preparation of each stock solution, we choose degas-mode (150 W, 37 kHz) for sonicating the mixture to be fully dissolved. When it comes to the reaction time, 2.8 L water is recommended to be added into the tank, and sweep-mode (150 W, 37 kHz) is chosen to form even sound field distribution in the ultrasonic bath. The beakers should be put into the PLA Models to avoid moving during reaction process, and the temperature controller is just used to monitor the real-time temperature of reaction surroundings (Details see Figures 1B and 1C).
Note: Those two ultrasonic modes have the same power and frequency, but different sound field distributions.
Note: The 0.1 M CuCl 2 solution can be stored at 20 C for two weeks.
Note: The 2.0 M NaOH/0.5 mM SDS solution can be stored at 20 C for one day. CRITICAL: SDS has an irritating effect on mucous membranes and upper respiratory tract. Be sure to wear masks when using it.
Alternatives: In our experiments, we use beakers and storage vials for the synthesis of nano-Cu 2 O and Cu 2 O@ZIF-8 composites. However, alternative containers such as conical flasks with plug can also be used.
Optional: The resources listed above were only based on our experience. The chemicals and resources obtained from reliable commercial sources are feasible.

Synthesis of Nano-Cu 2 O
Timing: 14 h The initial step for the synthesis of nano-Cu 2 O (Process i in Scheme 1) is the SDS-stabilized Cu(OH) 2 synthesis. Subsequently, NH 2 OH$HCl act as both reductant and morphology control agent to modify the exposed facets of nano-Cu 2 O.
Note: We fabricated nano-Cu 2 O with six different morphologies by controlling the usage of NH 2 OH$HCl solution, according to which, the usage of distilled water should be adjusted. In short, the total solvent volume should be controlled at 100 mL.
3. Sonicate for 10 min until the SDS is fully dissolved. 4. Add 5 mL 0.1 M CuCl 2 solution into above SDS solution and sonicate for another 10 min to form homogenous solution. 5. Add 2 mL 2.0 M NaOH/0.5 mM SDS solution into CuCl 2 /SDS solution dropwise (1 drop every 2 s) with shaking the beaker (see Methods video S1). 6. Sonicate for another 10 min to form blue SDS-stabilized Cu(OH) 2 flocs at 25 C-40 C.
Note: The subsequent reductant-controlled experiment is operated following this step.
CRITICAL: The NaOH/SDS solution is strong alkaline and corrosive. Avoid dripping on hands! 7. Add 0.25 mL 1.0 M NH 2 OH$HCl solution dropwise with shaking the beaker (see Methods video S2), and the flocs will turn from blue to green.
Note: We modified the exposed facets of nano-Cu 2 O by adjusting the usage of NH 2 OH$HCl. In this step, 0.25 mL 1.0 M NH 2 OH$HCl will help in forming cubic Cu 2 O nanoparticles ( Figures  2A1, 2B1 and 2C1). By increasing the usage of 1.0 M NH 2 OH$HCl solution to 0.50 mL, 0.75 mL, 1.00 mL, 1.25 mL and 2.00 mL, the morphology of nano-Cu 2 O will turn to cornercut cube, corner-cut octahedron, truncated octahedron, octahedron, and concave octahedron, respectively (see Figures A2-A6, B2-B6 and C2-C6). 8. Sonicate for 1 h at 30 C-40 C. The green flocs will disappear forming uniform dispersion, and the color will turn from green to orange or brown gradually (see Methods video S3).
Note: No more than 3 experimental groups that are carried out simultaneously. During the sonicating process, ice cubes can be added into ultrasonic cleaner tank to avoid overheating.

Timing: 6 h
After the successful preparation of nano-Cu 2 O with six morphologies, scanning electron microscopy (SEM) and transmission electron microscope (TEM) characterizations are adopted to evaluate the surface conformation of nano-Cu 2 O.
13. Disperse 1 mg Cu 2 O nanoparticles into 1 mL ethanol and sonicate 5 min to form homogeneous dispersion. 14. Drop 10 mL nano-Cu 2 O dispersion on the polysilicon wafer, then transfer the polysilicon wafer to oven, vacuum drying for 2 h at 65 C for further SEM characterization.
Note: SEM characterization, TEM sample preparation and TEM characterization are operated by professional engineer.

