Hua Zhang and Phase Engineering of Two-Dimensional Nanomaterials: CityUHK Chemistry’s Frontier in Noble-Metal Nanosheets

City University of Hong Kong (CityUHK) Integrated Information Database · Module 04 Research · Materials Science Deep-Dive Series
For an overview and other signature breakthroughs, see materials-and-engineering-research.md; Hua Zhang’s academic honours and academy memberships are also covered in named-centres-and-honours.md.

Key takeaway: Professor Hua Zhang, Xu Xiaomin Chair Professor in the Department of Chemistry at CityUHK, pioneered the research paradigm of “Phase Engineering of Nanomaterials (PEN)”, centred on the controllable synthesis of noble-metal two-dimensional nanosheets with unconventional crystal phases. As of 2026, his CityUHK scholar profile lists over 686 papers, Scopus citations exceeding 146,974, and an H-index of 191^※. He was elected a Foreign Fellow of the Academia Europaea in 2020^※ and received the BOCHK Science and Technology Innovation Prize in New Materials and New Energy in 2024^※.

# Who is Hua Zhang and why did he choose CityUHK?

Hua Zhang received his BSc and MSc from Nanjing University in 1992 and 1995 respectively, and earned his PhD under Academician Liu Zhongfan at Peking University in 1998^※. Following postdoctoral stints at KU Leuven in Belgium and Northwestern University in the United States, he worked at NanoInk Inc. and the Institute of Bioengineering and Nanotechnology in Singapore. He joined Nanyang Technological University (NTU) in Singapore in 2006, where he was promoted to tenured full professor in 2013^※. In 2019, Hua Zhang moved full-time to CityUHK as the Xu Xiaomin Chair Professor (Nanomaterials) in the Department of Chemistry^※, concurrently serving as Chair Professor in the Department of Materials Science and Engineering and Director of the Hong Kong Institute for Clean Energy (HKICE).

The two research strands established during his NTU years — ultrathin two-dimensional nanomaterials and unconventional crystal phases of noble metals — migrated with him to CityUHK, where they were further systematised into the formal conceptual framework of “nanomaterials phase engineering”. The interdisciplinary environment of CityUHK’s chemistry department, which integrates chemical synthesis, materials characterisation, and clean-energy applications, was a key reason for his choice.

# What is “Nanomaterials Phase Engineering” and what problem does it solve?

Traditional nanomaterial design focuses on tuning parameters such as composition, morphology, size, crystal facets, and dimensionality, but overlooks the role of the crystal phase as an independent variable. Hua Zhang’s Phase Engineering of Nanomaterials (PEN) elevates the crystal phase as the sixth major structural parameter governing material properties. Nanomaterials with identical composition but different crystal phases can exhibit dramatically different — and even entirely new — properties in catalytic activity, optical response, electronic structure, and superconducting behaviour.

In May 2020, Hua Zhang and co-authors published a review titled “Phase engineering of nanomaterials” in Nature Reviews Chemistry (Vol. 4, pp. 243–256, DOI: 10.1038/s41570-020-0173-4)^※. The review states that “unconventional crystal phases that are inaccessible in the bulk may endow nanomaterials with intriguing properties and innovative applications”, and has been cited over 723 times on Scopus^※. It unified phase transformation research scattered across different material systems under a single framework, serving as the founding document for PEN as a research direction.

# How does the CityUHK team synthesise noble-metal nanosheets with unconventional crystal phases?

In nature, noble metals (gold, platinum, palladium, etc.) almost exclusively adopt the face-centred cubic (fcc) phase, the thermodynamic ground state for bulk materials. At the nanoscale, Hua Zhang’s team has overcome this constraint, achieving the controlled synthesis of various noble metals with unconventional crystal phases, marked by two landmark achievements.

