Low-dimensional surface science has recently experienced an epoch-scale change in paradigm with the isolation of graphene, a single layer with 100% exposed surface atoms that exhibits exceptional physical properties with wide applications in cata- lysts,[1] chemical sensors,[2] and optoelectronic devices.[3] Other inorganic analogues of graphene have also been  discovered and their high-percentage of surface atoms has brought forth   a wealth of innovative applications,[4] demonstrating the key role of surface atoms in unlocking the properties of functional
 materials. Just as the graphene challenges the 2D limit of one-atom-thickness, the all- surface-atomic materials are approaching the ultimate upper limit of specific surface area. This could reasonably trigger break- throughs in manipulating the  physical and chemical properties of many func- tional materials and effectively provide new applications in solving the grand global problems of energy and environ- ment. However, these species with all sur- face atoms are still only limited to a few materials, i.e., the insulating monolayer hexagonal boron nitride,[5] along with the conducting graphene and single-walled carbon nanotubes.[2] Despite recent pro- gress in fabricating ultrathin 2D semi- conductor nanosheets,[4] their thickness with more than two atomic layers impedes their atoms to be completely exposed on the surfaces. Particularly, recent study reports the synthesis of severely agglomer-
ated SnSe nanosheets with four atomic thickness of 1 nm,[6]
but their X-ray diffraction (XRD) pattern resembles that  of bulk form, which is, in fact, in  contrast  to  the  characteris- tics of single-layers due to the lack of long-range order in the third dimensionality for ultrathin nanosheets. In addition, the stubborn surface non-volatile  organic  solvents  along  with the severe agglomeration unfortunately cause difficulty in pro- cessing them into devices. Therefore, freestanding all-surface- atomic semiconductor materials with clean surfaces remain almost entirely unexplored.
Generally, the light-harvesting semiconductors can mimic
 
   the photosynthetic process within a leaf by splitting water to
 
Dr. Y. F. Sun, S. Gao, F. C. Lei, Prof. Y. Xie
Hefei National Laboratory for Physical Sciences at Microscale
University of Science and Technology of China Hefei, 230026, P. R. China
E-mail: yxie@ustc.edu.cn
Dr. Z. H. Sun, H. Cheng, Dr. Q. H. Liu, Prof. S. Q. Wei National Synchrotron Radiation Laboratory
University of Science and Technology of China Hefei, Anhui 230029, P. R. China
E-mail: sqwei@ustc.edu.cn
DOI: 10.1002/aenm.201300611
 
produce H2 fuels,[7] which makes it possible to provide a clean and efficient pathway to overcome the limited supply of fossil fuels and the greenhouse effect. Unfortunately, the low effi- ciency and poor stability in silicon and other semiconductor thin films seriously impede their practical applications.[8] Our recent studies show that the atomically thin graphene-like sheets contribute to improve the water splitting efficiencies becuase of their atomic thickness and large surface area;[4b,d] however, their efficiencies are retained at relatively low values, less than 43%, which could be mainly attributed to the fact that only surface atoms are involved in the following water-splitting