Synthesis of Cu 2 O@ZIF-8
Timing: 14 h In this process, PVP (M w = 10,000) is used as an armor to protect nano-Cu 2 O from being etched, and as a surfactant to help ZIF-8 crystal in-situ coating on nano-Cu 2 O to form Cu 2 O@ZIF-8 core-shell composites. All of the six morphological Cu 2 O nanoparticles can be coated with ZIF-8 shell through this method (Figures 3 and 4).
15. Weigh 5 mg Cu 2 O nanoparticles, 200 mg PVP (M w = 10,000) to a 20 mL vial. Add 1.0 mL methanol and sonicate for 10 min with degas mode until all the mixture dissolve. 16. Pump N 2 to Cu 2 O/PVP dispersion for 10 min.
Note: Pumping N 2 into the dispersion through a PTFE hose with the gas flow rate being controlled to 20 sccm by a Mass flow controller ( Figure 5). Notably, N 2 is just used to remove . Shake storage vial with the lid on for 10 s, and then set quietly for 1 h at around 25 C. 20. After 1 h, the supernatant should be colorless transparent. 21. Pour off the supernatant. Centrifuge at 697.6 RCF (3 g)for 3 min to collect the precipitates and wash with methanol (50 mL 3 3). 22. Vacuum drying for 12 h at 65 C in oven.

Characterization of Cu 2 O@ZIF-8
Timing: 6 h After the successful preparation of six types of Cu 2 O@ZIF-8 composites, scanning electron microscopy (SEM), transmission electron microscope (TEM) and elemental mapping are carried to characterize the morphology of Cu 2 O@ZIF-8 (Figures 3 and 4).
23. Disperse 1 mg Cu 2 O@ZIF-8 composites into 1 mL ethanol and sonicate 5 min to form homogeneous dispersion. 24. Drop 10 mL Cu 2 O@ZIF-8 dispersion on the polysilicon wafer, then transfer the polysilicon wafer to oven, vacuum drying for 2 h at 65 C for further SEM characterization.

EXPECTED OUTCOMES
This protocol provides systematic strategies for modifying the surface conformation of nano-Cu 2 O with reductant-controlled and facet-directed precipitation methods. By adjusting the usage of reductant, Cu 2 O nanoparticles with different exposed facets ranging from low index to high index were synthesized, and with the molar ratio of the usage of NH 2 OH$HCl to CuCl 2 increasing to 4:1, nano-Cu 2 O with high index facets {332} (a ''terraces and steps'' structure) (Wang et al., 2013) was fabricated ( Figure 2B). Furthermore, the ZIF-8 shell was in-situ introduced on each nano-Cu 2 O under stationary condition with PVP as both protector and surfactant, and the contour profiles of ZIF-8 shell were highly dependent on the morphologies of nano-Cu 2 O.

LIMITATIONS
For the reductant-controlled methods to modify, only nano-Cu 2 O was chosen as an example to construct different exposed facets, whether it is suitable for other metal oxide or not, is still requiring further research. The slow growth rate of alkaline metal oxides in weak acid reduction surroundings, might contribute to form high index facets (Ng and Fan, 2006). Besides, ZIF-8 has high crystallinity, high stability and importantly, mild synthesis condition, in which Cu 2 O can be stable for a long time.
In this case, MOFs prepared under high temperature and strong acid conditions may not be suitable for this method.

Problem 2
After adding NH 2 OH$HCl, the color of the dispersion changes lightly, and when sonication reaction is performed, precipitates can be seen obviously which can't be dispersed (steps 7 and 8 in ''synthesis of Nano-Cu 2 O'').

Potential solution
Prepare new 1.0 M NH 2 OH$HCl solution or purchase new NH 2 OH$HCl reagent. Additionally, the color of the synthesized nano-Cu 2 O distinguish with each other, ranging from orange to dark brown in the sort from A-to F-Cu 2 O (Figure 7).  Problem 3 The as-prepared F-Cu 2 O nanoparticles are not in concave octahedral shape.

Potential solution
In step 7 in ''synthesis of Nano-Cu 2 O'', the dropping speed of 1.0 M NH 2 OH$HCl solution is significant. Try to ensure that the last drop is fully dispersed by shaking before adding the next drop.

Potential solution
Keep the reaction temperature lower than 25 C, and saturate stock solutions with N 2 .

Potential solution
Remove O 2 gas in Zn(NO 3 ) 2 and C 4 H 6 N 2 methanol solutions with N 2 pumping for at least 20 min before use.

RESOURCE AVAILABILITY
Lead contact Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Jian-Gong Ma (mvbasten@nankai.edu.cn).

Materials availability
All reagents generated in this study are available from the lead contact.

Data and code availability
The published article includes all datasets/code generated or analyzed during this study. Original data have been deposited to Mendeley Data: https://onlinelibrary.wiley.com/doi/10.1002/anie. 202116736.