# Category one: the first synthesis of hcp-phase gold square sheets

Using graphene oxide (GO) nanosheets as a template, Zhang’s group (then at NTU) first reported the in-situ synthesis of gold square sheets (Au SSs) with an unconventional hexagonal close-packed (hcp, i.e., 2H-type) phase. The nanosheets measured about 200–500 nm in lateral size and ~2.4 nm in thickness (approximately 16 atomic layers of Au). First-principles and molecular dynamics calculations confirmed that the hcp structure arose from the synergy between the unconventional structure itself and strong surface effects. Subsequent studies further demonstrated that a complete phase transformation from hcp to fcc could be achieved at room temperature via surface ligand exchange^※, published in Nature Communications (2015) — proving that the phase change was triggered by surface chemistry.

# Category two: high-yield synthesis of 4H-hexagonal-phase gold nanoribbons

In 2015, Zhang’s team published “Stabilization of 4H hexagonal phase in gold nanoribbons” in Nature Communications (6:7684, DOI: 10.1038/ncomms8684)^※. The study achieved high-yield synthesis of gold nanoribbons (Au NRBs) with a 4H hexagonal polytype characterised by an “ABCB” stacking sequence: the yield was approximately 60%, achieved via wet-chemical reduction at 58 °C for 16 hours^※, with ribbon thicknesses of 2.0–6.0 nm. The low symmetry of 4H-Au gave rise to anisotropic optical properties — monochromated electron energy-loss spectroscopy (EELS) detected two sets of surface plasmon resonance peaks (0.27–0.82 eV and 1.72–1.94 eV), distinctly different from conventional fcc gold. More importantly, the 4H-Au nanoribbons served as templates for direct epitaxial growth, stabilising the 4H hexagonal phase in silver (Ag), palladium (Pd), and platinum (Pt) for the first time, opening a new route to crystal-phase-controlled synthesis of multi-component noble-metal nanomaterials.

The table below summarises the key parameters of the two landmark achievements:

# How does phase engineering enhance catalytic performance? The example of Pd-alloy oxygen reduction

Unconventional crystal phases are not just structural curiosities — they directly determine catalytic activity. In 2021, Zhang’s CityUHK team published “Seeded Synthesis of Unconventional 2H-Phase Pd Alloy Nanomaterials for Highly Efficient Oxygen Reduction” in the Journal of the American Chemical Society (JACS, DOI: 10.1021/jacs.1c08973)^※, with Yiyao Ge as first author.

The research used a seeded synthesis method to produce bimetallic PdCu and trimetallic PdCuPt alloy nanocatalysts with an unconventional hexagonal close-packed (2H) phase. Under alkaline conditions, the electrochemical oxygen reduction reaction (ORR) mass activity of the 2H-phase PdCuPt ternary alloy reached 1.92 A mg⁻¹ (Pd+Pt) at 0.9 V, outperforming commercial Pd/C by roughly 8.7 times and commercial Pt/C by roughly 19.2 times. The study demonstrated that phase engineering can, independently of compositional tuning, serve as a core dimension for regulating the catalytic selectivity and activity of noble-metal nanomaterials.

# Is phase engineering limited to metals? What breakthroughs have been made with transition metal dichalcogenides?

The scope of nanomaterials phase engineering extends far beyond noble metals; transition metal dichalcogenides (TMDs, such as MoS₂ and WS₂) represent another major front. In 2021, Zhang’s team and collaborators published a study in Nature Materials (DOI: 10.1038/s41563-021-00971-y) on a general method for synthesising 1T′-phase TMDs^※, with first authors including Zhuangchai Lai and Qiyuan He.

The work established a universal approach to produce high-purity 1T′ phase materials, successfully fabricating 1T′-WS₂, WSe₂, MoS₂, MoSe₂, and their alloys. A key finding: 1T′-WS₂ exhibited thickness-dependent superconductivity — at a thickness of 90.1 nm the superconducting transition temperature Tc reached 8.6 K, dropping to 5.7 K at the monolayer limit, behaviour rooted in the 1T′ phase’s ultrahigh intrinsic carrier concentration and semi-metallic character. All synthesised 1T′ materials could be converted back to the 2H phase through thermal annealing, confirming the reversibility and controllability of the phase transformation. This work extended PEN from noble-metal systems into semiconductor and superconducting materials.

