matters (H2 in Figure 14) on the surface of the ZnO will lead
to the introduction of free charge carriers to the nanowires,
the polarization screening effect will be modified and result
in the decrease of output voltage to the external circuit
from the nanogenerators [74, 75]. The investigation of such
active sensors may largely promote the research on selfpowered
sensor system and the development of modern
sensor networks.
3.4.3. For Driving Self-Powered Sensors. The output voltage
and current can be used not only as the active sensing
parameters in self-powered sensors, but also as the driving
source in several kinds of sensors, in which the input voltage
or current will be changed with the sensing parameters
of the sensors, especially the resistance-type semiconductor
sensors. For example, Xu and coworkers have integrated
the ZnO VINGs with a nanowire-based pH sensor and
UV sensor, respectively. The partial voltage on the sensor
part generated by the VING changed with the resistance
of the sensing nanowire both in different pH environment
and under the irradiation of the UV light [33]. Wu et al.
have also reported the self-powered UV sensor based on
nanogenerators with PZT textiles and nanowire, respectively
[47, 76].
The difference between the above-mentioned selfpowered
sensors and the nanogenerator-driven electronic
devices is that the former one did not need high electric
power density which is essential for the operation in
latter ones. By integrating both parts with the output
of nanogenerator as both the power source and sensing
parameter, the researchers have fabricated completed
self-powered sensor systems. For example, Lee et al. have
demonstrated a self-powered environment sensor system
driven by a nanogenerator [67]. The generated electrical
power can light up a LED indicator when the sensor part
based on single wall carbon nanotubes is exposed to high
concentration of mercury ions in water solution. Li and Xia
have integrated ZnO VINGs with both the sensor unit and
a wireless transmitter unit [77]. In this system, the signal
detected by a phototransistor can be transmitted wirelessly
by a single transistor RF transmitter under the driving of the
nanogenerator.
3.4.4. Hybrid Cells for Harvesting Multiple Energies. Under
the demand of long-term energy needs and sustainable
development, devices that can harvest multiple types of
energies have also been developed. For example, Hansen
and coauthors have integrated the piezoelectric nanogenerators
with the enzymatic biofuel cell which can harvest the
biochemical energy in biofluid. Both unit can be used for
harvesting the energy available in vivo and can work simultaneously
or individually for boosting output and lifetime [12].
Moreover, Xu and Wang have developed a fully integrated
solid-state compact hybrid cell that consisted of an organic
solid-state dye-sensitized solar cell (DSSC) and piezoelectric
nanogenerator in one compact structure for concurrently
harvesting both solar and mechanical energy using a single
device. In addition to enhancing the open-circuit voltage, the