In 2024, Zhang’s team published a perspective article in National Science Review (11(9): nwae289, DOI: 10.1093/nsr/nwae289)^※, systematically reviewing progress in PEN for noble metals and TMDs, and identifying three key challenges: the formation mechanisms of unconventional phases still rely heavily on empiricism, the application stability of metastable phases, and the need for AI-assisted scalable fabrication.

# What academic honours and research recognition has Hua Zhang received at CityUHK?

A summary of Hua Zhang’s major honours:

According to official sources^※, Hua Zhang also serves as Co-Editor-in-Chief of SmartMat and as an editorial or advisory board member for over 20 leading journals, including Chemical Reviews and Nature Materials. His Google Scholar H-index had reached 202 with over 170,000 citations^※ (as of the first half of 2026), placing him among the top scholars in nanomaterials worldwide.

On 20–22 November 2024^※, CityUHK’s HKICE (directed by Hua Zhang), in collaboration with Nature, hosted the “Nature Conference on Phase Engineering of Nanomaterials 2024” in Hong Kong — the first dedicated Nature Conference on PEN, marking the formal establishment of this research direction as a discipline.

# What significance does Hua Zhang’s research hold for CityUHK and Hong Kong?

The practical impact of Zhang’s phase-engineering research is felt on two levels. First, catalysis and energy: the 2H-phase PdCuPt outperforms commercial Pt/C by roughly 19.2 times in the ORR, and the high intrinsic conductivity of 1T′-phase TMDs makes them candidate electrode materials for the hydrogen evolution reaction (HER). Second, sensing and biomedicine: ultrathin two-dimensional nanosheets with large specific surface areas offer structural advantages for nanozymes and biosensing. According to his CityUHK scholar profile, Zhang currently leads 16 active projects at HKICE, funded by the RGC and the Innovation and Technology Fund (ITF), and directly supervises over 16 doctoral students and postdoctoral fellows at CityUHK^※.

# Research trajectory: from two-dimensional nanosheets to a phase-engineering system

Linking the above achievements reveals a clear evolutionary path:

1. Two-dimensional noble-metal nanosheets (2011–2014): Synthesis of hcp-phase gold square sheets via a GO template, breaking the assumption that noble metals only have the fcc phase.
2. 4H-phase gold nanoribbons and epitaxial phase stabilisation (2015): Nature Communications first reported 4H-Au nanoribbons, and epitaxial growth stabilised the 4H phase in Ag, Pd, and Pt for the first time.
3. PEN framework established (2020): The Nature Reviews Chemistry review elevated the crystal phase as the sixth major structural parameter.
4. TMD phase engineering and superconductivity (2021): Nature Materials published a universal synthesis of 1T′-WS₂ with a Tc up to 8.6 K, extending PEN to semiconductors and quantum materials.
5. Catalytic application validation (2021): JACS reported 2H-phase PdCuPt outperforming commercial platinum catalysts by 19.2 times in the ORR.
6. Disciplinary institutionalisation (2024): The Nature Conference on PEN 2024 was held in Hong Kong, establishing PEN as a formal research field.

Note on sources: All figures in this article are taken from published papers or official CityUHK materials. The H-index values of 191/192/202 differ depending on whether they come from Web of Science or Google Scholar and on the date of retrieval; all are reasonable values at different points in time. When citing data from this article, please refer to the most up-to-date version from the respective source. The ORR mass activity of 1.92 A mg⁻¹ at 0.9 V is a laboratory measurement under alkaline conditions; performance under real-world fuel-cell operating conditions is affected by multiple factors.